SOME FACTORS AFFECTING RHYTHMIC PRECIPITATION BY ALFRED E. KOENIG
The phenomena of rhythmic precipitation, since Liesegang’s1 discussion of his discovery in 1898, have attracted considerable attention and created much interest. A very excellent historical account of the work of earlier writers is given by Stansfield,2 so that it does not seem necessary to present this phase of the subject here. This,paper gives the results of a number of experiments that throw some light on the conditions that affect the formation of rhythmic precipitates. Most of the experiments described in this paper were performed with a gel prepared in the following way.3 Equal volumes of a sodium silicate solution of I .05 specific gravity and N/2 acetic acid were thoroughly mixed. The resulting mixture set within five minutes to a stiff gel. The sodium silicate solution was such that it was not quite acidified by an equal volume of half normal acid. It was found convenient to use normal acetic acid solution and to dilute it with an equal volume of water, K2Cr04solution, or solutions of other substances whose effect on the rhythmic precipitation was to be studied. The gel was made l/go molecular with respect to K2Cr04,and after it had set, a ‘/z molecular solution of CuS04 was poured upon it. If the CuS04 solution is poured upon the gel as soon as it has set, there will form within 24 hours, first, a dense band of basic copper chromate about one centimeter deep, in which there may often be distinguished many bands crowded close together. Then at intervals of 5 cm. or more there will be regular thin bands of the copper chromate. The intervals
-
1
“Chemical Reactions in Gelatine,” Diisseldorf (1898).
Am. Jour,. Sci., [4]43, I (1917). * This procedure is somewhat similar t o t h a t described by Holmes: Jour. Am. Chem. SOC., 40, 1187 (1918).
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between the bands increase toward the bottom of the test tube. The spacing is evidently logarithmic as has been shown by 0kaya.l If the gel impregnated with K2Cr04 was allowed to stand for some time before pouring on the CuS04 solution, the bands that resulted were more and more irregular the longer the gel had stood. Finally, in a gel that had stood for a week or more, the copper chromate precipitate consisted of warped bands and fragments in all sorts of fantastic shapes. To all appearances the gel was as uniform as that which was freshly prepared. It seems likely that some change in the structure of the gel took place on standing open to the air that hindered the uniform diffusion of the interacting salts. This effect is shown in Fig. I . A remarkable effect on the rhythmic precipitation of the copper chromate is that of certain alcohols :-methyl, ethyl, propyl, and glycerin. Somewhat similar are the effects of urea and sugar. Photographs of some of these precipitates are shown in Fig. 2 . Five cc of each of the liquids was added in place of the corresponding volume of water in the preparation of 50 cc of the gel. In all of these the most apparent effect was to make the bands of copper chromate more sharply defined. It also caused a difference in the distances between the bands. The alcohols and urea caused them to be closer together, whereas the sugar and glycerin caused them to be farther apart. The intervals between bands was especially large where the gel contained sugar. The test tubes containing the sugar were tightly corked and stood over the summer vacation. Then there were formed faint but very sharp layers between those of the copper chromate which had the appearance, as if the gel had been cut with a sharp knife and then put together again. It has not been ascertained what the substance of the secondary rings was. They will be mentioned again in another connection. The effect of concentration of the ethyl alcohol was studied from one to nine cc of the alcohol to 50 cc of the gel. The effect on the narrowing of the intervals between the bands was progressive with Proc. Tokyo Math. Phys. Soc.,
[2]
9, 442 (1918).
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Alfred E. Koenig
increase in the concentration of the alcohol. With t e a o r more cc of alcohol the gel set too quickly, so that it-could’not be poured into test tubes before setting.
Fig.
I
Ig-CuSOr solution poured on 3 hrs. after the gel had set. PO-CUSO~ solution poured on 5 days after the gel had set. PI-CUSO~ solution poured on I O days after the gel had set.
There are two things that these added substances may do, both of which would influence the distribution and sharpness of the bands. The presence in the gel of another substance would alter the rates of diffusion of the two reacting subsstances. This would bring the places, where sufficient con-
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centration is reached for precipitation, either nearer together or farther apart according as the rates of diffusion are retarded or accelerated. Another important factor in the formation of the bands is the concentration of the reacting substances necessary for precipitation to begin. This would certainly
Fig. 2 Precipitations in silicic acid gel. ]-Precipitate formed by m/z CuSO4 and m/80 KsS04, the latter being in the gel. 2-Same as I with 5 cc of CeHjOH per 50 cc of gel. 3-Same as I with 5 cc of CsH70H per 50 cc of gel. -+-Same as I with 5 cc of CHICOCHI per 50 cc of gel. 5-Same as I with I gm. of urea per 50 cc of gel.
be modified by the presence in the gel of a substance which would increase or decrease the solubility of the precipitating substance. The change in the spacing of the bands in the gel is then due to' a combination of the two factors, the change in the rates of diffusion of the reacting substances and in the solubility of the resulting precipitate.. As far as was possi-
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ble to ascertain, there was no change in the nature of the precipitate. In a gel where 5 cc of ethyl acetate were present in 50 cc, there resulted in place of sharp 'uniform bandsof copper chromate, all sorts of fragmentary bands and queer scraps of precipitate. Some of these were remarkably symmetrical in outline and some bent and curled into fantastic shapes. There is often in this case and in others, a tendency to form beautiful spirals that wind about in the test tube like the threads of a screw. It was found that the ethyl acetate did not all dissolve in the acidified silicate solution and was present both within the resulting gel and on its surface in the form of small globules. The drops on the surface were lifted off by the CuS04 solution, thus giving it an uneven surface from which diffusion started. In fact, this etching of the surface was plainly visible. The droplets within the body of the gel caused an uneven distribution of the interacting salts. The result was that there were very irregular bands and pieces of bands bent into various strange shapes, the nature of which can be seen in the photograph in Fig. 3 . The mere mutilation of the surface from which the diffusion takes place did not necessarily result in a deformation of the bands. So it would seem that the enclosed droplets are the chief cause of these deformations. Gels containing 0 . 2 to 0 . 3 gm of finely powdered kaoline, BaS04, PbOz, fluwers of sulphur, charcoal, coke, bone black, and pumice in 50 cc were prepared. These gave sharp uniform bands provided the powder was fairly uniformly distributed in the gel. This was best accomplished by soaking the powders for some time in water before mixing with the silicate and acid. If the dry powders were stirred up with the mixture, it was easy to see that they were not uniformly distributed in the resulting gel. In these cases, the bands of copper chromate were always very irregular and broken up. This was especially true of charcoal, bone black, and kaolin where adsorption would make the diffusion all the more irregular. Sapo venetus was very effective in producing ir-
Some Factors a$ecting
Rhythmic Precipitation
47 I
regular bands, for this produced little centers where the CuS04 was used up in the formation of insoluble soap. This was evident by the dots of the deep blue copper soap scattered irregularly throdgh the gel. It was also noticed that when the mixture of water-glass solution and acid was shaken for some time and poured just before it would set, then there
Fig. 3 Copper chromate precipitate where 5 cc of ethyl acetate was added to 50 cc of the gel.
were streaming cloud-like precipitates with curved bands of the copper chromate running parallel to their outer boundaries. Some of these are shown in Fig. 4. In all these cases where either some added substance or the structure of the gel interferes with the uniform diffusion of the reacting substances, there result irregularities in the rhythmic bands that are formed.
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Stansfieldl has made a considerable number of experiments to show the effects of the relative concentrations of the reacting' substances and their relative rates of diffusion on the phenomena of rhythmic precipitation. He comes to
Fig. 4
Copper chromate precipitate in a gel that was poured just before it set.
the conclusion that the greater the difference in the rate of diffusion of the two interdiffusing substances the farther apart will be the bands of precipitate. This difference in the rates of diffusion may be due to the nature of the substances, the
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medium in which they move, and their relative concentrations. This can be illustrated by allowing a copper sulphate solution to diffuse into a slightly alkaline silicic acid gel containing various amounts of potassium chromate. The smaller the concentration of the chromate the farther apart will be the bands of copper chromate. If the amount of potassium chromate in the gel is kept constant, the bands become closer together as the concentration of the overlying copper sulphate solution is decreased. When the concentrations of the Cut304 and the K2Cr04 become nearly equimolecular, the bands come so close together as to form a continuous precipitate and finally as the concentration of the CuS04 is farther decreased there is no precipitate in the gel a t all. This is true in the case of other combinations of precipitate forming solutions. These experiments are best done in the following way: The acidified water-glass containing the K&r04 is sucked up into ordinary glass tubes. After the gel has set, the tube is cut into convenient lengths and put into the solution of CuS04. The bands are very sharp in such tubes and are convenient for comparison and measurement. Stansfield shows how all these phenomena may be explained by the ordinary laws of diffusion. The following experiments are given because they are certainly due to simple diffusion and precipitation and their study throws light on the things that take place in the rhythmic precipitation of substances in gels. When a liter flask of hydrochloric acid gas is connected with a similar flask of ammonia gas by means of a glass tube several meters long and about 4 mm internal diameter, the N H G l formed is deposited in a series of sharp bands on the wall of the tube. These bands run around the tube a t right angles to the direction of the diffusion. They vary in width from 0 . 5 mm to 5 mm. Between them is a faint deposit of NH4Cl with an occasional sharp clear band.* At the bottom of the tube the loose NHK1 gathers into little heaps and valleys like the cork dust in a vibrating tube. The bands are always built toward the gas that diffuses less rapidly.
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The formation of the bands may be explained thus. Suppose that we were t o start with the two flasks containing equal volumes of ammonia and hydrochloric acid a t the same temperature and pressure. The gases diffuse out into the air of the connecting tube till they meet. In the case cited this would be a little beyond the middle of the connecting tube nearer the hydrochloric acid containing flask. They will diffuse into each other till the concentration is such that solid NH4C1 is formed. The sharp heavy band is deposited where the ammonia and hydrochloric acid have the greatest concentration. Small amounts of HC1 and NH3 on either side of the band rise to the light fog that is between the bands of NH4C1. The space for some distance is cleared of the interacting gases. They again diffuse toward each other and meet a t some point nearer the less rapidly diffusing gas, the HC1 in this case. They diffuse into each other. The point of greatest concentration will be where they met the second time and here the second ring will be deposited with its thin fog on either side. And so the process continues. When H2S and C12 were put into the flasks and connected by a tube as before, the bands'of sulphur formed were very wide, 2 0 to 30 cm, and shaded off gradually into the intermediate clear space. This is probably due to the fact that these gases react more slowly than the HCl and NH3 and require a greater concentration before the precipitation of sulphur occurs. With SO2 and H2S there was no distinct banding of the precipitated sulphur because their rate of reaction is still slower than that of the CZz and H2S. NH3 and HBr make very sharp bands as also do NH3 and HF. In the latter case the NH3 was at a lower concentration than the HF. The flask containing the H F was lined with paraffin but the connecting tube was left uncovered. It showed no evidence of having been etched by the HF. When the flasks containing the NH3 and HC1 were connected by tubes in a vertical position, the bands were always built downward whether the NH3 or the HCl was in the upper flask. The flasks may also be placed side by side and con-
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475
nected by a tube bent back on itself, that is into a long U-tube. This offers the most convenient form for keeping the two flasks at the same temperature conditions. A farther study of these phenomena is in progress. These experiments show that the chief factor in the formation of rhythmic precipitates is a difference in the velocity of diffusion of the interacting substances. It is not necessary to assume, as is suggested by HolmesJ1 that the precipitate formed acts ds a retarding membrane. In fact, it is evident from the examination of a large number of different substances that were deposited as rhythmic bands, that the precipitated substance occupies only a small portion of the cross-section of the gel. Another important factor in the formation of banded precipitates, is the relative concentration of the reacting substances that must be attained before the solid phase is deposited. I n the case of the interaction of gases, the rate of the reaction that forms the solid and in the gels, the degree of supersaturation that may obtain, also play important roles. These factors have not been studied t o any extent from a quantitative standpoint. The differences in the precipitation of the NH4C1 and sulphur, described in this paper, are illustrations of the effects of the precipitation concentration and the speed of interaction of the substances brought together. An interesting example of the effect of supersaturation is the following: A half molecular CuS04 solution was allowed t o diffuse into a slightly alkaline silicic acid gel, prepared as described before and containing I cc of pyridine in 50 cc of the gel. The progress of the CuS04 could be readily followed by the deep blue color resulting from its combination with the pyridine. The gel femained clear for some weeks. Then bands of greenish blue crystals of the copper sulphatepyridine compound were formed, which became more distinct as the crystals grew a t the expense of the compound which was in solution in the gel. It is possible that the forma1
Jour. Am. Chem. SOC.,40, 1187 (1918).
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Alfred E. Koenig
tion of the secondary bands between those of copper chromate in the gel containing cane sugar, mentioned elsewhere in this paper, may have a similar explanation. In a gel that was very nearly neutral and which contained molecular K2Cr04,the bands of copper chromate were always formed in the midst of the gel which was already blue with the CuS04. That is, the CuSO4 had already penetrated a considerable distance beyond the place where (the formation of a band of copper chromate took place, and where the gel was most supersaturated with respect to copper chromate.
Summary
8
A number of experiments are described which show that the uniformity of the bands formed in rhythmic precipitation is influenced by the structure of the gel. This non-uniform structure may be brought about by allowing the gel to stand for some time before the solution is poured upon it or by pouring tHe mixture of sodium silicate and acid just before it sets. The bands of the precipitate may be modified by the presence in the gel of substances such as alcohols, urea and sugar, which modify the rates of diffusion of the reacting substances and the solubility of the precipitated substances. The rhythmic bands may be broken up more or less by the presence in the gel of various inert powders unevenly distributed and by the presence of small particles of soap, all of which tend to interfere with the uniform diffusion of the reacting substances. A series of experiments on the formation of rhythmic bands by the interaction in narrow tubes of such gases as NH3 with HC1, HBr, or HF; and HPS with Clz or SOZ, have been described and their relation to the phenomena in gels pointed out. In addition to the effects of diffusion and the factors that modify it, there are also the effects of the concentrations necessary for precipitation to take place, the velocity
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of the reaction that takes place which forms the precipitate, and the degree of supersaturation which the resulting substance may reach in a given medium, before precipitation takes place. University of Wisconsin Madison, Wisconsin
