Hydrous Oxides. V

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HYDROUS OXIDES. V BY HARRY B. W€?ISER

Hydrous Cupric Oxide The gelatinous precipitate obtained by the addition of dilute alkali to a cupric salt is usually considered to have the composition C U ( O H )or ~ Cu0.H20. This is because the precipitate washed rapidly until free from the mother liquor and dried over H2S04 has the composition corresponding to a monohydrate. Spring and Lucionl recognized that the freshly precipitated oxide contains more water and assigned it the formula Cu0.2H20 or Cu0.3Hz0. Van Bemmelen2 found however that the oxide forms neither a di- nor a trihydrate. The freshly precipitated blue substance is highly hydrous, containing more than 20 mols of water to one of CuO even after pressing i t between porous earthenware for two hours. The water, at least in excess of one HzO, is adsorbed and not chemically combined in the usual sense. When the precipitate is exposed at ordinary temperatures to an artificially dried atmosphere, it loses water continuously until the vapor pressure is equal to that of the aqueous vapor in the atmosphere. From * 2H20 to * lHzO the water is held more firmly than above * 2H20 and on standing at a pressure of zero the composition approaches the properties and composition of a crystalline hydrate Cu0.H20. Van Bemmelen considers the evidence insufficient to establish the existence of a definite amorphous oxide of the composition CuO.HzO;so that the gelatinous substance must be looked upon as hydrous cupric oxide rather than hydrous hydrated cupric oxide. However, a number of investigators have claimed to get a crystalline hydrate. BecZeit. anorg. Chem., 2, 195 (1892). Tbid., 5, 466 (1894).

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querel' prepared i t by the action of dilute KOH on a basic cupric nitrate and Bottger2 obtained it by the action of concentrated NaOH on crystalline basic cupric s ~ l p h a t e . ~ P41igot4 hydrolyzed crystalline blue-violet copper ammonium nitrate ; while Villiers5 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 noted above that the amorphous hydrous oxide goes over gradually to a crystalline monohydrate on standing. When the blue hydrous oxide is allowed to dehydrate slowly in the cold or rapidly at 100" a brown substance is obtained to which S a b a t i d assigned the formula 4Cu0.H20. This loses water slowly in dry air and approaches the composition of 7Cu0.Hz0. Schaffner and Rose' thought the brown oxide was a hydrate of the formula 3Cu0.H20. In the light of van Bemmelen's observations on the dehydration of the hydrous oxide, it seems altogether unlikely that the brown oxide is a definite hydrate. This view is strengthened by the more recent work of de Forcrands and of C h e m a n ~ v . ~The former obtained several so-called hydrates, among them a brown oxide which had a composition corresponding to the formula Cu0.0.35H20; while the latter found that the mineral atacamite was changed by KOH into a dark brown substance of the approximate composition 4Cu0.H20,while soluble copper salts CuCl2, CuS04, etc. are changed by alkali, according to the conditions, to one or the other of the following compounds; 7Cu0.Hz0, 8.5Cu0.Hz0 and 9Cu0.Hz0. Of course none of these substances was a definite hydrate. Comptes rendus, 34, 573 (1852). Jour. prakt. Chem., 73, 491 (1839). Cf. Habermann: Zeit. anorg. Chem., 30, 318 (1906). Comptes rendus, 53, 209 (1861). Ibid., 120, 322 (1895). Ibid., 125, 301 (1897). 7 Schaffner and Rose: Grnelin-Kraut Handbuch des Chemie, 3, 598. 8 Comptes rendus, 157, 442 (1913). 9 Jour. Russ. Phys. Chem. SOC., 47, 1268 (1915). 1

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Lorentz’ devised the interesting method of preparing the blue hydrous oxide by electrolyzing a solution of an alkali nitrate or sulphate with a copper anode. Muller and Spitzer2 electrolyzed an alkaline copper ammonium solution with a platinum anode and obtained a black deposit that contained 95.5’33 CuO, 4.2y0 HzO and 0.5% higher oxides of copper. I n the latter case dehydration took place a t the same time as the precipitation. Muller and Spitzer showed further that the blue oxide could be suspended in alkali solution for an hour without darkening; but, if a current was passed through the solution, the particles moved to the anode and darkened at once. I n one experiment the black oxide had the composition Cu0.0.128Ha0. This is an interesting example of partial dehydration of a hydrous oxide at the anode by electric endosmose. The experiments were repeated with the blue so-called “crystalline” cupric oxide and a similar dehydration took place though it required about three times as long. Muller and Spitzer do not believe that chemically combined water can be removed by electrical endosmose and suggest that the chemical hydrate, so-called, goes over to an adsorption compound that does dehydrate; or that an unstable peroxide results at first which later decomposes to the ordinary oxide containing less water. It seems to me that neither suggestion is very plausible. What seems more likely is that the so-called crystalline hydrate is not a true hydrate a t all but is a granular hydrous oxide in which the particles are matted closer together giving a denser structure that retains the adsorbed water more strongly than a gelatinous precipitate. In line with this Kohlschiitter and Tuscher3 showed that solid copper salts as the basic nitrate, vitriol and Schonit can undergo pseudomorphous transformations to hydrous oxide or hydroxide, as they say, and finally to oxide. The transformation products from a given crystal of salt were shown conclusively to have the same shape as the Zeit. anorg. Chem., 12,438 (1896). Cf. Elbs: Zeit. angew. Chem., 1903, 291. Zeit. Kolloidchemie, 1, 44 (1906); Zeit. anorg. Chem., 50,322 (1906). Zeit. anorg. Chem., 111, 193 (1920).

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original, a circumstance that indicates that the so-called crystals of copper oxide are really pseudo-crystals that do not result from its own power of crysta1lization.l Kohlschiitter and Tuscher claim however that both the gelatinous and granular products are made up of particles of cupric hydroxide, C U ( O H ) ~ , and not of hydrous cupric oxide as I have suggested. The real difficulty is that the vapor pressure of hydrous copper oxide becomes practically zero a t a composition of approximately C u 0 . H 2 0 so that i t cannot be determined by pressure measurements whether or not a definite hydrate exists. Bancroft2 has pointed out that if a definite compound, Cu0.H20, existed with a practically zero pressure, it should form from cupric oxide in the presence of water; but it is well known that the reverse process is the one that actually takes place. Kohlschutter and Tuscher consider that the failure of CuO to take up water is due to the chemical mechanism of the process of dehydration. Their view is that the dehydration is not due to a molecular splitting off of water that may be represented by the equation CU(OH)~ CuO H 2 0 but depends on intramolecular neutralization of the H’ and OH’ ions resulting from an amphoteric dissociation. The process is represented by the equations:

+

----f

-

++

Cu(OH)2 CU” 20” Cu(0H)z A CUOZ” 2H’ OH’ CUOZ”

+ H’ +

CU” =

H2O

2cuo

I shall come back to this again in a subsequent paragraph. Kohlschutter and Tuscher electrolyzed solutions of KN03 with a copper anode using varying concentrations of solution a t varying current densities and found that “the use of a small current density corresponds to precipitation in dilute solutions. As usual smaller particles are obtained in this way which can form a denser clump so t h a t the precipitate as a whole settles down further and i t can be shown that a t a current density of 2

Cf. Kohlschiitter: Zeit. anorg. Chem., 105, 1 (1919). “Applied Colloid Chemistry,” 246 (1921).

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0.04 amp/crn2 larger ‘primary particles’ and so more voluminous ‘secondary particles’ are formed whereas the smaller particles formed a t 0.01 arnp/cm2 deposit denser packed larger individuals. On the other hand, continuing the electrolysis leads in both cases to larger and looser secondary bodies and flocks since the first-formed smaller particles serve as kernels or centers of accumulation for the condensation of the particles formed later on.” The Colloidal Oxide.-Wi th but one exception, stable colloidal solutions of hydrous cupric oxide have becn prepared only by the aid of a protective colloid. Graham1 obtained a colloid by adding KOH t o a cupric chloride solution containing sugar. The solution was deep blue at first but changed to green after dialyzing. Most of the KCl was removed and the colloid contained approximately one part of copper to two of sugar together with some COz from the air. Graham called the preparation “copper saccharate.” It was precipitated readily by boiling or by adding salts and acids forming a bluish green precipitate. A colloid that is much more stable than Graham’s colloid can be prepared using Paa1’s2 sodium salts of protalbinic or lysalbinic acids as protective colloid. The colloid moved t o the cathode under electrical stress and was therefore pwitively charged. The oxide can be obtained in the colloidal state also in the presence of ~ a s e i n . ~ Biltz4 attempted the preparation of the colloidal oxide by dialysis of a solution of copper nitrate; but found that the salt was too little hydrolyzed and passed unchanged through the dialyzer. Ley5 hydrolyzed the copper salt of succinimide and obtained a very satisfactory colloid that changes in color slowly a t room temperature but rapidly a t 70” from a blue-green to yellow-brown and finally dark brown. The course of the hydrolysis was followed by cryoscopic observations and found 1

Phil. Trans., 151,183 (1861); Comptes rendus, 59,174 (1864). Ber. deutsch. chem. Ges., 39, 1550 (1906). Rittenhausen: Jour. prakt. Chem., [21 5, 215 (1873); 7, 361 (1874). Ber. deutsch. chem. Ges., 45, 4431 (1902). Ibid., 38, 2199 (1905).

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t o be complete after l/z hour a t 70". If the succinimide formed by the hydrolysis was removed by dialysis the colloid preiipitated; so i t is probable that succinimide acts as a protective colloid. Kohlschiitter has prepared a colloidal solution without the use of a protective colloid by what he terms "discharge electrolysis."l I n this process a passivifying layer of oxide was deposited on the anode and the layer was dispersed in the liquid by a rapidly oscillating discharge. In this way a very finely divided uniform colloid was obtained which was perfectly clear in transmitted light. The color was bluish green to green at the outset but later on went over to brown oxide. Ultramicroscopic observations of the latter disclosed particles varying in size from 0.05 to 0 . 0 8 ~ . The transformation to brown oxide took place more slowly the diluter the colloid and the lower the temperature. At 12" a fairly high concentration of the green colloid was obtained but this gradually clouded up and settled out. By heating to 40" the colloid became brown quickly and the degree of dispersion increased. At lower temperatures the sedimentation not only came to a standstill as the green oxide (hydroxide) changed to brown but the latter formed a very stable colloid that did not coagulate in a month. The Color of Cupric Oxides.--It is well known that a clear blue hydrous cupric oxide can be obtained only by precipitation in the cold. I t was demonstrated by Schaffner,2 harm^,^ T o m m a ~ iSpring ,~ and Lucion5 and by van Bemmelen6 that the pure blue oxide could be obtained only at temperatures below 18" and even a t low temperatures i t retained its color only when freed from the mother liquor as rapidly as possible. The color alters slowly a t room temperature but very rapidly a t high tem1

2 3 4 5 6

Zeit. Elektrochemie, 25, 309 (1919). Ann. Chem. Pharm., 51, 168. Arch. Pharm., 121 89, 35 (1858). Bull. SOC.chim. France, 37, 197 (1882). LOC.cit. Zeit. anorg. Chem., 5,468 (1894).

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peratures going from blue through various shades of green to brown and black. The transformation t o the black oxide is accompanied by a decrease in volume of the precipitate and a loss of most of the water. Van Bemmelen' prepared 5 samples of the hydrous oxide below 15" that varied in color from skyblue through dark blue and green to dark green, the green 'colors resulting from too slow washing. He noted that the composition of the several samples dried over H2SO4was approximately the same in spite of a rather marked variation in color. Unlike the amorphous hydrous oxide the blue granular oxide prepared by Becquerel, Biittger and PCligot is said by these investigators to be stable in boiling water and to maintain its blue color when heated at 100°.2 De Forcrand3 studied the effect of heat on Pbligot's blue oxide and found that it changed in color from blue to green at 85' without loss of water. Dehydration did not start until after it had become green andde Forcrand concludes that the blue compound is a metastable substance that does not lose water. At 100' the green oxide loses water becoming olive at a composition Cu0.0.8H20 and brown a t Cu0.0.35Hz0. It is difficult to reconcile de Forcrand's observation that P6ligot's oxide changes color a t 85 " with the observations of Becquerel, Bottger, and Pkligot that the blue compound does not change color even a t 100". De Forcrand's oxides seemed to behave more like the amorphous precipitated substance. De Forcrand dissolved the blue, green, olive and brown hydrates, so-called, and the black anhydrous oxide prepared a t 140" in sufficient HN03 to form the nitrate and Sabatier4 and Joannis5 carried out similar experiments on the oxides dehydrated at 440' and at red heat. From these data de Forcrand concludes that the blue and green hydrates are isomers, the transformation from the former to the latter involving 1

LOC.cit. PCligot: Comptes rendus, 53, 213 (1861).

* Comptes rendus,

157,441 (1913).

LOC.cit. Comptes rendus, 102, 1161 (1895).

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$0.261 Cal. Between 85' and 140' the oxide loses water and undergoes progressive polymerization. H e concludes further that there is a second isomeric change between 140' and 440 ' involving $0.181 Cal. and a third between 440' and red heat involving +2 Cal. It seems to us that thermochemical evidence of this sort is insufficient to establish the existence of definite isomers. The oxide undergoes a progressive change and the data do not establish definite inversion temperatures: Wohlerl has shown that the sudden decrease in the surface energy of a number of hydrous oxides when heated quickly is sufficient t o raise the temperature 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. I think i t quite unlikely that the different colored cupric oxides are definite allotropic or isomeric forms of the oxide. The slow change in color of the hydrous oxide on standing a t ordinary temperatures is doubtless caused by a change in the character and size of the particles. Kohlschiitter and Tiischer are of the opinion that the blue compound is amorphous or pseudocrystalline CU(OH)~ while the black compound is CuO. I have already called attention t o their observation that the blue colloidal hydrous oxide consists of rather large particles whereas dehydration of these particles by heating or by standing for some time results in the formation of a stable colloid made up of distinctly smaller particles of CuO. They point out further that the blue compound kept under cold water gradually becomes denser whereas the opposite is true with the anhydrous product. On the basis of these observations and the further fact noted by Berczeller2 that the black oxide adsorbs more alkali than the blue hydroxide under certain conditions, Kohlschii tter and Tiischer conclude that the blue substance is made up of larger particles than the black. This conclusion seems t o have been reached without taking all the 1Zeit. Kolloidchemie, 11, 241 (1913); Cf. Wdiser: Jour. Phys. Chem., 26,404 (1922). 2

Biochem. Zeit., 93, 230 (1919).

.

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facts into consideration. Every one knows that copper oxide can be obtaiped in quite large particles which are black and the hydrous oxide in very much smaller particles which are blue, Moreover, it is very probable that particles of the blue hydrous colloid observed by Kohlschutter and Tuscher, were in reality clumps of very finely divided blue particles that had adsorbed water strongly forming a gelatinous mass. On heating, this relatively large gelatinous clump is broken down and the particles constituting it agglomerate to larger particles that appear black. These larger black particles are small only in comparison with the gelatinous clump and, because of their physical character, are more readily peptized than the continuous mass of hydrous particles. Unfortunately Kohlschutter and Tiischer have given the wrong impression by calling attention to Berczeller’s experiments which tend to show that the black compound adsorbs more alkali than the blue without mentioning the further fact observed by Berczeller, that the blae compound adsorbs more iodate ions than the black. The work of Bancroft on the color of cupric oxides is merely mentioned by Kohlschutter and Tiischer and the results are entirely disregarded. The former believes that the dehydration of hydrous cupric oxide is attended by an increase in size of particles with the resulting change in color from blue through various shades of green t o brown and finally black. From this point of view anhydrous cupric oxide would be blue and not black if agglomeration during dehydration were prevented. I n support of this Schenckl observed that a mixture of the hydrous oxides of Cu and A1 containing 5yo CuO remained blue even after ignition. His explanation was that agglomeration of the blue oxide was prevented by the large excess of alumina. These experiments have been repeated recently3 in Bancroft’s laboratory by Parsell who dissolved out the excess of alumina from the blue mixture of CuO and A1203 with NaOH. I n this way a distinctly blue powder was obtained containing CuO and A1203in the proportion of approxiJour. Phys. Chem., 23, 283 (1919). 26, 501 (1922).

* Ibid.,

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mately 4 to 1. I n the light of these observations it is evident that Kohlschutter and Tiischer are not justified-in their conclusion that the individual particles of black oxide are more finely divided than the blue. As a matter of fact it seems very unlikely that peptization of a light blue substance should yield a black product. The other hypothesis that the blue compound is a definite hydrate while the black is anhydrous CuO stands or falls on the correctness of the unproven assumptions that there i s a definite amorphous hydrate and that this dehydrates not by the molecular splitting off of water in the usual way but by a process termed “internal neutralization as a result of amphoteric dissociation,” to which I have already referred. The Dehydration o j Hydrous Cupric Oxide and the E j e c t of Salts on the Process.-Attention has been called to the fact that the pure hydrous oxide maintains its blue color only when precipitated below 18” and freed from the mother liquor as rapidly as possible. Tommasi’ precipitated and washed the oxide at 0 ” and then suspended i t in a number of salt solutions which were subsequently heated. He made the interesting observation that in a number of cases salts retarded rather than hastened the blackening. As a notable example the suspended precipitate did not blacken even a t 100” in contact with a dilute solution of manganous sulphate. Bancroft2 considers that the blue hydrous copper oxide is stabilized by the hydrous oxide of manganese so that the former does not blacken a t 100”. Since wool fibre is colored green by boiling with a solution of copper salts Knecht, Rawson and Lowentha13 are positive that a hydrate is not formed in mordanting wool with copper salts since a copper hydrate would blacken at the boiling point. But Bancroft answers this argument by pointing out the stabilizing action of manganese sulphate on hydrous copper oxide and concludes that wool has a similar stabilizing action on the hydrous oxide. Reasoning from analogy BanBull. SOC.chim. France, 47, 197 (1882). Jour. Phys. Chem., 18,149 (1914); 26,512 (1922). 3 “A Manual of Dyeing,” 1, 59 (1910). 1 2

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croft arrives at the important conclusion that the instability in solution of a free color base like that of fuchsine does not mean that it is instable when adsorbed by silk. Blucher and Farnau’ repeated and extended Tommasi’s experiments and report that hydrous copper oxide remains blue in the presence of small amounts of the sulphates of Mn, Zn, Ni, Al, Cr and the chlorides of Co and Mg. Since such small amounts of the salts are effective it is concluded that hydrous cupric oxide adsorbs the hydrous oxides of Mn, Al, etc., and that these adsorbed oxides act as protective colloids preventing agglomeration and the consequent blackening of the blue oxide. Kohlschiitter and Tiischer discuss the effect of salts on hydrous cupric oxide in the light of their theory of the mechanism of the dehydration process. I n order to present their theory in accurate detail as well as t o emphasize the absence of experimental proof of their fundamental assumptions, I quote a t some length from their paper: “The results of our observations have lead t o the view that the intermolecular splitting off of water by inner neutralization does not take place with a single molecule of C U ( O H )or ~ between two simple Cu(OH)2 molecules but between complex molecples already of colloidal dimensions. This is subject to the further limitation that a definite degree of dispersity must be assumed to account for the reaction taking place, for it does not take place a t all or is retarded both with very finely divided and with coarse varieties; and so in common with many other properties of disperse systems it has a maximum in a mean region of dispersity. Further, a certain concentration or agglomeration of the particles is necessary since obvioiisly the distance between them must be reduced to such an extent that they come within the common sphere of action. From this viewpoint then, the nature of the subdivision and grouping or in other words the structure of a form is occasionally of paramount importance. Jour. Phys. Chem., 18, 629 (1914).

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Hurry B. Weiser

“With the aid of these conceptions the observed phenomena will be understood and in order to show their accuracy, the single results which, in part, were obtained by detailed comparison and observation, will be summarized briefly ; while at the same time, a few facts already known will be fitted into the bounds of the theory and certain conclusions of a general character will be drawn. “In addition to the chemical considerations, the formation of CuO by oppositely directed migration of particles in connection with the cataphoresis of finely divided hydroxide sols, as well as the permanence of a preparation that has once dried, speaks for the conception that the dehydration comes about as a result of neutralization based on the amphoteric nature of Cu(OH)?. That it takes place between particles follows from its non-existence under conditions where CU(OH)~exists in a molecular state of division. Thus CuO never separates out from salt solutions in which are present Cu (0H)z molecules formed by hydrolysis, and i t never results from crystalline basic salts which contain hydroxide if these, either dry or under water, are subjected to temperatures and other influences which give rise to the splitting off of water from hydroxides of a colloidal degree of dispersity. “However, alkali solutions in which either only a little hydroxide is dissolved at first or the separation of oxide has taken place until a definite amount is present, give no oxide although they unquestionably contain CU(OH)~ which, in part, is molecularly dissolved in equilibrium with an anion containing Cu and, in part, is in a highly dispersed form. “In this an indication can be seen that a very high colloidal degree of dispersion likewise suppresses the reaction. More conclusive proof of this is that hydroxide sols prepared with the high frequency discharge at high potential remain chemically unchanged at concentrations where sols prepared by other methods change rapidly and spontaneously into oxide at ordinary temperatures. “On the other hand a dispersed hydroxide is stable in which, considering the method of preparation, larger particles

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can or must be assumed. Such obtains in the case of the transformation product of basic nitrate in consequence of the close packing of the reacting material from the outset; also in the hydroxide frozen according to Villiers ; and in the precipitation from alkali solution. “Between the extremes of least reactivity the stability first increases and then decreases with the degree of dispersity. Of particular interest in this connection is the increase in the capacity t o dehydrate in the series of polymorphic hydroxides from basic nitrate, vitriol and Schonit and also its increase with decreasing dispersity in the electrically prepared sols. “Likewise one observes an increasing tendency to split off water in consequence of decreasing dispersity in the adsorption layers according as they are formed with or without cooling ; although here the second factor, the concentration of the particles, plays a r6le. “This influence of the concentration of the particles is particularly marked in the electrical colloids. However the action of adsorption in the boundary surface is at the basis of this and shows itself further in the condensing b y cataphoresis of the colloidal hydroxide in the highly dispersed alkaline solution and in the reaction at Cu anodes where the rapid increase of potential caused by the formation of CuO occurs, the more rapidly the greater the strength of the current, that is when the more particles in the same time are deposited on the electrode. “The differences in the possibility of reaction at a given concentration must not be traced exclusively to the size of the particles as such, for one has to deal with various orders of magnitude of the particles which may be designated as primary and secondary. The structure therefore plays a r61e. “Like primary particles can in different ways build up larger individuals in which they are grouped together in a looser or denser fashion. A loose structure obviously promotes the dehydration since it affords room for the forcing out of the layer of solvent and therewith provides the opportunity for amphoteric electrolytic dissociation and reaction

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Harry B. Weiser

to the particles themselves, present in sufficient amount; while a dense grouping prevents both. “In the pseudomorphous bodies one must perhaps attribute the difference in behavior more to these factors than to the size of the particles; at all events i t appears decisive in the electrolytically produced precipitates. A small current leads to the formation of small primary but to denser secondary particles, and thereby increases the stability of the hydroxide. On the other hand, products of greater looseness result if, under otherwise constant conditions, a larger amount of precipitate is obtained by longer electrolysis ; the dehydration capacity is increased in consequence of this. “The structural relations must play a large r61e also in the promoting of the reaction by raising of the temperature and addition of electrolytes and these have therefore been considered from this point of view. Here appears to be a connection with the investigation especially of Spring and Lucion whose qualitative results were that the presence of a salt calls forth an action which is comparable to a raising of temperature while from a quantitative view the process may be described thus: that time has a resistance t o overcome, as it were, before the reaction can take its course. “It is suggested to look upon the process which this action of time adjusts, as a swelling which the hydroxides can undergo in different amounts and with different velocity according to their structure. The swelling is hastened by warming and by the addition of electrolytes; its effect consists in increasing the dispersity by loosening up of the secondary particles. I n this way is obtained the freedom for reaction of primary particles since they are unable to come in contact with the layers of solvent which are necessary for their transformation by means of amphoteric dissociation and self -neutralization. And so is explained the action which temperature and salt solutions have on the formation of CuO. To follow this influence with a characteristic series of ions and thereby to prove it without objection as the promoting of a process of swelling was unfortunately not feasible on account of a secondary

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chemical reaction with the swelling material; still the investigations of Spring and Lucion contain indications for the validity of such a view. “Finally the point of view that has been followed clears up the counterpart to the accelerating action of many materials, namely, the retarding and suppressing of the splitting off of water by the presence of others. “The latter may be treated in two groups: first, those materials which can react chemically with the Cu(OH12 forming difficultly soluble products on the surface; and second, adsorbed colloidal substances which include everything from a pure surface adsorption to almost homogeneous mixtures of the nature of adsorption compounds. “Thus, carbon dioxide, especially, prevents the formation of oxide to a marked degree by the surface formation of basic salts; and traces of different anions often act in a quite similar manner as is occasionally observed, immediately on the appearance of the hydroxide precipitate. “The already long known stabilizing influence which the presence of larger amounts of other heavy metal salts has on precipitated C U ( O H ) ~ rests, on the other hand, on the adsorption or mixing of foreign hydroxides. These manifestly suppress the amphoteric dissociation and neutralization of the surface molecules of the copper oxide in the aqueous adsorption layer since they hold apart the particles capable of reaction or make the greater part inaccessible for a swelling. The action can be shown very clearly if the electrically prepared green colloid consisting essentially of hydroxide particles is treated with a protective colloid. I t remains green after the time when an unprotected portion has changed to brown by the formation of oxide; and its particles are dehydrated by electric cataphoresis and by warming.” This survey of the work that has been done on the effect of salts on the stability of hydrous copper oxide raises a number of questions. Blucher and Farnau found that small amounts of certain salts are effective in preventing blackening at 100 O thus supporting Bancroft’s hypothesis that hydrous cupric

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Harry B. Weiser

oxide adsorbs the hydrous oxides of manganese, aluminium, etc. and that these adsorbed oxides act as a protective colloid preventing agglomeration and the consequent blackening of the blue oxide. When wool is brought in contact with a copper salt it adsorbs hydrous cupric oxide and according to Bancroft the latter is stabilized by adsorption so that it does not blacken a t 100". These two cases are not the same. I n the first instance the hydrous cupric oxide is the adsorbing phase and in the second, the adsorbed phase. While there is no a priori reason why hydrous cupric oxide may not be stabilized both by being adsorbed and by adsorbing a protective agent, it seems to me that neither conclusion necessarily follows from the other. It will be recalled that Kohlschutter and Tuscher accept Bancroft's hypothesis that heavy metal salts stabilize C U ( O H ) by ~ adsorption of the hydrous oxides of the metals; but they seem to have gotten the impression that a fairly large concentration of salts is necessary. It should be pointed out that we have no independent confirmation of the hypothesis that the hydrous oxides of chromium, aluminium and manganese are strongly adsorbed by hydrous copper oxide and that such adsorption stabilizes the blue form. Kohlschutter and Tuscher conclude that the presence of electrolytes in small amounts favors dehydration by promoting the swelling and consequent peptization of the larger clumps of hydrous oxide. I n this connection I have been unable to find any reference to direct experimental evidence of such swelling or peptization. It would seem that if such a phenomenon took place it would not be difficult to detect. Some experiments bearing on these points were accordingly undertaken. I was assisted in carrying out the experiments described in the next section by Mr. Allen Bloxsom a student in my laboratory to whom I am greatly indebted. Experimental The Stability of the Granular Oxide.-As noted above P4ligot, Bottger, and Villiers claimed to get a crystalline hydrate that remained blue on heating to 100'; whereas de Forcrand obtained a similar compound that darkened gradually on heat-

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ing. I n order to determine whether a compound exists that will stand a temperature of 100" for an appreciable length of time without darkening, the following experiment was carried out : Bonsdorffl claims t o get a crystalline hydrate by precipitating a solution of copper ammonium nitrate with sodium hydroxide. I have followed his procedure. Crystals of copper ammonium nitrate were first prepared and a solution of the salt was precipitated by the addition of slightly more than the equivalent amount of NaOH free from carbonate. This precipitate stood in a corked flask in contact with the mother liquor for two weeks without darkening. A sample of this preparation in contact with the mother liquor was placed in a test-tube which was suspended in boiling water. Darkening was observed in ten minutes. A second sample was washed thoroughly by the aid of the centrifuge and suspended in pure water. This sample remained blue after heating 2 hours at 100". A third sample was washed thoroughly, filtered and placed in an oven a t 100". No blackening was observed after heating 3 hours. The product was distinctly granular in character but repeated microscopic observations failed to reveal a definite crystalline form. From the observations it is evident that a granular cupric oxide can be prepared that remains blue after prolonged heating a t 100". De Forcrand's sample seemed to possess properties intermediate between the usual hydrous oxide which blackens promptly a t 100" and the granular compound that is stable a t 100". There are doubtless an indefinite number of such intermediate compounds since Villiers and van Bemmelen have each observed the gradual transformation from the precipitated gelatinous hydrous oxide to the granular product on standing. The observations support Kohlschiitter and Tiischer's view that the granular product is not truly crystalline. The absence of definite crystalline structure eliminates the proof that has usually been offered in support of the exisZeit. anorg. Chem., 41, 132 (1904).

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tence of a definite hydrate of copper oxide having a zero vapor pressure. In the course of these experiments it was observed that the hydrolysis of a small amount of the copper ammonium salt in the presence of a large excess of water resulted in the formation of a highly dispersed oxide that appeared very much lighter than the usual blue precipitate. By standing or by centrifuging the precipitate was agglomerated into larger masses that were distinctly blue. The results show that increasing the degree of dispersion of the blue gelatinous compound results in the formation of a lighter product rather than a black one. It will be recalled that this is the reverse of Kohlschiitter and Tiischer's conclusion. The Action of Salts on Hydrous Copper Oxide.-Blucher and Farnau' took 1 cc of N CuS04, diluted i t t o 8-10 cc with distilled water and then added 1 cc NaOH from a burette. To the suspensions of hydrous cupric oxide prepared in this way were added normal solutions of several salts in drops from a pipette. The test-tube was shaken thoroughly and then placed in a beaker of boiling water. Observations were made with varying amounts of the "stabilizers," so-called. I n the interest of greater accuracy, the method of procedure followed' by Blucher and Farnau was modified slightly. N/10 solutions rather than normal were employed, the precipitation was carried out between 5" and 8" rather than a t room temperature and the precipitate was washed to remove excess sulphate. The method was as follows: 10 cc of exactly N/10 NaOH was mixed with 10 cc of N/10 CuS04 a t 5-8" in a hard glass test-tube. This test-tube was placed in a centrifuge immediately and the precipitate thrown down by centrifuging for one minute at 3000 r.p.m. The supernatant liquid was poured off and 20 cc of cold water added after which the precipitate was shaken thoroughly and the centrifuging repeated. By this procedure most of the sulphate was removed. 1

LOC.cit.

,

Hydrous Oxides. V

519

Finally cold water was pipetted on the precipitate and the salt solution added. The total volume of solution was 10 cc. The salt solutions which were N/10 were measured with a 3 cc Ostwald pipette graduated in tenths of a cc. After thorough shaking, the test-tube was clamped in an upright position in boiling water. Observations were made each minute for 5 minutes and then a t the end of 10 minutes. The results for MnS04 that are significant are recorded in Table I.

TABLE I Experiments with Manganese Sulphate

I ‘I*;...-

min.

Observations

.

Hydrous cupric oxide suspended in 10 cc. of Solution containing Water

1 3 5 10

0.25 cc N/10 MnS04

0.50 cc N/10 MnSOa

Light greenish blue Light greenish blue Greenish gray Dark gray

Light greenish blue 1,ight greenish blue 1,ight greenish blue Light greenish blue

Light greenish blue Dark gray Dark gray Brown

The above results confirm the observations of Tommasi and of Blucher and Farnau that the blackening of hydrous cupric oxide is prevented by the presence of only a small amount of MnS04. The necessary concentration is less than half that found by Blucher and Farnau. The latter investigators recorded “no change” in the hydrous oxide in the presence of a suitable amount of MnS04. This notation should have been “no blackening” for there is a very distinct change in color. The usual clear blue gelatinous precipitate first becomes light blue (baby blue) and later greenish blue as the heating is continued. At the same time the character of the precipitate changes from a highly gelatinous form to a less bulky, granular condition. The experiments were repeated with the sulphates of Cr, Al, Zn, and Co with the results recorded in Tables 11, 111, IV, and v.

Harry B. Weiser

520

TABLE I1 Experiments with Chromic Sulphate Observations Time, min.

1 3 5 10

Hydrous CuO suspended in 10 cc solutions containing 0.25 cc N/10 Crz(S04)3

0.50 cc N/10 Cri!(SO&

Light bluish green Gray Dark gray Dark gray

Light bluish green Light bluish green Light green Light green

TABLE111 Experiments with Aluminium Sulphate Observations Time, min.

Hvdrous CuO sumended in 10 cc solutions containinn

0.25 cc N/10 Alz(S04)s

1 3 5 10

I

Light greenish blue Light greenish blue 1,ight gray Gray

0.50 cc N/10 Ah(S0a)s

Light Light Light Light

greenish greenish greenish greenish

blue blue blue blue

Observations Time, min.

Hydrous CuO suspended in 10 cc solution containing

1 3 5 10

0.25 cc N/10 ZnSOc

Greenish blue Light brownish green Brown Chocolate brown

0.50 cc N/10 ZnSO4

Light greenish blue Light greenish blue Light greenish blue Light bluish gray

0.75 cc N/10 %SOl



blue blue blue blue

Hydrous Oxides.

V

52 1

TABLE V Experiments with Cobalt Sulphate Observations Tifne, min.

1 3 5 10

Hydrous CuO suspended in 10 cc solution containing 0.25 cc N/10 cos04

0.50 cc N/lO CoSOa

Light greenish blue Light greenish blue Grayish green Dark gray

1,ight greenish 1,ight greenish Light greenish Light greenish

blue blue

blue blue

The results recorded in the preceding tables show that the limiting concentration of the various sulphates necessary to prevent blackening of hydrous cupric oxide is very nearly the same. If the hydrous oxides of Al, Cr, Zn, Mn, and Co stabilize the blue oxide it means that the hydrous oxides have the same stabilizing power, which is interesting. To test this point some experiments were carried out with colloidal solutions of the hydrous oxides of Al, Cr and Sn. Colloidal alumina was prepared by prolonged boiling of a solution of hydrous aluminium oxide peptized by acetic acid colloidal chromic oxide by the hot dialysis of a solution of hydrous chromic oxide peptized by CrC13;2and colloidal stannic oxide by boiling a solution of hydrous stannic oxide peptized by NH40H.3 The colloidal alumina and chrome were positive colloids while the stannic oxide was negative. As previously noted, the evidence seemed to indicate that the hydrous oxides of the metals stabilized the blue oxide. Since the metallic salts are effective in such low concentration i t was thought that but little of the colloidal oxides would be required. It was found however that the presence of the colloidal oxides had but little effect in preventing darkening even when present in high concentrations. I n every case the colloid precipitated on heating and the color of the copper oxide was modified. 1

Weiser: Jour. Phys. Chem., 24, 638 (1920). Ibid., 26, 417 (1922). Ibid., 26, 682 (1922).

Harry B. Weiser

522

Observations Hydrous CuO suspended in 10 cc of colloidal solutions of

Time, min.

Hydrous Alp03 1.2 g per liter

1 3 5 10

Light bluish green Light gray Dark gray Dark gray

Hydrous SnOz 4.32 g per liter

Hydrous CrnOs 3.65 g per liter

~

Bluish green Grayish green Dull grayish green Dull grayish green

Light greenish blue Light greenish blue Light grayish green Gray



0.25 cc N/10 CUSO~ ,

1

3 5 10

Light greenish blue Light gray Gray Dark gray

0.50 cc N/10 C U S O ~



blue blue blue blue

Hydrous Oxides. V

523

consideration the effect of the hydrous oxides of the metals; for by no stretch of the imagination can blue hydrous cupric oxide be considered to prevent blue hydrous cupric oxide from darkening a t 100'. With the effect of the hydrous oxides eliminated, i t seemed possible that the anion was the important thing. If so, then alkali sulphates should behave like the sulphates of Mn, Cu, etc. This seemed unlikely in view of the observations of Harms, Spring and Lucion and others that alkali salts favor rather than retard the spontaneous dehydration of hydrous cupric oxide. This is borne out by the results recorded in Table VIII. TABLE VI11 Experiments with Sodium Sulphate

I Time, miri.

Observations

'


2 cc N/10 NakQ

1 3 5 10


6 cc N/10 Na2S04


10 cc N/10 NaZS04

Light greenish blue Greenish gray Gray Dark gray

Some experiments were next carried out on the effect of heating hydrous cupric oxide in the presence of chlorides. The results are similar to those with sulphates as indicated by the data for CuC12, CoClz and MgC12 recorded in Tables IX, X and XI. TABLEIX Experiments with Cobalt Chloride Observations Time, min.

1 3 5 10

Hydrous CuO suspended in 10 cc solutions containing. -

-_

1.25 cc N/10 CoClz

1.50 cc N/10 COCI?

Light greenish blue Light greenish blue Gray Dark gray

Light greenish blue Light greenish blue Light greenish blue Greenish gray

Harry B. Weiser

524

TABLE X Experiments with Cupric Chloride Observations Time, min.


0.50 cc N/10 CuCla

1 3 5 10

Light greenish blue Light gray Gray Darkgray

0.75 cc N/10 CuClz

Light greenish blue Light bluish green Light bluish green Light bluish green

1.00 cc N/10 CUClz

Light greenish blue Light bluish green Light bluish green Light green (color of Paris green)

TABLE XI Experiments with Magnesium Chloride 0bservations Time, rnin.

5 cc N/10 MgClz

1 3 5

so

Light greenish blue Greenish gray Gray Dark gray

10 cc N/10 MgClz

Light greenish blue Grayish green Light gray Light gray

The chlorides of cobalt and copper prevent the blackening of hydrous cupric oxide at 100" in much the same manner as the corresponding sulphates. A slightly higher concentration is required and the oxide takes on a distinctly greener color than was observed in the presence of sulphates. Contrary to the observations of Blucher and Farnau, MgCh does not prevent the darkening at 100" even when an amount equivalent t o the copper oxide is present. It was thought that Blucher and Farnau might have used the sulphate instead of the chloride; but this was not the case since preliminary experiments carried out in the usual way with MgS04 showed that between 8 and 9 cc of N/10 solution in 10 cc was required to prevent darkening.

Hydrous Oxides.

V

525

Discussion of Results At the outset of this investigation there seemed to be two possible explanations of the action of small quantities of salts in preventing the darkening of hydrous cupric oxide at 100”. The first of these, that the blue oxide is stabilized by adsorption of the hydrous oxides of the metals, was shown t o be untenable in the light of the investigations described above. It is a significant fact however that the solutions of those salts which are most effective are hydrolyzed giving an acid reaction. The extent of this hydrolysis is increased by shaklng with hydrous cupric oxide since Tommas’ found that solutions of alkali salts like sodium chloride and sodium sulphate show an alkaline reaction after such treatment. This is due to the fact that hydrous cupric oxide adsorbs acids more strongly than bases thereby displacing to the right the equilibrium represented by the equation 2Na: SO4” 2H20 2Na’ 20” 2H’ Sod”, and leaving ,an excess of OH’ ions in the solution. This tendency of hydrous cupric oxide to adsorb acids more strongly than bases is in line with our conclusions that the effect of certain salts on gelatinous cupric oxide is not due to stabilization of the blue compound’ by adsorption of hydrous oxides. According to Spring and Lucion2 and Mailhe3 basic copper salts are formed in the presence of alkalies. Having eliminated the hypothesis of stabilization by adsorption of hydrous oxides, we can consider the second hypothesis t h a t the blue copper oxide reacts with certain salts forming basic copper salts that retain the blue color when heated to 100”. I n support of this hypothesis, I have noted that the bright blue color of hydrous cupric oxide changes when heated with a suitable concentration of certain salts to a greenish blue, bluish green, or grayish green, the color varying slightly with the nature of the salt employed. Spring and

+

+

+

+

Comptes rendus, 92, 453 (1881).

LOC.cit. Bull. SOC. chim. France, [31 27, 167 (1902).

+

Harry B. Weiser

526

.

Lucion' investigated the action on blue cupric oxide of solutions containing an excess of sulphate and halogen salts; and was convinced that definite basic salts are formed to which he assigned the formulas: Cu2X2.CuO . 3 H 2 0 where X = chlorine, bromine, or iodine; and CuS04.2CuO. 2H20. As noted above Spring and Lucion claim that Tommasi also obtained basic salts of this character. While we cannot say with certainty that Spring and Lucion's products were definite basic saltsJ2there is no doubt that such compounds can be prepared. Thus Sabatier3 prepared crystalline compounds having the formulas: C U ( N O ~. ~) ~ C U ( O H CuCl2. )~; ~CU(OH)~; CuBr2.3Cu(OH),; C ~ ( c 1 0 ~ )C~U. ( O H ) CuS206. ~; ~CU(OH)~; C U S O ~ . ~ C U ( O H ) ~ . HMiller ~ O . and Kenrick4 by the aid of the phase rule established the existence of a basic chloride CuC&.3Cu0.2H20 formed by adding N/5 KOH to N/5 CuC12 at 85 O ; and Werner5 obtained basic salts of the general formula [ C U ( C U ( O H ) ~ ) ~ ]Although X~. hydrous cupric oxide may react with an excess of certain salts, e. g., sulphates to form definite basic sulphates which do not darken at loo", i t seems to us altogether improbable that this explanation will account for our observations. It will be recalled that 0.50 cc of N/10 sulphates of Zn, Mn, Cu, Co, AI and Cr in 10 cc was found sufficient to prevent the hydrous oxide prepared from 10 cc of N/10 copper salt from turning black when heated. If all the sulphate reacted with the hydrous oxide to form a basic salt the composition would be represented by the formula (Cu0)20.SO4.X H 2 0 . The improbability that such a compound should exist disproves the hypothesis that the entire precipitate is converted over to basic salt. On the other hand i t does not eliminate the possibility that a film of protective basic salt forms on the surface of the oxide. This explanation is untenable since relatively large concentrations of alkali salts Zeit. anorg. Chem.. 2, 209 (1892). Cf. Young and Stearn: Jour. Am. Chem. SOC.,38, 194 (1916). Comptes rendus, 125, 101 (1887). 4 Trans. Roy. SOC., Canada, [II] 8, part 3,35 (1901-2); Jour. Phys. Chem., 7, 259 (1903). 6 Ber. deutsch. chem. Ges., 40,444 (1917). 1

2

Hydrous Oxides.

V

527

are not effective and Spring and Lucion found such solutions to act on the hydrous oxide in much the same way as solutions of salts of the heavier metals. Since the experimental results fail to establish either the adsorption-or the basic salt hypothesis we were led to conclude that the stability of the cupric oxide may be due in large measure to a change in the physical character of the compound in contact with certain salts. I have pointed out that the oxide can be prepared either in a gelatinous form which darkens on heating or in a denser, granular form that does not darken readily at 100". That the gelatinous form actually does go over into a less bulky granular form when heated with dilute solutions of certain salts was noted a t the outset of' this investigation and I have called attention to i t in an earlier paragraph. The significance of this transformation was not appreciated at the time; but i t seems that therein lies the explanation of the stabilizing action of certain salts. I n an earlier communication' I have pointed out'that a gelatinous precipitate may be destroyed and the particles agglomerated into denser clumps in the presence of a substance having a slight solvent action. One might expect to get the necessary solvent action on hydrous cupric oxide with a solution having a small hydrogen ion concentration such as obtains with dilute solutions of salts of strong acids that undergo appreciable hydrolytic dissociation. I n line with this we find that the blue oxide does not darken when heated with relatively low concentrations of the sulphates of Co, Cr, AI, Zn and Cu, and the chlorides of Co and Cu. Other similar salts are doubtless equally effective if one wishes to extend the list. I n contrast to the behavior of the salts listed above are salts of the alkalies and the alkaline earths, sodium sulphate, magnesium chloride, and magnesium sulphate which are hydrolyzed much less and so are not effective in low concentrations. From Table VI1 i t is seen that 10 cc of an N/200 solution of cupric sulphate is sufficient t o prevent the darkening of the hydrous oxide obtained from 10 cc of N/10 copper salt. With Weiser: Jour. Phys. Chem., 24, 306 (1920).

Harry B. Weiser

528

sulphates more highly hydrolyzed than cupric sulphate, the latter will tend t o form so that it is not surprising that the sulphates of aluminium and chromium which are more hydrolyzed than cupric sulphate must be present in approximately the same concentration. It follows further that the concentration of sulphuric acid and of hydrochloric acid necessary t o prevent darkening will be the same as that for the corresponding copper salts. This is verified by the results in Table XI1 and XIII. '

TABLE XI1

Experiments with Sulphuric Acid

I

Observations

Time, min.

0.25 cc N/10 His04

1 3 5 10

Light greenish blue Light greenish blue Gray Dark gray

0.50 cc N/10 &SO4



blue blue blue blue


1 3 5 10

Hydrous CuO suspended in 10 cc solutions containing

0.25 cc N/lO HCl

0.50 cc N/10 HC1

0.75 cc N/10 HC1

bight greenish blue Light greenish gray Gray Darkgray

Light greenish blue Light grayish blue Light grayish green Light grayish green

Light greenish blue Light bluish green Light bluish green Light bluish green

In view of the change in physical charatter of the hydrous oxide which is not identical in all cases and the slight interaction of salt and oxide, it is not surprising that the final products should show slight variations in color.

Hydrous Oxides. V

529

I n the nature of things it is quite evident that the free hydrogen ion concentration must vary a t different stages of the process. N/200 CuS04 say, has a definite H' ion concentration a t the moment of formation; but when this solution is added to the hydrous oxide, a part of the H' ions is adsorbed. Under constant conditions the amount adsorbed is constant; but the physical character of the precipitated oxide changes during the experiments from a highly gelatinous to a granular form and this affects the adsorbability of the compound. Wagner1 has shown that when salts of aluminium, iron, etc. are hydrolyzed, the oxides adsorb the free acid; but that as the oxides age this adsorption decreases so much that practically all the acid is set free.2 Gilbert3 has observed that the physical character of cupric oxide has a marked influence on its adsorption of eosin. It will be recalled that Harms, Spring and Lucion, and van Bemmelen found that solutions of alkali salt favor rather than retard the spontaneous dehydration of hydrous cupric oxide. I n order to account for this, Kohlschutter and Tiischer assume that the salts cause a swelling or peptization of the precipitate. I have been unable to detect such an effect; on the contrary, the reverse appears to be the case from the moment the salt solution is brought in contact with the hydrous oxide. Kohlschiitter and Tiischer cite no evidence t o support their assumption and point out that the swelling cannot be detected because other factors come in. It is necessary to make some such assumption however, in order t o explain the facts in terms of their theory of the mechanism of the dehydration process. As a matter of fact, the effect of salt solutions on the hydrous oxide is exactly what one would expect from the well-known properties of the oxide. It is known that the finely divided particles adsorb water sufficiently strongly t o give a gelatinous precipitate but not so strongly but t h a t spontaneous dehydration takes place very rapidly unless

a

Monatsheft fiir Chemie., 34, 95 (1913). Cf. Freundlich and Schucht: Zeit. phys. Chem., 10,290 (1906). Jour. Phys. Chem., 18, 586 (1914).

530

Harry B . Weiser

the compound is kept at a low temperature. It is known also that the hydrous oxide adsorbs salts very strongly indeed so that they cannot be removed completely from the precipitate by even prolonged washing. In the presence of a salt solution then, the adsorption of the salt by the oxide decreases the capacity to adsorb water so that the loss of water is slightly more rapid in the presence than in the absence of a salt solution. It seems quite unnecessary to resort t o the interesting but rather fantastic theory of Kohlschiitter and Tiischer to account for this and other properties of hydrous cupric oxide. Summary The results of this investigation may be summarized as follows : 1. No hydrates of cupric oxide are known with certainty. Since a hydrous oxide having approximately zero vapor pressure approaches the composition CuO.Ht0, i t is usually concluded that a monohydrate exists. This conclusion is negatived by the fact that the anhydrous oxide does not take up water at ordinary temperatures in contact with water; but, on the contrary, the hydrous oxide loses water under these conditions. A “crystalline hydrate” has also been described but this has been found to consist of pseudo-crystals or granular particles that do not result from its own power of crystallization. Loss of water takes place more readily from the gelatinous oxide than from the granular because of the compact structure of the latter. 2. Highly dispersed hydrous cupric oxide is very light blue in color. The gelatinous oxide in mass possesses a characteristic clear blue color which alters slowly a t room temperature but rapidly at higher temperatures, going through various shades of blue to green, brown and finally black. The continuous change in color is due t o agglomeration of the particles which accompanies the spontaneous loss of water. 3. Blue gelatinous cupric oxide kept at 0 ” goes over spontaneously to a denser and bluer granular product. It is thus

Hydrous Oxides. V

53 1

possible to obtain any number of hydrous oxides that vary continuously in composition from CuO. * 20H20 (van Bemmelen) t o CuO. * HzO. 4. Hydrous cupric oxide adsorbs ions strongly. If shaken with solutions of neutral salts like NaCl or Na2S04, hydrolysis takes place and the solution becomes distinctly alkaline owing to stronger adsorption of acid than of base. 5. 0n.account of the strong adsorption of hydrous cupric oxide for certain ions the presence of certain salts frequently accelerates slightly the spontaneous loss of adsorbed water. 6. Hydrous cupric oxide may be heated to 100" without darkening in the presence of very small amounts of salts such as MnS04, CoS04, A12(S04)3,Cr2S04, ZnSO4, CuS04, ZnClz and CuC12. 7. The stability (absence of darkening) of hydrous cupric oxide at 100" in the presence of certain salts is not due t o adsorption of the hydrous oxides of their metals as suggested by Bancroft and reaffirmed by Blucher and Farnau. The evidence against this stabilization theory is (1) that hydrous cupric oxides adsorb acids more strongly than bases, (2) that relatively high concentrations of the colloidal hydrous oxides are not effective and (3) that CuS04 is as effective as MnS04, Cr2SO4, etc. This latter observation is conclusive since i t is inconceivable that adsorption of blue hydrous cupric oxide should stabilize blue hydrous cupric oxide. 8. The absence of darkening cannot be due to the formation of basic cupric salts since the effective concentration of electrolyte is so low that the ratio of oxide to sulphate in the salts could be no less than * 2OCuO : 1so4,which is altogether improbable; and i t is not due to the formation of a protecting film of.basic salt since alkali salts which give basic salts under certain conditions hasten rather than retard the blackening. 9. Darkening of the hydrous oxide at 100" does not take place in the presence of suitable concentrations of certain salts owing t o a change in the physical character from the highly gelatinous to the granular form of the oxide. Only

532

Harry B. Weiser

those salts which hydrolyze appreciably are effective in low concentrations since the slight solvent action of the H + ion destroys the gelatinous structure and the denser, granular modification which forms, loses water and darkens less readily than the loose voluminous mass. Department of Chemistry The Rice Institute Houston, Texas

Hydrous Oxides. V

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