LIESEGANG PHENOMENON I N SILICIC ACID GEL AZARIAH T. LIKCOLN
AND
JOHN C. HILLYER
Department of Chemistry, Carleton College, Northjeld, Minnesota Received September 50, 1933
Since the description of rhythmic formation of precipitates by Liesegang (10) in 1896, there have appeared several hundred articles (6) presenting numerous specific examples of this very common phenomenon, as well as many theories to account for the same. If all salts precipitated uniformly in this manner, the diffusion wave theory of Wo. Ostwald (12) would probably suffice as an explanation. The failure of many compounds to behave in this manner and the numerous irregularities and varieties of periodic structure obtained have led to the propounding of numerous other theories. The theory of Bradford (1) involves the adsorption of the precipitate, that of Holmes (9) emphasizes the part played by the diffusion of the two reactants, and that of Dhar and Chatterji (3) introduces the idea of coagulation and peptization of the precipitate, which was further amplified by Hedges and Henley (7); all these theories are of considerable importance. The studies on periodic precipitation in silicic acid gels which we have made emphasize the importance of a careful quantitative study of the phenomenon and indicate that a still more comprehensive study is desirable. A number of anomalous formations have been found which are not easily explained by any of the present theories, but not enough data have been assembled to allow an explanation to be.attempted. I n the present investigation, we have confined ourselves largely to a study of the effect of concentration on the phenomenon, that is, indirectly to a study of the effect of diffusion rate. Only incidentally will reference be made to any of the other factors which have been shown to influence banding. The work is being continued in the hope that sufficient data can be accumulated so that a satisfactory picture of the rBle played by diffusion rates may be formulated. I. COPPER CHROMATE I N SILICIC ACID GEL
Stansfield (13) has reported the results of his work on the formation of bands of silver chromate in gelatin. His findings showed clearly that the greater the ratio between the concentrations of the salt in the solution and that in the gel, the farther apart were the bands. Although his study was 907
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AZARIAH T. LIKCOLN A S D J O H N C. HILLYER
carefully done, it mas felt that with his data available a still more detailed investigation in another system would be of value. The precipitation of copper chromate in silicic acid gel was selected, as it is a very suitable reaction to study. The silicic acid gel was prepared by mixing sodium silicate and acetic acid solutions. The sodium silicate solution was prepared by dilution of commercial sodium silicate, after settling and filtering, to a specific gravity of 1.060 f 0.002. Upon titration against standard sulfuric acid, it was found to be 0.60 N f.0.05. Acetic acid of 1.0 M strength was used, prepared in situ as described below. Test tubes were cleaned with hot chromic acid, then alcoholic sodium hydroxide, rinsed with distilled mater, and dried. Into each was introduced with a graduated pipet 0.50 ml. of glacial acetic acid. To each was then added the required volume of standard potassium chromate solution (0.50 M ) . The volume was then made up with water to 12.5 ml., giving a TABLE 1 Concentrations o,f the salt in the gels and of the solutions o n top of the gels CONCENTRATION OF SALT I N GEL
CONCENTRATION RATIOS W H E N T H E CONCENTRATION O F THE S O L U T I O N O N T O P OF THE GEL IS
05
.u
0.4M
0.3 M
0.2 M
0.1 ?il
1:20
1:lO 1:3.3 1:2 1:1.2 1:l
'11
0.01 0.03 0.05 0.08 0.10
1:50
1:40
1:16.7 1: 10 1:6.3 1:s
1:13.3
1:30 1: 10
1:5 1:4
1:3.8 1:3
1
1:G.7 1:4 1:2.5 1:2
solution containing. the desired concentration of salt and having an acid strength of 0.6 M (in the precipitation of all other salts, 0.75 ml. of acid was used, making a solution of 1.0 M strength). To each tube was then added 12.5 ml. of sodium silicate solution from a graduated pipet. The tubes were then stoppered and inverted. The air bubble was allowed to traverse the tube twenty times. The gels were very slightly basic, and sei. in about three minutes. After firm gels had been obtained (some hours), 5 ml. of a standard solution of copper sulfate was placed on top of each. The tubes were then tightly stoppered and the reaction proceeded. In this and most of the following studies, a series of twenty-five tubes was used. The concentration in the gel varied from 0.01 M to 0.10 M potassium chromate, while the concentrations on top were from 0.1 M to 0.5 M copper sulfate. Hence, the values of the concentration ratio lay between 1 and 50. This is more clearly indicated in table 1. Observations were made daily on these earlier sets of tubes, until the
LIESEGANG PHENOMENON I N SILICIC ACID G E L
909
reaction appeared to be complete. The liquid was then removed from the top and the cork tightly sealed in with shellac. The tubes were preserved practically unchanged for over three years. A confirmatory series was later prepared by a modified technique which consisted of dissolving the weighed quantities of the salt in the standard acetic acid solution (0.5 M in this case). A series of these solutions was made up and 10-ml. portions placed in clean dry test tubes by means of a buret. To each was added 10 ml. of the standard sodium silicate solution and the contents thoroughly mixed as before described. These systems were allowed to stand, and when the gels became firm at the end of about twelve hours, 5 ml. of the other solution was placed on top of each gel by means of a pipet. The tubes were stoppered and allowed to stand. I n figure 1 is shown a series prepared by this modified technique and photographed after about one year. In this the effect of changing the concentration ratio can be seen. The number of bands decreased with the decrease of this ratio, as did also the space between corresponding bands, while the width of the bands increased. Marked irregularities are to be noted. These take the form of spirals in most cases, but well-formed washer-like bands were formed with the small center precipitated at a point slightly farther down the tube. Stated in another way, as the concentration of the salt in the gel was increased and that on top held constant, the number and spacing of the bands decreased. Keeping the concentration in the gel constant, the effect of an increase in the concentration on top was to cause an increase in the number and spacing of the bands. These effects were noted throughout the entire series studied. Hence it becomes evident that in this case the concentration ratio may be varied by changing the concentration of either salt with exactly the same effect. 11. MERCURIC IODIDE I N SILICIC ACID G E L
This salt was prepared by diffusion of potassium iodide into gels containing mercuric chloride. A series of tu-enty-five tubes was prepared by the original technique. The concentrations used in the gel were from 0.001 M to 0.01 M , and those on top from 0.01 31' to 0.05 M . The values used were one-tenth of the usual values, to prevent the formation of a large amount of precipitate, which tends to grow into long needles and obscure the banding. In figure 2 are shown some of these tubes after reacting two months. The photograph was made three years later. The spacing is not as regular as with other systems. One very distinct effect does stand out, however, and that is that the space between the bands decreases toward the bottom of the tube. This effect has been observed in few other cases. Formation of the soluble double salt KI.HgI2 in the presence of excess potassium
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AZARIAH T. LINCOLN AND J O H N C. HILLYER
iodide accounts for the absence of precipitate in the upper portion of the tube. Reference to figure 2 shows also that the best bands are obtained when the concentration ratio is highest, and that the distance over which banding occurs is also greatest. This effect is produced by varying the concentration on top. Exactly the same effect is obtained by varying the concentration of mercuric chloride in the gel. This is shown in figure 3. Dhar and Chatterji describe this salt as one belonging to their second class, and therefore there should be a decrease in the number and spacing of bands with increasing concentration. We find this to be true with increasing concentration in the gel, but not in the solution on top. It is still a question as to whether the effect of potassium iodide in redissolving the precipitate is suficient to account for this. In the hope of obtaining more information about this system, a new series was prepared by the newer technique described above. Much the same results were obtained. The investigation is being continued. 111. COLLOIDAL MERCURY IN SILICIC ACID
Banding of colloidal mercury was first reported by Davis (2), who reduced mercurous nitrate with sodium formate in agar gel a t a temperature of 53°C. Bands were evident, though not good. Orlowski (11) studied the reaction of ammonium hydroxide on mercurous nitrate in gelatin gels. He reports colored layers, followed by remarkable spiral bands. No references have been found to any study of this element in silicic acid gels, nor have either of these reported studies in other gels given any results comparable to the bands formed with common inorganic salts. The results of Davis, if interpreted in the light of ‘some of the theories of banding, point FIG. 1. COPPERCHROMATE PRECIPITATES I n these tubes the concentration of the copper sulfate solution was 0.5 M , while the concentration of the potassium chromate in the gel was, reading from left t o right, 0.01, 0.02, 0.03, 0.04, 0.05,O.lO M. FIG.2. POTASSIUM IODIDE-MERCURIC CHLORIDE-SILICIC ACIDGEL SYSTEM The concentration of mercuric chloride in the gel was constant a t 0.003 M. The concentration of the potassium iodide solution on top of the gel decreased, from left t o right, from 0.05 t o 0.02 M . F I G . 3. POTASSIUM IODIDE-~~~ERCUR CHLORIDE-SILICIC IC ACID GELSYSTEM The concentration of the potassium iodide solution on top of the gel was constant a t 0.04 M. The concentration of mercuric chloride, from left to right, was0.01,0.005, 0.003, 0.002, and 0.001 M. FIG. 4. MERCURY-SILICIC ACID GEL SYSTEM The concentration of the mercuric chloride in the gel was 0.1 N . Several portions of 0.2 N stannous chloride were used on top.
FIG.5. LEADACETATE-POTASSIUM CHROMATE-SILICIC ACID GEL SYSTEM The concentration of lead acetate on top of the gel was, from left to right, 0.4, 0.5, 0.4, 0.5, 0.02, and 0.02 M . The concentration of potassium chromate in the gel was, from left t o right, 0.03, 0.04, 0.04,0.03, 0.10, and 0.05 M .
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AZARIAH T. LINCOLN AND JOHN C. HILLYER
toward the possibility of the supersaturation of a colloidal solution for which other evidence is lacking. Further, if the mercury be colloidal, the dispersoid is a liquid; diffusion of this liquid m7ould result in annihilation of any bands which were formed. We selected the reaction between mercuric chloride and stannous chloride as the most satisfactory procedure. Gels were made as described, except that 3 N sulfuric acid was used in place of N acetic acid, thus furnishing a strongly acid medium. Thirteen days mere required for setting the gel. Saturated mercuric chloride was added to four times its volume of the silicate-acid mixture and 0.2 N stannous chloride was employed on top. During thirty-eight days of reaction the stannous chloride was renewed several times. White bands of what was evidently mercurous chloride progressed down the tube. These later became bands of black mercury and mercurous chloride alternately. Finally, bands of black mercury were all that could be observed. The spaces were very narrow in some parts of the tubes, but very distinct. These bands are shown in figure 4. The photograph was taken three years later, and the tubes are somewhat damaged by drying. The dense region near the top contains fine bands which do not show in the figure. Reduction of mercurous nitrate with sodium formate at room temperature proved unsuccessful. Stannous chloride reduced mercurous nitrate to mercury, but without the formation of bands. IV. LEAD COMPOUNDS I N SILICIC ACID GEL
1 . Bunds of lead chromate
It is well known that lead chromate and silver dichromate had not until recently been made t o band in silicic acid gel. Dhar and Chatterji (4), upon the basis of their theories, attempted to show why these two did not form bands in silicic acid gel. A zone of peptizing values was found inside of which banding did not occur readily. The values for these two salts were believed to lie outside this zone, Le., the peptizing influence of the gel was too strong to allow banding to occur. Later, Hatschek ( 5 ) showed that silver dichromate could be made to band in silicic acid gel. We have now been able to produce bands of lead chromate in silicic acid gel, Several series of tubes were prepared by the first technique. After several months reaction they were sealed and preserved. The bands in these tubes were perfectly clear and definite. They were very narrow, and made up of long needles which tended to obscure the bands. For these reasons it was impossible to secure satisfactory definition in photographs of them. Again, when the concentration in the gel was constant, the distance over
LIESEGANG PHEh-0MEI';ON
I N SILICIC ACID G E L
913
which banding occurred varied directly as the ratio of concentrations. Since in practically all tubes the amount of solution on top was in excess, the constant concentration in the gel should have predetermined a fixed and constant amount of precipitate. The spacing between the bands was wider as the ratio was increased, as found in the other cases reported. When the concentration on top was held constant (and in excess), the decrease in ratio caused a decrease in the spacing of the bands and in the region in which they were formed. I n this case the total quantity of precipitate varies with the concentration in the gel, and is greatest in those tubes in which the distance in which bands were formed is least. Another feature noted throughout the whole series was that bands were closer together near the bottom of the tubes. It was found difficult to repeat the banding by this technique. The uncertainty of obtaining bands, and their close packing and indistinctness, lend some support to the theory of Dhar and Chatterji that the peptization value for this salt in silicic acid lies near the boundary line. But our observations show that the actual value is probably just within rather than just without the precipitation value. Later several series of tubes were prepared by the newer technique. The banding was more pronounced and was easy to duplicate. A number of these tubes are shown in figure 5 . 2, Lead iodide
A series of tubes was made as above, with potassium iodide in the gel and lead acetate on top. The concentration ratios were the same as for lead chromate. The lead iodide formed large hexagons as usual, but these were arranged in quite definite bands. Measurement of distances was impossible but the banded appearance is unmistakable, as shown in figure 6. In those tubes in which the concentration of potassium iodide in the gel was less than 0.03 M there was no precipitation of lead iodide, although some white crystals appeared in the liquid or imbedded in the surface of the gel. When the concentration reached 0.05 M, some crystals appeared in the lower portion of the tube, and it was not until a concentration of 0.1U -Ifhad been reached that good crystals were formed. The same effects of changing concentration ratio were noted as in the other cases. By means of the new technique, more series were prepared, and again precipitates occurred in the most concentrated members. Finally, a third series was prepared in which the position of the reactants was reversed. These gave very much better results, forming very large beautiful crystals, but no evidence of bands. This is shown in figure 7. However in all cases the abundance and depth t o which the crystals formed was proportional to the increase in the concentration of potassium iodide.
6
L I E S E G A S G PHENOMENON IK SILICIC ACID GEL
915
Again, since this was in excess in almost all tubes the amount of precipitate should depend upon the amount of lead acetate present, which was constant. We conclude from this, that although these crystals grow large and obscure any evidence of bands, one of the factors is still operating, namely that which governs the quantity and distribution of the precipitate. Apparently, either this factor is independent of that which produces the banded structure, or in this case some other factor operates to prevent the formation of small crystals in bands. 3. Lead bromide
Lead bromide has been found to band in gelatin, but in silicic acid gel Holmes (8) reports that under the conditions of his experiments only white twin crystals were formed. We have prepared a concentration series, and among the tubes were found a number exhibiting marked banding, This is shown in figure 8. The bands were, however, almost “opacity” bands, the density of precipitate being very low. It was thought that the potassium acetate formed in the reaction acted to dissolve the lead bromide. By means of the newer technique, other series were prepared which exhibited only the white twin crystals. Just as in the case of lead iodide, the depth to which the precipitate forms varies directly with the concentration of the solution on top, whereas the quantity should be determined by the much smaller amount of the salt contained in the gel.
4. Lead
sulfate
There seems to be no definite information about the precipitation of this salt in silicic acid. We found that a tube containing 0.1 iM lead acetate in contact with 0.2 M ammonium sulfate solution showed excellent bands after two months. One of these tubes photographed three years after it, was prepared is shown in figure 9. Following this a number of concentration series were prepared by the usual method, using potassium sulfate in the gel reacting against lead acetate. There was a marked tendency to band, but the crystals were large and scattered, and the banding was a rhythmic concentration of crystals. The quantity and distribution of the precipitate varied exactly as did that of lead iodide and bromide. Recent experiments shorn that the use of ammonium sulfate produces much better banding than potassium sulfate. 5 . Lead formate
Concentrated formic acid was allowed to diffuse into a gel 0.1 M witla, respect to lead acetate. Nothing at all appeared for several days. Then suddenly, areas of precipitation appeared throughout the tubes, particularly near the bottom. These grew out into long branching masses in a
916
AZARIAH T. LINCOLN AND J O H S C. HILLYER 0
few hours. During the course of a week these crystals continued to grow, becoming thick and club-like. The reaction was repeated a number of times with similar results. Two of these tubes are shown in figure 10. This sudden appearance of the precipitate in all parts of the tube at once makes the system of particular interest. Diffusion throughout the whole tube must have taken place with the formation of a supersaturated solution, which suddenly precipitates. Since no evidence of bands can be noted, it appears that this is strong evidence against the precipitation of a supersaturated solution yielding a rhythmic structure, as Osttvald contends. V. BISMUTH COMPOUNDS I N SILICIC ACID G E L
The citrate, oxalate, iodide, and basic dichromate were prepared in silicic acid gel. No evidence of bands was observed in the case of the first two. Strong citric acid diffusing into a gel N / 2 5 with respect to bismuth nitrate gave after several days bunches of crystals resembling cotton and distributed throughout the tube. Supersaturation was again indicated. Saturated oxalic acid diffusing into a gel prepared in the same may gave only tree-like formations of flat, silky crystals. The basic bismuth dichromate was prepared in a gel formed from two parts of the silicate solution to one of N acetic acid. A small crystal of potassium dichromate was added. The gel set in two minutes and N / 5 bismuth nitrate was placed on top immediately. A bright yellow coloration progressed down the tube. Following this, bands of a yellow precipitate formed. They were perfectly plane, wellformed, and very thin. There were many holes in them. I n all tubes, five rings formed in a space of about 3 cm. Potassium iodide diffusing into a gel containing bismuth nitrate produced a heavy reddish brown precipitate. Reversing the reagents gave a yellow ring of color which progressed down the tube. It was followed by a heayy brown precipitate, and finally by well-formed black crystals which appeared to be iodine. A concentration series was attempted, and in every case these black crystals formed, and in many cases grew to be long feathery trees. A great deal of gas mas liberated also. V I . SUMMARY AND CONCLUSIONS
A study of the most favorable ranges of concentration for the formation of banded precipitates of copper chromate, mercuric iodide, mercury, lead chromate, lead iodide, lead sulfate, and basic bismuth dichromate in silicic acid gel has been made. No bands were, however, obtained of lead bromide, lead formate, bismuth oxalate, bismuth citrate, and bismuth iodide. The pairs of salts were so arranged that in one series one of the pair of reacting salts was in the gel, and in another series the other salt mas in the gel. In many cases the results were markedly different.
L I E S E G d N G PHENOMENON I N SILICIC ACID G E L
917
I n all of the salts which were prepared in a concentration series, it mas found that if the concentration in the gel was kept constant and that of the solution varied, the depth to which the precipitate formed and the number of bands formed was in proportion to the increase in concentration of the salt in the solution. It was also found that if the concentration in the solution on top was kept constant and that in the gel varied, the depth to which the precipitate formed and the number of bands decreased with the increase in concentration of the salt in the gel. In general, the width of the bands and the spacing between them increased as the bottom of the tube was approached. We have found two marked exceptions to this, namely, mercuric iodide and lead chromate. Both of these gave numerous bands composed of fine crystals, in which the width of the spaces between the bands decreased toward the bottom of the tube. It has been found that in those systems in which the concentration ratio was greatest, the banding was best and extended farthest into the tube. I n those systems of lower concentration ratios there was rarely any banding. I n addition to this, it was found that in those systems in which banding did not occur, the depth to which precipitates were formed was likewise proportional to the magnitude of the concentration ratio. Therefore it appears that the rate of diffusion, which is approximately proportional to this ratio, is the controlling factor in the distribution of the precipitate. I n the case of lead chloride, lead bromide, and mercuric iodide the additional factor of re-solution of the precipitate by one of the soluble salts seems to be important. The idea that banding is a result of precipitation of a supersaturated solution does not seem to be confirmed by our results on lead formate. The quantitative studies of the concentration relations now in progress in this Laboratory will add some valuable data and me hope ill help in the solution of some of these problems. The interesting phenomenon of the banding of liquid metallic mercury has been recorded. REFERENCES (1) BRADFORD: Biochem. J. 10, 169 (1916); 11, 14 (1917); 14,29, 474 (1920). (2) DAVIS,H. S.: J. Am. Chem. SOC.39, 1312 (1917). (3) DHARAND CHATTERJI:Kolloid-Z. 31, 15 (1922); 37, 2, 89 (1925). (4) DHARA N D CHATTERJI:Kolloid-Z. 37, 89 (1925). (5) HATSCHEK: Kolloid-Z. 38, 151 (1926). (6) HEDGES:Liesegang Rings and Other Periodic Structures. Chapman-Hall, Ltd., London (1932). (7) HEDGESAND HENLEY:J. Chem. SOC.1928, 2714. (8) HOLMES:J. Phys. Chem. 21, 709 (1917). (9) HOLMES:J. Am. Chem. SOC.40, 1187 (1918); J. Franklin Inst. 184,743 (1917). (10) LIESEGAXQ: Phot. Arch., p. 221 (1896). (11) ORLOWSKI: Kolloid-Z. 39, 48 (1926). (12) OSTWALD, WOLFGAXG: Kolloid-Z. 36, 380 (1925). (13) STANSFIELD: Am. J Sei. [4] 43, 1 (1917).
