REESE LIBRARY OK mi. UNIVERSITY OF CALIFOPx n n rt, NIA QUANTITATIVE CHEMICAL ANALYSIS BY ELECTROLYSIS BY DR. ALEXANDER CLASSEN PRIVY COUNCILLOR "' Professor of Petrochemistry and Inorganic Chemistry in the Royal School of Technology at Aachen IN CO-OPERATION WITH DR. WALTER LOB Lecturer orrflectrochemistry in the Royal School of Technology at Aachen AUTHORIZED TRANSLATION TH1D ENGLISH FROM THE REVISED AND GREATLY ENLARGED FOURTH GERMAN EDITION BY WILLIAM HALE HERRICK, A.M. Formerly Profeor of Chemistry in Iowa College and in the Pennsylvania State College AND BERTRAM B. BOLTWOOD, PH.D. Instructor in j,alytical Chemistry in the Sheffield Scientific School of Yale University NEW YORK JOHN WILEY & SONS LONDON : CHAPMAN & HALL, LIMITED 1898 QUANTITATIVE CHEMICAL ANALYSIS BY ELECTROLYSIS BY DR. ALEXANDER CLASSEN PRIVY COUNCILLOR '^ Professor of Electrochemistry and Inorganic Chemistry in the Royal School of Technology at Aachen IN CO-OPERATION WITH DR. WALTER LOB Lecturer on Electrochemistry in the Royal School of Technology at Aachen AUTHORIZED TRANSLATION THIRD ENGLISH FROM THE REVISED AND GREATLY ENLARGED FOURTH GERMAN EDITION WILLIAM HALE HERRIOK, A.M. Formerly Professor of Chemistry in Iowa College and in the Pennsylvania State College AND BERTRAM B. BOLTWOOD, PH.D. Instructor in Analytical Chemistry in the Sheffield Scientific School of Yale University NEW YORK JOHN WILEY & SONS LONDON : CHAPMAN & HALL, LIMITED 1898 QTM15 C (o Copyright, 1898, BY WILLIAM HALE HERRICK AND BERTRAM B. BOLTWOOD. f 32. BOBKRT DRUMMOND, ELECTROTYPER AND PRINTER, NEW YORK. PREFACE TO FOURTH EDITION. THE present edition, revised with the assistance of Dr. Lob, differs from the previous editions in that the Introduc- tion has been augmented by the insertion of a section devoted to theory. This was made the more necessary, since the in- vestigations of recent years have been chiefly devoted to the explanation of the reactions in solutions, and the determina- tion of the electrical magnitudes. The necessity of specific directions concerning electrode tension, current strength and decomposition tension has been demonstrated. The author, with the co-operation of his assistants, has experimentally determined these electrical magnitudes, not only for his own methods for the determination and separation of the metals, but also for a number of other methods, and has incorporated them in the text. Additional methods by other authors, in which directions concerning these important factors are want- ing, have been omitted, in consideration of the fact that these are either uncertain or entirely impractical ; mention of them has, however, been made in the references to the literature. The book has been made more complete by the description of various measuring instruments, sources of current and apparatus ; together with an explanation of simple and com- plete appliances for carrying out electrolytic experiments. These have been illustrated by a large number of new cuts, in the text and in the appended tables. The publishers have spared neither pains nor expense to make these new illustrations as perfect as possible; I feel called upon to express here my full appreciation of this fact. A. CLASSEN. AACHEN, January 18, 1897. iii TRANSLATORS 1 PREFACE. THE Author's Preface to the Fourth Edition points out so fully the improvements over preceding editions, that the translators need add nothing. It is plainly a more complete, scientific and logically arranged work than heretofore. The translators have made some additions, as had their senior in previous editions; and have corrected some errors in the German edition, apparently the result of hasty com- pilation. The " Special Part" of former German editions (which is omitted in the fourth) has been retained in the form of an appendix, and has been revised and brought up to date. In addition to this a carefully prepared index has been added, and the translators believe that the value and con- venience of the work is thereby much enhanced. WILLIAM HALE HERRICK. BERTRAM B. BOLTWOOD. January, 1898. CONTENTS. SECTION I. GENERAL PART. PAGE INTRODUCTION 1 ION THEORY 6 FARADAY'S LAW 10 OHM'S LAW 12 TENSION AND ITS SIGNIFICANCE 13 SIGNIFICANCE OF CURRENT STRENGTH 17 SIGNIFICANCE OF RESISTANCE 19 THEORY OF ELECTROLYTIC PRECIPITATION , 21 DETERMINATION OF 'THE CURRENT MAGNITUDES : 1. MEASUREMENT OF THE CURRENT STRENGTH 23 Oxyhydrogeu-gas Voltameter 24 Weight Voltameter 26 Tangent Galvanometer 26 Sine Galvanometer 28 Other Forms of Galvanometers 29 Spring Galvanometer 31 Amperemeter 31 2. MEASUREMENT OF THE TENSION 32 Voltmeter .... 32 Torsion Galvanometer 33 Lippmanu Capillary Electrometer 35 Quadrant Electrometer 37 SOURCES OF CURRENT 38 PRIMARY GALVANIC ELEMENTS : Leclanche Cell 39 Meidinger Cell 41 Daniell Cell 43 Gravity Cell 43 vii viii CONTENTS. PAGE Grove Cell 44 Bunsen Cell 45 Cupron Element 46 Edison-Lalande Element 47 SECONDARY GALVANIC ELEMENTS (ACCUMULATORS, OR STORAGE BATTERIES) ... 47 General Rules for the Handling of Accumulators 54 PHYSICAL METHODS OF PRODUCING THE CURRENT : Electromagnetic Machines 60 Thermo-electric Piles 64 REGULATION OF THE CURRENT 73 PROCESS OF ANALYSIS 83 HISTORICAL .. 101 ARRANGEMENTS FOR ANALYSIS 107 Arrangement for Smaller Experiments 108 Former Equipment of the Electrochemical Institute at Aachen.. . . Ill Present Equipment of the Electrochemical Institute at Aachen. ... 124 SECTION II. SPECIAL PART. QUANTITATIVE DETERMINATION OF THE METALS. IRON 137 COBALT 141 NICKEL 143 ZINC 144 MANGANESE 148 ALUMINIUM, URANIUM, CHROMIUM, BERYLLIUM 153 COPPER 153 BISMUTH 162 CADMIUM , 163 LEAD . 166 THALLIUM 170 SILVER 172 MERCURY 174 GOLD 177 ANTIMONY 178 PLATINUM 182 PALLADIUM 183 TIN.. . 183 CONTENTS. PAGE ARSENIC. . . ...... ...... ........... . . ................. ........... 188 POTASSIUM, AMMONIUM (NITROGEN) ........ ... ..................... 188 DETERMINATION OF NITRIC ACID IN NITRATES ............ . ....... 189 DETERMINATION OF THE HALOGENS. CHLORINE, BROMINE, IODINE ...................... . .............. 190 SEPARATION OF THE METALS. IRON .................................................. . ...... 191-204 Iron Cobalt .................................................. 191 Iron Nickel ...... ........................................... 192 Iron Zinc .................................................. 193 Iron Manganese ....................... ...................... 194 Iron Aluminium ............................................. 196 Iron Uranium ............................................... 198 Iron Chromium .......................................... ... 199 Iron Aluminium Chromium ....... . ......................... 200 Iron Chromium Uranium ............................ . ....... 200 Iron Beryllium . ............................................. 201 Iron Beryllium Aluminium .................................. 201 Iron Copper ................................................. 202 Iron Lead ................................................... 204 COBALT ...................................................... 204-206 Cobalt Zinc ............................................... 204 Cobalt Aluminium ........................................... 204 Cobalt Uranium ............................................ 204 Cobalt Chromium . . ........................................ 204 Cobalt Uranium Chromium ............. .................... 204 Cobalt Copper ............................................... 205 Cobalt Bismuth ........ ... .................................. 205 Cobalt Lead ................................................ 206 Cobalt Mercury ....... . ...................................... 206 NICKEL ....... . ............ . .............................. 206,207 Nickel Manganese ......................................... 206 Nickel Aluminium ................. '. ......................... 206 Nickel Uranium ...... , ..................................... 206 Nickel Chromium ......... .................................. 206 Nickel Copper .............................................. 206 Nickel Lead ............................................... 207 Nickel Mercury ................... ......................... 207 S CONTENTS. PAGE ZINC 208-211 Zinc Manganese 208 Zinc Aluminium 208 Zinc Copper 208 Zinc Cadmium 209 Zinc Lead 210 Zinc Silver 210 Zinc Mercury 210 MANGANESE 211, 212 Manganese Copper 211 Manganese Cadmium 212 COPPER 3i2-21? Copper Cadmium 212 Copper Lead 213 Copper Silver 215 Copper Mercury 216 Copper Arsenic 217 CADMIUM 217,218 Cadmium Lead 217 Cadmium Mercury 218 LEAD 218,219 Lead Silver 218 Lead Mercury 218 Lead Antimony 219 SILVER 220 Silver Antimony , 220 Silver Arsenic 220 MERCURY 220, 221 Mercury Antimony 220 Mercury Arsenic 221 ANTIMONY 221-228 Antimony Tin 221 Antimony Arsenic 224 Antimony Tin Arsenic 225 TIN PHOSPHORIC ACID 228 PLATINUM IRIDIUM 228 SEPARATION OF GOLD FROM OTHER METALS 228 POTASSIUM SODIUM 229 SODIUM AMMONIUM. . . 229 CONTENTS. XI APPENDIX. SOME APPLIED EXAMPLES OF ELECTROCHEMICAL ANALYSIS. PAGE BRASS 231 SILVER COIN 233 NICKEL COIN 233 GERMAN SILVER 234 BRONZE , 235 PHOSPHOR-BRONZE 235 MANGANESE PHOSPHOR-BRONZE 236 SOLDER 236 WOOD'S METAL 237 HARD LEAD, TYPE-METAL. 237 ALLOY OF ANTIMONY AND TIN 238 ALLOY OF ANTIMONY AND ARSENIC 238 ALLOY OF ANTIMONY, TIN AND ARSENIC 239 SPATHIC IRON-ORE 239 HEMATITE 240 LIMONITE 241 CLAY IRON-ORE , 242 BOG IRON-ORE 242 CHROME IRON-ORE 242 PSILOMELANE 244 SPHALERITE (ZINC BLENDE) 247 CALAMINE AND SMITHSONITE 249 ULTRAMARINE , 249 REFINERY SLAG 250 COPPER AND LEAD SLAGS 250 BLAST-FURNACE, CUPOLA, AND BESSEMER SLAGS 252 ZIRCON 253 ARSENOPYRITE 253 CHALCOPYRITE 254 NICKEL MATTE, COPPER MATTE 255 COPPER SPEISS, LEAD SPEISS . . 256 PYRARGYRITE 257 TETRAHEDRITE 257 FURNACE "Sows" - 258 STIBNITE (ANTIMONY GLANCE). 259 ULLMANITE 259 BOURNONITE . 260 Xll CONTENTS. PAGE ZlNKENITE 260 LlNN^EITE 261 CoBALTITE . 261 COBALTIFEROUS ARSENOPYRITE 262 CERUSSITE 263 GALENA ... 263 PYROMORPHITE 264 LEAD MATTE 264 CINNABAR , 265 SOFT LEAD (CRUDE LEAD) 265 ANTIMONY 268 SPELTER (CRUDE ZINC) 268 BLISTER COPPER 269 REFINED COPPER 272 TIN . 272 SILVER . . 273 COMMERCIAL NICKEL 274 PIG IRON, STEEL, SPIEGEL, FERROMANGANESE 275 TABLES FOR CALCULATION OF ANALYSES 280-283 REAGENTS 284-287 POTASSIUM OXALATE 284 AMMONIUM OXALATE 284 OXALIC ACID 285 AMMONIUM SULPHATE 285 SODIUM SULPHIDE 285 ALCOHOL 286 INDEX OF AUTHORS 287 INDEX OF SUBJECTS.. 293 QUANTITATIVE ANALYSIS BY ELECTROLYSIS. PAET L-GEKEKAL PAET. INTRODUCTION.* WATER acidified with sulphuric acid is decomposed into its elements, hydrogen and oxygen, when a galvanic current is passed through it; a large number of compound sub- stances conduct themselves in a similar manner. This gal- vanic decomposition is called electrolysis, and the substances which are decomposed by the electric current are known as electrolytes. The substances into which electrolytes are separated by the electric current are naturally divided into two groups : Those which separate at the positive electrode, or anode (connected with the + pole of the source of the current), and which are therefore the electro-negative con- stituents, are called anions; those which separate at the negative electrode, or cathode (connected with the pole of the source of the current), the electro-positive constitu- ents, are called cathions. The metalloids, or electro-negative acid groups, therefore appear at the positive electrode, while the metals are sepa- rated at the negative electrode. * An elementary knowledge of galvanic action is assumed. 2 QUANTITATIVE ANALYSIS BY ELECTROLYSIS. For instance, if the electric current is passed through the solution of a haloid salt, the halogen is separated at the anode, the metal at the cathode. CuCl 2 = C1 2 + Cu, ZnCl 2 = C1 2 + Zn. Oxygen salts act in a similar manner. CuS0 4 = S0 4 + Cu, Cu(N0 3 ) 2 = (N0 3 ) 2 + Cu. Many acids are decomposed in a similar manner.* H 2 S0 4 = S0 4 + H 2 , 2HC1 = C1 2 + H 2 . The substances formed by electrolytic decomposition, however, generally undergo further chemical change, or are acted on by the electrodes ; various secondary reactions take place. In the electrolysis of a solution of copper sulphate between platinum electrodes, the secondary process consists in the re- action with water of the group SO 4 , which cannot exist uncombined. SO 4 + H 2 O = H 2 S0 4 + O. The evolution of oxygen gas, which is partially due to this secondary reaction, is observed at the positive pole. Pri- marily the water itself splits off oxygen in the electrolysis of aqueous solutions. * Some acids are not decomposed by the electric current ; e.g., silicic, carbonic, and boric acids. INTRODUCTION. 3 In the electrolysis of hydrochloric acid, the chlorine set free at the anode reacts with water, forming hypochlorous acid, chloric acid, perchloric acid, etc. Similar secondary reactions are observed in the electrolysis of chlorides. If a solution of ammonium chloride, for example, is submitted to electrolysis the nascent chlorine acts on the un decomposed salt, with the production, among other substances, of nitro- gen, or nitrogen chloride. Haloid salts of the alkaline earths show similar phenomena. Nitric acid, on electrolysis, gives in the first place SHINTO, = 4H 2 (cathion) + 8E"O 3 (anion). The latter then splits up further: 4N 2 O 6 = 4^1,0. + 2O 2 (anion). The oxygen is given off, while the anhydride forms nitric acid again with water: 4N A + 4H 2 o = SHNO,. The hydrogen, on the contrary, which appears as cathion, is not set free but acts reducingly on the nitric acid present : 4H, + mro s = NH 3 + 3H a O. In the presence of sulphuric acid, or a sulphate, this de- composition is complete, the final product being ammonium sulphate. This decomposition of nitric acid is of practical importance in chemical analysis. From a nitric acid solution which con- tains copper and zinc, the former metal only is reduced ; this fact can be utilized for the separation of the two metals. If, 4 QUANTITATIVE ANALYSIS BY ELECTROLYSIS. now, the current is allowed to pass for a long time after the reduction of the copper, the nitric acid is gradually converted into ammonia, and the zinc then separates from the solution. If the salts of metals which decompose water at ordinary temperatures (alkalies and alkaline earths) are electrolyzed, secondary reactions occur at the negative electrode : K 2 SO 4 = SO 4 (anode), K 2 (cathode), (S0 4 + H 2 = H 2 S0 4 + 0), K a + 2H.O = 2KOH + H a . The decomposition products, then, are sulphuric acid and oxygen at one electrode, potassium hydroxide and hydrogen at the other. It must be borne in mind, however, that a small portion of the hydrogen and oxygen formed owes its existence to the primary electrolysis of the water. The metals disengaged at the negative electrode may yield secondary products by acting on the substances in solution. So, for instance, in the electrolysis of cupric chloride, the separated copper reacts with the cupric chloride to form cuprous chloride; copper acetate yields, at the cathode, a mixture of copper and cupric (or cuprous) oxide. In the electrolysis of organic compounds, the groups set free at an electrode may be decomposed in a manner analo- gous to that noted in inorganic compounds, and yield various products. The electrolysis of potassium acetate should yield, as iinal products, potassium (potassium hydroxide) and acetic acid. Instead of this, the acetic acid splits either into carbon dioxide and ethane, or ethylene is formed by the action of oxygen on the ethane. Potassium valerate yields, in addition to valeric acid, INTRODUCTION. 5 carbon dioxide and octane; the latter is oxidized by con- tinued electrolysis to isobutylene and water. Sodium snccinate yields, among other products, ethylene and carbon dioxide ; potassium lactate breaks up into carbon dioxide and acetaldehyde. For the purposes of quantitative chemical analysis, only such solutions are adapted, as indicated by the foregoing, as are decomposed completely by the current without the forma- tion of injurious intermediate products. Solutions which contain a free inorganic acid are well adapted to electrolysis, because of their high conductivity. Of all compounds of the metals, the double oxalates are the best adapted to quantitative analysis.* Oxalic acid is decomposed by the electric current : C 3 H 2 O 4 = 2CO 2 (anode), H 2 (cathode). When potassium oxalate is subjected to electrolysis, the principal decomposition -products are: K 3 C,O 4 = 2CO 3 (anode), K 2 (cathode), K 3 + 2H a O = 2KOH + H 2 (cathode), 2KHO + 2CO, = 2KHCO 3 . When ammonium oxalate is used, the decomposed solu- tion contains hydrogen ammonium carbonate. The latter partly decomposes into ammonia and carbon dioxide. In the electrolysis of double oxalates, e.g., of zinc ammo- * Classen, Ber. d. ch. Ges., 14, 1622, 2771; 17, 2467; 18, 1104, 1687; 19, 323 ; 20, 504 ; 21, 2900. 6 QUANTITATIVE ANALYSIS BY ELECTROLYSIS. nium oxalate, decomposition takes place as follows : Zinc oxalate breaks up into zinc and carbon dioxide, and ammo- nium oxalate into ammonium and carbon dioxide. The car- bon dioxide, which separates at the positive pole, combines with the ammonium to form hydrogen ammonium carbonate, as above explained. In the decomposition of oxalates there are no unfavora- ble secondary reactions. All oxalates are decomposed by the electric current with greater or less ease, and the reduced metals are not attacked by the decomposition-products, even when the current becomes weaker during the reaction. When the reaction is complete, the solution can be poured off at once, and the weight of the separated metal determined. (See further details later.) THE ION THEORY. Before proceeding to a description of the appliances and methods of quantitative analysis, the reactions which take place in solutions during their decomposition, together with the magnitudes which here come into consideration, should be made perfectly clear. The ion theory, proposed in connection with the researches of van't Hoff, by the Swedish investigator Arrhenius, furnishes us with a comprehensive picture of the same. According to this theory, a partial splitting up of the dissolved compounds into their component parts takes place in aqueous solutions ; a dissociation which, in contradistinction to the ordinary, is called electrolytic dissociation. In tlie case of a sodium chloride solution, for example, many phenomena, such as the osmotic pressure, the lowering of the freezing- point, and others, necessitate the assumption that, besides the particles of undecomposed Nad, separate. THE ION THEORY. 7 particles of "N& and Cl are present in the solution. The latter are entirely different from atomic Na and Cl, since it is of course impossible to conceive of a Na atom, which reacts violently with water, as existing free in an aqueous solution. The difference between these electrolytic dissociation products and atoms lies in an unlike content of energy. This of course materially affects the other properties. While the atom in itself must be considered non-electric (containing as much positive as negative electricity), it is necessary to attribute a certain electric charge to the products of electrolytic dissociation. These electrically charged par- ticles are called ions (iovres, the wandering), a name given to them by Faraday. The phenomena of the osmotic pressure and the depression of the freezing-point, already mentioned, have identified with electrolytic dissociation, and accordingly with ions, certain classes of chemical compounds, namely, acids, bases, and salts, but not indifferent organic compounds. Since it has been shown that the former compounds, and indeed only these and no others, conduct the electric current in aqueous solu- tions, the existence of ions and the characteristic of being decom- posed by the electric current have been brought into causal relation. The substances which are electrolytically dissociated in solution, and therefore conduct the electric current, are called electrolytes ; those which are not dissociated into ions and do not permit the passage of the current, non-electrolytes. Acids, bases, and salts are accordingly electrolytes ; all other substances, such as chloroform, benzene, ether, sugar, etc., non-electrolytes. Relative to the formation of ions, all acids exhibit a com- mon characteristic in yielding hydrogen ions ; correspondingly all bases give hydroxyl ions. QUANTITATIVE ANALYSIS BY ELECTROLYSIS. An acid is accordingly dissociated as follows : HC1 into H Cl . H 2 S0 4 into H 2 S0 4 HN0 3 into H N0 3 C 2 4 H 2 into H 2 C,0 4 CH 3 COOH into H CH 3 COO, etc. By the electrolytic dissociation of acids, therefore, all of those hydrogen atoms, which are replaceable by metals on neutralization with bases, are brought into the state of ions ; at the same time the corresponding acid radicals pass over into the ionic condition. The dissociation of bases is analogous : JSaOH into Na OH NH 4 OH into NH 4 OH Ca(OH) 2 into Ca (OH),, etc. All the hydroxyl groups, which are replaceable by acid radicals on neutralization with acids, change to the ion con- dition; and simultaneously the basic radicals, i.e., the metals. Salts are accordingly dissociated into metal and acid ions. When the current passes, a part of the ions migrate to the positive electrode, a part to the negative electrode. Since the ions possess an electric charge, those attracted by the positively charged electrode must be negatively charged (anions), and those attracted by the negative electrode positively charged (cat/lions). Hydrogen and all metals are electro-positive ; halogens and acid radicals, electro-negative. The former are cathions, the latter anions. Therefore when a salt of a metal is electrolyzed. the metal separates at the negative electrode, the acid radical at the positive electrode. THE ION THEORY/S^f TAH ai\K^^ 9 It h&s been demonstrated that the electrolytic dissociation increases with the dilution, and proportionally to it the elec- trical conductivity of the ions also. Arrhenius therefore drew the conclusion that the ions alone conduct the current, and that the undissociated portion takes no part in the electrolysis. This assumption has been proved correct, and through it elec- trolytic dissociation presents an entirely different aspect. The primary products which are set free at the electrodes are not separated by the current , but existed previously in the solution in the form of cathions and anions. Since the substances separate at the electrodes in an atomic, non-electric condition, while the ions possess an electric charge, a discharge of the ions must take place at the electrodes. Such is in fact the case. The negatively charged electrode attracts the positively charged cathions, and on their coming into contact a neutralization of equivalent parts of positive and negative electricities takes place, accompanied by the dis- appearance of electricity. The separation of the substance in an atomic form is then possible. The work done by the elec- tric current consists in the attraction and discharge of the ions, but not in the decomposition of the dissolved compound. What has been said of the cathion naturally applies in a similar manner to the anion. The degree of dissociation at a certain concentration [and temperature] is a fixed magnitude for every substance. Here the objection might be raised, for example, that in a copper sulphate solution in which ^ of the copper sulphate is split up into Cu and SO 4 ions and f is present as undissociated salt, the electrolysis must come to a standstill after the third of the copper has been removed, since there are no more copper ions present. Experiment shows that all the copper may be separated. This phenomenon is explained by the law of mass action, according to which the product of the ion con- 10 QUANTITATIVE ANALYSIS BY ELECTROLYSIS. centrations remains constant at a fixed concentration of the solution. In other words, if copper ions disappear from the solution owing to the removal of the metal, the undissociated salt present dissociates and furnishes new copper ions. This process continues until all the copper ions have been removed by precipitation as atoms. What is true of copper sulphate is true of all acids, bases, and salts, and anions behave similarly to cathions. FARADAY'S LAW. This law, which is named from its discoverer and is the basis of all electrolytic phenomena, includes the two following propositions : 1. The quantities of ions separated at the electrodes during equal intervals of time are directly proportional to the current strength. 2. Equal quantities of electricity separate the ions in pro- portion to their chemical equivalent weights. The truth of the first statement may be readily demon- strated by electrolyzing a copper sulphate solution for ten minutes with a current of certain strength and determining the weight of the separated copper. If now in a second operation a current of twice the former strength be passed through the same solution for an equal length of time, the weight of the copper separated in the second case will be twice as great as that precipitated in the first experiment. The second proposition, called in brief Faraday's Law, is proved experimentally by passing the same current simul- taneously through a series of solutions of metallic salts and weighing the quantities of metals which separate. It will then be found that the weights are proportional to the chemical equivalent weights of the metals. Accordingly solutions of silver nitrate, cupric chloride, and ferric chloride when decom- FARADAY'S LAW. 11 posed by the same current, yield precipitates of metals, the weights of which bear the following ratio to one another : 108 - 63 - 55 ' 9 8 -" -~- The ratio when silver nitrate, cuprous chloride, and fer- rous chloride are used is correspondingly 108:63:^, so that the equivalent weight is dependent upon the valence of the metal in the compound employed. That which may be conveniently carried out in the case of the metals, i.e., the cathions, holds true similarly in the case of the anions. The law of Faraday, regarded in the light of the ion theory, leads to a series of new conclusions. It declares that equal currents of electricity always separate equal quantities of univalent, half the quantities of bivalent, and one- third the quantities of trivalent ions. This separation consists in the discharge of the electrically charged ions ; therefore it follows that equal quantities of univalent ions are the carriers of equal electric charges, that the same quantities of bivalent ions bear twice, of trivalent ions three times, as great charges. Consequently all univalent ions, independent of their chemical nature, bear equal quantities of electricity, all biva- lent ions twice the quantity, etc. The magnitude of this charge is considerable. Experiment has shown that by the action of 96,500 coulombs of electricity on an electrolyte, a quantity of ions equal in grams to their atomic weight divided by their valence always passes over into the atomic condition, or, as it may be stated, by 96,500 coulombs a gram equiva- lent of ions will be discharged, or, better, neutralized on the electrode. For this neutralization the ions must carry a 12 QUANTITATIVE ANALYSIS BY ELECTROLYSIS. quantity of electricity equal, but opposite in sign, to that supplied by the source of current to the electrodes. A gram equivalent of ions must therefore be the bearer of 96,500 coulombs.* This conclusion, arrived at by v. Hehnholtz, leads to the assumption that every bond of valence of an ele- mentary or complex ion is charged with the same quantity of positive or negative electricity, which, similarly to an electric atom, cannot be further divided. OHM'S LAW. Ohm's Law holds good for solutions as it does for metals: C = |, E = 0-K. "When suitable units are chosen, the current strength is equal to the quotient of the electromotive force divided by the resistance. The ampere serves as the unit of current strength, and is that current strength by which, in one second, 0.328 nag. of copper will be precipitated, f The unit of resistance is the resistance (at 0) of a column of mercury having a length of 106.3 cm. and a cross-section of 1 sq. mm. It is called the ohm. The unit of electromotive force is the volt, and is defined by the equation 1 ampere X 1 ohm = 1 volt, volt ampere = , . ohm * " The quantity of electricity necessary for the separation of a gram equivalent, i.e., 96,540 coulombs, is to be denoted by the symbol F, in remembrance of Faraday." Report of Commission on Electrical Units, Deutsche Elektrochemische Gesellschaft. Zeit. f. Elektrochemie, 1897-98, p. 36. Trans. f An ampere may be also defined as the strength of the current which flows through a resistance of 1 ohm when the electromotive force is equal to 1 volt. Trans. TENSION AND ITS SIGNIFICANCE. 13 An electromotive force of one volt with a resistance of one ohm gives a current strength of one ampere. Every source of current furnishes a certain electromotive force ; it possesses a certain tension. If the two poles of a source of current are connected by a conductor, there takes place along this connection a fall of potential which is pro- portional to its resistance. If an electrolyte, into which ex- tend two electrodes, is included in the circuit, there arises a difference of potential between the electrodes which is called the electrode tension. This tension at the electrodes is of con- siderable importance in electrolysis, since it denotes that elec- tromotive force which comes into action in the cell itself. Each of the three factors given in Ohm's Law is of signifi- cance for quantitative electrolysis ; we will next proceed to their consideration. TENSION AND ITS SIGNIFICANCE FOR ELECTROLYSIS. As quantitative electrolysis is employed chiefly for the determination of metals, it will be w r ell to here consider some of the general properties of solutions of metallic salts. In accordance with present theory, the origin of an electro- motive force is explained as follows : If a strip of metal, zinc for example, be dipped into an electrolyte, say zinc chloride, the zinc ions present in the solution will have a tendency to discharge their electricity upon the zinc and to pass over into an atomic condition. This tendency may be considered as a pressure directed from the liquid toward the metal, and is known as the osmotic pressure of the ions. The metallic zinc, however, exerts a pressure in the opposite direction, which is due to the tendency of the zinc atoms to pass into the solution and assume the condition of ions, and is opposed to the sepa- ration of the ions already present, which strive to leave the 14 QUANTITATIVE ANALYSIS BY ELECTROLYSIS. .electrolyte as atoms. This latter pressure is known as the electrolytic solution pressure. Since the ions are bearers of electric charges, it is evident that the simultaneous action of these two pressure-forces is intimately connected with the production of electricity, and experience has taught that the electromotive force between the liquid and the metal may be considered a function of these two pressures. Osmotic pressure of the ions and electrolytic solution pres- sure cause currents in opposite directions. The positively charged ions of the salt solution tend, as a result of the osmotic pressure, to give up their charges to the exposed metal and to charge this positive ; while, on the other hand, the electrolytic solution pressure forces positive ions out from the metal and into the solution, leaving an equivalent negative charge upon the metal itself. If a similar course of reasoning be applied to the case where an electric current is passed between two platinum electrodes which dip into a solution of a metallic salt, the following conditions will be recognized. A definite difference of potential will exist between the electrodes, as a result of which metal will be separated on the negative pole, and, in general, oxygen on the positive pole. As soon, however, as the metal and oxygen have passed over into the atomic condi- tion, the electrolytic solution pressure of each comes into action and operates to drive them back into the form of ions. An electromotive force opposed to that of the primary cur- rent is thereby set up. This electromotive force, which may under certain conditions have a tension higher than that of the primary current, is the cause of a current called the polarization current. Polarization must always appear when unattacked electrodes, as those of platinum, which are exclu- sively used in quantitative electrolysis, are employed. TENSION AND ITS SIGNIFICANCE. 15 The tension required for electrolysis may always be deter- mined from the consideration of the above conditions. It must in all cases be greater than the resulting polarization current, for otherwise, at the commencement of decomposition, the electromotive force of the primary current would be counterbalanced by the polarization tension and electrolysis would be entirely prevented. Le Blanc, who made a careful study of the values of the tensions required for the decomposition of various solutions, directed attention to the fact that for the continued electrolysis of any solution a definite minimum tension, dependent directly upon the polarization phenomena, is required. This so-called decomposition tension value is often given as a measure in quantitative electrolysis, and by it is denoted that electromotive force at which the current is just able to pass through the cell. If the electromotive force of the primary current be denoted by E, the current strength by C, the resistance by R, and the polarization tension by P, then E must satisfy this equation without C being equal to ; then only can the current continuously decompose the solution. Le Blanc determined the following values for the deeoin- ,ST position tension of y solutions : ZnSO 4 .............. 2.35 volts. Cd(NO,) a ............ 1.98 volts. ZnBr a .............. 1.80 " CdSO 4 .............. 2.03 " NiS0 4 .............. 2.09 " CdCl, .............. 1.88 " NiCL, .............. 1.85 " CoS0 4 .............. 1.92 " Pb(NO,) a ........... 1.52 " CoCl a ............... 1.78 " AgNO, ............. 0.70 " 16 QUANTITATIVE ANALYSIS BY ELECTROLYSIS. ACIDS. Sulphuric acid 1.67 volts. Pyroracemic acid 1.57 volts* Nitric acid 1.69 Phosphoric acid. ...1.70 Monochloracetic acid 1.72 Dichloracetic acid .. 1.66 Malonic acid 1.69 Perchloric acid 1.65 Dextrotartaric acid . . 1.62 Trichloracetic acid... 1.51 Hydrochloric acid 1.31 Hydrazoic acid 1.29 Oxalic acid 0.95 Hydrobromic acid 0.94 Hydriodic acid 0.52 BASES. Sodium hydroxide. . . . 1.69 volts. Methylamine 1.75 volts. Potassium hydroxide. . 1.67 " Diethylamiue 1.68 " N Ammonium hydroxide 1.74 " ^- Tetramethyl ammo- nium hydroxide.. 1.74 " The sulphates and nitrates of the alkalies and alkaline earths have all nearly the same decomposition tension value, namely, about 2.20 volts. The values of the decomposition tension for solutions of metallic salts are of decided importance for quantitative electrolysis, since by them is given the minimum tension re- quired for the precipitation of a metal, and also the conditions under which several metals may be quantitatively precipitated from the same solution by simply altering the tension of the current. For example, zinc is not precipitated from a IS" r ZnSO 4 solution by a current having a tension of less than N 2.35 volts, while a silver nitrate solution is decomposed at 0.70 volts. The silver may therefore be separated at a ten- sion of less than 2.35 volts, the zinc remaining meanwhile in solution. After the separation of the silver, the electromotive- SIGNIFICANCE OF CURRENT STRENGTH. 17 force may be increased to over 2.35 volts and the zinc precipitated as metal. Kiliani, whose early death is to be greatly lamented, was the first to point out the importance of the tension for elec- trolytic separations. Somewhat later, Freudeiiberg, basing his work on Le Blanc's studies, carried out, in Ostwald's laboratory, a careful investigation of the exact relations. The results which he obtained will be given in the Special Part, in connection with the discussion of the determination and separation of the respective metals. SIGNIFICANCE OF CURRENT STRENGTH. Although the choice of tension makes the electrolytic separation of a metal possible, the condition of the resulting precipitate is first of all dependent upon the strength of the current which flows through the cell. This follows from Faraday's law, since the number of ions w T hich, by discharging on the electrodes, separate in the atomic condition, in the unit time, depends solely on the cur- rent strength. Irrespective of the tension employed, a current of double strength will precipitate twice the quantity of metal in the same time. The current strength therefore determines the number of ions which will discharge within a given time, and correspondingly the rate of deposit on the electrode. With relation to the latter, however, a second factor, namely, the size and si i ape of the electrodes, is of decided importance, since the manner in which the metal is deposited by a certain current strength depends entirely on this. If the area of the electrode surface is small and the current density great, the individual atoms of metal are deposited one upon the other in such rapid succession that the precipitate does not adhere firmly to the electrode, but scales off. In quantitative elec- IS QUANTITATIVE ANALYSIS BY ELECTROLYSIS. trolysis, the firm adherence of the precipitate to the electrode is most essential for the determination of the weight of the separated material. When, on the other hand, a very large surface is offered for the deposition, and a current of low current strength is employed, it is impossible for a compact layer to form and the metal will coat the surface in isolated patches. These conditions of precipitation are also useless for quantitative determinations. The real significance of the current strength lies in its ratio to the area of the electrode surfaces. This is known as the current density, and as unit, a current strength of one ampere for 100 square centimeters electrode surface, has been chosen. In this book, therefore, the current density will always be given with reference to 100 square centimeters of electrode surface, and will be expressed by the symbol ND ]00 . If, for example, a current of 3 amperes flows through a cell, the electrode surface of which is equal to 250 sq.cm., an equal distribution of the lines of the electric current being as- sumed, the current of 3 amperes will be distributed over the 250 sq.cm., and therefore every 100 sq.cm. receives a current o of ampere. The current density, therefore, ND 100 = 1.2 2 . 5 ampere. Although the current strength is the same at every point in the circuit, the current density of the cathode and anode have the same value only when the two electrodes have exactly equal dimensions. In the determination of metals it is usually sufficient to know the current density at the cathode alone. On the other hand, to determine the halogens, for example, a knowledge of the current density at the anode is required. SIGNIFICANCE OF THE KESISTANCE. 19 The current strength, in the form of the current density, accordingly occupies an important place in quantitative elec- trolysis. SIGNIFICANCE OF THE RESISTANCE. The third factor in Ohm's Law, the resistance, is to be considered chiefly in the selection of the solvent which will he most suitable for the experiment, and the proper substances to be added to the electrolyte. It is evident that with a certain given tension, which in very electrolysis may vary within certain limits, the speed of the operation depends upon the resistance of the solution, since by this the current strength is determined according to the equation E It is therefore necessary to have the conductivity of the solution as high as possible. Since aqueous solutions are ex- clusively employed in quantitative electrolysis, this is accom- plished by the addition of certain substances, the natures of which are dependent upon the chemical properties of the metals in the solution. In some cases acids are used ; in others, bases or salts. The proper substances can only be determined by experience. A fundamental requirement of the substance added and one which is independent of the chemical properties of the metal to be precipitated may, however, be stated. It must be a good conductor of the current and must form no decom- position products which are insoluble or are detrimental to the analysis. Alkalies and acids, which after their decomposition are again regenerated at the electrodes, are therefore suitable, 20 QUANTITATIVE ANALYSIS BY ELECTROLYSIS. as are also organic acids, the decomposition products of which are given off in a gaseous form. This last condition is fulfilled by oxalic acid, which, on account of its great solubility, is of special importance in the electrolysis of metals, particularly in the form of double salts. [A few words on the theory of the conductivity of solu- tions will perhaps be in place here. Since the passage of electricity through an electrolyte is always accompanied by the transference of material, the power which a solution has for conducting the electric current must depend directly upon the nature of the substances which are held in solution. As has already been stated, the ions alone are bearers of electric charges, the undissociated molecules taking no part in the transportation of electricity. The conductivity of a solution therefore depends upon the number of ions which it con- tains, and upon the nature of the ions themselves. If, in two equal volumes of a solution of the same substance, one contains twice as many ions as the other, the other physical conditions of both being the same, the conductivity of the former will be twice that of the latter. The ratio of the dissociated part to the total amount of substance present is called the degree of dissociation. The degree of dissociation varies for different substances, but in all cases for fairly concentrated solutions increases with dilution. For very concentrated solutions the degree of dis- sociation is very low. Concentrated sulphuric acid, for example, is practically a non-conductor, although a dilute solution of the same possesses a relatively high conductivity. The ease with which a current can pass through a cell containing two electrodes immersed in an electrolyte depends then upon the distance by which the electrodes are separated, and upon the number and nature of the ions which are be- tween them. THEORY OF ELECTROLYTIC PRECIPITATION. 21 The resistance of the cell may be decreased in a variety of ways. For example, a substance possessing a high degree of dissociation may be added to the solution, and, provided that it does not materially influence the degree of dissociation of the substance already present, the number of ions between the electrodes will be accordingly increased, and thereby the conductivity of the solution. The solution may also be warmed. The effect of this is generally to slightly increase the degree of dissociation, but more especially to decrease the viscosity of the solvent, as a result of which the ions experi- ence less resistance to their movements through the solution, and the passage of the current is thus expedited. Another means at hand is to diminish the distance between, or to in- crease the size of, the electrodes. The effect of the former is readily understood, and by the latter a greater number of ions are brought within the sphere of action. A word more as to the influence of ions themselves. Since the electricity is transported by the ions, the fate of migration of the same must be ar^ important factor of the conductivity. Hydrogen of all ions has tke highest velocity of migration. Accordingly all highly dissociated acids are good conductors. The hydroxyl ion comes next, which explains the relatively high conductivity of the bases. The conduc- tivity of a solution is indeed nothing more than a function of the rates of migration of the cathion and anion. Trans.~\ THEORY OF ELECTROLYTIC PRECIPITATION. When viewed from the standpoint of the theory of elec- trolytic dissociation the processes of quantitative electrolysis may be generalized as follows. Quantitative determinations may be divided into two classes according to whether the de- termination of a cathion (metal) or an anion (halogen or 22 QUANTITATIVE ANALYSIS BY ELECTROLYSIS. metal peroxide) is concerned. What takes place in the former case is evident without further explanation. The metal ions. migrate in the direction of the positive current, to the cathode ; discharge, and separate on the electrode in the form of a smooth metallic coating. The halogens may be separated in a similar manner ; but since some of them are gaseous or liquid, it is not practical to weigh them directly. Instead, therefore, of using inert platinum electrodes as are used with metals, silver electrodes are employed. With these the halogen atoms combine, at the moment of discharge, to form halogen silver compounds which adhere firmly to the elec- trode. The increase in weight of the electrode gives directly the quantity of the halogen which has separated. The process in the separation of metal peroxides (only lead and manganese peroxides will be here considered) is. somewhat more complicated. Formerly the formation of PbO a and MnO 2 was attributed to an oxidation brought about by the electrolytically generated oxygen. The investigations, of Liebenow* and Lobf have made it appear that lead peroxide ions and manganese peroxide ions are already present in the solutions. Since the peroxides separate from strong nitric acid solutions, it must be assumed that through the oxidizing power of this acid oxygen ions are formed in the solutions, and that these combine with the lead or manganese ions to- form peroxide ions. Since in the peroxides of the bivalent metals the two positive charges of the metal are combined with the four negative charges of the two oxygen atoms, the resulting peroxide ion therefore possesses two negative charges and consequently behaves like a bivalent anion. It is precipi- tated on the positive electrode as a smooth, adherent coating * Zeitschr. f. Electrochemie, 1895-96, pp. 420, 653. fZeitschr. f. Eleclrocbemie, 1896-97, p. 100. DETERMINATION OF THE CURRENT MAGNITUDES. 23 in a form similar to that of a metal. The details of the reac- tions will be given in the Special Part. From what has been said, the necessity of accurate data in the performance of electro-analyses is obvious, for unless all the important conditions are determined and recorded the ex- periment cannot be accurately repeated. Since the determination of the resistance of the liquid in the cell is beyond the scope of analytical work, therefore, in- stead of this, the exact volume and composition of the solu- tion, as well as the size and shape of the electrodes, must be stated. In addition to this the tension at the electrodes, the current strength as read directly on the amperemeter, and the calculation of the current density from the current strength, for the electrode on which the quantitative precipitation has taken place, must be given. All electrical relations are influ- enced by the temperature, so that an exact knowledge of this is most essential. The length of time required for the elec- trolysis and the nature of the source of current having been specified, all adequate and necessary data are at hand to enable every one to repeat the analysis under exactly similar condi- tions. DETERMINATION OF THE CURRENT MAGNITUDES. 1. MEASUREMENT OF THE CURRENT STRENGTH. The current strength is measured either by means of the chemical or the electromagnetic action of the current. The chemical instruments are the oxyhydrogen gas voltameter and the weight voltameter ; the first of which depends upon the volume of gas produced, the second upon the weight of metal precipitated. QUANTITATIVE ANALYSIS BY ELECTROLYSIS. THE OXYHYDROGEN GAS VOLTAMETER. The construction of the apparatus is shown in Fig. 1. The cylindrical vessel g is partly filled with pure dilute 33 FIG. 1. per cent sulphuric acid. The platinum wires d and d' welded to the platinum strips p and p* are fused into the walls of the vessel g. This latter stands in a large cylinder C of water which serves to cool it. The platinum wires end in the screws *In the apparatus used by the author, the platinum electrodes are 31 X 13 mm., aiid are distant from each other 20 mm. THE OXYHYDBOGEN GAS VOLTAMETER. 25 s and s', wliich are connected with the battery. The oxy- hydrogen gas, as it is formed, passes through the tube /, which contains a little water, and is then collected in the measuring- tube K, which is graduated into -^ cc, and filled with water. To measure an electric current with the voltameter, the water over which the gas is collected is first saturated with oxyhy- drogen gas, and then, by the use of a watch with second-hand, the volume of gas is observed which the current yields in a minute, or, if the current is weak, in a longer time. To compare observations, the volume should be reduced to and 760 rnm. pressure. v = observed volume of oxy hydrogen gas ; v l = normal volume (at and 760 mm.); t observed temperature ; h pressure reckoned in mm. of mercury. 1 + 0.00367* '760* Let I indicate the height of the column of liquid, s the density of the liquid, and I the barometric height ; then h = b - I -x- 13.6* The oxyhydrogen gas voltameter, which unfortunately is still frequently used, is quite unsuited to the purpose for which it is intended, since, among other disadvantages, it possesses a high tension which under some circumstances may be much greater than that of the experiment. In addition to this, the comparison of measurements made with different oxy- hydrogen gas voltameters is only possible when the instru- * 13.6 sp. g. of mercury ""HF^ UNIVERSITY ^CALIFOR^, 26 QUANTITATIVE ANALYSIS BY ELECTROLYSIS. ments correspond absolutely with one another in construction, a condition not possible in practice. THE WEIGHT VOLTAMETER. The weight voltameter, which is used in the form of a copper or silver voltameter, has found quite as little application in electrolytic laboratories. Its con- struction is given in Fig. 2. Both methods have been sup- planted by the electromagnetic measuring instruments, some of which make use of the deflection of the magnetic needle caused by the current, and others of the magnetic properties induced in a soft iron core. To the former class belong the sine and tangent galvanometers ; to the latter, the instrument most used in practice, i.e., the amperemeter. FIG. 2. THE TANGENT GALVANOMETER. In the tangent galvanometer (Fig. 3) there is a small mag- net which has its plane of swing horizontal and at right angles to the plane of a ring shaped circuit. The actual position of the magnet when at rest, as well as that of the plane of the windings of the circuit, is the magnetic meridian. When a current flows through the wire ring, there results a deflection of the magnetic needle, the amount of which depends upon the strength of the current and the number of windings. If H denotes the horizontal component of THE TANGENT GALVANOMETER. 27 the earth's magnetic field, n the number of turns of wire in the ring, r the radius of the ring, and the angle of deflection FIG. 3. caused by the current, then the current strength C = H tan0. n is a constant for each instrument, which may be determined by connecting it witli a source of known current strength, according to the equation r C n H tan 28 QUANTITATIVE ANALYSIS BY ELECTROLYSIS. The current strength to be determined may then be easily found ; it is C = K H tan (p. THE SINE GALVANOMETER. The sine galvanometer (Fig. 4) differs from the tangent galvanometer in that the plane of the windings is not fixed in the meridian, but is turned in the direction of the needle dis- FIG. 4. placed by the current, until the position of the latter again corresponds with the plane of the circuit. The angle through which the current circle has been turned from its original position in the magnetic meridian is then read off. The cur- rent strength is C = K - H - sin 0, where K denotes the constant reduction factor of the instru- ment. OTHER FORMS OF GALVANOMETERS. OTHER FORMS OF GALVANOMETERS. The galvanometers which are used for making the most accurate measurements depend likewise upon the displacement FIG. 5. of the magnetic needle by a circular current. Here, however, the needle is suspended by a fine cocoon fiber, between spools containing a very large number of turns of wire. In order to remove the magnet from the influence of the earth's magnet- ism, a pair of so-called astatic needles are frequently em- 30 QUANTITATIVE ANALYSIS BY ELECTROLYSIS. ployed. Two exactly similar magnets are rigidly connected with one another, so that the north pole of the one is situated exactly over the south pole of the other, and correspondingly the south pole of the first above the north pole of the second. By this arrangement the effect of the earth's magnetism is neutralized by the two needles. FIG. 6. FIG. 7. The angle of displacement is read by means of a telescope and a mirror which swings with the needle. The scale, which is rigidly clamped to the telescope, shows the divisions which correspond to the deflection of the needle. Figures 5 and 6 show two practical galvanometers. The effect of the magnetism induced in a soft-iron core is made use of in the two instruments, most useful for electrol- SPRING GALVANOMETER AMPEREMETER. 31 jsis; the Kolilrauscli spring galvanometer and the ampere- meter (often called ammeter). THE SPRING GALVANOMETER. In this apparatus of Kohlrausch (Fig. 7), a hollow cylin- der of sheet iron is suspended within a vertical solenoid by a spiral spring. When a current is passed through the instrument, the iron cylinder is drawn down into the solenoid until the force of attraction is equalized by the tension of the spring. A small pointer attached to the spring moves over a scale which is empirically graduated and gives the current strength directly in amperes. THE AMPEREMETER. In the amperemeter (Fig. 8) the solenoid is usually placed horizontal and carries eccentrically a bent piece of thin sheet FIG. 8. iron which is provided with a long pointer. This pointer moves over a scale. These instruments under suitable con- ditions are extraordinarily sensitive. 32 QUANTITATIVE ANALYSIS BY ELECTROLYSIS. 2. MEASUREMENT OF THE TENSION. For measuring the tension a great number of instruments are in use, the application of which depends upon the fineness of the measurement which is to be carried out. Two instru- ments are employed in electrolysis, the voltmeter and the torsion galvanometer, while for exact determinations of dif- ferences of potential, especially small ones, the Lippmann capillary electrometer and the quadrant electrometer, lately in the form improved by Nernst, have been generally adopted. THE VOLTMETER. Since, according to Ohm's Law, E = C E, every measure- ment of tension may be referred to a measurement of the current strength provided that the resistance R remains con- stant. This is done in the voltmeter, which has the external form of an amperemeter, by giving the solenoid a very high resistance, 1000 ohms for example. In this way, the resist- ance of the connecting wire may be left out of consideration, and the fall in tension is confined practically to the coils of the solenoid alone. Further, the voltmeter is always connected on a shunt circuit. If the resistance in this shunt is low, then the greater part of the current passes through it, causing a fall to take place in the tension at the cell under measurement. The cor- responding value obtained for the tension will therefore be too low. If, however, the resistance of the voltmeter is very high,, then the current will pass through the cell with almost un- altered tension and only an extremely small fraction will go through the measuring instrument itself. The scale is so THE TORSIOX GALVONOMETEK. 33 constructed with reference to the resistance that the amperes are converted directly into volts, according to the equation, which for the solenoid mentioned would be E = 1000 -C. A most excellent form of apparatus is the voltmeter of Weston (Fig. 9). This gives the value accurately to -^ volt and allows of approximation to T fg- volt. A mirror, placed below the scale over which the pointer moves, prevents parallax in reading. THE TORSION GALVANOMETER. The principle of this instrument is electromagnetic. A light bell magnet swings between two parallel and perpen- dicular coils containing many windings of wire. These two spools are so connected with one another that the current flowing through each of them tends to deflect the magnetic needle in the same direction. The magnet, the swinging of which is usually retarded by copper damping, is suspended from the cover of the case by a spiral spring. To this spring a horizontally moving pointer, which is just beneath the glass cover of the instrument, is attached. A second pointer is fastened directly to the magnet. The instrument is used as follows : By revolving the case the needle is brought into the magnetic meridian, so that the two pointers correspond to the zero point of the scale on the glass cover. Care must be taken that the magnet swings entirely free, which is insured by setting the instrument exactly horizontal by means of the foot-screws. If the 34 QUANTITATIVE ANALYSIS BY ELECTROLYSIS. binding-screws of the galvanometer are now connected with the source of current or with the cell, the tension of which is to be determined, the magnet will be deflected from its position of rest and the pointer attached to the spiral spring must be turned a certain distance in order to bring the pointer rigidly connected to the magnet back to the zero point on FIG. 10. the scale, and thereby to return the magnet itself to its former position. In this operation the pointer fastened to the top of the spiral spring has been moved through a certain number of divisions on the scale, and this scale is empirically calibrated, so that by the position of the pointer the tension is given in _i_ volt. By interposing resistance, the sensitiveness may be THE LIPPMANN CAPILLARY ELECTROMETER. 35 so decreased that the divisions correspond to tenths or to whole volts. Fig. 10 shows a torsion galvanometer of the type manufactured by Siemens & Halske (Berlin). The principle of the construction of the instrument is similar to that of the voltmeter. The deflection of the needle is of course proportional to the current strength, but since the resistance of the spool windings is very high and remains constant, a strict proportion exists between the intensity and the tension, so that the direct reading in volts is made possible by the use of the equation THE LIPPMANN CAPILLARY ELECTROMETER. This instrument is chiefly employed in the measurement of electromotive forces by the Poggendorf compensation method, most practical in the arrangement described by Ostwald. In this a known electromotive force is opposed to the one which is to be measured, and the former is modified through alterations of the resistance by certain known amounts, until the two electromotive forces are equal and compensate one another. In practice, an element of known electromotive force is so connected with a resistance-box, which contains for example 1000 ohms, that the whole fall in potential takes place through the 1000 ohms. Every resistance of 10 ohms is provided with a clamp to which a wire may be connected. If the known electromotive force is for example 1 volt, this will be distributed over the resistance in such a way that the 1000 ohms will represent a fall of 1 volt, every 100 ohms one of 0.1 volt, and every 10 ohms one of 0.01 volt. The electromotive force to be measured is now connected with the 36 QUANTITATIVE ANALYSIS BY ELECTROLYSIS. two clamps of the resistance-box in such a manner that it will be opposed to the known electromotive force and the resist- ance between the two clamps is varied, until the two electro- motive forces are equal and compensate one another. If, for example, the resistance between the clamps is 100 ohms, then the electromotive force to be measured is equal to 0.1 volt, for 110 ohms it would be 0.11 volt, for 120 ohms 0.12 volt, etc. In order to determine when the electromotive force which is being measured is equal to the opposed electromotive force, a Lippmann electrometer of the form given by Ostwald * is. employed. (Fig. 11.) FIG. 11. ' ' A platinum wire, partly encased in a glass capillary, leads from an insulated binding- screw and extends into the mercury at the bottom of the bulb 5, which also contains above the mercury a 10 per cent sulphuric acid solution. The capillary tube c opening into the bulb ~b is filled in its upper part with acid ; its lower part contains mercury, like- wise the tube d, which is in connection with a second binding- screw. The position of the mercury in the capillary tube o may be regulated through altering the inclination of the capil- lary by means of the screw at /. That this apparatus may *Ztschr. f. pbys. Chem., 1890, p. 471. THE QUADRANT ELECTROMETER. 37 give satisfactory results it should be short- circuited just before use, and consequently it was connected with a switch so con- structed that on breaking the current the electrometer was always short-circuited, and on making the current this connection within itself was destroyed . In measuring electromotive forces, so much of the resistance of the box was brought between the movable clamps that the mercury remained at rest on closing the circuit. A millimeter scale placed beneath the capillary, and a lens above it, aided in the reading. It w r as possible to approximately estimate to a thousandth volt. One hun- dredth volt corresponded to 3 divisions on the scale." The Lippmann capillary electrometer, the theory of which cannot be entered upon here, depends upon the fact that the surface tension of mercury alters under varying electrical con- ditions. When the two opposed electromotive forces are equal, then the mercury is electrically neutral and the menis- cus returns to its normal position. THE QUADRANT ELECTROMETER. The quadrant electrometer, which was constructed in the most varied forms by "W. Thomson and 'is very generally used for the measurement of potentials, has the following general construction. Four separated sectors of a flat, cylindrical metal box rest upon four insulating glass supports. Each of these sectors is called a quadrant. Each pair of oppositely located quadrants is in metallic connection. Within the hollow space formed by the four quadrants the so-called needle, a thin, horizontal plate of aluminium, is suspended by a fine wire which also carries a mirror for reading. A wire leads from the needle to a vessel filled with sulphuric acid, situated beneath the quadrants. In using the instrument, the aluminium needle is charged to a comparatively high potential by connecting it with some 38 QUANTITATIVE ANALYSIS BY ELECTROLYSIS. FlG. 12. source of electricity, usually a Leyden jar, through the wire dipping into the sulphuric acid. One pair of quadrants is then grounded, and the other pair is connected to one pole of the electromotive force to be measured, the second pole of the electromotive force being connected with the earth. So long as all the quadrants have the same potential, the aluminium needle remains at rest ; the difference between the potential of the earth and the one under consideration is deter- mined from the deflection of the needle measured by means of the mirror and scale. Nernst and Dolezalek * have made the employment of this instrument more simple and accurate. They avoid the operation of charging the aluminium needle before use, by employing a small perpendicularly hung Zamboni pile having a tension of about 1400 volts. (Figs. 12 and 13.) A small Zamboni pile Z, suspended by the quartz fiber /*, has fastened to its two- poles the electrometer needles N, and N 2 , which swing in the quadrant boxes Q, and Q 3 , placed one above the other. The measurement of a difference of potential is carried out by comparing the instrument with a normal tension, or by the compensation method. SOURCES OF CURRENT. Two classes of current supply are employed in electrolysis,, chemical and physical. The first class is represented by the *Ztschr. f. Electrochemie, 1896-97, p. 1. FIG. 13. LECLANCHE CELL. 39 galvanic elements, which are further divided into primary and secondary elements, according as the difference of poten- tial is directly due to a chemical reaction or to a polarization current (accumulators). To the second class belong the electromagnetic machines and thermopiles. The most important apparatus will be briefly described in the following section. 1. PRIMARY GALVANIC ELEMENTS. LECLANCHE CELL. This is a one-fluid cell using a solution of ammonium chloride, which surrounds the negative pole, the zinc. The FIG. 14. cell is much used in the form shown in Fig. 14:. In the jar, which is square in section, with a rounded projection at 40 QUANTITATIVE ANALYSIS BY ELECTROLYSIS. one corner, stands a porous clay cup, from which projects a block of carbon K surrounded by coarsely pulverized man- ganese dioxide, or a mixture of manganese dioxide and retort carbon. In the projecting rounded corner is a stout rod Z of amalgamated zinc. The carbon and zinc are both provided with binding screws, and are immersed in a concentrated solution of ammonium chloride. Leclanche* also uses, in place of the powdered man- ganese dioxide, compressed prisms (shown in Fig. 15) con- FIG. 15. sisting of 40 parts manganese dioxide, 55 parts gas carbon, arid 5 parts shellac ; a little potassium sulphate is also added to increase the conductivity. The porous cup is thus dispensed with. This cell has an electromotive force of 1.48 volts. MEIDINGER CELL. 41 MEIDINGER CELL. In contrast to the Leclanche* cell, that of Meidinger con- tains two liquids, solutions of magnesium and copper sul- phates. The element is constructed as follows : In the glass vessel G (Fig. 16) stands a smaller glass #, and in this a copper cylinder K to which an insulated copper wire D is fastened. A second cylinder Z of zinc, to which the projecting wire D 1 is fastened, is placed in the upper part of the vessel G. The balloon-shaped glass B, filled with crystals of copper sulphate, closes the cell. The cell is filled to about three-fourths of its capacity w r ith a solution of 1 part crystallized mag- nesium sulphate in 7 parts of water ; and the balloon- shaped flask containing copper sulphate is filled up with water, closed with a stopper fitted with the glass tube r, and, as the FIG. 16. cut shows, inverted in the cell. Electromotive force about 1 volt. 42 QUANTITATIVE ANALYSIS BY ELECTROLYSIS. DANIELL CELL. In a jar of glass (Fig. IT) is a porous clay cup T, and in this a cylinder of cast zinc, the negative pole (Fig. 18). The FIG. 17. FIG. 18. porous cup is surrounded by a cylinder of sheet-copper K> the positive pole. The cylinder of amalgamated zinc * stands in dilute sul- * The zinc is easily amalgamated by plunging it into mercury, on the surface of which a little hydrochloric acid has been poured. The amalga- mated cylinder is then placed in a vessel of water to remove the hydrochloric acid, and allow the excess of mercury to drop off. DANIELL CELL. 43 plmric acid (1 : 20), and the copper cylinder in a solution of copper sulphate ; the sulphuric acid may be replaced by a solution of zinc sulphate. The element has an electromotive force of 1.079 volts. [The modification of the Daniell cell known as the gravity cell is the form commonly in use for telegraph batteries in this country, and is the cheapest and most convenient cell for constant batteries to yield currents of moderate strength in scientific laboratories. It is very generally thus used. The copper is placed at the bottom of the jar ; an insulated copper wire is riveted to it, long enough to pass up through the solutions and connect with a binding screw on the zinc of an adjacent cell, or with the wire which serves to conduct the current to the solution for electrolysis. The bottom of the jar, about the copper, is filled with copper sulphate ; the zinc, a heavy casting with large surface, is suspended a few inches below the top ; and the jar is filled with water some- times acidulated with sulphuric acid. After standing a few hours, the copper 'sulphate dissolves ; copper is precipitated, and zinc dissolved ; and the jar, in its normal working state, thus contains two solutions ; the heavier, of copper sulphate, below, and the lighter, of zinc sulphate, above. The porous cup of the Daniell cell is thus dispensed with, and the zinc does not require amalgamation. The cut (Fig. 19) shows one of the simplest gravity cells, having the zinc in the so-called " crow-foot " shape, hanging directly on the edge of the jar, and furnished with a binding- screw. The outfit of the chemical laboratory of the Pennsylvania State College, while under the translator's charge, was found 44 QUANTITATIVE ANALYSIS BY ELECTROLYSIS. convenient, and sufficient for the needs of an ordinary labora- tory for instruction. Some twenty "crow-foot" gravity cells were kept in working condition, and eight Grove cells could be set up if needed for a strong current. Four sets of con- necting-wires were run from the battery-room to the labora- tory desk set apart for electrolytic work, each set being so arranged with binding-screws as to be quickly connected with any desired number of cells. (See under " Secondary Bat- teries," p. 47.) Trans.'] FIG. 19. GROVE CELL. The positive pole is a sheet of platinum foil of the form shown in Fig. 20 ; this is placed in a porous cup filled with nitric acid. The negative pole is a cylinder of amalgamated zinc placed in a glass jar containing dilute sulphuric acid (1 : 20). Fig. 21 shows the arrangement of the cell. Electromotive force 1.81 volts. BUNSEN CELL. 45 FIG. 20. FIG. 21. BUNSEN CELL. In the Bunsen cell, the platinum is replaced by a prism of retort carbon (Fig. 22) standing in a porous cup filled with nitric acid. The negative electrode, as in the Grove cell, is a cylinder of amalgamated zinc placed in a glass jar filled with dilute sulphuric acid (1 : 20). The screw-clamp shown in Fig. 23 is often used to fasten a metallic connection to the carbon prism. It has, however, the disadvantage that the clamp is quickly oxidized by the decomposition products of the nitric acid, and the contact thus broken. It is better, there- fore, to insert in the carbon a metallic socket (Fig. 24), the stem of which is closely covered with platinum foil. 46 QUANTITATIVE ANALYSIS BY ELECTROLYSIS. Fig. 25 shows the Bimsen cell in its most common form. Electromotive force 1.80 volts. FIG. 22. FIG. 23. FIG. 24. FIG. 25. CUPRON ELEMENT. A copper oxide plate and a zinc plate dip into an aqueous sodium hydroxide solution, as shown in Fig. 26. When a current is produced the following re - actions take place : II. CuO + 2H = Cu + H 2 O. The current ceases as soon as all 5 the copper oxide is reduced or all the zinc is dissolved. In the type intro- FIG. 26. duced on the market by TJmbreit & Matthews (Leipzig), when the copper oxide plate has been FIG. 27. GALVANIC SECONDARY ELEMENTS. 47 reduced it may be reconverted into copper oxide by allowing the plate to stand for 15 hours in a warm place. The element has an electromotive force of 0.8 volt, with an internal resistance of 0.05 ohm. [EDISON-LALANDE ELEMENT. This element, having a form differing somewhat from the above cuprori element, but depending for the production of the current upon a similar chemical reaction, has come largely into use in the United States. Elements of this type, with capacities of from 50 to 600 ampere-hours, are manufactured, and furnish a very convenient primary source of electricity. (Fig. 27.)] GALVANIC SECONDARY ELEMENTS. (ACCUMULATORS, OR STORAGE BATTERIES.) While the primary elements previously described furnish electrical energy through chemical reactions which involve a gradual using up of their component parts, the characteristic of accumulators lies in the fact that by passing an electric current through them they are brought into a condition which makes it possible for them to furnish a polarization current, and thereby to return to their original condition. Elements of this class are called reversible elements. Accumulators are therefore instruments which alternately convert chemical energy into electrical energy, and electrical energy into chemical energy.* The principle of their construction depends upon the behavior of lead plates in dilute sulphuric acid on the passage of a current. If we have two such plates dipped in sul- * Elbs, Die Akkumulatoren, 1896. 48 QUANTITATIVE ANALYSIS BY ELECTROLYSIS. phuric acid, one serving as cathode, the other as anode, it will be observed, upon closing the current, that a brown coat- ing forms upon the positive electrode, spongy lead separating out at the same time on the negative plate. This phenome- non is due to the formation, at both poles, of a saturated solu- tion of lead sulphate, the lead ions of which discharge upon the cathode as spongy lead, while at the anode the lead and oxygen ions separate in the form of lead peroxide. If the primary current be broken and the polarization current allowed to discharge by establishing metallic connection between the two plates, the following reaction takes place : The sulphuric acid dissolves the spongy lead at the negative pole ; the hydrogen ions thus made available migrate to the anode with their posi- tive charges and reduce the lead peroxide there present to lead oxide, which again forms lead sulphate with the sulphuric acid. As soon as all the spongy lead is dissolved and all the lead peroxide is reduced, the polarization current ceases, the accumulator is discharged, and the original condition is again reproduced. By means of a new primary current (charging current) the accumulator may be again brought into an avail- able condition. Jar. Anode. Anode Plates. Cathode Plates. Cathode. FIG. 28. In the construction of accumulators, the longest possible continuation of the polarization current is aimed at, together with the lowest possible internal resistance of the element. The electromotive force between lead and lead peroxide in dilute sulphuric acid is approximately 2 volts. In order to GALVANIC SECONDARY ELEMENTS. 49 arrive at the most practical construction, a number of parallel plates are metallically connected together and hung as cathodes in a trough containing sulphuric acid. Similarly, an equal number of anode plates are hung in and so arranged that each cathode plate is between two anode plates and vice versa, each anode between two cathode plates. (Fig. 28. View from above.) The time required for charging depends upon the area of the plates and the condition of the spongy lead and lead per- oxide. In order to satisfy all requirements the so-called PIG. 29. FIG. 30. " active material " (i.e., the spongy lead and lead peroxide) is attached to a solid lead frame by the employment of a series of processes (''forming," etc.), which cannot be discussed here. Fig. 29 shows a negative, Fig. 30 a positive plate, while Fig. 31 gives an accumu- lator of the form commonly used. The first experiments with accumu- lators for the purposes of quantitative analysis were conducted in the Aachen laboratory, with apparatus especially constructed by Professors Farbaky and Scheneck * in Schemnitz (Hungary). Fio. 81. * Compare Ueber die elektrischeu Akkumulatoren von Farbaky und 50 QUANTITATIVE ANALYSIS BY ELECTROLYSIS. These gentlemen had the kindness to place two accumu- lators at the author's service for testing, and he made his first experiments in 1888, working with R. Schelle, at that time professor in the Schemnitz Royal School of Mines. These accumulators have 6 negative and 5 positive lead-plate electrodes, each 6 mm. thick. The weight of the electrodes is 15.5 kg., the volume of the 33$ sulphuric acid 3.5 1., the total weight of each cell 35 kg. The active surface of the electrodes is 3133 sq. cm., so that the internal resistance is very low, measuring between 0.0166 and 0.017 ohm. The accumulators can be charged at a 20 to 25 ampere rate, and yield in discharge at 23, 30, 40, and 60 ampere rates respec- tively 150, 148, 140, and 125 ampere-hours, with a fall of not over 10$ in the voltage. If the discharge is lighter, and the fall in the electromotive force less than for lighting pur- poses, as in electrolytic analyses, an accumulator may yield over 250 ampere-hours. Two such accumulators were fully charged, until OH gas was obviously disengaged, by a current of 20 to 25 amperes from the dynamo. The current was measured by a Kohlrausch galvanometer, made by Hartmann & Braun, Bockenheim, Frankfurt a. M. , the scale of which read from to 60 amperes. A second Kohlrausch amperemeter, divided from to 15 am- peres, was used to measure the current taken from the accu- mulators for the analyses. A Siemens torsion galvanometer showed a tension, for each charged accumulator, of 2.05 volts. By the use of these two accumulators, four to eight analyses were carried on simultaneously, and the accumulators kept in Scheneck (Dingier polyt. Journ., 257, 357); further, Bericht tiber die Ak- kumulatoren von Farbaky und Scheneck von A. v. Waltenhofen. Zeitschr. f. Elektrotechnik, 1886. GALVANIC SECONDARY ELEMENTS. 51 constant use day and night, except for the short intervals needed to change the solutions for analysis. The results of analyses extending over a period of six days are subjoined. FIRST DAY. Tension 2.55 volts. Determination of Copper from Nitric-acid Solution. Taken CuSO 4) 5H 2 O. Found Cu. 4.0140 g 1.0170 g = 25.33^ 4.1376 " 1.0480 " = 25.33 2.2340 " 0.5661 " = 25.34 2.3575 " 0.5978 " = 25.35 Tin from the Acid Ammonium Double Oxalate.* Taken 8nCl 4 2NH 4 Cl. Found Sn. 1.8450 g. 0.5964 g. =32.33 2.0210 0.6548 " =32.39 Antimony from Solution in Sodium Sulphide. f Taken Sb 2 S 3 . Found Sb. 0.2404 g. 0.1720 g. = 71.50 0.2551 " 0.1827 = *1.60 * Classen's method : see Tin. t " ""... " Antimony. 52 QUANTITATIVE ANALYSIS BY ELECTROLYSIS. SECOND DAY. Tension 1.95 volts. 2.0490 g. NiS0 4 +(NH 4 ) 2 S0 4 ,6H 2 gave 0.3053 g. Ni = 14.90^* 2.0180 " " 0.3000 " " =14.91 2.3400 CoSO 4 + K 2 SO 4 ,6H 2 O " 0.3440 " Co = 14.70f 2.1200 " " 0.3120 " " =14.71 1.8920 " FeSO 4 +(NH 4 ) 2 SO 4 ,6H 2 O " 0.2697 " Fe = 14.25^ 2.1240 " " 0.3027 " " =14.25 1.0 CuSO 4 ,5H 2 O " 0.2533 " Cu = 25.33^ 1.0 " 0.2533 " " =25.33 1.0 " 02534 " " =25.34 1.0 " 0.2537 " " =25.37 1.9210 " SnCl 4 + 2NH 4 Cl " 0.6219 " Sn = 32.371 2.1320 " " 0.6900 " " =32.36 THIRD DAY. Tension 1.95 volts. (Six simultaneous analyses.) 1.0050 g. CuS0 4 ,5H 2 O gave 0.2550 g. Cu = 25.37 1.0170 " " 0.2580 " " =25.36 1.0006 " " 0.2539 " " =25.37 1.0013 " " 0.2540 " " =25.37 1.5680 " SnCl 4 +2NH 4 Cl " 0.5070 " Sn = 32.34 2.4520 " " 0.7946 " " = 32.40 * Classen's method : see Nickel. f " "Cobalt. \ " " " Iron. From the acid double oxalate, Classen's method. | From the acid ammonium double oxalate. GALVANIC SECONDARY ELEMENTS. FOURTH DAY. 53 1.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 2.20 2.45 2.1340 2.4350 g. CuSO 4 ,5H a O Tension 1.95 volts. gave NiS0 4 -KNH 4 ) 2 S0 4 ,6H 2 < < 1 1 CoSO 4 +K 2 SO 4 ,6H 2 O 0.2532 0.2535 0.2532 0.2536 0.2535 0.2538 0.2539 0.2537 0.3277 0.3650 0.3148 0.3587 g. Cu = Ni Co 25.32? 25.35 25.32 25.36 25.35 25.38 25.39 25.37 1489 14.89 14.75 14.73 FIFTH DAY. Tension 1.95 volts. 1.0 g. CuS0 4 ,5H 2 1.0 2.4120 2.2130 FeSO 4 -f(NH 4 ) 2 S0 4 ,6H 2 gave 0.2537 g. Cu = 25.37^ " 0.2537 " " =25.37 " 0.3438 " Fe = 14.25 " 0.3156 " " =14.26 SIXTH DAY. Tension 1.92 volts. (Eight simultaneous copper determinations.) 1.0 g. CuSO 4 ,5H 2 O gave 0.2533 g. Cu = 25.33$ 1.0 " 1.0 " 1.0 " " 1.0 " 1.0 " 1.0 1.0 " (( 0.2534 " " = 25.34 ( 0.2536 tt <=> <=> A resistance-box for the regulation of the current is required for each separate experi- ment. The description will be limited to four simultaneous electrolyses. The rheostats w^ w^ w t , w 41 of the form designated in the sketch, are then placed in front of the block 7, and a second block, 77, having four mercury-cups on one side and one on the other, is added. An amperemeter and a voltmeter com- plete the outfit. The connections are made as follows (Fig. 79) : Four short wires extend from the negative pole into the mercury-cups 1, 2, 3, 4, of board /, the fifth cup of which, #, is connected directly with the amperemeter. The second binding-screw of the amperemeter is connected by a wire to the negative pole. Four short wires lead from the corresponding mercury- cups of board 7, 1', 2', 3', 4', to the resistance- boxes w^ w w w 4 , the other binding-posts of which are connected both with the electrolytic cells and also with the mercury-cups s l9 *> * 8 ? *45 of board 77, in the corresponding manner. The mer- cury cup s, situated by itself on board 77, opposite the four mercury-cups just mentioned, is connected with the instru- ment for measuring the tension ; and the second binding-post of the latter is connected by a wire to the positive pole. In order to complete the circuit it is only necessary to connect the cups 1-1', 2-2', 3-3', 4-4' by short bent wires. For the cell 1, for example, beginning at the negative pole, the cur- rent pursues the following path: Negative pole, 11', 0,, cell 1, positive pole. The current travels likewise in the other experiments. In order at the same time to measure the current strength, the wire connection is laid from a to 1' (2', 110 QUANTITATIVE ANALYSIS BY ELECTROLYSIS. 3', 4/) and the connection 1-1 ' is broken (correspondingly 2-2', 3-3', 4-4/). By this arrangement the current, in passing from the negative pole, is forced to flow through the ampere- meter, thence over a-V to the rheostat, etc. In order that the correct value for the current strength may be obtained by this operation, the wires which make the connections 1-1', 2-2', etc., must have a resistance equal to that of the am. peremeter, while the connections a-V ', #-2', etc., must be made with wires having practically no resistance. (For fur- ther details see p. 122.) ' From the electrolytic cells the circuit is completed, on the one hand by wires having extremely low resistances run- ning to the positive pole, and on the other hand by similar wires connected to the binding-posts of the resistance -boxes, or what amounts to the same thing, to the cups $ 1? s 2 , s 3 , s 4 . Between one of the latter and the positive pole the tension must be measured, since here the fall in tension is due to the resistance of the cell only. It is accordingly sufficient to make the connection s 1 s, s 2 s, etc., in order to imme- diately obtain the difference of potential in the corresponding cell. The 'simultaneous measurement of several cells is of course out of the question. This simple appliance, the principles of which recur in the following descriptions, can be prepared by anyone from the simplest materials, so that, as already stated, it is very suit- able for students, since by working with it they become ac- quainted with the methods of making connections and the manipulation of more elaborate apparatus. The resistance- boxes permit of a variation of the tension and current strength sufficient for most purposes. THE ELECTRO-CHEMICAL INSTITUTE AT AACHEN. Ill FORMER EQUIPMENT OF THE ELECTRO-CHEMICAL INSTITUTE AT AACHEN. This system is based upon the employment of a dynamo of the type described on page 62, the current from which can be employed both directly and for charging accumulators. As already stated, the machine described has a tension of 10 volts with 1,000 revolutions. The tension, while the machine is in use, is measured by a galvanometer or other instrument which shows the tension directly. In Fig. 81, the tension indicator marked G is connected with both ends of the brass resistance MM r Siemens & Halske describe the tension indicator as follows : It consists of an electro-magnet, beside one pole of which stands on edge a piece of iron which has the same polarity, and is therefore repelled by it in proportion to the strength of the magnetism, and so of the electric current which passes around the instrument. The extent of the repulsion is measured on a scale on which plays an index attached to the piece of iron which is repelled. The indica- tions of the instrument are not entirely independent of the residual magnetism ; the direction of the current in the instrument must therefore be alwaj^s the same. This result- is accomplished by a small adjustable permanent magnet in front of the lower pole of the electro-magnet ; this shows the direction of the current in the instrument, and stops the index if the current is in the wrong direction. (See Z, Fig. 81.) If this occurs, the wires leading the current to the instrument must be interchanged. The instrument is supplied with a brass ring, which, before the current passes, is placed on the round weight of the index, and must then turn the index to the zero point. If this is not the case, the instrument is not plumb. When the instrument is in use, the ring is removed. 112 QUANTITATIVE ANALYSIS BY ELECTROLYSIS. As already stated, the laboratory apparatus constructed by Siemens & Halske is capable of carrying on, at the same time, a large number of electrolytic determinations on the small scale, requiring currents, differing in strength and ten- sion, so that each determination is independent of the rest. According to the description of Siemens & Halske, this THE ELECTROCHEMICAL INSTITUTE AT AACHEN. 113 result is obtained essentially by passing far the greater part of the current through a brass wire-gauze resistance,* the individual determinations being made by small branch cur- rents which may be independently varied in intensity by attaching their conductors to different portions of the wire- gauze resistance. The dynamo machine is connected by short heavy con- ductors to the ends M Mj of the zigzag brass wire-gauze resistance. These ends of the resistance are also connected, by smaller wires, with the instrument which shows directly the tension at the resistance. Care must be taken that this instrument always shows the same tension, i.e., that the velocity of the machine is uniform. If the tension at the ends of the resistance is 6 volts, and the resistance is made up of 24 equal parts, the ends of which are connected with binding-screws, the difference in tension between any two adjacent binding-screws is ^ = ^ volt. If the tension at the first screw is 0, the tensions at the following screws are i> f i f > 1? f > etc., volts ; that is, the whole interval of 6 volts is divided into portions of J volt each. If, now, a current, small in proportion to the current passing through the resistance, is taken out between any two binding-screws for an electrolytic determination, the tension between the screws is not materially changed ; the wires carrying this current can be connected with any binding screws without any change in the main current ; moreover, * When the same source of current is used for carrying on a number of dissimilar experiments simultaneously, the employment of resistances and the loss of a part of the energy is unavoidable. The apparatus con- structed by Siemens and Halske has the advantage that it makes use of only a single resistance, while with all other arrangements as many sepa- rate resistances are required as experiments are conducted. 114 QUANTITATIVE ANALYSIS BY ELECTROLYSIS. the introduction of a number of such currents does not materially change the tension, and the tension for any given determination can be varied at will without affecting the others, In the apparatus used by the author, Fig. 81 (one- twentieth natural size), the brass wire gauze resistance is divided into 20 equal parts marked 1, 2, 3, etc. As already stated, the machine, at 1 5 000 revolutions, has a current- strength of 60 amperes and a tension of 10 volts. Of the 60 amperes, 40 are conducted through the resistance, so that 20 remain for electrolytic determinations. The difference of tension between two adjacent binding- screws is J = volt. The tension, that is, at the screw marked 19, is volt, at 18 = 1, at 17 = 1J, at 16 = 2, at = 10 volts. The current from the machine enters by a heavy copper conductor at the screw marked 0, and passes out at that marked 20. On the board BBBB are fastened 6 T-shaped galvanized- iron strips, S x , S 2 , S 3 , S 4 , S 5 , S 6 , six resistances of 0.1 ohm each, W x , W 2 , W 3 , W 4 , W 5 , W 6 (to allow the strength of current in single experiments to be measured), and the brass strip M 2 . S 1 is connected by a wire with W x , S 2 with W 2 , S 3 with W 3 ,' S 4 with W 4 , S 5 with W 5 , and S 6 with W 6 . The iron strips may be connected with the binding-screws 1, 2, 3, etc., by means of wires and the brass screws K 1? K 2 , etc. If the apparatus is used as shown in the cut, and 1, 2, or 3 is connected with S 1? 4, 5, or 6 with S 2 , 7, 8, or 9 with S 3 , 10, 11, or 12 with S 4 , 13, 14, 15, or 16 with S 5 , and one of the others with S 6 , the strongest current is at W v and the weakest at W 6 . Any strip may, of course, be connected with any binding-screw. In performing electrolysis, the solutions to be acted on THE ELECTRO-CHEMICAL INSTITUTE AT AACHEN. 115 are placed in connection with a negative pole n v n v or w 3 , etc. (on the resistances W v W 2 , or W 3 , etc.), and a positive pole p v p^ or p%, etc., on the brass strip M 2 , the connections being made according to the strength of current desired. Moreover, as shown by the examples given later, several determinations requiring the same strength of current may be connected with any pair of poles, n^ and p v n 2 and p^, etc. In order to connect more conveniently with the platinum dishes containing the solutions for electrolysis, n^ and p v for instance, may be connected with a brass strip Z (the con- nection with n^ only is shown in the cut), to which are attached a number of binding-screws, z v 2 2 , etc. The tension and the strength of the current may be measured at each dish. For example, if the tension at the dish connected with W 2 is to be measured, the plugs from the galvanometer are inserted at 5 2 and c 2 ; if they are inserted at 2 and 5 2 , the tension in the resistance is meas^ ured, which, multiplied by 10, gives, in amperes, the strength of the current acting on the solution connected with W 2 . In order to test the working of the apparatus, the tension at the divisions of the wire-gauze resistance was directly measured by a torsion galvanometer, with the following results : 116 QUANTITATIVE ANALYSIS BY ELECTROLYSIS. Resistance marked Connected with Binding Screw, marked Tension in Volts. w, 1 10.300 w, 2 9.900 w, 3 9.400 W 2 4 8.950 W 2 5 8.300 w a 6 7.750 W 8 7 7.200 W 8 8 6.650 w $ 9 5.950 W 4 10 5.500 W 4 11 5.050 w. 12 4.500 W 6 13 4.000 W 6 14 3.450 W 6 15 2.850 W 6 16 2.300 W 6 17 1.700 W 6 18 1.100 w. 19 0.560 W 6 20 0.007 For the measurement of the strength of the current at the screws 1 to 20, a cell was used which had a copper elec- THE ELECTRO-CHEMICAL INSTITUTE AT AACHEN. 117 trode,* and contained 150 cc. of a 15 per cent solution of copper sulphate ; this cell was connected to the resistance W 6 (binding-screws, n e and p Q ). The screws 1 to 20 were then successively connected with the bar S 6 , and the deflection of the galvanometer read, the plugs connecting it being placed in # 6 and b 6 . After this reading, the tension in the cell was read, for each screw connection, by placing the plugs in > 6 and c Q . In order to control the rate of the machine during the -experiment, p l and n l on the resistance W 1 were connected through a rheostat ; and the tension at the binding-screw 1 (connected with Sj) was determined by a second torsion galvanometer, the plugs from which were inserted at b^ and c r The results of these experiments are given in the following table in the columns included under I. A second series of experiments was conducted to deter- mine the strength of the current by the quantity of copper precipitated. Six platinum dishes, as nearly alike as possible, were filled with 150 cc each of a 15 per cent solution of copper sulphate, supplied with copper eiectrodes (see note below), and different quantities of copper precipitated in the same time. These experiments were conducted in three series, as follows : Series 1. I., IV., VIII., XII., XVI., XIX. Series 2. II., V., IX., XIII., XVII., XX. Series 3. III., VI., X., XL, XIV., XVIII. * The cell consisted of a platinum dish, and the positive electrode was a round piece of sheet-copper (of the form of the platinum electrode shown in Fig. 55), 6 cm. in diameter and 2 mm. thick. The electrodes were 2.5 cm. apart. 118 QUANTITATIVE ANALYSIS BY ELECTROLYSIS. Of the columns included under II., A gives the strength of the current as determined from the precipitated copper ; B, the results, in a few cases, of the measurement of the strength of current by a torsion galvanometer ; and C, the tension measured at the same time with the torsion galvanometer. Binding Screw. I. II. Amperes. Volts, Experi- ment. Volts, Machine. A, Amperes. B, Am- peres. c, Volts. I. II. 18.018 15.352 7.900 7.400 10.90 9.90 15.97 14.04 9.200 9.000 III. 13.231 7.100 10.10 10.86 10.800 7.900 IV. 11.615 6.650 10.10 8.87 - 7.400 V. 10.302 6.350 10.30 8.00 - 7.100 VI. 9.595 6.010 10.40 6.04 - 5.500 VII. VIII. 8.383 6,565 5.710 5.300 10.50 10.60 4.97 _ 5.000 IX. 5.757 5.100 10.60 4.21 3.800 4.500 X. 4.747 4.700 11.1-0 4.03 - 3.800 XI. 4.040 4.250 10.90 3.75 3.700 3.100 XII. XIII. 3.838 3.535 3.800 3.400 11.00 10.90 3.54 3.47 : 2.900 2.500 XIV. 3.030 2.850 10.90 3.09 2.700 2.300 XV. 2.520 2.400 11.05 - - - XVI. 2.120 1.900 11.00 1.85 - 1.200 XVII. 1.560 1.500 11.00 1.35 - 1.050 XVIII. 0.759 0.890 10.90 0.76 0.605 0.600 XIX. 0.396 0.290 11.00 0.54 - 0.360 XX. 0.000 . ^ 0.007 11.10 ^^ ^ ^ 0.00 0.007 i ^~m THE ELECTRO-CHEMICAL INSTITUTE AT AACHEN. 119 The following sixteen experiments were made simulta- neously under the same conditions as before. The numbers in column A express the quantities of copper precipitated in 6.5 minutes ; those under B, the tensions measured with the torsion galvanometer. A, Copper. B, Volts. f 0.7616 g ] Binding screw I. to W l 1 0.7415 " I 0.8286 " 7.10 Screw IV. to W 2 .... ( 0.6021 " 0.5716 " 5.30 1 0.4788 " J 0.4155 " Screw VIII. to W 3 . . . . < 0.3510 " 3.30 . 0.3535 " C.2648 Screw XII. to W 4 .... < 0.2963 " 1.80 . 0.2652 " Screw XVI. to W 5 .... j 0.1435 " | ( 0.1470 " f 0.90 Screw XIX. to W 6 . . . . ( 0.0363 " ) ( 0.0260 4t ) 0.23 In order to reach a conclusion as to the value of the apparatus for the purposes of quantitative analysis, twelve determinations were carried on simultaneously, at the author's request, by Dr. Kobert Ludwig, formerly assistant in the In- organic Laboratory. The solutions used for these experi- ments were of iron, cobalt, tin, antimony, and copper, metals 120 QUANTITATIVE ANALYSIS BY ELECTROLYSIS. which, as will be shown later, require currents of widely different strengths for their separation. The results of one series of these experiments are subjoined. Taken. Found. I. 0.3546 g Fe 2 O 3 0.2479 g Fe = 0.3541 g Fe 2 O 3 II. 0.3836 " Fe 2 3 0.2691 " Fe = 0.3844 " Fe 2 O 3 III. 0.2624 " Co 0.2619 " Co IY. 0.2234 " Co 0.2231 " Co V. 0.1145 " Sn 0.1142 " Sn VI. 0.2290 " Sn 0.2290 " Sn VII. 0.2025 " Sb 2 S 3 0.1444 " Sb = 0.2019 " Sb 2 S 3 VIII. 0.1890 " Sb 2 S 3 0.1348 " Sb = 0.1885 " Sb 2 S 3 IX. 0.1670 " Sb 2 S 3 0.1189 " Sb = 0.1663 " Sb 2 S 3 X. 0.8374 " CuSO 4 0.2133 " Cu = 25.47 % Cu XI. 0.8768 " CuSO 4 0.2225 " Cu = 25.31 % Cu XII. 0.7905 " CuSO 4 ( 0.1991 " Cu = 25.29 % Cu I Calculated 25.39 % Cu In general the current of the dynamo was not directly employed, bu{; was used to charge four accumulators, which sent their current of 8 volts to the electrolytic work-bench. The current thus transformed was employed in the following manner (Table I). The connection of the cells with the positive conductor, which carries the current of the four accumulators to the electrolytic table, is effected by means of six binding-screws (marked 1, 2, 3, 4, 5, 6). For connecting the cells with the negative pole of the source of current, wooden blocks bearing separate binding-posts and mercury-cups (in the diagram 6) are made use of. The arrangement of such a board is shown in Fig. 82 (f actual size). The cups marked 1, 2, 3, 4 are connected with the four binding-posts JT, cups 5 and 6 with the negative conductor THE ELECTRO-CHEMICAL INSTITUTE AT AACHEN. 121 from the source of current, and cup 7 with one conductor from the amperemeter. The connections between the cups are made by plain copper forks while the current is being measured, and otherwise by forks upon which resistances O K 1-7 Binding posts. Mercury cups. Cup 7 is connected through the measuring circuit with the amperemeter. O FIG. 82. equal to the resistance of the measuring instrument are rolled. A fork of this description (resistance-roll) is shown separately in Fig. 83. An instrument made by Hartmann & Braun (Bockenheim -Frankfurt a. M.) serves for measuring the cur- rent strength. This instrument, especially constructed for the laboratory of instruction, is provided with two scales and pointers (one on fech side), which allow of its being observed from all points on the work-bench. The pointer of the 122 QUANTITATIVE ANALYSIS BY ELECTROLYSIS. amperemeter moves over a scale having a radius of 16 cm. The instrument permits of the measurement of currents up to 2 amperes in decimals of 0.05 ampere. By the use of a resistance which may be connected in shunt, the range of measurement is increased tenfold. The resistance of the instrument itself is 0.32 ohm. An exactly equal resistance is contained in the resistance-roll. The measurement of the current strength in the cell is conducted as follows : A moderately high resistance of, say, 60 ohms is 'inserted in the circuit of the connected rheostat and a wire from a positive binding-screw is connected with Resistance-roll having a resistance equal to that of the amperemeter. FIG. 83. the anode of the cell. A wire running to one of the neg- ative binding- screws (for example, 5, Table I, Fig. 1) i& now attached through the rheostat to the cathode. The arrangement is seen from Table I, Fig. 1. It only remains to connect the mercury-cups 4 and 7 (the latter connected with the amperemeter) by means of a copper fork. With a resistance of 60 ohms in the circuit the amperemeter will show only a small deflection, which may be increased to the required value by reducing the resistance in the rheostat. This having been done, a resistance- roll is inserted between the cups 4 and 6 and the copper fork between 4 and 7 is removed, which breaks the connection with the amperemeter. Since the resistance of the amperemeter and roll are equal, a current 1-6 POSITIVE BINDING POS1 . SEPARATE NEGATIVE B MERCURY CUPS. . POSITIVE CONDUCTORS NEGATIVE CONDUCTORS I CIRCUIT CONNECTED W \ MEASURING INSTRUMEN PLAN OF WORK-BENCH 1 tf 1 00 p ' j 1 +fTo TABLE I [NG ARRANGEMENT FOR MEASURING THE CURRENT THE USE OF A SINGLE J FIG. 1. r^ H !" !^ ' I O 1 5* L c * O > I^Ptf&f E /p i cor > 1 3) -t- , > 1 .,., CONNECTION FOR MEASUREMENT CABLE FROM THE DYNAMO WITH T( J AMPER ^ METER OR ACCUMULATORS j 1 WITH THE RESISTANCE \ % ELECTROLYTIC CELL CELL BORATORY OF THE ROYAL TECHNICAL HIGH SCHOOL AT AACHEN. i 4-j 1 O li ENGTH OF EACH SEPARATE ELECTROLYSIS WIT REMETER. ^J' _tF ;y : l OF THE UNIVERSITY THE ELECTRO-CHEMICAL INSTITUTE AT AACHEN. 128 corresponding to the one measured flows through the cell. To observe the current strength during the electrolysis, the copper fork is placed in the cups 4 and 7, and the resistance- roll is removed. It is, therefore, possible to measure the current strength at any time without interrupting the current through the cell. As is evident from Table I, Fig. 1, the construc- tion of the electrolytic table allows 24 separate electrolyses to be conducted simultaneously. If at least four accumulators, with a tension of 8 volts, are used, a considerable number of experiments may be carried out at the same time quite in- dependently of one another. The former equipment of the private laboratory is, with- out further comment, evident from Table II. It includes a special connection for using the current from the dynamo directly, as well as for working with the 8 accumulators. The wire-gauze resistance, described on p. 113, serves to reduce the current when charging the accumulators, or in the direct employment of the same. The conductors from the dynamo and accumulators pass from the private laboratory to the electrolytic tables in the laboratory of instruction. An amperemeter shows the current which is there being used, while another amperemeter serves to control the current used for charging the accumulators. The complete arrangement of the plant is explained by Table I, Fig. 2. That accumulators furnish the most suitable source of current for electrolysis is to-day beyond question. These instruments, since they can also be charged with a thermopile, are more practical and convenient for small laboratories than primary batteries, which furnish either insufficient or incon- stant currents. The torsion galvanometer has long served in this labora- tory for measuring the tension at the electrodes. Since a knowledge of the tension is of great importance, both for the 124 QUANTITATIVE ANALYSIS BY ELECTROLYSIS. quality of the precipitated metal and for the separation of different metals, it must always be possible, as has been repeatedly stated, to determine the tension as. well as the cur- rent strength. How this may be accomplished with a single voltmeter, in the performance of several simultaneous ex- periments by means of a common source of current, is clear from the diagram, Table I, Fig. 2. When this method is used, special care must be taken that the rheostat is connected between the cell and the negative pole, since otherwise the tension of the accumulators and not that of the cell will be measured. In carrying out the measurement, only the cathode should be connected with the voltmeter circuit, if a deflection at the voltmeter is expected. The manner of attaching the cell is readily seen from the sketch. Since the measurement of several tensions cannot be conducted at the same time, owing to one interfering with the other, it must always be ascertained, before switching in the voltmeter, that it is not in use elsewhere. PRESENT EQUIPMENT OF THE ELECTROCHEMICAL INSTITUTE OF THE TECHNICAL HIGH SCHOOL. Although the electric current of the former equipment was furnished by an independent generating plant, such is not the case in the present one. It was considered desirable to be as independent of a power plant as possible, since these are by nature uneconomical, and moreover are not always ready for use. The solution of the problem was made possible by the fact that the city of Aachen has an electric-power station, the cables of which extend to the Technical High School. It was decided, therefore, to take the electricity for the new installation from the city mains. THE ELECTRO-CHEMICAL INSTITUTE AT AACHEN. 125 The current, as taken from the city system, could not, of course, be used for all experiments without modification. As is well known, for carrying out many electro-analyses, only very low tensions, included within the limits 0.5-8 volts, are required. The current furnished by the Aachen Electrical Works, operating on the three- wire direct-current system, has a ten- sion of about 108 volts between the middle wire and an out- side wire, and a tension of about 216 volts between the two outside wires. It was therefore necessary, in connection with the experi- ments previously mentioned, to reduce the high tension of the power wire in some suitable manner to the low tension required for experiment. Moreover it should at all times be possible, without special preparation, to carry out experiments with high tension, as for example in experiments where the current must be forced through materials having a high resistance, or for performing experiments with the electric arc. For the reduction of the high tension to a low tension in the case at hand, a direct- cur rent transformer was considered, The economical working of a double- dynamo combination of this description, its quiet and convenient operation, together with the small space which it occupies, all speak for the choice of the direct-current transformer. The question as to the method of obtaining the low tension required for electrolytic experiments was thus solved. Before proceeding to the description of the plant installed by the firm of Schuckert & Co., proprietors of the Aachen Electrical Works, the nature of the different experiments and investigations carried out will be briefly sketched in order that what follows may be more readily comprehended. 126 QUANTITATIVE ANALYSIS BY ELECTROLYSIS. 1. EXPERIMENTS WITH Low TENSION. The experiments with low tension are chiefly confined to the electro-analysis of solutions of metallic salts. In addition to this the precipitation of metals on a large scale is also undertaken as an introduction to the study of electroplating. 2. EXPERIMENTS WITH HIGH TENSION. The experiments with high tension begin at about 45 volts, required for the production of the Davy arc ; the high- est available tension of the supply circuit, that between the two outside wires, is about 216 volts. The high-tension current is chiefly employed for fusion experiments, as well as for the decomposition of gases and other bodies of high resistance. In addition to the above purposes, the current is also used for running an electric projection lantern, as well as a number of arc and incandescent lights. The distribution of the current to the various rooms, and also the operation of the transformer, is controlled from a central switchboard. The centralization of the whole plant was desirable for many reasons, chief among which was the fact that a valuable and complicated switchboard might thus be placed under competent supervision in a room not open to every one. This would prevent unauthorized persons from taking off current, and besides would allow a general super- vision of the whole plant. From the central switchboard currents are carried to the following places: 1. Private laboratory. 2. Large lecture-room. 3. Laboratory for electro-analysis. THE ELECTRO-CHEMICAL INSTITUTE AT AACHEN. 127 4. Laboratory for experiments on a large scale with high and low tension. The circuits running to the different rooms are distin- guished, according to the purpose for which the current they carry is intended, as : a. Lighting circuits, 1). High-tension circuits, c. Low-tension circuits, and are entirely independent of one another. The lighting circuits run to the private laboratory and to the large lecture-room. The circuits for the high-tension current extend to the private laboratory, to the large lecture-room, and to the laboratory for experiments with high- and low-tension. The circuits for low-tension current extend to. the private laboratory, the large lecture-room, the laboratory for electro- analysis, and the research room for high and low tension, In addition to the above circuits, which are intended for direct current transmission, there are also to be mentioned the circuit for charging the two batteries of accumulators and the circuit for running the transformer. The switches, resistances, controlling, and measuring apparatus belonging to the different circuits are located on the central switchboard. 1. PRIVATE LABORATORY. Concerning the special arrangements, the private labora- tory will next be mentioned. As already stated, the central switchboard, with the apparatus for the control of the whole plant, is placed in the private laboratory. In tins room there is also located a battery of accumulators. Table III gives a photographic view showing the arrange- ment of this laboratory. In the middle may be seen the 128 QUANTITATIVE ANALYSIS BY ELECTROLYSIS. central switchboard upon which the various instruments are mounted; to the left stands the glass closet containing the battery of accumulators. On the wall by the window are two work-benches, one intended especially for electro-analytical work with low tensions and small currents, and the other for experiments with high tensions and large currents. The arrangement for electro-analysis is as follows: On the back of the corner bench is a slanting wooden frame, on the face of which are fastened the switches and branch bind- ing-posts, while the connecting wires are attached to the back There are altogether five work-places on this bench, each of which will permit of the performance of two analyses simul- taneously, so that in all ten experiments may be carried on at the same time. The installation of these work-places, as well as of the second work-bench, is in accordance with the scheme for current distribution shown in Table IY. Each work-place is connected in parallel to the positive and negative conductors, which are run through the work- bench. The current for every analysis can be independently varied by means of the regulating resistance at the work-place. A single amperemeter, which can be thrown into the circuit of any analysis by means of a switch placed at each work-place, serves for measuring the current strength. When the am- peremeter is cut out, its place is taken by a resistance, in or- der that the current strength may not be altered (see p. 122). The measurement of the tension is carried out in a similar manner by a single voltmeter, which may at will be switched into the circuit of any analysis in operation. A lead safety fuse is inserted in the circuit of each of the ten branches, to guard against the possibility of too great cur- rent strength. 32 0: i - II II II II X * " 2. 2 3 P P PT S' * 3 5> ?> ir?i^e rS 3~ _: . GO H H 3 H o , <1 tr 1 ' > | t* THE ELECTRO-CHEMICAL INSTITUTE AT AACHEN. 129 The connections of the electrolytic apparatus to the small switchboards of the work-bench are made with very flexible rubber-insulated copper leads, the ends of which are provided with small copper links to allow of a more convenient attach- ment to the apparatus. For setting up experiments where large currents of high or low tension are required, two cases furnished with locks are affixed to the second work-bench. That for low tension, contains two branch plates which carry a number of binding- posts, thus allowing several different pieces of apparatus to be connected at the same time. The case for high tension contains three plates, connected with the two outside leads and the middle lead of the three- wire system respectively, whereby a maximum tension of about 216 volts is obtainable. These plates also carry several binding-posts, which permit the use of several pieces of appa- ratus at one time. The two accumulator batteries are comprised of four cells each. One battery, with the cells connected in series, requires a charging current of 90 amperes ; the other, similarly con- nected, requires 25 amperes. The batteries are charged from the transformer. The small battery furnishes current to the private labora- tory only, while the large one supplies the rest of the plant. Each of the batteries is provided with a cell switchboard for four cells, so that by cutting out separate cells the tension of the current may be reduced and the use of higli external re- sistances avoided. As a protection against the possibility of the current re- versing, during the process of charging, and flowing back through the transformer, each battery circuit is provided with an automatic cut-out. The tension of the separate cells is controlled by a special 130 QUANTITATIVE ANALYSIS BY ELECTROLYSIS. voltmeter having contact plugs, which allows the tension of each cell to be independently measured at the cell switch- board. For the measurement of the battery tension and the strength of the charging and discharging currents a special voltmeter and amperemeter are provided. Further, that the operation of charging and discharging may be more closely observed, indicators for showing the direction of the current are attached to the corresponding circuits. 2. LARGE LECTURE-ROOM. The installation of the large lecture-room is especially in- tended for the performance of lecture experiments, which comprise the demonstration of electrolysis, the decomposition of gases and liquids by the Davy arc, and fusion experiments. Besides this, provision is made for the running of an electric projecting lantern, as well as for a number of incan- descent and arc lamps. 3. LABORATORY FOR THE ELECTRO- ANALYSIS OF METALS. In this room the transformer is placed. It also contains a large experiment table having ten work -places, for carrying out electro-analytical experiments with low tensions. (Gf. Table Y.) The transformer will next be described. This consists of a combination of two direct-current dynamos, with their shafts coupled directly together. One of the dynamos, ar- ranged as a motor, is driven by the current from the two outside wires of the three-wire system, by a tension, there- fore, of about 216 volts. The circuit is run to the trans- former from the central switchboard. The dynamo which is coupled to the motor, and which furnishes the low-tension THE ELECTRO-CHEMICAL INSTITUTE AT AACHEN. 131 current, is so arranged that the tension at the poles may be varied from about 4.5-9 volts, the corresponding current strengths being respectively 360 and 180 amperes. The con- ductors carrying the low-tension current from the dynamo run to the central switchboard. The tension of 9 volts is the one generally used, the lower tension "of 4.5 volts being employed for larger electrolytic experiments, such as the prep- aration of pure metals. The alteration in the tension of the current is brought about by connecting the two halves of the double armature, with which the transformer is provided, either in series or in parallel. This is done by merely changing the corresponding connections on the frame of the transformer. Further concerning the construction of the transformer, it should be mentioned that the machine is very solidly cast, and the magnets protected within the frame, so that a mechanical injury to the magnet-coils is out of the question. The lubri- cation of all parts is carried out by means of ring-lubrication, which has proved very satisfactory. Such delays as often occur when other mechanical contrivances are employed are here impossible. Owing to its construction, the transformer, which for protection is enclosed in a special covering, can run for hours without particular attention. The action of the transformer, in spite of its speed of about 1300 revolutions per minute, is so quiet and free from any jarring or shaking, that its running can scarcely be detected even in- the immediate neighborhood. It should be stated that there is a switchboard near the transformer, by which direct currents of low tension can be taken off in this room, without making use of the central switchboard. Such currents are required when experiments with high current strength and low tension are performed ; 132 QUANTITATIVE ANALYSIS BY ELECTROLYSIS. and in such cases short cables are run from this switchboard to the nearest work-bench, where the apparatus is set up. The arrangement of the large work-bench, a photograph of which is given in Table Y, corresponds in general to that of the table for conducting analyses in the private labora- tory. Here, on either side of the bench, there are five work- places, each of which allows of the simultaneous performance of two analyses, so that in all twenty experiments may be carried on at the same time.* Table IY shows the method employed for measuring the current strength and tension of an analysis. The ampere- meter and voltmeter are above. The current strengths are regulated by means of the rheostats (I, II, III, and IY). These consist of slate slabs into which are fixed metal knobs, which are attached to separate resistance spirals. By turning the lever in the direction indicated by the arrow, the resist- ance is cut out and the current strength correspondingly increased. The switches for the amperemeter A (I( n> m IV) , f or the electrolyses i E (I| IIt IIIt IV) , and the safety-fuses B (Ii n> IIIf IV) are located under bronzed metal cases. %, ii, in, iv) are the binding-posts to which the electrolyses are connected, Y is a double-pole switch used in measuring the tension. In the position o the voltmeter is cut out ; at i, n, in, iv the corresponding electrolysis is connected with the voltmeter. As already stated, there is only one ampere- meter and one vo.ltmeter to every table with 10-20 dishes, and therefore only one electrolysis can be measured at a time. The four figures on Table IY are designed to make the explanations clearer. * Two other work-benches have been recently added, so that there are now twenty work-places for electro-analysis. THE ELECTRO-CHEMICAL INSTITUTE AT AACHEN. 133 In position I, where the keys A r and E r are horizontal, the circuit is closed. In II, A n is perpendicular; the amperemeter is in circuit. The lever of the rheostat at II may be turned in the direction of the arrow until the meas- uring apparatus registers the desired current strength. A is then brought into the position A m and E to E m . The cur- rent now flows no longer through the amperemeter, but through a roll of wire, the resistance of which is equal to that of the amperemeter. The current strength remains the same as that previously shown by the amperemeter. Y serves for measuring the tension at the poles of the electrolytic vessel, as shown at Y IV . In this operation the position of A and E is the same as in III. The two metal strips (SS) are pushed to the right or left (in the figure to the right, iv), whereupon the voltmeter shows the tension existing at that time at the poles of the corresponding elec- trolysis. The instruments should be cut out immediately after use. 4t. LABORATORY FOR PERFORMING EXPERIMENTS ON A LARGE SCALE WITH Low AND HIGH TENSIONS. As already mentioned, special cases w r hich receive their currents from separate conductors running from the central switchboard are arranged for high and low tension. Within the case for high tension there are three separate plates corresponding to the three wires of the three- wire system, providing currents at tensions of 108 and 216 volts accordingly. The case for low tension contains two connections, with possible tension at the poles up to 9 volts. From both of the cases separate branch circuits run to the four work-benches, where they end in terminal boxes pro- 134 QUANTITATIVE ANALYSIS BY ELECTEOLYSIS. vided with locks. By this arrangement each table is pro- vided with both high and low tension. Each of the branches running to the tables is supplied with a safety fuse and a switch; each table is therefore independent of the others. A set of transportable resistances and measuring instru- ments for regulating the current is used in carrying out experiments. Large and cumbersome resistances are required to produce appreciable variations in the tension. A simple appliance in use in the Aachen laboratory overcomes this difficulty in the case of experiments of short duration, where economical use of the current is not an essential feature. This scheme, originated by Lob and Kaufmann,* permits the convenient splitting up of the current of 216 or 108 volts into separate independent currents having the required lower tension. A number of lead plates are hung parallel to one another in a large porcelain trough filled with sulphuric acid (Fig. 84), in such a manner that they cut all the lines of the current. They must therefore almost touch the sides and bottom of the trough. When the current passes, these lead plates act as intermediate conductors, the sum of their separate tensions being equal to the tension of the main current. The arrangement is of course impractical as an accumulator, since the polarized plates immediately short- circuit through the electrolyte and are reduced to the poten- tial of the electrodes. FIG. 84. * Zeitschr. f. Elektroch., 1895-96, p. 345. Ibid., p. 664. THE ELECTRO-CHEMICAL INSTITUTE AT AACHEN. 135 The immersed lead plates can be slid along the length of the trough on the glass rod by which they are hung. By moving the plates toward or away from the electrodes the tension is varied, and any desired tension may be obtained by making a connection between a terminal electrode and one of the plates. The arrangement is given in Fig. 84. E de- notes the source of current ; T, the trough filled with sul- phuric acid ; A and K, anode and cathode ; M, the five plates. The wires to S show the removal of three separate currents of different tensions. A large number of such connections are possible. On account of the gases given off, the trough should be kept under a hood. In addition to the details of the equipment which have been described, some general facts in connection with the management of the entire plant should be stated. Since the apparatus is much used, and is not always placed in experienced hands, it was considered desirable to have all parts solidly constructed and intended for continu- ous use. The switches and regulating instruments, as well as the branch plates, are all mounted on bases of fire-proof material. All connections are made with the best rubber-covered wire, fastened to large porcelain brackets, so that most perfect insulation of the conductors is assured. To secure against improper use, all switch-cases are pro- vided with safety-locks, so that currents can nowhere be taken off without the permission of the director of the laboratory. SECTION II. SPECIAL PART. QUANTITATIVE DETERMINATION OF THE METALS. * IRON. LITEEATUBE I Wrightson, Zeit. f. analyt. Chem., 15, 305. Luckow, Zeit. f. analyt. Chem., 19, 18. Classen and v. Keiss, Ber. deutsch. chem. Ges., 14, 1622. Classen, Zeit. f. Elektrochemie, vol. I. Moore, Chem. News, 53, 209. Smith, Amer. Chem. Jour., 10, 330. Brand, Zeib. f. analyt. Chem., 28, 581. Drown and Meckenna, J. of Analyt. and Applied Chem., 5, 627. Smith and Muhr, ibid., 5, 488. Rudorff, Zeit. f . angew. Chem., 15, 198. Vortmann, Monatshefte f. Chem., 14, 542. Heidenreich, Ber. deutsch. chem. Ges., 29, 1585. If the solution of a ferrous salt f is treated with potassium * Of the methods existing in the literature, reference will be made only to those which give the necessar} r and complete details concerning the conditions of experiment. f As stated on p. 5, sulphates are best adapted to this treatment, chlo- rides less so, while nitrates must be avoided. The presence of phosphoric acid is not harmful. 137 138 QUANTITATIVE ANALYSIS BY ELECTROLYSIS. or ammonium oxalate, there is produced an intensely yellow- ish-red precipitate of ferrous oxalate, soluble in an excess of the reagent to a yellowish-red solution of the double salt. The above-named oxalates do not precipitate ferric salts ; but, if added in sufficient quantity, a solution of the double ferric salt is produced having a more or less green color. If this solution is submitted to electrolysis, there is first produced the double ferrous, salt, which is then decomposed with separa- tion of metallic iron ; the green liquid therefore becomes first red, and then colorless. Because of this action, the determina- tion of iron is more rapidly performed in solutions of ferrous than of ferric salts. Potassium iron oxalate is not adapted to electrolysis, because the potassium carbonate which is pro- duced precipitates iron carbonate, and thus complete reduc- tion is prevented. The electrolysis of the ammonium double salt, when ammonium oxalate is in sufficient excess, proceeds smoothly, with no separation of an iron compound. If the solution contains free hydrochloric acid, it is best to remove it by evaporation on the water- bath. Free sulphuric acid may be neutralized with ammonia, since the ammonium sulphate thus produced only increases the conductivity of the solution. Nitrates are converted by evaporation with sulphuric acid into sulphates, or by repeated evaporation with hydrochloric acid into chlorides. The determination is conducted as follows: Assuming that 1 g of iron may be present in the solution to be elec- trolyzed, 68 g of ammonium oxalate are dissolved by heat in as little water as possible, and the iron solution is gradually added, with constant agitation.* The solution is then diluted * It is not desirable to add ammonium oxalate solution to a ferrous solution, as difficultly soluble ferrous oxalate separates, and can be dis solved to the double salt only by long heating. With a ferric solution this precaution is unnecessary. IRON. 139 with water to 100-150 cc, and the positive electrode is im- mersed in the liquid until it is just covered by the solution. The electrolysis is conducted according to the special direc- tions which are given below. The end of the reaction is determined by taking out a small portion of the colorless solution with a capillary tube, acidifying strongly with hydrochloric acid, and testing with potassium sulphocyanate. When the reaction is ended the positive electrode is removed from the solution, which is poured off, and the dish washed three times with cold water (about 5 cc each time), and three times with absolute alcohol, dried a few moments in the air-bath at a temperature of 70 to 90, and weighed after cooling. The separated iron has a steel-gray color and brilliant lustre, is firmly attached to the dish, and can be preserved in the air without oxidation for a full day. CONDITIONS OF EXPERIMENT.* Temperature of the liquid: Although the maintenance of a certain uniform temperature is not essential to the suc- cess of the experiment, it has been found in practice that the ordinary temperature of the solution (20-40) is the most favorable to the rapid completion of the analysis. Current density, ND 100 : For solutions at ordinary tem- perature, 1-1.5 amp.; for warm solutions (40-65), 0.5-1 amp. Electrode tension : For warm solutions, with the stated current density, 2.0-3.5 volts; otherwise, 3.6-4.3 volts. * Method of the author. In all of the author's methods the statements refer to the use of the electrodes described on p. 85; the current densities refer only to the dish given in Fig. 54. 140 QUANTITATIVE ANALYSIS BY ELECTROLYSIS. For the quality of the precipitated metal, polished or roughened dishes answer equally well. EXPERIMENT. Used 2.1-2.5 g FeSO 4 (NH 4 ) 2 SO 4 .6H 2 O, 6-8 g ammonium oxalate, 120 cc of liquid. Current Density, Amperes. 1 -1.5 Electrode Tension, Volts. 3.85-4.3 Temp. 20-40 Time. 2 hr. 15 m. Found.* 14.21 % 1 -1.05 3.6 -4.2 36 3 " 50 " 14.21 " 1 -1.08 3.05-3.52 65 2 " 30 " 14.28 " 0.5-0.55 2.0 -2.3 50-52 3 " 30 " 14.24 " Used 2.6-2.8 g ferric potassium oxalate (Fe a (C 2 O 4 ),. 3K 2 C 2 O 4 .6H 2 O), 6-7 g ammonium oxalate. 1.5-1.7 3.55-4.25 35-40 2 hr. 54 m. 11.39 %'\ 1.0-1.1 3.9 -4.0 30-40 3 " 15 " 11.35 " 0.5-0.8 2.4 -2.8 50 6 " 15 " 11.25 " Edgar F. Smith precipitates iron from a solution of am- monium citrate to which a few drops of citric acid have been added. The author's experiments in earlier years on the separation of iron from other metals in citric and tartaric acid solution, demonstrated that in the presence of fixed organic acids the precipitated metal always contains carbon. Heidenreich has recently shown, by experiments conducted in the Aachen laboratory, that iron may be quantitatively determined under certain conditions, namely: 0.2 g ferrous ammonium sulphate, 50 cc of a 10 per cent solution of sodium citrate, 2 cc of a saturated solution of citric acid; entire volume of liquid, 120 cc; temperature of room; ND 100 = 0.75-0.9 amp.; electrode tension, 5 volts; time, 4r-6 hours. The iron, however, always contains carbon. [Theory 14.29*.] t [Theory 11.40*.] COBALT. 141 COBALT. LITEKATUKE I Gibbs, Zeit. f. anal. Chem., 3, 336 ; 11, 10 ; 22, 548. Merrick, Amer. Chemist, 2, 136. Wrightson, Zeit. f. anal. Chem., 15, 300, 303. 333. Schweder, ibid., 16, 344. Cheney and Richards, Amer. Jour, of Science and Arts, [3] 14, 178. Ohl, Zeit. f. anal. Chem., 18, 523. Luckow, ibid., 19, 314. Riche, ibid., 21, 116. Classen and v. Reiss, Ber. deutsch. chem. Ges., 14, 1622. 2771. Classen, ibid., 27, 2061 ; Zeit. f. Elektrochemie, 1894-95, Heft 1. Schucht, Zeit. f. anal. Chem., 21, 493. Eohn and Woodgate, Journ. Soc. Chem. Indust., 8, 256. Riidorff, Zeit. f. angew. Chemie, 1892, p. 6. Brand, Zeit. f. anal. Chemie, 28, 588. Le Roy, Compt. rend., 112, 722. Vortmann, Monatsh. f. Chem., 14, 536. Oettel, Zeit. f. Elektrochemie, 1894-95, p. 195. Fresenius and Bergmann, Zeit. f. anal. Chem., 19, 329. Cobalt may be very easily precipitated from a solution of cobalt ammonium oxalate containing an excess of ammonium oxalate (method of the author). The metal separates rapidly at the negative electrode, in a compact adherent coating, showing its characteristic metallic properties. The operation is performed as in the determination of iron. 4-5 g am- monium oxalate are dissolved by heating in the solution, the volume of which should be about 25 cc ; it is then diluted to 100-120 cc, warmed, and electrolyzed at 60-70. CONDITIONS OF EXPERIMENT. Temperature of the liquid : The period of electrolysis is considerably shortened by warming, so that a temperature of 60-70 is suitable. 142 QUANTITATIVE ANALYSIS BY ELECTROLYSIS. Current density : The proper current density for warmed solutions is KD 100 = 1 amp. Electrode tension : 3.1-3. 8 volts. The condition of the surface of the cathode has no effect upon the quality of the precipitated metal. EXPERIMENT. Used 2.2-2.6 g CoSO 4 .K 1 SO 4 .6H,O, 4-5 g ammonium oxalate, 120 cc solution. A.mperes. jtLiieciroue lensi Volts. Temp. Time. Found.* 1 -1.1 3.1 -3.78 60-65 2 br. 15 ra. 13.36 0.5-0.52 2.7 -2.95 60-65 3 " 30 " 13.49" 1 -1.2 3.9 -4.1 15-35 4 " 30 ' 13.43" 0.5-0.53 3.46-3.9 15-27 6 " 35 " 1325" According to a method given by Fresenius and Bergmann, the cobalt solution, after the addition of 15-20 cc of an ammonium sulphate solution (300 g (NH 4 ),SO 4 to the liter) and 40 cc ammonia sp. g. 0.96 (where more than 0.5 g cobalt is present in the solution, 50-60 cc NH 4 OH), is diluted with water to 150-170 cc, and electrolyzed with a current of I^D 100 = 0.7 as a maximum at ordinary tempera- tures. The presence of chlorides and nitrates is unfavorable to the reduction. Fixed organic acids (citric acid, tartaric acid) and also magnesium compounds act injuriously. CONDITIONS OF EXPERIMENT. Temperature of the liquid : The separation is not hastened by warming. Current density: JSTD IOO = 0.5-0.7 amp. Electrode tension : With the given current density and at ordinary temperatures, this equals 2.8-3.3 volts. F. Oettel proposes the following method for the determina- tion of cobalt : The salt is dissolved in water and a quantity * [Theory 13.43^.] NICKEL. 143 of ammonium chloride, equal to four times the weight of the salt taken, is added. The volume of the liquid is 150 cc, -J- of which is an ammonia solution (sp. g. = 0.92). After electrolyzing for 14 hours the cobalt is quantitatively pre- cipitated if 100 cc of solution do not contain more than 0.25 g of the cobalt salt. NICKEL. LITERATURE I Gibbs, Zeit. f. anal. Chem., 3, 336 ; 11, 10 ; 22, 558. Merrick, Amer. Chemist, 2, 136. Wrightson, Zeit. f. anal. Chem., 15, 300, 303, 333. Schweder, ibid., 16, 344. Cheney and Richards, Amer. Journ. of Science and Arts, [3] 14, 178. Ohl, Zeit. f. anal. Chem., 18, 523. Luckow, ibid., 19, 314. Riche, ibid., 21, 116. Classen and v. Keiss, Ber. deutsch. chem. Ges., 14, 1622, 2771. Classen, ibid., 27, 2061 ; Zeit. f. Elektrochemie, 1894-95, Heft 1. Schucht, Zeit. f. anal. Chem., 21, 493. Kohn and Woodgate, Journ. Soc. Chem. Indust., 8, 256. Riidorff, Zeit. f. angew. Chem., 1892, p. 6. Brand, Zeit. f. anal. Chem., 28, 588. Le Roy, Compt. rend., 112, 722. Vortmann, Monatsh. f. Chemie, 14, 536. Campbell and Andrews, Journ. Am. Chem. Soc., 17, 125. Oettel, Zeit. f. Elektrochemie, 1894-95, p. 192. Fresenius and Bergmann, Zeit. f. anal. Chem., 19, 320. Nickel may be reduced under conditions similar to those requisite for cobalt ; the metal is precipitated from the solu- tion of the double oxalate containing ammonium oxalate in excess, by the action of a similar current, as a thick, bright coating on the negative electrode. The end of the reaction is ascertained by testing with ammonium sulphide or potas- sium sulphocarbonate, and the precipitate is treated as pre- viously directed. 144 QUANTITATIVE ANALYSIS BY ELECTROLYSIS. CONDITIONS OF EXPERIMENT . The conditions of experiment are the same as in the separation of cobalt. Here also polished or roughened dishes serve equally well. Used 1.2-2.1 g 'NiSO 4 .(]N T H 4 ) 2 SO 4 .6H 2 O, 4-5 g ammo- nium oxalate, 120 cc liquid. Current Density, Electrode Tension, Temp Time Found.* ArnpGrGS. vtMio. 0.9-1 3.1-2.9 65-70 2 hr. 50 m. 15.13 # 0.5-0.6 3.38-3.4 17 5" 15.11" 0.9-1 4.09-4.35 15-30 3 " 35 " 15.05" 05-0.53 2.7-2.85 60-65 4" 15.17" The condition of the precipitate is best when the elec- trolysis is conducted at a temperature of 60-70, with a current of ND 100 = 1 amp. According to Fresenius and Bergmann, nickel, like cobalt, may be precipitated completely from a solution treated with ammonium sulphate and ammonia (see Cobalt). The method given by Oettel for the determination of cobalt may also be used for nickel. For this purpose the nickel salt is dissolved in 20-40 cc ammonia (sp. g. 0.92) to which 10 g ammonium chloride are added, and after diluting to about 120 cc, the nickel is precipitated in 7-8 hours with a current of ND 100 = 0.45 amp. Campbell and Andrews dissolve nickel hydroxide in 30 cc of a 10 per cent solution of di-sodium phosphate, to which 30 cc of a concentrated ammonia solution are added, and, with a distance of 5 mm between the electrodes, separate out the nickel by the use of a current of KD 100 = 0.14 amp. ZINC. LITERATURE I Wrightson, Zeit. f. anal. Chem., 15, 303. Parodi and Mascazzini, Ber. deutsch. chem. Ges., 10, 1098 ; Zeit. f. anal. Chem., 18, 587. * [Theory 14.87$ Ni.] ZINC. 145 Riche, Zeit. f. anal. Chem., 17, 216. Beilstem and Jaweiu, Ber. deutsch. chem. Ges., 12, 446 ; Zeit. f. anal. Chein., 18, 588. Riche, Zeit. f. anal. Chem., 21, 119. Reinhardt and Ihle, Journ. f. prakt. Chem., 24, 193. Classen and v. Reiss, Ber. deutsch. chem. Ges., 14, 1622. Classen, ibid., 27, 2060. x- Gibbs, Zeit. f. anal. Chem/ 22, 558. Luckow, ibid., 25, 113. Brand, ibid., 28, 581. Warwick, Zeit, f. anorg. Chem., 1, 290. Vortmann, Ber. deutsch. chem. Ges., 24, 2753. Riidorff, Zeit. f. angew. Chem., 1892, p. 197. Vortmann, Monatsh. f. Chemie, 14, 536. Jordis, Zeit. f. Elektrochemie, 1895-96, pp. 138, 565, 655. Millot, Bull, de la Soc. chim., 37, 339. The metal may be easily and quickly separated from the double salts of zinc ammonium oxalate and zinc potassium oxalate (method of the author).* The reduced metal has a bluish-white color, and under proper conditions adheres firmly to the negative electrode. Indeed, the metallic zinc often adheres so firmly to the platinum dish that, after being cleaned with water and alco- hol, and dried, it is with difficulty dissolved by warming with acids. Generally, after this operation, a dark coating of platinum- black remains which can only be removed by ignit- ing the dish and again treating with acids. It is therefore desirable, before weighing the dish, to precipitate upon it a thin coating of copper, tin, or, better, silver. In laboratories * The reduction of zinc from a solution of zinc ammonium oxalate is very often credited to Reinhardt and Ihle. The author, however, described this method in Fehling's " Handworterbuch " before the research of the above-named investigators appeared in the Journal f lir praktische Chemie, to the editor of which, Kolbe, the author especially stated the facts at the time. 146 QUANTITATIVE ANALYSIS BY ELECTKOLYSIS. in which many zinc determinations are performed, silver dishes may be advantageously employed. A bright, thick coating of copper can be obtained in a few minutes if a saturated solution of copper sulphate is treated with an excess of ammonium oxalate to form the double salt, acidified with oxalic acid, warmed to 70-80, and decomposed by a current of 1 ampere. The preparation of the double salt in a beaker, and the transfer of the clear, hot solution to the platinum dish, is to be recommended. For silvering the dish it is best to precipitate the silver from a solution of the same in potassium cyanide (see Silver). In determining zinc by this method, the zinc salt is dis- solved in a little water by warming, about 4 g of potassium oxalate or an equal amount of ammonium oxalate is added, and the whole is brought into solution by warming and, if necessary, by the addition of small quantities of water.* The liquid is now transferred to a platinrtm dish coated with copper or silver, and electrolyzed. The author has demon- strated by experiments that the separation of the zinc in a dense, shiny condition is possible if the solution be kept acid during the process of analysis. For acidifying the solution, a cold saturated solution of oxalic acid, or, better, a solution, of tartaric acid (3 : 50) is em- ployed. At the start the solution is electrolyzed for about 3-5 minutes without addition of acid, and then the acid is permitted to flow in drops (about 10 drops per minute) from a burette with a fine outlet, upon the watch-glass covering the dish. The acid flows through the holes in the watch- glass into the dish itself. After the reduction is completed * If the alkali oxalate be added to a dilute solution of a zinc salt, there first forms a precipitate of zinc oxalate which is not completely converted into the soluble zinc double salt if the solution of the alkali oxalate is too dilute. ZINC. 147 (this is determined with potassium ferrocyanide), the metal must be washed without interrupting the current. CONDITIONS OF EXPERIMENT. Temperature of the liquid : This must be from 50-60. Current density : ND 100 = 0.5-1 amp. Electrode tension : 3.54.8 volts. Roughened or polished dishes answer equally well. Time : About 2 hours. EXPERIMENT. Used 1.8-2 g zinc ammonium sulphate, 4 g potassium oxalate, 120 cc liquid. CUr AmpSes 8ity ' Electr ^ ts Tension ' Temp. Time. Found.* 0.5-0.55 3.5-4.0 55-60 2 hr. 16.44# 0.9-1 4.7-4.8 60 1 " 50 m. 16.42 " According to v. Miller and Kiliani, 4 g potassium oxa- late and 3 g potassium sulphate are dissolved in water, the neutralized zinc solution (sulphate or nitrate containing not more than 0.3 g Zn) carefully added, and electrolysis effected without heat, by a current of ND 100 = 0.3-0.5 amperes. The reaction is complete in 2 to 3 hours. N. Eisenbergf obtained the following results by the above method : Current Electrode Condition Density, Tension, Temp. Time. Found. of en - Amperes. Volts. Metal. 1.8312 0.4-0.35 3.95-4.00 25 -26 4 hr. 16.35$ partly spongy 1.8312 0.40-0.35 4.15-4.25 28.5-30 4 " 15 m. 16.01 " spongy Remark: (1) Rough dish; (2) Polished dish. The agitation of the liquid by means of a stirring appli- ance is recommended for this method. According to Jordis, zinc, when present in the form of sulphate, chloride or nitrate, may be separated from a lactic * [Theory 16.29^ Zn.] \ Inaugural-Dissert. Heidelberg, 1895. 148 QUANTITATIVE ANALYSIS BY ELECTROLYSIS. acid solution. The ease with which this method may be car- ried out appears from the directions of the author, which read as follows :* " 2 g ammonium sulphate and 5-7 g ammo- nium lactate are added to the neutral solution containing not less than 0.3-0.5g zinc, and it is acidified with 'a few drops of lactic acid. A stirring attachment is employed, and the solution is electrolyzed with a current of !ND 100 = 1.0-1.5 amp. After 40-60 minutes the electrolyte is poured into a second dish and the separation completed in this. With a current of the above density this requires 20-25 minutes. A somewhat more concentrated solution of about 120-150 cc is advantageous. 4 c Since the lactic acid is but very slowly decomposed during the electrolysis, its regeneration resulting from the action of the sulphuric acid formed upon the ammonium lactate, the electrolyte remains acid until the end and requires no further attention." MANGANESE. LITERATURE I Riche, Ann. d. China, et Pliys., [5], 13, 508. Luckow, Zeit. f. anal. Chem., 19, 17. Schucht, ibid., 22, 493. Classen and v. Reiss, Ber. deutsch. chem. Ges., 14, 1622. Moore, Chem. News, 53, 209. Smith and Frankel, Journ. Anal. Chem., 3, 385 ; Chem. News, 60, 262. Brand, Zeit. f. anal. Chem., 28, 581. Riidorff, Zeit. f. angew. Chem., 15, 6. Classen, Ber. deutsch. chem. Ges., 27, 2060. Engels, Zeit. f. Elektrochemie, 1895-96, p. 413 ; 1896-97, p. 286. Groeger, Zeit. f. angew. Chem., 1895, p. 253. * Zeit. f . Elektrochemie, 1895-96, p. 656 MANGANESE. 149 From the results of experience in the Aachen laboratory, none of the methods long in use are applicable for the direct quantitative determination of this metal as peroxide. It is gen- erally assumed that the peroxide when dried at about 68 has the composition MnO a .H a O, an assumption which the author cannot confirm. If the attempt be made to convert the hyd rated peroxide into anhydrous peroxide by prolonged drying at a higher temperature, a strongly hygroscopic sub- stance results which rapidly increases in weight during the process of weighing. It is therefore necessary to convert the dried peroxide into mangano-manganic oxide by ignition, an operation conducted with ease and safety. After determin- ing the necessary conditions for the separation of large quan- tities of lead peroxide, the author was induced to assume that manganese behaved similarly to lead. Investigation proved, however, that strong inorganic acids interfere with complete precipitation, and even make it impossible. Of the organic acids, acetic acid alone is suitable, although the precipitation of large quantities, even when roughened dishes are used, cannot be successfully carried out, since it is impossible to obtain firmly adhering precipitates. As will be stated under lead, the separation of lead takes place from nitric acid solutions in the presence of other metals. The hope that manganese in the presence of iron might be separated and determined in an acetic acid solution has not been fulfilled. Innumerable experiments, conducted under the most varied conditions and with the most diverse substances, have given no satisfactory results. In view of the great importance which a method for the direct determina- tion of manganese in the presence of iron, etc., would pos- sess, this investigation will be continued. 150 QUANTITATIVE ANALYSIS BY ELECTROLYSIS. CONDITIONS OF EXPERIMENT. Temperature of the liquid : 50-70. Current density : ND ]00 = 0.3-0.35 amp. Electrode tension: 4.3-4.9 volts. Time: 3 hours. EXPERIMENT. Used about 0.5 g MnSO 4 .(NH 4 ) a SO 4 .6H a O, which was dissolved in about 75 cc water and eleetrolyzed after the ad- dition of 25 cc acetic acid sp. g. 1.069 (20). Current Density, Electrode Amperes. Tension, Temp. Time. Found. Volts. 0.3-0.3 4.4-4.9 50-68 3 hr. 0.1035 g Mn 3 O 4 14.94$* 0.3-0.35 4.3-4.6 56-62 3" 0.1045 g Mn 3 O 4 15-04'' An equally rapid and complete separation was secured by Engels, as a result of investigations conducted in the Aachen laboratory. The method is as follows : 1-2 g of the manga- nese salt is dissolved in about 125 cc of water, and 10 g am- monium acetate and 1 . 5-2 g chrome alum are also added. The clear solution is then eleetrolyzed. Chlorides must not be present, since the evolution of chlorine interferes with the separation of the manganese. If they are present, the pro- cess is carried out according to the directions given under the separation of manganese and copper. CONDITIONS OF EXPERIMENT. Temperature of liquid : 80. Current density: ND 100 = 0.6-1 amp. Electrode tension : 2.8-4 volts. Time : About 1 J hours. Note : Roughened dishes must be used. * [Theory 14.07$ Mn. Probably impure salt was used. Trans ] MANGANESE. 151 EXPERIMENT. In the determinations given below, 10 g ammonium ace- tate and 1.5-2 g chrome alum were added to the solution. TT \ /o/^ \ z-cr r\ Current Density Electrode m,^ T i mp Found* Mn(NH 4 ),(S0 4 ) a .6H,0. NDloo , Amp. Tension, Temp. Time. Mn,O 4 , en, , Volts. g. Per cent. 1.1522g 0.6-0.5 2.8-3.1 80 |br. 0.2235 19.39 1.2554" 0.6-0.5 2.8-3.1 80 " " 0.2436 19.40 1.2994" 0.6 3. 83 " " 0.2520 19.39 1.8099" 1.1 3.7-4.1 80 " " 0.3513 19.40 In the determination of manganese in the salts of perman- ganic acid, the solution of the latter is decomposed, accord- ing to Engels, with 5 cc acetic acid and enough hydrogen peroxide to completely decolorize it. Since the presence of even small quantities of hydrogen peroxide prevents the sepa- ration and the firm adherence of the precipitate, the excess of hydrogen peroxide must be removed. This may be most easily accomplished by the addition of small quantities of chromic acid, until further addition no longer causes the evo- lution of gas; generally 0.3-0.5 g is sufficient. EXPERIMENT. 50 cc of a potassium permanganate solution were decom- posed with 5 cc acetic acid and 10 cc of a weak solution of hydrogen peroxide. The excess of H a O 2 was removed with Cr0 3 . Current Density. Tension. Time. Temp. Mn 3 O 4 . I. 1.5 amp. 2.8 volts 1 hr. 85 0.1217 g II. " 1.65 " 3.15 " 1 " 85 0.1220" III. 1.78 " 3.4 " 1 " 80 0.1220" The current strength available varies between compara- tively wide limits. Weak currents also give rapid and satis- factory results. * [Theory 19.52# Mn 3 O 4 .] 152 QUANTITATIVE ANALYSIS BY ELECTROLYSIS. EXPERIMENT. Three dishes, each containing manganese sulphate solu- tion, 10 g ammonium acetate and 1 g chrome alum, were con- nected in parallel, and the current from a thermopile passed through. The tension at the electrodes at the beginning of the electrolysis was 3 . 2 volts, the entire current strength 1.5 amp., so that each dish received about 0.4 amp. The man- ganese salt used contained 20.45$ Mn 8 O 4 . Time. Found. 1.1955 0.22 3.2 80 2hrs. 30 min. 20.45 0.9009 0.22 3.2 80 2 " " " 20.44" 1.2012 0.22 3.2 80 2 " " " 20.40" Since manganese separates as peroxide from a cold solution to which ammonium acetate has been added, at 1.25 volts, and when warmed to 80 as low as 1-1.1 volts, the electroly- sis may therefore be conducted with low electromotive forces. The constancy of the latter may be assured by connecting in shunt (page 73). The lower the tension, the longer the time required for the separation. With the maximum tension of 1.8 volts it takes from 4 to 5 hours. For the firm adher- ence of the precipitate a temperature of 80 is essential. In those cases (i.e., in the presence of silver) where the chrome alum produces a precipitate in the solutions, it may be replaced by 10 cc of alcohol, which in general is not as effi- cient as the chrome alum for separating the manganese perox- ide. When alcohol is used, the electrolysis is conducted at a temperature of 75-80, with a maximum tension of 2 volts, which gives a current density ND JOO = about 0.15 amp. Time required for the electrolysis, about 5 hours. ALUMINIUM, URANIUM, CHROMIUM, BERYLLIUM. 153 ALUMINIUM, URANIUM, CHROMIUM, BERYLLIUM. If a solution of aluminium ammonium oxalate containing ammonium oxalate in excess is submitted to the action of the electric current, the ammonium oxalate is changed into car- bonate, and the aluminium separates as hydroxide. When the oxalate is decomposed, the solution is heated until there is only a faint odor of ammonia, the hydroxide filtered off, washed with water, and converted, by ignition, into A1 2 O 3 , Uranium is acted on in the same way as aluminium. Chromium ammonium oxalate is oxidized by the current with formation of ammonium chromate. To determine the chromic acid, the ammonium carbonate is decomposed by boiling, the* solution acidified with acetic acid, and the chromic acid determined as lead or barium chromate. When beryllium ammonium oxalate is subjected to elec- trolysis, the beryllium is kept in solution by the hydrogen ammonium carbonate produced, provided the solution is cold. The behavior of aluminium, chromium, uranium, and be- ryllium can be made use of, as explained later, to separate them from each other and from all metals which separate from their double oxalates in the metallic state at the nega- tive electrode. COPPER. LITERATURE : Gibbs, Zeit. f. anal. Chem., 3, 334. Boisbaudran, Bull. d. 1. Soc. Chiin., 1867, p. 468. Merrick, Amer. Chemist, 2, 136. Wrightson, Zeit. f. anal. Chem., 15, 299. Herpin, ibid,, 15, 335. Ohl, ibid., 18, 523. Classen, Ber. deutsch. chem. Ges., 14, 1622, 1627. 154 QUANTITATIVE ANALYSIS BY ELECTIIOLYS13. Classen and v. Keiss, Zeit. f. anal. Chem., 14, 246. Hampe, Berg- und Hiittenm. Ztg., 21, 220 ; 25, 113. Riche, Zeit. f. anal. Chem., 21, 116, Mackintosh, Am. Chem. Journ., 3, 354. Riidorff, Ber. deutsch. chem. Ges., 21, 3050 ; Zeit. f. angew. Chem., 1892, p. 5. Luckow, Zeit. f. anal. Chem., 8, 23. Warwick, Zeit. f. anorg. Chem., 1, 285. Smith, Am. Chem. Journ., 12, 329. Croasdale, Journ. of Anal, and Appl. Chem., 5, 133. Foote, Am. Chem. Journ., 6, 333. Meeker, Journ. of Anal, and Appl. Chem., 6. 267. Classen, Ber. deutsch. chem. Ges., 27, 2060. Heidenreich, ibid., 29, 1585. Regelsberger, Zeit. f. angew. Chemie (1891), 16, 473. Oettel, Chemiker-Zeitung, 1894, p. 879. Schweder, Berg- und Huttenmann. Ztg., 36 (5) 11, 21. / If copper be reduced from a solution containing an excess of ammonium oxalate, it is not always possible to obtain the metal in a compact form. For this reason the author, as long ago as 1888,* began experiments on the determination of this metal from a solution of the acid double oxalate. Further experiments in this direction have shown that coher- ent, bright red copper precipitates can be obtained when cop- per is reduced from such solutions at a temperature of about 80. The solution containing the copper is treated with a cold-saturated solution of ammonium oxalate, heated as di- rected, and at first electrolyzed for a few minutes without the addition of oxalic acid. A cold- saturated oxalic acid solu- tion is then run in from a burette. The method of proced- ure here is similar to that described under Zinc (page 147). In the analysis of substances low in copper, the solution may be made acid at the start ; in concentrated solutions, on the contrary, the electrolysis must be conducted in solutions *Ber. deutsch. chem. Ges., 21, 2898. COPPER. 155 which are as nearly neutral as possible, since otherwise diffi- cultly soluble oxalate of copper will separate out, owing to the free oxalic acid present. The end of the reaction is de- termined by testing with potassium ferrocyanide a small por- tion of the solution strongly acidified with hydrochloric acid. The precipitate must be washed without stopping the current. The metal is dried in an air-bath after treating with water and alcohol. The precipitated copper has a bright red color, adheres firmly to the dish, and has little resemblance to the copper precipitated from nitric acid solutions (see below). The chief advantage of this method is the rapidity with which it may be conducted. CONDITIONS OF EXPERIMENT. Temperature of the liquid : 80. Current density:, JSD 100 = 0.5-1 amp. Most favorable current density ND 100 = 1 amp. Electrode tension: 2.5-3.2 volts. Time of electrolysis; 2 hours. EXPERIMENT. * Used 1 g copper sulphate, 4 g ammonium oxalate, 120 cc liquid. CUr Amp?r e e n s Sity ' Elect ^J ensioa < Temp. Time. Found. Taken. 1.0-0.8. 2.8-3.2 80 2 hr. 0.2531 g 0.2529 g 0.45-0.35 2.5-2.8 80 2J " 0.2528 " 0.2529 " Copper precipitate bright red. As has been observed by Luckow, copper may also be precipitated from a solution to which nitric acid has been added. * Separation of copper from a solution of the ammonium double oxalate in the presence of free oxalic acid. 156 QUANTITATIVE ANALYSIS BY ELECTROLYSIS. The reduction of copper from a nitric acid solution de- pends upon the presence of a certain quantity of nitric acid and the absence of chlorides. To about 200ccof solution, containing the copper as sulphate, 20 cc of nitric acid * (sp. g. = 1.21) are added and the liquid is subjected to electrolysis. The end of the reaction is determined with ammonia. The presence of chlorides is to be avoided. In the pres- ence of antimony, arsenic, mercury, silver, tin, and bismuth, traces of these metals come down with the copper ; but iron, cobalt, nickel, cadmium, manganese, and zinc can be separated from copper by this method. According to the researches of Schroder large quantities of iron are detrimental, since a secondary reaction may take place between the ferric salt formed and the precipitated cop- per, which causes the copper to redissolve. Copper separates in a crystalline form from solutions warmed to 50-60 ; it is nevertheless impossible to separate the last traces of copper at this temperature. CONDITIONS OF EXPERIMENT. Temperature of solution : 20-30. Current density: ND 100 = 0.5-1 amp. The latter only when no other metal than copper is present in the solution. Electrode tension : 2.2-2.5 volts. Time : 4-5 hours. Agitating the solution with a stirring attachment hastens the operation. EXPERIMENT. Used about 1 g copper sulphate and 5$ by volume nitric acid. Entire volume of liquid 120 cc. * Such a large quantity of nitric acid is required only when the separa- tion of copper from other metals is to be carried out. If no other metal than copper is present in the solution, 2 or 3 per cent by volume of nitric acid is sufficient. COPPER. 157 Electro ^J t e s nsion ' Temp. Time. Found. Taken. 1.1-1.0 2.2-2.5 25-30 5 hr. 0.2490 g Cu 0.2495 gCu 1.0-0.95 2.25-2.3 30-32 5" 0.2505"" 0.2510"" A solution containing free nitric acid may also be used for separating such metals as are not reduced in the presence of this acid, or which are set free at the positive electrode in the form of peroxides. In such cases, however, it must be kept in mind that the nitric acid is gradually converted into ammonia, on account of which, after the current has acted for some time, nitric acid must be occasionally added. Copper may be separated from a solution containing am- monium oxalate or one containing free nitric acid, in the presence of small quantities of antimony and arsenic. If, however, the amounts of the latter are considerable, then, after continued action of the current, antimony and arsenic are deposited upon the copper, causing the negative electrode to appear more or less dark-colored. In order to determine the copper in such cases, the dried electrode is ignited for a short time, as a result of which the copper is oxidized and the antimony and arsenic are driven off. The residue of oxide is dissolved in nitric acid and again submitted to elec- trolysis.* In general the presence of chlorides causes the copper to separate in a spongy condition. To avert this action and to secure an adherent precipitate, Riidorff adds 2-3 g ammonium nitrate and 20 cc ammonia (sp. g. 0.96), dilutes with water to 100 cc, and electrolyzes this solution. At the close of the re- duction the solution is acidified with dilute acetic acid, the dish filled to overflowing with water, emptied, shaken to re- move the last drops of water, and dried at 100 in the air- bath. In the laboratory of the Technical High School at Munich * Mansfeld'scbe Hiittendirection. 158 QUANTITATIVE ANALYSIS BY ELECTROLYSIS. the preceding method is carried out under the following con- ditions : Ammonia is added in slight excess until the precipi- tate which at first appears is redissolved. Then 20-2 5 cc ammonia, sp. g. 0.96, are added, in case not more than 0.5g copper is present.* In this solution 3-5 g ammonium nitrate are dissolved and the electrolysis is conducted with a current of OT) JOU 2 amperes. The precipitate must he washed without interrupting the current. Oettel, who also carried out experiments on the quantita- tive determination of copper from ammoniacal solutions, found that by the addition of ammonium nitmte 0.2-0. 25 g of copper sulphate were quantitatively reduced in 6-8 hours at ordinary temperatures. The results of his investigation are : " 1 . That copper can be separated in a compact form from weakly ammoniacal solutions containing ammonium nitrate, by currents of ]STD 100 = 0.07-0.27 ampere. With too little ammonium nitrate, as well as in the presence of large quantities of free ammonia, the precipitate shows a tendency to a spongy structure. "2. The highest concentration of the solution is 0.8 g copper per 100 sq. cm. electrode surface, with the employ- ment of a wire-shaped positive electrode. "3. The presence of chlorine, zinc, arsenic, and small quantities of antimony is without detrimental action ; when the solution contains lead, bismuth, mercury, cadmium, or nickel, the results of the determinations are somewhat too high." E. F. Smith f precipitates copper from a solution which contains sodium phosphate and free phosphoric acid. Heidenreich, who tested the method in the Aachen laboratory, obtained the following results. * If as much as 1 g Cu is present, the quantity of ammonia is increased to 30-35 cc. f Electrochemical Analysis, p. 92. COPPER. 159 EXPERIMENT. 100 cc of a solution of NaJIPO 4 (1.0358 g) and 3.o cc of a solution of phosphoric acid (1.347 g) were diluted with water to 110 cc. To this solution the copper solution was added. Taken. Volts. Time. Found.* 0.3959 g 2.4-2.6 17 hr. 25.41 0.3982 " 2.4-2.6 17 " 25.26 " The copper separated at first brilliantly metallic, but as the electrolysis proceeded it became dark red and spongy. Variations of the conditions, such as increasing the tension, led to no better results. Owing to the spongy condition of the precipitate, the results came too high. For the special determination of copper, in copper-alumin- ium alloys, liegelsberger suggests dissolving 3-5 g of the alloy in nitric acid and evaporating the solution down to the consistency of sirup. The sample is diluted, and a measured quantity (corresponding to 0.6-1 g substance) is poured into the electrolytic cell. An excellent precipitate is obtained if the acid solution is neutralized with ammonia and 10 cc of dilute nitric acid (sp. g. 1.2) are added to 200 cc of the liquid. The clear solution is electrolyzed with a current den- sity ND IOO OA amp. When the solution is warmed the separation is completed in about three hours. A rapid and accurate method for the determination of copper has been worked out by Carl Engels in the Aachen laboratory. This method has the advantage over the use of nitric acid solutions that it can be more rapidly per- formed, and that, in separations, it also dispenses with the tedious conversion of the nitrates into sulphates. This method is based upon the addition of urea. The separation of copper from solutions containing sul- * [Theory 25.33# Cu.] 160 QUANTITATIVE ANALYSIS BY ELECTKOLYSIS. phuric acid is possible also if hydroxylamine be added. The method is as follows : If the separation is to be carried out with weak currents, say during the night, the addition of 2 cc concentrated sul- phuric acid and about J- g hydroxylamine sulphate is recom- mended. A fine crystalline precipitate and absolutely accurate results are obtained with a current strength of OT3 100 = 0.08-0.18 ampere. The tension at the poles of a shunt cir- cuit was 1.8-2.2 volts; after connecting the dish the tension sank to 1.1-1.3 volts, with a current of 0.1-0.2 amp. EXPERIMENT. Taken Current Density Tension, T , Found. p ^ CuS0 4 .5H 2 O. 'ND 100 . Volts. me ' Cu. 1.0130g. 0.1 amp. 1.1 Night 0.2574 25.41 1.7065 " 0.12 " 1.3 " 0.4335 25.40 1.1893 " 0.1 " 1.2 " 0.3021 25.41 If stronger currents are used, the amount of sulphuric acid must be increased. 1015 cc of cone, sulphuric acid are poured into the solution of the salt, it is diluted to 150 cc, and 1 g hydroxylamine sulphate is added. If 0.3-0.5 g Cu is present, with a current strength of ND JOO 1 amp., the separation is finished in 1^ to 2 hours. The condition of the precipitated copper is much better and much more suited for quantitative determination than the copper obtained under similar conditions without the addition of hydroxylamine. Urea exerts a far more satisfactory action than hydroxyl- amine upon the separation of copper from solutions contain- ing sulphuric acid. "With a current strength of OT) ]00 = 1 ampere, not the slightest tendency toward a spongy separa- tion is exhibited, but a bright-red crystalline coating is obtained on the negative electrode. The analysis, with the stated current density, is completed in 1J hours. 10-15 cc concentrated sulphuric acid and 1 g urea are * [Theory 25.33$ Cu.] .;urreui ueusity NDjooi Amp. JLVU81OU, Volts. Temp. Time. Found.* 1.05 3.1 25 1 hr. 15 m. 25.09 % 1.2 3.1 55 1 " 15 " 25.09" 0.75 2.7 65 1 " 45 " 25.09" COPPER. 161 added to the solution of the copper, which is then diluted to 150 cc. CONDITIONS OF EXPERIMENT. Temperature of liquid : Most suitable, 60-70. Electrode tension : 2.7-3.1 volts. Current density: KD 100 = 0.8-1 amp. Time : 1 J- hours. EXPERIMENT. Used CuSO 4 .5H 2 O. Quantsub 1.1364 0.9671 1.3972 The current may be interrupted in washing the precipitate. The separated copper contains traces of carbon, and also plat- inum which dissolves from the anode. These admixtures can be determined by dissolving the copper in dilute nitric acid (1 : 10). A thin dark coating remains on the dish, which may be washed with water, but not with alcohol, with- out becoming loosened. The weight of the dish, determined after washing and drying in the air-bath, is used as a basis for calculating the weight of the separated copper. With weaker currents the length of time required is of course greater. With a current density of ND 100 = 0.2 am- pere, the precipitation of from 0.3 to 0.4 g Cu is completed in 3- 4 hours. It is desirable in this case to add less sul- phuric acid to the solution; 5 cc cone. H a SO 4 to each 150 cc, is the proper proportion. Four dishes were connected in parallel, and for every 150 cc of solution of the copper salt which they contained 1 g urea and 5 cc cone. H,SO 4 were added. The four electroly- * [Theory 25.33^] 162 QUANTITATIVE ANALYSIS BY ELECTROLYSIS. ses were then conducted in the cold, with the current from a thermopile. The entire current strength was !ND 100 = 0.8 ampere, so that each dish received a current of ND 100 = 0.2 ampere. The analyses were completed in 4 hours. The per cent, of copper in the salt used was 25.08. Used CuSO 4 .5H 2 O. Found Cu. Found %. l.OlOlg 0.2533g 25.07 1.0815 " 0.2709 " 25.05 1.0320 " 0.2589 " 25.08 1.0111 " 0.2535 " 25.07 BISMUTH. LITERATURE : Luckow, Zeit. f. anal. Chem., 19, 16. Classen and v. Reiss, Ber. deutsch. chem. Ges., 14, 1622. Thomas and Smith, Am. Chem. Journ., 5, 114. Moore, Chem. News, 53, 209. Smith and Knerr, Am. Chem. Journ., 8, 206. Schucht, Zeit, f. anal. Chem., 22, 492. Eliasberg, Ber. deutsch. chem. Ges. , 19, 326. Brand, Zeit, f. anal. Chem., 28, 596. Vortmann, Ber. deutsch. chem. Ges., 24, 2749. Kiidorff, Zeit. f. angew. Chem., 1892, p. 199. Smith andi Saltar, Zeit. f. anorg. Chem., 3, 418. Smith and Moyer, Journ. of the Am. Chem. Soc., 15, 28, 101. Smith and Knerr, Am. Chem. Journ., 8, 206. Schmucker, Zeit. f. anorg. Chem., 5, 199. Up to the present time it has been found impossible to quantitatively precipitate bismuth in a compact metallic form. It separates in a more or less spongy form from all its com- pounds. A discussion of the directions given by the different investigators will therefore be omitted. G. Vortmann has attempted to separate bismuth as an amalgam. Since, however, the directions for the conditions of experiment are not given, the mere mention of this method will be sufficient. CADMIUM. 163 CADMIUM. LITERATURE : Smith, Am. Phil. Soc. Pr., 1878. Clarke, Zeit. f. anal. Chem., 18, 104. Beilstein and Jawein, Ber. deutsch. chem. Ges., 12, 759. Smith, Am. Chem. Journ., 2, 43. Luckow, Zeit. f. anal. Chem., 19, 16. Wrightson, ibid., 15, 303. Classen and v. Reiss, Ber. deutsch. chem. Ges., 14, 1638, Warwick, Zeit. f. anorg. Chem., 1, 258, 291. Moore, Chem. News, 53, 209. Smith, Am. Chem. Journ., 12, 329. Vortmann, Ber. deutsch. chem. Ges., 24, 2749. Eiidorff, Ztschr. f. angew. Chem., 1892. Classen, Ber. deutsch. chem. Ges., 27, 2060. Heidenreich, ibid., 29, 1586. The separation* of this metal in a compact brilliant form has been shown, by experiments carried out at the Aachen laboratory, not to be possible by any of the methods hitherto described. It may bo accomplished, however, by the elec- trolysis of a warm solution of the double oxalate which is kept acid with oxalic acid during the electrolysis. (A cold- saturated solution of oxalic acid is employed.) (See direc- tions for Zinc.) To prepare the double salt, the cadmium compound is dissolved in 20-25 cc water, by warming in a platinum dish ; a hot solution, which should be previously filtered, of 10 g ammonium oxalate in 80-100 cc water is added and the solution is electrolyzed. As soon as the action of the current has begun, several cubic centimeters of oxalic acid are poured upon the watch-glass covering the dish, and the liquid is kept * The metallic condition of the precipitated cadmium, all the conditions of the experiment being preserved, depends upon the absolute cleanliness of the surface of the cathode. 164 QUANTITATIVE ANALYSIS BY ELECTROLYSIS. weakly acid during the electrolysis. The end of the reaction is determined with hydrogen sulphide, by testing a small por- tion of the solution acidified with hydrochloric acid. The metal must be washed without interrupting the current. (Method of the author.) CONDITIONS OF EXPERIMENT. Temperature of the liquid : 70-75. Current density: !ND 100 = 0.5-1 amp. Electrode tension: 3-3.4 volts. Period of electrolysis : About 3 hours. Maximum quantity of metal which can be precipitated : 0.15-0.16 g. EXPERIMENT. Used 0.3-0.4 g cadmium sulphate, 10 g ammonium oxa- late, 120 cc liquid. CU Tmper e e n S Sity ' Electr ^ o e lt T s ension ' Temp. Time. Found. 0.6-0.5 2.75-3.4 73-76 3 hr. 30m. 0.1472 g 49.07 #* 1.0-0.8 3.0-3.4 68-73 3" 0.1474 g 49.13" Precipitate brilliantly lustrous. "With regard to further experiments by this method and for data on the use of polished and roughened dishes, of which the former are in this case to be preferred, the dissertation of N. Eisenberg f should be consulted. Smith and Luckow recommend the precipitation of cad- mium from a solution of the chloride or sulphate, which has been saturated with sodium acetate. Eliasberg, who tested this method in the Aachen laboratory, found that the reduc- tion took place readily when the solution, of about 100 cc volume, was treated with about 3 g sodium acetate and a few *[The salt taken was probably CdSO 4 .H 2 O containing 49.67$ Cd. Trans.] f Eisenberg, Inaugural-Dissertation, Heidelberg, 1895 CADMIUM. 165 drops of acetic acid, and the electrolysis wab carried out at a temperature of 40-50. In the laboratory of the Munich High School the forego- ing method is practised as follows : The solution, neutralized if necessary, containing not more than 0.5 g cadmium, is treated with 3 g sodium acetate, and made weakly acid with acetic acid. The solution is warmed to 45, and decomposed with a current of ]SD 10o 0.02-0.07 ampere. The metal is washed without interrupting the current, and quickly dried at 100. During the electrolysis the solution should not be warmed above 50, on account of the formation of basic salts. Cad- mium is only partly precipitated from solutions strongly acidified with acetic acid. By this method 0.2 g of cadmium may be separated in about five hours. The presence of nitrates is detrimental. According to Beilstein and Jawein, the determination of cadmium may be conducted from a solution of the double salt with potassium cyanide. Aside from the fact that the necessary directions are not given, this method possesses no advantages, the precipitation of 0.2 g cadmium requiring about 12 hours. Vortmann attempted the determination of cadmium by a method similar to that used for the determination of bismuth and zinc, by precipitation from a solution of the ammonium double salt in the form of amalgam. E. F. Smith determines cadmium by dissolving the oxide in acetic acid, evaporating, taking up in water, and electro- lyzing the solution thus obtained. Heidenreich, who carried out in the Aachen laboratory a series of varied researches on this subject, obtained no satisfactory results, either in the condition of the precipitate or in the quantitative separation of the metal. 166 QUANTITATIVE ANALYSIS BY ELECTROLYSIS." A further method by E. Smith depends upon the reduc- tion of cadmium from a solution to which sodium phosphate and free phosphoric acid have been added. Here also the quanti- tative separation of the cadmium does not take place, not even when the current is increased to 1 ampere. Finally, Smith * has proposed a method for the separation of cadmium from a solution containing free acetic acid. According to the statements of Heidenreich, cadmium is pre- cipitated from a solution containing 10 cc acetic acid (50$) to 120 cc of solution, by a current density of 0.4 ampere and a tension of 4.5 volts, in the form of small crystalline plates which cannot be washed without loss. Variations of the conditions of experiment (addition of less acetic acid [2-10 cc], employment of current densities of from 0.1 to 0.4 ampere and tensions of 4-7. 5 volts, as well as different temperatures) gave no satisfactory results. LEAD. * LITERATURE : Kiliani, Berg- u. Hiittenin.-Zeitung, 1883, p. 253. Luckow, Zeit. f. anal. Chem., 19, 215. Kiche, Ann. d. Chim. et Phys., 13, 508; Zeit. f.anal. Chem., 21, 117. Classen, Zeit. f. anal. Chem., 21, 257. Hampe, Zeit. f. anal. Chem., 13, 183. Parodi and Mascazzini, Ber. deutsch. chem. Ges., 10, 1098; Zeit. f. anal. Chem., 16, 469; 18, 588. Kiche, Zeit. f. anal. Chem,, 17, 219. Schucht, Zeit. f. anal. Chem., 21, 488. Tenny, Am. Chem. Journ., 5, 413. Smith, Am. Phil. Soc. Pr., 24, 428. Vortmann, Ber. deutsch. chem. Ges., 24, 2749. RMorff, Zeit. f. angew. Chem., 1892, p. 198. * Electrochemical Analysis, p. 95. LEAD. 167 Warwick, Zeit. f. anorg. Chem., 1, 258. Classen, Ber. deutsch. chem. Ges., 27, 163. Kreichgauer, ibid., 27, 315; Zeit. f. anorg. Chem., 9, 89. Classen, Ber. deutsch. chem. Ges., 27, 2060. Medicus, ibid., 25, 2490. Neumann, Chemiker-Zeitung, 1896, p. 381. If a solution of a lead salt containing an excess of ammo- nium oxalate be electrolyzed warm, the lead separates at the negative electrode, adheres closely, and shows its char- acteristic metallic properties ; but it oxidizes partially on wash- ing with water and alcohol, so that the results are always too high. The precipitation of lead as amalgam presents some difficulties, inasmuch as some lead peroxide separates at the positive electrode and must be dissolved. According to G. Yortmann, the aqueous solution of the lead salt, containing sufficient mercuric chloride to produce the amalgam, is treated with 3-5 g sodium acetate and a few cubic centimeters of concentrated potassium nitrite solution. The precipitate pro- duced by the latter reagent (which is added to prevent the formation of peroxide) is dissolved in acetic acid, and the clear yellow solution diluted and electrolyzed. If lead per- oxide appears on the positive electrode during the reaction, more potassium nitrite is added. The close of the reaction is determined by testing with ammonium sulphide. As lead amalgam oxidizes rather readily when moist, it is quickly washed with water, alcohol, and ether, dried by the warmth of the hand and by blowing upon it, and finally in the desic- cator. The amalgam may also be separated from an aqueous solu- tion acidified with nitric acid. However, as free nitric acid favors the formation of lead peroxide, more frequent addition of potassium nitrite is necessary, and complete precipitation is thereby seriously hindered. 168 QUANTITATIVE ANALYSIS BY ELECTROLYSIS. In a solution containing free nitric acid, lead is acted on like manganese; it is oxidized, and separates as hydrated peroxide at the positive electrode. If there is no other metal in the solution, it must contain at least 10 per cent free nitric acid, according to Luckow ; in the presence of other metals {mercury, copper, etc.), the oxidation is complete even in presence of little nitric acid. In the Munich laboratory experiments have been con- ducted as to the quantity of nitric acid, sp. gr. 1.36, and have demonstrated that this depends on the temperature and the current density which is used. The current density depends in turn on the condition of the surface of the positive electrode. If this is very smooth, a current of ND 100 = 0.05 is sufficient, otherwise one of NDi 00 = 0.5 is needed to pro- duce an adherent precipitate. When ND ]00 0.05 ampere, 2 per cent by volume of nitric acid should be added when the solution is heated, and 10 per cent by volume at ordinary temperatures. "When ED 100 = 0.5 the vohime-percentages are, respectively, 7 and 20 for heated and cool solutions. Heating the solution to about 50 materially assists the separation. ' The precipitate may be washed without loss, after the current is interrupted. Chlorine compounds urns*, not be present in the solution for electrolysis. Even when the stated conditions are observed, the quan- tity of lead which can be precipitated as peroxide in an adherent form is relatively small.* The rapid separation of large quantities of lead peroxide, firmly adherent like a metal, may only be carried out without difficulty, as the author's * From experience in the Aachen laboratory, the greatest possible quan- tity is 0.15 gPbO 2 per 100 sq. era. surface, while according to the statements of Dr. Cohen (Chem. Ztg., 1893, No. 98) as much as 0.3 g can be precip- itated. LEAD. 169 researches have shown, when the inside of the platinum dish serving as anode is roughened with a sand-blast.* By the use of such dishes, it is possible, with a current of 1.5 ampere, to precipitate in a few hours as much as 4 g of lead peroxide on 100 sq. cm. of surface. For conducting the determination of lead, after the solu- tion of the lead salt has been accomplished, 20 cc nitric acid (sp. g. 1.35-1.38) are added, the solution is diluted to about 100 cc, warmed to 60-65, and electrolyzed with a current of ND 100 = 1.5-1.7 amperes. If the warming is continued during the electrolysis, the precipitation of quantities up to 1 . 5 g lead peroxide is completed in about 3 hours ; with larger quantities in about 4-5 hours. Complete precipitation is insured by adding about 20 cc of water and observing whether the freshly wetted surface of the electrode becomes darker. Incase no blackening is observed at the end of 10-15 minutes, the current is stopped, and the precipitate is washed with water and alcohol, and dried at 180-190. The residue is anhydrous peroxide. CONDITIONS OF EXPERIMENT. Temperature of the liquid: 60-65. Current density: ND JOO = 1.51.7 amp. Electrode tension: 2.5 volts. The tension is without influence upon the condition of the peroxide, and may vary within wide limits. EXPERIMENT. Used Pb (NO 3 ) a dissolved in 100 cc water, with the ad- dition of 20 cc nitric acid (sp g. 1.35-1.38). * The platinum refinery of G. Siebert in Hanau has faultlessly carried out the roughening in the desired manner at the request of the author. Such roughened dishes are of course applicable to all other electrolytic determinations. 170 QUANTITATIVE ANALYSIS BY ELECTROLYSIS. Current Density, Amperes. Electrode Tension, Volts. Temp. Time. Found.* 1.55-1.45 2.43-2.4 60-65 1 hr. 5 m. 72.20 % 1.6 -1.58 2.48-2.43 60-65 1 " 10 " 72.19" 1.6 -1.65 2.41-2.36 60-65 1 " 5 " 72.20" The preceding method permits of the separation of lead from zinc, iron, nickel, cobalt, manganese, copper, cadmium, gold, mercury, antimony, and aluminium ; in the presence of silver and bismuth, traces of these metals in the form of peroxides pass over into the lead peroxide. THALLIUM. Tins metal may be completely precipitated from an am- monium oxalate solution. The properties of thallium, however, are similar to those of lead ; its determination therefore requires special consideration. G. Neumann, in connection with a research on certain double salts of thallium in the Aachen laboratory, has also investigated the quantitative determination of the metal. As his method is of value in the investigation of thallium compounds, it is here described. The process is based on precipitation of the thallium as metal, and determination of the volume of hydrogen set free by its solution in hydrochloric acid. The apparatus shown in Fig. K is a flask of about 100 cc FIG. 85. 85 is used for the process. * Theory 7221 THALLIUM. 171 capacity, containing platinum- foil electrodes of 9 sq. cm. surface, terminating in con tact- wires fused into the glass. The thallium salt and about 5 g ammonium oxalate are dis- solved in this flask, and electrolyzed, after dilution, with a current of 0.1 ampere. The completion of the reaction is ascertained by testing with ammonium sulphide. As the FIG. 86. ammonium oxalate is converted into carbonate by the current, and the measuring-tube would be insufficient to contain the disengaged carbon dioxide, the solution remaining in the flask is removed after the reaction. This may readily be done by the use of two siphons. Neumann's automatic arrangement for this purpose is shown in Fig. 86 ; it is very convenient where many determinations are to be per- 172 QUANTITATIVE ANALYSIS BY ELECTROLYSIS. formed, and its operation is easily seen from the figure. The washing is conducted without interrupting the current. To remove the gas-bubbles clinging to the electrode it is desirable to heat the flask a short time after the washing is complete. The flask is then connected to the measuring- tube, the thallium dissolved, and the hydrogen collected ani measured in the usual way, SILVER. LITERATURE : Luckow, Dingl. polyt. Journ., 178, 43 ; Zeit. f. anal. Chem., 19, 15. Fresenius and Bergmann, Zeit. f. anal. Chem., 19, 324, Krutwig, Ber. deutsch. chem. Ges., 15, 1267. Schucht, Zeit. f. anal. Chem., 22, 417. Einnicutt, Am. Chem. Journ., 4, 22. Riidorff, Zeit. f. angew. Chem., 1892, p. 5. Eisenberg, Dissertation. Heidelberg, 1895. Of the methods proposed for the determination of silver, the one suggested by Luckow (separation of the silver from the potassium double cyanide) is probably the most suitable. If insoluble silver compounds (silver chloride, silver oxalate) are to be analyzed, they are dissolved in potassium cyanide solution. For conducting the method, 3 g potassium cyanide, are added to the solution, which is then diluted to 100-120 cc. Eisenberg, who tested the method in the Aachen laboratory, was convinced that the success of the same, as well as the metallic condition of the precipitated silver, depends upon the purity of the potassium cyanide used. Even the so-called " purissimum " potassium cyanide of commerce is unsuited. It is therefore desirable to prepare pure potassium cyanide by passing hydrocyanic acid gas into an alcoholic solution of potassium hydroxide. SILVER. 173 CONDITIONS OF EXPERIMENT, Temperature of the liquid : 20-30. Current density: ND 100 0.2-0.5 amp. Electrode tension : 3.7-4.8 volts. Time of electrolysis : In the presence of equal quantities of silver, with current densities ND 100 = 0.2-0.5 ampere, this varies from 5 to 1 j- hours. For this determination, roughened dishes give best results. EXPERIMENT. (a) Experiment with the so-called " purissimum " potas- sium cyanide of commerce. Used silver sulphate. C,,KO* Current Electrode Condition Dsc - Density, Tension, Temp. Time. Found.* of Remark, taken. Amp ' Vokg Metal 0.8660 0.4-0.2 3.35 23-24 6 hr. 68. 91# } Not firmly j Rough dish. 0.8660 0.3-0.1 3.55 22-24 6" 68. 91 ") adherent ( Polished " (b) Experiment with pure potassium cyanide. Used silver sulphate. O,,K-- Current Electrode Condition Dsr - Density, Tens., Temp. Time. Found. of Remark, taken. Amp /' volts! Metal. 0.4369 0.3-0.23 3.72 22 5 hr. 69.08$ ) Firmly (Roughened 0.4370 0.52-0.54 4.6-4.8 20-30 1 " 40 m. 69.00 "J adherent 1 dish. 0.4369 0.32-0.21 3.70 22 5" 69.12") (( i Polished 0.8742 0.55-0.53 4.0 23 2 " 40 " 68.98 " f \ dish. J. Krutwig treats the solution of the silver salt with ammonia in slight excess, adds ammonium sulphate, and electrolyzes. In the Munich laboratory the following conditions have been determined for the preceding process. The solution, which must not contain more than 0.5 g silver, is treated * [Theory 69.22* Ag.] 174 QUANTITATIVE ANALYSIS BY ELECTROLYSIS. with 20 per cent by volume of ammonia (sp. gr. 0.96) and 5$ ammonium sulphate solution (1 : 10), warmed, and electrolyzed with a current of ND 100 = 0.02 0.05 ampere. After the current is stopped the precipitate must be very thoroughly washed to completely remove the ammonium sulphate. Fresenius and Bergmann have found that silver can also be precipitated in a dense form from a solution containing nitric acid: 20 cc of nitric acid (sp. g. 1.2) are added to the silver solution, which is then diluted with water to about 200 cc and electrolyzed. According to results in the Munich laboratory, it is desir- able to add to the solution, which may contain as much as OA g silver, 3 per cent by volume of nitric acid, sp. gr. 1.36, and to electrolyze the heated solution with a current of ND ]00 = 0.04-0.05 ampere. The silver must be carefully washed without interrupting the current, to prevent loss. An insuf- ficient quantity of nitric acid may lead to the formation of peroxide. MERCURY. LITERATURE : Clarke, Am. Journ. of Sci. and Arts, 6, 200. Classen and Ludwig, Ber. deutsch. chem. Ges., 19, 323. Hoskinson, Am. Chem. Journ., 8, 209. Smith and Knerr, ibid., 8, 206. Smith and Frankel, Am. Chem. Journ., 11, 264. Smith, Journ. Anal. Chem., 5. 202. Vortmann, Ber. deutsch. chem. Ges., 24, 2749. Brandt, Zeit. f. angew. Chem., 1891, p. 202. Riidorff, ibid., 1892, p. 5. Eisenberg, Dissertation. Heidelberg, 1895. Frankel, Journ. Franklin Inst., 131, 144. Rising and Lenher, Berg* und Hiittenm. Ztg., 55, 175. The metal can be readily separated from solutions of the mercuric salts to which 4-5 g ammonium oxalate have been MERCURY. 175 added (method of the author). If the mercury is present as chloride in the solution, the electrolysis is continued until mercurous chloride disappears from the positive electrode^ CONDITIONS OF EXPERIMENT. Temperature of liquid : Ordinary temperatures. Current density: NT) 100 = 0.1-1.0 amp. Electrode tension: 2.5-5.5 volts. Time : Dependent on the current density. Roughened dishes are preferable to polished, on account of the more uniform distribution and firmer adherence of the mercury to the cathode. On polished dishes the mercury separates in the form of small globules. EXPERIMENT. Subst. used. HgCl a . g- Current Density, Amperes. Electrode Tension, Voles. Temp. Time, hrs. m. Found '* Remark. 0.4068 0.2 -0.15 2.6 -3.35 30-23 5 15 73.74) 0.4073 1.02-0.93 4.05-4.75 29-37 1 30 73.63 0.4076 1.08-0.92 4.42-4.88 25-40 2 5 73.77 Roughened 0.4080 1.15-1.09 4.97-5.05 18-40.5 2 5 73.87 Dish. 0.4080 1.12-0.93 4.95-4.85 18-38 2 5 73.84 0.4080 1.52-0.48 3.65-4.65 16-27 3 55 73.67- 1 0.4070 0.2 -0.23 2.89-3.75 28^24 5 15 73.80 0.4073 1.06-0.95 4.45-5.00 30-39.5 1 30 73.29 0.4076 1.16-1.09 5.32-5.53 23-40 2 5 73.93 Polished 0.4080 1.20-0.99 4.70-4.90 18-43 3 73.55 Dish. 0.4075 1.51-0.48 3.87-4.50 16-30 3 55 73.85 Mercury may also be quantitatively precipitated from a solution containing nitric, sulphuric, or hydrochloric acid. If no other metal than mercury is present, 1-2 per cent by volume of nitric acid is sufficient ; while in the presence of other metals, which are not precipitated from solutions con- * [Theory 73.85# Hg.] 176 QUANTITATIVE ANALYSIS BY ELECTROLYSIS. taining free acid, 5 per cent by volume is required. In the latter case a current density not greater than 0.5 ampere is employed ; in the former the current density may be raised to 1 ampere. If hydrochloric acid is used, only a few drops are added, since larger quantities have a detrimental action on the separa- tion of the metal. Large quantities of chlorides have an action similar to that of large quantities of hydrochloric acid. Insoluble mercury compounds may be easily electrolyzed by suspending them in water slightly acidified with hydro- chloric acid, or in a dilute solution of sodium chloride (about 10 per cent). This process, originated by the author, is used at Almaden for determining the amount of mercury contained in cinnabar. Edgar F. Smith precipitates mercury from the solution of the same in potassium cyanide. The solution of the mercuric salt, which may contain about 0.2 g mercury, is decomposed with 0.25-2 g potassium cyanide, diluted with water to 175 cc and electrolyzed. Heidenreich, in the Aachen laboratory, determined the conditions of experiment for this method. CONDITIONS OF EXPERIMENT. Temperature of the liquid : Ordinary temperatures. Current density : KD 100 = 0.03-0.08. Electrode tension : 1 . 6 5-1 . T 5 volt. Time : Dependent on the current density. m , Current Dens. Tension, m._, & j * Taken - ND 100 , Amp. Volts. Time " Found.* 0.2501 g HgCU, 2-3 g KCN 0.08-0.04 1.65-1.69 5 hr. 73.61$ Hg 0.2655 " " " " " 0.03 1.75 14 " 73.50" " The metal reduced by this method must be washed with water only, and not with alcohol, since, on washing with * [Theory 73.85$ Hg.] GOLD. 177 the latter, small quantities of the mercury will become loos- ened and be carried away. GOLD. LITERATURE I Luckow, Zeit. f. anal. Chem., 19, 14. Brugnatelli, Phil. Magazin, 21, 187. Smith and Muhr, Ber. deutsch. chem. Ges., 23, 2175. Smith and "Wallace, Proceed. Chem. Soc. Franklin Inst., 3, 20. Smith, Am. Chem. Journ., 13, 206. Persoz, Annal. Chem. Pharm., 65, 164. Riidorff, Zeit. f. angew. Chem., 1892, p. 695. Gold may be separated in a compact form from solutions of gold salts in potassium cyanide. To form the double cyanide, about 3 g of potassium cyanide are added. The solution is then electrolyzed at ordinary temperatures or at temperatures between 50 and 60. Since the gold can be removed from the platinum dish with aqua regia only (an operation by which the platinum is also dissolved), platinum dishes coated with a thin deposit of silver have previously been used for this determination. According to a private commu- nication from Dr. "W. Dupre of Stassfurt, the gold may be readily removed from the platinum dishes by warming with a solution of chromic anhydride in saturated sodium chloride solution. The author can confirm this statement; in this operation gold only, and no platinum, goes into solution. Since the conditions of experiment for the separation of gold from double cyanides had not been previously determined, they were ascertained by Dr. v. Wirkner at the suggestion of the author. CONDITIONS OF EXPERIMENT. Temperature of the liquid: Ordinary temperatures; or better 50-60, since at ordinary temperatures a brownish decomposition product of potassium cyanide often separates. 178 QUANTITATIVE ANALYSIS BY ELECTROLYSIS. Current density : ND 100 = 0.3-0.8 amp. Electrode tension : 2.7-4 volts. EXPERIMENT. A solution of chloride of gold of unknown strength was used. The electrolyses were carried out in roughened plat- inum-iridium dishes without a coating of silver. Used 3 g potassium cyanide, 120 cc liquid. Taken, cc Gold Chlo- ride Sol. Current Density, Amperes. Electrode Tension, Volts. Temper- ature. Time, hr. m. 15 0.3 3.5-3.9 20-27 5 15 0.35 3.9-4.0 22-28 14 (overnight) 30 0.37 3.6-3.9 20-28 4 15 15 0.38 2.7-3.8 52-55 1 30 15 0.38 2.7-3.4 53-54 1 20 15 0.39 2.7-3.8 52-56 1 30 15 0.85 4.0-4.1 52-56 1 30 Found, g- 0.0545 0.0548 0.1099 0.0544 0.0546 0.0545 0.0544 ANTIMONY. LITERATURE I Wrightson, Zeit. f. anal. Chem., 15, 300. Parodi and Mascazzini, ibid., 18, 588. Luckow, ibid,, 19, 13. Classen and v. Reiss, Ber. deutsch. chem. Ges., 14, 1622 ; ibid., 17, 2467; 18, 1104. Lecrenier, Chemiker Zeitung, 13, 1219. Vortmann, Ber. deutsch. chem. Ges., 24, 2762. Rudorff, Zeit. f. angew. Chem., 1892, p. 199. Classen, Ber. deutsch. chem. Ges., 27, 2060. Antimony is precipitated from hydrochloric acid solution, but not in an adherent form. If potassium oxalate is added to the solution of the trichloride, antimony is easily reduced, but adheres even less closely than in the other case. An adherent metallic deposit can be obtained by adding potassium tartrate, but the separation is then too slow. ANTIMONY. 179 The precipitation of antimony from the solutions of its sulpho-salts is complete and satisfactory. If ammonium sul- phide is used to produce a double salt, it must contain neither free ammonia nor polysulphides. Ammonium hydrosulphide, therefore, is convenient for the determination ; it is kept in small, tightly corked bottles. When a solution of antimony containing ammonium sul- phide is electrolyzed, there is formed over the metal a coating of sulphur which cannot be washed off with water. When the metal is washed afterward with alcohol, the thin coating of sulphur can be removed by rubbing with the finger or a handkerchief moistened with alcohol, without danger of loss. The use of ammonium sulphide has the disadvantage that, when several determinations are made together, the odor becomes unbearable. For this reason the author has made a series of experiments with potassium and sodium monosul- phide and hydrosulphide, the results of which show that the precipitation of antimony from double salts with these com- pounds proceeds satisfactorily. As sodium sulphide (Na 2 S) is the one of the salts named which is most desirable for facilitat- ing the separation of antimony from tin and arsenic, the following particulars relate exclusively to the use of this salt * for the determination of antimony. The following equations probably represent the reactions which take place in the electrolysis of the antimony sulpho- salt. The current first decomposes water : 3H a O = 6H + 30. At the cathode : Sb a S, + 3Na a S +6H = 2Sb + 6NaHS. At the anode : 3O = 3Na 3 S a + 3H 2 O. * For the preparation of this salt, see section on Reagents. 180 QUANTITATIVE ANALYSIS BY ELECTROLYSIS. The reduction of antimony from the prepared sulpho-salt& can be carried out as well at ordinary as at higher tempera- tures. In the first case the determination requires 17-18 hours, in the latter about 2 hours. If the separation is con- ducted in polished dishes, only relatively small quantities of the metal can be made to adhere firmly to the dish, and the em- ployment of weak currents is necessary. In recent experi- ments roughened dishes have been used, and the reduction has been conducted from hot solutions and with stronger currents. To carry out the analysis, 80-100 cc of a solution of sodium monosulphide (sp. g. about 1.14) are added to the antimony solution, which is diluted with water to 120 cc, and electrolyzed. If the metal is precipitated from a warm solu- tion, it must be washed without interrupting the current. The end of the reaction can only be determined with certainty by the use of another electrode which is dipped into the liquid and brought into contact with the dish, i.e., the cathode. The dish with the separated antimony is treated in the usual way with water and perfectly pure absolute alcohol, dried for a short time in the air-bach at 80-90, and weighed. CONDITIONS OF EXPERIMENT. A. Temperature of the liquid : Ordinary temperatures. Current density: ND 100 = 0.3-0.35 ampere. Electrode tension: 1-1.8 volts. Remark : It is best to use roughened dishes only. EXPERIMENT. Used tartar emetic.* Subst. taken, Current Density, Electrode Tension, Time. Found, Condition of the g- Amp. Volts. %' Metal. 0.7892 0.7894 0.35 0.35 1.70-1.06 1.80-1.00 17 hr. 30 m. 17 " 30 " 37.84 37.80 ( bright rae- < tallic, adher- ( ent. * [Probably impure anhydrous, KSbC 4 H 4 O7 containing about 37.12$ Sb. Trans.] ANTIMONY. 181 B. Temperature of the liquid : 55-70. Current density: KD 100 = 1.0-1.5 amp. Electrode tension : 1-2 volts. Subst. taken, Current Density, Electrode Tension, Temp. Time. Found, Condition of the g. Amp. Volts. hr. m. % Metal. 0.7895 1. 00-1, 2 1 .45-1.25 55-60 2 5 37.64 ( 0.7895 1.06-1, 25 1 .35 65-70 2 15 37.57 1 bright 0.7898 1. 50 1 .42 70 1 45 37.85 J \ metallic 1.5873 1. 50 1 .80 70-80 2 30 37.84 The method of determining antimony in solutions of the polysulphides of the alkalies is very simple. The solution containing polysulphides is treated with an excess of hydrogen peroxide and heated till it becomes colorless. If a great excess of hydrogen peroxide is used, it may happen that the alkali sulphide is entirely decomposed and antimony sulphide precipitated. If the solution is entirely colorless, or if a pre- cipitate of antimony sulphide has already appeared, the solu- tion is cooled, 80 cc of a solution of sodium monosulphide are added, the whole is diluted with water to about 120-150 cc, and electrolyzed as above directed. [Chittenden and Blake * have applied the electrolytic method to the determination of very small quantities of anti- mony in a large amount of organic matter. In test experi- ments, 100 g of beef or liver were finely divided, a few cubic centimeters of a standard antimony solution added, the mixture thoroughly oxidized with hydrochloric acid and potassium chlorate, all free chlorine removed by heat, and the antimony precipitated by hydrogen sulphide. The precipitate containing, together with antimony sulphide, some sulphur and organic matter, was dissolved in cold sodium monosulphide, and directly submitted to the action of a current from four gravity cells of * Trans. Conn. Acad. Arts and Sci., 7, 276. 182 QUANTITATIVE ANALYSIS BY ELECTROLYSIS. moderate size. The electrolytic action was continued till all the organic matter and sulphur was oxidized (eighteen to forty- eight hours), and the deposited antimony washed without breaking the current. Results satisfactory, and much better than those obtained by any other process. Chittenden and Blake also found that antimony in small quantities was deposited quantitatively from urine by acidify- ing with sulphuric acid (1 cc dilute H 2 SO 4 to 25 cc urine), and submitting directly to electrolysis. The battery used was the same as before. Trans. ] PLATINUM. LITERATURE : Luckow, Zeit. f. anal. Chem., 19, 13. Classen, Ber. deutsch. cheni. Ges., 17, 2467. Smith, Am. Chem. Journ., 13, 206. Eudorff, Zeit. f. angew. Chem., 1892, p. 696. The compounds of platinum are very readily decomposed by the electric current, the metal being precipitated. Accord- ing to the determinations made by Dr. W. Gobbels in the Aachen laboratory, if a solution of a platinum salt containing 2-3 per cent by volume of sulphuric acid is used for the de- composition and is electrolyzed with a current of N"D ]00 = 0.1- 0.2 ampere, all the platinum separates in a short time in the form of platinum-black. If, however, a solution heated to 60-65 is electrolyzed with a current of ND 100 = 0.05 amp. and 1.2 volts tension, the platinum separates quantitatively and in a very compact form. The reduced metal is so dense that it cannot be distinguished from hammered platinum. If the quantity of platinum is about 0.4 g, the solution of the platinum salt, according to the practice in the Munich laboratory, is treated with 2 per cent by volume dilute sul- PALLADIUM TIN. 183 phuric acid (1:5), heated, and electrolyzed with a current of ND 100 = 0.01 0.03 ampere; the precipitation is complete in about 5 hours. Iridium is not reduced from its solutions by a current of ND 100 = 0.05 amp. and 1.2 volts tension: this property may be used for the quantitative separation of platinum, from irid- ium (Classen). PALLADIUM, LITER AT CTRE I Wohler, Lieb. Ann., 133, 357. Schucht, Zeit. f. anal. Chem., 22, 242. Smith and Knerr, Am. Chem. Journ., 12, 252. Smith, ibid., 8, 206 ; 14, 435. Palladium is determined in the same way as platinum. If a current of ND 100 = 0.05 ampere, with a tension of 1.2 volts, is used for the reduction, the palladium is obtained in an excellent metallic condition. TIN. LITERATURE I Luckow, Zeit. f. anal. Chem., 19, 13. Classen and v. Reiss, Ber. deutsch. chem. Ges., 14, 1622. Gibbs, Chem. News, 42, 291. Classen, Ber. deutsch. chem. Ges., 17, 2467 ; 18, 1104. Bongartz and Classen, ibid., 21, 2900. Rudorff, Zeit. f. angew. Chem., 1892, p. 196. Classen, Ber. deutsch. chem. Ges., 27, 2060. Engels, Zeit. f. Elektrochemie, 1895-96, p. 418. Freudenberg, Zeit. f. phys. Chem., 12, 121. Heideureich, Ber. deutsch. chem. Ges., 28, 1586. Tin separates completely from a solution containing the ammonium double oxalate, or from an ammonium sulphide solution. Sodium and potassium sulphides cannot be used, 184 QUANTITATIVE ANALYSIS BY ELECTROLYSIS. as tin separates only partially from a dilute solution of the corresponding sulpho-salt, and not at all from a concentrated solution. If tin is precipitated from the ammonium double oxalate, a separation of stannic acid readily occurs, especially when much tin is present, which must be redissolved by addition of oxalic acid. The reduction of tin may be carried out without difficulty, however, if acid ammonium oxalate is used instead of the neutral oxalate. The results obtained by this process are so accurate that the author has found it adapted to the determination of the atomic weight of tin.* The solution of tin is treated with a cold saturated solu- tion of acid ammonium oxalate in the proportion of 20 cc to 0.1 g tin. The solution is diluted to about 150 cc and elec- trolyzed. The tin is completely precipitated as a closely ad- herent, shining, silver- white metal, even when as much as 6 g is present. The current is interrupted, and the metal washed as usual with water and alcohol, and dried at 80-90. CONDITIONS OF EXPERIMENT. Temperature of liquid : Ordinary temperatures. Current density : ND 100 = 0.2-0.6 amp. Electrode tension: 2.7-3.8 volts. Time of experiment : 8-10 hours. EXPERIMENT. Used 0.9-1 g SnCl 4 .2NH 4 Cl[32.10# Sn], 120 cc of a satu- rated solution of acid ammonium oxalate. Current Density, Amperes. Electrode Tension, Volts. Temp. Time. Found. 02-0.3f 2.7-3.8 25 8 hr. 5. m. 32.062 0.3-0.6 2.8-3.8 30-35 9 " 45 " 32.00" * Bongartz and Classen, Ber. deutch. chem. Ges., 21, 2900. f Finally increased to 0.5 ampere. TIN. 185 If larger quantities of the tin salt are used, it is necessary to add acid ammonium oxalate from time to time, on account of the decomposition of the acid ammonium oxalate, which causes the solution to react alkaline. According to recent investigations, the determination of tin may be carried out by treating the solution of the tin salt with neutral ammonium oxalate to form the double salt, acidifying .with oxalic acid and electrolyzing warm. Heidenreich, who tested this method in the Aachen laboratory, found that the determination of tin can be com- pleted in 4-4^ hours. 4 g ammonium oxalate to every 0.3 g tin present are added to the solution, which is then acidified with 9-10 g oxalic acid, warmed to 60-65, and electrolyzed with a current of ND 100 = 1-1.5 amperes. The precipitate must be washed without interrupting the current. Instead of oxalic acid, acetic acid may be used ; it possesses, however, no advantages. 100 cc of a saturated solution of ammonium oxalate are added to the solution of the tin salt, which is then acidified with 25 cc acetic acid (sp. g. 1.0615; about 50$). The metal is precipitated in the form of radi- ated crystals, in contrast to the precipitate from acid ammo- nium oxalate solutions. Tin adheres better to roughened than to polished dishes. The following experiments were conducted by the acetic acid method : Current Density, Electrode Tom Timo i?rmn,i Ampere. Tens., Volts. 3mp ' 0.3 increased to 0.5 3.2-3.8 25 6 hr. 15 m. 32.00$ 0.5 " " 1.0 3.6-4.2 25-30 5 " 45 " 32.01" In these experiments the tin in the polished dishes ap- peared brilliantly crystalline, and in the roughened dishes silver- white. Since tin, like zinc, is dissolved with difficulty from the 186 QUANTITATIVE ANALYSIS BY ELECTROLYSIS. platinum dishes by acids, it is necessary to use fused acid potassium sulphate to remove it. It is therefore best to pre- cipitate the tin in coppered dishes (see Zinc). Engels worked out the following method in the Aachen laboratory : The tin salt is dissolved in water containing a few cubic centimeters of oxalic acid, and 0.3-0. 5 g hydroxyl- amine, 2 g ammonium acetate, and 2 g tartaric acid are added for every 0.5-1.2 g tin salt taken. The solution is then diluted to 150 cc. CONDITIONS OF EXPERIMENT. Temperature of liquid : 60-70. Current density: ND 100 = 0.99-1 amp. Electrode tension : 4-5 volts. Time : 3-5 hours. EXPERIMENT. SnCl 4 . 2NH 4 01, g- 0.9175 Current Density, Amp. 1-0.8 Electrode Tension, Volts. 5.2-5.6 Temp. 70 Time. 3hr. g. F Percent. Calculated. 0.2970 32.37 32.37# 0.9859 1-0.8 4.8-5.3 63 3 " 0.3195 32.40 0.9050 1-0.9 5.0-5.6 65 3 " 0.2931 32.39 1.1879 0.5 5.1-6.0 45 6 " 0.3847 32.38 1.0026 0.7 3.4 60 3 " 0.3238 32.36 0.9940 0.7 4.0 60 3" 0.3219 32.38 1.0024 0.8 4.6 60 3 " 0.3250 32.42 1.0022 0.8 4.2-4.4 60 3 " 0.3252 32.44 In the solution of the ammonium sulpho-salt tin behaves like antimony. The tin solution (if necessary after neutraliza- tion with ammonia) is treated with ammonium sulphide free from ammonia (no more is added than is needed to form the sulpho-salt), diluted to 150-175 cc, warmed to 50-60, and electrolyzed with a current of 1-2 amperes, at a tension of 3. 5- 4 volts. Under these conditions 0.3-0.4 g of tin can be reduced in an hour. Sometimes a deposit of sulphur adheres so strongly to the tin at the edge of the dish that it cannot be TIN. 187 washed off with water ; it may, however, be easily removed, after washing with alcohol, by gentle rubbing with a linen cloth. In gravimetric analysis tin is often separated from other metals by sodium sulphide instead of ammonium sulphide. In order to determine the tin electrolytically in such cases, the sodium sulphide must be converted into ammonium sul- phide.* To accomplish this, the solution is treated with about 25 g pure ammonium sulphate free from iron, and heated very carefully, with the dish covered, till the hydrogen sulphide has all escaped ; the solution is then kept in gentle ebullition for about fifteen minutes. Complete conversion into ammonium sulphide is shown by the greenish-yellow color of the solution. If the heating is continued too long, tin sulphide may separate ; it can be dissolved in ammonium sulphide. After it is completely cooled, any sodium sulphate that may have separated is dissolved by addition of water, and the solution electrolyzed. The determination of the tin is much more simply and easily accomplished by converting the solution of tin sulphide in sodium sulphide into the acid oxalate. This conversion may be accomplished in two ways ; either the sulpho-salt is decomposed with dilute sulphuric acid to remove the greater part of the sulphur as hydrogen sulphide, and the separated tin sulphide oxidized with hydrogen peroxide f until the stan- nic acid which is produced appears clear white, or the heated alkaline solution is treated directly with hydrogen peroxide (of which a great quantity is needed), then acidified with sul- phuric acid to precipitate stannic acid, neutralized with * Sodium sulphide cannot be replaced by potassium sulphide in the separation from other metals, because the latter produces difficultly soluble potassium sulphate when ammonium sulphide is formed. f Classen and Bauer, Ber. d. ch. Ges., 16, 1062. 188 QUANTITATIVE ANALYSIS BY ELECTROLYSIS. ammonia, and treated with more hydrogen peroxide. In either case the solution is heated to decompose the excess of hydrogen peroxide, and the stannic acid allowed to settle and then filtered off. The precipitate is washed with the oxalate solution from the filter into a beaker, the filter washed with hot oxalic acid solution, and the stannic acid in the beaker dissolved by heating. Sometimes there is a residue of sulphur, which is removed by filtration. The filtrate is collected in the weighed platinum dish to be used for the electrolysis, and the sulphur is washed with a cold saturated solution of am- monium oxalate or acid ammonium oxalate. The solution for electrolysis must contain at least 4 g of the oxalate. ARSENIC. LITERATURE I Luckow, Zeit. f. anal. Ohem., 19, 14. Classen and v. Reiss, Ber. deutsch. chem. Ges., 14, 1622. Moore, Chem. News, 53, 209. Vortmann, Ber. deutsch. chem. Ges., 24, 2764. Arsenic cannot be quantitatively separated either from aqueous solutions or from solutions containing hydrochloric acid, ammonium oxalate, or alkali sulphides. From aqueous as from oxalic acid solutions a part of the metal is reduced, while from hydrochloric acid solutions, if the current is al- lowed to act for a sufficient length of time, all of the arsenic passes off in the form of arseniuretted hydrogen. The behavior of arsenic (present as arsenic acid) in a con- centrated solution of sodium sulphide permits the separation of arsenic from antimony, as will be shown later. POTASSIUM, AMMONIUM. (NITROGEN.) Potassium and ammonium may be determined, as is well known, by converting them into potassium or ammonium pla- tinchloride, and weighing the precipitate, dried at 110, on a tared filter. This method, which is almost universally em- DETERMINATION OF NITKIC ACID IN NITEATES. 189 ployed in the separation of potassium from sodium, has many disadvantages. It is preferable, after precipitating and wash- ing the platinum salt as usual, to dissolve it in water, and determine the platinum as directed on p. 182. DETERMINATION OF NITRIC ACID IN NITRATES. As is well known, nitric acid is often converted into am- monia, and the latter determined. The action of the galvanic current converts nitric acid into ammonia, as explained in the Introduction (p. 3). If the solution of an alkali nitrate, acid- ified with dilute sulphuric acid, is exposed to the action of the galvanic current, no ammonia is formed. Luckow discovered that reduction of the nitric acid always takes place when a salt from which the metal is precipitated by the current is also present in the solution. Copper salts are best adapted for this purpose. G. Vortmann has deter- mined in the Aachen laboratory the conditions for the quan- titative determination of nitric acid in nitrates. The solution of the nitrate is treated with a sufficient quantity of copper sulphate (in the analysis of potassium nitrate, e.g., half as much crystallized copper sulphate as potassium nitrate), acidi- fied with dilute sulphuric acid, and electrolyzed cold. When the reaction is complete the solution is poured off, sodium hy- droxide solution is added, and the ammonia distilled off and determined volumetrically in the usual way. For this pur- pose one-fifth normal solutions of ammonia and sulphuric acid -are used. To standardize the sulphuric acid, a weighed quantity (0.5 g) of crystallized copper sulphate is decom- posed electrolytically, and the resulting free acid tit- rated with ammonia. G. Vortmann decomposed 0.4876 g CuSO 4 . 5H 2 O, and used, for the neutralization of the acid set free, 19.6 cc of ammonia of a strength equal to the one- fifth normal sulphuric acid. 1 cc of the latter corresponds therefore to 0. 0028017 g of nitrogen in the form of ammonia. 190 QUANTITATIVE ANALYSIS BY ELECTROLYSIS. DETERMINATION OF THE HALOGENS. Chlorine, Bromine, Iodine. LITERATURE : Vortmann, Monatshefte f. Chem., 15, 280; 16, 674 ; Elektrochem. Zeit., 1894 (1), p. 137. The method originated by Yortmann depends upon the principle that the halogens are set free from solutions of halogen salts by the electric current, and while in the ion state combine with a silver anode to form insoluble silver halide. The increase in weight of the anode gives directly the quantity of halogen which has separated. The comple- tion of the analysis is determined by replacing the original silver anode by a second weighed silver anode and noting its increase in weight. For an experimental test of the method, a weighed quantity of iodide is dissolved in water, 6-10 cc of a 10$ solution of sodium hydroxide added, and the solution diluted to 100-150 cc. The silver anode, having the form of a watch-glass 6 cm. in diameter, is fixed about 5 mm. from the bottom of the cop- per dish which serves as cathode. The cold solution is electrolyzed with a current strength of 0.03-0.07 ampere and a tension of 2 volts. After 4-5 hours the greater part of the iodine has been converted into silver iodide, and the remainder may be separated on a fresh silver anode, after the addition to the solution of sodium po- tassium tartrate. The liquid is warmed to 50-70 and elec- trolyzed with a current having a tension of 1.2-1.3 volts and a current strength of 0.01-0.02 ampere. SEPARATION OF METALS. 391 SEPARATION OF METALS. IRON. Iron Cobalt, LITERATURE. Classen, Ber. deutsch. chem. Ges., 27, 2060. The two metals may be determined by electrolyzing the solution of the double oxalates, as directed under Iron (p. 138), weighing the iron and cobalt together, and determin- ing the former volumetrically. After weighing the iron and cobalt, the deposit is dis- solved in dilute sulphuric acid (dilute sulphuric acid is poured over the metals, and concentrated acid gradually added, so that the solution becomes heated), and the iron is titrated in the platinum dish with potassium permanganate. To over- come the red color of cobalt sulphate, a sufficient amount of nickel sulphate is added before the titration. The end of the reaction is easily recognized. The residue of cobalt and iron may also be dissolved in hydrochloric acid, the iron oxidized with hydrogen peroxide, and titrated with stannous chloride, after removing the excess of hydrogen peroxide by boiling. EXPERIMENT. Used 1 g each of CoSO 4 .K,SO 4 .6H 9 O and Fe 3 (C a O 4 ),. 3K 2 C,O 4 .6H 2 O, and 8 g ammonium oxalate. Yolume of liquid, 120 cc. 192 QUANTITATIVE ANALYSIS BY ELECTROLYSIS. Current Electrode Found, Density, Tension, Temp. Time. & , c, n Calculated. Titrated. Amperes. Volts. 2.0-1.6 3.0-3.6 65-70 1 hr. 40 m. 0.2658 g 0.1141 g Fe* 0.1140 g Fe 0.1517 "Co 0.2658 g 1.55-1.43.2-3.6 62-65 1 " 20" 0.2650" 0.1138 g Fe 0.1140"" 0.1517 " Co 0.2655 g 1.0-0.85 2.85-3.1 60-65 2 " 30" 0.2585" 0.1137 g Fe 0.1140"" 0.1451 " Co 0.2586 g 0.5-0.4 2.0-2.7 60-67 4 " 0.2593" 0.1136 g Fe 0.1133"" 0.1452 " Co 0.2588 g 0.5-0.45 2.35-2.7 58-62 4 " 0.2617" 0.1189 g Fe 0.1141"" 0.1477 "Co 0.2616 g Iron Nickel. LITERATURE : Vortmann, Monatshefte f. Chem., 14, 536. Classen, Ber. deutsch. chem. Ges., 27, 2060. The method of determination is exactly like the preceding. Iron and nickel separate in the form of a beautiful white alloy scarcely distinguishable from the platinum. This alloy resists strongly the action of acids, and is only very slowly attacked by dilute sulphuric or hydrochloric acid. Since the precipitation of the last trace of nickel takes place very slowly, the use of a current of at least ND 100 = 1 ampere is to be recommended. Toward the end of the opera- tion the current strength should be increased to 1 ampere. To determine the iron, the precipitate in the dish must be heated with concentrated hydrochloric acid ; and if the iron is to be titrated with permanganate, the solution must be * The numbers placed under the heading " Calculated " are the quanti- ties of iron and cobalt in the two salts taken, which were separately deter- mined by electrolysis. IKON. 193 reduced by nascent hydrogen. It is more simple to oxidize with hydrogen peroxide, and, after removing the excess, titrate the ferric chloride with stannous chloride. EXPERIMENT. Used 1 g each of NiSO 4 .(NH 4 ) t SO 4 .6E t O and Fe,(C 2 O 4 ) 3 . 3K a C 2 O 4 .6H,O, and 8 g ammonium oxalate. Volume of liquid, 120 cc. Current Dens., Electrode Amperes. 2.2-1.75 Tens., Volts. 3.45-4.0 Temp. 70-65 Time, hr. m. Found. Fe4-Ni. 0.2760 g 2.0-1.75 335-3.9 69-67 2 0.2654 1.1-0.7 2.6-3.1 65-71 430 0.2675' 0.5-0.4 2.6-3.0 68-71 5 0.2664 Calculated. 0.1135 gFe* 0.1622" Ni 0.2757g 0.1135 gFe 0.1527"Ni 0.2662 g 0.1135 g Fe 0.1550"Ni 0.2683 g 0.2664 g Yortmann adds 4-6 g sodium potassium tartrate and an excess of sodium hydroxide to the solution, and precipitates the iron with a current of OT) 100 0.3-0.5 ampere in three to four hours, the nickel remaining in solution. Iron Zinc. LITERATURE : Vortmann, Monatshefte f. Chem., 14, 536. If the double oxalates of iron and zinc are submitted to electrolysis, an alloy of the two does not separate, but zinc, with a little iron, is first precipitated on the negative elec- trode. The electrolysis proceeds very satisfactorily, and the *The numbers placed under the heading " Calculated" are the quanti- ties of iron and cobalt in the two salts taken, which were separately deter- mined by electrolysis. 194 QUANTITATIVE ANALYSIS BY ELECTROLYSIS, united weight of the two metals may readily be determined if there is less than one-third as much zinc as iron in the solu- tion. If the proportion of zinc is greater, the zinc dissolves with the evolution of gas as the action proceeds, and a pre- cipitate of iron oxide is formed. Vortmann proposes the following method : Several grams of potassium sodium tartrate and an excess of a 10-20$ solu- tion of sodium hydroxide are added to the solution of the metals, and the liquid is electrolyzed at an electrode tension of 2 volts, with a current strength of KD 100 = 0.07-0.1 ampere. It is best to raise the temperature at the close of the operation to 50-60. After several hours the iron will be precipitated, the zinc remaining in solution. Iron Manganese. LITERATURE : Classen, Ber. deutsch. chem. Ges., 18, 1787. A solution of ammonium oxalate is decomposed by elec- trolysis, as stated in the introduction, mainly into hydrogen and hydrogen ammonium carbonate. The latter is partly decomposed into ammonia, most of which remains in solution, and carbon dioxide. In the electrolysis of a hot solution of ammonium oxalate, the ammonium carbonate produced by the current is partly neutralized as a result of dissociation of ammonium oxalate; carbon dioxide is rapidly liberated at the positive electrode. If a solution of the double oxalates of iron and manganese is subjected to electrolysis without the previous addition of a great excess of ammonium oxalate, the characteristic color of permanganic acid appears immediately at the positive elec trode, manganese dioxide gradually separates at the posi- tive electrode, and iron at the negative. If the electrolysis is conducted under these conditions, it is impossible to obtain IRON. 195 a quantitative separation of the two metals, since the manga- nese dioxide carries down witli it considerable quantities of ferric hydroxide. The complete separation of the metals is possible only when the separation of the manganese dioxide is delayed till most of the iron is precipitated. If a solution of the double oxalates of iron and manganese, which contains a great excess of ammonium oxalate, is electrolyzed in the cold, the greater part of the manganese dioxide is precipitated only after most of the ammonium oxalate is decomposed. In this case, however, the separation of the manganese dioxide is in- complete, because by the action of the current a considerable quantity of ammonium carbonate or ammonia is produced which acts on the manganese double salt, causing a portion of the precipitate (a mixture of dioxide and a lower oxide) to pass into solution. The rapid dissociation of ammonium oxalate when heated gives a simple means of delaying, or entirely preventing, the formation of a manganese precipitate during electrolysis. The double oxalate is prepared by the method given under iron, with the difference only that 8 to 1 g ammonium oxa- late are dissolved in the liquid, which is warmed to 70, and electrolyzed with a current of NT) 100 = 0.5 amp. When the reduction is complete, the solution is poured off, the dish washed repeatedly with water, and this, together with traces of the dioxide precipitate, removed by alcohol ; it is sometimes necessary to rub the dish gently with the finger. The preceding method gives satisfactory results when the percentage of manganese is not too high. For the analysis of manganiferous iron (f erro-manganese, for example) this method has no practical value, since the per cent of manganese is here required, while by this method the iron is determined directly and the manganese must be determined in the liquid from which the iron has been separated. 196 QUANTITATIVE ANALYSIS BY ELECTROLYSIS. To obtain a complete separation, the solution, containing suspended manganese dioxide, is heated with a solution of pure potassium or sodium hydroxide in a porcelain dish, till the ammonium carbonate produced by electrolysis is de- composed and the solution no longer has the odor of ammonia ; and then sodium carbonate and a small quantity of sodium hypochlorite, or, better, hydrogen peroxide, are added. The manganese dioxide quickly falls to the bottom, and can be filtered off. The precipitate is best washed with hot water to which a little ammonium nitrate has been added, and is either converted into mangano-manganic oxide (Mn 3 O 4 ) by ignition, or, better, into manganese sulphate (MnSO 4 ). The conversion into manganese sulphate is accomplished by moistening the precipitate in the crucible with a little pure concentrated sulphuric acid, arid igniting very gently, so that the bottom of the crucible is barely red. If it is desired to determine the manganese as manganese sulphide, the solution is boiled till the ammonium carbonate is decomposed, the remaining ammonia is neutralized with nitric acid, and ammonium sulphide added till the precip- itation is complete. The manganese sulphide is either deter- mined as such, by ignition in a stream of hydrogen, or, more simply, converted into manganese sulphate by heating with a few drops of sulphuric acid. Iron Aluminium. LITERATURE: Classen, Ber. deutsch. chem. Ges., 18, 1795 ; 27, 2060. When a solution containing the above-named metals and a great excess of ammonium oxalate is electrolyzed in the cold, iron is deposited on the negative electrode, while the aluminium remains in solution as long as ammonium oxalate is present in the solution in greater proportion than the ammo- IRON. 197 mum carbonate formed from it. If a precipitate of aluminium hydroxide finally appears, it is only when the solution is al- most free from iron. A small portion withdrawn by a capil- lary tube is tested, from time to time, with ammonium sul- phide or another reagent already mentioned, and the current is stopped as soon as no reaction is obtained. The process is as follows: The aqueous or weakly acid solution (in the latter case neutralized with ammonia) of the sulphates (the chlorides are not as well adapted to the process) is treated with ammonium oxalate in excess, and enough solid ammonium oxalate added (with gentle warming if necessary) to give the proportion of 2 3 g ammonium oxalate to 0.1 g of the metals. The entire volume of the solution should be 150-175 cc. If the temperature of the solution is not over 4:0, it may be submitted to electrolysis at once, since it grad- ually cools under the action of a current of the given strength. It is not best to continue the action of the current longer than , is necessary to reduce the iron ; for, otherwise, a large part of the aluminium is precipitated as hydroxide, and clings so closely to the negative electrode that it cannot be removed. In such a case it is necessary to bring the aluminium hydrox- ide into solution by acidifying with oxalic acid, and, in case too much acid has been added, to pass the current till the last traces of the redissolved iron have been again precipitated. The oxalic acid is poured gradually down the glass which covers the platinum dish, without interrupting the current, till there is no more ebullition, and the aluminium precipitate is dissolved. If the quantity of the aluminium is not greater than that of the iron, the method gives good results without further treat- ment. In other cases, the precipitate of aluminium hydrox- ide is dissolved, without interrupting the current, by careful 198 QUANTITATIVE ANALYSIS BY ELECTROLYSIS. addition of oxalic acid, and the electrolysis repeated until the iron is completely precipitated. To determine the alu- minium in the solution poured off from the iron, it is heated in a porcelain dish till the ammonia is driven off, filtered, and the aluminium hydroxide converted, by ignition, into A1.0.- EXPERIMENT. Usedl g each of Fe 2 (C 2 O 4 ) 3 .3K 2 C 2 O 4 .6H 2 O and A1 2 (SO 4 ) 3 . K 2 SO 4 .24rH 2 O, and 8 g ammonium oxalate. Yolume of liquid, 120 cc. Current Density, Amperes. Electrode Tension, Volts. Temp. Time hr. m. Found, g- Taken, 1.95-1.6 4 3 -4.4 31-42 2 35 0.1143 Fe 0.1135 Fe 1.65-1.35 3.8 -4.1 30-48 3 0.1159 " 0.1150 " 1.00-0.84 3.55-3.8 31-36 4 30 0.1138 " 0.1135 " 0.50-0.42 2.75-3.1 30-32 5 40 0.1139 " 0.1135 " In order to avoid the separation of aluminium hydroxide (small quantities of which often adhere to the iron) strong currents, which raise the temperature of the solution, should not be used. The effect of strong currents and high temperatures is illustrated in the above experiment. Iron Uranium. The separation of iron from uranium depends upon the same principle as the separation from aluminium. It is nec- essary to have a great excess (8 g) of ammonium oxalate present in the solution, in order to retain the uranium in the form of the double salt until all of the other metals are reduced. The process is conducted in the same manner as in the separa- tion of aluminium from iron. When a strong current is employed, especially when there is an insufficient quantity of ammonium oxalate present, it may happen that, as a result of IRON. 199 the decomposition of the hydrogen ammonium carbonate by the heat produced, the uranium is precipitated as hydroxide. The uranium solution, after the other metals have been separated, is freed from oxalic acid by further electrolysis, and finally the ammonium carbonate is decomposed by heating. To bring the finely divided precipitate of uranium hydrox- ide into suitable condition for filtration, nitric acid is added, the solution is heated till the precipitate is wholly dissolved, and ammonia is added to reprecipitate the hydroxide. The pre- cipitate is converted into uranium oxide by ignition in a stream of hydrogen. Iron Chromium. LITERATURE : Classen, Ber. deutsch. chem. Ges., 27, 2060. If a solution which contains an excess of ammonium oxa- late, and chromium as sesquioxide, that is, as chromium ammonium oxalate, be submitted to electrolysis, all of the chromium is converted into a chromate. If iron is also pres- ent, it is precipitated in the metallic state on the negative electrode ; the metal has a peculiarly characteristic lustre. When the precipitation is complete, the liquid is poured off from the precipitated metal and is boiled to decompose ammonium carbonate, and the chromic acid reduced by boiling with hydrochloric acid and alcohol. The chromium is then precipitated as hydroxide with ammonia. The hydroxide is converted into Cr 3 O, in the usual mariner, and weighed. EXPERIMENT. A. Used 1 g each of Fe,(C 2 O 4 ) s .3K,C a O 4 .6H a O and 3K,C,O 4 .Cr,(C,O 4 ) 3 .6H a O, and 8 g ammonium oxalate. So- lution diluted to 120 cc. 200 QUANTITATIVE ANALYSIS BY ELECTROLYSIS. Amperes. Electrode Tension, Volts. Temp, Time, hr. m. Found Fe. Taken Fe. 2.00-1.60 3.4-3.6 62-68 ^ 0.1123 g 0.1120 g 1.60-0.95 3.2-3.8 66-68 5 - 0.1135 " 0.1135 " 1.95-1.50 3.3-3.7 62-65 3 0.1130 " 0.1135 " B. Used 2 g chrome alum, 1.5890 g ferrous ammonium sulphate, and 8 g ammonium oxalate. 1.5 3 65 4 14.19$ Fe 14.28$ Fe C. Used 2 g chrome alum, 1 g Fe 2 (C,O 4 ) 3 .3K 2 C a O 4 .6H 2 O, and 8 g ammonium oxalate. 1.50-1.60 3.0-3.2 65 4 11.35* Fe 11.40$ Fe Iron Aluminium Chromium. LITERATURE : Classen, Ber. deutsch. chem. (res., 14, 2771. The separation is performed as above. To separate the aluminium from chromium, the solution poured off from the precipitated metals is boiled till it has only a weak odor of am- monia, the aluminium hydroxide filtered off, and the chro- mium precipitated as above. Iron Chromium Uranium. LITERATURE : Classen, Ber. deutsch. chem. Ges, 14, 2771; 17, 2483. The separation is accomplished by the precipitation of iron as metal, from the double oxalate solution, and the oxi- dation of chromium to chromic acid by the current. Uranium is separated as hydroxide, while chromium remains in solution as ammonium chromate. To accomplish the quantitative sep- aration of chromium from uranium, the electrolysis must be continued till the oxalic acid is completely oxidized. IRON. 201 The solution is boiled to decompose the resulting ammo- nium carbonate, and allowed to stand six hours. The chromi- um is determined, as above, in the filtrate from the uranium. Iron Beryllium, LITERATURE : Classen, Ber. deutsch. chem. Ges., 14, 2771, The separation of these two metals offers no difficulties whatever if the soluble double salts with ammonium oxalate are prepared, and if care is taken to have an excess of ammo- nium oxalate present. The iron is precipitated according to the directions given under the separation of aluminium from iron. Strong currents are not advisable lest the solution become heated, and thus the ammonium carbonate, which holds the beryllium in solution, be decomposed. The beryllium hy- droxide may, in any case, begin to precipitate before the iron is fully deposited. The determination of beryllium in the solution poured off from the iron is very simple; the solution is boiled to decompose the hydrogen ammonium carbonate, and the heating continued till the solution has only a weak odor of ammonia. The beryllium hydroxide is filtered, washed with hot water, and converted into BeO by ignition in a platinum crucible. Iron Beryllium Aluminium, LITERATURE : Classen, Ber. deutsch. chem. Ges., 14, 2771. The process is precisely like the foregoing. When the iron is reduced, the solution is poured into a second platinum dish, and the action of the current is continued till all the 202 QUANTITATIVE ANALYSIS BY ELECTROLYSIS. oxalic acid is decomposed, and the aluminium is precipitated as hydroxide. The beryllium is precipitated from the filtrate as hydroxide by boiling. It is advisable to redissolve the aluminium hydroxide, to convert it again into the double oxalate, and to repeat the electrolysis. Iron Copper. LITERATURE : Vortmann, Monatshefte f. Chem., 14, 536. Classen, Ber. deutsch. chem Ges., 27, 2060. The separation may be accomplished according to the method given by Luckow (p. 156), if the operation is conducted at ordinary temperatures. * To determine the iron in the solu- tion from which the copper has been removed, it is evaporated to dryness with the addition of sufficient sulphuric acid to convert the iron into sulphate, and the double oxalate is pre- pared by the method given on page 138. EXPERIMENT. Used about 1 g each of copper sulphate and ferrous am- monium sulphate and 5 cc nitric acid (sp. g. 1.35). Volume of liquid, 120 cc. Current Dens., Electrode T omn Time, Found Taken Amperes. Tens., Volts. ^ mp ' hr. m. Cu. Cu. 1.0-0.9 3.0-3.3 19-32 4 0.2518 g 0.2528 g 1.1-1.0 2.6-3.2 18-32 3 30 0.2430 " 0.2450 " The free sulphuric acid in the decanted liquid was neu- tralized with ammonium hydroxide, and 8 g ammonium oxalate were added. Current Dens., Electrode T Time, Found Taken Amperes. Tens., Volts. hr. m. Fe. Fe. 1.30-0.8 2.7-4.5 31-42 3 0.1416 g 0.1406 g 145-11 3.0-3.5 60 330 0.1438" 0.1435" IRON. 203 A similar separation may also be carried out in the pres- ence of sulphuric acid instead of nitric acid. Three cubic centimeters of the concentrated acid are used, the other con- ditions being the same. Current Dens., Electrode Tm^ Time, Found Taken Amperes. Tens , Volts. hr.m. Cu. Cu. 1.05-1.20 3.0-2.85 22-30 210 0.2534 g 0.2539 g 1.00-0.95 2.5-2.45 56-59 2 0.2504" 0.2510" The determination of the iron was conducted as before. Fe. Fe. 1.55-1.32 3.4-3.8 33-40 4 0.1419 g 0.1421 g 1.60-1.40 3.0-3.5 61-64 3 0.1625" 0.1675" The separation of iron and copper may be performed if the copper is precipitated from a hot solution of the double oxalate containing free oxalic, tartaric, or acetic acid. A saturated solution of oxalic acid is used, and one of tartaric acid which contains 6 g acid in every 100 cc. EXPERIMENT. Used about 1 g each of copper sulphate and ferric salt, 6 g ammonium oxalate. The copper must be washed without interrupting the current. Amperes. Volts. Temp. Time. 1.1-1.0 2.95-3.5 51-62 3 hr. 0.2525 g 0.2528 g 0.7-0.7 3.20-285 62 3" 0.2532" 0.2530" The iron was determined in the solution which was poured off from the copper, the free acid being first neutralized with ammonium hydroxide. Fe. Fe. 1.4-1.3 3.0-3.2 68-70 2} hr. 0.1431 g 0.1435 g 1.0-0.9 3.1-3.3 30-40 3 " 0.1425" 0.1429" 204 QUANTITATIVE ANALYSIS BY ELECTEOLYSIS. Yortmann dissolves the oxides of both metals in an am- moniacal solution, to- which are added several grams of ammo- nium sulphate, and electrolyzes with a current of OTD 100 = 0.1-0.6 ampere. Only copper is precipitated, the ferric hydroxide remaining unaltered in solution. Iron Lead. The separation is based on the separation of lead as per- oxide in the presence of nitric acid (p. 168). The iron is .determined as above. COBALT. Cobalt Zinc. LITERATURE I Vortmann, Elektrochem. Zeit., 1, 6. Smith and Wallace, Journ. of anal. Chem., 1893, p. 183. According to Yortmann, an excess of a 10-20$ solution of sodium hydroxide is added to the solution containing the metals. Several grams of sodium potassium tartrate are then added and the electrolysis is conducted with a current of NI) 100 = 0.07-0.1 ampere and an electrode tension of 2 volts. The cobalt is precipitated, but the addition of potas- sium iodide is necessaiy in order to prevent the separation of cobaltic oxide at the anode. Cobalt Aluminium. The method is carried out similarly to that of iron from aluminium. Cobalt Uranium ; Cobalt Chromium ; Cobalt Uranium Chromium The methods employed are similar to those of the corre- sponding separations from iron (p. 200). COBALT. 205 Cobalt Copper. LITERATURE : Classen, Ber. deutsch. chem. Ges., 27. 2060. Rudorff, Zeit. f. angew. Chem., 1894, p. 388. Warwick, Zeit. f. anorg. Chem., 1, 299. The separation of these two metals can only be satisfac- torily carried out by the electrolysis of solutions containing oxalic, tartaric, or dilute acetic acid, at a temperature of 50- 60, and at an electrode tension of not less than 1.1 or more than 1.3 volts. In order to have the tension constant and to be able to regulate it conveniently, it is best to insert the wire- gauze resistance described on page 113 in the main circuit, EXPERIMENT. Used 1 g copper sulphate, 1 g cobalt ammonium sulphate,, and 6 g ammonium oxalate. trorle Tension, Volts. Temp. Time, hr. m. Found* g Cu. % Cu. 1.24-1.30 50-60 3 50 0.2602 25.36 1.20-1.35 50-60 3 30 0.2531 25.29 1.20-1.29 50-60 4 0.2522 25.28 Cobalt Bismuth. LITERATURE : Smith and Wallace, Journ. of Anal. Chem., 1893, p. 183. Smith and Moyer, Zeit. f. anorg. Chem., 4, 268. According to Smith and Wallace, and also Smith, and Moyer, a separation of these metals may be satisfactorily con- ducted in a solution containing nitric acid. Since, however, the required conditions of experiment are not given in the respective publications, the methods will be here omitted. * [Theory 25 33g Cu. ] 206 QUANTITATIVE ANALYSIS BY ELECTROLYSIS. Cobalt Lead. The solution, to which nitric acid has been added, is electrolyzed (see Lead). Cobalt Mercury. Similar to the above. NICKEL. Nickel Manganese. What has been said with reference to the separation of iron from manganese applies also to the separation of nickel from manganese. Nickel Aluminium. Similar to the separation of iron from aluminium. Nickel Uranium ; Nickel Chromium. See Iron (pp. 198-199). Nickel Copper. LITERATURE I Classen, Ber. deutsch. chem. G-es., 27, 2060. The separation takes place under the same conditions as the separation of cobalt from copper. If 1 g each of copper sulphate and nickel sulphate are taken, 6 g ammonium oxalate are required. Larger quantities of metal require correspondingly greater quantities of the ammonium oxalate. NICKEL. 207 EXPERIMENT. Elec.Tens., Time, Found* Volts. hr. in. g Cu. % Cu. Remark. 1.11-1.3 3 50 0.2552 25.40 1.20-1.3 3 0.2559 25.37 Acidified with oxalic acid. 1.20-1.3 3 30 0.2591 25.38 Acidified with tartaric acid. J Acidified with acetic acid. The 1 copper contained nickel. 1 Q4_1 A^ Q ^O n 9^7Q 1.20-1.6 350 0.2595 25.33 The copper contained nickel. Nickel Lead. The separation corresponds to the method given under Cobalt. Nickel Mercury. LITERATURE I Kudorff, Zeit. f. angew. Chem., 1894, p. 388. Smith, Am. Chem. Jouru., 12, 104. Heidenreich, Ber. deutsch. chein. Ges., 28, 1585. The method for the separation of these two metals is similar to that of cobalt from mercury. According to the statements of Smith, the separation may be carried out from a solution of the double cyanides. Heidenreich, who de- termined in the Aachen laboratory the proper conditions of experiment, found that only the mercury is precipitated when the tension at the electrodes is 1.2-1.6 volts. EXPERIMENT. Used about 1 g nickel ammonium sulphate and 3 g potas- sium cyanide. Taken f HgCI 2 . Current Density, Amperes. El. Tension, Volts. Time. Found t per cent Hg. 03687 08-0.03 1.2-1.6 5 hr. 73.65 0.3702 0.05-0.93 1.4-1.5 overnight 73.62 0.3000 0.05-0.03 1.4-1.5 " 73.66 [Theory 25.33^ Cu.] f [Theory 73.80 Hg.] 208 QUANTITATIVE ANALYSIS BY ELECTROLYSIS. ZINC. Zinc Manganese. This separation, similar to that of copper from cobalt, takes place from hot solutions containing free oxalic acid, which prevents the separation of manganese peroxide. Zinc Aluminium. Conditions similar to the above. Zinc Copper. LITERATURE I Riidorff, Zeit. f. angew. Chem., 1893, p. 452. Smith and Wallace, Journ. of Anal. Chem., 1893, p. 183. Heidenreich, Ber. deutsch. chem. Ges., 28, 1585. For this separation, Smith and Wallace recommend the precipitation of the copper from a solution to which nitric acid has been added. Heidenreich, who determined in the Aachen laboratory the proper conditions of experiment, found that if the solution contains about 4 cc nitric acid (sp. g. = 1 . 3) to 120 cc of liquid, and the tension of 1.4 volts is not exceeded, the copper only is precipitated. The greater part of the copper separates in a short time, but the precipitation of the last trace proceeds very slowly. The analysis therefore requires from 18 to 20 hours. EXPERIMENT. Used copper sulphate (containing 25.29$ Cu) to which was added 0. 8 g zinc ammonium sulphate. ZINC. Taken CuS0 4 .5H 2 g- Current , Density, Amperes. Electrode Tension, Volts. Time, hr. m. Found Cu, % 0.4476 0.2 1.00-1.10 6 30 24.31 0.3857 0.2-0.3 1.00-1.20 8 25.00 0.4244 0.2 1.00-1.15 15 30 25.19 0.4689 0.2 1.00-1.15 15 30 25.25 0.4728 0.2-0.15 1.00-1.20 18 25.25 0.5049 0.20-0.15 1.13 18 25.31 0.4660 0.5 1.20 2 25.22 0.4775 1.05-0.9 1.50 2 25.84 ) contained 0.4826 1.00-0.8 1.35-1.98 18 25.80 ) zinc 0.4576 0.50-0.4 1.15-1.23 6 30 25.19 Zinc Cadmium. LITERATURE. Smith, Am. Chem. Journ., 11, 352. Yver, Bull. Soc. Chern., 34, 18. Eliasberg, Zeit. f. anal. Chem., 24, 550. Smith and Knerr, Am. Chem. Journ., 8, 210. A. Yver recommends the use of a solution of the ace- tates or sulphates treated with an excess of sodium acetate and a few drops of acetic acid ; the electrolysis to be con- ducted hot, using two Daniell cells. In the laboratory of the Technical High School in Munich the following directions are given for Tver's method : To the sulphuric acid solution of the two metals add sodium hydrox- ide solution until a permanent precipitate is obtained, dissolve the precipitate in the smallest possible quantity of dilute sul- phuric acid, dilute the solution to about 70 cc, and reduce the cadmium with a current of ND 100 = 0.07 ampere. When the greater part of the metal is precipitated, neutralize the free sulphuric acid with sodium hydroxide, add 3 g sodium acetate, heat to about 45, and subject to the action of a current of ND )00 =0.3 ampere. The latter direction assumes that the electromotive force is not over 3.6 volts; if more, it is to be reduced to about 2.4 volts. 210 QUANTITATIVE ANALYSIS BY ELECTROLYSIS. Zinc Lead. The separation is conducted from a nitric acid solution, the lead being precipitated as peroxide (see Lead, p. 168). To determine the zinc, it is converted into the sulphate in the manner described under the separation of iron from copper on page 202, and is precipitated by the method given on page 146. Zinc Silver. LITERATURE : Smith and Wallace, Zeit. f. Elektrochemie, 2, 312; Journ. of Anal. Chem., 1892, p. 87. Heidenreich, Ber. deutsch. chem. Ges., 28, 1585. The separation, according to Smith and Wallace, is con- ducted from a solution of the double cyanide. The proper experimental conditions were acertained by Heidenreich in the Aachen laboratory, with the result that the separation was best carried out at a temperature of 60-70, and with a ten- sion at the electrodes of 1.9-2 volts. EXPERIMENT. Taken Current Density, Electrode Tens., rp PTnn Timfk Found* gAgN0 3 . Amperes. Volts. % Ag. 0.4046 0.05 1.9-2.03 60 28 hr. 63.34 0.4149 0.03 2.1-2.05 " 22 " 63.31 0.3260 0.08 1.9 " 16 " 63.23 0.3739 0.08-0.05 3.0-2.15 " 15 " 63.31 0.2949 0.05-0.02 1.8-2.05 " 6 " 63.36 Zinc Mercury. LITERATURE I Smith and Wallace, Zeit. f. Elektrochemie, 2, 312. Heidenreich, Ber. deutsch. chem. Ges., 28, 1585. Smith and Wallace conduct the separation from a solution of the double cyanide. According to the experiments carried * [Theory 63.52$ Ag.J MANGANESE. 211 out in the Aachen laboratory by Heidenreich, the mercury is precipitated free from zinc. In performing the experiments, Heidenreich observed also that the dishes used suffer severely from the combined action of the mercury and potassium cyanide on the platinum. EXPERIMENT. Taken Current Density, Elec. Tension, m-^ Found* gHgCl 2 . gKCN. Ampere. ' Volts. Hg. 0.2501 2-3 0.08-0.04 1.65-1.69 5 hr. 73.61 0.2655 2-3 0.03 1.75 14 " 73.51 MANGANESE. Manganese Copper. The separation is conducted similarly to that of copper from cobalt. The copper is precipitated from a hot solution containing free oxalic acid which prevents the separation of manganese peroxide. The liquid containing the manganese is poured off from the copper. Generally this is not suited for direct electrolytic determination, since the substances previously added interfere with the precipitation of the manganese, and the volume of the liquid has become too great as a result of washing the copper without interrupt- ing the current. According to the directions of Jannasch the manganese is then precipitated with ammonia and hydro- gen peroxide. The precipitate is allowed to settle and the solution is filtered. The precipitate is dissolved in a mixture of 5 cc of acetic acid, 5 cc hydrogen peroxide, and 25 cc water, and this solution is submitted to electrolysis after the excess of hydrogen peroxide has been removed with chromic oxide. The same method is employed when the solution contains manganese chloride, since the presence of the chlorine also interferes with the separation of the peroxide. The following experiment was performed as above directed by Dr. Oarl Engels in the Aachen laboratory. * [Theory 73.80#Hg.] 212 QUANTITATIVE ANALYSIS BY ELECTROLYSIS. EXPERIMENT. The solution contained (NH 4 ) 2 MnCl 4 .7H 2 O. (NH 4 ) 2 MnCl4.7H a O Current Electrode ,. Found* taken, Density, Tension, Temp. . f Mn 3 O 4 , g. Amp. Volts. g. % 0.8619 0.63 2.8 80 1 45 0.2153 24.98 0.9550 0.62 2.8 82 1 45 0.2385 24.98 0.9562 0.70 2.9 83 1 45 0.2394 25.03 1.0131 0.72 3.1 80 1 30 0.2536 25.03 0.8580 0.80 3.1 80 1 15 0.2151 25.01 1.1383 0.78 3.1 85 1 15 0.2848 25.02 Manganese Cadmium. This is conducted similarly to the separation of manganese and copper, and the manganese is determined according to the directions of Engels, which are given above. COPPER. Copper Cadmium. LITERATURE I Freudenberg, Zeit. f. phys. Chem., 12, 97. Heidenreich, Ber. deutsch. chem. Ges., 28, 1585. Smith and Wallace, Journ. of Anal. Chem., 1893, p. 183. Smith and Moyer, Zeit. f. anorg. Chem., 1, 299. According to the statements of Freudenberg, the two metals may be separately precipitated from a sulphuric acid solution (1020 cc. of dilute sulphuric acid) by a variation of the tension. With a tension of 2 volts the copper is precipi- tated, all the cadmium remaining in solution. Heidenreich tested this method in the Aachen laboratory, and found that the separation is best conducted with a tension not exceeding 1.85 volts. * [It seems probable that the salt taken was not pure. MnCl 8 .2NH4Cl. 7H a O contains 21.270 of Mn 3 O 4 . Trans.] COPPER. 213 EXPERIMENT. The volume of the liquid was 120 cc, containing 15 cc dilute sulphuric acid (sp. gr. = 1.09). Taken Current Density Electrode ,. Found* CuS04.5H.jO, CdS0 4 .8H 2 O, ND 100 , Tension, L F^' Cu, g. g. Amperes. Volts. % 0.7078 0.4 0.07-0.05 1.7-1.76 24 25.27 Experiments in which it was attempted to replace the sulphuric acid by nitric acid yielded no satisfactory results. Copper Lead. LITERATURE : Classen, Ber. deutsch. chem. Ges., 27, 2060. Nissenson, Zeit. f. augew. Chem., 1893, p. 452. To separate copper from lead, 20 cc of nitric acid (sp. g. 1.35) are added to the solution, which is then diluted to 75 cc, warmed and electrolyzed with a current of 1.1-1.2 amperes (corresponding to ND 100 = 1.5-1.7 amperes). At the end of one hour the greater part of the lead has separated as peroxide (98-99$ when not more than 0.5 g is present in the solution), and the current is then interrupted, no trace of copper as yet appearing at the cathode. The liquid is then transferred to a second tared dish, the lead peroxide is washed with water, and after drying is weighed. The washings from the lead peroxide are added to the copper solution, which is then treated with ammonium hydroxide until the well-known deep blue color appears, and about 5 cc nitric acid are added. The platinum dish is connected with the negative pole of the source of current, and one of the perforated platinum bucket electrodes, described by the author, is employed as anode to * [Theory 25 33^ Cu.] 214 > QUANTITATIVE ANALYSIS BY ELECTROLYSIS. take up the remainder of the lead peroxide. This electrode should have a roughened surface. It is weighed before the experiment. After the solution has cooled, it is diluted to 120-150 cc, and electrolyzed with a current of OT3 100 1.0 1.2 amp. At the end of 3 to 4 hours the copper (if about 0.25 g is present), together with the rest of the lead, is pre- cipitated. This' method, which is of great value in technical work, is not only rapid (45 hours as compared to 14 hours or more), but allows of the complete precipitation of both metals, irrespective of the relative quantities present. When this method is employed for the analysis of sub- stances containing sulphur, the lead sulphate resulting from the oxidation is troublesome. The operation of dissolving this may often require more time than the analysis itself. Accordingly, if lead sulphate is formed, either as a result of the oxidation of sulphur or of double decomposition between lead nitrate and copper sulphate, a slight excess of ammonia is added and the solution is warmed for several minutes. The dense lead sulphate is hereby converted into porous lead hydroxide, The liquid is poured little by little into the platinum dish, which contains about 20 cc of warm nitric acid, and constantly stirred with the electrode. The lead sulphate which reappears either dissolves immediately, or if the quantity is large the greater part of it goes immediately into solution, and the remainder disappears on warming for a short time. The vessel in which the decomposition of the lead sulphate is conducted is first washed with a little nitric acid and then with water. EXPERIMENT. Usetf about 1 g each of lead nitrate and copper sulphate,, and 20 cc nitric acid. COPPER. 215 Current Density, Amperes. Electrode Tension, Volts. Beginning. End. Temp. Time, hr. Found PbO a , g- Taken PbO,, g- 1.1 -1.1 1.4 1.4 60-63 1 0.7266 0.7260 1.55-1.45 1.4 1.4 66-72 1 0.7310 0.7303 The liquid was poured off from the lead peroxide, made alkaline with ammonia, and 5 cc. nitric acid were added. The copper was then separated by electrolysis. Current Electrode Timp Found Taken Density, Tension, Temp. ^r c ' Cu, Cu, Amperes. Volts. g. g. 1.1-1.0 2.2 -2.5 25-30 5 0.2490 0.2495 1.0-0.95 2.25-2.3 30-32 5 0.2505 0.2510 H. Nissenson, who employed the preceding method for determining the copper and lead in copper matte, gives the following directions for carrying out the analysis : 1 g copper matte is dissolved in 30 cc nitric acid (sp. g. 1.4) and the solution is diluted to 180 cc. The electrolysis is so conducted that the lead is precipitated on the dish, a perforated platinum plate which serves as cathode receiving the copper. The electrolysis is started at ordinary tempera- tures with a current density of 0.5 ampere, which at the end of an hour is increased to 1.5-2 amperes. Both metals are completely precipitated in 6-7 hours. For technical analyses, where the determination is con- ducted from nitric acid solutions, the presence of small quantities of silver and bismuth may be neglected. Where lead is precipitated from nitric acid solutions containing arsenic, selenium, or manganese, even in very small quanti- ties, the results are not accurate. Copper Silver. LITERATUEE I Freudenberg, Zeit. f. phys. Chem., 12, 97. Smith and Wallace, Zeit. f. Elektrochemie, 5, 312. Heidenreich, Ber. deutsch. chem. Ges., 28, 1585. 216 QUANTITATIVE ANALYSIS BY ELECTROLYSIS. Freudenberg employs a solution containing a few cubic centimeters of nitric acid (sp. g. 1.2) for the separation of the silver, which is quantitatively precipitated at a tension of 1.3-1.4: volts. The copper remains in solution and is first decomposed at a higher tension (2-3 volts). According to E. Smith, these two metals may be sepa- rated from a solution of the double cyanides. 4. 5 g of potas- sium cyanide are added to a solution of about 0.4 g of the mixed metals. The solution is diluted to about 120 cc and electrolyzed. If the solution be warmed to 6575, the pre- cipitation of the silver is greatly hastened. M. Heidenreich tested this method in the Aachen laboratory and determined the following conditions of experiment. EXPERIMENT. Used silver nitrate containing 63.42$ silver, and copper sulphate. About 0.7 g of copper sulphate was added. Taken gAgN0 3 . gKCN. 0.2379 2 Current Density, Amperes. 0.07-0.03 Electrode Tension, Volts. 1.0-1.2 Time, Found hr. m. % Ag. 8 63.34 ^0.2303 2 0.04 1.0-1.28 8 63.43 3099 2 0.03 1.0-1.39 6 30 63.40 0.3327 2 0.09 1.2-1,3 4 warmed 63.27 0.6037 6 0.19-0.08 1.2-1.3 6 U3.33 Copper Mercury. LITER AT ORE : Smith, Journ. of Anal. Chem., 3, 254 ; 5, 489. Am. Chem. Journ., 11, 104, 264. Freudenberg, Zeit. f. phys. Chem., 12, 113. According to E. Smith, the separation may be conducted from a solution of the double cyanides. The temperature CADMIUM. 217 should be about 65. With the ordinary conditions of con- centration, about 2 g potassium cyanide are added, and the solution is electrolyzed with a current of ND 100 = 0.06-0.08 ampere. The decomposition requires about 4 hours for every 0.2 g of the combined metals. The copper remains in solu- tion, the mercury being deposited. Freudenberg found that at a tension of 2.5 volts the mercury, in the presence of 2-4 g potassium cyanide, sepa- rates brilliantly white and completely free from copper. Copper Arsenic. LITERATURE : Freudenberg, Zeit. f. phys. Chem., 12, 97. Schmucker, Zeit. f. anorg. Chem., 5, 199. Although formerly it was necessary to remove the arsenic before precipitating the copper, Freudenberg has shown that a separation may be satisfactorily conducted from a sulphuric acid solution (10-20 cc dilute sulphuric acid) if the tension is not allowed to exceed 1.9 volts. It is immaterial whether the arsenic is added in the form of trioxide or pentoxide. A second method of the same author is the following: Am- monia is added to the solution containing the metals in the form of higher oxides, until there is an excess of about 30 cc of a 10# ammonia solution. The electrolysis is conducted with a current tension of 1.9 volts, and is continued until the solution is completely decolorized, requiring generally 6-8 hours. This method is not suitable for the separation of copper and antimony. CADMIUM. Cadmium Lead. This process is the same as the separation of lead from copper. The lead is separated as peroxide from a nitric acid 218 QUANTITATIVE ANALYSIS BY ELECTROLYSIS. solution. To determine the cadmium in the solution from which the lead has been removed, the nitric acid is evapo- rated off on the water-bath, the cadmium converted into sul- phate, and treated according to the directions on page 164. Cadmium Mercury. LITEEATUEE I Freudenberg, Zeit. f. phys. Chem., 12, 97. According to Freudenberg, the separation proceeds best from a solution of the salts of both metals, containing 0.5-1 g potassium cyanide. With a tension of 1.8-1.9 volts mercury only is precipitated. After the separation of the mercury, the cadmium is precipitated from the solution by a current of higher tension. LEAD. Lead Silver. This separation is conducted like that of lead from copper (see page 213). To determine the silver, the solution is evaporated down on the water-bath, and the silver is precipi- tated according to the directions given on page 173. Lead Mercury. LITEEATUEE I Smith and Moyer, Zeit. f. anorg. Chem., 4, 267. Heidenreich, Ber. deutsch. chem. Ges., 28, 1585. The method corresponds to that used for the separation of copper from lead. Smith and Moyer attempt to deter- mine the lead and mercury at the same time. They add 5 cc nitric acid (sp. g. 1.3) to the solution of the two metals, and dilute the liquid to 180 cc. The electrolysis is con- LEAD. 219 ducted witli a current of 1.7 cc of oxy hydrogen gas per minute. Heidenreich determined the conditions of experiment for the preceding method, and found that 20-30 cc nitric acid (sp. g. 1.3-1.4) must be present for every 120 cc of the solution to be electrolyzed, since otherwise the lead peroxide scales off and cannot be accurately determined. A current of ND 100 = 0.2-0. 5 ampere may be used. The fact that greater quantities of lead could not be precipitated in an adherent form was due to the condition of the surface of the platinum disk which was used as anode. Lead Antimony. LITERATURE I Neumann and Nissenson, Chemiker Zeitung, 1895, No. 49. For the electrolytic determination of both metals in alloys (stereotype-metal, type-metal), Neumann and Nissenson rec- ommend that 2.5 g of the alloy be dissolved by warming with a mixture of 10 g tartaric acid, 4 cc nitric acid (sp.g. 1.4), and 15 cc water. 4 cc cone, sulphuric acid are then added, the solution is diluted with water, allowed to cool, and filled up to exactly one quarter liter. If the liquid is now filtered off from the separated lead sulphate, it will contain all of the antimony. 50 cc of this filtrate are made strongly alkaline with sodium hydroxide, 50 cc of a saturated solution of sodium monosulphide are added, the solution is filtered immediately, washed from the precipitate, and electrolyzed according to the method given on page 180. For the determination of the lead, the lead sulphate is treated as in the separation of lead from copper (page 213). 220 QUANTITATIVE ANALYSIS BY ELECTROLYSIS. SILVER. Silver Antimony. LITERATUEE I Freudenberg, Zeit. f . phys. Chem. , 12, 97. If the antimony is present as pentoxide, the separation may be carried out from an ammoniacal solution to which several grams of ammonium sulphate have been added. The silver is precipitated at a tension of 1.7-1.8 volts. Silver Arsenic. LITERATUEE I Freudenberg, Zeit. f. phys. Chem., 12, 97. According to Freudenberg, this separation is conducted in the same manner as the separation of silver from antimony. MERCURY. Mercury Antimony. LITEEATUEE I Freudenberg, Zeit. f. phys. Chem., 12, 97. The antimony must be added in the form of a pentavalent salt, since a reduction of the mercuric salt present would otherwise occur. A mixture of the chlorides of the two metals is brought into solution by the use of 0.5-1 g tartaric acid. The solution is diluted with water, made neutral with ammonia, and then about 20 cc of a 10# solution of ammonia are added until the solution is perfectly clear. The electrolysis is conducted at a tension of 1.6-1.7 volts. After the mercury ANTIMONY. 221 is deposited, the solution is made acid and hydrogen sulphide is passed in. The antimony sulphide may be either directly determined, i.e., weighed, or determined by electrolysis (see page 179). Mercury Arsenic. LITERATURE I Freudenberg, Zeit. f. phys. Chem., 12, 97. According to Freudenberg, the separation is conducted from a nitric acid solution (see page 175) from which the mercury is precipitated at a tension of 1.7-1.8 volts. ANTIMONY. Antimony Tin. LITERATURE : Classen, Ber. deutsch. chem. Ges., 17, 2245 ; 18, 1110 ; 28, 2060. The separation of antimony from tin by the ordinary gravimetric methods, which, as is well known, is difficult, and gives uncertain results, may be accomplished by electrolysis with ease and accuracy. Antimony may be completely pre- cipitated, in the presence of tin, from a concentrated solution of sodium sulphide, to which is added a certain amount of sodium hydroxide. The crystallized sodium monosulphide of commerce, aside from the fact that its purity is otherwise uncertain, is not pure monosulphide, but is a mixture of several sulphides with varying amounts of sodium hydroxide. This explains the large amount of alumina which it always contains. If, there- fore, commercial sodium sulphide is to be used, it must first be dissolved, and the solution, with exclusion of air, com- pletely saturated with pure hydrogen sulphide gas. It is then filtered from the precipitated impurities, and evaporated in a 222 QUANTITATIVE ANALYSIS BY ELECTROLYSIS. large platinum or porcelain dish. The further treatment is given in full in the chapter on reagents. As the condition of the sodium sulphide solution is of great importance to the success of the process, it is preferable to prepare the solution as directed in the chapter referred to. The process of separation is as follows : A mixture of the pure sulphides,* or the residue obtained by evaporating a solution of the two metals, is treated in a platinum dish with about 80 cc of a sodium sulphide solution saturated at ordinary .temperatures, and enough concentrated solution of pure sodium hydroxide f to furnish 1-2 g NaOH. If solution does not take place at once, it is hastened by heating over a low Aflame, the watch-glass covering the dish is rinsed with 10-15 cc water, and the solution is allowed to cool thoroughly. It is then submitted to electrolysis. When weak currents (ND 100 0.2 amp.) are employed, the separation of the antimony requires about 14 hours, so that the electrolysis must be continued through the night. Ex- periments recently undertaken by the author have shown that, for the precipitation of antimony in the presence of tin, the solution may be warmed to 50-60 and a current density of ND 100 0.5 ampere employed. It is thus possible to complete the precipitation within 2 hours. When the action begins, the whole surface of the dish, which is in contact with the solution, becomes quickly covered with a dark coating of antimony, which soon takes on a brilliant metallic appearance. * The solution of a mixture of the metallic sulphides and sulphur in sodium sulphide is to be treated like a solution of polysulphides (see further on). f The sodium hydroxide used must be absolutely pure, and must show no cloudiness when warmed with sodium sulphide. Otherwise the results obtained for the antimony will be too high, owing to the inclusion of the precipitate. ANTIMONY. 223 In the earlier part of the process, the entire solution ap- pears to be filled with small gas-bubbles which rise slowly, break at the surface, arid cover the watch-glass with minute portions of the solution. In the course of two hours the dis- engagement of gas is ended, and the solution is completely clear. To avoid loss, it is best, at this time, to wash re- peatedly the under surface of the watch-glass with a drop of water which is finally allowed to run down the positive elec- trode. When the reduction is completed, the antimony is washed without interrupting the current, and is treated accord- ing to the directions already given (p. 180). As tin cannot be reduced from a sodium sulphide solu- tion (as stated on p. 184), but can be completely precipitated from solution in ammonium sulphide, the sodium sulphide, after the separation of antimony, must be converted into ammonium sulphide according to the directions given on p. 187. If the two metals are to be determined in the yellow solution of polysulphides of the alkalies, the solution is decolorized with ammoniacal hydrogen peroxide (see Anti- mony, p. 181), and evaporated nearly to dryness; about 80 cc sodium sulphide solution and the necessary amount of sodium hydroxide are then added, and the process carried on as above directed. In the following experiment, antimony was precipitated from both warm and cold solutions containing tin. EXPERIMENT. Used about 1 g antimony potassium tartrate, an equal quantity of NH 4 CLSnCl 4 , 80 cc sodium sulphide solution, and about 2 g sodium hydroxide. Current Density, Amperes. Electrode Tension, Volts. Temp. hrs? 1.5-1.45 0.9-0.8 57-67 2 1.5-1.6 0.8-0.9 58-60 2 0.4-0.2 0.7-0.55 30-24 15 224 QUANTITATIVE ANALYSIS BY ELECTROLYSIS. Found Taken Sb, Sb, g. g. 0.3790 0.3780 0.3787 0.3780 0.3775 0.3780 The antimony precipitate appeared gray and shiny, and contained no tin. Antimony Arsenic. LITEKATUKE I Classen and Ludwig, Ber. deutsch. chem, Ges,, 19, 323. In an alkaline solution, arsenions acid is oxidized to arsenic acid by the galvanic current. If, however, a solution containing antimony and arsenious acid is clectrolyzed, a mixture of antimony with arsenic is deposited. The action is different if the arsenic is present in the solution as arsenic acid ; in the presence of a free alkali, the antimony alone is deposited from a concentrated sodium sulphide solution. The arsenic, therefore, if present as arsenious acid, must be oxi- dized to arsenic acid before the metals can be separated. It is heated with concentrated nitric acid or aqua regia, the acid completely removed by evaporation on the water-bath, the residue treated with 80 cc of a cold saturated sodium sul- phide solution, a concentrated solution of sodium hydroxide (containing about 1 2 g IsaOH) added, and the solution electrolyzed. The separation is conducted precisely like that of antimony from tin. The electrolysis may be conducted either warm or at ordi- nary temperatures. If antimony and arsenic are to be deter- mined in a solution of polysulphides of the alkalies, the solu- tion is treated as described on p. 181. To determine arsenic, the antimony-free solution is acidified with dilute sulphuric acid, heated in the water-bath to remove hydrogen sulphide. ANTIMONY. 225 filtered, and the precipitate dissolved in hydrochloric acid with the addition of potassium chlorate. This solution is treated with ammonia in excess, and the arsenic acid precip- itated as magnesium ammonium arsenate with magnesium mixture. The precipitate may be dried, at 110, on a weighed filter, and weighed, or converted into magnesium pyro-arsenate by careful ignition in a porcelain crucible. EXPEKIMENT. Used about 1 g of antimony potassium tartrate, 1 g sodium arsenate, 80 cc sodium sulphide solution, and 2.5 g sodium hydroxide. Current Density, Amperes. Electrode Tension, Volts. Temp. Time, hr. m. Found Sb, g. Taken Sb, g. 1.55-1.5 1.75-1.1 54-57 3 30 0.3778 0.3773 1.60-1.5 2.10-1.45 25-38 6 0.3770 0.3773 0.5 -0.4 1,75-0.8 21-24 overnight 0.3770 0.3770 Antimony Tin Arsenic . LITERATURE : Classen, Ber. deutsch. chem. Ges., 17, 2245; 18, 1110; 28, 2060. Classen and Ludwig, ibid., 19, 323. If arsenic is present as arsenic acid, antimony alone is precipitated from a concentrated alkaline solution of the three metals in sodium sulphide ; tin and arsenic remain in solution. The arsenic is converted into arsenic acid, and the antimony precipitated, exactly as heretofore described. For the separation of tin from arsenic, the solution poured off from the antimony is treated with dilute sulphuric or hydrochloric acid to decompose the sulpho-salts, the mixture of arsenic and tin sulphides and sulphur is filtered off and oxidized with hydrochloric acid and potassium chlorate, and 226 QUANTITATIVE ANALYSIS BY ELECTROLYSIS. the arsenic separated as described below. To determine the tin, the solution freed from arsenic is saturated with hy- drogen sulphide, filtered, and the tin sulphide dissolved in ammonium sulphide. The tin is determined electrolytically as directed p. 186. In the analysis of a substance which contains arsenic, antimony, and tin, the arsenic may also be first eliminated according to the method of E. Fischer-Hufschmidt simplified by R. Ludwig and the author,* and antimony and tin sepa- rated in the arsenic-free solution. If the sulphides of the metals are to be separated, they are oxidized with concentrated hydrochloric acid and potas- sium chlorate, and the acid evaporated on the water-bath. The residue is washed with fuming hydrochloric acid into a flask of 500-600 cc capacity, f treated with 20-25 cc of a saturated solution of ferrous chloride, or, better with about 25gof ammonium ferrous sulphate [FeSO 4 .(NH 4 ) 2 SO 4 .6H 2 O], and fuming hydrochloric acid added till the volume is 150 to 200 cc. A strong current of hydrochloric acid gas is now passed into the solution, and kept up for at least half an hour after the solution seems fully saturated. Then the solution is reduced to about 50 cc by distilling off the liquid, without a condenser, in a stream of hydrogen chloride gas. A flask of about 1 liter capacity, containing 400-500 cc water, is used as a receiver. If the flask is well cooled during the distillation, not a trace of arsenic passes over into a second receiver, even when as much as 0.5 g, reckoned as As 2 O 3 , is present. The arsenic in the distillate may either be saturated with sodium carbonate and titrated with iodine solution, or pre- * Ber. d. ch. Ges., 18, 1110. | A convenient apparatus is illustrated in the author's " Handbuch der Quantitative Analyse," 4th edition, p. 78. ANTIMONY. 227 cipitated as As 3 S 3 with hydrogen sulphide, and determined as such on a weighed filter, or the arsenic calculated from the amount of sulphur in the precipitate. The process, in the latter case, is as follows : The distillate is mixed with twice its volume of water, air expelled by a strong current of carbon dioxide, and the arsenic precipitated by passing in pure hydrogen sulphide gas. The excess of hydrogen sul- phide is removed by passing a strong current of carbon dioxide till lead acetate paper is not colored by the escaping gases. The arsenic sulphide is allowed to subside, and the clear solution siphoned off. The remaining strongly acid solution is saturated with ammonia, which dissolves the arsenic sulphide ; the solution is then boiled with an excess of hydrogen peroxide free from sulphuric acid. The solution is acidified with hydrochloric acid, and the sulphuric acid produced by the action of the hydrogen peroxide determined as barium sulphate in the usual way (Classen). To determine the antimony and tin, the strong acid solu- tion in the flask, which contains the iron, is diluted with three times its volume of water. Antimony and tin are pre- cipitated with hydrogen sulphide. After the precipitate has subsided, the clear solution is poured on a filter, the pre- cipitate washed several times by decantation, and afterwards, on the filter, with hot water, till free from hydrochloric acid. Portions of the sulphides often adhere to the walls of the flask in which the precipitation took place. These are washed out with concentrated sodium sulphide solution, and the solution is poured on the filter containing the sulphides. The filtrate is collected in a weighed platinum dish. The filter, on which some iron sulphide always remains after the solution of the antimony and tin sulphides, is washed with sodium sulphide solution, the necessary amount of sodium hydroxide is added 228 QUANTITATIVE ANALYSIS BY ELECTROLYSIS. to the filtrate, and the antimony and tin are separated electro- lytically as already directed. TIN PHOSPHORIC ACID. In the determination of metals, in the presence of phos- phoric acid, the latter is often removed as tin phosphate. The phosphoric acid is then usually determined in a separate portion, as its determination in the tin precipitate is too difficult and slow a process. The precipitate of tin oxide and tin phosphate may, however, be dissolved by digesting with ammonium sulphide, the solution diluted, the tin pre- cipitated by electrolysis, and the phosphoric acid determined as usual. PLATINUM IRIDIUM. As stated on page 182, platinum can be separated from a hydrochloric acid solution by a current of ]STD 100 = 0.05 am- pere and 1.2 volts. This property of platinum may be used for separating it from iridium, which under similar conditions remains in solu- tion. The platinum is deposited free from iridium. (Classen.) SEPARATION OF GOLD FROM OTHER METALS. LITERATURE : Smith and Muhr, Ber. deutsch. chem. Ges., 23, 2175. Smith and Wallace, ibid., 25, 779 ; Journ. of Anal. Chem., 1892, p. 87. As has already been frequently stated, Edgar F. Smith has made an exhaustive study of the action of the galvanic current on the cyanides of the metals, and has applied this SODIUM AMMONIA. 229 method to the separation of gold from palladium, copper, nickel, zinc, and platinum. The same conditions may also be employed for the separa- tion of silver from platinum and mercury from platinum. Smith gives but incompletely the conditions of experi- ment necessary for conducting these operations, and therefore a consideration of them in detail will be omitted. POTASSIUM SODIUM. The ordinary method of determining potassium and so- dium in the same solution is to weigh the mixed chlorides, and the potassium as platinchloride ; the sodium is thus deter- mined by difference. The errors of the work, therefore, all fall on the sodium. The potassium may be determined, as already directed (p. 188), by precipitating as potassium platin- chloride, and determining the platinum in the latter by elec- trolysis. To determine the sodium directly, the filtrate from the potassium platinchloride is evaporated on the water-bath to remove alcohol, the residue dissolved in water with the addition of a little hydrochloric acid, and the platinum re- moved by electrolysis. The sodium chloride in the solution poured off from the platinum is determined by evaporating to dry ness, and weighing the residue. SODIUM AMMONIUM. The direct determination of both is accomplished as with potassium and sodium ; the ammonium is precipitated as ammo- nium platinchloride, and the process conducted as described above. APPENDIX. SOME APPLIED EXAMPLES OF ELECTBO- CHEMICAL ANALYSIS.* BRASS. Alloy of Copper and Zinc (Lead, Tin, Iron). For the separation of the copper from the other metals, it is necessary to precipitate it from an acid solution. A nitric or sulphuric acid solution may be used. The employment of a solution containing free nitric acid has the disadvantage that if the action of the current is continued for too long a period after all the copper has been precipitated, the nitric acid is reduced to ammonia, and zinc is precipitated. It has the further disadvantage that enough ammonia is often formed to prevent the complete precipitation of the zinc by sodium carbonate, a method often employed in practice. The pres- ence of nitric acid or a nitrate also prevents the electrolytic separation of the zinc. If this acid is used, therefore, the solution, after removal of the copper, must be repeatedly * The applied examples of electro- analysis here given appeared in the third German and second English editions of this work, but are not con- tained in the fourth German edition. Owing to the practical advantages of these schematic outlines, the translators have thought it best to include them in the present edition, and have, at the same time, made such altera- tions as the recent advances along the variouj^Uww^wJd^eem to justify. 231 232 QUANTITATIVE ANALYSIS BY ELECTROLYSIS. evaporated to dryness with hydrochloric acid to convert the nitrates into chlorides. For the analysis of the alloy, 0.1-0.2 g is dissolved in as little dilute nitric acid as possible, and evaporated to dryness on the water-bath. The residue is then treated with a few cubic centimeters of water, and 20 cc nitric acid (sp. g. = 1.27) are added. The solution is now diluted to about 100 cc, and any stannic oxide which may be present is filtered off and de- termined gravimetrically. From the solution, the final vol- ume of which should be 120 cc, the copper is precipitated by electrolysis according to the directions given on page 156. The current is continued as long as a drop of the solution gives a blue color with ammonia. If lead is present in the alloy it may be determined at the same time as the copper, since it separates on the positive electrode in the form of peroxide. A weighed positive elec- trode is employed, and the precipitated peroxide is washed and treated according to the directions on page 169. The separated lead peroxide and copper are washed without inter- rupting the current. The zinc is best determined in the solution by adding about 5 cc dilute sulphuric acid and evaporating on the water- bath until no odor of nitric acid can be detected. The residue is dissolved in a small quantity of water, and a slight excess of ammonia added. If iron is present it will be precipitated as hydroxide, which may be filtered off from the solution and determined gravimetrically. Ammonium oxalate or lactate is now added, and the separation of the zinc conducted under the conditions * given on page 147. When a sulphuric acid solution is employed for the sepa- * The same electrode upon whicli the copper has been precipitated may be used for receiving the zinc. By this the necessity of especially prepar- ing a copper-plated electrode is avoided. APPENDIX. 233 ration of the copper, it is best to first dissolve the alloy in dilute nitric acid and filter off any stannic oxide as before, xln excess of sulphuric is then added, and the solution is evaporated until all nitric acid is driven off. The residue is now treated with water, any lead which is present being then found in the form of sulphate, which can be removed by filtering and determined gravimetrically. The solution is diluted to 115 cc, 5 cc nitric acid (sp. g. == 1.21) are added, and the precipitation of the copper conducted under the con- ditions given on page 156. After the copper has been sep- arated, the solution is evaporated to drive off nitric acid, and the separation of the zinc is carried out as in the previous case. SILVER COIN. Alloy of Copper and Silver. The alloy is analyzed by dissolving 0.1-0.2 g in dilute nitric acid, evaporating off the acid on the water-bath, dis- solving the residue in water, and treating the solution accord- ing to the directions on page 216. NICKEL COIN. Alloy of Copper and Nickel. About 0.4 g of the alloy, best in the form of small cut- tings, is dissolved in dilute nitric acid, 8 cc of dilute sulphuric acid (50 per cent) is added, and the solution is evaporated on the water-bath until all nitric acid is removed. The residue is then taken up in 150 cc of water, and electrolyzed with a current of ND 100 = 1 ampere, and an electrode tension of 2.75-3 volts. After the removal of the copper the solution is neutralized with ammonia, an excess of 40 cc ammonia (sp. g. 0.96) is 234 QUANTITATIVE ANALYSIS BY ELECTROLYSIS. added, and the nickel is precipitated by a current of OT3 100 = 0.5-1.5 amperes, and a tension at the electrodes of 2.8-3.3 volts. GERMAN SILVER. Alloy of Copper, Zinc, Nickel (Tin, Lead). For the analysis of this alloy about 0.3 g of the metal is dissolved in nitric acid, 10 cc concentrated nitric acid added, the solution diluted to 150 cc, and electrolyzed at ordinary temperatures with a current of !ND 100 = 0.5-1 ampere, and an electrode tension of 2.5-2.8 volts. The solution from which the copper has been removed is evaporated to dryness with sufficient sulphuric acid to convert the nitrates present into sulphates, and the residue is dissolved in water. To the solution containing the zinc and nickel 5 g potas- sium sodium tartrate is added, and the solution is made alka- line with sodium hydroxide. The zinc is now precipitated with a current of ND 100 = 0.3-0.6 ampere, and an electrode tension of 2 volts.* The zinc may be precipitated on the electrode bearing the copper precipitate. In this operation oxide of nickel may separate on the positive electrode, or may form in the solution in sufficient quantities to slightly discolor the precipitated zinc. This may be avoided by adding to the solution a small quantity of potassium iodide. The solution, containing now only nickel, is acidified with sulphuric acid, an excess of ammonia added, and the nickel separated according to the directions for cobalt given on page 142. Another method is to add 25 cc ammonia and 15-20 g ammonium carbonate directly to the nickel solution, and elec- trolyze with a current of KT> 100 = 0.8-1 ampere, at a temper- ature of 50-60. f * Vortmann, Monatsh. f. Chem., 14, 536. f Neumann, Analytiscben Elektrolyse, Halle, 1897. APPENDIX. 235 BRONZE. Alloy of Copper and Tin. The alloy in a finely divided form is treated with aqua regia, and the solution is evaporated to dryness. The residue is digested with a concentrated solution of sodium sulphide, the tin being dissolved. The copper sulphide which remains after filtering is washed thoroughly with sodium sulphide and then with hydrogen sulphide solution, dissolved in the proper quantity of nitric acid, and the copper precipitated under the conditions given on page 156. The solution of tin in sodium sulphide is brought to a volume of about 150 cc, 2530 g ammonium sulphate is added, and the solution is boiled for about one half hour to convert the sodium sulphide into ammonium sulphide (see page 187). The solution thus obtained is treated as described on page 186. . Accurate results may also be obtained* by treating 0.2-0.4 g of the alloy, best in the form of fine turnings, with 6 cc nitric acid (sp. g. = 1.5), and adding 3 cc water. When the reaction is over, the solution is heated to boiling, diluted with 15 cc boiling water, and the stannic oxide filtered off. To the solution containing the copper, 5-10 cc of nitric acid is added, and the copper is precipitated as directed on page 156. The stannic oxide is dissolved in ammonium sul- phide and determined electrolytically (page 186). PHOSPHOR-BRONZE. Alloy of Copper, Tin, Zinc, and Phosphorus. When the alloy is digested with concentrated nitric acid as stated under Bronze, a precipitate remains, which consists * Neumann, 1. c. 236 QUANTITATIVE ANALYSIS BY ELECTROLYSIS. of a mixture of tin oxide and tin phosphate, with small quan- tities of copper oxide. It is filtered off, washed with water containing nitric acid, and heated with a concentrated solution of sodium culphide. The residue of copper sulphide is dis- solved in nitric acid, and added to the principal solution. The tin is determined by converting the sodium sulphide into ammonium sulphide, and electrolyzing as directed p. 186. The phosphoric acid is determined in the filtrate in the usual manner. The nitric acid solution contains the copper and zinc. They are separated according to directions for the analysis of brass (p. 231). MANGANESE PHOSPHOR-BRONZE. Alloy of Copper, Tin, Zinc, Manganese, and Phosphorus. The process is similar to that given for Phosphor- Bronze ; the manganese remains with the zinc, and is finally separated as directed p. 208. SOLDER. Alloy of Tin and Lead. About 0.4 g of the alloy in the form of small pieces is treated with 6 cc nitric acid (sp. g. 1.5) and 3 cc water. When the reaction is completed the solution is heated to boil- ing, and diluted with 15 cc hot water, the precipitate of stan- nic oxide allowed to settle, filtered oft', and washed with water containing a little nitric acid. The stannic oxide may be determined gravimetrically, or may be dissolved in ammonium sulphide and determined by electrolysis according to the direc- tions given on page 186. The lead contained in the filtrate may be determined by the method given on page 169. APPENDIX. 237 WOOD'S METAL. Alloy of Tin, Lead, Bismuth, and Cadmium. The alloy is treated similarly to solder, the tin being sepa- rated and determined in the same manner. Since it is impos- sible to separate lead and bismuth by electrolysis, it is necessary to evaporate the solution to a sirup on the water-bath, add water and repeat the operation until the odor of nitric acid can be no longer detected. The solution is then treated with dilute ammonium nitrate solution, and the basic bismuth nitrate is filtered oft'.* A sufficient excess of nitric acid is added to the filtrate, and the lead is determined by electroly- sis. The cadmium is precipitated by one of the methods given under Cadmium. HARD LEAD. TYPE-METAL. Alloy of Lead and Antimony (Copper). The two metals may be separated, either by oxidizing with nitric acid, evaporating to dryness, and digesting the residue with sodium sulphide, or by heating the finely divided alloy with ten times its weight of anhydrous sodium thiosulphate in a covered porcelain crucible, over a very low fiame, till the mixture is sintered together, and extracting with water. In either case, lead sulphide remains undissolved, and is filtered off, and washed first with sodium sulphide, and then with hy- drogen sulphide, solution. It may be determined directly as sulphide, or as directed p. 169. The antimony is determined, in the filtrate from lead sul- phide, exactly as directed p. 180. The following method is recommended by Neumannf : 2.5 g of the alloy are brought into a 250-cc graduated flask, * Neumann, Analytisclien Elektrolyse, Halle, 1897. f Analytischen Elektrolyse, Halle, 1897. 238 QUANTITATIVE ANALYSIS BY ELECTKOLYSIS. 10 g tartaric acid, 15 cc water, and 4 cc strong nitric acid are added, and solution is effected by warming. To the clear solution 4 cc of concentrated sulphuric acid is added, it is di- luted somewhat, allowed to cool, and then diluted to the mark. 50 cc of the filtrate, corresponding to 0.5 g of the substance, is made strongly alkaline with sodium hydroxide, treated with 50 cc saturated sodium sulphide solution, heated to boiling, and immediately filtered. The filtrate, while still hot, is electrolyzed with a strong current according to the directions given on page 181. For the determination of the copper which is present, the residue remaining after treating with sodium sulphide is dissolved in nitric acid, the solution is di- luted, and the copper separated as given on page 156. If the percentage of lead is also required, 0.5 g of the alloy may be taken and the precipitated lead sulphate determined gravi- metrically ; it is more satisfactory, however, to treat the solu- tion of the metals directly with sodium hydroxide and sodium sulphide. The residue, consisting of the sulphides of lead and copper, is then dissolved in nitric acid, and the separation of the two metals is conducted under the conditions given on j)age 213. ALLOY OF ANTIMONY AND TIN. The method of analysis has been already given on p. 121. The alloy is oxidized with nitric acid, and the residue, after evaporation, dissolved in a concentrated solution of sodium sulphide, sodium hydroxide added, and the process followed throughout as given on p. 122. ALLOY OF ANTIMONY AND ARSENIC. It has already been stated (p. 224) that the two metals can be separated under conditions similar to those in the APPENDIX. 239 separation of antimony from tin ; the method requires the arsenic to be oxidized to arsenic acid. The alloy is digested with aqua regia, the acid removed by evaporation, the residue dissolved in concentrated sodium sulphide, sodium hydroxide added, and the directions given on p. 225 followed throughout. ALLOY OF ANTIMONY, TIN, AND ARSENIC. When this alloy is oxidized with aqua regia, and a solu- tion in sodium sulphide prepared as above, antimony alone is electrolytically deposited in presence of tin. The method is described on p. 225. SPATHIC IRON ORE. Constituents : Ferrous Carbonate, -with Manganese, Calcium, and Magnesium Carbonates (Gangue). All the constituents of the mineral may be determined in the same solution. About 0.5 g of the dry mineral is dissolved in a porcelain dish, in the least possible amount of hydrochloric acid, the acid removed by evaporation, and the residue taken up with water to which a little hydrochloric acid is added. If insoluble gangue is present, this is filtered off, washed with water, and weighed. The metals are con- verted into oxalates by treatment with potassium and ammo- nium oxalate, and the insoluble residue of calcium oxalate filtered off, and washed with hot water. If manganese is pres- ent, the calcium oxalate always carries down some manganese oxalate.* When the precipitate is ignited, a mixture of CaO and MTi 2 O 3 is obtained. It is weighed, and the manganese in it determined volumetrically.f The iron and manganese are separated as directed on p. 195, * Classen, Zts. anal. Ch., 16, 318. f Classen, Quant. Anal., 4th ed., p. 128. 240 QUANTITATIVE ANALYSIS BY ELECTKOLYSIS. the manganese finally precipitated as sulphide, and the mag- nesium in the filtrate as magnesium ammonium phosphate. If magnesium is absent, the manganese is determined as mangano-manganic oxide or sulphate (p. 196). HEMATITE. Constituents: Ferric Oxide, Manganic Oxide (Copper Oxide, Alumina, Lime. Magnesia), Phosphoric Acid, Sulphuric Acid. The iron, manganese, and calcium are determined a& above. If copper is present, it is first separated from the other metals by submitting the double oxalate solution to a very weak current. If, in addition to iron (copper, if present) and manganese, phosphoric and sulphuric acids are to be determined, the metals are converted into double oxalates, and iron and manganese completely removed (see separation of Iron and Manganese, p. 195); the two acids may now be determined in the solution entirely free from manganese. If only one acid is to be determined, the whole filtrate can be used ; otherwise it is diluted to a known volume, and aliquot portions taken for analysis. In the determination of either acid, the solution is first acidified with hydrochloric acid,* and then treated either with barium chloride, or with one-third its volume of ammonia, and mag- nesium mixture. About 1 g of the mineral is needed for the determination of sulphuric and phosphoric acids. If alumina, as well as phosphoric acid, is present in hematite (its presence is shown by a white turbidity f of * If the acid carbonates produced from the oxalates are not decomposed, small hard crystals of acid carbonates are precipitated together with am- monium magnesium phosphate. These crystals are difficultly soluble in ammonia, and may make the results too high. t A turbidity often appears when the solution is first heated, caused by the driving off of ammonium compounds. APPENDIX. 241 aluminium phosphate and hydroxide in the solution under- going electrolysis), the manganese must always be converted into sulphide. The iron-free solution is boiled to decompose hydrogen ammonium carbonate, tartaric acid or a solution of a tar tr ate added till the precipitate of aluminium hydroxide disappears, and the weakly ammoniacal solution precipitated hot with ammonium sulphide. The green manganous sulphide is determined as hereto- fore directed. The phosphoric acid may be determined with magnesium mixture, in the filtrate from the manganese sul- phide. To determine sulphuric acid in presence of alumina, iron and manganese are removed, by electrolysis, from a separate portion, the solution is poured off, the ammonium carbonate decomposed by heat, the solution acidified with hydrochloric acid, and the sulphuric acid determined with barium chloride. Determination of Iron, Manganese, Copper, Calcium, Magnesium, Phosphoric Acid, and Sulphuric Acid. The method of determining iron, manganese, etc., in the same solution has already been given. If it is desired to determine magnesium and phosphoric and sulphuric acids, in the filtrate from manganese peroxide, it is diluted to a known volume, magnesium is determined in an aliquot part with ammonium phosphate, and phosphoric and sulphuric acids in two other portions. LIMONITE. Constituents : Ferric Hydroxide, together with Manganese Oxide (Lime, Magnesia), Phosphoric Acid, Sulphuric Acid, Silica, and Gangue. The analysis may be conducted like those of hematite and spathic iron ; but care must be taken, at the outset, to 242 QUANTITATIVE ANALYSIS BY ELECTROLYSIS. convert the silica into the insoluble modification by evapo- rating the solution, and drying the residue. CLAY IRON-ORE. Constituents : Iron Oxide, Alumina, Manganese, and Water. The mineral is digested with concentrated hydrochloric acid till it is completely decomposed, the insoluble residue is filtered off, the filtrate evaporated to remove free acid, the residue dissolved in water with a few drops of hydrochloric acid, and the iron separated from aluminium and manganese as directed pp. 194-198. BOG IRON-ORE. Mixture of Ferric Hydroxide -with Ferrous and Ferric Silicates, Manganese, Alumina, Copper, Calcium, Magnesium, Sulphuric Acid, Phosphoric Acid, Arsenic Acid, Organic Matter, and Gangue. The analysis of the mineral is easily understood from the foregoing. Arsenic and copper are best determined by eliminating the former as chloride, as directed p. 226, and precipitating the copper with hydrogen sulphide in the greatly diluted residue left in the distillation flask. The copper sulphide is dissolved in nitric acid, and determined electrolytically as directed p. 156. CHROME IRON ORE. Constituents : Chromium Oxide, Ferrous and Ferric Oxides, Alumina, Manganese, Calcium, Silica. The finely powdered mineral is fused for a long time with sodium carbonate and potassium chlorate, and the fused mass APPENDIX. extracted with water. The residue contains oxides of iron., manganese, calcium, magnesium, and aluminium, and traces of chromium and silica ; the solution, chromic acid, silica, and some alumina and lime. The residue is dissolved in hydrochloric acid, the solution evaporated to dryness to separate silica, the residue treated with water and a little hydrochloric acid, and filtered. The metals in the filtrate are converted into double oxalates. If manganese is present, the precipitate of calcium oxalate must be treated as directed p. 239. The filtrate from the calcium oxalate, which contains iron, manganese, aluminium, and chromium, is treated as directed pp. 194199. The aqueous solution from the fused mass is evaporated to separate silica, the calcium precipitated as oxalate, and the aluminium and chromic acid separated accord- ing to previous directions. Edgar F. Smith recommends the use of the galvanic cur- rent for the decomposition of chrome iron ore. The process, according to his directions, is conducted as follows : Thirty or forty gin. potassium hydroxide are heated in a nickel crucible until the mass is in a condition of quiet fusion. The chrome iron ore for decomposition (about 0.5 gm.) is finely pulverized, weighed on a watch-glass, and gradually added, with the help of a camel's-hair pencil, to the crucible contain- ing the fused alkali. The crucible is then covered with a perforated watch-glass and connected with the anode of the battery or other source of current. The kathode employed is a thick platinum wire, which is plunged through the opening in the watch-glass into the fused mass. To regulate the current an amperemeter (p. 31) is inserted, and a switch is also placed in the circuit, so adjusted as readily to produce the reversal of the current, which is necessary toward the close of the process. The current strength must not exceed 1 ampere. After about 30 minutes the current is reversed by the switch, so that the 244 QUANTITATIVE ANALYSIS BY ELECTROLYSIS. crucible becomes the kathode, and the platinum wire the anode. The object of this reversal is to oxidize completely the last traces of the mineral, minute portions of which may have been protected by metallic iron which had been deposited by the current. After the current has acted in this direction for 10 minutes, the decomposition is complete. The fused mass of course contains the chromium as chromate. The author pursued similar researches some years since, and can confirm Smith's results. PSILOMELANE. Constituents : Manganous Oxide, Copper Oxide, Ferric Oxide, Nickel Oxide, Cobalt Oxide, Alumina, Lime, Potash, Soda, and Lithia. Determination of Manganese, Copper, Iron, Aluminium, Nickel, Cobalt, and Calcium. A weighed portion of the mineral is dissolved in hydro- chloric acid, evaporated to dryness, dissolved in water with a^ few drops of hydrochloric acid, converted into double oxalates, calcium oxalate filtered off, and the calcium and manganese in the precipitate determined as directed p. 239. In the filtrate, the copper is first determined electrolyti- cally (p. 155). After the precipitation of the copper is complete, the solution, which contains the other metals, is decanted from the copper precipitate, and is then again submitted to electrolysis for the precipitation of iron, co- balt, nickel, and manganese, the latter as dioxide at the positive electrode. After the electrolysis is completed, the solution is decanted from the precipitated metals, and the remaining manganese completely precipitated, according to directions given on p. 196. If only the weight of nickel and cobalt together is desired, the precipitate containing the APPENDIX. 245 three metals is dissolved in hydrochloric acid, and the iron determined by titration with potassium permanganate as directed p. 191. Otherwise the cobalt and nickel must first be separated from the iron. The precipitate of the metals is dissolved in hydrochloric acid, the acid removed by evapora- tion, the residue oxidized with hydrogen peroxide or bromine water, dissolved in water with a few drops of hydrochloric acid, and the metals converted into double oxalates by addi- tion of potassium oxalate in slight excess. From the boiling solution, which should have a volume of 80-100 cc, the cobalt and nickel are precipitated as oxalates by concen- trated acetic acid. A great excess of acetic acid must be used, and the solution, after the filtrate has subsided, must be tested with the reagent for a further precipitate. The filtrate from the cobalt and nickel oxalates contains all the iron as potassium iron oxalate.* The precipitate of nickel and cobalt oxalates is washed with a mixture of equal parts of alcohol, acetic acid, and water, and, after drying to remove acetic acid and alcohol, is dissolved on the filter with hot water containing potassium and ammonium oxalates. The solution is electrolyzed as directed p. 141. The sum of nickel and cobalt is determined, the metals dissolved in hydrochloric acid, evaporated to dry- ness, the residue dissolved in a few drops of water, potassium hydroxide added in slight excess, and the resulting precipi- tate dissolved in concentrated acetic acid. The cobalt is precipitated with a saturated solution of potassium nitrite acidified with acetic acid. The precipitate, after standing twenty-four hours,, is filtered off, washed with potassium nitrite, and dissolved in hydrochloric acid, the solution is * Classen, Zts. anal. Ch., 18, 189 246 QUANTITATIVE ANALYSIS BY ELECTROLYSIS. evaporated to dryness, and the residue converted into the double oxalate, and electrolyzed. The nickel is determined by difference. The nickel may also be determined, instead of, or in addition to, the cobalt, by precipitating nickel with potassium hydroxide, in the filtrate from the cobalt potas- sium nitrite, filtering, dissolving in hydrochloric acid, and separating nickel electrolytically as directed p. 144. To determine the iron in the filtrate from cobalt and nickel oxalates, the alcohol and acetic acid are completely removed by evaporation, the residue dissolved in water, and the iron electrolytically deposited from the solution of the double oxalate (p. 138). Determination of Potassium, Sodium, Lithium, Calcium, and Magnesium. The mineral is dissolved in hydrochloric acid, evaporated to remove acid, and treated with an excess of ammonium oxalate. The filtrate from calcium oxalate is electrolyzed, iron, nickel, cobalt, and copper separating as metals, man- ganese as dioxide, and aluminium as hydroxide. The filtrate from the manganese dioxide and aluminium hydroxide con- tains only alkalies, magnesium, and a little manganese. It is boiled to 'remove the hydrogen ammonium carbonate formed by the electrolytic decomposition of ammonium oxalate, con- centrated to about 50 cc ? heated to boiling, and at least an equal volume of concentrated acetic acid added. The pre- cipitate consists of manganese and magnesium oxalates. It is filtered off, washed with a mixture of equal volumes of alcohol, acetic acid, and water, and ignited. The residue is MgO + Mn 2 O 3 . It is weighed, dissolved in hydrochloric acid, and the manganese determined by electrolysis as dioxide (p. 150). APPENDIX. 247 The alkalies are determined in the filtrate from the man- ganese and magnesium oxalates. It is evaporated to dryness, the ammonium salts removed by gentle ignition, the residue dissolved in water, the solution filtered, and evaporated to t dryness after addition of a little hydrochloric acid. The residue is washed into a small stoppered flask with absolute alcohol, an equal volume of water-free ether added, and allowed to stand twenty-four hours. The solution is then filtered from the residue, the alcohol and ether evaporated, and the lithium chloride converted into sulphate and weighed. The residue of potassium and sodium chlorides is dissolved in water, and both metals directly determined as directed p. 229. SPHALERITE (ZINC BLENDE). Constituents : Zinc Sulphide, also Determinable Quantities of Iron, Manganese, Copper, Arsenic, Antimony, and Gangue. In most cases, it is only necessary to determine the zinc. The process is then as follows: About 0.5 g of the finely powdered mineral is digested with concentrated nitric acid till fully decomposed, the acid evaporated off, and the nitrates converted into chlorides by evaporation with hydrochloric acid. The residue is dissolved in about 25 cc water and 10 cc hydrochloric acid, and hydrogen sulphide passed through the solution. The precipitate of sulphides of lead, copper, etc., is filtered off, washed with water containing hydrogen sulphide and hydrochloric acid, and the, filtrate evaporated -to dryness. The residue contains chlorides of zinc, iron, manganese, calcium, and magnesium. It is dis- solved in water with a little hydrochloric acid, converted into double oxalates (p. 138), the calcium oxalate filtered oft', and the filtrate electrolyzed. Zinc and iron separate at the 248 QUANTITATIVE ANALYSIS BY ELECTROLYSIS. negative electrode, and manganese, as dioxide, at the positive. The two metals are weighed, dissolved in hydrochloric acid, and the iron determined by titration with potassium per- manganate (p. 191). It is stated on p. 194 that the precipitation of iron and zinc from the same solution is complete only when there is less than one-third us much zinc as iron, arid that it can be successfully performed, in other cases, by adding a weighed quantity of an iron salt before the electrolysis. Determination of Lead, Copper, Arsenic, Antimony, Zinc, Iron, Manganese, and Gangue. As when zinc alone is to be determined, the mineral is oxidized with nitric acid, the gangue filtered off, and the acid solution of chlorides treated with hydrogen sulphide. The precipitated sulphides are washed first with hydrogen sulphide water containing hydrochloric acid, and afterward with pure hydrogen sulphide water. The antimony and arsenic are separated from lead and copper by digestion with a concentrated solution of sodium sulphide ; the residue is washed with the same solution, and afterward with hydrogen sulphide solution. The sodium sulphide washings are added to the solution for determina- tion of arsenic and antimony, and the hydrogen sulphide washings separately collected. The necessary amount of sodium hydroxide is added to the sodium sulphide solution, and the antimony and arsenic separated and determined as directed p. 224. The sulphides of lead and copper are dissolved in nitric acid, and the metals determined as directed p. 213. Iron, zinc, and manganese are determined according to previous directions. APPENDIX. 249 CALAMINE AND SMITHSONITE. Constituents: Zinc (Cadmium), Copper, Lead, Arsenic, Antimony, Iron, Manganese, Calcium, Magnesium, Silica, Carbonic Acid, Water. Zinc and the other constituents are determined as already directed. If the mineral contains cadmium, copper and lead are first precipitated from the nitric acid solution, the decanted solution evaporated to dry ness, the cadmium nitrate converted into chloride, and cadmium determined as directed 1>. 103. ULTRAMARINE. Constituents : Alumina, Potassium, Sodium, Iron, Calcium, Sulphur, Silica, Sulphuric Acid, Chlorine. A weighed portion of the substance is dissolved in hydro- chloric acid, evaporated to dry ness to separate silica, the residue dissolved in water with a few drops of hydrochloric acid, filtered from the silica, the free acid neutralized with ammonia, and a great excess of ammonium oxalate added. The calcium oxalate is filtered off, iron and aluminium deter- mined electrolytically, the solution filtered from the alu- minium hydroxide, evaporated to dryness, the ammonium salts removed by gentle ignition, the residue dissolved in water, and the alkalies converted into chlorides by evapora- tion with hydrochloric acid. Potassium and sodium are determined as directed p. 229. 250 QUANTITATIVE ANALYSIS BY ELECTROLYSIS. REFINERY SLAG. Constituents: Ferrous and Ferric Oxides, Metallic Iron, Copper, Aluminium, Calcium, Magnesium, Silica, Sulphuric and Phosphoric Acids, A portion of the substance (0.5-1 g) is dissolved in hydrochloric acid, evaporated to remove silica, the residue dissolved in hydrochloric acid, evaporated to remove free acid, and the metals converted, as usual, into double oxalates. Calcium oxalate is filtered off, and the manganese in the precipitate determined as directed p. 239. The copper is separated by the action of a weak galvanic current (p. 156), and the iron, manganese, and aluminium separated in the copper-free solution as directed pp. 194198. For the determination of magnesium and sulphuric and phosphoric acids, see Hematite, p. 240. To determine the metallic iron, about 5 g of the finely powdered slag is placed in a small platinum or porcelain dish, and treated with an aqueous solution of copper sulphate. A quantity of metallic copper equivalent to the iron is precipi- tated (CuSO 4 + Fe = FeSO 4 -f Cu). The decomposition is hastened by frequent stirring ; the copper and undecom- posed slag are finally filtered off, washed thoroughly, and digested in the water-bath for a long time with nitric acid. In the solution, after filtration, the copper is electrolytically determined, and the quantity of iron calculated from it. COPPER AND LEAD SLAGS. Constituents : Copper, Lead, Iron, Manganese, Barium, Calcium, Magnesium, Silica, Sulphuric Acid, Sulphur, and ordinarily small quantities of Arsenic, Antimony, Bismuth, Cobalt, Nickel, and Zinc. The slag is decomposed by digestion with nitric acid, evaporated to dryness, the residue taken up with water and APPENDIX. 251 a little hydrochloric acid, and the solution filtered from the residue of silica and barium sulphate, which are separated as usual. The calcium is separated by adding ammonium oxa- late in great excess ; the calcium and the manganese it may contain are determined as directed p. 239. Copper is then precipitated (p. 155), and afterward iron and manganese (p. 194:), and magnesium and sulphuric acid are determined as directed p. 240. In the presence of arsenic, antimony, etc., the hydrochloric acid solution, after separation of silica, is treated, first hot and then cold, with hydrogen sulphide gas, and the precipi- tated sulphides are washed with hydrogen sulphide water, and treated with a concentrated solution of sodium sulphide. The insoluble sulphides of lead, copper, etc., are washed first with sodium sulphide, and then with hydrogen sulphide (see p. 237), and antimony and arsenic are separated in the solution as directed on p. 224. The residue of lead sulphide, etc., is digested with nitric acid till thoroughly decomposed, and lead and copper sepa- rated from the solution as directed p. 213. The nitric acid is evaporated off, and bismuth determined as directed p. 237. The solution filtered from the hydrogen sulphide precipi- tate, which contains iron, manganese, etc., is evaporated almost to dryness to remove hydrogen sulphide and most of the hydrochloric acid, and the metals finally converted into double oxalates. Calcium oxalate is filtered oif, and the precautions described on p. 239 are observed in its deter- mination. By electrolysis of the filtrate, iron, cobalt, nickel, and zinc are obtained as metals, and manganese, in part, as dioxide; magnesium remains in solution. The two latter are determined as directed p. 241. The iron, cobalt, etc. , are dissolved in concentrated hydro- chloric acid, the solution evaporated to dryness, the residue 252 QUANTITATIVE ANALYSIS BY ELECTROLYSIS. chloric acid, the solution evaporated to dryness, the residue dissolved in water with a few drops of acetic acid, potassium oxalate added in sufficient quantity to form the double oxa- lates, the solution diluted to 25-30 cc, and precipitated, at boiling heat, with concentrated acetic acid in great excess. After standing about six hours in a warm place, the oxalates of cobalt, nickel, and zinc are filtered off, washed with a mixture of equal volumes of acetic acid, alcohol, and water, and the oxalates converted, by very gentle heating, into oxides. The mixed oxides are dissolved in hydrochloric acid, and zinc separated from nickel and cobalt as directed on p. 234. Iron is determined, in the filtrate from the oxa- lates, as directed on p. 246. BLAST FURNACE, CUPOLA, AND BESSEMER SLAGS. Constituents : Ferrous and Ferric Oxides, Metallic Iron, Man- ganese, Aluminium, Copper, Lead, Zinc, Calcium, Magnesium, Alkalies, Silica, Sulphuric and Phosphoric Acids, Sulphur (as Calcium Sulphide). The method of analysis is so similar to the foregoing that it needs only brief mention. The slag is digested with fuming hydrochloric acid, or aqua regia, till completely decomposed, the solution evaporated on the water-bath to dryness, the residue dissolved in water and a little hydro- chloric acid, and the silica filtered off. After conversion into double oxalates, the calcium oxalate, which may contain manganese, is filtered off (p. 239), copper and lead first precipitated (p. 213), then iron and zinc with aluminium and the rest of the manganese ; iron and zinc are determined as directed p. 193, and manganese, aluminium, and magnesium as directed p. 241. The alkalies and sulphuric and phos- phoric acids are determined as heretofore directed. APPENDIX. 253 ZIRCON. Constituents : Zirconia, Iron Oxide, Lime, Silica. The mineral is decomposed by long-continued fusion with sodium carbonate, the fused mass dissolved in hydro- chloric acid, the solution evaporated to dryness, the residue taken up with water acidified with hydrochloric acid, the silica filtered off, and the filtrate treated with a great excess of ammonium oxalate. To overcome the injurious effect of sodium chloride, about 10 g ammonium oxalate must be dissolved by heating in the solution diluted to about 200 cc. The separation of iron and zirconium is carried out under con- ditions similar to those given for Iron-Beryllium, p. 201. If calcium is present, the calcium oxalate precipitate is, of course, to be filtered off before electrolysis, and determined. ARSENOPYRITE. Iron, Arsenic, Antimony, Sulphur, Gangue. A portion of the finely powdered mineral is oxidized with aqua regia till fully decomposed, the gangue filtered off, and the solution evaporated to dryness. The chlorides are con- verted into sulphates by moistening and heating with sul- phuric acid, water is added, the solution heated to 70-80, and hydrogen sulphide passed till it has cooled completely. After -standing some twelve hours at a moderate heat, the sulphides of arsenic and antimony are filtered off, and sepa- rated as directed p. 224. To determine the iron, the hydrogen sulphide is driven off from the solution, which is then treated as directed p. 138. 254 QUANTITATIVE ANALYSIS BY ELECTROLYSIS. CHALCOPTRITE (COPPER PYRITES). Constituents: Copper, Iron, Sulphur, Gangue. The mineral is oxidized with nitric acid, the gangue filtered off, and copper precipitated in the filtrate (p. 156). To determine iron, nitric acid is removed by evaporation, concentrated hydrochloric acid added, the solution again evaporated, and finally iron is precipitated, after formation of the double oxalate, according to directions on p. 138. Sulphur* may be determined in the same portion by pre- cipitating sulphuric acid with barium chloride, and removing the excess of the latter by careful addition of sulphuric acid. Copper is then separated from iron in sulphuric acid solution, and the latter determined as usual. As already stated on p. 157, copper cannot be precipitated from either nitric or sulphuric acid solution in the presence of any considerable quantity of arsenic and antimony without being contaminated by them. If only the copper is to be determined, the nitric acid solution of the mineral is evaporated to dry ness, the residue dissolved in water with a little acetic acid, and potassium oxalate added in excess. The solution is filtered hot from the gangue, the residue washed with water containing potas- sium oxalate, and the filtrate brought to a volume of about 50 cc. After cooling, almost all the copper crystallizes out as potassium copper oxalate ; the rest is precipitated by addition of much concentrated acetic acid. The precipitate is washed with a mixture of equal volumes of water, acetic acid, and alcohol, dissolved in ammonium oxalate, and elec- trolyzed. If arsenic and antimony are present in larger proportion, the finely pulverized mineral is mixed with four times its APPENDIX. 255 weight of ammonium chloride, and heated gently in a covered crucible. Arsenic and antimony, and the greater part of the iron are volatilized as chlorides.* The residue is dissolved in nitric acid, and treated as before. NICKEL MATTE. COPPER MATTE. Nickel, Cobalt, Zinc, Iron, Copper, Lead, Arsenic, Antimony, Sulphur, Gangue. The substance is decomposed with aqua regia, evaporated to dryness, the residue dissolved in hydrochloric acid, and filtered from the gangue. In this solution, the metals pre- cipitable by hydrogen sulphide are precipitated by heating to 70-80, and passing hydrogen sulphide gas till the solution becomes cold. The precipitate is filtered off, washed first with a solution containing hydrogen sulphide and hydro- chloric acid, then with pure hydrogen sulphide solution, and treated with a concentrated solution of sodium sulphide as directed p. 222, and the arsenic and antimony separated and determined as directed p. 224. The sulphides of lead and copper left undissolved by sodium sulphide are digested with nitric acid, and deter- mined as directed p. 213. The filtrate from the hydrogen sulphide precipitate is evaporated to dryness to remove hydrogen sulphide and hydrochloric acid, the residue dis- solved in water with a little acetic acid, potassium oxalate added in excess, and the solution of 50-100 cc precipitated boiling hot with a great excess of concentrated acetic acid (at least an equal volume). The precipitate of nickel, cobalt, and zinc oxalates is filtered off, washed with a mixture of equal volumes of alcohol, acetic acid, and water (p. 214), * Classen, Zts. anal. Ch., 18, 388. 256 QUANTITATIVE ANALYSIS BY ELECTROLYSIS. dried, and converted, by gentle ignition, into the oxides. The residue is dissolved in hydrochloric acid, and zinc, cobalt, and nickel separated, and determined as directed pp. 204 and 245. Iron is determined, in the filtrate from the mixed oxalates, as directed p. 245. COPPER SPEISS, LEAD SPEISS. Antimony or Arsenic Compounds of Iron, Cobalt, and Nickel, together with Sulphur Compounds of Copper, Lead, Silver, Bismuth, Iron, and Zinc. It is best to decompose the finely powdered substance in a suitable apparatus * with chlorine gas, volatilizing arsenic, antimony, iron, and zinc, as chlorides, and collecting them in a receiver containing equal volumes of hydrochloric and tartaric acids. The free chlorine is expelled, by heat, from the solution in the receiver, and hydrogen sulphide passed into the still hot solution until it cools. The sulphides are filtered, washed, treated with sodium sulphide, and arsenic and antimony determined in the solution, as directed p. 224. The insoluble sulphides of iron and zinc are dissolved in hydrochloric acid, evaporated to dryness, the residue dis- solved in water with a few drops of hydrochloric- acid, and iron and zinc determined as directed p. 193. After the decomposition with chlorine, the non-volatile chlorides of copper, lead, silver, bismuth, cobalt, and nickel, and a part of the iron and zinc, remain in the bulb. They are dissolved in dilute hydrochloric acid, and lead, copper, silver, and bismuth precipitated with hydrogen sulphide. The sulphides are digested with nitric acid till completely * Classen, Quantitative Analyse, 4th ed. p. 187. APPENDIX. 257 dissolved, and copper and silver precipitated as metals, and lead as peroxide, by electrolysis. Copper and silver are separated as directed p. 216, and bismuth from some residual lead as directed p. 237. The separation of cobalt and nickel from iron and zinc is given on pp. 204 and 244. PYRARGYRITE. Silver, Antimony (Arsenic), Sulphur, Gangue. The mineral may be decomposed by chlorine gas, or by heating with anhydrous sodium thiosulphate. In the former case, the chlorides of antimony and arsenic (and sulphur) go into solution, while silver chloride remains in the bulb tube. In the latter case, when the fused mass is treated with water, silver sulphide remains unclissolved, and may be dissolved in nitric acid, and the silver deposited, as metal, from the solu- tion (p. 174). To determine antimony, and separate it from arsenic, the solution of sodium pentasulphide is oxidized with hydrogen peroxide, evaporated, and treated as in the determination of antimony in presence of tin (p. 225). TETRAHEDRITE. Copper, Antimony, Arsenic, Silver, Lead, Iron, Zinc, Sulphur, Gangue. The mineral may be decomposed as heretofore described. When chlorine gas is used, the receiver contains chlorides of antimony, arsenic, iron, and zinc (and sulphur) ; the bulb- tube, copper, lead, silver, and gangue, with a portion of the iron and zinc. The metals are separated as already described* 258 QUANTITATIVE ANALYSIS BY ELECTROLYSIS. FURNACE "SOWS." Alloys of Iron (the principal constituent), Copper, Silver, Lead, Molybdenum, Vanadium, Cobalt, Nickel, and Zinc, with Sulphides and Phosphides of these Metals, and varying amounts of Carbonic Acid and Silica. The substance is best decomposed by chlorine gas. The quantity of iron is so great, however, that two bulb-tubes of the most infusible glass should be used, in the second of which is deposited most of the iron chloride. The substance is heated in a stream of chlorine as long as iron chloride sublimes ; then it is certain that all the molybdenum chloride will have been carried over into the receiver, which also contains vanadium, sulphur, and phosphorus chlorides. Hydrogen sulphide is passed into the solution collected in the receiver until the supernatant liquid is colorless. The precipitate of molybdenum sulphide is filtered off, washed, oxidized with nitric acid, the solution supersaturated with ammonia, and molybdenum oxide precipitated by electrolysis. The filtrate from molybdenum sulphide contains vana- dium and iron. Hydrogen sulphide and hydrochloric acid are evaporated off, double oxalates formed, and the two metals separated electrolytically, according to the method given for the separation of Beryllium-Iron, p. 201. To determine vanadium in the solution decanted from the iron, it is evaporated to dryness, the ammonium salts driven off by careful ignition, and the residue of vanadium oxide converted, by fusion with potassium nitrate, into potassium vanadate. The fused mass is dissolved in water, nitric acid added not to acid reaction, then a concentrated solution of ammonium chloride, and then alcohol in the proportion of one volume to three of the solution. After standing forty- eight hours, the ammonium vanadate is filtered off, and washed with a concentrated solution of ammonium chloride. APPENDIX. 259 and then with alcohol. The salt is heated first in the air, then in a stream of oxygen, and leaves a residue of pure vanadic acid which is weighed. The chlorides remaining in the bulb-tube are heated with hydrochloric acid ; a residue of silver chloride and carbon remains. It is heated with potassium cyanide, the carbon filtered off, and the silver determined by electrolysis. The methods of separation and determination of the metals in the hydrochloric acid solution have already been given. STIBNITE (ANTIMONY GLANCE). Constituents: Antimony and Sulphur, and usually small quantities of Iron, Lead, Copper, and Arsenic. The simplest method of analyzing the mineral is to mix with four or five times its weight of anhydrous sodium thiosulphate, and heat for a long time in a covered crucible (p. 237). The fused mass is extracted with water; the solution contains antimony and arsenic, and is treated for decomposition of sodium pentasulphide and determination of the two metals as directed p. 224 ; the undissolved sulphides of lead, copper, and iron are oxidized with nitric acid, and the metals separated according to foregoing directions. ULLMANITE. Antimony, Nickel, and Sulphur. The finely powdered mineral is decomposed in a stream of chlorine (p. 256), all the antimony passing into the receiver as chloride, and nickel chloride remaining in the bulb-tube. The latter is determined by dissolving the con- tents of the bulb in hydrochloric acid, evaporating, convert- ing into the double oxalate, and precipitating by electrolysis. 260 QUANTITATIVE ANALYSIS BY ELECTROLYSIS. Antimony is precipitated, as sulphide, by passing hydro- gen sulphide gas into the solution, in hydrochloric and tar- taric acids, dissolved in concentrated sodium sulphide, the solu- tion diluted with water and submitted to electrolysis (p. 179). If the mineral contains iron, it passes over, as chloride, into the receiver ; it may be determined, in the filtrate from antimony sulphide, after supersatu ration with ammonia, by precipitation as sulphide with ammonium sulphide. The sulphide thus obtained is dissolved, converted into the double oxalate, and iron determined electrolytically (p. 138). The analysis is made more simply if the mineral is decomposed by heating with sodium thiosulphate ; when the proportion of antimony is large, it is necessary to repeat the process with the residual nickel sulphide. Antimony is determined in the aqueous solution of the fused mass as directed p. 181. If, on treatment with hydrogen peroxide, or addition of sodium monosulphide, some nickel sulphide separates, it is added to the principal portion. The sulphides of iron and nickel are oxidized with nitric acid, the nitrates converted into chlorides, and the two metals separated as directed (p. 192). BOURNONITE. Antimony, Lead, Copper (Iron), and Sulphur. The finely powdered mineral is heated either with chlorine or anhydrous sodium thiosulphate, and the analysis conducted as already described. ZINKENITE. Antimony, Lead (Silver, Copper, Iron), Sulphur. The mineral is most simply decomposed by heating with anhydrous sodium thiosulphate. After extracting with APPENDIX. 261 water, the residue of undissolved sulphides is dried, the filter burnt, and fusion with thiosulphate repeated. Anti- mony is determined according to directions on p. 179. The sulphides of lead, silver, etc., are oxidized with nitric acid; copper and silver precipitated electrolytically, and separated as directed p. 216. A portion of the lead is separated, as peroxide, by the electrolysis of the nitric acid solution, and is determined as such. The rest is precipitated with hydro- gen sulphide, the filtrate neutralized with ammonia, ammo- nium oxalate added, and iron determined by electrolysis. LINN-3EIITB. Constituents : Cobalt and Sulphur. The analysis of this mineral is very simple. It is dis- solved in aqua regia, the free acid evaporated off, and chlorides formed by repeated evaporation with hydrochloric acid. The aqueous solution of the residue is treated with an excess of ammonium oxalate, and cobalt precipitated electro- lytically (p. 141). If iron is present, the two metals are separated as directed p. 191. In the solution decanted from the metallic cobalt, ammo- nium carbonate is decomposed by boiling, hydrochloric acid is added, and the sulphur determined by precipitation with barium chloride. COBALTITE. Cobalt, Iron (Copper, Antimony), Arsenic, and Sulphur. The mineral may be decomposed by heating with nitric acid, or with sodium thiosulphate. If nitric acid is used, the free acid is evaporated off, and the nitrates converted 262 QUANTITATIVE ANALYSIS BY ELECTROLYSIS. into chlorides. In the hydrochloric acid solution, arsenic, antimony, and copper are precipitated, as sulphides, by pass- ing hydrogen sulphide into the hot solution till it cools; the sulphides are digested with sodium sulphide, and the solution treated as directed p. 224. The residue of copper sulphide is dissolved in nitric acid, and the copper separated by elec- trolysis (p. 156). The filtrate from the hydrogen sulphide precipitate is freed from hydrogen sulphide and hydro- chloric acid, and iron and cobalt are separated as directed p. 191. If the mineral is heated with anhydrous sodium thio- sulphate, and extracted with water, antimony and arsenic go into solution, and are determined as directed p. 224. The sulphides insoluble in water are dissolved in nitric acid, and copper first precipitated (p. 156); the nitrates are then converted into chlorides, and cobalt and iron deter- mined (p. 191). Finally, arsenic and antimony may also be determined by removing the arsenic first. The nitric acid solution is heated with sulphuric acid to convert nitrates into sulphates. The arsenic is driven off from this, as chloride, by treatment with ferrous chloride or sulphate, and distillation in a stream of hydrochloric acid (p. 226). To determine antimony, the residue in the flask is saturated with hydrogen sulphide, and filtered ; the precipitate is washed, and treated with sodium sulphide (p. 227). COBALTIFEROUS ARSENOPYRITE. Cobalt, Iron, Arsenic, and Sulphur. The mineral is analyzed in the same manner as co- baltite. APPENDIX. 263 CERUSSITE. Lead, Iron, Calcium, Carbonic Acid. The pulverized mineral is dissolved by heating with nitric acid, and the lead determined, as peroxide, by connecting the platinum dish with the positive pole of the battery (p. 169V The solution decanted from the lead peroxide is evapo- rated to dryness with hydrochloric acid, the residue taken up with water and a few drops of hydrochloric acid, treated with ammonium oxalate in great excess, calcium oxalate filtered off, and iron determined electrolytically in the filtrate (p. 138). GALENA. Lead (Antimony, Arsenic, Copper, Silver, Gold, Zinc, Iron), Sulphur, Gangue. Galena rich in antimony is decomposed either by chlorine, or by heating with anhydrous sodium thiosulphate. When decomposed with chlorine, the receiver contains antimony, arsenic, iron, and zinc. These metals are separated as directed p. 256. The chlorides remaining in the bulb-tube are dissolved in hot dilute hydrochloric acid, and evaporated on the water-bath, with addition of sulphuric acid, till the hydrochloric acid is all driven off. The residue is diluted with water, one-third its volume of alcohol added to the solution, and the lead sulphate filtered off. In the filtrate, copper and silver are precipitated with hydrogen sulphide, the sulphides oxidized with nitric acid, and determined as directed p. 216.* The filtrate from the hydrogen sulphide * AS silver and gold are present only in small quantities, they are ordinarily determined by cupellation. 264 QUANTITATIVE ANALYSIS BY ELECTKOLYSIS. precipitate is evaporated, and iron and zinc determined as directed p. 193. By heating galena with sodium thiosulphate, and extract- ing with water, antimony and arsenic (and gold) are found in the solution, and are separated as already directed ; the sulphides of lead, silver, copper, zinc, and iron remain undis- solved. The proportion of lead is so great that it cannot well be determined, as dioxide, in -nitric acid solution ; it is converted into sulphate, and the analysis completed as before. PYROMORPHITE. Lead Phosphate and Chloride, sometimes Sulphate and Arsenate. The finely pulverized mineral is digested with nitric acid, and evaporated to dryness with hydrochloric acid. The residue is moistened with hydrochloric acid, dissolved in hot water, the clear nitrate poured off, and the lead chloride, which had crystallized out, brought into solution by repeated boiling with water. Lead and arsenic are precipitated by passing hydrogen sulphide into the hot solution till it cools, filtered hot after long standing, and the precipitate washed and digested with sodium sulphide. Arsenic is determined in the solution as directed p. 223. The lead sulphide is oxidized with nitric acid, and lead determined, as peroxide, as directed p. 168. Phosphoric acid is determined, in the usual way, in the filtrate from the hydrogen sulphide precipitate. LEAD MATTE. Lead, Copper, Iron (Silver, Antimony, Nickel, Zinc), Sulphur. If the mineral is decomposed by heating in chlorine, iron and antimony pass over into the receiver. The analysis is conducted according to directions for copper or lead speiss. APPENDIX. 265 CINNABAR. Constituents : Mercury, Manganese, Copper, Alumina, Iron, Calcium, Sulphur. The mineral is decomposed by heating with aqua regia, the solution evaporated on the water-bath, and the metals converted into nitrates by repeated evaporation with nitric acid. Mercury and copper are precipitated from the nitric acid solution (p. 175), the two metals redissolved in nitric acid, converted into the double cyanides, and determined according to the directions on p. 216. The small amount of manganese present is precipitated, as dioxide, in the elec- trolytic process, and may be weighed as such. To determine iron, aluminium, and calcium, the solution decanted from the metals is evaporated to dryness on the water-bath, the nitric acid removed by repeated evaporation with hydrochloric acid, the weak acid solution of the residue treated with ammonium oxalate in great excess, calcium oxalate filtered off, and iron and aluminium determined as directed p. 197. SOFT LEAD (CRUDE LEAD). In addition to Lead, small quantities of Silver, Copper, Bismuth, Anti- mony, Arsenic, Cadmium, Iron, Zinc, Cobalt, Nickel. According to the purity of the metal, 200 to 500 grams are taken. The weighed quantity, cleaned and rolled into thin plates, is digested with a mixture of about 250 cc con- centrated nitric acid, sp. gr. 1.4, and 500-600 cc water. The solution is hastened by careful heating on a sand or water bath. If the acid works very actively, the flask is removed from the bath, but not long enough for crystals of lead 266 QUANTITATIVE ANALYSIS BY ELECTROLYSIS. nitrate to separate from the cooled solution. If there is not more than 0.02-0.03 per cent of antimony, a perfectly clear solution is finally obtained. If the filtrate is turbid from the presence of lead antimonate, the precipitate is filtered off, and washed thoroughly with water (residue I). The nitric acid solution is transferred into a 2 -liter measuring-flask, about 170 cc of dilute sulphuric acid are added (1 part concentrated sulphuric acid and 2 parts water), thus precipitating all the lead as sulphate, and the flask is filled to the mark. The contents of the flask are thoroughly shaken, the precipitate allowed to settle, and the greater part of the solution siphoned off, taking care not to disturb the lead sulphate. 1,750 cc of the clear solution are evaporated till white fumes of sulphuric acid appear; 50-60 cc of water are added after cooling, and the small amount of lead sulphate that may remain undissolved is filtered off. As this latter may contain antimony, it is digested with concentrated sodium sulphide, and the solution siphoned off (solution I). The filtrate from lead sulphate is heated to about 70, and hydrogen sulphide passed in till it cools. When the precipitate, after long standing on the sand-bath, has com- pletely subsided, it is filtered off, washed thoroughly with water containing hydrogen sulphide, and digested with a concentrated solution of sodium sulphide. The residue marked I is also treated with sodium sulphide, and the dissolved portion, together with solution I, added to the principal solution. Antimony and- arsenic are then separated and determined as directed p. 224. The sulphides insoluble in sodium sulphide (copper, cadmium, etc.) are digested with nitric acid till completely oxidized, and copper and silver are separated from the solution, as metals, by electrolysis, and any remaining lead APPENDIX. 267 as peroxide. The copper and silver are separated and determined as directed p. 216. To determine bismuth arid cadmium, the nitric acid is completely removed by evaporation, the residue dissolved in water with a few drops of dilute hydrochloric acid, potassium cyanide added, and the solution gently heated on the water- bath ; the potassium bismuth cyanide is filtered off and washed with water. The bismuth may then be determined gravi- metrically. Cadmium can be directly electrolyzed from the solution of cadmium potassium cyanide (p. 165). The filtrate from the original hydrogen sulphide precipi- tate, which contains zinc, iron, cobalt, nickel, etc., is heated to boiling and oxidized with bromine- water. An excess of sodium hydroxide is added, and the metals, with the excep- tion of zinc, are precipitated as hydroxides. The solution is filtered off, and the zinc precipitated by electrolysis, either directly from the filtrate, or after being first converted into some other salt. The hydroxides are dissolved in dilute sul- phuric acid, the iron is precipitated with ammonium hydroxide and determined either gravimetrically or by electrolysis. The nickel and cobalt are determined in the solution, from which the iron has been removed, by electrolysis under the condi- tions given on p. 144. In calculating the analysis, the space occupied by the lead sulphate in the solution is to be taken into account. 100 g lead converted into sulphate occupy a space of 23 cc; 200 g, therefore, 46 cc. Accordingly in making the calculation, 1750 cc are to be reduced, not to 2000 cc, but to 2000 46 = 1954 cc, or to 179.12 g lead. Crude lead is also analyzed by the foregoing method; 10 to 50 g is a sufficient quantity for the analysis. 268 QUANTITATIVE ANALYSIS BY ELECTKOLYSIS. ANTIMONY. Metallic antimony may be treated in the same way as hard lead, p. 237. SPELTER (CRUDE ZINC). Zinc and determinable quantities of Lead, Iron, Cadmium, Arsenic, Antimony, Tin, and Copper. In the analysis of crude metals, the determination of the impurities is of more importance than that of the metal. As the quantity of other metals is so small, it is necessary to dissolve a large quantity of zinc. According to its purity, 25 to 100 g are taken, and dissolved, in a flask, by gradual addition of hydrochloric acid, some zinc, however, being left undissolved. If the zinc comes in sticks, a stick may be fastened to a platinum wire, and dipped partly into the solution, and the undissolved zinc removed, cleaned, and weighed. In both cases, zinc only goes into solution , the other metals, with the exception of arsenic and antimony, being left as spongy masses. It is necessary, however, to filter the solution of zinc at once, and to wash the residue. The latter is digested with nitric acid, and carbon and silica, with all the tin oxide and small quantities of antimony (most of it was volatilized during the solution in hydrochloric acid) and lead, remain undissolved. To determine the tin, the residue is heated with concentrated hydrochloric acid, carbon and silica are filtered off, the filtrate is evaporated to dry ness, and the residue digested with a concentrated solution of sodium sulphide. The antimony and tin in the filtered solution are separated as directed p. 221. The nitric acid solution of the metals is evaporated, the APPENDIX. 269 residue dissolved in dilute hydrochloric acid, diluted with water, and hydrogen sulphide passed into the hot solution till it lias thoroughly cooled. The precipitate, after settling, is filtered off, washed with water, and digested with concen- trated sodium sulphide. The sulphides of lead, copper, etc., remain, are dissolved in nitric acid, and separated as in the analysis of soft lead. The filtrate from the hydrogen sulphide precipitate is heated to boiling and treated with bromine- water ; and the metals present, iron, zinc, nickel, cobalt, etc., determined as in the analysis of soft lead. Antimony and arsenic must be determined in a separate portion of zinc, which is dissolved in aqua regia. The aqua regia is evaporated off, the residue treated with concentrated hydrochloric acid, and again evaporated, and finally dissolved in dilute hydrochloric acid. Hydrogen sulphide is passed into the solution as before, the sulphides are filtered off after long standing, washed thoroughly with water, and digested with a concentrated solution of sodium sulphide. Antimony and arsenic and tin, if present, are determined as directed pp. 225 ff. BLISTER COPPER* Copper, Iron, Lead, Silver, Antimony, Arsenic, Bismuth, Zinc, Nickel, Cobalt grams of blister copper must be taken to determine the impurities ; it is analyzed in two separate portions of * Process of analysis partly after W. Hampe ( " Beitriige zur Metal lurgie des Kupfers"), Zts. fiir Berg-, Hiltten- und Salinenwesen, 27, 205. 270 QUANTITATIVE ANALYSIS BY ELECTROLYSIS. 25 g each. Each portion of 25 g of bright copper cut- tings is digested with a mixture of about 175 cc nitric acid, sp. gr. 1.2, and 200 cc water, till no metallic residue is left ; and after cooling, whether the solution is clear or not, 25 cc of concentrated sulphuric acid are carefully added. The solution is evaporated on the water-bath, and heated on the sand-bath till the excess of sulphuric acid is driven off. After cooling, 20 cc nitric acid is added, the solution is diluted with 300 or 400 cc water, and heated to dissolve copper sulphate. This solution is treated with exactly enough* of a titrated solution of hydrochloric acid to precipitate the silver, and allowed to stand twenty-four hours, after which the precipi- tate (I.) of silver chloride, lead sulphate, antimony oxide, etc., is filtered off and washed with water. The filtrate is brought to a volume of 400-450 cc, and the copper separated by electrolysis. For this purpose, either a larger platinum dish is used, or the platinum cone shown in Fig. 61, p. 88; and the current is continued only so long as is necessary to remove the copper, as otherwise it might be contaminated with antimony and arsenic. If the copper is darkened by these metals, the process given on p. 157 must be followed. There is usually a slight deposit of lead peroxide on the positive electrode which is determined as directed p. 169. The precipitated copper contains bismuth. To determine the latter, the copper precipitated from both 25-g portions is dissolved in about 350 cc nitric acid, sp. gr. 1.2; a great excess of concentrated hydrochlowc acid added, and the solution boiled till all the nitric acid is driven off. It is evaporated on the water-bath till the residue has a brown * Silver must be previously determined in a separate portion of 25 g 1 . APPENDIX. 271 color, and then poured into a large quantity of boiling water to separate the bismuth as oxychloride. The bismuth oxy- chloride is generally contaminated with some basic copper salt. If the color shows the quantity of the latter to be considerable, the precipitate, after standing twenty-four hours, is filtered off, dissolved again in concentrated nitric acid, diluted with water, and copper precipitated electro- lytically (p. 156). The bismuth in the solution is determined gravimetrically. The solution siphoned off from the main portion of the copper is evaporated to dryness, and the sulphuric acid set free by the precipitation of copper removed by heating on the sand-bath, so that the residue contains only traces of acid. After cooling, it is dissolved in hydrochloric acid and water, any silica from the glass vessels filtered off, and hydrogen sulphide passed into the solution, heated to 70-80, till it is thoroughly cool. The precipitate, which consists mostly of arsenic and antimony, is filtered off after long standing on the sand-bath, and washed ; the filtrate, containing iron, cobalt, etc., is retained (II.). Another portion of the antimony is in residue I., which was left on the solution of the blister copper in nitric acid. Both precipitates are digested with a concentrated solution of sodium sulphide, filtered, and antimony and tin determined as directed p. 222. The sulphides insoluble in sodium sulphide are oxidized with nitric acid, silver (p. 172), and lead (p. 169) precipitated from the solution, the solution siphoned off, evaporated to remove nitric acid, and bismuth determined gravimetrically. Solution II. filtered from the hydrogen sulphide precipi- tate, which contains iron, cobalt, etc., is evaporated to remove hydrogen sulphide, etc., and the metals are deter- mined in the residue as directed p. 267. 272 QUANTITATIVE ANALYSIS BY ELECTROLYSIS. REFINED COPPER. This contains, in addition to the metals present in blister copper, cuprous oxide. The metals are determined as in blister copper. The determination of the cuprous oxide is based on the fact that it reacts with a dilute neutral silver solution, with the formation of metallic silver and basic copper nitrate, which precipitate, and normal copper nitrate, which remains in solution. 3Cu 2 O + 6AgNO 3 + 3H 2 O = 2Cu 2 (OH) 3 N0 3 + 2Cu(N0 3 ) 2 + 6Ag. The process is as follows: About 2 g silver nitrate is dissolved in 100 cc water, and about 1 g of the copper to be tested is added. When the reaction is ended in the cold, the precipitate is filtered off, and washed thoroughly with water ; either the copper or the silver in it may be deter- mined electrolytically. The nitric acid is removed by evaporation, and copper and silver separated as directed p. 216. If copper is to be determined, silver is precipitated as silver chloride from the aqueous solution of the residue, the excess of acid removed, and copper precipitated, by electrolysis, from solution of copper ammonium oxalate (p. 155). TIN. The Impurities are usually Copper, Lead, Bismuth, Iron, Zinc, Arsenic, and Antimony. By oxidation of the metal with nitric acid, the tin is completely converted into insoluble oxide, while the other APPENDIX.* 273 metals remain, for the most part, in solution. The tin oxide contains, however, detenninable quantities of lead, copper, antimony, and arsenic. The methods already described are used for their separation ; the tin oxide is digested with a concentrated solution of sodium sulphide, or fused with anhydrous sodium thiosulphite in a porcelain crucible. The insoluble sulphides of copper and lead are oxidized with nitric acid, and the solution added to the principal solution of the metals. The rest of the process is in accordance with previous directions. SILVER. Traces of Gold, also Lead, Copper, Antimony, and Arsenic. The gold remains undissolved when a large quantity of silver is dissolved in nitric acid entirely free from hydro- chloric acid. To determine copper and lead, the silver is precipitated from the largely diluted solution by hydro- chloric acid, the silver chloride filtered off, and copper and lead separated, after removal of hydrochloric acid, as directed p. 213. As antimony and arsenic can only be present in very small quantities, they are determined in a larger weight of silver. The silver is precipitated as chloride, and the metals precipitable by hydrogen sulphide by passing the gas into the hot filtrate. Antimony and arsenic are separated from the other metals by digestion with sodium sulphide, and determined as usual (p. 224). 274 QUANTITATIVE ANALYSIS BY ELECTROLYSIS. COMMERCIAL NICKEL. Nickel, Copper, Arsenic, Antimony, Iron, Cobalt (Carbon, Silica, Sulphur). The nickel is dissolved in nitric acid, the insoluble residue filtered off, the nitric acid removed by evaporation, the resi- due dissolved in hydrochloric acid, and hydrogen sulphide passed in to remove the metals which it will precipitate. It is best to redissolve the sulphides and repeat the precipitation. Antimony and arsenic are separated from copper by digest- ing the sulphide with sodium sulphide, and determined as usual* It is to be noted, in determining antimony, that the insoluble residue (silica, etc.) may contain antimony, and must be tested for it. To separate cobalt and nickel from iron, the filtrate from the hydrogen sulphide precipitate is evaporated to dryness, the residue oxidized with hydrogen peroxide or bromine water, and dissolved in water with addition of acetic acid. The metals are then converted into double oxalates by addition of potassium oxalate, and cobalt and nickel precipi- tated by a'cetic acid. The two metals, and the iron in the filtrate, are determined as directed p. 245. If only iron is to be determined, the three metals are precipitated from the double oxalate solution by electrolysis, the weight ascertained, and the iron determined volumetri- cally in hydrochloric acid solution (pp. 191-193). APPENDIX. 275 PIG IRON, STEEL, SPIEGEL, FERROMANGANESE. Constituents : Iron, Manganese, Copper, Zinc, Cobalt, Nickel, Chromium, Aluminium, Titanium, Arsenic, Antimony, Cal- cium, Magnesium, Silicon, Phosphorus, Sulphur, Carbon. If a complete analysis of iron is to be made, it is best to dissolve a large quantity, dilute to a known volume, and use aliquot parts of the solution. In many cases, only copper, or manganese, or certain other metals are to be determined. The complete analysis will first be described, and afterward the special determination of certain metals. 5 or 10 grams of the pure iron, in powder or turnings, are dissolved in hydrochloric acid in a capacious platinum or porcelain dish, and the solution evaporated to dryness. The residue is moistened with dilute hydrochloric acid, allowed to stand for a time that the acid may act, dissolved in water, and the insoluble residue of graphite, silica, and compounds of iron with titanium, chromium, phosphorus, and carbon, filtered off. The precipitate is ignited with the filter, fused with about its own weight of a mixture of equal parts of sodium carbonate and potassium nitrate, dissolved in water with addition of hydrochloric acid, and the solution evaporated on the water-bath. The residue is heated for a short time on the sand-bath to insure separation of silica, moistened, after cooling, with hydrochloric acid, treated with water, heated, and the silica filtered off, weighed, and tested for titanium. The filtrate contains chromium, together with the rest of the silica and titanium, and small quantities of iron and aluminium. To completely separate silica and titanium, the solution is evaporated to dryness, the residue treated with 276 QUANTITATIVE ANALYSIS BY ELECTROLYSIS. dilute sulphuric acid, and heated till all the hydrochloric acid is driven off; water is then added, silica filtered off, and titanic acid precipitated by long boiling. The filtrate from titanic acid is concentrated by evaporation, the free sulphuric acid neutralized with ammonia, iron, aluminium, and chromium converted into the double oxalates, and chromium separated as directed p. 153. For the determination of iron, aluminium, zinc, cobalt, nickel, manganese, copper, calcium, and magnesium, an aliquot part of the hydrochloric acid solution is saturated with hydrogen sulphide, and the precipitate filtered off after long standing in a warm place. Since arsenic and antimony are ordinarily present only in very small quantities, the copper sulphide can usually be oxidized with nitric acid, and the copper determined electro- lytically. If the precipitated copper is blackened by the presence of antimony or arsenic, it is treated as directed p. 157. The filtrate from the hydrogen sulphide precipitate is freed from hydrogen sulphide and hydrochloric acid by evaporation, oxidized with hydrogen peroxide or a little bromine water (by no means with nitric acid), dissolved in water with addition of a little acetic acid, and the metals converted into double oxalates by the use of potassium (not ammonium) oxalate. The insoluble calcium oxalate is filtered off, and separated from the manganese precipitated with it as directed p. 239. The filtrate is diluted * with water, heated to boiling, and an excess of concentrated acetic acid added, whereby all the zinc, cobalt, nickel, and magnesium, and a portion of the manganese, are precipitated as oxalates ; iron, aluminium, and the rest of the manganese remain in solution * Fifty cc of the dilute solution should contain 0.4-0.5 g iron. APPENDIX. 277 as double oxalates. The beaker is covered, and left standing in a warm place for six hours ; the precipitate is then filtered off, washed with a mixture of equal volumes of acetic acid, water, and alcohol, and dissolved, after drying, in ammonium oxalate. Zinc, cobalt, and nickel are separated from man- ganese and magnesium as directed p. 251. The filtrate from the oxalates is completely freed from alcohol and acetic acid by evaporation, and iron, aluminium, and manganese separated as directed pp. 194-198. As the quantity of zinc, cobalt, etc. , is generally very small, it is best, in order to facilitate the separation of the oxalates and the collection of the precipi- tate, to add about 0.2 g magnesium in the form of chloride,* so that magnesium oxalate is precipitated with the other oxalates. In this case, the magnesium in pig iron, if present at all, is determined in another portion, together with some other metal (e.g., copper). If magnesium is used, all the manganese is found in the precipitate produced by acetic acid. To determine manganese alone in pig iron, either an aliquot part of the hydrochloric acid solution, or a separate portion of 0.2-0.5 g iron may be taken, and the determina- tion conducted as directed under Spathic Iron Ore, p. 239. If copper is to be determined, the solution freed from acid and preferably oxidized is treated with ammonium oxalate in great excess, and electrolyzed as already directed. The hydrochloric acid solution may also be precipitated with hydrogen sulphide, and the copper determined in nitric acid solution (see p. 156). Determination of Arsenic and Antimony. Since these metals are present only in very small quantity, about 10 g pig iron are used for their determination, and * Dissolve magnesium oxide in hydrochloric acid, and remove the free acid by evaporation. 278 QUANTITATIVE ANALYSIS BY ELECTROLYSIS. digested with aqua regia. When solution is complete, the aqua regia is removed by evaporation, the residue treated with hydrochloric acid, and heated till no nitric acid remains. The solution is diluted, heated, and hydrogen sulphide passed into it until it is thoroughly cool (p. 253) ; the precipi- tated sulphides of arsenic, antimony, and copper are filtered off, thoroughly washed, and digested with sodium sulphide. The solution is treated like one containing polysulphides, and arsenic and antimony separated as directed p. 224. Determination of Phosphorus. About 2 g of iron is digested with nitric acid, sp. gr. 1.2, till decomposition is complete. If a carbonaceous residue is left, the nitric acid solution is poured off, and the residue heated with aqua regia. Nitric acid and aqua regia are completely removed by evaporation to dryness, and the nitrates converted into chlorides by repeatedly moistening with concentrated hydrochloric acid, and evaporating to dryness. ' The residue is treated with water, heated, and the iron brought into solution by the addition of the least possible quantity of hydrochloric acid. To convert the iron, etc., into double oxalates, six or eight times the weight of the iron, reckoned as oxide, of a mixture of 1 part potassium oxalate and 5-6 parts ammonium oxalate, is dissolved by heating in the solution, it is diluted to 250-300 cc, and electrolyzed at a temperature of about 80, The heating is maintained during the reaction ; the solution must by no means be heated to boiling, lest the iron scale off. The solu- tion is poured off when the reduction is complete, and phos- phoric acid determined as magnesium pyrophosphate. Two grams of iron are enough for tiie determination of APPENDIX. 279 phosphorus, even when the percentage is small. If a larger quantity is taken, it is best to divide the solution, after conversion into oxalates, and precipitate in several dishes. As it is not necessary to determine the iron, it may be precipitated just as well in a beaker; in this case, the negative electrode is a large piece of light platinum foil which is attached by a platinum wire to the negative pole of the source of current. Determination of Sulphur. About 2 grams of iron is oxidized, with aqua regia, to convert sulphur into sulphuric acid, and the insoluble resi- due filtered off. As a portion of the sulphur may be left in the residue, it is fused with a small quantity of a mixture of sodium carbonate and potassium nitrate, the fused mass dissolved in hydrochloric acid, and the solution thus obtained added to the other. The aqua regia is removed, the nitrates converted into chlorides, and the latter into double oxalates, as already directed. After removing the iron by electrolysis, the solution is poured off, boiled to remove ammonia, acidified with hydrochloric acid, and the sulphuric acid precipitated with barium chloride. 280 QUANTITATIVE ANALYSIS BY ELECTROLYSIS. TABLES FOR CALCULATION OF ANALYSES, A t.ATVI 1 f> Weight. Found. Required. Factor. Aluminium . 27.04 A1A Al 0.5304 Antimony . . 119.6 Sb Sb a O. 1.20017 Sb 2 S 3 1.40108 Arsenic . . . 74.9 As AsA 1.31962 AsA 1.53271 As. 2 S 3 1.64192 Barium 136.86 BaSO 4 Ba 0.58819 Ba00 3 Ba 0.69574 | BaO 0.77688 Beryllium . . 9.08 BeO Be 0.36262 Bismuth . . . 208.4 Bi BiA 1.11488 Boron . . . . 10.9 KBF 4 B 0.08639 BA 0.27613 Bromine! . . . 79.76 AgBr Br 0.42556 Cadmium . . . 111.7 Cd CdO 1.14288 CdS 1.28630 Caesium . 132.7 Calcium . 39.91 CaO Ca 0.71433 CaCO 8 Ca 0.40006 CaO 0.56004 Carbon 11.97 CO 2 C 0.272727 Ca00 3 C0 2 0.43995 Cerium . . . 141.2 Chlorine . 35.37 AgCl Cl 0.24729 Ag Cl 0.32853 Chromium . . 52.0 CrA Cr 0.81419 CrO 3 1.18581 1 TABLES FOR CALCULATION OF ANALYSES. 281 Atomic Weight. Found. Required. Factor Cobalt . . . 58.60 Co CoO L27116 Copper . . . 63.18 Cu CuO 1.25261 CuS Ic25309 Diclymium . . 145.0 Erbium . 166.0 Fluorine . 19.06 CaF 2 F 0.48853 Gold .... 196.7 Au Aa 2 O e 1.12171 Hydrogen 1 H 2 H 0,11136 Iodine. . 126.54 Agl I 0.54031 Ag I 1.17546 Iron .... 55.88 Fe FeO 1.28561 Fe 2 8 1.42842 Lanthanum . . 138.5 Lead .... 206.39 PbO 2 Pb 0.86605 PbO 0.93303 PbCl 2 1.16289 Lithium . . . 7.01 LiCl Li 0.165408 Li 2 0.35370 Li 3 P0 4 Li 0.18156 Li 2 O 0.38824 LiCl 1.09764 Magnesium . 23.94 Mg 2 P 2 7 Mg 0,21614 MgO 0.36024 Manganese . . 54.8 Mn 3 4 Mn 0.72029 MnO 0.93007 - Mn 2 O 8 1.03496 Mn0 2 Mn 0.63192 MuO 0.81596 Mn 2 O 8 0.90798 MnS0 4 Mn 0.36383 MnO 0.46979 Mn 2 O 8 0.52277 282 QUANTITATIVE ANALYSIS BY ELECTEOLYSIS. Atomic Weight. Found. Required. Factor. Mercury . . . 199.8 Hg Hg 2 1.03994 HgO 1.07988 HgCl 1.17703 Hg 2 S 1.08003 HgS 1.16006 Molybdenum 95.9 MoS 8 Mo 0.49989 Nickel. . . . 58.6 Ni NiO 1.27116 Niobium . . . 93.7 Nitrogen . 14.01 Pt N 0.14411 NH 3 0.17497 NH 4 0.18526 Osmium . 195 Palladium . . 106.2 Phosphorus . 30.96 Mg 2 P 2 O 7 P 0.27952 P 2 O 5 0.63976 Platinum . . . 194.43 Pt Pt0 2 1.16417 Potassium . . 39.03 Pt K 0.40129 K 2 O 0.48848 . 1 ". KC1 0.76495 K 2 S0 4 0.89389 Rhodium . . . 104.1 Rubidium 85.2 Ruthenium . . 103.5 Selenium . 78.87 Silicon 28 Si0 2 Si 0.46729 Silver .... 107.66 Ag Ag 2 1.07412 AgCl 1.32853 Sodium , 22.99 NaCl Na 0.39393 Na 2 O 0.53067 Na 2 SO 4 1.21488 Strontium 87.3 SrS0 4 Sr 0.47673 SrO 0.56389 TABLES FOR CALCULATION OF ANALYSES. 263 Atomic Weight. Found. Required. Factor. Sulphur . . . 31.98 BaSO 4 S 0.13744 S0 8 0.34322 S0 4 0.41181 Tantalum . . 182 Tellurium . . 127.7 Thallium . 203.7 T1 2 O Tl 0.9623 Thorium . 231.96 Tin .... 118.8 Sn SnO 2 1.26869 Titanium . . . 50.25 Ti0 2 Ti 0.61154 Tungsten . 183.6 W0 8 W 0.79316 Uranium . 239.8 UO 2 U 0.88249 U 8 8 1.03916 Vanadium 51.1 V 2 O 5 V 0.56154 Yttrium . 89.6 Zinc .... 65.1 Zn ZnO 1.24516 ZnS 1.49124 Zircon 90.4 Zr0 2 Zr 0.73904 284 QUANTITATIVE ANALYSIS BY ELECTROLYSIS. REAGENTS. POTASSIUM OXALATE. The crystallized potassium oxalate of commerce always contains determinable quantities of iron and lead. To purify it, one part of the salt is dissolved in three parts of water in a porcelain dish, and ammonium sulphide is added drop by drop, as long as a precipitate forms. The solution is now heated on the water-bath till the precipitate settles, and filtered through a plaited filter. To decompose the slight excess of ammonium sulphide, a current of air is conducted through the solution till it is perfectly colorless, and no longer gives a reaction with sodium nitroprusside. The separated sulphur is allowed to settle, and the clear solution siphoned off. AMMONIUM OXALATB. The same impurities are present as in potassium oxalate. The salt is purified by precipitating the hot saturated solution with ammonium sulphide. It is heated over a naked flame till the precipitate coheres together, and filtered hot by the use of a hot-water funnel. The greater part of the ammonium oxalate crystallizes from the filtrate on cooling. The solution is poured off, and the crystals dried by placing them in a funnel stopped with asbestos, and connecting with a filter- pump. REAGENTS. 285 OXALIC ACID. The impurities are similar to those of the alkali oxalates ; it is purified by repeated recrystallization. AMMONIUM SULPHATE. This salt is purified in the same way as ammonium oxalate. SODIUM SULPHIDE. The crystallized sodium sulphide of commerce is not only exceedingly impure, but is not inonosulphide at all, but a mixture of polysulphides and sodium hydroxide. The presence of the latter explains that of alumina, which is always found in abundance. If commercial sodium sulphide is used, its solution must first be completely saturated, without access of air, with hydrogen sulphide gas. It is better, however, to prepare the substance, in which case the process is as follows : Sodium hydroxide purified by alcohol is dissolved in water to a solution of sp. gr. 1.35. The solution is divided into two equal parts, and one half, with exclusion of air, saturated with the purest possible hydrogen sulphide gas till the volume ceases to increase. The hydrogen sul- phide is purified by passing it through a wash-bottle of water, and several tubes filled with cotton or wadding. When completely saturated, the solution is filtered from the pre- cipitate formed, and mixed with the other half of the, sodium hydroxide solution. Hydrogen sulphide is again passed into the mixture, with exclusion of air, and it is filtered again. The nearly colorless filtrate is evaporated in a capacious platinum or porcelain dish, over a strong free flame as quickly as possible. It boils without bumping if a platinum spiral is QUANTITATIVE ANALYSIS BY ELECTROLYSIS. placed in it. As soon as a thin crystalline pellicle forms on the surface, the boiling is stopped, and the solution poured while hot into small flasks with well-ground glass stoppers which must be filled full. It is best to completely exclude the air by melted paraffine. For the separation of antimony and tin, the solution should have a sp. gr. of 1.22-1.225. ALCOHOL. The alcohol used for washing metals must be free from acid, and, as nearly as possible, absolute. It is left standing in a large flask, for twelve hours, over quicklime, and then distilled off on a water or steam bath. The distillate must leave no residue on evaporation. INDEX OF AUTHORS. PAGE Andrews and Campbell 143, 144 Arrhenius 6, 9 Bauer and Classen 187 Becquerel 102 Bergmaun 104 and Fresenius 141, 142, 143, 144, 172, 174 Beilstein 104 and Jawein 145,163,165 Blake and Chittendeu 181 Bloxam 102 Boisbaudran 104-153 Bongartz and Classen 183 Brand 137, 141, 143, 145, 148, 162, 174 Brugnatelli 177 Bunseu 45 Campbell and Andrews. .. 143, 144 Cheney and Richards 141, 143 Chittenden and Blake 181 Clamoud 64 Clarke, F. W 104, 163, 174 Classen 105, 135, 137, 141, 143, 148, 153, 154, 163, 166, 167, 178, 182, 183, 191, 192, 194, 196, 199, 200, 201, 202, 205, 206, 213, 221, 224, 225, 2-27, 228, 239, 245, 255, 256 Classen and Bauer 187 and Bongartz 183 " and Lud wig , 174, 225 " and v. Reiss, 137, 141, 143, 145, 148, 154, 162, 163, 168, 178, 183, 188 Cozzi 102 Croasdule 154 Cruikshauk 102 287 288 INDEX OF AUTHORS. PAGk. Daniell 42 Danueel 70 Despretz 102 Dolezalek 38 Drown and Mackeima. 137 Duprfc 177 Eiseiiberg 147, 164, 172, 174 Elbs 47, 71 Eliasberg 162, 164, 209 Eugels 96, 148, 150, 151, 159, 183, 186, 211 Faraday 7, 10, 12 Farbaky 49 Fischer 102 Fischer-Hufschmidt 226 Foote 154 Fraukl 174 " and Smith 148 Fresenius 104 andBergmami 141, 142, 143, 144, 172, 174 Freudeuberg 17, 106, 183, 212, 215, 216, 217, 218, 220, 221 Gaultier 102 Gibbs 103, 106, 141, 143, 145, 153, 183 Gobbels 182 Groeger.... 148 Grove 44 Gillcher...;. 69 Hampe 154, 166, 270 Haunay 104 v. Helmholtz 12 Heidenreich 105, 137, 140, 154, 158, 163, 166, 176, 183, 185, 207, 208, 210, 212, 215, 216, 218 Herpiii 92, 153 Hofer 100 Hoskinson 174 Ikle 106 " and Reinhardt 145 Jannasch * 211 Jawein 104 and Beilslein 145,163,165 INDEX OF AUTHORS. 289 PAGE Jordis 145,147 Knufmann 134 Kiliani 17, 106, 166 " and v. Miller 147 Kinnicutt 172 v. Elobukow 97, 99 Knerr aud Smith 162, 174, 183, 209 Koliu and Woodgate 141, 143 Krcichgauer 167 Kriiger 107 Ki utvvig 1 73 Le Blanc 15, 106 Leclanche 40 Lecreuier 178 Leuher and Rising , . . . . 174 Le Roy 141 , 143 Lob 134 Luckow 103-106, 137, 141, 143, 145, 148,154, 155, 162-164, 166, 168, 172, 177, 178, 182, 183, 188. 189, 202 Ludwig 226 aud Classen 174,225 Mackintosh 154 v. Malapert . . . 89, 91 Mascazziui and Parodi 104, 144, 166, 178 Meckenna and Drown 137 Medicus 167 Meeker 154 Meidiuger . .- 41 Merrick 141, 143, 153 v. Miller and Kiliani 147 Millot 145 Moore 137, 148, 162, 163, 188 Morton: 102 Moyeraud Smith 162, 205, 212, 218 Muhrand Smith 137, 177, 228 Xernst 38 Neumann 167, 170, 284, 235, 237 andNissenson 213, 215, 219 Nickles.. . 102 290 INDEX OF AUTHORS. PAGE Nissenson and Neumann 213 215 219 Noe ' 66 Oettel 55, 141-144, 154, 158 Obi 141, 143, 153 Ohm 12 Ostwald 35 Paget 71 Parodi and Mascazzini 104, 144, 166, 178 Persoz 177 Regelsberger 154, 159 Reinhardt 106 " and Ihle 145 v. Reiss and Classen.... 137, 141, 143, 145, 148, 154, 162, 163, 168, 178, 183, 188 Richards and Cheney. 141, 143 Riche 92, 141, 143, 145, 148, 154, 166 Richert 104 Rising arid Lenher 174 Rudorff.... 137, 141, 143, 145, 148, 154, 157, 162, 166, 172, 174, 177, 178, 182, 183, 205, 207, 208 Saltar and Smith 162 Schelle, R 50 Scheneck 49 Schmucker 162, 217 Schroder 156 Schucht 141, 143, 148, 162. 166, 172, 183 Schweder 141, 143, 154 Smith, E. F. ... 79, 105, 137, 140, 154, 158, 163-166, 174, 176,177,182, 183, 207, 209, 216, 228, 243 and Frankel 145, 174 " Knerr 162,174,183,209 " Moyer 162, 205, 212, 218 " Muhr 137,177,228 " Saltar 162 " " Thomas 162 " Wallace 177, 204, 205. 208, 210, 212, 215, 228 Tenny..'. 166 Thomas and Smith 162 Thomson, W 37 INDEX OF AUTHORS. 291 PAGE van't Hoff 6 Vortraaun.... 106, 137, 141, 143, 145, 162, 163, 165-167, 174, 178, 180-190, 192-194, 202, 204, 234 Wallace and Smith 177, 204, 205, 208, 210, 212, 215, 228 v. Waltenhofeu 68 Warwick 145, 154, 163, 167, 205 Wirkner 177 Wohler 102, 183 Woodgate and Kohn 141, 143 Wrightson 103, 137, 141, 143, 144, 153, 163,178 104, 209 INDEX OF SUBJECTS. PAGE Accumulators 47 action of 48 charging of 56 general rules for the handling of 54 tests of 51 Acids, decomposition tension value for 16 dissociation of 8 Alcohol as reagent 286 Alloy, analysis of, containing antimony and arsenic 238 antimony and tin 238 antimony, tin, and arsenic 239 antimony and lead 237 copper and nickel 233 copper and silver 233 copper and tin 235 copper, tin, zinc, and phosphorus 235 copper, tin, zinc, manganese, and phos- phorus 236 copper and zinc 231 copper, zinc, and nickel 234 tin and lead 236 tin, lead, bismuth, and cadmium 237 Aluminium, determination of 153 separation from cobalt 204 iron 196 iron and beryllium 201 iron and chromium ...... 200 nickel 206 zinc 208 Ammonium, determination of. 188 separation from sodium 229 293 294 INDEX OF SUBJECTS. PAGE Ammonium, oxalate as reagent 284 sulphate as reagent 285 Ampere, definition of 12 Amperemeter 31 Analysis, arrangements for 107 ff process of 83 Anions 1 ? 8 Antimony, determination of 178 separation from arsenic 224 arsenic and tin 225 lead 219 mercury 220 silver 220 tin 221 glance, analysis of 259 metallic, analysis of 268 Arsenic, determination of 188 separation from antimony 224 antimony and tin 225 mercury 220 silver 220 Arsenopyrite, analysis of 253 Bases, decomposition tension value for 16 dissociation of 8 Beryllium, determination of 153 separation from aluminium 201 iron 201 Bismuth, determination of 162 separation from cobalt. 205 Bog-iron ore, analysis of 242 Bournonite, analysis of 260 Brass, analysis of 231 Bromine, determination of 190 Bronze, analysis of 235 Buusen cell 45 Cadmium, determination of 163 separation from copper 212 lead 217 manganese 212 mercury. 218 zinc.. . 209 INDEX OF SUBJECTS. 295 PAGE Calamiue, analysis of 249 Calculation of analyses, tables for 280-2S3 Cathions , 1, 8 Cerussite, analysis of 263 Chalcopyrite, analysis of 254 Chlorine, determination of 190 Chrome-iron ore, analysis of 242 Chromium, determination of 153 separation from iron 199 iron and aluminium 200 iron and uranium 200 iron, uranium, and cobalt 204 iron and nickel 206 Cinnabar, analysis of 265 Clay-iron ore, analysis of 242 Cobalt, determination of 141 separation from aluminium 204 bismuth 205 chromium 204 chromium and uranium 204 copper ..205 iron 191 lead 206 mercury 206 uranium 204 zinc 204 Cobaltiferous arsenopyrite, analysis of 262 Cobaltite, analysis of 261 Conductivity of solutions, theory of 20 Copper, determination of 153 separation from antimony and arsenic 157 arsenic 217 cadmium 212 cobalt 205 lead 213 manganese 211 mercury 217 nickel 206 silver 215 zinc 206 Copper, blister, analysis of 269 matte, analysis of 215, 255 296 INDEX OF SUBJECTS. PAGE Copper, refined, analysis of 272 speiss, analysis of 255 Cupron element 46 Current density, calculation of 18 specific directions concerning 139 distribution, scheme of 126 strength, measurement of 17, 28 during analysis 109, 122, 132 apparatus for regulation of 73, 75 Daniell cell 42 Decomposition, tension value of, for acids and bases 15 Double oxalates, general advantages of, for quantitative analysis 5 Dynamo, action of 62 Edison-Lalaude element , 47 Electrochemical equivalent 12 Institute at Aachen, former equipment of Ill present " " 124 Electrodes 84, 85, 88, 89, 92-94 Electrode tension 13 measurement of 110, 124 Electrolysis, influence of temperature on 94, 95 special apparatus for 100, 101 Electrolytic dissociation 6 of acids 8 bases 8 salts 8 Electrolytic dissociation , degree of 9, 20 precipitation, theory of 21 solution pressure 14 Electrometers 35-37 Faraday's law 10 Ferromangauese, analysis of 275 Furnace "sows," analysis of 258 Galena, analysis of 263 Galvanometers 29 German silver, analysis of 234 Gold, determination of 1?7 separation from other metals 228 Gravity cell 43 Grove cell.. 44 INDEX OF SUBJECTS. 297 PAGE Halogens, determination of 190 precipitation of 22 Heating, arrangements for 95, 96 Hematite, analysis of , , 240 High tension, laboratory for experiments with 133 Historical 101 Iodine, determination of , . . 190 Ions 7 " osmotic pressure of 13 Ion theory 6 Iridium, separation from platinum 228 Iron, determination of 137 separation from aluminium 196 aluminium and beryllium 201 aluminium and chromium 200 beryllium , 201 chromium 199 chromium and uranium 200 cobalt 191 copper 202 lead 204 manganese 194 nickel. ... 192 uranium 198 zinc 193 Laboratory, private 12? for the electro-analysis of metals 130 for experiments with high and low tensions 133 Lead, determination of 166 separation from antimony 219 cadmium 217 cobalt 206 copper 213 iron 204 mercury 218 silver ' 218 zinc 210 crude, analysis of 265 hard, " 219,237 matte, " " 264 soft, " " .265 298 INDEX OF SUBJECTS. PAGE Lead, speiss, analysis of 256 Leclanche cell 39 Lecture- room 130 Limonite, analysis of 241 Linnseite, " 261 Lippmann electrometer 35 Manganese, determination of 148 separation from cadmium 212 copper 211 iron 194 nickel 206 zinc 208 phosphor-bronze, analysis of 236 Meidinger cell 41 Mercury, determination of 174 separation from antimony 220 cadmium 218 copper 217 lead 218 nickel 207 zinc 210 Nickel, determination of 143 separation from aluminium 206 chromium 206 copper 206 iron 192 lead 207 manganese 206 mercury 206 uranium 206 coin, analysis of 233 commercial, analysis of 274 matte, " " 255 Nitric acid, electrolysis of 3 in nitrates, determination of 189 Ohm 12 Ohm's law 12 Organic compounds, electrolysis of 4 Osmotic pressure of the ions 13 Oxalic acid, as reagent 285 Oxyhydrogen gas voltameter 24 INDEX OF SUBJECTS. 299 PAGE Palladium, determination of 183 Peroxides, precipitation of 22 Phosphor-bronze 235 Phosphoric acid, separation from tin 228 Pig iron, analysis of 275 Platinum dishes as electrodes 84 determination of 182 separation from iridiuin 228 Poggendorf compensation method 35 Polarization current, explanation of 14 Potassium oxalate, as reagent 284 sulphate, electrolysis of 4 determination of 188 separation from sodium 229 Psilomelane, analysis of 244 Pyrargyrite, " " 257 Pyromorphite, " " 264 Quadrant electrometer 37 Reagents, preparation of 284-286 Resistance for high tensions 134 roll 122 significance of , 19 wire-gauze 113 Rheostats 81 Salts, decomposition tension value of 15 Secondary elements (see Accumulators) 47 reactions 3, 4 Separation of metals, directions for the 191 Shunt circuit, theory of 74 Silver, determination of 172 separation from antimony 220 arsenic 220 copper 215 lead 218 zinc 210 coin, analysis of. ... 233 commercial metal, analysis of 273 Sine galvanometer 28 Smithsonite, analysis of 249 Slag, blast-furnace, cupola, and bessemer 252 300 INDEX OF SUBJECTS. PAGE Slag, copper and lead 250 Slags, refinery 250 Sodium, separation from ammonium 229 potassium 229 sulphide as reagent 285 Solder, analysis of 236 Solutions, requirements of, for quantitative electrolysis 5 Spathic iron-ore, analysis of 239 Spelter, " " 268 Sphalerite, " " 247 Spiegel, " " 275 Spring galvanometer 31 Standards for electrolysis 86, 87 v. Klobukow's universal 98 Steel , analysis of 275 Stibnite, " " 259 Storage-batteries (see Accumulators). Tables for calculation of analyses 280 Tangent galvanometer 26 Tension 1*2 measurement of 32, 109, 124, 128, 182 Tetrahedrite, analysis of .- 257 Thallium, determination of 170 Thermopiles 64 Clamoud's 64 Noe's 66 Gulcher's 69 Paget's 71 regulator for 70 Tin, determination of 183 separation from antimony 221 arsenic 225 phosphoric acid 228 analysis of commercial 272 Torsion galvanometer 33 Transformer, direct-current 125 Transformation of current 125 Type metal, analysis of 219, 237 Ullmaiiite, analysis of 259 Ultramarine, " " 249 Uranium, determination of 153 INDEX OF SUBJECTS. 301 PAGE Uranium, separation from cobalt 204 cobalt and chromium 204 iron 198 iron ami chromium 200 nickel 206 Volt 12 Voltameter, oxy hydrogen gas 24 weight 26 Voltmeter 32 Wood's metal, analysis of 237 Zinc blende, analysis of 247 crude, " " 268 determination of 144 separation from alumin iuin 208 cadmium 209 cobalt 204 copper 208 iron 193 lead 210 manganese 208 mercury 210 silver 210 Zinkenite, analysis of 260 Zircon " . 253 SHORT-TITLE CATALOGUE OF THE PUBLICATIONS OP JOHN WILEY & SONS, NEW YORK, LONDON: CHAPMAN & HALL, LIMITED. 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