A TEXT-BOOK OF Volumetric Analysis WITH SPECIAL REFERENCE TO THE VOLUMETRIC PROCESSES OF THE PHARMACOPCE1A OF THE UNITED STATES. Designed for the Use of Pharmacists and Pharmaceutical Students. BY HENRY W. SCHIMPF, Ph.G., M.D., Professor of Inorganic Chemistry in the Brooklyn College of Pharmacy; Member of the American Chemical Society: of the American Association for the Advancement of Science: of the American Pharmaceutical Association • of the New York State Pharmaceutical Association; of the Kings County Pharmaceutical Society: of the German Apothecaries’ Society of New York City; Honorary Member of the Alumni Association of the Brooklyn College of Pharmacy, etc., etc. ®®itf) ffHts-nint Illustrations. THIRD EDITION, REVISED AND EN^JrsJSA FIRST THOUSAND. NEW YORK: john wiley & Sons. London; CHAPMAN & HAEL; L.TKnrET>r 1898. Copyright, 1898, BY H. W. SCHIMPF. ROBERT DRUMMOND, ELECTROTYPER AND PRINTER, NEW YORK. PREFACE TO THE THIRD EDITION. In submitting this new edition to the profession, the author hopes that the same generous reception will be given it as was accorded its predecessor. The main features of the latter have been retained in the present edition and much new matter added, which is intended to increase the usefulness of the book for the practical pharmacist and for the student. The present edition is divided into four parts. Part I is a systematic arrangement of volumetric processes and includes the pharmacopoeial methods for inorganic substances. This part has been revised and enlarged. In the chapter on apparatus used in volumetric anal- ysis some new forms of apparatus are described and illustrated. The chapter on the use of apparatus has been enlarged, several new cuts introduced, and methods for the calibration of graduated instruments described. There is also some additional matter concerning weights and measures. In the sixth chapter some unusual volumetric meth- ods are described, and under alkalimetry several meth- PREFACE TO THE FIRST EDITION. This book is designed for the use of pharmacists, and especially as a text-book for students in pharmacy. In the first portion of the book the author has at- tempted, in explaining the principles of volumetric analysis, to combine thoroughness with simplicity of expression. The United States Pharmacopoeia has been taken as the basis of the work, and the volumetric processes therein given are followed throughout, each step being carefully explained, and chemical equations inserted, wherever deemed necessary. The author has also added descriptions of processes not given in the Pharmacopoeia, but which are worthy of consideration. In teaching volumetric analysis to students in phar- macy the author discovered the necessity for a work especially designed for this class of students. Moreover, the requirements of the new edition of the United States Pharmacopoeia, in which many volumet- ric processes are given, necessitate on the part of the careful pharmacist a knowledge of this branch of ana- lytical chemistry; and no work that has as yet fallen into the hands of the author has seemed to be exactly suited to the needs of the practical pharmacist. Con- sequently the necessity for a book based upon the Pharmacopoeia and free from technicality is apparent. PREFACE. The latter portion of the book is devoted to descrip- tions of such special analytical processes as the phar- macist may be called upon to use, and such as are taught in the pharmaceutical colleges. The author has selected such processes as can be easily and quickly executed, and has given the gravi- metric only where volumetric processes cannot be employed. In the subject-matter of the book little originality is claimed, but the author has used his own judgment in its selection and arrangement. He has endeavored in the text to give credit wher- ever it was due, and especially acknowledges his indebt- edness to the United States Pharmacopoeia; Sutton’s Volumetric Analysis ; Bartley’s, Simon’s, and Attfield’s text-books ; Blythe’s Food Analysis ; Prescott’s Organic Analysis; Muter’s Analytical Chemistry (American edition) ; Lefmann and Beam’s Milk and Water Analy- sis ; and Witthaus’ and Holland’s Urine Analysis. He wishes to express his thanks to Dr, J, F. Gold- ing for the valued assistance he has rendered during the preparation of the book. He is also indebted to Richards & Co., of 41 Barclay Street, N. Y. City, manufacturers of chemical apparatus, from whom sev- eral of the cuts were borrowed. The author submits this work to the consideration of pharmacists, trusting its reception will be at least commensurate with the labor expended in its prepa- ration. Henry W. Schimpf. 365 Franklin Ave., Brooklyn, N. Y. TABLE OF CONTENTS. Table of the Elements and their Atomic Weights . xxviii Abbreviations and Signs xxix PAGE PART I. CHAPTER I. Introduction I Quantitative Analysis I The Gravimetric Method ........ I The Volumetric Method .2 CHAPTER II. General Principles .... 4 CHAPTER III. Apparatus used in Volumetric Analysis . . 7 The Burette 7 Mohr’s Burette 7 Glass-cock Burette ......... 8 Oblique-cock Burette ......... 9 Mohr’s Foot Burette with Rubber Ball. ..... 9 Swan’s-neck Burette . .... 0 ... 9 Gay-Lussac’s Burette . ........ 9 X TABLE OF CONTENTS. PAGE Sink’s Burette .......... 9 Foot Burette with Spindle Spout 9 Casamajor’s Burette ......... 10 Geissler’s Foot Burette . . , . . . . .11 Burette attached to Reservoir 11 Automatic Burette ......... 13 Pinch-cocks ........... 13 Burrette Supports 13 Clamps ............ 14 Pipettes 14 Litre Flasks 17 Graduated Cylinder 18 CHAPTER IV. Use of Apparatus .... 19 Cleaning the Instruments ........ 19 Filling the Burette ......... 19 Reading the Instruments ........ 22 Half-blackened Card ......... 22 Erdman’s Float .......... 23 “ “ with Projecting Points . . . . .24 Burette with Dark Longitudinal Stripe . . . . .25 “ “ White Enamelled Sides ... . . 25 Calibration of Instruments 26 CHAPTER V. Weights and Measures used in Volumetric Analysis . 29 Graduation of Instruments 29 CHAPTER VI. Some Unusual Volumetric Methods . . 31 CHAPTER VII. Standard and Normal Solutions . . .34 Normal Solutions 34 TABLE OF CONTENTS. XI PAGE Standard Solutions 35 “Standardized,” “Set,” or “Titrated” Solutions . . . 35. Decinormal Solutions ......... 35 Centinormal “ ....... j . 35 Semi-normal “ ......... 35 Double-normal “ ......... 35 Empirical “ 35 To “Titrate” .......... 39 Residual Titration 39 CHAPTER VIII. Indicators. Indicator defined .......... 42 Litmus Tincture .......... 43 Phenolphthalein T. S. . . . . . . . . .43 Methyl-Orange T. S. . . ' . . . . . -43 Rosolic Acid T. S. ......... 43 Turmeric T. S. . . . . . . . . *43 Cochineal T. S, . . . . . . . . .43 Eosin T. S. . . . . . . . . . , .44 Brazil-wood T. S. . . . . . . . . . .44 Fluorescein T. S. . . . • . . . . . .44 Potassium Chromate T. S 44 Potassium Ferricyanide T. S 44 Calculating Results . . . .46 Rules for finding Percentage 46 Factors or Coefficients 48 Table of Approximate Normal Factors for Alkalies and Acids . 50 CHAPTER IX. Analyses by Neutralization . . .51 Alkalimetry ........... 53 Preparation of Standard Oxalic-acid Solutions . . . -54 Preparation of Standard Sulphuric- and Hydrochloric-acid Solutions 55, 56 CHAPTER X. XII TABLE OF CONTENTS. PAGE Estimation of Alkaline Hydroxides ...... 58 Potassa ........... 58 Liquor Potassa .......... 60 Soda ............ 61 Liquor Soda . . . . . . . . . . 61 Aqua Ammonia .......... 62 “ “ Fortior ........ 63 Spirit of Ammonia ......... 63 Estimation of Alkaline Carbonates ...... 63 Potassium Carbonate ......... 64 Potassium Bicarbonate ......... 65 Sodium Carbonate ......... 65 Sodium Bicarbonate ......... 66 Ammonium Carbonate ......... 67 Lithium Carbonate ......... 67 Borax 70 Estimation of Mixed Alkaline Hydroxides and Carbonates . . 70 “ “ Alkaline Bicarbonates mixed with Carbonates . 72 “ “ Alkalies in the Presence of Sulphites . . .73 “ “ Mixed Potassium and Sodium Hydroxides . . 73 Estimation of Organic Salts of the A Ikalies ..... 73 Potassium Tartrate ......... 74 Potassium and Sodium Tartrate ....... 76 Potassium Bitartrate ......... 77 Lithium Citrate 78 Potassium Citrate 79 Potassium Acetate 80 Sodium Acetate . . . . . . . . . .81 Lithium Benzoate ......... 82 Sodium Benzoate .......... 83 Lithium Salicylate ......... 85 Sodium Salicylate ......... 86 Table showing Normal Factors for the Organic Salts of the Alkalies ..... .... 87 Acidimetry . . . . . . . . . . .87 Special Vessels for Preserving Alkali Solutions . . . .87 Preparation of Normal Alkali Solutions ..... 88 Acetic Acid ........... 92 “ “ Diluted ......... 93 “ “ Glacial ......... 93 Citric Acid - M TABLE OF CONTENTS. PAGE Oxalic Acid 94 Hydrobromic Acid ......... 94 Hydrochloric Acid ......... 95 Hypophosphorous Acid 96 Lactic Acid 97 Nitric Acid ........... 98 “ “ Diluted 99 Phosphoric Acid .......... 99 Sulphuric Acid .......... 101 “ “ Aromatic ........ 102 “ “ Diluted X02 Tartaric Acid .......... 102 Table showing Normal Factors, etc., for the Acids . . . 103 Estimation of Acids in Combination in Neutral Salts . . . 103 Estimation of Alkaline Earths ....... 104 Preparation of Normal Sodium Carbonate V. S. . . . . 105 Liquor Calcis .......... 106 Calcium Carbonate ......... 107 “ Bromide .......... xo8 Calcium Chloride ......... 109 Barium Chloride .......... 109 “ Nitrate 109 Strontium Lactate no CHAPTER XL Analysis by Precipitation . . .112 Estimation of Haloid Salts . . . . . . . • IX3 Preparation of Decinormal Silver Nitrate V. S. . . . .113 Ammonium Bromide IT5 Lithium Bromide XI7 Potassium Bromide . . . • . • • • • ri7 Sodium Bromide IJ8 Strontium Bromide IT9 Calcium Bromide ri9 Zinc Bromide 120 Potassium Iodide 121 “ “ Personne’s Method . . ° « . .122 Sodium Iodide I23 Strontium Iodide . I24 XIV TABLE OF CONTENTS. Zinc Iodide ........... 124 Ammonium Chloride ......... 125 Potassium Chloride ......... 125 Sodium Chloride .......... 126 Zinc Chloride .......... 126 Syrup of Hydriodic Acid ........ 127 “ “ Ferrous Iodide ........ 128 Preparation and Use of Standard Potassium Sulphocyanate V. S. (Volhard’s Solution) ........ 129 Saccharated Ferrous Iodide ........ 132 Syrup of Ferrous Bromide ........ 133 Hydrocyanic Acid ......... 133 Potassium Cyanide . . . . . . . . .136 Silver Nitrate 137 “ “ Fused 139 “ “ Diluted 139 “ Oxide . . . . . . . . . .139 Liquor Plumbi Subacetatis ........ 140 Table showing Factors of Substances estimated by Precipitation, 142 PAGE CHAPTER XII. Oxidimetry 143 Estimation of Ferrous Salts . . . . . . . .144 Preparation of Standard Solution of 2KMnOi and . 145 Estimation of Ferrous Salts by ..... 152 Saccharated Ferrous Carbonate . . . . . . .154 Ferrous Sulphate ........ 156 Estimation of Ferrous Salts by iKMnOt . . . . .158 Ferrum Reductum ......... 160 Ferrous Sulphate .......... 162 Estimation of other Oxidizable Substances ..... 163 Hypophosphorous Acid ........ 163 Calcium Hypophosphite ........ 165 Ferric Hypophosphite ......... 166 Potassium Hypophosphite ........ 167 Sodium Hypophosphite ........ 168 Hydrogen Peroxide 169 Barium Dioxide . . . . . . . . . .174 Oxalic Acid . . . . . . . . , . .175 Table of Substances which may be Estimated by Oxidation . 177 TABLE OF CONTENTS. XV CHAPTER XIII. PAGE Analysis by Indirect Oxidation . . .178 Preparation of Standard Solution of Iodine . . . . . 179 Arsenous Acid .......... 180 Liquor Acidi Arsenosi, U. S. P. . . . . . . . 181 Liquor Potassa Arsenitis, U. S. P. . . . . . . 182 Sulphurous Acid .......... 182 Sodium Sulphite .......... 183 Potassium Sulphite ......... 184 Sodium Bisulphite ......... 185 Sodium Thiosulphate ......... 185 Antimony and Potassium Tartrate ...... 186 Table of Substances which may be Estimated by Iodine . . . 188 CHAPTER XIV. Estimation of Substances Readily Reduced . 189 Preparation of Standard Solution of Sodium Thiosulphate . . 190 Estimation of Free Iodine ........ 192 Liquor lodi Compositus ........ 193 Tincture of Iodine ....... . , 194 Aqua Chlori ......... 194 Calx Chlorata .......... 195 The Arsenous-acid Process ........ 197 N Preparation of—Arsenous-acid Solution ..... 197 10 Liquor Sodae Chloratse ........ 199 Estimation of Ferric Salts ........ 201 Ferric Chloride .......... 202 Liquor and Tinctura Ferri Chloridi ...... 203 Ferric Citrate 204 Liq. Ferri Citratis ......... 205 Ferri et Ammonii Citras ........ 206 “ “ Potassii Tartras . ....... 206 “ “ Ammonii Tartras ........ 206 Ferri Phosphas ......... 206 Ferri et Quininae Citras ........ 207 Ferri et Strychninae Citras ........ 209 TABLE OF CONTENTS. XVI Ferri et Ammonii Sulphas . Ferri Pyrophosphas ......... 212 Ferri Valerianas .......... 213 Liq. Ferri Acetatis ......... 214 “ “ Nilratis ......... 215 “ “ Subsulphatis ........ 216 “ “ Tersulphatis . . . . . . . .217 Hydrogen Peroxide, estimation of, by Kingzett’s Method . . 218 N Table of Substances estimated by — Sodium Thiosulphate V. S. 219 10 PAGE PART II. CHAPTER XV. Acetic Acid and Acetates . . . 220 Vinegar ........... 220 “ Free Mineral Acids in ...... 221 “ Mohr’s Method ........ 222 “ Pettenkofer’s Method ....... 223 Alkaline Acetates ......... 223 Other Metallic Acetates 223 CHAPTER XVI. Boric Acid and Borates .... 225 Free Boric Acid, Will’s Method ....... 225 “ “ “ Thompson’s Method ..... 225 Boric Acid in Borax ......... 225 “ “ “ “ Smith’s Method ...... 226 CHAPTER XVII. Carbonic Acid and Carbonates . . . 227 Alkaline Carbonates ......... 227 Carbonic Acid in Insoluble Carbonates ..... 228 TABLE OF CONTENTS. XVII CHAPTER XVIII. Chlorates, Bromates, and Iodates . . 230 After Ignition .......... 230 lodometrically .......... 230 PAGE CHAPTER XIX. Citric Acid and Citrates . . . 232 Citrates of the Alkalies and Earths ...... 232 Lime and Lemon Juice ........ 232 CHAPTER XX. Hydrocyanic Acid and Cyanides . . . 234 CHAPTER XXL Ferrocyanides 238 Ferricyanides 239 CHAPTER XXII. Phosphoric Acid and Phosphates . . . 241 Phosphoric Acid, Stolba’s Method 241 “ “ Uranium “ ...... 243 “ “ Segalle’s “ ...... 245 “ “ Molybdic “ ...... 246 “ “ Pemberton-McDonald Method . . . 247 CHAPTER XXIII. Sulphates 250 CHAPTER XXIV. Sulphides 251 TABLE OF CONTENTS. CHAPTER XXV. PAGE Aluminum 252 Alum by Normal Alkali Solution 252 Ph. Germ. Method ......... 252 CHAPTER XXVI. Ammonium 254 CHAPTER XXVII. Antimony 256 Antimonous Oxide ......... 256 Antimonic Oxide .......... 256 CHAPTER XXVIII. Arsenicum 257 Arsenous Oxide .......... 257 “ “ by Oxidation with Dichromate .... 258 Arsenic Oxide by Reduction and then Titration with Iodine . 258 “ “ “ Magnesia Mixture ...... 258 “ “ “ Uranium Solution ...... 258 “ in Small Quantities ....... 259 CHAPTER XXIX. Barium 260 By Precipitation with Dichromate ...... 260 “ “ “ Sulphuric Acid 260 CHAPTER XXX. Bismuth . . . . .261 By Oxalic Acid and Permanganate . 0 , , 261 TABLE OF CONTENTS. XIX CHAPTER XXXI. PAGE Calcium 262 By Oxalic Acid and Permanganate 262 CHAPTER XXXII. Copper 264 By Precipitation as Iodide 264 CHAPTER XXXIII. Gold ...... 265 By Precipitation as Oxalate 265 CHAPTER XXXIV. Iron 266 Ferric Salts 266 Metallic Iron in Reduced Iron 267 CHAPTER XXXV. Lead ...... 269 Oxide or Carbonate, Acidimetrically ...... 269 By Precipitation with Dichromate ...... 270 “ “ as Oxalate 270 CHAPTER XXXVI. Magnesium 271 By Precipitation with Barium Chloride ..... 271 “ “ as Phosphate ....... 271 CHAPTER XXXVII. Manganese 273 By Permanganate 273 XX TABLE OF CONTENTS. CHAPTER XXXVIII. PAGE Mercury 276 Mercurous Nitrate 276 “ Chloride ......... 276 Mercuric Salts .......... 277 CHAPTER XXXIX. Silver 279 CHAPTER XL. Strontium 280 CHAPTER XLI. Tin 282 CHAPTER XLII. Zinc 284 PART III. Sanitary Analyses and Volumetric Analysis of Organic Medicinal Substances. CHAPTER XLIII. Sanitary Analysis of Water . . . 286 Collection of Sample 286 Color ............ 287 Odor 287 Reaction ........... 288 Suspended Matter . ....... 288 TABLE OF CONTENTS. XXI Total Solids ........... 288 Organic and Volatile Matter or Loss on Ignition .... 289 Chlorine 290 Ammonia .......... 291 Nessler's Solution .......... 291 Albuminoid Ammonia ......... 294 Nitrates ........... 295 Nitrites ............ 298 Oxygen-consuming Power ........ 300 Phosphates ........... 301 Hardness, Temporary and Permanent ...... 303 Interpretation of Results ........ 308 PAGE CHAPTER XLIV. Milk 317 Average Composition 317 Colostrum ........... 318 Reaction ........... 318 Specific Gravity . . . . . . . . .318 Lactometer ........... 319 Table for Correcting the Sp. Gr. of Milk according to Temper- ature ........... 320 Adulterations of Milk ......... 321 Total Solids and Water . . . . , . . .321 Fat, Adam's Method 322 Werner-Schmidt Method . . . . . . , , 323 Calculation Method ......... 324 Calculation of Per Cent of Added Water ..... 325 Total Proteids .......... 326 Milk Sugar 326 CHAPTER XLV. Butter , 327 Reichert’s Process for the Detection of Foreign Fats . . . 327 Rapid Method for the Detection of Oleomargarine „ . . 333 TABLE OF CONTENTS. CHAPTER XLVI. PAGE Estimation of C02 in the Atmosphere . . 334 Table showing Volume of .001 gm. of C02 at Various Tempera- tures 338 CHAPTER XLVH. Estimation of Alcohol in Tinctures and Beverages . 339 Table for Ascertaining the Percentages of Alcohol in Spirit from the Specific Gravity ...... 341, 342 CHAPTER XLVIII. Analysis of Soap .... 343 Geissler’s Method ......... 345 CHAPTER XLIX. Estimation of Starch in Cereals, etc. . . 347 Estimation of Starch in Baking-powders 350 CHAPTER L. Estimation of Sugars .... 351 Fehling’s Solution 351 Pavy’s Method . . . ■* . . . . . >353 CHAPTER LI. Pepsin 355 Valuation of Pepsin, U. S. P. Method ...... 357 Bartley’s Method 358 CHAPTER LII. Estimation of Peptone .... 361 TABLE OF CONTENTS. CHAPTER L111. PAGE Determination of the Diastasic Value of Malt and Pancreatic Extracts .... 362 Robert’s Method .......... 362 Park, Davis & Co.’s Method ....... 364 By Means of Fehling’s Solution ....... 365 CHAPTER LIV. Urine 366 Reaction ........... 366 Composition ........... 366 Specific Gravity .......... 368 Total Solids ........... 369 Chlorides ........... 370 Phosphates ........... 371 Sulphates ........... 372 Total Acidity 373 Urea 374 Uric Acid ........... 374 Abnormal Constituents ........ 375 Albumen ........... 375 Blood ............ 378 Pus 378 Sugar 379 Bile 383 Examination of Urinary Deposits ...... 384 CHAPTER LV. Determination of the Melting-point of Fats . 386 CHAPTER LVI. Estimation of Oil or Fat in Emulsions and Ointments. 387 Soxhlet Apparatus ......... 388 XXIV TABLE OF CONTENTS. CHAPTER EVIL PAGE Estimation of Fatty Acids . . . 390 Oleic Acid ........... 390 Free Fatty Acids in Lard 39° CHAPTER LVIII. Estimation of Tannin .... 391 G. Fleury’s Method 391 Ldwenthal’s Method 392 CHAPTER LIX. Estimation of Glycerin .... 395 Estimation of Glycerin in Fluid Extracts 398 CHAPTER LX. Estimation of Phenol .... 400 Preparation of Standard Bromine Solution ..... 400 By Koppeschaar’s Method ........ 402 Dr. Waller’s Method ......... 406 Assay of Crude Carbolic Acid 407 CHAPTER LXI. Volumetric Estimation of Alkaloids. . . 409 Table showing the Behavior of Some of the Alkaloids with Indi- cators 415 Table showing Factor for Various Alkaloids when Titrating N with — Acid V. S. . . . . . . . . . 416 10 Estimation by Mayer’s Reagent 416 “ of Alkaloids by Wagner’s Reagent .... 418 “ “ Caffeine “ “ “ .... 420 TABLE OF CONTENTS. XXV CHAPTER LX11. Volumetric Assaying of Vegetable Drugs . . 421 Alkaloidal Assay by Immiscible Solvents ..... 423 Assay of Aconite Root ......... 426 *' “ “ Leaves 427 " “ Belladonna Leaves ....... 427 *' “ " Root 428 “ “ Cinchona, U. S. P. . . . . . . . 428 “ “ Fluid Extract of Cinchona . . . . . .431 “ “ Coca Leaves ......... 431 “ “ Fluid Extract of Coca . . . . . . .431 “ “ “ “ " Ipecac. ...... 432 “ “ Ipecac Root ......... 433 “ “ Nux Vomica ......... 434 “ “ Extract of Nux Vomica ...... 434 “ “ “ “ Opium ........ 436 “ “ Tincture of Opium ....... 438 “ " Opium 43g “ “ Wild-cherry Bark ........ 440 Estimatiou of Caffeine in Crude Drugs . 441 “ “ Strength of Resinous Drugs ..... 442 “ “ Alkaloidal Strength of Scale Salts .... 443 PAGE CHAPTER LXIII. Glucosides 445 CHAPTER LXIV. Assaying of Surgical Dressings . . . 446 Carbolic Dressings 447 Salicylic-acid Dressings ........ 449 Boric “ “ 449 Sublimate “ ........ 450 Iodoform “ 453 Styptic Cotton 454 XXVI TABLE OF CONTENTS. CHAPTER LXV. Estimation of Formaldehyde . , . 455 PAGE CHAPTER LXVI. Estimation of Chloroform and Chloral Hydrate . 456 CHAPTER LXVII. Estimation of Compound Ethers . . . 458 Spirit of Nitrous Ether 459 PART IV. Gasometric Analysis . . . .461 CHAPTER LXVIII. The Nitrometer 461 Charles’ and Boyle’s Law 463 CHAPTER LXIX. Assay of Spirit of Nitrous Ether . . . 465 Assay of Amyl Nitrite 468 “ “ Sodium Nitrite ........ 469 Estimation of Nitric Acid in Nitrates ...... 469 CHAPTER LXX. Estimation of Soluble Carbonates . . , 471 TABLE OF CONTENTS. CHAPTER LXXI. PAGE Estimation of Urea in Urine . . . 472 I. By Doremus’ Ureometer. II. By the Gas-tube Method. III. By Squibb’s Urea Apparatus 472 CHAPTER LXXII. Estimation of Hydrogen Dioxide . . . 476 APPENDIX. Indicators ........... 479 Reagents and Test Solutions 489 A LIST OF ELEMENTS OCCURRING IN VOLUMETRIC METHODS, THEIR SYMBOLS, AND ATOMIC WEIGHTS. Name. Exact Atomic Weights according to Meyer and Seubert, adopted by the U. S. P. Approximate Atomic Weights. Aluminium A1 27.04 27.O Antimony Sb 119.6 120.0 Arsenic As 74-9 75-0 Barium Ba 136.9 136.9 Bismuth Bi 208.9 208.0 Boron B 10.9 11.0 Bromine Br 79.76 80.0 Cadmium Cd in.5 III.5 Calcium Ca 39-91 40.0 Carbon C 11.97 12.0 Chlorine Cl 35-37 35-4 Chromium Cr 52.0 52.0 Copper Cu 62.18 63.0 Gold Au 196.7 196.7 Hydrogen H 1.0 1.0 Iodine I 126.53 126 5 Iron Fe 55-88 56.0 Lead Pb 206.4 206.4 Lithium Li 7.01 7.0 Magnesium Mg 24-3 24.0 Manganese Mn 54-8 55-o Mercury Hg 199.8 200.0 Nitrogen N 14.01 14.0 Oxygen O 15.96 16.0 Phosphorus P 30.96 31.0 Platinum Pt 194-3 194-3 Potassium K 39-03 39-o Silver Ag 107.66 107.7 Sodium Na 23.0 23.0 Strontium Sr 87-3 87-3 Sulphur S 31.98 32.0 Tin Sn 118.8 118.0 Zinc Zn 65.1 65.0 ABBREVIATIONS AND SIGNS. Cc. ..... . cubic centimetre. Gm gramme, 15.43235 grains. Gr grain. At. wt. . . . atomic weight. V. S volumetric solution. T. S test solution, according to U. S. P, U. P. . . . United States Pharmacopoeia. — normal. 1 — decinormal. 10 centinormal. 100 — semi-normal. 2 2 — or 2N . . double-normal. N * means that the figure is approximate. A TEXT-BOOK OF VOLUMETRIC ANALYSIS. PART I. CHAPTER I. INTRODUCTION. In a chemical analysis the aim is to determine the nature of the chemical substances contained in a given compound or to ascertain their quantities. In the former case the analysis is a qualitative, in the latter a quantitative, one. The quantitative analysis of a substance may be made either by the gravimetric or the volumetric method. The Gravimetric Method consists in separating and weighing the constituents either in their natural states or in the form of new and definite compounds, the composition of which is known to the analyst. From the weights of these new compounds the analyst can calculate the quantities of the original constit- uents. Example.—To determine the quantity of silver in a solution by the gravimetric method we proceed as follows: 2 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. Add hydrochloric acid to the solution in slight but distinct excess, i.e., sufficient to cause the precipita- tion of all the silver in the form of silver chloride. The precipitate is thoroughly washed, dried, and then carefully weighed. 143.03 grammes of silver chloride represent 107.66 grammes of silver. The Volumetric Method.—In this method the quantity of the substance analyzed is ascertained by paying attention to the volume of a solution of known strength (standard solution) which must be added to it to perform a certain reaction. Example.■—If a silver solution is to be analyzed by this method it is treated with a standard solution of sodium chloride, added slowly from a burette until no more silver chloride is precipitated. Each cc. of this standard solution will precipitate a certain weight of silver as silver chloride, and hence by noting the number of cc. used to complete the precipitation, the weight of the silver in the solution analyzed is easily ascertained. N The — sodium chloride solution is generally used for this purpose. It is made by dissolving y of the molecular weight of the salt (in grammes) (5.837 gms.) in water sufficient to make 1000 cc. 1000 cc, of this solution will precipitate yL of the atomic weight of silver (in grammes) (10.766 gms.), and hence each cc. of the sodium chloride solution represents 0.010766 gramme of metallic silver, and by multiplying this figure by the number of cc. used, the quantity of silver in the solution is found. If in the above analysis N 100 cc. of the — sodium chloride solution were used, 10 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 3 then 0.010766 X 100 = 1.0766 gms. of metallic silver. The reaction is illustrated by this equation: AgNO, + NaCl = AgCl + NaN03. 10)107.66 10)58.37 N 1000) 10.766 gms. 1000)5.837 gms. = 1000 cc. — V. S. 10 0.010766 gm. 0.005837 gm. = 1 “ “ “ From the examples given it will be seen that the gravimetric operations consume no little time, and require the exercise of considerable skill. The wash- ing of the precipitate must be thoroughly performed in order that it be freed from all adhering matter. The drying also is a matter of some consequence and must be performed in such a manner as to prevent the admixture of dust or the decomposition of the pre- cipitate by excessive heat. A very accurate balance is also required. The volumetric operations, on the other hand, do not require that the substance to be determined be separated in the form of a compound of known com- position and weighed in the dry state; in fact, the substance may be accurately estimated when mixed with many others. It therefore obviates the necessity for the frequent separations and weighings which the gravimetric method demands, and enables the analyst to do the work in a very short time. The instruments needed for volumetric work are few and simple, and comparatively little skill is required. Furthermore the results obtained are in most in- stances more accurate. 4 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. CHAPTER II. GENERAL PRINCIPLES OF CHEMICAL COM- BINATION UPON WHICH VOLUMETRIC ANALYSIS IS BASED. I. When substances unite chemically the union always takes place in definite and invariable proportions. Thus when silver nitrate and sodium chloride are brought together, 169.55 parts (by weight) of silver ni- trate and 58.37 parts (by weight) of sodium chloride will react with each other, producing 143.05 parts of a curdy white precipitate (silver chloride). These substances will react with each other in these proportions only. If a greater proportion of silver nitrate than that above stated be added to the sodium chloride, only the above proportion will react, the excess remaining unchanged. The same is true if sodium chloride be added in excess of the above proportions. For instance, if 200 parts of silver nitrate be mixed with 58.37 parts of sodium chloride 169.55 parts only will react with the sodium chloride, while 3045 parts of silver nitrate will remain unchanged. Again, when potassium hydroxide and sulphuric acid are mixed potassium sulphate is formed, 111.98 parts of potassium hydroxide and 97.82 parts of sulphuric acid being required for complete neutralization. These two substances unite chemically in these proportions only. A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 5 The equation is 2K0H + H2S04 = K,S04 + 2H30. m.98 97.82 In other words, 111.98 parts of KOH will neutralize 97.82 parts of H2S04, and consequently 97.82 parts of H2S04 will neutralize 111.98 parts of KOH. Oxalic acid and sodium carbonate react upon each other in the proportions shown in the equation H.CA • 2H20 + Na9COs = Na2C204 + COa + 3H20 125.7 105.85 125.7 parts of crystallized oxalic acid are neutralized by 105.85 parts of anhydrous sodium carbonate. 2. Definite chemical compounds always contain the same elements in exactly the same proportions, the proportions being those of their atomic weights, or some multiple of these weights. Thus sodium chloride (NaCl) contains 23 parts of metallic sodium and 35.37 parts of chlorine, these be- ing the atomic weights of sodium and chlorine, respec- tively. Potassium sulphate (K2S04) contains twice 39.03 — 78.06 parts of potassium, 31.98 parts of sulphur, and four times 15.96 — 63.84 parts of oxygen. Potassium hydroxide (KOH) contains 39.03 parts of potassium, 15.96 parts of oxygen, and one part of hy- drogen. Hydrochloric acid (HC1) contains one part of hydrogen and 35.37 parts of chlorine. Upon these facts the volumetric methods of analysis are based. It has been shown that 97.82 grammes of sulphuric acid will neutralize 111.98 grammes of potassium hy- 6 A TEXT-HOOK OF VOLUMETRIC ANALYSIS. droxide ; it is therefore evident that if a solution of sul- phuric acid be made containing 48.91 grammes of the pure acid in 1000 cc. that one cc. of this solution will neutralize 0.056 gm. of potassium hydroxide. In esti- mating alkalies with this acid solution the latter is added from a burette, in small portions, until the alkali is neutralized, as shown by its reaction with some indi- cator. Each cc. of the acid solution required before neutra- lization is complete indicates 0.056 gm. of KOH, and the number of cc. used multiplied by 0.056 gm. gives the quantity of pure KOH in the sample analyzed. One cc. of the same solution will neutralize 0.03996 gm. of sodium hydroxide (NaOH), 0.052925 gm. of anhydrous sodium carbonate (Na2COs), etc. If a solution of crystallized oxalic acid be made by dissolving 62.85 gm- in sufficient water to make 1000 cc., we will have a normal solution, the neutralizing power of which is exactly equivalent to the above-mentioned normal sulphuric-acid solution. The strength of acids is estimated by alkali volumet- ric solutions. A normal solution of potassium hydrox- ide containing 55.99 gm, in the litre will neutralize exactly 1 litre of the normal acid solution ; 1 cc. of this normal alkali will neutralize 0.03637 gm. of HCl, 0.06285 gm. of H2C204, or 0.04891 gm. of H2S04, etc. A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 7 CHAPTER III. APPARATUS USED IN VOLUMETRIC ANALYSES. The Burette is a graduated glass tube which holds from 25 to 100 cc. and is graduated in fifths or tenths of a cc., and provided at the lower end with a rubber tube and pinch - cock. The use of this instrument is to accu- rately measure quantities of standard solutions used in an analysis. It is in an upright position when in use, and the flow of the solution can be regu- lated so as to run out in a stream or flow in drops by pressing the pinch- cock between the thumb and forefinger. The quantity of solution used can be read from the graduation on the out- side of the tube. This is the simplest and most common form of burette, and is known as Mohr’s (Fig. 1). The use of the pinch-cock in Mohr’s burette may be dispensed with by introducing into the rubber tube a small piece of glass rod, which must not fit too tightly. By firmly squeezing the rubber tube surrounding the glass rod a small canal is opened, through which the liquid escapes. A very delicate action can in this way be obtained, and the Fig, i. 8 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. flow of the liquid is completely under the control of the operator. (See Fig. 2.) The greatest drawback to this burette is that it cannot be used for permanganate or other solutions that act upon the rubber. Closed Open This defect can be overcome by the use of a burette having a glass stop-cock in place of the rubber tubing and pinch-cock. This form has the additional advantage of being capable of delivering Fig. 2. Fig. 3. Fig. 4. Fig. 5. Fig. 6. the solution in drops while both hands of the operator are disengaged (Fig. 3). A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 9 Another good arrangement is that in which the tap is placed in an oblique position, so that it will not easily drop out of place (Fig. 4). These glass stop-cock burettes should be emptied and washed immediately after use, especially if soda or potassa solution has been used; for these act upon the glass, and often cause the stopper to stick so firmly that it cannot be turned or removed without danger of breaking the instrument. The most satisfactory form of glass stop-cock is that shown in Fig. 6. Other forms of burettes are Mohr s Foot Burette, with rubber ball (Fig. 5). There is a hole in the rubber ball, and by placing the thumb over the hole and gently squeezing, the flow of the liquid may be regulated. Bink's Burette (Fig, 7) is used by holding in the hand and inclining sufficiently to allow the liquid to flow, then placing in an upright position, and reading when the surface of the liquid has settled. Gay-Lussacs (Fig. 8) must also be inclined when used, A wooden foot is generally provided, into which this burette is placed to rest in an upright position. By inserting a tightly fitting cork into the open end and passing through this cork a small bent glass tube, the flow of the solution from the exit-tube can be nicely regulated by blowing through the small glass tube. The necessity for inclining the burette is thus obviated. See Fig. 9. The burette shown in Fig. 10 with the spindle- shaped spout is used in the same manner as Bink’s. It is claimed for the dilated spout that it more readily admits of the delivery of single drops and prevents 10 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. the too sudden back-dropping of the solution upon returning the burette to the upright position. Casamajor s Burette (Fig. 11), described in Ameri- can Chemist, vol. VII. p. 213, has the advantage of portability and may be handled without heating the liquid. In using it the tube is grasped at the top and inclined; then by turning it the flow may be nicely controlled. Fig. 7. Fig. 8. Fig. 9. The four latter burettes being held in the hand when in use, there is a chance of increasing the bulk A TEXT-BOOK OF VOLUMETRIC ANALYSIS. of the fluid by the heat of the hand, thus leading to errors in measurement. Fig. xo. Fig. ii. Fig. 12. Geissler s Foot Burette (Fig. 12) needs no further description. When a number of estimations are to be made in which the same volumetric solution is employed, the arrangement shown in Figs. 13 and 14 is very service- able. A T-shaped glass tube is inserted between the lower 12 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. end of the burette and the pinck-cock and connected by a rubber tube with a reservoir containing the volu- metric solution. The tube which communicates with Fig. 13. Fig. 14. the reservoir is provided with a pinch-cock, which when open allows the solution to flow into and fill the A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 13 burette in so gradual a manner that no bubbles are formed. The burette is emptied in the usual manner. E. & A. Automatic Burette (Fig. 15). This is used for the same purpose as the fore- going. It is provided with a side tube for connection with reservoir, and has an over- flow reservoir which prevents its being filled to above the zero mark. The three-way stop-cock is so arranged that if turned one way the inlet is opened and the liquid from the reservoir flows into and fills the burette. If turned the other way the inlet is closed and the outlet is opened and the burette may be emptied. If the handle of the stop-cock is turned half-way round, both openings are closed. There are many other forms of automatic burettes. Pinch-cocks used with Mohr’s burettes Fig. 15. Fig. 16. are of various kinds (see Figs. 16 and 17). That shown in Fig. 16 is to be preferred. Burette-supports are of various forms; one of the best for one or two burettes is shown in Fig. 18. It is made of iron, can stand firmly upon an uneven sur- face, and does not easily tip over. The burettes are 14 A TEXT-BOOK OF VOLUMETRIC ANALYSIS fastened to it by means of clamps, illustrated in Figs. 19 and 20. Fig. 17. Fig. 19. Fig. 18. Fig. 20. A revolving burette-holder for eight burettes is shown in Fig. 21. Burette-supports are also made with white porcelain base which enables the operator the more readily to see the change of color in the liquid titrated. Pipettes are of two kinds—those which are marked to deliver one quantity only, and those which are A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 15 graduated on the stem like burettes. Their use is to measure out portions of solutions with exactness. Pipettes are filled by applying the mouth to the upper end and sucking the liquid up to the mark, then by closing the upper opening with the forefinger the liquid is prevented from running out, but may be delivered in drops or allowed to run out to any mark by lessening the pressure of the finger over the opening. Fig. 2r. 16 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. In using the pipettes of the first class (Fig. 22) the finger is raised and the instrument allowed to empty itself entirely. A drop or two, however, usually remains in the lower portion of the instrument, which may be blown out. By inclining the pipette and placing the point against the side of the vessel which is to receive the liquid, the instrument may be emptied more satisfactorily. Pipettes of the second class (Fig. 23) are never emptied completely when in use. The flow of the liquid is regulated by the pressure of the finger over the upper opening, and stopped at the desired point. A very convenient form of pipette is one which has attached to its upper end a piece of rubber tubing, into which a short piece of glass rod has been inserted. By squeezing the rubber surrounding the glass bead firmly between the fingers, a canal is opened and the liquid can be drawn up into the pipette by suction with the lips and run out again. By removing the pressure the canal closes and the flow of the liquid is stopped at any point (Fig. 24). The Nipple Pipette is very convenient for measuring small quantities of liquids, such as 1 or 2 cc. (Fig. 25). When a volatile or highly poisonous solution is to be measured it is not advisable to suck it up with the mouth. The pipette in this case is filled by dipping it into the liquid contained in a long, narrow vessel, until the liquid reaches the proper mark on the pipette, then closing the upper opening and withdrawing. When this is done the liquid which adheres to the outside of the pipette should be dried off before the measured liquid is delivered. The Measuring-flask is a vessel made of thin glass A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 17 having a narrow neck, and so constructed as to hold a definite amount of liquid when filled up to the mark on the neck. These flasks are of various sizes, hold- Fig. 22. Fig. 23. Fig. 24. Fig. 25. ing 100, 250, 500, 1000 cc., etc., but are generally called “ Litre Flasks. ” (Fig. 26.) They are used for making volumetric solutions. Those which have the mark below the middle of the neck are to be preferred, because the contents can be more easily shaken. Litre flasks are sometimes made with two marks A TEXT-BOOK OF VOLUMETRIC ANALYSIS. on the neck very near together; the lower one is the litre mark. If the flask is required to deliver a litre, it must be filled to the upper mark; the difference Fig. 26. Fig., 27. between the two measures being the equivalent of the liquid which remains in the flask, adhering to the sides. The Test Mixer, or Graduated Cylinder (Fig. 27), is for measuring and mixing smaller quantities of solutions. They are made of different sizes, holding 100, 250, 500, and 1000 cc., and graduated in fifths or tenths of a cc. A TEXT-BOOK OF VOLUMETRIC ANALYSIS. CHAPTER IV. ON THE USE OF APPARATUS. It is important that all apparatus used in volumetric analysis should be perfectly clean. Even new appara- tus should be cleansed by passing dilute hydrochloric acid through them and then rinsing with distilled water. If the burette, pipette, or other instrument is even slightly greasy, the liquid will not flow smoothly, and drops of the liquid will remain adhering to the sides, thus leading to inaccurate results. Greasiness may be removed with dilute soda solution. If this fails the instrument should be allowed to remain for some little time in a solution containing sulphuric acid and potassium dichromate, which will radically remove all traces of grease. The burette or other measuring instruments should never be filled with volumetric solution without first rinsing, even if the burette be perfectly dry. It is well to wash the inside of the instrument with two or three small portions of the solution with which it is to be filled. The burette may be filled with the aid of a funnel, the stem of which should be placed against the inner wall of the burette so that the solution will flow down the side and thus prevent the formation of bubbles. The burette should be filled to above the zero mark, 20 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. and the air-bubbles, if there are any, removed by gently tapping with the finger. A portion of the liquid should then be allowed to run out in a stream so that no air-bubbles remain in the lower part of the burette. In the glass tap burette it can be easily seen if any air is present, but in the pinch-cock burette it is sometimes necessary to take hold of the rubber tube between the thumb and fore- finger and gently stroke upward. Or the glass nip at the lower end of the burette may be pointed upward, and the pinch-cock opened wide so that a stream of the liquid will pass through and force out any air that may be inclosed. If the titration is to be conducted at a high tem- perature, as in the estimation of carbonates, when litmus is used as the indicator, or in the estimation of sugar by copper solution, a long rubber tube should be attached to the lower end of the burette. The boiling can then be done at a little distance, and the expansion of the liquid in the burette avoided. The pinch-cock is fixed about midway on the tube. Hart calls attention to the fact that if the fluid in a burette or pipette be run out rapidly at one time and slowly at another, different amounts of fluid are obtained. This is due to the adhesion of the fluid to the inner sides of the instrument, and reading before it has settled down. It is therefore advisable always to deliver burettes slowly, as more constant results are obtained. Solutions which are measured by means of pipettes should be dilute, since concentrated solutions adhere to glass with different degrees of tenacity, and hence A TEXT-BOOK OF VOLUMETRIC ANALYSIS 21 the amount of fluid delivered is slightly less than that measured. The temperature of the solutions measured should be taken into account, since all liquids are affected by change of temperature, expanding and contracting as the temperature is increased or reduced. This change of volume in the case of standard solu- tions does not exactly correspond to that in pure water; in fact some of them differ widely. The cor- rection of the volume of a standard solution for the temperature by the expansion coefficient of water is not entirely satisfactory, but in the case of very dilute solutions this may be done. Casamajor (C. N., XXXV. 160) gives the follow- ing figures showing the relative contraction and expansion of water below and above 150 C.: Degree C. Degree C. 8 — .OOO59O 17 + .000305 9 — .0005 50 18 + .OOO473 IO — .OOO492 19 + .000652 n — .000420 20 -f- .OOO84I 12 — .OOO334 21 4- .001039 13 — .OOO236 22 4“ .OOI246 14 — .OOOI24 23 4- .001462 15 — normal 24 4- .001686 16 .000147 25 4- .001919 By means of these numbers it is easy to calculate the volume of liquid at 150 C. corresponding to any volume observed at any temperature between 8° C. and 250 C. If 25 cc. of solution had been used at 20° C, the table shows that 1 cc. of water passing from 150 to 20° is increased to 1.000841 cc. Therefore, by dividing 22 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 25 cc. by 1.000841, the quotient, 24.97 cc., is ob- tained, which represents the volume at 150 C. corre- sponding to 25 cc. at 200 C. These corrections are of value only for very dilute solutions and for water, but useless for concentrated solutions. Slight variations of atmospheric pressure may be disregarded. ON THE READING OF INSTRUMENTS. In narrow vessels the surfaces of liquids are never level. This is owing to the capillary attrac- tion exerted by the sides of the vessel upon the liquid, drawing the edge up and forming a saucerlike concavity (Fig. 28). All liquids present this concave surface except mercury, the surface of which is convex. This behavior of liquids makes it difficult to find a distinct level, and in reading the measure either the upper meniscus (a) or the lower meniscus (b) may be used (Fig. 29). Fig. 28. The most satisfactory results are obtained if the low- est point of the curve (b) is used, especially with light- colored solutions. But if dark-colored or opaque solu- tions are measured, it is necessary to use the upper meniscus (a) for reading. In all cases the eye should be brought on a level with the surface of the liquid in reading the gradua- tion. The eye is very much assisted by using a small card, the lower half of which is black and the upper half white. This card is held behind the burette, the dividing line between white and black being about an A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 23 eighth of an inch below the surface of the liquid. The eye is then brought on a level with it, and the lower meniscus can be distinctly seen as a sharply defined black line against the white background (Fig. 30). Erdman’s Float, Fig, 31, is an elongated glass bulb, which is weighted at its lower end with mercury, to keep it in an upright position when floating. It is of Fig. 29. Fig. 30. Fig. 31. such diameter that it will slide easily up and down inside of a burette. There is a ring at the top by which it can be lifted in or out by means of a bent wire. Around its centre a line is marked. At this line instead of at the meniscus the reading is taken. These floats are sometimes provided with a ther- 24 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. mometer, and they then register the temperature as well as the volume. Others are provided with projecting points along the sides, the object of which is to prevent adhering to the walls of the burette. See Fig. 32. Fig. 32 Fig. 33. For the purpose of facilitating the reading, special forms of burettes are constructed which are provided with a dark longitudinal stripe on a white enamelled A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 25 background (Fig. 33); the reflection of the dark stripe with the meniscus produces the peculiar appearance shown in Fig. 34. The narrowest point is at the middle of the meniscus, and by reading from this point very accurate measurements are obtained. The same effect can be produced by holding behind an Fig. 34. Fig. 35- ordinary burette a white flexible card having a heavy black longitudinal stripe, about one-eighth inch in width. Another form of burette designed for the purpose of facilitating reading is that provided with white 26 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. enamelled sides, leaving a strip of clear glass in front and back (Fig. 35). This form is especially adapted for use with dark-colored liquids such as iodine and permanganate. CALIBRATION OF INSTRUMENTS. Burettes are made from tubes of nearly uniform width. They are filled with distilled water at 150 C. (590 F.) to the o° mark, and then 25, 50, or 100 cc. run out, and another mark made at the surface of the liquid. The distance between these two marks is then divided into 25, 50, or 100 equal parts, and the spaces again subdivided into fifths and tenths. Now it is very rarely possible to obtain tubes of exactly the same calibre throughout, and the divisions made as above do not always represent exactly what they are intended to do. If the tube is wider at one point the divisions at that point will contain more, and if it is narrower they will contain less, than they should. Hence before using a new burette, or in fact any other measuring instrument, it is essential that the error, if any, should be determined. This is done as follows: Fill the burette to the o mark with distilled water at 150 C. (590 F.) and run out 10 cc. at a time into a small weighed flask, and weigh after each addition of 10 cc. Each 10 cc. should weigh exactly 10 gms., and every deviation found should be noted and taken into consideration in using the instrument. A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 27 Example. Flask weighed 20.0000 grammes “ -f- 10 cc. “ 30.1005 ( i “ -j- 20 CC. “ 40.0499 6 < “ +30 CC. “ 49.8000 i i “ +40 CC. “ 59.9700 i C “ +50 CC, “ 70.0100 i i Thus the 1st 10 cc. weighed 10.1005 grammes. 2d IO cc. (I 9.9494 < i 3d 10 cc. < i 9.7501 i i 4th 10 cc. 4 i 10.1700 (( 5th 10 cc. i 4 10.0400 (i Having obtained these data, a table like the follow- ing may be constructed and kept in some convenient place where it can be readily consulted whenever the No. of cc. as read on Burette. No. of cc. as Corrected. No. of cc. as read on Burette. No. of cc. as Corrected. No. of cc. as read on Burette. No. of cc. as Corrected. I I.OI 14 14.06 27 26.79 2 2.02 15 15-05 28 27.76 3 3-03 16 16.04 29 28.73 4 4.04 17 17.03 30 29.70 5 5-05 18 18.02 31 30.71 6 6.06 19 19.OI 32 31.72 7 7.07 20 20.00 33 32.73 8 8.08 21 20-97 34 33-74 9 9.09 22 21.94 35 34-75 TO 10.10 23 22.9I 36 35-76 II 11.09 24 23.88 37 36.77 12 12.08 25 24.85 38 37-78 13 13.07 26 25.82 39 38.79 burette it represents is being used. It is not neces- sary to carry the figure beyond the second decimal place. A burette which deviates as much as is represented 28 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. by the foregoing table is best not used. There should be no greater deviation than 0.15 cc. In the fore- going table there is a deviation of 0.30 cc. at one point. In order to test the accuracy of a pipette, fill to the mark with distilled water at 150 C. (590 F.); empty into a previously weighed flask, weigh again and thus determine the weight of the water measured. 1 gramme is equal to 1 cc. Litre flasks are tested as follows: The flask, perfectly dry and clean, is counterpoised on a balance capable of turning with .005 when carry- ing about 2000 grammes; it is then filled to the mark with distilled water at 150 C. (590 F.), and the increase in weight should be exactly the number of grammes as the cc. indicated at the mark. A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 29 CHAPTER V. WEIGHTS AND MEASURES USED IN VOLUMETRIC ANALYSIS. The metric or decimal system is used in this country and on the continent in Europe, but in England the grain system is used. The unit of weight in the metric system is the gramme (gm.). A gramme of distilled water at its maximum density, 4° C. (390 F.), measures one cubic centimetre (cc.). A kilogram is 1000 gms. A litre is 1000 cubic centimetres. Volumetric instruments are graduated in the metric system, but not at 40 C. If they were, it would neces- sitate the carrying out of all volumetric operations at that temperature, and it would be impossible to do careful volumetric work except for two or three months of the year, unless troublesome calculations for the cor- rection of volume were made. For this reason the temperature of 150 C, (590 F.) was taken as the standard, and at this temperature most volumetric instruments are graduated. In making very careful examinations the work should be done at this temperature. One gramme of distilled water at I5°C. measures one cc. as used in volumetric analysis. The true cc. weighs at 150 C. only 0.999 gm- the grain system used in England, 10,000 grains is 30 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. taken as the standard of measurement. Sutton in his Handbook of Volumetric Analysis proposes that ten- grain measures be called a decern, or for shortness dm,; this term corresponding to the cubic centimetre, and bearing the same relation to the 10,000-grain measure that the cubic centimetre does to the litre. A 10,000-grain measure contains 10,000 fluid grains, or 1000 decerns. The flasks used in working by this system are graduated to hold 10,000, 5000, 2500, and 1000 grain measures. Burettes are graduated in 300- grain measures with 1-grain divisions, 600 grains in 1 or 2 grain divisions, 1100 grains in 5 or 10 grain divisions, etc. The system based upon the imperial-gallon measure of 70,000 grains is still to some extent in use. In this the decimillem (7 grains) bears the same relation to the pound (7000 grains) that the cubic centimetre does to the litre, or the decern to the 10,000-grain measure. A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 31 CHAPTER VI. SOME UNUSUAL VOLUMETRIC METHODS. VOLUMETRIC ANALYSIS WITHOUT WEIGHTS AND STANDARD SOLUTIONS. This is a matter of curiosity rather than of value, but under certain circumstances it might prove useful. The way in which this is carried out is best explained by an example. Suppose we wish to determine the proportion of pure sodium chloride in an impure specimen of salt. A portion of the latter is placed upon one pan of a balance and exactly counterpoised by placing on the other pan sufficient of the pure sodium chloride. The samples are then dissolved in water and each titrated with a solution of silver nitrate of unknown strength and the calculation made as follows: If the pure salt required 60 cc. of the silver solution, and the impure specimen 45 cc., then 6o : 45 :: 100 : x-, x = 75, the percentage of pure sodium chloride in the salt analyzed. This process it will be seen can be applied only to such substances of which pure specimens can be had, though in some instances a pure specimen of some 32 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. other salt may be used as a substitute, and the result obtained by calculation. For instance, suppose it is required to estimate sodium carbonate, and we have only pure calcium carbonate on hand to use as a weight. Equal weights are taken, and each titrated with an acid solution. It is now necessary to find out how many cc. of acid solution would be required if pure sodium carbonate were used, instead of pure calcium carbonate, as a counterpoise. The molecular weights of calcium carbonate and sodium carbonate are 100 and 106 respectively, and thus sodium carbonate would require i oo —>, the amount of acid solution as calcium carbonate. 106 We will assume that the calcium carbonate required 60 cc. of the acid solution and the impure sal soda 40 cc. 60 X = 56.6, the number of cc. which an equal weight of sodium carbonate will require. Then 56.6 : 4° ” 100 • x = 70.67, the percentage of pure sodium carbonate in the speci- men analyzed. With the exercise of a little ingenuity the method may be extended to a number of substances. Koningh and Peacock have devised a method by which the same end is attained without the aid of a pure substance as a standard. If impure sodium chloride is to be examined, an equal weight of silver nitrate is taken and dissolved in sufficient water to make 100 cc. of solution; this is placed in a burette, and the sodium chloride titrated after the latter is dissolved. A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 33 Assuming that 10 cc. of the silver solution were required, then 169.7 • 58.4 :: 10 ; jr = 3.44$. In the estimation of sodium carbonate an equal weight of oxalic acid i-s taken and us-ed in the same manner. The standard solutions are weighed instead of measured. This method is often resorted to where great accuracy is desired, for variations in temperature do not influence the result. It is, however, a slow process. VOLUMETRIC ANALYSIS WITHOUT A BURETTE. The standard solution is placed in a suitable flask (see Fig. 36), and the whole weighed on a delicate balance. The solution is then carefully run into the beaker containing the substance to be ana- lyzed, and when the end reaction is obtained the flask is again weighed, and the difference in weight is the amount of solution used. The standard solution should of course be standardized by weight. Fig. 36. 34 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. CHAPTER VII. Standard and Normal Solutions.—When volu- metric analysis first came into use the solutions were so made that each substance to be estimated had its own special volumetric solution, and this was generally of such strength as to give the result in percentages. Thus a certain strength of solution was used for testing soda, another for potassa, and a third for am- monia. These solutions were known as normal solutions, and since they are still to some extent in use it is im- portant that no misconception should exist as to what a normal solution is. It is to be regretted that some authors define a normal solution as one having the molecular weight in grammes of the active reagent in a litre. A Normal Solution is one which contains in a litre a quantity of the active reagent, expressed in grammes, and chemically equivalent to one atom of hydrogen. According to the U. S. P., Normal solutions are those which contain in one litre (1000 cc.) the mo- lecular weight of the active reagent in grammes, and reduced to the valence corresponding to one atom of replaceable hydrogen or its equivalent. Thus oxalic acid H2C204 2H20 = 125.7, having two replaceable H atoms. One half of its molecular weight in grammes is contained in a litre of its normal A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 35 solution, while hydrochloric acid HC1 = 36.37, which has but one replaceable H atom, has its full molecular weight in grammes in a litre of its normal solution. Sulphuric acid H2S04 has two replaceable H atoms, so its normal solution contains one half of its molecu- lar weight in grammes in a litre. NaOH and KOH being monobasic, a litre of a normal solution of either contains the full molecular weight of the salt in grammes. N Decinormal Solutions, —, are one tenth the 10 strength of normal solutions. N Centinormal Solutions,—are one hundredth IOO the strength of normal solutions. N Seminormal Solutions, —, are one half the 2 strength of normal solutions. 2 . . Double-normal Solutions, are twice the strength of the normal. Empirical Solutions are those which do not contain an exact atomic proportion of reagent, but are generally of such strength that 1 cc. = 0.01 gm. of the substance upon which it acts. A Standard Solution is any solution employed in volumetric analysis for the purpose of estimating the strength of substances—that is, any solution the strength or chemical power of which has been deter- mined. It may be normal, decinormal, or of any strength so long as its strength is known. Such a so- lution is said to be “titrated ” (French titre — title or power), sometimes called a “ set ” solution or “ stan- dardized ” solution. 36 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. Standard solutions for use in volumetric analysis are usually solutions of acids, bases, or salts, and in two cases elements, namely, iodine and bromine. A standard solution of a base is usually used for the estimation of free acids. A standard solution of an acid is usually used for the estimation of a free base, or the basic part of a salt, the acid of which can be completely expelled by the acid used in the standard solution. Example, carbo- nates. A standard solution of a salt may be used, as a pre- cipitant, or it may be used as an oxidizing or reducing agent. That part of the reagent in a standard solution which reacts with the substance under analysis is the active constituent of the solution. As Ag in AgNOa is the active constituent of the standard solution of silver nitrate, AgNO. + NaCl = AgCl + NaN03, or Cl in NaCI, is the active constituent of the standard solution of sodium chloride. If the reagent is a base, as KOH, the basic part K is the active constituent. If the reagent is an acid, the active constituent is the acidulous part, as S04 in h,so4. If the action of the reagent is oxidizing, then that part of the reagent which produces the oxidation is the active constituent. The valence of an acid is shown by the number of replaceable hydrogen atoms it contains. Thus, HC1 is univalent, HaS04 is bivalent; which means that a mole- cule of HC1 is chemically equivalent to one atom of A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 37 hydrogen, and a molecule of H2S04 is chemically equivalent to two atoms of hydrogen. The valence of a base is shown by the number of hydroxyls it is combined with. As KOH is univalent, Ca(OH)2 is bivalent. The valence of a salt is shown by the equivalent of base which has replaced the hydrogen of the corre- sponding acid. Thus NaCl, in which Na has replaced H of HC1, is univalent. K2S04, in which K2 has replaced H2 of H2S04, is bivalent. If a normal solution is to be made for a special pur- pose, its reaction in that special case is to be consid- ered. As, when K2Cr207 is to be used as a precipitat- ing agent its reaction is as follows: 2Ba(CaH3Oa)s + K2CraO, + HaO = 2BaCr04 + 2KC,H302 + 2HC3H3Oa. It is thus seen that one molecule of K2Cr207 will cause the precipitation of two atoms of barium in the form of chromate. Each atom of barium is chemically equivalent to two atoms of hydrogen; therefore one fourth of a molecule of K2Cr207 is equivalent to one atom of hydrogen. And therefore a normal solution of this salt when used as a precipitating agent must contain in one litre one fourth of its molecular weight in grammes. If K2Cr20, is to be used as an oxidizing agent, the three atoms of oxygen which it yields for oxidizing purposes must be taken into account. When this salt oxidizes it splits up into KaO -f- Cr203 -f- 03. The three atoms of oxygen combine with and oxidize the 38 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. salt acted upon, or they combine with an equivalent quantity of the hydrogen of an acid and liberate the acidulous part, which then combines with the salt. As the equations show, 6FeO + K2Cr207 = KaO + Cr203 + 3FeaOa; 6FeS04 + KaCraO, + 7 HaS04 = 7HaO+KaS04 + Cr2(S04)3 + 3Fea(S04)s; 7HaS04 + KaCr207 = 3S04 + 7HaO + KaS04 + Cra(S04)3. Each of these atoms of oxygen are equivalent to two atoms of hydrogen. Thus Oa is equivalent to H„. Hence a litre of a normal solution of K2Cr207, when used as an oxidizing agent, contains one sixth of its molecular weight in grammes. The same may be said of potassium permanganate when used as an oxidizing agent. 2KMn04 has five atoms of oxygen which are avail- able for oxidizing purposes, and each of these is capa- ble of taking two atoms of hydrogen from an acid and liberating the acidulous part. The hydrogen equiva- lent of this salt may therefore be said to be one tenth of the weight of 2KMn04, and a normal solution of this salt contains 51.554 gm. in a litre. Sodium Thiosulphate (Hyposulphite), Na2S2Os, is another instance. The molecule of this salt has two atoms of sodium, which have replaced two atoms of hydrogen of thiosulphuric acid. Thus it would seem that a normal solution should contain one half of the molecular weight in grammes. But the particular re- action of this salt with iodine is taken into account. A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 39 One molecule reacts with one atom of iodine, as seen by the equation 2Na2S203, 5HaO + I. = 2NaI -f Na2S4Oe + ioH20. Since iodine is univalent, a molecule of the salt is equivalent to one atom of hydrogen. A normal solution of this salt therefore contains the molecular weight in grammes in a litre. To Titrate a substance means to test it volu- metrically for the amount of pure substance it con- tains. The term is used in preference to “ tested ” or “analyzed,” because these terms may relate to quali- tative examinations as well as quantitative, whereas ti- tration applies only to volumetric analysis. Residual Titration, Re-titration, sometimes called Back Titration, consists in treating the substance under examination with standard solution in a quan- tity known to be in excess of that actually required ; the excess (or residue) is then ascertained by residual titration with another standard solution. Thus the quantity of the first solution which went into combination is found. Example.—Ammonium carbonate is treated first with N —H2S04 in excess, and the excess then found by ti- N tration with — KOH. i N The quantity of the —KOH used is then deducted N from the quantity of —H2S04 added, which gives the quantity of the latter which was neutralized by the ammonium carbonate. 40 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. In preparing standard solutions It must be remem- bered that most salts when dissolved in water reduce the temperature, while some, for instance sulphuric acid and alkaline hydroxides, cause a rise in tempera- ture. Therefore the solutions should be allowed to stand a while so that they may attain the temperature of the air before being measured. Furthermore, most salts cause a condensation in volume when dissolved in water; this must also be borne in mind. It is always best to weigh out a little more of the salt than the amount required by theory; dissolve in water less than required for the finished solution, determine its strength, and then dilute to the proper strength. After dilution it should always be again carefully titrated, and proved normal. To prepare solutions of exactly normal or deci- normal strength is a tedious process and often incon- venient. A solution may be made of approximately normal strength and, its exact strength having been deter- mined, used as it is. For instance, an approximately normal solution of potassium hydroxide is made and its strength determined as follows: io cc. of normal oxalic acid are put into a beaker, and after having added a suitable indicator the potassium hydroxide solution is run in from a burette, and we will assume that 10.4 cc. of the latter are required. Then we calculate thus: 10 cc. of normal oxalic acid solution contain 0.63 gramme of the acid; hence 10.4 cc. of the hydroxide solution are equiva- lent to 0.63 gramme of the acid, and 1 cc. is equiva- lent to 0.0605 gramme. A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 41 Then in estimating the strength of any solution of oxalic acid with this hydroxide solution, the number of cc. of the latter used must be multiplied by the factor 0.0605 gm* A handier way is as follows: A normal hydroxide N solution will neutralize an equal volume of — oxalic acid solution. In the case of the approximate solution, it was shown that 10.4 cc. were required for 10 cc. of N . . . 100 — oxalic acid solution ; hence its strength is or 1 104 • • 0.9615 that of the strictly normal solution, and the number of cc. used of it in any estimation must be 100 multiplied by or 0.9615, and then by the normal factor for the substance analyzed. It is a good plan to have the factor marked on the label of the bottle containing such an approximate solution. In this case it would be X 0.9615 = normal. 42 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. CHAPTER VIII. INDICATORS. In volumetric analysis the substance to be analyzed in the state of solution is placed in a beaker and the standard solution is added from a burette until a certain reaction is produced. The exact moment when a sufficient quantity of the standard solution has been added is known by certain visible changes, which differ according to the substance analyzed and the standard solution used. When such a visible change occurs the “end reaction ’’ is reached. The end reaction manifests itself in various ways, as follows: 1. Cessation of precipitation. 2. First appearance of a precipitate 3. Change of color. In some cases, however, the addition of the standard solution to the substance under analysis does not pro- duce either a precipitate or a change of color; in such cases we must resort to the use of an indicator. An indicator is a substance which is used in volu- metric analysis, and which indicates by change of color, or some other visible effect, the exact point at which a given reaction is complete. Generally the indicator is added to the substance under examination, but in a few cases it is used along- side, a drop of the substance being occasionally brought in contact with a drop of the indicator. A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 43 Thus in estimating, an alkali with an acid-volumetric solution the alkali is shown to be completely neutral- ized when the litmus tincture which was added becomes faintly red or the phenolphthalein colorless. Again, when haloid salts are estimated with nitrate-of-silver solution, chromate of potassium is added as indicator. A white precipitate is produced as long as any halogen is present to combine with the silver, and when all is precipitated the chromate of potassium acts upon the silver nitrate, forming the red-silver chromate, this color thus showing that all the halogen has been pre- cipitated. INDICATORS COMMONLY USED. The principal indicators used are : Tincture of Litmus, which shows acidity by turn- ing red and alkalinity by becoming blue. Phenolphthalein Solution, which is colorless in acid solutions and red in alkaline solutions, but is not reliable for alkaline phosphates, bicarbonates, or am- monia. Methyl-orange Solution turns red with acids and yellow with alkalies. It is not affected by carbonic acid, and is therefore adapted for the titration of alka- line carbonates. Rosolic-acid Solution is yellow with acids and vio- let-red with alkalies. It is very sensitive to ammonia. Tincture of Turmeric turns brown with alkalies, and the yellow color is restored by acids. Cochineal Solution turns violet with alkalies and yellowish with acids. It is used chiefly in the presence of ammonia or alkaline earths. 44 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. Eosin Solution is red by transmitted light, and shows a strong green fluorescence by reflected light. Acids destroy this fluorescence and alkalies restore it. Brazilwood Test-solution turns purplish red with alkalies and yellow with acids. Fluorescein Test-solution shows a strong green fluorescence by reflected light in the presence of the least excess of an alkali. Neutral Potassium-chromate Test-solution is used in the titration of haloid salts with silver-nitrate solution. It indicates that all the halogen has com- bined with the silver by producing a red-colored pre- cipitate (silver chromate). Potassium-ferricyanide Test-solution is used in the estimation of ferrous salts with potassium-dichro- mate solution. It gives a blue color to a drop of the solution on a white slab as long as any iron salt is present which has not been oxidized to ferric. Many other indicators are also used. PRECISION IN DETERMINING END REACTIONS. In most volumetric precipitation processes no direct reading of the end-point is possible; filtration and trial with small quantities of the clear filtrate being usually necessary. P. N. Rakow (Client. Zeit.) has found that many precipitates which remain obstinately suspended under ordinary conditions, and cause, in the liquid being titrated, an unmanageable turbidity, can be induced to collect and subside by the addition of some immiscible liquid heavier than water; for example, carbon disulphide or chloroform. Such liquids, although exerting no solvent action on the A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 45 precipitate, mix intimately with it and carry it down, leaving the supernatant liquid sufficiently clear for the observation of any turbidity produced by the addition of a further quantity of the precipitating solution. Carbon disulphide and chloroform are usually but not invariably effective. Thus the former carries down silver chloride rapidly and completely, but has no influence on the precipitation of barium sulphate. The author has found the method to work fairly well in the few cases he tried. In the titration of chlorides by means of silver nitrate with neutral chromate as indicator, the end reaction is more distinctly seen by gaslight than by daylight; and if very dilute solutions of chloride are estimated the titration is best performed by gaslight, and even then the change of color from yellow to red is not easily perceived. In order to overcome this difficulty Dupre suggests the following simple method : The chloride solution is placed in a white porcelain dish, a small quantity of neutral chromate added (sufficient to make the liquid yellow). Then the titra- tion is begun and watched by looking through a flat glass cell containing some of the neutral chromate. If the solution in the cell corresponds fairly with the tint of the liquid in the porcelain dish, the latter will appear to be perfectly colorless, like pure water, and the first faint appearance of red becomes strikingly manifest, and no mistake can be made. The same plan may be followed in other titrations, where the end reaction depends upon the perception of color changes. 46 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. CHAPTER IX. METHODS OF CALCULATING RESULTS. N Each cc. of a — univalent volumetric solution con- i tains joVc °f the molecular weight in grammes of its reagent, and will neutralize the molecular weight of a univalent substance, or of the molec- ular weight of a bivalent substance. N Each cc. of a — bivalent volumetric solution contains i Wro the molecular weight in grammes of its reagent, and will neutralize or combine with °f the molec- ular weight of a bivalent salt, or T-oVo the molec- ular weight of a univalent salt. N A — is only yL the strength of a normal solution and will neutralize only Tx¥ the quantity of salt, etc. Normal and decinormal solutions of acids should neutralize normal and decinormal solutions of alkalis, volume for volume. Decinormal solution of silver ni- trate and decinormal solution of hydrochloric acid or sodium chloride should combine volume for volume, etc. The Rules for Obtaining the Percentage of pure substance in any commercial article, such as acids, al- kalis, and various salts, are : A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 47 I. With normal solutions y or (according to its atomicity) of the molecular weight in grammes of the substance is weighed for titration, and the number of cc. of the V. S. required to produce the desired reaction is the percentage of the substance whose molecular weight has been used. Thus, if sodium hydroxide (NaOH) is to be exam- ined by titration with a normal acid solution of its molecular weight in grammes, 4 gms. is weighed out, and each cc. of the acid solution required represents 1_This salt, when heated, is decomposed, inflammable vapors and an odor of phenol being given off, and a residue of sodium carbonate and free carbon being left. No volumetric process is given in the U. S. P. for the estimation of this salt. The foregoing processes, however, may be applied to it, the alkaline carbonate which is left being titrated with sulphuric acid V. S., ' N each cc. of y H2S04 V. S. representing 0.15967 gm., or approximately 0.160 gm., of the pure salicylate. 2NaC7H6Os + 140, = Na2C03 + 5H20 + i3C02; 319.34 106 then Na2C03 + H2S04 = Na2S04 -f H20 + C02; 106 98 therefore 2NaC,H603 = Na2COg = H2S04. 2)319.34 2)106 2)98 159.67 gms. 53 gms. 49 gms. or 1000 cc. A TEXT-BOOK OF VOLUMETRIC ANALYSIS, 87 Table Showing the Approximate Normal Factors, etc., of the Organic Salts of the Alkaline Metals. Substance. Formula. Molecular Weight. Equivalent Weight in Carbonate. Normal Factor.* Lithium benzoate ... LiC7 H gOa 128 37 0.128 “ citrate Li3C6H 5O7 210 III O 070 “ salicylate LiC7 H 603 I44 37 O.144 Sodium acetate NaC2H802.3H20 136 53 0.136 “ benzoate NaC,H602 144 53 O.144 “ salicylate NaC7H503 l6o 53 0.160 Potassium acetate kc2h3o2 98 69 0.098 “ bitartrate khc4ha 188 69 0.188 “ citrate k3c6h6o7.h2o 324 207 0.108 “ tartrate “ and sodium tar- k2c4h4o6.h2o 244 138 O. 122 trate KNaC4H406-4H20 282 122 O. X4I * This is the coefficient by which the number of cc. of normal solu- tion used is to be multiplied in order to obtain the quantity of pure substance present in the material examined. ACIDIMETRY.—ESTIMATION OF ACIDS BY NEUTRALI- ZATION. In the previous experiments it has been shown how alkalies are estimated by the use of acid solutions of known neutralizing power. In the estimation of acids, which will now be described, the order is reversed, al- kaline solutions of known power being used in deter- mining the strength of acids. Either an alkaline carbonate or an alkaline hydrox- ide may be used in the form of standard solution for this purpose. The hydroxide, however, is to be preferred, for the carbonate when used for titrating an acid gives off car- bonic-acid gas (COa), which interferes to a great extent with the indicators. 88 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. In the U. S. P., 1890, volumetric solutions of both potassium and sodium hydroxides are official. The former, however, is preferable, because it attacks glass more slowly and less energetically, and also foams much less than does the sodium hydroxide. The neutralizing power of each is, however, the same. The caustic alkalies and their solutions are very prone to absorb carbon dioxide from the atmosphere. Therefore the solutions often contain some carbonates, the presence of which will occasion errors when used with most indicators, especially litmus and phenolph- thalein. Hence when these indicators or others which are affected by carbon dioxide are used gentle heat should be employed toward the close of each titration to drive off the liberated gas. The standard solutions of alkaline hydroxides should always be preserved in small vials provided with well- fitting cork or rubber stoppers. In order to keep solutions of this kind special ves- sels have been devised (see Fig. 39), The bottle is provided with a well-fitting rubber stopper through which a tube passes, which is filled with a mixture of soda and lime, which absorbs COa and prevents its ac- cess to the solution. An improvement upon this is shown in Fig. 40, since it allows of the burette being filled without removing the stopper, and consequently without any access of C02 whatever. Preparation of Normal Potassium Hydroxide Vol- umetric Solution, KOH = | *55*99 contains *55-99 | gms. in 1 litre.—Potassium hydroxide is so prone to ab- sorb carbon dioxide that the pure substance is not A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 89 readily obtained in commerce. If pure potassa were easily obtained it would only be necessary to dissolve 56 gms. in sufficient water to make a litre. But since it always contains some COs and H20, it is necessary Fig, 39. Fig. 40. to take a slight excess and dilute the solution to the proper volume after having determined its strength. The U. S. P. process is as follows: Dissolve 75 gms. of potassa in sufficient water to make about 1050 cc. at 15° C. (59° F.), and fill a burette with a portion of this solution. Dissolve 0.63 gm. of pure oxalic acid in about 10 cc. of water in a beaker or flask, add a few drops of phe- nolphthalein T, S., and then carefully add from the A TEXT-BOOK OF VOLUMETRIC ANALYSIS. burette the potassium-hydroxide solution, agitating frequently and regulating the flow to drops towards the end of the operation until a permanent pale-pink color is obtained. Note the number of cc. of the po- tassa solution consumed, and then dilute the remainder so that exactly 10 cc. of the diluted liquid will be re- quired to neutralize 0.63 gm. of oxalic acid. Instead of weighing off 0.63 gm. of the acid, 10 cc. of its nor- mal solution may be used. Example.—Assuming that 8 cc. of the stronger po- tassa solution had been consumed ,in the trial, then each 8 cc. must be diluted to 10 cc., or the whole of the remaining solution in the same proportion. Thus if 8 cc. must be diluted to 10 cc., 1000 cc. must be di- luted to 1250 cc. 8 : 10 :: 1000 : x x — 1250 cc. It is always advisable to make another trial after diluting. 10 cc. should then neutralize 0.63 gm. of pure oxalic acid. Centinormal Potassium Hydroxide V. S., KOH = \ 55-99 contains I in I litre.—This is ( *56 (0.56 gm. made by diluting 10 cc. of the normal solution with enough distilled water to make 1000 cc. Normal Sodium Hydroxide V. S., NaOH = \ contains 39-96 gms. | r jjtre—Dissolve 54 ( *40 40 gms. j gras, of sodium hydroxide in enough water to make about 1050 cc. at 150 C. (59° F.), and fill a burette with a portion of this solution. Dissolve 0.63 gm. of pure oxalic acid in about 10 cc. of water in a flask or beaker, add a few drops of phe- nolphthalein T. S., and then carefully add from a burette A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 91 the soda solution, agitating the flask or beaker fre- N quently, as directed under — KOH V. S., until a per- manent pale-pink color is produced. Note the number of cc. of soda solution consumed, and then dilute the remainder of the solution so that exactly 10 cc. will be required to neutralize 0.63 gm. of pure oxalic acid. Example. — If 8 cc. of the stronger soda solution had been consumed in the trial, then each 8 cc. must be diluted to 10 cc., or the whole of the remaining so- lution in the same proportion. Thus if 980 cc. should be still remaining, this must be diluted with water to make 1225 cc. Now make a new trial with the diluted solution to see whether 10 cc. will be required to neutralize 0.63 N gm. of oxalic acid (or 10 cc. of ~ oxalic acid V. S.). The neutralizing power of this solution is exactly N the same as that of — potassium hydroxide V. S., and may be employed in place of the latter, volume for volume. The following acids may be tested with either o. these alkaline solutions: Acidum aceticum. “ “ dilutum. “ “ glaciale. “ citricum. “ hydrobromicum dilutum. “ hydrochloricum. “ “ dilutum. hypophosphorosum dilutum. “ lacticum. 92 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. Acidum nitricum. “ “ dilutum. “ phosphoricum. “ “ dilutum. “ sulphuricum. “ “ aromaticum. “ “ dilutum. “ tartaricum. Acidum Aceticum, HC2H302 = | — The U. S. P. acetic acid contains 36/0, by weight, of absolute HC2H302 and 64$ of water. Mix 3 gms. of the acid with a small quantity of water, add a few drops of phenolphthalein T. S., and titrate with normal potassium hydroxide V. S. until a permanent pale-pink color is obtained, and apply the following equation: HC.H.O, + KOH = KQH3O3 + H3O. 60 56 N Thus 56 gms. or 1000cc. of — KOH V. S. will neutral- ize. 60 gms. of acetic acid; therefore each cc. of N — KOH V. S. represents .060 gm. of acetic acid. If 18 cc. are required to neutralize 3 gms. of the acid, it contains 18 X .060 — 1.08 gms. of absolute acetic acid. i.o8 X ioo —— = 36^. According to the U. S. P., 6 gms. of the acid should re- N quire 36 cc. of — KOH V. S. for complete neutraliza- tion. A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 93 Acidum Aceticum Dilutum.—A solution contain- ing 6$, by weight, of absolute acetic acid. The estimation is conducted exactly as the above. The diluted acetic acid U. S. P. should contain 6$ of absolute acid. 24 gms. should require 24 cc. of N — KOH V. S. 24 X .060 — 1.440 1.440 X 100 _ 24 Acidum Aceticum Glaciale.—Three gms. of glacial acetic acid are mixed with a small quantity of water, a few drops of phenolphthalein T. S. added, and the solu- N tion titrated with — potassium hydroxide V. S. until a very pale pink color appears. Each cc. represents .06 gm. of absolute acetic acid. 49.5 cc. are required by 3 gms. of the U. S. P. acid. 49-5 X .o6 = 2.970 gms. 2.970 X 100 —— — 99% 3 Acidum Citricum, H3C6H507.H20 = | '-h— 3.5 gms. of citric acid are dissolved in a sufficient quantity of water, a few drops of phenolphthalein added, N and the solution titrated with — potassium hydroxide V. S. until a very pale pink color appears. Each cc. of N — potassium hydroxide consumed before neutralization 94 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. is effected represents .070 gm. of the pure acid, and 50 cc. should be required. The reaction is expressed by the following equation; H,C.H.0,-H,0 + 3KOH = K.C.H.O, + 4H.0. 3)210 3)l68 ~70 56 Thus 56 gms. of KOH or 1000 cc. of its normal solu- tion represent 70 gms. of pure crystallized acid, and each cc. represents .070 gm. Therefore 50 X .o/o = 3-5 gms. 3,5 X 100 , — = 100$ 3-5 Acidum Oxalicum, H2C204+2H,0 | This acid may be estimated by oxidation with potassium permanganate V. S. as described under oxidimetry, but it is more conveniently tested by neutralization with an alkaline V. S. I gm. of the acid is placed in a beaker, a sufficient quantity of water is added to dissolve it, and then a few drops of phenolphthalein T. S., and the solution titrated with a normal alkali solution. Each cc. of the normal alkali solution represents 0.063 gm. of crystallized oxalic acid as the equation shows: HaCa04.2Ha0 -f 2KOH = KaCa04 + 4Ha0. 2)126 2)112 63 56 = 1000 cc. — V. S. 1 Acidum Hydrobromicum Dilutum (Diluted Hydro- bromic Acid), HBr= I *g° —A liquid containing 10 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 95 per cent, of pure hydrobromic acid (HBr) and 90 per cent, of water. 8.1 gms. of the acid are diluted with a small quan- tity of water, a few drops of phenolphthalein T. S, N added, and then — potassium hydroxide V. S. added from a burette, until a very faint pink color is pro- N duced. Note the quantity of — alkali used, and mul- tiply this by the factor .081 gm. to obtain the weight of HBr in the diluted acid taken. The reaction is expressed by the following equation : HBr -f- KOH = KBr + HaO. N 81 gms. 56 gms. = 1000 cc. of — V. S. Each cc. therefore represents .081 gm., or 1 per cent, of HBr. If this acid is made with tartaric acid and potassium bromide, a white, crystalline precipitate will be pro- N duced upon the addition of the y alkali, some of which will be neutralized by the dissolved potassium bitartrate and the excess of tartaric acid, and an incor- rect indication will be given. Acidum Hydrochloricum (Muriatic Acid), HC1 = ( 36.37 *364' — A liquid containing 31.9 per cent., by- weight, of absolute HC1 and 68.1 per cent, of water. 3 gms. of hydrochloric acid are diluted with a little water, a few drops of phenolphthalein added, and then N Y potassium hydroxide V. S. from a burette, until a 96 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. N faint pink color is produced. Note the quantity of — alkali used, and apply the following equation: HC1 + KOH = KC1 + H20. N 36.4 gms. 56 gtns. = 1000 cc. — V. S. N Each cc. of -- alkali required before the acid is neu- tralized represents .0364 gm. of pure HC1. 3.64 gms. of the U.S.P. acid should require for com- N plete neutralization 31.9 cc. of — KOH V. S. Diluted hydrochloric acid, U. S. P., contains 10 per cent, of absolute HCL 3.64 gins, of the diluted acid N should require for neutralization 10 cc. of ~ KOH V. S. Let us assume that the 3 gms. of hydrochloric acid N required 20 cc. of - KOH V. S. Ihen 20 X .0364 = .7280 gm. of pure HCl in 3 gms. of the acid. .7280 X ioo ,,, = 24.26$ Acidum Hypophosphorosum Dilutum (Diluted Hypophosphorous Acid).—The U. S. P. acid contains 10 per cent, of absolute HPH2Oa = | A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 97 This acid is estimated in exactly the same way as the acids previously noticed : HPH A + KOH = KPH A + HA N 66 gms. = 56 gms. = 1000 cc. — alkali. N Thus each cc. of — alkali represents .066 gm. of HPHaOa. Take 5 gms. of the acid, dilute it with a small quan- tity of water, add a few drops of phenolphthalein T. S., N and titrate with — KOH V. S. until a very faint pink N color appears. If 8 cc. of the y- alkali are used, the 5 gms. contain 8 X .066 = .528 gm. 5 : .528 :: 100 ; x. x — 10.56$ 6.6 gms. of the U. S. P. acid should require for neu N tralization 10 cc. of — KOH V. S. i Acidum Lacticum, HC3H A = | or" ganic acid containing 75 per cent., by weight, of abso- lute lactic acid and 25 per cent, of water. 5 gms. of lactic acid are slightly diluted with water, a few drops of phenolphthalein T. S. added, and then the N - KOH V. S. from a burette, until a pale-pink color is produced. Note the quantity of normal alkali used, 98 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. and multiply that number by .090 gm. to get the quan- tity of absolute acid in the 5 gms. taken. HC.H.O.+ KOII = KC.H.O.+ H,0, N go gms. = 56 gms. = 1000 cc. ~ KOH V. S. and i cc. of - KOH = .090 gm. of HC3H503. N If 40 cc. of KOH are required for neutralization of the 5 gms. of the lactic acid, then X 4° -°9 = 3-6o gms. 5 : 3.6 :: 100 : x. x = 72$ Acidum Nitricum (Nitric Acid), HN03= j —The U. S. P. acid contains 68 per cent., by weight, of absolute nitric acid and 32 per cent, of water. Take 3 gins, of nitric acid, dilute with a little water, add a few drops of phenolphthalein T. S., and then N pass into the mixture from a burette — potassium hydroxide V. S. until neutralization is effected, and the liquid acquires a faint pink color. Apply the following equation ; HNOa + KOH = KN03 + H20. N 63 gms. 56 gms. = 1000 cc. — KOH V. S. N Thus each cc. of — KOH V. S. required before neu- tralization is effected represents 0.063 gm. of absolute nitric acid. A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 99 N If 30 cc. of the y alkali are required, then the 3 gms. contain .063 X 30 = 1.890 gms. 3 : 1.89 : : ioo : x. x — 6344-54 199-43 110.65 215.40 308.94 O.OI275 Ammonium bromide “ chloride NH„Br NH4C1 nh4i 4 l 0.009777 0.005338 0.014454 Calcium bromide CaBr2 CaCl2 FeBr2 Fel2 44 41 O.OO997I5 0.005532 u O.OIO77 — AgNOa and 10 N — KSCN 10 0.015447 Lead acetate. Pb(C2H302)2*3H2 0 378.0 — h2so4 0.189 Pb20(C2H302)2 546.48 14 0.13662 N -AgNO, 10 11 Lithium bromide LiBr 86.77 0.008677 KBr KC1 KCN KI KSCN 118.79 74.40 65.01 165.56 96.99 0.011879 u “ cyanide “ iodide “ sulphocyanide (t (4 (4 — NaCl or IO N — KSCN O.OI3OO 0.016556 0.009699 Aftu 215-32 0.010766 AgN03 Ag20 NaBr 169.5s 231.28 102.76 10 44 0.016955 0.011564 0.010276 * t4 - AgNO» IO 44 NaCl Nal SrBr2.6H20 SrI2.6H2C ZnBr2 ZnCl2 Znl2 sS-37 >49-53 354-58 448.12 224.62 >35-84 318.16 0.005837 44 Strontium bromide 44 44 °*OI4953 0.012341 0.022406 44 44 0.006792 0.015908 44 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 143 CHAPTER XII. OXIDIMETRY—ANALYSIS BY OXIDATION. An extensive series of analyses are made by this method, with extremely accurate results—in fact, the results are generally more accurate than any which can be obtained by weighing. The principle involved in this method is extremely simple. Substances which are capable of absorbing oxygen or are susceptible of an equivalent action are subjected to the action of an oxidizing agent of known power, and the quantity of the latter required for complete oxidation ascertained. The substances which are used as oxidizing agents in volumetric analysis are potassium dichromate, po- tassium permanganate, iodine, etc. The reducing agents, or deoxidizers, are sodium thio- sulphate, oxalic acid, arsenous oxide, stannous chloride, metallic zinc, and magnesium. Thus ferrous oxide (FeO), an oxidizable substance, is ever ready and willing to take up oxygen, while potassium dichromate and permanganate are always ready to give up some of their oxygen. When po- tassium permanganate gives up its oxygen in this way it loses its color, and in volumetric analysis advantage is taken of this fact. When the permanganate, which is added in drops from a burette, is no longer decolor- 144 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. ized, the iron salt is completely oxidized. The reac- tion is as follows: loFeO + 2KMn04 = 5Fe2Os + 2MnO + KsO. Ferrous oxide. Ferric oxide. The oxidation of ferrous oxide by potassium dichro- mate is shown by the following equation : 6FeO + K2CrA = 3FetO, + CraOs + KaO. An oxidation is always accompanied by a reduction, the oxidizing agent being itself reduced in the opera- tion. As seen in the above equations, the manganic compound is reduced to a manganous compound, and the chromic to a chromous compound. ESTIMATION OF FERROUS SALTS. Ferrous salts are estimated by oxidizing them either with potassium dichromate or potassium perman- ganate. In some respects the dichromate possesses advan- tages over permanganate. 2. Its solution does not deteriorate upon standing as does that of permanganate. I. It may be obtained in a pure state. 3. It is not decomposed by contact with rubber as the permanganate is, and may therefore be used in Mohr’s burette. Its great disadvantage, however, is that when used in the estimation of ferrous salts the end reaction can only be found by using an external indicator. The indicator which must be used is freshly A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 145 prepared potassium ferricyanide T. S., a drop of which is brought in contact with a drop of the solution being tested, on a white slab, at intervals during the titration, the end of the reaction being the cessation of the production of a blue color, when the two liquids are brought together. Thus the estimation by potas- sium dichromate is cumbersome, and very exact results are not easily obtained. If potassium-permanganate solution is used for the estimation of these salts the end of the reaction is easily found without the use of an indicator. The permanganate is decomposed the instant it is brought in contact with a ferrous salt in an acid solu- tion ; therefore as long as any ferrous salt remains in solution the permanganate is decolorized, and when it ceases to lose its color the reaction is complete. Preparation of Standard Solution Decinormal Potassium Dichromate V. S., K2Cr20, = | | Sms* in 1 litre.—4.896 gms. (*4 9 gms.) of pure potassium dichromate are dissolved in sufficient water to make, at the ordinary temperature of the atmos- phere, exactly 1000 cc. Pure Potassium Dichromate for use in volumetric analysis should respond to all the tests for purity given in the text of the U. S. P. (under Potassii Bichromate), as well as to the following: A solution of 0.5 gm. of the salt in 10 cc. of water, rendered acid by 0.5 cc. of nitric acid, should produce no visible change when treated with barium chloride T. S. (absence of sulphate), nor with silver nitrate T. S. (absence of chloride'). If a mixture of 10 cc. of an aqueous solution of the 146 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. salt (i—20) with I cc. of ammonia water be treated with ammonium oxalate T. S., no precipitate should be pro- duced (absence of calcium). Standard solution 0/ potassium dichromate is some- times used as a neutralizing solution for estimating alkalies, phenolphthalein being used as indicator. When used for this purpose the normal solution contains 146.89 gms. in 1 litre (one half the molecular weight in grammes). It is then the exact equivalent of any normal acid V, S. 2KOH -f K2Cr20, = 2K2Cr04 + H20. 2)112 2)293.78 56 gms. 146.89 gms., or 1000 cc. normal V. S. Decinormal potassium dichromate V. S. may also be used in conjunction with potassium iodide and sul- phuric acid for standardizing sodium thiosulphate V. S. Iodine is liberated from potassium iodide in this reac- tion. The reaction is expressed by the equation K,CrA + 6KI + 7H2S04 = 4K2S04 + Cr2(S04)3 + 7H20 + 3It. When used as an oxidizing agent to convert ferrous into ferric salts, or to liberate iodine from potassium N iodide, the — solution of potassium dichromate must contain 4.689 gms. in I litre. If the decinormal solution containing 14.689 gms. in 1 litre is used, it has the effect r 3N , . of a — solution. 10 The decinormal solution which is used as an oxidizing agent is chemically equivalent to decinormal potassium permanganate. When used for the purpose of liberating iodine from potassium iodide, it is the equivalent of an equal volume of decinormal sodium thiosulphate. For titrating ferrous salts the decinormal solution of dichromate is used in the following manner: A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 147 Make an aqueous solution of the ferrous salt, intro- duce it into a flask, and acidulate it with sulphuric or hydrochloric acid. Now add gradually from a burette the decinormal potassium dichromate V. S. until a drop taken out upon a white slab no longer shows a blue color with a drop of freshly prepared potassium ferricyanide T. S. Note the number of cc. of the standard solution used, multiply this number by the factor, and thus obtain the quantity of pure salt in the sample taken. Ferrous salts strike a blue color with potassium ferricyanide T, S ; but as the quantity of ferrous salts gradually diminishes during the titration, the blue be- comes somewhat turbid, acquiring first a green, then a gray, and lastly a brown shade. The process is finished when the greenish-blue tint has entirely disappeared. The reaction of potassium dichromate with ferrous salts always takes place in the presence of free sul- phuric or hydrochloric acid at ordinary temperatures. Nitric acid should not be used. If it is desired to estimate ferric salts by this standard solution it is necessary to first reduce them. This may be done by metallic zinc, magnesium, sul- phurous acid, the alkali sulphites, or by stannous chloride. Standard potassium dichromate may be checked in the same way as standard permanganate, with pure metallic iron, as described below. Decinormal Potassium Permanganate V. S., 2KMn04 = I It: contains *3* | Sms* in 1 litre.—This solution may be prepared by dissolving the pure crystals in fresh distilled water. If the salt can be 148 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. obtained perfectly pure and dry, a decinormal solution will be obtained if 3.1534 gms. are dissolved in distilled water, sufficient to make 1000 cc. at the ordinary atmos- pheric temperature ; but nevertheless it is always well to verify it as described below. The solution will retain its strength for several weeks if well kept, but it should always be checked by titration before it is used. The standardization of permanganate solution may be effected as follows: With Metallic Iron —Thin annealed binding-wire, free from rust, is one of the purest forms of iron. o.i gm. of such iron is placed in a flask which is provided with a cork through which a piece of glass tubing passes, to the top of which a piece of rubber tubing is at- tached, which has a vertical slit about one inch long in its side, and which is closed at its upper end by a piece of glass rod (see Fig. 41). Diluted sulphuric acid is added and gentle heat ap- plied. The iron dissolves and the steam and liberated hy- drogen escape through the slit under slight pressure. The air is thus prevented from enter- ing and the ferrous solution protected from oxidation. Fig. 41, When the iron is completely dissolved a small quan- tity of cold, recently boiled, distilled water should be added, and the titration with potassium permanganate at once begun and continued until a faint permanent A TEXT-BOOK OF VOLUMETRIC ANALYSIS. red color is produced. If the solution is decinormal, exactly 17.85 cc. will be required to produce this result. The iron is converted by the sulphuric acid into ferrous sulphate, Fe2 -f- 2H2S04 = 2FeS04 + 2H2. This ferrous sulphate is easily oxidized by the air, and therefore it is directed that access of air should be prevented, and the distilled water, with which the solu- tion is diluted, previously boiled in order to drive off any dissolved free oxygen. ioFeS04 + 2KMn04 + 8H2S04 100)558.8 100)315.34 5.588 gms. 3.1534 gms. or 1000 cc. — V. S. 10 = 5Fea(S04)3 + KsS04 + 2MnS04 + 8H20. N This equation, etc., shows that each cc. of — 2KMn04 V. S. represents .005588 gm. of metallic iron. With Oxalic Acid.—0.063 gm, of the pure crystal- lized acid is weighed (or 10 cc. of decinormal oxalic acid V. S. carefully measured) and placed in a flask, with some dilute sulphuric acid and considerable water, the mixture warmed to about 6o° C. (140° F.), and the permanganate added from a burette. The action is in this case less decisive and rapid than in the titration with iron, and more care should be used. The color disappears slowly at first, but afterwards more rapidly. Note the number of cc. of the permanganate solu- tion used, and then dilute the remainder so that equal volumes of decinormal oxalic acid and decinormal permanganate solution will exactly correspond. 150 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. Example.—Assuming that 9 cc. of the perman- ganate solution first prepared had been required to produce a permanent pink tint when titrated into 10 N cc. of — oxalic-acid solution, then the permanganate must be diluted in the proportion of 9 of permanganate and 1 of distilled water, or 900 and 100. The U. S. P. gives the following method for the preparation of this solution : A stronger and a weaker solution is made and mixed in certain proportions to form a solution of the proper strength. It is said that when thus prepared the solu- tion will keep its titre for months if properly preserved. The Stronger Solution.—3.5 gms. of pure crystallized permanganate are dissolved in 1000 cc. of water by the aid of heat, and the solution then set aside in a closed flask for two days, so that any suspended mat- ters may deposit. The Weaker Solution.—Dissolve 6.6 gms. of the salt in 2200 cc. of water in the same manner as above, and set this solution aside for two days. These two solutions are then separately titrated in the following manner; Introduce 10 cc. of decinormal oxalic-acid solution into a flask, add I cc. of pure concentrated sulphuric acid, and before the mixture cools add the perman- ganate solution slowly from a burette, shaking the flask after each addition, and towards the end of the operation reducing the flow to drops. When the last drop is no longer decolorized, but imparts a pinkish tint to the liquid, the reaction is completed. Note the number of cc. consumed. Finally, mix the two solu- tions in such proportions that equal volumes of the A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 151 N mixture and of — oxalic acid V. S. will exactly corre- 10 J spond. To obtain the accurate proportions for mixing the two solutions, deduct 10 from the number of cc. of the weaker solution consumed in the above titration ; with this difference multiply the number of cc. of the stronger solution consumed : the product shows the number of cc. of the stronger solution needed for the mixture. Then deduct the number of cc. of the stronger solu- tion consumed in the titration from 10, and with the difference multiply the number of cc. of the weaker solution consumed : the product shows the number of cc. of the weaker solution needed for the mixture. Or, designating the number of cc. of the stronger so- lution by S, and the number of cc. of the weaker solu- tion by W, and using the following formula, the proportions in which the solutions must be mixed are obtained: Stronger Solution. Weaker Solution. (IV-io)S + (10 — S) W. Example.—Assuming that 9 cc. of the stronger and 10.5 cc. of weaker had been consumed in decomposing N 10 cc. of — oxalic acid V. S. ; then, substituting these values in the above formula, we obtain (io.5 - io)9 +(10-9)10.5, or 4-5 + 10.5, making 15 cc. of final solution. 152 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. The bulk of the two solutions is now mixed in the same proportion : 450 cc. of the stronger and 1050 cc. of the weaker, or 900 cc. of the stronger and 2100 cc. of the weaker. After the solutions are thus mixed a new trial should be made, when 10 cc. of the solution should exactly N decompose 10 cc. of — oxalic acid V. S. The reaction between potassium permanganate and oxalic acid is illustrated by the following equation: 2KMn04 + 5(HaCs04.2H20) + 3H2S04 = KsS04 + 2MnS04 + ioCOa + i8HaO. ESTIMATION OF FERROUS SALTS WITH POTASSIUM BICHROMATE. One molecule of potassium dichromate yields, under favorable circumstances, three atoms of oxygen for oxidizing purposes. This is shown by the following equation : 6FeO + K2Cr20, = 3Fe2Os + Cr203 + K20. Here it is seen that the three liberated atoms of oxygen combine at once with the ferrous oxide, con- verting it into ferric oxide : 6FeO + Os = Fe609 or 3Fe2Oa. In the oxidation of a ferrous salt, the reaction takes place only in the presence of an acid. The dichromate then gives up its oxygen. Four of A TEXT-BOOK OF VOLUMETRIC ANALYSIS. its oxygen atoms combine at once with the replaceable hydrogen of the accompanying acid, the other three being liberated. The three oxygen atoms thus set free are available either for direct oxidation or for combination with the hydrogen of more acid. In the latter case a corresponding quantity of acidulous radi- cals is set free. The following equation indicates this reaction : K2Cr A + 4H2S04 = K2S04 + Cr2(S04)3 + 4H20 + O, In this case four of the liberated atoms of oxygen combine with eight of the atoms of hydrogen of sul- phuric acid and liberate four S04 radicals, which at once combine with the K2 and Cr2 of the dichromate. The other three atoms are set free. If seven sulphuric- acid molecules are used instead of four molecules, the three free atoms of oxygen will liberate 3(S04): K,Cr,0,+7H,S0.=K.SO.+ Cr,(S0,).+7H,0 + (SO.),. If this liberation of 3(S04) takes place in the pres- ence of a ferrous salt, the 3(S04) will combine with six molecules of the ferrous salt, converting it into a ferric salt ; 6FeSO. + 3SO, = Fe.(SO.). = 3Fe,(SO.).; 6FeSO. + K,Cr,0, + 7H.S0, = K.SO. + Cr,(SO.). + /H,0 + (sFe/SOJ,). If in the above case hydrochloric acid is used instead of sulphuric, fourteen molecules of the former must be taken to supply the necessary hydrogen. 154 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. The seven liberated atoms of oxygen must have fourteen atoms of hydrogen to combine with. Three of these atoms of oxygen liberate six uni- valent, or three bivalent, acidulous radicals. Therefore, since one molecule of K2Cr207 will give up for oxidizing purposes three atoms of oxygen, which are equivalent chemically to six atoms of hydro- gen, one sixth of the molecular weight in grammes of the dichromate, dissolved in sufficient water to make one litre, constitutes a normal solution, and one tenth of this quantity of K2Cr207 in a litre, a decinormal solution. Thus the estimation of ferrous salts is effected by oxidizing them to ferric with an oxidizing agent of known power, the strength of the ferrous salt being determined by the quantity of the oxidizing agent required to convert it to ferric. Ferri Carbonas Saccharatus (Saccharated Ferrous Carbonate), FeCOs = | *1.16 (1.1573) gms. of saccharated ferrous carbonate are dissolved in 10 cc. of diluted sulphuric acid and the solution diluted with water to about 100 cc. The decinormal potassium dichromate is carefully added, until a drop of the solu- tion taken out and brought in contact with a drop of freshly prepared solution of potassium ferricyanide ceases to give a blue color. The number of cc. of the dichromate solution is read off and the following equations applied : 6FeC03 + 6HaS0, = 6FeS04 + 6H,0 + 6CO,; H5-73 151-7 6 6 694.38 qio.2 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 155 then 6FeCOs or 6FeS04 + KaCr207 + ;H2S04 = 6)694.38 6)glo. 2 6)293.78 10)115.73 10)151-7 IQ) 48.96 ix.573 gms. 15.17 gms. 4.896 gms., or 1000 cc. F K2Cr207 V.S. IO KaS04 + Cr2(S04)3 + 7 H20 + 3Fe,(S04)3. N Thus each cc. of — K2Cr207 represents 0.011573 gm. of pure ferrous carbonate or 0.005588 gm. of metallic iron. The U. S. P. saccharated ferrous carbonate requires N about 15 cc. of — K2Cr207 V. S. for complete neutral- ization, corresponding to about 15$. -oil573 x 15 = 0.173585 gm. 0.173585 X 100^ I-I573 5 If strong sulphuric acid is added to saccharated fer- rous carbonate it will char the sugar, and a black mass of burnt sugar is obtained. This may be prevented by adding water first and then, slowly, the sulphuric acid. Instead of sulphuric acid, hydrochloric acid may be used. This will not char the sugar; but the ferrous chloride which is then formed is too readily oxidized by the air. It has also been suggested that as hydrochloric acid so rapidly converts ordinary sugar into invert sugar as to render it easily attacked by the dichromate, it should be cautiously used, if at all. Phosphoric acid 156 A TEXT-BOOK OF VOLUMETRIC ANALYSIS has none of these disadvantages, and may be employed with good results. In making estimations of ferrous salts with potas- sium dichromate, care should be taken to avoid atmos- pheric oxidation. It is good practice to calculate approximately how much of the standard solution will probably be required to complete the oxidation, and then add almost enough of the standard solution at once, instead of adding it slowly. A white porcelain slab is then got ready, and placed alongside of the flask in which the titration is to be performed. Upon this slab is placed a number of drops of the freshly prepared solution of potassium ferricyanide, and at intervals during the titration a drop is taken from the flask on a glass rod and brought in contact with one of the drops on the slab. The glass rod should always be dipped in clean water after having been brought in contact with a drop of the indicator. See Fig. 42. When a drop of the solution ceases to give a blue color on contact with the indicator, the reaction is complete. Ferrous Sulphate, FeSO, -f 7H20 = | 42-— Dissolve about one gramme of crystallized ferrous sul- phate in a little water, add a good excess of sulphuric or hydrochloric acid, titrate with the decinormal potas- sium dichromate V, S. as directed under Ferrous Car- bonate, and apply the following equation : 6(FeS04.7Ha0) + KaCraO, + 7HaS04 = 6)1668 6)293.78 10) 278 10) 48.96 1 N 27.8 gms. 4.896 gms., or 1000 cc. - K2CroOT V. S. 10 3Fe,(SO.), + K,SO. + Crs(SO,), + 49H,0. A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 157 Fig. 42. 158 A TEXT-BOOK OF VOLUMETRIC ANALYSIS, N Thus each cc. of the — K2Cr207 V. S. represents 0.0278 gm, of crystallized ferrous sulphate or 0.0152 anhydrous. If 1 gin. of the salt is taken and dissolved as above, it should require about 37 cc. of the standard solution, equivalent to about 100$. Anhydrous Ferrous Sulphate.— 6FeS04 + K2Cr207 + ;H2S04 = 6)912 6)293.78 10)152 10)48-96 N 15.2 gms. 4.896 gms., or 1000 cc. — K2Cr207 V. S. 3Fe,(S0.). + K.SO. + Cr„(SO.). + 7H,0. Each cc. of the standard solution represents 0.0152 gm. of real ferrous sulphate or *.0056 gm. of metallic iron. Dried (Exsiccated) Ferrous Sulphate of the U. S. P. has the approximate composition FeS04 -\- 3H20. It is tested in the same manner as the anhydrous ferrous sulphate. Granulated Ferrous Sulphate, FeS04 7H20, is tested in the same manner as crystallized ferrous sul- phate, with which it should correspond in strength. ESTIMATION OF FERROUS SALTS WITH POTASSIUM- PERMANGANATE SOLUTION. The action of potassium permanganate in oxidation is very similar to that of the dichromate. The molecule 2KMn04 has 8 atoms of oxygen, which it gives up in the process of oxidation. These 8 atoms of oxygen unite with the replaceable hydrogen A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 159 of an accompanying acid, liberating an equivalent amount of acidulous radical. Three of these atoms of oxygen liberate sufficient acidulous radical to combine with the potassium and manganese of the permanganate, while the other five atoms are available either for direct oxidation or 2KMn04 + 3HaS04=KaS04 + 2MnS04 + 3H20 -f 5O. For combination with the hydrogen of more acid, more acidulous radical being liberated to combine with the salt acted upon, 2KMn04+8H3S04=K2S04+2MnS04+8H20+5(S04). 5(S04) when combined with ioFeS04 forms Fe10 (S04)16 or 5Fe2(S04)3, ferric sulphate. It is thus seen that one molecule of potassium per- manganate 2KMn04 has the power of converting 10 molecules of a ferrous salt into the ferric state. The equation in full is ioFeS04 + 2KMn04 + 8H2S04 = K2S04 + 2MnS04 + 8H20 + 5Fea(S04)s. We have seen that 2KMn04 has 5 atoms of oxygen available for oxidizing purposes, and that each of these will combine with 2 atoms of hydrogen. 2KMn04 is consequently chemically equivalent to 10 atoms of re- placeable hydrogen, and a normal solution of this salt when used as an oxidizing agent is one that contains in 1 litre one tenth of the molecular weight of 2KMn04, and a decinormal solution one which contains one hundredth of the molecular weight. A TEXT-BOOK OF VOLUMETRIC ANALYSIS. When potassium permanganate is brought in con- tact with a ferrous salt or other oxidizable substance, it is decomposed and decolorized. Hence when titrat- ing with a standard solution of this salt it is decolorized so long as an oxidizable substance is present; as soon, however, as the oxidation is completed the standard solution retains its color when added to the substance, and the first appearance of a faint red color is the end reaction, and the oxidation is known to be completed. In titrating with potassium permanganate it must be remembered that free acid should always be present in the solution titrated, in order to keep the resulting manganous oxide in solution. Diluted sul- phuric acid is generally used for this purpose. Hydro- chloric acid may also be employed, but in that case the titration must be conducted at a low temperature, otherwise chlorine will be evolved and the analysis spoiled. Ferrum Reductum is estimated for metallic iron, according to the U. S. P., in the following manner: 0.56 (0.559) gm- °f reduced iron is introduced into a glass-stoppered bottle, 50 cc. of mercuric chloride T. S. are added, and the bottle heated on a water-bath for one hour, agitating frequently, but keeping the bottle well stoppered. 2HgCl2 -f Fe2 = 2FeCl2 + 2Hg. Then allow it to cool, dilute the contents with water to 100 cc., and filter. Take 10 cc. of the filtrate, add to it 10 cc. of diluted sulphuric acid, introduce the mixture into a glass-stoppered bottle (having a capacity of about 100 cc.), and titrate the mixture with decinormal potassium permanganate V. S. until a permanent red color is produced. A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 161 Each cc. of the standard solution represents *0.0056 gm. of metallic iron, or 10$. ioFeS04 + 2KMn04 + 8H2S04 = K2S04 + 2MnS04 + 5Fe2(S04)3 + 8H20. To confirm the assay, add a few drops of alcohol to decolorize (or decompose) the excess of permanga- nate, then add I gm. of potassium iodide, and digest for half an hour at a temperature of 40° C. (104° F.). Fe,(S04)3 + 2KI = 2FeS04 + Is -f K2S04. 2)H2 2)254 IQ) 56 10)127 5-6 12.7 The cooled solution is mixed with a few drops of starch test solution, which gives it a dark-blue color, because of the formation of iodide of starch. Then add carefully, from a burette, decinormal sodium thiosul- phate V. S. until the blue color is discharged. I2 + 2(Na,S.O,.5H.O) = 2NaI + NaaS4Oe + ioHaO. 2)254 2)495.28 10)127 10)247.64 N 12.7 gms. 24 76 gms. or 1000 cc. — Na2S203 V. S. Thus each cc. of the standard thiosulphate repre- sents 0.0127 gm. of iodine, or 0.0056 gm. of metallic iron. In both of these estimations the quantity of standard solution used should be the same. The U. S. P. requirement is 8 cc. 0.0056 X 8 = 0.0448 gm, 0.044s X .00 = 8o^ O.O56 162 a text-book of volumetric analysis. Ferrous Sulphate (Crystallized), FeSO, -J- 7HsO = | *278'^2‘—***39 §ms- °f ferrous sulphate are dissolved in about 25 cc. of water, and the solution acidulated with sulphuric acid. Decinormal potassium permanganate V. S. is then delivered in from a burette until a permanent pink color is obtained, indicating the complete oxidation of the ferrous salt. io(FeS04 + ;H20) + 2KMn04 + 8H2SQ4 = 100)2774.2 100)315.34 N 27.742 gms. 3.1534 gms. or 1000 cc. — stand- ard solution. 5Fe2(S04)3 + K2S04 + 2MnS04 + 8H20. Thus each cc. of the standard solution represents 0.027742 gm. of crystallized ferrous sulphate. Not less than 50 cc. should be used before the po- tassium permanganate ceases to be decolorized. 0.027742 X 5° = 1.387100 gms. 1.387100 X 100 , —— = 100$ 1.3871 Granulated Ferrous Sulphate, FeS04-f- 7H20, is esti- mated in the same way as the foregoing, and should correspond with it in strength. Exsiccated {Dried) Ferrous Sulphate.—This salt is tested in the same manner as the other two sulphates. It contains a larger percentage of ferrous sulphate than the other two, having less water of crystallization. Its composition is approximately FeSQ4 -)-3H20. In estimating ferrous sulphate in this salt the water of crystallization is not taken into account. Then by A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 163 deducting the percentage of ferrous sulphate from 100 the percentage of water of crystallization is obtained. ioFeS04 + 2KMn04 + 8H2S04 100)1520 100)315.34 N 15.20 gms. 3-1534 gms. or 1000 cc. — standard solution. = 5Fe3(S04)3 + K2S04 + 2MnS04 + 8H20. Each cc. of the standard solution represents o.oi52 gm. of anhydrous (real) ferrous sulphate. If one gin. of the dried salt, treated as above described, requires N 48 cc. of — permanganate solution, it contains 0.0152 X 48 = 0.7296 gm., or 72.96$ of real ferrous sulphate, and 100.00 — 72.96 = 27.04$ of water of crystallization. Any salt may be analyzed in this way for water of crystallization. If the salt is pure, the difference be- tween the percentage of real salt and 100 always repre- sents the percentage of water of crystallization. ESTIMATION OF HYPOPHOSPHOROUS ACID, HYPOPHOS- PHITES, AND OTHER OXIDIZABLE SUBSTANCES. Acidum Hypophosphorosum Dilutum.—An aque- ous solution containing about 10 per cent, by weight, of absolute hypophosphorous acid. (H.PO.) HPHA=|*^'88- N This acid may be tested by neutralization with ~ potassium hydrate V. S., as described on page 9C, 164 A text-book of volumetric analysis. The U. S. P. also directs the estimation by residual titration, given below. 0.5 gm. of diluted hypophosphorous acid is mixed with 7 cc. of sulphuric acid, and 35 cc. of decinormal potassium permanganate V. S., and the mixture boiled for fifteen minutes. The potassium permanganate, in the presence of sulphuric acid, oxidizes the hypophosphorous acid to phosphoric, as the equation shows: 5HPH202 + 6HaS04 + 2(2KMn04) 2)329 40 2)630.68 100)164.7 100)315.34 N 1.647 gms. 3-1534 gms. or 1000 cc. — V. S. 10 = 5H3P04 + 6 H20 + 2K2S04 + 4MnS04. Each cc. of the decinormal V. S. represents 0.001647 gm. of absolute hypophosphorous acid. The quantity of permanganate solution directed to be added is slightly in excess. The excess is then ascertained by retitration with decinormal oxalic acid V. S. Each cc. of oxalic acid required corresponds to one cc. of deci- normal permanganate V. S., which has been added in excess of the quantity actually required for the oxida- tion. The excess of permanganate colors the solution red, and the oxalic acid V. S. is then added until the red color just disappears, which indicates that the excess of permanganate is decomposed. 2KMnO. + 5(H,C,0,2H,0) + 3H.SO. = K,SO. + 2M11SO. + 18H O + 10CO.. If 4.7 cc. of decinormal oxalic acid V, S. are required, it indicates that 35 cc. — 4.7 cc. = 30.3 cc. of deci- A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 165 normal permanganate were actually used up in oxidizing the hypophosphorous acid. Therefore 0.001647 gm. X 30.3 = 0.0499 8m-> 0.0499 X 100 or = 9.98$ of HPH202. In the above process the boiling facilitates the oxida- tion, but if the acid is boiled before it is completely oxidized it will decompose. Hence the necessity for adding an excess of the permanganate and retitrating. Calcium Hypophosphite, Ca(PH202)!I = | —o.i gm. of the salt is dissolved in 10 cc. of water, then 10 cc. of sulphuric acid and 50 cc. of decinormal potassium permanganate V. S. are added, and the mix- ture boiled for fifteen minutes. The excess of permanganate is then found by reti- trating with decinormal oxalic-acid solution. The reactions which take place are expressed by the following equations: 5Ca(PH.O,), + 5H.SO, = sCaSO, + ioHPH.O, ; (i) ioHPHA + i2HsS04 + 4(2KMn04) = ioH8P04 + I2H20 + 4K2S04 + 8MnS04. (2) These two reactions may be written together thus: 5Ca(PH,Os)a+ i7HaS04+4(2KMn04) 4)848-35 4)1261.36 100)212.08 100) 315.34 [ard V. S. 2.1208 gms. 3-1534 gms. or 1000 cc. stand- = 5CaS04 + 4KsS04+8MnS04+ ioH3P04+i2HaO. 166 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. Thus each cc. of the standard permanganate repre- sents 0.0021208 gm. of pure Ca(PH2Oa)J. 50 cc. of decinormal potassium permanganate are about 3 cc. more than is necessary to oxidize o. 1 gm. of pure cal- cium hypophosphite. Therefore not more than 3 cc. of the standard oxalic-acid solution should be required to decolorize the solution to which 50 cc. of perman- ganate has been added. Then 0.0021208 gm. X 47 — -09968 gin. 0.9968 X 100 _ — = 99.68$ pure salt. Ferric Hypophosphite, Fe.(PH.O.). = | —This salt is estimated in the same manner as the fore- going. o. 1 gm. is dissolved in 10 cc. of water, then 10 cc. of sulphuric acid and 50 cc. of decinormal potassium per- manganate V. S. are added, and the mixture boiled for 15 minutes. The quantity of permanganate solution here added is slightly in excess of the quantity actually required to oxidize the hypophosphite. The excess is deter- mined by retitrating with decinormal oxalic acid V. S., which corresponds volume for volume with the per- manganate. Not more than 3 cc. of the standard oxalic acid so- lution should be required to decolorize the excess of permanganate, which means that 47 c.c. of the per- A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 167 manganate should actually be required to oxidize the 0.1 gm. of hypophosphite taken. The reaction is illustrated by the following equation: 5Fe.(FH,0,).+5iH,S0.+ i2(2KMn0.) I2)2505.20 12)3784.08 100) 208.77 100) 315.34 N vs 2.0877 gm. 3.i534gms.or xooocc.io = 5Fe,(S04).+KiS04+24MnS01+3oHiP04+36HaO. N This allows that each cc. of — potassium permanga- nate V. S. represents 0.0020877 gm. of ferric hypophos- phite. If 47 cc. are required to oxidize 0.1 gm. of the salt, the latter contains 0.0020877 X 47 = 0.0981-(- gm., or 98.1 -f- This corresponds to 3 per cent, by weight, of H202. .ooi7 X 3° = -OS1 gm. .051 X 100 = 3 i 1.7 Estimation of Volume Strength.—Let us look at the above equation in a different light. We there see that when potassium permanganate and hydrogen peroxide react, 10 atoms of oxygen are liberated. The permanganate itself when decomposed liberates five atoms of oxygen. Therefore of the above ten atoms only five come from the peroxide of hydrogen. 5Ha0s=5H,0 + 50; 2KMnO,+3HaS04 = K2S04 + 2MnS04 + 3H20 + 5O. In order to find the factor for volume of available oxygen, see the following equation, etc.: 5H809 + 2KMn04 + 3H2S04 100)315.34 N 3*I534gms- or 1000 cc. of ~ V. S. = K,S04 + 2MnS04 + 8H20 + 5O + 5O. 100)79.8 .798 gm. 100) 80 *.80 gm. A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 173 N Thus it is seen that each cc. of — potassium per- manganate represents .000798 (*.0008) gm. of oxygen. But we wish to find the volume of oxygen, not the N weight represented by I cc. of the — permanganate. 1000 cc. of oxygen at o° C. and 760 mm. pressure weigh 1.424488 grammes, *(1.43 gms.). Therefore, if 1.43 gms. measure 1000 cc., .0008 gin, will measure x. x = 0-5594 cc. 1000 X .0008 = 0.5594 cc. 1.43 The factor, then, for volume of oxygen, liberated N when peroxide of hydrogen is titrated with — potas- sium permanganate is 0.5594, and the number of cc. N of the — potassium permanganate consumed in the titration gives the volume of oxygen liberated by the quantity of hydrogen peroxide taken. N Thus if 30 cc. of the — V. S. were required, 0-5594 X 30 — 16.782 cc. of oxygen, 1.7)16.782 9.87 volume strength. or the number of cc. of oxygen liberated by 1 cc. of the peroxide solution tested. It is convenient to operate upon 1 cc. hydrogen- peroxide solution. Then each cc. of potassium per- 174 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. manganate V. S. used will represent 0.5594 cc. of avail- able oxygen, or 0.0008 gm. of oxygen, and it is only necessary to multiply the cc. by these numbers to obtain the volume or weight of available oxygen. Hydrogen-peroxide solution may also be volumetri- cally assayed by Kingzett’s method, which is described in the chapter on lodimetry. The gasometric estimation is also described further on. Barium Dioxide (Barium Peroxide), BaO, = | *168 9^*—This substance is assayed by treating it with an acid, and then estimating the liberated hydro- gen dioxide, as follows: Weigh off 2.11 gms. of the coarse powder, put it in a porcelain capsule, add about 10 cc. of ice-cold water, then 7.5 cc. of phosphoric acid, U. S. P., and sufficient ice-cold water to make 25 cc. Stir and break up the particles with the end of the stirrer until a clear or nearly clear solution is obtained, and all that is soluble is dissolved. 5 cc. of this solution (which corresponds to 0.422 gm. of barium dioxide) is measured off for assay. Drop into this from a burette, with constant stirring, decinormal potassium permanganate V. S. until a final drop gives the solution a permanent pink tint. Not less than 40 cc. of the decinormal permanganate V. S. should be required to produce this result. In this process, the first step is the formation of hy- drogen peroxide by treating the barium peroxide with phosphoric acid, as illustrated by the following equa- tion : BaOa + H3P04 = BaHPO, + HaOa. A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 175 The hydrogen peroxide is then estimated with deci- normal permanganate V. S. 5H202 + 2KMn04 + 3H2S04 100)169.6 100)315.34 1.699 gms. 3.1534 gms. or 1000 cc. — permanganate V. S. 100)1.70 *1.70 = K2S04 + 2MnS04 -f 8HaO + sOa. N Thus each cc. of — potassium permanganate V. S. represents 0.001696 gm. (*0.0017 gm.) of H202; and since 169.6 gins, of H202 are equivalent to 844.1 gms. , „ 0 / 5Ba02= 5H202 \ N of BaU2, gms. 169.6 gms. • o* the perman- ganate solution corresponds to 0.008441 gm. of BaO„. Not less than 40 cc. of the decinormal solution should be required. Thus .008441 X 4° = 0.3376 gm. •_3376.x 100 _ of pure Ba0 .422 Oxalic Acid, HaCs04 + 2HsO — | — Oxalic acid may be estimated either by neutralization with an alkaline V. S., or by oxidation with potassium perman- ganate V. S. The permanganate is generally used when the acid is in combination as oxalate. 176 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. The reaction is illustrated as follows: 5(HaC204 + 2HaO) + 3H2S04 + 2KMn04 100)628.5 100)315.34 6.285 gms. 3-1534 gms. or 1000 cc. — permanganate V. S. 10 = KsS04 + 2MnS04 + i8HaO + ioC02. N Thus each cc. of the — permanganate represents 0.006285 gin. of pure oxalic acid (crystallized). Note.—It must be remembered, in titrating with per- manganate, that an excess of sulphuric acid is always necessary, in order to keep the resulting manganous compound in solution, by forming a soluble manganous sulphate. If hydrochloric acid is used the solution must be very dilute, and the temperature not raised too high, other- wise chlorine will be liberated, which will spoil the analysis. It should be borne in mind that the solution of potassium permanganate should not be filtered through paper, as it is decomposed by organic matter. It may, however, be filtered through gun-cotton or glass-wool. It should never be used in a Mohr’s burette. A TEXT-BOOK OF VOLUMETRIC ANALYSIS. Table of Substances which may be Estimated by Oxidation, by Potassium Dichromate or Potassium Permanganate, showing Formula, Molecular Weight, Factor, etc. Name of Substance. Formula. Mole- cular Wt. -V. S. 10 Used. Factor (exact). hph2o2 65.88 ‘25-7 .OO1647 .O06285 OC844I .0021209 0020877 •OI1573 H 2 C 2 O 4 ~fl 2 O BaOj 2KMn04 2KMn04 Ca(P H202)2 Fe2(PH202)6 FeC03 169.67 501.04 115.64 2KMn04* j K.Cr.O*, ( | 2KMii04 f j K2O20t f 1 2KM1104 f FeO 71.84 .007195 Ferrous sulphate (anhydrous) — FeSG4 151-58 j K2Cr207 ( 1 2KMn04 r .015170 Ferrous sulphate (dried) 2FeS04-|-3H20 357-28 j K2Cr207 [ 1 2KMnG4 ( .017864 Ferrous sulphate (crystallized)... FeS04 7H20 277.42 j K2Cr207 ( ) 2KMn04 ) .027742 Fe2 111.76 ) K2Lr2U7 [ 1 2KMnG4 f .005588 h2o2 33-Q2 31 93 2KMn04 .001696 .000798 Oxygen,wt. of available, in H202 2KMn04 “ volume “ “ “ o2 2KMn04 •5594 kph2o2 103 Qt 105.84 2KMn04* .002598 .OO2646 Sodium hypophosphite NaPH202-)-H20 2KMn0j* * Determined by residual titration with decinormal oxalic acid V. S. The factors given in this table are calculated upon the revised atomic weights, which are indorsed by the U. S. P. 178 A TEXTBOOK OF VOLUMETRIC ANALYSIS. CHAPTER XIII. ANALYSIS BY INDIRECT OXIDATION. THIS method of analysis is based upon the oxidizing power of iodine. Iodine acts upon- the elements of water, forming hydriodic acid with the hydrogen, and liberating oxy- gen in a nascent state. Nascent oxygen is a very active agent, and readily combines with and oxidizes many substances, such as arsenous oxide, sulphurous acid, sulphites, thiosul- phates, etc. As2Os -f- 2H20 -(- 2l3 — 4HI -|- As205 ; HaS03 + HaO + I, = 2HI + h,so4. Therefore iodine is said to be an indirect oxidizer, and may be used for the estimation of a great variety of substances, with extreme accuracy. The end of the reaction in an analysis by this method is ascertained by the use of starch test solution, which produces, with the slightest trace of free iodine, a dis- tinct blue color. In making an analysis with standard iodine solution, the substance under examination is brought into dilute solution, the starch solution added, and then the iodine, in the form of a decinormal solution, is delivered in from a burette, stirring or shaking constantly, until a final A TEXT-BOOK OF VOLUMETRIC ANALYSIS. drop colors the solution blue—a sign that a slight ex- cess of iodine has been added. Decinormal Iodine V. S., I = | } in i litre.—12.653 gms. of pure iodine are dissolved in 300 cc. of distilled water containing 18 gms. of pure potassium iodide. Then enough water is added to make the solution measure, at 150 C. (590 F.), exactly 1000 cc. The solution should be kept in small glass-stoppered vials, in a dark place. The potassium iodide used in this solution acts merely as a solvent for the iodine. If pure iodine be not at hand, it may be prepared from the commercial article as follows : Powder the iodine and heat it in a porcelain dish placed over a water-bath, stirring constantly with a glass rod for 20 minutes. Any adhering moisture, to- gether with any cyanogen iodide, and most of the iodine bromide and iodine chloride, is thus vaporized. Then triturate the iodine with about 5 per cent, of its weight of pure, dry potassium iodide. The iodine bromide and chloride are thereby decomposed, potas- sium bromide and chloride being formed, and iodine liberated from the potassium iodide. The mixture is then returned to the porcelain dish, covered with a clean glass funnel, and heated on a sand- bath. A pure resublimed iodine is then obtained. If pure iodine is used in%making this solution, there is no necessity for checking (standardizing) it. But if desired, the solution may be checked against pure arsenous acid or pure sodium thiosulphate. A TEXT-BOOK OF VOLUMETRIC ANALYSIS. ESTIMATION OF ARSENOUS ACID. Arsenous Anhydride, Arsenic Trioxide, As2Os = | *198—When arsenous acid is brought in contact with iodine in the presence of water and an alkali, it is oxidized into arsenic acid, and the iodine is decolorized. The reaction is: As)03 -f- 2l2 -(- 2H20 — As2Ob -f- 4HI; NaHC03 + HI = Nal + H20 + COa. The alkali should be in sufficient quantity to combine with the hydriodic acid formed, and must be in the form of potassium or sodium bicarbonate. The hydroxides or carbonates should not be used, as they interfere with the indicator. Starch solution is used as the indicator, a blue color being formed as soon as the arsenous acid is entirely oxidized into arsenic acid. 0.1 gm. of arsenous acid is accurately weighed and dissolved, together with about I gm, of sodium bicar- bonate, in 20 cc. of water heated to boiling. Allow the liquid to cool, add a few drops of starch T. S., and allow the decinormal iodine V. S. to flow in, shaking or stir- ring the mixture constantly, until a permanent blue color is produced. The following equation illustrates the reaction : As,0, -f 5H20 + 2la = 4HI + 2HsAsO,. 4)197.68 4)5Q6 xo) 49-42 10)126.5 N 4.942 gms. 12.65 gms. or 1000 cc, — Iodine V. S. A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 181 N Thus it is seen that each cc. of the — standard so- 10 lution represents 0.004942 gm. of pure As203. If 20 cc. are consumed, then O.OQ4942 X 20 = 0.09884 gm. •°9884ix 100 = 98.84$ The U. S. P. requirement is 98.8$ of As303. The starch T. S. is not used in the U. S. P. process, and the end of the reaction is known by the iodine being no longer decolorized. But with starch the indication is exceedingly delicate, and it should always be used. Liquor Acidi Arsenosi, U. S. P.—Measure accu- rately 10 cc. of the solution, add to it 1 gm. of sodium bicarbonate, and boil for a few minutes. Then allow the liquid to cool, and dilute it to 50 cc. with water. A little starch T. S. is then added and the decinormal iodine V. S. run in from a burette, until a final drop produces the blue color of starch iodide. N Each cc. of — I. V. S. represents 0.004942 gm. of As303. (See Estimation of Arsenous Acid.) The U. S. P. requirement is that 24.7 cc. of the liquor acidi arsenosi, when treated as above, will con- sume 49.4 to 50 cc. of decinormal iodine V. S. Use 2 gms. of the bicarbonate. 0.004942 X 5° = 0.2471 gm. 0.2471 X IOO — = I ms* *n 1 —Sodium thiosulphate is a salt of thiosulphuric acid in which two atoms of hydrogen have been replaced by sodium ; it therefore seems that a A TEXT-BOOK OF VOLUMETRIC ANALYSIS. normal solution of this salt should contain one half the molecular weight in grammes in one litre. But this salt is used chiefly for the estimation of iodine, and, as stated before, one full molecular weight reacts with and decolorizes one atomic weight of io- dine; and since one atom of iodine is chemically equiv- alent to one atom of hydrogen, a full molecular weight of sodium thiosulphate, must be contained in a litre of its normal solution. Sodium thiosulphate is easily obtained in a pure state, and therefore the proper weight of the salt, re- duced to powder and dried between sheets of blotting- paper, may be dissolved directly in water, and made up to one litre. The U. S. P. directs that a stronger solution than necessary be made, its titer found by iodine, and then the solution diluted to the proper measure. 30 gms. of selected crystals of the salt are dissolved in enough water to make, at or near 150 C. (590 F.) 1100 cc. Transfer 10 cc. of this solution into a flask or beaker, add a few drops of starch T. S., and then gradually deliver into it from a burette decinormal iodine solu- solution, in small portions at a time, shaking the flask after each addition, and regulating the flow to drops toward the end of the operation. As soon as a blue color is produced which does not disappear upon shak- ing, but is not deeper than pale blue, the reaction is completed. Note the number of cc. of iodine solution used, and then dilute the thiosulphate solution so that equal volumes of it and the decinormal iodine V. S. will exactly correspond to each other, under the above- mentioned conditions. A TEXT-BOOK OF VOLUMETRIC ANALYSIS. Example.—The 10 cc. of sodium thiosulphate, we will assume, require 10.7 cc. of decinormal iodine V. S. The sodium-thiosulphate solutioi must then be diluted in the proportion of 10 cc. to 10.7 cc., or 1000 cc. to 1070 cc. After the solution is thus diluted a new trial should be made, in the manner above described, in which 50 cc. of the thiosulphate solution should require exactly 50 cc. of the decinormal iodine V. S. to produce a faint-blue color. The solution should be kept in small dark amber- colored, glass-stoppered bottles, carefully protected from dust and air. One cc. of this solution is the equivalent of: Iodine 0.012653 gramme. Bromine 0.007976 “ Chlorine 0.003537 “ Iron in ferric salts 0.005588 “ Iodine, I = | *^265^’—Dissolve °-32 gm* °f iodine in 20 cc. of water, in a beaker or flask, with the aid of 1 gm. of potassium iodide; the solution is mixed with a few drops of starch T. S., and then the deci- normal sodium thiosulphate V. S. gradually delivered in from a burette, in small portions at a time, shaking the flask after each addition, and regulating the flow to drops toward the end of the reaction, until a final drop just discharges the blue color. Note the number of cc. of decinormal sodium thio- sulphate V. S. consumed, and multiply this number by the factor for iodine. A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 2(NaaSA + 5H20) + I. = NaaS4Oe + 2NaI + ioHaO. 2)4q6_ 2)253 10)248 10)126.5 24.8 gms. or 12.65 gms. N 1000 cc. — V. S. 10 Thus the factor for iodine, that is, the quantity equiv- N alent to I cc. of — thiosulphate V. S., is 0.01265 gm. 0.32 gm. of iodine, which answers to the tests of the N U. S. P., requires at least 25 cc. of the — V. S. 0.01265 X 25 = 0.31625 gm. , .31625 X too _ _ . ... —— = 98.8$ pure iodine. Liquor lodi Compositus (Lugol’s Solution).—This is an aqueous solution of iodine and potassium iodide. It is estimated for iodine in the same way as the foregoing. The potassium iodide acts merely as a sol- vent for free iodine, and does not enter into the re- action. 10 or 12 gms. of the solution is a convenient quan- tity to operate upon. Starch T. S. is the indicator. The U. S. P. states that 12.66 gms. of the solution should require for complete decoloration from 49.3 to 50 cc. of decinormal sodium thiosulphate V. S. N As shown by the above equation, each cc. of the — V. S. represents 0.01265 gm. of pure iodine. There- fore 50 cc. represent 0.01265 X 50 = .6325 gm. .6325 X ioo , . — c% pure iodine, ubout* 12.66 j r 194 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. Tinctura lodi (Tincture of Iodine).—This is an al- coholic solution of free iodine, and must be diluted with a solution of potassium iodide, before titration, in order to provide sufficient liquid to keep the resulting salts in solution. Aqua Chlori (Chlorine Water)—This is an aqueous solution of chlorine, Cl = ] .«35-37 containing at least ( x35-4 b 0.4$ of the gas. The estimation of chlorine is effected in an indirect way, namely, by determining the quantity of iodine which it liberates from potassium iodide. A definite quantity of chlorine will liberate a definite quantity of iodine from an iodide ; these quantities are in exact proportion to their atomic weights, as the equation shows: 2KI + Cla = 2KC1 + I2 2)70-74 2)253 io)35-37 io)i26.5 3-537 gms. 12,65 gms. Thus it is seen that by estimating the liberated io- dine the quantity of chlorine may be determined with accuracy. Ten gms. is a convenient quantity to operate upon. To this about half a gramme of potassium iodide is added. A little starch T. S. is then introduced, and the titration is begun, with decinormal sodium thiosul- phate V. S. When the blue color of starch iodide has entirely disappeared the reaction is finished. The reaction between iodine and sodium thiosulphate is illustrated by the following equation • A TEXT-BOOK OF VOLUMETRIC ANALYSIS. I, + 2(NaaS103 + 5HiO) = 2NaI+Na2S406+ioH30. 2)253 2)496 10)126.5 10)248 N 12.65 gms. 24.8 gms. or 1000 cc. —V. S. 10 N Ihus we see that 1000 cc. of — NaaS203 5HaO repre- sent 12.65 gms. of iodine, which are equivalent to 3.537 gms. of chlorine. Each cc. therefore is equivalent to .003537 gm* of chlorine. This number is the factor which, when mul- N tiplied by the number of cc. of — thiosulphate V. S. used, gives the weight in grammes of chlorine, contained in the quantity of chlorine water acted upon. The U. S. P. requirement is that 17.7 gms. of chlorine water, when mixed with 1 gm. of potassium iodide dis- N solved in 10 cc. of water, and titrated with — sodium 10 thiosulphate V. S. should consume not less than 20 cc. of the latter in decolorizing the solution. .003537 X 20 = .07074 gm. .07074 X 100 . f , , . —— = 0.4% of chlorine. 17.7 Chlorinated Lime (Calx Chlorata, Chloride of Lime, Bleaching-powder).—This substance was formerly sup- posed to be a compound of lime and chlorine, CaOCl3, and hence the name chloride of lime. It is now gener- ally considered to be a mixture of calcium chloride and 196 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. calcium hypochlorite, CaCl, -f- Ca(ClO), or 2(CaOCl3). The hypochlorite is the active constituent. This is a very unstable salt, and is readily decomposed even by carbonic acid. When treated with hydrochloric acid it gives off chlorine. The value of chlorinated lime as a bleaching or dis- infecting agent depends upon its available chlorine, that is, the chlorine which the hypochlorite yields when treated with an acid. In estimating the available chlorine, the latter is liberated with hydrochloric acid. This liberated gas, then, acting upon potassium iodide, sets free an equiva- lent amount of iodine. The quantity of iodine is then determined, and thus the amount of available chlorine found. .1 to .2 gm. is a convenient quantity to oper- ate upon. The U. S. P. directs to weigh off *0.35 (0.354) gm. of chlorinated lime. This is to be thoroughly triturated with 50 cc. of water and carefully transferred, together with the washings into a flask. 0.8 gm. or more of po- tassium iodide and 5 cc. of diluted hydrochloric acid are then added, and into the resulting reddish-brown liquid, N the — sodium thiosulphate V. S. is delivered from a 10 r burette. Towards the end of the titration, when the brownish color of the liquid is very faint, a few drops of starch T. S. are added and the titration continued until the bluish or greenish color produced by the starch has entirely disappeared. Not less than 35 cc. of the volumetric solution should be required to pro- duce this result. The reactions which take place in this process are illustrated by the following equations : A TEXT-BOOK OF VOLUMETRIC ANALYSIS. CaCl„Ca(C10), -f 4HCI = 2CaCls + 2HsO + 2Cla 2C1, + 4KI = 4KCI -f- 2la. 4)i4i-48 4)5°6 10) 35-37 10)126.5 3-537 gras. 12.65 gms. 2l, + 4(Na,SaO.+5H,0)=4NaI+2NaaS,Oe+2oH30. 40)506 40)496 N 12.65 gms. 24.8 gms. or 1000 cc. — thiosulphate V. S. We thus see that 1 cc. of the decinormal volu- metric solution represents 0.01265 gm. of iodine, which is equivalent to 0.003537 gm- °f available chlorine. Then 0.003537 X 35 = 0.12379 gm. 0.12379 X 100 ....... • = 35% of available chlorine. •35 This is a very rapid method for estimating chlorine ; but when calcium chlorate is present in the bleaching- powder (and it often is, through imperfect manufact- ure) the chlorine from it, is recorded, as well as that from the hypochlorite, the chlorate being decomposed into chlorine, etc., by hydrochloric acid. The chlorate, however,is of no value in bleaching; its chlorine is not available. Hence, unless the powder is known to be free from chlorate, the analysis should be made by means of arsenous-acid solution. The Arsenous-acid Process. — 0.35 gm. of the bleaching-powder is rubbed to a smooth paste with 50 cc. of water, as described above. A measured excess of decinormal arsenous acid V. S. is then added ; this 198 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. is followed by a little starch T. S., and then decinormal iodine V. S. added until the blue color appears. De- duct the number of cc. of the standard iodine solution used from those of standard arsenous-acid solution, and the quantity of the latter which went into combi- nation is found. N Each cc. of — As2Os V. S. represents .003537 gm. of available chlorine. 2CaOCla + As2Os = AssOt + 2CaCl, 4)141-48 4)iq8_ 10) 35-37 10) 49-5 N 3-537 gms. 4.95 gms. or 1000 cc. V. S. Decinormal Arsenous-acid Solution is made by dissolving 4.95 gms. of the purest sublimed arsenous anhydride (As2Os) in about 250 cc. of distilled water with the aid of about 20 gms. of pure potassium bicar- bonate. The acid should be in fine powder, and the mixture warmed, to effect complete solution. The solution is checked with decinormal iodine V. S., using starch as indicator. Decinormal arsenous-acid solution and decinormal iodine solution should correspond, volume for volume. The strength of bleaching powder is expressed in per cent of available chlorine, or in degrees (Gay- Lussac). The latter represents the number of litres of chlorine, at o° C. and 760 mm. pressure, available from one kilogramme of the bleaching powder. The relation between these two methods is shown in the table following: A TEXT-BOOK OF VOLUMETRIC ANALYSIS 199 Degrees. Gay- Lussac. Per Cent Chlorine Degrees. Gay- Lussac. Per Cent Chlorine. Degrees, Gay- Lussac. Per Cent Chlorine. 65 20.65 80 25.42 95 30.19 66 20.97 8l 25-74 96 30.51 67 21.29 82 26.06 97 30.82 68 2I.6l 83 26.37 98 31-14 69 21.93 84 26.69 99 31.46 70 22.24 85 27.OI 100 3I-78 71 22.56 86 27-33 IOI 31.09 7? 22.88 87 27.65 102 31 41 73 23.20 88 27.96 103 32-73 74 23-51 89 28.28 104 33-05 75 23-83 90 28.60 105 33-36 76 24.15 9i 28.92 106 33-68 77 24.47 92 29-33 107 34.00 78 24.79 93 29-55 108 34-32 79 25.10 94 29.87 109 34-64 The various bleaching preparations of the market which depend upon their available chlorine are all salts of hypochlorous acid (HCIO) or solutions of such salts. Eau de Javelle, Javelle' s Water, is a solution of potassium hypochlorite and potassium chloride, A solution of magnesium hypochlorite is known in com- merce as Ramsay's or Gronvelle's Bleaching Fluid. The solution known as Wilson s Bleaching Fluid con- tains aluminium hypochlorite. Liquor Sodae Chloratae (Solution of Chlorinated Soda; Labarraque’s Solution).—This is an aqueous solution of several chlorine compounds of sodium, principally sodium chloride and hypochlorite, contain- ing at least 2.6/0 of available chlorine. In this solution, as in chlorinated lime, it is the available chlorine which is estimated. The chlorine is first liberated with hydrochloric or sulphuric acid ; this then liberates iodine from potassium iodide, and the 200 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. free iodine is then determined by standard solution of thiosulphate. *6.7 (6.74) gms. of chlorinated soda solution are mixed with 50 cc. of water, 2 gms. of potassium iodide, and 10 cc. of hydrochloric acid, together with a few drops of starch T. S. Then pass into the mixture from a burette sufficient decinormal sodium thiosulphate V. S. to just discharge the blue or greenish tint of the liquid. The reaction is illustrated by the following equation. Hydrochloric acid liberates chlorine from the salts in the solution : NaCl,NaC10 + 2HCI = 2NaCl -f H20 -f Cl2. 70.74 The chlorine then liberates iodine from potassium iodide; Cl, + 2KI = 2KCI + I2. 20)70-74 20)253 3-537 12.65 The iodine is then determined by sodium thiosul- phate V. S.: I2 -)- 2(NaaS303+5Ha0)—2NaI-j-Na!!S408-[-ioH,10. 2)253 2)40 i o) 126.5 10)248 N 12.65 gms. 24.8 gms. or 1000 cc. — V. S. Thus each cc. of standard solution represents .01265 gm. of iodine, which is equivalent to .063537 gm, of available chlorine. In practice the potassium iodide should always be added before the hydrochloric acid is, so that the A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 201 chlorine has potassium iodide to act upon, as soon as it is itself liberated, and thus any loss of chlorine is obviated. In the pharmacopceial test above given not less than N so cc. of the — V. S. should be required. J 10 0.003537 X 50 = 0.17785 gm. OI7785JOOO = 2£ % availaMe c,_ 6.7 Instead of weighing off the U. S. P. quantity, any other convenient weight may be taken. ESTIMATION OF FERRIC SALTS. When a ferric salt in an acidulated solution is di- gested with an excess of potassium iodide the salt is reduced to the ferrous state, and iodine is set free. Fe3Cl6 + 2KI = 2FeCl3 + 2KCI + I, One atom of iodine is liberated for each atom of iron in the ferric state. The liberated iodine is then determined by sodium thiosulphate, in the usual way. 12.65 gins, of iodine = 5.6 gms. of metallic iron. This is the method of the U. S. P. ; it is given in detail here. *0.56 (0.5588) gm. of the salt is dissolved in 10 or 15 cc. of water and 2 cc. of hydrochloric acid in a glass- stoppered bottle having a capacity of about 100 cc. 1 gm. of potassium iodide is then added, and the mix- ture digested for half an hour at a temperature of 40° C. (104° F.). During the digestion the stopper should be 202 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. left in the bottle, and the heat not allowed to rise too high, otherwise the liberated iodine will be volatilized. When cool a few drops of starch T. S. are added. N It is now ready for titrating with — sodium thiosul- phate. Each cc. corresponds to I per cent, of metallic iron. When the quantity of metallic iron and the chemical formula for the ferric salt under estimation are known, the quantity of pure salt is easily found by calculation. In all the estimations of ferric iron it is convenient to take 0.56 gm. of the salt. Each cc, of the volumetric solution used will then represent 1$ of metallic iron, assuming the atomic weight of iron to be 56, Ferric salts may be tested in many other ways; for instance: A ferric salt in solution may be filtered through a column of zinc dust, which reduces it to the ferrous state. This is then estimated with potassium perman- ganate V. S. in the usual method, or the ferric solution is treated with a few small pieces of zinc or magnesium coarsely powdered, until complete reduction is effected. When a red color is no longer produced by sulphocy- anate of potassium the ferric salt is completely re- duced, and may be estimated with potassium perman- ganate V. S. Stannous chloride, ammonium bisulphite, and other substances may also be used as reducing agents. Ferric Chloride, FeaCl. -f i2HaO = { — 1 361 a 540.4 *0.56 (0.5588) gm. of the salt is dissolved in a glass- stoppered bottle (having a capacity of about 100 cc.) in 10 cc. of water and 2 cc. of hydrochloric acid, and A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 203 after the addition of 1 gm. of potassium iodide, is kept for half an hour at a temperature of 40° C. (104° F.), then cooled, mixed with a few drops of starch T. S., and titrated with decinormal sodium thiosulphate V. S. until the blue or greenish color of the liquid is dis- charged. Each cc. represents *0.0056 gm. or 1$ of metallic iron, or 0.026975 gm. of pure ferric chloride. The following equations illustrate the reactions: FeaClB+i2H20+2KI =1 2FeCl24-2KCl+ I, + i2HsO. 20)539-5 20)253 26.975 gms. 12.65 gms. Then I. + 2(Na,S.O. + 5HsO) 20)253 20)496 N 12.65 gms. 24.S gms. or 1000 cc. — V. S. 10 = 2NaI + Na2S4Oe + ioH20. N 20 cc. of the — V. S. should be required, which rep- 10 resents 20$ of metallic iron, or 96.34$ of pure ferric chloride (crystallized): 0.026975 X 20 = 0.5395 grn. 0.5395 X 100 . — = 93-34* Liquor Ferri Chloridi (Solution of Ferric Chloride). —This is an aqueous solution of ferric chloride, Fe2Cl6 = | contain*ng about 37.8 per cent, of the an- hydrous salt or about 13 per cent, of metallic iron. 204 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 0.56 (or .5588) gm. of the solution is introduced into a glass-stoppered bottle (having a capacity of about 100 cc.), together with 15 cc. of water and 2 cc. of hydro- chloric acid. 1 gm. of potassium iodide is then added, and the mixture kept for half an hour at 40° C. (104° F.), then cooled, and mixed with a few drops of starch N T. S. and titrated with — sodium thiosulphate V. S. 10 until the blue or greenish color of the liquid is dis- charged. 0.56 gm. of the solution having been taken, each cc. of the standard solution represents 1 per cent, and 13 cc. should be required. If 1.12 gms. are taken, as the U. S. P. directs, each cc. represents 0.5 per cent, and 26 cc. should be required. The reactions are the same as in ferric chloride, each cc. representing 0.026975 gm. of crystallized ferric chloride, or 0.016199 gm. of anhydrous ferric chloride, or .0056 gm. of metal- lic iron. To find percentage: Multiply by number of cc. used, then multiply the result by 100 and divide by the quantity of solution taken. Tinctura Ferri Chloridi (Tincture of Ferric Chlo- ride).—A hydro-alcoholic solution of ferric chloride, Fe2Cl6 = | containing about 13.6 per cent. of anhydrous ferric chloride, and corresponding to about 4.7 (4.69) per cent, of metallic iron. To estimate this tincture follow the directions given for liquor ferri chloridi. Ferric Citrate, Fe2(C6H607)3 = | fc4^48-—*°-S6 (0.5588) gm. of the salt is dissolved in a glass-stop- pered bottle (having a capacity of 100 cc.) in 15 cc. of water and 2 cc, of hydrochloric acid, with the aid of gentle heat. 1 gm. of potassium iodide is then added, A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 205 and the mixture kept for half an hour at a temperature of 40° C. (104° F.). It is then cooled, and a few drops of starch T. S. added. The decinormal sodium thio- sulphate V. S. is then delivered in from a burette, until the blue or greenish color of the liquid just disappears. Each cc. of the decinormal solution represents 1 per cent, or 0.0056 gm. of metallic iron, corresponding to 0.024424 gm. of ferric citrate. 3Fe.(C.H.O,),+6KI=2Fe.(C.H.O,),+2K.C.H.O,+3l.- Ferric citrate. Ferrous citrate. 3Fe., \ 488.48 6)759 6)335-28 3 10)126.5 10) 55-88 6)1465.44 12.65 gms, 5.588 gms. 10) 244.24 (*5.6 gms.) 24.424 gms. I, + 2(NaaSA, 5H40) = 2NaI+Na2SiOs+ ioH,0. 2)253 2)496 io)i26.5 10)248 12.65 gms. 24.8 gms. or 1000 cc. — sodium thiosulphate. Thus each cc. represents 0.01265 gm. of iodine, which corresponds to 0.024424 gm. of ferric citrate or *0.0056 gm. metallic iron. 16 cc. = 16 X 0.0056 = .0896 gm. metallic iron. :o826x roo = lg 0.56 i6 X 0.024424 = 0.390784 gm. ferric citrate. 0390784 x_ioo = O.56 * * Liquor Ferri Citratis (Solution of Ferric Citrate). —This is an aqueous solution of ferric citrate, corre- sponding to about 7.5 per cent, of metallic iron. 206 a text-book of volumetric analysis. *0.56 (0.5588) gm. of the solution is introduced into a glass-stoppered bottle (having a capacity of about 100 cc.), together with 1 5 cc. of water and 2 cc. of hy- drochloric acid. 1 gm. of potassium iodide is then added, and the mixture kept at a temperature of 40° C. (104° F.) for half an hour; it is then cooled and mixed with a few drops of starch T, S., and deci- normal thiosulphate V. S. delivered in from a burette until the blue or greenish color of the liquid is dis- charged. Each cc. of the volumetric solution indicates 1 $ of metallic iron. If *1.12 (1.1176) gms. of the liquor are taken, as the U. S. P. directs, each cc. of the V. S. used represents 0.5$ of metallic iron. Iron and Ammonium Citrate (Ferri et Ammonii Citras).—The precise chemical constitution of this preparation is not determined. Therefore the metallic iron only is estimated, of which it should contain 16 per cent. Ammonio-ferric Tartrate (Ferri et Ammonii Tar- tras).—The exact chemical composition of this com- pound is not known. It is, theoretically, 2(FeO)- NH4C4H406.3H20). It should contain 17 per cent, of metallic iron. Potassio-ferric Tartrate (Ferri et Potassii Tartras). —There is some difference of opinion as to the com- position of this salt. It is probably a double salt, con- sisting of one molecule of ferric tartrate, Fes(C4H406)3 and one of potassium tartrate, K2C4H406, with one of H20. It should contain 15 percent, of metallic iron. Soluble Ferric Phosphate (Ferri Phosphas Solu- bilis).—This salt is called soluble ferric phosphate in order to distinguish it from the true ferric phosphate. It is not a definite chemical compound, but a mixture A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 207 of citrate and phosphate of sodium and iron It should contain 12 per cent of metallic iron. The foregoing four salts being of indefinite chemical composition, are tested for metallic iron only, as follows : o.$6 (0.5588) gm. of the salt is dissolved in a glass- stoppered bottle (having a capacity of 100 cc.) in 15 cc. of water and 2 cc. of hydrochloric acid. 1 gm. of po- tassium iodide is then added, and the mixture kept at 40° C. (104° F.) for half an hour, then cooled, a few drops of starch T. S. added, and decinormal sodium thiosulphate V. S. delivered in slowly from a burette until the blue or greenish color of the liquid is com- N pletely discharged. Each cc. of — V. S. represents 1 per cent, of metallic iron, if 0.56 (0.5588) gm. of the salt is taken. Iron and Quinine Citrate (Ferri et Quininse Cit- ras).—The U. S. P. gives an assay process for quinine and one for iron to be applied to this salt. ESTIMATION OF THE QUININE. 1.12 (1.1176) gms. of the salt are dissolved in a capsule in 20 cc. of water, with the aid of gentle heat. The solution is poured into a separator, the capsule is rinsed with a little water, and the rinsings added to the liquid in the separator; when this has become cool, add 5 cc. of ammonia water and 10 cc. of chloroform, and shake. Allow the liquids to separate, draw off the chloroformic layer, and add to the residual liquid a second and a third portion of 10 cc. of chloroform added, shaking after each addition, and drawing off the chloroformic solution. The combined chloroformic a text-book of volumetric analysis. solutions are evaporated spontaneously in a tared cap- sule, and the residue dried at ioo° C. (212° F.) to a constant weight. It should weigh not less than 0.1288 gm. 0.1288 X ioo , f , . , YYij6—= ° dned quinine. In the above assay the ammonia water precipitates the quinine and the chloroform dissolves it. Then by evaporating the chloroformic solution the quinine is obtained. ESTIMATION OF THE IRON. The aqueous liquid from which the quinine has been, removed, as above described, is heated on a water-bath until the odor of chloroform and ammonia has disap- peared; allow the liquid to cool, and dilute it with water to the volume of 50 cc. Take 25 cc. of this, put it in a glass-stoppered bottle (having a capacity of 100 cc.), add 2 cc. of hydrochloric acid and 1 gm. of potassium iodide, and digest at 40° C. (104° F.) for half an hour. Allow it to cool, add a few drops of starch T. S. and titrate with decinormal sodium thiosulphate V. S. until the blue or greenish color is discharged. Each cc. of the volumetric solution represents 0.0056 (.005588) gm. of metallic iron, or 1 per cent. 14.5 cc. should be required. 0.0056 X 14-5 = o.o8i2 gm. 0.0812 X IOO , —— = 14.5$ 0.56 Soluble Citrate of Iron and Quinine (Fern et Quininae Citras Solubilis).—This salt is assayed for A TEXT-BOOK OF VOLUMETRIC ANALYSIS. quinine and iron in the manner above described under Ferri et Quinince Citras, and should respond to the requirements for the latter. Iron and Strychnine Citrate (Ferri et Strychninae Citras).—This salt should be tested quantitatively for strychnine and iron. ESTIMATION OF THE STRYCHNINE. *2.24 (2.2352) gms. of the salt are dissolved in a sep- arator in 15 cc. of water, 5 cc. of ammonia water are then added and 10 cc. of chloroform, and the mixture shaken. Set aside so as to allow the liquids to separate, draw off the chloroformic layer, add a second and a third portion of 10 cc. of chloroform, shaking each time and drawing off the chloroformic solution. The chloroformic extracts are then mixed, and allowed to evaporate spontaneously in a tared capsule. The resi- due is then dried at ioo° C. (2120 F.) to a constant weight. This residue should not weigh less than 0.02 gm. nor more than 0.0224 gm., corresponding to not less than 0.9 nor more than 1 per cent, of strychnine. .0224 X IOO — 2.24 ESTIMATION OF THE IRON. The aqueous liquid from which the strychnine has been removed in the manner described above, is heated on a water-bath until the chloroform and ammonia are entirely volatilized. This is then allowed to cool, and diluted with water to the volume of 100 cc. 25 cc. of 210 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. this are transferred to a glass-stoppered bottle (having a capacity of ioo cc.), 2 gms. of hydrochloric acid and I gm, of potassium iodide are then added, and the mix- ture kept at a temperature of 40° C. (104° F.) for half an hour. After it has been allowed to cool add a few drops of starch T. S., and titrate with decinormal so- dium thiosulphate V. S. until the blue or greenish color N of the liquid is entirely discharged. 16 cc. of the — V. S. should be required to produce this result, each cc. corresponding to 1 per cent, or 0.0056 gm. of metallic iron. 0.0056 x 16 = 0.0896 gm. 0.0896 X ioo . . f —— = i6% of Fe. 0.56 Ammonio-ferric Sulphate (Ferri et Ammonii Sul- phas ; Ammonio-ferric Alum), Fes(S04)3.(NH4)aS04 -j- 24H20 = | **'—This salt has a definite chemical composition, and therefore by determining the quan- tity of metallic iron the quantity of pure salt may be found by calculation. The U. S. P. process for assay is as follows: 0.56 (0.5588) gm. of the salt is dissolved in a glass- stoppered bottle (having a capacity of 100 cc.) in 15 cc. of water and 2 cc. of hydrochloric acid, 1 gm. of potas- sium iodide is then added, and the mixture kept at a temperature of 40° C. (104° F.) for half an hour. It is then allowed to cool, and mixed with a few drops of starch T. S., and titrated with decinormal sodium thio- sulphate V. S. until the blue or greenish color of the A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 211 N liquid is discharged. Not less than n.6 cc. of the — V. S. should be required, each cc. corresponding to 1 per cent, or .0056 gm. of metallic iron, or 0.0482 gm. of the salt. See the following equations: (Fe.) Fei(S04)3.(NH4)aS04.24H?0 + 2KI V v * 2) 112 2)*9&4 10) 56 10) 482 5.6 gms. 48.2 gms. = 2 FeS04+ KaS04 + (NH4)aS04 + Ia + 24HaO. 2)253 10)126.5 12.65 gms. Then 2)253 2)496 10)126.5 10)248 12.65 gms. 24.8 gms. or 1000 cc. — V. S. 10 I, + 2(NaaSa03.5H10)=2NaI+NaaS40.+ ioHaO. N Thus it is seen that 1 cc. of — V. S. represents 0.01265 gm. of iodine, and this corresponds to 0.04.82 gm. of ammonio-ferric sulphate, or 0.0056 gm. of metallic iron. 0.0482 X 11.6 = 0.55912 gm. 0.55912 X 100 0 , , ,, l4. I = of the pure salt. 0.0056 X H.6 = .06496 gm. .06496 X ioo - . — 11.656 of Fe. 0.56 7 212 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. Soluble Ferric Pyrophosphate (Ferri Pyrophos- phas Solublilis).—This is estimated according to the U. S. P. in the following manner: 0.56 (0.5588) gm. of the salt is dissolved in a glass- stoppered bottle (having a capacity of 100 cc.) in 10 cc of water, then 10 cc. of hydrochloric acid and subse- quently 40 cc. of water are added. Then 1 gm. of potassium iodide is put into the solution and the tem- perature kept at 40° C. (104° F.) for half an hour. The liquid is then cooled and a few drops of starch T. S. N added, and the — sodium thiosulphate V. S. delivered in from a burette, until the blue or greenish color is N completely discharged. Each cc. of the V. S. repre- sents 1 per cent, or 0.0056 gm. of metallic iron. True ferric pyrophosphate has the chemical compo- sition Fe4(P,07)3-|-qH20. The soluble ferric pyro- phosphate of the U. S. P. is a mixture of ferric pyro- phosphate and sodium citrate. The reaction with potassium iodide is expressed as follows: Fe4(PA)3 + 4KI = 2FeaPA + K4PA + 2l. 4)746 4)506 10)186.5 10)126.5 18,65 gms. 12.65 gms. Thus 18.65 gms. of ferric pyrophosphate cause the liberation of 12.65 gms. of iodine, and since each cc. of N — sodium thiosulphate V. S. will absorb, and con- sequently represent, .01265 gm. of iodine, it corre- sponds to 0.01865 gm. of pure ferric pyrophosphate. A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 213 10 cc. of the decinormal solution is the quantity which the U. S. P. requires should be used. 0.01865 X io = 0.1865 gm. 0.1865 X ioo —056 = 33-3* of ferric pyrophosphate, which corresponds to 10$ of metallic iron in the U. S. P. salt. Ferric Valerianate (Ferri Valerianas), Fea(C5H9Oa)e — 1*718.—The true ferric valerianate is illustrated by the above formula, but the U. S. P. salt is of variable composition, and should contain not less than 15$, nor more than 20$, of iron in combination. The estimation is conducted as follows: *0.56(0.5588) gm. of the salt is dissolved in a glass-stoppered bottle (having a capacity of 100 cc.) in 2 cc. of hydrochloric acid. This decomposes the salt, forming ferric chlo- ride and liberating valerianic acid. 15 cc. of water are now added, together with 1 gm. of potassium iodide, and the mixture heated to 40° C (104° F.) and kept at that temperature for half an hour; it is then cooled, and the liberated iodine estimated with decinormal sodium thiosulphate V. S., using starch T. S. as indi- cator. N Not less than 15 cc. nor more than 20 cc. of the — 10 V. S. should be required to discharge the color of starch iodide. Each cc. corresponds to i corre- sponding to about 1.4$ of metallic iron. Introduce into a glass-stoppered bottle (having a capacity of 100 cc.) 1.12 (1.1176) gms. of the solution, together with 15 cc. of water and 2 cc. of hydrochloric acid. Then add to the mixture 1 gm. of potassium iodide, and keep it at a temperature of 40° C. (104° F.) for half an hour. Allow the mixture to cool, and esti- mate the liberated iodine with decinormal sodium thio- sulphate V. S., using starch T. S. as indicator. When the blue or greenish color of starch iodide has entirely disappeared, the reaction is completed. 216 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 2.8 cc. of the — V. S. should be required, each cc. 10 corresponding to 0.5$ of metallic iron. The reaction between the ferric nitrate and potas- sium iodide is as follows: Fe2(N03)8 + 2KI = 2Fe(N0.)a + 2KNO, + I3. 2)483-1 2)253 10)241.5 10)126.5 24.15 gms. 12.65' gms. N „ c or 1000 cc. — V. b. 10 Thus each cc. of the decinormal sodium thiosulphate V. S. represents 0.02415 gm. of ferric nitrate. Liquor Ferri Subsulphatis (Solution of Basic Ferric Sulphate; Monsel’s Solution).—An aqueous solution of basic ferric sulphate of variable composition, chemi- cally corresponding to about 13.6$ of metallic iron. 1.12 (1.1 i/)gms. of the solution are introduced into a flask (having a capacity of 100 cc.), together with 15 cc. of water and 2 cc. of hydrochloric acid. 1 gm. of potassium iodide is then added and the mixture digested for half an hour at a temperature of 40° C. (104° F.). It is then cooled, and after adding a few drops of starch T. S., it is titrated with decinormal sodium thiosulphate V. S. When the blue or greenish color of the liquid disappears, the reaction is completed. 27.2 cc. should be required to complete the reaction, each cc. corresponding to 0.5$ or 0.0056 gm. of metal- lic iron. 0.0056 X 27.2 = 0.15232 gm. 015232X100 1.12 J A A TEXT-BOOK OF VOLUMETRIC ANALYSIS. Liquor Ferri Tersulphatis (Solution of Ferric Sul- phate).—An aqueous solution of normal ferric sulphate 2(S04)3= | containing about 28.7 per cent. of the salt, and corresponding to about 8 per cent, of metallic iron. 1.12 (i.i 176) gms. of the solution are introduced into a 100-cc. glass-stoppered bottle, together with 15 cc. of water and 2 cc. of hydrochloric acid ; 1 gm. of potas- sium iodide is then added, and the mixture kept at a temperature of 40° C. (104° F.) for half an hour, then allowed to cool, and the liberated iodine estimated in N the usual way with — sodium thiosulphate V. S., using starch T. S. as indicator. N About 16 cc. of the — V. S. should be required. The following equation illustrates the reaction : Fe2(S04)3 + 2KI - 2FeS04 + K2S04 + I, 2)399-2 2)253 Io)i99.6 10)126.5 19.96 gras. 12.65 Sms< or N the equivalent of 1000 cc. of — thiosulphate V. S. Thus each cc. represents 0.01996 gm. of ferric sul- phate, which corresponds to 0.5 per cent, or 0.0056 gm. of metallic iron. If 16 cc. are used, the solution of ferric sulphate con- tains 0.01996 X 16 = 0.31936 gm. °-3'936 X loo = 2g , 1.12 218 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. of pure ferric sulphate, and 0.0056 X 16 = 0.0896 gm., .0896 X 100 _ u 1.12 of metallic iron. Hydrogen Peroxide, H.O, = | *^'92.—The iodo- metric method, which originated with Kingzett, is based upon the fact that iodine is liberated from po- tassium iodide by hydrogen peroxide, in the presence of sulphuric acid, and that this liberation of iodine is in direct proportion to the available oxygen contained in the peroxide. Then by determining the amount of iodine liberated, the available oxygen is readily found. HA + H2S04 + 2KI = K2S04 + 2HaO + I2. 2)34 2)i6 2)253 17 = 1 available O = — 126.5 This shows that 126.5 gms. of iodine are liberated by 17 gms. of absolute peroxide, which are equivalent to 8 gms. of available oxygen. N Thus 1000 cc. of — sodium thiosulphate V. S., which absorb and consequently represent 12.65 gms. of iodine, are equivalent to 1.7 gms. of H.,0., or 0.8 gm. of avail- able oxygen. N Each cc. of this V. S., then, represents, of H202 *0.0017 gm., of available oxygen *0.0008 gm. The coefficients for weight of H202 and of oxygen, it is seen, are identical with those used in the perman- ganate process. Therefore the coefficient for volume is also the same in this method as in the other, namely, 0.5594. A TEXT-BOOK OF VOLUMETRIC ANALYSIS. The process is carried out as follows: Take 2 or 3 cc. of sulphuric acid, dilute it with about 30 cc, of water, add an excess of potassium iodide (about 1 gm.), and then 1 cc. of hydrogen peroxide. After the mixture has been allowed to stand five minutes starch T. S. is added, N and the titration with — sodium thiosulphate begun. Note the number of cc. required to discharge the blue color, and multiply this number: by 0.0017 gm. to find the quantity, by weight, of H202; by 0.0008 gm. to find the weight of available oxygen ; by 0.5594 cc. to find the volume of available oxygen. If 18 cc.are required, the solution is of 0.5594 X 18 = 10.0683 volume strength. 0.0017 X 18 = .0306 or 3.06$ H202. o.oooS X iB = .0144 or 1.44$ of oxygen. With this method the author has always obtained satisfactory results. The lack of uniformity in the re- action, which is frequently reported, is doubtless due to the use of insufficient acid. Table of Substances, Estimated by Decinormal Sodium Thio- sulphate V. S, Name. Formula. Molecular Weight. Factors. Chlorine Cla 70.68 Gm. .003537 Ferric acetate F e*»(C3 H 302)« Fe2Cl6 +12 H 20 464.92 .023446 Ferric chloride 539-5 .026975 Ferric citrate Fe2(C6H5(J7 )2 488.48 .024424 Ferric nitrate Fes(N03)g 483.1 .02415 Ferric phosphate Fe2(P04 )2 *402 .0151 Ferric pyrophosphate Fe4(P207)3 anhydrous *746 .01865 Ferric sulphate P e2(S04)3 399-2 .01996 Ferric and ammonium sulphate. €2(N H4)2(b>04)4-j-24H 20 *964.0 .0482 Ferric valerianate Fe2(C5H902)6 *718 •0359 Hydrogen peroxide h2o2 33-92 .OO1696 Iodine 253 .OI265 Iron, in ferric salts Fe2 111.76 .005588 Oxygen, available, weight o2 *32 .0008 Oxygen, available volume ,. o2 *32 •5594 cc- PART II. CHAPTER XV. ACETIC ACID AND ACETATES. Vinegar.—Vinegar is impure diluted acetic acid. Its strength may be estimated in the same manner as acetic acid. Phenolphthalein must be used as an indi- cator. Litmus will give only approximate results, be- cause potassium and sodium acetate both have a slightly alkaline reaction with litmus, but show no reaction with phenolphthalein.* The absence of mineral acids must be assured before the volumetric test is applied. The strength of vinegar may also be estimated by distilling no cc. until 100 cc. come over. The too cc. will contain 80$ of the whole acetic acid present in the 110 cc., and may be titrated ; or the specific gravity of the distillate may be taken, and, by consulting the table below, the per cent strength of the distillate found. By adding 20$ to this the strength of the original vinegar is obtained. Vinegar usually contains from 3$ to 632 58 gm. of Zn ; “ “ “ “ =.004058 “ofZnO; “ “ “ “ =.008058 “ of ZnS04. Zinc Dust.—Zinc dust is generally a mixture of metallic zinc, zinc oxide, and often some zinc car- bonate. It is largely used as a reducing agent, and its value in this respect is proportionate to the metallic zinc it contains. Hence it is important to be able to estimate the quantity of free metal in a sample. This may be done as follows: A weighed portion of the zinc dust, free from lumps, is introduced into a flask provided with a ground-glass stopper, and a measured excess of a centinormal solu- tion of iodine added and the mixture digested for some time. The metallic zinc is acted upon by the iodine, and zinc iodide is formed; the oxide is not affected. When the reaction is completed, the excess of iodine is determined by retitration with centinormal sodium thiosulphate solution, and the quantity re- quired deducted from the quantity of iodine added. N Each cc. of iodine V. S. = 0.0003244 gm. of metallic zinc. PART III. CHAPTER XLIII. SANITARY ANALYSIS OF WATER. In collecting samples of water great care must be exercised in order to secure a fair representation of the water and to avoid the introduction of foreign matters. The samples should be collected in clean glass-stop- pered bottles having a capacity of from 2 to 5 pints. It is well to completely fill the bottle with water, then empty it, and again fill with the water to be ana- lyzed. In taking samples from lakes, reservoirs, or slow streams the bottle should be submerged, so as to avoid collecting any water that has been in direct contact with the air. In collecting from pump-wells a few gallons should be pumped out before taking the sample in order to remove that which has been standing in the pump. If the public water-supply is to be analyzed, take the water from a hydrant communicating directly with the street main, and not from a cistern. At the time of collecting, a record should be made of those surroundings and conditions which might influ- A TEXT-BOOK OF VOLUMETRIC ANALYSIS. ence the character of the water, such as proximity of cesspools, sewers, stables, and factories. It should also be noted whether the sample is from a deep or shallow well, a river, spring, or artesian well. The nature of the soil and the different strata of the locality must also be taken into account. The sample should be kept in the dark and analyzed with as little delay as possible. Color.—This may be taken by looking down through a column of water in a colorless glass tube about two feet long, standing upon a piece of white paper. A comparison is made with a second tube containing distilled water. Another way of determining the color is by the use of a colorless glass tube two feet long and two inches in diameter, closed at each end, with disks of colorless glass cemented on, but having a small opening at one end for filling and emptying the tube. To use this tube, it is half filled with the water to be examined and placed in a horizontal position. A piece of white paper is held at one end of the tube, and then by looking through from the other end the color of the liquid is observed, and a comparison of tint made be- tween the lower half of the tube containing the water and the upper half containing air. Odor.—Three or four ounces of water are placed in a small flask fitted with a cork through which is passed a thermometer; the flask is placed in a water-bath and heated to iOO° F. The flask is then shaken, the cork withdrawn, and the odor immediately observed. In this way, satisfactory and uniform tests are ob- tained, and a practised nose can frequently detect pollution. 288 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. Reaction.—This may be determined by the use of a neutral solution of litmus. If an acid reaction is ob- tained, the water should be boiled in order to determine if it is due to carbonic acid ; if the red color disappears upon boiling, the acid reaction is due to carbonic acid. Phenolphthalein or lacmoid may also be used for this purpose. Suspended Matter.—A litre of the turbid water is passed through a dried and weighed filter. The filter is then again dried and weighed, and the increase in weight represents the suspended matter in one litre of the water. TOTAL SOLIDS. A platinum dish having a capacity of about 120 cc, is heated to redness, then cooled under a desiccator, and weighed. 100 cc. of the water is then intro- duced and evaporated over a low-temperature burner at a moderate heat. When the residue appears dry the heat may be increased by placing the dish in an air oven kept at a temperature of about 212° F. until it ceases to lose weight; finally cool under a desic- cator, and weigh. In waters of exceptional purity it may be advisable to use larger quantities, such as 250 cc. The increase in weight of tire dish represents approxi- mately the total solids contained in the water taken. If the solid residue does not exceed 57 parts per 100,000, no reason is afforded for rejecting the water for domestic use. It has been found that the figure for total solids obtained thus, does not truly represent the sum of the organic and mineral matters in all cases. Experiments have been made with urea dissolved in A TEXT-BOOK OB' VOLUMETRIC ANALYSIS. 289 varying quantities of water. Where the solution con- tained 1 gm. of urea the residue after evaporation varied from 0.98 to 0.007 grn. Besides the possible loss of organic matter during the evaporation, some of the mineral constituents may retain with great obstinacy, large quantities of water in the form of water of crystallization, which would cause an error in the opposite direction. Thus the determination of total solids is only an ap- proximation. ORGANIC AND VOLATILE MATTER—LOSS ON IGNITION. Though the mineral matter in a water must to some extent be taken into account in judging of a water, the organic matter is of far greater importance. The really injurious matters are more probably the organic. It is therefore important to determine as near as possible their quantity and nature. It was naturally supposed that by igniting the resi- due obtained from evaporation of the water, the or- ganic matter would be burned out, and that the loss of weight would then represent the organic matter. But as waters ordinarily contain some earthy carbo- nates, which upon ignition are deprived of carbonic-acid gas and converted into oxides, it was customary to add a few drops of carbonic-acid water or ammonium car- bonate to the ash, and then dry and weigh the residue. Ignition, however, decomposes other salts which may be contained in water, and may even volatilize some wholly; therefore the loss on ignition cannot be truly called the organic matter. Hence the expressions “ Or- ganic and Volatile Matter,” and “ Loss on Ignition.” Frankland recommends ignition as a rough qualita- A TEXT-BOOK OF VOLUMETRIC ANALYSIS. tive test for the presence of organic matter, the degree of blackening which takes place, giving some idea of the probable amounts of organic matter present. CHLORINE may be estimated by the use of decinormal or centi- normal silver-nitrate solution; but analysts generally use a solution of such strength that 1 cc. will represent 0.001 gm. of chlorine. Standard Silver-nitrate Solution. — Dissolve 4.794 gms. of pure recrystallized silver nitrate in sufficient water to make 1000 cc. Potassium-chromate Solution.—Five gms. of neutral potassium chromate are dissolved in 100 cc. of water and a weak solution of silver nitrate added, drop by drop, until a slight permanent red precipitate is pro- duced, which is allowed to settle in the bottle, or sepa- rated by filtration. The Process.—Measure out 100 cc. of the water to be analyzed into a beaker or white basin ; add a few drops of the potassium-chromate solution ; then run in slowly from a burette, the silver-nitrate solution until a slight red tint appears. Note the number of cc. of silver solution used. Each cc. represents 0.001 gm. (1 milligramme). If the chlorine is present in small quantity, about 250 cc, of the water should be evapo- rated to about one fifth before titrating with the silver-nitrate solution. Example.—100 cc. of water taken, 4 cc. of silver solution consumed ; thus showing that the 100 cc. of water contained 0.004 gm. of chlorine, or 100,000 cc. contained 4 gms. Multiplied by 10 gives parts per million. A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 291 The water must be perfectly neutral before titration. If acid, it must be shaken with a little pure precipi- tated calcium carbonate. AMMONIA. When organic matter decomposes spontaneously, it first forms ammonia, then nitrites, and finally nitrates. Thus the presence of ammonia in water is generally conceded to indicate decomposing organic matter, and hence its determination is an important part of the sanitary examination of water. The ammonia is generally spoken of as free am- monia and albuminoid ammonia, or, more properly, as ammonium salts and ammonia from organic nitrogen. The sanitary examination of a water should always include a quantitative determination of nitrogen in both compounds. The process requires several solutions and consider- able care in manipulation. The solutions required are : 1. Nessler's Solution, made by dissolving 35 gms. of potassium iodide in 100 cc. of water and 17 gms. of mercuric chloride in 300 cc. of water. The liquids may be heated to aid solution, but if so, must be again cooled. When solution is complete, add the latter to the former until a permanent precipitate is pro- duced ; then dilute with a 20% solution of sodium hydroxide to 1000 cc. Now add mercuric-chloride solution again until a permanent precipitate is formed. Let the mixture stand until settled, then decant off the clear solution for use. The bulk of the solution should be kept in a well-stoppered bottle, and a small quantity transferred from time to time to a small bottle, from which it should be used. This solution A TEXT-BOOK OF VOLUMETRIC ANALYSIS. improves on keeping, and reacts with extremely min- ute quantities of ammonia. 2. Sodium-carbonate Solution.— A 20$ solution of pure freshly-ignited sodium carbonate in water free from ammonia. 3. Standard Ammonium-chloride Solution.—Dissolve 0.3138 (*.314) gm. of pure ammonium chloride in water to 100 cc. For use dilute 1 cc. of this solution with 99 cc. of distilled water free from ammonia. Each cc. of this solution contains 0.00001 gm. of ammonia. 4. Alkaline Potassium-permanganate Solution.—Dis- solve 200 gms. of pure potassium hydroxide and 8 gms. of pure potassium permanganate in sufficient ammonia- free water to make 1000 cc. 5. Ammonia-free Water.—If the distilled water of the laboratory gives a reaction with Nessler’s solution, it should be treated with sodium carbonate, about I gm. to the litre, and boiled until one fourth has been evaporated. A good clear hydrant water when treated with so- dium carbonate and distilled yields ammonia-free water. The first portion which comes over has of course some ammonia in it, and small portions of the distil- late should be tested with Nessler’s reagent until no more reaction is obtained ; the remainder, except the very last portion, should be collected. Ammonia-free water may also be obtained by distil- ling water acidulated with sulphuric acid. In the first two processes the ammonia is converted into a volatile salt and is easily dissipated, or appears in the first distil- late ; in the last process it is converted into a non-vol- atile salt, which does not distil over. Apparatus Required.—A still, consisting of a glass A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 293 retort, having a capacity of about 700 cc., which is con- nected with a Liebig’s condenser by an air-tight joint. The heat is applied by means of a low-temperature burner, the iron ring of which is removed, so that the retort rests directly upon the gauze. (See Fig. 45.) Fig. 45. Cylinders for Comparative-color Tests.—These cylin- ders are made of pure colorless glass, about one inch in diameter, having a capacity of about 100 cc. and graduated at 50 cc. These should either have a milk- glass foot, or should stand upon white paper. Two or more of these are required. 294 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. The Process.—The retort and condenser are thor- oughly rinsed out with ammonia-free water. Then 500 cc. of the water to be tested are introduced, and about 5 cc. of sodium-carbonate solution added to make the water alkaline. The water is then gently boiled until 50 cc. of distillate are obtained. This distillate is transferred to one of the color-comparison cylinders and 2 cc. of Nessler’s reagent added ; a yellow color is produced, which develops more fully in 3 or 5 minutes, and the intensity of which is proportionate to the amount of ammonia present. The color produced is exactly matched by introduc- ing into another cylinder 50 cc. of ammonia-free water and an accurately measured quantity of the standard ammonium-chloride solution, and 2 cc. of Nessler’s reagent, as before. According as the color so produced is deeper or lighter than that obtained from the water, other solu- tions are prepared for comparison, containing smaller or larger proportions of the ammonium chloride, until the proper color is produced. The distillation is continued, and successive portions of 50 cc. of the distillate taken and tested, until the liquid no longer reacts with Nessler’s reagent. The sum of the figures obtained from the several distillates gives the total ammonia, existing in ammonium com- pounds, in the 500 cc. of water taken. The residue in the retort serves for the determina- tion of the nitrogen of the organic matter, which is converted by the alkaline permanganate into ammonia (albuminoid ammonia). 50 cc. of the alkaline permanganate are placed in a porcelain dish of about 150 cc. capacity, the dish nearly A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 295 filled with distilled water, and then the liquid boiled down to 50 cc. This is added to the residue in the retort, the distil- lation resumed, and the ammonia estimated in each 50 cc. of the distillate, as before described. It is the practice of some analysts to mix the distil- lates of each of the above operations, and thus make determinations merely of the total quantity of ammo- nia and albuminoid ammonia. By so doing valuable information may be lost, since it has been pointed out that the ammonia may be differently distributed in the distillates, according to the state, decomposing or otherwise, in which the ammonia exists in the water. If the ammonia distils over very rapidly, it indicates that the organic matter is in a putrescent or decom- posing condition. If, on the other hand, it distils gradually, it indicates the presence of organic matter in a comparatively stable or fresh condition. It is best, therefore, to keep the record of each distillate, so that the rapidity with which the ammonia is set free, as well as the actual amount, may be known. The greatest care should be exercised in order to avoid the introduction of ammonia in any way during the course of the analysis, since small quantities of ammonia compounds and nitrogenous matters are everywhere present. All measuring-vessels, cylinders, etc., should be thoroughly rinsed before using. NITROGEN AS NITRATES. Solutions Required.—Acid Phenyl Sulphate.—18.5 cc. of strong sulphuric acid are added to 1.5 cc. of 296 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. water and 3 gms. of pure phenol. This should be pre- served in a tightly-stoppered bottle. Standard Potassium Nitrate.—0.722 gm. of pure potassium nitrate, previously heated to a temperature just sufficient to fuse it, is dissolved in water, and the solution made up to 1000 cc. 1 cc. of this solution will contain .0001 gm. of nitrogen. The Process.—A measured volume of water is evap- orated just to dryness in a platinum or porcelain dish, 1 cc. of the acid phenyl sulphate added and thoroughly mixed with the residue by means of a glass rod, then 1 cc. of water and three drops of strong sulphuric acid, and the dish gently warmed. ,The liquid is then diluted with about 25 cc. of water, a slight excess of ammonium hydroxide added, and the solution made up to 100 cc. The reactions are: HC6HbS04+3HN03=HC6H2(N02)30+H2S04+2H30 Acid phenyl Trinitrophenol sulphate. (picric acid). HCeH9(N0,)30 + NH4OH = NH4C6H2(N02)304- H20. Ammonium picrate. The nitric acid used in the above equation is derived from the potassium nitrate by the action of sulphuric acid. The ammonium picrate imparts a yellow color to the solution, the intenstiy of which is proportional to the amount present. Five cc. of the standard potassium-nitrate solution are now similarly evaporated in a platinum basin, treated as above, and made up to 100 cc. The color produced A TEXT-BOOK OF VOLUMETRIC ANALYSIS. is compared to that given by the water; and one or the other of the two solutions diluted until the tints agree. The comparative volumes of the liquids furnish the necessary data for determining the amount of nitrate present, as the following example will show. Five cc. of the standard nitrate are treated as above, and made up to 100 cc. Each cc. represents 0.0001 gm. of nitrogen. .OOOI 5 .0005 gm. N per 100 cc. 10 .0050 gm. N per 1000 cc. Suppose 100 cc. of water similarly treated are found to require dilution to 150CC. before the tint will match that of the standard ; then ioo : 150 :: .005 : x. x = 0.0075. That is, the water contains 7.5 milligrams of nitrogen as nitrate per litre. Care should be taken that the same quantity of acid phenyl sulphate is used for the water and for the com- parison liquid, otherwise different tints instead of depths of tints are produced. With river or spring waters 25 to 100 cc. should be evaporated for the test, but with subsoil and other waters which probably contain much nitrates 10 cc. will be sufficient. 298 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. The Copper-zinc Process.—500 cc. of the water are acidulated with oxalic acid, and half of this is poured into each of two wide-mouthed bottles. Into one of these is put a copper-zinc couple, made by taking a piece of sheet zinc (4X6 in.) and rolling it into a loose coil and immersing it in a 1.5-per-cent, solution of copper sulphate until the surface is covered with an even layer of copper. Cork both bottles and let stand 24 hours. Remove 50 cc. from each bottle and Nesslerize as directed under Ammonia. The difference between the two readings gives the ammonia due to the reduction of the nitrates and nitrites present. The nitrogen in the nitrites, which is separately determined, must be subtracted, when the remaining nitrogen will be that from the nitrates. NITROGEN AS NITRITES. Solutions Required.—1. Naphthylammonium Chlo- ride (Naphthalamin Hydrochlorate').—Saturated solu- tion in water free from nitrites. It should be colorless (0.5 gm. dissolved in 100 cc. of boiling water). This solution should be kept in a glass-stoppered bottle with a little animal charcoal, which will keep the solution colorless. 2, Snlphanilic Acid {Para-aviido-bcnzenc—Sulphonic Acid).—A saturated solution in water free from nitrites (1 gm. in 100 cc. of hot water). . Hydrochloric Acid.—25 cc. of concentrated pure hy- drochloric acid mixed with 75 cc. of water free from nitrites. A TEXT-BOOK OF VOLUMETRIC ANALYSIS. Standard Sodium Nitrite. — 0.275 gm. pure silver nitrite is dissolved in pure water, and a dilute solution of pure sodium chloride added until a precipitate ceases to form. It is then diluted with pure water to 250 cc. and allowed to stand until clear. For use 10 cc. of this solution are diluted to 100 cc. It must he kept dark. One cc. of the dilute solution is equivalent to .00001 gm. of nitrogen. A standard solution of silver nitrite is used by some chemists, but the above is said to give better results. The Process.—100 cc. of the water is placed in one of the color-comparison cylinders, the measuring-vessels and cylinder having previously been rinsed with the water to be tested. By means of a pipette introduce into the water 1 cc. each of the solutions of sulphanilic acid, dilute hydrochloric acid, and naphthylammonium- chloride solution in the order named. It is convenient to have three pipettes—one for each of these solutions, and to use them for no other purpose. In all cases the pipettes should be rinsed with ammonia-free water before using them. Into another clean comparison- cylinder introduce 1 cc. of the standard nitrite solution and make up to 100 cc. with pure water; then add the same reagents as were added to the water in the other cylinder. A pink color is produced in the presence of nitrites, which requires in dilute solutions half an hour for com- plete development. At the end of that time the darker solution is diluted with water until the tints are matched, and the calculation made as explained under nitrates. The reactions are explained by the following equa- tions : 300 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. C.H4NH,HS01 + HN02 = C6H4N2S03 + 2H20; Sulphanilic acid. Para-diazo-benzene-sulphonic acid. C,H,NtSO, + C10HtNHfCl Naphthammonium chloride. = C10H6(NH,)NNC6H4HSO3 + HC1. v * Azo-alpha-amido-naphthalene- parazo-benzene-sulphonic acid. The last-named body gives the color to the liquid. Example.—Suppose that 100 cc. of the water re- quire dilution to 125 cc. in order to bring it to the same tint as that produced by 1 cc. of the standard nitrite solution, which contains .00001 gm. of nitrogen as nitrite. 100 : 125 :: .00001 : x. 0.0000125 gm. in 100. That is, 100 cc. of water contain 0.0000125 grm of N ; 0.0125 gm, in 100,000 cc. OXYGEN-CONSUMING POWER. Potassium permanganate readily yields up ics oxy- gen, especially in the presence of a strong mineral acid, as sulphuric. It oxidizes many salts, and organic mat- ter. This property led to the idea that this salt may be used for burning up (chemically speaking) the organic matter in water, and that the quantity of permanganate used could be relied upon as a means of measuring the organic matter in water. This method does not distinguish between ani- mal and vegetable matter, nor does the quantity of A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 301 permanganate consumed represent only the organic matter. The organic matters in water are very variable in character and condition, and their oxidability is subject to much difference. Nevertheless as a high oxygen-consuming power certainly indicates pollution by organic matter, the process is of considerable value. The following is a convenient method for approxi- mating the oxygen-consuming power of a water: Solutions Required.—Potassium Permanganate.— 0.395 gm. of pure potassium permanganate is dissolved in distilled water, and the solution made up to 1000 cc. 1 cc. of this solution will yield under favorable circum- stances 0.0001 gm. of oxygen. Diluted Sulphuric Acid.—50 cc. of pure sulphuric acid are mixed with 100 cc. of water, and then just sufficient of the permanganate solution added to give the mixture a faint pink color, which remains after standing in a warm place four hours. The Process.—Five stoppered bottles having a capac- ity of 500 cc. are thoroughly cleansed with strong sul- phuric acid and then carefully rinsed with pure water, and 250 cc. of the water to be tested put into each one. 10 cc. of the dilute sulphuric acid is then added to each, together with regularly increasing quantities of the standard permanganate, say 2, 4, 6, 8, and 10 cc., respec- tively. At the end of an hour they should be examined, to see which, if any, are decolorized. At the end of the fourth hour they should again be examined, and again at the expiration of twenty-four hours. If all of the bottles are decolorized at or before the 302 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. fourth hour an additional 10 cc. of the permanganate solution should be added to each bottle. With ordinary waters the first and probably the second bottle will be decolorized, while a little color will remain in the third, and the color in the fourth and fifth will be but little diminished. In this way an ap- proximate figure for the oxygen-consuming power of the water may be obtained, which in most cases is all that is necessary. If a closer figure is desired, the ex- periment may be repeated, using quantities of perman- ganate intermediate between those marking the limits of the reaction. Thus if the second bottle is decolorized and a faint color still remains in the third, repeat the experiment with 5 cc. of the permanganate. This method of procedure has an advantage over some of the other processes, because the rate of oxida- tion can easily be seen. This is considered by some to be of more importance than the actual amount of oxy- gen consumed. It must be remembered that nitrites, ferrous salts, sulphides, etc., consume oxygen as well as organic mat- ter. It is therefore important to boil water containing hydrogen sulphide in order to drive the latter off. Nitrites may be removed by treating the water with sulphuric acid, and boiling. The nitrite is thus con- verted into nitrous acid, which is driven off by the heat. Or the oxygen required to convert the nitrites pres- ent into nitrates may be deducted from the total amount of oxygen consumed. 14 parts of nitrogen as nitrite require 16 parts of oxygen for oxidation into nitrate. A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 303 PHOSPHATES. Solutions Required.—Ammonium Molybdate.—Made by dissolving 10 gins, of molybdic anhydride in a mix- ture of 15 cc. of concentrated ammonia (sp. gr. .900) and 25 cc. of water. This solution is poured slowly, and with constant stirring, into a mixture of 65 cc. of concen- trated nitric acid (sp. gr. 1.4) and 65 cc. of water, and allowed to stand until clear. It should be kept dark. The Process.—One litre of the water is evaporated to about 50 cc.; a few drops of a dilute solution of ferric chloride are added, followed by a slight excess of am- monia. Ferric hydroxide is thus precipitated, which carries down with it all the phosphate. This precipi- tate is separated by filtration, dissolved on the filter in the smallest possible quantity of hot dilute nitric acid, and a little water passed through the filter. The filtrate and washings should not exceed 5 cc., and should, if more, be evaporated to this bulk. The solution is now heated nearly to boiling and 2 cc. of the ammonium-molybdate solution added. If after half an hour an appreciable precipitate is formed, it is collected on a small weighed filter and its weight found after thorough drying. This weight, multiplied by 0.05 gives the amount of P04. If the quantity is too small to be collected and weighed in this manner, it is usually reported as “ traces,” “ heavy traces,” or “ very heavy traces.” HARDNESS. The hardness of water, that is, its soap-destroying power, is due principally to the presence of calcium salts; but salts of magnesium, iron, and other metals may also contribute to this effect. A TEXT-BOOK OF VOLUMETRIC ANALYSIS. Two kinds of hardness are recognized : i. “Temporary,” which is due to the presence in water of the acid carbonates of calcium, magnesium, etc. By boiling, these salts are decomposed, the car- bonic-acid gas being driven off, and the neutral carbon- ate formed, which is precipitated. Thus the water loses its hardness upon boiling. CaH2(C03)2 = CaC03 + H20 + C02. 2. “ Permanent ” hardness is due to the presence in water of salts of the above-mentioned metals which are not removed by boiling, such as the sulphates. Hardness is estimated by means of a standard soap solution. Many samples of water possess both temporary and permanent hardness, and it is sometimes desirable to estimate them separately. The total hardness is estimated in one sample, and the hardness in another sample is determined after boiling and filtering off the precipitated calcium car- bonate. The hardness found after boiling is the permanent hardness, and is the most objectionable form. The difference between the total and permanent hardness is the temporary hardness. To express the hardness in some tangible form, the usual custom in this coun- try and in England is to give results in the correspond- ing amounts of calcium carbonate, i.e., practically to determine the amount of soap destroyed by a meas- ured quantity of water, and then to state the results as the amount of calcium carbonate which would de- stroy that quantity of soap. A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 305 The reaction which takes place when soap is added to a hard water, is illustrated in the following equa- tions ; CaH2(C03)s + 2NaC18H3B02 Acid calcium Sodium stearate carbonate. (Soap). = Ca(C,.H..O,), + Na,CO, + H,0 + CO,; Calcium stearate. or, CaS04 + 2NaC18HsA = Ca(C18H3fA), + NaaS04. Calcium sulphate. The calcium stearate, which is an insoluble calcium soap, is precipitated in both cases as a white curd-like mass. The method for estimating hardness in water by the use of soap solution is known as Clark’s method. Solutions Required.—Standard Soap Solution.— Dissolve 10 gms. of shavings of air dried Castile soap in a litre of dilute alcohol. Filter the solution if it is not clear, and keep it in a tightly-stoppered bottle. Standard Calcium Chloride Solution.—Dissolve 1 gm. of pure calcium carbonate in the smallest excess of hydrochloric acid, then carefully neutralize with am- monia water, and add sufficient water to make up to one litre. One cc. of this solution will contain the equivalent of O.OOi gm. of calcium carbonate. This solution is used for determining the strength of the soap solution, which is done as follows: Measure out 10 cc. of this solution, add 90 cc. of dis- tilled water, and run in the soap solution, drop by drop, from a burette until a lather is formed, which remains 306 a text-book of volumetric analysis. for five minutes. Note the number of cc. of soap so- lution used. We now repeat the experiment with 100 cc. of dis- tilled water. The amount of the soap solution re- quired to produce a permanent lather with the distilled water must be deducted from the amount used in the first test. Usually it will be about one half or one cc. The io cc. of the calcium chloride solution contained the equivalent of o.oio gm. of CaC03. Suppose in the above-mentioned test 8.5 cc. of the soap solution were used to produce a permanent lather, and 0.5 cc. were used by the distilled water. Then 8 cc. were used to precipitate 0.010 gm. of CaC03. Thus each cc. of this soap solution will represent £ of .010 gm. = .00125 of calcium carbonate. The soap solution may either be used as it is, or it may be diluted with dilute alcohol so that about 10.5 or 11 cc. of it will be required to produce a permanent lather with 10 cc. of the standard calcium chloride so- lution. If so diluted each cc. will represent 0.001 gm. of CaC03. This is a convenient strength, because if 100 cc. of water are operated upon, each cc. of the soap solution used will represent 1 part of CaC03 in 100,000 parts of water. Measure 100 cc. of the water into a well-stoppered bottle having a capacity of about 250 or 300 cc. Add the soap solution gradually from a burette, one cc. at a time at first, and smaller quantities towards the end of the operation, shaking well after each addition until a soft lather is obtained, which if the bottle is placed at rest on its side, remains continuous over the whole surface for five minutes. A TEXT-BOOK OE VOLUMETRIC ANALYSIS. 307 The soap should not be added in large quantities at a time, even if the volume required is approximately known. If magnesium salts are present, a kind of scum (simulating a lather) will be seen before the reaction is completed. The character of this scum must be care- fully watched, and the soap solution added very care- fully, with an increased amount of shaking after each addition. The point when the false lather due to the magnesium salt ceases and the true persistent lather is produced is comparatively easy to distinguish. If more than 23 cc. of the soap solution are con- sumed by the 100 cc. of water, a smaller quantity of water should be taken (say 50 or 25 cc.) and made up to 100 cc. with distilled water, recently boiled. In such case the quantity of soap solution used must be multiplied by 2 or 4. If the first-mentioned soap solution is used each cc. represents 000125 gm. If the second solution is used each cc. represents 0.001 gm, of CaC03, and if 100 cc. of water are acted upon each cc. represents 1 part of CaCO, in 100,000. If 70 cc. of water are acted upon, instead of 100 cc., each cc. of soap solution used represents 1 gm. per 70,000 cc., which corresponds to 1 gr. per imperial gallon (70,000 grs.) or 1 degree of hardness. These estimations are, however, only approximate, for the lather does not form until the reaction between the soap and the calcium in the water is completed, and then the quantity of soap solution required to produce the lather depends upon its strength. Dr. Clark, the originator of this method, has shown that 1000 grains of distilled water (free from hardness) 308 a text-book of volumetric analysis. require 1.4 measures of soap solution, each measure being the volume of 10 grains of distilled water at 160 C. For Permanent Hardness.—To determine the hard- ness after boiling, a measured quantity of water must be boiled briskly for half an hour, adding distilled water from time to time to make up the loss by evaporation. At the end of the half-hour allow the water to cool, make up to its original volume with recently boiled and cooled distilled water, filter rapidly, and test in the same manner as described above. One half or one cc. is de- ducted from the soap solution used, for the calculation. Among German chemists it is customary to desig- nate the soap-destroying power equivalent to 1 part of CaO in 100,000, as one degree of hardness. Among French chemists each degree of hardness represents 1 part of CaC03 in 100,000. INTERPRETATION OF RESULTS. Statement of Analysis.—The composition of water is generally expressed in terms of a unit of weight in a definite volume of liquid, but no fixed standard is used, the proportions being expressed by some analysts in parts per million, by others in parts per hundred thou- sand. Sometimes, generally by English chemists, the figures are given in grains per imperial gallon of 70,000 grains; less frequently, in grains per U. S. gallon of 58,328 grains. In order to pass judgment upon the analytical re- sults from a sample of water, the analyst must know to which class of water it belongs—whether river-water, well-water, or artesian-well water. He must know something of the soil and geological character of the locality from which the water is obtained, as well as A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 309 other conditions of the locality which might affect the quality of the water, such as proximity of stables, cess- pools, sewers, factories, etc. Color.—Water of the highest purity is clear, color less, odorless, and nearly tasteless. But the color of water is no indication of its quality. A turbid or col- ored water is not necessarily a dangerous one, neither is a clear, colorless water always a safe water. Odor.—For comfort, if for nothing else, potable water should be free from odor. Water sometimes has an unpleasant odor and taste, yet it may be used with perfect safety for domestic purposes. At other times the odor may give rise to suspicions which a subsequent examination may confirm. Thus by the odor alone the safety of the water cannot be told. Total Solids.—This is intended to represent the total solid matters dissolved in the water; but since much of the organic matter as well as some of the in- organic matter is volatilized by evaporation, the total solids obtained by this method are only the total non- volatile solids. The indication is thus lower than it should be. On the other hand, certain salts, especially calcium sulphate, retain water of crystallization, thus producing an effect in the opposite direction. The total solids so obtained, contain both organic and inorganic matters, either of which may be injurious or not. Mineral waters contain large quantities of in- organic salts. Much smaller quantities of total solids in other waters might indicate pollution. Large quantities of mineral solids, especially of marked physiological action, are known to render water non-potable; but no absolute maximum or minimum can be assigned as the limit of safety. An arbitrary 310 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. limit has, however, been fixed by sanitary authorities of 60 parts per 100,000 ; and if the solid residue does not exceed 57 parts per 100,000, there is no reason for rejecting a water. Many waters, especially artesian waters, which are in constant use, contain much larger quantities. The loss on ignition should never reach 50 per cent of the total residue. Chlorine in potable waters is very largely derived from sodium and potassium chlorides of urine and sewage. Food contains considerable amounts of chlorides, and still more is added by way of condiment in the shape of salt. The chlorine thus taken into the system is again thrown off in the excreta, and thus ap- pears in the sewage ; hence the presence of large quan- tities of chlorine in water is taken as an indication of pollution. Urine contains about 500 parts of chlorine per 100,000. The average quantity found in sewage is about 11.5 parts per 100,000. Over 5 parts per 100,000 of chlorine in a water may be considered, in most cases, to be due to pollution of the water by sewage or animal excretions. The chlorine itself is not a dan- gerous constituent of water, but its presence in large quantities is an unfavorable indication. Nevertheless too much dependence must not be put upon the amount of chlorine in water as a means of judging of its purity, for dangerous vegetable matter may exist in it without its presence being indicated by chlorine. The maximum amount of chlorine per 100,000, given by the Rivers Pollution Commission, is 21.5 parts, the minimum 6.5 parts. Various conditions, however, which affect the proportion of chlorine, such as the A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 311 nature of the strata through which the water passes, proximity to the sea, etc., must be taken into account. Nitrogen in Ammonia.—Ammonium compounds are usually the result of the spontaneous putrefactive fermentation of nitrogenous organic matter; nitrites are then formed, and finally nitrates. Ammonium com- pounds may also result from the reduction of nitrites and nitrates in the presence of excess of organic matter. Therefore in either case the presence of ammonia sug- gests contamination. This fact is so generally conceded that the estima- tion of ammonia in water, is a very important part of the sanitary examination. In the water from deep wells an excess of ammonia is nearly always found, but its presence here cannot always be considered an adverse condition, since it is derived largely from the decomposition of nitrates, and shows previous contamination; but the water hav- ing undergone extensive filtration and oxidation, its organic matter is presumably converted into harmless bodies. Rain-water often contains large proportions of am- monium compounds, which it dissolves out of the air in its descent; but here also, this fact cannot condemn the water, since it does not indicate contamination with dangerous organic matter. An average of 71 samples of rain-water collected in England contained 0.05 parts per 100,000, including an exceptional maximum of 0.21 parts. Fischer (“ Chemische Technologic des Wassers”) gives two analyses of typically good wells, containing respectively 0.048 and 0.044 parts per 100,000, and of 312 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. two typically bad shallow wells, containing respec- tively 0.084 and 2.227 parts per 100,000. Albuminoid Ammonia.—If water yields no albu- minoid ammonia it is free from recently added organic contamination. If it contain more than 0.01 part per 100,000 it is looked upon as suspicious, and when it reaches 0.015 parts per 100,000 it is to be condemned. When free ammonia is present in considerable quan- tity, then the albuminoid ammonia becomes suspicious when it reaches 0.005 parts per 100,000. An opinion should not, however, be formulated without a knowl- edge of the source of the water; for, as has been said before, free ammonia may exist in large quantities in deep wells without indicating contamination. Wanklyn gives the following standards : High purity ooo to .0041 of albuminoid ammonia per 100,000 Satisfactory purity .0041 to .0082 “ “ “ “ “ Impure over .0082 “ In the absence of free ammonia he does not condemn a water unless the albuminoid ammonia exceeds .0082 parts per 100,000; but he condemns a water yielding 0.0123 parts of albuminoid ammonia, under all circum- stances. Nitrogen as Nitrates.—Nitrates are normally pres- ent in all natural waters, and are derived chiefly from the oxidation of animal matters. The nitrogen of organic matters liberated by putrefaction, is first converted into ammonia; then this is oxidized into nitrous, and finally into nitric acid. These changes are due partially to direct oxidation and partially to certain micro- organisms which have the power of converting nitro- genous organic matter into nitrites and nitrates. A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 313 Nitrates and nitrites in themselves, in the quantity in which they exist in water, are perfectly harmless. They are, however, an indication of previous contamination ; and many analysts believe that a water which has once been contaminated is always open to suspicion. Others consider them of little importance in determin- ing the impurity of a water. Water which is laden with organic matter is purified by percolating through the ground, the nitrogenous matter being converted into nitrates; therefore deep wells may contain large quantities of nitrates without being essentially impure, while the water from shallow wells should be con- demned if the nitrates are excessive. Certain strata, as the chalk formation, yield large amounts of nitrates to water. If the nitrogen as nitrates exceeds 0.6 parts per 100,000 the water is suspected. Nitrogen as Nitrites.—Some chemists regard the presence of nitrites as an indication that the oxidation of the dangerous compounds has probably been in- complete, and accordingly condemn water in which nitrites are found. Leeds places the nitrites in Ameri- can rivers at .03 per 100,000. The average in good waters is placed at about .0014 per 100,000. When the quantity exceeds .02 parts per 100,000 it is con- sidered an indication of previous contamination. Oxygen Consuming Power.—This is intended to represent the oxidizable organic matter in the water. But there are other substances in water besides organic matters which absorb oxygen, namely, nitrites, which are thus oxidized to nitrates; ferrous salts, which are oxidized into ferric salts; etc. Thus the oxygen-con- suming power does not represent the organic matter 314 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. alone. However, a water having a high oxygen-con- suming power may be considered as polluted. The following basis for interpreting results of this method are given by Frankland and Tidy: Oxgen absorbed in 3 hours. High organic purity .05 parts per 100,000 Medium purity 05 to .15 “ “ “ Doubtful 15 to .21 “ “ “ Impure over .21 “ “ “ Phosphates.—Sewage contains large amounts of phosphates, but water usually contains alkaline or earthy carbonates, which precipitate the phosphates; therefore the absence of phosphates does not indicate purity. But their presence may indicate sewage con- tamination. .06 parts per 100,000 is regarded with sus- picion (calculated as P04). Hardness.—On account of the presence of con- siderable amounts of calcium compounds in our food sewage is usually very hard, containing especially calcium sulphate. The hardness of water, therefore, has some bearing upon the question as to whether the water is probably polluted with sewage or not. But water may be hard, yet otherwise perfectly pure. The test for the degree of hardness is therefore of little importance in determining sewage, as the figures below show that water uncontaminated by sewage may be very hard. Temporary. Permanent. Rain-water, average 0.3 1.7 Highest from different geological formations ... 38.6 48.5 From 272 samples of water from shallow and polluted wells : Minimum o 3.8 Maximum 52 164.3 Average 19 31.5 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 315 The above figures are parts in 100,000. The hard- ness has, however, much significance from an economic point of view. Hard water is objectionable for domes- tic purposes in washing, because of its soap-destroying power, and for manufacturing purposes in boilers. It has no bad effect upon the health, but is by some con- sidered wholesome. Standards.—Certain standards have been fixed by some chemists for determining the purity or impurity of water, according to which if certain figures are ex- ceeded the water is to be condemned. Dr. Tidy's classification depends upon the amount of oxygen consumed, from potassium permanganate, after standing three hours. 1. Great organic purity o to 0.05 2. Medium purity 0.05 “0.15 3. Doubtful 0.15 “0.21 4. Impure over 0.21 These standards are applied to waters other than upland surface-waters, in which larger quantities of oxygen may be absorbed. Wanklyris standard is based upon the indications of the amounts of free and albuminoid ammonia, as follows: 1. Extraordinary purity 6 to 0.005 part albuminoid NH3. 2. Satisfactory purity 0.005100.010 “ “ “ 3. Dirty over 0.010 “ “ “ If the albuminoid ammonia exceeds 0.005 parts per 100,000 the free ammonia must be taken into account. If the free ammonia is in large quantity it is a sus- picious sign. If it is in small quantity or altogether absent, the water should not be condemned, unless the 316 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. albuminoid ammonia is something like o.oio parts pet 100,000; while over 0.015 should condemn the water absolutely. The following is a list of analyses of waters which were pronounced good. The results are given in parts per 100,000. I. II. III. IV. Chlorine 0.877 t-333 9.294 i-578 Free ammonia 0.0004 none 0.002 0.0002 Albuminoid ammonia none 0.0006 0.005 0.0022 Oxygen absorbed (in 3 hours) 0.0054 0.0016 0.0255 0.0008 N in nitrates and nitrites.... 0.2525 0.3376 0.0107 0 2633 Total hardness 19.23 14.0000 13-33 2.079 Permanent hardness 3-715 3-934 3.060 1.980 Organic and volatile matters r-5 i-7 trace 2.100 Total solids (dried at 230“ F.) 24.4 27 37-40 9.40 I. IT. in. IV, Chlorine 0.316 62.43 4.208 28.230 Free ammonia 0.0196 0.278 none 0.0105 Albuminoid ammonia 0.0678 0.0030 0.0105 0.0395 Oxygen absorbed (in 3 hours) 0.2912 0.133 0.0165 0.2130 N in nitrates and nitrites 0.0283 none 0.247 0.6210 Total hardness 6.940 27.72 13.068 50.00 Permanent hardness 3-5 23.76 2.574 32.670 Organic and volatile matters 0.5 19.5 trace 8.00 Total solids (dried at 230° F.) 15.60 156.20 30.50 146.50 I, Back of slaughter-house; II, Drive-well on beach ; III, Well; IV, Well 30 feet deep. A TEXT-BOOK OF VOLUMETRIC ANALYSIS. CHAPTER XLIV. MILK. Milk is the nutritive secretion of glands (the mam- mary glands) which are characteristic of the mam- malia. This secretion takes place as a result of pregnancy and delivery, and continues for a variable period, con- stituting the entire food of the young animal until it is able to live upon other foods. The milk of different animals contains qualitatively identical or analogous ingredients to that of the cow, namely, fat (which is held in suspension), nitrogenous matters such as casein and albumen, milk sugar, in- organic salts, and water. The average composition of cow’s milk is as follows : Fat 3.65 per cent. Proteids 4-40 “ “ Lactose 4.25 “ “ Inorganic salts 0.75 “ “ Total solids 13.05 “ “ Water 86.95 “ “ 100.00 In the milk of different animals, however, these in- gredients are in different proportions, as the following table shows: 318 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. Human. Goat. Mare. Ass. per cent. per cent. per cent. per cent. Fat 3-40 5-2 1.1 1.0 Proteids 2-45 3-8 2.2 2.7 Lactose 5-75 4-3 5-8 5-3 Inorganic salts 0-35 0.7 0-3 0.4 Water 88.05 86.0 90.6 90.6 100.00 100.0 100.0 100.0 Total solids n-95 14.0 9-4 9.4 Milk is a perfect natural emulsion. The casein appears to be the emulsifying agent, a film of which envelops each globule of fat, thus preventing cohesion. The inorganic salts are chiefly the phosphates of sodium and calcium, and the chlorides of sodium and potassium, but magnesium and iron are also generally present. The proteids consist mainly of casein with some al- bumen, the proportion being about as 6 to I. Besides the above-mentioned constituents milk also contains a very small quantity of peptone, kreatin, leucin, etc. Also gases, such as CO,2, O, and N. Colostrum is the milk secreted in the early stages of lactation ; it is rich in proteids, often containing as much as 20 per cent, and contains a few corpuscles of a peculiar character, which look like epithelium-cells, called colostrum corpuscles. Reaction.—The reaction of the milk of herbivorous animals is generally alkaline, that of carnivora is gener- ally acid. The reaction of cow’s milk is generally neutral, sometimes slightly acid, rarely alkaline. Specific Gravity.—This varies in normal cow’s milk from 1.029 to 1.035. It should not be below 1.029. A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 319 An excess of fat lowers the specific gravity and the removal of fat raises it. Thus skimmed milk has a higher specific gravity than normal milk. These facts use of for the detection of the ordinary adulterations. Determinations of the specific gravity of milk should always be made at the temperature of 6o° F., and may be made by any of the ordinary methods. See table of corrections for temperatures other than 6o° F., page 320. A special hydrometer known as the lactometer is, however, generally used. The lactometer is graded from o° at the top to 120° at the bottom. In taking the specific gravity with this instrument the tempera- ture of the milk must be 6o° F. For every of temperature above the 6o° standard, one degree is to be added to the reading of the lactom- eter; below 6o° F. a similar subtraction is to be made. On the lactometer scale o° = 1.000, the specific gravity of pure water; at 6o° F. 100= 1.029, the specific gravity of the poorest possible milk at the same temperature. If in a sample of milk the lactometer stands at 8o° the sample contains about 80 per cent of standard milk and 20 per cent of water. If the lactometer stands at 90°, the sample contains about 10 per cent of water. Lactometer Reading. Specific Gravity. Lactometer Reading. Specific Gravity. 0 1.0000 70 I.0203 10 1.0029 80 I.O232 20 1.0058 QO 1.0261 30 1.0087 100 I.O29O 40 I.0116 no 1.0319 50 1.0145 120 I.0348 60 1.0174 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. (From Muier's Analytical Chemistry.) Directions for Use.—Find the temperature of the milk in the uppermost horizontal line, and the observed specific gravity in the first and last vertical lines. In the same line with the latter, under the temperature is given thz co? reeled specific gravity. Example.—Supposing the temperature to be 590 and the observed specific gravity 1.032.0, at 6o° the specific gravity will be 1.031.9. A specific gravity of 1,032.0 at 66° will be 1.032.9 at 6o°. Observed Specific Degrees of Thermometer iFahr.). Gravity. 50° 5 ° 52 53° 54° 55° 56° 57 58° 59° 60° 61° 62 63° 64° 65° 66° 67° 68° 69° 70° 1.020.0 19.a 19 3 19 4 19.4 19.5 19,6 19.7 19 8 19,9 19.9 1.020.0 20.T 20 2 20.2 20-3 20-4 20 5 20.6 20.7 20.9 21 .O 1.021.0 20.2 20 3 20 3 20.4 20.5 20.6 20.7 20 8 20.9 20.9 1.021.0 21.1 21 2 2I.3 21.4 21.5 21 6 21.7 21.8 22.0 22. I 1.022.0 21.2 2 T 3 21 3 21.4 21.5 21.6 21.7 21 8 21.9 21.9 1.022.0 22.1 22 2 22 3 22.4 22.5 22 6 22,7 22.8 23.0 23-r 1.023.0 22.2 22 3 22 3 22.4 22.5 22.6 22.7 22 8 22.8 22-9 1.023.0 23,1 23 2 23-3 23-4 23-5 23 6 23-7 23.8 24.O 24.I 1.024.0 23.2 23 3 23 3 23.4 23-5 23.6 23.6 23 7 23.8 23-9 1.024.0 24.1 24 2 24-3 24.4 24-5 24 6 24.7 24.9 25.0 251 1.025.0 24.1 24 2 24 3 24.4 24-5 24.6 24.6 24 7 24.8 24.9 1.025.0 25-1 25 2 25-3 25-4 255 25 6 25.7 2S-9 26.0 26.1 1.026.0 2^.1 25 2 25 2 2S-3 25.4 25-5 25.6 25 7 25-8 25.9 1.026.0 26.1 26 2 26.3 26.5 26.6 26 7 26.8 27.0 27.I 27.2 1.027.0 26. I 26 2 26 2 26.3 26.4 26.5 26.6 26 7 26.8 26.9 1.027.0 27.1 27 3 27.4 27-5 27.6 27 7 27.8 28.0 28. I 28.2 1.028.0 27.O 27.I 27 2 27-3 27.4 27.5 27.6 27 7 27.8 27.9 1.028.0 28.1 28 3 28.4 28.5 28.6 28 7 28.8 29.0 29.I 29.2 1.029.0 28.0 28.1 28 2 28.3 28.4 28.5 28.6 28 7 28.8 28.9 1.029.0 29.1 29 3 29.4 29S 29 6 29 8 29.9 30.1 30.2 303 1.030.0 29.O 2Q.I 29.I 29.2 29.3 29.4 29.6 29 7 29.8 29.9 1.030.0 3°.r 3° 3 3°-4 3°-5 3°-7 3° 8 3°-9 31-’ 31.2 31-3 1.031.0 29.9 30 O 3° I 30.2 3°-3 3°-4 3°-5 30 6 3°.8 3°-9 1.031.0 31-2 31 3 31 -4 3i-5 3T-7 31 8 32.0 32.2 32.2 324 1.032.0 3°-9 31 O 31 •1 31.2 31 -3 31-4 31 • 5 3T 6 31-? 31 -9 1.032.0 32.2 32 3 32-5 32.6 32-7 32 9 33-° 33-2 33-3'33-4 1.033.0 31.8 31 9 32 O 32.1 32-3 32-4 32-S 32 6 32.7 32.9 1.033.0 33-2 33 3 33-5 33-6 33-8 33 9 34.0 34.2 34-3 34-S 1.034.0 32,7 32 9 33 O 33-i 33 2 33-3 33-5 33 6 33-7 33-9 1.034.0 34-2 34 3 34-5 34-6 34.8 34 9 35-o 35-2 35-3 35-S 1.035.0 33 6 33 8 33 9 34-2 34-3 34 5 34 6 34-7 34-9 1.035.0 35-2 35 3 35-5 35-6 35-8 35 v6-1 36.2 36-4 36-S Table for Correcting the Specific Gravity of Milk according to Temperature. A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 321 The Adulterations of Milk.—The adulterations usually practised are the abstraction of cream (skim- ming) or the addition of water, or both. Occasion- ally the addition of some foreign substance, as sodium carbonate, borax, common salt, or sugar, is met with ; or preservatives, as boric or salicylic acids. The detection of adulterations usually depends upon the determination of the specific gravity, the fat, total solids, and the ash. These ingredients are, however, present in milk in varying proportions, and hence certain limits of allow- able variations have been determined upon from time to time. The standard adopted in many States in this country is, for specific gravity, not less than 1.029, for total sol- ids, not less than 12 per cent., for fat 3 per cent. The total solids may vary legally from 12 to 13.13 per cent., and the solids not fat, from 8.5 to 9.5 per cent. Estimation of Total Solids and Water.—A small, shallow platinum or porcelain dish about inches in diameter is heated to redness, allowed to cool, and weighed. About 5 cc. of milk are then put in, and again weighed. The difference between the two weighings gives the weight of the milk taken. Now place the dish on a water-bath and heat until the milk ceases to lose weight. Then cool again and weigh. The weight of the dry residue minus the tare of the dish equals the total solids. Then by multiplying this by 100, and dividing by the weight of milk taken, the percentage of total solids is found. Thus, total solids X 100 , f , . , ... — = per-cent, of total solids. weight of milk 322 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. This deducted from 100 gives the per-cent, of water. Fat.—Where great accuracy is unnecessary the fat may be determined by treating the total solid residue with successive portions of warm ether until the fat is completely dissolved out. The ethereal solution is then evaporated and the fat which remains behind is weighed, or the residue in the dish may be again weighed. The loss of weight then represents the fat. The results so obtained are 0.3 to 0.5$ too low. Adams Method is the officially recognized method for the accurate estimation of fat in milk. This consists essentially in spreading the milk over absorbent paper, drying, and extracting the fat. The paper used for this purpose must previously have been thoroughly exhausted with alcohol and ether, and should be in long narrow strips. The procedure is as follows: 5 cc. of the milk are put into a small beaker and weighed. A strip of the absorbent paper which has been rolled into a coil is thrust into the beaker containing the milk. In a few minutes nearly the whole of the milk will be absorbed ; the coil is then withdrawn, and stood dry end down upon a sheet of glass. It is important to take up the whole of the milk from the beaker, as the paper has a selective action, removing the watery constituents by preference over the fat. The beaker is again weighed, and the milk taken found by difference. The paper charged with the milk is now dried in a water-oven and placed in a Soxhlet • extraction apparatus (Fig. 53). About 75 cc. of ether are introduced into the tared flask of the apparatus, and heat applied by means of a A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 323 water-bath and continued until exhaustion is com- plete. The flask is then detached, the ether removed by distillation, and the fat which remains is weighed. The paper may be charged with the milk by spread- ing the latter over the surface of the paper by means of a pipette. The Werner-Schmid Method.— This is a satisfactory and at the same time a rapid method for the determin- ation of fat, and is especially suitable for sour milk. 10 cc. of the milk are put into a long tube having a capacity of about 50 cc., and 10 cc. of strong hydrochloric acid are added; or the milk may be weighed in a small beaker and washed into the tube with the acid. The liquids are mixed and boiled for minutes, or until the liquid turns dark brown, but not black. The tube and contents are then cooled, and 30 cc. of ether are added, shaken, and allowed to stand until the acid liquid and ether have separated. The cork is now taken out and the wash-bottle arrangement inserted (see Fig. 52). The stopper of this should be of cork, not of rubber, since the ether has a solvent action upon the latter. The lower end of the exit tube is adjusted so as to rest immediately above the junction of the two liquids. The ethereal solution of fat is then blown off, and received in a weighed beaker. Two more portions of 10 cc. each are shaken successively with the acid liquid, blown out, and added to the first. The ethereal solution is then heated on a water-bath, and the residue of fat weighed. The results agree quite closely with the Adams method. Calculation Method.—This rests upon the assumption that every per-cent, of solids not fat, raises the specific gravity by a definite amount, while every per-cent, of TEXT-BOOK OF VOLUMETRIC ANALYSIS. fat lowers it by a definite amount. An accurate de- termination of the per-cent of total solids and of the specific gravity therefore furnish the necessary data for calculating the amount of fat. Hehner and Richmond have devised the following formula: F — 0.859?" — 0.2186(7.; in which F — fat, T — total solids, and G = the last two units of the specific gravity and any decimal. Thus if the specific gravity is 1029, G will be 29; if 1029.5, G will be 29.5. Example.—Let us assume in the examination of a certain milk that the specific gravity was 1030, and that it contained 12 per cent, of total solids. We then have Fat = 0.859 X 12 — 0.2186 X 30 = 3.75$ When the per-cent, of fat is known, the formula may be transposed so as to calculate the total solids, as follows ; j. _ F 0.2186G 0.859 Example.—A sample of milk is found to contain 3.75 per cent, of fat, and its specific gravity is 1030; then « ... 3.75 +0.2186 X 30 Total solids = - / — = \2% 0.859 Ash.—The ash may be determined by igniting at a dull-red heat the residue left after the fat has been extracted from the total solids. The organic matter is thus all burned off, and the residue is weighed and cal- culated as ash. The ash should be about 0.75 per cent, never below 0.67. To Calculate the Per cent, of Pure Milk and of A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 325 Added Water, the following formula maybe adopted, which is based upon the legal standard of the State of New York, which is based upon the poorest possible natural milk, viz., 3 per cent, of fat, 12 per cent, of total solids, and 9 per cent, of “ solids not fat.” If, however, a milk has 3 per cent, of fat and only 8.5 per cent, of “ solids not fat,” it need not be considered as definitely proved to be adulterated. The quantity of added water should, however, always be calculated upon the average standard of 9 per cent “solids not fat,” provided the milk is certainly well below the limit of 8.5 per cent. The “solids not fat” are used as a basis for the cal- culation because they are a fairly constant quantity, the fat being variable. The calculation is made thus : “ Solids not fat” X 100 , ... = p. c. of pure milk present; and the difference between this result and 100 will of course give the added water. Example. — A sample of milk upon analysis was found to contain 8.1 per cent of solids not fat; then B.i X ioo 810.0 , = = 90 ic 9 9 of pure standard milk and 10 per cent of water. Total Proteids.—Rittenhausens Copper Process.— The solutions required are: (1) Copper-sulphate solu- tion, 34.64 gms. in 500 cc.; (2) Sodium-hydroxide solu- tion, 12 gms. to 500 cc. 10 gms. of the milk are diluted to 100 cc. with dis- tilled water and placed in a beaker; 5 cc. of the cop- per-sulphate solution are now added and thoroughly mixed. 326 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. The sodium-hydroxide solution is now added drop by drop, stirring constantly until the precipitate settles quickly, and the liquid is neutral or feebly acid. It should never be alkaline, as an excess of alkali pre- vents the precipitation of some of the proteids. The precipitate which includes the fat carries down all of the copper. It is washed by decantation and collected upon a weighed dry filter, the contents of the filter being washed until the total filtrate measures about 250 cc. This filtrate, which contains no copper, is reserved for the determination of the sugar by Feh- ling’s Solution. The precipitate is washed once by strong alcohol to remove adhering water; it is then washed several times with ether to remove the fat. The residue on the filter, which consists of the proteids and copper hy- droxide, is dried at 265° F. in the air-bath and weighed. It is then transferred to a porcelain crucible and incin- erated, and the residue weighed. The weight of the filter and contents less the weight of the filter and residue after ignition, gives the weight of the proteids. The Milk-sugar is estimated in the mixed filtrate from the precipitated proteids by the use of Fehling’s Solution in the usual way. (See Estimation of Sugar.) A TEXT-BOOK OF VOLUMETRIC ANALYSIS. CHAPTER XLV. BUTTER. An analysis of butter comprises the estimation of water, fat, casein, ash, sodium chloride, and also volatile fatty acids. The water is determined by drying to a constant weight: the fat, by extraction with ether; the casein, by heating the residue, after extraction of fat, to just below redness, until the former becomes white. The loss of weight represents casein and the residue ash. In the ash the chlorine may be determined by dissolving in water and titrating with standard silver nitrate solution. Salt is estimated by washing the butter with several portions of hot water and titrating the aqueous solution, with standard silver nitrate solution. THE ESTIMATION OF VOLATILE ACIDS. Reichert’s Process.—This is undoubtedly the best process for detecting the admixture of foreign fats with butter. This process depends upon the fact that butter contains certain constituents which when appropriately treated yield volatile acids in much larger quantity than is obtained from any of the practicable substitutes for butter. These acids are 328 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. principally butyric and caproic. The process consists in saponifying the fat with an alkali, then separating the fatty acids by neutralization, and distilling off the volatile acids for titration with standard alkali. “ Reichert’s number” is the number of cc. of deci- normal alkali solution required to neutralize the acids distilled from 2.5 gms, of fat. The results are, however, often specified for 5 or 10 gms. of the fat. The operations involved in this process do not admit of any arbitrary variation, and reliable and comparable results can only be obtained by strictly adhering to the prescribed details. The solutions required are: Sodium Hydroxide Solution. —100 gms. of sodium hydroxide are dissolved in 100 cc. of distilled water. The alkali should be as free as possible from car- bonates. and be preserved out of contact with the air. Alcohol of about 95 per cent, redistilled with caustic soda. Diluted Sulphuric Acid.—25 cc. of the strongest sulphuric acid, in water to make 1000 cc. Standard Barium Hydroxide Solution.—Accurately standardized, approximately decinormal. The apparatus required are: Saponification-flasks of from 250 to 300 cc. capacity, of hard, well-annealed glass, capable of resisting the tension of alcohol vapor at ioo° C. A Pipette, Distilling Apparatus, and an Accurately Calibrated Burette. The following method of manipulation, as drawn up by the Association of Official Agricultural Chemists, is recommended as giving accurate results. In this approximately 5 gms. of the butter are taken. A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 329 The process: Weighing the Fat.—The butter or fat to be ex- amined should be melted and kept in a dry, warm place at about 6o° F. for two or three hours, until the water and curd have entirely settled out. The clear supernatant fat is poured off and filtered through a dry filter-paper in a jacketed funnel containing boiling water. Should the filtered fat in a fused state not be perfectly clear, it must be filtered a second time. This is to remove all foreign matter and any trace of moisture. The saponification-flasks are prepared by thoroughly washing with water, alcohol, and ether, wiping perfectly dry on the outside, and heating for one hour at the temperature of boiling water. The flasks should then be placed in a tray by the side of the balance and covered with a silk handkerchief until they are perfectly cool. They must not be wiped with a silk handkerchief within 15 or 20 minutes of the time they are weighed. The weight of the flasks having been accurately determined, they are charged with the melted fat in the following way: A pipette with a long stem, marked to deliver 5.75 cc., is warmed to a temperature of about 50° C. The fat having been poured back and forth once or twice into a dry beaker in order to thoroughly mix it, is then taken up in the pipette and the nozzde of the pipette carried to near the bottom of the flask, having been previously wiped to remove any adhering fat, and 5.75 cc. of fat are allowed to flow into the flask. After the flasks have been charged in this way they should be recovered with the silk handkerchief and allowed to stand 15 or 20 minutes, when they are again weighed. 330 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. Saponification. —10 cc. of 95 per cent alcohol are added to the fat in the flask, and then 2 cc. of the caustic soda solution. A soft cork stopper is now inserted in the flask and tied down with a piece of twine. The saponification is then completed by- placing the flask upon the water or steam bath (see Fig. 46). The flask during the saponification, which should last one hour, should be gently rotated from time to time, being careful not to project the soap for any distance up its sides. At the end of an hour the flask, after having been cooled to near the room temperature, is opened. Removal of the A leohol. — The stoppers having been laid loosely in the mouth of the flask, the alcohol is removed by dipping the flask into a steam-bath. The steam should cover the whole of the flask except the neck. After the alcohol is nearly removed, frothing may be noticed in the soap, and to avoid any loss from this cause or any creeping of the soap up the sides of the flask, it should be removed from the bath and shaken to and fro until the frothing dis- appears. The last traces of alcohol vapor may be removed from the flask by waving it briskly, mouth down, to and fro. Fig. 46. A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 331 Dissolving the Soap.—After the removal of the alcohol the soap should be dissolved by adding 100 cc. of recently boiled distilled water, warming on the steam-bath, with occasional shaking until solution of the soap is complete. Setting Free the Fatty Acids.—When the soap solu- tion has cooled to about 6o° or 70°, the fatty acids are separated by adding 40 cc. of the dilute sulphuric acid solution mentioned above. Melting the Fatty Acid Emulsion.—The flask should now be restoppered as in the first instance, and the fatty acid emulsion melted by replacing the flask on the steam-bath. According to the nature of the fat examined, the time required for the fusion of the fatty acid emulsions may vary from a few minutes to several hours. The Distillation.—After the fatty acids are com- pletely melted, which can be determined by their forming a transparent oily layer on the surface of the water, the flask is cooled to room temperature, and a few pieces of pumice-stone added. The pumice-stone is prepared by throwing it, at a white heat, into dis- tilled water, and keeping it under water until used. The flask is now connected with a glass condenser, slowly heated with a naked flame until ebullition begins, and then the distillation continued by regulat- ing the flame in such a way as to collect 110 cc. of the distillate in, as nearly as possible, 30 minutes (see Fig. 47). The distillate should be received in a flask accurately graded at 110 cc. Titration of the Volatile Acids.—The 110 cc. of distillate, after thorough mixing, are filtered through perfectly dry filter-paper, 100 cc. of the filtered dis- 332 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. tillate poured into a beaker holding from 200 to 250 cc., 0.5 cc. phenolphthalein solution added, and decinormal barium hydrate run in until a red color is produced. The contents of the beaker are then returned to the measuring-flask to remove any acid remaining therein, poured again into the beaker, and the titration continued until the red color produced remains apparently unchanged for two or three min- utes. The number of cubic centimetres of deci- normal barium hydroxide required should be increased by one tenth. Fig. 47. When treated as above described, 5 gms. of genuine butter never yields less acidity than is represented by N 24 cc. of — alkali. It is true, however, that butter 10 made from the milk of a single cow, especially towards the end of her period of lactation, has been known to fall slightly below this figure, but the average butter, as produced from the mixed milk of a herd, usually requires 27 cc. or more. Oleomargarine requires A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 333 about I cc. beef-fat, and lam about the same. Cacao butter requires about 7 cc. The percentage of butter-fat in a mixture of fats, 5 gms. being taken: {n — 0.6) X 3.65 = percentage of true butter-fat. A rapid method for detecting oleomargarine or an admixture of it with butter is to heat the suspected substance in a small tin dish directly over a gas-flame. If it melts quietly, foams, and runs over the dish, it is butter; if it sputters noisily as soon as heated and foams but little, it is oleomargarine. Another way is to heat the butter for a moment with an alcoholic solution of sodium hydroxide and then empty into cold water. It gives a distinct odor of pineapples (due to ethyl butyrate), while oleomar- garine gives only an alcoholic odor. 334 A text-book of volumetric analysis. CHAPTER XLVI. ESTIMATION OF CARBONIC-ACID GAS IN THE ATMOSPHERE. This is done by Pettenkofer’s method. A glass globe or bottle holding from 5 to 10 litres is filled with the air to be tested, by means of a bellows ; baryta water of known strength is then introduced in con- venient quantity. The bottle is then securely closed and set aside for about one hour, rotating it at intervals, so that the liq uid is spread over the entire inner wall of the bottle. When the time is up the baryta water is emptied out quickly into a beaker, covered carefully with a watch- glass, and when the barium carbonate has subsided a portion of the clear liquid is withdrawn and titrated N with — oxalic-acid solution. The difference between 10 the quantity of oxalic-acid solution required to neu- tralize the barium-hydroxide solution, before and after contact with the air, is the quantity equivalent to the carbonic-acid gas absorbed. The Baryta-water is made by dissolving about 7 gms. of pure crystallized barium hydroxide in 1000 cc. of distilled water. This solution, being prone to absorb COa out of the air, must be kept in a special bottle, such as is illus- trated in Fig. 40, which prevents access of COa and A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 335 admits of the withdrawal of any quantity of solution without inverting the bottle. The Bottle which is used to collect the air should hold from 5 to 10 litres, and its exact capacity must be known. This may be found by filling the bottle to the bottom of the cork with water and then accurately measuring the water. Before using the bottle it must be absolutely dry. 7'he Analysis.—Into the bottle, the capacity of which is exactly known,—we will assume it to be 7100 cc.,— is blown the air to be tested, by means of a bellows. 100 cc. of the baryta-water are then introduced, thus leaving 7000 cc. of air in the bottle. The bottle is now securely closed and set aside for about half an hour, rotating it occasionally so as to spread the liquid over the entire inner wall of the bottle. While waiting for the half-hour to expire, a convenient quantity of baryta-water is taken and its strength compared to decinormal oxalic-acid solution by titrating with the latter, using phenolphthalein as indicator. 50 cc. of baryta-water is a convenient quantity. This is placed in a beaker, a few drops of phenolphtha- N lein added, and then titrated with the — acid solution io until the color just disappears. Let us assume that 40 cc. of the latter were con- sumed ; 80 cc. will then be consumed by 100 cc. of baryta-water. Ba(OH), + H2C204 + 2H20 = BaC204 + 4HaO ; 2)170,9 2)126 10) 85-45 10) 63 N 8.545 gms. 6.3 gms. or 1000 cc. — V. S. 336 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. Ba(OH), + CO, = BaCOs+ H,0. 2)1709 2)44 10) 85.45 10)22 8.545 gms. 2.2 gms. These equations show that 2.2 gms. of carbon di- oxide will neutralize as much barium hydroxide as N 1000 cc. of — oxalic-acid solution. And thus each cc. 10 of the ~ oxalic-acid solution is chemically equivalent 10 to 0.0022 gm, of carbon dioxide ; therefore 100 cc. of the baryta-water is capable of absorbing 80 X .0022 gm. = 0.176 gm. of C02. The next step is to determine the quantity of CO, that was absorbed by the 100 cc. of baryta-water, which was introduced into the bottle of air. The liquid is poured out of the bottle into a small beaker, carefully covered with a watch-glass, and the barium carbonate allowed to settle. Then 50 cc. of the clear supernatant liquid are drawn out of the beaker by means of a pipette, treated with a few drops of N phenolphthalein T. S., and titrated with the — oxalic acid V. S. until the red color is just discharged. Note the number of cc. consumed, double it, and deduct this number from 80, the quantity which 100 cc. of baryta- water consumed before being brought in contact with CO,. Example.—Assuming that 30 cc. of the oxalic-acid solution were required by the 50 cc. of the baryta- water after exposure, the 100 cc. then would require 60. There is thus a loss of alkalinity equivalent to 20 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 337 N cc. of — oxalic acid V. S. This is due to the absorp- tion of carbon dioxide, which neutralizes the hydroxide by forming a carbonate. N Now since each cc. of — oxalic acid V. S. is chemi- 10 cally equivalent to 0.0022 gm. of C02, the baryta- water must have absorbed 20 x 0.0022 gm. = 0.044 gm- °f COa. Therefore the .7000 cc. of air which the bottle held contained 0.044 gm- of C03. In stating the result of an analysis the quantity of COa by volume in 10,000 cc. of air is generally given. In the above case 7000 cc. of air contained 0.044 gm* of COa; 10,000 cc. of this same air, then, contains 0.044 X io,ooo 0.044 X io „ ——t or = 0.0628 gm. 7000 7 b If several bottles are in use it is convenient to mark upon them the multiplier and divisor; thus; 10,000 IO or —. 7000 7 In calculating the volume of a gas, the temperature and pressure must be taken into account. By referring to the following table the volume occu- pied by 0.001 gm. of COa at different temperatures can be seen. The volume of 0.0628 gm. of COa at 160 C. is 0.0628 x 0.53843 o — = 33.81 cc. 0.001 J 338 A TEXT-BOOK OF VOLUMETRIC ANALYSIS, Table showing Volume of .ooi Gm. of Carbon Dioxide at Various Temperatures. C ° F.° Cc. C.° F.° Cc. o 32 0.50863 13 55-4 0.53314 I 33-3 0.51049 14 57-2 0.53471 2 35-6 0.51235 15 59 0.53657 3 37-4 0.51451 16 60.8 0.53843 4 39-2 0.51608 17 62.6 O.54030 5 4i 0.51794 18 64,4 0.54216 6 42.8 0.51980 19 66.2 0.54402 7 44.6 0.52167 20 68 0.54589 8 46.4 0.52353 21 69.8 0-54775 9 4S.2 0.52539 22 71.6 0.54961 IO 50 0.52726 23 73-4 0.55177 ii 51.8 0.52912 24 75-2 0-55334 12 53-6 0.53098 If the pressure remains constant, the volume of a gas increases regularly as the temperature increases, and decreases as the temperature decreases. (Charles’ Law.) This expansion or contraction amounts toof the volume of the gas for each degree centigrade. Thus by calculation the volume of .001 gm. C02 (0.50863 cc.) at any temperature may be found. 0f *50863 = 0.001863. Then to find the volume at any given C. temperature multiply the degree of temperature by O.OOl863, and add the answer to 0.50863. A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 339 CHAPTER XLVII. ESTIMATION OF ALCOHOL IN TINCTURES AND BEVERAGES. The quantity of alcohol contained in dilute spirit, which leaves no residue upon evaporation, may be ascertained by taking the sp. gr. and referring to the alcohol table. When taking the specific gravity, the temperature of the liquid should be i5f° C. (6o° F.). In Wines, Beer, Tinctures, and other alcoholic liquids containing vegetable matter, the sp. gr. of the sample is taken at i5f° C. (6o° F.) and noted. A cer- tain quantity (say ioo cc.) is measured off and evapo- rated to one half, or till all odor of alcohol has passed off, the evaporation being conducted without ebullition, in order that particles of the material may not be car- ried off by the steam. The liquid left is then diluted with distilled water, cooled to 6o° F. and made up to the original volume (ioo cc.), and the sp. gr. taken. Lastly, we calculate: the sp. gr. before evaporating is divided by the sp. gr. after evaporating, and the quo- tient will be the sp. gr. of the water and alcohol only of the liquor. Then by referring to the alcohol table the percentage of alcohol contained in the liquor is obtained. Example.—The liquor before evaporating had a sp. gr. of 0.9951 ; after evaporation and dilution to 100 cc. the sp. gr. was found to be 1.0081. 340 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. —= 0.987, the sp. gr. of the contained spirit. Then by referring to the table we will find that this sp. gr. corresponds to 7.33 per cent., by weight, of abso- lute alcohol. Another Way is to boil the liquid in a retort, con- dense the vapor, and when all the alcohol has passed over add sufficient water to the distillate to make up the original volume, at the temperature of I5|° C. (6o° F.). Then, by taking the sp. gr. of this diluted distillate, the quantity of absolute alcohol is found by reference to the table. This latter method requires the taking of the sp. gr. but once and gives more ac- curate results. A TEXT-BOOK OF VOLUMETRIC ANALYSIS. Table for Ascertaining the Percentages respectively of Alcohol by Weight, by Volume, and as Proof Spirit, from the Specific Gravity. Condensed from the excellent Alcohol Tables of Mr. Hehner in the “Analystvol. v. pp. 43-63. Specific Gravity 15-5°. Absolute Alcohol by w’ght. Per cent. Absolute Alcohol by vol’me Per cent. Proof Spirit. Per cent. Specific Gravity iS-5*- Absolute Alcohol by w’ght. Per cent. Absolute Alcohol by vol’me Per cent. Proofj Spirit. Percent. s 1.0000 0.00 0.00 0.00 •9489 35-os 41.90 73-43 •9999 0.05 0.07 0.12 •9479 35-55 42.45 74-39 .9989 0.58 0.73 1.28 •9469 36.06 43-oi 75-37 •9979 1.12 1.42 2.48 •9459 36.61 43-63 76.45 .9969 i-75 2.20 3-85 •9449 37-17 44.24 77-53 •9959 2-33 2.93 5-13 •9439 37-72 44.86 78.61 •9949 2.89 3.62 6.34 .9429 38.28 45-47 79-68 •9939 3-47 4-34 7.61 .9419 38.83 46.08 80.75 .9929 4.06 5.08 8.90 .9409 39 35 46.64 81.74 .9919 4.69 5-86 10.26 •9399 39-85 47.18 82.69 .9909 5-31 6.63 n.62 •9389 40.35 47.72 83.64 .9899 5-94 7.40 12.97 •9379 40.85 48.26 84.58 .9889 6.64 8.27 14.50 •9369 41-35 48.80 85-53 .9879 7-33 9.13 15-99 •9359 41-85 49-34 86.47 .9869 8.00 9-95 17-43 •9349 42.33 49.86 87-37 •9859 8.71 10.82 18.96 •9339 42.81 50-37 88.26 .9849 9-43 11.70 20.50 •9329 43-29 50.87 89-15 •9839 10.15 12.58 22.06 •9319 43-76 51-38 90.03 .9829 10.92 13-52 23.70 •9309 44 23 51-87 90.89 .9819 11.69 14.46 25-34 .9299 44 68 52-34 9i-73 .9809 12.46 15.40 26.99 .9289 45.14 52.82 9256 •9799 i3-23 16.33 28.62 .9279 45-59 53-29 93 39 .9789 14.00 17.26 30.26 .9269 46.05 53-77 94-22 •9779 14.91 18.36 32.19 •9259 46.50 54-24 95-os .9769 iS-75 19.39 33-96 .9249 46.96 54-71 95.88 •9759 16.54 20.33 35-63 •9239 47-41 55-i8 96.70 •9749 17-33 21.29 37-30 .9229 47.86 5S-6s 97.52 •9739 18.15 22.27 39-°3 .9219 48.32 56.11 98.34 .9729 18.92 23.19 40.64 .9209 48.77 56-58 99.16 .9719 19-75 24.18 42.38 .9199 49-20 57.02 99-93 20.58 44.12 .9699 .9689 .9679 21.38 22.15 26.13 27.04 45-79 47-39 48.98 •9198 49.24 57.06 ioo.ooPs 27-95 .9669 23.69 28.86 So-57 .9189 49.68 57-49 100.76 •9659 24.46 29.76 52.16 .9179 5013 57-97 101.59 .9649 25.21 3° 65 53-71 .9169 50-57 58.41 102.35 .9639 25-93 3!.48 55-i8 •9T59 51.00 S8.85 103.12 .9629 26.60 32.27 56.55 .9149 Si-42 59.26 103.85 .9619 27.29 33-°6 57-94 •9139 51-83 59.68 104.58 ,9600 28.00 33-89 59 4o .9129 52.27 60.12 105-35 •9599 28.62 34-6i 60 66 9"9 52.73 60.56 106.15 .9589 29.27 35-35 61.95 9109 53-17 61.02 106.93 •9579 29 93 36.12 63.30 .9099 S3-6i 61.45 107.69 •9569 30-50 36.76 64-43 .9089 54-05 61.88 108.45 •9559 31.06 37-41 6S-55 •9079 54-52 62.36 109.28 •9549 31.69 3811 66.80 .9069 55-oo 62.84 110.12 •9539 32.31 38.82 68.04 •9059 55-45 63.28 IIO.92 •9529 32.94 39-54 69.29 •9049 55-91 63-73 III.7X •9519 33-53 40.20 70.46 •9039 56.36 64.18 112.49 •9S°9 34.10 40.84 71-58 .9029 56.82 64.63 113.26 •9499 34-57 41.37 72-50 .9019 57-25 65.05 113-99 342 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. Specific Gravity I5-5°- Absolute Alcohol by w’gbt. Per cent. Absolute Alcohol by vol’me Per cent. Proof Spirit. Percent. Specific Gravity 15-5°- Absolute Alcohol by w’ght. Per cent. Absolute Alcohol by vol’me Per cent. Proof Spirit. Percent. .gOOQ S7-67 65-45 114.69 .8429 82.19 87.27 152.95 .8999 58.09 65-85 115-41 .8419 82.58 87,58 153-48 .8989 58.55 66,29 116.18 .8409 82.96 87.88 154.01 .8979 59.00 66.74 116.96 •8399 83-35 88.ig 154-54 .8969 59-43 67-15 117.68 .8389 8.3-73 88.49 155-07 •8959 59.87 67.57 118.41 •8379 84.12 88 79 155 61 .8949 60.29 67.97 II9.I2 .8369 84.52 89.II 156.16 •8939 60.7I 68.36 119.80 •8359 84.92 89.42 156.71 .8929 61.13 68.76 120.49 •8349 8531 89.72 157-24 .8919 61.54 69.15 I2I.l8 •8339 85.69 90.02 157-76 .8909 61.96 69-54 121.86 ■8329 86.08 90.32 158.28 .8899 62.41 69.96 122.61 .8319 86.46 90.61 158 79 .8889 62.86 70.40 123-36 .8309 86.85 9O.9O •59-31 .8879 63-30 70.81 124.09 .8299 8723 91.20 159.82 .8869 63-74 71.22 124.80 .8289 87.62 91.49 160,33 .8859 64.17 71.62 125.51 •8279 88.00 91-78 160.84 .8849 64.61 72.02 126.22 .8269 88.40 92.08 161.37 .8839 65.04 72.42 126.92 .8259 88.80 92.39 161.91 .8829 65.46 72.80 127.59 .8249 89.10 92.68 162.43 .8819 65.88 73.19 128.25 .8239 89.58 92.97 162.93 .8809 66.30 73-57 128.94 .8229 89.96 93.26 16343 •8799 66.74 73-97 129.64 .8219 90.32 93-52 163.88 .8789 67.17 74-37 130.33 .8209 90.68 93-77 164-33 .8779 67.58 74-74 130.98 .8199 91.04 9403 164.78 .8769 68.00 75-12 131.64 .8189 91-39 94.28 165.23 •8759 68.42 75-49 132.30 .8179 91-75 94-53 165.67 .8749 68.83 75-8? 132-95 .8169 92.11 94-79 166.12 •8739 69.25 76.24 133.60 .8159 92.48 95.06 166.58 .8729 69.67 76.61 134.25 .8149 92.85 95-32 167.04 .8719 70.08 76.08 134.90 •8139 93.22 9S58 167.50 .8709 70.48 77-32 135-51 .8129 93-59 95-84 167.96 .8699 70.88 77.67 136.13 .8119 93-96 96.11 168.24 .8689 71.29 78.04 136.76 .8109 94-31 96.34 168.84 .8679 71.71 78.40 137-40 .8099 94.66 96.57 169.24 .8669 72.I3 78.77 !38.o5 .8089 9S-oo 96.80 160.65 .8659 72-S7 79.16 138.72 .8079 95 36 97-os 170.07 .8649 73.00 79-54 139-39 .8069 95-71 97.29 170.50 .8639 73-42 79.90 140.02 .8059 96.07 97-53 170.99 .8629 73-83 80 26 140.65 .8049 96.40 97-75 171.30 .8619 74.27 80.64 Mi-33 .8039 96.73 97.96 171.68 .8609 74-73 81.04 142.03 .8029 97.07 98.18 I72-°5 ■8599 7S-i8 81.44 142.73 .8019 97.40 98-39 172.43 .8589 75 64 81.84 143.42 .8009 97-73 98.61 172.80 •8579 76.08 82.23 I44..IO •7999 98.06 98.82 173-17 .8569 76.50 82.58 144.72 •7989 98.37 99 00 •73-50 •8559 76.92 82.93 MS 34 • 7979 98.69 99.18 173-84 .8549 77-33 83.28 14S-96 .7969 99.00 99-37 174.17 •8539 77-75 83.64 146-57 •7959 99-32 99-57 174-52 .8529 78.16 83.98 147.17 •7949 99.6s 99-77 174.87 •8519 78.56 84-31 147-75 • 7939 99-97 99.98 175.22 .8509 78.96 84.64 148.32 .8499 79-36 84.97 148.90 .8489 79.76 85.29 149.44 Absolute Alcohol. .8479 80.17 85-63 150.06 .8469 80.58 8597 150.67 •8459 81.00 86.32 151.27 .8449 81.40 86.64 151.83 •8439 81.80 86.96 152.40 ■7938 100.00 100.00 1 •75-25 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 343 CHAPTER XLVIII. ANALYSIS OF SOAP. Estimation of Water and Volatile Matters.— (a) 10 gms. of the soap are dried to a constant weight at ioo° C. and carefully weighed ; the loss of weight = water. (b) Free Fats.—The dried soap obtained as above, is exhausted with petroleum ether of low boiling-point. The petroleum ether is then evaporated off and the residue weighed : this is the weight of the fat contained in 10 gm. of the soap. (c) Fatty Acids.—The residue from (b) which is free from fat and which represents 10 gms. of the soap, is weighed and half of it dissolved in water. Normal nitric acid is then added in excess to liberate the fatty acids. These are collected on a tared filter, dried and weighed. This weight when doubled gives the amount of fatty acids in 10 gms. of the soap. The reaction is illustrated by this equation : NaC]8H33Oa + HNOa= HC18H33Os + NaNO,. Sodium oleate. Oleic acid. The acid filtrate is now titrated with normal soda or potash, using phenolphthalein as an indicator. The difference between the volumes of acid and alkali solu- tions used gives roughly the quantity of total alkali. (d) Chlorides and Sulphates.—The residual neutral 344 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. liquid from the above, is divided into two equal parts, N in one of which chlorine is estimated by—AgNOs, using potassium chromate. In the other sulphuric acid is estimated with barium chloride. (e) Free Alkali, (i.e., the alkali which does not exist as soap).—Ten grammes of the soap are dissolved in hot al- cohol,and one drop of phenolphthalein T.S. added; then carbonic-acid gas is passed through the solution until the color disappears. The free alkali is thus converted into sodium carbonate, which is insoluble in alcohol and may be separated by filtration. The residue on the filter is washed with hot alcohol, and then dissolved N in a little water and titrated with — acid in the pres- to r ence of methyl-orange. The number of cc. used multi- plied by 0.0031 gives the grammes of free alkali, as NaaO, in the to gms. of soap. Combined Alkali.—The alcoholic solution from the above which contains the combined alkali and the fatty acids, is diluted with a little water, methyl-orange added, and the mixture titrated with decinormal acid. The quantity of combined alkali is thus found. The num- ber of cc. of acid consumed multiplied by 0.0031 gives the quantity as Na20. Another Way is to evaporate the alcoholic solution to dryness, the residue then ignited, and the soap thus converted into alkali carbonate. This is dissolved in water and titrated with normal or decinormal acid in the presence of methyl-orange. The fatty acids are found by using the factor 0.0282 or 0.282, The number of cc. of decinormal acid used A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 345 in the above titration when multiplied by 0.0282, or of normal acid when multiplied by 0.282, gives the quan- tity of fatty acid as oleic. Soaps, however, contain var- ious fatty acids the molecular weights of which differ. Therefore in estimating the fatty acids volumet- rically, the neutralizing power of the acids liberated from soap, expressed in cc. of standard alkali and called the saponification equivalent, is employed. Geissler determines the free and combined alkali in soap as follows: 10 gms. of the soap are dissolved in 100 cc. of water, phenolphthalein T. S. added, and the solution N titrated with — hydrochloric acid solution until the N color is just discharged. The quantity of — hydro- chloric acid solution used represents the free alkali and is calculated as carbonate. N Each cc. of — acid = 0.053 gm. °f Na2COs or 0.069 gm- °f K2C03. The — acid is now added in excess in order to 1 liberate the fatty acids, and the mixture is heated to melt the fatty acids and cause them to form a clear oily layer on the surface. After the mixture has cooled off, the watery layer is separated and the fatty acids washed with water. The washings are added to , . . . N the aqueous liquid and titrated with — potassium hydroxide until the red color reappears; this gives the 346 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. excess of acid, and when deducted from the quantity of acid added after decolorization of the phenolphtha- lein gives the quantity of the acid which combined with and hence represents the combined alkali of the soap. This is also calculated as carbonate, using the same factors as given above. A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 347 CHAPTER XLIX. ESTIMATION OF STARCH IN CEREALS, ETC. The method about to be described depends upon the fact that when barium hydroxide is brought in contact with starch, an insoluble compound is formed, the formula of which is Ca4H40O20BaO. This combina- tion takes place in definite proportions, so that if an excess of barium hydroxide solution is added to the starchy substance, and then the excess estimated, the quantity which combined with and which consequently represents the amount of starch present, is found. Solutions Required.—1. Decinormal Hydrochloric Acid. See page 40 (3.637 gm. to 1 liter.) Each cc. represents .00765 gm. of BaO. 2. Baryta-water (barium hydroxide solution), made by dissolving about 7 gms. of pure crystallized barium hydroxide in 1000 cc. of water. Should be kept in a special vessel such as is illustrated in Fig. 40. The Process.—The sample is finely powdered, and 1 gm. weighed out for analysis. This is rubbed up with successive portions of water (using not more than 50 cc.) and transferred to a flask having a capacity of about 150 cc. The flask and contents are now heated upon a water-bath for half an hour to thoroughly gelatinize the starch. If the substance analyzed con- tains oil, this must first be extracted in a “ Soxhlet ” apparatus before the water is added. 348 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. If free starch is to be experimented with, 0.2 or 0.3 gm. instead of 1 gm. should be taken. When the starch is gelatinized, the solution is cooled, and 25 cc. of the baryta-water are added. The flask is corked, and well shaken for two minutes; proof spirit is then added to make about 125 cc., the flask again corked, thoroughly shaken, and set aside to settle. While settling a check is made upon 10 cc. of the baryta-water mixed with 50 cc. of recently boiled dis- tilled water, by titrating with decinormal hydrochloric acid, using phenolphtalein as indicator. The number N of cc. of hydrochloric acid V. S. used, is noted, and 10 when multiplied by the total strength of the 25 cc. of the baryta-water employed in the analysis is ob- tained. When the settling of the insoluble compound is completed, 25 cc. of the clear liquid is drawn off (this is jf of the entire quantity) with a pipette and rapidly N . titrated with the — acid V, S. in the presence oi a few 10 drops of phenolphtalein T. S. The number of cc. consumed is noted, multiplied by 5, and then deducted from the number representing the total strength of 25 cc. baryta-water. The difference is the quantity which went into combination with the starch. N Each cc. of the — hydrochloric acid V. S. represents 0.00765 gm. of BaOa, which is equivalent to 0.0324 gm. of starch. Therefore by multiplying the number of cc, repre- senting the quantity of baryta which combined with A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 349 the starch by 0.0324 gm., the quantity of starch pres- ent in the sample is obtained. Example.—1 gm. of substance was taken, mixed with 50 cc. of water, 25 cc. of baryta-water, and sufficient proof spirit to make 125 cc. This is set aside and allowed to settle. The reaction which takes place is as follows : 2C12H20O10 -f- BaO,H20 — C24 H40O20.BaO -f- H20. Starch, '—<—' 2)648 2)153.0 324 76.5 While settling, the strength of the baryta-water is determined by titrating with decinormal hydrochloric acid V. S., the following equation being applied; BaO,H.O + 2HC1 = BaCl, + 2H80. 2)72.74 10) 76.5 10)36-37 N 7.65 gms. 3.67 gms. or 1000 cc. — V. S. Thus each cc. represents 0.00765 gm. of BaO. 10 cc. of the baryta-water are taken, and 8 cc. of the N — acid solution are required to neutralize this. There- 10 n fore 25 cc. of baryta-water will require X 8 cc. = 20 cc. of — acid V. S. 10 When the settling is completed, 25 cc. of the clear N solution is drawn off and titrated with — acid V. S. 10 N We will assume that 2.5 cc. of the — acid V. S. are 10 350 A TEXT-BOOK OF VOLUMETRIC ANALYSTS. required ; therefore the entire quantity of solution will neutralize 5 X 2.5 cc. = 12.5 cc. The difference between 12.5 cc. and 20 cc. = 7.5 cc., N which is the loss of alkalinity expressed in cc. of — N acid V. S. Each cc. of alkalinity lost, expressed as — acid V. S., indicates that 0.00765 gm. of BaO went into combination with starch; and since 0.00765 gm. of BaO represents 0.0324 gm. of starch, the substance analyzed contains 7.5 X 0.0324 gm. or O.243 gm. of starch. o.243 X ioo , = 24.3^ Another Method for Estimating Starch consists in converting it into glucose and then estimating the glucose with Fehling’s Solution. The starch is weighed and boiled in a flask with water containing hydrochloric acid for several hours ; the solution is then cooled, neutralized with potassium hydroxide, and diluted so that I part of starch, or rather sugar, shall be contained in 200 parts of water. This is put into a burette and titrated into 10 cc. of Fehling’s Solution, as described below under Sugar. In estimating the starch in baking powder, 2 to 5 gms. of the powder are introduced into an Erlen- meyer flask, 150 to 200 cc. of a 4 per cent solution of hydrochloric acid are added and the solution gently boiled for four hours, after which the flask and con- tents are cooled, neutralized by adding sodium hy- droxide, and made up to a definite volume. It is then ready for testing with Fehling’s solution. A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 351 CHAPTER L. ESTIMATION OF SUGARS. Fehling’s Solution.—(a) The Copper Solution.— 34.64 gms. of carefully selected small crystals of pure cupric sulphate are dissolved in sufficient water to make, at or near 150 C. (590 F.), exactly 500 cc. Keep in small well-stoppered bottles. (b) The Alkaline-tartrate Solution.—173 gms. of po- tassium and sodium tartrate (Rochelle salt) and 125 gms. of potassium hydroxide, U. S. P., are dissolved in sufficient water to make, at or near 150 C. (590 F.), ex- actly 500 cc. Keep in small rubber-stoppered bottles. For use, equal quantities of the two solutions should be mixed at the time required. io cc. of the mixed solution is equivalent to Glucose 050 Maltose 082 Inverted cane-sugar 0475 Inverted starch. 045 The Process.—0.5 gm. or less of the sugar is dis- solved in 100 cc. of water. This liquid is placed in a burette. 10 cc. of the Fehling’s Solution are mixed with 50 cc. of water and placed in a porcelain dish over a Bunsen burner and heated to boiling. The sugar solution is then run in from the burette, until all blue color is destroyed. 352 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. It is always somewhat difficult to determine the exact point at which the blue color disappears, owing to the presence of the precipitated suboxide of copper. This difficulty may be overcome by the addition of some substance which will prevent the precipitation of the cuprous oxide, such as ammonium hydroxide or potassium ferrocyanide. The disappearance of the blue color can then be readily seen, as the solution re- mains clear to the end, turning from blue to green, and finally brown, which indicates the end of the reaction. Professor Bartley reports this method as accurate, reliable, and rapid, provided the solution be not boiled during the reduction. He recommends to add to the Fehling’s Solution in the porcelain basin 10 cc. of a 10$ freshly prepared solution of potassium ferrocy- anide and 30 cc. of water. The ferrocyanide does not precipitate the copper in alkaline solution. If the sugar to be examined be either glucose, malt- ose, or lactose, it may be titrated directly; but if it be cane-sugar, it must first be inverted. This is done by dissolving the sugar (0.475 gm.) in about 100 cc. of water, adding 3 or 4 drops of strong hydrochloric acid, and boiling briskly for ten or fifteen minutes. This is then allowed to cool, neutralized with potassium hy- droxide, and made up to 100 cc. with water. The Calculation.—10 cc. of Fehling’s Solution are always taken ; and whatever the quantity of glucose or sugar solution is required to effect reduction, that quantity contains the equivalent of 10 cc. of Fehling’s Solution. Thus if 12 cc. of the sugar solution were required to reduce 10 cc. of Fehling’s Solution, the 12 cc. contain 0.05 gm. of glucose or 0.082 gm. of maltose, A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 353 etc. ioo cc. of the solution therefore contain x gm. of glucose. .05 X 100 ~ , — = 0.410 gm. glucose. The sugar in urine may be estimated by this process. The urine is placed in the burette and run into the boiling Fehling’s Solution in the usual manner. If it contain a large quantity of sugar, it must be diluted two or three times. In estimating with Fehling’s Solution it is well to attach a rubber tube 8 to 12 inches in length to the lower end of the burette, so that the boiling need not be done directly under the burette, and thus cause in- correct readings through the expansion of the liquid therein. Pavy s Method.—This consists in adding ammonia- water to the ordinary Fehling’s solution, in order to prevent the precipitation of cuprous oxide, which has a tendency to hide the end reaction. Thus the dis- appearance of the blue color which constitutes the end reaction is distinctly seen. Pavy’s solution is made by dissolving 170 gms. of Rochelle salt and 170 gms. of potassium hydroxide in sufficient water. Then 34.65 gms. of copper sulphate are separately dissolved in water with the aid of heat, and the two solutions are mixed and diluted to 1 litre. 120 cc. of this solution are now taken and mixed with 400 cc. of ammonia-water (sp. gr. 0.88) and diluted with water to 1 litre. This constitutes Pavy’s solution, or rather Pavy-Fehling’s solution, of which 10 cc. = 1 cc. of Fehling’s solution, i.e., 10 cc. of Pavy’s solution = 0.005 °f glucose. 354 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. The process is conducted as follows: 10 cc. of the Pavy’s solution are diluted with 20 cc. of water and placed in a small flask, and heated to and kept at the boiling-point, while the glucose solution properly diluted is added from a burette. The glucose solution should be added at about the rate of 100 drops per minute until the blue color is just destroyed. The sugar solution should be so diluted that not less than 4 nor more than 7 cc. are required to produce the decoloration. In order to avoid the nuisance of filling the labora- tory with ammoniacal vapors, the titration may be performed in a small flask provided with a well-fitting cork, having two holes, through one of which the spit of the burette is passed, and through the other an escape-tube which conducts the vapors into a vessel containing water or diluted hydrochloric acid. Several titrations should always be made in order to obtain exact results, and it is advisable to check the solution against a sugar solution of known strength, since the ratio of reduction is seriously influenced by the amount of potassium hydroxide present and the strength of the ammonia-water. The calculation is exactly the same as that in the use of Fehling’s solution, except that 10 cc. of Pavy rr: 0.005 gm. gluCOSC. A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 355 CHAPTER LI. PEPSIN. Pepsin, the active constituent of the gastric juice, is an albuminous principle secreted by glands imbedded in the lining membrane of the stomach. Pepsin has never been isolated in a pure state, and its exact chemical composition is not known, therefore pepsin cannot be quantitatively estimated ; but the digestive strength of pepsin or its preparations is meas- ured by the amount of egg-albumen it will digest under certain conditions. A good pepsin should digest 2000 times its own weight of albumen. The different tests for ascertaining the digestive power of pepsin do not give the actual strength, but serve to show whether a sample is above, below, or near the standard. All the known tests are compara- tive tests, and must be conducted under like conditions, as slight variations in the manipulation will frequently occasion very different results even with the same pepsin. In testing pepsin, it is generally assumed that the sample which will so change the largest amount of egg- albumen as to render it soluble is the best. Coagulated egg-albumen is not readily soluble, but when acted upon by pepsin it is converted into a sub- stance which is soluble. 356 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. The value of pepsin as a digestive agent does not, however, lie in its power to convert albumen into a soluble substance, but rather in the amount of a certain soluble and diffusible principle (peptone) which it pro- duces in a given time and under certain conditions. The function of the gastric juice in the animal econ- omy consists in reducing the proteids of the food to a condition in which they are easily absorbed into the system, and not reducing them to a soluble condition. This conversion of the indiffusible proteids into solu- ble and diffusible peptone does not take place at once, but occurs only after they have passed through several successive stages. The first step in the digestive action of pepsin upon coagulated egg-albumen is the conversion of the latter into soluble acid-albumen, or syntonin, from which state it is subsequently converted into parapeptone, metapeptene, and finally peptone. The latter is the only one of these products which is highly diffusible, hence the albumen is not digested until it is converted into peptone. Thus it is seen that in the tests in which the dissolv- ing power of a pepsin is alone taken into account the actual digestive power is not ascertained. A weak pepsin may dissolve a large quantity of albu- men and convert it into syntonin, but will carry the digestion no further, while a stronger pepsin may in the same time convert the same amount of albumen not only into syntonin, but also into peptone. Ap- parently both samples have done equal work, the albumen being dissolved in both cases, while in reality one is double the strength of the other. When pepsin is brought in contact with more albu- A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 357 men than it can thoroughly digest, the latter is con- verted principally into syntonin, and little or no pep- tone is formed ; thus in order to determine the real digestive power of a pepsin, it is necessary to find out how much peptone it produces in a certain period. This may be accomplished by boiling the solution when the time is up, to prevent further action of the pepsin ; the solution is then filtered while still hot, and neutralized with sodium carbonate ; the syntonin will then be precipitated. This precipitate should be dried to a constant weight, and weighed ; the difference between this weight and the weight of the albumen originally taken will give approximately the quantity of peptone produced. If, however, the albumen was not completely dissolved, that remaining must also be deducted from the quan- tity first taken. It must not be forgotten that the conditions of tem- perature, acidity, time, amount of agitation, etc., must be the same in all cases. The U. S. P. method for the valuation of pepsin is as follows: Solutions Required.—{a) To 294 cc. of water add 6 cc. of diluted hydrochloric acid. (b) In 100 cc. of solution (a) dissolve 0.067 gm. (1 gr.) of the pepsin to be tested. (c) To 95 cc. of solution (a) brought to a temperature of 40° C. (104° F.) add 5 cc. of solution (b). The resulting 100 cc. of liquid will contain 0.21 gm. (0.2 cc.) of absolute hydrochloric acid, 0.00335 gm- °f the pepsin to be tested, and 98 cc. of water. Immerse and keep a fresh hen’s egg for fifteen min- utes in boiling water. Then remove it and place in 358 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. cold water. When it is cold, separate the white, coag- ulated albumen, and rub it through a clean sieve having 30 meshes to the linear inch. Reject the first portion passing through the sieve. Weigh off 10 gms. of the second clean portion, place in a flask of about 200 cc. capacity, and add one half of solution (c), and shake to distribute the albumen evenly through the liquid. Then add the other half of solution (c) and shake. Place the flask on a water-bath and keep the temperature at about 40° C. (104° F.) for six hours, shaking gently every fifteen minutes. At the expiration of this time the albumen should have disappeared, leaving at most only a few thin, insoluble flakes. The U. S. P. re- quirement is that the pepsin should be capable of digesting (dissolving) 3000 times its own weight of egg- albumen, coagulated and disintegrated as described above. The relative proteolytic power of pepsin stronger or weaker than that above described may be determined by ascertaining how much of solution (b) made up to too cc. with solution (a) will be required to exactly dissolve 10 gms. of coagulated and disintegrated albu- men under the conditions given above. This method is somewhat cumbersome and tedious. The following is Professor Bartley’s favorite method. In the hands of the author it has given entirely satis- factory results. Solutions Required.—{a) To 25 gms. of the well- mixed whites of several eggs add enough distilled water to make exactly 250 cc. Mix well by thor- oughly shaking with clean fine gravel, and boil for 5 minutes. After cooling, make up the solution to the original volume with *vater. This solution contains A TEXT-BOOK OF VOLUMETRIC ANALYSIS. 359 about 10$ of egg-albumen, or about 1.22 gms. of the dry albumen in 100 cc. ((b) One gm. of the pepsin to be tested is dissolved in 25 cc. of water. 2 cc. of diluted hydrochloric acid (U. S. P.) are added, and enough water to bring the solution up to 50 cc. Procedure.—Measure out into a beaker or bottle 50 cc. of the albumen solution, and warm on a water-bath to about 40° C. (104° F.). Add to this 2 cc. of diluted hydrochloric acid, and from 0.5 to 5.0 cc. of the pepsin solution. The more active the pepsin the less the quantity of the pepsin solution is to be taken. It is sometimes necessary with a pepsin of unknown strength to make a preliminary test, to determine the approxi- mate time required by the digestion, as it is best to so regulate the quantity of pepsin and albumen that the digestion shall be complete in two hours or less. The time when the pepsin is added must be carefully noted, and the temperature kept at about 350 to 40° C. (950 to 104° F.). At intervals of 10 minutes a few drops of the solution are drawn out with an ordinary dropper, and floated upon a few drops of pure nitric acid in a nar- row test-tube. Note the time when the nitric acid ceases to give a coagulum of albumen, or when the albumen disappears. We thus get for the calculation the weight of the egg- albumen, A ; the weight of the pepsin taken, P\ and the time consumed, T. We next assume the standard time of 3 hours, the average time of stomach digestion. The relation between the quantities of albumen and a pepsin is expressed by the fraction —; that is, it is 360 A TEXT-BOOK OF VOLUMETRIC ANALYSIS. found by dividing the amount of albumen by the weight of the pepsin. This result gives the amount of albumen digested by one part of pepsin, in the time observed in the experi- ment. To calculate what this would digest in the standard time, we must multiply the above ratio by the ratio of the observed time to the standard time; or, to put this in the form of an equation, we have D (or digest- ive power) = — X Suppose 50 cc. of solution (a) containing 5 gms. of egg-white be taken, and that 1 cc. of solution (