T H E FORMATION OF LIESEGANG RINGS I N THE PRESENCE OF PRECIPITATES BINAYENDRA NATH SEN Chemical Laboratory, Presidency College, Calcutta, I n d i a Received September 18, i956
Much work has been done on the formation of periodic bands of precipitates of different substances in the presence of gels. It has been almost generally believed that gels are almost essential for such types of periodic precipitation, though the number of instances in which the formation of Liesegang rings is discernible even in the absence of gels has increased (4, 5, 6, 8) comparatively recently. It is the object of the present paper to report the formation of such periodic bands of precipitates, not in a gel but in the presence of certain other precipitates where the specific characteristics of gel-structure are absent and where the probability of the coexistence of sol and the precipitate of the same substance (1, 2, 3, 4), if not absent, would appear to play no prominent function. It is very difficult to explain the occurrence of such bands in the presence of these insoluble substances unless adequate experimental data are available, but it may be suggested that selective adsorption of some of the ionic species in the system by these precipitates may in all probability play a very important r6le in such a phenomenon. EXPERIMENTAL
The calcium sulfate and the barium sulfate were powdered to a very fine state of subdivision. Into thoroughly cleaned test tubes known volumes of a solution of ferric chloride of known concentration were introduced and then 10 g. of the extremely fine powder of each of the precipitates was allowed t o drop in. A further known volume of the solution of ferric chloride was next introduced into each of the test tubes. The exact volume of the solution to be added was found by trial, and such a volume was selected that when the 10 g. of powdered precipitate was placed in the solution, the mass of the precipitate would completely absorb the solution on standing. The precipitates in contact with the solution were allowed to stand for eight, hours. The contents of the tubes were next contrifuged and kept overnight. It was then found that the mass of precipitate, after absorbing the solution completely, appeared t o present some resemblance to a thickly set uniform gel of agar. It was, however, not transparent. 369
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A knoxn volume of the solution of potassium ferrocyanide of known concentration was then carefully added t o the precipitate without causing any disturbance of the solid particles. The test tubes with their contents were then allowed to stand for days, and the changes were noted each day until the Prussian blue precipitate had reached the bottom of each tube. As it has been brought out by the experiments of Lloyd and Moravek ( 7 ) that carrying out thp reaction in a small space exerts a marked favorable influence on the formation of Liesegang rings, experiments were also performed in much the same way in tube7 of narrow bore, having internal diameters of 0.5 and 0.25 em. However, these experiments yielded no positire results. It may be mentioned that 8 cc. of the ferric chloride solution was necessary for 10 g. of the calcium sulfate precipitate for complete absorption by the said precipitate; 4 cc. of the ferric chloride was added befoie and another 4 cc. after the introduction of the calcium sulfate into the test tube. In the case of barium sulfate, however, 6 cc. of the solution was necessary, of nhich 3 cc. was added before and 3 cc. after the introduction of the
FIG.1. Liesegang rings obtained by using 10 g. of calcium sulfate, 8 cc. of 0.385 A' ferrir chloride, and 6 cc. of 1.15 A' potassium ferrocyanide
sulfate precipitate. The purpose in introducing some of the solution before the introduction of the precipitate was t o minimize the possibility of the air bubbles being kept enclosed in the bodies of the solid precipitates. Care was always taken to ensure that no air bubbles could have a chance to remain thus enclosed. RESULTS WITH CALCIUM SULFATE
The number of rings formed increased with the dilution of the ferric chloride solution, and the distance between the rings decreased as the concentration of the ferric chloride was increased. The concentration of the potassium ferrocyanide appeared to have no favorable influence on the formation of the rings (1.15 N solution was used in the experiments). The first ring appeared after about forty hours; then another ring came in the course of a day. The distance between these two rings was quite considerable. The thickness of the first ring was, however, greater than that of the second. The distance of the first ring from the surface of separation of the solid and the
FORMATION OF LIESEGANG RINGS
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solution was about one-fourth the distance between the first and the second rings at favorable concentrations of ferric chloride. After the appearance of the second ring the blue color proceeded downward, uniform in tint but weaker in shade. The progress continued for several days and then ceased. This position was maintained for about three days. At the e$d of that time there appeared a distinctly blue ring, much deeper in shade than the blue color which had just ceased moving, and uniformly thick, its lower edge coinciding with the edge of the blue tint which had ceased to move downward. The blue tint above the ring became lighter and lighter in shade; after a day a ring appeared above it making the intervening space colorless. Another ring appeared again abovethis second ring in the course of a day, and the intervening space again lost its color. In this way, a ring appeared every day until there were five successive rings. After the formation of the fifth ring in the series, no fresh rings appeared. The rings were stable and distinct for about a week, after which they began losing their distinctness. As soon as the rings began losing their distinctness, the blue color started travelling down from the lowermost ring, slowly and gradually, until it came to the bottom. This occurred in the course of another week. Even when the blue color had reached the bottom, the rings previously formed, though indistinct, were yet discernible and about a month more was required for them to lose their identity completely. It may be mentioned that the distance between the rings in the above series increased only slightly in the order of the sequence of appearance of the rings. It is also of interest to note that no periodicity in the precipitation was observed in the region below the above series of rings, through which the blue tint ultimately travelled down. At very high concentrations of ferric chloride (above 1.5 N ) , two semicircular highly colored patches appeared just below the surface of separation of the liquid and the solid. No ring, however, was found to appear, even after keeping the system for about a week, but at the end of the week several periodic bands were observed in these highly colored spots in the form of arcs of circles. At moderately high concentrations of ferric chloride (0.75-1.5 N ) , however, these two deeply colored patches also appeared but with increased areas, and a highly colored ring appeared 8 mm. below these two areas. In the course of about three days another ring appeared just above; this was succeeded in its upper region by three fully developed rings and three partly developed rings at equal distances apart, the last partly developed ring touching the lower borders of these spots. I n the meantime, there appeared several periodic bands in the form of arcs of circles in each of the highly colored semicircular patches. The most favorable concentrations of ferric chloride for ring formation are in the range 0.3-0.6 N .
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BINAYEKDRA NATH SEN RESULTS WITH BARIUM SULFATE
In the case of barium sulfate, however, it was somewhat difficult to obtain the periodic bands. It appeared to be difficult as well to study the probability of the formation of the rings with reference to the concentration of the ferric chloride, the concentration of the potassium ferrocyanide having apparently no influence on the ring formation. I n most concentrations no rings were obtained; if formed a t all at some concentration, they were comparatively indistinct. The range of concentrations in which the ring formation was favored was also very narrow (0.15-0.25 iV). It was only in this small range of concentration of the ferric chloride that comparatively distinct rings, though lighter in shade than those obtained in the body of the calcium sulfate precipitate, were obtained. I n 0.2 N ferric chloride the most distinct rings were obtained, and three rings were discernible. The distances between the rings were approximately equal, but the thickness of the first ring was greater than that of the second and that of the second was greater than that of the third. The thickness of the rings was, however, considerably greater than that of those obtained in the body of calcium sulfate; the rate of travel of the blue color through the precipitate was also comparatively rapid. SUMMARY
The formation of Liesegang rings of Prussian blue in the bodies of calcium sulfate and barium sulfate, moistened with ferric chloride solution, has been recorded. The influence of the concentration of the ferric chloride on the characteristics of the rings has been studied. hIy best thanks are due to Dr. P. Neogi for giving me facilities for carrying on this piece of work. REFERENCES DHARAND CHATTEKJEE: J. Phys Chem. 28,41 (1924). D H ~ AND R CHATTERJEE: Kolloid-Z. 37, 3, 89 (1925). DHARAND CHATTERJEE: Z. anorg. Chem. 169, 129, 186 (1926) DHARAND MITRA:Nature 761, May 21, 1932. FISCHER . ~ N DSCHMIDT: Rocznilri Chem. 6, 404 (1926). HEDGES:J. Chem. Soc. 1929, 2779. (7)LLOYDA N D ;\/IoR.~vEK: J. Phys. Chem. 36, 1514 (1931). (8) MORSE:J. Phys. Chcm. 34, 1554 (1930).
(1) (2) (3) (4) (5) (6)
