AOE AVTCORET vAUteAT te 4 % “ghayas fee 4 we f { ays Pee pede sacred Ata be i? wat 148 Shree Gy > : Sth Mut Pirienncante rates = ae eS acl = aims oS ss = 7 =a can a f= — ow > = — PiStALLIEEAVEHELE © Raymond Pettibon RESEARCH LIBRARY eee ey ry RESEARCH INSTITUTE JOHN MOORE ANDREAS COLOR CHEMISTRY LIBRARY FOUNDATION INTERNATIONAL CHEMICAL SERIES H. P. TALBOT, Pu.D., Sc.D., Consuztine Eprror THE HYDROUS OXIDES INTERNATIONAL CHEMICAL SERIES (H. P. Tatsor, Px.D., Sc.D., Consuttina EpiTor) Bancroft— APPLIED COLLOID CHEM- ISTRY Second Edition Bingham— FLUIDITY AND PLASTICITY Cady— INORGANIC CHEMISTRY Cady— GENERAL CHEMISTRY Second Edition Grifin— TECHNICAL METHODS OF ANALYSIS As Employed in the Labora- tories of Arthur D. Little, Inc. Hall and Williams— CHEMICAL AND METALLO- GRAPHIC EXAMINATION OF IRON, STEEL AND BRASS Hamilton and Simpson— CALCULATIONS OF QUAN- TITATIVE CHEMICAL ANALYSIS Loeb— PROTEINS AND THE THEORY OF COLLOIDAL BEHAVIOR Second Edition Lord and Demorest— cat EN prarte oct ar ANALY- SI Fifth Edition Mahin— QUANTITATIVE ANALYSIS Third Edition Mahin and Carr— QUANTITATIVE AGRICUL- TURAL ANALYSIS Millard— PHYSICAL CHEMISTRY FOR COLLEGES Second Edition Moore— HISTORY OF CHEMISTRY Norris— TEXTBOOK OF INORGANIC CHEMISTRY FOR COL- LEGES Norris and Mark— LABORATORY EXERCISES eae EE eda CHEMIS- Norris— ORGANIC CHEMISTRY Second Edition Norris— EXPERIMENTAL ORGANIC CHEMISTRY Second Edition Parr— ANALYSIS OF FUEL, GAS, WATER AND LUBRICANTS Third Edition Robinson— THE ELEMENTS OF FRAC- TIONAL DISTILLATION White— TECHNICAL GAS AND FUEL ANALYSIS Second Edition Williams— PRINCIPLES OF METALLO- GRAPHY W oodman— FOOD ANALYSIS Second Edition Long and Anderson— CHEMICAL CALCULATIONS Bogue— THE THEORY AND APPLI- CATION OF COLLOIDAL BEHAVIOR Two Volumes Reedy— ELEMENTARY QUALITA- TIVE ANALYSIS FOR COLLEGE STUDENTS Leighou— CHEMISTRY OF ENGINEER- ING MATERIALS Second Edition Adkins and McElvain— PRACTICE OF ORGANIC CHEMISTRY Eucken, Jette and LaMer— FUNDAMENTALS OF PHY- SICAL CHEMISTRY ' Underwood— PROBLEMS IN ORGANIC CHEMISTRY Schorger— THE CHEMISTRY OF CELLU- LOSE AND WOOD W eiser— THE HYDROUS OXIDES THE HYDROUS OXIDES BY HARRY BOYER WEISER Professor of Chemistry at the Rice Institute First EDITION McGRAW-HILL BOOK COMPANY, Inc. NEW YORK: 370 SEVENTH AVENUE LONDON: 6 & 8 BOUVERIE ST., E. C. 4 1926 ‘> 4 ‘ nN : yi ij 44 TP 6 a seppnne? ptt a CopyriGHT, 1926, BY THE McGraw-Hitt Book Company, PRINTED IN THE UNITED STATES OF THE MAPLE PRESS. COMPANY, YO! THE GETTY RESEAR PREFACE The scientific foundation of modern colloid chemistry was laid by Thomas Graham more than three score years ago as a result of his basic researches on the colloidal behavior of albumin, gums, and gelatin, and of the hydrous oxides of silicon, iron, aluminum, chromium, tin, titanium, molybdenum, and tungsten. Since Graham’s time a great many investigators, van Bemmelen in particular, have studied the colloidal character and application of the hydrous oxides. So far as the author is aware, the present volume represents the first endeavor to correlate systematically and summarize critically the numerous scattered facts in an old but increasingly important field. No group of substances presents a greater variety of colloidal properties than the hydrous oxides. For this reason they have been employed frequently in the investigation of colloid chemical phenomena and applied in widely diversified ways to the indus- trial arts. There is little doubt that a more intimate acquaint- anceship with this group of substances will serve to extend their field of usefulness rapidly. It is hoped, therefore, that the book may prove of value alike to scientist and industrialist. Portions of the manuscript of the book have been read and criticized by several gentlemen. Special acknowledgment of this sort is gratefully made to W. D. Bancroft of Cornell University, k. H. Bogue of the Bureau of Standards, F. L. Browne of the United States Forest Products Laboratory, E. M. Chamot of Cornell University, P. L. Gile of the U. S. Department of Agri- culture, and C. L. Parsons, Secretary of the American Chemical Society. Harry B. WEISER. Houston, TEXAS. Feb. 1, 1926 . CONTENTS PAGE Rete eM a oe lw ke we te Vv ERTRODUCTION 2.1... .. Oh Or EN oe oy Fae rn ee es 1 CHAPTER I JELLIES AND GELATINOUS PRECIPITATES ............. 33 Se ne 3 IC etre dO rs dg So a Sack bok ook ein oaks 15 DOMME CUTO CLE TIONS fF)... ous oo ade as ve be bbe cee vbedces 30 CHAPTER II MeEMSSSIPOURTORIDNS OF IRON... . ... :. .. ss . sw oe 34 Bette eer OXIIG ka ecm se ccd sense nu sbveuecta 34 TOT er yj, occas ac ale cash bw dpe bas 38 The Precipitation of Sols by Electrolytes................... 55 Basorpuon py tHydrous Ferric Oxide.........0...00e 260s ae 67 Pret morornyarous Perric Oxide... ........:.6.c00.00ees 70 I COR ORT OT os id vs ema s Pisa eens gcaleone es 74 CHAPTER III PeITIPRCCHOMICHAIXIDE: . 2 6k he ek eee SB Pea RU RRO a shee eae wg d SP alate we Pelee mela os 82 Pree PUGUAOSOIPtlON 2 oo. dees cde ca eee cave wb aleas 91 CHAPTER IV Tue Hyprovs OxiIpEs oF ALUMINUM, GALLIUM, INDIUM, AND THALLIUM 103 emer epee INU, CXICG sy f505).) cine ca Gls cds vies we coe ee ee ae 103 De BReNMTH ERR ULE DIOLS kD, oy ae Pode dis RY « vay wd Sea 112 Adsorption by Hydrous Aluminum Oxide.................. 122 IKI, ho 1G. es dinin cool pin woe dw cay ne wie pape Par oek 129 RE MMEPMTNOE XIC OS cs fs kl we 225 oie 4 spare W Oa arlene a vm 131 MMR EOC INUCLG Ca yh sa cas wpe glace a abeniinn SNe eee 132 CHAPTER V Ture Hyprovus OxipEs or Coprmr, CoBALt, NICKEL, SILVER, AND GOLD 134 Dee mrTeme Re CYKICG 2 i-5 eer excel ots os ft Da enw es es awe ee 134 Piste AEUIDIOUS, OXIGG . ies se ee acsce eles Hae wee OS Pi a 145 vill CONTENTS PAGE Hydrous Cobaltous Oxide...........0. 45... «s/o 147 Hydrous Cobaltic Oxide... ...V.css. 0.0 @.- . . 0 er 173 CHAPTER VII Tue Hyprovus OxIpEs oF SILICON AND GERMANIUM. ....... 175 Hydrous Silicon Dioxide. ............2.:05.5)) one 175 Silica Gel... Oo ee ates we oe lo 175 Silica Sols... 06.0.6 ssc 0 ge ae ws pw on cis oe Ogee Seen 193 Silicate of Soda... 2... 0.2 vn sm om dee 6 een 196 The Hydrous Oxides of Germanium...:>..... 20.56) 199 CHAPTER VIII Tue Hyprovus Oxiprs or Tin anp LEAb, 2 eee Me Hydrous Stannic Oxide.:......0.1....... ©, ae 202 Precipitated Hydrous Stannic Oxide... ., 7-235 202 Stannic Oxide Sols... 2... 0655 <2 1-6 2 215 Hydrous Stannous Oxide....../.: 2.2... «0 |e 224 Hydrous Lead Monoxide...... Fr aes re 225 Hydrous Lead Peroxide.....: 2.0...» «4 5: 230 CHAPTER IX THE Hyprovus OxIpEs OF TITANIUM, ZIRCONIUM, AND THORIUM . . . 233 Hydrous Titanium Dioxide... .°.....25.. 772) Pee 2 Other Oxides of Titanium.......*....:..0...50 236 Hydrous Zirconium Dioxide. :...........-. 5) eee 237 Zirconium Dioxide Sols... .... 2... 1.0... 5 vee 241 Adsorption by Hydrous Zirconia. ....../ 244 Hydrous Zirconium Peroxide. ......... s co, api td 4 ee eet aoe aaa ee MORDANTS..... . er ererin creme ett ee PN Tee oe es cc uics ah alah Daa eee 1X x CONTENTS Tannin » rec in oss wisic fhe cco ee ee Fixing Agents. ... . oss <4. < > 2... THE HYDROUS OXIDES INTRODUCTION When a solution of a ferric salt is treated with an alkali, there is. formed a voluminous, gelatinous precipitate which is commonly called ferric hydroxide and assigned the formula Fe(OH)s. The extent to which this terminology is fixed in our chemical literature is evidenced by its almost universal use in our text- books, although four decades ago van Bemmelen! showed not only that there is no definite hydrate of the formula Fe(OH); or Fe.03:3H.O, but that no other hydrate is formed by the usual method of precipitating the oxide. The viscous voluminous precipitate when first formed may be represented approximately by the formula Fe.03;- +2O0H,O, but it loses water, gradually attaining a composition that varies with the time, the tempera- ture, and the pressure of the water vapor in contact with it. A composition corresponding to a definite hydrate is, therefore, purely accidental, depending as it does on the exact method of formation, the method of drying, the temperature, and the age of the sample. Precipitated oxides like ferric oxide which contain varying amounts of water adsorbed by the oxide particles are called hydrous oxides to distinguish them from hydrates, in which the water is chemically combined in definite stoichiometric proportions. There are a few hydrated oxides, such as Al.QOs- 3H,O and BeO:H;0, which may adsorb varying amounts of water, depending on the conditions of formation. Such prepara- tions may be termed hydrous hydrated oxides. On standing, the primary colloidal particles of the hydrous oxides grow and lose water spontaneously, causing the mass to assume a less gelatinous and more granular character. This spontaneous transformation from a loose voluminous precipitate to a granular mass is accom- panied by a decrease in the solubility, the adsorbability, and the peptizability of the compounds. 1 Rec. trav. chim., 7, 106 (1888). 1 2 THE HYDROUS OXIDES Although the rapid precipitation of a hydrous oxide usually gives a gelatinous mass with a supernatant liquid, it is frequently possible to bring about uniform precipitation throughout the entire solution with the formation of a jelly which differs from a gelatinous precipitate in that all the liquid is enclosed by the precipitated phase. Since the hydrous oxides are obtained so frequently in the form of gelatinous precipitates, and since the latter are produced when jellies contract spontaneously or are broken up by stirring, the first chapter deals in a general way with the structure, preparation, and properties of gels. This is followed by separate chapters devoted to the typical oxides of iron, chromium, and aluminum, after which the remaining oxides are taken up by families in the approximate order in which the elements appear in the periodic table. The last five chapters are concerned with some of the more important industrial applica- tions of the hydrous oxides. CHAPTER I JELLIES AND GELATINOUS PRECIPITATES Gelatinous precipitates and jellies are the two forms of solid or semisolid colloids that are commonly included under the term gel. Gels of the hydrous oxides, such as ferric oxide and chromic oxide, which lose their elasticity and become powdery on drying, are called rigid or non-elastic gels in contradistinction to the elastic gels, such as gelatin, albumin, and agar, which are charac- terized by perfect elasticity through certain narrow limits and by retaining their elasticity and coherence on drying. Although a detailed discussion of the properties of elastic gels les beyond the scope of this book, any adequate theory of gels must take them into account. Moreover, the vast majority of the work on gel structure has been done with gelatin, and a survey of the results of these investigations throws considerable light on the nature of gels of the hydrous oxides. STRUCTURE Since a working theory of the structure of gels is necessary for a systematic discussion of their preparation and properties, we shall take up first the question of the structure of the two forms, beginning with jellies. This question has doubtless received more attention at the hands of investigators than any other single problem in the field of colloid chemistry; but, in spite of this, opinions differ as to the exact nature of a jelly. Thus Robertson, Procter,! and Katz? regard jellies as homogeneous single-phase systems, solid solutions, or semisolid solutions “of the exterior solution in the colloid in which both constituents are within the range of the molecular attraction of the mass.” . 1J. Chem. Soc., 105, 313 (1914). * Kollotdchem. Bethefte, 9, 1 (1918). 5) 4 THE HY DROUS OXIDES Wolfgang Ostwald! considers gels to be two-phase liquid-liquid systems possessing an interfacial tension. The vast majority of investigators, however, incline to the view that jellies are two- phase solid-liquid systems in which there is a network or cellular arrangement of solid phase permeated by liquid. The Solid-solution Theory.—The evidence in support of the solid-solution theory of jelly structure has been drawn largely from investigations on the swelling of substances. Thus in an exhaustive monograph, published in 1917, Katz points out the close similarity between the phenomena associated with swelling and the changes which accompany the formation of binary liquid mixtures. This parallelism would indicate that the swelling process is simply the formation of a solid solution between water and the swelling substance. Later, however, Katz? studied the effect of swelling on the x-ray spectrum of a number of substances to determine whether the taking up of liquid is intermicellar or intramolecular. If the liquid is held between the particles, the crystal lattice should not be altered by swelling, whereas if a solid solution is formed, the dimensions of the lattice should be increased. In practically all cases investigated, Katz observed no change in the x-ray spectrum, indicating that, as a rule, the swelling process is not a solid-solution phenomenon. From a study of the swelling of gelatin in acid solution, Procter concludes that gelatin combines with acid, forming easily soluble, highly-ionized salts, and that the volume of a swollen jelly under equilibrium conditions is determined by the osmotic pressure of the salts and the Donnan equilibrium. This view seems inade- quate to account for the marked increase in viscosity and the loss of mobility when a warm gelatin solution is cooled. In order to get around this difficulty, Procter postulates the forma- tion of tenuous and possibly flexible crystals which interlace and anastomose when a warm solution sets to a jelly on cooling. These crystals are assumed to be so very minute and the network so extremely fine that both solvent and crystals are within the 1 Pfliiger’s Arch., 109, 277 (1905); 111, 581 (1906). ‘‘Theoretical and Applied Colloid Chemistry,” translated by Fischer, 103 (1917). 2 Koninklijke Akad. Wetenschappen Amsterdam, 33, 281 (1924); Physik. Z., 25, 321 (1924); Karz and Mark: Jbid., 33, 294 (1924); Chem. Zentr., I, 442 (1924); Z. physik. Chem., 115, 385 (1925). JELLIES AND GELATINOUS PRECIPITATES 5 range of each other’s molecular attraction. From these cond- siderations, it would appear that the only essential difference _ between the solid-solution theory and the two-phase solid-liquid theory is in the size of the particles constituting the network. Since these particles are not infrequently of microscopic dimen- sions, the solid-solution theory cannot be of general application. The Emulsion Theory.— Wolfgang Ostwald’s theory that jellies are simple emulsions of spherical or more or less distorted glob- ules in a liquid medium meets with serious objection at the - outset, since there are no emulsions known that have really the properties of jellies. The inorganic jellies certainly could not be looked upon as emulsions particularly in those cases where a rigid crystalline structure has been detected. Recalling the applicability of Boltzmann’s theory! which considers molecules to be completely elastic material particles incapable of much deformation, and van der Waals’ view” that the properties of molecules must be compared with those of solids, Zsigmondy? assumes, as seems necessary, that the larger ultramicrons of a solid are themselves solid. The liquid properties of gels rich in water are explained by assuming that the ultramicrons are surrounded by water layers and have a certain free path and motion. Hatschek* examined the emulsion hypothesis critically and found it untenable if the assumptions necessary to allow of mathematical treatment are granted. The Cellular or Honeycomb Theory.—The oldest theories of jelly structure were alike in picturing the bodies as two-phase solid-liquid systems; but there has long existed a fundamental difference of opinion as to the exact nature of the solid framework which is assumed to entrain the liquid phase and the manner in which this framework is formed. From an extended investigation first on foams and emulsions and later on gelatin, agar, and silicic acid jellies, Biitschli® concluded that the droplets of liquid were held in a cell-like 1 “‘Vorlesungen tiber Gastheorie,’’ Leipsig, 34 (1896). 2 “Die Kontinuitit des gasférmigen und flussigen Zustandes,”’ Leipsig, 34 (1899). 3 ““Chemistry of Colloids,” translated by Spear, 138 (1917). 4 Trans. Faraday Soc., 12, 17 (1916). 5 “Untersuchungen tiber Strukturen,”’ Leipsig (1898). 6 THE HYDROUS OXIDES framework comparable to a honeycomb, an idea suggested, in all probability, by the cellular structure of the stems of young plants which enclose a relatively high percentage of water and still possess considerable rigidity. The walls of the cells in a silica jelly appeared to be about 0.3 » in diameter and the pockets which held the liquid from 1 to 1.54 in diameter. Gelatin jellies that appeared homogeneous under the microscope were hardened with alcohol or chromic acid to make their structure visible, and these likewise appeared to be made up of thin films. Bitschli’s general concept of jelly structure was supported by van Bemmelen,! Quincke,? and Hardy.* According to the latter, gelatin consists of two phases separated by a well-defined surface; one phase a solid solution of gelatin in water and the other a solution of water in gelatine. Like van Bemmelen, he assumes that both phases are liquid at first; but with fall of temperature, one becomes solid. The solid solution forms on the concave side of the surface of separation when the proportion of gelatin is small and on the convex side when the proportion of gelatin is large. In the latter case the drops of liquid are held in a solid gelatin-rich phase. As Bancroft* points out, such a jelly consists merely of a viscous medium in which liquid is dispersed and so does not have a honeycomb structure in the same sense that an emulsion has a honeycomb structure. The view entertained by Bancroft is that both phases in a gelatin jelly are colloidal rather than solid solutions. Since water peptizes gelatin under certain conditions, there is no reason why gelatin or a gelatin- rich phase should not peptize water. The separate phases will, therefore, in the nature of things, never be homogeneous. The investigations of Biitschli, van Bemmelen, and Hardy seemed so conclusive that a decade ago the honeycomb theory was generally looked upon as established.®> But later investigations of Zsigmondy and his pupils disclosed errors in the optical obser- vations of Biitschli and Hardy and showed the heterogeneity of jellies to be of an entirely different order of magnitude from that 1Z. anorg. Chem., 18, 14 (1898). 2 Drude’s Ann., 9, 793, 969 (1902); 10, 478, 673 (1903). 3Z. physik. Chem., 33, 326 (1900). 4“ Applied Colloid Chemistry,’ 241 (1921). 5 Cf, FrEUNDLICHY ‘ Kapillarchemie,” 475 (1909). JELLIES AND GELATINOUS PRECIPITATES ‘s which the latter supposed. By applying the laws of capillarity to van Bemmelen’s! results on the hydration and dehydration of silica gel, Zsigmondy” estimated the diameter of the pores to be 0.5yu, that is, 200 or 300 times smaller than Biitschli observed. This was confirmed by Anderson* who showed that the pores vary in size, some being as small as 10uy in diameter. Working by the same method, Bachmann‘ found that gelatin jellies hard- ened by alcohol or chromic acid contained very much finer spaces than Biitschli supposed. Apparently the structure observed by Biitschli and Hardy were artifacts produced by the action of the hardening agents on the much finer structure already existing.® In the light of the work of Zsigmondy and his pupils, Lloyd® postulates a porous but continuous, solid cellular framework to enclose the liquid. The gelatin is assumed to exist in two chemi- cal states: gelatin, per se, and gelatin in the form of soluble salts. On cooling a solution containing isoelectric gelatin and gelatin salts in equilibrium with free electrolytes, the insoluble isoelectric gelatin is believed to precipitate not as crystals but in a state of suspended crystallization forming a solid framework which is kept extended by the osmotic pressure of the soluble gelatin salts in solution. In support of this hypothesis, isoelectric gelatin and water, in the absence of so-called gelatin salts in solution, were found to form an unstable clot that contracted and squeezed out liquid. It would seem, therefore, that an electrolyte must be present to form a stable gelatin jelly in accord with the view of Jordis.? J. Alexander® suggests that what Lloyd calls “‘suspended crystallization’? may be a manifestation of the protective or crystal-inhibiting action of a portion of the gelatin solution. This would account for the fact that a jelly 1“T)ie Absorption,” 198 (1910). 2Z. anorg. Chem., 71, 356 (1911). 3Z. physik. Chem., 88, 191 (1914). 4Z. anorg. Chem., 100, 1 (1917). 5 Cf. Pauuti: ‘Der kolloidale Zustand und die Vorgiinge in der Lebendigen Substanz,”’ Braunschweig (1902); A. Fiscuer: “Fixerung, Farbung, und Bau des Protoplasms,’’ 312 (1899). 6 Biochem. J., 14, 165 (1920), cf. THomson: J. Soc. Leather Trades’ Chem., 3, 299 (1919). 7Z. Elektrochem., 8, 677 (1902). 8 “Glue and Gelatin,” 71 (1923). 8 THE HYDROUS OXIDES formed of isoelectric gelatin and water alone is apparently unstable in the sense that it contracts and squeezes out some of the water. But because of the slight inherent tendency of gelatin to crystallize, it is doubtful whether the alleged increase in stability of a jelly in the presence of a trace of electrolyte is due to inhibition of the crystallization of the gelatin phase. It seems more probable that the presence of an adsorbed ion may influence the nature and size! of the agglomerated particles and so may have an effect on the stability. If an electrolyte is necessary to form a stable jelly, the amount is apparently very slight indeed, since Field? prepared such a jelly from a very highly purified gelatin. Sheppard and Elliott* see no need of postulating the existence of osmotic pressure to keep the jelly extended, if the isoelectric gelatin forms a rigid solid framework. The Micellar Theory.—The investigations of Zsigmondy and Bachmann which disproved the observations of Bitschli and Hardy resulted in a resuscitation of the micellar theory of Frank- enheim‘ and Nageli.» According to this, the earliest theory of jelly structure, distensible bodies were assumed to consist of small anisotropic erystal-like molecular aggregates which retain their identity even when the substance goes into (colloidal) solution. The micelles, as Nageli called the molecular aggre- gates, take up water in such a manner that they are surrounded by a water layer, the thickness of which is determined by the relative intensity of the attraction of the micelles for water and for each other. Zsigmondy’s earliest investigations with the ultramicroscope led him to conclude with Nageli that the jelly structure is granular or flocculent; but later, Zsigmondy and Bachmann® observed a fibrilar structure in addition to the apparently grainy structure met with in diluted gels of gelatin, agar, and hydrous silica. ‘The fibrils or threads are quite sharply defined in soap jellies studied by Bachmann and later by McBain 1WeisEer: J. Phys. Chem., 21, 314 (1917). 2 J. Am. Chem. Soc., 43, 667 (1921). 3 J. Am. Chem. Soc., 44, 373 (1922). 4 “Tie Lehre von der Kohasion,’’ Breslau (1835). 5 ““Pfanzenphysiologischen Untersuchungen,” Zurich (1858); ‘Theorie der Garung,’’ Munich (1879). 6 Kolloid-Z., 11, 150 (1912). JELLIES AND GELATINOUS PRECIPITATES 9 and his coworkers,' and in barium malonate jellies studied by Flade.* The latter noted the crystalline character of the fibrils and suggested that jellies in general probably consist of a net- work of crystalline threads.* Gortner* prepared a jelly of di-benzoyl-l-cystine which was found to consist of minute crystalline needle-like fibrils. Biichner®> showed that jellies, obtained from myricyl alcohol dissolved in chloroform and in amyl alcohol, consist of a conglomerate of very fine crystals which retain a large amount of liquid in the meshes. Bradford? champions the theory that the reversible sol-gel transformation is merely an extreme case of crystallization. Ultramicroscopic examination of a gelatin jelly reveals the presence of spherites which Bradford believes are made up of crystalline particles. Moeller’ likewise believes gelatinization to be a kind of crystal- lization in which there is formed a lattice of crystal threads that entrains the liquid; and von Weimarn® concludes from his inves- tigations that a jelly is a sponge composed of highly dispersed. crystalline granules soaked in dispersive medium. While Bradford, Moeller, and von Weimarn may have suffi- cient evidence to convince them of the crystalline character of all jellies, it is difficult for me to accept the view that there is no such thing as an amorphous precipitate of the flocculent, gelati- nous or jelly-like type. The theory that jelly formation is merely a process of crystallization seems to be contradicted by the work of Bogue, McBain, and Barratt, although all of the latter are strong supporters of a filamentousstructure. Bogue?® believes the elastic jellies such as gelatin to be made up of streptococcal 1Lainc and McBain: J. Chem. Soc., 117, 1506 (1920); Darke, McBain, and Saumon: Proc. Roy. Soc. (London), 98A, 395 (1921). 2Z. anorg. Chem., 82, 173 (1913). 3 Cf. Sripei: Pfliiger’s Arch., 156, 361 (1914); HowEuu: Am. J. Physiol., 40, 526 (1916). 4 J. Am. Chem. Soc., 43, 2199 (1921). 5 Rec. trav. chim., 42, 787 (1923). 6 Cf. Fiscurr and Bosertaa: Jahresber. schles. Ges. vaterl. Kultur, 86, 33 (1909). Chem. Zentr., I, 262 (1909). 7 Biochem. J., 12, 351 (1918); 14, 91 (1920); 15, 553 (1921). 8 Kolloid-Z., 23, 11 (1918). 9 J. Russ. Phys.-Chem. Soc., 47, 2163 (1915). 1 Chem. Met. Eng., 23, 61 (1920); J. Am. Chem. Soc., 44, 1343 (1922). 10 THE HYDROUS OXIDES threads of molecules. According to his view, the catenary threads are very short and but slightly swollen in the sol condition, but elongate and absorb a great deal of water as the temperature falls and the sol starts to gel. A solid jelly results when the relative volume occupied by the swollen molecular threads is so great that freedom of motion is lost and the adjacent, heavily swollen aggregates cohere. Although it is possible for colloidal particles to possess the thread-like characteristics essential for forming an entangling mesh in which each particle is discrete, it seems more probable that in most cases the micelles actually become stuck together or orientated into loose aggregates which may take the form of chance granules, threads, or chains. Such a linking together of the particles to form an enmeshing network seems essential in some of the extremely dilute hydrous oxide jellies to which I shall refer later on. Laing and McBain! consider the gelatiniza- tion of soap to result from the linking up of colloidal particles to form a filamentous structure. ‘‘The colloidal particles in soap and gel are the same; but whereas in the former they are independent, in a fully formed gel they become linked up prob- ably to form a filamentous structure.” The formation of the soap curd is looked upon as a phenomenon analogous to crystal- lization that is distinct from the process of jelly formation.? The conception of micellar orientation in the process of gelation is supported by a number of observations mentioned by Laing and McBain, among which are the following: the identity in sol and gel of the electrical conductivity,? and the lowering of the vapor pressure; the intensifying of the molecular movement by heat which overcomes the forces holding the particles and causes melting of the gel; the transformation of certain jellies, such as nitrocotton into sol, by mechanical stirring which breaks down the orienting bonds between the particles;* the absence of Brownian movement in soap or gelatin jellies;> the dependence of the apparent viscosity of sols on their previous treatment and 1 J. Chem. Soc., 117, 1506 (1920). 2 Cf. Piper and GRINDLEY: Proc. Phys. Soc. (London), 35, 269; 36, 31 (1923). 3 Cf. ARRHENIUS: Oefvers. Stockholm Akad., 6, 121 (1887). 4 Cf. ALEXANDER: ‘‘Glue and Gelatin,” 75 (1923). 5 BACHMANN: Z. anorg. Chem., 73, 125 (1912). JELLIES AND GELATINOUS PRECIPITATES 11 history which influence the degree of orientation of their particles ;! the tendency of the jelly structure to shrink and exude liquid— Synerize—as a result of the component of attraction in the orienting force between the particles; and the frequent occurrence of supersaturation and hysteresis with regard to gelation. To these should be added the observation of Walpole? that the refractive index of a gelatin-water system is a linear function of the concentration, and when plotted against the temperature, no break occurs at the point of gelation; and the findings of Bogue? that the viscosity-plasticity change in the sol-gel transformation is gradual and regular. | Barratt* observed in fibrin jellies a non-crystalline fibrillary structure which formed an enmeshing network. When the jelly was first formed by gelatinization of a fibrinogen sol, no fibrils could be detected, but later they became visible in the ultramicroscope. This growth of particles in jellies has been observed frequently and in some cases is unquestionably due to growth of crystals, notably with barium malonate and some of the arsenate jellies® and with the dyes, benzopurpurine and chrys- ophenene;® but in other cases, it is the result of the agglomeration of amorphous particles. In accord with this view Scherrer’ showed that certain rigid jellies like hydrous silicon dioxide and hydrous stannic oxide showed well-defined crystalline inter- ference figures as well as the characteristics of amorphous bodies, whereas gelatin jellies showed no signs of a crystalline structure. Harrison® obtained spherical coagulation forms of starch which resembled Bradford’s spherites; but he does not regard them as crystalline. In the course of their investigations, Zsigmondy and Bach- mann observed ultramicroscopically the formation of gelatin, 1Cf. HarscHEK: Kolloid-Z., 13, 881 (1913). 2 Kolloid-Z., 18, 241 (1913). 3 J. Am. Chem. Soc., 44, 1313 (1922). 4 Biochem. J., 14, 189 (1920). 5 Dersz: Kolloid-Z., 14, 139 (1914). 6 Harrison: ‘‘The Physics and Chemistry of Colloids and Their Bearing on Industrial Questions,’ report of a general discussion held jointly by the Faraday Society and Physical Societies of London, Oct. 25, 57 (1920). 7 Nachr. Kgl. Ges. Wiss. Gottingen, 96 (1918). 8 J. Soc. Dyers Colourists, 32, 40 (1916). 12 THE HYDROUS OXIDES agar, and silica jellies by agglomeration into flaky groups of freely movable ultramicrons of unknown structure. It is thus implied that all jellies are not necessarily filamentous in structure. This is supported by recent ultramicroscopic observations carried out by Harrison! on gelatin and cellulose jellies which were found to consist of minute portions joined together in a somewhat irreg- ular manner. Alexander? believes that the formation of chains or threads is not essential to gelation, although chain-like struc- tures may form as a result of orientation of the polar molecules. Whatever may be the exact structure of jellies, most of the experimental evidence supports the micellar or sponge theory rather than the cellular or honeycomb theory. The presence of definite threads or filaments leaves little room to doubt the exist- ence of an interlacing network structure in certain jellies. It would, of course, be highly interesting, if jellies of widely different substances were all essentially identical in structure. Such a condition seems altogether unlikely; but investigators have apparently sought to establish such an identity. Studies on specific jellies have led some to conclude that all jellies are made up of a framework of amorphous threads; other that they are composed of crystalline threads; and still others who fail to find any threads or filaments at all but observe an irregular grouping of particles. Doubtless all are right in specific cases. Indeed, it is not unlikely that there are various arrangements of molecular aggregates in different jellies and perhaps in the same jelly. In a heterogeneous mixture of complex groups such as are found in gelatin sol or jelly, it is probable that the process of gelation and the jelly structure are more complex than in the inorganic jellies or in soap jellies. The orientation of the particles may result in fibrils in certain cases and in more or less irregular arrangements in others. In certain cases the fibrils may consist of definite crystals, while in others the crystalline characteristics may be entirely lacking. In all cases it seems probable that the particles are highly hydrous as a result of adsorption or absorp- tion and that they are linked together, forming an irregular mesh or network in the interstices of which liquid is entrained. 1“ The Physics and Chemistry of Colloids and Their Bearing on Industrial Problems,” 57 (1920). 2 “Glue and Gelatin,” 84 (1923). JELLIES AND GELATINOUS PRECIPITATES 13 We may next inquire into the structure of gelatinous precipi- tates. Since a gelatinous precipitate differs from a jelly only in having undergone contraction with the consequent excretion of liquid, the two types of gels are generally considered to be quite similar in structure. Recent investigations of the physical character of bodies by means of x-rays confirm von Weimarn’s contention that many gelatinous precipitates, such as hydrous alumina and ferric oxide, which we used to think were amorphous, are, in reality, made of myriads of tiny crystals. This naturally raises the question whether the submicroscopic crystals are themselves gelatinous and so impart the gelatinous property to the mass. Unfortunately, von Weimarn does not enlighten us on this point; but it is apparently possible to have gelatinous crystals. Thus Harrison! speaks of aqueous solutions of benzo- purpurine and chrysophenene setting to jellies containing gelat- inous crystals, some of them so fine that they can pass unbroken through a filter paper. Similarly, cholic acid gives a blue precipi- tate with iodine which may form in clusters of needle crystals possessing rigidity. Under other conditions needle-shaped crystals are formed which are gelatinous and can be bent in all kinds of shapes by moving the cover glass on the microscope slide. Some of these so-called gelatinous crystals show remarkable vibrations due to the impact of the molecules and move about like the spiral bacteria present on the teeth. Harrison’s observa- tions seem to throw some light on the problem of what constitutes a gelatinous crystal or aggregate and hence on therelated problem of what is a gelatinous precipitate. Le Chatelier? succeeded in polishing metal with colloidal silicic acid and hence concluded that the gelatinous precipitate consists of anhydrous silica and water. Bancroft*® considers this evidence inconclusive since anhydrous silica may have been formed as a result of pressure during polishing, and suggests that a better method of attack is to consider whether grains of sand mixed with water will give a gelatinous precipitate. Since this does not happen, as a rule, Bancroft concludes: 1“The Physics and Chemistry of Colloids and their Bearing on Industrial Problems,’’ 58 (1920). 2 “Tia Silice et les Silicates,”’ 76 (1914). 3“ Applied Colloid Chemistry,” 236 (1921). 14 THE HYDROUS OXIDES We must therefore assume one of two things. Either the sand grains are held together extraordinarily firmly by water when they are very fine, or there is some other factor comes in. ‘The first explanation cannot be the right one because, if it were, one ought then to be able to get a gelatinous precipitate of any colloid at ordinary temperatures without much difficulty, which is not the case. We never get gelat- inous gold, and while we can get gelatinous calcium carbonate, we have to do it in a very special way. Consequently, Le Chatelier’s hypothesis cannot be accepted without modification. ; As previously noted, Zsigmondy! explains the liquid character of gels rich in water by assuming the ultramicrons to be sur- rounded by water layers and to have a certain free path and motion. The objection to this view is that Zsigmondy does not show why it should be so. Harrison’s observations on gelati- nous crystals bear on this point. Gelatinous crystals are appar- ently extremely fine, needle-shaped masses so thin that they lack rigidity and so flexible that they can be bent and twisted into various shapes and may move under the bombardment of water molecules. A cluster or network of such needle-shaped, flexible crystals that adsorb water strongly would form a viscous or plastic mass, usually known as a gelatinous precipitate. If the crystals are compact and rigid rather than thin and flexible, they would not form a gelatinous precipitate unless they united into threads or strings possessing the flexibility and elasticity which characterizes a thin needle crystal. Obviously the particles need not be crystalline, and as a rule they probably are not. A gelatinous precipitate 1s apparently a network composed of extremely finely divided particles which have coalesced to form flexible filaments or chains and which adsorb water very strongly and so are highly hydrous. Where the particles do not adsorb water particularly strongly and where the tendency to coalesce into filaments or threads is not great, a high concentration of the finely divided particles is necessary, as in the case of calcium car- bonate and barium sulfate. It is probable that neither tendency is very marked in the case of gold, which accounts for the fact that no one has prepared a gold jelly. I am not aware, however, that anyone has attempted to precipitate a fairly large amount 1 ZstigMonpy: ‘‘Chemistry of Colloids,” translated by Spear 138 (1917). JELLIES AND GELATINOUS PRECIPITATES 15 of gold in a small volume, as von Weimarn does with barium sul- fate. While a gelatinous precipitate of gold has not yet been prepared, this might be a fairly simple process if the metal were dispersed in some liquid, other than water, which is very strongly adsorbed by gold. Bdérjeson! working in Svedberg’s laboratory, prepared a cadmium jelly by allowing a very dilute sol of cad- mium in alcohol to stand for some time in a glass bottle. In this case the particles were only 5upy in radius and the concen- tration but 0.2 to 0.5 per cent. Barium sulfate is readily obtained in a gelatinous form by precipitation in selenium oxy- -chloride.2 The physical character of the precipitate is due to very strong adsorption of selenium oxychloride by the minute particles which form as a result of the extreme insolubility of the sulfate in the liquid medium. PREPARATION If we start out with the assumption that a gel consists of myri- ads of particles enmeshed into a network which entrains liquid, it follows that any substance should form a gel, provided a suit- able amount of a highly dispersed substance is precipitated and provided the particles adsorb the dispersing medium very strongly. The amount of the dispersed phase that must be present to form a firm jelly by a precipitation method will depend on the size and nature of the orientation of the particles and the extent to which they adsorb the dispersing liquid. The methods of procedure which have been employed will be con- sidered separately. Cooling of Sol.—Certain substances such as gelatin and agar- agar swell in water at ordinary temperatures but are not pep- tized, forming a sol, until the temperature is raised. At the higher temperature, the liquid phase serves the double role of peptizing agent and dispersing medium. On cooling such a sol, a jelly is formed provided the concentration is suitable. 1“The Physics and Chemistry of Colloids and their Bearing on Industrial Problems,” 55 (1920). 2 LENHER and Tayuor: J. Phys. Chem., 28, 962 (1924). 16 THE HYDROUS OXIDES Thus a sol containing 1 per cent of pure gelatin does not gel until around 10°, and gelation does not take place at any concen- tration above +35°. According to Bachmann,! pure warm solutions of gelatin are almost homogeneous, but on cooling, a new phase appears, as evidenced by a heterogeneity that is amicroscopic or submicroscopic, depending on the concentration. This process is similar in certain respects to crystallization but differs from it in that microns, submicrons, ultramicrons, and amicrons are formed according to the concentration. The appearance of visible particles is not dependent on the formation of a jelly, as these may be seen before the jelly sets and in dilute solutions that do not set. When a jelly results on cooling a sol, the process apparently consists in the formation of highly hydrous molecular aggregates which are linked together to form a more or less rigid network. Bogue believes that the aggregates not only grow but become more hydrous on cooling. This might’ be expected in view of the rapid increase in adsorp- tion which usually results from lowering the temperature. The sol-gel transformation in a given system does not occur at a definite transition point, but the transition is continuous and reversible over a somewhat indefinite period.? Swelling. Non-aqueous Gels.—Practically all substances which form the so-called elastic gels show the capacity of swelling in a suitable liquid. Thus dry gelatin, fibrin, and starch will swell in water at ordinary temperature, forming jellies that are peptized at higher temperatures giving sols. Similarly, albumin swells in water but not in alcohol, benzene, ether, or turpentine. Vulcanized india rubber swells in various organic solvents such as benzene, toluene, and xylene but not in water; and soaps swell in water and in many organic solvents. Numer- ous theories? have been advanced to explain the phenomenon, but there is as yet no explanation to account for the fact that certain substances swell in only a limited number of liquids. The swelling of gelatin has been studied most extensively and has been found to depend on a number of factors, among which 1Z. anorg. Chem., 73, 125 (1911). 2 Bocur: J. Am. Chem. Soc., 44, 13813 (1922). 3'These theories have been summarized and their limitations pointed out in a paper by BarTEuy and Sims: J. Am. Chem. Soc., 44, 289 (1922). JELLIES AND GELATINOUS PRECIPITATES ee may be mentioned the hydrogen ion concentration;! the addition of neutral. salts;? the temperature; and the structure.* The importance of the hydrogen ion concentration on the swell- ing phenomenon was suggested by Ostwald and has been empha- sized particularly by Procter and Wilson and by Loeb, who have applied Donnan’s theory of membrane equilibria in interpreting the mechanism of the swelling process. Before taking up the Procter-Wilson theory of swelling, the theory of membrane equilibria on which the former is based will be considered briefly. Donnan’s theory of membrane equilibria’ deals with the equilib- ria resulting when a membrane separates two electrolytes containing 1 ion which cannot diffuse through the membrane. Starting with two completely ionized electrolytes, (1) NaCl and (2) NaR, separated by a membrane impermeable to the ion R’, Donnan shows that equilibrium will be established only when the product of the concentration of sodium and chloride ions has the same value on both sides of the membranes, thus, [Na’]i X[Cl’], = [Na’}e X[CI’]2 the brackets signifying concentration in mols per liter, and the subscripts 1 and 2 referring to solution 1 and solution 2, respee- tively. This is the so-called equation of products based on the distribution law. For the specific case cited above, the equation of products may take a somewhat different form. Thus, at the outset, the system of two solutions separated by a membrane may be repre- sented as follows: ‘Solution 1 Solution 2 [Na’ ][Cl’] [Na ][R’] 1 CurtAri: Biochem. Z., 38, 167 (1911); Procrmr: J. Chem. Soc., 106, 313 (1914); Lozs: J. Gen. Physiol., 1, 41 (1918). 2 HorMeiIsTer: Arch. exptl. Path. Pharmakol., 27, 395 (1890); 28, 210 (1891); Pau: Pfliiger’s Arch., 67, 219 (1897); 71, 333 (1898); Sprro: Beitrége zur chem. Physiol., 5, 276 (1904); Wotreane OstwaLp: PYltiger’s Arch., 108, 563 (1905); Fiscnmr: ‘‘Hdema,” New York (1910). 3ProcteR and Burton: J. Soc. Chem. Ind., 35, 404 (1916); Arisz: Kolloidchem. Bethefte, T, 42 (1915). 4Z. Elektrochem., 17, 572 (1911). 18 THE HYDROUS OXIDES On allowing the system to stand, the diffusible sodium and chlo- ride ions distribute themselves until equilibrium is established. At equilibrium in solution 1, let a teso| NG ue Ce and at equilibrium in solution 2 let y = [Cl] and a =| hye hence, | (y +z) = [Na’] Thus we have Solution 1 Solution 2 — £Na-* Lov le + 2)Na:* Yor’ 2p and the equation of products is P= ty are 2) In solution 1, zy,- = “cv, while in solution 2, yng: + 2na: = Yer but since the product of the concentrations in solution 1 is the same as the product of the concentrations in solution 2, it must follow that Na + Lov < YNa + YCv + 2na- or 22 <2y + 2 In other words, at equilibrium, the concentration of diffusible ions in solution 2 is greater than in solution 1. Now, if we let e= (2y+ 2) — 22 then 2yte=e+ 2x and the equation of products becomes e=y t+ Vey which shows again that sodium chloride does not distribute itself equally, but the concentration of the ionized sodium chloride at equilibrium is greater in solution 1 than in solution 2. This gives rise to an osmotic-pressure difference as well as to a difference in — potential across the membrane. The equation for this potential difference was derived by Donnan in the following way: Let 7, and ze be the potential for positive electricity in solu- tion 1 and solution 2, respectively, in the above mentioned system; JELLIES AND GELATINOUS PRECIPITATES 19 _ and let the minute amount of positive electricity Fdn be trans- ferred isothermally from solution 2 to solution 1. This process involves a change in free electrical energy represented by Fdn (7; — m2) and the simultaneous transfer of udn mols of Na’ from solution 2 to solution 1 and of vdn mols of Cl’ from solution 1 to solution 2, where u and v are the transport numbers of the respec- tive ions. The maximum osmotic work involved in the transfer of the ions is given by the expression [Na ]2 [Na]: [Cl]: [Cl’]e udn RT log. + vdn RT log. Since the system is in equilibrium, the electrical work is equivalent to the osmotic work, or Fdn (a1 — m2) = udn RT loge tN : +vdn RT log. are 2 praises ACM): « at Bu MN: an = cl; =jandu +1 = Hence, if Cs Ty <— 19 RT = - log | It may be shown that this equation is valid even when other ions of any valence are added to the system. Donnan has tested - the accuracy of the equation in the following cases: (1) Congo red and sodium chloride; (2) potassium chloride and potassium ferricyanide; (3) sodium arsenate and sodium chloride. In every instance there is fairly good agreement between the calcu- lated and experimental values. Donnan has also applied the same general principles to such cases as NaA and KA, and NaA and CaAz, in which the membrane is impermeable to the ion A’. The Procter-Wilson Theory of Swelling..—To account for the swelling of gelatin, Procter and Wilson assume that hydrochloric acid, say, combines with gelatin forming a readily soluble high- ionized salt, gelatin chloride, and that the resulting equilibrium is a special case of the Donnan membrane equilibria. To make the 1 Procter: J. Chem. Soc., 105, 313 (1914); Kolloidchem. Bethefte, 2, 243 (1911); Proctrr and Wiuson: J. Chem. Soc., 109, 307 (1916); Winson and Witson: J. Am. Chem. Soc., 40, 886 (1918). 20 THE HYDROUS OXIDES reasoning general, a protein G is supposed to react with an acid HA in accord with the following equation: G+ H’ + A’ = GH’+ A’ Hence, if a millimol, say, of G is immersed in a solution of HA, the solution penetrates the jelly which combines with a part of the H‘ions giving GH’. In this way the concentration of H’ ions within the jelly is reduced below that of the A’ ion; whereas in the solution surrounding the jelly, the concentrations of the two ions are necessarily equal. ‘Thus the solution is separated into two phases, an external phase with the two diffusible ions H’° and A’ and a jelly phase containing the diffusible ion A’ and the ion GH" which is a part of the elastic jelly structure and so cannot diffuse. This constraint imposes a restraint on the equal distribution of ions within and without the jelly. When equilib- rium is established, in the external phase, let x = (H' | ={Aq and in the jelly let yl Hy and PA =map eet From which [A’]=yt+e Since the product [H’] x [A’] will have the same value in both phases at equilibrium, it follows that x? = y(y + 2) If, as before, we let e= 2y +2— 2x then x=yt /ey This shows z to be greater than y, which means that [H ‘]is greater — outside the jelly than in it. From this it follows that [A’] is greater in the jelly than in the external solution. For this reason the anions of the protein salt will tend to diffuse outward into the external phase. This exerts a pull on the cations GH° forming part of the protein framework, and causes an increase in the volume of the jelly directly proportional to e, the excess of concentration of diffusible ions of the jelly over that of the external solution. Procter and Wilson have tested this theory experimentally in the case of gelatin and hydrochloric acid, and have found good JELLIES AND GELATINOUS PRECIPITATES 21 agreement between observed and calculated values. Moreover, Loeb and Kunitz! showed that all monobasic acids produce approximately the same degree of swelling at the same hydrogen ion concentration, as the theory predicts. The addition to acid-swollen jellies of neutral salts, such as MA, neither of whose ions combine with the protein as hydrogen ion is supposed to do, increases the cation concentration y in the jelly but not the alleged gelatin cation concentration z. This decreases the excess of diffusible ions inside the jelly over that outside and so decreases the swelling, as observed experimentally. The application of the Donnan theory of membrane equilibria to the swelling of gelatin is certainly a step forward in explaining the mechanism of the swelling process, although it is apparently inapplicable to such cases as the swelling of rubber in benzene or xylene, where the existence of a Donnan equilibrium is pre- cluded by the absence of dissociation. Moreover, it is a serious mistake to conclude, as some have done, that prediction of results by means of a formula proves the assumptions on which the for- mula is based. Procter, Wilson, and Loeb assume a definite chemical combination between gelatin and hydrochloric acid with the formation of a highly ionized salt, gelatin chloride, which gives a non-diffusible cation GH’. The mathematical formulas deduced from this hypothesis do not prove its correctness, for one can get exactly the same formulas and make exactly the same predictions by making the more probable assumption that hydrogen ion is preferentially adsorbed on the surface of gelatin particles rather than entering into definite chemical combination with the particles. This is recognized clearly by Donnan:? Very many interesting investigations based on this simple theory have been made by Jacques Loeb and his collaborators. In this work, among other things, the effects of acids, alkalies, and salts on the osmotic pressures and membrane potentials of the amphoteric proteins have been studied. Loeb has shown that the simple theory of membrane equilibria is capable of accounting fairly quantitatively for a great many of his experimental results, and regards this as a proof that the phenomena exhibited by the protein ampholytes are due to simple chemical reactions and not to the adsorption of ions by colloid aggre- 1 J. Gen. Physiol., 5, 665, 693 (1923). 2 Chem. Reviews, 1, 87 (1924). 22 THE HYDROUS OXIDES gates or micelles. While this view may be correct in many instances, it is necessary to remember that the theory of membrane equilibria depends simply on two assumptions: (a) the existence of equilibrium; (b) the existence of certain constraints which restrict the free diffusion of one or more electrically charged or ionized constituents; and that the equations which result from the theory will hold equally well whether we have to deal with ‘‘colloid units’? which have acquired an ionic character (electrical charge) by adsorption of ions, or with simple molecules which have become ionized by the loss or gain of electrons. All that is necessary for the theory is that the simple ionized molecules of the ionic micelles be subjected to the same constraint, namely, inabil- ity to diffuse freely through the membrane. This constraint then imposes a restraint on the equal distribution on both sides of the mem- brane of otherwise freely diffusible ions, thus giving rise to the concen- tration, osmotic, and electrical effects with which the theory deals. The investigations of Loeb led him to conclude that only the anions of neutral salts are taken up by gelatin on the acid side of the so-called isoelectric point of gelatin (pH = 4.6) and only cations on the alkaline side. This conclusion is hardly justified by Loeb’s experiments since, throughout most of the range investigated, he was working with relatively low concentrations of salts and so detected no effect of cations other than hydrogen on the acid side and of anions other than hydroxyl on the basic side. At relatively high concentrations of neutral salts, the specific effect of cations other than hydrogen and of anions other than hydroxyl would doubtless appear. This inference is supported by work carried out in the author’s laboratory on the adsorption of anions by hydrous chromic oxide on the alkaline side of the isoelectric point. If the concentration of the anion under consideration is very large relatively to that of hydroxyl, the effect of the latter is negligible, whereas if the hydroxyl ion concentration is appreciable, the adsorption of the other ion is cut down enormously or completely nullified.! As noted above, the dehydration and swelling of a gelatin jelly is reversible over a considerable range. ‘This is not the case with hydrous oxide jellies such as silica. Van Bemmelen? 1Cf. MicHaEuis: Colloid Symposium Monograph, 2, 1 (1924); Strasny: Kolloid-Z., 35, 353 (1924). 2 “Die Absorption” (1910). JELLIES AND GELATINOUS PRECIPITATES 23 showed that a silica gel containing a great deal of water shrinks very much when the water is removed; and, while it will take up some water again, the volume change is not reversible. If the drying is carried sufficiently far, pores are developed that are filled with air, and these pores can then be filled with a liquid other than water; but there is no appreciable swelling. When gelatin is dried, such pores are not developed and a dry gel of natural gelatin will not adsorb benzene. Although the porous mass formed by drying a non-elastic gel will not swell in organic liquids, Graham found that such liquids will replace the water in a jelly. Thus a silica gel con- taining 11 per cent SiO». was suspended repeatedly in alcohol, and an alcogel was formed having approximately the same volume as the original gel. In a similar way the water was replaced by inorganic and organic acids. Van Bemmelen substituted ace- tone for the water and Bachmann! put in benzene. Neuhausen and Patrick? found that the replacement of water was not quite so complete as Graham reported on repeated immersions of a silica jelly in anhydrous alcohol or benzol. Elastic jellies show a similar behavior. Thus Biitschli? found it comparatively easy to replace the water in a gelatin jelly with alcohol and this again by chloroform, turpentine, or xylene, even popes dry gelatin does not swell in these liquids. Concentrated Gels.—Many difficultly soluble salts that ordi- narily precipitate in relatively large crystals can be thrown out in the form of a gelatinous precipitate or jelly from very concen- trated solutions. This phenomenon was observed by Hartung,’ Biichner,® Biedermann,® Neuberg,’ and particularly by von Wei- marn.® The latter? made a systematic study of the form in which substances precipitate from solution. He calls attention to a 1Z. anorg. Chem., 78, 125 (1912). 2 J. Am. Chem. Soc., 48, 1844 (1921). 3 “Uber den Bau quellbarer Korper,” Gottingen, 22 (1896). | 4 “Recherches de morphologic synthétique sur la production artificielle de quelques formations calcaries organiques,’’ Amsterdam (1872). 5 Chem. Ztg., 17, 878 (1893). 6 Z. allgem. Physiol., 1, 154 (1902). 7 Siteb. Akad. Wiss. Berlin, 820 (1907). 8 “Zur Lehre von den Zustiinden der Materie’”’ (1914). 9 Von WermARN: ‘‘Grundziige der Dispersoidchemie,” 39 (1911). 24 THE HYDROUS OXIDES number of very different factors on which precipitation depends: the solubility of the substance; the latent heat of precipitation; the concentration at which the precipitation takes place; the normal pressure at the surface of the solvent; and the molecular weights of the solvent and the solute. He points out the impossi- bility of taking all of these factors into account and simplifies the problem by considering, first, but two of the factors: the solubility of the precipitating substances, and the concentration at which precipitation begins. The effect of viscosity is discussed briefly in a later work.! The process of condensation (precipita- tion) is divided into two parts: the first stage, in which the mole- cules condense to invisible or ultramicroscopic crystals; and the second, which is concerned with the growth of the particles as a result of diffusion. The velocity at the important first moment of the first stage of the precipitation is formulated thus: Condensation pressure es neat Ee KE ae a W=K Condensation resistance L L where W is the initial rate of precipitation; Q the total concen- tration of the substance that is to precipitate; L the solubility of coarse crystals of the substance; Q — L = P the amount of super- rele ; saturation. The ratio jcoe U is the precentage supersatura- tion at the moment precipitation begins. The velocity of the second stage is given by the Nernst-Noyes equation: Ve me O-(C —1D) as where D is the diffusion coefficient; S the thickness of the adherent film; O the surface; C the concentration of the surrounding solu- tion; and / the solubility of the dispersed phase for a given degree of dispersity. C — 1 may be termed the absolute supersaturation. From these general formulations, von Weimarn arrives at the conclusion that jellies are obtained only when the ratio ee that is, the percentage supersaturation U, can be made enormous. It is pointed out that the nature of a precipitate is quite different, 1Von WEIMARN: Kolloidchem. Bethefte, 4, 101 (1912). - JELLIES AND GELATINOUS PRECIPITATES 25 depending on whether a given value of U is obtained by a large Por byasmall Zl. If a large U is obtained by a high value of P, a large amount of disperse phase is produced and a gel forms, while if P is small and L very small, a relatively small amount of disperse phase is produced and a sol is formed. Von Weimarn has demonstrated the accuracy of his deductions in a large num- ber of cases, using reacting solutions of high concentrations; and it is apparently true that any salt can be obtained in a gelatinous form if the concentration of the reacting solutions and so the velocity of precipitation is sufficiently high. Thus, vonWeimarn! prepared gelatinous precipitates of barium sulfate which usually comes down in the form of crystals, by mixing 1 to 3 N solutions of manganese sulfate and barium thiocyanate. By using solu- tions of sufficiently high concentration (3 to 7N) all the solute was enclosed, forming jellies. These are not the conditions under which jellies are usually obtained, and their existence is temporary. By mixing very high concentrations of materials that react to form an insoluble precipitate, a very large number of relatively small particles are formed, because of the high degree of super- saturation.2 Each of these minute particles adsorbs a little water and as they are very close together, a semisolid mass results that entrains all the liquid phase, thus forming what has been termed a jelly. These so-called jellies break down on standing, on account of growth of the particles and the consequent liberation of adsorbed water. I do not believe that precipitates in which the ratio of mols of water to mols of salt is, say, 20:1 or 25:1 should be considered as jellies in the same sense as precipitates in which this ratio is two or three hundred times as great. Very finely divided sand or fuller’s earth may be matted in the bottom of a test tube, and this solid will take up a great deal of water before a supernatant water layer is observed; but I should not call such a preparation a jelly. It seems to me that von Weimarn’s barium sulfate jelly may be similar except that the particles are much smaller, and so a given amount will take up more water. On the other hand, with true jellies where the amount of enclosed water may be relatively enormous, time must be allowed for 1 ‘Zur Lehre von den Zustiinden der Materie,” 21 (1914). 2 BancrorT: J. Phys, Chem., 24, 100 (1920). 26 THE HYDROUS OXIDES formation of a definite structure. As a matter of fact, von Weimarn! recognized a difference between a barium sulfate jelly . prepared by his method and a jelly formed by uniform gelatiniza- tion of a liquid throughout its mass, as in the case of gelatin jelly. The former he terms a ‘‘coarsely cellular gel’? and the latter a ‘“‘reticulated gel.” | Precipitation of Sol.—Since finely divided particles that adsorb — water strongly are of primary importance for the formation of a hydrous jelly, it would seem that the most promising method of preparing dilute jellies would be to precipitate hydrous substances from colloidal solution. The von Weimarn theory would tell us, of course, that this precipitation would have to take place at a suitable rate under conditions that are not conducive to growth of the individual particles; but it does not enable us to predict the optimum rate of coagulation, the effect of salts on jelly formation, or the conditions that favor the formation of a jelly rather than a gelatinous precipitate. As a result of recent investigations in the author’s laboratory on the formation of typical dilute inorganic jellies, the hydrous oxides particularly, it is possible to outline the general conditions of jelly formation and the effect on the process of various factors other than the percentage supersaturation ‘‘at the important first moment of the first stage of condensation” from molecules to invisible particles. Jellies would be expected to form from colloidal solution if a suitable amount is precipitated at a suitable rate -without agitation in the absence of a medium that exerts an appreciable solvent or peptizing action. If the concentration of the colloid is too low, no jelly or only a very soft jelly can result. If the velocity of precipitation is too great, contraction is likely to occur with the formation of a gelatinous precipitate instead of a jelly. The effect of the presence of salts on jelly formation is, therefore, determined in large measure by the precipitating and stabilizing action of the ions in so far as these affect the rate of precipitation. In general, a slow rate of precipitation favors the formation of a jelly rather than a gelatinous precipitate if there is little or no tendency of the particles to grow as a result of the solvent action of the electrolyte. The favorable concentration for different electrolytes is in the immediate region of their 1 J, Russ. Phys.-Chem. Soc., 47, 2163 (1915). JELLIES AND GELATINOUS PRECIPITATES 27 precipitation concentration. A little below this value, no precipi- tation or only a slight precipitation takes place; while above this value, coagulation is usually so rapid that a gelatinous precipitate , is formed instead of a jelly. The reason is that time is not allowed for the uniform mixing of the colloid with coagulant, and the slow uniform precipitation necessary for the building of a uniform jelly structure is replaced by rapid uneven coagulation and the consequent contraction that distinguishes a gelatinous precipitate from a jelly. The accuracy of these deductions has been demonstrated repeatedly, and frequent reference will be made to them in later chapters. In many cases, these jellies may be obtained in rela- tively low concentrations. A notable example is the case of hydrous chromic oxide which formed a firm jelly containing but 0.18 per cent Cr2O; and a soft jelly containing 0.09 per cent Cr2QO3. The formation of such dilute jellies can result only when the particles are very hydrous and when the conditions of precipitation allow time for the building up of an enmeshing network. In ease the particles are but slightly hydrous and show but little tendency to link together into threads, extremely high concen- trations must be present, as von Weimarn found. Dialysis of Sols.—Prolonged dialysis of colloidal solutions frequently leads to the precipitation of a part of the suspended phase as a gelatinous precipitate. When this process was carried out in a suitable way on a colloidal solution of ferric arsenate peptized by ferric chloride, Grimaux! obtained a firm, transparent jelly. This observation has been confirmed and extended by Holmes and his pupils. Similar observations have been made in the author’s laboratory with hydrous oxides of chromium and aluminum, and the method is probably a general one. From the point of view outlined in the foregoing section, the formation of jellies by dialysis of a colloidal hydrous substance is readily understood. Dialysis merely removes the stabilizing ion slowly and uniformly below the critical value necessary for peptization; and precipitation results just as if the adsorption of the stabilizing 1 Compt. rend., 98, 1540 (1884). 2 Hotmes and Rinprusz: J. Am. Chem. Soc., 38, 1970 (1916); Hotmes and Arnoup: Jbid., 40, 1014 (1918); Hotmes and Fatt: Jbid., 41, 763 (1919). 28 THE HYDROUS OXIDES , ion were compensated for or neutralized by the addition of an electrolyte having a suitable precipitating ion. The accuracy of these deductions has been demonstrated conclusively in a series of investigations on the arsenates of iron and aluminum.! Dilute Jellies by Metathesis.—According to the von Weimarn theory, mixing dilute solutions that interact at once may give a gelatinous precipitate but. not a jelly, since the percentage supersaturation = U is too small because of the small value of P. As a matter of fact, however, jellies have been obtained under certain conditions by mixing very dilute solutions in which L is sufficiently large that precipitation is slow and quantitative precipitation impossible, so that - = U is very small. Such cases are apparently not covered by the von Weimarn theory. It is quite possible to obtain a gelatinous precipitate by mixing dilute solutions of two salts which precipitate immediately (P small, but Z very small); but a jelly will not form under these conditions. The reason is evident when we consider the impossibility of getting the instantaneous mixing of the solutions which is essential for uniform precipitation throughout the mixture. One part is precipitated before another is mixed with the precipitant, and the uniformity characteristic of a jelly is lost. Moreover, the mixing itself will tend to destroy the jelly structure. The results are, therefore, not unlike those obtained when a colloid capable of forming a jelly by slow precip- itation is coagulated too rapidly by the addition of excess elec- trolyte. To obtain a jelly from a colloidal solution, it is necessary to add such an amount of electrolyte that thorough mixing is possible before appreciable coagulation takes place. From these considerations, it follows that precipitation of a hydrous substance as a result of double decomposition might form a jelly instead of a gelatinous precipitate in case the thorough mixing of the solutions could be effected before precipitation begins and in case the precipitation, once started, proceeds at a suitable rate. Such conditions do not obtain as a rule; but they are entirely possible theoretically. Thus the precipitation 1 Weiser and Bioxsom: J. Phys. Chem., 28, 26 (1924). JELLIES AND GELATINOUS PRECIPITATES 29 may be the result of a stepwise process, one step of which proceeds at a suitably slow rate. It is further possible to have a reaction that goes very slowly at low temperatures but with marked velocity at higher temperatures. This would not only allow of mixing without precipitation but would enable one to control the sub- sequent rate of reaction by a suitable regulation of the tempera- ture. Such a favorable combination of circumstances apparently obtains when a manganese salt of a strong acid and KH,AsO, are mixed. The latter salt ionizes thus: KH.AsO s K’ + H.AsOy.’; but on account of the solubility of Mn(H2AsO,)2, no Mn” ions are removed from solution by interaction with H,AsO,’. The latter ion, however, undergoes secondary ionization to a slight degree as follows: H2AsQO.’ reports considerable success in the treatment of anemia by injection of colloidal hydrous ferric oxide. Powis® prepared a stable negative sol without a protective colloid by allowing 100 cubic centimeters of 0.01N ferric chloride to run slowly, with constant shaking, into 150 cubic centimeters of 0.01N sodium hydroxide. The sol was of clear brownish- yellow color and showed no sign of precipitation after standing 3 weeks, although a trace of barium chloride caused immediate coagulation.” A ferric oxide sol prepared by electrical disintegration of iron electrodes under water® is usually positively charged; but it becomes less positive, neutral, and finally negative by repeated filtration through such substances as filter paper, glass wool, cotton, or sand, which are negatively charged in the presence of water. According to Malarski,® the reversal of charge on the particles is brought about by contact with the negative filtering media. While this explanation seems plausible, the experiments should be repeated to determine to what extent the properties of 1 THorPE: “ Dictionary of Applied Chemistry,” 3, 176 (1912). 2 Compt. rend., 101, 321 (1885). 3 Biochem. Z., 27, 223, 238 (1910). 4 FiscHer and Kuznitzky: Biochem. Z., 27, 311 (1910). 5 Gazz. ospedali clin., 41, 182 (1920). 6 J. Chem. Soc., 107, 818 (1915). 7Cf. Keuumr: Kolloid-Z., 26, 173 (1920). 8’ Brepia: Z. Elektrochem., 4, 514 (1898); Z. physik. Chem., 34, 258 (1899). ® Kolloid-Z., 23, 113 (1918). 46 THE HYDROUS OXIDES the sol are altered by adsorption of stabilizing ions during repeated filtration. Composition of Ferric Oxide Sol.—Since but one precipitated hydrate of ferric oxide exists—the yellow monohydrate—it seems altogether improbable that the red colloidal solutions of Péan de St. Gilles or Graham contain definite hydrates. The variation in properties between the Graham sol and the red and yellow Péan de St. Gilles sol is not due to chemical structure or the existence of hydrates, but is the result of differences in the size-distribution curve of the primary colloidal particles. For the Graham sol, the maximum is in the region of exceedingly small particle size, the position of the maximum shifting toward larger size particles as we pass to the red and the yellow Péan de St. Gilles sols.!. The conditions under which the sols are formed favor this view. Thus the percentage supersaturation of ferric oxide is highest for the conditions which give the Gra- ham sol and lowest for those which give the yellow Péan de St. Gilles sol. Differences in hydration of the ferric oxide sols are due to differences in specific surface, the Graham sols possessing the greatest specific surface and, therefore, the greatest amount of aasorbed water. If the Graham sol is impure and dilute, there is a gradual growth of primary particles accompanied by a decrease in the amount of adsorbed water. The absence of chemical combination between water and ferric oxide in sols prepared by hot dialysis was confirmed recently by means of freezing-point determinations carried out by Browne.? Although the colloid contains ferric oxide in a highly hydrous condition, the effect of dextrose on the freezing point of the sol showed that the water associated with the oxide was adsorbed, as all the water present in the sol acted as solvent for dextrose or for any other soluble substance. While the particles of the red sol are hydrous oxides, it is possible that the yellow Péan de St. Gilles sol may contain par- ticles of monohydrate or of hydrous monohydrate. . Colloidal ferric oxides can be prepared fairly free from electro- lytes but it has been demonstrated repeatedly that at least some 1 ZsIGMONDY: ‘‘Chemistry of Colloids,”’ translated by Spear, 163 (1917). 2 J. Am. Chem. Soc., 45, 297 (1923). THE HYDROUS OXIDES OF IRON 47 electrolyte must be present in such sols to ensure their stability.! Thus the Graham sol, peptized by ferric chloride or hydrochloric acid, always contains traces of chlorides, however long the dialysis may be continued.* On this account, a number of investigators consider the various dialyzed colloids to be chlorides of condensed ferric hydroxides like Feo(OH).14 9Fe:Cle or as oxychlorides of variable composition.*? This conception of the nature of col- loidal solutions meets with serious objection at the outset, since Fischer* and others’ have shown that definite chemical oxychlor- ides of iron do not exist at ordinary temperatures. Naturally, investigators who assume the existence of such definite compounds in ferric oxide sols are unable to agree on their composition. Thus, Nicolardot claims that the sols are made up of mixtures of two compounds in which the ratios of iron to chlorine in equiv- alents are 6 and 125, respectively. Neidle showed these ratios to be purely accidental; but believes there is a compound in which the ratio is 21. Recently Thomas and Frieden have arrived at the conclusion that 1 mol of ferric chloride is necessary to keep 21 mols of ferric oxide (ratio of iron to chlorine in equiv- 1 KastTNER: Ann. chim. phys., (3) 57, 231 (1859); DeBray: Compt. rend., 59, 174 (1864); MaGnier DE LASouRCE: [bid., 90, 13852 (1880); Hanrzscu and Descu: Liebig’s Ann. Chem., 323, 28 (1903); LinpER and Picton: J. Chem. Soc., 87, 1920 (1905); Wyrousorr: Ann. chim. phys., 7, 449 (1905); Rumr: Z. anorg. Chem., 48, 85 (1905); Nuiwue: J. Am. Chem. Soc., 39, 2334 (1917). 2 Cf. Urrr: ‘Uber kolloides Eisenoxyd.,’’ Dissertation, Dresden (1915). 3 WyRoOuUBOFF and VERNEUIL: Bull. soc. chim., (8) 21, 187 (1899); Jorpis: Z. anorg. Chem., 35, 16 (1903); Z. Hlektrochem., 10, 509 (1904); Ductaux: Compt. rend., 138, 144, 809 (1904); 140, 1468, 1544 (1905); 143, 296, 344 (1906); J. chim. phys., 5, 29 (1907); LinpER and Picron: J. Chem. Soc., 87, 1919 (1905); Nrcouarpot: Ann. chim. phys., 6, 334 (1905); Maurirano: Compt. rend., 139, 1221 (1904); 140, 1245 (1905); 141, 660, 680 (1905); 143, 172, 1141 (1906); Z. physik. Chem., 68, 232 (1910); Maurirano and MIcHEL: Compt. rend., 145, 185, 1275 (1907); MicuEu: Compt. rend., 147, 1052, 1288 (1908); Dumanskt: Kolloid-Z., 8, 232 (1911); Neipuu: J. Am. Chem. Soc., 39, 2334 (1917); Maruua: Kolloid-Z., 21, 49 (1917); Tuomas and FRIEDEN: J. Am. Chem. Soc., 45, 2522 (1923); Pautrand Roaan: Kolloid-Z., 35, 131 (1924); Pautiand WautsErR: Kolloidchem. Bethefte, 17, 256 (1923); Ktunu and Pauuti: Jbid., 20, 319 (1925). 4Z. anorg. Chem., 66, 38 (1910). 5 CAMERON and Rosinson: J. Phys. Chem., 11, 690 (1907); Groxirtt: Gazz. chim. ital., 36, 1157 (1906); Smrru and Griesy: J. Am. Pharm. Assoc., 12, 855 (1923). 48 THE HYDROUS OXIDES alents = 42) dispersed in the colloidal condition, irrespective of the concentration of the sol. These observations and conclu- sions are not in accord with Neidle, who showed that the maxi- mum purity obtainable before precipitation sets in increases appreciably with decreasing iron content. Neidle prepared a sol, approximately 0.05 N with respect to iron, in which the ratio equivalents Fe:--: equivalents Cl’ was 84; while the maximum purity obtained by Thomas and Frieden at this concentration was only about half as great. Bradfield! prepared a ferric oxide sol by washing by the use of the centrifuge, in which the ratio was 396. In the purest sol Ufer? was able to prepare by dialysis, the ratio was approximately 2700. In the light of the wide variation in the iron: chlorine ratio obtained by different investigators, there seems no room to doubt but that, within reasonable limits, the composition of a dialyzed colloid depends upon the condition of formation. Everybody knows that precipitation of a colloid will take place if the dialysis is carried too far; and that consistent results can be obtained only by a very careful control of the experimental conditions. Since the method of procedure followed by different investigators is likely to vary widely, we might expect the wide variation in the results which the records show. Some people require very little evidence to convince them of the existence of chemical compounds and these assign definite formulas to the dialyzed sol. Others who have observed the passing of many cherished and time- honored ‘‘compounds” content themselves with postulating the formation of a series of “‘indefinite’”? compounds in order to explain their observations. This course is of questionable value because of the complexity and variability of the systems that are encoun- tered and the consequent complexity of the hypothetical com- pounds that must be assumed to exist. Recently, attempts have been made to determine the number of molecules in a single collodial particle per unit of electrical charge, from electrical conductivity and transport measurements on the sols themselves and on the ultrafiltrates from the 1J. Am. Chem. Soc., 44, 965 (1920). . 2 “Uber kolloides Eisenoxyd.,’’ Dissertation, Dresden (1915); cf. FrmuNp- LicH; ‘‘Kapillarchemie,” 511 (1922). THE HYDROUS OXIDES OF IRON 49 sols.1. According to Duclaux? the specific conductance K,, of the colloidal particles or micelles of a sol is given by the expression ee eee where K, and K; are the specific conductances of the sol and the ultrafiltrate from the sol, respectively. If a quantity of electricity H, is passed through the sol, the part carried by the micelles E,, will be nie als (1) If one knows the mass of the micelle ions m transferred by a quantity of electricity H,, together with the mobility of the micelle ion U,. and of accompanying anion U,, one can calculate the charge on the mass Ae corresponding to the electrochemi- eal equivalent, from the equation, Ae = Gems (2) where F is 1 faraday. Substituting for H,, its value from Eq. (1), Kq. (2) becomes FK, 4, UetU; Ae = RK mM 9 (3) If EH, is made identical with F, then m becomes S, the mass of the micelle ions in mols of dispersed substance, Fe2.O3. Ae thus becomes A, and since Ut Us _utv_A Ol. Bie ip U Eq. (8) may be written SK,.A Ap. = eer (4) Representing the sum of the mobilities of the cation and anion from conductivity measurements in the usual way by wu + 0; 1 WiInTGEN: Z. physik. Chem., 103, 250 (1922); Wintcamn and BI.rTz: Ibid., 107, 403 (1923); WintGEN and Léwentuat: I[bid., 109, 378 (1924); cf. Putacart: ‘First American Congress of Chemistry” (1924); Chem. Abstracts, 19, 1518 (1925). | 2 Compt. rend., 140, 1468, 1544 (1905). 50 THE HYDROUS OXIDES the molecular weight of the colloidal component, Fe2O;, by M; and the weight in grams of Fe2QOs in a liter of sol by g, then Ae, which becomes Az, is Mieoee Thee lt) % i 1000. (5) Wintgen has developed this formula from Kohlrausch’s law of the independent migration of the ions, in the following way: For a ferric oxide sol prepared by dialysis of ferric chloride, let M = molecular weight of Fe.203; g = concentration in grams of Fes,Os; per liter; Z = number of elementary charges carried by 1 micelle; and n = number of FeO; molecules in 1 micelle. The electro- chemical equivalent weight W of the micelle is given by nM wae 6) which corresponds to the electrochemical equivalent weight of an ordinary ion; and so gives the weight in grams of Fe.0; which carries 1 faraday. The value Ae=—=5 : (7) gives the number of mols of Fe,O3 in a micelle ion carrying 1 faraday. The equivalent concentration C,. of the micelles is then Cee — es (8) If uw and v are the mobilities of the micelle ion and chloride ion, respectively, from conductivity measurements, then 1000Km = (u+0)Cae = (u +0) Ee (9) from which _ gu +2) Wi aan (10) Substituting for W its value MAe from Kq. (7), Eq. (10) becomes ono A, = Ae = 77 Top0K, - THE HYDROUS OXIDES OF IRON ol The value Ae is spoken of as the ‘‘equivalent aggregate” or as the electrochemical equivalent of the micelle. If, in a liter of sol, there are m; mols of Fe.O3 and mz gram atoms of chlorine (ionized and combined); and if the concentration of chlorine in the intermicellar liquid (that is, in the ultrafiltrate) is [Cl,], then the gram atoms of chlorine corresponding to Ae mols of FeO; are Te ne [Cli] my -Ae Of this, unit amount is split off from the micelle and the remainder E is a part of the micelle; therefore ma —(Ch] | mi ase Ae = 1 Using these formulas, Wintgen! calculated the composition of the micelles of a number of sols. For example, assuming the micelles of an aged iron oxide sol (containing 1.601 grams Fe.O3; and 0.06014 gram Cl in 100 grams of sol) to be all the same size, the average composition of the micelles is represented by the formula (175.35F e203; 7.86HCI1; zH20 FeO ) 10,230 a 10,2380Cl’ Lottermoser? found the hydrogen ion concentration of the ultra- filtrates from an aged sol to be the same as that of the sol. The micelles are believed to contain neutral chloride as well as chloride ion. Since the positive charge on the particles is probably due to stronger adsorption of hydrogen ion than of chloride ion, their composition may be represented by the general formula (| #F e203: yHCl-2H.0 |H’)a:(n — g)Cl’ where gCl’ represents the chloride ion corresponding to the excess of adsorbed hydrogen ion to which the particle owes its free charge. Lottermoser* found the specific conductance of the sol to be higher than that of the ultrafiltrates, the difference being regarded as the true conductivity of the micelles. If 1 WINTGEN and LOWENTHAL: Z. phystk. Chem., 109, 378 (1924). 2 Z. Elektrochem., 30, 391 (1924). 3 Cf. KopaczEwskI: Compt. rend., 179, 628 (1924). oO2 THE HYDROUS OXIDES the micelles P are considered to be complex electrolytes, the equivalent conductivity at infinite dilution may be calculated from the equation _ 1000K> Pico a Ka The mobility of the micelle was found to rise abnormally with increasing dilution in purified sols containing but small amounts of chlorine. This fact necessitates the assumption that the micelles are adsorption complexes, the abnormality being due to the displacing of the adsorption and hydrolysis equilibria by dilution. The value Ap., approaches a constant value only in sols rich in chlorine. From this, it would appear that nothing is gained by looking upon ferric oxide sols as electrolytes with complex cations. But if one insists on regarding them in this light, there is no particular objection provided one recognizes clearly that there is a fundamental difference between sols and non-colloidal, complex electrolytes. In the latter, there exists a simple, stoichiometric ratio between the neutral component and the complex-forming ion; while in sols, the ratio of neutral constitutent to what Lottermoser calls sol-forming ions is indefi- nite and changes continuously. A simpler and probably quite as exact an interpretation of the variable properties and composition of the sols may be given from the point of view of specific adsorption: Any number of hydrous ferric oxides are possible, differing among themselves in the size of the particles, and hence in the amount of salt or ion adsorption. The colloid prepared by the Graham method is formed in the presence of ferric chloride, hydrochloric acid, ferric ions, hydro- gen ions, and chloride ions.! Accordingly, we might expect the colloidal particles to adsorb some ferric chloride and hydrochloric acid, and they will always adsorb ferric, hydrogen, and chloride © ions in amounts depending on the nature of the colloid, the specific adsorbability, and the concentration.2 Now it is well known that a substance always shows a strong tendency to adsorb its own ions, and hydrogen ion is usually very strongly adsorbed; on the other hand, chloride ion is not usually adsorbed so 1 BROWNE: J. Am. Chem. Soc., 45, 297 (1928). 2 Cf. Marria: Kolloidchem. Bethefte, 3, 85 (1911). THE HYDROUS OXIDES OF IRON 53 strongly, and this preferential adsorption results in a stable positive colloid. Since there is an equilibrium between the amount of a substance adsorbed and the amount in solution, prolonged dialysis will result in the loss of part of the adsorbed cations (together with an equivalent amount of anions) and this will decrease the stability of the sol. Adsorbed chloride, either as salt or as ion, will not give a test with silver nitrate; and small amounts of unadsorbed chloride in the presence of colloidal iron oxide cannot be detected by precipitation with silver nitrate, since the protecting action of the hydrous oxide does not allow the particles of silver chloride to become large enough to cause tur- bidity.!. Moreover, adsorbed chloride will have a negligible effect on a chlorine electrode and will not be detected potentio- metrically; hence it is not surprising to learn that the amount of chlorine as ion is less than the total chlorine content of the sol.” From this point of view, it is obviously unnecessary to postulate the existence of oxychlorides of varying composition to account for the observation that only a part of the chlorine present appears to exist as ion.? Since colloidal solutions, in general, are instable in the absence of some soluble substance that is strongly adsorbed by the col- loidal particles, it follows that a colloidal solution will show a slight osmotic pressure and freezing-point lowering. This has been observed by a number of investigators with Graham’s colloidal ferric oxide. Duclaux‘* found that the osmotic pressure increases with the concentration of sol but is not proportional to it. He demonstrated also that the osmotic pressure falls off slightly with rise in temperature, a result that was confirmed by Zsigmondy.®> Both Duclaux and Malfitano® observed that the osmotic pressure of ferric oxide sols does not vary directly with their conductivity, the latter decreasing more rapidly than the former with dilution of the sol. It has been customary to interpret these results qualitatively by postulating the presence 1RuER: Z. anorg. Chem., 48, 85 (1905). 2Pauti and Matuta: Kolloid-Z., 21, 49 (1917) 3 See DumanskI: Kolloid-Z., 8, 232 (1911). 4 Compt. rend., 140, 1544 (1905). 5 ““Chemistry of Colloids,’ translated by Spear, 167 (1917). 6 Compt. rend., 189, 1221 (1904). 54 THE HYDROUS OXIDES in the sol of complex oxy-salts having all the necessary properties;! but such an explanation is not particularly helpful. If we have a suspension that is altogether insoluble and con- tains no impurities, it will give rise to no osmotic pressure. The osmotic pressure of a well-dialyzed Graham sol is due partly to the colloidal particles which have adsorbed ions; but chiefly to the ions of ferric chloride and hydrochloric acid. Since the behavior of an adsorbed ion will depend on the size and nature of the adsorbing particle, it follows that any factor affecting the physical character of the particles or the adsorption of ions by them will influence the osmotic pressure of the sol. Moreover, since the osmotic pressure and freezing-point lowering in a well- purified sol are necessarily small, molecular weights deduced there- from may be absurdly large.” In the nature of things, it is wrong to attribute the observed osmotic pressure and freezing-point lowering of any sol to the insoluble suspended material, and molecular weights deduced from such data are meaningless. The experiments of Duclaux and Malfitano should be repeated, and observations made of the effect of dilution and temperature on the number, physical character, adsorbability, and mobility of the colloidal particles of hydrous oxide. Optical Properties—Majorana* made the interesting observa- tion that a sol exhibits pronounced double refraction when placed in the field of a powerful electromagnet and traversed by a light ray at right angles to the lines of force. This property is undoubtedly due to orientation of the particles of sol by the electric field;+ for this orientation and the concomitant double refraction can be observed directly by working with a sol con- taining particles large enough to see with an ordinary microscope. Moreover, a gel formed by coagulation of a sol in an electric field exhibits permanent double refraction, whereas coagulation under ordinary conditions gives an optically inactive gel. Large 1 MauriTano: Compt. rend., 139, 1221 (1904); Ducuaux: J. chim. phys., 7, 405 (1919). . 2TLINDER and Picton: J. Chem. Soc., 87, 1920 (1905); Dumanskr1: Kolloid-Z., 8, 232 (1911). | 3 Atti accad. Lincet, 11, (1) 374, 463, 531; 12, (1) 90, 139 (1902). 4Scumauss: Drude’s Ann., 12, 186 (1903); CoTtron and Movuron: Compt. rend., 141, 317, 349 (1905); ‘“‘Les Ultramicroscopes,’” Paris, Chap. VIII (1906). THE HYDROUS OXIDES OF IRON 55 particles cause a greater effect than small ultramicrons since the Brownian movement of the latter prevents sufficient orientation to cause pronounced double refraction. Cotton and Mouton attribute the optical phenomenon to the particles themselves and not to their position alone. This is in accord with Freund- lich’s! observation that colloidal solutions of ferric oxide and vanadium pentoxide” showing the Majorana phenomenon exhibit double refraction when stirred mechanically* or when a current of electricity is passed through the sols. These observations lend support to Nageli’s* view that the particles of certain sols consist of anisotropic ultramicrons having a resemblance to tiny crystals. THE PRECIPITATION OF SOLS BY ELECTROLYTES Investigations on the precipitation of ferric oxide sols by elec- trolytes have been confined pretty largely to Graham’s sol. Duclaux, working with a colloid of this type containing 203 x 10-® equivalents of iron and 16.6 < 10~° equivalents of chlorine per 10 cubic centimeters, found the critical coagulation concen- tration of sodium sulfate, citrate, chromate, carbonate, phosphate, hydroxide, and ferrocyanide to vary from 13 X10~° in the case of ferrocyanide to 19 X10~°® equivalents in the case of phosphate. These observations were believed to show that equivalent amounts of the various ions cause the same effect, and further- more, that the amount necessary for precipitation is the same as the chloride content of the colloid, within the limits of the experimental errors. He thus came to regard the precipitation process as a definite stoichiometric chemical action, a double decomposition of the ordinary type. A marked variation from the equivalence rule was observed with sodium chloride and sodium nitrate which required 2000 < 10-* and 1880 * 10~° gram equiv- alents, respectively, to precipitate the same amount of colloid as the seven salts above referred to. Freundlich® found a wide 1Z. Elektrochem., 22, 27 (1916). 2 See p. 266. 3 Cf. QuINcKE: Drude’s Ann., (4) 15, 28 (1904); Treri: Atte accad. Lincet, (5) 19, 470 (1910). 4 “Theorie der Garung,’”’ Miinchen, 121 (1879). ® “ Kapillarchemie,”’ 352, 358 (1909). 56 THE HYDROUS OXIDES variation from equivalence in the precipitation concentration of various salts, which he attributed to a difference in the adsorb- ability of the precipitating anions. From Freundlich’s observa- tions the order of precipitating power of the anions is: dichro- mate > sulfate > hydroxide > salicylate > benzoate > chloride > nitrate > bromide > iodide. The results of some experiments! on the precipitation of a Péan de St. Gilles sol (1.54 grams Fe,O; per liter) with various potassium salts give the following series, beginning with ferro- cyanide which has the greatest precipitating power: ferrocyanide > ferricyanide > dichromate >tartrate>sulfate>oxalate> chromate > iodate > bromate > thiocyanate > chloride > chlorate > nitrate > bromide>iodide. As one should expect, the order of ions is identical with that deduced from Freundlich’s data for the ions common to both series. Owing to differences in nature and purity? of the Péan de St. Gilles sol and Duclaux’s sol, the precipitation concentrations of electrolytes were higher for the former than for the latter; but the magnitude of the variation is relatively unimportant compared with the fact that the precipita- tion values are not the same, as Duclaux believed. The Graham sol is stabilized by preferential adsorption of hydrogen ion and probably some ferric ion. The unadsorbed chloride ion present in an extremely pure sol is a measure of the excess cationic adsorption which gives the colloid particles their charge and the colloid its stability. Different concentrations of electrolytes are necessary to neutralize the adsorbed ions and precipitate the sol. The concentration of anion necessary to effect neutralization will approximate the chloride ion concentration only in so far as its adsorption tendency approaches that of the adsorbed cations. The precipitation concentrations of acids are uniformly higher than those of potassium salts since the stabilizing hydrogen ion is more strongly adsorbed than potassium ion. The order of precipitating power of electrolytes changes but little with varia- . tion in the hydrogen ion concentration of the sol.* 1 WEISER and MippietTon: J. Phys. Chem., 24, 641 (1920). 2See p. 91 for a discussion of the influence of purity of sols on the pre- cipitation values. 3 Rona and Lipmann: Biochem. Z., 147, 163 Wb To € . | THK HYDROUS OXIDES OF IRON o7 The change in dispersity of ferric oxide during coagulation does not involve a measurable heat effect. The precise investiga- tions of Mathews and Browne! show that the heat effects during precipitations of sols of low purity, are due to dilution of the ferric chloride and hydrochloric acid in the sols; to mixing of these electrolytes with the coagulating electrolyte; and to changes in the adsorption equilibria. The absence of heat effect on coagulation indicates either (1) a very low interfacial tension between hydrous ferric oxide and water, or (2) no appreciable change in specific surface during coagulation. In support of the latter viewpoint Bradfield? showed that so-called “irreversi- ble” coagula could be repeptized by thorough washing in the centrifuge. Apparently, coagulation by electrolytes is not accompanied by a growth of the primary colloidal particles, but the latter merely agglomerate into loose clumps without occa- sioning any marked decrease in the specific surface. Effect of Concentration of Sol.—The early observations of Freundlich? on the precipitation of colloidal arsenious sulfide led him to the erroneous conclusion that the precipitation values of electrolytes for colloids of different concentrations bear a constant ratio to each other. Thus Kruyt and van der Spek? found that the precipitation value of potassium chloride for colloidal arsenious sulfide increases, and of aluminum chloride falls off, with decreasing concentration of colloid; while the precipitation value of barium chloride does not change appreci- ably with the dilution. Similar results obtained by Burton and Bishop® with colloidal arsenious sulfide and mastic® led to the formulation of the following rule: The precipitating action of univalent ions increases, that of divalent ions remains unchanged, and that of trivalent ions decreases with diminishing concentra- 1J. Am. Chem. Soc., 43, 2336 (1921); Browne: Ibid., 45, 297 (1923). 2 J. Am. Chem. Soc., 44, 965 (1922). | 3 Z. physik. Chem., 44, 129 (1903). 4 Kolloid-Z., 25, 3 (1919); cf. MukHopapHyaya: J. Am. Chem. Soc., 37, 2024 (1915). 5 J. Phys. Chem., 24, 701 (1920); Burton and MaclInnus: [bid., 25, 517 (1921). ’ 6 Cf. Neisser and FrrepEMANN: Mtinch. med. Wochenschr., 51, 827 (1904); cf. Becuuo.p: Z. physik. Chem., 48, 385 (1904); Bacu: J. chim. phys., 18, 52 (1920). 08 THE HYDROUS OXIDES tion of sol. Some investigations! were carried out in the author’s laboratory using colloidal chromic oxide, Prussian blue, Péan de St. Gilles ferric oxide, and arsenious sulfide. The results of a a ZB e 3 EF evel Ratio of Precipitation Values 0 _ Concentration of Colloid, per cent Fig. 3.—Precipitation of colloidal hydrous ferric oxides. er a Batt aa [ a Eee E eke eee rc aa Yaa et - 0.9 SSS Seal | eee s | [| |e) 2 £07 ae :| | | aa oO ers ee) 03 0 35 50) te 100 Concentration of Colloid, per cent Fic. 4.—Precipitation of colloidal arsenious sulfide sols. series of experiements on colloidal ferric oxide and arsenious sulfide are given in Table II and shown graphically in Figs. 3 and 4. The concentrations of the sols are expressed in per 1 WEISER and Nicuouas: J. Phys. Chem., 25, 742 (1921). — a Gt PORT. CE ip bere hts a Saag ot gg rh ONT ee Oe AS é THE HYDROUS OXIDES OF IRON 59 cent, taking the most concentrated as 100 per cent. The curves in the figures were obtained by plotting concentration against ratio of each precipitation value for a given electrolyte to that of the strongest sol. In the case of ferric oxide sol, the effect of dilution on the precipitation value of electrolytes is clearly not in accord with Burton and Bishop’s rule. ! TaBLeE II Precipitation of Ferric Oxide Sols Concentration of colloid, | Precipitation values of econ’ | KBrO; K,80. K.Fe(CN), 100 40.1 0.68 0.57 (1.7 grams per liter) 50 34.4 0.41 0.30 Zo 28 .0 0.25 0.16 1255 25.0 0.16 0.08 Precipitation of Arsenious Sulfide Sols ; . Precipitati ] t Concentration of colloid, recipitation values 0 Noa | KCl BaCl: AICI; 100 68.3 1.940 0.513 (6.24 grams per liter) a 75 5; 68.3 1.877 0.473 50 70.0 1.800 0.380 25 76.7 1.733 0.333 10 80.0 1.683 0.260 Similarly with colloidal Prussian blue? and chromic oxide, the precipitation value of all electrolytes diminishes as the concen- tration of the colloid falls off irrespective of the valence of the precipitating ion. In the case of colloidal arsenious sulfide, the precipitation value of potassium ion increases and of aluminum ion decreases with dilution of the sol, in accord with Burton and ° 1 Burton and Bisnop: J. Phys. Chem., 24, 701 (1920). 2 Of., however, Guosu and Duar: J. Phys. Chem., 29, 663 (1925). 60 THE HYDROUS OXIDES Bishop’s rule. However, from the slope of the curve in Fig. 2, it is obviously incorrect to say that the precipitating action of divalent barium ion is independent of the concentration of the colloid; this is no more true of barium ion than of potassium ion. According to Kruyt and van der Spek, two factors determine the effect of dilution of a colloid on the precipitation value of electrolytes: First, the smaller number of particles requires less electrolyte to lower the charge on the particles to the point of agelomeration; and, second, the greater distance between par- ticles making collision less probable, a further reduction in particle charge must be effected through the addition of more electrolyte. Since these two factors have opposite effects on the precipitation value, it is only necessary to assume the pre- dominating influence of one or the other in order to account for the results in a given case. Thus, Kruyt and van der Spek assume that the predominating influence in the precipitation of arsenious sulfide with potassium ion is the changing chance of collision, while the more important factor in the precipitation of ferric oxide with chloride ion is the alteration in the required amount to be adsorbed. The difference in behavior with pre- cipitating ions of the same valence is attributed to the lyophile properties of hydrous ferric oxide. Although both of the factors recognized by Kruyt and van der Spek unquestionably have an influence in determining the effect on the precipitation value of changing the concentration of sol, it would seem that these factors alone are inadequate to account for all the experimental results. The explanation sug- gested for the difference in behavior of colloidal arsenious sul- fide and hydrous ferric oxide with univalent precipitating ions is of doubtful value, particularly since mastic emulsion! behaves much like colloidal arsenious sulfide although certainly possessing more lyophile properties than Péan de St. Gilles’ ferric oxide. Furthermore, if the decreased chance of collision is the pre- dominating factor in preventing a weaker arsenious sulfide sol from coagulating in a given time in the presence of enough potas- - sium chloride to coagulate a stronger sol, it would seem that 1 NEISSER and FRIEDEMANN: Mtinch. med: Wochenschr. 61, 827 (1904); Burton and Bisuop: J. Phys. Chem., 24, 701 (1920). + i _——le THE HYDROUS OXIDES OF IRON 61 complete coagulation of the weaker sol should result if sufficient time were allowed. As a matter of fact, however, enough potas- sium chloride to precipitate in 2 hours a colloid containing 5 grams per liter will not precipitate a colloid one-fourth as strong in several weeks. Other observations indicate that Kruyt and van der Spek attach too much importance to the decreased chance of collision of the particles resulting from dilution of sols. Thus, everyone finds that the precipitation concentration varies almost directly with the concentration of sol in case the precipitating ion is of high valence. That the theory of Kruyt and van der Spek should be inade- quate in certain respects might be expected, since these investi- gators concerned themselves only with the precipitating ions of electrolytes, disregarding entirely the effect of adsorption of the stabilizing ions having the same charge as the colloid. If there is no adsorption of the stabilizing ion and if the adsorption of the precipitating ion is very great, there will be a tendency for the precipitation concentration to vary directly with the concentra- tion of the sol. On the other hand, if the stabilizing ion is adsorbed, a greater concentration of precipitating ion will be required to produce coagulation. This effect will be more pro- nounced the greater the dilution of the sol since the decreased chance both of collision and of coalescence will combine to render the sol proportionately more stable, so that correspondingly more of the precipitating ion must be added for complete precipi- tation. These conclusions are in accord with experimental results. With electrolytes sae multivalent precipitating ions, the influence of the stabilizing ion is frequently very small, since the adsorption is so slight at the very low precipitation coreenteenienn Under these conditions, the precipitation value diminishes to a greater or lesser extent as the concentration of the colloid decreases. As might be expected, the greater the valence of the precipitating ion and hence the lower the precipitation value, the more nearly we find the latter varying directly with the concentration of the sol. With electrolytes having univalent precipitating ions, the precipitation value is usually quite large. Although this is generally attributed to weak adsorption of the precipitating ion, 62 THE HYDROUS OXIDES at the high concentration necessary for coagulation, the adsorp- tion of the stabilizing ion cannot be disregarded. In fact, if the adsorption of the two ions is of the same order of magnitude, both may be taken up fairly strongly and a high precipitation value will result. Considerable experimental evidence! indicates that potassium ion and lithium ion are fairly strongly adsorbed by arsenious sulfide, the high percipitation value of potassium chloride or lithium chloride for this colloid arising from appreci- able adsorption of chloride ion. On the other hand, the high precipitation value of potassium chloride for colloidal hydrous ferric oxide is due to relatively weak adsorption of the precipi- tating ion, the stabilizing ion having much less effect than with colloidal arsenious sulfide.? In general, it may be said that the adsorption of the stabilizing ion varies widely but is never negligible for electrolytes which precipitate only in the high concentration characteristic of uni-univalent electrolytes. This adsorption of the ion having the same charge as the sol renders the latter more stable, and proportionately more of the precipi- tating ion is required for coagulation than in those cases where the influence of the stabilizing ion is negligible. Under these - conditions we may expect the precipitation value to fall off much less sharply or even to increase as the colloid concentration is reduced, the increase being greater the higher the valence of the stabilizing ion. Mutual Precipitation of Sols.—Biltz’? investigated the pre- cipitation’ of positive sols, including hydrous ferric oxide by negative colloids, such as platinum, selenium, silica, stannic oxide, molybdenum blue, Mo;Os, tungsten blue, W203, and the sulfides of arsenic, antimony, and cadmium. Complete precip- itation occurs when a sol of one sign is neutralized by adsorption of an amount of colloid carrying an equivalent quantity of ion of opposite sign. The amount of various colloids necessary to effect mutual precipitation will depend on their nature. Thus a certain colloidal ferric oxide is more effective than cerium oxide and less effective than thorium oxide in precipitating colloidal gold; while both thorium oxide and cerium oxide are more effec- 1 WEISER: J. Phys. Chem., 25, 665 (1921); Ibid., 28, 232 (1924). > WeIsER: Loc. cit.; cf. FREUNDLICH: Z. physik. Chem., 44, 157 (1903). ’ Ber., 37, 1095 (1904). | te Oe ote THE HYDROUS OXIDES OF IRON 63 tive than ferric oxide in precipitating colloidal antimony sulfide and arsenious sulfide. Similarly, a red colloidal gold, prepared by reduction of gold chloride with formaldehyde, requires for complete precipitation considerably less of a given ferric oxide sol than a blue-gold sol prepared with phosphorus as reducing agent.' Recently, Freundlich and Nathanson? found colloidal arsenious sulfide sol and Oden’s sulfur sol to be instable in the presence of each other. Since both sols are negatively charged, this instability cannot be due to neutralization by adsorption, but was found to result from interaction between the stabilizing agents of the two sols, hydrogen sulfide and pentathionic acid. This observation led Thomas and Johnson* to attribute the mutual precipitation of sols of opposite sign to chemical inter- action of the stabilizing electrolytes in the sols. Thus, the precipitation of Graham’s colloidal ferric oxide, stabilized by hydrogen ion, and colloidal stannic oxide, stabilized by hydroxyl ion, was attributed to chemical neutralization. This view was supported by the observation that mutual precipitation was effected over a limited range of purity of sols, when the hydro- chloric acid and sodium hydroxide concentrations in the sols were approximately equivalent. The variation from equivalence was quite marked in case the sols were fairly pure. Thus a silica sol containing 16 SiO, to 1 NaOH was precipitated at various dilutions with a sol containing 13 Fe.O; to 1 FeCl. At the highest dilution possible for obtaining accurate data, mutual precipitation was observed when an amount of colloidal silica was added corresponding to but 50 per cent of the hydrochloric acid. This variation was attributed to the metastability of pure sols, which causes them to precipitate with a subnormal disturbance. This does not seem quite convincing since, in the absence of contamination other than that mentioned, the purity of the sols would scarcely be great enough to make them abnor- mally sensitive. Erratic results were also obtained when the amount of peptizing agent was too large, say three times as much as in the case referred to above. Thus, to obtain data to support a purely chemical mechanism involving the stabilizing agents, 1 GaLEecKI and Kastovski: Kolloid-Z., 18, 143 (1913). ? Kolloid-Z., 28, 258 (1921); 29, 16 (1921). * J. Am. Chem. Soc., 45, 2532 (1923). 64 THE HYDROUS OXIDES it seems necessary to choose the experimental conditions to fit the case. While everyone will agree that the peptizing agents of two sols may interact under certain conditions, thus affecting the stability of each, such an interpretation of the mechanism of the mutual precipitation process would not account for the repeated observation of mutual precipitation of sols where inter- action between the peptizing agents is impossible or improbable. A sol peptized by hydrogen ion will be precipitated by a base or by a salt of a weak acid. From such observations alone, we might conclude that the precipitation of the sol was a result of chemical neutralization of the stabilizing agent. But the same sol will, in general, be precipitated with a small amount of an acid having a multivalent ion where chemical neutralization of hydrogen ion is impossible. If there are no disturbing influences, such as interaction between peptizing agents, Wintgen and Lowenthal! found the reciprocal precipitation of oppositely charged sols to be a maximum when the concentrations of the sols, expressed in ‘‘equivalent aggre- gates,’ are the same; that is, when equal numbers of charges of opposite sign are mixed. This rule does not hold in certain cases where a highly dispersed sol of one sign is mixed with a coarser sol of opposite sign, possibly because the smaller particles penetrate the larger ones and are precipitated by the electrolyte contained in the latter. Billitzer? found that gelatin in acid or neutral solution is a positive sol and so precipitates negative sols, but not positive ones such as hydrous ferric oxide; whereas gelatin in ammoniacal solution is a negative sol and precipitates hydrous ferric oxide. No precipitate is thrown down, however, if gelatin is first added to colloidal ferric oxide, followed by the addition of ammonia. In the latter case, we get a stable mixture of positive sols changed simultaneously to a stable mixture of negative sols by the addi- tion of hydroxy] ions. Brossa and Freundlich? studied the precipitation and repeptiza- tion of colloidal albumin by means of colloidal ferric oxide in the presence of electrolytes. The amount of albumin thrown 1Z. physik. Chem., 109, 391 (1924). 2Z. physik. Chem., 51, 148 (1905). 3 Z. phystk. Chem., 89, 306 (1915). THE HYDROUS OXIDES OF IRON 65 down by the ferric oxide sol decreases with decreasing concen- tration of electrolytes until eventually only a slight turbidity results, which disappears on adding a sufficient amount of ferric | oxide sol. Obviously, the colloidal ferric oxide adsorbs, and so keeps the colloidal albumin in solution. The ferric oxide-albu- min sol formed in this way is positively charged but is much more sensitive than the original sol. The sensitivity is at its maximum when the ferric oxide has adsorbed all the negative albumin sol that it can hold, without precipitation taking place. With increasing concentrations of ferric oxide sol, the sensitivity falls off, approaching that of the pure positive sol. If instead of adding an electrolyte to a ferric oxide-albumin sol, an albumin sol containing an electrolyte is precipitated with ferric oxide sol, the relationships are identical in many respects, particularly in the amount of albumin adsorbed by the ferric oxide. The presence of non-electrolytes such as urethane, camphor, and thymol have likewise been shown to increase the sensitivity of ferric oxide sol toward electrolytes.!. Freundlich attributes this to a lowering of the surface charge on the particles as a result of adsorption of a substance having a lower dielectric constant than water; but Michaelis? failed to detect any adsorption of non- electrolytes by hydrous ferric oxide or any effect of their presence on the adsorption of electrolytes. This failure to confirm Freund- lich’s hypothesis may be due to the limitations of the experimen- tal method in the systems investigated. The author* has observed a marked antagonistic action of phenol and isoamyl alcohol on the adsorption of barium ion by colloidal arsenious sulfide. Ferric Oxide Jellies —Although hydrous ferric oxide is usually thrown down as a gelatinous precipitate, jellies may be pre- pared by coagulation of a sol under suitable conditions. Thus, Grimaux‘ added to an excess of water an alcoholic solution of ferric ethylate which hydrolyzed very rapidly, forming a col- loidal ferric oxide. The sol was similar to Graham’s, but the 1 FREUNDLICH and Rona: Biochem. Z., 81, 87 (1915); cf. Matsuno: Biochem. Z., 150, 139 (1924). 2 MIcHAELIS and Rona: Biochem. Z., 102, 268 (1920). 8 WeIsER: J. Phys. Chem., 28, 1253 (1924). 4 Compt. rend., 98, 105, 1434 (1884). 66 THE HYDROUS OXIDES particles were probably much smaller on account of the more rapid rate of hydrolysis. The sol coagulated spontaneously on standing for some time at room temperature; and more rapidly on heating or by the addition of electrolytes such as carbonice, - sulfuric, and tartaric acids; the nitrate, chloride, and bromide of potassium; the chlorides of sodium and barium, etc. The coagulum formed in every case was a transparent jelly, provided the sol was not agitated during the process of coagulation. Even with quite dilute sols, the jelly was firm; but contraction took place, very slowly in the cold and more rapidly at high temperature. With colloidal ferric oxide as with a number of other sols, slow uniform precipitation throughout the entire solution pro- duces a jelly, while rapid uneven precipitation results in contrac- tion and the consequent formation of a gelatinous precipitate. As compared with the usual Graham sol, Grimaux’s colloid is much more easily thrown down in the form of a jelly. This is accounted for by the fact that a sol formed by rapid hydrolysis in the cold will contain finer and more hydrous particles than one formed by prolonged dialysis in the cold or shorter dialysis in the hot. For the same reason, the coagulum from the Graham sol is much more hydrous and bulky than that obtained from a Péan de St. Gilles sol. The usual Graham sol can be precipi- tated as a jelly, provided the concentration is sufficiently high. Schalek and Szegvary? added electrolytes in amounts below their precipitation values to colloidal solutions containing 6 to 10 per cent of ferric oxide and allowed the sols to stand quietly. After a time, the mixture set to a jelly that was no more cloudy than the original sol. This jelly solidified slowly after shaking up. The logarithm of the time required for solidification after shaking was found to be inversely proportional to the tempera- ture and to the concentration of coagulating electrolyte. Ultra- microscopic observation of the liquefaction process showed no change in the average distance between the particles and no formation of secondary particles. I am inclined to attribute the reversible sol-gel transformation in such a system to the breaking 1 Cf. WaaneER: Kolloid-Z., 14, 150 (1914). 2 Kolloid-Z., 32, 318 (1923); 33, 326 (1923); FREUNDLICH and ROSENTHAL: Ibid., 37, 129 (1925). THE HYDROUS OXIDES OF IRON 67 up and subsequent realignment of the orienting forces among the particles. Ferric oxide jellies may be prepared also, by slow removal of the peptizing agent by dialysis. Thus Grimaux! obtained a firm jelly by dialysis of a negative sol prepared by peptization of the hydrous oxide with alkali in the presence of glycerin. If ammonia were used instead of alkali, and the sol exposed to the air, the slow loss of peptizing agent by evaporation resulted in the precip- itation of a jelly. Grimaux’s observations were confirmed by Fischer,? who prepared a firm jelly on prolonged dialysis of a sol containing but 1 per cent of iron. Unlike the more concentrated jellies of Schalek and Szegvary, this preparation broke down into a gelatinous precipitate when it was warmed, stirred, or frozen, Browne® obtained a jelly simply by allowing part of the water to evaporate Bey from a concentrated Graham sol of high purity. ADSORPTION BY HYDROUS FERRIC OXIDE Hydrous ferrric oxide as a technical adsorbent finds its most important use as a mordant in the dye industry and in the purification of municipal water supplies. These applications are considered in Chaps. XVI and XVII, respectively. Adsorption of Arsenious Acid.—Ninety years ago Bunsen‘ made the important discovery that freshly precipitated hydrous ferric oxide is an antidote for arsenic poisoning. As might be expected, this action was attributed by Bunsen to stoichiometric chemical union of ferric oxide and arsenious acid. While some people’ still maintain that iron arsenites of varying degrees of complexity® are formed when hydrous ferric oxide and arsenious acid are brought together under varying conditions, the investiga- tions of Biltz’ show the apparent interaction to be an adsorption 1Compt. rend., 98, 1485 (1884). 2 Biochem. Z., 27, 223 (1910). 3 Private communication. 4BunseEN and Berruorp: ‘‘Hydrated Ferric Oxide, an Antidote for Arsenious Acid,’’ G6ttingen (1834); cf. GurmpourtT: Arch. Pharm., (2) 28, 69 (1840). 5 REYCHLER: J. chim. phys., 7, 362 (1909); 8, 10 (1910). 6 Oryna: Kolloid-Z., 22, 149 (1918). 7 Ber., 37, 3138 (1904); cf. Kolloid-Z., 26, 179 (1920). 63 THE HYDROUS OXIDES process in which the arsenic content of the hydrous oxide varies continuously with the concentration of arsenious acid in contact with it, giving a typical adsorption isotherm without a break or an evidence of discontinuity. Lockemann and Paucke! made a quantitative study of the adsorption of arsenious acid by charcoal, aluminum oxide, ferric oxide, and albumin. With ferric oxide, they find most complete adsorption when the iron is precipitated with stoichiometric quantities of ammonia; excess of ammonia or precipitation by potassium or sodium hydroxide decreases the adsorbability. This accords with Bradfield’s? observation that the most finely divided and most readily peptized particles of hydrous ferric oxide are formed by precipitation with but a very slight excess of ammonia. ‘The amount of hydrous oxide necessary to adsorb a given amount of arsenic can be calculated by means of the formula H = BAp where HE = milligrams ferric oxide, A = milligrams arsenic, and 6 and p are constants which vary with the temperature;’ but it should be pointed out that this equation — serves only as a simple approximation to the course of the adsorption.‘ Fischer and Juznitzky® injected colloidal ferric oxide simultane- ously with arsenious acid, under the skins of mice, and obtained partial protection from a fatal dose of arsenic. The negative colloidal hydrous oxide formed by peptizing ferric oxide with dilute alkali and glycerin was more effective than a positive Graham sol. Since it was thought improbable that a negative colloid would adsorb a negative ion, Fischer advanced the more improbable hypothesis that the antidotal effect was due to the formation of an iron-arsenic complex of some sort. These observations should be confirmed and a plausible explanation formulated. Catalytic Action.—Slightly hydrous or anhydous ferric oxide seems to have a relatively high adsorption capacity for gases 1 Kolloid-Z., 8, 273 (1911). 2 J. Am. Chem. Soc., 44, 965 (1922). 3 Cf. LockKEMANN and Lucius: Z. physik. Chem., 83, 735 (1913). 4 BoswELL and Dickson: J. Am. Chem. Soc., 40, 1793 (1918); cf. Mpcx- LENBURG: Z. physik. Chem., 83, 609 (1918). 5 Biochem. Z., 27, 311 (1911). THE HYDROUS OXIDES OF IRON 69 even at elevated temperatures since it is used as a catalyst in such industrial operations as the burning of hydrogen sulfide in the Chance-Claus process for recovering sulfur from alkali waste ;! in the Hargreaves and Robinson process for making salt cake;? and in the manufacture of sulfuric acid by the contact process.* In the latter process the conversion of sulfur dioxide to trioxide is 98 per cent using platinum as catalyst at 425°, dropping to 91 per cent at 500° owing to dissociation of: the trioxide. The velocity with which sulfur dioxide and oxygen combine is less in the presence of ferric oxide so that it is necessary to work at a higher temperature when this catalyst is employed. On this account, the efficiency does not rise much over 60 per cent. It ought to be possible to make a ferric oxide catalyst that would work at as low a temperature as platinum if adsorption were the sole criterion of catalytic efficiency. Unfortunately this does not appear to be the case, as evidenced by such cases as charcoal which has a high adsorptive capacity but relatively poor catalytic properties. FREUNDLICH: Z. physik. Chem., 44, 143 (1908). ’ H6BER and Gorpon: Beitr. chem. Physiol. Path., 5, 436 (1904). 4 Weiser: J. Phys. Chem., 25, 413 (1921); 30, 20 (1926). 6 Dammer: “ Handbuch anorg. Chem.,” 8, 304 (1893). THE HYDROUS OXIDES OF IRON FA pigment was the magnetic oxide of iron commonly called magnetite.! The variations in color of the anhydrous oxide appear to be due to the size of the particles. Thus Andersen? found plates of hematite as thin as 0.1u to be yellow by transmitted light, the color varying with increasing thickness through reddish brown to deep brown red or blood red; similarly, Wohler and Condrea? prepared anhydrous oxides that vary in color from yellow to red by simply varying the size of the particles, the red being the largest. Keane‘ attributes the yellow color of the so-called Mars pigments to finely divided ferric oxide which is kept from agglom- erating by the presence of aluminum oxide; and the yellow color which iron imparts to bricks, to sufficiently finely divided anhy- drous ferric oxide; when the particles are too large, the color is red rather than yellow.> Mott® obtained anhydrous red and yellow ferric oxide by volatilization in the electric arc; the yellow particles were the smaller. The hydrous oxide can be prepared in a variety of colors so similar to those of the anhydrous oxide that it seems reasonable to attribute the difference in color to the same cause—a differ- ence in the size of the hydrous particles. As a matter of fact, the variation in color from brown through yellow to red was shown by Malfitano and by Fischer to be associated with an increase in the size of the particles although they did not recog- nize the possible connection between the two. That there is a definite connection between particle size and color was shown by a series of experiments’ on the hydrolysis of ferric chloride solu- tions. The very finely divided brown particles may be trans- formed either into the larger yellow or the still larger brick red, by heating under suitable conditions. Since a very dilute solution of ferric chloride is colorless at the outset, changing spontaneously to yellow and then to reddish 1 GERMANN: Science, 30, 20 (1926). 2 Am. J. Sci., (4) 40, 370 (1913). 3 Z. angew. Chem., 21, 481 (1908). 4 J. Phys. Chem., 20, 734 (1916). ‘Cf. ScurEtz: J. Phys. Chem., 21, 576 (1917); Yor: Ibid., 25, 196 (1921). 6 Trans. Am. Electrochem. Soc., 34, 292 (1918). 7 J. Phys. Chem., 28, 313 (1920), 72 THE HYDROUS OXIDES brown,! it would appear that yellow hydrous particles are smaller than brown. This conclusion is unwarranted, since the color of a dilute colloidal solution is not necessarily determined by the color of the particles. Thus colloidal solutions of gold have been obtained which are red, violet, or blue by transmitted light ;? but this does not tell us the color of light reflected from the particles in the respective sols. Asa matter of fact, massive gold reflects yellow when compact and brown to black when porous. Small particles of gold which do not resonate are yellow to brown by reflected light and transmit blue. The surface color of gold is red by multiple reflection and very thin films are green by transmitted light.? Colloidal solutions with very fine particles of gold reflect green and transmit red. Hence, we conclude that the particles in the blue sol are yellow to brown, and in the red sol they are green.4 A deep-red Graham colloid from which can be thrown down a red-brown gelatinous precipitate appears distinctly yellow when diluted sufficiently. A 5-year-old brick- red Péan de St. Gilles sol appears yellower on dilution, although the reddish color persists. It is possible that the reddish-brown particles in a red Graham colloid transmit more yellow than red when sufficiently highly dispersed. At any rate, there seems no reason for believing the yellow colloid formed by hydrolysis of. ferric chloride to be other than a highly diluted Graham sol. The color of such a solution becomes redder with age, owing to the formation of more red-brown colloidal hydrous oxide. A thousandth normal solution which Goodwin found to be com- pletely hydrolyzed in a few hours, is very much redder than a fiftieth normal or hundredth normal solution after 24 hours. It appears that a colloidal solution of hydrous ferric oxide contains varying amounts of small highly hydrous red-brown particles and larger less hydrous yellowish-brown particles, both of which may be converted into still larger and less hydrous brick-red particles by heating at 100°. If the conditions are such that the red particles remain in colloidal solution, we have the 1AnTONY and Giauio: Gazz. chim. ital., 25, 1 (1895); Goopwin: Z. physik. Chem., 21, 1 (1896); cf. Waaner: Kolloid-Z., 14, 150 (1914). 2? FaraDay: Phil. Trans., 147, 145 (1857). 3’ BemtBy: Proc. Roy. Soc., 72, 226 (1913). 4Cf. Bancrort; ‘Applied Colloid Chemistry,’ 204 (1921). THE HYDROUS OXIDES OF IRON 73 brick-red Péan de St. Gilles colloid. Bradfield! demonstrated conclusively that the reddish-brown precipitate formed by adding ammonia to ferric chloride solution until minute floc- cules are barely visible, contains both very small highly hydrous dark-brown particles and larger less hydrous yellowish-brown particles which can be separated rather sharply from each other by centrifuging the suspended precipitate. Both the reddish and yellowish particles in a sol formed by heating a 1 per cent solution of ferric chloride from room temperature to the boiling point appear to be transformed to larger less hydrous bright- red particles by heating at 100°. The granular ocher-yellow particles formed by heating a more concentrated solution slowly are not converted into the red at this temperature. This differ- ence might be ascribed to the dense granular character of the particles which precipitate on heating the more concentrated solutions; but it will be recalled that a yellow Péan de St. Gilles colloid formed by slow hydrolysis of ferric acetate is not changed to red by prolonged boiling of the sol. The yellow particles formed under certain conditions lose water much less readily at 100° than the reddish brown; and this seems to account for the difference in behavior. As previously pointed out, Keane and Scheetz have shown the yellow color of bricks to be due to finely divided anhydrous ferric oxide which is kept from agglomerating by alumina and probably by certain other substances as well. This requires a rather high percentage of alumina. In the so-called Mars pigments which are yellow, the ferric oxide is in the hydrous state; and in this condition it agglomerates less readily to the red oxide, and less alumina is required to prevent the transformation. Since the yellow oxide retains its water more tenaciously than the brown, it is natural to inquire into the cause of the increased stability. In view of the synthesis of a yellow monohydrate of ferric oxide by the slow hydrolysis of ferric sul- fate,” it would appear reasonable to conclude that the yellow oxide which does not lose water and become red at 100° is ferric oxide monohydrate. The yellow oxide that apparently loses water and agglomerates to red at 100° may be regarded as hydrous ferric oxide in which the particles are somewhat ee and less hydrous 1 J. Am. Chem. Soc., 44, 965 (1922). 2 PosNJAK and Nearer: J. Am. Chem. Soc., 44, 1965 (1922). 74 THE HYDROUS OXIDES than the brown. But as I am aware of no case in which yellow hydrous particles free from brown appear to be transformed into red by heating at 100°, it is open to anyone to assume that the yellow particles are really never transformed into red; but that the bright-red color formed by agglomeration of the brown oxide masks the yellow monohydrate. If one objects to the assumption that the yellow colloid is a hydrous monohydrate, another alternative is to attribute its stability at 100° to adsorption of some salt. Bancroft! suggested that the yellow color of the oxide is due to the presence of adsorbed ferric salt. This suggestion was based on Fischer’s observation that the brown colloid goes over into red in the pres- ence of hydrochloric acid; on Malfitano’s experiment, that the brown colloid is transformed into the yellow by boiling with ferric chloride; and on Phillips’ method of preparing the yellow oxide by oxidation of ferrous carbonate. Malfitano’s observa- tion is inconclusive, since boiling a ferric chloride solution alone will give a yellow colloid. Moreover, the author precipitated the hydrous oxide in a gelatinous form in the presence of a large excess of ferric chloride, a condition favorable to adsorption of ferric salt; and yet the oxide was distinctly red. Hence, there seems no reason for attributing the color of the yellow colloid to adsorbed iron salt. Bancroft’s hypothesis was the outgrowth of the observation that the yellow colloid is formed when the adsorption of an iron salt is a possibility. The converse appears not to be the case, namely, that the adsorption of an iron salt always results in the formation of a yellow hydrous oxide. Although the adsorption of an iron salt does not impart a yellow color to a hydrous ferric oxide, it is possible that the yellow oxide which is not converted to red by heating at 100° is stabilized by adsorbed iron salt. LOWER OXIDES OF IRON Hydrous Ferrous Oxide.—On account of its relatively low solubility, hydrous ferrous oxide comes down in a highly gela- tinous form when a solution of ferrous salt is treated with potas- sium or sodium hydroxide.? The gel is white when absolutely 1 J. Phys. Chem., 19, 282 (1916). 2Scumipt: Liebig’s Ann. Chem., 36, 101 (1840). THE HYDROUS OXIDES OF IRON 75 pure; but owing to the difficulty in excluding all air during pre- cipitating and washing, it is usually obtained as a green hydrous mass. Even when dried, the gel oxidizes so readily in the air that the whole mass sometimes becomes incandescent. As ordinarily prepared, the gel is hydrous FeO; but de Schulten! obtained the monohydrate or hydroxide by crystallization from solution in strong caustic soda. The crystals were small green prisms which oxidized very rapidly in the air even after they were washed with alcohol and ether, and dried in hydrogen. Owing to its strong affinity for oxygen, the oxide is a powerful reducing agent, converting nitrites and nitrates to ammonia, a reaction that may be used for the quantitative estimation of the substances.’ Whitman, Russell, and Davis? find that the rate of corrosion of iron in salt solutions parallels the solubility of ferrous hydroxide in these solutions. It is suggested that this is due to changes in film protectivity with the solubility of the ferrous salt. Hydrous Ferro-ferric Oxide.—The gel of ferro-ferric oxide is obtained by adding alkali to a solution containing equivalent amounts of ferrous and ferric salts. If washed and dried out of contact with air, it is a magnetic brownish-black mass containing an indefinite amount of water.‘ 1 Compt. rend., 109, 266 (1889). 2 MIYAMOTO: v2 One Soc. Japan, 48, 397 (1922). 3 J. Am. Chem. Soc., 47, 70 (1925); cf. Frrmnp: J. Chem. Soc., 119, 932 (1921). 4 WountER: Liebig’s Ann. Chem., 22, 56 (1838); Lerort: Compt. rend., 69, 179 (1869). CHAPTER III HYDROUS CHROMIC OXIDE Composition.—The addition of ammonia or an alkali to a solution of chromic salt precipitates chromic oxide as a highly hydrous gel, the composition and properties of which depend on the conditions of precipitation and the subsequent treatment. The gel is frequently designated chromic hydroxide and assigned the formula Cr (OH); or Cr.03:3H.O, although 35 years ago van Bemmelen! determined the isotherm for chromic oxide and water between 15 and 280° and found no evidence of any definite hydrate. As van Bemmelen’s observations have been confirmed by von Baikow,? it is altogether likely that the various so-called hydrates described from time to time* were merely hydrous chromic oxides dried to a composition expressible by a Dalton formula. Férée* claims to have obtained the compound Cr.0;: HO by electrolysis of a neutral solution of chromium chloride with a platinum cathode. The brownish-black amorphous powder loses water on heating to 80°; but it is questionable whether this is a definite inversion temperature at which all the water is lost. It is also claimed by some that a green hydrate, Guignet’s green,’ is formed by fusing 1 part of bichromate of sodium, potas- sium, or ammonium with 3 parts of boric acid; but there is a 1 Rec. trav. chim., 7, 37 (1888). 2 J. Russ. Phys.-Chem. Soc., 39, 660 (1907). 3 ScHAFFNER: Liebig’s Ann. Chem., 61, 169 (1844); Stmwert: Jahresber., 242 (1861); Lomweu: J. Pharm., (8) 7, 328, 401, 424 (1845); Fremy: Compt. rend., 27, 269; 30, 415 (1847); 47, 883 (1858); LErortT: J. Pharm., (3) 18, 27 (1850); Vincent: Phil. Mag., (4) 18, 191 (1850). 4 Bull. soc. chim., (3) 25, 620 (1901); cf. Bunsmn: Pogg. Ann., 91, 619 (1854); GeuruEeR: Liebig’s Ann. Chem., 118, 66 (1861). 5 GUIGNET; Jahresber., 761 (1859). 76 HYDROUS CHROMIC OXIDE 77 difference of opinion as to the formula.! Wohler and Becker? obtained a similar green pigment by heating the ordinary oxide in an autoclave at 180 to 250°. It retains its color when dried at 80° but darkens gradually and loses water above this temperature. The oxide was taken to be a definite hydrate, since its composition on drying at 80° may be represented by the formula 2Cr,0;-38H,O. The green pigment prepared in any way is amorphous in character and, like the ordinary pre- cipitated oxide, loses water continuously as the temperature is raised. Of course, it is entirely possible to dry the pigment under such conditions that the percentage composition may be expressed by a simple formula, but that does not prove that a true hydrate is formed. Ageing.—Hydrous chromic oxide, freshly precipitated from a cold chromic salt solution with an alkali or ammonia, is readily soluble in acids giving the corresponding salts and is peptized by alkali hydroxides with the formation of a colloidal solution. On standing, the oxide undergoes a change in physical character accompanied by a marked decrease in solubility and reactivity. This process called ‘‘ageing” is probably due to the growth and agglomeration of primary colloidal particles, since the velocity of change increases rapidly with rising temperature and is hastened in a medium possessing a slight solvent action. Recoura? followed the change by determining the molar heat of solution in hydrochloric acid of the oxide precipitated with acid from the colloidal solution in alkali, after definite intervals of time. From his results given in Table III, it will be noted that the change in the heat of solution is quite marked during the first few minutes. This change is accompanied by a similar decrease in solubility. Since the ageing is more rapid at higher temperatures, the oxide precipitated at 100° is much less.soluble than that thrown down at room temperature. 1S8atveTar: Compt. rend., 48, 295 (1859); ScunuRER-Kestner: Dinglers polytech. J., 176, 386 (1865); EnneR and Hus: Farbezig., 15, 2106, 2157, 2213, 2268, 2319 (1910). 2Z. angew. Chem., 21, 1600 (1908); 24, 484 (1911). 3 Compt. rend., 120, 1335 (1895); cf. FricKkm and WINDHAUSEN: Z. physik. Chem., 118, 248 (1924); Z. anorg. Chem., 1382, 273 (1924), 78 THE HYDROUS OXIDES TaBLE II].—Mouar Heat or SoLutTion or Hyprous CHROMIC OXIDES PRECIPITATED FROM SOLUTION IN ALKALI : Molar heat of : Molar heat of Time i 5 Time ‘pee : solution, calories solution, calories 0 20.70 7 hours 2.40 10 minutes 7.90 1 day 1,75 1 hour 5.80 7 days . 1.20 2 hours 3.90 30 days 0:75 4 hours 2.85 60 days 0.50 Solutions of hydrous chromic oxide in alkali were found by Bourion and Senechal! to lose their reducing power toward hydrogen peroxide on standing. The reaction (loss of reducing power) with a solution containing 0.938 gram chromic oxide and 58 grams sodium hydroxide per liter appeared to be approximately tetramolecular for the first 8 hours. The results were attributed to the transformation of the original oxide into complexes of decreasing chemical activity, the tetramolecular order being only apparent. Bourion and Senechal evidently believe that hydrous chromic oxide dissolves in alkali with the formation of chromite; but in reality it is held in colloidal solution, for the most part. The decreased activity on standing is due to a gradual change in the physical character of the particles, a change that is sufficiently marked with a concentrated sol to cause partial precipitation in a short time. This transformation from a very soluble to a less soluble and less reactive form of hydrous chromic oxide has very naturally been attributed to the existence of definite allotropic or isomeric modifications. This is very unlikely, particularly since there is no inversion point for a soluble and an insoluble modification. Between these two extremes of solubility, it is possible to prepare an indefinite number of hydrous oxides, each differing slightly from the others in water content, in size of particles, in structure of the mass, and consequently, in reactivity with acids and alkalies.” The Glow Phenomenon.—When hydrous chromic oxide is heated at a suitable rate to temperatures around 500°, it evolves 1 Compt. rend., 168, 59, 89 (1919). 2 FRICKE and WINDHAUSEN: Z. anorg. Chem., 132, 273 (1924). HYDROUS CHROMIC OXIDE 79 enough heat to cause it to become incandescent. The tempera- ture at which the glowing takes place varies with the sample and with the method of heating. Berzelius, Wohler,! and Endell and Rieke? give approximately 500° for the glow temperature; Le Chatelier* gives 900°, and Rothaug? finds it to vary between 420 and 680°, depending on whether the precipitate is in a pow- dery or granular form. The glow is regarded by some® as an accompaniment of the transformation of one allotropic modifica- tion of the oxide to another; but this seems unlikely, since the glowing depends on the rate of heating® and since the glow temperature varies with the size of the particles. Moreover, the phenomenon is observed with a number’ of hydrous oxides as well as other substances; and it is improbable that all of them should exist in two forms. Wohler found that the glowing is increased by all conditions which favor hydrosol formation in the preparation of the oxide, for example, the use of dilute solu- tions of reagents, the use of chloride rather than sulfate, and of potassium hydroxide rather than ammonium hydroxide. More- over, the glow was found to be greater, the greater the adsorption capacity of the precipitate, indicating that the phenomenon is connected closely with the surface area. Under the same conditions of heating, the heat evolved by 1 gram of oxide was sufficient to raise its temperature anywhere from 50 to 100°, depending altogether on the extent of surface. In the light of Wohler’s observations, there is little doubt but that the glow is due to a very sudden decrease in the large sur- face of the oxides prepared by precipitation. The oxides thrown down under different conditions vary in the size of the particles and the amount of enclosed water and hence in the extent of surface. The maximum glow and heat evolution are obtained when the sample, made up of finest particles, is heated rapidly 1 Kolloid-Z., 11, 241 (1913). 2 Zentr. Min. Geol., 246 (1914). 3 Bull. soc. chim., (2) 47, 303 (1887). 4Z. anorg. Chem., 84, 165 (1913). 5 Moissan: Bull. soc. chim., (2) 34, 70 (1880); Ann. chim. phys., (5) 21, 199 (1880); Le CuatenipR: Bull. soc. chim., (2) 47, 303 (1887); Mrxmr: Am. J. Sci., (4) 26, 125 (1908); 39, 295 (1915). 6 Stmwert: Jahresber., 243 (1861); cf. Mixer: Loc. cit. 7 Wouter: Kolloid-Z., 11, 241 (1913); Enpeui and Rieke: Loc. cit. 80 THE HYDROUS OXIDES to the glow temperature, which is in the neighborhood of 500.° If a fine-grained precipitate is heated very slowly or kept for some time below the glow temperature, there is a gradual, instead of a sudden, diminution of surface, which is not accom- panied by incandescence. Thus, glowing at elevated tempera- tures is the visible manifestation of the coalescence of primary colloidal particles into larger masses, involving a marked decrease in specific surface. Similarly, at ordinary temperature the gradual change in solubility, in reactivity, and in molal heat of solution in hydrochloric acid is due to coalescence of the small primary particles into larger primary particles with the concomi- tant diminution in specific surface. This change is a truly irre- versible process differing from ordinary coagulation in which the primary particles merely form secondary aggregates with very little change in specific surface. Color.—Hydrous chromic oxide can be obtained in various shades from a clear gray blue to a dark green. Certain of these colors, such as chrome green and Guignet’s green, constitute the most permanent green pigments. The color of the oxide freshly precipitated in the cold is variously described by different people as bluish, violet blue, clear blue, clear gray blue, and gray violet. The shade differs somewhat, depending on whether it is precip- itated from a green or violet chromic salt. On drying the precipitate, the color changes to a distinct green, and the dry amorphous oxide is described as vivid green. Mention has been made of the transformation of the ordinary precipitated oxide into Guignet’s green by ageing in an autoclave at 180 to 250°. The rate of precipitation seems to have a marked effect on the color. Thus, Casthelez and Leune? claim to have pre- pared an oxide with a richer and purer color than Guignet’s green, simply by slow precipitation at ordinary temperatures of a green solution of a chromic salt with aluminum hydroxide, zine carbonate, zinc sulfide, or zinc. This observation was confirmed? by adding mossy zine to a solution of green chromic chloride and allowing to stand at 25° for several days. ‘The clear dark-green oxide which formed was much more granular than the gray-blue 1 See p. 57. 2 Bull. soc. chim., (2) 10, 170 (1868). 3 WEISER: J. Phys. Chem., 26, 410 (1922). HYDROUS CHROMIC OXIDE 81 gelatinous oxide obtained by rapid precipitation; moreover, it was quite insoluble in normal sulfuric acid. Berzelius! believed the oxides precipitated from violet and green solutions to be isomers, since they redissolve in acids giving solutions with the original colors. This, however, seems to depend altogether on the method of procedure. Thus Recoura? added alkali to a green solution until a precipitate was formed which was dissolved at once in hydrochloric acid giving a violet solution; while the hydrous oxide precipitated from what Recoura claimed to be Cr2.OCl, gave a green solution. It would appear, therefore, that the hydrous oxides from different-colored solutions are the same in chemical structure, the individual variation in color and solubility arising from the difference in the physical character of the hydrous particles’ and the structure of the mass. The wide difference in color between the gray-blue precipitated oxide and Guignet’s green causes Wohler and Becker to regard the two substances as hydrate isomers bearing a relation to each other similar to the relationship between blue and green chromic chloride. In support of this view, they show that two prepara- tions with the same water content have a different vapor pressure; and that the ordinary oxide can be convered into Guignet’s green by heating in an autoclave. These evidences are alto- gether inconclusive. In the first place, the vapor pressure of a hydrous oxide is determined not only by the amount of water it contains but by its structure;* and since the conditions of forming Guignet’s green and the ordinary oxide are so different, it is not surprising to find variation in the size of the particles and the structure of the masses of each, as is evidenced not only by difference in vapor pressure but by difference in color. In the second place, Wohler and Becker were unable to find an inversion temperature of gray-blue oxide to Guignet’s green, and the fol- lowing experiments® indicate that a definite transition point does not exist: 20-cubic-centimeter portions of a solution con- 1 “Lehrbuch,” 5th ed., 2, 315 (1848). 2 Compt. rend., 104, 1227 (1887); Ann. chim. phys., (6) 10, 1 (1887); cf. Oute: Z. anorg. Chem., 52, 48 (1907). 3 Cf., however, Recoura: Loc. cit.; LonwEeu: J. Pharm., (3) 7, 323, 401, 424 (1845); Fremy: Ann. chim. phys., (3) 28, 388 (1848). 4 Van BEMMELEN: ‘Die Absorption,” 239 et seq. (1910). 6 Weiser: J. Phys. Chem., 26, 409 (1922). 82 THE HYDROUS OXIDES taining 0.2 gram of chromium chloride were treated with just enough sodium hydroxide solution to cause complete precipita- tion at the various temperatures shown in Table IV; and the precipitates were kept at this temperature for a definite length of time. For temperatures above 100° the precipitations were carried out in an autoclave. The color varies continuously from TaBLE IV.—EFrrect oF TEMPERATURE OF PRECIPITATION ON THE COLOR oF Hyprovus CHRomMIC OxIDE EDL oes Time of heating Color of precipitate degrees 0 30 minutes Gray blue 50 30 minutes Greenish blue 100 30 minutes Bluish green 150 30 minutes Green with faint tinge of blue 200 30 minutes Clear green 200-225 15 hours Bright green gray glue to clear green with increasing temperature of precipita- tion. This indicates that the various colors are not due to iso- mers but to a difference in the size of the particles, the structure of the mass, and the amount of water enclosed under the different conditions of formation. As the color changes from blue to clear green with increasing temperature of precipitation, the oxide becomes less gelatinous, less soluble in acids, and less readily peptized by alkalies. CHROMIC OXIDE SOLS The Positive Sol Formed by Peptization Methods.—Graham! prepared colloidal hydrous chromic oxide by peptizing the freshly precipitated oxide with chromic chloride and dialyzing to remove excess electrolyte. The colloidal solution is dark green, and can be diluted with water or heated; but is very instable in the presence of salts. Neidle and Barab? investigated the dialysis of a colloidal solution prepared by the Graham method. The sol was placed 21 Phil. Trans., 151, (1), 183 (1861). J. Am. Chem. Soc., 38, 1961 (1916). — HYDROUS CHROMIC OXIDE 83 in a parchment membrane surrounded by water. In one series of experiments the water was changed at intervals; while in a second series, a continuous flow of water through the dialyzer was maintained. Colloidal particles diffused through the mem- brane in both cases. In the intermittent dialysis, the sol continued to diffuse until but little remained within the mem- brane; whereas in the continuous process, the passage of the sol ceased after a time, and 75 per cent remained within the mem- brane. The growth of the colloidal particles during dialysis was influenced by two factors: agglomeration following removal of peptizing agent, and growth of nuclei by hydrolysis of adsorbed chloride by adsorbed water. In the intermittent process, the removal of peptizing agent was not rapid enough to cause sufh- cient agglomeration to prevent the passage of the colloid through the particular membrane; while in the continuous process, a gradual growth of the particles resulted finally in their retention by the membrane. By continuous dialysis at a high tempera- ture,! the time required to get a colloidal solution containing a minimum amount of peptizing agent may be shortened by weeks. Bjerrum? obtained small amounts of basic chlorides having the formulas Cr(OH)Cl, and Cr(OH)2Cl on adding alkali to chro- mic chloride,* and Recoura‘ claimed to get CreOCl, by the oxida- tion of CrCl, in the air;> but it is unlikely that any quantity of basic salt is present in the well-dialyzed solution of hydrous chromic oxide in chromic chloride. Neidle and Barab dialyzed such a colloidal solution in the hot until the ratio, equivalents Cr: equivalents Cl, was above 1500. It seems absurd to regard such a solution as a basic salt; on the other hand, it does not pre- clude the possible presence of a trace of basic salt in a highly purified sol. For the most part, however, the sol consists of hydrous chromic oxide peptized by preferential ‘adsorption of chromium and hydrogen ions. 1 J. Am. Chem. Soc., 39, 71 (1917). 2Z. physik. Chem., 73, 724 (1910); cf. also DenHam: J. Chem. Soc., 93, 41 (1908). 3 Cf. Fiscnoer: Z. anorg. Chem., 40, 39 (1904). 4 Ann. chim. phys., (6) 10, 1 (1887). 5 See also Moserea: J. prakt. Chem., 29, 175 (1843); Lonwe.u: J. Pharm., 4, 424 (1843); P&iicot: Compt. rend., 21, 24 (1845); Orpway: Am. J. Sci., (2) 26, 202 (1858); Ouie: Z. anorg. Chem., 52, 62 (1906). | 84 THE HYDROUS OXIDES If it were possible to dialyze the sol until all the chromic chloride were hydrolyzed and practically all of the hydrogen were adsorbed either as hydrogen chloride or as hydrogen ion, the composition of the. sol might be represented by the general formula [(jaCr2O3-yHCl-2H.0) H’),-(n — q)CV] + gCl’ where qg represents the excess of adsorbed hydrogen ions over adsorbed chloride ion, that is, the charge on the colloidal particles. Actually, the solution as well as the sol particles will contain hydrogen ions and may contain chromium ions; and the particles may contain adsorbed chromium. If hydrochloric acid is placed on one side and a well-dialyzed sol on the other side of a mem- brane permeable to hydrogen and chloride ions but not to the colloidal particles holding an excess of adsorbed hydrogen ion, a Donnan equilibrium will be set up with the attending con- centration, osmotic, and electrical effects. Bjerrum? placed a chromic oxide sol in a collodion bag and surrounded it by solutions of hydrochloric acid of varying concentration. The outside solu- tion was renewed daily until equilibrium was established, and the osmotic pressure and membrane potential were measured in a special apparatus: Some observations are recorded in Table V. The concentration cz of HCl in the outer solution and the con- centration [Cr.03] of the sol are expressed in mols per liter. The TaBLeE V.—Osmortic PRESSURE AND MEMBRANE POTENTIAL OF A CHROMIC Ox1pE Sou aT 18°. Tue ‘ EQuivALENT AGGREGATE” OF THE SOL HCI (ee ee | m = 1000 | m= 900 | m = 250 x [Cr203] Pi Ey. |= oie TT Sa en Ae | VEN | Ae Ver | Ae Py; Ae 0.010 0.042 THe 5 3 ne 0) is eal: 14 4.1 18 0.010 0.038 6.0 6.4 12 0.9 4853 1.9 15 oe re 20 0.005 0.038 9.7 10.5 14 0.9 14 1.9 15 3 yall ilg 0.005 OF027 bape Olle ee 14 0.6 14 1.4 16 Oe Fi 20 0.005 0.027 4.8 9.2 14 0.6 14 1.4 16 2.6 20 0.005 0.026 4.4 9.1 14 0.6 15 1.3 16 2.5 21 0.0025 0.026 iO 16.1 14 0.6 15 i123 16 DAES 18 0.001 - 0.026. 17.8 28.2 14 0.6 14 L38 14 ANE 15 0.005 0.025 4,2 q04 14 0.6 14 133 16 2.4 20 0.010 0.025 20) 5.2 12 0.6 14 Les 18 2.4 100 Of apr 2Z. physik. Chem., 110, 656 (1924). HY DROUS CHROMIC OXIDE 85 osmotic pressure P; is given in centimeters of water and the membrane potential H,, in millivolts. The measured osmotic pressured P is the sum of the pressure P;, of the colloidal particles and the pressure P2, caused by the difference in the number of dialyzed particles within and without the membrane, that is, P=P,+P, (1) According to Avogadro P, = rr. (C203) (2) m and according to Avogadro and Donnan, — pr. [Gr20s}? Reaalt lh sera as, (3) where RT is 24,700 at 18°; m is the number of Cr.03 molecules in a colloidal particle, and Ae the equivalent aggregate, that is, the number of Cr.O; molecules carrying one electrical charge.! P is determined directly for the different values of cz and [Cr2O3] as given in the table. Corresponding values of P; are calculated from (2) for various assumed values of m; and from these P; values, P.. values can be gotten from (1) and Ae values from (3). Bjerrum took the values of m which give the most constant values of Ae as the correct m; and the average value of Ae as the correct Ae. One would conclude from the table that m is greater than 250; but the true value is quite indefinite. Bjerrum says m is approximately 500 and Ae approximately 15;? in other words, the colloidal particle contains something like 1,000 chro- mium atoms and carries 30 free positive charges. From the osmotic-pressure measurements, Bjerrum also calcu- lates the amount of free chloride ion in the sol. Subtracting this from the total chloride concentration is said to give the adsorbed chloride ion. From such considerations, the conclu- sion is reached that the colloidal particle contains 1,000 chro- mium atoms, carrying a total of 240 positive charges, 210 of which are neutralized by adsorbed chloride ion. This is prob- 10f. Zstiamonpy: ‘‘ Kolloidchemie,’’ 206 (1925). 2 Cf., however, WINTGEN and LOWENTHAL: Z. physik. Chem., 109, 378 (1924), 86 THE HYDROUS OXIDES ably incorrect, as a part of the chlorine is doubtless adsorbed as chloride and not as ion. It is of interest to compare the value m = 500 for a Cr2Q; sol, aged by prolonged boiling, with m = 750,000 for an aged Fe.O; sol, as reported by Wintgen and Biltz.! I doubt very much whether there are 1500 times as many molecules in an aged iron sol as in an aged chromium sol. It is more likely that the limits of the experimental methods employed by both Bjerrum and Wintgen render the values for both sols of doubtful accuracy. Richards and Bonnet? digested hydrous chromic oxide with chromium sulfate on the steam bath for several hours, obtaining a green solution which appeared to them to be a basic salt, Cr(OH)SO,. Na > Li.t From the curves in Fig. 7 it will be seen that the precipita- tion value of barium chloride is increased by like amounts of alkali chlorides in the order LiCl > NaCl > KCI; while in the presence of HCl, the precipitation value of barium salt first rises to a point just below that in the presence of a like amount of lithium chloride and then drops off rather sharply. The follow- ing explanation of these phenomena is suggested: For a given alkali chloride concentration, precipitation will take place when the combined adsorption of the two cations neutralizes the combined adsorption of chloride and hydrosulfide ions. The combined adsorption will be equivalent for different pairs of cations; but the relative amounts of each that make up this equivalent adsorption will vary, depending as it does on the relative adsorbability of the two cations. If one may disregard for the moment the slight variation in the amounts of chloride added with barium chloride as compared with the relatively large amount of this ion added with the alkali chloride, it follows that, for a given concentration of different alkali chlorides, the vary- - ing amounts of barium that must be added will depend on the effect of each cation on the adsorption of the other. Thus, the adsorption of barium is cut down by lithium ion less than by potassium ion, tending to make the precipitation concentration of barium chloride less in the presence of lithium chloride than of potassium chloride. Hand in hand with this is the decrease in- the adsorption of alkali by barium, which will tend to make the precipitation concentration of barium chloride higher in the presence of lithium. From this point of view, the latter factor appears to predominate with the alkali chlorides. With hydro- chloric acid, however, the cutting down of the adsorption of barium by hydrogen ion is the determining factor with lower 1 Weiser: J. Phys. Chem., 29, 955 (1925); cf. p. 125. 100 THE HYDROUS OXIDES concentrations of hydrochloric acid; while with higher concen- trations of acid, the second factor appears to predominate. Precipitation of Negative Sol—In Table XII are given the precipitation values of several electrolytes for a negative sol prepared by mixing 5 cubic centimeters of chromic chloride con- taining 40 grams Cr.O; per liter with 45 cubic centimeters of 0.2N KOH. The precipitation value is that concentration of electrolyte which will just cause complete coagulation in 10 min- utes. It will be noted that the precipitating power of cations follows the usual order: barium > lithium > sodium > potassium; and the stabilizing action of the an‘ons is: sulfate > chloride > acetate. TABLE XIJ.—PRECIPITATION VALUES OF SALTS Precipitation value, Salt milliequivalents gil x, nf erliee precipitate Barium, chloride... 4.0 ee See Gelatinous Potassium ehlorides:. .. ace. oe 500.0 Gelatinous BOUL ChlOTIde. <4 5 fee a eee 210.0 Gelatinous Lithnim chioridet:s ter fee 51.0 Gelatinous sodium: sulfates 5 5a. eee een 315.0 Gelatinous Sodim-acetate.. 10) oo ee 220.0 Gelatinous Chromic Oxide Jellies—Mention has been made of Reinit- zer’s observation that a solution of chromic salt boiled with sodium acetate and rendered alkaline with caustic alkalies or ammonia sets to a jelly. Bunce and Finch! confirmed this observation and showed further that a jelly is formed by adding excess sodium hydroxide or potassium hydroxide to chrome alum and allowing the solution to stand. They were unable to obtain a jelly from chromic sulfate, nitrate, or chloride; but Nagel? succeeded in getting a jelly with sulfate by keeping down the concentration of alkali. From these observations it was logical to conclude that acetate or sulfate ions are necessary for the formation of a chromic oxide jelly. That such is not the case 1Cf. J. Phys. Chem., 17, 769 (1918). 2 J. Phys. Chem., 19, 331 (1914). 3 BancroFT: ‘Applied Colloid Chemistry,’’ 244 (1921). ——s1 = HYDROUS CHROMIC OXIDE 101 is evident from the series of experiments recorded in Table XIII, using chromic chloride instead of sulfate or acetate. The experi- ments bear out the general conclusions regarding jelly formation previously considered in detail.!. The rapid addition of a slight excess of alkali to a chromic chloride solution produces a negative colloidal oxide that is instable and precipitates slowly, forming a jelly (Table XIII). If this precipitation is hastened by heating or by addition of a suitable amount of electrolyte, the precipitate forms so rapidly that it is gelatinous and not jelly-like (Table XII). Finally, if the hydrous oxide has been peptized by too great a concentration of alkali, the precipitate comes down very slowly and is almost granular in character, as observed by Nagel. TaBLE XIII.—Curomic Ox1DE JELLIES FROM NEGATIVE COLLOID Solutions (cubic centi- meters) mixed Alkali Observations aes Alkali | icy, | Total AA 0.6 N volume ING OE eee 10.0 5.0 25 Peptization incomplete Gelatinous Na Ho. heres 11.5 5.0 De Peptization incomplete Firm green jelly Na OMe tes 11.75 5.0 25 Peptization almost complete | Firm green jelly NOE a pnantl 12.0 5.0 25 Peptization complete Firm green jelly [OIEL ANS es eee 10.0 5.0 25 Peptization incomplete Firm green jelly CO Has tees 10:75 on0 25 Peptization almost complete | Firm green jelly HCO epee aes 11.0 5.0 20 Peptization complete Firm green jelly Ba(OH) seemed. 20.0 baO Do No peptization Gelatinous ISAO sae vs ae 24.0 1.0 25 No peptization Gelatinous INS OH. fhe > 5. 1355 5.0 50 Peptization almost complete | Soft green jelly INS OH ae aaaes eve 5.0 50 Peptization complete Soft green jelly i OT ete ee aes 12.0 5.0 50 Peptization almost complete | Soft green jelly OMT a leg: ashes. | Pagers a0 50 Peptization complete Soft green jelly The experiments under consideration corroborate the observa- tion of Fischer and Herz that the peptizing power of potassium hydroxide is slightly greater than that of sodium hydroxide. On the other hand, they disprove the statement that hydrous chromic oxide is peptized by barium hydroxide and that the pep- tizing power of alkalies depends on the absolute amount present and not on the concentration. The hydroxides arranged in order of peptizing power are potassium hydroxide > sodium hydroxide > barium hydroxide. As would be expected, this is 1 See p. 26, et seq. 102 THE HYDROUS OXIDES the reverse of the order of precipitating power of the cations (Table XII). Knowing the conditions favorable to jelly formation by pre- cipitation of a negative colloidal hydrous chromic oxide, it is a simple matter to precipitate the positive sol as a jelly. All that is necessary is to add just enough electrolyte to cause com- plete coagulation in an hour or two. If too little electrolyte is used, precipitation is incomplete and the results are unsatisfac- tory; while if too great an excess is added, the precipitation is so rapid that a gelatinous precipitate is formed. From the results recorded in Table X, it is quite evident that jellies will form in the presence of any precipitating ion. Moreover, the hydrogen ion concentration within which jellies will form, can vary over a wide range; thus, they are obtained from strongly alkaline solu- tion and from a colloid stabilized by hydrogen ion. A typical jelly containing but 0.18 per cent chromic oxide will stand for days without undergoing noticeable syneresis. Shaking destroys the jelly structure, which does not re-form as in the case of more concentrated jellies.! 1 Of, ScHALEK and Szecvary: Kolloid-Z., 32, 318; 33, 326 (1923). CHAPTER IV THE HYDROUS OXIDES OF ALUMINUM, GALLIUM, INDIUM, AND THALLIUM Hyprovus ALUMINUM OXIDE The Gelatinous Oxide.—The addition of ammonia to an aluminum salt solution throws down a very highly gelatinous precipitate of hydrous aluminum oxide. An x-radiogram of the precipitate formed in the cold with not too dilute solutions shows it to possess no crystalline character! even after prolonged drying at room temperature.? The precipitate exhibits a wide variation in properties depending on the conditions of formation and the age and history of the sample. Thus Tommasi’ found the newly formed oxide to be quite soluble in acids and alkalies, whereas the aged product was sparingly soluble. Recently Willstatter and Kraut* described a number of hydrous oxides differing in reactivity and adsorptive power, by precipitating aluminum sulfate with ammonia: With concentrated ammonia, and boiling for a long time, the precipitate was a pale yellow plastic mass A; without prolonged boiling, it was a very pale yellow plastic mass B; with dilute ammonia it was a pure white, very voluminous, and very finely divided substance C. An intermediate variety 6 prepared by the dialysis of aluminum chloride with frequent additions of small quantities of ammonia, was claimed to be related chemically to B but resembled A in adsorptive capacity; and a modified form of C precipitated at 60° had an adsorptive capacity similar to B. Specimens of A were entirely different in properties, depending on whether they 1Haser: Ber., 55, 1727 (1922); cf. Frickz and WEAVER: Z. anorg. Chem., 136, 320 (1924). 2 Boum and Nicuassen: Z. anorg. Chem., 182, 1 (1924); Boum: Jbid., 149, 203 (1925). 3 Compt. rend., 91, 231 (1880); cf. PHituips: Phil. Mag., (3) 38, 357 (1848). 4 Ber,, 66, 149, 1117 (1923); 57, 58, 1082 (1924), 103 104 THE HYDROUS OXIDES were still moist or subjected to a rapid preliminary drying in a high vacuum over P,O;. As a result of desiccation experiments, Willstatter concluded that the different gels contained a variety of different hydrates. This brings to mind earlier papers on hydrous aluminum oxide in which are described such hydrates as Al,O3; - H.O! corresponding to the crystalline mineral diaspore, Al,O; -2H,O? corresponding to amorphous bauxite, and Al.O3:- 3H.O* corresponding to crystalline gibbsite; but the existence of hydrates in gelatinous alumina is rendered doubtful by the work of Carnelley and Walker® and of van Bemmelen.* The latter showed that at constant temperature the precipitated oxide takes up or gives off water until the vapor tension of the substance is the same as that of the surroundings; hence, change in tempera- ture causes a continuous change in the water content of the substance by varying its vapor tension. Moreover, the vapor pressure of the hydrous oxide is influenced by the conditions of precipitation and the subsequent treatment. Thus an oxide adsorbs water more strongly if thrown down from a dilute solu- tion of aluminum chloride than from a concentrated solution. The precipitate decreases in solubility in alkali and acids in pro- portion to the quantity of water lost by heating; after heating at various temperatures, the different oxides adsorb smaller quan- tities of water when placed in a saturated atmosphere, and they retain less in dry air in proportion to the water lost. By standing under water, the capacity to adsorb water and the solubility in acids and alkalies alters in proportion to the time of standing An ‘“‘amorphous” hygroscopic oxide formed by ageing the gelat- inous precipitate for 6 months under water and drying in air, 1 MirscHERLicu: J. prakt. Chem., 83, 468 (1861); BecqurrEt: Jahresber., 87 (1868); Ramsay: J. Chem. Soc., 32, 395 (1877). 2 Lowen: Z. fiir Chemie, 3, 247 (1864); P&an de St. GinuEs: Ann. chim. phys., (3) 46, 57 (1856); Crum: Liebig’s Ann. Chem., 89, 156 (1853). 3 ALLEN: Chem. News, 82, 75 (1900); Cossa: Z. fiir Chemie, 18, 4438 (1873); Tommasi: Compt. rend., 91, 231 (1880). 4Other hydrates have been described by ZuNINOo: Gazz. chim. ital., 30, 194 (1900); Ramsay: J. Chem. Soc., 32, 395 (1877); ScHLUMBERGER: Bull. . soc. chim., (3) 18, 41 (1895). 6 J. Chem. Soc., 58, 87 (1888). 6 Rec. trav. chim., 7, 75 (1888); cf. Surper: Mem. Coll. Sci., Kyoto, 9a, 42 (1924). aa ee | a a ay ALUMINUM, GALLIUM, INDIUM, AND THALLIUM 105 corresponds to a trihydrate in composition. Van Bemmelen’s general conclusions were confirmed by observations on gelatinous alumina by Martin! and Kohlschiitter? and on ‘‘fibrous”’ alumina? _by von Zehman;* but Willstatter claims that vapor-pressure data do not show the absence of hydrates in preparations dried in vacuum over P.O; or treated with acetone, operations which are tacitly assumed to remove all adsorbed water. On the contrary, Willstatter’s predried preparations give certain temperature intervals of almost constant water content which are cited to prove the existence of hydrates. Thus the acetone-dried oxide precipitated from aluminum sulfate at low hydroxyl ion con- centration analyzes approximately for a trihydrate (van Bem- melen); and the precipitates obtained with excess ammonia in the hot give what are assumed to be polyaluminum hydroxides, such as 2Al1(OH)3 - H2O, as a result of intermolecular dehydration. Willstatter believes fresh gels to be hydrates; but his arguments are vague and unconvincing. Thus he says: It is not known whether the hydrates found after desiccation existed originally in the gelatinous suspension with the same amount of chem- ically combined water. Of course, there is no need of assuming the existence of single hydrates. The formulas calculated for hydrates of alumina in the cases described are complex; they have little significance, for they can usually be looked upon as mixtures of different hydrates. Whatever may be the water content and the degree of hydration of the gel suspended in water, it follows from the drying curve of prepa- ration C, (Al(OH); -nH:O which dries to Al(OH);), that desiccated preparations with values between (A103). - H20 and Al.O3 - H2O could not be Al(OH); in the original moist condition, but are probably mix- tures of compounds of the composition AI,(OH)3, - #H.0, 12.e., polymetahydroxides. Willstatter’s observations on a variety of oxides would seem to disprove the existence of hydrates with the possible exception of van Bemmelen’s trihydrate. Wide variations in the conditions of forming the oxides cause differences in their physical character and structure that determine not only the behavior toward 1 Mon. sci., (5) 5, 225 (1915). 2Z. anorg. Chem., 105, 1 (1919); Z. Elektrochem., 29, 246 (1923). 3 WIsLICENUS: Z. angew. Chem., 17, 805 (1904). 4 Kolloid-Z., 27, 233 (1920). 106 THE HYDROUS OXIDES reagents and their adsorption capacity for dyes and enzymes, but also the amount of water they retain under given conditions. The nature and the location of the kinks or bends in the tempera- ture-dehydration curves depend on the previous history of the sample and so are different for each sample. As might be expected, the composition of a preparation treated in a certain definite way may sometimes be represented by a Dalton formula; but this does not prove the existence of a definite compound. Van Bemmelen! obtained breaks in the vapor-pressure curves Pressure WwW pape oes Si ey es ey Temperature (pe) Oo oO Minutes Fia. 8.— Dehydration of hydrous alumina prepared at 15°. for gels of lower water content; but these were not due to the presence of hydrates, for the location of the breaks varied with the history of the sample, and similar breaks were observed when alcohol or benzene was substituted for water. Guichard? followed the continuous dehydration of hydrous aluminas with increasing temperature by means of a specially designed hydrostatic compensation balance.* The form of the curves is well illustrated by Figs. 8 and 9 showing the results of experiments carried out on the gelatinous oxide precipitated (1) 1 “T)ie Absorption,’’ 257 et seq. (1910). 2 Bull. soc. chim., 37, 381 (1925). 3 GUICHARD: Bull. soc. chim., 37, 251 (1925). ALUMINUM, GALLIUM, INDIUM, AND THALLIUM 107 in the cold and (2) at the boiling point. With regular increase in temperature there appears a slight slowing down of the dehydration of the oxide formed in the cold, between 150 and 200°, corresponding to a composition between 3H.O and 2H,0. The ‘‘pseudo flat” is interpreted to indicate the existence of Al,03:3H2O with adsorbed water and possibly Al,O3-2H2O with adsorbed water. Contrary to what one might have expected, the Pressure Temperature Minutes Fia. 9.—Dehydration curve of hydrous alumina prepared at 100°. aged crystalline oxide formed in the hot gives no indication of being Al.O3;-3H2O or any other hydrate. | The Crystalline Hydrate.—Although the oxide formed by precipitation of aluminum chloride with ammonium hydroxide contains no definite hydrates, crystals of artificial gibbsite have been prepared in a number of ways. Bornsdorff! obtained such a compound by saturating a sodium hydroxide solution with ' gelatinous alumina and allowing the solution to stand in a closed 1Pogg. Ann., 27, 275 (1833); cf. VAN BEMMELEN: Rec. trav. chim., 7, 75 (1888); Bayer: Chem. Ztg., 12, 1209 (1889); Dirre: Compt. rend., 116, 183 (1893); AtLEN: Chem. News, 82, 75 (1900); Russ: Z. anorg. Chem., 41, 216 (1904), 108 THE HYDROUS OXIDES vessel.! A similar compound is formed by passing carbon dioxide through a boiling solution of alkali aluminate; by boiling alumi- num in water for many hours; by the action of hydrogen peroxide on aluminum;? by the action of water on aluminum amalgam;? by allowing potassium aluminate and aluminum chloride to mix slowly through a diaphragm ;* by calcining hydrated alumi- num nitrate;> and by electrolysis of an aqueous solution of alum.* According to Milligan? the composition of this com- pound remains constant up to 145°, when it starts losing water continuously with increasing temperature. All but 8 per cent of the water is driven off below 200°, and there is no evidence whatsoever of another hydrate. Alumina dried at as low a temperature as 275° takes up water by adsorption but does not combine to reform the hydrate. The higher the temperature of ignition the less the adsorption capacity of the oxide, doubtless on account of decreased porosity resulting from sintering. Alumina prepared from amalgamated aluminum is much denser than the ordinary precipitated hydrous oxide.° The gelatinous oxide freshly precipitated in the cold dissolves in acids and alkalies forming salts and is readily peptized by certain dilute acids and salts. The precipitate thrown down from the hot solution is less reactive and less easily peptized. The newly formed oxide ages fairly rapidly in the hot and more slowly in the cold even under water. Two modifications of alumina have, therefore, been recognized, the so-called ordinary or alpha and meta or beta modifications, representing the two extremes of reactivity. But there is no temperature of inversion from the soluble alpha to the insoluble beta form; on the con- trary, between these two extremes one may have all possible variation in solubility, reactivity, and adsorbability depending on the structure of the mass which in turn is determined by the conditions of precipitation and the subsequent method of treat- 1 WoutER: Liebig’s Ann. Chem., 118, 249 (1859). 2 WELTZIEN: Liebig’s Ann. Chem., 138, 130 (1866). 3 Cossa: Z. fiir Chemie, 18, 448 (1873). 4 BECQUEREL: Compt. rend., 67, 1081 (1868). 5 ScHLésine: ‘Traite d’analyse,”’ Paris, 105 (1877). 6 Duxtuo: Bull. soc. chim., (2) 5, 78 (1866). 7 J. Phys. Chem., 26, 247 (1922); cf. Martin: Mon. sci., (5) 5, 225 (1915). 8 Haun and Tureuer: Ber., 57, 671 (1924). ALUMINUM, GALLIUM, INDIUM, AND THALLIUM. 109 ment.! These conclusions have been confirmed by Kohlschiitter? with different pseudo crystals of hydrous aluminas formed by the action of ammonia on crystals of aluminum salts. Unlike the fresh gelatinous oxide, the crystalline trihydrate is almost insoluble in cold acids and alkalies; it is very slowly soluble in hot concentrated HCl but it dissolves readily in con- centrated H.SO,. It is, therefore, similar in properties to the aged gelatinous oxide. By means of x-radiograms Bohm and Niclassen* observed the gradual transformation from an amor- phous to a crystalline oxide during ageing. Naturally, this raises the question whether the ageing process consists essentially in the gradual formation of crystalline trihydrate. X-radio- grams would appear to answer this in the negative, for aged oxides, obtained by precipitation in the hot or by precipitation in the cold and heating to 100° for an hour,‘ gave interference patterns corresponding to the indefinite mineral bauxite’ and not to definite crystalline gibbsite.6 However, from observations of the magnetic susceptibilities of a number of freshly precipi- tated and aged gels, as well as of crystalline trihydrate from potassium aluminate, Pascal’ concludes that newly formed gels consist solely of anhydrous Al,O3; with adsorbed water. On long standing, the gels go over to van Bemmelen’s unstable trihydrate, which appears to be quite distinct from the crystalline trihydrate of the same composition obtained from aluminate. ~ We know definitely that gelatinous alumina, aged in the presence of alkali, gives trihydrate crystals identical with gibbsite; but neither x-ray nor magnetic analyses furnish conclusive evidence as to whether van Bemmelen’s submicrocrystalline oxide, formed by ageing the ammonia-precipitated oxide in the cold, is really a trihydrate, and if so, whether it is identical with artificial gibbsite or an allotropic modification of the latter. For the present I am inclined to attribute the difference between the crystalline oxides aged by long standing in cold water and aged in dilute 1 WaisEeR: J. Phys. Chem., 24, 505 (1920). 2Z. anorg. Chem., 105, 1 (1919). $Z. anorg. Chem., 182, 1 (1924). 4Miuuican: J. Phys. Chem., 26, 254 (1922). 6 Fricke and Weaver: Z. anorg. Chem., 186, 321 (1924). 6 RINNE: Z. anorg. Chem., 136, 322 (1922), 7 Compt. rend., 178, 481 (1924), 110 THE HYDROUS OXIDES alkali to a difference in specific surface rather than in composi- tion or chemical structure. Five years ago, before x-ray analysis established the crystalline character of aged aluminum oxides, Fricke! observed a marked difference in the physical character and solubility of trihydrates obtained from aluminate under varying conditions. The adsorption capacity of a gelatinous oxide aged for a short time is much greater than that of crystalline trihydrate from aluminate; and the heating curves of the two are quite distinct.? On heating the crystalline trihydrate, there appears a diminution corresponding to an endothermal change below 360°; while gelatinous alumina gives a curve with a decided hump at 850°, corresponding to an exothermal change. Mellor and Holdcroft* suggest the term calorescence for the exothermic phenomenon. This calorescence or glow phenomenon, like that observed by calcining hydrous chromic oxide, is a manifestation of the energy lost by a sudden diminution of surface at some tempera- ture. The relatively large trihydrate crystals which precipitate from alkali aluminate do not caloresce when heated, since they possess a much smaller surface for a given mass than the aged gelatinous oxide. The diminution in surface of the heated oxide is accompanied by a decrease in hygroscopicity, specific gravity, reactivity, and adsorbability. X-radiograms* show that ' the ignited oxide is not a different allotropic modification. Anhydrous Alumina. Corundum Gems.—While there is little evidence of the existence of a and 6 hydrous oxides of aluminum, the anhydrous oxide has been prepared in two dis- tinct forms:> a aluminum oxide, the usual trigonal, crystalline form represented by corundum; and 8 aluminum oxide formed in hexagonal crystals or appearing in groups of overlapping triangular plates when a aluminum oxide is melted and allowed to cool slowly. The presence of a small amount of MgO (0.5 per cent) materially assists the formation of 6 aluminum oxide while small amounts of either calcium oxide or silicon diox- 1Z, Elektrochem., 26, 143 (1920). 2 Le CHATELIER: Compt. rend., 104, 1517 (1887). 3 Trans. Ceram. Soc., 10, 169 (1912); 138, 83 (1914). 4 HepvALL: Z. anorg. Chem., 120, 327 (1922). 5 RANKIN and Merwin: J. Am. Chem. Soc., 38, 568 (1916). ALUMINUM, GALLIUM, INDIUM, AND THALLIUM 111 ide favor the formation of the a variety. _Since the 8 form does not revert to the a form even when held at temperatures above or below the melting point, it is suggested that 6 aluminum oxide may be monotropic with respect to the a form; but this con- clusion does not follow. The existence of these two modifications of alumina is of inter- est in connection with the color of corundum gems. ‘The color of pure Al,O; is white or water clear. Natural corundum occurs as blue, green, violet, yellow, and white sapphires and as ruby which varies in color from pale rose to carmine red or bluish red. The yellow, purple, and green sapphires are sometimes called oriental topaz, amethyst, and emerald, respectively. The pleochroism is marked in some gems. Thus the ruby may be deep red in the direction of the vertical axis and lighter color or colorless at right angles to this direction. Similarly, the sapphire may be deep blue in the direction of the vertical axis and greenish blue to bluish white when viewed at right angles. The various tints are due to the presence of colored oxides. By fusing alu- minum and chromium oxides Fremy and Verneuil! synthesized ruby and also obtained crystals which, in parts, had the color of blue sapphire.” The difference in color was attributed to a difference in the state of oxidation of the chromic oxide. If such is the case, it would appear that the ruby may owe its color to CrO3 and the sapphire to Cr.O3. In line with this, a red color can be obtained only in an oxidizing atmosphere; moreover, by heating a ruby in a reducing atmosphere it may become green or even color- less,? owing to the low tinctorial power of the green oxide. There are, however, two difficulties with this hypothesis. In the first place, CrO; is instable at the temperature of molten alumina, and so we must make the unproved assumption that the oxide is stabilized by alumina; and in the second place, we do not know whether CrO3; when highly dispersed will give a red color to alumina. An alternative hypothesis is that the different colors of gems with chromic oxide as pigment are due to variation in the size of the particles of Cr203. While this would account for 1 Compt. rend., 111, 667 (1890). 2 Cf. Deviiute and Caron: Compt. rend., 66, 765 (1858). 3 BortTer: “ Edelsteinkinde,”’ Leipsig, 88 (1893); KENNGoTT: Neues Jahrb. Mineral, Geol., 313 (1867); Rinne: [bid., I, 170 (1900); II, 47 (1906). 112 THE HYDROUS OXIDES variations in color from light blue to dark green,! it seems unlikely that this explanation can be extended to include the red color. The Norton Company found that. artificial gems made with a Al,O3; and chromic oxide are red, while those made with 8 oxide are green. Bancroft suggests, therefore, that the different colors are due to different allotropic modifications of Cr2Q3. Since a alumina is only partly converted into 8 alumina by melt- ing and slowly cooling the oxide, Bancroft’s explanation might account for red and blue patches in the same crystals, both natural and artificial. Morozenwicz? claimed to get rose, yellow, greenish-yellow, red, and pale-blue corundum with iron oxide and so suggested that the coloring agent in certain gems is due to iron instead of chromium oxides. Verneuil’s* most recent work on the synthesis of sapphires leads him to attribute the coloration of natural sapphires to iron and titanium oxides. However this may be, there is no denying that artificial cee may owe their color to chromium oxide. ALUMINUM OXIDE SOLS Since it is possible to prepare an indefinite number of hydrous aluminas differing in the size and structure of the particles and the amount of water they contain, it is possible to obtain col- loidal solutions of alumina having widely varying properties depending on the method of formation. Two general methods of preparation are employed: hydrolysis of aluminum salts, and peptization of the hydrous oxide by acids and salts. Hydrolysis of Aluminum Salts.—Gay Lussac‘* boiled a con- centrated solution of aluminum acetate and obtained a precipi- tate of hydrous alumina which redissolved when the temperature was lowered. Crum heated a more dilute and more basic solu- tion than Gay Lussace, first in a closed vessel and subsequently in an open one, to drive off the excess acetic acid. In this way a 1 Weiser: J. Phys. Chem., 26, 417 (1922). 2 Tschermak’s mineralog. petrog. Mitt., (2) 18, 456 (1899), 3 Compt. rend., 150, 185 (1910). 4 Ann. chim. phys., (1) 74, 193 (1810). * Liebig’s Ann. Chem., 89, 168 (1854), ALUMINUM, GALLIUM, INDIUM, AND THALLIUM 113 stable but opalescent colloidal solution was formed, containing alumina and acetic acid in the ratio of 5.5:1. The conditions of formation, namely prolonged digestion at high temperature with subsequent boiling in a medium having a slight solvent action, were conducive to the formation of relatively large dense non-reactive primary particles. Accordingly, the oxide thrown down from the sol by electrolytes was an aged coagulum made up of crystalline particles that were not very soluble in acids or alkalies and had no mordanting action. Graham! prepared a sol having properties similar to Crum’s by heating an acetate solution for several days and then dialyzing in the cold. The time required for making Crum’s sol may be materially shortened by peptizing freshly precipitated hydrous alumina with the smallest possible amount of acetic acid, diluting, and boiling to remove the excess acid.2,_ Minachi and Okazaka* diluted a saturated solution of aluminum acetate in dilute acetic acid, added hydrogen dioxide, and dialyzed at 50 to 80°. Attempts to prepare colloidal alumina by dialysis of the chloride and nitrate’ in the cold have not proved successful, owing to the relatively low degree of hydrolysis of even !4999 M solutions.°® Since the temperature coefficient of the hydrolysis is quite high,*® Neidle’ was able to get a 9.5 per cent conversion of a 0.05 M solution of AlCl; by dialyzing for 37 hours at 75 to 80°. Peptization of Hydrous Alumina.—Graham! peptized freshly prepared and thoroughly washed hydrous alumina in a solution of aluminum chloride and then dialyzed out the excess of the peptizing agent in the cold. By this method a positively charged sol results that is very sensitive to the action of electrolytes. The precipitate formed on coagulation is highly gelatinous, is readily soluble in acids and alkalies, and isa mordant. The sol, therefore, bears the same relation to Crum’s colloidal alumina 1Phil. Mag., (4) 28, 290 (1862); see also ScHLUMBERGER: Bull. soc. chim., (3) 18, 62 (1895). . 2 Weiser: J. Phys. Chem., 24, 525 (1920). 3 Japanese Patent 41726 (1922). 4 Bintz: Ber., 35, 4432 (1902). 5 Ley: Z. physik. Chem., 30, 219 (1899). 6 BserRuM: Z. physik. Chem., 59, 343 (1907). 7 J. Am. Chem. Soc., 39, 71 (1917). 8 Liebig’s Ann. Chem., 121, 41 (1862). 114 THE HYDROUS OXIDES that Graham’s colloidal ferric oxide bears to the Péan de St. Gilles sol. Analogous to ferric oxide sols, the difference in prop- erties of the two colloidal aluminas is closely associated with the size and physical character of the hydrous particles. Peptization of highly gelatinous alumina in the cold favors the formation of small highly hydrous primary particles that are more reactive and have a higher adsorption capacity than the more granular and denser particles formed during prolonged boiling in a medium possessing a slight solvent action. The peptization of an alumina gel by AICI; does not take place very readily; but Hantzsch and Desch! got around this difficulty by adding ammonia to an aluminum chloride solution until the precipitate first formed failed to dissolve, and then dialyzing the sol. By evaporating the transparent purified sol on the water bath, a glassy mass was obtained which was readily repeptized by water; but the new colloid was quite opalescent owing to the formation of larger crystalline? particles during the process of evaporation. The sol prepared by hot dialysis was also slightly opalescent, possessing properties intermediate between Graham’s and Crum’s sols. Highly purified sols cannot be prepared by adding ammonia to aluminum sulfate and dialyzing, because of the precipitating action of sulfate ion. However, one may add NaeCO; and Al,(SO,)3 in the approximate ratio of 3:5 without any precipi- tation taking place; when the ratio is 7.5:5, half the alumina is thrown down; and when it is 12:5, all the alumina precipitates.’ Schneider‘ first peptized gelatinous alumina with a dilute solution of HCl. The excess acid was removed by evaporating to dryness and repeptizing with water. The sol gave no test for chloride ion with AgNO; in the cold, but AgCl precipitated out on heating; with silver oxide, both AgCl and the sol were thrown down. The failure to get a test for chloride ion in the cold was doubtless due to inhibition of the growth of AgCl particles by the protecting power of hydrous alumina. Heating caused the AgCl to show up, owing to partial agglomeration of the particles. 1 Tiebig’s Ann. Chem., 328, 30 (1902). 2 Boum and NicLassENn: Z. anorg. Chem., 182, 1 (1924). 3 Miuits and Barr: J. Chem. Soc., 41, 341 (1882). 4 Tiebig’s Ann. Chem., 257, 359 (1890). ALUMINUM, GALLIUM, INDIUM, AND THALLIUM 115 The addition of silver oxide introduced the strongly adsorbed hydroxyl ion which neutralized the charge on the particles precipitating the colloidal ‘oxide together with AgCl. Miller! boiled freshly prepared hydrous alumina with N/20 HCl and found the amount of acid required for complete peptization to be one-seventy-second of that necessary to form AICl;; Pauli? used one-ninth of the theoretical amount; and Kohlschiitter? showed that the quantity of acid required was determined by the history of the sample. The dissolution of hydrous alumina in concentrated HCl is always preceded by sol formation; but H.2SO, does not form a sol. Hydrous alumina is peptized by ferric chloride or nitrate but not by ferric sulfate. The peptizing action of the chloride and nitrate is due to strong adsorption of ferric ions and of hydrogen ions resulting from hydrolysis of the salts. Such sols contain both hydrous alumina and hydrous ferric oxide. With ferric sulfate, the peptizing action of the cations is neutralized by strong adsorption of sulfate ion and no sol is formed. The order of peptizing power of different acids and salts on an aged gel thrown down from a boiling solution is: HNO; > HCl > FeCl > AICl, > HC2H30..* If we assume, as Lottermoser does, that a peptizer must contain one of the ions of the disperse phase, then the first step in the peptization of alumina by an acid or salt would be interaction with the formation of some aluminum ion. This would seem to be an unnecessary step in view of the stronger peptizing action of hydrogen ion than of aluminum ion. On account of the relatively small ionization of acetic acid, its peptizing power is less than that of HCl or HNO;. Bentley and Rose® have reported many anomalies in the behavior of the sol formed by peptizing alumina with acetic acid; but for the most part, these are the result either of experimental error or of misinterpretation of data.® 1Z. anorg. Chem., 57, 311 (1908); cf. SCHLUMBERGER: Bull. soc. chim., (3) 13, 60 (1895). 2 Kolloid-Z., 29, 281 (1921). 3 Z. Elektrochem., 29, 253 (1923). 4 Weiser: J. Phys. .Chem., 24, 521 (1920). 5 J. Am. Chem. Soc., 35, 1490 (1913); Ros: Kolloid-Z., 16, 1 (1914). 6 Weiser: J. Phys. Chem., 24, 522, 527 (1920). 116 THE HYDROUS OXIDES On account of the marked tendency of aluminum salts to hydrolyze, one is not surprised to encounter a very large number of basic aluminum chlorides, sulfates, and acetates. While there may be some definite salts of this type, it is certain that by far the most of them are mixtures of indefinite composition. ' Pauli and his collaborators? champion the view that the various alumina sols are highly complex basic salts of variable composi- tion. While one cannot deny the possible existence in a sol of such compounds as Pauli describes, there seems no reason for - postulating their existence until someone shows that such definite compounds are formed and defines their limits of stability. Action of Alkalies and Ammonia.— Private communication to BLum: J. Am. Chem. Soc., 35, 1503 (1913). 6 FRESENIUS: “Quant. Chem. Analysis,” 2, 807 (1916); WepenuorsT: “‘Beitrage zur Quant. Bestimmung und Trennung des Aluminiums,”’ Gottingen (1921); Bum: Z. analyt. Chem., 27, 19 (1888); von WEIMARN: Kolloid-Z., 4, 38 (1909); JANDER and WEBER: Z. anorg. Chem., 131, 266 (1923). ALUMINUM, GALLIUM, INDIUM, AND THALLIUM 119 solution. Blum observed a small but appreciable solvent action when the solution is just alkaline to phenolphthalein at a hydro- gen ion concentration of 10-®. Whether this is due wholly or in part to peptization of the oxide is open to some question. When potassium aluminate is precipitated with the calculated amount of ammonium chloride and an excess of ammonia added rapidly, all the alumina redissolves. Renz! mixed the calculated amount of ammonium sulfate with barium aluminate to which an excess of ammonia was added, and after filtering off the barium sulfate, obtained a clear solution containing 2.0 grams Al,O3 per liter. On evaporating to dryness, there was found a white gummy mass of hydrous oxide insoluble in ammonia. The solvent action of ammonia is most pronounced at the moment of formation, and from analogy with the alkali aluminates it is probable that aluminate is formed.? Blum was unable to detect the presence of this salt by determing the change in hydrogen ion concentra- tion during the precipitation of hydrous alumina, because of the low alkalinity of the aqueous solution. If the salt exists, the maximum quantity that can be held in solution will be deter- mined by the alkalinity of the resulting solution and its ability to repress the hydrolysis of the salt. According to Archibald and Habasian, the solubility of alumina in ammonia rises to a maximum of approximately 0.45 gram per liter at a concentra- tion of about 0.5 N, and then decreases owing to a change in the physical character of the hydrous oxide. The solubility of Renz’s preparation in excess ammonia was more than four times this maximum. Ammonium nitrate decreases the solubility in ammonia while potassium nitrate apparently increases it. Lottermoser and Friedrich*® prepared a very readily peptized hydrous oxide by adding AICI; in small increments to a solution of N/10 NH,OH cooled to 0° and stirred by air saturated with ammonia. After thorough purification by dialysis, the oxide was peptized by AICI; slowly in the cold and rapidly at 60 to 70°. Traces of ammonia peptized the gel forming a negative sol that was not very stable on heating. In the light of this work, Renz’s experiments should be repeated to determine the 1 Ber., 36, 2751 (1903). 2 Of. ARCHIBALD and Hagpasian: Trans. Roy. Soc. Canada, 10, 69 (1916). 3 Ber., 57, 808 (1924). 120 THE HYDROUS OXIDES nature of his solutions. Jander and Weber! found no evidence of sol formation on shaking precipitated alumina with ammonia solutions. For a given concentration of ammonia, the solubility was the same in the presence of monovalent and univalent anions; organic solvents have no precipitating effect;? and the solution passes readily through an ultrafilter. Alumina is not precipitated from an alum solution by ammonia in the presence of a tartrate owing to the formation of a complex aluminum tartrate.* A sol results by precipitation in the pres- ence of glucose. In some preliminary experiments on grinding alumina in a colloid mill with glucose, Utzino* claimed to get a sol, the maximum stability of which does not occur with the finest state of subdivision. These observations should be repeated. Coagulation of Sol.—The precipitating power of electrolytes for colloidal aluminum oxide sols has been studied repeatedly.°® While the absolute precipitation values of electrolytes vary with the concentration and purity of the sol and with the experimental method, the order is always approximately the same. On account of the transparency of the gelatinous oxide, some diffi- culty is experienced in determining the critical precipitation concentration of electrolytes. Kawamura took advantage of the change in viscosity which the sol undergoes on coagulation, and this method was adopted by Ishazaka and Gann. The latter in collaboration with Freundlich, followed the slow coagulation of colloidal alumina by the addition of electrolytes containing uni- valent precipitating ions. The process which was found to be autocatalytically accelerated, takes place in accordance with aoe ite : : the equation y = k(1 + bir) (1 — x) where z is the increase in viscosity after time ¢ expressed as a fraction of the total increase; 1Z. anorg. Chem., 181, 266 (1923). 2 YANEK: Ann. ecole mines Oural, 1, 45 (1919); Chem. Abstracts, 15, 1239 (1921). 3 Hakomori: J. Chem. Soc. Japan, 48, 629 (1922). 4 Kolloid-Z., 32, 149 (1923). 6 Kawamura: J. Coll. Sci., Imp. Univ. Tokyo, 28, Art. 8 (1908); IsHazaKa: Z. physik. Chem., 83, 97 (1913); Gann: Kolloidchem. Bethefte, 8, 125 (1916); Weiser and Mippueton: J. Phys. Chem., 24, 639 (1920); IwanrrzKasa: Kolloidchem. Bethefte, 18, 24 (1923). Cy a i ALUMINUM, GALLIUM, INDIUM, AND THALLIUM 121 and k and 6; are constants. For concentrated sols the coagula- tion process is more nearly represented by the equation - = ki(1 — x)’. The coefficient k; increases rapidly with the con- centration of the electrolyte during slow coagulation, while for very rapid coagulation the velocity is independent of the nature of the electrolyte. In rapid coagulation, Smoluchowski! assumes that all the collisions of particles are inelastic because of the great attractive forces existing between particles; and in slow coagula- tion, only a portion of the collisions result in immediate union, because the mutual attraction is not always great enough to overcome the repulsive effect of more highly charged particles. Freundlich turns this around and assumes a constant force of attraction for a given concentration of electrolyte below that necessary for rapid coagulation; but because of repulsion between charged particles, only those collisions are inelastic in which the particles collide with sufficient force. Obviously, the greater the charge on the particles, the greater must be the velocity of collision in order to overcome the repulsive effect and so to bring about coalescence and agglomeration. The rapid increase in the velocity of slow coagulation is due to the proportionately larger number of inelastic collisions that result when the charge on the particles is reduced by adsorption of precipitating ions. Aluminum Oxide Jellies.—A sol formed by peptizing sufficient hydrous alumina to form a viscous liquid sets to a jelly on stand- — ing.? If this jelly is broken up by shaking, a gelatinous precipi- tate settles out which is not repeptized by the acid and so cannot be reconverted into a jelly. Schalek and Szegvary* prepared a so by Crum’s method which set to a jelly on the addition of a sulitable amount of electrolyte just below the precipitation value. This jelly was broken up on shaking, but instead of giving a gelatinous precipitate, a sol was re-formed that would again set to a jelly on standing. The reversible sol-gel transformation has been observed only with relatively concentrated sols of the hydrous oxide. A jelly may be formed by coagulating a dilute sol prepared by peptizing hydrous alumina with acetic acid; 1Z. physik. Chem., 92, 129 (1917); Kolloid-Z., 21, 98 (1917). 2 ScHLUMBERGER: Bull. soc. chim., (3) 13, 56 (1895). 3 Kolloid-Z., 33, 326 (1923). 122 THE HYDROUS OXIDES but shaking converts the jelly into a gelatinous precipitate that is not repeptized. ADSORPTION BY HYDROUS ALUMINUM OXIDE If an electrolyte is added to a sol stabilized by preferential adsorption of cations, precipitation will take place when the anions of the electrolyte are adsorbed sufficiently to reduce the charge on the particles below a critical value. Whitney and Ober! first showed that the amount of various ions carried down during the precipitation of arsenious sulfide sol are not far from equivalent. This conclusion was upheld by Freundlich? as a result of similar observations on adsorption during the precipitation of other sols. The results with alumina given in Table XIV are frequently offered as proof of equivalent adsorption TaBLE XIV Adsorption at the precipitation Precipitation values Ions value, millimols per liter Th yailltaeere In milli- equivalents Salicylate... J... + 8.0 0.30 0.30 Pictates 4.0 0.18 0.18 Oxalatese. 2. ee 0.36 0.18 0.36 Ferricyanide...... 0.10 0.09 0:27 Ferrocyanide...... 0.08 0.073 0.29 during the precipitation of sols with precipitating ions of varying valence although the variation from equivalence is quite appreci- able. The investigations of Freundlich on aluminum oxide sol have been extended by Middleton’ with the results given in Table XV. The adsorption for different ions is not even approxi- mately equivalent, and the variation cannot be attributed to experimental errors, as Freundlich assumes. While the latter is doubtless right in concluding that neutralization of the charge is accomplished by adsorption of equivalent amounts, the 1 J. Am. Chem. Soc., 23, 1842 (1901). 2 “ Kapillarchemie,” 579 et seq. (1922). 8 WEISER and MippLeTon: J. Phys. Chem., 24, 630 (1920). ALUMINUM, GALLIUM, INDIUM, AND THALLIUM 123 TABLE XV Adsorption, Precipitation value Anion milliequivalents per | of salt, milliequiva- liter lents per liter (OC Gch Oe en 1.280 0.375 Merrieyanide: si, v./,.....-.. oiLeQi4 0.400 Se eee a ae 0.997 0.538 Ci ene e: Aes) Mischa ia 1, 142 0.700 SENCUIOALO te ee Sa. . 0.870 1.300 POA eey ee ee ke. es 0.657 1.625 LO OTe See en? a a 0.269 ier are: actual amount carried down is determined (a) by adsorption of the electrically charged particles during neutralization and (b) by adsorption of salt by electrically neutral particles during the agglomeration process. The amounts of (a) will be approxi- mately equivalent, but the amounts of (b) will vary with the nature and concentration of the electrolyte. Owing to salt adsorption by neutralized particles, Freundlich’s conclusion that equivalent amounts are adsorbed at the precipitation con- centration cannot be generally true, since this would mean either that the neutralized particles do not act as an adsorbent or adsorb all ions to the same extent. Moreover, the variability of the precipitation concentration will necessarily result in varia- tion in the degree of saturation of the adsorbent by the adsorbed phase. One should expect the adsorption value to approach equivalence more nearly, the less the adsorption capacity of the precipitated particles. This probably accounts for the values being more nearly equivalent with an arsenious sufide sol than with a hydrous oxide sol having many times the adsorption capacity. If the variation from equivalence arises from adsorption after neutralization, the adsorption values might appear a priori to give directly the order of adsorption of the ions. This is not necessarily true, however, because there are variable factors other than the adsorbability of the precipitating ions that deter- mine the amount of adsorption after neutralization; for example, the nature and degree of ionization and the degree of hydrolysis of the salt; the hydrogen ion concentration; the effect of different 124 THE HYDROUS OXIDES salts on the physical character of the precipitate; ete. From the observations recorded in Table XV, the order of adsorbability expressed in equivalents would appear to be as follows: ferro- cyanide > ferricyanide > oxalate > sulfate > chromate > dithionate > dichromate. Considering the precipitation value of the several potassium salts, we find the order of precipitating power beginning at the greatest to be: ferrocyanide > ferri- cyanide > sulfate > oxalate > chromate > dithionate > dichromate. The order of adsorption determined directly is the same as the order deduced from precipitation data with the exception of oxalate and sulfate, which are reversed. The cause of this exception is not known; but in this connection, attention may be called to some unpublished work of Everett E. Porter which disclosed that the order of precipitating power of oxalate and sulfate for chromic oxide sol is determined by the hydrogen ion concentration of the precipitating solution. If the adsorption value is expressed in equivalents, as seems logical, since neutralization is determined by the number of adsorbed charges, the results given in Table XV are in accord with the usual interpretation of Schulze’s law that the ion of highest valence is most readily adsorbed. At the same time, the qualitative nature of the rule is indicated by the different adsorp- tion value for ions of the same valence. Schilow! found a wide variation in the adsorption of cations of the same valence by ignited alumina; but the ions of highest valence were most strongly adsorbed. It is unfortunate that a comparison of the relationship between the precipitation values of electrolytes and the adsorption of precipitating ions cannot be made directly with salts containing univalent precipitating ions since the precipitation values and adsorption values for multivalent ions are likely to be so close together that it is hazardous to draw conclusions, particularly when the differences may be of the same order of magnitude as the errors inherent in the experimental method. The direct determination of adsorption of univalent ions that precipitate only in high concentration, is impracticable since the change in concentration resulting from adsorption is too low to measure accurately. It is possible, however, to determine the relative 1Z, physik. Chem., 100, 425 (1922). a ay ALUMINUM, GALLIUM, INDIUM, AND THALLIUM 125 adsorbability of univalent precipitating ions during the precipi- tation of sols by an indirect method consisting essentially in determining the extent to which the presence of different uni- valent ions cuts down the adsorption of an easily estimated multivalent ion.! This is illustrated by the results recorded in Table XVI. The extent to which the adsorption of barium ion TaBLE XVI : : Barium adsorbed Cubic centimeters electrolyte added to Precipitation 100 cubic centimeters sol, total volume Millienaivalents value, milliequiva- 200 cubic centimeters. Grams lents per liter per gram SUN ASTOS VEMTILES ELS 5. ric i rr 0.0109 0.058 BaCle 274. 30 N/50 BaCle + 30 N/2 LiCl........... 0.0037 0.019 LiCl 88.7 30 NV /50 BaCle + 30 N/2 NaCl.......... 0.0025 0.014 NaCl (ono a0 750 BaCle -— 30 .N/2 KCl........... 0.0018 0.009 KCl 63.7 30/50 BaCle-F 30):NV/2 HCl........... 0.0013 0.007 HCl SPAS by arsenious sulfide is cut down by the presence of the same amount of different alkali chlorides is in the order Li < Na < K < H. Since, under otherwise constant conditions, one should expect the adsorption of a given cation to be cut down by the presence of a second, in proportion to the adsorbability of the latter, it follows that the order of adsorbability of univalent ions is H>K>WNa>Ii. This is exactly the same as the order deduced from the precipitation values of the salts, assum- ing that the salt containing the most readily adsorbed cation precipitates in lowest concentration. Weak adsorption of the precipitating ions of electrolytes requiring a high concentration to effect neutralization, is indi- cated by the ease of reversibility of precipitation. Thus hydrous alumina precipitated from a sol with a relatively high concentra- tion of KCl, NaCl, or NaC.H3O: is readily carried back into colloidal solution by washing, whereas the precipitation is more nearly irreversible if K:SO, is the precipitating electrolyte. Similarly, a precipitated alumina thrown down from an alum solution with alkali can be washed until most of the sulfate is removed before peptization begins,? whereas the precipitate from chloride solution is very easily dispersed by washing. ‘The 1 Weiser: J. Phys. Chem., 29, 963 (1925). 2 BRADFIELD: J. Am. Chem. Soc., 44, 969 (1922). 126 THE HYDROUS OXIDES difference in degree of reversibility of precipitation is determined by the relatively weak adsorption of univalent chloride as com- — pared with bivalent sulfate. Rakuzin' reports that hydrous alumina adsorbs gum arabic reversibly; but the adsorption from sodium and potassium silicate is partly reversible. The adsorption of chromate by hydrous alumina is sufficiently strong to impart a yellow color to the precipitate formed in the presence of alkali chromate or precipitated and subsequently shaken with alkali chromate solutions. Charriou? found little alkali metal in the precipitate and so attributed the color to the formation of aluminum chromate on the surface of the alumina. There is no justification for this conclusion and it is probably erroneous. If well-washed alumina is shaken with alkali chro- mate, the solution becomes alkaline owing to stronger adsorption of acid than of base. The yellow color is due to chromic acid rather than aluminum chromate. Ishazaka* found that potas- sium dichromate was converted to chromate in the presence of powdered alumina. The explanation of this phenomenon is as follows: The equilibrium in solution between dichromate and chromate ion may be represented by the equation Cr20,’’ + H,0 @ 2H’ + 2CrO,’’.. Alumina shows such a strong preferen- tial adsorption for hydrogen ion that the presence of the oxide in a finely divided condition shifts the equilibrium to the right with the formation of chromate ion at the expense of dichromate. Colloidal alumina stabilized by preferential adsorption of hydrogen ion has a comparatively slight effect on the equilibrium. * Adsorbed chromate is displaced but slightly by washing with 5 per cent solutions of the more weakly adsorbed chloride, bromide, iodide, nitrate, or acetate; while chromate is displaced by ions the adsorption of which is of the same order of magnitude, such as carbonate, sulfate, sulfide, oxalate, tartrate, phosphate, or arsenate. Similarly, sulfate is not displaced by weakly adsorbed univalent ions but is displaced by bivalent ions. Charriou® generalized that an adsorbed ion is displaced by one of the same 1 J. Russ. Phys.-Chem. Soc., 58, 357 (1921). 2 Compt. rend., 176, 679, 1890 (1923). 3Z. physik. Chem., 88, 97 (1918). 4 Weiser and Mippieton: J. Phys. Chem., 24, 648 (1920), 5 Compt. rend., 176, 1890 (1923), ALUMINUM, GALLIUM, INDIUM, AND THALLIUM 127 kind having the same or a higher valence; but is not displaced by one of lower valence. With two ions of the same valence the less concentrated is displaced the most. These generalizations may be approximately true in certain cases, but they are neces- sarily not quantitative, since they are based on the erroneous impression that ions of like valences are all adsorbed to the same extent and that trivalent ions are always more strongly adsorbed than bivalent and bivalent ions always more strongly adsorbed than univalent. Generalizations based on Schulze’s law are of value only in so far as the limitations of Schulze’s law are fully recognized. ! : From the practical point of view, in the Pieniative estimation of alumina one may avoid the contamination by such ions as chromate by carrying out the precipitation with NH,HCOs; instead of NaOH, or one may remove the adsorbed ion by wash- ing the precipitate with NH,HCOs. Miller? investigated the simultaneous adsorption of sulfate and oxalate ions during the precipitation of alumina. The adsorbabilities of the two ions are not far apart, and it is claimed that the sum of the adsorption expressed in mols per mol of hydrous alumina may be considered constant, although the observed values really show variations of more than 20 per cent both above and below the mean value. The distribution ratios ion in solution ion in precipitate values for the same ion were considered to be of the same order of magnitude, although here the variations from a constant value were more than 100 per cent from the mean. As a result of these observations, the taking up of anions by the hydrous oxide was considered to be a solid-solution phenomenon. ‘This con- clusion seems hardly justified by the evidence. Of course, one cannot expect too much from data obtained on such a complex system where such factors as the hydrogen ion concentration and the physical character of the precipitate are not subject to control. But one cannot be certain of the effect of eliminating all variable factors other than the relative amounts of oxalate and sulfate. In case the adsorbabilities of the two ions are 1 Weiser: J. Phys. Chem., 29, 963 (1925). 2U. S, Pub, Health Repts., 39, 1502 (1924), at equilibrium were also calculated, and the 128 THE HYDROUS OXIDES very similar, one should expect the total adsorption to be approx- imately equivalent, irrespective of the relative amounts of each; but if the adsorbabilities of the ions are widely different, there is likely to be an antagonistic action between the two which will cause the total adsorption to vary with the relative amounts of each in the solution. But even should the adsorp- tion be equivalent and the displacement follow the law of dis- tribution between solutions, it does not follow that the taking up of ions is a true solid-solution phenomenon rather than a surface phenomenon. If in certain cases there should be a reciprocal. displacement of adsorbed ions, there is probably no real objec- tion to calling the system a homogeneous single-phase solid solution as Miller does, provided one recognizes that this designa- tion is probably not strictly accurate. If a small amount of ferric salt is added to the test tube con- taining the precipitate thrown down from an alumina sol by the required amount of ferrocyanide, no Prussian blue is formed until after an appreciable interval of time.! This is not due to the slow rate of reaction between ferrocyanide and ferric ions as a result of the colloidal nature of ferric salt solutions;? but is due to the very strong adsorption of ferrocyanide ion which removes it from the field of action. If another strongly adsorbed precip- itating ion is added to the sol either before or after precipitation, the ferrocyanide is displaced and the time necessary for the appearance of Prussian blue is diminished appreciably. In the same way, the transformation of Congo blue to Congo red by dilute alkali is slowed down in the presence of hydrous alumina on account of the strong adsorption of Congo blue by the hydrous oxide.’ The selective adsorption of alumina seems to offer a great many possibilities to the biochemist,* although the observations to date are somewhat fragmentary. Rakuzin® reports that casein is adsorbed by alumina without splitting the molecule, 1 RerrstoTrer: Kolloid-Z., 21, 197 (1917); Freunpuicw and Resrr- sTOTTER: Ibid., 23, 23 (1918). 2 VORLANDER: Kolloid-Z., 22, 103 (1918). ’ Bayuiss: Proc. Roy. Soc. London, 84, 81 (1912). 4 KuLer-and Erikson: Z. physiol. Chem., 128, 1, 9 (1923). > Ber., 56, 1385 (19238); Z. Immunitdts., 34, 155 (1922), ALUMINUM, GALLIUM, INDIUM, AND THALLIUM 129 whereas most proteins are broken up. Thus egg albumin is separated into two components differing in optical rotatory power; chondrin is separated from chondroitin sulfuric acid which remains in solution, while the colloidal chrondrin residue is adsorbed reversibly. Koch’s tuberculin and Deny’s tuber- culin can be distinguished by their difference in adsorbability. Alumina is also recommended for the: purification of pepsin and of diphtheria antitoxin; therapeutically, it is suggested for use in intestinal infections. It cannot be emphasized too strongly that comparative data on adsorption by hydrous alumina or any other substance cannot be obtained unless particular attention is paid to the physical character of the adsorbent. To make the most rapid progress it would seem to be essential for biochemists to get together on some well-defined arbitrary methods of procedure for making a series of preparations that could serve as standards. Thesystems with which the biochemists deal are so complicated at best that there seems no justification for carrying out adsorption experi- ments with adsorbents that are not standardized in some way. The important réle which hydrous aluminum oxide plays in the soil and in such important technical processes as water puri- fication and dyeing will be considered in later chapters. Hyprovus GALLIUM OXIDE Hydrous gallium oxide requires a very slight hydroxyl ion concentration for its precipitation and is thrown down in a highly gelatinous form not only by both strong and weak alkalies but by salts of weak acids,” such as carbonate, bicarbonate, sulfide, sulfite,* etc. ‘Tartaric acid prevents the precipitation, presumably because complex tartrates are formed.* Unlike hydrous alumina, the gel is fairly soluble in an excess of strong ammonia, doubtless owing to the formation of a complex gal- lium-ammonium ion; like alumina, it is very soluble in alkalies apparently forming gallates.> From the alkali solution, the oxide 1EuLER and Nitsson: Z. physiol. Chem., 131, 107 (1923); 134, 22 (1924). 2 Lecog DE BoisBAUDRAN: Chem. News, 35, 148, 157, 167 (1877). 3 Dennis and BripGeMAN: J. Am. Chem. Soc., 40, 1531 (1918). 4 Lmcog DE BoIsBAUDRAN: Compt. rend., 98, 293, 329, 815 (1881). 5 Fricke and BLeNcKE: Z. anorg. Chem., 1438, 183 (1925). 130 THE HYDROUS OXIDES precipitates very slowly out of contact with air, but it is readily thrown down by carbon dioxide as a flocculent mass entirely different from the granular crystals of AleO3:3H2O which precip- itate from aluminate solution.! Owing to the highly gelatinous nature of the precipitate, one should expect sol formation to result from thorough washing of the gel. Moreover, it:is not unlikely that a small amount of gallic chloride or hydrochloric acid would peptize the gel, forming a positive sol; or a slight excess of alkali, a negative sol; but there is no record of such experiments having been performed. There is some evidence that in the presence of excess alkali a part, at least, of the hydrous oxide is in the sol form. Thus, if COse is conducted into an alkali solution newly formed in the cold, one obtains a very voluminous gel, quite different in appearance and properties from the flocculent precipitate thrown down from an old alkali solution. Moreover, the solubility of the hydrous oxide in KOH solution is appreciably less than in NaOH solution,' as would be expected from the higher precipitating power of K’ ion than of Na’ ion for negative sols. Hydrous gallium oxide, like hydrous alumina, ages fairly rapidly even at ordinary temperatures, as evidenced by a pro- gressive loss of adsorbed water and a decrease in the solubility in alkalies.! As already intimated, the ageing progresses more rapidly in the presence of alkali. No definite hydrate of Ga2O3 has been established, and the available evidence indicates the non-existence of such com- pounds. The hydrous oxide precipitated from ammonia contains less water than corresponds to the hydrate Gae2O3 - 3H2O when dried in a vacuum desiccator over H2SQO, or heated on the water bath. The water content of an oxide obtained from an old alkali solution and dried over H2SOQ, is greater than required for a trihydrate; but the more gelatinous precipitate from a newly formed alkali solution falls considerably below that for a tri- hydrate, even when dried in the air at ordinary temperatures. ! Apparently, the hydrous precipitate is GazO; with adsorbed water in amount depending on the conditions of formation and the method of drying. 1FRIcKE: Z. Elektrochem., 30, 393 (1924). ALUMINUM, GALLIUM, INDIUM, AND THALLIUM 181 Hyprovus INDIUM OXIDE Hydrous indium oxide in a highly gelatinous form is precipi- tated by adding alkali, ammonia, hydroxylamine,! or dimethyl- amine? to a solution of an indium salt. The oxide loses water continuously on heating, and there is no indication of the existence of a hydrate.* The last traces of the adsorbed water are not removed until a temperature of 650° is reached; but the oxide undergoes no appreciable decomposition below 850°.‘ Like hydrous gallia and alumina, the precipitate ages slowly at ordi- nary temperature but rapidly at the boiling point, particularly in the presence of alkali. The newly formed oxide dissolves in the cold in excess alkali but soon precipitates out in a much less reactive form.> This precipitation is almost quantitative in the hot. Unfortunately we do not know whether in the cold the oxide is dissolved by excess alkali forming an indate or whether it is peptized forming a negative sol, although the latter seems more likely. Nor do we know whether the precipitate which comes down on standing is an aged hydrous oxide as in the case of chromic oxide or a definite hydrate as in the case of alumina. These problems should be investigated. As ordinarily prepared, hydrous indium oxide is but slightly soluble in ammonia. Renz’ claimed at one time to have obtained an ammonia-soluble form of the gel; but later he was not sure about it. It is, of course, altogether possible that the hydrous oxide may be thrown down under special conditions in a more soluble or more readily peptizable form than that ordinarily obtained; and if so, there should be little difficulty in determining whether a sol is formed, as has been suspected.® Whatever may be the nature of the alkali solution of hydrous indium oxide, the gel is readily converted into a sol by thorough washing with distilled water. Even the denser gel thrown 1 Dennis and Guer: Ber., 37, 961 (1904). 2 Renz: Ber., 34, 2763 (1901); 36, 1847, 2751, 4394 (1904); 87, 2111 1904). 3 eee and WALKER: J. Chem. Soc., 58, 88 (1888). 4THiEL and Kornscu: Z. anorg. Chem., 66, 288 (1910). 5 Mryer: Liebig’s Ann. Chem., 150, 137 (1869). 6 RicHaRps and Borer: J. Am. Chem. Soc., 41, 133 (1919). 7 Ber., 36, 1848, 2754 (1903). 8TuieL: Z. anorg. Chem., 40, 322 (1904); cf., however, Ture, and Korxscu: [bid., 66, 300 (1910). 132 THE HYDROUS OXIDES down at 100° is peptized in this way. The colloidal solutions obtained by Thiel precipitated out in a few weeks’ time; but there is no doubt that very stable sols could be formed by supercentrif- ugal washing. To prevent sol formation during quantitative washing, it is only necessary to follow the time-honored practice of adding a little ammonium salt to the wash water. A stable colloid is easily obtained by passing air through a cold solution of indium monoiodide.t The reaction 2InI + xH2O + Oe = In20; - cH2O + 2HI goes slowly, practically all of the oxide remaining colloidally dissolved. If desired, there seems no reason why this sol should not be purified by dialysis. By carrying out the oxidation of indium monoiodide in the hot, 99 per cent of the indium is converted into hydrous oxide most of which precipitates out. Obviously, the oxide ages quite rapidly, or it would not precipitate from dilute acid solution in which the newly formed gel is very soluble. Indeed, the oxide thrown down in this way is almost insoluble in the cold in dilute acids and dissolves but slowly in concentrated ones, a behavior analogous to the ageing of the better-known hydrous oxides of aluminum and chromium. Anhydrous indium oxide likewise furnishes a good example of the influence of the physical character of an oxide on its chemical properties. Not only is an oxide heated to 850° acted on much more readily than one ignited at 1200°;? but a newly formed oxide decomposes into In304 and Oe» between 1200 and 1500° much more rapidly than a dense preparation aged by long igni- tion’ at a low red heat. Hyprovus THALLIC OXIDE The most hydrous form of thallic oxide is obtained by adding ammonia or alkali in slight excess to a thallic salt solution in the cold. The very insoluble* voluminous precipitate is reddish brown in color like hydrous ferric oxide; it adsorbs alkali strongly, and in consequence, ammonia is always used as the precipitant in the estimation of thallium as trioxide.® If the solution after 1 Turret and Koruscu: Z. anorg. Chem., 66, 300, 304 (1910). 2 Renz: Ber., 36, 1848 (1903); Ture, and Koruscu: Z. anorg. Chem., 66, 296 (1910). 3 THIEL: Z. anorg. Chem., 40, 322 (1904). 4 ApnaG and Spencer: Z. anorg. Chem., 44, 379 (1905). 6 Mpyer: Z. anorg. Chem., 24, 364 (1900). ALUMINUM, GALLIUM, INDIUM, AND THALLIUM 1338 precipitation is heated to boiling, the brown mass of hydrous oxide loses practically all its water becoming a dark granular powder; in this respect it behaves like blue hydrous cupric oxide. The oxide precipitated in the cold and dried in the air has a composition approaching Tl,O3;-H2O.! It has, therefore, been tacitly assumed that the red-brown slimy precipitate is a mono- hydrate. While this may be true, it is probably purely accidental that the composition at room temperature can be formulated Tl,03-H2O, particularly since it loses water continuously above room temperature, becoming almost anhydrous at 100°. By treating an alkaline solution of a thallous salt with hydrogen peroxide at room temperature,* one obtains a dark-brown floc- culent precipitate of hydrous oxide which changes slowly to small lustrous crystals of almost anhydrous Tl.O3. If the reac- tion is carried out at 80 to 100°, the oxide is a black sandy pow- der. ‘The density of the black oxide is 5.6 per cent higher than the brown; and the latter dissolves much more readily in acids and is more readily reduced to the thallous state by boiling water. It is probable that these difference in properties are due entirely to variations in the physical character of the mass, as determined by the conditions of formation, and not to allotropy as assumed by Rabe.* Indeed, by heating the brown oxide to 500° the pri- mary particles sinter together and assume permanently the prop- erties of black oxide. A crystalline hydrate of thallic oxide, ThO3:3H2O or TI1(OH)s;,° stable to a temperature of 340°, is said to be formed by prolonged fusion of Tl,O3; with KOH and subsequently treating the yellow mass with water. ‘These observations should be repeated, as the formula was derived from thallium analyses the accuracy of which is not known. Thallous oxide forms a definite crystalline hydrate, Tl,O -H2O or TIOH, soluble in water and possessing basic properties of the same order of magnitude as the caustic alkalies. 1 CARNELLEY and WALKER: J. Chem. Soc., 58, 88 (1888). 2 BIRNBAUM: Liebig’s Ann. Chem., 138, 133 (1866); WmrruEr: J. prakt. Chem., (1) 91, 385 (1864); cf., however, Lamy: Compt. rend., 54, 1255 (1862) ; 58, 442 (1863). 3 RaBe: Z. anorg. Chem., 48, 427 (1906); 50, 158 (1906). 4Z. anorg. Chem., 55, 130 (1907). 6 CARNEGIE; Chem, News, 60, 113 (1889). CHAPTER V THE HYDROUS OXIDES OF COPPER, COBALT, NICKEL, SILVER, AND GOLD | Although the compounds of cobalt and nickel are usually studied in connection with those of iron, it seems advisable to consider their hydrous oxides along with copper. ‘Thus, the most common oxide of iron is ferric oxide, whereas the most common oxides of nickel and cobalt are the ‘‘ous”’ oxides which are more nearly related to cupric oxide than to ferrous oxide. Moreover, the relationship between the blue and rose oxides of ‘cobalt is similar in certain respects to that between the blue and black oxides of copper. Hyprovus Cupric OXIDE The gelatinous mass obtained by the addition of dilute alkali to a cuprous salt is usually considered to have the composition Cu(OH.)z or CuO: H20. This is because the precipitate, washed rapidly until free from the mother liquor and dried over H2SQOu, has the composition corresponding to a monohydrate. Van Bemmelen! found the freshly precipitated blue substance to be highly hydrous, containing more than 20 mols of water to 1 of cupric oxide even after pressing between porous earthenware for 2 hours. When the precipitate is exposed at ordinary tempera- tures to an artificially dried atmosphere, it loses water con- tinuously until the vapor pressure is equal to that of the atmosphere. From +2H2O to +1H20O, the water is held more firmly than above +2H,0O, and at a pressure of zero, the oxide approaches the properties and composition of a crystalline hydrate, CuO-H.O. The ease with which water is eliminated decreases with the age of the sample; but even the freshly pre- pared oxide does not lose all its water at 100°. Neither a dihy- drate nor a trihydrate is formed,” and van Bemmelen considers 1Z. anorg. Chem., 5, 466 (1894). 2 Cf, Sprina: Z. anorg. Chem., 2, 195 (1892). 134 COPPER, COBALT, NICKEL, SILVER, AND GOLD 135 the evidence insufficient to establish the existence of a definite amorphous oxide of the composition CuO : H20; hence, the gelat- -inous body must be looked upon as hydrous ale oxide rather than hydrous hydrated cupric oxide. Besides the familiar blue gelatinous oxide, a crystalline com- pound can be obtained in a number of ways. Becquerel! pre- pared the latter by the action of dilute potassium hydroxide on a basic cupric nitrate, and Bottger,? by the action of concentrated sodium hydroxide on crystalline basic cupric sulfate. Péligot? hydrolyzed crystalline blue-violet copper ammonium nitrate; while Villiers* claimed that a crystalline hydrate was formed from the amorphous hydrous oxide by suspending the latter in water which was subsequently frozen and allowed to stand several hours. Villiers’ observation supports van Bemmelen’s view that the amorphous hydrous oxide goes over gradually to a crystalline monohydrate on standing. While the transformation of the oxide from the amorphous to the crystalline state on standing is an established fact, there is a difference in opinion as to whether the crystals are monohydrate. By electrolyzing a solution of alkali nitrate,» one obtains blue hydrous copper oxide, the physical character and hence the properties of which depend on the concentration of solution and the current density.°® Miller and Spitzer’ electrolyzed an alkaline copper ammonium salt solution with a platinum anode and obtained a black deposit containing 95 per cent CuO. In the latter case, dehydration took place at the same time as the precipitation. The blue oxide did not darken appreciably when suspended for an hour in alkali; but if a current was passed through the solution, the particles moved to the anode, where partial dehydration and darkening 1 Compt. rend., 34, 573 (1852); cf. VAN BEMMELEN: Z. anorg. Chem., 5, 468 (1894). 2 Jahresber., 198 (1858); HaBERMANN: Z. anorg. Chem., 50, 318 (1906). 3’Compt. rend., 53, 209 (1861); cf. BonsporFr: Z. anorg. Chem., 41, 132 (1904). 4 Compt. rend., 120, 322 (1895). 5 LorRENZ: Z. anorg. Chem., 12, 438 (1896); Exss: Z. angew. Chem., 17, 291 (1903). 6 KoHLSCHUTTER and TiiscHerR: Z. anorg. Chem., 111, 193 (1920); Kout- SCHUTTER and SEDELINOvVICH: Z. Elektrochem., 29, 30 (1923). 7 Kolloid-Z., 1, 44 (1906); Z. anorg. Chem., 50, 322 (1906). 136 THE HYDROUS OXIDES took place by electrical endosmose. Similar observations were made with the blue crystalline oxide but the dehydration was considerably slower. Miiller and Spitzer believe that definite hydrate water would not be removed by electrical endosmose and suggest that the chemical hydrate, so called, goes over to an unstable adsorption compound or, that an unstable peroxide results at first which later decomposes to the ordinary oxide containing less water. A more plausible guess is that the crystal- line compound is not a definite monohydrate at all but a hydrous oxide possessing a dense structure that retains adsorbed water more tenaciously than a gelatinous mass.! Since the vapor — pressure of hydrous copper oxide becomes practically zero at a composition closely approximating CuO-H.O, it cannot be determined by vapor-pressure measurements whether a definite hydrate exists. This could probably be decided by comparing x-radiograms of the black oxide and the blue crystalline com- | pound. Hedvall? made such a comparison of black oxides prepared in a variety of ways and found them to be identical. He also obtained an x-radiogram of the blue compound, but no comparison of the latter with the black oxide was recorded. Bancroft® points out that, if a definite compound, CuO: H.O, exists with a practically zero vapor pressure, it should form from cupric oxide in the presence of water; but the reverse process is the one that actually takes place. Kohlschiitter and Tiischer+ get around this by assuming that the dehydration is not simply a molecular splitting off of water, thus: Cu(OH)e = CuO + H.O; but depends on intramolecular neutralization of the H° and OH’ ions resulting from amphoteric dissociation of Cu(OH). as follows: Cu(OH)e = Cu” + 20H’ Cu(OH)e2 = CuOe” + 2H” OH) Be eG CuO.” + Cu = 2Cu0 This interpretation of the mechanism of the dehydration process is superfluous, if the blue compound having zero vapor pressure 1 KoHLSCHUTTER and SEDELINOvICH: Z. Elektrochem., 29, 30 (1923). 2Z. anorg. Chem., 120, 327 (1922). 3 “¢ Applied Colloid Chemistry,’’ 246 (1921). 4Z. anorg. Chem., 111, 193 (1920). COPPER, COBALT, NICKEL, SILVER, AND GOLD 137 is not Cu(OH)s, and may be open to serious question even should a monohydrate exist. Stability of Blue Cupric Oxide.—The instability of gelatinous cupric oxide is one of its most characteristic properties. If allowed to stand in contact with its mother liquor, it loses water and changes in color from blue to green, brown, and finally black,! the process going on slowly at room temperature and with increasing velocity as the temperature is raised.” To obtain a clear blue product, precipitation should be carried out at 0° and the mother liquor removed by washing with iced water as rapidly as possible. Analogous to the hydrous oxides of iron and chromium, the blue oxide aged at low temperatures holds on to its water more strongly than the newly formed product. Unlike the gelatinous oxide, the blue crystalline compound is stable in boiling water and maintains its blue color when heated for hours at 100°. However, it darkens gradually on long stand- ing even at room temperature. Thus, 10-year-old samples made by the methods of Béttger and Péligot were reported by Fowles* to have a bluish-slate tint, while a 12-year-old sample was quite black. In the light of these observations Fowles concludes that the crystalline oxide is a highly stable form of the blue oxide in a state of suspended transformation. While alkalies and certain salts tend to decrease the stability of the blue gelatinous oxide, Tommasi® found a retardation of the blackening in the presence of a number of salts, notably MnSO,. Bancroft® attributed the stability to adsorption of the hydrous oxide of manganese which acts as a protective col- loid. Although this conclusion was reaffirmed by Blucher and Farnau’ as a result of observations with salts of a number of heavy metals other than manganese, it seems questionable, since (1) relatively high concentrations of colloidal hydrous oxides are 1 ScHaFFNER: Liebig’s Ann. Chem., 61, 168 (1844); Harms: Arch. Pharm., (2) 89, 35 (1857); Bull. soc. chim., (2) 87, 197 (1882). 2Sprine and Lucion: Z. anorg. Chem., 2, 195 (1892); vaN BEMMELEN: Ibid., 5, 468 (1894); Euter and EvieEr: Z. anorg. Chem., 124, 70 (1922). 3 VituiERS: Compt. rend., 120, 322 (1895). 4 Chem. News, 128, 2 (1924). 5 Bull. soc. chim., (2) 37, 197 (1882); Compt. rend., 99, 38 (1884). 6 J. Phys. Chem., 18, 118 (1914). 7J. Phys. Chem., 18, 629 (1914). 138 THE HYDROUS OXIDES not effective, and (2) copper sulfate is as effective as manganous sulfate or chromic sulfate. The first experiment is not especially impressive, since there is no necessary reason why the adsorp- tion of a colloidal oxide by the separately precipitated copper oxide gel should give the same result as adsorption of the oxide from a salt solution; but the second observation is fairly con- clusive, since it is inconceivable that blue hydrous cupric oxide should stabilize itself. Inasmuch as the only salts effective in low concentration are those which give an acid reaction by hydrolysis there still remains the possibility that the stabilizing agent is a basic salt? or an adsorption complex. Since there is a marked change in the physical character of hydrous cupric oxide during heating in the presence of salts which hydrolyze to give an acid reaction, I postulated! a slight solvent action which was supposed to destroy the gelatinous structure, giving a denser modification that loses water and darkens less readily than a loose voluminous mass. Fowles? accepts the essential part of this view, that the stability is a result of change in ‘the physical character of the hydrous oxide; but he very properly rules out any solvent action as an important factor in the process. Instead, he believes the heavy metal salts remove adsorbed alkali‘ as basic salts; thus, the stabilization consists in removing alkali, a catalyzer, and allowing the unstable gelatinous substance to pass to the more stable crystalline form. Fowles’ hypothesis is not particularly helpful or constructive, since it does not attempt to define the nature of the alleged catalytic action of alkali on the dehydration process. vs Like bases, certain alkali salts increase rather than retard the spontaneous dehydration of hydrous cupric oxide. Since the effect is not appreciable except with relatively high concentra- tions of the salts, it probably results from their dehydrating action.» An alternative hypothesis is suggested by the behavior of hydrous cupric oxide toward alkali salts. Tommasi® found 1 Weiser: J. Phys. Chem., 27, 501 (1923). 2 Krier: J. prakt. Chem., 108, 278 (1924). 3 Chem. News, 128, 5 (1924). 4 Jorpis: Z. Hlektrochem., 18, 553 (1912). 5 Poma and Patront: Z. physik. Chem., 87, 196 (1914). 6 Bull. soc. chim., (2) 37, 197 (1882). a COPPER, COBALT, NICKEL, SILVER, AND GOLD 139 that solutions of sodium chloride and sodium sulfate show an alkaline reaction after shaking with hydrous cupric oxide. This is due to hydrolysis of the salts owing to stronger adsorption of acid than of base. The slight decrease in stability in the pres- ence of alkali salts may be due to the slight alkalinity of the solution in which the particles are suspended. Recently, hydrogen peroxide has been found by Quartaroli! to accelerate the darkening of hydrous cupric oxide suspended in a definite amount of alkali at 50°. This action is still per- ceptible with 1 part of peroxide in 200 million of water. In view of the presence of minute traces of hydrogen peroxide in ordinary distilled water, this compound is believed to bring about the spontaneous decomposition of the hydrous oxide or, at least, to accelerate the process. Such sensitive action of extremely minute amounts of substance has been found only in the action of copper in provoking the oxidation of sulfites and in the quantity of substance required to break up metastable states. Various electrolytes, especially magnesium salts, retard the blackening when present in amounts hundreds of times less than that of the hydrous oxide; but the action of such electrolytes exhibits striking irregularities. Quartaroli concludes that the blackening of the oxide suspended in alkali solution is not a simple dehydration process but is a phenomenon connected with oxidations and reductions with the formation of saline hydrates containing copper atoms with various grades of oxidation. This conclusion is so hopelessly vague and indefinite that the obser- vations should be confirmed and extended. By dehydrating the blue oxides under suitable conditions, compositions approximating the formulas for hydrates have been obtained.2 Recently, Losana* obtained temperature-composi- tion, vapor-pressure, and electromotive-force curves indicating the formation of hydrates in which the ratios CuO: H:20 are 3, 4, and 8:1 when the dehydration takes place in the presence of liquid and 3, 4, 6, 7, and 8:1 when the compound has been dried before dehydrating. The presence of alkali and other 1 Gazz. chim. ital., 55, 264 (1925). 2See Mertuor: ‘Treatise on Inorganic and Theoretical Chemistry,’’ 3, 142 (1923). 3 Gazz. chim. ital., 58, 75 (1923). 140 THE HYDROUS OXIDES salts influences the dehydration temperature, and in some instances, loss of water occurs below what was regarded as the true inversion point. Such behavior is not characteristic of well-defined stable hydrates. Color.—As already noted, hydrous copper oxide may be blue, green, olive, brown, or black in color. The change in color is not necessarily associated with loss of water, as De Forcrand! observed a change in Peligot’s oxide at 85° from blue to green without loss of weight. De Forcrand dissolved the blue, green, olive, and brown hydrous oxides and the black anhydrous oxide in sufficient nitric acid to form the nitrate; and Sabatier? and Joannis® carried out similar experiments on the oxides dehydrated at 440° and atred heat. From thesedata De Forcrand concludes that the different colored oxides are isomers involving definite heat changes in the transformation from one form to another. It would seem, however, that thermochemical evidence of this sort is altogether insufficient to establish the existence of definite isomers. Many hydrous oxides, when heated quickly, undergo a decrease in surface energy which is sufficient to raise the tem- perature of the whole mass to incandescence. Thermochemical data obtained before and after glowing might be interpreted to mean that an isomeric transformation had taken place; but such a conclusion would be erroneous. It is much more probable that the differences in color result from differences in the physical character and size of particles. Kohlschiitter and Tischer believe the blue compound to be amorphous or pseudocrystalline Cu(OH)., consisting of rather large particles, and the black compound to be CuO made up of distinctly smaller particles. The conclusion that the blue particles are larger than the black seems to have been reached without taking all the facts into consideration. Everyone knows that copper oxide can be obtained in quite large particles which are black and the hydrous oxide in very much more highly dispersed, gelatinous particles which are blue. A clump of the blue hydrous oxide consists of very finely divided primary particles that have adsorbed water strongly, forming a gelatinous mass. On heating, the relatively 1 Compt. rend., 157, 441 (1913). 2 Compt. rend., 125, 301 (1897). 3 Compt. rend., 102, 1161 (1886). COPPER, COBALT, NICKEL, SILVER, AND GOLD 141 large gelatinous clump is broken up and the particles constituting it coalesce to larger particles that appear black. From this point of view, anhydrous cupric oxide would be blue and not black if coalescence during dehydration were prevented. In support of this, Schenck,! in Bancroft’s laboratory, observed that a mixture of the hydrous oxides of copper and aluminum containing 5 per cent cupric oxide remained blue after ignition. In one instance” the excess of alumina was dissolved out with alkali, giving a distinctly blue powder containing CuO and Al,O3 in the ratio of approximately 4:1. The hypothesis of Kohlschiitter that the blue and black oxides are Cu(OH)2. and CuO, respectively, stands or falls on the unproved assumption that there is a definite amorphous hydrate and that this dehydrates not by the molecular splitting off of water but by “internal neutralization as a result of amphoteric dissociation.” The wide variation in color of anhydrous CuO is utilized in coloring glass and pottery glazes. That cupric oxide imparts a -blue or green color to glass under certain conditions was known to the ancients and the later alchemists. An old blue Venetian glass contained 1.382 per cent CuO.* Artificial emeralds have been prepared with this pigment. ‘The color imparted to a glaze by CuO depends on the constituents of the glaze and the condi- tions of firing. Ina reducing atmosphere, a red glaze is obtained consisting probably of colloidal copper or cuprous oxide; blue and green glazes develop in an oxidizing atmosphere.* Cupric Oxide Sols.—To prepare sols of hydrous cupric oxide, it is usually necessary to employ some protective agent. Gra- ham? prepared a fairly stable sol by adding alkali to a solution of cupric chloride containing cane sugar. It was deep blue at first but changed to green on dialyzing; the precipitate obtained on boiling the sol or on adding salts or acids was bluish green in color. A more stable sol can be prepared using Paal’s® sodium 1 J. Phys. Chem., 23, 283 (1919). 2 PARSELL: J. Phys. Chem., 26, 501 (1922). 3 Scuwarz: Dinglers polytech. J., 205, 425 (1872). 4RavuTER: Z. angew. Chem., 14, 753 (1901). 5 Phil. Trans., 161, 183 (1861); Compt. rend., 59, 174 (1864). 6 Ber., 39, 1550 (1906); Kolloid-Z., 30, 1 (1922). 142 THE HYDROUS OXIDES salts of lysalbinic or protalbinic acids as protective colloid. Other stabilizing agents that have proved effective are agar,! casein,’ milk and grape sugar,* and soap. Thorium and uranyl nitrate* peptize the gelatinous oxide owing to strong adsorption of the salt cations. Biltz> attempted to prepare the colloidal oxide by dialysis of a solution of cupric nitrate; but the salt passed unchanged through the dialyzer owing to its low hydrolysis constant. Ley® hydrolyzed the copper salt of succinimide, obtaining a very satisfactory sol that changed in color, slowly at room temperature but rapidly at 70°, from blue green to yellow brown and finally dark brown. Succinimide is a protector, since its removal by dialysis causes agglomeration of the sol. ! Cupric oxide sols have been prepared without the use of a protective colloid by oxidizing a copper sol’ and by what Kohl- schiitter called ‘discharge electrolysis.”’® In the latter process, a passivifying layer of oxide was deposited on the anode and sub- sequently dispersed in the liquid by a rapidly oscillating discharge. The sol was perfectly clear in transmitted light; it was bluish: green in color at the outset but changed to brown on standing. By means of the ultramicroscope, Kohlschiitter® observed the formation of blue hydrous cupric oxide sol when a current of relatively high density was passed between copper electrodes dipped in dilute CuSO, solution or in water. By passing a spark between copper wires under water, there is formed a positive sol of cupric oxide instead of copper.’® Stirring accelerates the velocity of coagulation of this sol by electrolytes, particularly when ions of high coagulating power are employed and when the concentration of electrolyte is in the region of the 1Lupwic: Brandes Archiv, 82, 157 (1855). 2 RITTENHAUSEN: J. prakt. Chem., [2] 5, 215 (1873); 7, 361 (1874). 3 Saukowsky: Pfliiger’s Arch., 6, 221 (1872); Sen and Duar: Kolloid-Z., 38, 193 (1923). 4 SztLarD: J. chim. phys., 5, 636 (1907). 5 Ber., 35, 4431 (1902). 6 Ber., 38, 2199 (1906); Ley and WernzER: Jbid., 39, 2178 (1906). 7 LoTTERMOSER: “ Anorganische Kolloide,’”’ Stuttgart (1901). 8 KOHLSCHUTTER: Z. Elektrochem., 25, 309 (1919). 9Z. Elektrochem., 30, 164 (1924). 10 Burton: Phil. Mag., [6] 11, 436 (1906). COPPER, COBALT, NICKEL, SILVER, AND GOLD 148 precipitation value. Prolonged stirring alone, without the addi- tion of precipitating electrolyte, lowers the charge on the par- ticles sufficiently to allow agglomeration and finally complete precipitation. ! Hooker prepared a very satisfactory blue sol? by thorough washing of the gelatinous oxide formed by adding alkali to copper sulfate solution until the supernatant liquid was just colorless. The washing may be carried out by sedimentation, but a more stable sol results by repeated washing with the supercentrifuge.* The colloidal oxide has high fungicidal action against apple scab and apple black in concentrations of 1 part of hydrous oxides to 5000 of water; at this concentration it causes very slight burning. This sol possesses excellent sticking properties due to its positive charge and can be used in conjunction with lead arsenate and nicotine sulfate, if desired. Since the sol can be prepared at relatively low cost, it is suggested as a substitute for Bordeaux mixture and lime sulfur. Hydrous cupric oxide dissolves but slightly in dilute alkali, but is appreciably soluble in concentrated alkali forming deep-blue solutions.4 As might be expected, the unstable blue gelatinous oxide is more soluble than the black compound, and the latter separates gradually from a solution of the former, provided the alkali (NaOH) concentration does not exceed 17 NV. Both cop- per, in the presence of air, and anhydrous CuO dissolve in alka- lies, forming blue solutions which are stable on boiling and do not precipitate out on standing.® The bulk of the evidence supports the view that the blue coloration is due to CuO,’”” ion and not to colloidal cupric oxide. Fischer® added alkali to copper salts short of precipitation and obtained blue solutions which were supposed to be colloidal, because hydrous copper oxide settled 1 FREUNDLICH and Basu: Z. physik. Chem., 115, 203 (1925). 2J. Ind. Eng. Chem., 15, 1177 (1923). 3 BRADFIELD: J. Am. Chem. Soc., 44, 965 (1922). 4 Low: Z. anal. Chem., 9, 463 (1870); Donatu: [bid., 40, 137 (1901). 5 MULurR: Z. physik. Chem., 105, 73 (1923). 6 CREIGHTON: J. Am. Chem. Soc., 45, 1237 (1923). 7MeELBye: Meddel. Vetenskapsakad. Nobelinst., 4, 8 (1922); Chem. Abstracts, 17, 1572 (1923). 8 Z. anorg. Chem., 40, 39 (1904); CHATTERJI and Duar: Chem. News, 121, 253 (1924). 144 THE HYDROUS OXIDES out on standing or could be filtered out. The discoloration of the solution by filtering was believed by Creighton to result from interaction between the cellulose of the filter and the blue com- ponent, since the filtrate was blue after several portions were passed through the same filter. This observation is not quite conclusive, since the filter may have become more porous owing to peptization of the filter paper by the alkali. However, Miller prepared a cobalt-blue crystalline cuprite from the alkali solution; so there is no doubt of the existence of such a salt. It is altogether probable that sol formation precedes cuprite forma- tion when alkali acts on the gelatinous oxide. The solubility of hydrous cupric oxide in alkali may be increased enormously by the presence of other substances. Thus, the addition of alkali to copper sulfate in the presence of tartrate forms the deep-blue solution known as Fehling’s solution, so widely used in detecting small amounts of reducing sugar. ‘The copper in this solution is not present as cupric oxide sol but as a cupric tartrate complex. The same is apparently true for the alkali solution formed in the presence of higher valent alcohols, such as glycerin and mannite, and of certain amines.” Gelatinous copper oxide is adsorbed strongly by hydrous chromic oxide, and the colloidal solution of the latter in alkali carries a consider- able amount of the former into colloidal solution.* Cupric Oxide Jellies.—A cupric oxide jelly forms on hydrolysis of a solution of cupric ammonium acetate of the formula Cu- (CeH302)2-2NH3.4 It is unnecessary to start with this salt, and much more stable jellies are obtained by precipitation of a suitable amount of colloidal oxide at a suitable rate. The desired conditions may be realized® by adding ammonia to cupric acetate in the presence of a small amount of sulfate® and allowing the instable colloidal solution to precipitate spontaneously. 1 Kuster: Z. Elektrochem., 4, 112 (1897); Masson and STEELE: J. Chem. Soc., 75, 725 (1899); KAHLENBERG: Z. physik. Chem., 17, 577 (1895). 2 TRAUBE: Ber., 55, 1899 (1922); 56, 1653 (1923); Donnan: Abegg’s ‘‘Handbuch anorg. Chem.,’’ 2, 547 (1908). 3Cf. Knecut, Rawson, and Léwrentua.: ‘A Manual of Dyeing,” 1, 241 (1916); Prup’HomME: Bull. soc. chim., (2) 17, 253 (1872). 4 ForrstTEer: Ber., 25, 3416 (1892); Frncu: J. Phys. Chem., 18, 26 (1914). 5 WEISER: J. Phys. Chem., 27, 685 (1923). 6 Fincu: J. Phys. Chem., 18, 26 (1914). COPPER, COBALT, NICKEL, SILVER, AND GOLD 145 The solution obtained is perfectly clear at the outset, but pre- cipitation starts after intervals varying from a few seconds to several minutes depending on the relative amounts of the three components. In view of the great importance of rate of pre- cipitation on jelly formation, the most favorable conditions are pretty sharply defined. A firm jelly that remained unbroken for weeks is obtained by mixing 5 cubic centimeters of 3 N NH,OH to 25 cubic centimeters of 0.75 N Cu(C2H302)2 con- taining 2 cubic centimeters of N K,SO,. The presence of sulfate is necessary in order to get a sol of sufficient concentration. Gelatinous precipitates instead of jellies are obtained by adding ammonia directly to copper sulfate, chloride, or nitrate, on account of the high velocity of precipitation. Hyprovus Cuprous OXIDE The yellow or orange precipitate thrown down from a cuprous salt solution by sodium hydroxide! or carbonate? is not CuOH but hydrous cuprous oxide in an amorphous state.’ The yellow compound is best prepared by reduction of Cu’’ in the presence of OH’ or by electrolysis of alkali salts in the cold with a copper anode.* For the chemical reduction, a variety of reducing agents may be used, such as dextrose,° maltose,* and phenylhydrazine ;’ hydroxylamine hydrochloride® is particularly satisfactory. The yellow oxide is formed in the cold by the reduction of Fehling’s solution with dextrose or in the hot when the amount of tartrate is too small to convert all the Cu’ into a complex. The clear- yellow amorphous product goes over rapidly to orange or reddish yellow, probably with the loss of water. By drying in the absence of air, a stable product is obtained. This is changed 1 MirTscHERLICH: J. prakt. Chem., 19, 450 (1840); Proust: J. phys., 61, 182 (1800); Kuason: Svensk Kem. Tid., 36, 202 (1924). 2FremMy: Ann. chim. phys., [3] 28, 391 (1848). j 3 GROGER: Z. anorg. Chem., 81, 326 (1902); Mosmr: /bid., 105, 112 (1919). 4 LORENZ: Z. anorg. Chem., 12, 488 (1898). 5 SANDMEYER: Ber., 20, 1494 (1887); Miniter and Haaren: Pfltiger’s Arch., 28, 221 (1880). 6 GLENDENNING: J. Chem. Soc., 67, 999 (1895). 7 CHATTAWAY: Chem. News, 97, 19 (1908). 8 Moser: Z. anorg. Chem., 105, 112 (1919); cf. Epuer: Ber., 35, 3055 (1902). 146 THE HYDROUS OXIDES to a brick-red amorphous powder by heating for 60 hours at 500°; and by igniting in a stream of nitrogen, it goes over to the familiar crystalline red form which is commonly obtained by reducing Fehling’s solution at 100°. Similarly, by electrolyzing alkali salts, the oxide is yellow at room temperature, bright red at 100°, and intermediate colors in between.! Red cuprous oxide is prepared on a commercial scale by electrolysis of a hot solution of sodium chloride with copper anodes. Red crystalline Cuz,O appears to bear the same relation to the yellow hydrous oxide that black CuO bears to the blue gelatinous oxide. The change in the color of hydrous cuprous oxide from yellow, through orange, brick red, and bright red results from a gradual increase in particle size and loss of absorbed water. With this change from the finely divided hydrous precipitate having a high specific surface to the granular red product having a low specific surface, the reactivity with oxygen and the solu- bility in acids and alkalies fall off. The varicolored products are not definite isomeric modifications of Cu20O. The colorless solution of hydrous cuprous oxide in ammonia turns blue in the air owing to the formation of Cu(NHs).’’, thereby furnishing a delicate test for oxygen. Red cuprous oxide is largely used in coloring glass red, a property known to the ancients and in the middle ages. The manufacture of this ancient red glass was revived by Bontemps in France and Englehardt in Germany about 1827. The oxide is also used for the production of a red glaze on pottery? and as a poisonous pigment in antifouling compositions for painting the bottoms of vessels. Cuprous Oxide Sols.—A sol of hydrous cuprous oxide is nearly always obtained in the reduction of alkali solutions of copper salts. * Similarly, during the reduction of cupric sulfate solution with. SnCle* or hydrazine, yellow hydrous® cuprous oxide first appears, which goes over to the red oxide and finally to black 1 Mitumr: J. Phys. Chem., 18, 256 (1909); Moser: Z. anorg. Chem., 105, 112 (1919). : *Louts and Dutatuity: Mon. Ceram. et Verr., 19, 237. 3 Cf. Ruoss: Z. anal. Chem., 68, 193 (1919). 4 LOTTERMOSER: J. prakt. Chem., (2) 59, 492 (1899). 5GuTBIER and HormeierR: Z. anorg. Chem., 32, 355 (1902); 44, 227 (1905); cf. PAau and Lruze: Ber., 39, 1550 (1906). COPPER, COBALT, NICKEL, SILVER, AND GOLD 147 copper sol. Reduction of a neutral solution of copper sulfate in the presence of gum arabic or gelatin! gives a cuprous oxide sol which is stable in the absence of air. Grdéger? obtained a stable sol on attempting to purify the orange hydrous oxide by pro- longed washing in the absence of air. Hyprovus CoBALTOUS OXIDE If alkali is added in excess to a solution of rose-colored cobalt- ous salt, there is formed, at first, a blue hydrous mass which goes over into rose hydrous cobalt oxide or hydroxide, the transforma- tion taking place more rapidly the higher the temperature.? The gradual transformation can be observed by boiling cobaltous carbonate with a solution of potassium hydroxide. The volu- minous blue oxide formed at first turns to violet and then to rose red.* On the other hand, if insufficient akali to react with all the cobalt ion is used, the precipitate retains its blue color indefinitely. At one time, the blue compound was believed to be a basic salt which was decomposed by excess alkali, forming rose-colored hydrate. Hantzsch* disproved this view by showing that the blue oxide, precipitated from sulfate or acetate solution with insufficient alkali, absorbs the respective salts or basic salts strongly; but most of the latter can be removed by repeated boiling of the precipitate with water free from air, without alter- ing the blue color in any way. Hence, the blue color is that of the oxide and not of a basic salt. The blue and rose precipitates differ quite appreciably in their chemical properties. Thus, the blue compound loses practically all its water at 150° and is completely dehydrated at 170°; whereas the rose compound still retains some water after heating for several hours at 300°. Moreover, the blue oxide reacts slowly with acetyl chloride, while the red reacts so rapidly that the chloride boils. After drying the preparations in a desiccator, each analyzes experimentally for a monohydrate or hydroxide. 1 Lopry Dr Bruyn: Rec. trav. chim., 19, 236 (1900). 2Z. anorg. Chem., 31, 326 (1902). 3 WINKELBLECH: Liebig’s Ann. Chem., 18, 148 (1835); Brrz: Pogg. Ann., 61, 472 (1844). 4Fremy: Jahresber., 637 (1851). 5Z. anorg. Chem., 78, 304 (1911). 148 THE HYDROUS OXIDES Hantzsch concludes, therefore, that the two compounds are hydrate isomers differing in the way in which the water is held, the blue being CoO: H.20 and the red Co __ ae As has been pointed out, the gelatinous oxide does not go over to rose in the absence of alkali or in the presence of a little cobalt salt. Benedict! observed that the change in color from blue to rose in the presence of excess alkali is retarded by the addition of a small amount of nickel salt. The retardation is sufficiently marked to serve as a delicate test for nickel. To account for this behavior, Benedict postulates the formation of a deep-blue nickel cobaltite which masks the rose-colored hydrate. This assumption cannot be correct, since increasing the amount of nickel salt does not increase the intensity of the blue color. Apparently, hydrous nickel oxide is adsorbed by the blue oxide, thereby stabilizing it to a certain extent. The phenomenon recalls the stabilization of blue hydrous cupric oxide by salts, but it differs from the latter in that salts of metals other than nickel have little or no effect. Thus the presence of the sulfates of iron(ous), zinc, manganese, magnesium, chromium, copper, and aluminum; the chlorides:of tin and calcium; and the nitrates of lead, cadmium, thorium, and strontium produces no marked retardation.” The specific effect of nickel oxide may be due to its having the same crystal lattice as cobalt oxide. Both the blue and rose compounds thrown down by mixing solutions of cobalt salt and alkali are gelatinous and appear amorphous. It is possible that both are hydrous oxides when first formed but, like cadmium oxide, go over into microcrystal- line hydrates on standing. This is certainly true of the rose compound, as shown by x-ray examination.* Large crystals of rose hydroxide come down from a solution made by adding 250 grams of potassium hydroxide to 10 grams of cobalt chloride hexahydrate in 60 cubic centimeters of water, and heating in the absence of air until solution is complete. The crystals are elongated rhombic prisms that analyze for Co(OH):.4 They are 1 J. Am. Chem. Soc., 26, 695 (1904). 2 CHATTERJI and Duar: Chem. News, 121, 253 (1920). 3 HEDVALL: Z. anorg. Chem., 120, 327, 338 (1924). 4 Dr SCHULTEN: Compt. rend., 109, 266 (1889). COPPER, COBALT, NICKEL, SILVER, AND GOLD 149 pleochroic, appearing rose colored along n,, rose yellow along Nm, and pale brownish yellow along n,. Unlike the gelatinous oxide, the large crystals are not altered by contact with air. Since a crystalline rose-colored oxide is known with certainty, and there is only an analysis of an apparently amorphous mass to indicate the nature of the blue preparation, it might be concluded that the blue compound is a hydrous. oxide and the rose, a hydroxide. ‘This hypothesis, like that of Hantzsch’s, cannot be correct, since it is based on the manner in which water is held by the oxide, and apparently we may have either a blue or a rose oxide in the absence of water. Thus cobalt glass owes its color to the blue anhydrous oxide; and the brown anhydrous powder obtained by drying the precipitated hydrate melts without decomposition and gives rose-colored crystals on cooling.t This suggests the possibility of the color of cobalt oxide and hydroxide being determined by the size of particles. In glass the particles are obviously highly dispersed and appear blue, while the oxide in mass is red. Similarly, the precipitated oxide is most finely divided when first formed and so is blue; but in the presence of a slight excess of alkali, the highly hydrous mass ages, losing water and becoming denser, the color at the same time changing from blue through lavender to rose. The rate of this transformation is, of course, hastened by raising the temperature, and is retarded or stopped by the presence of basic cobalt salts or hydrous nickel oxide. If one objects to attributing the difference in color to the size of particles, an alternative hypothesis is that there are two allo- tropic forms of cobalt oxide, an instable blue oné and a stable rose. As a matter of fact, the transformation of the blue gelati- nous oxide to rose in the presence of alkali has led people to regard the former as the alkali instable modification and the latter as the alkali stable modification.? The existence or non-existence of allotropic forms could probably be settled by comparing x-radiograms of the blue and rose oxides or hydrates.* 1 Morssan: Ann. chim. phys., (7), 4, 186 (1895); Hepvauu: Z. anorg. Chem., 86, 210 (1914); Hantzscu: Z. anorg. Chem., 73, 304 (1912). 2 HantzscH: Z. anorg. Chem., 73, 304 (1912); Farnav and WITTEVEEN: J. Ind. Eng. Chem., 18, 1060 (1921). 3 Cf. HEDVALL: Z. anorg. Chem., 120, 338 (1922). 150 THE HYDROUS OXIDES Both blue and rose cobalt oxide dissolve in concentrated alkali, giving a solution with a deep-blue color. A similar color results on electrolyzing a solution of alkali, 4 N or stronger, with a cobalt anode. The blue solution was thought by Tubandt! to be colloidal cobalt oxide; but the results of exact potential measure- ments of cobalt against the blue solutions containing different amounts of cobalt in 8 N potassium hydroxide show conclusively that the blue color is due to potassium cobaltite, KeCoOzs, and not to colloidal cobalt oxide.2 Thus the behavior of cobalt oxide in excess alkali is similar to that of cupric oxide. In the light of these observations, it is unlikely that the alkaline solution in the presence of glycerin is colloidal.* Positive sols have been formed both by peptization of the blue oxide with dilute hydro- chloric acid? and by thorough washing of the fresh gelatinous precipitate;> but they are quite instable, settling out in the course of a few hours. Liesegang rings of Co(OH)> are formed by pouring ammonia on a gelatin gel containing cobalt chloride. Under certain condi- tions a spiral is obtained instead of a series of rhythmic bands.°® Cobaltous oxide is used by enamelers and porcelain manu- facturers for the production of the finest blue glaze and color in porcelain, glass, and other vitrifiable substances; 1 part of oxide in 100,000 imparts a faint blue while 1 part in 1000 gives a deep blue. When heated with certain oxides, colored compounds are formed, and with others, colored solid solutions which are widely used as pigments. With aluminum oxide, a blue com- pound CoO-Al.03 known as cobalt blue or Thenard’s blue is formed at 1100°, and above this temperature a green one, said to have the composition 4CoO:3AI,03.7 The exact tint of cobalt blue depends on the conditions of formation and on the relative amounts of the two oxides fused. A similar valuable 1Z. anorg. Chem., 45, 368 (1905); cf. Donatu: Monatschefte fiir Chemie, 14, 93 (1893). 2 GRuBE and Frucnut: Z. Elektrochem., 28, 568 (1922). 3 Cf., however, SEN and Duar: Kolloid-Z., 33, 193 (1923). 4 MUuuer: Z. anorg. Chem., 57, 311 (1908). 5 Tower and Cooke: J. Phys. Chem., 26, 733 (1922). 6 WoLFGANG OsTWALD: Kolloid-Z. (Zsigmondy Festschrift), 36, 390 (1925). 7 HEeDvALL: Archiv Kemi, Mineral. Geol., 5, 18, 1 (1914); Z. anorg. Chem., 96, 71 (1916). COPPER, COBALT, NICKEL, SILVER, AND GOLD 151 pigment known as cobalt green or Renneman’s green is obtained by fusing cobalt oxide with zinc oxide. The green color is due to cobalt zincate which forms solid solutions with excess zinc oxide.t Stannic oxide likewise forms a green stannate,? and chromic oxide, a green chromite*® on fusion with cobalt oxide. The chromite dissolves in excess of either oxide, giving various shades of blue. Many combinations with other oxides have been reported, but in the majority of cases these are either mixtures or solid solutions. Magnesium oxide forms mixed crystals varying in color from light to dark red, depending on the relative proportions of the two oxides. Mixed erystals are also formed with the isomorphous oxides of nickel and manga- nese.® It is probable that cobalt oxide is dissolved or dispersed by silica although violet cobalt orthosilicate and ruby-red meta- silicate have been reported.’ Obviously, the deep-blue color of cobalt glass is not due to the formation of these alleged compounds. Cobalt oxide proves to be a very good ‘‘dryer”’ for paints.*® Since the action of dryers is to catalyze the oxidation of the oil, ‘this behavior is in line with the observations that the spontaneous oxidation of cobaltous hydrate induces the oxidation of the stable nickelous hydrate. ® Hyprovus CoBALTIC OXIDE Cobaltice oxide, Co20s;, in a highly hydrous state is precipitated on treating a solution of a cobaltous salt with alkaline hypo- chlorite!® or persulfate;!! or by electrolysis of an alkaline solution 1 HEDVALL: Z. anorg. Chem., 93, 313 (1915); 96, 71 (1916); cf., however, [bid., 86, 201 (1914). 2 HEDVALL: Archiv Kemi, Mineral. Geol., 5, 18, 1 (1914). 3 BuuioT: ‘On the Magnetic Combinations,’ Gottingen, 33 (1862). 4Farnavu and WITTEVEEN: J. Ind. Eng. Chem., 13, 1061 (1921). 5 HEDVALL: Z. anorg. Chem., 86, 296 (1914). 6 HepvALt: [bid., 92, 381 (1915). 7RtaerR: Keram. Rundschau, 31, 79, 87, 99, 110 (1923); C. A., 18, 156 (1924). 8 GARDNER and Parks: Paint Mfrs.’ Assoc. U. S., Circ. 186 (1923). 9 Mirrra and Duar: Z. anorg. Chem., 122, 146 (1922). 10 Carnot: Compt. rend., 108, 610 (1889); ScuR6pER: J. Chem. Soc., 58, 1213 (1890). 11 Mawrow: Z. anorg. Chem., 24, 263 (1900); Htrrner: Jbid., 27, 81 (1901). 152 THE HYDROUS OXIDES of cobalt sulfate.! It forms a brownish-black mass that loses water readily, the composition depending on the method of drying. The dark-brown anhydrous powder is transformed into black cobaltous cobaltic oxide, Co304, corresponding to magnetic oxide of iron, by heating below 910°.? Hydrous Co30, results when cobaltous hydroxide oxidizes in the air. A fairly pure preparation is obtained by warming cobaltous hydroxide with an excess of ammonium persulfate, washing and heating the product with dilute nitric acid. The substance obtained by fusing an oxide of cobalt with caustic potash which was thought to be a potassium cobaltite, CogOi6- Ke:3H20,* is probable cobalto-cobaltic oxide with adsorbed potash.® Hyprovus NICKELOUS OXIDE The addition of potash or soda to a solution of nickel salt throws down a voluminous apple-green precipitate of hydrous nickel oxide which goes over to crystalline® Ni(OH):.? The purest preparation is obtained by using nickel nitrate or nickel ammonium nitrate rather than sulfate or chloride, since nitrate ion is said to be least strongly adsorbed by the hydrous precipi- tate.* The gelatinous oxide is readily soluble in ammonia form- ing a deep-blue solution from which a green crystalline powder is precipitated by boiling. Anhydrous NiO is an olive-green powder which becomes deep yellow on heating but returns to the original green on cooling to room temperature.® Like cobalt oxide, it forms a variety of 1 CorHN and GLAsER: Z. anorg. Chem., 33, 9 (1903). 2 BURGSTALLER: Chem. Zentr., II, 1525 (1912). 3 Mawrow: Z. anorg. Chem., 24, 263 (1900). 4 SCHWARZENBERG: Liebig’s Ann. Chem., 97, 212 (1856); PrsBau: Jbid., 100, 257 (1856); Mayr: /bid., 101, 266 (1857). 5 McConne.u and Hanus: J. Chem. Soc., 71, 585 (1897). 6 HEDVALL: Z. anorg. Chem., 120, 338 (1922). 7 TowErR: J. Phys. Chem., 28, 176 (1924). 8 BonsporFF: Z. anorg. Chem., 41, 136 (1904); TrrcumMann: Liebig’s Ann. Chem., 156, 17 (1870). 9 Morssan: Ann. chim. phys., [5] 21, 238 (1880); Zimmerman: Liebig’s Ann. Chem., 232, 344 (1880). COPPER, COBALT, NICKEL, SILVER, AND GOLD 158 pigment colors for glazes! when fused with other metallic oxides. Thus alumina gives a blue aluminate, and zine oxide, a blue zincate; but a variety of tints is possible, as the compounds form solid solutions with the excess of either oxide. Mixed crystals result on fusing nickel oxide with the isomorphous oxides of magnesium, nickel, and cobalt.? To prepare the active form of Ni(OH)e for the Edison storage battery, the oxide is precipitated from sulfate solution with an excess of sodium hydroxide. The gel is dried slowly with the - enclosed salts and alkali which are subsequently washed out. Excess of alkali increases the porosity of the product.’ As early as 1906, Ipatiev+ employed nickel oxides as catalytic agents for hydrogenation, working at temperatures around 250° and at 100 atmospheres pressure. Later Bedford and Erdmann? found nickel oxides to be efficient catalyst for the hydrogenation of oils ‘The oxides were considered to be superior to metallic nickel: first, because the velocity of hydrogenation is more rapid with the former; and second, because the oxides are much less sensitive to the action of poisons such as sulfur, chlorine, and carbon monoxide. The latter gas is especially poisonous to nickel, but in technical hydrogenation with oxides, it can be allowed to accumulate in the system without having any effect except to dilute the system. Nickelous and nickelic oxides are effective at 250°; at temperatures as low as 180°, the most effi- cient catalyst appears to be suboxide, possibly NiO, which forms colloidal solutions in oil. Indeed, the increased activity of the higher oxides, after using for a short time, is attributed to the formation of a colloidal solution of nickel suboxide. Since nickel oxide is reduced by hydrogen at 190°, it is claimed by some that the actual hydrogen carrier in Erdmann’s experi- ments was metallic nickel. This does not seem to be the case, since the catalyst freed from the hardened oil is a strongly mag- 1Cf. Wuirner: J. Am. Ceram. Soc., 4, 357 (1921). 2 HepvaLu: Z. anorg. Chem., 103, 249 (1918). 3 Epison: U.S. Patents 1083355—-1083356 (1914); 1167484 (1916). 4 J. Russ. Phys.-Chem. Soc., 38, 75 (1906); 39, 693 (1907); 40, 1 (1908). 5 J. prakt. Chem., (2) 87, 245 (1913); J. Russ. Phys.-Chem. Soc., 46, 616 (1913); British Patent 29612 (1910); 18122 (1913). 6 MEIGIN and Bartg.s; J, prakt. Chem., (2) 89, 290 (1914). 154 THE HYDROUS OXIDES netic black powder which does not form a carbonyl and does not conduct the current as does nickel. Moreover, finely divided nickel is a hydrogenation catalyst either in the presence or absence of moisture, whereas the suboxide is inactive except in the presence of moisture.! According to Erdmann, the suboxide forms an additive product with the oil which assists in preventing reduction to metallic nickel. Sabatier and Espel? prepared what they took to be NuO by reducing NiO with hydrogen at 220°; but this differs from the catalyst, as it forms a carbonyl. Erdmann prepared an oxide very similar in properties to that obtained from the hardened oil by electrical reduction of potassium nickel cyanide.* ‘The product was colloidally dispersed by oil and proved to be a good catalyst for hydrogenation. While the evidence points to the existence of a catalytically active suboxide of nickel, its composi- tion has not been established with certainty.* A sol of nickel hydroxide results on mixing solutions containing equivalent amounts of nickel tartrate and potassium hydroxide. If the solutions are as concentrated as normal, precipitation takes place slowly giving a transparent green jelly; but if the solutions are dilute, say N/10, a sol forms which can be purified by dialysis. The gel precipitated, from nickel chloride solution by alkali is peptized by washing. Using N/10 solutions, six or seven washings by decantation suffice. ‘Tower® attributes the stabili- zation to potassium chloride, since sol formation is retarded or prevented by washing either too little or too much. One mol of KCl to 200 mols of Ni(OH): was found to be the limiting ratio for a stable sol.® 1 SENDERENS and ABOULENC: Bull. soc. chim., (4) 17, 14 (1915). 2 Compt. rend., 158, 668 (1914). 3 Cf. Moors: Chem. News, 71, 82 (1895). 4 MUuuer: Pogg. Ann., 136, 59 (1869); GuasmR: Z. anorg. Chem., 36, 18 (1903); TscHucarv and IcHLoPINE: Compt. rend., 159, 62 (1914); BercErR: Compt. rend., 158, 1798 (1914); 174, 1341 (1922); cf., however, WOHLER and Bauz: Z. Elektrochem., 27, 406 (1921); Levi and TaccuHin1: Gazz. chim. ital., 55, 28 (1925). ’ TowreR and Cooke: J. Phys. Chem., 26, 728 (1922); Towsmr: Ibid., 28, 176 (1924). 6 PaaL and Brunuems: Ber., 47, 2200 (1914). COPPER, COBALT, NICKEL, SILVER, AND GOLD 155 Hyprovus NIcKELIC Ox1IpE AND NIcKEL PEROXIDE By passing chlorine or bromine through a suspension of nickel hydroxide or by warming a nickel salt with an alkali hypochlorite or hypobromite, a black precipitate is thrown down which was assigned the formula, Ni2O;:3H,0.! This is in error in two respects: Not only is the water content indefinite,? but the degree of oxidation of the nickel varies with the nature of the oxidizing agent, the rapidity of oxidation, and the temperature.* Under no conditions is pure hydrous Ni2O; precipitated; and with bromine at 0°, the ratio of nickel to oxygen approaches 1:2 Howell* showed that both hydrous NiO; and NiO2 are formed simultaneously during the action of alkali and hypochlorite on Ni(OH)e. Since NieOsz is not oxidized, there is a limit to the oxygen content of the precipitate. Moreover, unlike cobalt dioxide, NiOz is instable, decomposing to NiO without the inter- mediate formation of Ni,O3. The rate of decomposition of the dioxide is accelerated by heat; but excess alkali stabilizes it by adsorption. NiO; has the structure represented by NiO -NiOsz.°® A greenish-gray compound having the composition NiO2:xH2O is obtained by mixing 30 per cent hydrogen peroxide with a dilute alcoholic solution of nickel chloride cooled to 50°, followed by the addition of alcoholic potassium hydroxide.* Unlike the black dioxide, the green compound behaves like hydrogen peroxide. The latter is, therefore, regarded as a true peroxide O ye Ni¢ | ; and the former as Ni 7 A peroxide of nickel is O No formed by the electrolytic oxidation of the metal and plays a part in the Edison battery.® 1WacutTerR: J. prakt. Chem., 30, 327 (1843); Veit: Compt. rend., 180, 21111925). 2 CARNELLEY and WALKER: J. Chem. Soc., 58, 91 (1888). 3 BeLLuccr and Ciavari: Atti accad. Lincei, 14, II, 234 (1905). 4 J. Chem. Soc., 123, 669, 1772 (1923). 6’ CLark, Asspury, and Wick: J. Am. Chem. Soc., 47, 2661 (1925). 6 PELLINI and MENEGHINI: Z. anorg. Chem., 60, 178 (1908). 7 TuBANDT and RIEDEL: Ber., 44, 2565 (1911); Z. anorg. Chem., 72, 219 (1911); cf., however, TanaTar: Ber., 42, 1516 (1909). 8 Forrster: Z. Elektrochem., 18, 414 (1907); 14, 17 (1908); RimsENFELD: Ibid., 12, 621 (1906); cf., however, ZepNER: [bid., 11, 809 (1905); 12, 463 (1906); 18, 752 (1907). 156 THE HYDROUS OXIDES The black hydrous dioxide of nickel is peptized by small amounts of organic acids, such as acetic, citric, and tartaric, forming a very stable colloid. Peptization results simply on washing the hydrous oxide with cold water, but the sol obtained in this way is not stable.! HybDROUS SILVER OXIDE By mixing a dilute solution of silver nitrate and KOH in 90 per cent alcohol at —45°, hydrous silver oxide comes down as a flocculent mass almost pure white in color.? As the temperature rises, it changes in color from pale brown to brown, owing to loss of adsorbed water and agglomeration of the particles. The hydrous oxide precipitated at room temperature is brown, but becomes black on drying at temperatures as low as 50 to 60°. Pure AgeO decomposes slightly even at 100° and it does not give up all its adsorbed water until a temperature of 280° is reached; accordingly, pure AgeO cannot be obtained.4 A silver oxide sol is formed both by heating silver wire to red- ness and plunging it suddenly into water,’ and by mixing a dilute N/40 solution of AgNO; with a slight excess of KOH of similar concentration.® KLEEBERG: Kolloid-Z., 87, 17 (1925). 6 Z. anorg. Chem., 132, 5 (1924). 164 THE HYDROUS OXIDES to the collodion for mantle coating to increase the protection given the mantle.! The crystals of beryllia obtained from an electric are furnace are almost as hard as corundum, and so it is sometimes mixed with other substances as an abrasive.! The oxide would also seem to possess certain advantages over magnesia as a refractory for crucibles. It has a high melting point, 2450°, and after cal- cination it resists acid corrosion much more effectively than magnesia.2 The oxide has also shown some promise as a body in paints and in the manufacture of certain dental products and synthetic gems. Hyprous Maanesium HypRoxiIpE Magnesium hydroxide in a flocculent. hydrous condition is formed by the action of water on magnesia obtained from the naturally occurring carbonate, magnesia alba.* The precipi- tated mass is not a hydrous oxide as is so frequently the case, but is a hydrous hydrate* made up of very finely divided particles which adsorb alkali so strongly® that its presence prevents the adsorption of sulfate and chloride. The oxide is more soluble when first formed, going over to a less soluble crystalline form quickly when the magnesium ion concentration is high, and more slowly when it is low.’ -‘X-radiograms show the microcrystalline particles to possess a structure identical with natural brucite.*® Large crystals of the hydrate are formed by heating magnesium chloride with an excess of potash in a limited volume of water.?® The flocculent hydroxide dried over sulfuric acid at 100 to 200° adsorbs more than 1.5 H20, which it gives up in a dry atmos- 1Cf, James: Metal Ind., 11, 66 (1917). 2 BERZELIUS: Sioeigere J., 15, 236 (1815); Leprav: ‘Com rend., 123, 818 (1896); Ann. chim. phys., (7) 16, 457 (1899). 3 DEviLLE: Compt. rend., 61, 975 (1865); Drrrn: Ibid., 73, 191 (1871). 4VaN BEMMELEN: J. prakt. Chem., 26, 238 (1882). 5 GROUVELLE: Ann. chim. phys., (2) 17, 354 (1821); MarcHuanp and ScutiRer: J. prakt. Chem., (1) 50, 385 (1850). 6 Parren: J. Am. Chem. Soc., 25, 186 (1903). 7 GJALDBAEK: Z. anorg. Chem., 144, 145, 269 (1925). 8 BOum and NicuassEen: Z. anorg. Chem., 132, 6 (1924). 9 Dre ScHULTEN: Compt, rend., 101, 72 (1885), BERYLLIUM, MAGNESIUM, ZINC, CADMIUM, MERCURY 165 phere.t The amount of adsorbed water taken up decreases with the temperature of ignition and the anhydrous oxide obtained at high temperatures hydrates very slowly. Campbell? burned magnesite between 600 and 800°, obtaining an impure oxide which hydrates completely in 3 days. Between 1000 and 1100°, the magnesia was said to undergo a change resulting in a marked decrease in the rate of hydration until at 1450°, about the tem- perature used in burning Portland cement, the oxide, immersed in water for 18 months, combines with 60 per cent of that neces- sary to form MgO-H.O. Le Chatelier* gave 1600° as the trans- formation temperature, and Parravano and Mazzetti* placed it at 800°, at the same time calling attention to the effect of impur- ities on the transformation temperatures; thus, ferric oxide hastens it. Mellor® pointed out the absence of a definite transformation temperature and showed the change to proceed more quickly the higher the temperature of calcination. Mellor attributed the change to a conversion from amorphous to crystal- line periclase; but this cannot be the case, as Hedvall® found the oxide formed at various temperatures to have a _ cubic- lattic crystal structure which underwent no change on heating. The specific gravity of calcined magnesia varies, however, between 3.0 and 3.6, depending not only on the method of preparation but on the temperature of calcination. The low-temperature — low-specific-gravity oxide not only reacts much more rapidly with water than the oxide formed at high temperatures, but the former possesses a greater adsorption capacity for gases and moisture, and dissolves more rapidly in acids.’ Although the melting point of magnesia is in the neighborhood of 2500°, it undoubtedly sinters at a much lower temperature, and this change in physical character probably accounts for the difference in reactivity of the oxide ignited at different temperatures. 1 Van BemMME EN: “ Die Absorption,’’ 369 (1910). 2 J. Ind. Eng. Chem., 1, 665 (1909). 3 Compt. rend., 102, 1248 (1883). 4 Atti accad. Lincei, (5) 30 I, 63 (1921). 5 Trans. Ceram. Soc., 16, 85 (1917). 6 Z. anorg. Chem., 120, 327 (1922). 7 Drrre: Compt. rend., 78, 111, 191, 220 (1871); ANpERson: J. Chem. Soc., 87, 257 (1905). 166 THE HYDROUS OXIDES Magnesia Cement.—It is an interesting fact that magnesia prepared by heating the chloride or nitrate to redness possesses hydraulic properties similar to Portland cement in that it sets to a rigid mass when mixed with a limited quantity of water.! If the nitrate is calcined at as low a temperature as 350°, the resulting magnesia will not set; if calcined at 440 to 500°. the magnesia hardens under water and at the end of 2 months is like polished marble; but if heated to 1200° or more, the oxide loses its power to set. The oxide obtained by gentle ignition of natural magnesite also possesses hydraulic properties but that obtained from synthetic carbonates will not set, although it appears to react readily with water. This difference cannot be due to the presence of impurities in the natural product, since an hydraulic oxide is formed by converting the synthetic car- bonate to nitrate and igniting the latter. As willbe discussed in Chap. XVIII, the setting of such substances as Portland cement and plaster of Paris involves the formation of a gel structure,’ and the same is probably true in the setting of magnesia. The temperature of ignition and the structure of the calcined sub- stance determine the physical character of the oxide, and these, in turn, determine the rate of hydration and the nature of the resulting product. As in the preparation of jellies by precipi- tation, a suitable rate of agglomeration of highly hydrous par- ticles is essential for obtaining a firm jelly structure. Magnesia possessing setting properties is sometimes used in conjunction with lime for mortar making in districts where only magnesium limestone is available. Similarly, gently calcined magnesia is mixed with crushed dead-burnt magnesia in manu- facturing firebricks so widely used in the basic Bessemer steel process. The hydraulic magnesia gives plasticity to the paste formed by mixing the materials with water to permit of molding. Sorel’s magnesia cement consists of a mixture® of magnesia with a concentrated solution of magnesium chloride, sp. gr. 1.16 to 1.26. This sets in a short time to a con mass made up of 1 DevitueE: Compt. rend., 61, 975 (1865); ScHWARZ: ingles polytech. J., 186, 25 (1867); Knapp: Ibid. 202, 513 (1872). 2 Cf. MICHAELIS: Kollota-Z., 5, 9 (1909); 7, 320 (1910); KriseRMANN: Kolloidchem. Bethefte, 1, 423 (1910). 3 SOREL: Compt. rend., 65, 102 (1867), BERYLLIUM, MAGNESIUM, ZINC, CADMIUM, MERCURY 167 minute interlacing crystals! of what has been assumed to be basic chloride. People are unable to agree on the formula of the hypothetical salt, and it is probably only an indefinite solid solution of magnesium oxide and chloride. The chloride may be dissolved out completely with boiling water, leaving hard mag- nesium oxide. ‘The cement can be prepared by adding water to a suitable mixture of dry components.? It possesses marked mechanical strength and is used for cementing glass and metal and for making artificial stones; e.g., xylolith is made from saw- dust, cement, and water. The tendency of calcined magnesia to take up water and expand is of importance in the cement industry, since the presence of as much as 2 to 3 per cent of uncombined magnesia would give a concrete that would disintegrate from excessive expansion.? In addition to the applications mentioned, hydrous magnesium hydroxide has been substituted for charcoal as a clarifier in the refining of sugar.4 Its mild basic action has been utilized in pharmaceutical preparations as an antacid. Milk of magnesia is a fairly stable suspension of the hydrous oxide that is widely employed as a mouth wash; in the preparation of modified milk for infants; and in combating hyperacidity of the stomach. Rhythmic Bands.—The precipitation of magnesium hydroxide in gelatin in the form of rhythmic bands has been investigated quantitatively by Popp. When ammonia diffuses into gelatin containing magnesium chloride, it is found that with increasing concentration of magnesium salt, the rings increase in number and thickness, and the space between them decreases; with dimin- ishing ammonia concentration, the rings decrease in number and thickness, and the space between them increases; adding ammo- nium chloride causes the number and thickness of the rings to decrease and the space between them to increase; with diminishing gelatin concentration, both the rings and the space between them increase, the number remaining the same. The rhythmic precip- 1 LUHMANN: Chem. Ztg., 25, 345 (1901); Krinaer: [bid., 34, 246 (1910). 2 Cf. KRANER: German Patent 143933 (1902); Lyre and Tarrers: Brit- ish Patent 11545 (1890). 3 CAMPBELL and WuitTe: J. Am. Chem. Soc., 28, 1273 (1906); CAMPBELL: J. Ind. Eng. Chem., 1, 665 (1909). 4 Hake: J. Soc. Chem. Ind., 2, 149 (1883). 5 Kolloid-Z., 36, 208 (1925), 168 THE HYDROUS OXIDES itation takes place also in clay, agar, silica gel, fine sand, and glass beads in water. To account for these and other Liesegang phenomena, Wolfgang Ostwald! postulates the existence of three principal diffusion waves in all reacting systems giving typical periodic precipitates: The added electrolyte diffuses into the gel; the electrolyte in the gel diffuses outward; and the electrolyte produced by the reaction may diffuse in both directions. In many instances the soluble reaction product possesses a higher rate of diffusion than one or both of the reactants. Ostwald assumes further that many and probably all reactions giving Liesegang rings are balanced reactions. Precipitation, therefore, depends on certain critical concentrations of reactants which vary over wide ranges through the interference of diffusion waves. In support of the theory, it was shown that many Liesegang rings are destroyed by subsequent introduction, by diffusion, of the electrolyte produced in the reaction. ‘Thus, bands of mag- nesium hydroxide are destroyed by allowing ammonium chloride to diffuse into the gel supporting them. The converse of rhyth- mic precipitation, namely rhythmic solution, may sometimes be produced by adding a reaction product. Thus a uniform precipitate of lead sulfate in gelatin gel containing ammonia is converted into rings by the interdiffusion of concentrated ammonium chloride. Continuous precipitation results if one reactant is replaced by a compound not giving a balanced reaction, as evidenced by the failure to get bands when alkali is substituted for ammonia in the precipitation of magnesium hydroxide in gelatin. The distribution of chloride ions in a gelatin jelly containing magnesium chloride was found after the diffusion of ammonia, to show periodic variation between values much higher and much lower than those in the original gel. Wolfgang Ostwald’s theory of rhythmic banding is merely an extension of Holmes’? diffusion theory based on Frick’s law of diffusion. The influence of such phenomena as supersatura- tion,*® peptization and coagulation of the precipitate,* adsorption 1 Kolloid-Z. (Zsigmondy Festschrift), 36, 380 (1925). 2 J. Am. Chem. Soc., 40, 1187 (1918). 3 OstwaLD: ‘‘Lehrbuch allgem. Chemie,”’ 2d ed., 2, 778. 4 FREUNDLICH: ‘‘ Kapillarchemie,”’ 2d ed., 1009 (1922); Sen and Duar: Kolloid-Z., 34, 270 (1924). . BERYLLIUM, MAGNESIUM, ZINC, CADMIUM, MERCURY 169 of reacting solutes by the precipitate,' etc. is looked upon as a secondary factor in the banding process. Stable sols of hydrous magnesium hydroxide in water have not been prepared without the aid of a protective colloid; but a typical sol of great stability is formed by shaking magnesia with methyl! alcohol. Hyprovus ZINc OXIDE The voluminous precipitate obtained by adding the calculated amount of ammonia or alkali to a solution of zinc salt is hydrous zinc oxide, the amount of adsorbed water depending on the exact method of formation, the temperature, and the age of the sample.* If the precipitation is carried out at 100°, it contains less than 1 per cent of adsorbed water. Although the oxide newly formed in the cold is a transparent gel,* it quickly becomes flocculent and later powdery, the change being accompanied by a gradual transformation into the crystalline state. The hydrous oxide ages more rapidly if precipitated from chloride rather than from nitrate; and in the presence of alkali rather than water. As in the case of hydrous beryllium oxide, the microcrystalline mass formed on standing in the cold always contains more water than corresponds to ZnO-H,O, and in this state, it is probably hydrous zinc hydroxide. Kaufmann® observed a gradual loss of water on heating a precipitated hydrous zine oxide to 125°, where it had the composition Zn(OH). which it maintained to 180°, and then broke down gradually, giving anhydrous ZnO atalowred heat. There are no hydrates of Zn(OH)., as assumed by De Forcrand’ and Boedecker.® 1 BraDForD: Biochem. J., 10, 169 (1905). 2 NEUBERG and REewa.p: Kolloid-Z., 2, 354 (1908). 3 GouDRIAAN: Rec. trav. chim., 39, 505 (1920). 4LinpDER and Picton: J. Chem. Soc., 61, 130 (1892). -5 Fricke and AHRNDTs: Z. anorg. Chem., 134, 344 (1924); Frickn: Jbid., 136, 48 (1924); BGum and Nicuassen: [bid., 182, 1 (1924). 6 Dissertation, Miinchen, 69 (1913); cf. Pascau: Compt. rend., 177, 765 (1923). 7 Compt. rend., 135, 36 (1902). 8 Liebig’s Ann, Chem., 94, 358 (1855), 170 THE HYDROUS OXIDES The hydrous precipitates adsorb chloride, nitrate, and espe- cially sulfate! so strongly that they cannot be purified completely by washing.? Large crystals of Zn(OH)s, exhibiting a very slight adsorption capacity, precipitate spontaneously from the alkali solution prepared in a variety of ways. ‘Thus, Goudriaan obtained long prismatic needles from a solution of normal zinc | sulfate to which normal potassium hydroxide was added until the precipitate first formed just failed to redissolve; and Fricke and Ahrndts obtained the usual dense rhombic crystals by diluting a solution of the hydroxide in strong alkali. The newly formed gel dissolves readily in alkali, 1 atom of Zn" being taken up by approximately 6 of OH’.* On account of the ageing of the oxide, the solubility in alkali and ammonia is less the older and less hydrous the preparation. ‘The variation in the solubility has naturally led to the assumption that the oxide exists in different polymerized forms or allotropic modifica- tions. Klein® recognizes an easily soluble form 2ZnO- H,O0 and two insoluble forms having the composition, Zn(OH):, analogous to Bleyer and Kaufmann’s A, B, and C beryllium oxides. But as in the latter case, the solubility is not definite but varies con- tinuously from the loose highly gelatinous to the most massive granular form. Although the alkali solution of hydrous Zn(OH). has been the subject of repeated investigations during the past 25 years, there is still a difference of opinion as to the exact nature of such solutions. On account of the very weak acidic character of Zn(OH)e, Hantzsch® believed that alkalies peptize the latter, forming an insoluble sol from which most of the hydroxide precipitates on standing, leaving the remainder in solution as 1 Kuritorr: Chem. Zentr., 1222 (1901). 2 GoUDRIAAN: Rec. trav. chim., 39, 505 (1920); Frickm and AHRNDTs: Z. anorg. Chem., 134, 344 (1924); Lorenz: Jbid., 12, 489 (1896); Hatt: Am. Chem. J., 19, 901 (1897). 3’ RUBENBAUER: Z. anorg. Chem., 30, 331 (1902); Hmrz: Ibid., 28, 274 (1901). 4 Hantzscu: Z. anorg. Chem., 30, 289 (1902); Kunscuert: [bid., 41, 337 (1904). . 5Z. anorg. Chem., 74, 157 (1912); cf. De Forcranp: Compt. rend., 134, 1426 (1902); 135, 36 (1902); Masson: Bull. soc. chim., [3] 15, 1104 (1896). 6 Z. anorg. Chem., 30, 300 (1902). BERYLLIUM, MAGNESIUM, ZINC, CADMIUM, MERCURY 171 zincate. As Hantzsch worked with dilute alkali solutions, he was probably right in concluding that most of the hydroxide was peptized; but diffusion experiments! and electrometric measurements” on solutions in concentrated alkali showed the presence of alkali zincate. The more concentrated the alkali, the more hydroxide it will take up and the more zincate will form.* Goudriaan* determined the 30° isotherm for the system, Naz2O-ZnO-H,0. The saturation concentration increases rapidly to the triple point, ZnO-Na2ZnO,-4H.O, where the com- position of the solution in weight per cent is 27.8 per cent Na2O and 16.5 per cent ZnO. The zincate forms well-developed crystals decomposed by water and is stable from the triple point to the quadruple point, NaZnO,:4H.,O-Na,O-3H.0-H.0, at 39.2 per cent Na2O and 9.7 per cent ZnO. Sodium zincate forms an incongruent solution, the addition of water to the solid salt or the dilution of the solution causing ZnO to precipitate. This accounts for a number of so-called sodium zincates®> which are either metastable or non-existent. While Na,ZnOz appears to be the stable salt in strong alkali solution, electromotive determin- ations of fixed H'ion, on adding sodium hydroxide to solutions of zine salts, indicate the formation of acid zincate in rela- tively dilute alkali. Such solutions always contain colloidal zine hydroxide stabilized by preferential adsorption of hydroxyl ion. Fricke and Ahrndts claim that potassium hydroxide forms chiefly KHZnO, even in concentrations above 8 N. By dipping red-hot zine into water, a sol is formed consisting of both colloidal zinc and zine hydroxide.’ A dilute sol results by allowing zinc to stand in water for a long time in contact with 1 CorrrELu: Z. physik. Chem., 42, 418 (1902); Kaurmann: Dissertation, Minchen, 45 (1913); KremMann: Z. anorg. Chem., 35, 48 (1903). 2Duroir and Groset: J. Chim. phys., 19, 324 (1921); Fricke and AuRNDTS: Z. anorg. Chem., 134, 344 (1924). 3 KEIN: Z. anorg. Chem., 74, 157 (1912); Rupensaver: Jbid., 30, 331 (1902); Woop: J. Chem. Soc., 97, 878 (1910). 4 Rec. trav. chim., 39, 505 (1920). 5 H.g., see Comey and Jackson: Am. Chem. J., 11, 145 (1889). 6 HILDEBRAND and Bowers: J. Am. Chem. Soc., 38, 785 (1916); cf. also Kunscuert: Z. anorg. Chem., 41, 337 (1904); Forrster: Z. Elektrochem., 6, 301 (1899). 7 Kimura: Mem, Coll. Sci., Kyoto Imp. Univ., 6, 211 (1913). 172 THE HYDROUS OXIDES air.! With the exception of alkali-peptized colloids, concentrated sols have been obtained only in the presence of protective col- loids, such as potassium soaps? and sodium _ protalbinate.* Zine oxide in the finely divided or colloidal state finds its most important application in the anhydrous rather than the hydrous condition. Thus, zine white alone, or mixed with finely ground silica or calcium carbonate and ground with linseed oil, forms a white paint that does not discolor in the presence of H2S. A suitable mixture of zinc white and of finely divided zine hydrox- ide precipitated in the cold is said to form a useful enamel pigment.4 Zine oxide has a mild antiseptic action, and a sol consisting of the oxide, gutta percha, and Venice turpentine is applied to cloth in the manufacture of surgeons’ adhesive tape. Like magnesia, a wet mixture of zine oxide and chloride sets to a solid gel. A strong dental cement consists of a mixture of zine oxide and aluminum phosphate. The oxide also finds some applications in face powders, in glazes, and as a filler in oilcloth and celluloid; but by far the greatest demand is as a filler and pigment in rubber goods, especially automobile tires. Hyprous CADMIUM OXIDE Hydrous cadmium oxide precipitates in a very voluminous and highly hydrous form when a concentrated solution of cadmium salt is treated with alkali. The precipitate loses water on heating, becoming a flocculent microcrystalline mass of hydrous Cd(OH)»2. The purest form is obtained from nitrate solution, since it adsorbs nitrate less strongly than chloride or sulfate. Like the corresponding zine compound, cadmium hydroxide is soluble in excess ammonia; but unlike the former, it is only slightly soluble in dilute alkalies. Hot, highly concen- trated solutions of potassium hydroxide carry considerable amounts into solution from which hexagonal plates of Cd(OH), 1 TRAUBE-MENGARINI and Scaua: Kolloid-Z., 10, 115 (1912); NorpENSON: Kolloidchem. Bethefte, T, 106 (1915). 2 KurRILoFF: Z. Hlektrochem., 12, 213 (1906); Rora: German Patent 228139 (1908). 3 PaaLt and HartMann: Ber., 51, 894 (1918); AmpeRGcER: German Patent 229306 (1909). 4 Joannis: J. Soc. Chem. Ind., 25, 486 (1906). 6 FoLuLENIvs: Z, anal. Chem., 13, 272 (1874). BERYLLIUM, MAGNESIUM, ZINC, CADMIUM, MERCURY 173 erystallize.! | Alkali sulfides react with the voluminous oxide formed in the cold, giving yellow cadmium sulfide, and with the aged oxide formed in the hot, giving red cadmium sulfide. Since the yellow and red sulfides were thought to be polymers, Biichner? assumed the existence of two forms of Cd(OH)2; but it now appears that the difference in color of the sulfides is not due to polymorphism or to crystal structure, but to a difference in the size and nature of the surface of the particles. Rapid action of the voluminous compound with alkali sulfides gives small yellow particles, while slower action with the denser aged hydroxide gives larger particles that appear red. Hyprovus OxIpEs oF MERCURY Mercuric Oxide.—Hydrous mercuric oxide is thrown down as a yellow flocculent mass on adding alkali to a cold mercuric solution. It does not form the monohydrate or hydroxide HgO- H20, as claimed by Carnelley and Walker,‘ nor does it retain its adsorbed water very strongly, but is readily dried to the anhydrous oxide.® If the yellow oxide is boiled with aqueous solutions of salts or the dried oxide is heated, the color changes to orange red. This red compound is formed directly by the thermal decomposition of mercuric nitrate. As usually obtained, the yellow oxide decomposes at a lower temperature, is more soluble in water, and reacts more readily with acids, alkalies, and salts than the red compound. ‘These distinct differences in physical and chemical properties were attributed by Gay Lussac® and later by W. Ostwald’ and others to a difference in the degree of fineness of the particles, the greater activity of the yellow oxide resulting from the greater surface of the smaller 1 Dr ScHuLTEeN: Compt. rend., 101, 72 (1885). 2 Ber., 20, 681 (1887); cf. KuosuLKkorr: J. prakt. Chem., (2) 39, 412 (1887). 3 ALLEN and CrensHaw: Am. J. Sci., (4) 34, 341 (1912). 4 J. Chem. Soc., 58, 59 (1888); cf. ScHarrNneR: Liebig’s Ann. Chem., 61, 182 (1844). ’Scnocu: Am. Chem. J., 29, 321 (1902); cf. Mitton: Ann. chim. phys., (3) 18, 33 (1846). 6 Compt. rend., 16, 309 (1843). 7Z. physik. Chem., 18, 159 (1895); 34, 495 (1900); Scuick: /bid., 42, 155 (1903); Varer; Compt. rend., 120, 622 (1895). 174 THE HYDROUS OXIDES particles. This view was called in question by Glazebrook and Skinner! and by Cohen? who showed that the E.M.F. of the chain: Hg| HgO red, KOH, HgO yellow|Hg, was 0.685 millivolt,. indicating the existence of two isomeric modifications of the oxide; but Ostwald and Allmand?® traced these results to the variation in solubility of particles of different size. Schoch® attributed the difference in properties to a difference in crystal structure, the yeilow oxide consisting of quadratic plates and the red of prisms. Allmand confirmed Schoch’s observation but showed conclusively that either type of crystal may be yellow or red, depending altogether on the state of subdivision of the particles. A stable yellow sol is obtained by precipitating hydrous mercuric oxide in the presence of Paal’s® sodium salt, of protalbinic and lysalbinic acids which act as protective colloids. After dialysis, this is agglomerated by acids and certain salts, giving a gelatinous precipitate. By adding mercuric chloride to a normal solution of potassium hydroxide containing 40 cubic centimeters of acetone, a sol is obtained which sets to a firm jelly on standing, the time required depending on the concentration of sol.? The setting may be hastened by adding a small amount of acid or by heating; but too much heating causes agglomera- tion to a gelatinous precipitate. For some unknown reason, the presence of even a small amount of mercurous salt seems to retard or prevent jelly formation. Mercurous Oxide.—Hydrous mercurous oxide, obtained by adding alkali to a mercurous salt solution, cannot be obtained free from mercuric oxide. Bird’ claims to get mercurous hydrox- ide by mixing mercurous nitrate with alcoholic potassium hydrox- ide at —42°; but this has not been proved. 1 Proc. Roy. Soc., 51, 60 (1892). 2Z. physik. Chem., 34, 69 (1900). 3 Z. Hlektrochem., 16, 254 (1910). 4 Huuett: Z. physik. Chem., 37, 385 (1901). 5 Am. Chem. J., 29, 321 (1902). 6 Ber., 35, 2219 (1902); cf. KaLtuE and Co.: Z. angew. Chem., 20, 1374 (1907); May: German Patent 248526 (1911). 7 Bunce: J. Phys. Chem., 18, 269 (1914); Reynoups: Proc. Roy. Soc., 19, 431 (1871). 8 Am. Chem. J., 8, 426 (1886); cf. Rercuarn: Ber., 30, 1914 (1887), CHAPTER VII THE HYDROUS OXIDES OF SILICON AND GERMANIUM Hyprovus SILICON DIOXIDE SILICA GEL Composition.—The classic investigations of van Bemmelen on the composition of the hydrous oxides were climaxed by his exhaustive study of the hydration and dehydration of hydrous silica! thrown down from alkali silica solutions with dilute hydro- chloric acid. A silica jelly containing 300 mols of water to 1 of silica is very soft, and when broken into pieces, it flows together like a viscous liquid. A gel with a water content of 30 to 40 mols is brittle; and with 6 mols, it can be pulverized, giving an apparently dry powder. On further dehydration, the vapor- pressure curve drops continuously, giving no indication of a definite hydrate. The highly hydrous oxide is almost perfectly clear, but when the water content drops to a point usually between 1.5 and 3.0 mols, depending on the method of preparation and the history of the sample, the gel becomes opaque and chalky but clears up once more when the water content is reduced to 0.5 to 1.0 mol. The clouding is due to the appearance of air bubbles in the pores of the gel and lasts until the pores are com- pletely filled with air. Owing to capillary action, the water which evaporates from the outer surface of the capillaries is replaced from the inside of the gel leaving a vapor space in the center of the jelly and thus producing an opacity which lasts until the pores are free from capillary water. The remaining 0.5 to 1.0 mol is adsorbed very strongly on the surface of the particles and can be removed only by heating to a relatively high temperature. In Fig. 11 is given van Bemmelen’s schematic representation of the pressure-concentration relations at 15° for a freshly formed hydrous silica. The A curves represent the first dehydration ‘Van BEMMELEN: “Die Absorption,” 196, 214, 232 (1910). 175 176 THE HYDROUS OXIDES over sulfuric acid; and the Z curves are for an oxide which has _ been dehydrated once, more or less completely. The direction of the arrows shows whether water is being taken up or given off. Starting with a fresh gel, the vapor pressure falls below that of pure water and decreases along the curve A, the volume decreasing simultaneously. There is no actual break at O where the gel begins to cloud. The volume does not change much after reaching O, and the loss of water along the curve AaG causes the capillaries to fill up with air, the gel becoming cloudy. At O,, the capillaries are filled with air except for a small amount of very strongly adsorbed water and the gel is clear again. Saturated Water-Vapor 0; Pressure Grams Water per Gram Silica Fig. 11.—Vapor pressure diagram for hydrous silica. Along Aa, the last trace of adsorbed water is driven off. If the dehydration is stopped at some point along the curve A@ and the gel is subsequently subjected to a higher partial pressure of water vapor, the hydration is not reversible, but a curve Zy is obtained. ‘This is because the gel shrinks along AB and as it does not swell to any marked extent, the water is not taken up under the same conditions. The Z curves represent reversible phenom- ena at least until they cut the curve A@. If dehydration is stopped at any point along OOs, hydration curves like Zy and Zy are obtained which usually meet in the point Oe. From Oz to O3 and from Oz to O, the pressure-concentration curves are HYDROUS OXIDES OF SILICON AND GERMANIUM 177 reversible. It is possible to pass along the path 0010.0 as often as one pleases but only in the one direction indicated. The existence of this hysteresis loop was confirmed by Anderson with the systems gel-water, gel-alcohol, and gel-benzene. Both van Bemmelen and Anderson! explained the hysteresis from the known fact that a liquid in a capillary tube has a greater vapor pressure when being filled than when being emptied, as in the former there is a diminution of the curvature of the liquid meniscus, due to incomplete wetting. Zsigmondy? attributed the marked hysteresis to adsorbed air which prevents the capillaries from being wetted readily. As a matter of fact, Patrick and McGavack? found no hysteresis in the adsorption of sulfur dioxide by silica gel when special precautions were taken to remove all air from the system. Moreover, no hysteresis was observed in the adsorption of sulfur dioxide, alcohol, carbon tetrachloride, and benzene by a dynamic method which consists in passing a mixture of air and the vapor in question over the adsorbent until equilibrium is attained.4 On the other hand, Patrick and Opdycke* were unable to eliminate the hysteresis with water by removal of all air. They ascribed the phenomenon to an increase in the viscosity of adsorbed water due to the decrease in internal pressure brought about by capillary and surface-tension forces. Since the point O may represent approximately 2 mols of water to 1 of silica, and the point O,, approximately 1 mol of water to 1 of silica, there is a temptation to conclude that the dehydration process consists in the decomposition of a hydrate. Van Bem- melen showed this point of view to be untenable, since the points O and O, do not correspond in the vast majority of cases with 2 mols and 1 mol of water, respectively, but vary with the history of the sample between 1.5 and 3 with the former and 0.5 and 1 with the latter. Moreover, one gets the same form of curves and optical phenomena by substituting for water such liquids as aleohol, benzene, and carbon tetrachloride. Van Bemmelen’s work has been confirmed and extended and his conclusions reaffirmed by a number of investigators, among whom may be 1Z. physik. Chem., 88, 191 (1914). 2 “Kolloidchemie,”’ 161 (1912). 3 J. Am. Chem. Soc., 42, 946 (1920). 4 PaTrick and OppykE: J. Phys. Chem., 29, 601 (1925). 178 THE HYDROUS OXIDES mentioned Léwenstein,'! Zsigmondy,? Thiele,? Anderson,* Bach- mann,* Vanzette,® Lenher,’? and Behr and Urban.’ Tschermak,? on the other hand, champions the view that the action of hydro- chloric acid on mineral silicates yields definite silicic acids corresponding to the salts from which they are obtained. Tscher- mak’s conclusions from dehydration experiments were shown to be altogether unwarranted, by Jordis,'° van Bemmelen,!! Mugge,!? Serra,'? and Thiele,'* since the breaks in the composi- tion curves are determined by the temperature at which the drying takes place, the nature of the drying agent, and the age and history of the sample. In spite of the evidence piled up against the existence of definite silicic acids, people are still attempting to establish their identity. Thus Schwarz and Menner! claim to remove adsorbed water by Willstatter’s method of washing the gelatinous oxide with alcohol and ace- tone. By a suitable choice of the conditions of preparation and dehydration, the existence of H2SiO3, HeSizOs, HeSiz07, and H,4S8i30g is regarded as definitely established; and the individual- ity of 128i102-10H2O and 12Si0,-9H2O is believed probable. As a matter of fact, these observations merely confirm what everybody knows, that one can get a composition for a gelatinous body corresponding to almost any desired formula provided one 1Z. anorg. Chem., 68, 69 (1910). 2 ZSIGMONDY, BACHMANN, and STEVENSON: Z. anorg. Chem., 76, 189 (1912). 8 Dissertation, Leipsig (1913). 4Z. physik. Chem., 88, 191 (1914). 5 Z. anorg. Chem., 100, 77 (1917). 6 Atti ist. Veneto, 75, 621 (1915-1916); Gazz. chim. ital., 47, I, 167 (1917). 7 J. Am. Chem. Soc., 48, 391 (1912). 8 Z. angew. Chem., 36, 57 (1928). 9Z. physik. Chane 53, 351 (1905); Zentr. Moe Geol., 225 (1908); Z. anorg. Chem., 63, 230 (1910); 87, 300 (1914); Nowe and Rotu: J. Am. Chem. Soc., 19, 832 (1897); cf. HILLEBRAND: Sitzb. Akad. Wiss. Wien, 115, 697 (1906). 10 Z. angew. Chem., 19, 1697 (1906). 11 Z, anorg. Chem., 59, 225 (1908). 12 Zentr. Mineral., Geol., 129, 326 (1908). 13 Attt accad. Lincet, 19, I, 202 (1910). 14 Dissertation, Leipsig (1913). 15 Ber., 67 B, 1233 (1924); 58 B, 73 (1925). HY DROUS OXIDES OF SILICON AND GERMANIUM 179 chooses the conditions properly. Pascal! analyzed three types of hydrous silicon dioxide magnetically and found all of them to behave like a mixture of anhydrous oxide and water.’ Structure.—Silica gel consists of minute hydrous particles joined together into an enmeshing network which holds water in the fine pores or capillaries.* Precipitated silica is completely amorphous, giving no x-ray interference pattern. Quartz is, of course, crystalline, but quartz glass is amorphous. When precipitated gelatinous silica is heated to 1300°, interference rings appear, indicating a partial conversion to. crystobalite.‘ The properties of hydrous silica show the usual variations with the temperature of precipitation. The gel precipitated by hydrolysis of silicon fluoride at 0° is much more readily soluble in hydrofluoric acid and sodium hydroxide and has a much stronger adsorptive capacity for methylene blue than the oxide formed at 100°. Schwarz and Leide® consider the two oxides to be distinct modifications of silica, but there is no justification for this assumption. ‘The difference is due to the size and physical character of the particles, and any member of intermediate prod- ucts between the 0 and 100° oxide could be made by a suitable choice of the conditions of formation. Indeed, Schwarz and Leide® have studied the gradual spontaneous loss of water and agglomeration of oxide particles and find it to be a continuous process. They regard the ageing as a definite chemical condensa- tion from (SiO2)z to (SiOz)ez. Until there is some definite proof of polymerization, I prefer the more. probable assumption that the ageing is a physical process involving a growth of the primary colloidal particles with the attending decrease in specific surface and loss of adsorbed water. Although silica is classified as a non-elastic ae the freshly formed jelly possesses an elasticity’ of the same order of magnitude 1 Compt. rend., 175, 814 (1922). 2 Le CuHaTELier: Compt. rend., 147, 660 (1908). SCT, p: 12. 4 Kyroupoutos: Z. anorg. Chem., 99, 197 (1900); Gross: Umschau, 34, 510 (1920). 5 Ber,, 53 B, 1680 (1920). 6 Ber., 58 B, 1509 (1920); Scowarz and STOWENER: Kolloidchem. Bethefte, 19, 171 (1924), 7 PrasaD: Kolloid-Z., 33, 279 (1923), 180 THE HY DROUS OXIDES as that of a gelatin jelly.’ Like the latter, the elasticity modulus varies greatly with the water content of the sample. Silica jellies possess the interesting property of vibrating like a rigid body under certain conditions.? Holmes, Kaufmann, and Nicholas? obtained jellies in a glass tube that gave a tone two octaves above middle C when the vessel was struck. If the jellies were prevented from touching the walls of the glass tube by coating the latter with vaseline, the vibration frequency was much lower than for similar jellies adhering to the walls. The vibration frequency is increased by decreasing the concentration of silica and by the presence of excess mineral acid, factors which increase the tension and thus the effective rigidity. The same factors increase the tendency of the jelly to synerize, thus showing that both vibration and syneresis have a direct relation to tension. Holmes believed the vibration to be transverse, the vibration frequency varying approximately inversely as the diameter of the cylinder of jelly. Prasad‘ failed to confirm these conclusions for gels removed from the vessel in which they were made. The tone emitted by a given jelly showed wide variation depending. on how it was held. Moreover, by applying Newton’s formula for the velocity of propagation of a longitudinal wave, the vibra- tions were shown to be longitudinal rather than transverse. On account of the ease with which electrolytes diffuse into silica jellies, a number of interesting reactions have been carried out in this medium. The usual method of procedure consists in adding one electrolyte to the silica sol before it sets, after which a solution of a second electrolyte is poured on the jelly and allowed to diffuse into the mass where interaction takes place. If a crystalline precipitate is formed by the reaction, the crystals will be much larger and better formed than if the solu- tions are mixed directly. In this way, Hatschek and Simon® prepared large gold crystals by reducing gold salts with several 1Lnicx: Drude’s Ann., 14, 189 (1904); SHePPARD and SwEsET: J. Am. Chem. Soc., 48, 539 (1921). 2 Konkel Z. physik. Chem., 12, 773 (1893). 3 J. Am. Chem. Soc., 41, 1829 (1919). 4 Kolloid-Z., 33, 279 (1923). 5 J. Soc. Chem. Ind., 31, 489 (1912); Mining Eng. World, 37, 280 (1912); cf. HatscuEexk: Kolloid-Z., 10, 77 (1912). HY DROUS OXIDES OF SILICON AND GERMANIUM 181 reducing agents; and Holmes! prepared magnificent crystals of a number of metals and salts. ‘The function of the jelly is to prevent rapid mixing of the interacting solutions, thereby avoid- ing rapid precipitation and the consequent formation of amor- phous particles or small crystals. Under certain conditions, reactions in jellies give rhythmic bands or Liesegang? rings of precipitates instead of large crystals. What actually happens appears to be determined in large measure by the nature of the jelly. Thus silver chromate forms bands in gelatin but not in agar, lead chromate forms bands in agar but not in gelatin; while neither silver nor lead chromate forms bands in silica jelly, although copper chromate does. The varicolored bands of gold in silica described by Holmes* are obtained only in changing light and are not true Liesegang rings; in the dark, large crystals only are formed.® Reactions in jellies are important as offering a plausible explanation of certain formations innature. Thus gold salts may have been reduced to gold in gelatinous silica which subsequently become quartz. Similarly, agate has probably been produced from gelatinous silica into which iron and other salts have diffused and deposited rhythmic bands.°® Adsorption of Gases.—The adsorption of gases by silica gel has been studied in great detail by Patrick and his collaborators. ‘The absorbent used in these investigations was prepared by mixing suitable concentrations of a solution of silicate of soda and hydrochloric acid under violent agitation. After setting to a firm jelly, the material was thoroughly washed and dried in vacuo at a temperature varying from 110 to 300°, the most active samples being obtained by heating at 250 and 300° for a half hour or more.’ 1 J. Phys. Chem., 21, 709 (1917). 2 “‘Chemische Reaktionen in Gallerten”’ (1898). 3 Cf. p. 167. 4 J. Am. Chem. Soc., 40, 1187 (1918). 5 Davis: J. Am. Chem. Soc., 44, 2700 (1922); 45, 2261 (1923). 6 LimseGaAne: Zentr. Mineral., Geol., 593 (1910); 497 (1911); cf. Kolloid-Z., 10, 273 (1912). 7 Parrick and GREIDER: J. Phys, Chem., 29, 1031 (1925). 182 THE HYDROUS OXIDES Adsorption isotherms for sulfur dioxide were obtained at varying temperatures between —80 and +100°.! The empirical equation of Freundlich, xe de 1/n ms kP (1) where x is the amount adsorbed by the mass of adsorbent m at pressure P, and k and n are constants, was found to hold over almost the entire range studied, exceptions being at points where the saturation pressure was appreciable.? The straight lines obtained by plotting logarithm S against logarithm P at various temperatures were separated widely. Patrick considers the adsorption by a porous substance, such as silica gel, to be a condensation in the capillaries that is inde- pendent of the chemical nature of the adsorbent. Capillary adsorbents will differ, therefore, in the extent of their total inter- nal volume and also in the dimensions of the pores that make up the internal volume. If such be the case, the form of the adsorption isotherm merely expresses the distribution of the internal volume as a function of the dimensions of the pores. From this point of view, it would appear logical to seek a relation between the volume occupied by the adsorbed gas and the equilib- rium pressure rather than between the weight of adsorbed gas and the pressure. As a matter of fact, when the logarithms of the volume V of liquid sulfur dioxide obtained by dividing the weight of adsorbed gas by the density of liquid sulfur dioxide at the corresponding pressure are used as ordinates, the curves are brought closer together. The next step is to plot logarithm V against logarithm of the ‘“‘corresponding pressure”’ a where P, is the vapor pressure of the condensed gas at the temperature in question. In this way it was found that greater volumes were taken up at lower temperatures at the same partial pressures, probably because the condensed phase is more compressible at the higher temperatures, the surface: tension being smaller.*. 1 PatTrick and McGavack: J. Am. Chem. Soc., 42, 946 (1920). 2Cf. Ray: J. Phys. Chem., 29, 74 (1925). 3 Parrick and McGavack: J. Am. Chem. Soc., 42, 976 (1920). HYDROUS OXIDES OF SILICON AND GERMANIUM 183 As an empirical relationship, dividing the volume of condensed sulfur dioxide by the value of the surface tension o raised to a ‘fractional power gives a correction in the right direction. The Freundlich equation thus takes the form P 1/n v=k( 2 _1/n @) ( ) o 1 ie or assuming the same value of F to hold for P. and o, o 1/n ro) ; This equation appears to be a general one for capillary adsorb- ents! and has been applied by Patrick and his pupils to the adsorption of sulfur dioxide, butane,? benzene, carbon tetra- chloride, alcohol,* and ammonia,‘ after correcting for the amount. dissolved in the gel water.® Since adsorption of gases takes place above the critical tem- perature where no condensation to liquid occurs under ordinary conditions, Patrick, Preston, and Owen® studied the adsorption of carbon dioxide and nitrous oxide in the region of the critical Po\\/" temperature. When the equation V =k P. was applied to the experimental results, it was found that k at 0° was not equal to k at higher temperatures near the critical point. This variation in k was attributed to an increase in surface tension of the liquid in the capillaries at temperatures near the critical temperature, owing to capillary forces. After correcting the surface tension, the equation was found to apply, indicating ; 1 that in all cases the constants k and * depend only on the struc- 1 Recently Munro and Johnson [J. Phys. Chem., 30, 172 (1926)] found the equation to hold for the adsorption of water vapor by alumina except when the partial pressure approaches the vapor pressure of the liquid at the temperature of the adsorbent. 2 PaTRicK and Lorn: J. Phys. Chem., 29, 336 (1925). 3’ Patrick and OppykeE: J. Phys. Chem., 29, 601 (1925). 4 DAVIDHEISER and Patrick: J. Am. Chem. Soc., 44, 1 (1922). 5 Cf. NEUHAUSEN and Patrick: J. Phys. Chem., 25, 693 (1921). 6 J. Phys. Chem., 29, 419 (1925). | 184 THE HYDROUS OXIDES ture of the silica gel. Since the theory applies even above the critical temperatures for CO, and N2O, Patrick suggests that the critical temperature is raised in the pores of the gel. Although the most active gel is obtained by heating in vacuo to 250 to 300°, the oxide calcined at 1000° still adsorbs more, water than charcoal up to 70 per cent humidity. The cutting down of adsorption by calcining at 1000° is due in part to coales- cence of primary particles and in part to crystallization. By igniting at a high temperature, the adsorption capacity for water is reduced to zero.” The heat of wetting of silica gel by water is 19.22 calories per gram of gel at 25°. It has been usual to attribute the heat of wetting of adsorbents by liquids to com- pression of the adsorbed liquids.4 Patrick, on the other hand, attributes the heat of wetting of silica gel to the filling up of the pores with water, whereby the water surface is reduced from its original very large value to practically zero. In support of this view he showed the net heat of adsorption (mean heat of adsorption minus heat of liquefaction) at 0° to be positive and to be equal to the heat of wetting, within the limits of experimental error.® Attention has been called to the absence of hysteresis in the adsorption of vapors other than water when care is taken to exclude air from the system. Adsorption of Liquids from Solution.—The adsorption from solution by silica gel was investigated in Patrick’s® laboratory for the following systems: formic acid, butyric acid, acetic acid, benzoic acid, and iodine from a series of solvents; nitrobenzene from kerosene; and acetic acid from carbon bisulfide throughout the entire range of concentration. Recently Schwarz! peptized a fresh silica gel with ammonia and removed the excess peptizing agent in a vacuum desiccator containing sulfuric acid. All of these preparations are similar to the sol formed by Graham’s method. Bradfield® obtained a sol with somewhat different properties by washing gelatinous silica with the supercentrifuge until it was practically free from electrolyte. If the washing is repeated a sufficient number of times, the hydrogen ion concentration of the sol becomes constant at pH = 6.5, whether approached from the acid or alkaline side. Hardy’ attributes this slight acidity to the ability of certain of the adsorbed water molecules to ionize. The highly purified sol can be concentrated on the water bath to a syrupy consistency which can be brought back to the original sol condition by adding water. If the boiling is carried too far, minute crystals of hydrous silica separate from the sol. Even 1 Ber., 39, 116 (1906). 2 Cf. Schwarz and ST6wWENER: Kolloidchem. Bethefte, 19, 171 (1924); Scuwarz and Leipe: Ber., 58, 1509, 1512, 1680 (1920); ScHwarz and LeonarpD: Kolloid-Z., 28, 77 (1921); GrunpMANnN: Kolloidchem. Beihefte, 18, 197 (1923). 3 GRIMAUX: Compt. rend., 98, 1484 (1884). 4 Espier and Frevitner: Ber., 44, 1915 (1911). 5 Kolloid-Z., 34, 23 (1924). 6 J. Am. Chem. Soc., 44, 965 (1922). 7 J. Phys. Chem., 30, 262 (1926). 8 Cf. BACHMANN: Z. anorg. Chem., 100, 1 (1917); Zstamonpy-SpPEarR: ‘‘Chemistry of Colloids,’ 137 (1917); Schwarz and St6wENER: Kolloid- chem. Beihefte, 19, 171 (1924), HYDROUS OXIDES OF SILICON AND GERMANIUM 195 the most concentrated sols show no tendency to gel, probably because the secondary aggregates have been broken up by repeated centrifuging and repeptization, leaving small groups of primary particles that entangle relatively little water. Lenher! prepared silica sol by grinding Ottawa sand for several days until the particles are less than 0.004 millimeter in diameter. When such finely divided silica is heated with an excess of water in a pressure bomb at 300 to 450°, gels are formed containing 15 to 18 per cent of water. Ray? claims that crystalline quartz is partly converted into amorphous silica by prolonged grinding; but the claim appears to be without experimental foundation. Colloidal silica has been recommended for the treatment of pulmonary tuberculosis. It is administered along with protein in the form of tablets or better by subcutaneous or intramuscular injection.* Great care must be taken not only in the preparation of the sol® but in its administration. The treatment appears to be of questionable value.’ Kramer® finds that the addition of animal or vegetable oils to a 0.2 per cent solution of sodium silicate gives a fine stable emul- sion in which many of the drops exhibit Brownian movement. _ The fatty acid of the oil combines with the alkali to form soap, liberating colloidal silica which acts as a protective colloid for the emulsion. The careful addition of dilute hydrochloric acid produces a silica gel emulsion, while the addition of lime water causes coagulation forming a cheese-like coagulum and a thin liquid. ‘Theseexperiments are said to reproduce synthetically the changes in the tissue which take place in tuberculosis: Alkali silicate forms a fat emulsion in the tissues. The hydrous silica in the emulsion has a strong affinity for lime which is with- - drawn from the blood and causes the caseation of the emulsified 1 J, Am. Chem. Soc., 48, 391 (1921). 2 Proc. Roy. Soc., 101A, 509 (1922). 3 SosmMaANn and Merwin: J. Wash. Acad. Sci., 14, 117 (1924). 4Kiun: Minch. med. Wochschr., 67, 253 (1920); Z. Tuberk., 32, 320 (1920); Kauue: Beitr. Klin. Tuberkulose, 47, 296 (1921); GoNNERMANN: Z. physiol. Chem., 99, 255 (1917). 5 Chem. Ztg., 45, 1249 (1921). 6 Gyr and Purpy: Brit. J. Expil. Paih., 3, 75, 86 (1922). 7 Kauiscu: Beitr. Klin. Tuberkulose, 58, 111 (1922). 8 Kolloid-Z., 31, 149 (1922). 196 THE HYDROUS OXIDES fat as in the experiments referred to above. Carbonic acid then acts slowly on the ‘‘silica cheese,” converting the lime into car- bonate, a process designated by the pathologists as calcification. There remains in the tissues the small amount of hydrous silica which served originally as the protective colloid for the emulsion. In line with this, Neyland found in tubercular lymph glands of oxen, a silica content of 0.27 gram SiOz in | kilogram of dry tissue while a calcified lymph gland contained 1.54 gram SiOz per kilo- gram of tissue. + SILICATE OF SODA The commercial ‘‘silicate of soda’’ or water-glass solutions so widely used as an adhesive or cementing agent, are colloidal solutions containing negatively charged particles of silica and soda stabilized by preferential adsorption of hydroxyl ion.? When soda ash is fused with more than one equivalent of silica, a glass results. If but slightly more than one equivalent is used, the glass may crystallize partially, giving a definite sodium metasilicate. Such fusions are slowly soluble in cold water and readily soluble in hot water; but the solubility decreases — as the proportion of silica increases. When the ratio is approxi- mately 1Na,O to 28102, complete solution is obtained with difficulty; and when it reaches 1Na2,O to 48102, special methods must be employed to effect solution. The commercial ‘‘silicates of soda” are not definite chemical individuals; but are variable systems of sodium oxide, silica, and water. The solution most commonly employed in this country consists approximately of 1Na.,O to 3.38102 To prepare the solution, the molten fusion of soda ash and sand is run into large revolving bins partially filled with water. By this procedure, the melt is shattered, giving a spongy mass that is fairly readily peptized by water. In a preparation containing between 18 and 35 per cent Na2O-3.358102, the ultramicroscope reveals myriads of particles so clearly distinguishable that they cannot be greatly hydrated. However, the increase in viscosity with increasing concentration is typical of emulsoid colloids, 1 Kane: Beitr. Klin. Tuberkulose, 47, 316 (1921). 2 STERICKER: Chem. Met. Eng., 25, 61 (1921). HY DROUS OXIDES OF SILICON AND GERMANIUM 197 The viscosity increases only slightly with the concentration for low values of the latter, but rises very rapidly when the concen- tration reaches a critical value. The slope of the viscosity- concentration curve is dependent on the sodium-oxide-silica ratio. ‘Thus a change in concentration of but 1 per cent in a solution of Na,O-:3.98iO.2 causes an increase in viscosity from 379 to 7000 centipoises; while an 8 per cent change in concentra- tion is necessary for asimilar increase, in a solution of NazO- 2S8iOx.. The rate of change of viscosity is important as a measure of the rate of set when the silicate is used as an adhesive. Gels formed by concentrating silicate sols containing a high percentage of silica are very elastic. Stericker! reports that balls of the gel dropped 40 feet will rebound two-thirds of the distance; and yet, like fluids, they will take the shape of the con- tainer in which they are placed. When a solution of water glass is neutralized by an acid, it sets to a jelly sooner or later, provided the concentration is not too low. ‘The speed of gelation is determined by a number of factors among which may be mentioned the concentration, the excess of hydrogen or hydroxyl ions present, the impurities, the kind of acid used, and the temperature.2. Holmes gives directions for preparing various types of jellies setting in any required time. The addition of concentrated sodium chloride to water-glass solutions throws down a gelatinous precipitate which tends to become granular and hard; an excess of brine causes repeptiza- tion of the gel. Malcolmson’ took advantage of this behavior to increase the volume of silicate solution without altering its viscosity appreciably. By proper adjustment of the concen- tration of brine, it was possible to extend the volume approxi- mately 21 per cent. Unfortunately, the adhesive properties of the extended solution are not so good as those of the original silicate; and the cost of mixing is usually greater than that of the silicate replaced.* 1 Bogue’s ‘Colloidal Behavior,” 2, 565 (1924). 2 FLEMMING: Z. physik. Chem., 41, 427 (1903); Houmus: J. Phys. Chem., 22, 510 (1918). 3 J. Ind. Eng. Chem., 12, 174 (1920). 4 STERICKER: Bogue’s ‘Colloidal Behavior,” 2, 569 (1924). 198 THE HYDROUS OXIDES The addition of iron or aluminum salts to water glass yields a gelatinous precipitate of variable composition containing hydrous silica and hydrous ferric or aluminum oxide as the case may be. The action between alum and water glass is made use of in the mineral sizing of paper.t The gelatinous precipitate is adsorbed by the paper, giving it a smoother and harder finish than is obtained in its absence. The precipitate sizes for printers’ ink which has an oil base; and it increases the resistance to aque- ous inks, possibly because of an increased retention of resin in the paper. When small crystals of various metallic salts are dropped into an 18 per cent solution of commercial water glass, growths resembling plant shoots spring up, giving rise to the so-called “artificial vegetation” or ‘‘colloidal forest.”? The growths are colored when colored salts are used, but the water-glass solution does not become colored except in the case of manganese salts. The form of the growths is different with different metals. For example, hair-like filaments result with cadmium salts and thick fungoid growths with nickel salts. The growths are tubular and act as semipermeable membranes. Applications—Commercial water glass finds application in a great many branches of industry. Attention has been called to its use in paper sizing and asan adhesive. For the latter purpose it is said to be the only substance employed in the fiber-container industry for gluing together the components of both double- faced corrugated board and laminated solid fiber board.* It is also used for impregnating sandstone and other porous stones as a protection against weathering. ‘This is accomplished by treat- ing the stone with water glass followed by the application of a solution of calcium or aluminum sulfate, which precipitates an insoluble gel in the pores of the stone, greatly increasing its hardness and durability. It is also employed as a cement in the manufacture of artificial stone from sand and lime. A mixture of 2 parts fluorspar and 1 part powdered glass, made into 1VaiL: Chem. Met. Eng., 25, 823 (1921); Srmrickrer: Paper Ind., 5, 1398 (1923). 2 Do.uiries: Compt. rend., 143, 1148 (1906); Ross: Proc. Roy. Soc. New South Wales, 44, 583 (1910); cf. J. Chem. Soc., 102, II, 49 (1912). 3 Mautcotmson: J, Ind. Eng. Chem., 12, 174 (1920). HYDROUS OXIDES OF SILICON AND GERMANIUM 199 a thick paste with water glass, gives a cement for glass and porcelain. On account of its detergent properties, water glass is frequently added to cheap soap.! It is employed in the calico-printing and dyeing industry and in fixing fresco colors by the process of stereochromy. It is also used for rendering wood, paper, etc. inflammable; and to a limited extent, in preventing wood from rotting and in the preservation of eggs. Because of its peptizing or deflocculating action, silicate of soda may be employed to produce the clay ‘‘slip”’ from which the casts are made in the manufacture of pottery and sanitary ware. An undeflocculated slip containing around 20 per cent of water is a stiff plastic mass. By working into it about 0.1 per cent each of Na,O- 3.3810. and sodium carbonate, the mixture becomes sufficiently fluid that it can be pumped readily through l-inch pipes. This casting slip is then run into plaster molds which adsorb the water and flocculate the clay. Silicate of soda or some similar agent must also be used to prepare the slip in the electrical casting of clay.? The deflocculating action of silicate of soda on siliceous and argillaceous material has been applied in concentrating ore by flotation. Sulman* found that less of the gangue constituents are carried into the froth, the greater their degree of dispersion. The importance of silicate of soda solutions in flotation is due to the wide range of their deflocculating action on the gangue. On the other hand, some ores suchas copper sulfide are flocculated, thereby facilitating their flotation. Another method of ore concentration consists in deflocculating the gangue and removing it from the settled concentrate by decantation.* THe Hyprous OXIDES oF GERMANIUM Germanium Dioxide——Hydrous GeO, is precipitated in a gelatinous form by the hydrolysis of germanium tetrachloride® 1 Cf, RicHARDSON: J. Ind. Eng. Chem., 15, 241 (1923); SrmrickEr: [bid., 15, 244 (1923). 2 KLEEMAN: Phys. Rev., 20, 212 (1922). 3 Bull. Inst. Mining Met., 29, 49 (1920). 4 BorcHerpT: U.S. Patents 1446375 to 1446378; 1448514, 1448515 (1923). 5 WINKLER: J. prakt. Chem., (2) 34, 211 (1886); DmnNis and JOHNSON: J. Am. Chem. Soc., 45, 1380 (1923). 200 THE HYDROUS OXIDES or tetrabromide.'! It is also obtained by passing carbon dioxide into a solution of the oxide in alkali.2 It forms no hydrates,’ but it holds on to the last trace of adsorbed water quite strongly, complete dehydration requiring a temperature of 950°.4 The precipitated oxide is fairly soluble in water, giving an acid solu- tion® from which microscopic rhombic crystals® separate on evaporation. Miiller and Blank’ recognize three distinct preparations: (1) The hydrolyzed oxide obtained by hydrolysis of GeCl,. This gel forms with cold water a milky suspension which clears up on boiling. It is readily soluble in hydrofluoric and hydrochloric acids. (2) The ‘‘evaporated”’ oxide resulting from evaporation of the aqueous solution. ‘This preparation is but slowly soluble in cold water but dissolves in hot water after a short time. (3) The ‘insoluble’ oxide prepared by heating the evaporated oxide to any temperature between 200° and its melting point (about 1100°) and then boiling the mass thoroughly with water to remove the unconverted ‘‘evaporated”’ oxide. This prepara- tion is insoluble in water and in boiling hydrochloric and hydro- fluoric acids, alkali, and ammonia, but becomes soluble on fusion. Analogous to the behavior of different preparation of silica and stannic oxide, the varying properties of the three germanium oxides might be due to differences in size of the primary particles and physical character of the precipitates. Against this hypoth- esis, it was shown: that the yield of ‘‘insoluble” oxide formed by heating the evaporated oxide at different temperatures for the same period of time increased up to 380° and then decreased to the melting point. This suggests that 380° may represent the temperature of maximum velocity of transformation of the evaporated oxide into the ‘‘insoluble” form. When the time of heating of the evaporated oxide was varied at the constant temperature of 280°, the yield of the ‘‘insoluble” form increased 1 Dennis and Hance: J. Am. Chem. Soc., 44, 299 (1922). 2 WINKLER: J. prakt. Chem., (6) 34, 177 (1886). 3 Van BEMMELEN: Rec. trav. chim., 6, 205 (1887). 4 DENNIs, TRESSLER, and Hancn: J. Am. Chem. Soc., 45, 2033 (1923). 5 WINKLER: J. prakt. Chem., (2) 34, 211 (1886); Mt.Lurr and Iszarp: J. Med. Sci., 163, 364 (1922). | 6 HausHorer: Sitzb. Akad. Miinchen, 1, 133 (1887). 7 J. Am. Chem. Soc., 46, 2358 (1924). HYDROUS OXIDES OF SILICON AND GERMANIUM 201 in such a manner as to suggest that the conversion could never reach 100 per cent. For this reason and because the yield of the insoluble form varied greatly with different preparations of evaporated oxide, Miiller and Blank suggest that three allo- tropic forms of germanium oxide may exist, the evaporated oxide being a mixture of two of them. | Germanium dioxide is said to be of value in the treatment of secondary and pernicious anemia.! Germanous Oxide.—Unlike silicon but like tin, germanium forms an ‘‘ous”’ oxide. ‘This is precipitated in a gelatinous form by the action of alkalies on a solution of GeCl, or by the hydrolysis of germanium chloroform, GeHCl;. When thrown down in the cold, the precipitate is yellow but it becomes yellowish red by boiling in the mother liquor. It is peptized by boiling water, giving a yellow sol. According to Hantzsch,? it is very slightly soluble in water, acting as a weak monobasic acid of the constitu- tion HGeO: OH, analogous to formic acid. 1Lenker: Penn. Med. J., 26, 86 (1922); Kast, Crouti, and Scumitz, J. Lab. Clin. Med., 7, 643 (1922); MititumrR and Iszarp: J. Med. Sci., 163, 364 (1922); J. Metabolic Research, 3, 181 (1923); cf., however, Minor and Sampson: Boston Med. Surg. J., 189, 629 (1923). 2Z. anorg. Chem., 30, 289 (1902). CHAPTER VIII THE HYDROUS OXIDES OF TIN AND LEAD HyprRous STANNIC OXIDE ‘As early as 1812 Berzelius' called attention to differences between the hydrous oxide formed by precipitation of stannic chloride with alkali and the product resulting from the action of nitric acid on tin. Berzelius thought at first that he was dealing with two degrees of oxidation; but this was disproved by subse- quent investigations of Davy, Gay Lussac, and Berzelius? himself. Thus, Berzelius was led to conclude that the two preparations, having widely different properties, were simply modifications of the same oxide. In this way the term isomer or isomeric modifica- tion was introduced in chemistry.® PRECIPITATED HYDROUS STANNIC OXIDE Since the oxides formed by precipitation of stannic salts and by the action of nitric acid on tin both give a very slight acid reaction when shaken with water, they are commonly designated as orthostannic and metastannic acids, respectively. The earlier chemists regarded them as distinct chemical individuals and recognized the similarities and differences between the two that are listed in Table XVII. In the light of what is now known of the colloidal state of matter, the statements of earlier chemists concerning the properties of these bodies are inaccurate in many respects. Since both substances are more properly termed hydrous oxides rather than acids, I shall designate them as alpha . oxide and beta oxide, respectively, instead of as orthostannic and metastannic acid. 1“Tehrbuch,” 5th ed., 2, 596 (1812). 2 Ann. chim. phys., [2] 5, 149 (1817). 3 Apeaa@: ‘Handbuch anorg, Chemie,”’ [2] 3, 593 (1909), 202 THE HYDROUS OXIDES OF TIN AND LEAD TaBLe XVII 203 Orthostannic acid Metastannie acid Preparation.......| Precipitation from solu-| Action of concentrated tion of stannic salt HNO; on tin Lin ECW aS ae H.SnO; H.SnO; Action of HNO;...{ Easily soluble Insoluble Action of HCl..... Easily soluble; not precipi- | Insoluble. Product treated : tated by excess acid with concentrated acid and filtered dissolves in water but precipitates again with excess acid Insoluble but swells in con- centrated acid, forming a mass that is soluble in water Soluble when freshly pre- pared; precipitated by excess alkali Yellow precipitate solution in HCl Action of H2SO,...| Easily soluble Action of caustic | Easily soluble; not precipi- alkalies. tated by excess alkali from Action of SnCl,....| No action Formation.—The typical a oxide is prepared by precipita- tion of SnCl, or SnBr, with alkali! or with an excess of the car- bonate of barium or calcium;? and by precipitating a solution of soluble crystalline stannate having the formula M.Sn(OH),' with mineral acid.* Rose® claimed to get the a oxide by hydrol- ysis of a dilute solution of SnCl, at the boiling point. This is unquestionably incorrect, since it has been observed repeatedly that a oxide, formed by hydrolysis of SnCl, at low temperatures, goes over to @ oxide gradually on standing or very rapidly at the boiling point. Similar observations have been made with SnBr,’ and with Sn(NO3),. Lorenz® obtained the a oxide by 1 BerzeEvivs: “Lehrbuch,” 5th ed., 2, 1596 (1812). 2 Scuirr: Liebig’s Ann. Chem., 120, 47 (1861). 3 BeLLucci and ParRravaNno: Z. anorg. Chem., 45, 142 (1902). 4Fremy: Ann. chim. phys., (3) 12, 463 (1844); 23, 385 (1848); Kita: Pharm. Zig., 538, 49 (1908). 5 Pogg. Ann., 75, 1 (1848). 6 BarrorD: J. prakt. Chem., 101, 368 (1867); EncEL: Compt. rend., 124, 765 (1897); 125, 464, 651, 709 (1897); Zstamonpy: Liebig’s Ann. Chem., 301, 368 (1898). 7 Lorenz: Z. anorg. Chem., 9, 371 (1896). 8 Z. anorg. Chem., 12, 436 (1896), 204 THE HYDROUS OXIDES electrolyzing an alkali chloride, nitrate, or sulfate solution using a platinum cathode and a tin anode. The typical 6 oxide is prepared by the oxidation of tin with moderately concentrated HNO3. Weber! claimed that acid of 1.2 sp. gr. gave both a and B oxides, while acid of 1.35 sp. gr. produced a clear solution from which @ oxide was obtained by warming. Hay? and Scott’? likewise observed the complete dissolution of tin in moderately dilute nitric acid (1:1) at 2°, from which 8 oxide precipitated by warming or by standing at ordinary temperatures. ‘The solution contained stannous nitrate stannic nitrate,* and doubtless colloidal stannic oxide® in varying amounts depending on the concentration of acid and the tem- perature. As before noted, 8 oxide is produced whenever a dilute solution of a crystalline tin salt undergoes hydrolysis at the boiling temperature. A solution of amorphous sodium meta- stannate, so called, likewise precipitates 6 oxide when heated. From this survey, it is evident that either oxide may be pre- pared by hydrolysis of stannic salts under suitable conditions. In all probability the first product of this hydrolysis is always a oxide, which subsequently goes over to 8 oxide quite slowly at ordinary temperatures but with increasing rapidity as the tem- perature is raised. ; Composition.—By drying different precipitated oxides under the proper conditions, earlier investigators have reported the preparation of a wide variety of supposedly definite hydrates and hydrated acids. An extensive study of the change in vapor pressure of different preparations with the temperature led van Bemmelen to conclude that such compositions were purely acci- dental, depending on the method of formation, the method of drying, the temperature, and the age of the sample. Van Bemmelen’s observations were confirmed and extended, and his 1Pogg. Ann., 122, 358 (1864). 2 Chem. News, 22, 298 (1870). 3 Chem. News, 22, 322 (1870). 4 WaLkER: J. Chem. Soc., 68, 845 (1893). 5 MECKLENBURG: Z. anorg. Chem., 64, 370 (1909). 6 Fremy: Ann. chim. phys., (3) 12, 463 (1844); 23, 393 (1848); cf. WEBER: Pogg. Ann., 122, 358 (1864); cf. Granam: Liebig’s Ann. Chem., 18, 146 (1835); Scuarrner: Jbid., 61, 168 (1844); CaRNELLEY and WALKER: J. Chem. Soc., 58, 83 (1888). THE HYDROUS OXIDES OF TIN AND LEAD 205 conclusions reaffirmed by Lorenz' and Mecklenburg.? Recently, however, Willstatter and his collaborators? adopted the older view that the behavior of the variety of oxides could be explained best by assuming the existence of more or less stable hydrates. Willstatter claimed to remove all the adsorbed water from a compound by drying rapidly in vacuum or by leaching with acetone. The composition of a gel formed in a special way and dried by the acetone method at —35° to +10° was represented by the formula Sn(OH),: HO; but when dried at room temperature the analysis showed a composition Sn(OH),4, which was regarded as the first member of a series of a stannic acids. In an aqueous medium, Sn(OH).4 was supposed to go over into other less basic members of the series. Thus by suitable conditions of precipi- tation and drying with acetone at 0 to 10°, orthodistannic acid was supposedly formed; at 35 to 46°, orthotristannic acid; and soon. Different so-called 8 stannic acid were likewise prepared and many of them assigned formulas. As proof of hydrate formation, Willstitter cites the regions of almost constant water content in the temperature-composition curves of acetone-dried preparations. Such evidence is alto- gether inconclusive, particularly when the nature and location of the “flats” in the curves are determined almost exclusively by the history of the sample. The same may be said of the ‘flats’ in the temperature-vapor-pressure curves of van Bemme- len. The adsorptive capacity of a hydrous oxide for water at different stages of dehydration is determined by the physical character of the preparation; hence a “‘flat”’ corresponding to a definite hydrate is purely accidental and can be duplicated only by following a set method of procedure in precipitation, ageing, and drying. Willstatter’s comparison of the behavior of hypo- thetical high-molecular hydrated stannic acids with their groups Sn:O and Sn: OH, to that of carbohydrates with their groups C:O and C- OH, appears highly fantastic and illusionary. Action of Acids.—F reshly prepared a oxide is readily soluble in dilute HCl and is not precpitated by an excess of acid even at the boiling point; whereas 6 oxide is insoluble in both dilute and 1Z. anorg. Chem., 9, 369 (1895). 2Z. anorg. Chem., 64, 368 (1909); 74, 207 (1912); 84, 121 (1914). 8 WILLSTATTER, Kraut, and Fremery: Ber., 57 B, 63, 1491 (1924). 206 THE HYDROUS OXIDES concentrated acid. However, if @ oxide is treated with concen- trated HCl, a gelatinous mass is formed which Engel! believes to be a salt, metastanyl chloride. This product is taken up by water but is reprecipitated by boiling or by adding concentrated HCl. In view of the variety of acids that are supposed to be derived from stannic oxide, it is not surprising to encounter a number of basic salts of tin. Thus Engel claims to get SnCh, SnsOsCl.:4H.O, and Sn;O9Cle:2H2O corresponding to his ortho, meta, and para acids.2_ While SnCl, is obtained by the action of concentrated HCl on the @ oxide* and H2SnCl,* is formed by passing gaseous HCl into a solution of stannic chloride, there is little or no evidence to support the view that the amorphous precipitates, obtained from solutions of a@ and 8 oxide under varying conditions, are definite compounds. Van Bemmelen® was the first to recognize the real nature of such solutions. He proved it to be incorrect to speak of “solubility” of the oxides in acid by showing that the acid which holds the a oxide in what was thought to be a true solution may be neutralized almost entirely without the oxide precipitating; that the salt formed may be removed by dialysis without precipitation taking place; and finally, that the solutions may be boiled, converting a oxide into B, which likewise does not precipitate unless the boiling is continued too long. Van Bemmelen also observed the adsorp- tion of HCl by both oxides. Below the concentration which causes peptization, the adsorption isotherms have the usual form, indicating that the amount adsorbed depends on the concentration of acid in contact with the oxide. The adsorption was found to be less with 6 oxide than with a, and the older and denser the 8 oxide, the less was the adsorption. The action of hydrochloric acid on the different oxides can now be explained. The newly formed oxide possesses a softer and 1 Compt. rend., 724, 765 (1897); 125, 464, 657, 709 (1897). 2 Cf. TSCHERMAK: J. prakt. Chem., 86, 334 (1862); Mauuet: J. Chem. Soc., 35, 524 (1879); ScomurER-KeEstnER: Ann. chim. phys. (3) 58, 471 (1860); Orpway: Am. J. Sci., (2) 28, 220 (1857); cf. also Rosz: Pogg. Ann., 75, 1 (1848); WirrstrIn: Jahresber., 1850, 321. 3 Barrorp: J. prakt. Chem., 101, 368 (1867). 4 Kowatwsky: Inaugural Dissertation, Breslau (1902). 5 “Die Absorption,” 56, 393 (1910); Z. anorg. Chem., 23, 111 (1900). 6 GRAHAM: Liebig’s Ann, Chem., 121, 1 (1861). THE HYDROUS OXIDES OF TIN AND LEAD 207 looser structure than the 6 oxide, and so the former is readily peptized by dilute acid and the colloid is stable even in the pres- ence of a very small amount of acid. A high concentration of acid converts it into a true solution of SnCl. The coarser, denser particles of @ oxide are insoluble and are not peptized by dilute HCl. Concentrated acid, on the other hand, peptizes the oxide; and if the excess of acid is poured off, the particles will go into solution in water (dilute hydrochloric acid) from _ which they are precipitated by excess acid or by boiling. Since a oxide changes to @ even at ordinary temperatures and in contact with water, we should expect the colloidal solution of a oxide formed by hydrolysis of a dilute solution of SnCl, gradually to assume the properties of the dilute hydrochloric acid solution of 8 oxide, as observed by Fremy, Rose, L6wenthal,! and others.? In the light of the experiments of van Bemmelen, it is unlikely that the amorphous masses obtained by Engel and others are definite chlorides. Further doubt is thrown on this by Biron,’ who obtained products similar to Engel’s meta and para chlor- ides, but found their composition to be indefinite. Mecklenburg investigated the properties of the oxides obtained by the simultaneous action of various mixtures of hydrochloric and nitric acid on tin. The products adsorbed both acids in proportion to the relative concentrations in the original solution; the ratio, total acid:SnOe, remaining approximately 0.5:1. The greater the hydrochloric acid content of the hydrous oxide, the more readily it was peptized by water, a circumstance which led Mecklenburg to attribute to this acid a protecting action similar to that of a protective colloid. Collins and Wood‘ regard the various stannic oxides as salt- like complexes formed by continued condensation between molecules of stannic hydroxide acting as acid and base, respec- tively. The peptization by hydrochloric acid is looked upon as essentially a chemical process, although the first stage in the process is recognized as adsorption of hydrochloric acid by the 1 J. prakt. Chem., 77, 321 (1859). 2 BaRFOED: Loc. cit.; ALLEN: J. Chem. Soc., 25, 274 (1872); Lorenz: Z. anorg. Chem., 9, 369 (1895). 3 J. Russ. Phys.-Chem. Soc., 36, 933 (1904). 4 J. Chem. Soc., 121, 441 (1922). 208 THE HYDROUS OXIDES oxide particles in varying amounts, depending on the extent of surface which in turn is proportional to the degree of condensa- tion. Following this adsorption, a reaction is thought to take place between the adsorbent and the adsorbed acid, due to neu- tralization of the latter by some of the basic affinities of the original stannic hydroxide still possessed by the condensed acid. The resulting salt will yield a positive complex ion and chloride ion, if the ionization is not prevented by the presence of too high a concentration of chloride ion from ionization of hydrochloric acid. While the behavior of oxides prepared in different ways has been interpreted by the aid of these assumptions, there seems to be no real justification for postulating the existence of a wide variety of condensed stannic acids and complex basic salts. It seems much more likely, particularly in the light of the recent observations of Pascal! and Yamada,” that the various products are simply hydrous stannic oxides that have adsorbed hydrogen and chloride ions, the positive charge on the colloidal particle arising from preferential adsorption of hydrogen ion. Sulfuric acid acts on both a and 8 oxide in much the same way as HCl. Dilute HNO; peptizes a oxide quite readily when the latter is freshly prepared; but 6 oxide is neither dissolved nor peptized by even the most concentrated acid. However, 6 oxide adsorbs HNO; to a certain degree and the adsorbed acid can be removed only by prolonged washing.*® The adsorbing power of 8 oxide for phosphoric acid deserves special mention, since a standard analytical procedure for sepa- rating this acid from mixtures consists in adding tin foil to the nitric acid solution, the resulting 6 oxide carrying down the H3PQO,.4 For the complete precipitation of 1 mol of phosphoric acid, 6 to 7 atoms of tin must be present according to Classen;° 7 according to Antony and Mondolfo;® and about 13 according 1 Compt. rend., 175, 1063 (1922). 2 J. Chem. Soc. Tokyo, 44, 175 (1928). 3 JORGENSEN: Z. anorg. Chem., 57, 353 (1908). 4 Reyonoso: J. prakt. Chem., 54, 261 (1851); Roscoz and ScHORLEMMER: “Treatise on Chemistry,” 2, 899 (1923); Reisia: Liebig’s Ann. Chem., 98, 339 (1856); Girarp: Compt. rend., 54, 468 (1862). 5 “ Angew. Methoden analyt. Chem.,”’ 2, 555 (1903). 6 Gazz. chim. ital., (2) 38, 145 (1898). THE HYDROUS OXIDES OF TIN AND LEAD 209 to Wobling.t| Mecklenburg? showed conclusively that the removal of H3;PO, by precipitated stannic oxide was not due to the formation of a definite stannic pyrophosphate, but to adsorp- tion, the amount of acid carried down depending not only on its concentration but on the nature of the oxide. The adsorption isotherms for five oxides, prepared at different temperatures, showed a decreasing adsorption capacity with increasing temper- ature of formation. Action of Alkalies——Dilute caustic alkalies carry both the a and 6 oxides into solution. For a long time this solution was believed to taak place by virtue of the formation of definite alkali stannates and metastannates, largely because evaporation of solu- tions of a oxide in strong alkali yield definite crystals having the formula M.Sn0O3-3H.O? or M2Sn(OH)..4 However, if the solu- tion of the a oxide in dilute alkali is not evaporated but treated with alcohol, a precipitate is formed, varying in composition from 5 to 17 mols of SnO, to 1 mol of K,O, depending altogether on the relative amounts of the two substances in solution. In like manner, the precipitate obtained from a solution of 8 oxide in alkali varies very widely, depending as it does on the condi- tions of formation.® Twenty-five years ago, van Bemmelen® called attention to the colloidal nature of the solutions of both a and @ oxides in alkalies. He agitated the same amounts of a oxide with like volumes of cold dilute KOH of various concentrations; when the concentration of alkali was less than 8.8 mols of K,O to 100 of SnO., the peptized oxide precipitated spontaneously, carrying down with it a greater part of the alkali. Asis usual, the amount 1 “Tehrbuch anal. Chem.,’”’ 405 (1911). 2Z. anorg. Chem., 74, 215 (1912). 3 Orpway: Am. J. Sci., (2) 40, 173 (1865); Marianac: Ann. mines, (3) 15, 277 (1859); Mosere: J. prakt. Chem., 28, 230 (1848). 4Beutuuccr and PaRRAvANO: Z. anorg. Chem., 45, 142 (1902). Other hydrates are also known; HanrriEy: Dinglers polytech. J., 144, 66 (1867); Jonas: Chem. Zentr., 607 (1865). 5 Wremy: Ann. chim. phys., (8) 12, 460 (1844); 23, 385 (1848); Rosp: Pogg. Ann., 75, 1 (1848); 105, 564 (1858); Weper: Jbid., 122, 358 (1864); Muscuuus: Compt. rend., 65, 961 (1867); Mosura: Berzelius’ Jahresber., 22, 144 (1848). 6 “Die Absorption,” 57 (1910), 210 THE HYDROUS OXIDES of KOH adsorbed by a given amount of oxide varied with the temperature and concentration, and there was no indication of the formation of definite stannates. He thus accounted for the wide variation in the composition of the precipitate thrown out by alcohol from the colloidal solution of the @ oxide in alkali, as observed by Ordway and others. Van Bemmelen obtained similar results with the more difficultly peptizable 8 oxide. The first action of dilute NaOH was to produce an opalescent solution that, in itself, showed the oxide to be in the colloidal state. The relative amounts of oxide and alkali in the solution were varied widely; and, as in the case of a oxide, spontaneous precipitation took place the more rapidly, the greater the relative amount of SnOe. These colloids were coagulated by excess alkali, which was not the case with colloidal a oxide. The precipitated oxides obtained in any case, adsorbed alkali in varying amounts, depending on the alkali concentra- tion and the physical character of the precipitate.! Some observations of Heinz? and of Franz® give some idea ys the relative ease of peptization of different oxides by alkali. The former prepared a colloidal solution of an a oxide in which the ratio K,0: SnO, was 1:200; and the latter obtained colloidal B oxides in which this ratio varied from 1:25 to 1:50. As is usual, the peptizing action of potassium hydroxide is greater than that of sodium hydroxide, the precipitating power of K’ ion being appre- ciably smaller than that of Na’ ion. Mordanting Action.—Tin salts, particularly SnCl, are some- times used as mordants in dyeing cotton, wool, and silk. The salt is adsorbed by the dye fiber and subsequently hydrolyzes, giving hydrous stannic oxide which forms lakes with certain dyes that are distinguished by their brilliancy.* Vignon® studied the action of both 6 and a oxide on a basic dye, pheno- safranine; with the former, a brilliant-red lake was formed, while with the latter no dye was taken up. Thus the dye is readily 1Cf. WinTGEN: Z. physik. Chem., 108, 238 (1923). 2 Dissertation, Géttingen (1914). 3 Dissertation, Géttingen (1913). 4WeisER: J. Phys. Chem., 26, 424-427 (1922); cf. Cottins and Woop: J. Chem. Soc., 121, 2760 (1922). ® HerzFeLp: ‘‘Das Farben und Bleichen der Textifasern,” 1, 73, (1904). 6 Compt. rend., 112, 580 (1891), THE HYDROUS OXIDES OF TIN AND LEAD 211 adsorbed by the loose finely divided particles of a oxide, while the larger denser particles of 8 oxide have comparatively little adsorbing power.! This behavior is general and has led to the statement: “The formation of metastannic acid during the prep- aration of tin mordants is called firing; it must be avoided, since this substance has no mordanting power and its generation involves loss of tin.’’2 | The Question of Isomers.—A survey of the properties of the so-called a and 8 oxides discloses marked differences in their solubility, adsorbability, and ease of peptization. The typical a oxide is quite soluble in concentrated acids and alkalies forming definite salts under suitable conditions; it possesses a marked capacity for adsorption and is readily peptized. The typical 6 oxide, however, is very difficultly soluble, has a comparatively sight capacity for adsorption, and is not peptized by dilute acids or alkalies. If these two oxides are definite isomers, then any product having properties intermediate between the two might be looked upon as a mixture. If the difference in the properties of the two, however, is due to variation in the size of the primary particles and the compactness of the hydrous mass, then any product with intermediate properties must be a chemical individual and not a mixture. Van Bemmelen came out definitely against the view that the difference between the two oxides is due to allotropy rather than to physical structure. He called attention to the absence of a definite inversion point at which a oxide goes over to 8, and demonstrated the slow but continuous transformation at ordinary temperatures even under water. Mecklenburg? comes to a similar conclusion: ‘‘The a@ and £6 stannic acids are hydrous oxides that are little if at all soluble in water, and differ from each other in the size of the particles.” Mecklenburg‘ prepared five distinct oxides by hydrolysis of stannic sulfate at 0, 25, 50, 75, and 100°. ‘These oxides were dried in the air and ground toa powder. Each product was differ- 1Cf, Moruey and Woop: J. Soc. Dyers Colourisis, 39, 105 (1923). 2 KnEcHT, Rawson, and LOwentuHa.: ‘‘A Manual of Dyeing,’ 1, 272 (1916). 3 Z. anorg. Chem., 64, 368 (1909); 74, 207 (1912); 84, 121 (1914). 4Z. anorg. Chem., 74, 207 (1912). 212 THE HYDROUS OXIDES ent from the others, and with few exceptions, the properties approached more nearly to those usually attributed to 6 oxide, the higher the temperature of formation. There was no apparent connection between the compactness of the dried powder and the temperature of preparation; thus, the 100° oxide contained the least water and was most voluminous. ‘This was possibly due to some variation in the conditions of drying and grinding of the several products, for the volume of the precipitated oxide depends on the temperature of formation in a perfectly regular fashion,! the oxide formed at the highest temperature being the most compact. Mecklenburg found little difference in the ease of peptization of his oxides with concentrated HCl, except the 50° oxide which appeared most difficult to peptize. It seems to me altogether unlikely that 0 and 100° oxides should be peptized with equal facility. It is more probable that the difference in peptizibility was not detectable with concentrated acid. Meck- lenburg observed an increase in the precipitation concentration of sodium sulfate for the 100° oxide peptized by HCl on mixing with it the 0° oxide or one freshly prepared by hydrolysis of stannic chloride. From this he concludes that the different oxides cannot be mixtures of definite a and 8 isomers. While the conclusion is doubtless correct, it is certainly not justified from his precipitation experiments; on the contrary, these would seem to support the view that the oxides are mixtures. Thus, the so-called ‘‘sulfate value” of 100° oxide alone is 0.04 cubic centimeter; but when mixed with 10 to 90 per cent of a freshly prepared oxide it varies from 0.15 to 1.8 cubic centimeters. Since certain ones of his 50 and 75° oxides have sulfate values within the limits found for the mixtures, it might be argued that all of his preparations are mixtures. It should be noted in passing, that Mecklenburg’s observations are exactly what one should expect. The adsorption of the sulfate ion by the fresh oxide is much greater than by the 100° oxide; hence, the initial amounts added are all taken up by the former, and the precipitation of a given amount of the latter cannot take place until a higher sulfate concentration is reached. In order to determine the relative peptizability of hydrous stannic oxides formed at different temperatures, some experi- 1 Weiser; J. Phys. Chem., 26, 667 (1922). THE HYDROUS OXIDES OF TIN AND LEAD 213 ments were carried out! on the moist instead of the dried oxides, using dilute instead of concentrated acid. In these experiments dilute nitric acid was used, since it is known that this acid peptizes a oxide, whereas it has neither a peptizing nor a solvent action on the typical 6 oxide. Accordingly, the behavior with dilute nitric acid of the oxides formed under different conditions should give not only a measure of the relative peptizability but should indicate whether the oxides are definite individuals TaBLE XVIII.—PEpTIzATION oF Hyprous STANNIC OXIDES WITH Nitric ActIp Age of | Temperature of samples,| precipitation, Observations minutes degrees 5 : 23 Peptized rapidly; solution cloudy after 10 min- utes but clear in 15 minutes’ 40 Peptized more slowly; solution very cloudy after 15 minutes, quite cloudy after 30 minutes but clear with only a slight opalescence in 45 minutes 58 Peptized very slowly; solution very cloudy after 1 hour, clearing in 2 hours, and clear with slight opalescence after 3 hours 100 Peptization far from complete after 8 hours 10 Be Peptized slowly; solution quite cloudy after 2 hours and slightly opalescent after 4 hours 39 Peptized very slowly; no residue, but solution very cloudy after 5 hours; transparent but cloudy after 10 hours 58 Most peptized in 15 hours but solution opaque 100 But little peptized differing in solubility, adsorbability, and peptizability, or whether they are mixtures of a definite a oxide peptizable by nitric acid with a definite B isomer not peptizable by this acid. One-gram samples of SnO:, were precipitated at varying tem- peratures, the mixtures centrifuged, and the supernatant liquid discarded. The precipitates were shaken with 100 cubic centi- 1 Weiser: J. Phys. Chem., 26, 654 (1922). 214 THE HYDROUS OXIDES meters of 1.25 N nitric acid either’5 or 10 minutes after precipita- tion, as recorded in Table XVIII. It will be noted that there is a distinct difference in the peptizability of the oxides prepared at different temperatures. The loose, finely divided and highly hydrous particles of the oxide formed at room temperature are peptized readily by nitric acid; whereas the more compact, coarser and less hydrous particles formed at higher temperatures are less readily peptized. Moreover, the 40 and 60° oxides are not readily peptized by dilute HNO; and so would not be designated as a oxides; but they are peptized after a time, which proves them to be neither 6 oxide nor mixtures of a and B oxides. Conclusions as to the relationships among the various stannic oxides, deduced from investigations of their behavior with chemi- cal reagents, are supported in a striking way by recent studies of their physical characteristics. Thus Pascal’ compared the theoretical values of the molecular magnetic susceptibilities for the hypothetical acids Sn(OH), and SnO(OH). with the values for the hydrous oxides obtained by various methods. ‘The results show that the ‘‘acids” are not definite compounds but are mixtures of anhydrous stannic oxide with water in varying amounts, depending on their history. Quite similar conclusions were reached by Yamada? from x-ray analysis of natural cassiter- ite and of ten samples of hydrous oxides prepared by the methods of (a) Zsigmondy,* (b) Schneider,* (c) Collins and Wood,® from SnCl, and marble (SnO.-4.2H20), (d) Graham,® (e) Rose,’ (f) Collins and Wood, oxidation of tin by HNOs:, (g) Engel,® (h) desiccating sample (f) in a vacuum, (z) drying sample (f) at 100° (SnQz2 - 1.1H2O), (7) heating sample (f) to redness. From the photographs were measured the distances of the lines from the center, their angles, and their intensities. All the samples, irrespective of their history, contained a similar central nucleus; hence, the physical difference among them is due not to chemi- 1 Compt. rend., 175, 1063 (1922). 2 J. Chem. Soc. Japan, 44, 210 (1923); Cf. Posnsax: J. Phys. Chem., 30, 1073 (1926). 3 Tnebig’s Ann. Chem., 301, 361 (1898). 4Z. anorg. Chem., 5, 82 (1894). 5 J. Chem. Soc., 121, 441 (1922). 6 Pogg. Ann., 123, 538 (1864). 7 Pogg. Ann., 75, 1 (1848). 8 Ann. chim. phys., (3) 12, 463 (1844), THE HYDROUS OXIDES OF TIN AND LEAD 215 eal differences but to the physical structure and to the manner in which water adheres to the surface of the oxide granules. STANNIC OXIDE SOLS Formation.—Colloidal stannic oxide almost free from electro- lytes was first prepared by Graham! by adding alkali to stannic chloride solution or hydrochloric acid to sodium stannate solu- tion short of precipitation and dialyzing the resulting solutions. In both cases a gel was first formed on the dialyzer, but this went into colloidal solution again as the purification was continued. The sol was negatively charged, doubtless owing to the presence of a small amount of free alkali. Excess of the latter was removed by the addition of a few drops of tincture of iodine. As noted previously, Graham was able to boil the colloid without precipitating it, thereby forming colloidal 6 oxide. - His prepa- ‘rations were fairly pure and so were readily coagulated by salts and acids. Schneider? dialyzed the sol formed by adding ammonia to stannic chloride short of precipitation; and Zsigmondy? peptized with ammonia the thoroughly washed oxide formed by hydrolysis of a dilute solution of stannic chloride. The amount of ammonia required was very small; in one experiment, a single drop contain- ing approximately 0.03 gram of ammonia sufficed for the peptiza- tion of 1.45 grams of oxide. Any excess ammonia was removed by heating the colloid to boiling, thus doing away with the neces- sity for dialysis. Sols prepared in this way were negatively . charged and were readily precipitated by electrolytes, particu- larly those having strongly adsorbed cations. ‘The properties of the precipitated oxides lay between those of the typical a and 6 oxides, and Zsigmondy believed them to be mixtures of the two forms, the usual properties of each being modified by the presence of the other. This view is probably incorrect. As previously noted, hydrous stannic oxide freshly prepared at room temperature, is readily peptized by dilute mineral acids; while the aged oxide is peptized by concentrated HCl and H2SO, under suitable conditions, but not by HNO;. The sols are posi- 1 Phil. Trans., 121, 213 (1861). 2Z. anorg. Chem., 5, 82 (1894). 3 Liebig’s Ann. Chem., 301, 361 (1898). 216 THE HYDROUS OXIDES tively charged owing to preferential adsorption of hydrogen ion,! as evidenced by the low precipitation value of sulfate ion in the presence of considerable excess of hydrogen ion.? Biltz® obtained a fairly pure positive sol by dialysis of stannic nitrate prepared by metathesis of stannic chloride and lead nitrate. Metallic tin melted in an electric arc furnace and blown with air gives very finely divided SnO, which can be peptized by 0.02 to 0.01 N hydrochloric acid. Ageing.—Attention has been called to the transformation of a oxide peptized by dilute HClinto the 8form. This transforma- tion has been followed in a number of ways, a few of which will be mentioned: Léwenthal®> found that potassium ferrocyanide could be removed from solution completely by the addition of a dilute solution of stannic chloride, but that the older the tin solution, the more was necessary to precipitate a definite amount of ferrocyanide and the greater was the relative amount of tin in the precipitate. This is shown in Table XIX. L6éwenthal’s observations were confirmed by Lorenz,® who assumed that the ferrocyanide was removed as SnFe(CN)., and that more old stannic chloride solution was necessary on account of the lower TaBLE XIX C ition of ipi- Amount of SnCl, solu- omposition OF precip tate Age of SnCl, solution, | tion to precipitate 0.5 Mole ore j H days gram K4Fe(CN)6., cubic Oe eae centimeters mols of K,Fe(CN)¢ 0 6 1.5 7 105 2.3 21 27 6.5 126 32 7.5 1Cf. ZocHER: Z. anorg. Chem., 112, 46 (1920). 2 LOWENTHAL: J. prakt. Chem., 56, 366 (1852); MmcKLENBURG: Z. anorg. Chem., 74, 207 (1912). 3 Ber., 35, 443 (1902). 4 GoLpscHMipT and KonuscuHtrrer: British Patent 189706 (1922). 6 J. prakt. Chem., TT, 321 (1859). 6Z. anorg. Chem., 9, 369 (1895), THE HYDROUS OXIDES OF TIN AND LEAD 217 concentration of stannic ion resulting from slow hydrolysis. This explanation is unsatisfactory! for two reasons: first, because the hydrolysis of inorganic salts takes place much more rapidly than Lorenz assumed; and, second, because the composition of the precipitate is not SnFe(CN). but is variable, containing more and more tin the older the solution. The true explanation of Léwen- thal’s observations les in the ageing of the colloidal hydrous oxide. ‘The addition of K,Fe(CN). causes coagulation of the colloid. Since the particles of a newly formed colloid are smaller and have a greater adsorption capacity than those of an older colloid, less of the former is necessary to adsorb completely a given amount of ferrocyanide and the ratio of tin to ferrocyanide in the precipitate is relatively low. As the colloid ages, it becomes less stable; and the adsorption capacity falls off so that more colloid is necessary to adsorb a definite amount of ferro- cyanide, and the ratio of tin to ferrocyanide becomes quite large.? Tartaric acid was found by Lowenthal to prevent the ageing of colloidal hydrous stannic oxide. While this may be due to some specific action of tartrate ion, I am inclined to attribute it to the formation of a definite complex, obtainable in crystalline form if desired.’ The age of colloidal stannic oxide may be determined roughly by treating with stannous chloride. The colloid prepared from newly formed stannic oxide is not precipitated by stannous chloride,t whereas the aged colloid is thrown down as a yellow precipitate by this reagent. The precipitate is variable in com- position,® consisting of hydrous stannic oxide that has adsorbed varying amounts of stannous chloride under the different condi- tions of precipitation. Collins and Wood observed a small 1 MECKLENBURG: Z. anorg. Chem., 65, 372 (1909). 2 Cf. Barrorep: J. prakt. Chem., 101, 368 (1867). 3 ROSENHEIM and Aron: Z. anorg. Chem., 39, 170 (1904). 4LOwENTHAL: J. prakt. Chem., T7, 321 (1859); Brron: J. Russ. Phys.- Chem. Soc., 37, 933 (1905). 5 Fremy: Ann. chim. phys., (3) 12, 462 (1844); 23, 393 (1848); Scuirr: Liebig’s Ann. Chem., 120, 47 (1861); TscuermMakK: J. prakt. Chem., 86, 334 (1862). 6 Weiser: J. Phys. Chem., 26, 674 (1922); Cotuins and Woop: J. Chem. Soc., 123, 452 (1923). 218 THE HYDROUS OXIDES increase in adsorption with increasing 6 character of the hydrous oxide, indicating that some factor other than size of grain is involved. As is usual, stannic oxide shows a stronger _tendency to adsorb tin ions than chloride ions. On account of the usual strong adsorption of hydrogen ion, the adsorption of stannous chloride is somewhat less in the presence of hydrochloric acid.! It may be mentioned in passing that hydrogen sulfide precipi- tates stannous sulfide from a colloidal solution of the fresh oxide in dilute hydrochloric acid; whereas, from the aged col- loid, hydrogen sulfide precipitates hydrous stannic oxide that is converted only very slowly into stannous sulfide. The explana- tion of this behavior is evident when we consider the difference in solubility of the new and old oxide. In the new colloid pre- pared by peptization with hydrochloric acid there is some stannic ion, the removal of which by precipitation as stannous sulfide results in further solution and subsequent precipitation until all is thrown down as sulfide; while the aged colloid contains but a negligible amount of stannic ion and the precipitate with hydro- gen sulfide is almost entirely the hydrous oxide. It thus appears that we may have colloidal solutions of any number of hydrous stannic oxides, each differing from the others in the size of the primary hydrous particles and, hence, in their reactivity, adsorbability, and stability under given conditions. As a rule, the particles tend to agglomerate into denser and less reactive secondary aggregates on standing, but the reverse process goes on in the presence of fairly concentrated hydrochloric acid or alkali. As with the precipitated oxide, there is no ground for assuming that the different colloidal solutions are mixtures of colloidal a with colloidal 6 particles in varying proportions. Behavior with Colloidal Metals.—One of the most characteris- tic properties of colloidal hydrous stannic oxide is its protective action on colloidal metals. It is well known that a gold solution treated with stannous chloride first gives a red coloration fol- lowed by the settling out of a purple or brown precipitate known as gold purple of Cassius from its discoverer, Andreas Cassius, 1Cf., however, CoLLins and Woop: J. Chem. Soc., 123, 452 (1923). * JORGENSEN: Z. anorg. Chem., 28, 140 (1901); Barone J. praki. aer 101, 368 (1867). THE HYDROUS OXIDES OF TIN AND LEAD 219 of Leyden. Because of its wide use as a pigment in the ceramic industry, a number of recipes have been given for its preparation. The substance varies in color and composition with the method of formation. Certain earlier investigators, as Richter and Gay Lussac, believed purple of Cassius to be a mixture; but Berzelius thought it must be a definite compound. The latter view is supported by several facts: Purple of Cassius is purple in color, while a mixture of gold and stannic oxide is brick red; gold is not separated from purple of Cassius with aqua regia, whereas it is from a mixture; mercury does not extract gold from the purple as it does from a mixture; and finally, the freshly prepared purple is dissolved by ammonia, forming a purple liquid. In spite of this evidence we now know that Berzelius’ view is incorrect. Debray' believed that gold forms a kind of color Jake with stannic oxide, which is soluble in ammonia. Schneider? emphasized the colloidal character of the purple and concludes rightly that its ammoniacal solution is a mixture of colloidal gold with colloidal hydrous stannic oxide. Supporting Schneider’s view, Zsigmondy* showed that a mere trace of ammonia will dissolve a large amount of freshly precipitated purple and that this purple solution will not pass through parchment during electrolysis, as electrolytes do. He settled the question once for all by precipitating with nitric acid suitable mixtures of colloidal gold with colloidal stan- nic oxide, obtaining purples almost identical with those prepared in other ways. The gold does not combine chemically with stannic oxide, but the usual properties of the former are masked by the protective action of the latter. Colloidal gold was found by Miiller+ to impart a red coloration to a number of substances® and Moissan® obtained purples by distilling gold with tin, alumina, magnesia, zirconia, silica, and lime. Substances similar to gold purples have been prepared with other metals. Thus Wohler’? obtained silver purples similar to the gold pigment by mixing silver nitrate with stannous © 1 Compt. rend., 75, 1025 (1872). 2Z. anorg. Chem., 5, 80 (1894). 3 Liebig’s Ann. Chem., 301, 361 (1898). 4 J. prakt. Chem., (2) 30, 252 (1884). 5 ANTony and Luccusst: Gazz. chim. ival., (2) 26, 195 (1896). 6 Compt. rend., 141, 977 (1905). 7 Kolloid-Z., T, 248 (1910). 220 THE HYDROUS OXIDES nitrate; and Lottermoser' prepared the former synthetically in the same manner as Zsigmondy prepared the latter. Wohler? has also made an analogous platinum combination. All of these so-called ‘‘purples’” are colloidal in nature, the composition varying with the conditions of formation. Their colloid chemis- try is chiefly that of the hydrous oxide. When freshly prepared, the purples are readily peptized by ammonia or dilute hydro- chloric acid; but when dried, there is little or no peptizing action even by concentrated ammonia or hydrochloric acid. Behavior with Other Hydrous Oxides.—In analytical chemis- try, the usual method of estimating tin consists in oxidizing it to insoluble stannic oxide and weighing it as such. Itis well known, however, that the oxide formed in this way is always contam- inated by other substances present in the solution, such as iron, bismuth, copper, and lead. Rose*® observed that when iron is present in small amounts, the stannic oxide precipitated from nitric acid solution is contaminated by it; but that when any considerable quantity of iron is present, both the iron and tin remain in solution. Lepez and Storch‘ digested tin with nitric acid containing iron, and obtained solutions of variable stability depending on the relative amounts of the two metals present; solutions containing 2 atoms or less of tin to 1 of iron could be boiled and even evaporated to dryness in a vacuum. Concen- trated nitric acid threw out of the solutions a yellowish precipi- tate that redissolved on dilution; sulfuric acid. and sulfates caused a permanent precipitate; while acetic acid and alkali chlorides and nitrates caused no precipitation. By evaporating different solutions, the authors claimed to get compounds having such formulas as 1.8Sn0;2 ¥ HO : Fe.O3 ° 1.8N.0; and 4SnO0, : H.O = Feo- QO3:1.1N20;. When a mixture of hydrous ferric and stannic oxides was thrown down from the mixed nitrates by a slight excess of ammonia and the precipitate washed free from ammo- ‘nium nitrate, this precipitate, still containing a trace of ammonia, dissolved in water to a clear solution. Removal of ammonia by dialysis resulted in precipitation; but the addition of a trace of 1“ Anorganische Kolloide,’”’ 53 (1901). 2 Kolloid-Z., 2nd Supplement, III (1907). 3 Rose: Pogg. Ann., 112, 164 (1861). 4 Monaishefte, 10, 283 (1889). THE HYDROUS OXIDES OF TIN AND LEAD 221 ammonia again caused complete solution. Chromic nitrate behaved like ferric nitrate; but aluminum, uranium, cobalt, nickel, and copper nitrates did not cause solution of stannic oxide. The phenomena described by Lepez and Storch strongly suggest that their ferric-stannic mixtures were not salt solutions but were either colloidal solutions of hydrous ferric oxide pep- tized by hydrous stannic oxide or colloidal hydrous stannic oxide peptized by ferric and hydrogen ions. ‘The real nature of the mixtures was shown by two series of experiments.! In the first experiments, mixtures of freshly prepared oxides were treated with 100 cubic centimeters of 0.01 N ammonium hydrox- ide as shown in Table XX. The results are quite conclusive: -Hydrous stannic oxide is peptized by hydroxyl ion, while hydrous ferric oxide is not. However, the colloidal stannic oxide adsorbs ferric oxide and carries it into colloidal solution as long as tin is present in excess. At the same time, hydrous ferric oxide adsorbs stannic oxide and tends to take it out of colloidal solu- tion so that no tin remains peptized when the former is present in large excess. This is quite analogous to the behavior of TABLE XX.—PEPTIZATION OF MrIxTuURES oF HypRovUS STANNIC OXIDE AND Hyprovus Ferric OxipE witH 0.01 N AmMMontum HypROXIDE Mixed oxides prepared from ; N SnCl, + N FeCl;, Observations cubic centimeters 9.5 0.5 Clear colorless colloidal solution 9.0 1.0 Clear colloidal solution with yellow tinge 8.5 He Clear yellow colloidal solution 8.0 20 Clear yellow colloidal solution 7.5 2.5 Clear reddish-yellow colloidal solution 6.0 4.0 But little ferric oxide peptized; supernatant liquid cloudy 5.0 5.0 No ferric oxide peptized; supernatant liquid clear and colorless 2.0 ao. No peptization of either hydrous oxide 1 WerIsER: J. Phys. Chem., 26, 678 (1922). 222 THE HYDROUS OXIDES hydrous chromic oxide with the hydrous oxides of iron, manga- nese, cobalt, nickel, copper, and magnesium.! On account of the mutual adsorption of hydrous stannic oxide and hydrous ferric oxide, we should expect the precipitate obtained by mixing positive ferric oxide sol with negative stannic oxide sol to contain appreciable amounts of both oxides. It would be interesting to know whether the precipitate obtained by mixing yellow colloidal ferric oxide? with colloidal stannic oxide under suitable conditions can be ignited without becoming red.* In a second series of experiments, freshly prepared samples of hydrous stannic oxide were treated with mixtures of ferric nitrate and nitric acid, as shown in Table XXI. The explanation of the observations is fairly simple. As previously noted, hydrous stannic oxide peptized by nitric acid coagulates spontaneously, * since the aged oxide is neither peptized nor dissolved by this acid. Ferric nitrate peptizes this oxide both when newly formed and when aged. Accordingly, if freshly prepared hydrous stannic oxide is peptized either by ferric nitrate or by a suitable mixture TaBLE XXI.—CoLiompaAL STANNIC OXIDE PEpTIZED BY FE(NO3)3 AND HNO; 0.38 gram SnO; + xH.0 peptized Observations by N HNO, + N Fe(NOs)3 + H.O, cubic centimeters After 1 day After 1 week 40 0 10 All precipitated 39 1 10 Very cloudy; part- | All precipitated ly precipitated 38 2 10 Slightly cloudy Very cloudy 37 3 10 Slightly opalescent) Slightly cloudy 36 4 10 Clear Slightly opalescent 35 5 10 Clear Slightly opalescent 34 6 10 Clear Clear 0 40 10 Cloudy Clear 1 NorTHCOTE and Cuurcu: J. Chem. Soc., 6, 54 (1854); Naas: J. Phys. Chem., 19, 331 (1915). a Wanreuh: J. Phys. Chem., 24, 322 (1920). 3 KEANE: J. Phys. Chemin: 734 (1916); Scoretz: Ibid., 21, 570 (1917); Yor: [bid., 25, 196 (1921). THE HYDROUS OXIDES OF TIN AND LEAD 223 of ferric nitrate and nitric acid, coagulation does not take place on standing or boiling, on account of the stabilizing action of the strongly adsorbed ferric ion; but if the concentration of ferric ion in the nitric acid solution is too low, partial coagulation takes place as shown in the table. ! Stannic Oxide Jellies—When a colloidal solution of hydrous stannic oxide is evaporated, a transparent jelly is obtained; while precipitation with electrolytes is said always to give a gelatinous precipitate and not a jelly. Since hydrous stannic oxide apparently possesses the desired properties, one should expect to get stannic oxide jellies by precipitation from colloidal solution under suitable conditions. This conclusion was con- TaBLeE X XII.—PRECIPITATION OF COLLOIDAL STANNIC OXIDE BY ELECTROLYTES Electrolyte Amount Concentration, Observations Formula | added, cubic | milliequivalents centimeters per liter BaCl, 3.00 N/100 3.00 Clear transparent jelly BaCl, | 3.50 N/100 3.50 Clear transparent jelly; very firm BaCl, 3.75 N/100 Shaves Jelly somewhat cloudy and slightly synerized BaCl, 4.25 N/100 4.25. Gelatinous precipitate SrCl, 3.50 N/100 3.50 Clear; somewhat viscous SrCl, 4.00 N/100 4.00 Clear transparent jelly SrCl, 4.50 N/100 4.50 Clear transparent jelly; very ‘y firm | SrCl, 5.00 N/100 5.00 Cloudy jelly; synerized slightly NaCl 2.00 N/10 20.00 Clear; viscous NaCl 2.20 IV 710 22.50 Soft cloudy jelly NaCl 2.50 N/10 25 .00 Soft cloudy jelly NaCl 2.75 N/10 27.50 Gelatinous precipitate HCl 1.50 N/50 3.00 Clear HCl 1.75 N/50 3.50 Clear transparent jelly HCl 2.00 N/50 4.00 Clear transparent jelly HCl 2.25 N/50 4.50 Cloudy jelly; synerized slightly HCl 2.50 N/50 5.00 Gelatinous precipitate 1 Zsiamonpy: ‘“‘Chemistry of Colloids,’ translated by Spear, 155 (1917). 224 THE HYDROUS OXIDES firmed! by precipitating a sol prepared by Zsigmondy’s method containing 28 grams SnO, per liter. The results as recorded in Table X XII are in accord with the general theory.” The jellies formed under the most favorable conditions were very firm and stable, remaining unbroken after standing several months. If just the right amount of electrolyte is added, the jelly may be converted into a sol by shaking and on standing will again set to a jelly.’ As might be expected, jellies are not formed by adding an excess of alkali to a stannic salt and allowing the sol to stand, the reason being that alkalies have a slight solvent action as well as a pep- tizing action on the hydrous oxide. This solvent action causes the precipitate which comes out spontaneously to consist of large granular particles instead of the fine chains or filaments that make up a jelly structure. | Hyprous STANNOUS OXIDE Hydrous stannous oxide is precipitated as a yellow highly gelatinous mass by adding alkali hydroxide or carbonate to a solution of stannous chloride. Ditte* assigned the formula 38nO:2H.O to the compound precipitated with alkali and dried at 110°; and Schaffner® claimed to get 25nO-H2O by precipitat- ing with carbonate and drying below 80°. Bury and Partington® prepared five different samples, using ammonia, carbonate, and alkali as precipitants, both in the air and in an atmosphere of carbon dioxide. With alkali and carbonate, the samples pos- sessed a yellow tinge from the start; but with ammonia, they were white when first prepared, becoming yellow on drying. After drying over phosphorus pentoxide, the samples gave an analysis for tin corresponding to the compound 3Sn0:2H,0; but the water content varied from 7.11 to 8.82 per cent, the cal- culated value for the compound being 8.16 per cent. These data are insufficient to establish the identity of the alleged hydrate. — 1 Weiser: J. Phys. Chem., 26, 681 (1922). 2° Chapa; ps 26: 3 ScHALEK and SzEavaRi: Kolloid-Z., 33, 326 (1923). 4 Ann. chim. phys., (5) 27, 145 (1882). 6 Tniebig’s Ann. Chem., 51, 168 (1844). 6 J. Chem, Soc., 121, 1998 (1922), THE HYDROUS OXIDES OF TIN AND LEAD 225 The gelatinous oxide is very difficult to wash free from the mother liquor, and if the washing is carried too far, the oxide passes through the filter, forming a sol. Bury and Partington observed the slow transformation of the hydrous oxide sol into a crystalline oxide, a part of which precipitated on the walls of the flask and a part remained in suspension, giving a creamy-yellow liquid that glistened on shaking. The hydrous oxide loses water even in contact with water, going over to the anhydrous state. In the presence of alkali, the rate of dehydration is acclerated and the precipitate darkens. The effect of a trace of alkali is evidenced by the darkening of samples of hydrous oxide in contact with the walls of glass vessels and the absence of darkening in samples stored in quartz vessels. ! The color of the anhydrous oxide varies from dark gray to black, depending on the method of preparation. It is not likely that the different shades represent different modifications. Roth? claims to get a red crystalline oxide by the action of an acetic acid solution of SnO, on the gelatinous oxide, but Bury and Partington were unable to confirm this result. Hydrous stannic oxide is dissolved by alkali forming NaHSnOg, unless the alkali is quite concentrated when Na2SnOsz is obtained.* Schneider‘ reports the preparation of a colloidal solution of Sn2.O3 by adding a very dilute solution of stannous chloride to a sol of hydrous stannic oxide and dialyzing in the absence of air. The sol is a yellow clear neutral liquid with strong reducing properties; thus, the addition of gold chloride gives gold purple. In all probability, the sol is not Sn2O3 but a mixture of hydrous stannic oxide and basic stannous chloride® stabilized by the hydrous oxide. Hyprovus LEAD MONOXIDE It is somewhat surprising not to find a reference to an analysis of precipitated lead monoxide which corresponds to the formula 1 Bury and Partineton: J. Chem. Soc., 121, 1998 (1922). 2 Abege’s “Handbuch anorg. Chem.,” 4, IJ, 573 (1909). 3 Hantzscu: Z. anorg. Chem., 30, 289 (1902); GoLtpscumipT and EcKarpr: Z. physik. Chem., 56, 385 (1906); KorticHEen: [bid., 33, 129 (1900). 47Z. anorg. Chem., 5, 83 (1894). 5 Carson: J. Am. Chem. Soc., 41, 1969 (1919). 226 ‘ THE HYDROUS OXIDES Pb(OH)s. However, it is tacitly assumed by Ditte'! and by Wood? that the compound freshly precipitated from lead nitrate solution by alkali and ammonia is lead hydroxide, when, as a matter of fact, the mass is either a basic salt’ or contains adsorbed alkali, unless it is digested repeatedly with sodium hydroxide solution which converts it to pure hydrous oxide.’ Hydrates having the formula 2PbO - H.O4 and 3PbO - H2O® have been reported; but Glasstone® repeated the experiments of various authors and failed to get any product which could be described as either of these compounds. The water content of the oxide precipitated from lead acetate solution with alkali was lower the greater the dilution, and the higher the temperature of the ~ reacting solutions. A variety of hydrous products were heated in a current of air at 105 to 110° until decomposition set in, as evidenced by a slight change in color, and the water content was determined. In every case the oxides contained 3.08 to 3.13 per cent of water, corresponding approximately to the com- position 5PbO:2H.O. From this, Glasstone assumes that every form of the oxide, whether crystalline or amorphous, is the same chemical entity with varying amounts of adsorbed water; but whether the substance is a single hydrate or a solid solution of two hydrates is left undecided. Glasstone rules out the possi- bility of the precipitated oxide being hydrous PbO, since at 105° there is always a color change when the water content is reduced to approximately 3.18 per cent. It would seem that the cause and nature of this color change should be investigated further. If the temperature were higher, it might be ascribed to the transformation of yellow to red oxide or to the formation of minium. Glasstone quotes Winkelblech’ as reporting a loss of 1 Compt. rend., 94, 1310 (1882). 2 Woon: J. Chem. Soc., 97, 878 (1910). 3 WINKELBLECH: Liebig’s Ann. Chem., 21, 21 (1837). 4ScHAFFNER: Liebig’s Ann. Chem., 51, 175 (1844); Ltprexine: Am. Chem. J., 18, 120 (1891); Ocata and Karun: J. Pharm. Soc. Japan, 492, 75 (1923). 5 PavEn: Ann. chim. phys., (4) 8, 302 (1866); Mutprr: Dammer’s ‘‘ Hand- buch anorg. Chem.,”’ 2, II, 524; PLetissnreR and AUERBACH: Abegg’s ‘‘ Hand- buch anorg. Chem.,’’ 3, II, 677 (1909); cf. also BOrreER: Z. physik. Chem., 46, 580 (1903); Lorenz: [bid., 12, 436 (1897). 6 J. Chem. Soc., 121, 58 (1922). 7 Liebig’s Ann, Chem., 21, 25 (1837). THE HYDROUS OXIDES OF TIN AND LEAD 227 0.6 per cent of water at 105° before decomposition started; whereas what Winkelblech actually observed was a decomposition of basic nitrate on heating to some unrecorded temperature, red fumes being evolved and minium being formed.! Lead monoxide occurs in nature in two crystalline forms, litharge and massicot, which are yellow and red, respectively. Hydrous lead oxide precipitated in the cold is white, but heating with 10 per cent alkali converts it to yellow or yellowish-green oxide. If the precipitation is carried out in boiling alkali solu- tion, red anhydrous oxide is obtained.’ The relationships among the various forms worked out by Ruer? may be represented as shown in Table XXIII.* TaBLeE XXIII alkali Reddish brown 5% NaOH lead acetate-———————>white hydrous Heat to 620° — — solution oxide and cool => 50 se 2% KOH ~ lhe 7 5% KOH boiling ’ Lox alkalis heat to 720° and coo] ms 1 ‘| ee red 50 % alkali hot yellow or eg re a tt, a —— > yellowish 50 % alkali ho po Treen ‘3 ‘ | Yellow —> 1 Commercial reddish brown The difference between the two forms of the oxide is commonly attributed to polymorphism, the red being regarded as the more stable form at the ordinary temperature and at all temperatures up to a transition point that has not been determined.* This view is supported by the observed differences in crystal structure,° density, and solubility® of the two forms. The yellow crystals 1 Cf. Burton: Dinglers polytech. J., 167, 361 (1863). 2 Ruger: Z. anorg. Chem., 50, 265 (1906). 3 GLASSTONE: J. Chem. Soc., 119, 1689 (1921). 4 JamceR and Germs [Z. anorg. Chem., 119, 147 (1922)] give 587°. 6’ NORDENSKIOLD: Pogg. Ann., 114, 619 (1861); Larsen: U. S. Geol. Survey Bull., 679, 105 (1921). 6 GruTHER: Liebig’s Ann. Chem., 219, 56 (1883); RuER: Z. anorg. Chem., 50, 265 (1906). 228 THE HYDROUS OXIDES are rhombic, biaxial, and positive in action on polarized light, while the red are tetragonal, uniaxial, and negative in action on polarized light. | Recently, however, Glasstone suggests a closer relationship between the two forms than that of allotropy. He calls atten- tion to the transformation of all red forms to brownish yellow on grinding and all yellow forms to red on heating with con- centrated alkali. These observations indicate a close connection between color and size of particles, the large particles appearing red and the small ones yellow. In further support of this view, Glasstone! made solubility determinations both gravimetric and electrometric, on eight different preparations varying in color from lemon yellow through reddish brown to red and found approximately the same value, irrespective of the color. Micro- scopic examinations likewise indicate that the red forms are made up of larger particles, the yellow samples being agglomerates of small particles which are almost identical with the finely divided red forms. ‘These data are misleading, however, as Appleby and Reid? succeeded in making well-defined crystals of the two forms which differ not only in crystal structure but in solubility, the yellow being 1.8 times as soluble as the red. Glasstone’s products were mixtures, and the constant solubility he observed was the value for the more soluble yellow form. Appleby and Reed’s conclusions were confirmed by Kohlschiitter and Scherrer* who examined the two forms with x-rays and found them struc- turally different. There is, therefore, no doubt of the poly- morphism of lead. oxide; but it is not improbable that each crystalline form should show variations in color between yellow and red by varying the size of the particles. Indeed, this is what we find with the two crystalline forms of mercuric oxide;* and it is known that grinding red lead oxide crystals changes them to brownish yellow, probably without changing the crystal structure. 1 J. Chem. Soc., 119, 1689, 1914 (1921). 2J. Chem. Soc., 121, 2129 (1922). ° Helvetica chim. Acta, 7, 387 (1924); cf. KonLtscHiiTTER and Rosgstt: Ber., 56, 275 (1923). 4 Page 173, THE HYDROUS OXIDES OF TIN AND LEAD 229 Lead oxide dissolves slightly in dilute alkali but appreciably in concentrated alkali, forming plumbite.? The alkali plumbites are strongly adsorbed by cotton. By washing the fiber thor- oughly, the salts are hydrolyzed, giving alkali that dissolves out and lead oxide that is retained by the fiber and acts as a mordant.* Although the compound Pb(OH), is not known, it is an inter- esting fact that boiling lead sulfate or chloride with aqueous Milligrams of Lead in!00 ce. PbO 80 60 A0 20 0 0 20 40 60 80 —- PbC0, Molecular Percentages in Solid Phases Fria. 14.—Composition of white lead. sodium carbonate gives Pb(OH).- 2PbCOs, basic lead carbonate or white lead. The same compound is formed by bringing - anhydrous PbO and lead carbonate together in a sodium acetate solution. In Fig. 14 are given the results of allowing various mixtures of PbO and PbCO; to stand in contact with 20 per cent sodium acetate solution at 75° for 12 hours and subsequently analyzing both solutions and precipitates. ‘The diagram shows 1 Bert and AusTERWEIL: Z. Elektrochem., 13, 165 (1907). 2 HantzscH: Z. anorg. Chem., 30, 305 (1902); Herz and Fiscusr: [bid., $1, 454 (1902). 3 BONNET: Compt. rend., 117, 518 (1893). 4 SALVADORI: Gazz. chim. ital., 24, I, 87 (1904). 6 Hawuey: J. Phys. Chem., 10, 654 (1906). 230 THE HYDROUS OXIDES that PbO and PbCO; do not form solid solutions but a compound containing 1 mol of the former to 2 of the latter. Water deter- minations show the formula to be Pb(OH).2: 2PbCQ3.! Hydrous plumbic oxide and thorium oxide mutually adsorb each other. The former, suspended in water, is carried into colloidal solution by thorium acetate which hydrolyzes to give colloidal thorium oxide; but if an excess of hydrous lead oxide is shaken with a thorium acetate solution, thorium oxide is carried down along with the lead.’ Hyprous LEAD PEROXIDE Electrolysis of a weak alkaline solution of lead sodium tartrate gives a black lustrous compound reported to be PbO2: H20.* The hydrate is also said to form during the electrolysis of a sodium chloride solution in which litharge is suspended.* Electrolysis of acid or neutral solutions of lead salts usually gives the anhy- drous oxide;®> but Wernicke reports the formation of lead peroxide with variable quantities of water by electrolyzing dilute solutions of lead nitrate for varying lengths of time. It is probable, there- fore, that the so-called monohydrate or metaplumbiec acid is really a hydrous oxide. ‘There are well-defined metaplumbates as well as orthoplumbates, however, the latter derived from the hypothetical acid H4PbO, or PbO2.: 2H20. According to Bellucci and Parravano® the metaplumbates such as K2PbO3-3H2O should be regarded as salts of an acid HePb(OH)., both be- cause of isomerism with the corresponding potassium stannate and platinate and because they cannot be dehydrated without decomposition. Alkali metaplumbates hydrolyze strongly in water,’ giving colloidal hydrous lead peroxide® together with potassium 1Cf. PueissNER and AvERBACH: Abegg’s ‘‘Handbuch anorg. Chem.,”’ 4, II, 726 (1909). 2 $ziLaRD: J. chim. phys., 5, 645 (1907). 3 WERNICKE: Pogg. Ann., 141, 109 (1870). 4Chemische Fabrik. Griesheim-Elektron, German Patent 124512; Chem. Zenir., II, 1101 (1901). 5 W6HLER: Liebig’s Ann. Chem., 90, 383 (1854); GruruER: Ibid., 96, 382 (1865); FEHRMANN: Ber., 15, 1882 (1882). 6 Z. anorg. Chem., 60, 107 (1906); Atti accad. Lincei, 14, I, 378, 457 (1905). 7 PARRAVANO and Caucaanti: Gazz. chim. ital., 37, II, 264 (1907). 8 BELLUCCI and PARRAVANO: Atti accad. Lincet, 15, II, 542, 631 (1906). THE HYDROUS OXIDES OF TIN AND LEAD 231 hydroxide, a great part of which can be dialyzed out without precipitation taking place; but if the dialysis is carried too far, the hydrous oxide gelatinizes on the dialyzer. A sol containing 0.32 gram PbO: and 0.008 gram K;O, that is, 175 mols of per- oxide to 1 of alkali, is neutral in reaction and gives no depression of the freezing point of water. It possesses a chestnut-brown color, and is perfectly clear in transmitted light but cloudy by reflected light; it can be diluted, heated to boiling, frozen or evaporated on the water bath without coagulation. By evapor- ating off the excess water, a jelly is formed which can be repep- tized by water; drying the jelly renders it non-peptizable. The sol is negatively charged and is quite sensitive to the action of certain electrolytes, particularly those having multivalent cations. The order of precipitating power of chlorides, begin- ning with the greatest, is: Fe’, Al’, Ca’, Sr’, Ba’, Mg”, oeeiie vin | Ni’, Co’, Cu’, NHi, Cs’, li, K’, Rb, Na. The effect of stabilizing ions having the same charge as the sol is quite marked. Thus, potassium salts of AsO4’”’, CO3”, C204” and IO,’ do not coagulate the sol; FeCNe’’””, ClO’, ClO3’, MnO,’ Cr,0,7", Br Oe CNS’, SO4”, NO3’, 10,3’, C.H;30,’ precipitate it partially; and F’, C4H,0,"’, Fe(CN).’’", and I’ precipitate it completely. The slight stabilizing action of ferrocyanide and tartrate ions is anomalous and should be reinvestigated. If an alkali solution of PbOz is treated with potassium or cal- cium plumbate, an amorphous orange-yellow powder is obtained which analyzes approximately for Pb.O3;-2H,O! after drying over sulfuric acid. It loses only about one-third of its water at 170° but at higher temperatures it can be dried completely, apparently without decomposition. The sesquioxide can be broken up by acids into PbO and PbO, and is regarded as a compound of the two, PbO: PbO, or Pb(PbO:2), lead meta- plumbate. Bellucci and Parravano consider the hydrate to be Pb[Pb(OH).]. Similarly, red lead or minium Pb3;0,4 can be decomposed by certain acids into soluble plumbous salts and PbO, and is, therefore, regarded as 2PbO, PbOs, or Pb(PbO:): -lead orthoplumbate. No hydrate or hydrous form of Pb3Qxz is reported. 1Se1weu: J. prakt. Chem., (2) 20, 200 (1879); BrLiuccr and PARRAVANO: Z. anorg. Chem., 50, 107 (1906). 232 THE HYDROUS OXIDES While the hydrous oxides and the alleged hydrates of lead are not important commercially, the anhydrous compounds are widely used in the arts. ‘Thus, litharge is used in the manufac- ture of flint glass and as a glaze for earthenware. It is also used in making the very important pigments, white lead and min- ium, in the manufacture of plates for the lead accumulator, and as a “dryer” in oils. Minium, like litharge, is employed in making flint glass and battery plates, but its widest use is as a pigment. The peroxide has a strong oxidizing action and is frequently employed as an oxidizing agent. The mixture of nitrate and dioxide (oxidized red lead), obtained by heating red lead with nitric acid, is used in the manufacture of lucifer matches. CHAPTER IX THE HYDROUS OXIDES OF TITANIUM, ZIRCONIUM, AND THORIUM The hydrous oxides of titanium, zirconium, and thorium are always described as existing in both an alpha or ortha and a beta or meta modification. In every case the relationship between these two forms is the same as between the so-called a and 6 stannic oxides whose colloid chemistry has been consid- ered in detail in the preceding chapter. Accordingly, in this chapter will be given but a brief survey of this phase of the chemistry of the hydrous oxides under consideration. Hyprovus TITANIuM DIoxIDE The addition of ammonia or alkali hydroxide or carbonate to a cold solution of titanium dioxide in hydrochloric or sulfuric acid throws down the so-called orthotitanic acid as a voluminous white mass easily soluble in dilute acids. The product forms no hydrates! but is a typical hydrous oxide whose water content is determined by the method of precipitation and drying.? Dried in the air, the compound gives an x-ray interference pattern indicating its crystalline character.* If heated rapidly, the oxide exhibits the glow phenomenon; but as usual, if the heating is slow or the temperature is held for some time below the glow temperature, there is a gradual sintering and loss of surface - energy without incandescence.* 1 CARNELLEY and WALKER: J. Chem. Soc., 53, 81 (1888). 2 Rose: Pogg. Ann., 65, 507 (1844); Demoty: Compt. rend., 20, 325 (1845) Merz: Jahresber., 197 (1866); Turrscunw: Liebig’s Ann. Chem., 141, 111 (1867). 3 HeDvALL: Z. anorg. Chem., 120, 327 (1922). 4See p. 79, 233 234 THE HYDROUS OXIDES Heating the acid solution of titanium dioxide to boiling pre- cipitates the typical betatitanic acid as a white powder. If obtained from hydrochloric acid solution, the oxide cannot be washed without undergoing peptization, forming a positive sol; but the oxide from sulfuric acid solution is not peptized by wash- ing, on account of the precipitating power of sulfate ion. The typical 8 oxide is distinctly less hydrous than the a; it does not exhibit the glow phenomenon on heating; and it is almost insol- uble in acids with the exception of concentrated sulfuric acid. Hydrous titania is only slightly soluble in alkalies, the solu- bility varying from 2 milligrams per 100 cubic centimeters in 10 per cent sodium hydroxide to 120 milligrams in 100 cubic centimeters in 40 per cent potassium hydroxide. ‘The statement that NaeTiO; - 4H2O and K,T103- 4H.O! can be erystallized from alkaline solution of alkali titanate is obviously erroneous.? The a and 8 modifications of titanic acid are not chemical individuals but are hydrous oxides differing in the size and physi- cal character of the particles and in the amount of adsorbed water. The soluble highly gelatinous oxide ages gradually at ordinary temperatures*® and more rapidly at higher temperatures, forming a continuous series of products that approach the char- acter of the granular insoluble 8 oxide as a limit. This conclu- sion was confirmed by Morley and Wood by observations on the varying adsorption capacity for dyes® and on the varying solu- bility and peptizability by hydrochloric acid,® of the hydrous oxides prepared in different ways. ‘There seems no real justifica- tion for assuming, as Morley and Wood do, that the change in physical character of the gelatinous oxide on ageing is due to the formation of complex salt-like condensation products by the molecules of hydrous oxide functioning both as acid and base. Titanium Dioxide Sol and Jelly—Graham’ obtained a sol of hydrous titanium dioxide by dialysis of a 1 per cent solution of 1 Demo.uy: Jahresber., 271 (1849). 2 AUGER: Compt. rend., 177, 1802 (1923). 3 WaGNER: Ber., 21, 960 (1888). 4 LorrerMosER: Abegg’s ‘“‘Handbuch anorg. Chem.,” 3, (2), 883 (1909). 5 J. Soc. Dyers Colourists, 39, 100 (1923). 6 J. Chem. Soc., 125, 1626 (1924). 7 Phil. Trans., 151, 213 (1861), TITANIUM, ZIRCONIUM, AND THORIUM 235 the oxide in dilute hydrochloric acid. With more concentrated solutions, jellies are formed on the dialyzer during the purification process. ‘The water in an aged jelly can be replaced by alcohol, ether, benzene, glycerin, or concentrated sulfuric acid in the same way as in the corresponding silica jelly. More than a century ago, Rose! reported the formation of a soft titania jelly. He treated a fusion of titania and sodium car- bonate with hydrochloric acid, filtered the solution, and allowed it to stand, whereupon the hydrous oxide aged and precipitated out as a jelly. Later, Knop? obtained a jelly in an interesting way: A strong hydrochloric acid solution of magnetic oxide of iron was treated with tartaric acid and then neutralized with ammonia. ‘The iron remained in solution and the titania came down as a white precipitate. On filtering and attempting to wash the oxide, it swelled up in much the same manner as gelatin, forming a colorless transparent jelly which was transformed into a gelatinous precipitate by heating. Recently, Klosky and Mar- zano® prepared firm transparent jellies by neutralizing slowly an acid solution of titanium dioxide with the carbonates of sodium, potassium, or ammonium. Hydrous titanium dioxide probably finds its most important use as a mordant. If leather or textile goods are immersed in a solution of titanium salt and then steamed, the hydrous dioxide is precipitated. This adsorbs certain dyes forming permanent brilliantly colored lakes. As a mordant for alizarin orange, coerulein and alizarin blue, titania is superior to chrome.* For delicate fabrics, titanium salts of organic acids are employed in order to avoid the injurious action of mineral acids. Both tri- valent and tervalent salts are used for this purpose. If anhydrous titanium tetrachloride is sprayed into air, it takes up moisture, giving a dense smoke composed of fine particles of the solid hydrate, TiCl,- 5H2O. The chloride was used suc- cessfully during the war for producing smoke screens. In case 1Gilbert’s Ann., 73, 76 (1823); cf. ProrpTEN: Liebig’s Ann. Chem., 237, 213 (1887). 2 Liebig’s Ann. Chem., 123, 351 (1862). 3 J. Phys. Chem., 29, 1125 (1925). 4 Barnes: J. Soc. Dyers Colourists, 12, 174 (1896); 35, 59 (1919); Ham- MEL: [bid., 20, 65 (1904). 236 THE HYDROUS OXIDES the air is too moist, hydrolysis takes place, giving hydrochloric acid and hydrous titanium dioxide which forms a smoke, but with less obscuring power than the chloride hydrate. The cloud may be increased in moist air by the presence of ammonia which forms ammonium chloride. If the air is not quite moist, however, ammonia must be avoided, otherwise the chloride forms an ammonate, TiCl,- 6NH;, which has little obscuring power. On this account, it is usually planned to disperse the tetrachloride and ammonia separately. Precipitated titania makes a particularly good pigment in paints on account of its permanence, great opacity, and non- poisonous nature. It has been employed with barium sulfate in place of zinc oxide, giving a titanium lithopone. Titania paints are not affected by sea water; have no saponifying action on linseed oil; and have more than a third more covering power than white lead paints. OTHER OXIDES OF TITANIUM Titanium Monoxide.—The hydrous oxide of divalent titanium is thrown down as a black precipitate by adding hydroxyl ion to a solution of titanous salt. It is very unstable in the air, oxidizing first to blue hydrous titanium sesquioxide and finally to the white dioxide. Titanium Sesquioxide——The hydrous oxide of trivalent tita- nium has been variously described as black, dark blue, cherry red and brown red, depending upon the exact conditions of formation. It is prepared by digesting a solution of dioxide in hydrochloric acid with metallic copper at 20 to 40° until the solution attains a violet-blue color, followed by the addition of ammonia. It is also thrown down directly from titanium trichloride solution with ammonia. If the hydrous oxide is shaken with milk of lime in the presence of oxygen, it is oxidized to the dioxide and at the same time an equivalent amount of hydrogen peroxide is formed. In the same way, when the sesquioxide is oxidized by a solution of chromic acid in the presence of potassium iodide, or by potassium permanganate in the presence of tartaric acid, hydrous titanium dioxide is formed, and simultaneously, oxidation of the potassium iodide TITANIUM, ZIRCONIUM, AND THORIUM 237 or tartaric acid is brought about.! These are typical cases of auto-oxidation.? Titanium Peroxide.—The addition of hydrogen peroxide to a neutral or acid titanium solution produces an intense yellow coloration, owing to the formation of a hexavalent titanium compound. Since the color is quite distinct, even in the presence of less than 0.01 per cent of titanium, the reaction affords a delicate test both for titanium and hydrogen peroxide.? If gelatinous titania is treated with an excess of hydrogen peroxide, it is converted into yellow titanium peroxide. The latter com- pound is best obtained by dropping titanium tetrachloride slowly into dilute alcohol; adding a large excess of hydrogen peroxide; and finally treating with ammonia, ammonium car- bonate, or alkali.t| The yellow hydrous oxide adsorbs salts very strongly, and so it is difficult to obtain pure.’ When freshly formed, the composition can be represented by the formula TiO;-2H20, but on drying over phosphorus pentoxide, it becomes a horny mass containing less oxygen than corresponds to a tri- oxide. The freshly precipitated hydrous peroxide appears to be considerably more soluble in alkali than the dioxide. This may be due in part to peptization, since the alkali peroxide solutions are instable, depositing an aged granular oxide in the course of a few days. It is usually assumed, however, that the alkali solutions contain alkali pertitanate.°® Hyprovus ZIRCONIUM DIOXIDE The most gelatinous form of hydrous zirconia is obtained by precipitating a solution of a zirconium salt with ammonia or 1 Mancnort and Ricuter: Ber., 39, 320, 488 (1906); Mancuor and WIL- HELMS: Liebig’s Ann. Chem., 325, 105 (1902); Haser: Z. EHlektrochem., 7, 441 (1900). 2 ScHONBEIN: J. prakt. Chem., 98, 24 (1864); Trause: Ber., 26, 1471 (1893); Van’r Horr: Z. physik. Chem., 16, 411 (1895); Eneier: Ber., 30, 1669 (1897). 3 Ricuarz and Lonnss: Z. physik. Chem., 20, 145 (1896); Haaser and GRINBERG: Z. anorg. Chem., 18, 37 (1898). 4 Levy: Compt. rend., 110, 1368 (1890); Ann. chim. phys., (6) 25, 433 (1892). 5 CLassEN: Ber., 21, 370 (1888); cf. WELLER: Ber., 15, 2592 (1882). 6 MevrkorrF and PissarJewski: Ber., 31, 678, 953 (1898); Z. anorg. Chem., 18, 59 (1898); cf. Bruty; Compt, rend., 172, 1411 (1921). 238 THE HYDROUS OXIDES alkali hydroxide. The latter is adsorbed so strongly that the former must be employed if a pure gel is desired. This hydrous oxide, the so-called a zirconic acid, bears a marked resemblance to alumina both in its appearance and in its capacity to adsorb water and salts. Like alumina, also, it is almost entirely insolu- ble in water.!_ When dried at 100°, the gel is reported to be a monohydrate, ZrO.-H2.O or ZrO(OH).;? but van Bemmelen® showed that the minimum temperature necessary for attaining this composition depends on the previous history of the sample. Thus, the water content of van Bemmelen’s gels was not reduced to the point corresponding to a monohydrate until a temperature of 140° or more was reached. Between 140 and 200° the composi- tion was approximately constant. The latter observation might be taken to mean the existence of a definite hydrate of zirconia like the crystalline hydrates of beryllia and alumina. Van Bemmelen found, however, that the adsorption capacity for water and salts, of zirconia containing 1 mol of water was similar to that of hydrous alumina and beryllia and not like that of the crystalline hydrates. He concludes, therefore, that the water in the alleged hydrate of zirconia is adsorbed in capillaries and not chemically combined in the ordinary sense. This view receives strong support from recent investigations of the structure of zirconia sols and gels, using the method of x-ray interference. Haber and his pupils* find that hydrous zirconia possesses no crystalline structure whatsoever either when freshly precipitated or when thoroughly dried below 400°. As has been pointed out repeatedly in these pages, hydrous oxides, amorphous when first prepared, usually assume a microcrystalline form on ageing; and all the definitely established oxide-hydrates are crystalline. If zirconia forms an amorphous hydrate, it is an outstanding exception. When formed in the cold, the hydrous oxide is more gelatinous and more reactive than when formed in the hot. Either prep- aration heated to approximately 300° glows very brightly, pro- 1 VENABLE and Betpen: J. Am. Chem. Soc., 20, 273 (1898). 2 RuER: Z. anorg. Chem., 48, 297 (1905). 3Z. anorg. Chem., 49, 125 (1906). 4 Haper: Ber., 65 B, 1717 (1922); Boum and NicuassEn: Z. anorg. Chem., 132, 1 (1924). TITANIUM, ZIRCONIUM, AND THORIUM 239 vided the water content is not reduced below 1.9 per cent.! If the hydrous mass is heated rapidly above 300°, the glowing is accompanied by small explosions, caused, in all probability, by expulsion of some of the adsorbed water. It is an interesting fact that the oxide retains considerable water even after the glow- ing. Ruer looks upon the glow phenomenon as a manifestation of the transformation of ordinary zirconia into isomeric meta- zirconia; but Wohler? showed it to result from a sudden diminu- tion in surface energy accompanying the change from a gelatinous structure to a granular powder. Ruer prepared an aged hydrous zirconia by boiling down repeatedly a solution of zirconium oxychloride. The sol obtained by this process was precipitated by hydrochloric acid giving what Ruer called a metachloride. After centrifuging out the precipitate, it was peptized in water and thrown down again with ammonia. The hydrous oxide, still containing considerable chloride, was dried over caustic alkali and then heated to 100°, where its water content corresponded approximately to ZrOs.:- 22H2O. On account of its relatively slight solubility in acids and its failure to glow on heating, Ruer believed it to be an isomeric form of zirconic acid which he designated metazirconic acid. Van Bemmelen found, however, that oxides prepared by Ruer’s method lost water continuously without any evidence of the existence of a hydrate. A composition corresponding to Ruer’s 100° hydrate was obtained by van Bemmelen at 85°; and, at every observed temperature, the composition showed consider- able variation with different samples. Van Bemmelen showed further that Ruer’s metachloride was merely an aged zirconia with adsorbed chloride. By evaporating the oxychloride to dryness and replacing the water repeatedly, a product was obtained which retained but a trace of chloride. Prolonged boiling of hydrous zirconia in a medium possessing a slight solvent action gives a dense structure that is not only less reactive chemically but has a much lower adsorption capacity than the gelatinous precipitated oxide. This change in structure is a gradual process, involving the formation of a continuous series of products intermediate between the typical ortho and 1 Van BEMMELEN: Z. anorg. Chem., 45, 83 (1905). * Kolloid-Z., 11, 241 (1918). 240 THE HYDROUS OXIDES meta. oxides. It is unnecessary to start with zirconium oxy- chloride to prepare the so-called meta oxide. A sol of the ordinary oxide is aged by boiling, the amorphous particles gradu- ally becoming denser and at the same time tending to orient themselves into crystals.! Zirconium dioxide is the most important compound of zirco- nium from the technical viewpoint. Its very high melting point, low heat conductivity, low coefficient of expansion, low porosity, and high resistance to corrosion even at elevated temperatures, combine to make it an almost ideal refractory. The only difficulty is that very small amounts of certain materials modify its properties, and the removal of these is very expensive. ‘Thus, iron, which acts as a flux, can be removed entirely only by com- plete solution of the oxide in hydrofluoric acid. Moreover, the oxide prepared by igniting compounds such as the hydrous oxide or nitrate is a very loose powder that shrinks enormously when highly heated. Accordingly, high-temperature utensils such as muffles, crucibles, etc. must be made from zirconia which has been fused and subsequently ground to a powder. On account of its gelatinous character, hydrous zirconia makes a good binder for holding together the particles of fused zirconia, thus giving a paste that may be molded into the desired shape. In addition to its use as a refractory, anhydrous zirconia has been used for almost a century in connection with the problems of artificial ighting, because of the brilliant light emitted when it is heated to incandescence. The first -Welsbach mantles were made largely of zirconia, but this was later replaced by thoria, since the latter oxide glows at a much lower temperature. It is also employed for coating the lime or magnesia pencils in the Drummond light where it is distinctly advantageous, not only because of the brilliant light it emits but because it does not absorb carbon dioxide or moisture from the air as do lime and magnesia. ‘The Bleriot lamps used for automobile headlights | consist of zirconia rods heated to incandescence. Nernst employed rods of pure zirconia in his early attempts to obtain a means of illumination, by use of the electric current, which would be superior to the carbon filament lamp. Later, he obtained a 1 Boum and NIcuLassEen: Z. anorg. Chem., 182, 6 (1924), TITANIUM, ZIRCONIUM, AND THORIUM 241 more intense light with mixtures of the oxides of zirconium, thorium, yttrium, and sometimes cerium. Prepared in various ways, zirconia is used as a toilet powder, as a polishing powder, and as a substitute for bismuthyl] nitrate in the diagnosis of gastrointestinal disease by means of x-rays. For the latter purpose, it is distinctly advantageous because of its non-poisonous character. Zirconia is also used as an opacify- ing agent in enamels and a clouding agent in glass, instead of the more costly stannic oxide and the poisonous compounds of antimony and arsenic. Asa pigment, it possesses good covering power, mixes readily with paint vehicles, is permanent, and is unaffected by hydrogen sulfide, acids, or alkalies. ZIRCONIUM DIOXIDE SOLS Hydrolysis of Zirconium Salts.—Biltz! dialyzed a solution of zirconium nitrate for several days, obtaining a rather impure sol of hydrous zirconium dioxide which was slightly acid and gave a distinct test for nitrate. The sol possessed a positive charge which was neutralized by the addition of negative sols, the particles of opposite sign mutually precipitating each other. Ruer? dialyzed solutions of zirconium oxychloride both without heating and after heating for 2 hours. Like Biltz’s preparation, the sols were clear in transmitted light but cloudy by reflected light. Addition of sodium or ammonium chloride caused precip- itation, the amount required being less the more thorough the purification by dialysis. A transparent glass was obtained by evaporation on the water bath. The addition of 10 cubic centi- meters of N sulfuric acid to 2.5 cubic centimeters of sol containing 0.015 gram of ZrO, gave a precipitate that dissolved in 144 hour provided the solution was not heated before dialysis; the pre- cipitate from the preheated solutions did not dissolve for approxi- mately 6 hours under similar conditions. This decrease in solubility was the manifestation of growth of primary particles which proceeded quite gradually at ordinary temperature but more rapidly at the boiling point. As we have seen, prolonged heating of the oxychloride gave a slightly hydrous mass, insoluble © 1 Ber., 35, 4436 (1902); 37, 1100 (1904). 2Z. anorg. Chem., 48, 282 (1905). 242 THE HYDROUS OXIDES in both hydrochloric and nitric acid but readily peptized on washing with water. By dialysis of the so-called metachloride, Ruer obtained a milky white sol which left on evaporation an amorphous white residue instead of a glassy mass. The chlorine content was reduced to 0.026 atom Cl per mol of ZrOs. On account of the relatively slight adsorption capacity of the particles, the precipitation by electrolytes with univalent anions is readily reversible. Adolf and Pauli! attempted to establish the composition of equilibrium solutions of zirconium oxychloride of varying con- centrations up to 0.5 N by observations of the freezing-point lowering, conductivity, and directions of migration under elec- trical stress, as well as the hydrogen and chloride ion concen- trations using the hydrogen and calomel electrodes, respectively. The hydrolysis does not change materially with the dilution. The curves for hydrogen ion and chloride ion concentrations against the concentrations of ZrOCl, are S shaped and intersect each other at three points, so that, at very low and again at moderate concentrations, the hydrogen ion concentration appears to be greater than that of chloride, indicating the presence of complex ions containing zirconium. The osmotic concentration at the higher concentrations is less than the molar concentration of oxychloride and does not greatly exceed it even at the greatest dilutions. By subtracting from the total conductivity, the conductivity due to the hydrogen and chloride ions present, the conductivity due to the alleged complex zirconium ions is obtained. This appears to constitute a large part of the total conductivity and to vary with the concentration of oxychloride. These obser- vations are explained by assuming the formation of complex cations and anions such as 2[Zr(OH)4: ZrOCl,: ZrO] and 2[Zr- (OH),Cl.]’’.. Migration experiments indicate a migration of zirconium to both anode and cathode, more going to the anode than to the cathode when the hydrogen ion concentration exceeds the chloride ion concentration and vice versa. It is very difficult to make head or tail of the conglomeration of facts and speculations given in the preceding paragraph. ‘This difficulty increases when we reflect that Adolf and Pauli’s con- ductivity and electrometric measurements do not give what 1 Kolloid-Z., 29, 173 (1921). TITANIUM, ZIRCONIUM, AND THORIUM 243 they assumed them to give. Leaving out any complex ions, there are in any given solution: undecomposed oxychloride, hydrous zirconium dioxide, hydrogen ions, and chloride ions. The hydrous oxide adsorbs some undecomposed zirconium oxy- chloride and possibly stabilizes it to a certain degree. It also adsorbs both hydrogen ions and chloride ions in amounts depend- ing on the experimental conditions. The conductivity is due to the unadsorbed ions and to the hydrous oxide particles which _have adsorbed ions and which move with a velocity somewhat less than that of the free ions. Thus, the adsorbed ions contrib- ute to the conductance of the solution, but they behave abnor- mally as regards electrometric measurements. Adsorbed chloride ion gives no test with silver nitrate, and its effect on the calomel electrode will be negligible. To assume that all the hydrogen and chloride ions which do not show up in electrometric measure- ments exist in complex ions will necessarily lead to erroneous conclusions. Until we know definitely what Adolf and Pauli’s conductivity and electrometric measurements actually mean, it seems idle to speculate as to the real nature of solutions of zirconium oxychloride, whether dialyzed or undialyzed. Adolf and Pauli assume the existence of complex ions in sols where the ratio ZrO, to Clis 3 or 4:1. This would seem to be a far-fetched assumption in a sol such as Ruer’s, where the ratio is 40: 1 or more. A very satisfactory sol was prepared by Rosenheim and Hertzmann! by the dialysis for a week of a 1.5 per cent solution of zirconium acetate. The colloid was perfectly clear in both trans- mitted and reflected light and contained but a trace of acetate. Heating on the water bath converted the sol into a clear trans- parent jelly. It was very sensitive to the action of electrolytes, dilute potassium chloride precipitating it quantitatively. Peptization of Hydrous Zirconia.—Miiller? prepared sols both by adding freshly precipitated and washed zirconia to a solution of zirconium nitrate and by adding ammonia drop by drop to the nitrate solution until the precipitate first formed just failed to redissolve. Evaporating the sol to dryness gave a gummy residue that swelled in water and was then repeptized. The oftener this process was repeated, the smaller the nitrate content 1 Ber., 40, 813 (1907). 2Z. anorg. Chem., 52, 316 (1907). 244 THE HYDROUS OXIDES became. The sols were precipitated by low concentrations of electrolytes containing multivalent anions. Zirconium sulfate like the nitrate, peptizes hydrous zirconia. Hauser' showed conclusively that the products of the peptizations are sols and not basic salts, ZrOSO,and ZrO(NOs)e, as asummed by Berzelius? and Paykull.’ Szilard* added ammonia to a zirconium nitrate solution and washed the resulting gel thoroughly, using the centrifuge, until complete peptization took place. In this way, a highly purified zirconia sol was obtained which was quite sensitive to the action of electrolytes, carbon dioxide from the air being sufficient to induce coagulation. Szilard® also peptized the purified gelati- nous oxide with the nitrates of zirconium, thorium, and uranyl, obtaining sols similar to those of Miiller. — A zirconia sol of suitable concentration is converted into a jelly by adding enough electrolyte to cause slow coagulation. The jelly can be broken up by shaking, giving a sol which will set again to a jelly; but the process cannot be repeated very often without throwing down a gelatinous precipitate.°® ADSORPTION BY HYDROUS ZIRCONIA On account of its highly gelatinous character, hydrous zirconia possesses a marked adsorption capacity for many substances.7 The taking up of iodine and ammonia by the hydrous oxide care- fully purified by dialysis does not follow the ordinary adsorption rule. Instead, the amount taken up increases with the concen- tration of the solutions without approaching a constant value, thus indicating the formation of a solid solution. Colloidal solutions of ferric oxide, molybdenum blue, zirconium, and silver are quickly decolorized by shaking with a paste of hydrous zirconia. The blue starch-iodine sol is taken up, giving a blue zirconia gel which is decolorized by heating and becomes blue 1Z. anorg. Chem., 54, 208 (1907). 2 Pogg. Ann., 4, 117 (1825). 3 Ber., 6, 1467 (1873). 4 J. chim. phys., 5, 488 (1907). ® J. chim. phys., 5, 636 (1907). 6 SCHALEK and Szravari: Kolloid-Z., 38, 326 (1923). 7 WEDEKIND and RHEINBOLDT: Ber., 47, 2142 (1914), TITANIUM, ZIRCONIUM, AND THORIUM 245 again on cooling, just as the original sol. Colloidal Congo red is strongly adsorbed, giving a blue adsorption compound which is converted into a red salt by warming.! The last-mentioned phenomenon has been observed in a number of instances by Wedekind and Wilke.? Thus, arsenic acid is adsorbed in the cold by hydrous zirconia; but on standing or boiling, Zr(HAsOu.)> is formed. A similar thing was observed with phosphoric acid; but adsorption only takes place with the following acids: arseni- ous, monochloracetic, hydrochloric, and perchloric. Obviously, the tendency to form salts following adsorption is not a question of the strength of the acids. Zirconia gel rapidly catalyzes the decomposition of hydrogen peroxide, especially in concentrated solutions; but the removal of hydrogen peroxide by the gel during very short periods of contact with dilute solutions can be represented by the usual adsorption equation. After prolonged contact, the hydrogen peroxide in solution is almost completely decomposed, but large amounts remain in the gel. Not all the peroxide taken up by the gel can be titrated by permanganate in 8 per cent sulfuric acid. ‘This is taken to indicate the formation of a complex peroxide following the initial adsorption. The adsorption capacity of hydrous zirconia for certain dyes suggests the use of zirconium salts as mordants? and in the prepara- tion of lac dyes.* For these purposes the hydrous oxide poss- esses no properties that are distinctive and so it finds but limited application. Hyprovus ZIRCONIUM PEROXIDE A hydrous peroxide of zirconium was first obtained by adding ammonia to a solution containing zirconium sulfate and hydrogen peroxide.’ Such gels contain both dioxide and peroxide; but Bailey® added hydrogen peroxide alone to solutions of zirconium salts, obtaining gelatinous precipitates which analyzed approxi- 1Cf. Bayuiss: Chem. Zenir. II, 1095 (1911). 2 Kolloid-Z., 34, 83, 283; 35, 23 (1924). 3 BarRNEs: J. Soc. Chem. Ind., 15, 420 (1896); Wenarar: Fdrber Ztg., 25, 277 (1914). 4 ScHEURER and BrytiuskI: Bull. soc. ind. Mulhouse, 68, 124 (1898). 5 Curve: Bull. soc. chim., (2) 48, 57 (1885). 6 J. Chem. Soc., 49, 149, 481 (1886); Proc. Roy. Soc., 46, 74 (1890); cf. Hermann: J. prakt, Chem., 97, 331 (1866). 246 THE HYDROUS OXIDES mately for ZrO3;:3H.O0 when dried over phosphorus pentoxide at ordinary temperature. The oxide loses oxygen on heating, the composition approaching Zr.O; at 100°. Pissarjewsky! obtained hydrous ZrO; by electrolyzing a sodium chloride solu- tion in which hydrous ZrO. was suspended. Irrespective of the method of preparation, the higher oxide behaves as a true peroxide, giving off oxygen on standing and yielding hydrogen peroxide when treated with dilute sulfuric acid. The gelatinous oxide is fairly soluble in alkali, and perzirconates are said to form. Hyprovus THORIUM DIOoxIDE The ordinary gelatinous form of hydrous thorium dioxide is precipitated by adding ammonia or alkalies to a cold solution of thorium salt. The gel is readily soluble in mineral acids but is insoluble in alkalies. The anhydrous oxide obtained by igniting thorium nitrate, sulfate, or the hydrous oxide is not attacked by acids, whereas that prepared by ignition of the oxalate under suitable conditions is a loose insoluble powder which is rendered soluble by boiling to dryness with hydrochloric or nitric acid. As in the case of zirconia, people have assumed that the product obtained by ignition of thorium oxalate is a meta oxide and that the product of the action of hydrochloric acid, say, is a meta chloride.? These assumptions are erroneous, since the chlorine content of the alleged compound varies through wide limits and the solution in water is a typical case of sol formation. Thoria Sols.—By dialyzing a 14 per cent solution of thorium nitrate for several days, Biltz? obtained a dilute, water-clear thoria sol containing a small amount of nitrate ion. This sol is stabilized by preferential adsorption of Th’*** and H’ ions and so is precipitated by suitable amounts of negatively charged sols. Under the influence of electrical stress, the colloidal particles migrate to the cathode, where they precipitate as a jelly contain- ing bubbles of gas. Miiller* obtained similar sols containing as much as 15 grams ThO; in 100 cubic centimeters by peptizing 1Z. anorg. Chem., 25, 378 (1900); 31, 359 (1902). 2 CLEVE: Bull. soc. chim., (2) 21, 115 (1874); Stevens: Z. anorg. Chem., 27, 41 (1901). 3 Ber., 35, 4436 (1902); 37, 1095 (1904). 4 Ber., 39, 2857 (1906); Z. anorg. Chem., 57, 314 (1908), TITANIUM, ZIRCONIUM, AND THORIUM 247 freshly precipitated and washed hydrous thoria with thorium nitrate, hydrochloric acid, aluminum chloride, ferric chloride, and uranyl nitrate. The sols are slightly cloudy, but they can be boiled without precipitating. The particles in the newly formed sols are completely amorphous; but the ageing which accom- panies boiling, results gradually in the appearance of a crystal- line structure detectable by x-ray analysis.!_ Evaporating to dryness gives a glistening brittle varnish-like residue which swells in water and finally is dispersed into a distinctly opalescent sol. All of Miuller’s sols are quite sensitive to the action of electrolytes, particularly those with multivalent precipitating ions. By shaking with benzene,’ the hydrous oxide is precipitated at the benzene-water interface as a gel containing bubbles of air. The amount of electrolyte required to peptize a given quantity of hydrous oxide depends on the history of the sample. Szilard® peptized the fresh oxide precipitated from thorium nitrate solu- tion with ammonia, by thorough washing to remove the excess of ammonium nitrate. If the oxide is allowed to age even under water, it is not peptized by washing. As already noted, ignition of the hydrous oxide renders it non-peptizable; and the oxide from thorium oxalate is peptized only after boiling to dryness with a mineral acid, such as hydrochloric. By the latter process, Bahr‘ first prepared a so-called metachloride which was described as forming an opalescent solution in water. By repeated evapo- ration and repeptization in water, Cleve® obtained a preparation containing less than 1 per cent of chlorine. Cleve also observed the instability of the supposed solutions in the presence of various electrolytes. Stevens® found that the hydrous oxide, ignited until it is completed’ dehydrated, no longer forms a ‘‘soluble”’ chloride with hydrochloric acid. He attributed the observed variation in the thorium-chlorine ratio in the alleged compounds to the existence of several oxychlorides.’ Moreover, the failure _ 1B6um and Nicuassen: Z. anorg. Chem., 182, 6 (1924). 2 WINKELBLECH: Z. angew. Chem., 19, 1953 (1906). 3 J. chim. phys., 5, 488, 636 (1907). 4 Liebig’s Ann. Chem., 132, 227 (1864). 5 Bull. soc. chim., (2) 21, 117 (1874). 6 Z. anorg. Chem., 27, 41 (1901). 7 Cf. Wyrousorr and VERNEUIL: Compt. rend., 127, 863 (1898). 248 THE HYDROUS OXIDES to obtain a test for chloride with silver nitrate was believed to furnish conclusive proof of compound formation. All of the observations on thoria gels and sols are readily inter- preted in the light of van Bemmelen’s' investigations on hydrous zirconia. nor is it known just what role the ceria plays in pro- moting the combustion of electrolytic gas. Swan* suggests that 1 Rusens: Ann. Physik, (3) 20, 583 (1906); Ives, Kinaspury, and Karrer: J. Franklin Inst., 186, 401, 585 (1918). 2 Popszus: Z. Physik., 18, 212 (1923). 3 Swan: J. Chem. Soc., 125, 780 (1924). 4 STEINMETZ: “Radiation, Light, and [lumination,” 92 (1909). 5 WuitTE and Traver: J. Soc. Chem. Ind., 21, 1012 (1902), 250 THE HYDROUS OXIDES it may act as an oxygen carrier! or may increase the electron emission of thorium and thus bring about a greater ionization of the gases. Small pencils of the Welsbach mixture of thoria and ceria become brilliantly luminous like the incandescent mantle when heated to a moderate temperature. Lamps of this kind are of use for searchlight and projection lanterns for moving pictures wherever the electric current is not available. In addition to its use in artificial lighting and as a refractory, thoria has been employed for defining the digestive tract in clinical examinations by means of x-rays.” It is also used as a catalyst in the synthesis of many organic compounds. For example both symmetrical and unsymmetrical ketones are prepared directly from monocarboxylic acids;* alcohols are con- verted into ethers and olefines, depending on the temperature employed; and ammonia and alcohols yield olefines and primary amines at 360°.4 Hyprovus THoRIUuM PEROXIDE Hydrous thorium peroxide is thrown down in a gelatinous form by adding hydrogen peroxide to a solution of thorium acetate sulfate or nitrate.’ The gel adsorbs acids quite strongly; hence, it is very difficult to obtain in a pure state. Wyrouboff and Verneuil attempted to avoid this contamination by carrying out the precipitation in the presence of an excess of ammonia; but under these conditions, the precipitate contained nitric acid resulting from the action of hydrogen peroxide on the ammonia. The peroxide is formed by the action of hydrogen peroxide on hydrous thorium dioxide and also by electrolysis of a sodium chloride solution in which the dioxide is suspended. The latter method of formation indicates that the product is a true 1 Mryer and Anscniirz: Ber., 40, 2639 (1907). 2 KaESTLE: Miinch. med. Wochschr., 56, 919 (1909). 3 SENDERENS: Compt. rend., 148, 927 (1909); Ka@uuErR: Bull. soc. chim., (4) 15, 647 (1914). 4 MaruHE: Chem. Zig., 34, 1173 (1911). 5 WyRouBOFF and VERNEUIL: Ann. chim. phys., (8) 6, 441 (1906). 6 Lecog DE BoisBAuDRAN: Compt. rend., 100, 605 (1885); Cumve: Bull. soc. chim., (2) 48, 53 (1885). TITANIUM, ZIRCONIUM, AND THORIUM 251 peroxide and not an addition compound of hydrous thorium- dioxide and hydrogen peroxide. The freshly prepared peroxide appears to be hydrous TheO;, but this is quite unstable, going over on standing to the much stabler ThO3.! Dilute sulfuric acid reacts with it, giving hydro- gen peroxide; and strong sulfuric acid gives ozone. Unlike the corresponding compounds of titanium and zirconium, it is not attacked by alkalies. 1 PISSARJEWSKY: Z. anorg. Chem., 31, 359 (1902); 25, 378 (1900). CHAPTER X THE HYDROUS OXIDES OF THE RARE EARTHS The term rare earths is applied to a group of closely related trivalent metals forming basic oxides with oxalates insoluble in dilute mineral acids. The rare-earth group includes scandium, yttrium, and lanthanum, together with all the elements between cerium, atomic number 58, and lutecium, atomic number 71, inclusive. These elements are frequently divided into three families, the basis for the arbitrary classification being the solu- bility of the double alkali sulfates.1 The elements of the cerium family, scandium, lanthanum, cerium, praseodymium, neody- mium, and samarium, form quite insoluble double sulfates; and the elements of the yttrium family, dysprosium, holmium, erbium, thulium, yttrium, ytterbium, and lutecium, form quite soluble double sulfates. On the border line between these two families are the terbium family elements, europium, gadolinium, and terbium, whose double sulfates are but moderately soluble. The hydrous oxides of the cerium group are the best known and will be considered separately, beginning with hydrous ceric oxide. Tur Hyprovus OXIDES OF THE CERIUM FAMILY Hydrous Ceric Oxide.—Cerium differs from all the other members of the rare-earth family in forming a definite series of ceric salts derived from the most stable oxide of cerium, CeQOs. It is only as a trivalent metal that cerium exhibits the proper- ties of a typical rare earth. Hydrous ceric oxide is precipitated as a yellowish highly gelatinous mass by adding ammonia or alkali to a solution of ceric salt. It is also formed by oxidizing hydrous cerous oxide suspended in water, either by the oxygen of the air or by adding an oxidizing agent such as chlorine, bromine, alkali hypochlorite, 1 Ursain: Ann. chim. phys., (7) 119, 184 (1900), 252 THE HY DROUS OXIDES OF THE RARE EARTHS 253 or sodium peroxide. Like most gelatinous precipitates, it adsorbs alkali salts and hydroxide strongly and so is best obtained pure by precipitating cold ceric ammonium nitrate with ammonia, allowing the washed precipitate to dry partially, and finally rewashing to remove all ammonium nitrate. The precipitate dried over potassium hydroxide has the formula CeO2: 1.5H20,! but it is-altogether unlikely that this is a definite hydrate. Indeed, B6hm and Niclassen? found the ammonia precipitated oxide to be crystalline, the x-radiogram showing it to be CeQOs. On the other hand, the hydrous gel obtained by dialysis of ceric ammonium nitrate is amorphous. The hydrous oxide gives up a great deal of adsorbed water on standing and is transformed into a fibrous or granular mass. If dried below 120°, it dissolves in acids and alkalies, but the ignited oxide is quite insoluble.* Although the highly dispersed gelatinous oxide free from praseodymium,‘ is white;> when calcined at a high temperature, it assumes a citron-yellow color, becoming white or a lighter yellow again on cooling. The tint assumed on ignition depends on the mode of preparation; that obtained by igniting the hydrous oxide is darker than that from the sulfate; but according to Wyrouboff and Verneuil,® the tint of neither is definite enough to be described other than as a shade of white. Spencer’ attributes the yellow color to polymerization, and Sterba® sug- gests that it may be due to a higher oxide. There seems to be no experimental justification for either of these assumptions, and I am inclined to believe that the color assumed on heating is due to coalescence of particles which appear white in a finer state of subdivision. It is well known that zinc oxide is yellow when hot, due to coalescence of particles; but disintegration 1 Wryrovusorr and VERNEUIL: Ann. chim. phys., (8) 9, 289 (1906); Ram- MELSBERG: Pogg. Ann., 108, 40 (1859); Erx: Z. Chem., (2) 7, 100 (1871); cf., however, CARNELLEY and Waker: J. Chem. Soc., 58, 59 (1888). 2Z. anorg. Chem., 132, 1 (1924). 3 MENGEL: Z. anorg. Chem., 19, 71 (1899). 4 Wir: Chem. Ind., 19, 156 (1896). 5 Cf., however, BRAUNER: Z. anorg. Chem., 34, 207 (1903). 6 Ann. chim. phys., (8) 9, 356 (1906). 7 J. Chem. Soc., 107, 1272 (1915); Muymr: Z. anorg. Chem., 37, 378 (1903). 8 Compt. rend., 188, 221 (1901); Ann, chim. phys., (8) 2, 193 (1904). 254 THE HYDROUS OXIDES takes place on cooling, accompanied by a return to the white color. However, a thoroughly sintered mass of zinc oxide remains yellow indefinitely, even on cooling.! Similarly, the citron-yellow color of hot hydrous ceric oxide becomes white or light yellow on cooling, depending on the time and temperature of ignition. The oxide has been suggested as a yellow opacifying agent for glass and enamel.? Cerium salts may be used more or less successfully for tanning leather? and as a mordant in dyeing cotton.4 In both of these processes, the hydrous oxide plays an important role. By far the most important use to which the oxide has been put is in the manufacture of incandescent mantels. This application has been referred to already, in connection with thorium oxide. The precipitate of hydrous ceric oxide obtained in the cold by adding sodium peroxide to a solution of cerous salt is reddish brown in color; but on boiling, oxygen is evolved and the color disappears.® The red-brown color may be due to a higher oxide of cerium, possibly hydrous CeQO;° which is instable at 100°. Ceric Oxide Sols —Hydrous ceric oxide sol is best prepared by dialysis of a solution of ceric ammonium nitrate.’ The sol may be evaporated to dryness on the water bath, giving a gummy mass which goes into colloidal solution again on shaking with water. There is no evidence of crystal structure in the hydrous oxide formed in’this way.® Like hydrous chromic oxide and ferric arsenate, the sol pre- pared by dialysis of ceric ammonium nitrate sets to a firm jelly if the dialysis is carried too far. This is particularly noticeable 1Farnau: J. Phys. Chem., 17, 653 (1918). 2 RICKMANN and Rappe: British Patent 203773 (1908). 3 KitnER: Gerber, 37, 199, 213 (1911); GaRELLI: Collegiwm, 418 (1912); PareEnzo: [bid., 121 (1910). 4 Matscuak: Chem. Ind., 21, 150 (1898); Wirt: Jbid., 19, 156 (1896); WarGNneErR and Mtuumr: Z. Farben- u. Textil Chem., 15, 290 (1903); BAskER- VILLE and Fouts: J. Soc. Chem. Ind., 28, 104 (1904). 6 MmNGEL: Z. anorg. Chem., 19, 71 (1899). 6 Lecog DE BorsBAUDRAN: Compt. rend., 100, 605 (1885); CiEvE: Bull. soc. chim., (2) 48, 53 (1885); Knorr: Z. angew. Chem., 11, 687, 717 (1897). 7 Biuttz: Ber., 36, 4431 (1902). 8 B6uM and NicuassEN: Z. anorg. Chem., 182, 6 (1924), 9 Farnav and Pautt: Kolloid-Z., 20, 20 (1917), THE HYDROUS OXIDES OF THE RARE EARTHS 259 if the initital concentration of the peptizing agent, nitric acid, falls below a critical value that is determined in part by the pres- ence in the sol of the precipitating electrolyte, ammonium nitrate.! Thus a jelly returns to the sol conditions if shaken up with a quantity of fresh undialyzed sol; and the concentration of electrolyte necessary to precipitate the hydrous oxide as a jelly is increased by adding a small amount of nitric acid. Kruyt and van der Made studied the effect of different elec- trolytes on the nature of the precipitate, obtaining stable jellies that do not contract in some instances and undergo rapid synere- sis or coagulation in others. As shown,’ this is purely a question of rate of precipitation of the sol, which in turn, is determined by the concentration of added electrolyte. Jellies with almost identical properties should result with any precipitating elec- trolyte that does not react with the particles, provided the con- centration is such as to allow a suitable slow rate of precipitation. The order of concentration of ions necessary for jelly formation fieeeenours is. br > ClO, > Cl > NO; > CNS > I > SOQ, > HPO,. Alcohol likewise decreases the stability of the sol and in concentrations of 40 to 50 per cent precipitates it as a jelly. Like the precipitated hydrous oxide, the primary particles in the sol condition coalesce and lose water more rapidly than is usual with sols of the hydrous oxides. This ageing is readily followed viscosimetrically, since the loss of adsorbed water by the dispersed particles is accompanied by a marked decrease in viscosity. A solution of ceric ammonium nitrate dialyzed short of the appearance of any gel on the dialyzer gives a viscosity- time curve having the general form represented in Fig. 15. This curve is for a sol containing 1.28 grams CeO, which was prepared by continuous dialysis for 30 hours of a 6 per cent cerlum ammonium nitrate solution. The initial increase in viscosity is a manifestation of gelation; with a relatively strong sol, this may proceed to the point where the time of flow can no longer be measured, followed in the course of a few weeks by a decrease in viscosity until the value of an aged sol is reached. If gelation has already started in the dialyzer before the viscosity measurements are begun, the maximum in the viscosity curve is 1Cf. Kruyt and vAN DER Maps; Rec, trav, chim., (4) 42, 277 (1923). 2 See p. 26, 256 THE HYDROUS OXIDES missed. Nor is it observed after the sol has been heated to 50° which causes rapid ageing, or when the concentration of the sol is too low to admit of marked coalescence of the hydrous particles. Thus, the character of a hydrous ceric oxide sol is influenced to a marked degree by relatively slight variations in the method of dialysis, concentration, temperature, and time. The particles of a heated sol or one aged by long standing in the cold are no longer sufficiently hydrous to give a jelly on precipitation, at least in concentrations as low as 1.5 per cent CeQs. 2.00 on e) mM on Relative Viscosity Time,days Fig. 15.—Change in viscosity of CeOzg sols with time. Farnau and Pauli! added to a fresh sol insufficient salt to pro- duce coagulation and observed an immediate drop in the viscosity of the sol, followed by a gradual increase in viscosity, the final result being a jelly; with still less salt, the initial diminution in viscosity was followed by an increase to a maximum and there- after by a slow decrease as indicated by the dotted curve in Fig. 15. 8 and y rays from radium act on the sol in much the same manner as electrolytes. Prolonged action produces a firm stable jelly, while shorter action results in a viscosity-time curve readily distinguished from the electrolyte curve by a much steeper rise and fall on opposite sides of the maximum, as shown by the results of observations of Farnau and Pauli represented in Fig. 16. 1 Kolloid-Z., 20, 20 (1917). THE HYDROUS OXIDES OF THE RARE EARTHS = 257 The sols used in these experiments contained 0.96 per cent CeO. and had already begun to decrease in viscosity when the measure- ments were started. On the fifth day the sol was subjected, for 13 hours only, to the action of 8 rays from radium. This brought about a sharp rise in viscosity, which reached a maximum on the twentieth day, followed by a sharp fall. On the fifty-first day the 1.9 4 =Rays Applied t=Rays Removed Vis cosity \ hs : ig 0 10 20 50 40 50 60 Time, days Fic. 16.—Effect of B-rays from radium on CeO: sol. 8B rays were applied continuously until gelation took place on the fifty-fifth day. Under the influence of a suitable amount of electrolytes or prolonged action of radiations, the charge on the particles is neutralized. This is apparently accompanied by a loss of adsorbed water and a consequent lowering of the viscosity, gradual under the influence of @ and y rays but immediately when an electrolyte is added. The subsequent increase in viscosity 258 THE HYDROUS OXIDES is due to aggregation of the electrically neutral particles forming a jelly. The attainment of a maximum viscosity and the sub- sequent fall, when the added electrolyte is small in amount or’ the time of exposure to the rays is comparatively brief, is attrib- uted by Farnau and Pauli to the peptizing action of electrically charged particles entangled in the jelly. Since hydrogen ion is the stabilizing ion of the sol, observations of the changes in the hydrogen ion concentration might throw some light on the anomalous behavior during the ageing process. Kruyt and van der Made! peptized hydrous ceric oxide with dilute hydrochloric acid; but an excess of peptizing agent was ‘required. Wyrouboff and Verneuil? decomposed cerium oxalate at as low a temperature as possible and heated the oxide with 2 per cent nitric acid on the water bath. The resulting product, dried at 100°, was assigned the formula (CeOe)4- 4HNO3. It dissolved in water and on dialysis gave a precipitate that was represented as (CeOz)40° 10H2O. The soluble product was not a definite compound as Wyrouboff and Verneuil supposed, and the apparent solution was simply an aged CeQOz sol peptized by nitric acid. Hydrous Cerous Oxide.—This compound is obtained as a pure white,*® gelatinous precipitate by treating a cerous salt solution with ammonia or alkali in the absence of air. It oxidizes readily in the air especially in the presence of alkali,* the color changing to violet and finally light yellow, owing to the formation of hydrous CeO,. A similar color is obtained by heating ceric carbonate, nitrate, oxalate, or oxide in hydrogen. As one should not expect a mixture of two light bodies to be violet, the colored body is probably a cero-ceric oxide to which Chase® and Meyer® assign the formula C407 or 2CeQ2 - Ce2O3; and Wyrouboff and Verneuil’ the formula Ce7;Oy. or 3CeQO2- 2Ce203. The latter 1 Rec. trav. chim., (4) 42, 278 (1923). 2 Compt. rend., 124, 1300 (1897); Bull. soc. chim., (3) 17, 679 (1897). 3 Dennis and Maazn: J. Am. Chem. Soc., 16, 649 (1894); DaAmiEns: Ann. chim., (9) 10, 137 (1918). 4 SpENcER: J. Chem. Soc., 107, 1265 (1915). 5 J. Am. Chem. Soc., 39, 1576 (1917). 6 Z. anorg. Chem., 37, 378 (1903); cf. SteRBA: Ann. chim. phys., (8) 2, 193 (1904). 7 Ann. chim. phys., (8) 9, 289 (1906); Compt, rend,, 128, 501 (1899). THE HYDROUS OXIDES OF THE RARE EARTHS — 259 investigators obtained the violet product directly by adding alkali to a mixed solution of cerous and ceric salts, the maximum intensity resulting when the ratio of cerous to ceric ion was 2 to 1. Hydrous Praseodymium Oxide.—The gelatinous mass of hydrous Pr.O3, precipitated from a praseodymium salt by alka- lies, is bright green in color and can be dried to a green powder which has, probably erroneously, been assumed to be a tri- hydrate.! If the hydrous oxide, the oxalate, or the nitrate of _ praseodymium is heated in air, a black powder is obtained, intermediate between PreO; and PrO2;? but the exact composi- tion depends on the substance calcined and the temperature of calcination.* In the presence of a small amount of CeOs, which appears to act as an oxygen carrier, the product approaches near the limit PrO:.4 It is probable that products of intermediate composition are not definite chemical individuals but are mix- tures representing intermediate stages in the oxidation of the lower oxide. In the present state of our knowledge, it is, of course, open to anyone to postulate an intermediate oxide, such as seems necessary to account for the color changes accompany- ing the oxidation of Ce.,.03;. By adding hydrogen peroxide to a praseodymium salt before precipitating, Braesner® claims to get Pr.O; - xH20. | Hydrous Scandium Oxide.—Alkalies and ammonia precipitate hydrous Sc2O03 as a white voluminous mass, insoluble in excess of precipitant. Like hydrous alumina, it is amorphous when first precipitated, but after ageing for some time, an x-radio- gram shows a transformation to a crystalline structure. When dried in the air at room temperature, it forms a hard horny mass which analyzes approximately for a trihydrate, Sc2O; - H.0;° but there is no definite evidence that such a hydrate exists. By dialyzing a solution of SnCl; to which ammonia is added short 1 Cf. Damiens: Ann. chim., (9) 10, 181 (1918). 2 WELsBACcH: Monatsh., 6, 477 (1885); Jones: Am. Chem. J., 20, 345 (1898) ; ScHOTTLANDER: Ber., 25, 569 (1892); Mryur: Z. anorg. Chem., 41, 97 (1904). 3 ScHhELE: Ber., 32, 409 (1899). 4 JAamR: Proc. Acad. Sci. Amsterdam, 16, 1095 (1914); Marc: Ber., 35, 2382 (1902). 5 Proc. Chem. Soc., 17, 66 (1901); cf. Mevixorr and Kuimenxo: J. Russ. Phys.-Chem. Soc., 33, 663, 739 (1901). 6 Crooks: Phil. Trans., 209A, 15 (1909). 260 THE HYDROUS OXIDES of precipitation, a hydrous sol results which sets to a jelly when treated with a suitable amount of electrolyte.! Under favor- able conditions this jelly is broken up by shaking, forming a limpid sol which will set again on being allowed to stand quietly.’ Hydrous Lanthanum Oxide.—The oxide La2O3 reacts with water with the evolution of heat like lime, giving a voluminous snow-white powder which has the formula La(OH)3; when dried at 100°. Although it dissolves slightly in water, the hydrous oxide thrown down by alkalies is almost as gelatinous as hydrous alumina; but the adsorption capacity of the latter for saccharose is appreciably greater.t The basic reaction of lanthanum hydrox- ide is comparable to that of ammonia;> hence, the gelatinous oxide absorbs CO, from the air and even the ignited oxide is readily soluble in acids. The basicity of the oxide would seem to preclude the formation of lanthanates, although Baskerville and Catlett® claim to have prepared complex compounds of this type by fusing lanthana with potassium hydroxide or by digest- ing the oxide with strong solutions of alkali. Undoubtedly, the products were hydrous lanthanum oxide with adsorbed alkali.” A transparent sol is obtained by peptizing the freshly formed hydrous oxide with a small amount of dilute hydrochloric acid.® Hydrous Neodymium Oxide.—The gelatinous oxide precipi- tated from a highly purified solution of a neodymium salt is blue and gives blue Nd,O3 on ignition. The blue color may be modified by the presence of impurities. By heating neodymium oxalate to a red heat in a stream of oxygen, Waegner® obtained a rose-colored product which gave a distinctly different reflection spectrum from Nd:O; and which appeared to be a higher oxide of the formula Nd,O;. By suitable choice of conditions, mixed spectra of Nd:O; and the so-called Nd4sO; were obtained. Similar observations were made on heating the hydrous oxide 1 Boum and NicuassEen: Z. anorg. Chem., 132, 6 (1924). 2 ScHALEK and Szecvanrt: Kolloid-Z., 33, 326 (1923). 3 CLEVE: Bull. soc. chim., (2) 21, 196 (1874). 4 EKuLER and Nitsson: Z. physiol. Chem., 181, 107 (1923). ’ VESTERBERG: Z. anorg. Chem., 94, 371 (1916). 6 J. Am. Chem. Soc., 26, 75 (1904). 7 Cf. ZAMBONINI and CARosBi: Gazz. chim. ital., 54, 46, 53 (1924). 8 Boum and NiciassEn: Z. anorg. Chem., 182, 6 (1924). 9Z,. anorg. Chem., 42, 118 (1904). THE HYDROUS OXIDES OF THE RARE EARTHS 261 and the anhydrous nitrate and carbonate. Joye and Garnier! claim that the different-colored products are not due to the oxy- gen content but to the degree of hydration of Nd,O3. Thus the hydrous oxide dried in air was taken to be Nd(OH);; on heating this to 320°, it has a formula corresponding to Nd2O3;- 1.5H.2O and gives a reflection spectrum corresponding to that of a similarly colored oxide described by Waegner; on further heating to 520°, the oxide has the composition Nd2O3;-H.2O and gives a reflection spectrum identical with Waegner’s Nd.O;. These data are interpreted to establish the existence of three hydrates of Nd.O3 and the non-existence of a higher oxide; but they are not conclusive. Thus, Garnier claims to get the same reflection spectrum by heating the hydrous oxide that Waegner does by heating what he says is an anhydrous salt, thereby precluding the formation of a hydrate. Of course, it may be argued that Waegner’s salts decomposed during dehydration, but this cannot be true, at least in the case of the carbonate which gives up all its hydrate water below 200°? and does not start to decompose until above 300°. Moreover, the view that the rose-colored product is a hydrate does not fit in with Waegner’s observa- tion that gentle heating in a current of hydrogen converts it into clear-blue Nd2,O3. Obviously, the whole problem should be reinvestigated. The blue gelatinous precipitate of the hydrous oxide is readily peptized by dilute HCl, forming a beautiful blue sol. Hydrous Samarium Oxide.—Gelatinous hydrous Sa,Q3 is almost white with a pale-yellow tinge which is not appreciably intensified on ignition to Sa2QO3. According to Cleve,‘ if the precipitation with ammonia is carried out in the presence of hydrogen peroxide, a hydrous oxide of the formula Sa4Op» - tH2.O results which is similar in appearance to hydrous S8a2Q3. Tur Hyprovus OXIDES OF THE TERBIUM FAMILY The hydrous oxides of europium, gadolinium, and terbium are obtained in the same way as the corresponding compounds of 1Compt. rend., 134, 510 (1912); GarnimrR: Arch. sct. phys. nat., (6) 40, 98, 199 (1915). 2 Preiss and Rainer: Z. anorg. Chem., 131, 287 (1923). 3’ Boum and NiciassEen: Z. anorg. Chem,, 132, 6 (1923), 4 Bull. soc. chim., (2) 48, 53 (1885). 262 THE HYDROUS OXIDES the cerium family, by the action of alkali or ammonia on solutions of their salts. When freshly prepared, the gelatinous oxides rapidly absorb carbon dioxide when exposed to the air. Anhy- drous Gd2,03 and Tb2O3 are white solids, while Ku2O3 possesses a reddish-yellow tinge.! All of the oxides are soluble in acids; but Gd.O; dissolves very slowly at the start, the velocity increas- ing as the action proceeds.2, When terbium oxalate is ignited, it gives a dark-brown peroxide which approaches the composition required for TbOs. If a mixture of air and coal gas is passed over TbO:, or a mixture of Gd2O3 and TbO2 heated almost to redness, the whole mass immediately becomes incandescent, and the gas often takes fire.® THE Hyprovus OXIDES OF THE YTTRIUM FAMILY Dysprosium, holmium, erbium, thulium, yttrium, ytterbium, and lutecium all form highly gelatinous oxides when thrown down from their salt solutions with ammonia. Like hydrous alumina, the gels of Er.03 and Y203 become microcrystalline on standing,‘ and it is probable that the other oxides behave similarly. Ho2O3 has a pale-yellow color; Er.Os is rose red; Tm2O3 is white with a greenish tinge; and Dy203, Y2O3, Yb2O3:, and Lu2O; are white. Hydrous peroxides of yttrium Y,4O,- #H.O and of erbium ErQ, :- «H,O are formed by adding hydrogen peroxide and ammonia to solutions of their respective salts.® On account of the gelatinous character of the precipitated oxides, it is probable that all of them will form sols; but so far only two have been described. Bohm and Niclassen® dialyzed a solution of erbium nitrate to which ammonia was added short of precipitation. This sol set to a jelly on adding a suitable amount of precipitating electrolyte. Miller’ peptized the hydrous oxide of yttrium with dilute hydrochloric acid, aluminum chloride, and ferric chloride; and Szilard® employed thorium acetate. 1PRANDTL: Ber., 55B, 692 (1922). 2 BENEDICKs: Z. anorg. Chem., 22, 392 (1900). 3 BissEL and JamEsS: J. Am. Chem. Soc., 38, 873 (1916). 4 Boum and NICLASSEN: Z. anorg. Chem., 182, 1 (1924). 5 CLeve: Bull. soc. chim., (2) 21, 196 (1874). 6 Z. anorg. Chem., 182, 6 (1924). 7Z. anorg. Chem., 57, 314 (1908). 8 J. chim. phys., 5, 488, 636 (1907). CHAPTER XI THE HYDROUS OXIDES OF THE FIFTH GROUP The elements of the fifth group which form hydrous oxides are vanadium, columbium, tantalum, antimony, and bismuth. These will be taken up in the order named. Hyprovus VANADIUM PENTOXIDE The addition of a mineral acid to a concentrated solution of an alkali or alkaline earth vanadate throws down V.O; as a red- brown amorphous hydrous mass, closely resembling hydrous ferric oxide. A similar precipitate results from the hydrolysis of vana- dium oxychloride. The gel is made up of very fine particles which cannot be washed free from the mother liquor without undergoing peptization. By drying in the air, von Hauer? real- ized a composition approaching that of a dihydrate which was taken to be pyrovanadic acid, H4V.O,;, analogous to the corre- sponding phosphorus compound. Continuing the drying over sulfuric acid until another molecule of water is lost, gives the correct formula for metavanadic acid, HVO;.2 The exact inves- tigations of Dullberg* show, however, that the red-brown gel is not an acid but is hydrous vanadium pentoxide whose water content depends on the condition of drying. The so-called pyro- and metavanadic acids not only do not occur as solids but are incapable of existing in solution, although both pyro- and metavana dates are known. The stronger hexavanadic acid, H4V,O17 or 6V20; - 2H.O, does exist in dilute solution, but the solid acid is unknown. 1 MorssaNn: Bull. soc. chim., (3) 15, 1278 (1896). 2 J. prakt. Chem., 80, 324 (1860). 3 FRITZSCHE: J. prakt. Chem., 58, 93 (1851); Manassrm: Liebig’s Ann. Chem., 240, 23 (1887). 4Z, physik, Chem., 45, 129 (1908). 263 264 THE HYDROUS OXIDES VANADIUM PENTOXIDE SOLS Biltz! treated ammonium vanadate with a dilute solution of hydrochloric acid, obtaining vanadium pentoxide as a brownish- red powder. After thorough washing to remove excess elec- — trolyte, the oxide peptizes completely in water, giving a clear reddish-yellow sol. The colloidal particles are negatively charged and are highly hydrous. Addition of ammonium chloride to a concentrated sol causes it to set to a jelly; while a dilute sol is precipitated as reddish-yellow highly gelatinous flocs that settle very slowly. If the washed oxide is dried before being peptized, the particles in the sol are larger and are precipitated in less voluminous floes which settle more rapidly. Wegelin? prepared vanadium pentoxide by hydrolysis of a boiling solution of VOCI;. This was peptized by washing; but the particles are larger and less hydrous than those in the Biltz sol. When treated with electrolytes, the particles agglomerate into dense clumps that settle out rapidly. The precipitate from a boiled Biltz sol is likewise much denser and darker than from an unboiled sol. | Miiller* obtained a sol by triturating the granular mass pro- duced by sudden cooling of molten vanadium pentoxide either by plunging the containing vessel of platinum into cold water or by pouring the melt into cold water. Sols formed in this way are reddish brown in color. The precipitation by ammonium chloride is reversible; but the dense brown residue obtained by evaporating the sol to dryness on the water bath is not repeptized by water, whereas the looser yellow mass resulting from evapora- tion of the Biltz sol is easily repeptized. Freundlich and Leonhardt® peptized an amorphous, ocher- yellow oxide obtained by gentle ignition of ammonium vanadate. This takes up water from air saturated with moisture, the color becoming reddish yellow. The sol formed by triturating with a little water, followed by shaking with an excess of water, is 1 Ber., 37, 1098 (1904). 2 Rong. Z., 11, 25:(1912). 8 Himeeneice and LEONHARDT: Rollsidehern Bethefte, T, 193 (1915). 4 Kolloid-Z., 8, 302 (1911). 6 Kouchner Bethefte, 7, 187 (1915); cf. Dirrn: Compt. rend., 101, 699 (1885). THE HYDROUS OXIDES OF THE FIFTH GROUP 265 orange yellow in color; but on standing for several days with occasional shaking, it changes to a yellowish red. The esters of orthovanadic acid are readily hydrolyzed by water, and Prandtl and Hess! took advantage of this reaction to prepare “‘electrolyte-free’”? vanadium pentoxide sols. For this purpose, the tertiary butyl ester is particularly satisfactory, both because it is a stable salt and because the hydrolysis product, tertiary butyl alcohol, can be removed from the sol almost com- pletely by boiling. The sols are orange when first prepared, but are changed to yellowish red by heating. While two modifications of vanadium pentoxide have been described—a yellow amorphous form and a red crystalline form— the observations recorded in the preceding experiments indicate that the color is influenced to a marked extent by the degree of dispersion. ‘The most highly dispersed oxide appears yellow, the color changing to reddish brown as the particles become larger and denser. If this view is correct, the reddish crystalline oxide should be yellow if sufficiently finely divided. As a matter of fact, Wegelin? prepared a canary-yellow sol by prolonged tri- turation, in an agate mortar, of red-brown crystals of vanadium pentoxide obtained by allowing the molten oxide to cool slowly. If this sol is coagulated by the addition of a small amount of sodium chloride, the yellow precipitate shows little change of color on keeping; but if a larger amount of sodium chloride is used in the coagulation, the resulting precipitate changes its color in the course of a few days from yellow to reddish brown. This change in color is due to growth of the particles, a process which Freundlich*® has found to increase rapidly with increasing concentration of electrolyte in contact with a precipitate. In vanadium pentoxide sols there is always a small amount of the oxide in molecular solution. This portion, yellow in color, passed through a dialyzing membrane and is not thrown down by electrolytes. As the sol is slightly acid, the yellow solution is a vanadic acid, possibly hexavanadic, H4V6QO17,*+ which yields a yellow anion, [HV.Q.7]’’. The presence of a tervalent anion 1Z. anorg. Chem., 82, 116 (1913); cf. RiepeL: Pharm. J., 92, 648 (1914). 2 Kolloid-Z., 14, 65 (1914). 3 FREUNDLICH and Haske: Z. physik. Chem., 89, 446 (1915). 4 DuLLBERG: Z. physik. Chem., 45, 175 (1903). 266 THE HYDROUS OXIDES which is likely to be strongly adsorbed accounts for the negative charge on the colloidal particles.1 As might be expected, the solubility determinations of different investigators show wide variations owing to the influence of particle size on solubility. Moreover, the usual measurements made on the supernatant solution after agglomeration of the sol are necessarily wrong, since they fail to take into account the amount adsorbed by the hydrous particles during precipitation. Optical Properties.—Probably the most interesting property of vanadium pentoxide sol is its double refraction on stirring, a phenomenon first observed by Freundlich and his pupils.? If stirred with a glass rod and viewed in reflected light, an aged sol appears to be filled with yellow glittering streaks as if there were fine crystals suspended in it. In transmitted light, the sol remains clear, but dark streaks can be observed. Viewed between crossed nicols, the field remains dark as long as the sol is not dis- turbed; but stirring causes the field to become bright at once. By allowing the sol to flow between crossed nicols in convergent light parallel to the line connecting the nicols, an image is ob- tained of a crossed axis with concentric rings. Observed with a quarter-wave mica plate, the flowing sol behaves like a positive uniaxial crystal. Freundlich pictures the sol at rest as made up of elongated particles possessing the usual unordered Brownian movement which can give no double refraction. The setting up of directed motion causes the sol to lose its isotropic nature and to become double refracting. A section cut from the sol may be looked upon as having a space lattice somewhat similar to a plate from an optically monoaxial crystal, the long axis of the sol particles coinciding in direction with the optical axis. If the sol is rotated between two cylindrical walls and viewed between crossed nicols, four minima of brightness are seen, giving the appearance of a dark cross,* the arms of which form an angle with the direction of polarization. The angle is independent 1 OSTERMAN: Wissench. u. Ind., 7, 17 (1922); Chem. Zentr., I, 396 (1923); cf., however, Dumanskt: Kolloid-Z., 38, 147 (1923). 2 FREUNDLICH and LronnuarpDT: Kolloidchem. Bethefte, 7, 207 (1915); DresseLHorstT and Freunpiicu: Physik. Z., 16, 422 (1915); Freunp.icu: Z. Elektrochem., 22, 27 (1916). 3 ZOCHER: Z. physik. Chem., 98, 293 (1921). THE HYDROUS OXIDES OF THE FIFTH GROUP 267 of the concentration of sol but increases rapidly with increasing age of sol-and decreases with rise of temperature. In a slowly moving fresh sol, the angle has the value of 45°, and in a rapidly moving aged sol, it approaches 90°. This behavior of the so-called vortex cross has been explained by Freundlich! in terms of the elasticity of the sol.? In fresh, slowly moving sols the elastic deformation of the sol elements is small; and so the -sol behaves like a rigid body and the cross-angle is 45°. In an aged rapidly moving sol, the angle is close to 90°. From this point of view, the cross-angle is identical with the angle of maximum deformation; and the direction of maximum deformation cor- responds with the direction of the velocity gradient. Hence, the colloidal particles do not arrange themselves along the line of motion because of friction between adjacent liquid layers of different velocities, but place themselves in the direction of maximum deformation. Only in an aged sol moving with high velocity does the direction practically coincide with the direction of flow, giving a vortex cross with 90° angles. The double refraction in an aged sol is so strong that it can be demonstrated by allowing the sol to flow through a prismatic trough with a triangular cross-section and using this as a prism to decompose spectrum lines. In this way, the red hydrogen line is resolved into two oppositely polarized lines. The more strongly refracted ray vibrates parallel to the direction of flow of the sol, and in accordance with Babinet’s rule, this extraordi- nary ray is more strongly absorbed than the other.‘ As already noted, the double refraction is not observed in a freshly prepared vanadium pentoxide sol. Freundlich®investi- gated quantitatively the influence of age of sol on its double refraction and found the velocity of ageing at constant streaming velocity and temperature, to be given by the equation dA/dt = kA 1 FREUNDLICH, STAPELFELDT, and ZocHEeR: Z. physik. Chem., 114, 161, 190 (1924); cf., however, MortsmirH and Lanemutir: Phys. Rev., (2) 20, 95 (1922). 2 FREUNDLICH and Sgrirritz: Z. physik. Chem., 104, 233 (1923). 3 Scowestorr: J. phys., (3) 1, 49 (1892). 4Cf. also HumpHrey: Proc. Phys. Soc. London, 35, 217 (1923). ’ FREUNDLICH, STAPELFELDT, and ZocHER: Z. physik. Chem., 114, 161 (1924); cf. GessNER: Kolloidchem, Bethefte, 19, 283 (1924). 268 THE HYDROUS OXIDES (Ac — A)?, where A is the double refraction. The magnitude of the velocity of ageing is very sensitive to the action of impuri- ties which may have either a stabilizing or peptizing action on the sol. With rising temperature the anisotropy decreases in a linear fashion. The double refraction of the sol corresponds approximately to that of the vanadium pentoxide content. Examination of an aged sol with the cardioid ultramicroscope! reveals rod-like structures whose length is approximately thirty times the diameter. In a slit ultramicroscope, their axis deviates by less than 30° from a line perpendiculr to the axis of the illum- inating beam. Reinders? believes the appearance of birefring- ence on ageing is due to the formation of ultramicroscopic needles, since he succeeded in demonstrating a similar birefringence in sols of mercurous chloride and lead iodide which ordinarily form microscopic crystals. Later, Zocher? established the cry- stalline character of the particles in an aged vanadium pentoxide sol by means of x-radiograms. The interference lines are broad, indicating the very small size of the crystals in the sol; but the arrangement of the lines is the same as observed with crystals obtained by cooling the molten pentoxide. The effect of ageing on the dielectric constant of the sol* indicates that the growth of rod-shaped particles during the ageing process is not an ordinary case of crystallization.> Freundlich® attributes the appearance of double refraction, on adding electrolytes to a benzopurpurin sol, to the development of longer particles by ordered coagulation and not to the growth of needle crystals. Just why we should get ordered coagulation into rod-shaped particles only in certain cases 1s not obvious. The phenomenon of streaming double refraction is not confined to sols of vanadium pentoxide, but has been observed with an aged ferric oxide sol, soap solutions, clay suspensions, silver cyanate, and a number of dyes such as benzopurpurin, alizarin, 1Kruyt: Proc. akad. Wetenschappen, 18, 1625 (1916); Kolloid-Z., 19, 161 (1916). 2 Proc. akad. Wetenschappen, 19, 189 (1916). 3Z. physik. Chem., 98, 312 (1921). 4 HERRERA: Kolloid-Z., 31, 59 (1922); 32, 373 (1928). 5 SzEGVARI and WIGNER: Kolloid-Z., 33, 218 (1923). 6 FREUNDLICH, SCHUSTER, and ZocHER: Z. physik. Chem., 106, 119 (1923). THE HYDROUS OXIDES OF THE FIFTH GROUP 269 and aniline blue.! In most of these cases, the double refraction appears to be due in large méasure to the anisotropic nature of the particles themselves rather than to the lattice-like arrange- ment of rod-shaped isotropic particles. On the other hand, with tungsten trioxide sol, the form and size of the particles is the important factor in changing the nature of the Tyndall lght, while the anisotropy of the particles is negligible. Attention has been called to the greater adsorption of the extraordinary ray than of the ordinary ray by a streaming vana- dium pentoxide sol. This gives rise to dichroism which may be termed streaming dichroism. When the intensity of transmitted polarized light whose electric victor vibrates perpendicular to the direction of flow of the sol is decreased by allowing the sol to flow, the light appears redder. As a matter of fact, the spectrum of the flowing sol extends from 710 to 582uu; while at rest, it extends only to 558uun. With parallel electric victor, the light appears yellower when the sol flows, the spectrum extending only to 542uy.? | Coagulation of Sol. Jellies—Vanadium pentoxide is quite sensitive to the action of most electrolytes as evidenced by the relatively low concentration necessary to cause clouding of the sol within 5 minutes, the so-called ‘‘clouding value,’’ Table XXIV.* With but few exceptions the clouding value is lower, the greater the valence of the precipitating ion; but ions of the same valence show the usual large variations from a constant value. The precipitated gel is very readily repeptized by washing, pro- vided the precipitating ions are not too strongly adsorbed. A sol containing 5 grams V.O; per liter sets to a stiff jelly when coagulated by suitable concentrations of electrolytes. This might be expected in view of the strong tendency of the oxide in mass to adsorb water and swell. A fresh sol contains smaller and more hydrous primary particles and gives a more gelatinous precipitate than an aged sol. It is probable that the relatively rapid coagulation in the presence of electrolytes gives fibrils just as the slow agglomeration during a long interval gives the 1 ZocunEr: Z. physik. Chem., 98, 293 (1921); FreunpLicH, SCHUSTER, and ZocHER: [bid., 105, 119 (1923). 2 DigssELHORST and FrREuNDLICH: Physik. Z., 16, 419 (1925). 3 FREUNDLICH and LEONHARDT: Kolloidchem, Bethefte, 7, 195 (1915). 270 THE HYDROUS OXIDES ' TasBLE XXIV.—CLoupING VALUE OF VANADIUM PENTOXIDE SOLS Clouding Clouding Electrolyte value, ee Electrolyte ra aN equivalents equivalents per liter per liter Ti Clase Sy er ek eas 130.0 Sr(NOs)> ois" lhe arate ee Rea 0.562 INS ts eee eae: © 50.0 BaCl:. &. 227 eee 0.46 NH,Cl.. 2040 WnSOiws | i, ee 1.68 Oe a eas 17.0 VO8S0O4... 5.4 7 eee 1.26 id ® Ea Sete RO TE Pb(C,H;03):. 3s 0.62 AGING) See ets ae cares Onl CuS0¢ oS eee 0.78 T1804 SWetenatne: Sushi ctis Remain stats 0.51 HgCl, a pau leteeedhaeeaelas ia oom 0: 726 Guanadine nitrate...... 4.0 Ce(NOs)6§:..- The hydrous mass has a composition approximating Bi(OH)3; only when dried in air. When dried at 100°, it is usu- ally assumed to form metahydroxide, BiOOH or Bi,0;-2H,0,° 1J. Chem. Soc., 123, 259 (1923). 2 Lowe: Z. anal. Chem., 22, 498 (1883). 3 THIBAULT: J. Pharm., (6) 12, 559 (1900). 4 PripEAux and Hewis: J. Soc. Chem. Ind., 41, 167 (1922). 5 Motes and Portituo: Chem. Zentr., I, 33 (1924). 6 ArpPPE: Pogg. Ann., 64, 237 (1845); Rupp: Z. anal. Chem., 42, 732 (1903); Moser: Z. anorg. Chem., 61, 379 (1909); Pripkaux and Hewis: J. Soc. Chem. Ind., 41, 167 (1922). 278 THE HYDROUS OXIDES but this could not be confirmed.! Apparently the so-called ortho- and metahydroxides merely represent stages in the continuous dehydration of a hydrous gel. A positive sol of hydrous bismuth trioxide results on dialyzing a dilute solution of bismuth nitrate containing nitric acid.? The sol is but slightly opalescent, is almost neutral, and gives only the faintest test for nitrate. A very stable sol is formed by adding alkali to a solution of bismuth nitrate in glycerin contain- ing Paal’s sodium salts of lysalbinic and protalbinic acids. After purification by dialysis, this sol can be evaporated in vacuum: at 60°, giving a gel that can be repeptized in water. The hydrous gel precipitated from antimony nitrate solution with concentrated alkali can be peptized by thorough washing.‘ The resulting sol is more stable in the presence of sucrose, man- nite, glycerin, and lactose.> The last two substances appear to react chemically with the hydrous oxide. While the hydrous oxide of bismuth is white, the anhydrous oxide is yellow when cold and red when hot. It is used for stain- ing glass and porcelain and for neutralizing undesirable colors in certain fluxes. Higher Oxides of Bismuth.—If a current of chlorine is passed into an alkali in which hydrous bismuth trioxide is suspended, a reddish powder results which is supposed to be anhydrous or hydrous Bi.O.. Similarly, highly oxidized products are formed by the electrolytic oxidation of the trioxide and by the action of persulfates, hydrogen peroxide, and potassium ferricyanide on the trioxide in the presence.of alkali. According to Gutbier and Biinz,’ none of these reactions gives a definite homogeneous product. 1 CoRFIELD and Woopwarp: Pharm. J., 113, 83, 128 (1924). 2 Bittz: Ber., 35, 4434 (1902). 3 Paau: Pharm. Ztg., 48, 594 (1903). 4 Ktun and Pirscu: Kolloid-Z. (Zsigmondy Festschrift), 36, 310 (1925). 5 Cf. Spen and Duar: Kolloid-Z., 33, 193 (1923). § DEICHLER: Z. anorg. Chem., 20, 81 (1899); Hausmr and Vanino: Jbid., 39, 381 (1904); Rurr: [bid., 57, 220 (1908); Mossr: Ibid., 50, 33 (1906); Murr: J. Chem. Soc., 51, 77 (1887). 7Z. anorg. Chem., 48, 162, 294; 49, 482; 50, 210 (1907); 52, 124 (1907); 59, 143 (1908), THE HYDROUS OXIDES OF THE FIFTH GROUP 279 Worsely and Robertson! claim to have obtained the tetroxide pure, by oxidizing the trioxide suspended in dilute alkali and freeing the resulting product from trioxide and alkali by tritu- rating with glacial acetic acid. ‘Two isomeric monohydrates are described, one brown and the other purplish black. Using con- centrated alkali and chlorine, a mixture of yellow tetroxide dihydrate and red or brown pentoxide monohydrate is said to form. Ammonium persulfate is said to give some hexoxide. These observations are incomplete if not inaccurate in many respects and should be repeated. It seems altogether unlikely that the alleged hydrates are anything but indefinite hydrous oxides. Unlike the amphoteric oxides of arsenic and antimony, the oxides of bismuth are not acid forming in character. tJ. Chem. Soc., 117, 63 (1920). CHAPTER XII THE HYDROUS OXIDES OF MOLYBDENUM, TUNGSTEN, AND URANIUM Tur Hyprovus OxipEs oF MOLYBDENUM Molybdenum Trioxide.—Molybdenum trioxide forms two and only two! crystalline hydrates, MoO3;-2H.O and MoO;-H.0. The dihydrate separates at room temperature in yellow crusts from a nitric acid solution of ammonium molybdate such as is used in the estimation of phosphorus. By heating to 40 to 50° a solution of the dihydrate or the solid suspended in water, Rosen- heim and Davidson? obtained what they called a MoO;3-2H:2O to distinguish it from 8 MoO3:H2O which comes down at 65 to 70°. Both preparations crystallize in fine white needles, but the so-called a oxide differs from the 8 in forming with water a stable milky suspension or sol and in losing all its hydrate water at a lower temperature. It is probable that the differences between the two preparations are due to variations in the size and physical character of the primary particles thrown down at different temperatures, rather than to allotropy. Doubtless this could be settled by an x-ray study of the crystal structure of the two preparations such as Burger? used to establish the chemical individuality of MoO3:H20 and MoOs3. , Graham‘ first recognized the existence of a sol of the trioxide which he prepared by dialysis of a 5 per cent solution of sodium molybdate acidified with a slight excess of HCl. During dialysis this sol behaves like hydrous ceric oxide in settling to a jelly which subsequently liquefies as the dialysis is continued. The sol is very stable toward electrolytes, has a yellow color and an astringent taste, and is acid to litmus. By evaporating in vac- 1 HUTTie and Kourre: Z. anorg. Chem., 126, 167 (1928). 2Z. anorg. Chem., 37, 314 (1903). 3Z. anorg. Chem., 121, 240 (1922). 4 Tiebig’s Ann. Chem., 186, 65 (1865). 280 ~ MOLYBDENUM, TUNGSTEN, AND URANIUM 281 uum over sulfuric acid, a glassy hydrous mass is obtained which is readily taken up again by water.! Graham’s observations were confirmed by Sabanejeff,? Linebarger,? and Lottermoser;? but Bruni and Pappada? failed to get a sol by dialysis of a nitric acid solution of ammonium molybdate having the composition of the phosphoric acid reagent. Rosenheim and Bertheim® likewise claimed that the solutions formed by shaking MoO; -- 2H.O with water are not colloidal since at every temperature, the oxide possesses a definite solubility. However, such solu- tions saturated at high temperature do not crystallize out on cool- ing even when stirred for a long time with crystals of dihydrate. Indeed, a solution saturated at 100° is fortyfold supersaturated on cooling to room temperature. Cryoscopic determinations on such a solution indicate a molecular weight for the trioxide in solution of approximately 600, which is of the same order as Sabanejeff obtained for the Graham sol. This fact, together with the observed high conductivity and high hydrogen ion concentration of the dihydrate solution, led to the conclusion that solutions of MoO;-2H.0, whether prepared directly or by the method of Graham, are not colloidal. This conclusion was called in question by Wohler and Engels,’ who demonstrated the heterogeneity not only of Graham’s sol but of the nitric acid solution of ammonium molybdate and the aqueous solutions of MoO;-2H,0 saturated in the hot or in the cold. All of these solutions contained particles clearly visible in the ultramicro- scope, which were precipitated by the addition of gelatin but not by electrolytes. In the light of their observations, Wohler and Engels classify the solutions as semicolloidal, since they appear to lie in the borderland between true crystalloidal solu- tions and hydrophile sols. This disposition of the matter fails to emphasize the important fact that MoO;-2H:,0 is soluble to a certain extent and, hence, under different conditions, it may 1Cf. Utuick: Sitzb. Akad. Wiss. Wien, 60, 302 (1870). 2 Ber., 23, 87 (1890). 3 Am. of. Sct., (3) 48, 222 (1892). 4 “Uber anorg. Kolloide,”’ Stuttgart, 11 (1901). 5 Gazz. chim. ital., 31, I, 244 (1901). 6 Z. anorg. Chem., 34, 427 (1903); 37, 314 (1904). 7 Kolloidchem. Bethefte, 1, 466 (1910). 282 THE HYDROUS OXIDES be chiefly in solution or chiefly colloidal. A newly formed solution of sodium molybdate acidified with hydrochloric acid will contain more oxide in solution than an old preparation, since ageing brings about an increase in size and a decrease in solubility of the colloidal particles. In all preparations the particles are relatively small and the solution pressure sufficiently large to make the mixture distinctly acid and a good conductor. But a part of the oxide is suspended in the liquid, and this con- tributes neither to the acidity nor the conductivity of the solu- tion. The lowering of the vapor pressure of such mixtures is due in large measure to the dissolved oxide and not to the suspended particles; hence, molecular weights deduced from cryoscopic measurements under the assumption that all the oxide is dis- solved are necessarily erroneous. Rosenheim heated to 50° a solution containing hydrochloric acid and sodium molybdate in the ratio of 4:1. From this solution there precipitated gradually, a slimy hydrous mass which was readily peptized by water after thorough washing. The particles in this sol are larger and less hydrous than in the Graham sol; and, unlike the latter, it is shightly opalescent and is precipitated in flocs by the addition of solutions of neutral salts and weak acids. Dimolybdenum Pentoxide.—The hydrous oxide of pentavalent molybdenum is thrown down by adding ammonia to a hydro- chloric acid solution of MoO; previously reduced with metallic molybdenum! or with hydriodic acid.2, At one time these were thought to be oxides of tetravalent molydenum, but Klason® obtained a compound with identical properties by adding ammo- nia to a dilute solution of (NH,4)eMoOCl; in which the molyb- denum is pentavalent. The precipitate is distinctly gelatinous; but it is said to have the composition MoO(OH); when dried over phosphorus pentoxide. When newly formed, it is very similar in physical character and color to hydrous ferric oxide. — Like the latter, it is peptized by thorough washing, forming a clear sol which varies in color from yellow to dark red depending 1 BeRzELIUS: Pogg. Ann., 6, 366, 389 (1826). 2 P&CHARD: Compt. rend., 118, 804 (1894); Ann. chim. phys., (6) 28, 537 (1893). | 3 Ber., 34, 148 (1901). be MOLYBDENUM, TUNGSTEN, AND URANIUM 283 on the concentration. The sol gives a gelatinous precipitate with electrolytes, but it is probable that it could be precipitated as a jelly since Berzelius obtained a transparent jelly simply by allowing a dark red sol to stand for a month in a closed vessel. Freundlich and Leonhardt! studied the properties of a sol pre- pared by peptizing the hydrous oxide thrown down with ammonia from a very dilute solution containing pentavalent molybdenum. The precipitate as well as the sol oxidizes to molybdenum blue more readily than Berzelius’ preparation, probably on account of the difference in size of the particles. The negatively charged sol agglomerates fractionally on adding electrolytes, and the last traces of sol are thrown down only by relatively high concentra- tions of multivalent cations that ordinarily precipitate in low concentrations. This behavior is accounted for by the wide variation in the size of the colloidal particles; the larger particles agglomerate first, leaving a very dilute, highly dispersed sol which is not readily precipitated. In this respect, it is like Péan de St. Gilles’ ferric oxide sol.? Like the latter also, the precipitation is readily reversible when the precipitating ions are weakly ad- sorbed, and almost irreversible when the DATA ate ions are strongly adsorbed. Molybdenum Dioxide and sangtiocide: —Al]though the oxide MoO: can be prepared by oxidizing molybdenum or by reducing MoOs3 under suitable conditions, it is claimed that the hydrous oxide or hydroxide of tetravalent molybdenum does not exist, the reported preparations being hydrous pentoxide.* Paal and his coworkers,* however, claim to have prepared the compound by reduction, at room temperature, of an ammonium molybdate solution with hydrogen in the presence of a little colloidal pal- ladium. By stopping the reduction when the theoretical amount of hydrogen is used up, there is obtained a greenish-black mass which has adsorbed most of the colloidal palladium. If dried under suitable conditions, the composition can be made to approximate MoO(OH),.; but there is no evidence that this is a 1 Kolloidchem. Bethefte, 7, 172 (1915). 2 Weiser: J. Phys. Chem., 25, 672 (1921); 24, 312 (1920). 3 Kiason: Ber., 34, 153 (1901); GuicHarp: Compt. rend., 143, 744 (1906). 4Paau and Briniss: Ber., 47, 2214 (1914); Paat and BiTrner: 48, 220 (1915). 284 THE HYDROUS OXIDES definite hydrate. or that all the molybdenum is tetravalent. If the reduction is carried out in the presence of Paal’s sodium pro- talbinate, there results a stable sol, black in reflected light and reddish brown in transmitted light. By a similar procedure Paal claimed to get the precipitated hydrous oxide and sol of trivalent molybdenum. The precipitate is an amorphous black mass! which cannot be dehydrated to Mo.03 without oxidation taking place.’ Molybdenum Blue.—When a solution of Mo0; or an acidified molybdate is reduced by hydrogen sulfide, sulfur dioxide, stan- nous chloride, metallic molybdenum, zinc, or other reducing agent, a deep-blue solution results, which deposits a blue hydrous precipitate known as molybdenum blue. A similar product is obtained by the oxidation of lower oxides such as MoOs, and by adding a cold dilute solution of MoO, in hydrochloric acid to a solution of ammonium molybdate. The last method of forma- tion suggests that the blue compound is a molybdenum molyb- date such as MoO2:2Mo0O;; but much doubt exists as to its composition. Marchetti® believes it to be Mo30.5-5H2O; Gui- chard,* Mo;0Oi4-6H20; Junius,> Mo7O2; while Bailhache® and Klason’ believe theré are a number of blues which Klason regards as complex derivations of Mo.O0; and MoO; analogous to phos- phomolybdic acid. The evidence for the existence of different compounds is based largely on analytical differences that are probably of the same order of magnitude as the experimental errors inherent in analyzing a colloidal mass; hence, whether there is an individual blue or a number of related compounds, the composition is known only approximately. Marchetti® claims 1 MuTHMANN and Naas: Ber., 31, 2009 (1898); Cuitesorti: Atti accad. Lincei, (5) 12, II 22, 67 (1903); SmirH and Hosxinson: Am. Chem. J., 7, 90 (1885). 2 GUICHARD: Ann. chim. phys., (7) 23, 498 (1901). 3Z. anorg. Chem., 19, 390 (1899) ; MurumMann: Liebig’s Ann. Chem., 238, 108 (1887). 4 Ann. chim. phys., (7) 28, 498 (1901); Burzmiius: Pogg. Ann., 6, 385 (1826). 5>Z. anorg. Chem., 46, 428 (1905). 6 Compt. rend., 183, 1210 (1901). 7 Ber., 34, 153 (1901). 8 Z. anorg. Chem., 19, 391 (1899). —— MOLYBDENUM, TUNGSTEN, AND URANIUM 285 to have prepared a crystalline oxide having the formula Mo30s- - 5H.O by the cathodic reduction of a hydrochloric acid solution of MoO; and subsequent evaporation to crystallization. Cryo- scopic investigations on a solution of this oxide in water indicate a molecular weight of 460 as compared to 416 for Mo3Os. Bultz! was unable to confirm these observations, and Koppel? questions the existence of a crystalline molybdenum blue. Biltz prepared a stable sol by reduction with hydrogen sulfide of an ammonium molybdate solution acidified with sulfuric acid, and subsequent dialysis. Dtumanski made _ ultramicro- scopic observations on a fairly pure sol prepared in this way and observed submicrons in rapid motion, if the sol was not too dilute; but on high dilution, it appeared optically empty. Dumanski also prepared an electrolyte-free solution by reducing pure MoQO3;, suspended in water, with metallic molybdenum. This appeared optically empty in the ultramicroscope, and from the freezing-point lowering, the molecular weight was calculated to be 440(Mo303 = 416). The addition of small amounts of electrolytes caused submicrons to appear. Dumanski concludes, therefore, that very pure molybdenum blue dissolves in water forming a true solution, but the presence of a small amount of electrolyte polymerizes the molecules. Since the blue as usually prepared contains adsorbed impurities, its solutions are colloidal. These observations of Dumanski should be confirmed, for if the facts are as stated, they raise the question of the mechanism of the agglomeration of monomolecular molecules by the presence of small amounts of a variety of electrolytes. The sol of molybdenum blue, prepared by Biltz’s method, is negatively charged and so is precipitated by positively charged hydrous oxide sols of iron, aluminum, chromium, thorium, zirconium, and cerium. Particularly beautiful and stable color lakes with the hydrous oxides of aluminum, thorium, and cerium are formed by the mutual precipitation of the oppositely charged sols. Sols of molybdenum blue act as a dye bath, imparting a blue color to various fibers. Builtz* studied the influence of concen- 1 Ber., 37, 1095 (1904). : bette ‘‘Handbuch anorg. Chem.,”’ 4, (2) 626 (1921). 3 Ber., 37, 1766 (1904); 38, 2963 (1905). 286 THE HYDROUS OXIDES tration of sol on the amount of blue oxide taken up by silk, cotton, and hydrous aluminum oxide. The results of these observations are given in Fig. 18. The uniform nature of the curves shows that the colored fibers and lake are ‘‘adsorption - compounds,”’ the composition of which varies continuously with the concentration of sol. The isotherms of adsorption are very similar to those obtained for organic colloidal dyestuffs such as benzidine! and benzopurpurine? and for dyes in true 0 0 0 0.25 0.50 0.75 1.00 1.25 150 tS eee Concentration of Molybdenum Blue, per cent Grams Molybden um Blue Adsorbed by One Gram of Alumina Grams Molybdenum Blue Adsorbed by One Gram of Fibers Fic. 18.—Adsorption of molybdenum blue by silk, cotton and hydrous alumina. solution such as picric acid’ and Congo red. In view of these observations, there is no longer any question but that dyeing by many organic and inorganic dyestuffs is essentially an adsorption process rather than a chemical process of the usual type, or a solid-solution phenomenon. With true solutions of dyes, one is not surprised to find a continuous increase in the amount of dye taken up with increasing concentration of the dye bath, in accord with the adsorption isotherm. But in colloidal solutions of dyes, the particles con- 1 Groraievics: Monatshefte fiir Chemie, 15, 705 (1894); 16, 345 (1895). 2 FREUNDLICH and LosEv: Z. physik. Chem., 59, 284 (1907). 3 APPLEYARD and WauLkER: J. Chem. Soc., 69, 13834 (1896). MOLYBDENUM, TUNGSTEN, AND URANIUM 287 stitute a second phase in the ordinary sense of the term; and it is not obvious why they should fail to act like a phase of constant composition instead of the adsorption varying continuously with the apparent concentration of the bath. Bancroft' has gotten - around this difficulty by accepting with reservations, the physicist view that, according to the kinetic molecular theory, a suspended particle should behave like a molecule in true solution. If this is true, it will account for the marked similarity in the isotherms of adsorption for colloidal and molecular solutions. While recognizing the value, at some times, of treating a colloidal solution as having some of the properties of a true solu- tion, Bancroft emphasizes the importance of distinguishing between the two at other times. In the same way, it is useful to treat a solid solute as behaving in certain respects like an ideal gas, but this does not mean either that the solute behaves in all respects like an ideal gas or that it is an ideal gas. In most cases the distinction between true and colloidal solutions can be made by applying the criterion of Gibbs. According to Gibbs, an apparent phase is not a one-phase system unless the properties are definitely defined when the temperature, pressure, and con- centration are fixed. By applying this test, it may be shown readily that most colloidal solutions are two-phase systems. The difficulty comes with solutions of such substances as tannin and soaps which appear to satisfy Gibbs’ criterion for a one-phase system when in reality they are two-phase systems. A similar situation is encountered with a mixture of two gases which in the last analysis is neither physically nor chemically homogeneous but which is a one-phase system, nevertheless. To take care of these cases, Bancroft assumes that any gas or vapor will pass through any pore through which any other gas or vapor will pass. If this unproved but reasonable assumption is granted, it leads directly to the conclusion that any substance which can be filtered out by an ultrafilter is in colloidal solution and not in true solution, an ultrafilter being defined as a porous membrane which shows no marked negative adsorption, that is, specific adsorption of the solvent. This criterion puts soap and tannin in the list of colloidal solutions and would undoubtedly take care of the highly dispersed solutions of molybdenum trioxide and 1 J. Phys. Chem., 29, 966 (1925). 288 THE HYDROUS OXIDES molybdenum blue. It should be pointed out, however, that any apparent solution which will pass through the finest ultrafilter is not necessarily in true solution. Tur Hyprovus OxIpEs or TUNGSTEN Anhydrous tungsten trioxide does not combine with water; but an insoluble monohydrate usually known as tungstic acid, H.WO,, is formed by precipitating a solution of a tungstate with excess mineral acid at the boiling point. The hydrate comes ENS Mols Water to One Mol WO, oO om 0 —— 0 | 62.5 125.0 187.5 250.0 312.5 Temperature Fig. 19.—Dehydration curves of yellow and white tungsten trioxide. down as a yellow powder possessing a definite crystalline structure distinctly different from that of WO;.! By carrying out the reaction at low temperatures, the oxide comes down as a white voluminous mass that cannot be washed completely free from adsorbed salts. The white oxide was believed to be WQO3-- 2H.O; but this seems improbable in the light of Hiittig and Kurre’s” recent investigations of the change in vapor pressure of different preparations with changing temperature, using a spe- 1 BurGcerR: Z. anorg. Chem., 121, 240 (1922). 2Z. anorg. Chem., 122, 44 (1922). MOLYBDENUM, TUNGSTEN, AND URANIUM 289 cially designed tensi-eudiometer.! The results of observations on both yellow and white preparations are given in the temperature- composition curves reproduced in Fig. 19. The yellow oxide forms one and only one hydrate WO;3:-H.2O, and the white voluminous compound is a hydrous oxide, the water content varying continuously with change in temperature. The curve for the yellow oxide is reproducible; but the ease with which the white compound gives up its water is determined by the size of the hydrous particles. Thus, the curve for the very highly dis- persed 6 oxide lies above that for the coarser a oxide throughout the entire range of the investigation. The crystal structure of the white hydrous oxide is apparently different from that of the yellow monohydrate or yellow WO3. If this be true, it means that the oxide exists in two forms and not that the white com- pound is a hydrate, as Burger? supposed. The white oxide becomes yellow on standing;? and the yellow oxide sometimes takes on a greenish color which has been traced to the presence of a lower oxide, possibly W203. Graham’? first prepared a sol of tungstic acid, so called, by dialysis of a 5 per cent solution of sodium tungstate acidified with only a slight excess of dilute hydrochloric acid. On evaporating the sol to dryness, a glassy mass was obtained which could be heated to 200° without losing its sol-forming property. An 80 per cent sol with a density of 3.25 was obtained by peptizing the glassy mass with one-fourth its weight of water. The sol is less stable than the corresponding molybdenum trioxide sol and precipi- tates out in the form of beautiful quadratic prisms,® after standing several months. The Graham sol cannot be prepared free from alkali, which led Sabanejeff’ to conclude that it is a solution of a sodium salt of the formula Na,.O:4WO;3. This seems not to be the case, since Biltz and Vegesack® found the ratio Na2O : WO; to be 2 : 11 1 Hirrie: [bid., 114, 161 (1920). 2Z. anorg. Chem., 121, 240 (1922). 3 Moser and Euruicu: Kdel-Erden u.-Erze, 3, 49, 65 (1922). 4Van Liempt: Z. anorg. Chem., 119, 310 (1921). 5 Pogg. Ann., 123, 539 (1864). 6. WoOuLER and Enaets: Kolloidchem. Bethefte, 1, 472 (1910). 7Z. anorg. Chem., 14, 354 (1897). 8 Z. phys. Chem., 68, 376 (1910), 290 THE HYDROUS OXIDES in a well-dialyzed sol. Wohler and Engels! confirmed Graham’s observations and demonstrated the optical heterogeneity of the preparation. Unlike colloidal MoQOs, the sol is not precipitated by adding gelatin, but an adsorption complex is formed which is thrown down by the addition of a little ammonium chloride. The presence of a small amount of tungsten trioxide in colloidal molybdenum trioxide seems to act as a protective colloid,? preventing the precipitation of the latter by gelatin. Similarly, molybdenum trioxide seems to exert a protecting action on tung- sten trioxide. As has been pointed out, molybdenum trioxide comes down only very slowly from a strongly acidified molybdate solution in the cold, whereas tungsten trioxide precipitates readily from a strongly acidified tungstate. If a mixture containing a small amount of tungstate and a large amount of molybdate is acidified, no precipitate forms for a long time unless the mixture is warmed, the time required for its appearance at a given tem- perature being determined by the composition of the mixture. The precipitate is an adsorption complex since the two oxides exhibit a mutual adsorption for each other. ‘The more stable molybdenum trioxide sol adsorbs and so holds tungsten trioxide in colloidal solution until the particles of the latter grow to the point of precipitation, carrying down with them adsorbed molyb- denum trioxide. Although the nature of Graham’s dialyzed solutions has been questioned, there can be no doubt as to the colloidal character of the solution of the yellow oxide formed by mechanical dis- integration® or of the white hydrous oxide peptized by washing. The white oxide always comes down as a gelatinous precipitate when sodium tungstate is treated in the cold with excess acid. The velocity of precipitation and the nature of the precipitate depend on the hydrogen ion concentration of the acid used. Contrary to von Weimarn’s theory, Lottermoser® found the precip- 1 Kolloidchem. Bethefte, 1, 472 (1910); cf. PappapA: Gazz. chim. ital., 32, (2), 22 (1902). 2 Cf. KréaeEr: Kolloid-Z., 30, 18 (1922). 3 WEGELIN: Kolloid-Z., 14, 65 (1914). 4LOTTERMOSER: Kolloid-Z., 15, 145 (1914); cf. vAN LizmptT: Rec. trav. chim., 43, 30 (1924). 5 Van Bemmelen’s: ‘‘Gedenkboek,”’ 152 (1910). MOLYBDENUM, TUNGSTEN, AND URANIUM 291 itate to be more voluminous the slower the rate of precipitation. On washing the gel by decantation, it gradually becomes less voluminous and yellow in color, and finally is peptized completely, forming a yellow very cloudy sol in which the particles appear rod shaped.! Lottermoser compares Graham’s clear stable tungsten trioxide sol with the latter’s clear ferric oxide sol formed in a similar way, and the cloudy tungsten trioxide sol with Péan de St. Gilles’ ferric oxide sol. There is some doubt as to whether the comparison is justified. The difference between the two ferric oxide sols is due to a difference in the size and hydrous character of the particles, whereas the tungsten oxide sols may be different chemically. The x-ray investigations which indicate the chemical individuality of white and yellow tungsten trioxide should be confirmed. Pappada? prepared a very sensitive sol by peptizing the tri- oxide with oxalic acid and purifying by dialysis; and Miiller? obtained a sol highly sensitive to electrolytes by diluting with water an alcohol-ether solution of the oxide. Tungsten Blue.—The first product formed on reducing tung- sten trioxide or a tungstate is a blue substance similar to molyb- denum blue and known as tungsten blue. It may be formed also by partial oxidation of tungsten dioxide or of the hydrolysis product of tungsten pentachloride and pentabromide. The composition of the blue has been represented by a number of formulas ranging from W.20;* to W;O.4;> but the bulk of the evidence indicates that it is a mixture of variable composition and not a single chemical individual. Depending on the method of preparation, the color varies from purple bronze to deep blue; but it is not known whether this is due to differences in composition or physical structure or both. A sol of tungsten blue is formed by neutralization with ammo- nia of a solution of metatungstic acid saturated with hydrogen 1 DimsseLHorst and Freunpuicu: Phystk. Z., 17, 117 (1916). 2 Gazz. chim. ital., 32, II 22 (1902). 3 Van Bemmelen’s: ‘‘Gedenkboek,”’ 416 (1910). 4 CHAUDRION: Compt. rend., 170, 1056 (1920); Ann. chim., 16, 221 (1921); VAN Liempt: Z. anorg. Chem., 126, 183 (1923). 5’ ALLEN and GorrscHaLK: Am. Chem. J., 27, 328 (1902). 6 WOHLER and Batz: Z. Elektrochem., 27, 406 (1921); RernpERS and VeRvVLOET: Rec. trav. chim., 42, 625 (1928). 292 THE HYDROUS OXIDES sulfide,! or by electrolytic reduction of an acidified tungstate solution.2 In the purification by dialysis, appreciable amounts of the blue substance pass through the dialyzing membrane; the sol remaining has a deep sky-blue color in marked contrast to the slate-blue color of the corresponding molybdenum sol. Like the latter, however, it is negatively charged and is precipi- tated by electrolytes and positively charged sols. Freshly prepared sols dye silk, cotton, and wool directly, imparting a clear-blue color to the fiber.® THE Hyprovus OxipEs oF URANIUM Uranium Trioxide.—Graham/ first prepared a sol of uranium trioxide by adding potassium carbonate to a uranyl salt solution containing sugar, and dialyzing. ‘The deep orange-yellow col- loid was very stable but was readily agglomerated by electro- lytes. It is not clear why Graham found it necessary to use sugar in the preparation of his sol, for Szilard® found that uranyl nitrate peptizes the oxide directly. To get the oxide inasuitable form, Szilard mixed a solution of uranyl acetate with ether and exposed the mixture to light, thereby obtaining a hydrous violet precipitate® analogous to molybdenum blue but having a definite composition U;Os. On allowing the thoroughly washed oxide to stand for a day, it oxidized to the yellow trioxide which was suspended in water and added to a hot solution of uranyl nitrate as long as it was peptized easily. The orange-yellow sol was quite stable in the presence of an appreciable excess of uranyl salt; but if too much oxide was added, it agglomerated spontane- ously in a form that was not readily repeptized. It is altogether probable that the sol is a hydrate rather than a hydrous oxide since the anhydrous oxide takes up water at 1 ScHEIBLER: J. prakt. Chem., 80, 204 (1860); 88, 273 (1861); Bizz: Ber., 37, 1095 (1904). > LetserR: Z. Elektrochem., 18, 690 (1907); Kroger: Kolloid-Z., 30, 16 (1922). § BrutTz: Ber., 37, 1771 (1904). 4 Phil. Trans., 151 I, 183 (1861). 5 J. chim. phys., 5, 488, 495, 6386 (1907). ® Atoy: Bull. soc. chim., (2) 25, 344 (1901); ALoy and Roprmr: Jbid., (4) 27, 101 (1920). MOLYBDENUM, TUNGSTEN, AND URANIUM 293 room temperature,! forming a dihydrate that is converted into a monohydrate at a temperature of 80° and a water-vapor pres- sure of 15 millimeters.?, The hydrate is usually yellow in color, but an orange compound of the same composition is obtained by electrolysis of uranyl nitrate? or by suspending the violet hydrous U3;Ogs in water which is subsequently boiled in the air. The anhydrous oxide formed by decomposition of uranyl nitrate is yellow if the decomposition takes place slowly, but red if the decomposition is rapid.> It is probable that the variations in color of both the anhydrous and hydrated oxides are the result of differences in physical character of the same compound formed in different ways. Uranium Dioxide.—Hydrous uranium dioxide is thrown down as a voluminous red-brown mass by adding alkalies or ammonia to a cold green uranous solution. The gel loses water and becomes denser and darker on heating. It is probable that the newly formed hydrous mass would be peptized by washing, since Samsonow’ obtained a sol by washing the dark hydrous dioxide precipitated during the electrolytic reduction of 50 grams of uranyl chloride in 100 cubic centimeters of 2 N hydrochloric acid. Samsonow’s sol when freshly prepared contains small positively charged particles in brisk Brownian movement. The particles grow quite rapidly, however, and within 24 hours most of the oxide settles out. It is of particular interest that colloidal uranium trioxide appears to catalyze the formation of formaldehyde by the action of sunlight on a solution of carbon dioxide in water.® 1LEBEAu: Compt. rend., 154, 1808 (1912). 2 Hirria and ScHROEDER: Z. anorg. Chem., 121, 243 (1922). 3 OECHSNER DE ConinckK: Bull. Acad. Roy. Belg., 222 (1901). 4 OECHSNER DE CoNINCK: Compt. rend., 132, 204 (1901); Bull. Acad. Roy. Belg., 363, 448 (1904). 5 Anoy: Bull. soc. chim., (3) 28, 368 (1900). 6 Anoy: Bull. soc. chim., (3) 21, 613 (1899). 7 Kolloid-Z., 8, 96 (1911). 8 Moore and Wesster: Proc. Roy. Soc., 87B, 163 (19138); 90B, 168 (1919), CHAP TER XT THE HYDROUS OXIDES OF MANGANESE Hyprous MANGANESE DIOXIDE Hydrous manganese dioxide is obtained by the oxidation of a manganous salt with such oxidizing agents as permanganate, hypochlorite, chlorate, ammonium persulfate, nitric acid, and ozone. It is also obtained by hydrolysis of a salt of tetravalent manganese and by reduction of permanganate by hydrogen peroxide, glycerin, dextrose, potassium oxalate, etc. It is difficult, if not impossible, to obtain the hydrous oxide in a pure form, partly on account of its tendency to lose a portion of its oxygen giving mixtures of MnO and MnO,! and partly because of its high adsorption capacity. While definite hydrates of precipitated manganese dioxide have been described, the composition was found by van Bemme- len? to be indefinite, depending on the physical character, age, and conditions of drying the sample. Two widely different preparations were studied by van Bemmelen, one the ordinary black compound precipitated from a solution of manganous salt by alkali hypochlorite, and a second red variety obtained by hydrolysis of Mn(SO,)o.2 The red oxide is much more finely divided than the black and possesses a much higher adsorption capacity for water and dissolved electrolytes. Both oxides show a strong tendency to adsorb hydroxyl ion. This is evi- denced by the fact that dilute solutions of alkali peptize the gel forming a stable negative sol. Moreover, neutral salts such as potassium sulfate, chloride, and nitrate are hydrolyzed by the gel | 1Wricut and Menke: J. Chem. Soc., 37, 25 (1880); Goocu and Austin: Am. J. Sct., (4) 5, 260 (1898); Goraru: Compt. rend., 110, 1134 (1890); von Knorre: Z. angew. Chem., 14, 1149 (1901). 2 J. prakt. chem., (2) 28, 324, 379 (1881). 3 Fremy: Compt, rend., 82, 475, 1231 (1876), 294 THE HYDROUS OXIDES OF MANGANESE 295 owing to stronger adsorption of base than of acid, the hydrolytic decomposition being more complete when the salt solutions are dilute. Thus, 65 per cent of the potassium present in a 0.0025 N solution of potassium sulfate is adsorbed by the hydrous oxide but only 6 per cent of that from a 0.1 N solution. The total amount adsorbed, however, increases with increasing concen- tration of salt.1. This behavior accounts for Gorgeu’s? observa- tion that solutions of both alkali and alkaline earth salts become acid when brought in contact with the hydrous oxide. In view of this marked adsorption capacity for alkalies, it is probable that the so-called manganites* formed by precipitating hydrous manganese dioxide in the presence of basic oxides, are not definite compounds. In favor of this conclusion, it may be pointed out that the composition is determined by such variable factors as the physical character of the hydrous mass, the concentration of alkah, and the method of washing the precipitate. Manganese Dioxide Sols.—As previously mentioned, van Bemmelen‘ observed the ease with which hydrous manganese dioxide is peptized by dilute alkali. He also noted that the precipitated oxide is more or less completely peptized by washing out the excess of adsorbed alkali or salt. The voluminous red oxide is very readily peptized by washing, forming a clear-brown sol that is quite sensitive to the action of electrolytes. The most satisfactory method of preparing the sol consists in reducing potassium permanganate under such conditions that the precipitation concentration of no electrolyte in the solution is exceeded. The solution becomes alkaline during the reduction; but, as already noted, an appreciable concentration of hydroxyl ion is favorable to sol formation. Reducing agents that have been employed successfully are hydrogen peroxide, sodium thio- sulfate, arsenious acid, reducing sugars, and ammonia. 1 HUMMELCHEN and Kapprmn: Z. Pflanzenerndhr. Diingung, 3A, 289 (1924); Chem. Abstracts, 19, 1800 (1925). 2 Ann. chim. phys., (3) 66, 155 (1862). 3 Rousseau: Compt. rend., 104, 780, 1796 (1887); 114, 72 (1892); Sor- sTEIN: Pharm. Ztg., 32, 659 (1887); Stinet and Marawskt: J. prakt. Chem., (2) 18, 86 (1878); GuAsEeR: Monatshefte fiir Chemie, 6, 329 (1885); 7, 651 (1886). 4Cf. also GorarEu: Ann. chim. phys., (3) 66, 154 (1862); Serine: Ber., 16, 1142 (1883); Sprinc and Dr Borck: Bull. soc. chim., (2) 48, 170 (1867). 296 THE HYDROUS OXIDES Swiontkowski! first reported the formation of a coffee-colored sol of manganese dioxide by reducing a solution of KMnO, with pure neutral hydrogen peroxide. According to Bredig and Marck,’ a satisfactory sol results if a dilute hydrogen peroxide solution is added slowly with constant shaking to a potassium permanganate solution no stronger than M /10 until the color of the permanganate just disappears. By dialysis with conductivity water, the conductivity of the sol may be reduced to that of ordinary distilled water. A dilute sol is clear yellow in color, changing to dark brown as the concentration increases. If not too concentrated, the sol can be kept indefinitely without precipitating; but it is very sensitive to the action of electrolytes with the exception of potassium hydroxide and permanganate. Bredig and Marck made a quantitative study of the catalytic decomposition of hydrogen peroxide by colloidal manganese dioxide. The reaction is of the first order, but in most cases the constant increases as the decomposition progresses, possibly owing to the formation and subsequent decomposition of a hydrogen peroxide salt during the course of the reaction.? In alkaline solution, the velocity of decomposition increases to a maximum with increasing concentration of hydroxyl and then falls off just as Bredig found with colloidal metals. The catalytic activity of the oxide is increased by heating the sol for 4% hour, but prolonged heating causes precipitation. The presence of gelatin increases the stability of the sol and raises its catalytic activity slightly. Low concentrations of substances like hydro- gen sulfide, potassium cyanide, and carbon dioxide which have a marked poisoning action on a platinum catalyst, are without effect on manganese dioxide. On the other hand, phosphorus, its oxidation products, and sodium phosphate cut down the catalytic activity of the oxide, and mercuric chloride and potas- sium fluoride increase it. A mixture of perborate and permanganate may be added to bath water to make what is known as an “oxygen bath.”’ The reaction in solution gives hydrogen peroxide and colloidal man- ganese dioxide, and the latter catalyzes the decomposition of 1 Liebig’s Ann. Chem., 141, 205 (1867). 2 Van Bemmelen’s: ‘‘Gedenkboek,’’ 342 (1910). 3 Cf., however, LoOTTERMOSER and LEHMANN: Kolloid-Z., 29, 250 (1921). THE HYDROUS OXIDES OF MANGANESE 297 former, setting free oxygen which forms a supersaturated solution in the water and is subsequently evolved in small bubbles on the skin of the bather. The presence of electrolytes in the bath water was found to have such a marked effect on the rate of evolution of oxygen that Lottermoser! investigated quantitatively the influ- ence of various alkalies and salts on the decomposition of hydro- gen peroxide by manganese dioxide. ‘The impurity was added to the peroxide solution, after which the catalyst was prepared directly in the solution by adding permanganate and base in the order named. With salts of a common anion, different cations influenced the decomposition in the order Ba’ > Sr” > Ca’’- > Na’ > K’ > Li, barium ion accelerating it the most and lith- ium ion retarding it the most. Unfortunately, Lottermoser did not inquire into the reason for the accelerating action of certain electrolytes and the inhibiting action of others; but it is probable that this is very closely related to the adsorbability of the cations and to the variation in physical character of the hydrous parti- cles formed in the presence of different electrolytes. Mg’ and NH, ions were found to have a marked retarding action by cutting down the concentration of hydroxyl ion. For preparing, a satisfactory oxygen bath, it is obviously necessary to avoid waters containing magnesium salts. The reduction of permanganate by arsenious acid was shown by Deisz? to give a very stable sol, particularly if it is not sub- jected to dialysis. If evaporated to dryness over sulfuric acid, a residue is obtained which is again converted into a sol by shak- ing with water. If a bit of sol is allowed to drop into still water, beautiful vortex rings are formed. ‘The first ring increases to a certain size and breaks into several small rings and these in turn into others. All these rings are connected with each other by thin lines of hydrous manganese dioxide, thus giving the whole system a striking clustered or festooned appearance. If the sol is dropped into a salt solution, it is precipitated in the form of miniature rings; by using a very dilute solution, the system of rings will form before coagulation takes place. This phenomenon, first described by Thomson and Newall,’ is not limited to col- t Kolloid-Z., 29, 250 (1921). 2 Kolloid-Z., 6, 69 (1910); cf. Travers: Bull. soc, chim., 37, 456 (1925). 3 Proc. Roy. Soc., 39, 417 (1886). 298 THE HYDROUS OXIDES loids like milk, ink, blood, soap, etc., but is shown by many solutions of both electrolytes and non-electrolytes. Suitable concentrations of permanganate give strikingly beautiful rings. A stable sol may be obtained by the oxidation of a manganous salt in the presence of a protective colloid,! such as albumin, dextrin, gum arabic, sodium ‘‘salts” of albuminous products and starch. Low concentrations of positive hydrous ferric oxide sols precipitate the negative manganese dioxide sol; but in high concentrations they adsorb it and hold it in the suspended form. Thus, by dissolving a manganous salt in a neutralized ferric chloride solution and treating with potassium permanga- nate, a dark-brown hydrosol is obtained; similarly, a ferrous chloride solution can be oxidized with a potassium permanganate solution without any precipitation taking place. Witzemann? prepared colloidal manganese dioxide by incom- plete oxidation of a glucose solution in the presence of a little alkali. On adding slowly 100 cubic centimeters of 6 per cent potassium permanganate to a cold solution containing 5 grams of glucose in 20 cubic centimeters, together with a few cubic centimeters of 10 per cent sodium hydroxide, the mixture rapidly became viscous and in 5 to 10 minutes set to a stiff jelly. The jelly soon started to synerize, and after standing for a few days, it was transformed completely into a stable limpid sol. The rate of transformation from the jelly to the sol stage depends on the alkali concentration. With quite low concentrations, the jelly forms slowly and does not liquefy, while with relatively high concentrations, the jelly stage is not observed, the solution merely undergoing an initial increase in viscosity, followed by a rapid transformation to the limpid sol. This behavior of col- loidal manganese dioxide is very similar to that of colloidal hydrous ceric oxide*® except that in the latter case the trans- formation from a hydrous sol to a jelly and again to a less hydrous sol takes place in the absence of glucose. It is obvious that the newly formed particles of manganese dioxide are in an extremely finely divided and highly hydrous form, and in rela- 1'TrILLAT: Compt. rend., 188, 274 (1904); Bull. soc. chim., (3) 31, 811 (1904); German Patent 227491. 2 J. Am. Chem. Soc., 37, 1079 (1915); 39, 27 (1917). 3 See p. 255. THE HYDROUS OXIDES OF MANGANESE 299 tively high concentration, they adsorb and entangle all of the liquid, forming a jelly. Now, asa rule, inorganic jellies synerize, particularly in the presence of salts,! the hydrous particles losing water and growing until they settle out. This is particularly noticeable with barium sulfate and certain arsenate jellies. But in the case of hydrous manganese dioxide, the primary particles coalesce to form larger primary particles even though agglomera- tion and precipitation are prevented by the protective action of glucose and the peptizing action of adsorbed hydroxylion. The ageing of the primary particles in the presence of electrolytes has, however, left them relatively free from adsorbed water, the latter merely serving as the medium in which the slightly hydrous particles are suspended. Because of this irreversible change, neither an aged ceric oxide or manganese dioxide sol can be precipitated in the form of a jelly. Jellies of the hydrous oxides of iron and aluminum may be broken up by shaking, forming sols of relatively low viscosity which, on standing, set again to firm jellies. In these instances the primary particles retain their small highly hydrous character in contradistinction to the behavior of ceric oxide and manganese dioxide, and the structure broken up by shaking gradually re-forms, entangling all of the unadsorbed water. Probably the simplest method of preparing manganese dioxide sols consists in adding concentrated ammonia, one drop every 3 minutes, to an M/20 solution of potassium permanganate at 90° until all the permanganate is reduced.” The only impurity in the sol is potassium hydroxide, which has a marked stabilizing effect. Adsorption by Hydrous Manganese Dioxide.—Dhar and his collaborators? have studied the precipitation by electrolytes of a manganese dioxide sol in the presence of a protective colloid, gelatin. They have also made adsorption studies on the oxide precipitated in the presence of various salts. As a result of 1 Poma and Patront: Z. physik. Chem., 87, 196 (1914). 2 Guy: J. Phys. Chem., 25, 415 (1921). 3 CuHatTrerRsi: Proc. Eighth Ind. Sci. Congress, 17, 180 (1921); GaNauULY and Duar: J. Phys. Chem., 26, 701, 836 (1922); CHatrerst and Duar: Kolloid-Z., 38, 18 (1923); Duar, Sen, and Guosn: J. Phys. Chem., 28, 457 (1924); cf. Sprrinc and Dr Borckx: Bull. soc. chim., (2) 48, 170 (1887). 300 THE HYDROUS OXIDES these observations, the conclusion is reached that an ion which has a high precipitation value for a colloid is most strongly adsorbed by the colloid and vice versa. ‘‘'Thus, the monovalent ions—silver, sodium, and lithium—are more adsorbed (by MnO.) than any of the bivalent, trivalent, or tetravalent ions. These facts show that ions of higher valence which in general have greater coagulating power are adsorbed the least.”’ Dhar’s observations were not made on a purified sol but on hydrous manganese dioxide formed by mixing potassium permanganate and manganous sulfate in the presence of various electrolytes. In the solution from which the oxide separated, there were the two reacting electrolytes, the salt whose adsorption was meas- ured, together with the soluble products of the reaction, potas- slum acid sulfate and sulfuric acid. This makes an almost hopelessly complicated system; and it seems unsafe to draw any conclusions whatsoever from the observations until more is known of the effect of foreign electrolytes on the rate of precipitation and physical character of the precipitate, and until something is known of the influence of other salts in the system on the adsorption of the salt investigated. To cite but one example: Aluminum nitrate is adsorbed about eight times as strongly as aluminum sulfate, whereas the sulfates of cobalt, copper, and cadmium are each adsorbed somewhat more than their respective nitrates. Aluminum is not “far less adsorbed” than strontium, nickel, cobalt, zinc, barium, or cadmium if the values for the nitrates are compared. Pavlov! showed that the taking up of silver from nitrate solution by hydrous manganese dioxide is not a simple case of adsorption but is complicated by a chemical reaction between the adsorbent and the adsorbed salt. With colloidal manganese dioxide, as with other sols, it is probable that the most readily adsorbed ion will be found to precipitate in lowest concentrations, provided influences other than the adsorb- ability of the precipitating ions can be suppressed or eliminated.’ The adsorption of iron, nickel, and copper by hydrous manga- nese dioxide, formed in acid solution by the action of (NH4)2S20, on a manganese salt, follows the Freundlich adsorption iso- 1 Kolloid-Z., 35, 375-877 (1924). 2G Werte J. Phys. Chem., 29, 955 (1925); cf. ScHiLow: Z. nhyate. Chem., 100, 425 (1922). THE HYDROUS OXIDES OF MANGANESE 301 therm, the amounts of the several ions taken up being appreci- ably greater at lower concentrations. ! Manganese compounds have been found to play an important role in many biochemical actions, and in certain instances, this may be due to colloidal oxides of manganese.?, Thus manganese compounds stimulate alcoholic fermentation® and the growth of fungi.4 The stimulating effect on the growth of plants in general’ is evidenced by the fact that the production of wheat per acre may be increased 10 per cent by sprinkling a manganese compound on certain soils. For this purpose, manganese dioxide is one of the most effective compounds. Salts of manganese likewise appear to stimulate metabolism and to increase the hemogenetic power.® It is, therefore, proposed to administer manganese therapeutically along with iron to make the latter effective. Further, the addition of minute amounts of manga- nese increases the activity of the enzyme laccase,’ and colloidal manganese dioxide behaves like an oxidase toward guiac tincture, hydroquinone, etc.* As the processes mentioned are thought to be regulated by enzymes and enzymes are colloidal, Witzemann suggests that the effect of manganese on enzymic activity is due to the effect of the hydrous oxide on the physical character of the enzyme. Thus, if the colloidal oxide keeps the colloidal enzyme dispersed under conditions which would normally be unfavorable to this effect, then it might be expected to have a positive influence on the enzymic activity. 1GrLoso: Compt. rend., 176, 1884 (1923); 178, 1001 (1924); Bull. soc. chim., 37, 641 (1925). 2 WiTzEMANN: J. Am. Chem. Soc., 37, 1089 (1915). 3 Kayser and Marcuanp: Compt. rend., 145, 343 (1907). 4 BERTRAND and JAVILLIER: Bull. soc. chim., 11, 212 (1912); Burrranp: Ibid., 11, 494 (1912); Warerman: J. Chem. Soc., 104, I, 229 (1913). 5 Masoni: Staz. sper. agrar. ital., 44, 85 (1911); Monremartini: [bid., 44, 564 (1911); Ricct and Barpera: Ibid., 48, 677 (1915); BarTMann: J. agr. prat., (2) 20, 666 (1911); Skinner and Suuuivan: U. S. Dept. Agr. Bull. 42, (1913); Preirrer and Biancx: Landw. Vers.-Sta., TT, 33 (1912); 88, 257 (1914). 6 PrccinIN1: Eighth Int. Cong. Applied Chem., 19, 263 (1912); Biochem. terap. sper., 2, 885 (1910-1911); Chem. Abstracts., 7, 369 (1918). 7 BertTRAND: Bull. soc. chim., (3) 17, 619, 753 (1897); Ann. chim. phys., (7) 12, 115 (1897); cf. also Bacu: Ber., 43, 364 (1910). 8 SyotLEMA: Chem. Weekblad, 6, 287 (1909); Chem, Zentr., I, 496 (1911), 302 THE HYDROUS OXIDES In addition to its use in the manufacture of chlorine and as a depolarizer in the Le Clanche battery, manganese dioxide in the anhydrous or slightly hydrous state finds wide application as a dryer for oils in paints. The drying is a process of oxidation and the manganese dioxide serves as an efficient oxygen carrier or catalyst. It is sometimes used also in conjunction with other oxides to produce warmer shades in colored glass and to destroy the injurious tint produced in colorless glass and white enamels by the presence of ferrous compounds. The latter greenish compounds are oxidized to the nearly colorless ferric salts while the slight pink tint imparted by the manganese still further counteracts the bluish color. The latter effect seems to be the more important, as red lead and other oxidizing agents do not have this decolorizing power. In a very finely divided state or in thin layers, manganese dioxide has a purplish-red color. The purple color of amethyst is due to a trace of MnOz2 or Mn;0, as Impurity in quartz crystal. OTHER Hyprous OxIDES orf MANGANESE Manganous Oxide.—By adding alkali hydroxide to a solution of manganous salt, white manganous oxide precipitates in a highly gelatinous form. The hydrous oxide adsorbs chloride slightly and sulfate strongly, so that the former is not carried down in the presence of the latter. Manganous hydroxide can be obtained ~ in regular hexagonal prisms similar to the mineral pyrochroite, by adding manganous chloride to a boiled concentrated solution of potassium hydroxide in an atmosphere of hydrogen.? By heating to 160°, all the oxide is carried into solution from which it precipi- tates in transparent crystals having a reddish tint. When pure, the crystalline hydroxide is fairly stable in the air, but if it contains even a small amount of alkali, it oxidizes very readily. Like magnesium hydroxide, it dissolves in excess ammonium chloride.* , A sol of hydrous MnO is formed by treating a solution of a manganous salt with alkali in the presence of protective colloids 1 PatTren: J. Am. Chem. Soc., 25, 192 (1903). 2 Dr ScHULTEN: Compt. rend., 105, 1265 (1887). 3 Cf. Herz: Z, anorg. Chem., 21, 242 (1899); 22, 279 (1900). THE HYDROUS OXIDES OF MANGANESE 303 such as gelatin,! Paal’s ‘‘sodium protalbinate,’’? albumin,? and nuclein acid. Because of its fine state of subdivision, it oxidizes readily to dioxide. In the presence of certain reducing agents such as hydroquinone and gallic acid, hydrous MnO, will give up oxygen, again forming colloidal MnO,° the process coming to a standstill only when there is no further reduction to MnO by the reducing agent, that is, when the oxidation of the added substance is complete. This action as an oxidase or oxygen carrier probably accounts for the rapid drying of manga- nese oxide paints, varnishes, and siccatives. The drying oil, such as linseed, doubtless plays the double role of protective colloid and of oxygen-consuming reducing agent.°® Manganic Oxide-—Hydrous manganic oxide is best prepared by hydrolysis of manganic salts, but it is also obtained in a more or less pure condition by the partial oxidation of manganous salts. By drying at 100°, the composition is said to be repre- sented by the formula Mn.O; - H.O, corresponding to the mineral manganite.’ When formed by the hydrolysis of potassium man- ganic cyanide, it is a black gelatinous mass which becomes less hydrous on heating with the mother liquor, changing in color from black to brown.’ Meyer® suggests that the color change may be due either to the decomposition of a hydrate or to a change in the size and physical character of the particles; he leaves the matter for someone else to decide. Mangano-manganic Oxide.—The oxide Mn;0, or MnO, -- 2Mn0O is the most stable of all the oxides of manganese when heated in the air. Accordingly, higher oxides decompose around 1Lopry DE Bruyn: Z. physik. Chem., 29, 562 (1898); Rec. trav. chim., 19, 236 (1900). 2 Kalle and Company: German Patent 180729 (1901). , 3 TRILLAT: Bull. soc. chim., (8) 31, 811 (1904); Compt. rend., 138, 274 (1904). 4 SarAson: German Patent 272386 (1913). 5 BERTRAND: Compt. rend., 124, 1032, 1855 (1897); Vinurers: Jbid., 124, 1349 (1897). 6 Cf. Levacue: Compt. rend., 97, 1311 (1883); 124, 1520 (1897). 7 FRANKE: J. prakt. Chem., (2) 36, 31, 451 (1887); Mrymr: Z. anorg. Chem., 81, 385 (1913). § BERTHIER: Ann. chim. phys., (2) 20, 344 (1822); Hermann: Pogg. Ann., 74, 303 (1848); Gorcru: Compt. rend., 106, 948 (1888). 9Z. anorg. Chem., 81, 400 (1913). 304 THE HYDROUS OXIDES 940° and lower oxides oxidize in the air, forming Mn;0,. It is obtained in a hydrous condition more or less impure, by treating a mixture of manganous and manganic salts with alkali or by the oxidation of an ammonical solution of a manganous salt with oxygen. Christensen! obtained it by adding hydrous MnO, in small amounts at a time to a dilute solution of manganous chloride containing an excess of ammonium chloride. Depending on the exact condition of formation, the precipitate is yellow brown, red brown, to chocolate brown in color. 1Z. anorg. Chem., 27, 321 (1901). CHAPTER XIV THE HYDROUS OXIDES OF THE PLATINUM FAMILY The platinum family consists of two groups of three closely related elements following iron, nickel, and cobalt in the eighth group of the periodic table. The metals of the first group— ruthenium, rhodium, and palladium—have atomic weights near 100 and densities near 12; and the metals of the second group— osmium, iridium, and platinum—have atomic weights near 200 and densities near 21. From their position in the periodic table, it is not surprising that these elements should form a number of compounds with oxygen. Some of the oxides can be obtained only in the anhydrous state while others may be precipitated as flocculent or gelatinous masses containing varying proportions of adsorbed water. ‘The oxides of the several ele- ments will be taken up in order, beginning with ruthenium. Hyprovus OxipEs oF RUTHENIUM Ruthenium Monoxide.—Hydrous RuO is thrown down by alkali from the blue solution of RuCl,. The precipitate is very highly dispersed and the adsorbed alkali cannot be washed out without peptization taking place. Moreover, like hydrous fer- rous oxide, the gel oxidizes very readily in the air. There seems little doubt of the chemical individuality of hydrous RuO, since the freshly precipitated oxide dissolves in hydrochloric acid, re-forming the characteristic blue solution of RuCle.! Ruthenium Sesquioxide.—Hydrous Ru,O3; is precipitated by adding alkali to a solution of the corresponding chloride.? Prepared in this way, the oxide contains adsorbed alkali that 1 Remy: Z. anorg. Chem., 126, 185 (1923); cf., however, GuTBreR and RansouorFr: [bid., 45, 243 (1905). 2Ciaus: Liebig’s Ann. Chem., 59, 234 (1846); Jory: Compt. rend., 114, 291 (1892); Brizarp: Ann. chim. phys., (7) 21, 311 (1900); GuTsinr and Ransouorr; Z, anorg, Chem., 45, 253 (1905). 305 306 THE HYDROUS OXIDES cannot be removed by prolonged washing.! Krauss and Kiiken- thal? obtained a chloride- and alkali-free product by evaporating the hydrochloric acid solution of RuCl; to dryness, redissolving in water, and adding just enough potassium hydroxide to the dark-brown solution to cause complete precipitation of the oxide while the colorless liquid is still acid. The black flocculent pre- cipitate analyzed for trihydrate or hydroxide, after washing thoroughly and drying in an atmosphere of nitrogen at 120°. From these observations alone, it is impossible to say whether the compound is a true hydrate or a hydrous oxide. Since the pure preparation is soluble in acids, it furnishes a good starting point for making the most common ruthenium salts. The reddish-yellow solution of RuCl; deposits, slowly on standing but quickly when warmed, a black very finely divided precipitate said to be ruthenium oxychloride. This reaction is so delicate that 1 part of the metal imparts a distinct ink-like color to 100,000 parts of water. Ruthenium Dioxide.—Claus* prepared hydrous rahe dioxide which he formulated Ru(OH),:3H.2O, by evaporating a solution of Ru(SO,). with caustic potash. The precipitate is dark red in color and adsorbs alkali strongly. According to Gutbier, the hydrous oxide cannot be obtained pure since it goes into colloidal solution extremely readily on washing. The pure anhydrous oxide results on roasting Ru(SO,)2 in the air* or on burning the metal.> It crystallizes in small very hard tetragonal pyramids possessing a green metallic luster and a bluish or green- ish iridescence. Ruthenium Pentoxide.—Hydrous Ru2O; results, according to Remy,® when hydrous RuO is allowed to oxidize spontaneously; but its identity has not been established with absolute certainty. Debray and Joly’ prepared what they took to be Ru2O;-2H.2O by neutralizing an alkali ruthenate with nitric acid; but Gutbier 1 GuTBIER: Z. anorg. Chem., 95, 185 (1916); 109, 206 (1920). 2Z. anorg. Chem., 182, 315 (1923). 3 Liebig’s Ann. Chem., 59, 234 (1846). 4GutTBiER and Ransonorr: Z. anorg. Chem., 45, 243 (1905). 5’ GuTBIER, Leucus, WIkSssMANN, and Marscu: Z. anorg. Chem., 96, 182 (1916). 6 Z. anorg. Chem., 126, 185 (1923). 7 Compt. rend., 106, 328, 1494 (1888). THE HYDROUS OXIDES OF THE PLATINUM FAMILY 307 and Ransohoff! showed the alleged compound to be a mixture of hydrous Ru2O3 with some higher oxide. Salts containing hexavalent and heptavalent ruthenium are known, but the oxides RuO; and Ru,.O; have not been prepared. Ruthenium Tetroxide.—RuO, is not obtained in the hydrous state, but the anhydrous oxide is formed by passing chlorine into a solution of sodium ruthenate prepared by fusing ruthenium with sodium peroxide.? The heat of the reaction is sufficient to bring about the distillation of the oxide. The golden-yellow crystalline compound is fairly stable when dry, but decomposes quickly when moist. It dissolves slowly in water, giving a solution that is fairly stable, provided some free chlorine or hypochlorite is present. The oxide blackens organic matter readily and is reduced immediately by alcoholic potash with the separation of finely divided ruthenium. Serious explosions may occur if the solid oxide is treated with alcohol even in dilute solution. ? Hyprovus OxIpEs or RHODIUM Rhodium Sesquioxide.—Hydrous Rh.O; is precipitated as a black gelatinous mass by adding excess potassium hydroxide and a little alcohol to a solution of Na;RhCl,. If an excess of potash is not used, a sol is formed which deposits thin citron-yellow particles, said to be Rh(OH);-H.O.4 The gelatinous oxide is soluble in acids, forming the corresponding rhodium salts. Anhydrous Ru2O; only is formed by heating the finely divided metal in the air to 600 to 1000°. According to Gutbier,® the alleged formation of RhO* in this way was the result of incomplete combustion. Rhodium Dioxide.—Hydrous RhOz separates as a green pow- ’ der when chlorine is passed into a solution of Rh.O3; in alkali. If the flow of chlorine is continued, the green precipitate dis- 1Z. anorg. Chem., 45, 243 (1905); 95, 177 (1916). 2 Myuius: Ber., 31, 3187 (1898); Gursier: Z. angew. Chem., 22, 487 (1909). 3 Desray and Joty: Compt. rend., 118, 693 (1891). 4Cuaus: J. prakt. Chem., 76, 24 (1859); 80, 282 (1860); 85, 129 (1862). 5 Z. anorg. Chem., 95, 225 (1916). 6 Wim: Ber., 15, 2225 (1882). 308 THE HYDROUS OXIDES solves, giving a deep-blue solution resembling the ammoniacal copper solution. The blue color is attributed to the alkali salt of rhodous acid, H2RhO,,' which decomposes gradually, precip- itating hydrous RhO2. Hyprovus OxipEs or PALLADIUM Palladium Monoxide.—The addition of sodium carbonate to a solution of palladous salt precipitates hydrous PdO as a dark- brown mass.2. When thrown down in the cold, the oxide is readily soluble in alkalies, but it loses this property when dried or when precipitated from boiling solution, owing to coalescence of the particles. The hydrous oxide is best obtained pure by hydrolysis of palladous nitrate. A PdO sol in paraffin oil has been introduced as a therapeutic agent under the name ‘‘Lep- tynol.’”’? The oxide serves as a catalyst for the reduction of aldehydes to alcohols.+* Palladium Sesquioxide.—Hydrous Pd:2O; is best prepared by the electrolytic oxidation of a concentrated solution of palladous nitrate at 8° with a current density of 0.5 ampere per square centi- meter. If the electrolysis is prolonged, hydrous PdO, is formed. This is not a direct oxidation, but the sesquioxide decomposes into dioxide and monoxide, the latter dissolving in the free acid and undergoing further oxidation.> The sesquioxide is formed also by the action of ozone on palladous nitrate. It is chocolate brown in color when first prepared, but on washing, it gets darker owing to agglomeration of the particles and loss of adsorbed water. Palladium Dioxide.—An impure hydrous PdO, is precipitated on adding caustic soda to a solution of KePdCls. As mentioned above, it is obtained free from alkali and basic salt by the anodic oxidation of the nitrate, but is not quite free from hydrous PdO. The fresh oxide is soluble in acids, but like the monoxide, its reactivity decreases rapidly on standing. It cannot be dehy- 1 ALVAREZ: Compt. rend., 140, 1841 (1905). 2 WOHLER and KGnia: Z. anorg. Chem., 46, 323 (1905). ’ THorPE: ‘Dictionary of Chemistry,” 5, 51 (1924). 4 SHRINER and Apams: J. Am. Chem. Soc., 46, 1683 (1924). 6 WOuLER and Martin: Z, anorg. Chem., 57, 398 (1908). THE HYDROUS OXIDES OF THE PLATINUM FAMILY 309 drated,! as it decomposes at the ordinary temperature under an oxygen pressure of 80 atmospheres. It is, therefore, a vigorous oxidizing agent. Hyprovus OxIpES orf OSMIUM Osmium Monoxide and Sesquioxide.—Claus and Jacobi pre- pared hydrous OsO by the action of warm concentrated alkali on OsSO3 in an atmosphere of nitrogen. It is a blue-black precipi- tate which takes up oxygen very rapidly from the air. The same authors obtained hydrous Os,O3 as a brown-red precipitate on adding alkali to a solution of K;OsClg. Osmium Dioxide.—A very highly hydrous form of OsQOz is precipitated by the addition of alkali to K2OsCls, and by the action of alcohol or other reducing agent on an alkali osmate such as K,OsO,.2 The hydrous mass may be converted into a fine powder by prolonged heating on the water bath in contact with the mother liquor. If the gel is dried, it forms a horny body which loses water explosively and emits flashes of light when heated above 100°. The more granular oxide aged on the water bath becomes incandescent quietly at the glow temperature. It is obvious that the primary particles of the gel are extremely small, the coalescence on ignition causing a marked decrease in surface energy with the accompanying glow. ‘The gel isa typical hydrous oxide, the water content of which is determined by the conditions of drying. The compound formed by hydrolysis of K.OsO, in the presence of alcohol and hydrogen and by the action of sulfuric acid on K,OsO, is hydrous OsOe and not H,OsO,4, as claimed by Moraht and Wischin.* Since the gel formed by reduction of alkali osmates contains such small primary particles, it can be peptized by shaking with an excess of water‘ or by treating with a small amount of alkali 1 WOuLER and Konia: Z. anorg. Chem., 46, 323 (1905); 48, 203 (1906); 57, 398 (1908); Bexuucct: Gazz. chim. ital., 35, I, 343 (1905); Z. anorg. Chem., 47, 287 (1906). 2 Rurr and BorRNEMANN: Z. anorg. Chem., 65, 429 (1910); Rurr and Ratussura: Ber., 50, 484 (1917). 3 Z. anorg. Chem., 3, 153 (1893). 4Cxiaus and Jacost: J. prakt. Chem., 90, 65 (1863). 310 THE HYDROUS OXIDES or ammonia.! Freundlich and Baerwind? dissolved 1 gram of OsO, in 50 cubic centimeters of water, added 10 cubic centimeters of ethyl alcohol and allowed the mixture to stand 24 hours. The precipitate of the dark dioxide was washed with alcohol and then peptized by shaking for several days with 800 cubic centi- meters of water. The deep-blue-black sol is fairly stable, but its stability is greater in the presence of a little alcohol or protective colloid. The particles are negatively charged and are not spherical, as they appear alternately bright and dark when viewed with a cardioid ultramicroscope. Osmium Tetroxide.—OsQO,, erroneously called osmic acid, does not form a hydrous oxide. It is obtained in transparent glistening needles by burning the metal or by the action of oxidizing agents on the lower oxides. It dissolves readily in water, forming a colorless liquid possessing a caustic or burning taste. The solution is used for staining biological preparations and also for taking finger prints,* the oxide being reduced to metal. The fumes of the oxide are very poisonous, attacking the lungs and eyes. It also acts violently on the skin causing painful wounds.® It may be employed as a catalyst for many oxidation reactions, ® Hyprovus OxIpEs OF [RIDIUM Iridium Sesquioxide.—Hydrous Ir.O; is obtained in much the same way as the corresponding rhodium compound which it resembles closely. When a solution of IrCl; - 6NaCl - 24H2O is heated with alkali in a stream of carbon dioxide, an impure hydrous IreO; separates that is greenish white to black in color, depending on the alkali concentration. The light-colored prod- ucts come down from dilute alkali, while excess alkali gives 1 RurFr and Ratusspura: Ber., 50, 484 (1917). 2 Kolloid-Z., 33, 275 (1923). 3 Castro: Z. anorg. Chem., 41, 126 (1904); Paat and AmpErceR: Ber., 40, 1392 (1907); 49, 557 (1916); AmperGErR: Kolloid-Z., 17, 47 (1915). 4 DEVILLE and Drsray: Ann. chim. phys. (3) 56, 400 (1859); Compt. rend., 78, 1509 (1874). 5 MITCHELL: Analyst, 45, 125 (1920). 6 HormMann: Ber., 45, 3329 (1913); Hormann, EHRHART, and SCHNEIDER: Ibid., 46, 1657 (1913). THE HYDROUS OXIDES OF THE PLATINUM FAMILY 311 the black oxide containing relatively little water. This recalls the behavior of cupric oxide which dehydrates and darkens very quickly in the presence of excess alkali. Wohler and Witzmann! peptized the green oxide in dilute hydrochloric and sulfuric acids; concentrated acids dissolve it, giving reddish-yellow salts. Iridium Dioxide.—Hydrous IrQ,: is best prepared by adding alkali to a hot solution of NasIrCle, the sesquioxide first formed being oxidized to dioxide in a current of oxygen.? The fresh preparation is fairly soluble in acids and alkalies but it loses this property on drying.* The oxide can be gotten almost pure by drying the hydrous mass at 400° in carbon dioxide and then boil- ing with alkali and subsequently with sulfuric acid. The color of the oxide varies from light blue to black, depend- ing on the size of the particles and the structure and water content of the mass. Like the sesquioxide the precipitate is darker and less hydrous when it comes down from strong alkali solution. The solution obtained by the action of alkali on NageIrCl, in the cold has a violet color and contains hydrous IrQ, in sus- pension; after a time a violet modification of the oxide separates, which becomes blue on drying. Boiling the violet sol changes it to blue, probably owing to coalescence of the positively charged particles.4 Dilute hydrochloric acid peptizes the desiccator- dried preparation, giving a blue sol.® Iridium trioxide, IrO;, formed by fusing finely divided iridium with sodium peroxide or by the anodic oxidation of hydrous IrOz, is too instable to be isolated. Hyprovus OxIpEs oF PLATINUM Platinum Monoxide.—The black precipitate of hydrous PtO thrown down from PtCl. with caustic alkali cannot be washed free from chloride or alkali.* It is prepared in the pure state by adding the calculated amount of dilute caustic soda to 1Z. anorg. Chem., 57, 323 (1908). 2 Cuaus: J. prakt. Chem., 39, 104 (1846). 3 Jory and Lrempe: Compt. rend., 120, 1341 (1895). 4 WOuLER and WiTzMANn: Z. anorg. Chem., 57, 323 (1908). 5 Cf. also PaAL, BIEHLER, and StryYeER: Ber., 60, 722 (1917). 6 Limpia: Pogg. Ann., 17, 108 (1829). 312 THE HYDROUS OXIDES a solution of K.PtCly.! As the fresh oxide takes up oxygen readily from the air, the precipitation, washing, and drying must be carried out in an atmosphere of carbon dioxide. When newly formed, it is readily soluble in dilute halogen acids but is insoluble in bases and in oxy acids other than sulfurous. Dried in a vacuum desiccator, the water content corresponds approximately to the formula PtO-2H.O. It holds on to its water very strongly, one sample retaining 6.6 per cent water after heating several days at 400°. PtO is a stronger oxidizing agent than the ees and a stronger reducing agent than the metal. Platinum Sesquioxide.— Wohler and Martin? obtained hydrous Pt.O; for the first time in a pure condition by adding solid PtCl; to a solution of sodium carbonate or by dissolving the chloride in concentrated potassium hydroxide and precipitating with acetic acid. The latter method yields a product containing some PtO.. The precipitate obtained at room temperature is light brown in color and highly hydrous; by boiling with alkali, it becomes less hydrous and darker; the dried preparation is almost black. The freshly formed oxide is not oxidized by boiling with water through which a stream of oxygen is passed; but it cannot be dehydrated completely without decomposition taking place. In chemical behavior it occupies an intermediate position between hydrous PtO and PtOs. Platinum Dioxide.—Wohler* prepared pure hydrous PtO:, by boiling a solution of platinic chloride with caustic potash, which converts PtClh’’ to Pt(OH).”. When cold, this solution is neutralized with acetic acid, and the hydrous oxide is obtained as an almost white precipitate which becomes yellow on drying. Even when dried in the air, the water content is less than cor- responds to the tetrahydrate PtO.-4H.O or H.Pt(OH)..4 It loses water continuously by lowering the vapor pressure of the surroundings or by raising the temperature; and there is no evidence of the existence of any definite hydrate. The last 2.5 1 THOMSEN: J. prakt. Chem., (2) 15, 299 (1877); WOuuER: Z. anorg. Chem., 40, 456 (1904); W6uuErR and Frey: Z. Elektrochem., 15, 133 (1905). * Ber., 42, 3958 (1909); cf., however, DupLEy: Am. Chem. J., 28, 59 (1902); BLonpEL: Ann. chim. phys., (8) 6, 111 (1905). 3Z. anorg. Chem., 40, 434 (1904); TopsGn: Ber., 3, 462 (1870). 4 BeELLucctr: Atti accad. Lincei, (5) 12, 635 (1903). THE HYDROUS OXIDES OF THE PLATINUM FAMILY 3138 per cent of water cannot be removed without decomposing the oxide. ‘The freshly precipitated product is soluble in acids and alkalies; but the thoroughly dried substance is insoluble in all dilute and concentrated acids with the exception of hydrochloric and aqua regia. It is sometimes called platinic acid, since its reactions with alkalies yields platinates such as K.Pt(OH). isomorphous with the stannates.! Platinum Trioxide.—By the electrolysis of a solution of hydrous PtO, in 2 N potassium hydroxide, a brilliant golden-yellow body of the composition K.O-3PtO; separates at the anode. When this is treated with dilute acetic acid in the cold, it yields the trioxide PtO3, a reddish-brown substance which loses oxygen readily and evolves chlorine slowly from dilute hydrochloric acid.’ 1 BeLLuccr and PARRAVANO: Atti accad. Lincei, (5) 14, 459 (1905). 2 WoOuLER and Martin: Ber., 42, 3326 (1909). CHAPTER XV TANNING Tanning is the process whereby the skin or hide of animals is converted into leather. Before subjecting hides to the tanning process, they must be treated to remove the hair, epidermis, and fat and to get the remainder of the substance in suitable condition to take up the tanning agent. The dermis or leather-producing portion of the skin consists essentially of bundles of fine connec- tive-tissue fibers about 1uin diameter, bound together irregularly. The fibrils consist essentially of a protein material, collagen, which is converted into gelatin by boiling with water. To prepare the hide for tanning, it is first immersed in lime water to which is usually added sodium sulfide to “‘sharpen”’ or hasten the action of the lime. The liming process not only removes the hair and destroys the epidermis, but it swells the collagen fibers and removes the cementing material between them, thereby splitting the bundles into their constituent fibrils. Following the liming process, the alkali is neutralized with dilute acids, and the hide is subsequently ‘‘bated”’ by subjecting it to the action of tryptic ferments in conjunction with ammonium chloride to remove the last trace of lime. The enzymes digest off part of the remaining connecting and epidermal substance, and completely emulsify the fat. This digestive action is stopped by ‘‘drenching,”’ that is, by treating the hide with fermenting bran infusions which bring the skins to a slightly acid state in which tryptic ferments are not active. The starchy matters of the bran are first converted into glucose, which undergoes bac- terial fermentation by several types of lactic-, butyric-, and acetic-acid-forming bacteria. As these bacteria develop only in solutions of feeble acidity and are destroyed by the accumula- tion of their own acid products, the acidity of the drench is auto- matically self-regulating and tends to produce a very slight acid - swelling of the skins. For chrome tanning, a similar result is 314 TANNING 315 brought about by “pickling” the limed or bated skins in a bath consisting of a solution of sodium chloride and sulfuric acid in amount depending on the degree of basicity of the chrome liquor employed. After all undesirable impurities are removed and the collagen fibrils are brought to a flaccid slightly swollen condi- tion, the hide is ready for the tannage proper. In general, if the skin is soaked in infusions of barks, fruits, or galls which contain members of the class of compounds known as tannins, the process is called vegetable tanning; and if the tanning liquor is a mineral salt, it is known as mineraltanning. A consideration of the mechanism of these two processes will be taken up in order, even though any discussion of vegetable tanning might appear to be without the scope of this book. VEGETABLE TANNING It has been known for centuries that skin substance undergoes a marked change in properties when brought in contact with vegetable infusions, the active principle in which is now known to be tannin. Seguin regarded the tanning process as a reac- tion between hide, a base, and the tanning agent, an acid, giving leather, a salt. Berzelius and Dumas likewise considered leather to be a compound of hide and tanning agent without going into the mechanism of the process. Knapp,! whom Procter calls the father of leather chemistry, was the first to reason that leather could not be a definite »chemical compound since the amount of material taken up by the hide is not in any definite stoichiometrical proportion but depends on the concentration of the tan liquor. Moreover, since so many chemically different substances—tannin, alum, chromic sulfate, formaldehyde, stearic acid, etc.—can be used for tanning, Knapp concludes that the process must be essentially a physical one: “The two active substances are rendered insoluble in water by means of surface attraction (adsorption).’”’ The essential difference between hide and leather recognized by Knapp is that, in the latter, the fibers are no longer in the condition of a colloidal jelly, but may | be dried without adhesion, the substance remaining porous and flexible. 1 Dinglers polytech. J., 149, 305 (1859), 316 THE HYDROUS OXIDES Stiasny! was first to call attention to the similarity in the adsorbing capacity of carbon and hide powder. ‘Thus, both adsorb a wide variety of different substances; aromatic acids are adsorbed more strongly by both than are aliphatic acids; and with both adsorbents, acetic acid and the chloracetic acids possess approximately the same adsorbability in spite of their difference in strength. From such observations, Stiasny concludes that the taking up of tannin by hide powder is an adsorption process. Herzog and Adler? reached a similar conclusion from observa- tions of adsorption by hide powder of such substances as resorcin and pyrogallol, which are closely related to tannin but differ from the latter in forming molecular solutions in water. It remained for von Schréder* to demonstrate the adsorption of tannin from colloidal solution by carbon and hydrous alumina, as well as by gelatin and hide powder. Since the bacteria present, in hide powder cause decomposition of tannin, giving gallic acid, more consistent results can be obtained by sterilizing the adsorb- ent. Within the first hour after the adsorption, there exists an adsorption equilibrium between the tannin and the adsorbent, but the amount of tannin that can be removed by washing decreases with the time of standing. The effect of acids on the adsorption of tannin is less marked than that of alkalies. Thus 0.05 N (NH,4)2CO3 added to a tannin solution cut down the adsorption, expressed in millimols per gram of adsorbent, from 700 to 120. Since the collagen of hide is converted into gelatin by boiling, von Schréder compared the adsorption of tannin by hide powder to that of gelatin. The negatively charged particles of tannin sol are adsorbed by and precipitate a slightly acid and, therefore, positively charged gelatin sol. In slightly alkaline solution the gelatin particles are negatively charged, and adsorption by tannin with the accompanying mutual precipitation does not take place. As has been mentioned, hide powder in very dilute alkali likewise adsorbs tannin but slightly. Under comparable conditions, the adsorption capacity of gelatin and hide powder for tannin is very similar. As might be expected, a longer time is required for 1 Kolloid-Z., 2, 257 (1908); Collegium, 118 (1908). 2 Kolloid-Z., 2, 2d Supplement, III (1908). 3 Kolloidchem. Bethefte, 1, 1 (1909). TANNING rah attaining the maximum adsorption with hide powder-in mass than with gelatin in the sol form where adsorption and mutual precipitation is quite rapid. Just as with hide powder, there is at first an adsorption equilibrium between gelatin and tannin, but this gradually gives way to an irreversible change in the mass. Von Schréder’s observations led him to say: Tanning with tannin is characterized by adsorption of the tanning agent. However, the adsorption compound is not leather at first; but this results in the course of time by a change in the adsorption com- pound whereby the tannin is more firmly held . Considering what has been said concerning the precipitation of gelatin by tannin and the parallelism in the behavior of gelatin and hide powder, one reaches the conclusion that the adsorption of tannin by hide powder is a concealed colloidal precipitation. Before the hide powder can absorb tannin, it must obviously be brought by swelling to such a condition that it can be precipitated. Von Schréder thus comes out definitely in support of the view that the first step in the tanning process is the mutual colloidal precipitation of negatively charged tannin and positively charged hide substance. It is not obvious just wherein the Procter- Wilson theory of vegetable tanning differs from von Schréder’s. It seems that Procter,! Wilson,? and others* were not aware of the definiteness of von Schérder’s viewpoint, for Procter writes in 1924: Knapp’s theory of the purely physical nature of the combination in tanning has remained the popular one in Germany, where it has been strongly supported by Wolfgang Ostwald and others, and considered as a case of “adsorption,” whatever that may be, but a view has gained ground in America and England that the change is of a colloidal character, and dependent on the opposite electric charges of the hide fiber and the tannin particles, which combine and electrically neutralize each other. Recent investigations by R. J. Browne, at the Procter Research Laboratory at Leeds, go far to prove that all vegetable tan- nins are colloidal in character, since they can be entirely removed from solution by ultrafiltration, and it is known from cataphoresis experi- ments that they are negatively charged, while hide fiber on the acid 1 Bogue’s ‘‘Colloidal Behavior,” 2, 728 (1924). 2 “The Chemistry of Leather Manufacture,” Easton, 271 (1923), 3 THomas and Frrepen: J. Ind. Eng. Chem., 15, 839(1923), 318 THE HYDROUS OXIDES side of its isoelectric point has a positive charge in consequence of the Donnan equilibrium. If two colloid suspensions of opposite charges come in contact, they combine, much as two oppositely charged ions would do, and, if mixed in the right proportions for complete neutral- ization, the precipitation is complete. The Procter-Wilson theory of tannage holds that leather is such a combination, and with regard to what may be called the first stage of vegetable tannage, there is strong evidence in its favor. Tannage only takes place when the hide fiber is slightly swollen with acid and so possesses a positive Donnan charge, and this charge will naturally vary with the difference between the hydrogen ion concentrations of the pelt and the liquor which is in equilibrium with it, which is greatest when the acidity is very small. In alkaline liquors, tannage does not take place. So far as I can make out, von Schréder’s interpretation of the mechanism of the first step in the tanning process is the same as Procter and Wilson’s. What the latter have done in addition is to attempt to give the origin of the charge on the collagen of the hide. Thus it is assumed that in equilibrium with a tan liquor having a pH value lying in the range 2 to 5, collagen (represented by C) forms a compound, CHA, which is completely ionized into the positive ion CH’ and the negative ion A’. Since the hypothetical collagen cations are a part of an elastic structure which cannot diffuse, the conditions necessary for a Donnan equilibrium obtain.t When equilibrium is attained between the collagen and the acid: In the tan liquor let v= [Ht Ae and in the collagen jelly let y = [H’] and 2 =r from which [A] =yt+e The equation of products may now be written: a? = y(y + 2) in which the product of equals is equated to the product of unequals. It follows, therefore, that the sum of the unequals is greater than the sum of the equals, or that 2y + 2, the sum of the diffusible ions in the hide jelly, is greater than 22, the sum of 1Cf. Chap. I, p. 18, TANNING 319 the ions in the tan liquor. This gives rise to an electrical differ- ence of potential between the jelly phase and the external solu- tion, which may be formulated thus: Pie, RT —2+ V4e?+2? 22 E = —,~ log. steal log. By similar reasoning the electrical difference of potential H, between the surface film of the tannin particles and bulk of the solution is given by hes KT log, = KT log. BE a8 an Yi F ay pate Bae a where 2; is the concentration of the cations balancing the negative charge on the tannin particles and y; the concentration of the anions [A’] in the surface film. Since # and E, are of opposite sign, the Procter-Wilson theory assumes that the first step in the mechanism of tanning results from the tendency for E and EL, to neutralize each other. The equation for the difference of potential between a positive collagen jelly and the surrounding solution is deduced from the specific assumption that hydrochloric acid, say, combines with collagen, forming highly ionized collagen chloride in which the collagen is the constituent of a complex cation whose free diffusion is restricted. It should be emphasied that this does not furnish any proof whatsoever of the existence of a definite highly ionized compound, collagen chloride, yielding a collagen cation. One arrives at exactly the same equation by making the more probable - assumption that collagen, like the oxides of iron, chromium, and aluminum, adsorbs hydrogen ion more strongly than chloride ion and so possesses a positive charge in dilute hydrochloric acid solution. The hydrogen ion adsorbed by the jelly is not free to diffuse, thus imposing the constraint conditions necessary for a Donnan equilibrium. As Donnan! puts it: ‘An adsorption of hydrogen ions by colloidal aggregates or micelles (constituting the units of the molecular network) would lead to the same constraint conditions and the same general equations as the ionization of the amphoteric protein molecules assumed by Procter.” 1 Chemical Rev., 1, 89 (1924), 320 THE HYDROUS OXIDES The initial step in the tanning process would thus appear to be neutralization by adsorption of negative tannin by positive collagen, which owes its charge to preferential adsorption of hydrogen ion. The isoelectric point of collagen is claimed to be at pH = 5;! and the amount of tannin adsorbed by a given amount of hide powder increases with decreasing pH values, as would be expected. It caused considerable worry, however, to find an increase in the adsorption of tannin with increasing pH on the alkaline side of the alleged isoelectric point. This reaches a maximum around pH = 8, above which it falls off rapidly to zero.” A partial explanation of this was forthcoming when Wilson and Gallun’ found a second isoelectric point for collagen at pH = 7.7. Two tautomeric forms of collagen are, therefore, assumed to exist: one, C,, stable in acid solution with an iso- electric point at pH = 5; and a second, Cs, stable in alkaline solution with an isoelectric point at pH = 7.7. On increasing the pH value from 5 to 7.7, if the change from C, to Cy proceeds at a greater rate than positive C, changes to negative Ca, then the net result will be an increase in the amount of positive C, - and an increase in the amount of tannin adsorbed. But there is still some tannin adsorbed above pH = 7.7, more in commercial tanning extracts than in tannic acid. Thomas and Kelly® assume the existence of some complex organic reaction to account for this. While one cannot deny that the initial step in tanning by tannin may involve factors other than neutralization by adsorption of negatively charged tannin by positively charged hide, there seems no necessity, at least for the present, to postulate any other action to account for the fact that there appears to be some adsorption of tannin above pH = 7.7 and below pH = 2. For, if there are two modifications of collagen, Ca and Cy, with different isoelectric points in contact with each other, then each is certain to influence the other, and the values pH= 5 and 1 Porter: J. Soc. Leather Trades’ Chem., 5, 259 (1921); 6, 83 (1922); Tuomas and Ketiy: J. Am. Chem. Soc., 44, 195 (1922). 2 THomas and Ketty: J. Ind. Eng. Chem., 15, 1148 (1923). 8 J. Ind. Eng. Chem., 16, 71 {1923)% 4 THOMAS and Onin J. Ind. Eng. Chem., 16, 800 (1924). 5 J. Ind. Eng. Chem., 16, 31 (1924). TANNING 21 pH = 7.7 are not true isoelectric points at all; but each is an average or compromise value at which the mutual effect of two collagens of the same or opposite charge give a minimum. If the supposed Cy, could be isolated from the influence of the supposed Ca, the isoelectric point of the former might very well be at a higher value than pH = 7.7. Attention has been called to von Schréder’s observation that the adsorption of tannin by hide powder is completely reversible for a short time; but on standing, the process becomes irreversible. Not only can no tannin be extracted from the hide by water, but it resists the action of dilute alkalies; that is, leather is formed. Justin-Mueller! believes that the second stage in the tanning proc- ess following adsorption is some chemical reaction between tannin and hide. A similar view seems to be favored by Freundlich,? although he does not commit himself definitely. On the other hand, a number of people* come out squarely in support of the view that the process is physical throughout. Thus Moeller+ states ‘‘that the changes which tannin colloid undergoes after being taken up by hide substances were found to depend solely on irreversible colloidal changes of state. Simple chemical processes do not occur.”’ Moeller’ considers leather to be animal hide, the elementary particles of which are microcrystalline micelles protected from hydrolytic influences by a sheath of tan particles. Some attempt has been made by Meunier,’ Fahrion,’ and others to work out a purely chemical interpretation of vegetable tanning. Thus Meunier obtained a leather of remarkable per- manence by bringing skin in contact with hydroquinone. A por- tion of the quinone was reduced to quinol and Meunier concludes 1 Kolloid-Z., 6, 40 (1910). 2 “Blements of Colloid Chemistry,” translated by Burger, London, 186 1925). 3 Von Scurover: Kolloidchem. Bethefte, 1, 53 (1909); Strasny: Collegium, 118 (1908); Kolloid-Z., 2, 257 (1908); 31, 299 (1922); GoLDMANN: Collegium, 93 (1908). 4 MoE.LuER: Collegium, 39 (1917). 5 Z. Leder-Gerberei Chem., 1, 360 (1922). 6 Chimie & industrie, 1, 71, 272 (1918); J. Am. Leather Chem. Assoc., 13, 530 (1918); Meunrer and Sryewetz: Mon. sct., 28, 91 (1908). 7Z. angew. Chem., 22, 2083, 2135, 2187 (1909). 322 THE HYDROUS OXIDES that this reduction is accompanied by oxidation of the collagen, whereupon the oxidized collagen combines with the remaining quinone, giving leather. Meunier postulates the formation of quinones in vegetable tanning materials, which react with col- lagen to form leather. Powarnin!' objects to the assumption that the quinones result from oxidation and suggests that they are formed by a tautomeric change which, for tannin, is assumed to be: CH CH Yaa 4 HC C — OH = HC CH — O | | as | | | HC C — OH HC CH — O Bee Soe CH CH Enol form Keto form The enol form is supposed to be stable in alkaline solution and the keto form in acid solution. Only the latter form is assumed to have tanning properties. As yet, these views of the tanning process lack definite experimental foundation;? but they indicate the probable existence of a chemical as well as a physical action in vegetable tanning. Formaldehyde has tanning properties’ in solutions having a pH value greater than 4.8, the best practical results being obtained between pH = 5.5 to 10.0.4 Meunier believes that a definite chemical compound is formed between the aldehyde and oxidized collagen; but this appears doubtful, as the formaldehyde can be recovered quantitatively from formaldehyde leather simply by digesting with dilute hydrochloric acid. MINERAL TANNING Any mineral salt may be employed for tanning leather, pro- vided it undergoes hydrolytic dissociation forming a colloidal 1 Collegium, 634 (1914). 2Cf., however, THomMas and Keuiy: J. Ind. Eng. Chem., 16, 800 (1924); THomas and FostErR: J. Am. Chem. Soc., 48, 489 (1926). 3 British Patent 2872 (1898). 4 Hny: J. Soc. Leather Trades’ Chem., 6, 131 (1922); THomas and KELLy: J. Ind. Eng. Chem., 16, 925 (1924). TANNING 320d hydrous oxide or basic salt. Actually only the salts of iron, aluminum, and chromium have been employed, and of these the salts of chromium are by far the most important and so will be considered first. bee Chrome Tanning.—As early as 1858, Knapp! described a proc- ess for tanning hide with solutions of salts of aluminum, iron, and chromium; but the first successful method of mineral tanning was invented by Augustus Schultz in 1884. In Schultz’s two- bath process, the skins are treated with an acidified solution of potassium dichromate until the liquor penetrates them thoroughly after which they are put into a bath of acidified sodium thiosul- fate which reduces the chromate in the hide to chromic salt, the tanning agent. In 1893 Dennis revived and patented Knapp’s original single-bath tan liquor which consists of a partially neu- tralized solution of chromic chloride. Dennis prepared the bath by dissolving chromic oxide in hydrochloric acid and subsequently rendering this more basic by adding caustic soda. Later Procter? showed that good tanning liquors could be prepared by reducing bichromate solution with glucose in the presence of enough hydrochloric acid to leave the solution basic. Basic chromic sulfate was found to be superior to the chloride for one-bath tanning and is now almost universally employed.* A useful method of preparing a satisfactory bath consists in the reduction of a strong solution of sodium bichromate directly with sulfur dioxide. A concentrated liquor can be obtained in this way and diluted as required. In view of the relatively weak character of sulfurous acid, the liquor is sufficiently basic for many purposes. The equation for the reduction is usually written NaeCr2O7 -+- 350.2 + H.O = NaesO, + 2CrOHSO, This merely represents the relative basicity of the final liquor, but there is no assurance of the formation of a definite basic salt like that formulated. 1 “Nature and Essential Character of the Tanning Process and of Leather,”’ J. G. Cotta Buchandlung (1858); English translation, J. Am. Leather Chem. Assoc., 16, 658 (1921). 2 Leather Trades’ Rev., Jan. 12 (1897). 3 Wiuson: “‘The Chemistry of Leather Manufacture,”’ Easton, 278 (1923). 4 BatpERSTON: Shoe & Leather Rep., Oct. 18 (1917); Procter: J. Roy. Soc. Arts., 66, 747 (1918). 324 THE HYDROUS OXIDES That basic liquor is more satisfactory for tanning is well illus- trated by some observations of Thomas, Baldwin, and Kelly! on the rate of taking up of chromic oxide by hide powder from a commercial tan liquor and from a solution of chromic sulfate. The chrome liquor had a basicity corresponding to the formula Cr(OH)1.2(804)o.9 and contained 17 grams Cr.O; per liter. The chromic sulfate contained 164 grams Cr.O; per liter. The results are shown in Fig, 20. The time in hours covered by Time, hours Cr,03 Adsorbed, milligrams per gram of hide substance Fic. 20.—Adsorption of chromic oxide by hide substance. the experiment with the commercial chrome liquor is plotted on the top horizontal axis and the time in days covered by the experi- ments with chromic sulfate solution on the bottom horizontal axis. The amount adsorbed is determined by direct analysis of the leather. It will be seen that the rate of tanning is very much less in the chromic sulfate solution which has a hydrogen ion con- centration about twenty times greater than the commercial liquor. Moreover, the amount of chromic oxide taken up from ‘J. Am. Leather Chem. Assoc., 15, 147 (1920); THomas and Keuzy: J bid., 15, 487 (1920). TANNING 325 the commercial liquor has not reached a limiting value in 4 days; but is appreciably greater than the limiting value in the case of pure chromic sulfate. Since the amount of chromic oxide taken up and the velocity of the process is greater in the more basic solutions, it is obvious why such solutions are used in practice. The basicity must be subject to careful control, however, since if too basic, the bath is rendered turbid in the presence of hide, owing to precipitation of hydrous chromic oxide. This is of importance in connection with the theory of chrome tanning, which will next be considered. The most plausible theory of chrome tanning is that the hide fibrils adsorb from the tan liquor hydrous chromic oxide or basic salt which subsequently ages, giving a protective coating. This film not only keeps the fibrils separated and thereby prevents their coalescence on drying but protects them from the action of water and dilute alkali. As Rochelle salt dissolves even an aged hydrous chromic oxide, it is not surprising to learn that a chrome-tanned leather is detanned by soaking in a solution of this salt.' The detanned leather can be tanned once more by washing and soaking in fresh chrome liquor. Davison? determined the amount of chromic oxide taken up in 4 hours by a constant amount of hide powder, from various concentrations up to 1.5 grams Cr.O3; per liter, of a single-bath chrome-tanning solution. On plotting the chromic oxide taken up against the concentration of the residual solution a continuous curve is obtained which corresponds with the ordinary adsorption formula. This supports the view that the initial step in chrome tanning consists in adsorption of hydrous chromic oxide or basic _ salt.3 | Attempts have been made to interpret the chrome-tanning process as a mutual precipitation of oppositely charged particles just as in the case of vegetable tanning. The difficulty encoun- tered is that hydrous chromic oxide in acid solution takes a posi- tive charge just like the hide. Thompson and Atkin‘ suggest that 1 Procter and Wixson: J. Soc. Chem. Ind., 35, 156 (1916). 2 J. Phys. Chem., 21, 190 (1917). 3 BENNETT: J. Soc. Leather Trades’ Chem., 1, 130, 169 (1917); cf., how- ever, Witson, Tuomas, et al.: J. Am. Leather Chem. Assoc., 12, 450 (1917). 4 J. Soc. Leather Trades’ Chem., 6, 207 (1922). 326 THE HYDROUS OXIDES the active constituent of the chrome-tan liquor is a negative ion or colloidal particle having a composition such as Cr(OH);- - CrOCl- Cl’ which combines with positively charged collagen, forming leather. This view was called in question by Seymour- Jones! who found that hide was tanned in a normal fashion in a basic chromic chloride solution which showed no anodic migration of chromium whatsoever. Seymour-Jones” also attempted the ultrafiltration of a typical tanning bath containing 270 grams chromic oxide per liter prepared by reduction of a solution of sodium bichromate with sulfur dioxide. The solution passed unchanged through a collodion disk ultrafilter and through filter papers impregnated with 1 and 5 per cent gelatin solutions, respectively, and subsequently hardened. This was taken to mean that colloidal chromic oxide or basic salt plays no réle in chrome tanning. There is, however, no doubt of the presence of colloidal chromic oxide in certain technical tan liquors. Thus, Wintgen and Lowenthal? ultrafiltered a so-called one-third basic commercial tan liquor prepared by mixing 20 grams chrome alum in 170 cubic centimeters of water with 7 grams of crystalline sodium carbonate in 20 cubic centimeters of water. Using a very thick fine hardened ultrafilter and applying a pressure of 75 atmospheres, they obtained a filtrate consisting of chromium salt in molecular solution; and a residue possessing the appearance and properties of colloidal chromic oxide. It is altogether prob- able that Seymour-Jones could have ultrafiltered some colloidal chromic oxide from his tan liquor had he used a sufficiently dense filter. This is, however, more or less beside the point, as one can tan leather in a chromic salt solution containing but little colloidal oxide. If chromic sulfate is placed in solution, an equilibrium exists that may be represented thus: Cre(SO.4)s +- xH,.O —s Cr.03;7H.O fe 3H.SO, or, if preferred, by Cr2(SOx)s + 12H.O <= [Cr(OH)> . (H20) 4]2S04 ++ 2HSO, 1J. Ind. Eng. Chem., 15, 265 (1923). 2 J. Ind. Eng. Chem., 15, 75 (1923). 3 Kolloid-Z., 34, 294 (1924), TANNING 327 since Werner! has prepared a definite crystalline basic salt, insolu- ble in water, of the formula indicated. When hide is placed in such a solution, it adsorbs acid strongly, thus displacing the equilibrium to the right with the consequent precipitation of the insoluble hydrous oxide or basic salt on the surface of the particles of hide, where it is adsorbed. The amount deposited in the hide substance under these conditions is obviously small and so the tannage is relatively light. If, on the other hand, a por- tion of the acid is neutralized, the adsorption of acid by the hide brings about a correspondingly greater precipitation of hydrous oxide or basic salt, and the tannage is correspondingly heavy. As I have already pointed out, if the tan liquor is rendered too basic, the adsorption of acid by the hide causes precipitation of the hydrous oxide in the liquor rendering the latter cloudy. Obviously, a careful control of the conditions is necessary for successful tanning. In general, if the acidity is too high, the penetration is good, but the amount of chromic oxide deposited is slight; whereas if the basicity is too high, the bath contains hydrous chromic oxide in too coarse a state of subdivision to penetrate well. It should be emphasized that the displacement to the right of the hydrolytic decomposition of chromic salt is occasioned not only by adsorption of sulfuric acid but by adsorption of hydrous chromic oxide as well. To illustrate, let us consider the adsorp- tion phenomena which take place from solutions of chromate and dichromate with hydrous alumina. In such solutions, the follow- ing equilibrium exists: Cr.0 7” a H.O one PAS & oe 2CrO.” If a sample of highly purified “grown alumina’’? is added to a solution of red dichromate, the solution becomes yellow. This is because hydrous alumina adsorbs hydrogen ion, strongly shifting the equilibrium to the right. But the alumina also adsorbs chromate which can be determined quantitatively and can be detected qualitatively by the color it imparts to the adsorbent. — This likewise tends to displace the equilibrium to the right. 1 Ber., 41, 3447 (1909). 2 Pecanas: Z. angew. Chem., 18, 801 (1904); Kolloid-Z., 2d Supplement, XI (1908). 328 THE HYDROUS OXIDES If, instead of adding powdered alumina to a solution of dichro- mate, one adds aluminasol stabilized by preferential adsorption of hydrogen ion, the adsorption capacity of the alumina for hydro- gen is partially supplied and the equilibrium is not disturbed appreciably, the solution remaining red. Under these conditions, the adsorption of chromate is relatively small; and incidentally, the amount of dichromate carried down is less than that for most multivalent ions.' In the same way, when hide is placed in a chromic sulfate solution containing a relatively large amount of hydrogen ion, hydrous chromic oxide or basic salt is adsorbed, as well as sulfuric acid; but the adsorption of the former is much greater from more basic solutions. After the tannage is complete, the skin is left in a somewhat acid condition. In practice, it is rendered nearly neutral by treating with a dilute alkaline solution. Even after this treat- ment chrome leather is characterized by having a relatively high sulfuric acid content. Only a trace is free at any one time, but as soon as this trace is removed, more is immediately liberated. A part of this sulfuric acid is adsorbed by the hide and a part by the hydrous oxide, while some may exist in solid solution in the hydrous oxide or as a basic salt. The addition of neutral salts to a bath cuts down the adsorption of chromic oxide by the hide. In the case of chlorides, this may be due to the observed increase in the hydrogen ion concentra- tion ;? but sulfates decrease the hydrogen ion concentration which should favor increased adsorption of chromic oxide. To get around this difficulty, Wilson and Gallun’ postulate the formation of addition compounds between the chromium compounds and the added salt, which are supposed to be endowed with the prop- erty of tanning less readily than the original chromium com- pounds. It is probable that a great deal of the effect of neutral salts is due to their adsorption by the hide, which cuts down the adsorption of hydrous chromic oxide. 1 Weiser and Mippieton: J. Phys. Chem., 24, 647 (1920). 2 Poma: Z. physik. Chem., 88, 671 (1914); HaRNEpD: J. Am. Chem. Soc., 37, 2460 (1915); THomas and Batpwin: J. Am. Leather Chem. Assoc., 13, 248 (1918); J. Am. Chem. Soc., 41, 1981 (1919); THomas and Fostsr: J. Ind. Eng. Chem., 14,132 (1922). ° J. Am. Leather Chem. Assoc., 15, 273 (1920). TANNING 329 A purely chemical theory of chrome tanning receives its most enthusiastic support from Wilson and his collaborators.! It is the opinion of these investigators that even in acid solution, there are some negatively charged groups in the collagen structure. In the tanning process, Cr(OH),.’ ions or ions of similar structure are supposed to diffuse into the jelly composing the hide and to attach themselves to negatively charged groups wherever encountered, giving salts that have been designated chromium collagenates. Attempts have been made to establish the exist- ence of such salts by Baldwin? and by Thomas and Kelly.® Baldwin studied the fixing of chromic oxide from various liquors containing 0.38 to 66.4 grams of chromic oxide per liter and found that the amount taken up reaches a maximum in a bath con- taining 15 to 20 grams per liter. Davison failed to observe this maximum, as he worked with lower concentrations of tan liquor. Thomas and Kelly repeated Baldwin’s experiments with con- centrations varying from 0.36 to 202 grams chromic oxide per liter, and confirmed the result that the amount of chromic oxide taken up per gram of hide powder in 48 hours reached a maximum in a solution containing approximately 16 grams of chromic oxide per liter, after which the curve sloped downward, reaching a minimum when the concentration of chromic oxide in solution was approximately 150 grams per liter as shown in the lower curve, Fig. 21; the experiments were repeated, keeping the liquor in contact with the hide for 8.5 months, with the results given in the upper curve, Fig. 21. The conclusions drawn from these observations are the following: Wilson* found that 750 grams of a certain collagen take up 1 mol of hydrochloric acid, forming what he believes to be collagen chloride, a salt of a weak monoacid base. He, therefore, assumes the combining weight of collagen to be 750. Using this value, Thomas and Kelly calculate that 4 equivalents of chromium are combined with 1 of collagen at the maximum in the 48-hour curve which represents a definite compound, tetrachrome collagen. In the 8.5-month run, the 1 Witson: ‘‘The Chemistry of Leather Manufacturer,’ Easton, 278-308 (1923). 2 J. Am. Leather Chem. Assoc., 14, 433 (1919). 3 J. Ind. Eng. Chem., 18, 65 (1921); 14, 621 (1922). 4 J. Am. Leather Chem, Assoc., 12, 108 (1917). 330 THE HYDROUS OXIDES maximum is approximately twice as high as in the 48-hour run, a circumstance that is claimed to prove the existence of octa- chrome collagen. The octachrome curve shows a slight bend at a higher concentration of chrome liquor where the fixation of chromic oxide is believed to be sufficiently near the theoretical for tetrachrome collagen to justify postulating its formation. After championing the use of thermodynamic formulas to interpret tanning by tannin, one wonders why-no attention what- en >) Oo 250 Cr,0, Adsorbed milligrams pergram of hide substance Concentration, grams Cr.03 per liter Fig. 21.—Effect of concentration of chrome liquor on the adsorption of chromic oxide by hide substance. soever seems to have been paid to the phase rule in interpreting the results with chrome tanning. In the light of this generaliza- tion, the curves obtained by Thomas and Kelly certainly do not offer convincing proof of the formation of chromium collagenates. On the contrary, they indicate that quite the opposite is true. A maximum is observed repeatedly in the taking up of one sub- stance from solution by another. For example, the lower curve in Fig. 22 shows the adsorption of acetic acid from toluene solu- tion by animal charcoal! and the upper curve, the adsorption of 1 Scumipt-WaLTER: Kolloid-Z., 14, 242 (1914). TANNING | ax phenol from solution in ethyl alcohol by the same adsorbent.’ The maxima in these curves are no more indicative of compound formation than any other points on the curves. Freundlich’ observed maxima in the adsorption of strychnine nitrate from bed per Gram Carbon 04 05 Gram Solute per Gram of Solution Grams Adsor Fia. 22.—Adsorption by carbon of (1) phenol from ethyl alcohol and (2) acetic acid from toluene. aqueous solution by carbon, wool, and arsenious sulfide; and in the adsorption of crystal violet by carbon and fibers*. Slmilra observations were made by Biltz and Steiner* in the absorption of dyes, such as night blue and Victoria blue, by wool and car- 1 Gusrarson: Z. physik. Chem., 91, 397 (1916). 27. physik. Chem., 78, 400 (1910); FREUNDLICH and PosER: Kolloidchem. Beihefte, 6, 295 (1914). 3 Freunpuicu and Lossv: Z. physik. Chem., 59, 284 (1907). 4 Kolloid-Z., 7, 113 (1910). 332 THE HYDROUS OXIDES bon. Dreyer and Douglas! found that the adsorption of agglu- tinin by bacteria reached a maximum at a certain concentration and thereafter decreased. ‘‘In short,” says Freundlich,? ‘‘by far the majority of the adsorption curves that are not entirely regular show a maximum in the adsorbed mass with increasing concentration, followed by a falling off until the adsorption is negative.? A number of cases have been reported where a change in the physical character of the adsorbent leads to a maximum in the adsorption curve. Thus Lottermoser and Rothe* observed a decrease in the adsorption of potassium iodide by silver iodide above a certain concentration of electrolyte. This was traced to a change in the structure of the silver iodide, which became denser and more granular. Freundlich and Schucht? noted the spontaneous transformation of amorphous mercuric sulfide to a crystalline form that shows a decreased power of adsorbing dyes. Wagner® showed that when salts of many of the hydrous oxides are hydrolyzed, they absord the free acid to some extent and later give it up owing to a change in the physical character of the adsorbent. While the chromic-oxide hide-powder curves are typical of adsorption curves showing a maximum, it is prob- able that the irreversible change in state which hide powder undergoes in contact with tanning liquor is in part responsible for this maximum. ‘The increasing hydrogen ion concentration with increasing concentration of tan liquor likewise contributes to the cutting down of the adsorption of chromic oxide at higher concentrations. The important thing is that the maximum in the continuous curves should not be construed as indicating the formation of definite chromium collagenates any more than any other point on the curve. At suitable points on the curve, a whole series of definite salts from monochrome to octachrome collagenate may be assumed to exist; but this does not indicate, let alone prove, their existence. 1 Proc. Roy. Soc., 82B, 185 (1910). 2 “ Kapillarchemie,”’ 246 (1922). 3 Cf. WituiAMs: Med. fr. K. Vet. Akad. Nobelinst., (2) No. 27 (1913). 4Z. physik. Chem., 62, 359 (1908). 5 Z. physik. Chem., 86, 660 (1913). 6 Monatshefte frir Chemie, 34, 95, 931 (1913). TANNING 333 Alumina Tanning.—An alumina tan bath consists of basic aluminum sulfate together with enough sodium chloride to prevent undue swelling of the skin. ‘The hydrous oxide or basic salt appears to be adsorbed less strongly than in the case of chrome tanning, and the freshly treated hide cannot be washed without swelling. Moreover, hydrous chromic oxide ages much more rapidly than hydrous alumina, and so it is necessary to keep the alumina-treated skins in the dried state for weeks or months before a satisfactory leather is obtained. Even at best, alumina-tanned leather is not so permanent as chrome-tanned leather, probably because hydrous alumina becomes crystalline on ageing and so does not afford such good protection to the hide particles as does the amorphous film of hydrous chromic oxide which never assumes the crystalline form. Iron Tanning.—Ferric salts may be employed as tanning agents, but attempts to manufacture iron-tanned leathers have not met with success. According to Procter,! a part of the diffi- culty arises from the fact that ferric oxide acts as an oxygen carrier, causing slow oxidation of the hide and consequent deteri- oration. Moreover, difficulty is encountered in neutralizing the excess sulfuric acid after tanning. When the leather is treated with a dilute alkali solution, the adsorbed hydrous oxide is displaced, and any normal or basic salt is converted into col- loidal hydrous oxide and washed out of the skin.” Jackson and How? claim to have prepared a fairly good leather by adjusting the acidity so as to give a tan liquor in which the ratio of equiv- alents of hydroxide groups to equivalents of acid radical is never less than 1:5 nor more than 1:3. After tanning, the neu- tralization is effected very gradually. Silica Tanning.—Graham*‘ pointed out in 1862 that gelatin was precipitated by colloidal silica. The precipitate was insolu- ble in water and was not decomposed by washing; in other words, the gelatin was tanned. Hough® found that purified colloidal silica is much too instable to serve as a tanning agent. As would 1 “The Principles of Leather Manufacture,” 2d ed., p. 275. 2 JeTTMAR: Cuir, 8, 74, 106 (1919). 3 J. Am. Leather Chem. Assoc., 16, 63, 139, 202, 229 (1921). 4 J. Chem. Soc., 15, 246 (1862). 5 Cuir, 8, 209, 257, 314 (1919). 334 THE HYDROUS OXIDES be expected, the sol agglomerates before it has a chance to diffuse into the hide substance. By adding a 30 per cent solution of sodium silicate to a 30 per cent solution of hydrochloric acid until the concentration of free acid is reduced to tenth normal, a bath is obtained which diffuses into the hide and deposits a protecting layer of hydrous silica. A fully tanned leather usually contains from 17 to 24 per cent of silica. One of the difficulties of the process is to prevent too great an adsorption of silica by the hide. | The most serious fault with silica-tanned leather is that it tears very easily after keeping for a few months.! This is prob- ably due to a change in the physical character of the hydrous silica on ageing. Miscellaneous Tanning, Agents.—Basic ceric chloride? can be used for a tan bath, giving a fairly good leather; but salts of bismuth have not proved satisfactory.* It is a remarkable fact that freshly precipitated finely divided sulfur is adsorbed by hide substance, giving a white leather* which does not swell when left for 24 hours in water and can be dried without losing its stability. Apostolo claims that the sulfur is not extracted by carbon bisulfide; but this is disputed by Thomas?’ who finds that sulfur is not a true tanning agent. Colloidally dispersed insoluble sulfides, silicates, oxides, and phosphates of many metals appear to act as tanning agents.°® Indeed, Procter’ reports that finely divided insoluble powders, such as ultramarine, can convert hide into leather by mere mechanical drumming. There is no doubt of the essential physical nature of the latter process. At the opposite extreme is the tanning action of chlorine and bromine but not iodine, where the change is doubtless of a purely chemical nature.’ The leather obtained with halogens is imputrescible and resists the action of cold water but not boiling water. 1THuAv: Cuir, 9, 10, 80, 102 (1921). 2 GARELLI: Collegium, 418 (1912). 3 GARELLI and Apostouo: Collegiwm, 422 (1913). 4 AposToLo: Collegium, 420 (1913). 5 J. Ind. Eng. Chem., 18, 259 (1926). 6 SomMMERHOFF: Collegium, 381 (1913). 7 Bogue’s ‘‘Colloidal Behavior,’’ 2, 718 (1924). 8 MEUNIER and SEYEWETz: Collegium, 289, 373 (1911). TANNING 335 From this survey it is obvious that the term ‘‘tanning”’ has been applied to a wide variety of processes whereby hide fiber is converted into what is called “leather.’”’ It would be more proper to speak of ‘‘leathers,”’ for the commercial article shows marked variations in properties, depending on the method of manufacture. Procter! distinguishes the following general methods of tanning: 1. By mere dehydration of the separated fibrils in such a way that they can be dried without adhesion. 2. By actual changes in the chemical nature of the fibrils, which destroy their adhesive character. 3. By coating the fibers with fine powders or precipitates or, perhaps, fatty matters, which mechanically separate them. 1 Bogue’s “Colloidal Behavior,”’ 2, 730 (1924). CHAPTER XVI MORDANTS The adsorption of many dyes by wool, silk, and cotton is so weak that they are of value to the practical dyer only when used in conjunction with mordants. ‘The term mordant (from mordre, to bite or to corrode) was first applied by the French to metallic salts which were supposed to act by biting or opening a passage into the fibers of the cloth, giving access to the color. Thus, alum was believed to be effective in fixing certain dyes, owing to the solvent or corrosive action of sulfuric acid.t Itis now known that the real mordant is the hydrous oxide and not the acid derived from the salt. In general, a mordant may be defined as any substance that is adsorbed strongly by the cloth and, in turn, adsorbs the dye strongly. In dyeing a mordanted cloth, it is the mordant rather than the fiber which adsorbs the dye in most cases. When a mordant adsorbs a dye in the absence of a fiber, the product is called a lake. The lakes employed as pigments are usually prepared in contact with what are termed lake bases, such as barium sulfate, china clay, red lead, and lead sulfate, which modify the physical properties of the lakes in some desired way. In order to appreciate the importance of mordants in the art of dyeing, one needs but to recall that the first so-called direct or substantive dye, Congo red, was not discovered until 1884. Before this date it was impossible to dye cotton with acid and basic dyes except by the use of mordants. Moreover, substan- tive dyes on cotton are in general much less fast to light and washing than are the mordant colors. A typical example of a mordant dye is alizarin, the important coloring matter of the roots of rubia tinctorium, or madder, a plant of Indian origin which was cultivated largely in France and 1 Bancrort: “Philosophy of Permanent Colors,” 1, 341 (1813); Napier: ‘‘A Manual of Dyeing,”’ 186 (1875). 336 MORDANTS 337 Holland before the synthesis of alizarin from anthracene was accomplished in 1868. If a piece of cotton is dipped into an aqueous solution of alizarin, it assumes a yellow color that is easily removed by washing with soap and water; but if the cloth is first mordanted, it is dyed a fast color: red with alumina, reddish brown with chrome, orange with tin, and purple or black with iron. By treating the fiber with the so-called sul- fonated oils before mordanting with alumina, there results the brilliant Turkey red, a color remarkable for its fastness to light and to the action of soap and water. The dyeing of Turkey red is a very ancient process having been carried out centuries ago in India, using milk as fatty matter and munjeet, the Indian madder plant. The plant itself with its earthy incrustations furnished enough alumina to give the color lake. The art spread from the East through Persia and Turkey, reaching France and England in the latter part of the eighteenth century. Wool like cotton can be dyed with madder only by the aid of mordants. The scarlet trousers of the French soldiers, introduced by Louis Philippe to encourage madder culture, and the scarlet uniform of the British soldier of Revolutionary war days were made possible by the use of the mordant alumina. Two classes of mordants are generally recognized: acid and basic or metallic. The acid mordants are the tannins, the fatty acids, albumin, hydrous silica, arsenic acid, and phosphoric acid; while the basic mordants are the hydrous oxides of the heavy metals. The most important metallic mordants are the hydrous oxides of chromium, aluminum, iron, tin and copper, in the order named. Alumina was the first mordant used, and years ago, alumina and stannic oxide were the most important because people were inter- ested in getting the bright colors which these mordants yield. As might be expected, the mordanting action of nearly all the possible oxides has been investigated. Liebermann! reports that the oxides of yttrium, beryllium, thorium, cerium, zirconium, and copper hold dyes most tenaciously; while the oxides of zinc, cadmium, manganese, antimony, bismuth, lead, tin, and thal- lium are much less satisfactory; and the oxides of iron, aluminum, chromium, and uranium occupy an intermediate position. Such 1 Ber., 35, 1493 (1902). 338 THE HYDROUS OXIDES a classification is not generally applicable; thus Wingraf! finds zirconia to be a stronger mordant for certain dyes than alumina; while the reverse is true in other cases. In any event, oxides of metals other than aluminum, chromium, iron, and tin are used only in special cases. For example, titania is reported to be a particularly good mordant to use with leather.2, The more important mordants will be taken up in some detail. ALUMINA If an aluminum salt which we shall represent by AIA; is dis- solved in water, hydrolysis takes place in accord with the fol- lowing equation: 2AlA; + zH.0 @ Al.O;:2H20 + 6HA The reaction proceeds further to the right, the more dilute the solution, the weaker the acid formed, and the higher the tempera- ture. Whether the insoluble hydrous oxide precipitates out on heating or remains in colloidal solution depends on the concen- tration of the solution and the precipitating power of the anion. Crum prepared a positive sol of hydrous alumina by hydrolyzing the acetate and boiling off the excess acetic acid; and Neidle‘* obtained a sol by dialysis of a solution of aluminum chloride at elevated temperatures. A sol cannot be prepared by dialysis of the sulfate on account of the high precipitating power of sulfate ion. The amount of hydrous oxide formed in a given case is increased by removing hydrogen ion with alkali; but the range of hydrogen ion concentration in which the oxide precipitates is much wider in the case of salts with strongly adsorbed multi- valent ions, such as sulfate, than with salts of univalent ions. While the non-existence of definite basic salts of aluminum has not been established with certainty, it is probable that no definite basic compounds are formed by the hydrolysis of aluminum salts either alone or on the addition of alkali. Certainly, the vast majority of the alleged basic acetates described by Crum and of the basic nitrates, chlorides, sulfates, acetates, and sulfoacetates 1 Farber-Zig., 25, 277 (1914). 2 BaRNES: J. Soc. Dyers Colourists, 35, 59 (1919). 3 Tiebig’s Ann. Chem., 89, 168 (1854). 4 J. Am. Chem. Soc., 39, 71 (1917). MORDANTS . 0339 formulated by Liechti and Suida! are wholly without experi- mental foundation. By adding alkali to aluminum sulfate, a phase separates below pH = 5.5 having approximately the com- position 5A12.03 - 3803;? but the ease with which the sulfate can be displaced by a wide variety of inorganic and dye cations argues against its being a definite compound.’ In view of the fact that aluminum salts hydrolyze of them- selves, one should expect the hydrolysis in a given case to proceed further in the presence of a fiber which adsorbs hydrous aluminum oxide. This is actually the case, as will be shown in the sub- sequent paragraphs. Mordanting of Wool.—When wool is treated with solutions of aluminum sulfate, Al.(SO4)3- 18H2O, less than 5 per cent on the wool, the bath is exhausted completely, all the alumina and the sulfuric acid being adsorbed.* At higher salt concentrations, more and more remains in the bath. Knecht? believes that both hydrous oxides and true basic aluminum salts are deposited by the mordanting process, since the spent liquors on dyeing well-washed wool with alizarin always possess an acid reaction. This evidence of basic salt formation is inconclusive, since adsorbed sulfuric acid would be displaced quite as readily as acid in definite chemical combination. Ftirstenhagen and Apple- yard® give data to show that the amount of sulfate taken up by wool remains constant when the fiber is mordanted from potash alum solutions containing 10 to 20 per cent of alum referred to the wool. According to Havrez’?’ and to von Georgievics,°® the amount of alumina taken up by wool from relatively dilute potash alum solutions is greater than the amount of sulfuric acid; but with increasing salt concentrations, the amount of sulfuric 1J. Soc. Chem. Ind., 2, 537 (1883); cf. also ScotumBERGER: Bull. soc. chim., (3) 18, 41 (1895); BOrrincER: Liebig’s Ann. Chem., 244, 224 (1888). 2 Miuuer: U. S. Pub. Health Repts., 38, 1995 (1923); WiLiiamson: J. Phys. Chem., 27, 284 (1923). 3 See p. 379. 4ZLiecuti and Scuwitzer: Mitt. techn. Gewerbe-Museums in Wien, Sek- tion fiir Fdrberet, 3, 47 (1886). 6’ KNECHT, Rawson, and LowEntTHAL: ‘A Manual of Dyeing,’ 237 (1916). 6 J. Soc. Dyers Colourists, 105 (1888). 7 Chem. Zentr., 696 (1874). 8 J. Soc. Chem. Ind., 14, 653 (1895). 340 THE HYDROUS OXIDES acid taken up increases relatively to the alumina until at 24 per — cent alum referred to the wool, the alumina and acid are taken up in the same relative amounts as they occur in aluminum sul- fate.t Recently, Paddon,? in Bancroft’s laboratory, determined the amounts of both alumina and sulfuric acid removed from potash alum baths at different concentrations. In these experiments, — 2-cram samples of well-washed wool were boiled for 1 hour in the alum solutions, after which the wool was removed and aliquot portions of the several baths were analyzed in the usual gravimet- ric manner for aluminum and sulfate. The adsorption of alumina and sulfuric acid is given in Tables XX VI and X XVII and shown graphically in Figs. 23 and 24, respectively. Both curves are TABLE XX VI.—ADSORPTION OF ALUMINA BY WooL Per cent potash Seg onan oe repre Milligram mol : tration Al.Os, tration Al,Os, alum on weight Ree ae Al,O3 adsorbed milligram mols | milligram mols of wool ; 3 per gram of wool per liter per liter 5.12 0.4388 0.137 0.377 10.25 0.881 0.395 0.607 15.37 1.319 0.842 0.597 20.50 1.761 1.362 0.500 25.62 2.200 1.863 0.421 30.75 2.645 — 2.403 0.303 TABLE XX VII.—ApDSoORPTION OF SuLFURIC ACID By WooL Per cent potash Organ ae Hag act n Milligram mol : tration SOs, tration SOs, alum on weight pe he SO; adsorbed milligram mols | milligram mols of wool : : per gram of wool per liter per liter 5.12 1.730 1.120 0.068 10.25 3.460 2.550 0.113 15.37 5.185 4.075 0.139 20.50 6.915 5.705 0.153 29).08 8.650 7.305 0.168 30.75 10.375 9.055 0.165 1Cf. THeNARD and Roarp: Ann. Chim., 74, 267 (1810). 2 J. Phys. Chem., 26, 790 (1922). MORDANTS 341 Milligram Mots Adsorbed per Gram of Wool Concentration of Als0,, milligram mols per liter Fig. 23.—Adsorption of hydrous alumina by wool. Milligram Mols Adsorbed per Gram of Wool 0 4 Q | fs S 4 5 6 T tS) 9 Concentration of SO3 milligram mols per liter Fic. 24.—Adsorption of sulfate by wool. 342 THE HYDROUS OXIDES smooth and free from sudden breaks, indicating that the mor- danting of wool with potash alum does not lead to the formation of definite chemical compounds on the fiber; but that the proc- | ess is strictly an adsorption phenomenon, involving both alumina and sulfuric acid. It is probable that the acid is adsorbed both by the alumina and by the wool. The alumina curve passes through a maximum due to the pre- cipitation of considerable alumina on boiling the solutions of higher concentrations, thereby cutting down the concentration of alumina so far as the wool is concerned. The adsorption of SO; follows a continuous course, approximating saturation in the neighborhood of 20 per cent of alum on the weight of the wool. Above this concentration, the amount adsorbed is necessarily approximately constant; hence, this should not be construed as indicating the adsorption of a definite basic salt on the fiber. The purpose of the mordant is to have something on the fiber — which will adsorb and hold the coloring matter. It is, therefore, important to have the mordant taken up under such conditions that it will be held most tenaciously by the cloth, have the maximum transparency, and adsorb the greatest amount of dye. As Bancroft? points out, one would not ordinarily expect to obtain the mordant in such a form that it will satisfy all these require- ments to the maximum degree, simultaneously; but the aim should be to get the mordant in the form which is most generally useful. One objection to alum or aluminum sulfate for mordant- ing wool is that sulfate ion coagulates alumina too readily, thereby precipitating perceptible amounts of the hydrous oxide in the bath or superficially on the wool in a form that does not hold well. This is particularly true with more concentrated baths, as noted by Havrez? and by Paddon.* The former recommends a bath containing less than 10 per cent alum referred to the amount of wool, otherwise the mordant washes off readily and the wool is not dyed deeply when treated with the coloring matter. As one would expect, the so-called basic solutions of aluminum sulfate cannot be used at all with wool, since the precipitation of 1J. Phys. Chem., 18, 399 (1914). 2 Dinglers polytech. J., 205, 491 (1872). 3 J. Phys. Chem., 26, 791 (1922). MORDANTS 343 the hydrous oxide is altogether too rapid. Liechti and Suida! claim that alum does not give as good a mordant as aluminum sulfate. This may be due to one or more of the following causes: the increase in the hydrolysis of aluminum sulfate by the pres- ence of sodium sulfate;? the detrimental precipitating action of the excess sulfate in alum; and the increasing of the relative amount of sulfate adsorbed. Addition of sulfuric acid to alum causes the mordant to penetrate more thoroughly and to be fixed better than when the normal sulfate is used.* This is because the cutting down of the hydrolysis by the increased acidity is more pronounced than the agglomerating action of the increased con- centration of sulfate. The rapid precipitation of hydrous oxide in a loose condition on the surface of the cloth can be obviated by using an aluminum salt of a weak acid, such as aluminum oxalate, tartrate, or lactate. Although these salts hydrolyze more readily than sulfate, the resulting hydrous oxide is held in a more highly peptized state. Accordingly, the mordanting is deeper and more uniform from a solution of aluminum tartrate or oxalate; or from a solution of aluminum sulfate to which a mordanting assistant such as cream of tartar, tartaric acid, or oxalic acid is added. Some observa- tions of Miller* are of interest in this connection: Portions of a solution 0.005 M with respect to aluminum chloride and 0.0075 M with respect to potassium oxalate were treated with gradually increasing amounts of alkali. No precipitate was formed until the pH value of the solution reached 8.8. Below this value, a slightly opalescent sol was formed but no floc. In striking con- trast to this, a 0.0025 M solution of potash alum formed a good floc at as low a pH value as 4.3 and up to 8.9. Obviously, the tendency of hydrous alumina to agglomerate under these condi- tions is much less in the presence of oxalate than of sulfate. It is probable that the behavior of tartrate is similar to that of oxalate, since the mordant obtained in the presence of the former is even more satisfactory than in the presence of the latter. 1 J. Soc. Chem. Ind., 5, 526 (1886). 2 LigcutTi and Supa: J. Soc. Chem. Ind., 2, 587 (1888). 3 LiecuTi and Scuwitzer: J. Soc. Dyers Colourists, 161 (1886). 4U. 8, Pub. Health Repts., 40, 351 (1925). 344 THE HYDROUS OXIDES The beneficial influence of organic acids on the mordanting process has received widely diversified interpretations from time to time. Thus, Knecht, Rawson, and Lowenthal! claim that the aluminum salts of tartaric and oxalic acids possess a certain resist- ance to the dissociating action of wool; this is, of course, inaccu- rate, as the salts of the weaker acids hydrolyze more readily than sulfate. Beech? says that the addition of a little oxalic acid, cream of tartar, or tartaric acid to the mordanting bath helps in the decomposition of the metallic salt by the wool fiber; but this seems improbable, as the addition of an acid cuts down the hydrolysis. Herzfeld* offers no explanation of the phenomenon, but he recognizes clearly that the loose and uneven character of the mordant obtained with aluminum sulfate alone is due to the rapidity with which the salt decomposes; and that the presence of cream of tartar, oxalic acid, or lactic acid causes. the precipitation to take place more slowly and regularly, thereby giving a more satisfactory mordant. Mordanting of Silk.—Silk adsorbs hydrous alumina somewhat less strongly than wool and must, therefore, be mordanted from slightly more basic solutions. The solutions employed are alu- minum sulfate,* alum,* and the sulfate-acetate and nitrate-acetate mixtures. Hermann® has made observations which leave little room to doubt but that the real mordants are the hydrous oxides, at least in the case of silk. In these experiments, both raw and boiled-off’ silk were treated with various mordanting baths at 30°, and the mordanted fiber was analyzed for both metallic oxide and acid radical. The results have been collected in Table XXVIII. Hermann looks upon mordanting as a catalytic process in which the fiber decomposes the mordanting salts catalytically, giving hydrous oxides that become fixed on the fiber and acids 1“ A Manual of Dyeing,’ 236 (1910). 2 “The Principles and Practice of Wool Dyeing,”’ 71 (1902). 3 “Das Farben und Bleichen der Textilfasern,’’ 58 (1900). 4 GANSwINDT: “Theorie und Praxis der modernen FAarberei,”’ 2, 18 (1903). 5 KNECHT, Rawson, and LéwentHAL: ‘‘A Manual of Dyeing,”’ 238 (1916). 6 J. Soc. Chem. Ind., 28, 1148 (1904). 7 Immersed in a good neutral Marseilles or olein soap solution at 90 to 95°, in order to remove the silk gum or pericine from the fibers. MORDANTS 345 TaBLE XXVIII Ratio of adsorbed Mordanting solution SEM: of oxide to adsorbed silk : : acid radical BEBUNIG CUIOTIOG. 2. .4.0.5... 06000). Raw LoorenOsel Cl MOR IeTCH OTIGG. 2.0... ce ek eee Boiled off 143 SnO2:1 Cl Porricsiuate (Dasic).. o6....00....-- Raw 111 Fe,03:1 SOs; Merme suliate (DASIC)..../....5...... Boiled off 91 Fe.,O3:1 SO3 comic GhlOmie..c..5..-...-....:| Raw 40 Cr203:1 Cl Piivoiiiercnligniie..................-.| . Boiled off 44 Cr.O3:1 Cl PUR ChtALe. ... ss... ose eee Raw Al.O; only adsorbed PEN BOCLALC.. 25... ence se ses Boiled off Al.O3 only adsorbed that remain in the bath. While the hydrolysis of the mordanting salts is increased, owing to strong adsorption of the hydrous oxides by the fiber, the process is not catalytic, as a given amount of fiber can increase the decomposition of only a limited amount of salt, and the mordanted fiber is not in the same condition after the process as before. The mordanting of silk may be carried out satisfactorily at 15 to 20°. At as low a temperature as 0 to 5°, the mordanting salts do not penetrate the fiber well, and the adsorption of the hydrous oxides takes place slowly and irregularly. ! Mordanting of Cotton.—It has been recognized for a long time that normal aluminum sulfate and alum cannot be used as a mordant for cotton.? This is because the cotton adsorbs hydrous alumina much less strongly than wool or silk and so does not decompose solutions which are distinctly acid. If the acidity of the alum solutions is reduced by the addition of sodium car- bonate, they can be used to mordant cotton. Liechti and Suida’® showed that the amount of alumina fixed is greater the more basic the mordanting solutions. Since cotton adsorbs hydrous alumina less strongly than wool, the mordant is fixed less strongly by 1 Hermann: J. Soc. Chem. Ind., 22, 623 (1903); 28, 57 (1904). 2 Cf. Bancrort: “Philosophy of Permanent Colors,” 1, 357; 2, 148, 242 (1813). 3 J. Soc. Chem. Ind., 2, 538 (1883); cf. Kmrrscuera and Utz: Mitt. techn. Gerwerbe-Museums in Wien, Sektion fiir Fdrberet, 3, 110 (1886). 346 THE HYDROUS OXIDES cotton than by wool;! accordingly, we should expect the relatively large amounts of mordant taken up from highly basic solutions to rub off readily. Recently, Tingle* claimed that hydrous alumina is adsorbed neither from aluminum sulfate nor basic aluminum sulfate solutions. Hisresults with aluminum sulfate confirm those of everybody else, but the observations with basic aluminum sulfate cannot be generally true, since such solutions have been used in mordanting cotton without a fixing agent. Aluminum acetate appears to be the best mordanting bath for cotton. Fifty years ago, Napier* pointed out the advantages of acetate over sulfate: First, the acetic acid is not so hurtful in its action upon the vegetable coloring matters; second, it holds the alumina with much less force than sulfuric acid, and consequently yields it much more freely to the cloth; and third, being volatile, a great portion of the acid flies off during the process of drying. Another way of putting it is that aluminum acetate hydrolyzes readily, giving the hydrous oxide in the form of a finely divided sol which can penetrate into the fiber and be adsorbed. The use of aluminum formate® and aluminum lactate® in place of aluminum acetate has been suggested; but the principle is the same with all salts of weak organic acids. Cotton is not mordanted from a solution of ‘‘sodium aluminate,” but the latter is used to pad on hydrous alumina in calico printing.’ This is accomplished by precipitating hydrous alumina on the cloth by adding ammonium chloride to the aluminate bath. Cotton may be mordanted with alumina by first treating the fiber with a substance like tannin which is adsorbed strongly by the fiber and, in turn, adsorbs hydrous alumina strongly. This will be referred to again in the section on fixing agents. 1BancrorT: J. Phys. Chem., 26, 501 (1922). 2 J. Ind. Eng. Chem., 14, 198 (1922). 3 KNECHT, Rawson, and LOwrentuat: “A Manual of Dyeing,’’ 233 (1916). 4“ A Manual of Dyeing,” 121 (1875); cf. Bancrorr: ‘Philosophy of Permanent Colors,” 1, 365 (1813). > ScowaLBE: Kolloid-Z., 5, 129 (1907). 6 BOEHRINGER and Sons: Z. Fdrben-Ind., 9, 237, 253 (1910). 7 GanswinpT: “Theorie und Praxis der modernern Fiarberei,” 2, 212 (1903). MORDANTS 347 It is interesting to note that mercerized cotton adsorbs sub- stantive dyes! and takes up basic mordants? more strongly than ordinary cotton does. ‘This is not because the mercerized cotton is a definite chemical compound between cotton and sodium hydroxide*® as Ganswindt* assumes; but is probably due to the retention of sodium hydroxide in the channel of the cotton fiber or to a change in structure of the cotton as a result of the merceri- zation process. CHROME Mordanting of Wool.—Chrome is by far the most important mordant used with wool. More than twenty years ago Gans- windt® claimed that 98 per cent of all the mordanting of wool is done with chromic oxide; and Matthews® stated recently that ‘‘chrome mordant is used for dyeing practically all of the alizarin, mordant, and acid mordant or after-chromed dyes; it is also the principal mordant used in conjunction with the natural logwoods.”’ It is interesting to note that the mordanting bath most gener- ally used is an acid solution of bichromate instead of a chromic salt.” Before the war, the readily crystallized potassium bichro- mate was commonly used, but the demand for a cheaper product led to the development of a pure crystalline form of sodium bichromate which has displaced the potassium salt for mordant- ing purposes.® From the bichromate solution, wool adsorbs chromic acid which is subsequently reduced to chromic oxide, the real mor- dant. Chromic acid is not held very strongly® by the fiber and practically all of it can be removed by washing.!®° Wool itself 1 Marttruews: “Application of Dyestuffs,’ 165, 278 (1920). 2 SCHAPOSCHNIKOFF and Minaserr: Z. Fdrben-Ind., 3, 165 (1904); 4, 81 (1905). 3 LeicHTon: J. Phys. Chem., 20, 188 (1916). 4 “Theorie und Praxis der modernen Farberei,’”’ 2, 215 (1903). 5 “Theorie und Praxis der modernen Farberei,’’ 2, 69 (1903). 6 “ Application of Dyestuffs,’ 334 (1920). 7 Knecut, Rawson, and LOwentuat: ‘‘A Manual of Dyeing,” 255 (1916). 8 MatrueEws: ‘Application of Dyestuffs,’ 344 (1920). 9 Liecutt and Hume : J. Soc. Chem. Ind., 12, 244 (1893). 10 BancRoFT: J. Phys. Chem., 26, 737 (1922); cf., however, WHITELEY: J, Soc. Chem. Ind., 6, 131 (1887), 348 THE HYDROUS OXIDES has been shown to reduce chromic acid,! but this involves more or less waste,” so that a reducing agent is usually added either by itself or in the form of a dye, such as logwood? or alizarin;* and under these conditions, the wool is not attacked appreci- ably. Chromic acid mordants wool more strongly than either neutral chromate or bichromate,’ so that, in practice, a suitable amount of acid is added to the bichromate bath. Within limits, increasing the acid concentration increases the amount of chromic acid adsorbed.’ This is less marked with sulfuric acid than with either hydrochloric or nitric acid, probably because sulfuric acid is more strongly adsorbed by wool than hydrochloric or nitric acid and so is more effective in cutting down the adsorption of chromic acid. The importance of sulfate ion is further indicated by the fact that a mixture of sodium chloride and sulfuric acid behaves like sulfuric acid and not like hydrochloric. The presence of sulfuric acid is more effective than an equivalent amount of either hydrochloric or nitric acid in causing the oxidation of wool by chromic acid. Since the oxidizing power of chromic acid is greater the higher the concentration of acid, and since sulfuric acid is adsorbed by wool more strongly than hydrochloric or nitric acid, Bancroft’ attributes the greater effect of the former to higher acid concentration at the surface of the wool. A bichromate bath acidified with sulfuric acid is objectionable, not only because the reduction of chromic acid takes place at the expense of the wool, but because some chromic oxide remains in the mordant and oxidizes such colors as alizarin blue, aliz- arin yellow, etc., producing weak shades that may be undesir- able.2 As a matter of fact, the more customary thing is to use an organic acid or acid salt such as cream of tartar, tartaric acid, oxalic acid, formic acid,® and lactic acid.‘° As these 1Lrecuti and Humme.: J. Soc. Chem. Ind., 12, 244 (1893). 2 DurFEE: Am. Dyestuff Rep., 9, No. 10, Tech. Sec. 20-23 (1921). 3 Marruews: “Application of Dyestuffs,’ 477 (1920). 4 Liecuti and Humme.: J. Soc. Chem. Ind., 12, 244, 246 (1893). 5 HumMMEL and Garpner: J. Soc. Chem. Ind., 14, 452 (1895). 6 BancrortT: J. Phys. Chem., 26, 743 (1922). 7 J. Phys. Chem., 26, 744 (1922). 8 BeEcH: “The Principles and Practice of Wool Dyeing,’’ 116 (1902). 9 Kappr: Z. Fdrben-Ind., 4, 159 (1905); WuirraKeEr: “ Dyeing with Coal Tar Dyestuffs,’ 50 (1919). 10 KNECHT, Rawson, and LOwWENTHAL: ‘‘A Manual of Dyeing,” 173, 256 (1916). MORDANTS 349 so-called assistants are oxidized by chromic acid, it is probable that there is little, if any, oxidation of the wool in their presence. Moreover, they bring about a uniform deposit of the mordant in a form highly satisfactory for receiving the dye.! Solutions of chromium salts undergo hydrolysis to a greater or lesser degree, depending on the basicity of the solutions, the concentration, and the temperature. Wool adsorbs hydrous chromic oxide from such solutions in the same manner as hydrous aluminum oxide is adsorbed from aluminum salts. If chrome alum is used, the fiber takes up sulfuric acid as well as the hydrous oxide. Liechti and Hummel claim that a part of the acid is taken up as a basic salt having the formula 3Cr.03-2SQ3. Their data do not justify this conclusion, but the absence of a basic salt has not been proved. Williamson’ obtained a gel of approximately constant composition by precipitating chrome alum below a certain pH value which was not determined. The amorphous mass was assigned the formula 7Cr.03:4SO3. It will be recalled that Williamson* and Miller® obtained a gel of approxi- mately constant composition, 5Al,03:38O03, by adding alkali to alum below pH = 5.5. For reasons already given,® I do not consider the alumina sulfuric acid gel to be a definite basic salt and the same applies to the chrome sulfuric acid gel. However, at least one definite crystalline basic sulfate of the formula [Cr(OH)2(H.2O)4]2°SO4 has been prepared;’ so the formation of a basic salt on the fiber must be regarded as a possibility. It is claimed that chrome alum cannot be used for a mordant- ing bath, because the mordant is not adsorbed evenly and the subsequent dyeing is uneven. Since a good mordant can be obtained with aluminum alum, it would appear that the difficulty with chrome alum could be corrected by suitable adjustment of the temperature or of other conditions of mordanting. The addition of cream of tartar, oxalic acid, or tartaric acid to the 1KneEcHT, Rawson, and LOwentHau: “A Manual of Dyeing,” 256 (1916); Begcu: “The Principles and Practice of Wool Dyeing,” 117 (1902). 2 Liecuti and ScuwirzeEr: J. Soc. Chem. Ind., 4, 586 (1885). 3 J. Phys. Chem., 27, 384 (1923). 4 J. Phys. Chem., 27, 280 (1923). 5 U.S. Pub. Health Repts., 38, 1995 (1923). 6 Cf., pages 339, 379. 7 Werner: Ber., 41, 3447 (1909). 300 THE HYDROUS OXIDES alum bath gives a satisfactory mordant as does chromium oxalate! or chromium tartrate but not chromium acetate or chromium fluoride.? Liechti and Hummel* observed increased mordanting with increasing concentration of chrome alum, just as would be expected. They also claimed to get an increased amount of chro- mium taken up by increasing the sulfuric acid content of the alum bath; but this is improbable, if not impossible, unless the heating is conducted in such a manner that a precipitate forms in the bath and is padded on the fibers. The reported increase in adsorption with increasing sulfuric acid content is contradicted by the further observation of Liechti and Hummel that the bath is exhausted less completely the greater the concentration of sulfuric acid. ‘Wool is mordanted very slightly from solutions of chromic chloride or chromic nitrate,4 probably because the degree of hydrolysis is less and the peptizing action of these solutions for hydrous chromic oxide is too great to yield the mordant to the fiber. If this be true, the addition of a suitable amount of soda to chromic chloride HURUEOS should give a satisfactory mordant- ing bath. Mordanting of Silk.—Silk adsorbs chromic oxide less strongly than wool.’ In practice, it is mordanted from a bath of chrome alum® or chromic chloride but not from bichromate.’ To pre- ' serve the luster of silk, Whittaker® recommends mordanting the silk overnight in a cold bath of chromic chloride, followed by treating with sodium silicate, which fixes the mordant on the fiber. Mordanting of Cotton.—Cotton adsorbs hydrous chromic oxide very much less readily than either wool or silk, as evidenced by the observation that no mordanting whatsoever results on heating cotton with a 10 per cent chrome alum solution. Apparently, no completely satisfactory chrome mordant for dyeing cotton, 1 Tag iant: Color Trade J., 11, 158 (1922); Textile Colorist, 44, 650 (1922). — 2 Lrecuti and HuMMEL: i sun Chem. Ind., 13, 356 (LSE 3 J. Soc. Chem. Ind., 18, 222, 356 (1894). 4 Liecuti and Hosa J. foe Chem. Ind., 18, 224 (1894). 6’ GANSWINDT: ‘Theorie und Praxis der modernen Farberei,”’ 2, 19 (1903). 6 Liecuti and HummgE.: J. Soc. Chem. Ind., 13, 223 (1894). 7 KNECHT, Rawson, and LOwentnuat: ‘‘A Manual of Dyeing,’ 258 (1916). 8 “Dyeing with Coal Tar Dyestuffs,’ 50 (1919). MORDANTS 351 especially cotton yarns, has been found.! The most satisfactory bath is the colloidal solution of hydrous chromic oxide in alkali, the so-called alkali chromate bath.? This cannot be used for yarns on account of the caustic action on the hands of the work- men; nor can it be used on oiled material, since the oil would be stripped from the fiber. A bath of chromic acetate is fairly successful, as the acetic acid may be removed by heating. Iron MorpbDANTS Mordanting of Wool.—At one time, ferrous sulfate was widely used for mordanting wool; but it has been largely replaced by chrome mordants. The iron mordant is still of importance in dyeing logwood blacks, since the latter on chrome mordant are likely to turn green on exposure to light. Moreover, it is claimed that cloth mordanted with copperas posesses a ‘‘kinder’”’ and softer handle than cloth mordanted with chrome. In general, iron mordants tend to “‘sadden”’ or darken the shade of most dyes, and they are, therefore, used chiefly for dark colors, espe- cially browns and blacks. A copperas black may be obtained either by mordanting before dyeing or mordanting after dyeing. The latter process, which is usually. employed, consists essentially in boiling the wool in a decoction of dyewoods for a time and then adding copperas directly to the bath. When the fiber is mordanted before dyeing, it is necessary to add comparatively large quantities of tartar or oxalic acid to prevent unequal precipitation of the oxide of iron on the fiber. Before placing the mordanted cloth in the dye bath, better results are obtained by allowing it to le for several hours in the air, whereby hydrous ferrous oxide is oxidized more or less completely to the ferric state. From this, it would appear either that hydrous ferric oxide is a better adsorbent than fer- rous oxide or that the oxidation of the mordant following dyeing may have a detrimental effect on the final product. Mordanting of Silk.—Iron salts are quite extensively used in mordanting silk for dyeing black, especially with logwood. Alumina and tin mordants are of minor importance and chrome 1 KNECHT, RAwSsoN, and LowenrHan: “A Manual of Dyeing,” 252 (1916). 2 Korncuuin: Dinglers polytech. J., 254, 132 (1884). 302 THE HYDROUS OXIDES is seldom used as a mordant for logwood; nor is logwood used to produce any color on silk other than black. For the dyeing of silk, mordants are applied in sufficient amount not only to take up the dye but to add appreciably to the weight of the silk. Raw silks adsorb the hydrous oxide fairly strongly; but it is customary to impregnate the fiber with tannin before putting it in the iron bath which is usually ferrous acetate. By repeated treatment in the tannin and salt baths, the weight of the silk fiber may be increased as much as 400 per cent. If a ferric salt such as basic ferric sulfate is employed, the fiber is first mordanted with the hydrous oxide which is subsequently ‘‘fixed” in a tannin bath. While raw silk adsorbs and holds the hydrous iron oxides fairly strongly, boiled-off silk possesses but a slight adsorption capacity for the mordant. The latter is, therefore, dipped in the iron liquor and subsequently put into a boiling soap solution containing olein soap and a little soda, which precipitates hydrous ferric oxide on the fiber in an aged condition. This operation may be repeated several times according to the amount of weighting desired. Mordanting of Cotton.—Cotton shows a much weaker adsorp- tion for hydrous ferric oxide than either wool or raw silk. It is, therefore, mordanted by a process similar to that employed with boiled-off silk, namely by saturating in a solution of basic ferric sulfate followed by treating with lime water or soda solution, which precipitates the hydrous oxide in the cloth. If ferrous sulfate is employed, the fiber is first mordanted with tannin, which adsorbs the hydrous oxide strongly; and any sulfate adsorbed is subsequently removed by washing with lime water. After mordanting, the adsorbed hydrous oxide is allowed to oxidize in the air before placing in the dye bath. Tin Morpants! Mordanting of Wool.—Although wool is seldom mordanted with tin mordant, when this is done, the bath consists of stan- nous chloride in conjunction with oxalic acid or tartaric acid. Considerably more acid is said to be taken up from stannic salt baths than from stannous salt baths, which accounts for the use 1 Cf. p. 210, MORDANTS 353 of the latter in practice. As in the case of alumina mordanting, tin salts require the presence of an organic acid to prevent rapid and uneven deposition of the hydrous oxide on the fiber. Stan- nous tartrate and stannic tartrate alone are said to be unsatis- factory; but it is possible that the addition of an excess of either tartaric acid or oxalic would made a good mordanting bath if there were any point in avoiding the use of chloride. The hydrous oxide of tin is sometimes “‘fixed”’ with alum. Mordanting and Weighting of Silk.—The most important use of tin salts in the dyeing industry is in the mordanting and weight- ing of silk. For this purpose, stannic chloride is the salt gener- ally employed. ‘The cloth is first steeped in a solution of this salt, and after rinsing, is put into a bath of sodium phosphate and subsequently into one of sodium silicate. In order to give the silk the desired weight,” the process must be repeated several times. . If the silk is weighted excessively by the tin-phosphate-silicate process, serious faults may develop in the goods. Thus, heavily weighted silk frequently becomes quite tender when exposed even for a short time to direct sunlight.* Moreover, reddish- colored tender spots often appear in pieces, after storing. Gne- hm, Roth, and Thomann’ first attributed the formation of these tender spots to the action of perspiration; but this cannot be true, as unused goods frequently show the damaged spots. Sis- ley® pointed out that the only constituent of perspiration which has an injurious effect is the salt; and Meister® showed that the deterioration of the silk is due to active chlorine produced by the catalytic action of copper which is always present in small quan- tities as a result of careless handling during spinning and weaving. As a preventive, Knecht’ suggest padding the goods in a very weak solution of ammonium thiocyanate; but this is not infal- 1HEERMANN: J. Soc. Dyers Colourists, 1903-1906; NruHaAvs: Knecht, Rawson, and Léwenthal’s “‘A Manual of Dyeing,” 279 (1916). 2 GNEHM and BarnzicER: J. Soc. Dyers Colourists, 40 (1897). 3 KNECHT, Rawson, and LOwentHAL: “A Manual of Dyeing,” 279 (1916). 4 J. Soc. Dyers Colourists, 256 (1902). 5 J. Soc. Dyers Colourists, 276 (1902). 6 J. Soc. Dyers Colourists, 192 (1905). 354 THE HYDROUS OXIDES lible. The use of thiourea and its salts has been patented for the same purpose. ! Since silk adsorbs hydrous stannic oxide, leaving most of the hydrochloric acid in the bath, the latter becomes strongly acid by continued use. To keep the bath in good condition, stannic chloride must be replaced and the excess hydrochloric acid neu- tralized with ammonia from time to time. After the ammonium chloride content of the liquor becomes too high for satisfactory mordanting, a fresh bath must be employed. Mordanting of Cotton.—Stannic salts are sometimes used to mordant cotton; but on account of the usual weak adsorption of cotton for the hydrous oxides, the fiber must first be mordanted with tannin. When sodium stannate is used, the cloth is first impregnated with a solution of the salt and is then passed through a very dilute solution of sulfuric acid or of aluminum sulfate. Hydrous stannic oxide or, if an aluminum salt is employed, a mixture of the hydrous oxides of tin and aluminum are precipi- tated and constitute the mordant. TANNIN Having considered the most important basic mordants, it seems advisable to point out the essential characteristics of a typical acid mordant. The class of substances known as the tannins, to which tannic acid belongs, is seldom employed in mordanting wool but finds its chief use in mordanting cotton and linen, in “‘fixing”’ the hydrous oxide mordants on cotton, and in weighting silk with hydrous ferric oxide, as noted in an earlier paragraph. Both wool and cotton adsorb tannin from its colloidal solution in water, the amounts taken up varying continuously with the concentration of the sol, as shown by the curves in Fig. 25 con- structed from the data of Pelet-Jolivet? on the adsorption by wool and of Sanin® on the adsorption by cotton. The adsorption of tannin by wool is not very marked, especially at ordinary temperatures; but it increases with the temperature; 1J. Soc. Dyers Colourists, 51 (1907). 2“TDie Theorie des Farbeprozesses,’’ 79 (1910). 3 Kolloid-Z., 10, 82 (1912). MORDANTS 395 on the other hand, the adsorption by cotton apparently decreases with increasing temperature of the bath.!_ If mixed cotton goods containing wool are mordanted at ordinary temperature, the cot- ton only is mordanted to any appreciable extent. Since tannin is an acid mordant, one might expect the adsorp- tion to be reduced in alkaline solution and increased in acid solution. As a matter of fact, the adsorption of tannin is cut down almost to zero in the presence of alkali; and acetic acid increases the adsorption? which, however, passes through a Concentration of Tannin, grams per !00 cc. in Mordanting Cotton Adsorption, grams Tannin per gram Fiber 0.05 010 015 020 025 030 035 040 OAS Concentration of Tannin, grams per 100 ce. Fig. 25.—Adsorption of tannin by wool and cotton. maximum at high concentration.* Sulfuric acid, on the other hand, cuts down the adsorption, and hydrochloric acid has little effect. This behavior with different acids is probably due to the difference in the adsorption of the acids by cotton. We know, for example, that sulfuric acid is adsorbed by cotton more strongly than hydrochloric,* which would account for the adsorption of tannin being cut down more by the former than by the latter. Different salts added to the bath all seem to increase 1Knecut and Kersuaw: J. Soc. Dyers Colourists, 40 (1892); Gan- SwINDT: ‘Theorie und Praxis der modernen Fiarberei,’’ 2, 216 (1903). 2 Knecut, Rawson, and Léwentuau: “A Manual of Dyeing,” 1, 188 (1916). 3 DreaPeER: “The Chemistry and Physics of Dyeing,’ 161 (1906). 4LeicHtTon: J. Phys. Chem., 20, 188 (1916). 306 THE HYDROUS OXIDES the adsorption of tannin, possibly because they decrease the stability of the sol. Although tannin is adsorbed quite strongly by cotton, it must be ‘‘fixed’’? on the fiber before the dyeing process. The best fixing agents are antimony salts; but salts of tin, aluminum, and iron are used in special cases. Frxinc AGENTS Whenever a mordant is not fixed sufficiently strongly by a fiber, it is necessary to add a so-called fixing agent to bring about the desired results. For example, sodium phosphate is used for fixing alumina and tin; sodium arsenate, soap, and tannin for iron; sodium silicate and tannin for chrome and tin; salts of antimony, tin, and aluminum for tannin; etc. In other words, arsenates, silicates, phosphates, fatty acid salts, and tannin are used as fixing agents for the basic or metallic mordants while the latter are used for fixing the acid mordants, tannin, and the fatty acid compounds. ‘The question arises as to whether the fixing process consists in the formation of definite chemical compounds, such as antimony tannate, iron arsenate, tin phosphate, etc., as is generally assumed, or whether the fixed mordants are mix- tures of indefinite composition. The latter view seems much the more reasonable in the light of the evidence. For example, it is known that precipitated hydrous ferric oxide is peptized as a positive sol on washing and that tannin is peptized by water as a negative sol. If the two are brought in contact, there is mutual adsorption and each keeps the other from being peptized; in other words, there is a mutual “‘fixing.’”” The so-called iron tannates are not definite compounds. The case of the action between antimony salts and tannin has been studied by Sanin! who believes there are at least three definite antimony tannates. These cannot be obtained pure; but at first, Sanin preferred to regard different products as mixtures of two or more antimony tannates rather than as sub- stances of continuously varying compostion. Later,? he con- cluded that adsorption does occur when tannin and potassium 1%, Farben-Ind., 9, 2, 17, 49 (1910). 2 Sanin: Kolloid-Z., 18, 305 (1918). MORDANTS 357 antimony tartrate are mixed; but that tannates are formed during the technical procedure used in mordanting with tannin. While no one can question the possibility of forming a true anti- mony tannate under special conditions, it is altogether improbable that the varied conditions in technical practice are such as to yield a definite salt. Wislicenus and Mutte! studied the action of tannin on fibrous alumina. The amount taken up was found to increase rapidly at first, with increasing concentration of the sol and then to reach an approximately constant value. This constant value is the limiting value of adsorption? for the particular alumina and does not indicate the formation of aluminum tannate. Had a different alumina been used, the saturation value would have been found at a different point. Von Schréder* showed that the taking up of tannin from solution in alcohol and from the aqueous sol is a typical adsorption phenomenon and no tannate is formed. It is a moot question whether the fixing of iron oxide or alumina by oil mordants is due to the formation of salts of fatty acids. Knecht‘ says: The amount of iron which is taken up by the fiber depends less on the strength of the mordanting liquor than on the amount of oil that has already been fixed in the material; the oil attracts the oxide of iron with great energy, so that it is not readily stripped from the fiber even by comparatively concentrated sulfuric acid or hydrochloric acid. This behavior is more nearly what one would expect if the ferric oxide were adsorbed by the oil than if a ferric salt of a fatty acid were formed; but there is no proof either way.® Turning to the fixing of metallic mordants by phosphates, silicates, etc., we know that compounds are not formed, in many cases. Thus, hydrous aluminum oxide adsorbs arsenic acid;® 1 Kolloid-Z., 2, 2d Supplement, XVIII (1908). 2Scumipt: Z. physik. Chem., 74, 699 (1910). 3 Kolloidchem. Beihefte, 1, 1 (1909). 4Knecut, Rawson, and Lowentuat: ‘“‘A Manual of Dyeing,’ 2, 597 (1916). 5 Cf. BAncroFT: J. Phys. Chem., 19, 50 (1915). 6 LOCKEMANN and Paucke: Koilloid-Z., 8, 273 (1911). 358 THE HYDROUS OXIDES hydrous ferric oxide adsorbs arsenious acid! and arsenic acid; hydrous stannic oxide adsorbs phosphoric acid;? and beryllium oxide adsorbs arsenious acid.* In none of these cases is a definite compound formed; and it is probable that many more of the alleged phosphates, stannates, and silicates of the heavy metals are not obtained under ordinary conditions. However, this does not preclude the formation of definite compounds under special conditions. Thus, crystalline aluminum orthophosphate?* results on treating a concentrated solution of sodium aluminate with an excess of phosphoric acid and heating in a sealed tube at 250° for several hours. It was claimed for a long time, that alizarin would dye alumina only in the presence of lime salts, the color on the mordanted cloth being attributed to a calcium aluminum alizarate.> This is now known to be true only in case the hydrous oxide contains sulfate. Hydrous alumina prepared from aluminum acetate takes up alizarin in the absence of calcium salts.6 The purpose of the calcium is not to fix the alumina to the fiber or the dye to the mordant, but to remove sulfate, which cuts down the adsorption of the alizarin.’ Cotor LAKES In the dyeing of mordanted cloth, the color is taken up chiefly by the mordant giving a color lake on the fiber. For a long time, all lakes were believed to be definite compounds between the metallic oxide and the dye. This view was questioned by Biltz and Utescher,* who investigated the behavior of alizarin with hydrous chromic oxide and hydrous ferric oxide. With the former, the amount of dye taken up increases continuously with increasing concentration of solution, giving no indication what- soever of the formation of a chromium alizarate. On the other 1 Britz: Ber., 37, 3138, 3151 (1904). 2 MECKLENBURG: Z. physik. Chem., 74, 207 (1912). 3 BLEYER and Miitumr: Arch. Pharm., 251, 304 (1913). 4 Dr ScHuuTEen: Compt. rend., 98, 1853 (1884). 5 LigcutTi and Surpa: J. Soc. Chem. Ind., 5, 525 (1886). 6 Davison: J. Phys. Chem., 17, 737 (1913). 7 BancrortT: J. Phys. Chem., 18, 10 (1914). 8 Ber., 38, 4143 (1905). MORDANTS 359 hand, with hydrous ferric oxide, there is a rather marked increase in the amount of dye taken up with relatively small change in the concentration of the bath, leading Biltz to conclude that iron and alizarin combine in a definite ratio of 1 molecule of iron to 3 of alizarin.t But since the amount of alizarin taken up by the iron oxide is so far in excess of that necessary to form alizarate, one is confronted by the necessity of assuming either that ali- zarate adsorbs the excess dye or that the whole phenomenon is a case of adsorption of dye by the hydrous oxide. If sodium alizarate reacts with hydrous ferric oxide to form a ferric alizarate, it must follow that sodium hydroxide will be liberated by the reaction; while if the sodium alizarate is adsorbed by the hydrous oxide, there should be no accumulation of alkali in the solution. Bull and Adams? investigated the phenomenon quantitatively. It was necessary, first of all, to determine the adsorption of sodium hydroxide by hydrous ferric oxide, since ferric alizarate might form and still leave little alkali behind in case the latter were sufficiently strongly adsorbed by the excess oxide. Observations were then made on the amount of alkali remaining in solution on shaking the hydrous oxide with sodium alizarate prepared by dissolving resublimed alizarin in a very slight excess of the theoretical quantity of alkali. The relative quantities of dye and oxide were so chosen that practically com- plete adsorption of the dye occurred even at the highest concen- trations. The results are given in Tables X XIX and XXX and shown graphically in Fig. 26. | TABLE X XIX.—ApsorpPTION oF NAOH sBy Hyprovus FE,.O3 N/10 NaOH at start, N/10 NaOH at end, N/10 NaOH adsorbed, cubic centimeters cubic centimeters cubic centimeters 1.20 0.20 1.00 2.40 0.56 1.84 3.60 1.07 2800 4.80 Ba 4.) 6.00 2.13 Bereyé 1 Cf. also LizcutTsE and Surpa: J. Soc. Chem. Ind., 5, 523 (1886). 2 J. Phys. Chem., 25, 660 (1921). 360 THE HYDROUS OXIDES TABLE XX X.—ADSORPTION OF SopIuM ALIZARATE BY Hyprowus FER.O; N/10 NaOH Sodium alizarate : N/10 NaOH N/10 NaOH : equivalent to ; solution, in bath, adsorbed, : : the alizarate, : : . : cubic centimeters ; : cubie centimeters | cubic centimeters cubie centimeters 5.00 ya 0.15 1,20 10.00 PA EY 0.25 225 15.00 3.00 0.30 3.45 20.00 5.00 ou 4.65 25.00 6.25 0.35 5.90 If ferric alizarate were formed, alkali would be liberated as given in column 2 of Table XXX. Much smaller quantities of this are found, and there is also much less sodium hydroxide - present in the baths than would be found if the calculated amount Cc.of 0.1 N NaOH Adsorbed w eae it 7 a 1.5 1.8 “al 24. Cc. of 0.1 N NaOH in Solution Fic. 26.—Adsorption by hydrous ferric oxide. of alkali were formed and subsequently adsorbed by the ferric oxide as shown in Table X XIX. The small quantities of alkali recorded in column 3 of Table XXX are due to hydrolysis of the adsorbed sodium alizarate, producing in solution the amounts of MORDANTS O61 alkali shown, while the insoluble alizarin remains on the fiber. — The first two values are lower than the hydrolysis value because the amount of hydrolysis will be determined to some extent by the intensity of adsorption. Further experiments were carried out, which eliminated the possibility that the adsorption of alkali was increased by the presence of sodium alizarate. In the light of these observations and the continuous curve obtained by Biltz, it seems altogether probable that the iron-alizarin lake is an adsorption complex and not ferric alizarate. Liechti! claims that a definite aluminum alizarate is obtained when hydrous aluminum oxide and sodium or ammonium aliza- rate are brought together. This claim was found to be altogether without foundation by Williamson? who investigated the matter in much the same way as Bull and Adams did the iron-alizarin lakes. Marker and Gordon? studied the influence of hydrogen ion concentration on the amount of the basic dyes, crystal violet, and methylene blue, and the acid dyes, orange II, and metanyl yellow, taken up by hydrous ferric oxide, alumina, and silica. The different hydrogen ion concentrations were obtained by the addition of sulfuric acid or sodium hydroxide. Some data are given in Tables XX XI and XXXII. In all cases, it will be seen that the amount of dye taken up increases with increasing pH for basic dyes and decreases with increasing pH for acid dyes. Throughout the range investigated, the pH-adsorption curves appear continuous for the adsorption of crystal violet and metanyl yellow by alumina and of crystal violet for ferric oxide. On the other hand, the amount of methylene blue taken up increases greatly between pH = 11 and 12, and the same is true for orange II between pH = 3.2 and 2.3; so that these curves are drawn with a sharp break at approximately pH = 11 and pH = 3.2, respectively. Since crystalline salts of a number of metals, including iron and aluminum, can be obtained by the action of the sulfonic acid, orange II, on the respective oxides, it is con- cluded that the breaks in the pH adsorption curves show the lakes investigated to be definite compounds. While certain ones 1J. Soc. Chem. Ind., 4, 587 (1885); 5, 523 (1886). 2 J. Phys. Chem., 28, 891 (1924). 8 J. Ind. Eng. Chem., 16, 1186 (1924); 15, 818 (1928). 062 THE HYDROUS OXIDES TaBLE XXXI.—ApDsoRPTION oF Dyes By Hyprous FERRIC OxIDE (Adsorption in milligrams dye per gram of gel) Basic dyes Acid dyes Methylene blue Crystal violet | Orange II | Metanyl yellow Adsorp- Adsorp- Adsorp- Adsorp- Re tion no tion pH tion ne tion 1.96 27.6 2.06 | 23.2 2.30 | 429.0 1.92.40 5361-0 2.33 29.0 2.94 | 33.0 3.20 75.0 2.30 | 340.0 5.95 30.0 5.02 | 42.3 5:27 70.0 3.38 | 255.0 9.85 32 9.01 50.6 10.14 52.0 7.46 | 211.0 tT) .12 33.8 10.95 | 56.1 11.02 50.0 | 11.60 80.7 12.00 | 131.0 TABLE XX XII.—ApDSORPTION OF DyrEs BY Hyprous ALUMINA (Adsorption in milligrams dye per gram of gel) Basic dyes Acid dyes Methylene blue Crystal violet | Orange IT | Metanyl yellow Adsorp- Adsorp- Adsorp- Adsorp- pe tion i tion pe tion pH tion 1.96 65.6 1.50 3.0 2.30 | 452.0 1.92 | 703.0 2.23 66.1 5.44 8.0 3.20 186.0 2.30 | 460.0 5.95 67.5 9.18 45.0 5.27 179.0 7.46 | 276.0 9.85 77.0 10.70 | 282.0 10.14 162.0 9.67 | 226.0 11.12 82.7 Tia 4s. 0 11.02 136.0 11.60 115.0 12.00°| 27920 . of the lakes may be definite salts, I cannot see how this can be deduced from the adsorption data or the curves constructed therefrom. Consider the orange II lake with either alumina or ferric oxide: If the amount of dye taken up by a definite amount of oxide were independent of the concentration of the dye bath at a certain pH value, then the assumption that the lake is a definite salt might be justified. But the above data show only that a change in pH value from 5.2 to 3.2 causes a much smaller MORDANTS 363 increase in adsorption than a change in pH value from 3.2 to 2.3. If the equilibrium Fe20 lak 3 eee ee LK UX (where X is the acid dye anion) exists as assumed by Marker and Gordon, there is nothing to indicate why the velocity to the right should proceed regularly with decreasing pH to a certain point, and then jump abruptly. Had the adsorption been determined at two or three points between pH = 3.2 and 2.3, it is probable that the adsorption curve would prove to be a continuous one with a sharp bend, instead of a broken one that has no apparent significance. It is possible that the marked increase in adsorp- tion above pH = 3.2 is due to the concentration of hydrogen ion being sufficiently great to cause some peptization of the oxide on boiling for an hour. If the experiments were repeated with a freshly formed gel, I should expect the amount adsorbed for a given pH value to show considerable variation from the values obtained by Marker and Gordon. | The effect of the concentration of dye on the amount taken up at various constant pH values should be investigated. Marker and Gordon determined the amounts of dyes left over on treating different concentrations of orange II with an excess of hydrous oxide at constant pH. They fail to give the important thing, namely, the constant pH value of the solution; but one is pretty safe in assuming that it was low, as all the dye anion was taken up except a constant small amount, the equilibrium concentration. For methylene blue to form a salt, hydrous ferric oxide must function as an acid. Until someone shows that a very weak base like that of methylene blue will react with ferric oxide to give a stable salt even under special conditions, there seems no ground for assuming that the iron-methylene blue lake as ordinarily obtained is ever a definite compound. Pelet-Jolivet! showed that methylene blue is adsorbed by silica, the amount taken up depending upon the previous history of the hydrous oxide. That it is unsafe to assume a lake to be a true compound simply because the constituents in question can form a definite salt under special conditions is further emphasized by the work of Gilbert? 1“Die Theorie des Farbeprozesses,”’ 71, 205 (1910). 2 J. Phys. Chem., 18, 586 (1914). 364 THE HYDROUS OXIDES on the copper lakes of eosin. Gilbert prepared a definite crystal- line copper eosinate; but found it to be a different substance from the precipitate obtained by the interaction of copper sulfate and sodium eosinate. Although the precipitated lake has a fairly constant composition, it always contains an excess of copper when an excess of copper salt is employed. By shaking hydrous copper oxide with varying concentrations of an ether solution of eosin, a typical adsorption isotherm is obtained, showing no evidence of compound formation. The maximum amount of eosin adsorbed under these conditions is only about one-tenth of that necessary to form copper eosinate. Starting with colloidal hydrous copper oxide and colloidal eosin acid, lakes were obtained varying in composition between 2 molecules of copper to 1 of eosin and 2 molecules of eosin to 1 of copper. All the lakes behave like the one in which copper and eosin are in equivalent quantities, and all can be carried into colloidal solution. In the presence of ether, small amounts of certain salts decompose the lakes. This is because the adsorption of the anions of the salts by hydrous copper oxide is sufficiently great to displace the adsorbed eosin. The order of displacing power of the anions is the usual order of adsorption: SO4”” > Br’ > Cl’ > NO3’. The taking up of crystal ponceau by wool mordanted with alum was shown by Pelet-Jolivet! to be a clear case of adsorption when the process is carried out at room temperature; but at 90°, the amount taken up is practically independent of the concen- tration of the dye bath, when the latter contains more than 700 milligrams per liter, the lowest concentration employed. It is probable that, in this instance, a definite aluminum salt of crystal ponceau is formed. Pelet-Jolivet prepared such a salt in crys- talline form. Bayliss? found that hydrous alumina adsorbs Congo red acid from its deep-blue colloidal solution in water. If this adsorption complex is washed, suspended in water, and heated, the color changes from blue to red. Since Congo red salts are red, Bayliss attributed this change in color to the formation of an aluminum salt. The experiments were extended to the precipitates obtained by mixing the negative sol of Congo red acid with the positive sols 1 “Die Theorie des Farbeprozesses,”’ 213 (1910). * Proc. Roy. Soc., 84B, 881 (1911). MORDANTS 365 of the hydrous oxides of aluminum, zirconium, and thorium. The blue adsorption complex became red on heating in every case, provided the hydrous oxide sols were dialyzed until practi- cally free from acid. A small amount of acid is sufficient to pre- vent the color change. Assuming that the color change is due to the formation of a Congo red salt, Bancroft! fails to see why a trace of acid should prevent the change, provided there is an excess of hydrous oxide with which the Congo red can react. Blucher and Farnau? attempt to get around this difficulty by assuming that hydrous alumina adsorbs and stabilizes the free red Congo acid which is instable in aqueous suspension. This raises the question why a trace of free mineral acid should prevent the adsorption and alleged stabilization of the red Congo acid. The whole phenomenon should be reinvestigated quantitatively, paying attention to the relative concentrations of both the posi- tive and negative sols and their respective pH values. In this connection it may be mentioned that Schaposchnikoff and Bogo- jawlenski® have isolated the metastable red Congo acid by allow- ing the pyridine salt to effloresce. Considering the acid mordant, tannin, for a moment, we find Dreaper* stating that magenta and tannic acid form a definite salt; but no proof is offered for the statement, and the admission is made that 100 parts of the alleged salt will take up at least 160 parts of tannin when the latter is present in excess. Sanin® likewise states that basic dyes form definite salts with tannin when the dyes are in excess; but when tannin is in excess, the latter is adsorbed. In no case is any proof.of compound forma- tion presented except the fact that a formula for the alleged product can be written. In conclusion, one seems justified in saying that mordants function by adsorbing dyes in indefinite proportions depending on the conditions. In certain cases, definite salts may be formed, but these constitute the exceptions to the general rule. 1 J, Phys. Chem., 19, 57 (1915). 2 J. Phys. Chem., 18, 634 (1914). 8 J. Russ. Phys.-Chem. Soc., 44, 1813 (1918). 4“The Chemistry and Physics of Dyeing,” 244 (1906). ®Sanin;: Z, Farben-Ind., 10, 97 (1911). CHAPTER XVII WATER PURIFICATION All natural waters are contaminated to a greater or lesser degree by the materials with which they come in contact. Thus, waters from regions of old rocks like granite are relatively low in mineral content; while waters from regions of limestone are hard. Surface waters flowing through districts containing readily pep- tizable material like clay are more or less turbid, and those from swampy regions are highly colored. The purification of water on a large scale is carried out with one or more of the following objects in view: first, to render the supply safe and suitable for drinking; second, to reduce the amounts of mineral ingredients which are injurious to boilers; and third, to remove substances injurious to the machinery or the manufactured product in industrial processes. The col- loidal matter in surface waters consists of finely divided particles of clay, sand, organic coloring matter, and bacteria. Such material can usually be removed by agglomeration and filtering under suitable conditions. Undesirable dissolved substances such as the bicarbonates and sulfates of calcium and magnesium can be eliminated only by resorting to chemical precipitation. Many of the largest artificial water purification plants are operated solely to provide potable water without special atten- tion to its use for industrial purposes. In other instances, the water is not only rendered potable but is softened at the same time. A notable example of the latter is the purification plant at New Orleans where hard, colored, turbid, sewage-polluted water from the Mississippi River is rendered suitable for indus- trial as well as domestic consumption. The most important requirement in the purification of a municipal water supply is the elimination of bacteria, especially those causing disease, and the removal of turbidity; but a per- fectly acceptable drinking water is free from objectionable odor, 366 | WATER PURIFICATION 367 taste, and color. Small amounts of the mineral constituents commonly found in water are not objectionable, as a rule, but certain ones are highly undesirable. Thus, the presence of as little as 2 p.p.m. of iron renders the water unpalatable to some people and causes trouble by discoloring washbowls and tubs, and by producing rust stains on cloth. It is needless to say that drinking water must contain no more than a trace of salts of barium, copper, zinc, and lead, because of their poisonous character. Fortunately, the occurrence of harmful amounts of the latter salts in the ordinary water supply is quite rare, FILTRATION Surface water is rendered potable by filtration, sometimes accompanied by disinfection with ozone, chlorine, or hypochlorite which destroy disease-producing organisms, and by the addition of an algicide such as copper sulfate to kill organisms responsible for objectionable tastes or odors. Chemical treatment alone is not a substitute for purification by filtration, since it does not remove colloidal matter which causes turbidity and color, or dissolved organic matter which produces swampy tastes and odors. Two general types of filters are employed in purifying municipal water supplies: slow sand filters and mechanical filters. Slow Sand Filtration.—In slow sand filtration the water is caused to pass through a suitable layer of sand which removes the undesirable suspended matter. The method was inaugurated by Simpson in England in 1829 and is frequently referred to as the English system. The filter is a very large water basin con- taining filtering material 1.5 to 2 meters in thickness. The upper layer consists of fine sand approximately 1 meter in thickness supported on coarser sand, and this in turn, on a layer of graded gravel, the coarest material at the bottom. Drains are installed below the gravel to carry off the filtered water. The process of slow sand filtrations is about as follows: The raw water containing suspended material, together with colloidal clay, bacteria, microscopic plants, etc., is run into asedimentation basin where part of the impurities settle out under the influence of gravity. The removal of microorganisms by the filter is not 368 THE HYDROUS OXIDES very efficient until the surface layer of the sand becomes coated with a slimy protoplasmic deposit called the ‘‘schmutzdecke.”’ This protoplasmic filtering laver consists essentially of myriads of living forms—diatoms, fungi, blue and green algz, protozoa, and bacteria—together with silt, mud, and other colloidal matter. Although the greater part of the impurities are retained in the surface layer, thick filter beds have been found to be more efficient than thinner ones, indicating that each particle of sand contributes to the purification. Obviously, the rate at which the filtration is carried out has an important bearing on the effi- ciency of the process. In practice, from two to four million gal- lons of water per acre per day are rendered substantially free from suspended matter, including bacteria. When the proto- plasmic film has become clogged so that the rate of filtration is unduly retarded, the water is allowed to subside below the sur- face, about 144 inch of sand is. scraped off, and the filtering resumed. The sand is used over again after washing free from impurities. The slow sand filter is suited to purification of waters contain- ing relatively small amounts of color, suspended matter, and animal pollution. This type of filter has been in use in Europe for years and has proved most efficient; on the other hand, but few American waters can be treated successfully and economically by this process. In some places where the slow sand filter has been adopted and has not proved entirely satisfactory, the normal biological action of the filter is supplemented by the use of coagulants, such as aluminum sulfate. The trivalent alum- inum ion causes agglomeration of the negatively charged col- loidal particles and hydrous aluminum oxide, which subsequently settles out, carries down with it a large portion of the impurities. Thus, at Washington, the Potomac River water is treated with aluminum sulfate before it enters the Georgetown reservoir, which acts as a sedimentation basin. After partial clarification by sedimentation, the water is conducted to the filter bed, where the undesirable impurities are further reduced. Clark? suggests loading the sand with coagulant as a means of supplementing 1 PinrKe: Z. Hyg., 7, 115, 170 (1889). 2J. Am. Water Works Assoc., 36, 385 (1922); Public Works, 68, 197 (1922), WATER PURIFICATION 369 the action of the slow sand filter. From 75 to 225 tons of alum- inum sulfate per acre have been used in practice. Aeration, preferably by spraying, before filtration brings about the precipitation of dissolved iron as hydrous ferric oxide. Objectionable odors and tastes are likewise best removed by aeration, either before or after filtration. Mechanical Filtration—The method of rapid sand filtration was developed in America and is, therefore, referred to as the American system. ‘The process is characterized by the artificial formation of a surface filtering layer consisting essentially of hydrous aluminum or ferric oxide, by the method of cleaning the filters, and by the rapid rate of filtration which may be as much as fifty times that in slow filters. The method is eminently suited to the treatment of turbid and highly colored waters, and is commonly used where softening as well as filtration is necessary. The process is substantially as follows: The raw water passes through a meter which measures the volume of water passing and at the same time regulates the rate of addition of coagulant to the flow of water. If the water is to be softened, it is next passed to a set of weirs where it is divided, one small fraction receiving the charge of lime and a second the requisite amount of soda ash. ‘These portions are subsequently mixed with the main body of water, which is then allowed to stand until the body of the precipitate settles out. The water is next conducted to the filters, which consist of concrete or wooden basins having a filtering area of 50 to 120 square meters. The top layer of a filter is of fine sand about 75 centimeters thick followed by a 30- centimeter layer of graded gravel, which rests on perforated brass strainers connected with the drain system. The small residual amount of suspended hydrous oxide quickly forms a filtering layer on the sand, which entrains the remaining impuri- ties. Usually after 8 to 12 hours’ operation, the filters become clogged and must be washed. This is accomplished by forcing clean water up through the strainers, thus dislodging the impuri- ties which pass over the top of the filters with the wash water. Both gravity and pressure filters are in use, but the principle is the same in each. The precipitate of hydrous alumina or ferric oxide adsorbs and entangles practically all suspended matter including bacteria; 370 THE HYDROUS OXIDES but where the raw water has a very high bacterial count, it may be necessary to sterilize the water with chlorine, hypochlorite, or ozone as an added precaution against transmitting’such diseases as typhoid fever and Asiatic cholera. Flinn, Weston, and Bogert! summarize the applicability of slow sand and rapid sand filters, as follows: For a water having a turbidity? less than 30 p.p.m.? or a color? less than 20 p.p.m., slow filters without coagulation give excellent results. For waters having a turbidity of more than 50 p.p.m. or a color of more than 30 p.p.m., mechanical filters give unquestionably better results. They not only produce an equally safe water but one of far better appearance. Between these extremes is a region where either the mechanical filter or the slow filter with coagulants may be used equally well. Under ordinary conditions, the latter is far more expensive than the former, . Tue ACTION OF COAGULANTS When a coagulant such as aluminum sulfate is added to pol- luted water, several colloidal processes take place, most important of which are the neutralization of colloidal particles by adsorption of ions, followed by agglomeration; and the adsorptive action of the highly gelatinous aluminum oxide. The strongly adsorbed aluminum ion has a marked precipitating action on colloidal clay, bacteria, and coloring matter. Observations of the effect of multivalent cations on the sedimentation of clay and on the agglutination of bacteria date back to the pioneer work of Bodlander® and Bechhold,® respectively, More recently, Saville’ 1 “Waterworks Handbook,” 734 (1918). 2 The standard of turbidity is a water which contains 100 p.p.m. of pre- cipitated fuller’s earth in such a state of fineness that a bright platinum wire 1 millimeter in diameter can just be seen when the center of the wire is 100 millimeters below the surface of the water and the eye of the observer is 1.2 meters above the wire. The turbidity of this standard water is 100. 3 Parts per million. * The standard color solution, having a color of 500, contains 1.246 grams K.PtCle, 1 gram CoCl,-6H:O and 100 cubic centimeters of concentrated HCl in 1 liter. 5 Jahrber. Mineral., 2, 147 (1893). 6° Z. physik. Chem., 48, 385 (1904). 7J. New Engl. Weve Works Assoc., 31, 78 (1917). WATER PURIFICATION d7v1 showed that the color taken up by water originating in swamps or peaty soils is due almost exclusively to negatively charged colloidal particles which are coagulated by the cations of the coagulant. Miller! confirmed this result and demonstrated fur- ther that the decolorizing action on the so-called humic acid colors is due, for the most part, to the agglomerating action of aluminum ion, hydrous alumina alone playing an unimportant role in the process. The precipitating action of hydrogen ion on the negatively charged impurities is much less than that of aluminum ion of the same concentration.2. The high precipitat- ing power of sulfate ion neutralizes any positively charged colloids that may be present in the water; but the most important function of a multivalent negative ion is to prevent the formation of a positive sol of hydrous aluminum oxide. The highly gelatinous hydrous alumina which precipitates under suitable conditions adsorbs and entangles the finely divided impurities, leaving the water relatively clear and uncontaminated as it settles out. Because of the outstanding role of the hydrous _ oxide in the purification process, it is important to know what constitutes the most satisfactory floc and how the desired product may be obtained. Some waters contain sufficient iron to produce a good floc when lime or soda ash is added; others have normally sufficient alkali to precipitate hydrous alumina on the addition of aluminum sulfate; still others require the addition of both sulfate and alkali. If ferrous sulfate is added, lime must always be used to bring about satisfactory precipitation. In practice, the coagulant most used is commercial aluminum sulfate, com- monly called filter alum or alum. A great deal of empirical information regarding the use of coagulants has been collected; but the principles underlying their proper use have received but little attention until recent years. In this connection, the important work of Clark, Thierault, and Miller in the Hygienic Laboratory of the United States Public Health Service deserves special mention. | Formation of Alumina Floc.—It is well known that hydrous aluminum oxide does not separate from an aluminum sulfate solution when the final solution is either too acid or too alkaline. 1U.8. Pub. Health Repts., 40, 1472 (1925). 2 Of. BanEeRJI: Indian J. Med. Research, 11, 695 (1924). 372 THE HYDROUS OXIDES In other words, there is a comparatively narrow range of hydro- gen ion concentration in which a precipitate forms and the range of complete precipitation is still narrower. ‘The ideal conditions should result in the rapid and complete formation of a floc that settles readily. Rina | ieee ei ACE ) Time. minutes 40. «35 45 as we 65 70 pl : Fia. 27.—Relation between time required for the appearance of floc in solu- tions buffered at various pH values when the total salt concentration is con- stant and the alum concentration is varied: 1 = 400 p.p.m.; 2 = 300 p.p.m.; 3 = 200 p.p.m.; 4 = 100 p.p.m. In any precipitation process, the highest rate of precipitation will result under otherwise constant conditions, when the highest concentration of separable material above the equilibrium con- centration is attained. In the present instance, the degree of supersaturation of water with hydrous aluminum oxide can be varied either by increasing the total amount of aluminum sulfate added at a given final pH value or by varying the pH value with WATER PURIFICATION 373 a constant amount of aluminum sulfate. The maximum degree of supersaturation and rate of precipitation of hydrous alumina under varying conditions was first studied by Thierault and Clark. The procedure was as follows: A definite volume of a solution of known composition was treated with varying amounts of aluminum sulfate in dilute solution. After rapid mixing, the liquid was poured into a 100-cubic-centimeter cylinder, and the time necessary for the first appearance of a floc was noted. The visibility of the floc was increased by slight agitation of the cylinder. In Fig. 27 is given the time required ‘for the first appearance of a floc in solutions buffered at various pH values, when the total salt concentration is constant and the alum is varied. The concentration of aluminum sulfate in parts per million is 400, 300, 200, and 100 for curves 1, 2, 3, and 4, respec- tively. The optimum pH value for producing a floc in mini- mum time increases slightly as the concentration of alum decreases. In curves 1, 2, 3, and 4, it is at pH = 4.95, 5.10, 5.25, and 5.40, respectively. With less than 100 p.p.m. of aluminum sulfate, the optimum pH value is close to 5.5, which is significant since the amount used in practice is ordinarily con- siderably less than 100 p.p.m. Moreover, as the concentration of aluminum sulfate decreases, the width of the curves decreases, the optimum zone, using 100 p.p.m., being less than one pH unit. Attention is called to the fact that the floc which appears first in a series of experiments is always the best so far as the floccu- lent appearance is concerned. Also, the floc formed in mini- mum time is most abundant and settles most rapidly. The actual time required for the appearance of a floc depends on the size of the vessel. This difference is quite marked, a precipitate forming within a minute in a large vessel and often requiring hours to become visible in a small one. Apparently this ‘“‘volume effect’’ is only the effect of the volume-surface ratio upon circulation, since mechanical circulation decreases the time required for the appearance of a floc.?, Gentle agitation was found to influence only the time of flocculation and not the amount of precipitate or the optimum pH value for maximum rate. 1U. 8. Public Health Repts., 38, 181 (1923). 2 Cf. Hoover: J. Am. Water Works Assoc., 11, 582 (1924). 374 THE HYDROUS OXIDES Since the two branches of the curve relating pH value to floccu- lation time, tend to become parallel, no floc is likely to appear for a very long time in laboratory vessels if the pH value is beyond the asymptote to either branch. Moreover, since the two branches come closer together the more dilute the solutions, the region in which a floc appears may be quite narrow for extremely dilute solutions. Thierault and Clark found that if a floc does not appear within a few hours with slight occasional agitation, it will not appear within a greatly reduced time with mechanical agitation. This indicates the necessity for rigid control of the final pH values under large volume conditions, in order to secure floc formation in a reasonable time from highly dilute aluminum sulfate solutions. The above method of determining the optimum conditions for a satisfactory floc was found to be applicable to natural waters containing carbonates. A slight but definite buffer action is obtained in the region of pH = 5.5 with aluminum sulfate and a hydroxide. At as low a pH value as this, the carbon dioxide of the air is not so effective in disturbing the equilibrium of dilute solutions as it is in such solutions nearer neutrality. Accordingly, it is possible to obtain mixtures of aluminum sulfate and calcium hydroxide having definite pH values in the range 4.6 to 6.0. Using M/500 calcium hydroxide without the use of supplemen- tary buffers and varying the amount of aluminum sulfate, the optimum pH value is between 5 and 6; with M/500 sodium hydroxide and varying amounts of aluminum sulfate, the best floc is obtained at pH = 5.2; and with constant amount of aluminum sulfate and varying amounts of alkali, the optimum value is about pH = 5.8. These observations would indicate that a hydrogen ion concentration between pH = 5 and 6 is best suited for the coagulation of aluminum sulfate even in natural waters. This proves to be approximately true in many cases but not in all. A minimum in residual alum in filter effluents under commercial conditions was found by Buswell and Edwards! at pH = 6; by Baylis? between pH = 5.5 and 7.0; and by Hatfield? between pH = 5.8 and 7.5. The latter 1 Chem. Met. Eng., 26, 826 (1922). 2 J. Am. Water Works Assoc., 10, 365 (1928). 3 J. Am. Water Works Assoc., 11, 554 (1924). WATER PURIFICATION 375 values are for Lake St. Clair water in which the maximum rate of flocculation is between pH = 6.1 and 6.8. Dallyn and Dela- porte’ found the optimum condition for coagulation to be pH = 5.5 and 6.5 for soft colored, and for clear Great Lakes water, respectively; and Mum? obtained the most favorable results with Triliwong River water at pH = 5.5 to 6. On the other hand, Hatfield* obtained most satisfactory results at Highland Park, Mich., when the pH value of the treated water was between 7.2 and 7.3; and similar conditions exist in other places.4- Obviously, therefore, the hydrogen ion concentration most favorable for coagulation varies with the nature of the water. In this con- nection, the importance of research being carried out under the conditions which obtain in actual practice cannot be emphasized too strongly. Different waters present their individual problems which very frequently cannot be solved simply by referring to data obtained with pure chemicals. Some of the factors which influence the optimum hydrogen ion concentration for obtaining a good floc will be considered in order. The observations recorded in the preceding paragraphs were all carried out with aluminum sulfate. The range of hydrogen ion concentration over which flocculation occurs might be expected to vary with the nature of the anion. Thus, it will be recalled that colloidal hydrous alumina is formed by dialysis of a solution of aluminum chloride to which ammonium hydroxide is added short of precipitation; aluminum sulfate cannot be substituted for aluminum chloride on account of the strong precipitating action of sulfate ion. This is illustrated further by some observations of Miller® on the effect of various anions on the — zone of precipitation of hydrous alumina. For example, 0.005 M solutions of aluminum chloride and aluminum sulfate were treated with varying amounts of alkali, and after precipitation, the hydrogen ion concentration of the supernatant solution and the amount of aluminum in the precipitate were determined. 1 Contract Record, 37, 343 (1923); cf. CatteTT: J. Am. Water Works Assoc., 11, 887 (1924). | 2 Mededeel-Burgerlyken Geneeskund. Nederland-Indie, Part 1, 27 (1925). 3 J. Ind. Eng. Chem., 14, 1038 (1922). 4 BaANERJI: Indian J. Med. Research, 11, 695 (1924). 5 U.S. Pub. Health Repts., 40, 351 (1925). 376 THE HYDROUS OXIDES Referring to Fig. 28, it will be seen that with aluminum sulfate, practically complete precipitation occurs between pH values of about 5.3 and 8.7;! while with aluminum chloride, flocculation occurs only in the narrow range between pH = 7.8 and 8.6. It should be emphasized that these ranges represent zones of flocculation and not of insolubility. The insoluble hydrous oxide formed in the presence of chloride remains in colloidal solution throughout the lower pH values on account of the strong stabiliz- ing action of hydrogen ion and the weak precipitating power of =) S Oo as Mofs Al Precipitated = Cae | GS 3. 4 5 (GUN ti =?ihcnt pH Fria. 28.—Zones of hydrogen ion concentration in which flocculation occurs for alum and aluminum chloride. chloride ion. Similar observations were made by Miller with more dilute solutions approaching those used in the actual opera- tion of water purification. It is quite evident, therefore, that the nature and precipitating power of the anions present in solution are equal in importance to that of the hydrogen ion concentration in controlling the formation of a suitable precipitate of hydrous alumina, From the results of observations with various anions, Miller reports that sulfate yields a floc best suited to successful water clarification. The range of concentration over which 1 Cf. GREENFIELD and BuswEuu: J. Am. Chem. Soc., 44, 1485 (1922). WATER PURIFICATION Old precipitation occurs is broad and the floc is of good quality, rapid settling, and shows least tendency to become colloidally dispersed. Since the maximum rate of flocculation of pure aluminum sul- fate in dilute alkali is at pH = 5.5, Thierault and Clark suggest that this value may represent the isoelectric point! of hydrous aluminum oxide. Hatfield? likewise refers to his values of pH = 6.1 to 6.3 as indicating the ‘‘apparent”’ isoelectric point of the hydrous oxide. Asa matter of fact, the zone of maximum rate of flocculation is of no significance whatsoever as a direct experimental method of determining the isoelectric point of hydrous alumina, since the zone can be varied at will by varying the anions present in solution. Miller confirmed Thierault and Clark’s value of pH = 5.5 as the approximate point of maximum precipitation for aluminum sulfate; but the maximum is at pH = 8 for aluminum chloride. In addition to the effect of the negative ion content of natural waters, the presence of colloidal inorganic or organic matter, which may function as a protective colloid, will cause variation in the zone of hydrogen ion concentration in which flocculation occurs. ‘Thus, colloidal silica’ prevents the formation of hydrous alumina under certain conditions; and sewage-polluted water requires more coagulant than an unpolluted water having the same turbidity and color. Instead of adding aluminum sulfate directly to water, Coxe‘ suggests adding a colloidal solution® prepared by mixing 40 grams of crystalline aluminum sulfate in 80 cubic centimeters of water with 10 grams of sodium carbonate in 40 cubic centimeters of water. This sol is precipitated simply by dilution, and it is claimed to have certain advantages as a clarifying agent over aluminum sulfate and alkali added separately. Thus, clarifica- tion can be brought about without softening, if desired; and the carbon dioxide content of the water is not increased as a result of decomposition of aluminum sulfate in the water treated. The clearest advantage would appear to be in the very short time 1 Cf. Heyrovsky: J. Chem. Soc., 117, 11, 695, 1013 a 2 J. Am. Water Works Assoc., 11, 554 (1924), 3Smitu: J. Am. Chem. Soc., 42, 160 (1920). 4Chem. Met. Eng., 29, 279 (1923). 5 Cf. SpencER: Chem. Age, 32, 31 (1924). 3718 THE HYDROUS OXIDES necessary for the formation of the floc, as compared with the usual process. On account of the low concentration of aluminum ion, the sol would probably be unsuited for treating waters con- taining large amounts of negatively charged coloring matter. In view of the importance of aluminum ion in the coagulation and removal of coloring matter, it would appear advantageous to treat highly colored waters at a low pH value where aluminum > ions exist in solution as such, followed by increasing the pH value in order to precipitate all the aluminum. This is exactly what Norcom! does with Cape Fear River water at Wilmington, N. C. The desired result is accomplished by connecting two sedimentation basins in series, treating the water with alum at a low pH value in the first basin, and increasing the pH value in the second basin by the addition of alkali. Finally, it may be said that successful water purification by alum depends on the presence of a certain minimum quantity of aluminum ion; the presence of an anion of high precipitating power, such as sulfate; and the proper adjustment of the hydrogen ion concentration.’ Composition of the Alumina Floc.—When aluminum sulfate is added to water, an equilibrium is set up that may be represented by the following equation: Al2(SO4)3 -+ xH.O — Al,O3 7 xH2O + 6H’ aie 35804” The addition of alkali displaces this reaction to the right, com- plete precipitation of the aluminum resulting when approxi- mately 2.5 equivalents of hydroxyl to 1 of aluminum are added. Miller? determined the composition of the precipitate formed at various final pH values: Liter quantities of 0.005 M solution of potassium alum were added to varying quantities of sodium hydroxide. The pH values of the resulting solutions were determined, and the precipitates were analyzed for their alu- minum and sulfate content, after thorough washing. The results are given in Fig. 29. From the lowest pH value at which a pre- cipitate forms up to pH = 5.5, the composition of the precipitate 1J. Am. Water Works Assoc., 11, 97 (1924); cf. Mituer: U. S. Pub. Health Repts., 40, 1479 (1925). 2 Miuter: U.S. Pub. Health Repts., 40, 365 (1925). 3 U.S. Pub. Health Repts., 38, 1995 (19238). WATER PURIFICATION 379 remains constant and may be represented approximately by the formula 5AI,03-3S03.!. Above pH = 5.5, which corresponds to 2.4 equivalents of alkali to 1 of aluminum, the sulfate content of the precipitate decreases gradually, becoming zero at pH = 9 when exactly 3 equivalents of alkali to 1 of aluminum have been added. The constancy of composition of the precipitate thrown down below pH = 5.5 suggests that it may be a basic salt. This view Mol Ratio Al to SO4, Fig. 29.—Composition of the precipitate from alum at varying hydrogen ion concentration. is rendered improbable by the ease with which the sulfate is displaced by washing the hydrous oxide containing sulfate, with solutions of negative ions of equal or greater valence.” Dyes containing two or more acid groups such as the di-, tri-, and tetrapotassium sulfonates of indigo likewise displace sulfate. From a study of the reciprocal displacement of oxalate and sul- fate ions, Miller? suggests that the negative ions are in solid 1 Wiuiamson: J. Phys. Chem., 27, 284 (1923); cf., however, Hopkins: J. Am. Water Works Assoc., 12, 425 (1924). 2 CHarRiovu: Compt. rend., 176, 679, 1890 (1923). 3 U, S. Pub. Health Repts., 39, 1502 (1924). 380 THE HY DROUS OXIDES solution in the hydrous oxide. This question was considered in an earlier chapter! and the conclusion was reached that the carry- ing down of ions by hydrous alumina is an adsorption phenom- enon rather than a case of solid solution in which the ions form an integral part of the space lattice of the microcrystals. A possible explanation of the constancy of adsorption of sulfate ion below pH = 5.5 is that the adsorption of hydrogen ion by the hydrous oxide reaches the saturation value at approximately this point. If such be the case, the amount of sulfate ion which must be adsorbed to neutralize the adsorbed hydrogen ion will be constant below pH = 5.5. Above this value, the adsorption of hydrogen ion falls off, and there is a corresponding gradual decrease in the adsorption of sulfate until it becomes zero at pH = 9 and above. It will be recalled that Thierault and Clark obtained the best and most rapid flocculation in very dilute aluminum sulfate solutions near pH = 5.5, where the precipitation of aluminum first approaches completion on the addition of alkali and where the greatest proportion of sulfate is found in the precipitate. Miller likewise found the precipitate to be more dense, more rapid settling, more opaque, less gelatinous in appearance, and less voluminous in the more acid portion of the flocculation range than at the higher pH values. It would appear that the best floc for commercial water clarification should be sufficiently gelati- nous to adsorb and entangle all impurities but sufficiently dense to settle rapidly. The Ferric Oxide Floc.—Ferrous sulfate in conjunction with lime is a very good coagulant for turbid alkaline waters such as those of the Missouri and Ohio basins. If ferrous sulfate is added to such water, the action with calcium bicarbonate may be represented as follows: FeSO. + Ca(HCOs3)2 = Fe(HCOs)2 + CaSO. Ferrous bicarbonate oxidizes and precipitates too slowly for practical use, and so lime must be added which precipitates hydrous ferrous oxide thus: Fe(HCO3)2. + Ca(OH). + xH2,0 = FeO: xH20 + Ca(HCQs)e el agtte BA E WATER PURIFICATION 381 By adding sufficient lime, the calcium is also precipitated as carbonate. Hydrous ferrous oxide is slightly soluble; but it is oxidized by the oxygen dissolved in the water, giving hydrous ferric oxide, the coagulant desired. This oxide may be obtained directly from ferric sulfate but the latter salt is much more expensive than the ferrous salt. Hydrous ferric oxide forms a denser coagulum than hydrous alumina, and ferrous sulfate is considerably cheaper than alu- minum sulfate. On the other hand, ferrous sulfate must always be used in conjunction with lime and the mixture is not suitable for soft waters, because any surplus lime gives the water a caustic alkalinity. Moreover, hydrous ferric oxide does not remove coloring matter so well as hydrous alumina and so is not suitable for clarification of waters which are high in color or which are alternately turbid and colored. ‘The failure of ferrous sulfate to remove coloring is probably due to the relatively low precipitating power of ferrous ion as compared with aluminum ion and the rapidity with which the former is removed from solution when used in conjunction with lime. Miller! extended the study of aluminum compounds to the corresponding compounds of iron and found the same factors which determine the optimum conditions for forming an alumina floc to apply equally to ferric oxide floc. The floc from ferric alum is precipitated almost completely near pH = 3, approxi- mately 2.5 pH units below the zone of maximum precipitation of hydrous alumina from sulfate solution. Like aluminum chloride, ferric chloride forms a sol at lower pH values, complete floccula- tion occurring at approximately pH = 5.0; but unlike alumina, hydrous ferric oxide is insoluble at higher pH values. The zone of precipitation of hydrous ferric oxide is, therefore, much wider than that of hydrous alumina, a circumstance which may be of distinct advantage under certain conditions. 1U. 8. Pub. Health Repts., 40, 1413 (1925). CHAPTER XVIII CEMENT The term cement, as ordinarily used at the present time, refers to mortars which possess the property of hardening in water as well as in air. Reference has already been made to hydraulic cements in which magnesia or zinc oxide is the most important constituent, so that this chapter will deal for the most part, with what is known as Portland cement. PORTLAND CEMENT The need for a cementing material to bind sand and small stones together was recognized from the time man started to build. In some of their constructions, the Assyrians and Baby- lonians are known to have used moistened clay, which was probably the first cementing material ever used for building purposes. Such a binder is not sufficiently durable or hard for building massive constructions, and the next development appears to have been the discovery by the Egyptians of the cement now known as plaster of Paris, which was mixed with sand to make the mortar used in the construction of the Pyramids. The discovery that the application of heat to certain rock minerals, such as gypsum, would give a cementing substance was later employed by the Greeks in making lime from limestone or marble. The Greeks prepared some very satisfactory mortars by mixing lime with sand or with sand and volcanic earths known as pozzolana. The development of pozzolana mortars was brought to a high state of perfection by the Romans, as evidenced by many of their imposing structures which still exist. The so-called Roman or pozzolana cements were similar in many respects to the modern Portland cement. The art of cement making declined with the fall of Rome and was not revived until 1756 when John Smeaton discovered that a clayey limestone found in Cornwall would give an hydraulic lime, when burned, This product was mixed with pozzolana 382 CEMENT | 383 to prepare the mortar used in constructing the Eddystone light- house. Because of the scarcity of pozzolana, which is found only in a few volcanic regions, subsequent investigations were carried out in an attempt to produce an artificial Roman cement. The invention of a satisfactory process is attributed to Joseph Aspdin of Leeds, who took out a patent in 1824 for making a cement by heating an intimate mixture of limestone and clay at the temperature ordinarily used in burning lime. To this product, Aspdin gave the name Portland cement, since its color, after hardening, was similar to that of Portland stone, a famous English building stone. Aspdin’s original cement was not what is now known as Portland cement, as the temperature of burning was not high enough; but a year later, in 1825, the importance of heating the mass to incipient fusion was recognized. From this beginning, a century ago, there has developed the modern Portland cement industry, the importance of which in our present-day civilization is difficult to overestimate. Formation.—Portland cement is produced by heating a mixture of compounds containing suitable amounts of aluminum, calcium, and silicon together with small amounts of iron and magnesium. In the early stages of the development of the industry, the method of procedure employed in making a satisfactory cement was determined by the method of trial and error. Now, it is known that certain definite compounds impart the desired properties to cement and that a uniform product made up of these compounds _ results only when the raw material containing calcium, aluminum, and silicon in rather definite proportions is ground to a fine powder and the intimate mixture heated to a minimum temperature. Typical raw materials employed in cement manufacture are limestone and clay, both of which are found in large deposits of uniform composition. In some places, there exist deposits of clayey limestone, called cement rock, containing all three of the essential constituents; but as a rule, either limestone or clay must be added to get the desired composition for good Portland cement. The alumina and silica are sometimes derived from blast-furnace slag and the calcium oxide from sea shells. From whatever source the material is obtained, the separate constituents are first mixed in the proper proportions and then thoroughly pul- yverized. If the raw materials are rocks, the grinding is commonly 384 THE HYDROUS OXIDES carried out in the dry way. On the other hand, soft materials, such as marl and clayey mud which are gathered by dredging operations, are usually ground wet and are kept suspended until dried out in the kiln. The burning process is carried out in cylindrical kilns, 100 to 300 feet in length and 6 to 12 feet in diameter, built of steel plates and lined with highly refractory material. The drums are held in a slightly inclined position by friction rollers and are rotated slowly. The process is continuous, the raw mix entering at one end of the kiln and the cement clinker leaving it at the other. The heat is derived from pulverized coal, fuel oil, or gas which is blown into the lower end of the kiln by compressed air, giving a flame 30 to 40 feet in length. The time of passage through the kiln is from 1.5 to 2 hours, during which the raw material is subjected to a gradually increasing temperature that reaches a maximum of about 1425°. In the first stage of the burning proc- ess, the raw material is thoroughly dried; in the second stage, carbon dioxide and organic matter are driven off; and in the final stage, the alumina, silica, and lime react to form the cement clinker. The latter consists of partially sintered masses of particles from 0.5 to 6 centimeters in diameter. After adding a small amount of gypsum which regulates the rate of setting, the particles of clinker are ground to a fine powder which is the Portland cement of commerce. Composition.—The limits of composition within which cements of good quality usually fall are set down in Table XX XIII as given by Meade.! This, of course, gives only the percentage amount of the several components and does not indicate the nature of the compounds present. It will be seen that more than 90 per cent of the average Portland cement consists of calcium, aluminum, and silicon, referred to the oxides; hence, it is reasonable to suppose that its properties are due chiefly to compounds of these three constituents. As a matter of fact, Richardson? demonstrated that a good Portland cement can be prepared by starting with lime, silica, and alumina in a pure state. Many workers have been concerned with the constitution of Portland cement since Le Chatelier published the results of his 1 “Portland Cement’’ (1925). 2 Cement, 5, 314 (1904). CEMENT 385 TaBLE XX XIII.—Composition or PortTLAND CEMENT Limits of Average Constituent composition, | composition, per cent per cent EE Sy a ts 60.0 to 64.5 62.0 et ER WE chy os ca ca pi ee ees 20.0 to 24.0 22.0 NS it i 5.0 to 9.0 7.5 NS A A 1.0to 4.0 2.5 PED Seal aoe a 2.0to 4.0 2.5 ORIG ie hss ek a eee 1. O:too An 75 125 classic investigations four decades ago. In most of the work, the evidence offered in support of the alleged reactions which take place during the burning process and of the compounds formed is not convincing, since the criteria used to define a compound were either indefinite or insufficient. The solution of many questions connected with the constitution and setting of Portland cement has been brought about by the thorough systematic investigations carried out in the Geophysical Labor- atory and the United States Bureau of Standards. Thus Rankin and Wright? made a complete phase-rule study of the ternary system CaQ-Al.0;-SiO2 which necessitated the investigation of about 1000 different compositions and fully 7000 heat treat- ments and microscopical examinations. The results of these observations are summarized in the triangular concentration diagram shown in Fig. 30. In this diagram, the pure compo- nents are represented by the apices of the triangle; the binary mixtures, CaOQ—Al.O3, AlsO3-SiOe, and SiO;—-CaO, respectively, by points on the three sides; and ternary mixtures by points within the triangle. Each side of the triangle is divided into 100 parts and all compositions are given as percentage weights of the components. ‘The lines within the large triangle divide the latter into 14 small triangular spaces which enclose all possible mixtures of the three components whose compositions are repre- sented by the apices of the respective triangles. The com- 1“Fxperimental Researches on the Constitution of Hydraulic Mortars” (1887), translated by Hall (1905). | 2Am. J. Sci., (4) 39, 1 (1915); SHEPHERD, RANKIN, and WRicurT: J. Ind. Eng. Chem., 3, 211 (1911); Ranxtin: Jbid., 7, 466 (1915). 386 THE HYDROUS OXIDES pounds within each small triangle are represented by symbols in which C = CaO, A = Al.O3, and S = SiOs. Thus the compound 5CaQO:3Al1.03 is formulated: CsAs. Richardson finds that a good cement clinker can be made from mixtures of the three oxides in the proportion represented by the points at P in Fig. 30. Since all the points lie within the triangle whose apices are 8CaO: SiOQo, 3CaO- Al,Os, and 2CaO - SiOz, Ce DA C3A C5A 3 CA C3A Ls Fria. 30.—Diagram showing final products of crystallization of solutions of CaO, Al2O3 and SiO». it follows that a cement clinker made by burning the three pure components in this proportion until equilibrium is reached should consist of these three compounds only. If, however, equilib- rium were approached but not reached, there should be, in addition, only the compounds 5CaO - 3A1,03, and CaO, as these are the only other constituents present in the adjacent triangles. As will be shown subsequently, observations on commercial Portland cement clinker confirm these conclusions. CEMENT 387 As a result of precise investigations of the conditions of forma- tion and the optical properties of each of the four compounds individually, Rankin! deduces the mode and order of their forma- tion in the burning of a cement clinker made up from the pure oxides. The first step in the process is the union of lime with the other components to give the readily formed compounds, 5CaO - 3Al.03 and 2CaO - SiOe, probably in this order, since the melting point of the former is lower than the latter; subsequently, these compounds unite in part with more lime to give 3CaO -- SiOz and 8CaO - Al,O;. The formation of the last two compounds is a slow process in mixtures of their own composition but is hastened in the ternary mixtures by the circumstance that a portion of the charge has already melted and acts as a flux or solvent. Since 3CaO- S102 is the most important constituent in Portland cement, the necessity for burning at a high enough temperature to sinter the raw materials is readily understood. At a temperature somewhat above 1335°, the conversion of 5CaO - 3A1,03 to 3CaO - Al,O3? is complete and the important compound 3Ca0O - SiOz is forming rapidly. At 1475°, most of the lime has entered into combination but a complete melt is not obtained until around 1900°. ‘The final products of crystalli- zation of this melt are: 3CaO-: SiO, 2CaO- SiOz, and 3CaQO-- Al.Os. Investigations carried out on commercial Portland cements disclose that their composition is essentially the same as the clinker made from pure oxides. ‘This is best illustrated by the data recorded in Table 34 for (1) pure cement; (2) a commercial white cement; and (8) the more common gray variety of com- mercial Portland cement. All three are made up largely of the same constituents. It is of interest that the optical character- istics of the essential compounds persist even when these com- pounds are formed in the presence of small amounts of magnesia, iron oxide, etc. The magnesia and alkalies are apparently taken 1J. Ind. Eng. Chem., 7, 466 (1915). 2 Nore: CamMpsBeEy [J. Ind. Eng. Chem., 9, 943 (1917)] claims that tri- calcium aluminate should be regarded either as a metastable saturated solid solution of CaO in 5CaO - 3Al1,03 or as 5CaO - 3A1.03 with four molecules of CaO of crystallization rather than as a stable phase in the strict sense of the word. 388 THE HYDROUS OXIDES up in solid solution by tricalcium aluminate and dicalcium sili- cate,! and the iron oxide combines with lime to give ferrite.” The minor constituents play an important part in the burning of the cement clinker, since their presence results in the formation of a flux at a lower temperature, thereby hastening the combina- tion of lime with alumina and silica, This is evidenced by the TABLE XX XIV.—Compos!ITION OF PORTLAND CEMENTS! Relative to | Burning content of | temper- | Compounds in re- CaO, Al,Os3,-| ature, sulting cements SiO, degrees Type Actual constituents CaO —s 68.4. CaO 68.4 2CaO - SiO, SiO. 2a SiO, 23.6 3CaO - Al.O3 CaO 66.2 | CaO 67.9 2CaO - S102 Al.Os we 97.6| AlOs 6.5| 1525 3CaO - SiOz ‘|| MgO, Fe20s, Small-amount of Na.O, and eae: ; CaO K,O CaO 63.2 } CaO 66.7 2CaO - SiOz ALO: 7.7} 93:8) Al.Oe 070 [emus 3CaO - SiOz Gray. || SiOz _ 22 4] SiO, 24.3 3CaO : Al.Os "* |) MgO, Fe2Os, Small amounts of Na,O, K,O, 6.7 5CaO Al;Os, CaO, and SO3 and ferrites 1 RANKIN: J. Ind. Eng. Chem., 7, 466 (1915). data given in Table XXXIV. It should be pointed out, however, that the temperatures are not strictly comparable, since the reactions of the white and gray products are incomplete. In these cases, it is probable that the temperatures necessary for equilibrium would be somewhat higher than the values recorded. 1 RANKIN and Merwin: J. Am. Chem. Soc., 38, 568 (1916); cf. Kunin and Puituips: Highth Int. Cong. Applied Chem., 5, 81 (1912). ?Sosman and Merwin: J. Wash. Acad. Sci., 6, 15 (1916); CampBELu: J. Ind. Eng. Chem., 11, 116 (1919). CEMENT 389 Setting and Hardening.—When finely pulverized Portland cement is mixed with water, a plastic mass results which becomes solid in the course of a few hours. This process, which is called setting, is followed by a gradual increase in strength or hardening of the mass. While a good cement becomes very hard in the course of a few weeks, it may require years to attain its full strength. According to Le Chatelier,! the setting and hardening of Portland cement consist in the dissolution in water of the anhy- drous silicates and aluminates, which subsequently become hydrated. Since the hydrates are less soluble than the anhy- drous salts, the solutions become supersaturated with respect to the former and deposit an entangling mass of needles, thereby giving the cement its characteristic hardness. This theory of the hardening process was not questioned until Michaelis? recognized the formation not only of crystals but of a gel which increased gradually until it filled the interstices between the crystalline needles as well as those between the cement particles. The cementing gel was supposed to be calcium monosilicate and the crystals tricalcium aluminate and calcium hydroxide. Accord- ing to this hypothesis, the cement particles and crystals become embedded in a common sheath of gelatinous substance which imparts a degree of hardness that could not be attained by the felting of crystalline needles alone. More or less successful attempts were made to distinguish the various products of hydration of Portland cement by the use of organic dyes which stain colloidal and zeolitic minerals selectively. Such experiments led Blumenthal* to conclude that crystalline monocalcium silicate and tricalcium aluminate are among the first products of hydration and that a gelatinous silicate forms subsequently. From this, he concludes that the setting is due to crystallization alone, the later hardening process consisting of the binding together of the crystals and the filling of the pores by means of a gel. _ 1xperimental Researches on the Constitution of Hydraulic Mortars” (1887), translated by Hall (1905). 2 Kolloid-Z., 5, 9 (1909); 7, 320 (1910); Chem. Ztg., 17, 982 (1893). 3’ KEISERMANN: Kolloidchem. Bethefte, 1, 423 (1910). 4 Silikat Z., 2, 43 (1914); Thesis, Jena (1912). 390 THE HYDROUS OXIDES A systematic investigation of the setting and hardening process was possible only after the constitution of cement had been definitely established. Knowing that cement consists essentially of 3CaO- Al,O3, 8CaO-SiOs, and 2CaO- 8iOs, investigations were carried out by Klein, Phillips, and Bates! in the U. 8. Bureau of Standards to determine what happens when each of the constituents separately is brought in contact with water. The results of these and later observations are as follows: Tricalcium Aluminate—When 3CaQO: Al.O3 is mixed with water, a gelatinous hydrous material is first formed which sets so rapidly that it is almost impossible to make test pieces. It never attains a tensile strength much beyond 100 pounds per square inch. When mixed with silicate, it affects the latter more markedly in the time of set than in the strength, tending to hasten the former and retard the latter. With the limited amount of water in cement pastes, it is converted into and remains a gel during the first 24 hours, at least.2, With more water, crystalliza- tion takes place fairly rapidly. Pulfrich and Linck? isolated crystals having the composition 3CaO- Al,O3;:7H2O, which were claimed to be identical with those in cement. Duchez* claims that all calcium aluminates in cement hydrate to 3CaO -- Al,O;: 12H.O. If lime aluminates of lower basicity are present, a quantity of hydrous alumina is liberated which in turn combines with Ca(OH). from the hydrolysis of calcium silicates, forming the duodecahydrate. It is an interesting fact that 3CaO - Al,O3, the single aluminate present in Portland cement, is the only one that does not possess hydraulic properties.» This is probably because the action of water on aluminates other than 3CaO- Al.O3; gives some hydrous 3CaO - Al,O3, which later crystallizes, and gelatinous alumina which is the real cementing agent in aluminous cements. Dicalcium Silicate—The compound 2CaO: SiO» sets very slowly with water, and it is only after long intervals that sufficient 1KuEIn and Puitups: U. S. Bur. Standards, Techn. Paper 43 (1914); Batss and Kern: [bid., 78 (1916). 2 Bates: J. Am. Ceram. Soc., 2, 708 (1919). 3 Kolloid-Z., 34, 117 (1924). 4 Rock Products, 27, No. 18, 62 (1924). 5 Bates: J. Am. Ceram. Soc., 1, 679 (1918); U. S. Bur. Standards, Techn. Paper 197 (1921). CEMENT 391 gelatinous material is produced to cement the granules of com- pound together into a solid mass. After 14 days, a test piece broke at less than 60 pounds per square inch; but at the end of a year, a sample developed a tensile strength of 600 pounds per square inch. The cementing material is hydrous monocalcium silicate, which, together with calcium hydroxide, is formed by the slow hydrolysis of the silicate. Tricalcium Silicate-—3CaO - SiO» is the only one of the three major constituents which reacts with water within a reasonable time to give a mass comparable to Portland cement in hardness and strength. The end products of the reaction are hydrous monocalcium silicate and calcium hydroxide, as in the case of dicalcium silicate; but unlike the latter, tricalcium silicate hydrolyzes very readily to give the essential binding gel and so is the most important constituent in Portland cement. Summary.—When water is added to a mixture of the three constituents as they occur in Portland cement, the initial set is due to the formation of a gel of tricalcium aluminate and possibly a small amount of monocalcium silicate. Pulfrich and Linck? emphasized the importance of gel formation in the initial stage by showing that crystallization does not take place at the outset in the presence of the amount of water used in technical practice. Their observations were made in glycerin solutions in order to get the necessary dilution for microscopic examination; and the glycerin may have inhibited the crystallization. This, however, merely emphasizes the contention that setting is not necessarily occasioned by the formation of microscopically visible crystal needles. It is true, of course, that some crystallization takes place in time; but the only crystalline bodies which form ordina- rily? are calcium hydroxide* and a crystalline hydrate derived from tricalcium aluminate, probably the duodecahydrate. Whereas the initial set results primarily from the formation of tricalcium aluminate gel, the subsequent fairly rapid increase in 1 MicnaeEtts: Kolloid-Z., 5, 9 (1909); 7, 8320 (1910); DucuEz: Rock Prod- ucts, 27, 18, 62 (1924). 2 Kolloid-Z., 34, 117 (1924). 3 Cf., however, GLASENAPP: Zement, 11, 446 (1922); Ktun: [bid., 138, 362, 375 (1924.) 4 Cf. Barkov and Racozinski: J. Russ. Phys.-Chem. Soc., 47, 761 (1916). 392 THE HYDROUS OXIDES cohesive strength and hardness is due in large measure to the liberation of hydrous monocalcium silicate by the hydrolysis of the tricalcium silicate and dicalcium silicate. It is a pity that dicalcium silicate does not hydrolyze more rapidly, for it is formed at a lower temperature than tricalcium silicate, and yields ultimately a higher percentage of the important binding material, monocalcium silicate. As already noted, gypsum is added to cement clinker before erinding, in order to retard the time of set. The gypsum may function by diminishing the solubility of tricalctum aluminate or by precipitating calc1um sulfoaluminate,! thus removing lime from solution which would otherwise be available for the forma- tion of tricalcium aluminate gel. Tippermann? is of the opinion that the presence of gypsum serves two functions; it retards crystallization and aids the formation of colloids. This opinion is based on the observation that sulfate-free cements to which no gypsum is added, undergo rapid and extensive crystal formation, but no colloidal material is present at the end of a year. The addition of gypsum to such cements cuts down the rate of crystallization; but swelling, together with gel formation, takes place at once. Tippermann attributes the action of gypsum to the sulfate ion and not to calcium ion, since in the sulfate-free cement, the concentration of calcium ion varies from zero to saturated solution without gel formation entering in. ‘These observations should be confirmed and extended. Since gypsum is softer than clinker, it is probable that the gypsum particles are ground considerably finer than the cement particles. It has been suggested that the more finely divided gypsum particles coat the coarser cement particles, thereby ~ acting to some extent as a protecting film and so delaying the chemical process involved in setting.*® The addition of salts influences the time of set to a greater or lesser degree. Gadd‘ reports the results of recent observations with a large number of compounds including the carbonate, 1 Ktuu: Prot. ver. D. P. C. F., 45, 98 (1922). * Zement, 18, 1385, 147 (1924). 3 Fink: J. Phys. Chem., 21, 32 (1917); Briaes: Ibid., 22, 216 (1918). 4 British Portland Cement Research Assoc., Pamphlet 1 (1922). CEMENT 093 nitrate, chloride, sulfate, borate, and hydroxide of sodium, ammonium, aluminum, zinc, cobalt, and chromium. Of the various compounds studied, the nitrates appear to have little effect on the rate of setting, whereas all the other compounds except gypsum and plaster of Paris accelerate the set. In view of the influence of electrolytes on jelly formation,! it is not sur- prising to find that their presence has an effect on the rate of setting of cement. One would expect the addition of salts?® to have either a retarding or accelerating action, depending on whether they have a coagulating or stabilizing action on the colloids formed by the action of water on the cement particles. The addition to cement of calcium chloride or ‘‘Cal’’* materi- ally accelerates the rate of hardening of Portland cement mix- tures. This is probably due to the precipitation of a calcium chloraluminate of the composition 3CaO - Al,O3- CaCl,: 18H.0,° with an accompanying decrease in the pH value. This reduction in pH accelerates the hydrolysis of the silicates and so hastens the hardening process. Platzmann,® on the other hand, attributes the action mainly to the hygroscopicity of calcium chloride which, by absorption of moisture during the first few weeks, prevents the shrinking and cracking of the cement and protects it from too rapid a loss of moisture. CEMENTS RELATED TO PORTLAND CEMENT Iron-Portland Cement.—A cement may be prepared in which iron is substituted for aluminum. It is manufactured in much the same way as ordinary Portland cement; and as in the latter, the chief hydraulic constituent is tricalcrum silicate. The aréa occupied by cements rich in iron oxide in a triaxial diagram of the system lime-silica-iron oxide is in nearly the same position as 1 See p. 26. 2 RoHLAND: Kolloid-Z., 8, 251; 9, 21 (1911). 3 Cf. Benson, NEWHALL, and Trempsr: J. Ind. Eng. Chem., 6, 795 (1914). 4 A material resulting from the interaction of lime and calcium chloride in water. 6Laruma: “Le Ciment,’’ 174 (1925); cf., however, Kijtu~t and Unricu: Zement, 14, 859, 880, 898 (1925); GassnurR: Chem. Ztg., 48, 157 (1924). 6 Zement, 10, 499 (1921); 11, 137 (1922); Chimie & industrie, T, 943 (1922); 8, 614 (1922). 394 THE HYDROUS OXIDES that of Portland cement in the system lime-silica-alumina.! Iron-Portland cement with its lower lime content” contracts more on setting than does Portland cement. The addition of calcium chloride to iron-Portland or blast-furnace cements causes them to swell, so that the natural shrinkage is counteracted or takes place only after a long time. In Germany, up to 30 per cent of blast-furnace tae is added to Portland cement clinker giving what is called Eisen-Portland cement. This produces a superior product for sea-water con- struction, possibly because the added slag unites with any free lime, thereby preventing it from acting with the sea water to form calcium hydrosilicates* or such compounds as magnesium hydroxide® or calcium sulfoaluminate,® which are active in pro- ducing cracks. Aluminous Cement.—Cements in which the alumina content is equal to or greater than that of the silica content are known commercially as ‘‘aluminous,” ‘‘fused,”’ or ‘‘electrofused”’ cements. They are produced by fusion, because calcium alumi- nates soften readily, and clinkering is very difficult.’ As pre- viously mentioned, Bates* found that all the alumina compounds in the lime-alumina-silica system possess hydraulic properties except 3CaO- Al.O3. The compound 5CaO - 3Al.03 sets very rapidly indeed; while both 3CaO - 5Al.03 and CaO - Al.O3 set slowly but harden rapidly, developing great strength in 24 hours. Very good cements may be had with 55 to 75 per cent alumina in lime-alumina burns. Aluminous cements are manufactured extensively in France; but the chief drawback to their wide com- mercial use is the lack of a widely distributed supply of hydrous alumina and the consequent high cost of raw materials. 1 Kin: Zement, 10, 361, 374 (1921). 2 CAMPBELL: J. Ind. Eng. Chem., 11, 116 (1919). 3 GUTTMANN: Zement, 9, 310, 429 (1920). 4Cf. GassneR: Practical Questions Concerning Concrete in Sea Water, Zement, 14, Nos. 21 to 25 (1924). 5 Lewis: Engineering, 109, 626 (1920); Gary: Mitt. Material-prifungsamt, 37, 12 (1919). 6 Grtn: Zement, 12, 297, 307, 317, 326 (1924). 7 Brep: Techn. ees 14, 508 (1922); Rev. métal., 19, 759 (1922). 8 J. Am. Ceram. Soc., 1, 679 (1918); U. S. Bur. Standards, Techn. Paper 197 (1921); cf. ENDELL: Zement, 8, 319, 334, 347 (1919). CEMENT 395 The setting and subsequent hardening of aluminous cements result from the formation of tricalcium aluminate gel and hydrous alumina.! ‘The early hardening is due to the relatively high rate of hydrolysis of the aluminates. Advantages claimed for aluminous cement over Portland cement are: the more rapid rate of hardening; greater strength; and the higher temperature developed on setting, usually sufficient to permit normal harden- ing even in severe weather.’ 1 Ktuu and Tuurine: Zement, 18, 109, 243 (1924); PLatzmann: Rock Products, 27, No. 19, 23 (1924). 2 GUERITTE: Contract Record, 38, 1197 (1924); Anon.: Eng. News-Record, 94, 320 (1925). CHAPTER XIX THE SOIL In his classic work on adsorption by the hydrous oxides, van Bemmelen! advances the idea that the inorganic colloids in the soil are similar in general nature to the gelatinous oxides. ‘This idea has persisted, and according to Whitney :? ‘‘It is now coming to be quite generally believed that the inorganic colloidal material of the soil is essentially the same as the artificial gels of silica, iron, and alumina, which have been prepared.” ‘There are, however, a number of people? who champion the view that a con- siderable portion of soil colloids consists of complex acid alumino- silicates rather than a mixture of the hydrous oxides. In any event, it would seem that a volume devoted to the hydrous oxides would be incomplete without some reference to the col- loidal matter of the soil. COMPOSITION OF THE SOIL COLLOID The colloidal matter of the soil is derived from both organic and mineral sources. The organic colloidal matter consists of the remains of animal and vegetable life, together with the soil bacteria and fungi. In such organic soils as the so-called peats and mucks, the colloids are chiefly organic; but in most agricul- tural soils, the colloidal matter is of mineral origin, derived in large measure from the hydrolysis of silicates. It is difficult, if not impossible, to separate all the colloidal matter from a soil. The earlier investigators merely rubbed up the soils with a considerable amount of water and estimated as colloidal matter the amount that remained suspended for a 1 “Die Absorption,” 114 (1910); LANDER: Ber. Stat., 28, 265 (1879). 2 Bogue’s ‘‘Colloidal Behavior,” 2, 468 (1924). 3SHarP: Univ. Calif. Publ. Agr. hie 1, 291 (1916); BRADFIELD: Colloid Symposium Monograph, 1, 369 (1923); a oe Soc. Agron., 17, 253 (1925); Truoa: Colloid Symposium Monograph, 3, 228 (1925). 396 THE SOIL 397 given length of time. Schlésing! is of the opinion that the material which remains longest in suspension differs essentially from material which does not remain suspended so long and so estimates the colloid content of soils to be only 0.5 to 1.5 per cent.? Hilgard* and Williams‘ reported much higher percentages based on the amount of material that does not settle in a 24-hour period. Since the amount of soil that will remain suspended depends on the degree of peptization of a gel and the time of settling, methods of estimating the colloid content of soils based on such procedures® are necessarily inaccurate. Other methods that have been employed are based on determination of the adsorption capacity of the soil for malachite green,® water, and ammonia. Gile’ and his coworkers determine the adsorption capacity of a sample of soil and of the colloidal material extracted frem the soil, and from these data, calculate the percentage colloidal matter. After correcting for the possible alteration in adsorptive capacity of the colloid produced by extraction, the percentages of colloidal matter indicated by adsorption of mala- chite green, water, and ammonia show fairly good agreement among themselves® and with the percentages estimated gravi- metrically and microscopically. As would be expected, the colloidal content of different soils varies widely. Assuming that all particles less than lu in diameter are colloidal, the sandy soils contain but a few per cent of colloids; while the loam soils may contain 15 to 25 per cent, and the ote 40 to 50 and up to 90 per cent colloidal matter. The method of procedure employed in the Bureau of Soils, U. 8. Department of Agriculture, for separating samples of colloids from the rest of the soils is essentially as follows: The soil is suspended in distilled water or in water containing enough ammonia to impart a pH of 7 to 8. After allowing to settle for 1 Compt. rend., 70, 1345, 1870; 78, 1276; 79, 473 (1874). 2 Cf. EHRENBERG: ‘‘ Die Bodenkolloide,’”’ Dresden, 99 (1922). 3 Am. J. Sci. Arts, (3) 106, 288, 333 (1873); ‘Soils,’ New York, 333 (1919). 4 Forsch. Gebiete Agrikuitur-Physik., 18, 225 (1895). 5 ScaLtes and Marsu: J. Ind. Eng. Chem., 14, 52 (1922). 6 Asutey: U.S. Geol. Survey Bull. 388, 65 (1909). 7 Gite, MrppLeTon, Rosinson, Fry, and AnprERSON: U. S. Dept. Agr. Bull. 1193 (1924). 8 Cf. Davis: J. Am. Soc. Agron., 17, 277 (1925). 398 THE HYDROUS OXIDES 18 hours, the turbid supernatant liquid is passed through a supercentrifuge where each particle is exposed to a force of approximately 17,000 gravity for 3 minutes. The colloid which passes through the supercentrifuge is collected on the outside of a Pasteur-Chamberlain filter by sucking off the water. The average diameter of the particles obtained by this procedure is 0.1 to 0.15u, the largest being about 0.3u. The residue appears distinctly gelatinous and dries to a hard, horn-like mass. To give some idea of the composition of the soil colloids, there are given in Table X XX V the analyses of anumber of such colloids TABLE XXXV.—ComMPOSITION OF Soin CoLLoIDS Soil type Substance ‘|? |) 4) 5) oor iLO Pes Se gere nw 5 / 50.49 |50.13 |44.94 |48.04 |42.40 /36.26 [81.30 |15.86 Wee ea eee 0.51 | 0.46 | 0.47 | 0.65 | 0.56 | 0.65 | 1.01 | 3.54 AloO3..........{16.73 |21.70 |22.15 |25.19 |24.71 (32.85 |33.64 |34.38 Hee cave ei 10.77 | 8.70 | 8.91 | 8.80 {15.27 |12.44 |11.66 (22.67 MnO ee 0.121 (OA Me ee ae 2.00 NipO siete 5-82 |-2:54-1) 120506458 HO eee eae 2.24 86402507 sis ese 0.035} 0.126) 0.032) 0.138} 0.160; 0.070) 0.068 1 2 2 1 2. : Na,O...........| 0.54 | 0.24 | 0.19 | 0.88 | 0.51 | 0.47 | 0.58 | 0.33 0 0 3 3 8 7 .48 | 1.12 | 1.29 | 1.18 | 0.44 | 0.56 | 0.21 Jae As ter 0.37 09: 120707 7051s Organic matter.| 1.79 .83 | 7.94 | 4.52 9 Combined H,0.| 8.26 wf gto ron .23 .22 |12.73 |11.79 |15.63 . Ontario loam, subsoil, New York . Vega Baja clay loam, soil, Porto Rico . Cecil loamy fine sand, soil, Georgia . Aragon clay, deep subsoil, Costa Rica 1. Fallon loam, soil, Nevada 2. Sharkey clay, soil, Mississippi 3. Marshall silt loam, soil, New York 4. Carrington loam, subsoil, Iowa CONDON from widely different types of soils. This table was compiled from data obtained in the Bureau of Soils of the U. 8. Department of Agriculture.t Although the composition of the colloids from different sources may show wide variation, these differences may be relatively small in soils from similar climatic regions.” Inves- tigations on a large number of representative soils disclosed 1Cf. Gite: Colloid Symposium Monograph, 3, 218 (1925); Rosrnson and Houmss: U.S. Dept. Agr. Bull. 1311 (1924). 2 BRADFIELD: J. Am. Soc. Agron., 17, 253 (1925). THE SOIL 399 that the composition of the colloids, as compared with that of the whole soil, was much higher in alumina, ferric oxide, organic matter, water, magnesia, phosphorus, and sulfur, and lower in silica. In the ageing process to which the soil is subjected, there appears to be a tendency either for the silica to be transformed into secondary quartz or to move below the soil layer, while iron oxide and alumina accumulate in the soil colloids. The colloidal mineral matter of soils appears to be formed by the action of water on hydrated silicates of igneous origin. Whitney! believes the soil colloids are formed by the bombard- ment of soil particles by water molecules when the former are of the order of magnitude of 0.0001 millimeter in diameter; while Gordon? considers that the outer layer of all silicate parti- cles is constantly subjected to hydrolytic action. By these weathering influences, there are formed the insoluble hydrous oxides of iron, aluminum, and silicon, and the soluble salts of sodium, potassium, calcium, and magnesium, which are adsorbed in part by the hydrous oxide gels. There appears to be no con- clusive evidence as to whether the hydrous oxides remain as such in the soil, retaining more or less of the adsorbed soluble constitu- ents, or whether in the course of time there are formed complex aluminosilicates of definite composition. On account of the variability in the proportion of alumina to silica, it is obvious that no one fixed proportion would account for all the alumina or all the silica in every colloid. It is necessary, therefore, either to postulate the existence of several complex silicates or to assume that a portion of the hydrous oxides remain as such. Until the question is definitely settled, I subscribe to the simpler assumption that the inorganic soil colloids consist essentially of variable amounts of the hydrous oxides of iron, alumina, and silica with varying amounts of adsorbed salts. For the most part, the so-called aluminosilicates are adsorytion complexes of indefinite composition, formed by the mutual precipitation of negatively charged hydrous silica and positively charged hydrous alumina. It is probable that the organic material* and the 1 Science, 64, 656 (1921). 2 Science, 55, 676 (1922). 3’ EHRENBERG: Z. angew. Chem., 41, 2122 (1908); Wreaner: Kolloidchem. Bethefte, 2, 238 (1910); Fopor and ScuHornretp: Kolloidchem. Bethefte, 19, 1 (1924). ) 400 THE HYDROUS OXIDES hydrous silica keep the colloidal soil material in a dispersible state. As Gile! put it: “‘In most soils, colloidal material has probably persisted several thousand years, undergoing some changes, but remaining nevertheless a dispersible colloid. The experiment will never be performed; so it is safe to predict that pure inorganic gels would not preserve their characteristics over the same period of time.” Bradfield? points out that mixtures of artificial colloids having the same composition as colloids found in the soil do not have the same properties as the soil colloids. This, in itself, offers no proof that the soil colloids do not consist essen- tially of the hydrous oxides of iron, aluminum, and silicon together with colloidal organic matter and adsorbed salts. It would seem impossible to prepare a synthetic soil that even approaches the properties of a true soil until one can duplicate very closely the conditions of formation of the hydrous oxides and the influence of salts, organic matter, and other soil conditions which enter into soil formation. The organic colloids introduced into the soil in the form of plant and animal residues are subjected to the action of bacteria and other lower forms of life which cause porfound changes. Under aerobic conditions, that is, under conditions of good aeration such as exist in cultivated soils, the organic matter is oxidized fairly completely, giving water and carbon dioxide and the phosphates, carbonates, nitrates, and sulfates of sodium, potassium, calcium, and magnesium which are made available for new plant growth. On the other hand, under anaerobic conditions such as obtain in poorly drained and hence poorly aerated marshes and swamps, a part of the organic matter is decompsoed with the formation of a colloidal substance known as humic acid or humus. ‘This product is a dark, waxy mixture of many complex compounds.? -The so-called humic acids are formed also in drained prairie soils which are covered with a dense growth of grass. The sod provides what amounts to partial anaerobic conditions by pre- venting rapid aeration, so that dead roots, stems, and leaves of grass are in part converted into humic acid. In the presence of 1 Colloid Symposium Monograph, 3, 227 (1925). 2 Missouri Agr. Exp. Sta., Res. Bull. 60 (1923). 3 SCHREINER and SuHorey: U.S. Bur. Soils Bull. 74 (1910). THE SOIL 401 considerable calcium, the organic matter may be quite black. The dark color of prairie soils is probably due to a coating of the black organic substance on the particles of mineral matter. After the humus substance is once formed, it resists decomposi- tion when exposed to aerobic conditions, as evidenced by the fact that the cultivation of black prairie land for many years does not cause it to lose its black color. The bacteria and other living organisms constitute a very important part of the colloidal matter of the soil; but they repre- sent a very minute proportion of the total weight. RELATION BETWEEN PROPERTIES AND COMPOSITION OF SOIL COLLOIDS Although colloidal soil material contains a number of constitu- ents in variable proportions, the three major constituents in the colloids from soil other than peat soils are silica, alumina, and ferric oxide. As a result of investigations carried out in the U. 8. Bureau of Soils, it has been demonstrated that the prop- erties of soil colloids vary fairly regularly with the contents of the major constituents as expressed by the molecular ratio of silica to alumina plus ferric oxide. This is well illustrated in Table XXXVI, compiled by Anderson and Mattson.? In this table, a series of colloids extracted from different soils is arranged in ascending order of the molecular ratio, silica to alumina plus ferric oxide. In columns 3 and 4, respectively, are given the heats of wetting in calories per gram of colloid and the amounts of ammonia gas adsorbed per gram of colloid. In columns 4, 5, and 6, the data of 2, 3, and 4 are expressed relatively, in order to make the relationships more apparent and to bring out individual exceptions. The evidence indicates that the correlation between heat of wetting and ammonia adsorption will hold fairly well for practically all soil colloids,* whereas the relationship between the molecular ratio of silica to alumina plus ferric oxide and the 1 ANDERSON: J. Agr. Research, 28, 927 (1924); Grin, et al.: U. S. Dept. Agr. Bull. 1193 (1924); Rosinson and Houtmgs: U. S. Dept. Agr. Bull. 1131 (1924). 2 Science, 62, 114 (1925). 3 Cf. Bouroucos: Soil Science, 16, 320 (1924). 402 THE HYDROUS OXIDES TABLE XXX VI.—RELATION BETWEEN COMPOSITION AND PROPERTIES OF Sort CoLLoips Actual values for Relative values for Source of colloidal material Ps Heat of | NHsad- fed. wetting, | sorbed, Heat of | NH3;ad- Ale O3 + FeO; | calories | grams | AleO3 + Fe2O3 wetting | sorbed Cecil subsoil...... 1.20 4.5 0.0192 0 0 3 Cecil soil... .a Colloid Symposium Monograph, 2, 132 (1924); cf. Kina: Wis. Agr. Exp. Sta., Sixth Rept., 189 (1889); Auway and Kina: J. Agr. Research, 14, 27 (1917). ® Bouroucos and McCoou: Mich. Agr. Exp. Sta., Tech. Bull, 31 (1916); 36 (1917); Parker: J. Am. Chem. Soc., 48, 1011 (1921). 4 Foore and Saxton: J. Am. Chem. Soc., 38, 588 (1916). THE SOIL 409 in a granular structure, thus preventing them from being blown or washed away and providing for aeration. The quantity of colloidal matter in a soil does not differ greatly, as a rule, from the quantity of the “clay fraction” given by various systems of mechanical analysis. Guile! points out, however, that in certain cases the nature of the clay fraction may be a more important factor in determining how a soil will act than the quantity of this fraction.2 It is possible to increase the colloidal content of sandy soil by the direct addition of a plastic clay and to cut down the plasticity of a clay soil by the addition of sand; but this method of controlling the relative proportion of suitable colloidal to non-colloidal material is too expensive to use in the ordinary farming operations. Sand-clay roads, however, are constructed by mixing sand with plastic clay, which serves as a binder. Acidity of the Soil.—The so-called acidity of the soil is prob- ably due in large measure to selective adsorption. If one shakes fuller’s earth with distilled carbon-dioxide-free water and filters, the filtrate is neutral to litmus and to phenolphthalein, showing the absence of soluble base or acid. Now, if a dilute sodium chloride solution is shaken with fuller’s earth and filtered, the filtrate is acid to litmus or the phenolphthalein. Obviously, this is not because the fuller’s earth is acid, but because it adsorbs the base from the sodium chloride solution more strongly than the acid, giving the solution an acid reaction. Similarly, if a piece of litmus paper is pressed against moistened fuller’s earth, the paper turns red, and if fuller’s earth is added to a faintly alkaline solution of phenolphthalein, the red color disappears. Bancroft reports that the adsorbing power of fuller’s earth is so great that an acre-foot, as soil, would adsorb 30,000 pounds of lime, thus making the fuller’s earth about equivalent in acidity to a 2 per cent solution of sulfuric acid. Not only do clays and certain hydrous oxides, such as hydrous silica and manganese dioxide,‘ show this selective adsorption, but van Bemmelen® 1Cf. Proc. Am. Soc. Civil Eng., 51, 892 (1925). 2 MippLETON: J. Agr. Research, 28, 499 (1924). 3Campron: J. Phys. Chem., 14, 400 (1910); Bancrort: “Applied Col- loid Chemistry,” 121 (1921). 4Van BEMMELEN: “Die Absorption,” 445 (1910). 5 “Die Absorption,” 454 (1910). 410 THE HYDROUS OXIDES reports that colloidal humus substance decomposes small amounts of solutions of ammonium chloride, carbonate, phosphates, and borates, the base being adsorbed more strongly than the acid, giving the solution an acid reaction. The same results are obtained by digesting either a humus-rich or a clay-rich soil witha solution of ammonium chloride. Gile! showed conclusively that silica gel has a beneficial action on the growth of plants supplied with rock phosphate by increasing the quantity of phosphoric acid in solution. This is due to decomposition of the rock phos- phate by stronger adsorption of hydroxyl ion than of hydrogen ion by the silica gel. In the light of these observations, it appears evident that a part and possibly the larger part of the so-called soil acidity results from selective adsorption of the basic constituent of certain salts.2, This view has been supported by Salter and.Mor- gan® as a result of recent observations on the change in acidity of certain soils with variation in the soil-water ratio. In general, it was found that the variation in hydrogen ion concentration agrees with the distribution of hydrogen ions between soil and solution which could be expected if controlled by an adsorption mechanism. The conclusion is reached that the reaction of a soil is dependent on three factors: the total dissociated acid pres- ent; the adsorptive capacity of the soil for hydrogen ion; and the soil-water ratio. The selective adsorption theory of soil acidity is opposed by those who believe that the acidity is due to aluminosilicic acids and humic acid which are relatively insoluble but are soluble enough to give the soil solution an acid reaction. Bradfield gets a kind of end point on titrating dilute solutions of strong bases with acid colloidal clays by either the conductivity or hydrogen electrode method. This is considered as proof of a neutralization reaction between a strong base and weak soil acid, the anion of which is a particle of colloidal dimensions. 1 Gite and Smitu: J. Agr. Research, 31, 247 (1925). 2 Harris: J. Phys. Chem., 18, 335 (1914); Noyss: J. Ind. Eng. Chem., 11, 1040 (1919); Kappmn: Landw. Vers.-Sta., 96, 306 (1920); Matrson: Kolloidchem. Bethefte, 14, 296 (1922). oJ. Phys. Chem: 27, 117 (1928), 4J. Am. Chem. Soc., 45, 2669 (1923). THE SOIL 411 Returning to the case of fuller’s earth and salt referred to at the beginning of this section, one may write the equation for the hydrolysis of sodium chloride as follows: Na + Cl’ + H.0 5 Na’ + OH’ + H’ + Cl’ Since fuller’s earth adsorbs hydrxhyl ion more strongly than hydrogen ion, it displaces this equilibrium to the right, giving the solution an acid reaction. Now if one adds a base, it will tend to displace the equilibrium in the opposite direction, and one will obtain what amounts to an end point when the amount of alkali added just neutralizes the increased tendency of sodium chloride to hydrolyze as a result of preferential adsorption of hydroxyl ion by the fuller’s earth. But this is an entirely differ- ent thing from fuller’s earth itself being a weak acid that is neutralized by a strong base. Stating the matter in another way: In the presence of a certain concentration of hydroxy] ion,. the adsorption capacity of the fuller’s earth is satisfied for this ion, and the hydrolysis of the sodium chloride remains the same as in the absence of fuller’s earth. ‘The concentration of all ‘strong bases required to bring about this result would be the same, provided the cations of all bases are adsorbed equally and have the same effect on the adsorption of hydroxyl ion. Actu- ally, the cations of strong bases are not all adsorbed to the same extent, and the concentration will not be identical for different bases. For the present at least, there appears no reason for regarding the so-called end point in the titration of acid soils as proof of the existence of a definite soil acid which yields a colloidal anion. It should be mentioned in passing, that pseudo end points are not infrequently encountered in adsorption phenomena. ‘Thus, in the exchange adsorption with strongly polar adsorbents! such as kaolin, one gets what might be termed end points at quite similar concentrations of different salts of the same cation. Loe Moreover, when the value 7 the Freundlich adsorption formula? is small, the adsorption curve may bend relatively _ sharply and take a direction nearly parallel to the concentration 1 FREUNDLICH: “Kapillarchemie,”’ 279 (1922). 2 FREUNDLICH: “ Kapillarchemie,” 156 (1922). 412 “THE HYDROUS OXIDES axis, thereby giving what might be interpreted as an end point above which the adsorption increases but little with increasing concentration. Bradfield! determined the hydrogen ion concentration of vary- ing concentrations of colloidal clay and compared the results Normality of Acetic Acid 0.02 004 0.06 008 010 Ole 014 46 | i 5 ee pianos - Ei eee of fen Ch vote Fig. 31.—The effect of concentration of colloidal clay and acetic acid upon the hydrogen ion concentration. with similar determinations on acetic acid. Colloidal material was extracted from an acid clay by the aid of the supercentrifuge, and a sol was prepared containing 12.8 per cent of oven-dried material. From this stock solution, dilutions containing 6.4, 3.2, 1.2, 0.8, 0.4, 0.2, 0.05, and 0.25 per cent were prepared and the hydrogen ion concentration determined. Various concentrations 1J, Phys. Chem., 28, 170 (1924). THE SOIL 413 of acetic acid from 0.000025 to 0.1 N were also prepared and their hydrogen ion concentrations determined. ‘The results are plotted in Fig. 31. It will be seen that the relationship between concentration of acetic acid solution and its hydrogen ion con- centration is nearly linear at very low concentrations and becomes exponential at higher concentrations. Similarly, the relationship for colloidal clay is about linear at high dilutions, exponential at intermediate dilutions, and almost constant at higher concentrations. The similarity in the two curves leads Bradfield to regard the colloidal clay as a weak acid which behaves like acetic acid. Personally, I cannot see how the evidence justifies this conclusion. It would seem that with equal pro- priety one might assume that the acid or mixture of acids formed as a result of preferential adsorption of hydroxyl ion behaves similarly to acetic acid as regards change in pH with increasing concentration. + Truog? likewise supports the view that the acidity of soils is due to the presence of relatively insoluble aluminosilicic acids, but claims that the electrometric method is unsuitable for deter- mining the hydrogen ion concentrations of the very slightly buffered solutions such as are obtained with relatively low soil- water ratios. ‘The reason for this is that slight diffusion of potassium chloride from the connecting bridge, slight contamina- tion of alkali from glassware, slight impurities in the hydrogen, presence of nitrates in the soil, and shghtly contaminated or so-called poisoned electrodes can easily effect the reaction of slightly buffered soil suspensions and solutions.” He, therefore, determined the hydrogen ion concentration of soil-water extracts colorimetrically after filtering out all the colliodal matter with a special ultrafilter. When the soils were thoroughly washed to remove excess soluble salts, the hydrogen ion concentrations of the ultrafiltrates appeared to be fairly constant at soil-water ratios of 1 to 2, 1 to 20, and 1to50. Salter and Morgan failed to obtain a constant hydrogen ion concentration measured poten- tiometrically at varying salt-water ratios, and so concluded that soil acidity was not due to complex soil acids. The constancy in hydrogen ion concentration at varying soil-water ratios as 1Wawpo.e: J. Chem. Soc., 105, 2521 (1914). 2 Colloid Symposium Monograph, 3, 228 (1925). 414 THE HYDROUS OXIDES determined by Truog’s method was offered as proof that acidity of the soil is due to colloidal acids. Now if the acidity is due to relatively insoluble colloidal acids, the surface ionization will give hydrogen ion and a cation of colloidal dimensions, as claimed by Bradfield. When such a suspension is filtered through an ultrafilter which holds back all the negatively charged colloidal particles, it will obviously hold back their hydrogen ion equivalent, so that from this point of view, the hydrogen ion concentration determined as Truog does it is due entirely to the molecularly dissolved complex acid. Since the degree of dissociation of such an acid is probably very slight even at high dilutions, the solubility of “clay acid” necessary to get a pH value of 4 would be quite appreciable. Truog should make a careful investigation of his perfectly clear ultrafiltrates; for if he can show that these ultrafiltrates contain only complex aluminosilicic acid and humic acids in molecular solution, then the problem is solved. It is altogether unlikely that the alleged complex acids, if they exist, are as soluble as Truog’s data would suggest. Until we know more of the nature and composition of Truog’s ultrafiltrates, it 1s impossible to give an intelligent interpretation of his observations. . It should be mentioned, in conclusion, that Schreiner and Shorey,! Olin,? and others have demonstrated the existence in the soil of definite compounds possessing an acid character; — but the cases in which these compounds are present in sufficient quantities to give an acid reaction are rare. FLOCCULATION AND DEFLOCCULATION A suspension of soil colloid is made up of negatively charged particles and is, therefore, flocculated readily by the addition of salts containing cations that are relatively strongly adsorbed. As a result of his investigations on the flocculation of kaolin, Bodlander? introduced the term ‘‘threshold value” of electrolytes, which was defined as the concentration necessary to cause rapid flocculation. Hall and Mouson‘* determined the precipitation 1 U. 8. Bur. Soils, Bull. 47, 70, 74, 77, 80, 83, 87, 88, 98. 2 Ber., 45, 651 (1912). 3 Jahrb. Mineral., 2, 141 (1893). 4 J. Agr, Sct., 2, 251 (1907). THE SOIL 415 concentration of various chlorides, sulfates, and nitrates on colloidal clay. The order of precipitating power of the cations beginning at the greatest is: hydrogen, aluminum > calcium, barium, magnesium > potassium > sodium; and the order of stabilizing power of the anions is hydroxyl > sulfate > nitrate > chloride. ‘The order of a series of acids beginning with hydro- chloric, which has the greatest precipitating power, is: hydro- chloric > nitric > sulfuric > mono-, di-, and tri-chloracetic > acetic > oxalic, tartaric > amido acetic, citric, phenol. The last three exert no precipitating action. Since colloidal clays owe their charge to preferential adsorption of hydroxyl ion, one should expect the precipitation value of hydroxides to be higher than that of neutral salts. Bradfield! reports that 1.4 milliequivalents of potassium are required to coagulate a certain soil colloid when present as chloride, and 14 milliequivalents as hydroxide; while 10 milliequivalents are required with a mixture of 19 parts chloride and 1 part hydroxide; and 14 milliequivalents, with a mixture of 9 parts chloride and 1 part hydroxide. The precipitation value of an electrolyte for a sol is that con- centration which results in sufficient adsorption of the precipitat- ing ion to neutralize the combined adsorption of the original stabilizing ion and the stabilizing ion added with the precipitating electrolyte or mixture of electrolytes.2 The precipitation value of potassium chloride is much lower than of potassium hydroxide, since chloride ion is adsorbed much less strongly than hydroxyl ion by colloidal clay. Mixtures of potassium chloride and hydroxide cause coagulation at some value in between the values for the individual electrolytes. Obviously, the effect of hydroxyl ion will be much greater at relatively low concentrations on account of the relatively greater adsorption; and above the saturation value for the adsorption of hydroxyl ion which is reached fairly sharply in the case of a strong adsorbent for a strongly adsorbed ion such as clay for hydroxyl, the precipitation value of potassium ion is fairly constant. Another factor which may come in is that, above the normal saturation value, the presence of the strongly adsorbed hydroxyl ion may actually 1J. Am. Chem. Soc., 45, 1243 (1923). 2 WeIsEeR: J. Phys. Chem., 25, 680 (1921). 416 THE HY DROUS OXIDES increase the adsorption of the precipitating cation to such a degree that the rate of precipitation in the presence of the mixture is greater than that of the same concentration of salt without any added hydroxide. This is apparently what happens in certain cases as observed by Mattson! in Ehrenberg’s laboratory. Matt- son finds the order of precipitating power of calcium compounds for a negatively charged colloidal clay to be: calcium chloride > calcium sulfate > calcium bicarbonate > calcium hydroxide; but when a concentration a little above the precipitation value of calcium hydroxide is attained, the rate of flocculation is faster than is observed for salt concentrations considerably above their respective precipitation values. Similarly, when small amounts of sodium hydroxide are added to the clay sol, the stability is increased, as evidenced by the higher concentration of calcium sulfate required for flocculation. But when the initial concen- tration is increased to 0.002 N, in a 1 per cent clay, the rate of precipitation is appreciably greater than with calcium sulfate alone, even though the concentration of the latter is considerably above its precipitation value in the absence of sodium hydroxide. It appears obvious that, above a certain concentration, the influence of hydroxyl ion in increasing the adsorption of the precipitating calcium ion predominates over its own stabilizing action. Mattson showed that the presence of hydroxyl ion increases enormously the adsorption of calcium ion by quartz. Comber? attributes the abnormal flocculating power of calcium hydroxide above a certain concentration to its coagulating action on emulsoid matter that tends to stabilize the clay sol. Alkali hydroxides in low concentration have a stabilizing action on colloidal clay, while higher concentrations cause flocculation. In the ceramic industry, the so-called clay slip is prepared by deflocculating clay with sodium hydroxide, carbonate, or silicate. The slip can be readily poured or cast, even though it contains less water than a stiff mass of clay and water without alkali. Adding a little acid to a fluidified clay slip flocculates the mass which becomes so stiff that it will not fall from an inverted vessel. Clays carrying appreciable amounts of soluble salts, such as 1 Kolloidchem. Beihefte, 14, 241 et seq. (1922); cf. Compmr: J. Agr. Scz., 11, 450 (1922); Fopor and ScHoENFELD: Kolloidchem. Bethefte, 19, 1 (1924). 2 J. Agr. Research, 12, 372 (1922). THE SOIL 417 the sulfates of calcium and magnesium, are difficult to defloecu- late; while clays containing protective colloids, such as humus, are readily peptized. The deflocculating action of calcium hydroxide is not as marked as that of the alkali hydroxides, because of the relatively strong precipitating power of calcium ion. Nevertheless, it appears that calcium hydroxide in low concentrations may have an appre- ciable stabilizing influence on colloidal clay. The addition of lime to soil containing a large amount of deflocculated colloidal material is intended to impart a crumbly flocculent structure to the soil; but in certain instances, liming is reported to have an TasBLE XLIITIL—FLoccuLaTIon AND DEFLOCCULATION OF CLAY BY LIME Concentration in milliequivalents per liter of Character of superna- Ca(HCO;). Ca(OH), tant solution 2.02 ey Clear 2.02 1.35 Slightly cloudy 2.02 Le5r : Cloudy 2.02 1.80 Very cloudy 2.02 2.02 Very cloudy 2.02 2,25 Cloudy 2.02 2.47 Slightly cloudy oe 2.70 Clear 2.02 2.92 Clear 2.02 Boho Clear unfavorable effect on the structure. Mattson! flocculated a colloidal clay with calcium bicarbonate and then treated it with varying concentrations of lime water, with the results recorded in Table XLIII. It will be seen that calcium hydroxide in certain concentrations does have a peptizing action on clay containing bicarbonate, and it is probable that a similar condition may be encountered in a clayey soil if the lime has been used too spar- ingly. When lime is added to the soil, it is converted into the hydroxide, a part of which is adsorbed and another part of which is neutralized by the bicarbonate present. If the amount of lime added is sufficient to neutralize the deflocculating bicarbonate 1 Kolloidchem. Beihefte, 14, 276 (1922). 418 THE HYDROUS OXIDES and not enough to neutralize the adsorbed hydroxy] ion, then the lime will have an unfavorable influence on the soil structure. Under certain conditions, sodium salts! in the soil are converted in part into soda which has a strong deflocculating action on the colloidal material. If'the soil in question is permeable or sandy, the colloidal hydrous oxides and humus are washed down by the rain to a lower stratum, the depth of which is determined by the rainfall in the locality. There, the collected mass of colloidal material and fine sand hardens by desiccation forming an insolu- ble rock-like layer known as hardpan. This formation may shut off the soil beneath from air and water and, by interfering with drainage, may bring about swampy conditions. The addition of a suitable amount of gypsum to a soil containing soda, neutral- izes the deflocculating action of the latter, owing to strong adsorp- tion of calcium ion. 1 HinGarD: ‘‘Soils,’’? 62 (1906); Enrensmera: ‘Die Bodenkolloide,’’ 347 et seq. (1922). AUTHOR INDEX Abegg, 1382, 202 Aboulene, 154 Adams, 308, 359, 361 Adler, 316 Adolf, 116, 242, 243 Ahrndts, 169-171 Alexander, 7, 10, 12 Allen, A. H., 207 Allen, E. T., 104, 107, 118, 173, 291 Allmand, 174 Aloy, 292, 293 Alvarez, 308 Alway, 408 Amberger, 172, 310 Andersen, 71 Anderson, J. 8., 7, 177, 178 Anderson, M. 8., 397, 401, 402 ‘Anderson, W. C., 165 Anschiitz, 250 Antony, 72, 208, 219 Apostolo, 334 Appleby, 228 Appleyard, 286, 339 Archibald, 119 Arisz, 17 Arnold, 27 Aron, 217 Arppe, 277 Arrhenius, 10 Asbury, 155 Ashley, 397 Aspdin, 383 Atkin, 325 Atterberg, 160 Auerbach, 226, 230 Auger, 234 Austerweil, 229 Austin, 294 Avogadro, 85 Bach, 57, 301 Bachmann, 7, 8, 10, 11, 16, 23, 178, 194, 273 Baenziger, 353 Baerwind, 310 Bahr, 247 Baikov, 391 von Baikow, 76 Bailey, 245 Bailhache, 284 Balderston, 323 Baldwin, 324, 328, 329 Balz, 154, 291 Bancroft, E., 336, 345, 346 Bancroft, W. D., 6, 18, 25, 32, 74, 89, 100; 112,. 136," 137, 287; 340, 342, 346-348, 357, 358, 365, 409 Banerji, 371, 375 Barab, 43, 82, 83, 88 Barbera, 301 Barclay, 188 Barfoed, 203, 206, 207, 217, 218 Barnes, J., 235, 245, 338 Barnes, 8. K., 162 Barr, 114 Barratt, 9, 11 Bartell, 16, 189 Bartels, 153 Bartman, 301 Baskerville, 254, 260 Bassett, 86 Basu, 143 Bates, 390, 394 Bayer, 107 Baylis, 374 Bayliss, 128, 245, 364 Bechhold, 57, 370 Becker, 77, 81 419 420 Becquerel, 104, 108, 135 Bedford, 153 Beech, 344, 348, 349 Behr, 178, 184 Beilby, 72 Belden, 238 Bellucci, 155, 203, 209, 230, 231, 309, 312, 313 van Bemmelen, 1, 6, 7, 22, 23, 3C, 35-37, 42, 76, 81, 104, 106, 107, 134, 135, 137, 160, 164, 165, 175, 177, 178, 200, 204, 206, 207, 209, 211, 238, 239, 248, 2738, 275, 290, 294, 295, 396 Benedicks, 262 Benedict, 148 Bennett, 325 Benson, 393 Bentley, 115 Berger, 154 Berl, 229 Bertheim, 281 Berthier, 303 Berthold, 67 Bertrand, 301, 303 Berzelius, 79, 81, 157, 164, 202, 203, 219, 244, 282-284, 315 Betz, 147 Bied, 394 Biedermann, 23 Biehler, 311 Billitzer, 64 Billy, 237 Biltz, M., 49, 86 Biltz, W., 42, 62, 67, 89, 113, 216, 241, 246, 254, 264, 278, 285, 289, 292, 331, 358, 359, 361 Bird, 174 Birnbaum, 133 Biron, 207, 217 Bishop, 57, 59, 60 Bissel, 262 Bjerrum, 83-86, 89, 91, 118 Black, 90 Blanck, 301 Blank, 200, 201 Blencke, 129 THE HYDROUS OXIDES Bleriot, 240 Bleyer, 161, 163, 358 Blondel, 312 Bloxsom, 28 Blucher, 137, 365 Blum, L., 118 Blum, W., 116, 117, 119 Blumenthal, 389 Bobertag, 9 Bodlander, 370, 414 Boedecker, 169 Boehringer & Sons, 346 Boelter, 111 Bogaers, 190 Bogert, 370 Bogojawlenski, 365 Bogue, 9, 11, 16, 404 Bohm, 38, 103, 109, 114, 159, 163, 164, 169, 238, 240, 247, 258, 254, 260-262 Boisbaudran, Lecogq de, 129, 250, 254 Boltzmann, 5 Bonnet, A., 229 Bonnet, F., 86 Bonsdorff, P. A., 107 Bonsdorff, W., 135, 152 Bontemps, 146 Borcherdt, 199 Borjeson, 15 Bornemann, 309 Boswell, 68 Béttger, 135, 137, 226 Boéttinger, 339 Bourion, 78 Bouyoucos, 401, 407, 408 Bowers, 171 Boyer, 131 Bradfield, 48, 48, 57, 68, 73, 125, 148, 194, 396, 398, 400, 410-415 Bradford, 9, 11, 169 Braesner, 259 Brauner, 253 Bredig, 45, 296 Brescius, 34 Bridgeman, 129 Briggs, H., 192 Briggs, L. J., 407, AUTHOR INDEX Briggs, T. L., 392 Britton, 162 Brizard, 305 Brizzi, 271 Brossa, 64 Brown, 192 Browne, F. L., 43, 46, 52, 57, 67 Browne, R. J., 317 Bruce, 156 Bruni; 198, 281 Briinjes, 154, 283 Bruyn, Lobry de, 147, 303 Bryliuski, 245 Buchner, 9, 23, 32, 173 Bull, 359, 361 Bunce, 100, 174 Bunson, 67, 76 _ Binz, 278 Burger, 280, 288, 289 _ Burgstaller, 152 Burrell, 190 Burton, D., 17 Burton, E. F., 57, 59, 60, 142, 227 Bury, 224, 225 Buswell, 374, 376 Butschli, 5, 7, 8, 23 Bittner, 283 Caleagni, 230 Cameron, 47, 409 Campbell, 165, 167, 387, 388, 394 Carnegie, 133 Carnelley, 104, 131, 133, 155, 173, 204, 233, 253 Carnot, 151 Carobbi, 260 Caron, 111 . Carrara, 118 Carson, 225 Cassius, 218 Casthelez, 80 Castro, 310 Catlett, 260, 375 Chance, 69 Chaney, 188, 191 Charriou, 126, 379 Chase, 258 421 Chattaway, 145 Chatterji, 44, 89, 116, 148, 148, 299 Chaudrion, 291 Chen, 271 Chiari, 17 Chilesotti, 284 Chrétien, 48 Christenson, 304 Church, 89, 222 Clark, F. W., 276, 277 Ciat Creel oD Clark, W. M., 368, 371, 373, 374, 377, 380 Classen, 208, 237 Claus, C., 305-307, 309, 311 Claus, C. F., 69 Clavari, 155 Cleve, 245-247, 250, 254, 260-262 Coehn, 152 Coggeshall, 30 Cohen, 174 Collins, 207, 210, 214, 217, 218 Comber, 416 Comey, 171 Condrea, 71 Conrad, 274 Cooke, 150, 154 Coolidge, 184, 188 Corfield, 278 Cossa, 104, 108 Cotton, 54, 55 Cottrell, 171 Coxe, 377 Creighton, 148, 144 Crenshaw, 173 Croll, 201 Crombie, 43 Crooks, 259 Crow, 271 Crowther, 408 Crum, 104, 112, 338 Cushman, 30 Dallyn, 375 Damiens, 258, 259 Dammer, 70 Daniels, 191 422 D’ Ans, 69 Darke, 9 Daubrawa, 274 Davidheiser, 183 Davidson, 280 Davies, 34 Davis, E. C. H., 181 Davis, G. H. B., 75 Davis, R. O. E., 397 Davison, 325, 329, 358 Davy, 202 De Boeck, 295, 299 Debray, 41, 47, 219, 306, 307, 310 De Forcrand, 140, 169, 170 Deichler, 278 Deisz, 11, 29, 297 Delacroix, 274, 276 Delaporte, 375 Demoly, 238, 234 Denham, 83 Dennis, 129, 181, 199, 200, 258, 323 Desch, 47, 114, 248 Deville, 111, 164, 166, 310 Dhar, 44, 59, 89, 98, 116, 142, 148, 148, 150, 151, 168, 278, 299 Dickson, 68 Diesselhorst, 266, 269, 291 Ditte, 107, 164, 165, 224, 226, 264 Dollifies, 198 Donath, 148, 150 Donnan, 17-19, 21, 85, 144, 318, 319 Douglas, 332 Dozzie, 45 Dreaper, 355, 365 Dreyer, 332 Drummond, 240, 249 Duchez, 390, 391 Duclaux, 47, 49, 53-56 Dudley, 157, 312 Dullberg, 263, 265 Dullo, 108 Dumanski, 47, 53, 54, 266, 285 ° Dumas, 315 Durfee, 348 Dutailly, 146 Dutoit, 171 THE HYDROUS OXIDES Eberman, 184, 187 Kbler, 145, 194 Ebner, 77 Eckardt, 225 Edison, 153 Edwards, 271, 374 Ehrenberg, 397, 399, 418 Ehrhart, 310 Ehrlich, 289 Hitner, 254 Elbs, 135 Elliot, 151 Elliott, 8 Endell, 79, 394 Engel, 203, 205-207, 214 Engels, 281, 289, 290 Englehardt, 146 Engler, 237 Erdmann, 153, 154 Erikson, 128 Erk, 253 Errera, 268 Espel, 154 Kuler, H., 128, 129, 137, 260 Euler, U., 137 Fahrion, 321 Failyer, 406 Fall, 27 Faraday, 72 Farnau, A., 254, 256, 258 Farnau, E. F., 187, 149, 151, 254, 365 Fehrmann, 230 Fellner, 194 Fells, 192 Férée, 76 Feucht, 150 Field, 8 Fieldner, 190 Figuier, 157 Finch, 100, 144 Fink, 392 Firth, 192 Fischer, A., 7 Fischer, H. W., 35, 45, 47, 67, 68, 71 83, 89, 148 ; AUTHOR INDEX Fischer, M., 17 Fischer, W., 9, 89, 101, 229 Flade, 9 Flemming, 197 Flenner, 403, 405 Flinn, 370 Fodor, 399, 416 Foerster, 144, 155, 171 Follenium, 172 Foote, 37, 408 Foster, 322, 328 Fouts, 254 Fowles, 137, 138 Franke, 303 Frankenheim, 8 Franz, 210 Fremery, 205 Fremy, 76, 81, 111, 145, 147, 203, 204,. 207, 209, 217, 274, 294 Fresenius, 118 Freundlich, 6, 42, 48, 55, 62-66, 70, 94, 95, 120-122, 128, 148, 159, 168, 184, 264-267, 283, 286, 291, 310, 321, 331, 332, 411 Frey, A., 248 Frey, W., 312 Fricke, 77, 78, 103, 109, 110, 129, 130, 169-171 Friedemann, 57, 60 Frieden, 47, 48, 317 Friedrich, 119 Friend, 75 Fritzsche, 263 Fry, 397 Fuller, 162 Fulton, 191 Furness, 191 Firstenhagen, 339: Gadd, 392 Gain, 271 Galecki, 63 Gallun, 320, 328 Ganguly, 299 Gann, 120 Ganswindt, 344, 346, 347, 350, 355 Gardner, 151, 348 423 Garelli, 254, 334 Garnier, 261 Gary, 394 Gassner, 393, 394 Gaudechon, 407 Gaurilow, 438 Gay Lussac, 112, 173, 202, 219 Geer, 131 Geloso, 301 von Georgievics, 286, 339 Gerland, 270 Germann, 71 Germs, 227 Gessner, 267 Geuther, 76, 227, 230, 274 Ghosh, 59, 98, 299 Gibbs, 287 Giesy, 47 Giglio, 72 Gilbert, 189, 363, 364 Gile, 397, 398, 400, 401, 407, 409, 410 Giolitti, 39, 48, 47 Girard, 208 Gjaldbaek, 164 Glasenapp, 391 Glaser, F., 154 Glaser, M., 152, 295 Glasstone, 226-228 Glazebrook, 174 Glendenning, 145 Gmelin, 161 Gnehm, 353 Goldmann, 271, 321 Goldschmidt, 216, 225 Gonnerman, 195 Gooch, 294 Goodwin, 72, 88 Gordon, D., 70 Gordon, N. E., 361, 363, 399, 403-405 Gorgeu, 294, 295, 303 Goris, 190 Gortner, 9 Gottschalk, 291 Goudriaan, 118, 169-171 Graham, 28, 33, 42, 82, 118, 141, 193, 204, 206, 214, 215, 234, 280, 289, 333 424 Green, 191 Greenfield, 376 Greider, 181, 184 Grimaux, 27, 42, 44, 64, 67, 194 Grimm, 184 Grinberg, 237 Grindley, 10 Grobet, 118, 171 Groéger, 145, 147 Groschuff, 194 Gross, 179 Grouvelle, 164 Grover, 88 Grube, 150 Griin, 394 Grundemann, 194 Gueritte, 395 Guibourt, 67 Guichard, 106, 184, 283, 284 Guignet, 76 Gunz, 276 Gustafson, 331 Gutbier, 146, 278, 305-307 Guttman, 394 Guy, 299 Guyard, 271 Guzzmann, 88 Gye, 195 Habasian, 119 Haber, 103, 159, 161, 237, 238 Habermann, 135 Haeffley, 209 Hagen, 145 Hahn, 108 Hakamori, 120 Hake, 167 Hall, A. D., 414 Hall, V. J., 170 Hammel, 235 : Hance, 200 Hanes, 152 Hantzsch, 47, 89, 114, 117, 147-149, 161, 170, 171, 201, 225, 229, 248 Hardin, 156 Hardy, F., 194 Hardy, W. B., 6-8 THE HYDROUS OXIDES Hargreaves, 69 Harms, 137 Harned, 328 Harris, 405, 410 Harrison, 11—14 Hartmann, 172 Hartung, 23 Hase, 265 Hatfield, 374, 375, 377 Hatschek, 5, 11, 180 von Hauer, 263 Hauser, 244, 272, 278 Haushofer, 200 Havrez, 339, 342 Hawley, 229 Hay, 89, 204 Hedvall, 110, 136, 148, 149-153, 165, 233 Heermann, 353 Hefftner, 274 Heinz, 210 Hermann, 245, 273, 303, 344, 345 Hertzmann, 243 Herz, 89, 117, 170, 229, 302 Herzfeld, 210, 344 Herzog, 316 Hess, 265 Hewis, 277 Hey, 322 Heyer, 193 Heyrovsky, 118, 377 Hildebrand, 89, 117, 171 Hilgard, 397, 418 Hillebrand, 178 Hober, 70 Hofmann, 310 Hofmeister, 17, 146 Holdcroft, 110 Holleman, 190 Holmes, 27, 168, 180, 181, 192, 197, 398, 401, 402 Hooker, 143 Hoover, 373 Hopkins, 379 Hoskinson, 284 Hough, 333 How, 333 AUTHOR INDEX Howell, O. R., 155 Howell, W. H., 9 Hue, 77 Hulett, 174 Hummel, 347-350 Hiimmelchen, 295 Humphrey, 267 Hiittig, 30, 31, 37, 280, 288, 289, 293 Hittner, 151 Ichlopine, 154 Ipatiev, 153 Ishazaka, 120, 126 Iszard, 200, 201 Ives, 249 Iwanitzkaja, 120 Jackson, 171, 333 Jacobi, 309 Jaeger, 227 Jager, 259 James, 164, 262 Jander, 118, 120, 273, 275 Jandraschitsch, 116 Javillier, 301 Jettmar, 333 Joannis, 140 de Joannis, C. L., 172 Jobling, 69 _ Johnson, E. B., 199 Johnson, F. M. G., 183 Johnson, L., 63 Johnston, 118 Joly, 305-307, 311 Jonas, 209 Jones, D. C., 184 Jones, G. W., 190 Jones, H. C., 259 Jordis, 7, 47, 138, 178, 193 Jorgenson, 208, 218 Joye, 261 Junius, 284 Justin-Mueller, 321 Juznitzky, 68 Kaestle, 250 Kahle, 195, 196 Kahlenberg, 144 Kaiser, 69 Kaiun, 226 Kalisch, 195 Kalle and Co., 174, 3038 Kappen, 295, 410 Kappf, 348 Karrer, 249 Kast, 201 Kastner,47 Kastovski, 63 Katz, 3, 4, 31 Kaufmann, S. W., 161, 169, 171 Kaufmann, W. E., 180 Kawamura, 120 Kayser, 301 Keane, 71, 73, 222 Keen, 408 Keisermannn, 166, 389 Keitschera, 345 Keller, 45 Kelley, W., 192 Kelly, M. W., 320, 322, 324, 329, 330 Kenngott, 111 Keppeler, 69 Kershaw, 355 Kimura, 156, 171 King, 408 Kingsbury, 249 Klason, 145, 282, 283, 284 Kleeberg, 163 Kleeman, 199 Klein, A. A., 388, 390 Klein, O., 170, 171 Klimenko, 259 Klobbie, 36 Klobulkoff, 173 Klosky, 235 Knapp, 166, 315, 317, 323 Knecht, 144, 211, 344, 346-351, 355, 357 Knop, 235 von Knorre, 254, 294 Koechlin, 351 Koehler, 250 Koelichen, 225 Koelsch, 181, 132 426 THE HYDROUS OXIDES Kohlrausch, 50, 180 Kohlschiitter, 105, 109, 115, 118, 135, 186, 140-142, 216, 228, 248 K6nig, 308, 309 Kopaczewski, 51 Koppel, 271, 285 Kowalwsky, 206 Kramer, 195 Kraner, 167 Krantz, 192 Krauss, 306 Kraut, 103, 205 Krecke, 41, 42 Kremann, 89, 171 Kreps, 90 Krieger, 167 Kroch, 191 Kroéger, 193, 290, 292 Kriiger, 138 Kriss, 157, 161, 163 Kruyt, 57, 60, 61, 255, 258, 268 Kithl, Hans, 392-395 Kuhl, Hugo, 203 Kuhn, A., 278 Kiihn, A., 195 Kiihnl, 47 Kikenthal, 306 Kunitz, 21 Kunschert, 170, 171 Kuriloff, 170, 172 Kurre, 280, 288 Kiister, 144 Kuznitzky, 45 Kyropoulos, 179 Lafuma, 393 Laing, 9, 10 Lamb, 184 Lamy, 133 Lander, 396 Langmuir, 189, 267 Larsen, 227 Latshaw, 191 Lea, 156, 277 Lebeau, 164, 293 Le Chatelier, 18, 79, 110, 165, 179; 384, 389 Lefort, 34, 75, 76 Lehman, 296 Leick, 180 Leide, 179, 194, 311 Leighton, 347, 355 Leiser, 292 Lenher, 15, 157, 178, 195 Lenker, 201 Leonard, 194 Leonhardt, 264, 266, 269, 283 Lepez, 220, 221 Leuchs, 306 Leune, 80 Leuze, 146 Levache, 303 Levi, 154 Levy, 237 Lewis, 394 Lewite, 272 Ley, 113, 142, 161 Leyte, 117 Lichtenwalner, 403, 405 Liebermann, 337 Liebig, 311 Liechti, 116, 339, 348, 345, 347-350, 358, 359, 361 van Liempt, 289, 290, 291 Liesegang, 181 Linck, 390, 391 Lindenbaum, 270 Linder, 47, 54, 94, 169 Linebarger, 281 Lipmann, 56 Lloyd, 7 Locke, 271 Lockemann, 68, 357 Loeb, 17, 21, 22 Loewel, 76, 81, 83 Long, 276, 277 Lonnes, 237 Lorenz, 135, 145, 170, 203, 205, 207, 216, 217, 226 Lorn, 183 Losana, 139 Lésenbeck, 193 Losev, 286, 331 AUTHOR INDEX Lottermoser, 51, 52, 115, 119, 142, 146, 156, 220, 234, 281, 290, 291, 296, 297, 332 Louth, 146 Lovelace, 189 Léw, 148 Lowe, 104, 277 Léwenstein, 178 Lowenthal, H., 47, 51, 64, 85 Léwenthal, J., 207, 216, 217 Léwenthal, R., 144, 211, 326, 344, 346-351, 355, 357 Lucchesi, 219 Lucion, 137 Lucius, 68 Liideking, 32, 226 Ludwig, 142 Luhmann, 167 Lummis, 191 Lunge, 69 Lyte, 167 McBain, 8, 9, 10 McCollum, 191 McConnell, 152 McCool, 408 McGavack, 177, 182 MaclInnes, 57 van der Made, 255, 258 Madsen, 156 Maffia, 52 Magee, 258 Magnier de la Source, 47 Mahin, 116 Mailhe, 250 Maisch, 306 Majorana, 54 Malarski, 45 Malcolmson, 197, 198 Malfitano, 47, 58, 54, 71, 74 Mallet, 206 Malyschew, 94 Manasse, 263 Manchot, 237 Marawski, 295 Mare, 259 Marchand, 164, 301 427 Marchetti, 284 Marck, 296 Marignac, 209, 272 Mark, 4 Marker, 361, 363 Marsh, 397 Martin, F., 308, 312, 313 Martin, G., 105, 108 Marzano, 235 Mascetti, 91 Masoni, 301 Massol, 170 Masson, 144 Mathews, 57 Matschak, 254 Matsuno, 65 Matthews, 347, 348 Mattson, 401, 402, 410, 416, 417 Matula, 47, 53 Mawrow, 151, 152 May, 174 Mayer, 152 Mazzetti, 165 Meade, 384 Mecklenburg, 68, 204, 205, 207, 209, 211, 212, 216, 217, 358 Meigin, 153 Meister, 353 Melbye, 143 Melikoff, 237, 259 Mellor, 110, 189, 165 Meneghini, 155 Mengel, 253, 254 Menke, 294 Menner, 178 Merwin, 36, 38, 73, 110, 195, 388 Merz, 233 Meunier, 321, 322, 334 Meyer, J., 303 Meyer, R. E., 131 Meyer, R. J., 182, 250, 253, 258, 259 Michaelis, L., 22, 65 Michaelis, W., 166, 389, 391 Michel, 47 Middleton, E. B., 56, 69, 120, 122, 126, 328 Middleton, H. E., 397, 409 428 Miller, D., 146 Miller, E. B., 127, 128, 189-191, 339, 348, 349, 371, 375-380 Miller, W., 145 Milligan, 107, 109 Millon, 173 Mills, 114 Minachi, 113 Minajeff, 347 Minot, 201 Miolati, 91 Mitchell, 310 Mitscherlich, 104, 145 Mittra, 151 Mixer, 79 Miyamoto, 75 Moberg, 83, 209 Moeller, 9, 321 Moissan, 79, 149, 152, 219, 263 Moles, 277 Mondolfo, 69, 208 Montemartini, 301 Moore, B., 293 Moore, T., 154 Moraht, 161, 163, 309 Morgan, 410, 413 Morley, 211, 234 Morozenwicz, 112 Morris, 157 Moser, 145, 146, 270, 277, 278, 289 Mott, 71 Mottsmith, 267 Mouson, 414 Mouton, 54, 55 Muck, 35 Mugge, 178 Muir, 278 Mukhopadhyaya, 57 Mulder, 226 Miiller, A., 115, 150, 243, 244, 246, 254, 262, 291 Miiller, B., 163, 358 Miiller, E., 90, 135, 136, 143, 144, 264 Miiller, J. H., 200, 201 Miiller, M., 219 Miiller, W., 154 THE HYDROUS OXIDES \ Mulligan, 192 Mum, 375 Munro, 1838 Miintz, 407 Musculus, 209 Muthmann, 284 Mutte, 357 Mylius, 194, 307 Nagel, C. F., 89, 90, 100, 101, 222 Nagel, W., 284 Nageli, 8, 55 N N N N N N N N N N N apier, 336, 346 athanson, 63 Jeidle, 42, 43, 47, 48, 82, 83, 88, 113, 338 eisser, 57, 60 ernst, 240 euberg, 23, 169 Neuhaus, 353 euhausen, 23, 183 ewall, 297 ewhall, 393 eyland, 196 Nicholas, 58, 180 Niclassen, 103, 109, 114, 159, 163, 164, 169, 238, 240, 247, 253, 254, 260-262 Nicolardot, 47 Nilsson, 129, 260 Norcom, 378 Nordenskiold, 227 Nordenson, 172 N Norton, 178 N N orthcote, 89, 222 oyes, A, A., 117 oyes, H, A., 410 Ober, 122 Odén, 94 Oechsner de Coninck, 293 Ogata, 226 Okazaka, 113 Olie, 81, 83 Olin, 414 van Oordt, 159, 161 Opdyke, 177, 183 AUTHOR INDEX Ordway, 83, 116, 206, 209, 210 Oryng, 67 Osterman, 266 Ostwald, Wilhelm, 162, 173, 174 Ostwald, Wolfgang, 4, 17, 150, 168, 317 Owen, 1838 Paal, 87, 141, 146, 154, 156, 172, 174, 278, 288, 284, 310, 311 Paddon, 340, 342 Pappada, 198, 281, 290, 291 Parenzo, 254 Parker, 408 Parks, 151 Parravano, 165, 203, 209, 230, 231, 313 Parsell, 141 Parsons, 159, 162, 163 Partington, 224, 225 Pascal, 109, 118, 169, 179, 208, 214 Patrick, 23, 177, 181-189, 191, 192 Patroni, 188, 299 Patten, 164, 302 Paucke, 68, 357 Pauh, 7, 17, 47, 53, 115, 116, 242, 243, 254, 256, 258 Pavlov, 300 Payen, 226 Paykull, 244 Pazuski, 163 Péan de St. Gilles, 34, 38, 39, 104 Pebal, 152 Péchard, 282 Pelet-Jolivet, 354, 363, 364 Péligot, 83, 135, 1387 Pelletier, 157 Pellini, 155 Perquin, 190 Pfeiffer, 301 Pfordten, 235 Phillips, A. J., 888, 390 Phillips, R., 34, 74, 103 Piceini, 271, 301 Picton, 47, 54, 94, 169 Piefke, 368 Piper, 10 429 Pirsch, 278 Pissarjewski, 237, 246, 251 Platzmann, 393, 395 Pleissner, 226, 230 Pluddemann, 69 Podszus, 249 Polack, 117 Poma, 138, 299, 328 Popp, 167 Porter, E. C., 320 Porter, E. E., 91, 124 Portillo, 277 Poser, 331 Posnjak, 32, 36, 38, 73, 214 Pott, 193 Powarnin, 322 Powis, 45 Prandtl, 262, 265 Prasad, 179, 180 Prat, 157 Preiss, 261 Prescott, 117 Preston, 183 Prideaux, 277 Procter, 3, 4, 17, 19-21, 315, 317, 319, 328, 325, 333-335 Proust, 48, 145 Prud’homme, 90, 144, 161 Puiggari, 49 Pulfrich, 390, 391 Purdy, 195 Puri, 408 Quartaroli, 139 Quincke, 6, 55 Rabe, 1383 Ragozinskii, 391 Rainer, 261 Rakuzin, 126, 128 Rammelsberg, 253, 273 Ramsay, 34, 104 Rankin, 110, 385, 387, 388 Ransohoff, 305-807 Rappe, 254 Rathsburg, 309, 310 Rauter, 141 430 THE HYDROUS OXIDES Rawson, 144, 211, 344, 346-351, 355, Rodier, 292 357 Roesti, 228 Ray, 182, 188, 195 Rogan, 47 Recoura, 77, 81, 83 Rogers, 118 Reichard, 174 Rohland, 393 Reid, E, E., 192 Rona, 56, 65 Reid, R. D., 228 Roscoe, 208 Reinders, 268, 291 Rose, H., 44, 156, 203, 206, 207, 209, Reinhardt, 69 214, 220, 233, 235, 273 Reinitzer, 87, 88 Rose, R. P., 115 Reinke, 32 Rosenheim, 217, 243, 280-282 Reisig, 208 Rosenthal, 66 Reitstétter, 128 Ross, 198 Remy, 305, 306 Roth, 225 Renz, 119, 131, 132, 162 Roth, D. M., 178 Rewald, 169 Roth, K., 172 Reychler, 67 Roth, O., 353 Reyerson, 191 Rothaug, 79 Reynolds, 174 Rothe, 332 Reyonso, 208 Rousseau, 295 Rheinboldt, 244 Rubenbauer, 117, 161, 170, 171 Rhodes, 271 Rubens, 249 Ricci, 301 Ruer, 47, 53, 227, 238-240, 242, 248 Richards, 86, 131 Ruff, 35, 278, 309, 310 Richardson, A. 8., 199 Riiger, 151 Richardson, C., 384, 386 Ruoss, 146 Richarz, 237 Rupp, 277 Richter, 219, 237 Russ, 107 Rickmann, 254 Russell, 75 Rideal, 69 Riedel, J. D., 265 . Sabanejeff, 193, 281, 289 Riedel, W., 155, 156 Sabatier, 140, 154 Rieke, 79 St. John, 188 Riesenfeld, 155 Salkowsky, 142 Riffard, 44 Salmon, 9 Rindfusz, 27 Salter, 410, 413 Rinne, 109, 111 Salvadori, 229 Rittenhausen, 142 Salvétat, 77. Roard, 340 Sampson, 201 Roberts, 159 Samsonow, 293 Robertson, P. W., 279 Sandmeyer, 145 Robertson, T. B., 3 Sanin, 354, 356, 365 Robin, 45 Santesson, 272 Robinson, F., 69 Sarason, 303 Robinson, W. O., 47, 162, 397, 398, Saville, 370 401, 402 Saxton, 37, 408 Rodewald, 32 Seala, 172 AUTHOR INDEX Scales, 397 Schaffgotsch, 161 Schaffner, 34, 76, 137, 224, 226, 276 Schalek, 66, 67, 102, 121, 224, 244, 260 Schaposchnikoff, 347, 365 Schéele, 259 — . Scheetz, 71, 73, 222 Scheibler, 292 Schenck, 141 Scherrer, 11, 228 Scheurer, 245 Scheurer-Kestner, 41, 77, 206 Schick, 173 Schiff, 87, 203, 217 Schilow, 124, 300 Schlésing, 108, 397, 406 Schlumberger, 104, 1138, 115, 116, 121, 339 Schmauss, 54 Schmidt, G., 74 Schmidt, G. C., 357 Schmidt, T., 37 Schmidt-Walter, 330 Schmitz, 201 Schneider, E. A., 114, 214, 215, 219, 225 Schneider, O., 310 Schoch, 173, 174 Schoenfeld, 399, 416 Scholz, 94, 95 Sch6énbein, 237 Schorlemmer, 208 Schottlander, 157, 259 Schreiner, 400, 406, 414 Schréder, G., 151 von Schréder, J., 316, 317, 321, 357 von Schroder, P., 31, 32 von Schroeder, E., 293 Schucht, 332 de Schulten, 75, 148, 164, 173, 302, 358 Schultz, 323 Schulz, 273 Schulze, 124, 127 Schiirer, 164 173, 204, Schuster, 268, 269 Schwalbe, 346 Schwarz, H., 141, 166 Schwarz, R., 178, 179, 194 Schwarzenberg, 152 Schwestoff, 267 Schwitzer, 339, 343, 349 Scott, 204 Sedenlinovich, 135, 136 Seguin, 315 Seidel, 231 Seifritz, 267 Sen, 98, 142, 150, 168, 278, 299 Senderens, 154, 250, 274, 276 Senechal, 78 Sensburg, 156 Serono, 276 Serra, 178 Seyewetz, 321, 334 Seymour-Jones, 86, 326 Shantz, 407 Sharp, 396. Shepherd, 385 Sheppard, 8, 180 Shidei, 104 Shorey, 400, 414 Shriner, 308 Siewert, 76, 79 Simon, A., 275 Simon, A. L., 180 Simpson, 367 Sims, 16 Singer, 191 Sisley, 353 Sjollema, 301 Skinner, 174, 301 Slade, 117 Smeaton, 382 Smith, E. F., 284 Smith, J. G., 410 Smith, O:.M.~377 Smith, R. B., 47 Smoluchowski, 121 Solstein, 295 Sommerhoff, 334 Sorel, 166 Sosman, 195, 388 431 432 van der Spek, 57, 60, 61 Spencer, H. M., 377 Spencer, J. F., 132, 258, 258 Spiro, 17 Spitzer, 135, 136 Spring, 134, 137, 295, 299 Stallo, 276, 277 Stapelfeldt, 267 Steele, 144 Steiner, 331 Steinmetz, 249 Sterba, 253, 258 Stericker, 196-199 Stevens, 246, 247 Stevenson, 178, 273 Steyer, 311 Stiasny, 22, 316, 321 Stingl, 295 Storch, 220, 221 Stéwener, 179, 194 Stiibel, 9 Suida, 116, 339, 343, 345, 358, 359 Sullivan, 301 Sulman, 199 Sundell, 69 Swan, 249 Sweet, 180 Swiontkowski, 296 Szegvary, 66, 67, 102, 121, 224, 244, 260, 268, Szilard, 142, 230, 244, 247, 262, 292 Tacchini, 154 Tagliani, 350 Tanatar, 155 Tatters, 167 Taylor, Hy Gad Taylor, H. 8., 69 Taylor, W. A., 189 Teichmann, 152 Thenard, 340 Thibault, 277 Thiel, 131, 1382 Thiele, 178 Thieler, 108 Thierault, 371, 373, 374, 377, 380 Thomann, 353 THE HYDROUS OXIDES Thomas, 47, 48, 63, 317, 320, 322, 324, 325, 328-330, 334 Thompson, 325 Thomsen, 312 Thomson, F. C., 7 Thomson, J., 297 Thorpe, 45, 69, 308 Thuau, 334 Thuring, 395 Tieri, 55 Tingle, 346 Tippermann, 392 Tommasi, 34, 103, 104, 116, 137, 188 TopsGe, 312 Tower, 150, 152, 154 Traube, 144, 237 Traube-Mengarini, 172 Traver, 249 Travers, 297 Tremper, 393 Tressler, 200 Tribot, 43 Trillat, 298, 303 Truog, 396, 418, 414 Tschermak, 178, 206, 217 Tschugaev, 154 Tubandt, 150, 155, 156 Tischer, 135, 140, 316 Tuttschew, 233 Ufer, 47, 48 Ullick, 281 Ulrich, 393 Urbain, 191, 252 Urban, 178, 184 Utescher, 358 Utz, 345 Utzino, 120 Vail, 198 Vanino, 157, 278 Van’t Hoff, 237 Vanzette, 178 Varet, 173 Vegesack, 289 Veil, 155 Venable, 238 AUTHOR INDEX Verneuil, 47, 111, 247, 250, 253, 258 Vervloet, 291 Vespignani, 118 Vesterberg, 260 Vignon, 210 Villiers, 185, 137, 303 Vincent, 76 Vogel, 158 Vorlander, 128 Voss, 156 van der Waals, 5 Wachter, 155 Waegner, 254, 260, 261 Wagner, C. L., 66, 72, 332 Wagner, R. F., 234 Walden, 193 Walker, C. H. H., 204 Walker, J., 104, 131, 133, 155, 173, 204, 238, 253, 186 Walpole, 11, 4138 Walter, 47 Washburn, 32 Waterman, H. I., 190 Waterman, H. J., 301 Waw, 191 Weaver, 103, 109 Weber, B., 118, 120 Weber, R., 204, 209 Webster, 293 Wedekind, 244, 245 Wedenhorst, 118 Weeren, 162 Wegelin, 264, 265, 290 von Weimarn, 9, 138, 15, 23-27, 94, 118 Weinland, 87, 88 Weiser, 8, 28-30, 39, 41, 56, 58, 62, 65, 69, 70, 80, 81, 87, 91, 94, 97, 99, 109, 112, 113, 115, 120, 122, 125-127, 138, 144, 210, 212, 213,217, 221, 222, 224, 283, 300, 328, 415 Weiss-Landicker, 272 Weller, 237 Welsbach, 240, 249, 250, 259 Weltzien, 108 433 Wengraf, 245 Werner, 86, 87, 142, 193, 327, 349 Wernicke, 230 Werther, 133 Weston, 370 White, 167, 249 Whiteley, 347 Whitman, 75 Whitner, 153 Whitney, 117, 122, 396, 399 Whittaker, 348, 350 Wick, 155 Wiedemann, 32 Wiegner, 399 Wiessmann, 306 Wigner, 268 Wiley, 403 Wilhelms, 237 Wilke, 245 Williams, 191, 332, 397 Williamson, 339, 349, 361, 379 Willstatter, 103, 104, 105, 205 Wilm, 307 Wilson, J. A., 17, 19-21, 317, 320, 323, 325, 328, 329 Wilson, R. E., 188 Wilson, W. H., 19 Windhausen, 77, 78 Wingraf, 338 Winkelblech, 147, 226, 227, 247 Winkler, 199, 200 Wintgen, 49, 51, 64, 85, 86, 210, 326 Wischin, 309 Wislicenus, 105, 327, 357 Witt, 253, 254 Witteveen, 149, 151 Wittstein, 157, 206, 244 Witzemann, 298, 301 Witzmann, 311 Wobling, 209 Wohler, F., 75, 108, 280, 272 Wohler, L., 69, 71, 77, 79, 81, 154, 219, 220, 239, 281, 289-291, 308, 309, 311-313 Wohler, P., 69 Wolff, 32 434 Wood, 90, 118, 161, 171, 207, 210, 211, 214, 217, 218, 226, 234, 277 Woodward, 278 Worsely, 279 Wosnessensky, 42 Woudstra, 88, 89 Wright, C. R. A., 294 Wright, F. E., 385 Wright, L. T., 42 Wyrouboff, 47, 247, 250, 253, 258 Yamada, 208, 214 THE HYDROUS OXIDES Yanek, 120 Yoe, 71, 222 Zambonini, 260 Zedner, 155 von Zehman, 105 Zimmerman, 152 Zocher, 216, 266-269 Zsigmondy, 5-8, 11, 14, 40, 46, 53, 85, 117, 178, 194; 195,32: 214, 215, 219, 220, 228, 273 Zunino, 104 SUBJECT INDEX A ‘‘Acclimatization,”’ 69, 70 Adsorption (see also this heading under several hydrous oxides). capillary theory of, 183-185 effect of neutral salts, 328 influence of hydrogen ion concen- tration, 91-94, 316, 320, 321, 324 isotherms, 185, 186, 286, 324, 330, 331, 341, 355, 360 maxima in, 331, 332 mutual (see Mutual adsorption). preliminary to chemical reaction, 245 reversibility, 332 theory of composition of sol, 48, 52, 53 of mechanism of mineral tan- ning, 325, 328 selective, in the soil, 409-411 Agar jellies, 5, 8, 12 Agate, 181 Agglomeration, prevention of, 71, 73 Albumin, 16, 65, 68, 70, 128, 298, 303, 334 adsorption by hydrous alumina, 128 of arsenious acid, 68 sol, 65, 70 mutual precipitation of ferric oxide sol and, 65 swelling, 16 Albumin-ferric oxide sol, 65 Alizarin, 268, 336, 358-361 adsorption of, 358-361 iron-alizarin lake, 359, 360 streaming double refraction, 268 Alum as coagulant in water purifi- cation, 370-380 Alumina mordant, 338-347 Aluminum oxide, anhydrous, 110- 112 corundum gems, color of, 111, 112 modifications, 110 Aluminum oxide, hydrous, 103-129, 162, 285, 319, 337-346, 356, 361-363, 365, 369-380 adsorption by cotton, 345 wool, 340 adsorption of acid and basic dyes, 361-363 effect of hydrogen ion concen- tration, 362, 363 adsorption of arsenious acid, 68 | albumin, 128 alizarin, 358, 361 ammonium ion, 406 calcium, 403, 404 casein, 128 chondrin, 128 chromate, 123, 126 Congo blue, 128 red, 364 ferrocyanide, 128 gum arabic, 126 hide, 333 magnesium, 405 phosphate, 405 potassium, 405 precipitating ions, 122-124 order of, 123, 124 sulfate, 405 tannin, 357 tuberculin, 128 ageing, 109, 160 composition, 103-107 435 436 Aluminum oxide, hydrous, fibrous, 105 floc, 371-380 glow phenomenon, 110 in dyeing, 338-347 in intestinal infections, 129 in purification of diphtheria anti- toxin, 129 of pepsin, 129 of water, 370-380 jellies, 121 mordant for cotton, silk, and wool, 339-346 reversibility of precipitation, 125- 126 sol, 112-122 action of ammonia and alkalies, 116-119 formation of aluminate, 117, 118 coagulation, 120, 121 mechanism of, 121 velocity, 121 composition, 115, 116 formation, 112-120 in presence of glucose, 120 precipitation values of electro- lytes, 122, 123 sol-gel transformation, 121 temperature-dehydration curve, 105-107 Aluminum oxide, trihydrate, 107- 110, 118 preparation, 108 x-ray analysis, 109, 110 Amethyst, artificial, 111 manganese oxide in, 302 Anaemia, treatment of, 45, 67, 200 germanium dioxide in, 200 intravenous injection of ferric oxide 45, 67 Aniline blue, streaming refraction of, 268 Antagonism of ions, 94, 98 Antimony pentoxide, hydrous, 274- 276, 337 adsorption of alkali salts, 275 double THE HYDROUS OXIDES Antimony pentoxide, hydrous, ad- sorption of phosphoric acid, 276 gels, 276 optical phenomena during dehy- dration, 275 sol, 276 vapor-pressure isotherms, 275 sulfide sol, 63 tetroxide, 277 _trioxide, hydrous, 276, 277 Arsenate jellies, 11, 27-29 Arsenic poisoning, hydrous ferric oxide as antidote, 45, 67, 68 Arsenious acid, 45, 67, 68, 163, 245, 337 adsorption of, 45, 67, 68, 245 solid solution with beryllium oxide, 163 Arsenious sulfide sol, 63, 331 ‘“‘acclimatization,”’ 70 adsorption of alkalies, 99, 124, 125 of precipitating ions, 62, 125 order, 125 precipitation by electrolytes, 57, 61, 62 effect of concentration of sol, 57, 62 stabilizing ions, 61, 62 precipitation by mixtures of elec- trolytes, 97-99 factors determining, 97, 99 Artificial gems, 110-112, 141, 302 vegetation, 197 Assistants, mordanting, 343, 348, 349 Auric oxide, hydrous, 156, 157 Aurous oxide, hydrous, 157, 158 sols, 157, 158 B Bacteria in soils, 401 Barium, malonate jellies, 8, 11 sulfate, adsorption of selenium oxychloride by, 15 gelatinous, 15, 25 jellies, 25,29 SUBJECT INDEX Battery, Edison (see Edison storage batiery). Le Clanche, manganese dioxide in, 302 Bauxite, 109 Benzene, silica gel in recovery of, 190, 191 Benzopurpurine, gelatinous crystals, 13 jellies, 11, 13 streaming double refraction, 268 Beryllium hydroxide, crystalline, 160-162 effect of heat on, 160 pure, 162 solubility in salt solutions, 162 oxide, anhydrous, 163, 164 uses, 163, 164 oxide, hydrous, 159-164, 170, 334 adsorption by, 163 ageing, 159, 161 solid solution with arsenious oxide, 163 with boric acid, 163 transformation to crystalline, - 159 _ x-ray analysis ,159 Bismuth hexoxide, 279 tetroxide, 278, 279 trioxide, hydrous, pure, 277, 278 © in ceramics, 278 sols, 278 Bleriot lamp, 240 Boric acid, adsorption of, 163 solid solution with beryllium ox- ide, 163 Bricks, ferric oxide in, 71, 73 Brine, effect on silicate of soda, 197 Bromine in tanning, 334 C Cadmium hydroxide, 172 jellies, 15 oxide, hydrous, 172, 173 Calcification in tuberculosis, 196 Calcium, adsorption by silica gel, 177 by soil colloids, 403, 404 437 Calorescence (see Glow phenomenon). Carbon, adsorption by, 188, 316, 331 Casein, adsorption of, 128 Catalyst, ferric oxide as, 69, 70 nickel oxide as hydrogenation, 153, 154 silica gel, 192 thorium dioxide, 250 uranium dioxide, 293 vanadium pentoxide, 270 Cellular or honeycomb theory of jelly structure, 5-8 Cellulose jellies, 12 Cement, 382-395 aluminous, 394, 395 dental, 164, 172 glass, silicate of soda as, 199 iron-Portland, 393, 394 magnesia (see Magnesia cement). Portland, 382-393 CaO-Al1,03-SiO2 387 clinker, 387 composition, 384-389 dicalcium silicate, 390-391 tricalcium aluminate, 390 — tricalcium silicate, 391 discovery, 383 function of gypsum in, 384, 392 manufacture, 383, 384 setting and hardening, 389, 390 effect of calcium chloride, 393 effect of salts, 392, 393 theory, 393 Pozzolana, 383 Centrifugal methods of preparing sols, 43 Ceramic pigments (see Pigments). Ceric oxide, hydrous, 252-258, 280, 285, 337 color, 253 jellies, 255, 298, 299 mordant, 254 sol, 62 action of electrolytes, 255 of radium rays, 256, 257 ageing, 255 system, 386, 438 Ceric oxide, hydrous, sol, viscosity- time curve for, 255, 256 in tanning, 254 in Welsbach mantel, 254 x-ray analysis, 253 Cerium peroxide, 254 Cerous oxide, hydrous, 258, 259 Chance-Claus process, 69 Charcoal, adsorption of arsenious acid, 68 Chlorine in tanning, 334 Cholic acid, gelatinous crystals, 13 Chondrin, adsorption by hydrous alumina, 128 Chromate, adsorption by hydrous alumina, 123, 126, 327 Chrome green, 80 mordant, 335, 347-350 Chromic acetates, complex, 87, 88 Chromic oxide, hydrous, 30, 31, 76- 102, 222, 285, 319, 337, 338, 358 adsorption by hide, 324-327 by, influence of hydrogen ion concentration on, 91, 94 of alizarin, 358 other hydrous oxide, 90, 144 oxalate, 91-94, 96 ageing, 77, 78, 160 alkaline solution, colloidal nature, 78, 89, 90 color, 80-82 factors determining, 81, 82 composition, 76, 77 glow phenomenon, 78-80 cause of, 79, 80 jellies, 27, 91, 100-102 from negative sol, 101 positive sol, 91 mordant for cotton, silk, and wool, 347-351 peptization by alkalies, 89, 90 pigment, 76, 77, 80, 81 chrome green, 80 Guignet’s green, 76, 80, 81 sol, 57, 59, 82-100, 116 composition, 83-86, 88 intermittent dialysis, 83 THE HYDROUS OXIDES Chromic oxide, hydrous, sol, mole- cules in micelle, number from membrane potential measure- ments, 84-86 negative, 89, 100, 116 precipitation by mixtures of electrolytes, 94-95 factors determining, 97, 99 _ values of salts, 91, 100 preparation, 82-89 Chromic oxide, in gems, 111, 112 Chromic sulfate, basic, 86 Chromite, 90 Chrysophenene, gelatinous crystals, 13 jellies, 11, 13 Clay, colloidal, precipitation by electrolytes, 415, 416 slip, 199 | streaming double refraction of, 268 Coagulents in water purification, 375-381 Cobalt blue, 150 glass, cause of color, 151 green, 150 Cobaltic oxide, hydrous, 151, 152 Cobalto-cobaltic oxide, hydrous, 152 Cobaltous hydroxide, 148-150 Liesegang rings of, 150 plechroism in, 149 x-ray analysis, 148 oxide, anhydrous, 150, 151 ceramic pigment, 150 dryer for paints, 151 oxide, hydrous, 90, 147-151, 222 color, 147-151 change from blue to rose, effect of nickel salt on, 148 sols, 150 solubility in alkali, 150 Collagen of hide, 319-321 adsorption by, 319, 320 isoelectric point of, 320, 321 Colloidal and molecular solutions, 287 distinction between, 287 SUBJECT INDEX Colloidal forest, 197 matter in soils, 396-418 in surface waters, 366 Columbium pentoxide, 272, 273 separation from tantalum pent- oxide, 273 sol, 272 Composition of sols, complex theory of, 47, 83, 207 Congo blue, adsorption of, 128 red, 336, 364 adsorption of, 163, 364 Contact process for sulfuric acid, 191 Corrosion of iron, 75 Corundum gems, pigments in, 110- 112, 141, 302 rubies, 111, 112 sapphires, 111 amethyst, 111, 302 emerald, 111, 141 oriental topaz, 111 Cotton, mordants for, 345, 346, 350-352, 354 adsorption of molybdenum blue by, 286 Crystal violet, adsorption of, 331 Cupric oxide, anhydrous, in artificial emeralds, 141 ceramic pigment, 141 Cupric oxide, crystalline, 135, 136, 160 Cupric oxide, hydrous, 90, 134-145, 222, 277, 337, 364 adsorption by hydrous chromic oxide, 144 of eosin, 364 effect of salts on, 364 color, 140, 141 composition, 134, 139 ' dehydration, 136, 189, 141 darkening during, effect of alumina, 141 mechanism of, 136, 139 jellies, 144, 145 effect of sulfate on, 145 sols, 141-144 coagulation, 142, 143 439 Cupric oxide, hydrous, sols, coagula- tion, effect of stirring, 142, 143 ‘“‘discharge electrolysis,” 142 fungicidal action, 143 preparation, 141-143 stabilizing agents for, 141- 143 solubility in alkalies, 148, 144 effect of tartrate, glycerin, and mannite on, 143, 144 spontaneous dehydration of, 311 stability of blue, 137-140 effect of alkalies, 137 of hydrogen peroxide, 139 of salts, 137-139 Cuprous oxide, anhydrous, as ce- ramic pigment, 146 in antifouling paints, 146 Cuprous oxide, hydrous, 145-147 sols, 146, 147 D Deflocculating action of silicate of soda, 199 Deflocculation of soils, 414-418 Dehydration curves, 38, 106, 107, 175, 176, 288, 289 Dental cement, beryllium oxide in, 164 zine oxide in, 172 Dialysis of sols, 27, 28, 43, 83 electro, 43 intermittent, 83 Di-benzoyl-l-cystine jellies, 9 Dimolybdenum pentoxide, hydrous, 282, 283 color, 282 jelly, 283 oxidation to molybdenum blue, 283 Diphtheria antitoxin, hydrous alu- mina in purification of, 129 “Discharge electrolysis,” 142 Disinfection in water purification, 367, 370 440 Donnan theory of membrane equili- bria, 17-22, 318, 319 application to swelling of gela- tin, 19-22 Drummond light, 240 Dyeing, mordants in (see Mor- dants). Dysprosium oxide, hydrous, 262 E Edison storage battery, hydrous nickel peroxide in, 155 nickelous hydroxide for, 153 Elasticity of silica gel, 180 of vanadium pentoxide sols, 267 Electro dialysis, 43 Electrometric measurements, ad- sorbed chloride in, 243 Emerald, artificial, 111, 141 Emulsion theory of jelly structure, 5 Emulsions with silica gel, 195 Eosin, adsorption of, 364 “‘Equivalent aggregate,” 48, 51, 84- 86 Erbium oxide, hydrous, 262 jelly, 262 sol, 262 Europium oxide, hydrous, 261, 262 F Fehling’s solutions, nature of, 144 Fermentation, alcoholic, manganese dioxide in stimulating, 301 Ferric oxide-albumin sol, 65 Ferric oxide, hydrous, 13, 30, 31, 34- 74, 90, 192, 222, 263, 282, 285, 319, 337, 338, 358, 361-363, 369, 381 “acclimatization ”’ of, 69, 70 adsorption of acid and basic dyes by, 361, 362 alizarin, 359, 360 ammonia, 400 and hydrous stannic oxide, mutual, 221 THE HYDROUS OXIDES Ferric oxide, hydrous, adsorption of arsenious acid, 45, 67, 68 effect of hydrogen ion concen- tration on, 361, 362 magnesium, 405 phosphate, 405 potassium, 405 precipitating ions, 69, 70 sodium hydroxide, 359 sulfate, 405 agglomeration prevented by alumina, 71, 73 antidote for arsenic poisoning, 45, 67, 68 brown, 71-73 catalytic action, 68, 69 color, 34, 35, 44, 70-74 effect of size of particles, 71-73 pigment in bricks, 71, 73 composition, 34-38 fractional precipitation, 70 jellies, 65-67 by dialysis, 67 by precipitation of sol, 66 sol-gel transformation, 9, 11, 16, 66, 121 mordant for cotton, silk, and wool, 351, 352 red, 34, 71-74 relation to yellow, 34, 35 sols, 38-65, 193, 221, 244, 283 color, 44, 45 composition, 46—54, 86 adsorption theory of, 48, 52, 53 complex theory of, 47 purity, 48 ji effect of dextrose on freezing point, 46 ~ “equivalent aggregate,’ 48, 52 for intravenous injection in anaemia, 45 freezing-point lowering, 53, 54 Graham’s, 42-44, 291 mutual precipitation of and other sols, 62-65 negative, 44—46 protective colloids for, 44, 45 SUBJECT INDEX Ferric oxide, hydrous, sols, optical properties, 54, 55 Majorana phenomenon, 54, 55 osmotic pressure, 53, 54 Péan de St. Gilles’, 38-42, 58, 291 types of precipitates from, 39, 40 yellow, 41 precipitation of by electrolytes, 55-62 effect of concentration of sol, 57-62 effect of stabilizing ion, 61, 62 heat effect, 57 order of ions, 56 precipitation values of electro- lytes, 50-58 factors determining effect of concentration, 60-62 preparation, 39, 41-45 relation of yellow to red, 46 sensitivity of, effect of non-elec- trolytes on, 65 streaming double refraction, 54, 268 sugar, effect on crystallization of, tf temperature-composition curves of, 38 x-ray analysis, 37, 38 yellow, 34, 35, 41, 71-74 stability, 73, 74 Ferric oxide, in gems, 112 Ferric oxide, monohydrate, 36, 73 Ferro-ferric oxide, hydrous, 75 Ferrous oxide, hydrous, 74, 75, 380, 381 and rate of corrosion of iron, 75 in estimating nitrites and nitrates, 75 _ sulfate as coagulent in water puri- fication, 380, 381 Fibers, adsorption of dyes by, 331 Fibrillar structure of jellies, 8-10, 12 Fibrin jellies, 11 swelling of, 16 441 Filtering agent, silica gel as, 192 Filtration in water purification, 367- 381 mechanical, 369-381 slow sand, 367-369 Fire brick, use of magnesia in, 166 Fixing agents for mordants, 356-358 theory of action, 356-358 Floc, alumina, 317-380 ferric oxide, 380-381 Flocculation of soils, 414-418 Forest, colloidal, 197 Formaldehyde, catalyst in synthesis of, 293 : in tanning, 322 Fuller’s earth, adsorption of base by, 409, 411 G Gadolinium oxide, hydrous, 261, 262 Gallium oxide, hydrous, 129, 130 action of alkalies, 129, 130 ageing, 130 composition, 130 formation, 129 gelatinous character, 130 Gases, adsorption by silica gel, 181- 184 Gelatin, 3-33, 147, 290, 303, 316 jellies, 3-33 sol, 13, 14, 64 mutual precipitation of ferric oxide and, 64 swelling, 16-23 application of Donnan theory of membrane equilibrium, 19-22 effect of hydrogen ion concen- tration, 17 of neutral salts, 21 Procter- Wilson theory, 19-22 reversibility, 22 x-ray analysis, 11 Gelatinous crystals, 18, 14 of benzopurpurine, 13 of cholic acid, 13 of chrysophenene, 13 442 Gelatinous precipitates, 3-33 conditions favoring formation, 26 structure, 13 Gelation, micellar orientation in, 10, 12 Gels, 3, 9, 15, 16, 26-31 elastic, 3, 9, 16, 31 forms, 3 non-aqueous, 23 non-elastic, 3 preparation, 15-80 structure, 3-15 vapor pressure, 30-33 von Weimarn’s theory of forma- tion, 24, 25 Gems (see Artificial and corundum gems). Germanium chloroform, 201 dioxide, hydrous, 199-201 forms, 200 in treating anaemia, 201 Germanous oxide, hydrous, 201 Gibbsite, 109 Glass, cement for, silicate of soda as, 199 manganese dioxide as decolorizer for, 302 stains for (see Pigments, ceramic). Glow phenomenon, 78-80, 110, 233, 234, 239, 273, 309 Gold sol, 63, 72, 94, 219 color, 72 Gothite, 35, 36 Guignet’s green, 76, 80, 81 Gum arabic, 128, 147, 298 adsorption by hydrous alumina, 128 Gypsum in cement, 384, 392 H Hardening of cement, 389-393 Hardpan, formation of, 418 Hargreaves-Robinson process, 69 Hematite, 35, 36 as pigment, 70 color, 70, 71 THE HYDROUS OXIDES Hexavanadic acid, 263 Hide, 314-335 adsorption of hydrous chromic oxide, 324-330 effect of hydrogen ion con- centration, 324 effect of neutral salts, 328 reversibility, 332 of sulfuric acid, 328 of tannin, 316-320 effect of hydrogen ion concen- tration, 316, 320, 321 preparation of for tanning, 314 Holmium oxide, hydrous, 262 Honeycomb or cellular theory of jelly structure, 5-8 Humic acid in soils, 400 Hydrogen ion concentration, effect on adsorption by hydrous chro- mic oxide, 91-94, 324 formation of alumina floc, 372-375 of acid and basic dyes by hydrous alumina, 362, 363 of acid and basic dyes by hydrous ferric oxide, 361-363 of tannin, 316, 320, 321 on swelling of gelatin, 17 Hydrolysis, preparation of gels by slow, 29 of indium monoiodide in air, 132 of sols by, 87-89, 112, 113, 142, 194, 216, 265 Hysteresis in dehydration of silica gel, 177 : of tantalum pentoxide gel, 273 Lei Indium monoiodide, hydrolysis in air, 132 oxide, hydrous, 181, 1382. action of alkalies and ammonia, 131 ageing, 131, 132 - composition, 131 sol, 132 Intermittent dialysis, 83 SUBJECT INDEX Intestinal diagnosis, thorium dioxide in, 250 zirondium dioxide in, 241 infection, hydrous alumina in, 129 Tonic antagonism, 94, 98 Iridium dioxide, hydrous, 311, 312 color, 311 sol, 311 sesquioxide, hydrous, 310, 311 spontaneous dehydration, 310 Iron corrosion, hydrous ferrous oxide and rate of, 75 mordants, 351, 352 weighting of silk with, 352 tanning, 333 Jellies, 3-33 agar, 5, 8, 12 aluminum oxide, 30, 121 antimony pentoxide, 276 arsenate, 11, 27, 28, 29 barium malonate, 8, 11 sulfate, 25, 26 benzopurpurine, 11, 13 cadmium, 15 cellulose, 12 _ ceric oxide, 255, 298, 299 chromic oxide, 27, 91, 100, 102 chrysophenene, 11, 13 cupric oxide, 144, 145 di-benzoyl-l-cystine, 9 erbium oxide, 262 ferric oxide, 65-67 fibrin, 11, 16 formation, by dialysis of sols, 27, 28 by metathesis, 23-30 by precipitation of sol, 26, 27 conditions favoring, 26 effect of presence of salts on, 26 of rate of precipitation on, 28, 29 from negative sol, 101 from positive sol, 91 gelatin, 3-33 443 Jellies, manganese dioxide, 298, 299 mercuric oxide, 174 molybdenum pentoxide, 283 trioxide, 280 nickelous hydroxide, 154 scandium oxide, 260 silica, 5, 6, 8, 11, 12, 175-1938, 197 soap, 8, 10 stability, 7, 8 stannic oxide, 11, 224, 273 starch, 226 structure, 3-12 cellular or honeycomb theory, 5-8 emulsion theory, 5 fibrillar, 8-12 micellar or sponge theory, 8-12 solid-solution theory, 4 swelling of, 16, 32, 33 pressures, 32, 33 titanium dioxide, 11 vapor-pressure relations, 30-33 zirconium dioxide, 243, 244 L Lake, color, 336, 358-365 iron-alizarin, 359, 360 iron-methylene blue, 363 Lanthanum oxide, hydrous, 260 sol, 260 Le Clanche battery, 302 Lead monoxide, crystalline, 227, 228 color, 228 forms, 227 polymorphism, 228 x-ray analysis, 228 | Lead monoxide, hydrous, 225-230 color, 226 composition, 226 mordant, 229 mutual adsorption of and hydrous thorium oxide, 230 Lead peroxide, hydrous, 230-232 sesquioxide, 231 sol, 231 action of electrolytes, 231 444 Lepidocrocite, 35, 36 Liesegang rings, in silica gel, 181 of cobaltous hydroxide, 150 of magnesium hydroxide, 189 theory of formation of, 168 Limnite, 35, 70 Limonite, 35, 36, 70 Litharge, 232 Lutecium oxide, hydrous, 262 M 167, Magenta, adsorption by tannin, 365 Magnesia cement, 166, 167 setting of, 166 Sorel’s, 166 application, 167 use in manufacturing fire brick, 166 in mortar making, 166 Magnesium hydroxide, 164-169, 302 rhythmic bands, 167-169 sols, 169 x-ray analysis, 164 Magnesium oxide, anhydrous, 165- 167 effect of on concrete, 167 hydration of, 165 effect of ignition temperature on rate, 165 Magnesium oxide, hydrous, as an antacid, 167 as clarifier in refining of sugar, 167 Magnetic analysis, of hydrous stan- nic oxide, 214 of silica gel, 179 Magnetite, as pigment, 71 Majorana phenomenon, 54, 266-269 Manganese dioxide, decolorizer for glass, 302 dryer for oils, 302 hydrous, 90, 222, 294-302, 337 adsorption of hydroxyl ion, 294, 295 causes hydrolysis of neutral sols, 294, 295 hydrous, THE HYDROUS OXIDES Manganese dioxide, hydrous, color, 294 effect on enzymic activity, 301 in stimulating alcoholic fer- mentation, 301 growth of plants, 301 metabolism, 301 jelly, 298, 299 oxygen carrier, 303 sols, 295-299 adsorption of ions during precipitation, 299 catalytic decomposition of hydrogen peroxide, 296, 297 preparation, 295, 297, 298 vortex rings with, 297 in amethyst, 302 in Le Clanche battery, 302 Manganic oxide, hydrous, 303 Mangano-manganic oxide, hydrous, 303, 304 color, 304 Manganous hydroxide, 302 oxide, hydrous, 302, 303 sol, 302, 303 Mars pigment, 71, 73 Mastic sol, 57 Membrane equilibria, Donnan the- ory of, 17-22, 318, 319 application to swelling of gelatin, 19-22 potential, 18, 19 equation for, 19 measurements, 84, 86 Mercuric oxide, crystalline, 173, 174, 228 color, 173, 174, 228 effect of size of particles, 173 hydrous, 173, 174 jellies, 174 sol, 173 Mercurous oxide, hydrous, 174 Metathesis, formation of jellies by, 23-30 Methylene blue-iron lake, 363 SUBJECT INDEX Micellar or sponge theory of jelly structure, 8-12 orientation in gelation, 10, 12 Micelles, chromic oxide sol, 84-86 determined by membrane-poten- tial measurements, 84-86 “equivalent aggregate,” 51, 84 ferric oxide sol, 48—52 determined by electrical methods, 48-52 number of molecules in, 48-52, 84-86 Milk of magnesia, 167 Mineral tanning, 322-335 Minerals, iron oxide, 35 Minium, 231, 232 Mixture of electrolytes, precipita- tion of arsenious sulfide sol by, 97-99 chromic oxide sol by, 94, 95, 97, 99 Molecular and colloidal solutions, 287 distinction between, 287 Molybdenum blue sol, 62, 244, 284— 288 adsorption isotherms, 286 as dye bath, 285-287 mutual precipitation of and other hydrous oxides, 285 dioxide, hydrous, 283 = pentoxide (see Dimolybdenum pen- toxide). sesquioxide, 284 trioxide, hydrates, 30, 31, 280 x-ray analysis, 280 hydrous, 280-282 jelly, 280 protective action on tungsten trioxide, 290 sol, 280-282, 289. Mordanting, theory of, 349 assistants, 343, 348, 349 of cotton with alumina, 345, 346 with chrome, 350, 351 with iron, 352 with tin, 210, 354 of silk with alumina, 344, 345 445 Mordanting, of silk with alumina, 344, 345 with chrome, 350 with iron, 351 with tin, 210, 353, 354 of wool with alumina, 339-344 assistants in, 343 with chrome, 345-350 assistants in, 348, 349 with iron, 351 with tin, 210, 352 fixing of, 353 Mordants, 336-365 acid, 337 alumina, 338-347 basic or metallic, 337 ceric oxide, 254 chrome, 347-350 fixing agents for, 356-358 theory of action of, 356-358 for cotton, silk, and wool (see Mordanting). Iron, 351, 352 tannin, 354, 356 tin, 210, 352-354 titanium dioxide, 235 vanadium pentoxide, 271 zirconium dioxide, 245 Mutual adsorption of hydrous chro- mic and other oxides, 90, 144 of hydrous stannic and ferric oxides, 221 of hydrous thorium oxide and lead monoxide, 230 precipitation of sols, 62-65 ferric oxide and albumin sol, 65 and gelatin sol, 65 and other sols, 62—65 mechanism of, 63, 64 molybdenum blue and other sols, 285 N Negative sols, 44, 45, 89, 100, 101 formation of jellies from, 101 Neodymium oxide, hydrous, 261 5 260, 446 Neodymium oxide, hydrous, color, 261 reflection spectrum, 261 sol, 261 Nickel oxide, anhydrous, 152, 153 catalytic agent for hydrogen- ation, 153 ceramic pigment, 153 color, 152 peroxide, hydrous, 155 in Edison storage battery, 155 suboxide, 153-154 Nickelic oxide, hydrous, 155 Nickelous hydroxide, 152, 153 for Edison storage battery, 153 jelly, 154 sol, 154 oxide, hydrous, 90, 152-155, 222 Night blue, adsorption of, 331 Nitrates, adsorption by zine oxide, 170 by soil colloids, 403 estimation with ferrous oxide, 75 Nitrites, estimation with ferrous oxide, 75 Nitrogen fixation, silica gel in, 191 O Oil, dryer for, manganese dioxide as, 302 vanadium pentoxide as, 271 Opacifying agent, zirconia as, 241 Optical phenomena during dehydra- tion, 175, 275 properties of sols (see Majorana phenomenon). Osmium dioxide, hydrous, 309, 310 glow phenomenon, 309 sol, 310 monoxide, hydrous, 309 tetroxide, 310 stain for biological prepara- tions, 310 Osmotic pressure of ferric oxide sols, 53, 54 Oxygen bath, 296, 297 THE HYDROUS OXIDES P Paints (see Pigments). antifouling, use of anhydrous cuprous oxide in, 146 dryer for, anhydrous cobalt oxide as, 151 manganese dioxide as, 302 vanadium pentoxide as, 271 Palladium dioxide, hydrous, 308 monoxide, hydrous, 308 sols as therapeutic agent, 308 sesquioxide, hydrous, 308 Paper, sizing of, silicate of soda in, 198 , Péan de St. Gilles’ sol, 38-42, 58, 283, 291 Peptization, formation of sols by, 82-87, 118-120, 130, 194, 215, 225, 243, 244, 264 Perchloric acid, adsorption by zir- conium dioxide, 245 Petroleum refining, silica gel in, 189, 190 Phosphate, adsorption of, 405-406 Pigments, beryllium oxide, 163 ceramic, bismuth trioxide as, 278 cobalt or Renneman’s green, 150 or Thenard’s blue, 150 cupric oxide, 141 cuprous oxide, 146 ‘nickel oxide, 153 chrome green, 80 ferric oxide, 71, 73 in bricks, 71, 73 Guignet’s green, 76, 80, 81 hematite as, 70 magnetite as, 71 Mars, 71, 73 titanium dioxide, 236 zine oxide, 172 zirconia, 245 Plaster of Paris, 382 Plasticity in soils, 409 SUBJECT INDEX Platinum dioxide, hydrous, 312, 313 monoxide, 311 sesquioxide, hydrous, 312 sol, 62 trioxide, 313 Pleochroism, in cobaltous hydroxide, 149 streaming, in vanadium pentox- ide, 269 Polymorphorism of crystalline lead monoxide, 228 Porcelain, stains for (see Pigments, ceramic). Portland cement (see Cement). Potassium, adsorption of, 405 Pozzolana, 383 Praseodymium oxide, hydrous, 259 peroxide, hydrous, 259 Precipitating ions, adsorption of, 62, 122-125 Precipitation of sols by salt pairs, factors determining, 97—100 values of electrolytes, 55-65, 91, 223, 231, 270 factors determining effect of concentration on, 60-62 of ferric oxide sol, 56-58 relation between and adsorp- tion of precipitating ions, 122-125 Procter-Wilson theory of swelling of gelatin, 19-22 Prussian blue, 57, 59 Purple of Cassius, 218, 219 purples related to, 219, 220 R Radium rays, effect on ceric oxide sol, 256, 257 Rare earths, hydrous oxides of, 252- 262 | Red lead or minium, 231, 232 Refraction, streaming double, 54, — 266-269 Refractory, thorium dioxide as, 250 zirconia as, 240 Renneman’s green, 151 447 Rhodium dioxide, hydrous, 307, 308 sesquioxide, hydrous, 307 sol, 307 Rhythmic rings). solution, 168 Rubber, vulcanized. India, swelling of, 16 zine oxide in, 172 Rubies, artificial, 111, 112 Ruthenium dioxide, hydrous, 306 sol, 306 oxide, hydrous, 305 pentoxide, hydrous, 306, 307 sesquioxide, hydrous, 305, 306 tetroxide, anhydrous, 307 Ss bands (see Liesegang Samarium oxide, hydrous, 261 peroxide, hydrous, 261 Sapphires (see Artificial and corun- dum gems). Scandium oxide, hydrous, 259, 260 ageing, 259 jelly, 260 sol, 260 Schulze’s law, qualitative nature of, 124, 127 Selenium sol, 62 Setting of magnesia cement, 166 of Portland cement, 389-393 Silica gel (see Silicon dioxide, hydrous). Silica tanning, 333, 334 Silicate of soda, 196-199 applications, as an adhesive, 196, 198 cement for glass, 199 deflocculating action, 199° detergent properties, 199 preservation of eggs, 199 printing and dye industry, 199 sizing of paper, 198 effect of brine, 197 nature of commercial, 196 preparation, 196 viscosity, 197 448 Silicon dioxide, hydrous, 30, 31, 175- 196, 200, 273, 334, 361, 362, 377 adsorption, of calcium, 403-405 of gases, 177, 181-184 of liquids from solution, 184- 188 isotherms for, 185, 186 mechanism of, 187 of phosphate, 403-405 of solids from solution, 188, 189. ageing, 179, 194 applications, 189-192 as catalyst, 192 as filtering agent, 192 fixation of nitrogen, 191 in manufacture of sulfuric acid, 191 in petroleum refining, 189, 190 in recovery of benzene from coal gas, 190, 191 in vacuum refrigeration process, 191 composition, 175, 179 effect of conditions of formation, 178 dehydration, 176, 177 hysteresis in, 177 elasticity, 180 formation, 181, 192, 194-195 of crystals in, 181 gelatinous precipitate, 13 heat of wetting, 184 improved, 192 jellies, 5, 6, 8, 11, 12, 175-193, 197 magnetic analysis, 179 rhythmic bands in, 181 sols, 62, 193-196 as protective colloid for emul- sions, 195 in treatment of tuberculosis, 195 structure, 179-182 vibration, 180 x-ray analysis, 179 THE HYDROUS OXIDES Silk, adsorption of molybdenum blue by, 286 mordants for, 344, 345, 350, 351, 353, 354 weighting of, with tin mordant, 353, 354 Silver oxide, hydrous, 156 sol, 156 Smoke screens, with titanium tetrachloride, 235, 236 Soap, 287, 366 jellies, 8, 10 solutions, streaming double re- fraction, 268 Sodium ‘‘protalbinate’’ as protec- tive colloid, 87, 142, 174, 278, 298, 303 Soil, 396-418 acidity, 409-414 role of soil colloids in, 409-414 Soil colloids, 396-418 adsorption of ammonia gas, 401; 402 bacteria in, 401 color, effect of humic acid on, 401 composition, 396—402 determination, 397, 398 relation between composition and properties, 401, 402 flocculation and _ deflocculation, 414-418 effect of bases, 416, 417 formation, 399 heat of wetting, 401, 402 humic acid, 400 organic, 400 role of, 403-414 in adsorption of salts, 403-407 of water, 407-408 in plasticity, 409 in soil acidity, 409-414 complex-acid theory, 412-414 selective-adsorption theory, 409-411 Sol-gel transformation, 9, 11, 16, 66, 121 SUBJECT INDEX Solid-solution theory of jelly struc- ture, 4 Sorel’s magnesia cement, 166 Spectrum, reflection, of neodymium oxide, 261 Sponge or micellar theory of jelly structure, 8, 12 Stannic acid, meta and ortho (see Stannic oxide, hydrous). non-existence of, 201, 211-214 Stannic oxide, hydrous, 30, 200, 202— 225, 233, 273, 334, 338, 356 action of acids, 205-209 of alkalies, 209, 210 adsorption by silk, 254 of dyes (see Mordants). of hydrochloric acid by, 206 of hydrous ferric oxide, 220- 222 of phosphoric acid, 208, 209 of potassium ferrocyanide, 216 of stannic chloride, 217 ageing, 207 in presence of nitric acid, 208 alleged forms, 203 question of isomers, 211-215 relationships between, 204 relative peptizability of, 213, 214 composition, 204, 205 absence of hydrates, 205 complex theory of, 207 jellies, 11, 223, 224 magnetic analysis, 214 mordant for cotton, silk, and wool, 210, 352-354 mutual adsorption of and other hydrous oxides, 221, 222 pepization of, by alkalies, 210 by ferric nitrate, 222 by hydrochloric acid, 207 by nitric acid, 208, 213 by sulfuric acid, 208 by washing, 225 sols, 62, 96, 215-223 ageing, 216-218 effect of tartaric acid on, 217 449 Stannic oxide, hydrous, sols, behav- ior with colloidal metals, 218-220 with other hydrous oxides, 220-223 formation, 215 purple of Cassius, 218, 219 x-ray analysis, 11, 214 Stannous oxide, hydrous, 224, 225 Starch, sol, 221, 298 swelling of, 16 Starch-iodine, adsorption by zir- conium dioxide, 244 Sterilization in purification of water, 367, 370 ; Structure of gels, 3-15 Strychnine nitrate, adsorption of, 331 Sulfur in tanning, 334 sol, 94 Sulfuric acid, silica gel in manufac- ture, 191 Swelling, albumin, 16 application of Donnan theory of membrane equilibria, 19—22 effect of hydrogen ion concentra- tion on, 17-22 of neutral salts on, 21 fibrin, 16 gelatin, 16-23 preparation of gels by, 16 pressure of jellies, 32, 33 Procter- Wilson theory of, 19-22 reversibility of, 22 starch, 16 vulcanized india rubber, 16 it Tannin, 287, 315-317, 354-357 adsorption of, by hydrous alum- ina, 316, 357 cotton, 354 gelatin, 316 hide, 316-320 magenta, 365 wool, 355 mordant, 354-356 fixing of, 356 450 Tanning, 314-335 agents, miscellaneous, 334, 335 alumina, 333 ceric oxide, 254 chrome, 323-332 iron, 333 mineral, 315, 322-335 adsorption theory of, 325, 328 chemical theory, 329, 330 criticism, 330-332 preparation of hide for, 314 silica, 333, 334 vegetable, 315-322 adsorption theory, 316, 317, 320 chemical interpretation, 321, 322 Procter- Wilson theory, 318, 319 quinones in, 322 with bromine, 334 chlorine, 334 formaldehyde, 322 insoluble powders, 334 sulfur, 334 Tantalum pentoxide, hydrous, 273, 274 glow phenomenon, 273 separation from columbium pent- oxide, 272 sols, 274 vapor-tension isotherms, 273, 274 Terbium oxide, hydrous, 261, 262 Thallic oxide, hydrous, 132, 183, 337 Thallous hydroxide, 133 Thenard’s blue, 150 Thorium dioxide, 246-250 catalyst, 250 for gastro intestinal diagnosis, 250 hydrous, 246-250, 285, 337, 365 ageing, 247, 248 adsorption of Congo red by, 364 sols, 246, 247 in Welsbach mantel, 249 refractory, 250 Thorium peroxide, hydrous, 250, 251 Thulium oxide, hydrous, 262 Tin mordants, 352-354 weighing of silk with, 3538, 354 THE HYDROUS OXIDES Titanic acids, meta and ortho (see Titanium dioxide, hydrous). Titanium dioxide, 236 hydrous, 2383-236, 338 adsorption of dyes, 234 ageing, 234 alleged forms, 233, 234 glow phenomenon, 233, 234 jellies, 235 mordant, 235 sol, 234, 235 in corundum gems, 112 in paints, 236 monoxide, 236 peroxide, hydrous, 237 adsorption of salts by, 237 sesquioxide, hydrous, 236 tetrachloride, in producing smoke screens, 235, 236 Topaz, oriental, artificial, 111 Tuberculin, adsorption by hydrous alumina, 128 Tuberculosis, calcification in, 196 silica gel in treatment of, 195 Tungsten blue sol, 62, 292 as dye bath, 292 color, 292 trioxide, anhydrous, 288 hydrous, white, 288-291 dehydration curve, 288, 289 protective action of molyb- dium trioxide, 290 sol, 269, 289-291 monohydrate, yellow, 31, 288, 289 dehydration curve, 288, 289 Turgite, color of, 70 U Uranium dioxide, anhydrous, 293 catalyst for synthesis of formal- dehyde, 293 . hydrous, 293, 337 trioxide, dihydrate, 292, 293 color, 293 sol, 292, 293 SUBJECT INDEX V Vanadium bronze, 270 dioxide, hydrous, 271 pentoxide, hydrous, 263-271 as catalyst, 270 color, 265, 270 dryer for linseed oil, 271 jelly, 269, 270 mordant, 271 sol, 267-270 dielectric constant, effect of ageing on, 268 elasticity, 267 Majorana phenomenon, 565, 266-269 precipitating action of eclec- trolytes, 269, 270 preparation, 264, 265 © streaming double refraction, 266-269 streaming pleochroism, 269 x-ray analysis, 268 sesquioxide, hydrous, 271 Vapor pressure of gels, 30-33 Vegetable tanning (see Tanning). Vegetation, artificial, 197 Vibration in silica gel, 80 Victoria blue, adsorption of, 331 Viscosity-time curves of ceric oxide sol, 255, 256 von Schroeder’s paradox, 31 Vortex rings with manganese dioxide sol, 297 W Water glass (see Silicate of soda). Water purification, 366-381 aeration in, 369 alum in, 370-880 alumina floc in, 371-380 composition, 378-380 formation, effect of anion on, 375, 376 effect of colloidal matter on, 379 451 Water purification, alumina floc in, formation, effect of colloi- dal silica, 377 effect of hydrogen ion concen- tration on, 372, 373 effect of hydrogen ion con- centration on time for, 372, 373 effect of mechanical circula- tion on time for, 373 optimum conditions for obtain- ing, 380 by disinfection, 367, 381 by filtration, 367-380 mechanical, 369-381 applicability of, 370 coagulents in, 370-381 slow sand, 267-369 applicability of, 370 by sterilization, 367, 370 coagulents in, 370-381 color, removal of, aluminum ion in, 370, 371, 378 ferric oxide floc in, 380, 381 optimum conditions for forming, 381 optimum conditions for successful, 378 use of colloidal alumina, 377 Welsbach .mantel, 163, 249, 250, 254 theory of, 249, 250 White lead, 229 composition, 229 Wool, adsorption of hydrous alu- mina by, 340 mordants for, 339-344, 347-353 of sulfuric acid by, 341 x Xanthosiderite, 35 X-ray analysis of aluminum oxide trihydrate, 109, 110 beryllium oxide, 159 ceric oxide, 253 cobaltous hydroxide, 148 452 X-ray analysis of cupric oxide, 253 ferric oxide, 11 gelatin, 11 lead monoxide, 228 magnesium hydroxide, 164 molybdenum trioxide, 280 silicon dioxide, hydrous, 11, 179 stannic oxide, 11, 214 thorium dioxide, 247 vanadium pentoxide, 268 zirconium dioxide, 239 Y Ytterbium oxide, hydrous, 262 Yttrium oxide, hydrous, 252, 334, sol, 262 Z Zine hydroxide, 171 sol, 171 Zine oxide, anhydrous, antiseptic action of, 172 color, 253, 254 use in adhesive tape, 172 THE HYDROUS OXIDES Zinc oxide, anhydrous, use in dental cements, 172 enamel pigment, 172 paints, 172 rubber, 172 hydrous, 169-172, 337 action of alkalies,.170, 171 ageing, 169 color, 253 Zirconia, uses of, 240-241 Zirconium dioxide, hydrous, 237- 241, 285, 337, 338, 365 adsorption by, 244-245, 364 ageing, 160, 238, 239 alleged form, relationship between, 239, 240 glow phenomenon, 239 jelly, 248, 244 mordant, 245 sols, 241-244 by hydrolysis of zirconium salts, 241-243 by peptization, 243, 244 x-ray analysis, 238 peroxide, hydrous, 245, 246 oa. ats x . , 4 te 1 Tt} 3 3125 01143 0697 AN HA HHI | at i ie ~~ ee - - ee ne ne ae — — oe fog OE RR ATE er ee — eR Ae eee = ~~ aa ns — — a a 8 Tan pak onsien rn nistatnaapegeae aeons = - ——— aN a SO Ra : easyer = earns = —— —— = — — ~eoe cS nee en Am Fw rien Sasickcenenoatees Pree an PO ee — : eR NIELS AP aes eae paar Siarn ~— ——— eon a A Me nae are SF SS A er ON ~* ieee - —- = Doe =e SE ee PR oe a 7 oe em ~ IN — ———— pa ees oaneeeeetienbsranses ae —~> renee ———— a a ematniaparayoapenantio a SE ~~ = eae See eealee — =~ = = = =: an : —— em — —-<~ ~ =~ a —— = - rae a on es ee ee — ws pees ere eter Soe re nd a no GaSe ee reeset ae A SN Ee eho alam Seat es Sam DURES capscioameoreee ten — ITN TERNS ae an a an SS a —— aa ~ —s ~ od —o ee = A A rn nn eee one —— Se ere ene cng SO A an RN SR AAS oO ee ames, a A en — aS RR mem cei — ae : meee ase 2 = noo Se a a ae rao en a One nn wR rn a eT re eae —_ TB EIR A AN oe A CSD OE A NAR a AAT a some NO : — Semen = oe ~ ~ 6 In a a ar Y= Seceian cpdbaaencaenhtelbsy-besmaioapactaeeie toate etek pian =: —- = —- _ area = — nena maed on S an nennaeaiens a aN oe reer = eee a | ae ea * Re : oe — mere ee eS a ten oon PN Senseo ~ Sapte orator es ~ seroma amie ——— -~76 ae ea - = “es ie on = = mond narabecoee a. a = an ates een oe a: ee eee 4 erent. ee tene sas - ~ a ~ ~. ae —— — = = ~ : a = : = : ERNE = ~s = fom