C H R O M I C O X I D E JELLIES B Y E. f l . BUNCE A N D L. S . FIXCH
IT-hen a solution of sodium silicate is acidified with hydrochloric acid, the solution usually solidifies more or less rapidly t o a jelly. When caustic soda is added t o a solution of chromic, ferric, cupric ,or aluminum chloride we usually get a gelatinous precipitate which does not form a jelly. On the other hand, i t is possible t o obain jellies of hydrous chromic, ferric, cupric, or aluminum oxides. At present there is no satisfactory theory t o show why a gelatinous precipitate will form a jelly in one case and not in another. It is not entirely a question of high concentration, as von n‘eimarn’ assumes, because C m m ? claims t o have obtained hydrous aluminum oxide in the form of a jelly when the solution contained one part of alumina in six hundred of water and one part of sulphuric acid in seventy-five hundred of water. As a preliminary t o a theory, i t seemed desirable t o obtain some data as t o the conditions under which one gets jellies of hydrous inorganic oxides. IYe began with the chromic oxide jellies which had previously been studied by Reinitzer, from whom we quote: “If a solution of chromic sulphate or chloride is mixed with an excess of sodium acetate and boiled, no deposit is formed even after hours of ebullition, however concentrated the solution, and however much sodium acetate is added. If the solution is boiled for a short time only, it takes a violet color on cooling. The solution of a chromic salt thus treated is found t o have assumed decidedly new properties. Caustic alkalies, ammonia, ammonium sulphide and carbonate alkaline carbonates, sodium phosphate, baryta water, and barium carbonate, produce in the cold not the slightest turbidity, whether added in small quantities or in excess. Caustic Zeit. Kolloidchemie, 5 , 1 2 2 (1909); 7, 1 5 7 ( 1 9 1 0 ) ;9, 2 j (1911) Jour. Chem. SOC.,6 , 216 (1856). Chem. Xews, 48, 114 (1883).
E. H . Bzizice and L.S. Fiizcli
77 0
alkalies and baryta change the color of the solution, a t first to an olive-green, and then to an emerald-green, and after standing for twelve hours the liquid congeals to a green jelly. On adding ammonia no immediate change of color occurs; after about 48 hours the liquid is converted to a violet jelly. Ammonium sulphide and carbonate act in the same manner after the lapse of several days. If the mixture is boiled after the addition of the above-mentioned reagents, a precipitate appears sooner or later according to the strength of the alkaline solution and the quantity added ; sodium phosphate alone causes no deposit X very satisfactory jelly can be obtained in the following way: Dissolve one part of chromic sulphate or chloride in about twenty of water. Add approximately one gram sodium acetate per 2 0 cc solution; boil for one minute and allow to cool The solution is now x-iolet in color. Add concentrated caustic potash ( I . 3 ) or soda until the solution is distinctly alkaline The solution turns green and sets in 10-20 minutes to a stiff jelly If ammonium hydroxide is added instead of sodium hydroxide, the solution becomes distinctly purple' and sets to a jelly inside of twelve hours. The effect of \-arying the concentration of the chromium salt was next tried, keeping all the other conditions as nearly the same as possible. Caustic potash was added until the solutions were just alkaline to litmus paper. T h e concentrations varied from one of chromic sulphate jn one thousand of water to one in less than nine of water. With solutions containing less chromic sulphate than one part in one hundred of water. no jelly was obtained but merely a viscous solution, the viscosity decreasing with decreasing concentration of the chromic salt. One part of chromic sulphate per hundred of water gave a soft but clear jelly; with fifty parts of water a firm jelly was obtained, and a very stiff jelly with thirty. twenty or nine parts of water. A t higher concentrations the jelly is cloudy and less satisfactory. The concentration "
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119061
C'izrovzic Oxide Jellirs
771
of one t o twenty gave the best jelly and the jelly formed most rapidly. This concentration was, therefore, used in the subsequent experiments except when otherwise stated. The next \.-ariable to be studied n-as the concentration of the sodium acetate. The figures refer to grams of sodium acetate per 2 0 cc solution. n'ith 0.1gram sodium acetate a green gelatinous precipitate was obtained. n'ith 0 . 2 gram there was some tendency to form a jelly, especially if the alkali was added carefully. n'ith 0 . j gram, I gram, and 2 . 5 grams sodium acetate, good jellies were obtained. JVith three grams sodium acetate, crystallization took place, which spoiled the jellies. 113th solutions containing over one gram sodium acetate no precipitate is formed on adding caustic potash and a good jelly is obtained in about twenty-four hours. The sodium acetate apparently peptonizes the gelatinous precipitate and prevents its separation. X jelly is formed when the hydrous chromic oxide comes out slowly. The color of the jelly depends t o a certain extent on the amount of alkali added. If the solution is made only just alkaline, a violet jelly is formed. IITth increasing alkalinity the color varies from a \-iolet to a distinct olive-green. lTrhen ammonia is used, the jelly is always purple regardless of the amount of ammonia. It forms more slowly than when potassium hydroxide is added. If enough hydrochloric acid is added to the jell>-to make the mass distinctly acid to litmus, the jelly dissolves, the rate of solution being increased by heating. If the solution is made alkaline again with S a O H , KOH or SH,OH, i t jells on standing provided the amount of sodium acetate present is not too small. n'ith low concentrations of sodium acetate a precipitate is formed on neutralizing the hydrochloric acid solution. This can be prevented by adding more sodium acetate t o the acid solution before adding the alkali. Some experiments were next made to determine the effect of various salts. Fil-e solutions were made up containing one gram chromic chloride in 30 cc. water. To each was added 3 grams sodium acetate and then 2 , 3 , 4, j, 6 cc, respecti\-ely,
772
E. H . B m c c
aizd
L. S. Finch
of a solution of one part of S a C l in three of water. All solutions were heated to boiling, allowed to cool, and made alkaline with KOH ( I : I). All five solutions jelled on standing. Three solutions were made up containing 0 .j gram chromium sulphate and 2.5 grams sodium acetate per 2 0 cc water. To these were added I , 3 , and I O cc KCl solution ( I : 3). On treating as before a firm green jelly was obtained in each case. Four solutions were next made up containing 0.5 gram chromium sulphate in 2 0 cc water. Potassium chloride and sodium acetate were added as follows: 0.2 gram KCl 2 grams sodium acetate B. 2 . 0 grams KC1 2 grams sodium acetate C. 1 . 2 grams KCI, no sodium acetate D. 1 . 2 grams SaC1, no sodium acetate
+ +
With A and B firm green jellies were obtained. With C and D gelatinous precipitates were formed. This shows t h a t neither sodium chloride nor potassium chloride prevents the formation of a jelly provided sufficient sodium acetate be present] and also t h a t neither of them can replace the sodium acetate. JVhen these jellies are allowed t o stand in an open beaker, the potassium chloride eventually crystallizes and destroys the jelly, a coagulated mass forming a t the bottom of the beaker with a layer of greenish tinted water above it. Sodium oxalate, potassium bitartrate] and ciric acid were tried as substitutes for sodium acetate; but none of them made the solution set t o a jelly even on long standing. For a long time the 'presence of acetate in the solution seemed t o be essential to the formation of a jelly. Later work showed t h a t potassium sulphate could apparently be made t o work; and i t is very probable t h a t other salts may be effective when the proper conditions have been determined. Our earlier experiments seemed to show t h a t the time of heating must be regulated carefully. A solution containing 0.5 gram chromium sulphate and 2.5 grams sodium acetate per 2 0 cc water was acidified with hydrochloric acid, heated just t o boiling, cooled, and neutralized with alkali. A jelly was formed. A similar solution was boiled for fifteen minutes
c ' l z v o u k Oxide Jellics
7 73
and then treated as before. KO jelly was formed. This seemed fairly conclusive experimentally, but there was no apparent theoretical justification for it. The only conceivable reason for heating together sodium acetate and chromic sulphate is t o accelerate the hydrolysis of the resultant chromic acetate into colloidal chromic hydroxide and acetic acid. If prolonged heating in itself destroys the tendency of the solution to jell when made neutral, it must mean t h a t the hydrolysis must be stopped before equilibrium is reached, which did not seem probable. The other alternative was 'that prolonged heating was not detrimental except in so far as it volatilized the gcetic acid. To test this point a solution was made up containing one gram chromic sulphate and one gram sodium acetate per 2 0 cc water. Three hundred cc of this solution was heated to boiling for eight hours, using a return condenser. The solution remained violet in color. When cooled and made alkaline, a stiff jelly was formed. In another experiment the amount of sodium acetate was cut down to 0.5 gram per 2 0 cc and the solution was boiled with a return condenser for eleven hours. On cooling and making alkaline a stiff jelly was obtained. We made a solution of one part chromic sulphate to twenty of water. To 2 0 cc portions were added amounts of sodium acetate varying from 0.1 to 3.0 grams. These were boiled in open beakers for varying lengths of time, keeping the volume practically constant by adding water from time to time. From the data thus obtained i t was seen t h a t solutions containing more than 1 . 5 grams sodium acetate could be heated for a long time and could still be made to give a good jelly. Solutions containing less than 0.5 gram sodium acetate gave a precipitate with alkali after being heated to boiling for several hours. ,411 these results are what one would expect if the driving off of the acetic acid prevents the solutions from setting subsequently. The acetic acid would naturally pass off more quickly in the solutions acidified with hydrochloric acid. Since the object of heating is apparently t o hydrolyze the chromic salt, the same effect should be obtained cold if
774
E . H.Biinsc aiid L . S.Fi?zclz
one took time enough. X solution of one gram chromic sulphate and 1.5 grams sodium acetate per 2 0 cc was made up, neutralized with caustic potash and allowed to stand cold. In about two hours the solution turned violet and a firm jelly was formed after several hours more. A similar solution, t o which alkali was not added, did not form a jelly on standing. The effect of freezing was next tried, using ice and salt as the freezing mixture and leaving the solutions in it for five hours. jelly which had stood in a open beaker for seven days mas frozen solid. U'hen thawed a jelly was obtained which appeared t o be more granular than the original one. The experiment was repeated with a jelly which had stood for five days. The result was the same as before. A jelly, which had stood only two days was frozen and thawed to an amorphous mass which showed no tendency to form a jelly. V'hen freshly formed jelly was frozen, layer ice formed on the surface. U-hen thawed, this jelly gave an amorphous precipitate with supernatant liquid. T17hen solutions were frozen before they had time to jell, they thawed to a liquid layer and an amorphous mass which showed no tendency t o jell. ,After being frozen and thawed, the solutions do not form the film-like surface characteristic of the jellies. Any of the jellies which had been spoiled by freezing and thawing gave stiff jellies if the precipitates were dissolved in hydrochloric acid and the solutions then made alkaline. Freezing seems to disintegrate a jelly unless it is an old one and has had time to set and even then it has a noticeable effect. Freezing apparently breaks u p the structure of the jelly and separates the water from the jelly. On standing the water does nor recombine with the chromic oxide and consequently the solution does not jell. Currant jelly can be disintegrated by picking at it with a fork. It, therefore, seemed desirable to test the effect of stirring. Solutions were divided into two portions, one being set aside as a check, while the other was stirred by means of a bent glass rod, the other end of which was inserted into the shaft of a small electric motor set vertically. After add-
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1hick, gclatinouh precipitate Good jell>Good jelly Good jelly Good jelly : little tvater on top Good jelly : little water on top Good jelly: more n-ater on top Good jelly: more n-ater on top Soft jell!- a t bottom: clear water on Soft jelly a t bottom : clear water on Soft jelly a t liottom: clcar water on Soft jell?. at hottom : c3lc.w \vatcr o n (klatinous precipitate a t I)ottoiii ; to form a jell!-.
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tendency t o jell and which did not get viscous Adding 'I larger excess of ammonia had no good effect nor adding KOH to the ammoniacal solution 7T-e were not able to get a jelly with ammonia in the solution 7Ye did not have time to determine whether an ammonia chrome alum would jell with ammonia The solutions of chrome alum containing KOH gave the typical green color while solutions containing animonia had the characteristic violet color l y e also tried adding sodium acetate to the chronic. aluni solutions X jelly was obtained on adding ROH h u t it xis\ apparently no firmer nor was it formed any more quickly than without the sodium acetate. Even with sodium acetate it was not possible to get a jell!- li? adding ammonia The time a t our disposal did not permit us t o test within what ratios of potassium sulphate t o chromium sulphate i t was possible to get a jelly on adding caustic potash. nor havc we made ariv c y ~ e r i m c n t ito see whether these jellies behave as do the jellies made with acetate i t hen subjected t o heating, freezing, acids, etc 7Ye did t r \ one experiment with a solution of onc part chroniic iulphatcl in tcn of n ater On adding KOH a precipitate formed at fir\t hut this dibsolved in an excess of caustic potash yielding a clear green solution. After
standing for several hours this solution began to get viscous; the next day it was very viscous. U'hile the solution did not a t any time set to a jelly, it showed a decided tendency to do so and me think that it may be possible to cause a chromic sulphate solution to form a jelly simply by adding caustic potash The jellies containing acetate shrink on drying, eventually cracking into squares, mhich finall!. dry to a powder. At no time is there any formation of a supernatant liquid layer. The reverse experiment was also tried of pouring water over the jellies carefully so as not to break up the surface of the jelly After itanding several days the water shows a green or violet tirigc depending on the color of the jelly. The violet jellies, made with ammonia or with small amounts of caustic potash seem to be more soluble than the green jellies. II'hen the jellies are heated with water, the water becomes colored quite deeply green or violet as the case may be, and there is a sediment in the bottom of the beaker which does not dissolve even when heated for selyeral hours. Under the conditions t h a t we have tried, the chromic oxide jellies are, therefore, not reversible in the sense t h a t the gelatine jellies are. IT-e tried adding sodium acetate and then alkali to solutions of salts of manganese, aluminum, copper and cadmium. In no case were we able to get a jelly. The acetate method is therefore not a general one. The general results of this paper are: I If a sufficient amount of sodium acetate be added to a solution of chromic sulphate or chloride, the subsequent addition of a suitable amount of KaOH, KOH or ammonia will cause the solution to jell 2 The chromic oxide jell?- is violet if made with ammonia or with a slight excess of potassium or sodium hydroxide. I t is green if a considerable excess of potassium or sodium hydroxide be added. 3 The concentrations of the chromic salt and of the sodium acetate ma! vary within fairly n-ide limits without interfering with the formation of a good jelly.
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3. It is not necessary t o heat the solution after adding the acetate and before adding the alkali but doing so increases the rate of hydrolysis and, therefore. cuts d o n n the time 1iecessar~-to form a jelly. j . Prolonged heating of the solatioil has 110 effect on the iubsequent formation of a jell!-, uiiless acetic acid is driven off. 0 The chromic oxide jelly dissolves in hydrochloric acid and is formed again when the solution is neutralized. provided enough acetate he present. j . n'hen enough acetate is present, the concentration of sodium or potassium chloride scerns t o have no appreciable effect on the jelling of the solution, though it probably changes the limiting conccntrationi. S. Sodium oxalate, potassium bitartrate, and citric acid could not be sulistituted succesifully for sodium acetate. c). Freezing solutions prevents the formation of jellies I f a n old jelly be frozen, a coarser jelly i i obtained after t h a w ing; hut if a freshly formed jelly be frozen, thawing yields a gelatinous precipitate and a supernatant liquid layer. I O . Stirring also prevents the formation of jellies 1 1 . The jellies made with acetate dry without any cxtrusion of a liquid layer. 1 2 . n'hen heated with water, the jellies tircak down to '1 gelatinous precipitate and iupcrnatant liquid 13 A jelly, once formed, does not swell i n water t o a l l y appreciable extent. 14.'I'hc acetate method of obtaining jellies did not work with salts of manganese, aluminrim, copper or cadmium 15 A jelly can tie formed by the addition of potassium hydroxide to a chromic acetate iolution. 16. A jelly can he formed by adding potassium or sodium hydroxide to a chrome alum solution b u t not by adding ammonia. 17. Adding \odium acetate to a chrome alum solution seems t o have no effect one way or the other. It does not make possible the formation of a jelly on adding ammonia.
C h ow i ic ONideg Jel lies
779
18 KO cvperimcnls have been madc to determine hoxv far it i i posiihle to 1 ary the ratio of potasiium sulphatcl t o chromic sulphate or to kec whether these \ulphate jellies 1 ) c h u c'~ i n other respects like the acetate jellies 19 Xo jellies were obtained by adding potassium hydrouide to solutions of chrornic nitrate, chloride or sulphate. II'ith chromic sulphate a v u > - viscous solution was obtained on long standing and it seems possible that further itudy ~vould show how to obtain a ]ell>- with this salt 20 I n the ahsencc of any satisfactory theory of jr~lliei, ncgati1.e result5 hold only for the particular r.\pcriinetits tried I t is not safe to generalize from them This investigation was sup-geited b y I'rofcisor I3:incroft and has been carried or1 under his supervision ('(1 r n I
11 1-11 I - r i11 I (3
