LIESEGANG RINGS O F MANGANESE SULFIDE. I1 OLIN F. TOWER Morley Chemical Laboratory, Western Reserve University, Cleveland, Ohio Received November 81, 1936
Some years ago Chatterji and Dhar (1) published an account of a number of experiments on the formation of Liesegang rings in many gelatinous media, among them silica gel. They reportcd that they obtained rings of cadmium and antimony sulfides in this gel. However, they gave no detailed directions as to concentrations and other conditions under which the rings formed. For some years we have been interested in the formation of bands of metallic sulfides in gels, and have recently tried to form bands of such sulfides in silica gel. The experiments were not altogether successful, as only faint rings were obtained, and these under varying conditions which could not always be reproduced. The rings so formed were those of zinc, cadmium, and antimony sulfides. As we had obtained excellent bands of manganese sulfide in gelatin (4),we tried to form rings of this substance in silica gel. We found the best procedure to be the following: Commercial water glass was diluted to a density of 1.07 and was mixed with an equal volume of 0.5 N acetic acid. This mixture, which reacted acid toward litmus, was poured into test tubes 1 in. in diameter until they were threefourths full, and then the whole was saturated with hydrogen sulfide gas. The tubes were then allowed to stand until the contents set to a gel, after which the upper portion of the tube was filled with a solution of manganese chloride. The concentrations of manganese chloride giving the best bands were from 0.5 to 1 molar. The bands obtained are shown in figure 1. The rings usually obtained were like figure 1 a and b, but occasionally rings of the type shown in figure 1 c were produced. As far as the concentration of the manganese chloride is concerned, there seemed to be no difference whether the fine banded or coarsely banded ones were obtained, so the cause of this difference must have lain in the nature of the silica gel or in the concentration of the sulfide ion in it, as it is obvious from the method of preparation that the latter could not be accurately controlled. Attempts to control the concentration of the sulfide ion by using definite concentrations of sodium sulfide were unavailing, as this rendered the medium alkaline and the mixture failed to gel. 599
600
OLIN F. TOWER APPLICATION OF HUGHES’ DIFFUSION THEORY
E. B. Hughes (3) has recently developed a theory of the formation of Liesegang rings, based on diffusion. The theory is essentially in accord with that originally advanced by Wo. Ostwald, but Hughes has worked out mathematically the conditions for the formation of a new band on the basis of the upper electrolyte diffusing in and building up a sufficient concentration to attain the solubility product of the insoluble substance anew. I n order to apply the equations it is necessary to determine the distance between bands and the time of the formation of each band.
FIG. 1, Liesegang rings of manganese sulfide in silica gel
As bands of manganese sulfide could be reproduced very readily in gelatin, bands of this material and in this gel were used to teqt the theory. As seen in figure 1 a and b, a heavy precipitate of manganese wlfide was always produced a t the junction of the two electrolytes. The thickness of this precipitated area varied greatly in different cases, and in general was greater the greater the differencein concentration of the N n + + ion above from that of the S-- ion in the gel. The thicker this precipitated band, the more slowly thp Mn+ + ion diffused through it and the longer was the initial period before the first Liesegang ring formed. Another difficulty in measuring the time of formation of the bands was the long time consumed in completing the formation of the rings, viz., ten to twenty days. Naturally, therefore, some of the rings would form in the middle of the
601
LIESEOANG RINGS OF MANGANESE SULFIDE
night, and the time of their formation would not be exactly fixed. However, since the time between the formation of successive rings was long, this error was small. On account of the varying thickness of the initial banded precipitate, the time was measured from the end of the formation of this band. The appearance of the rings in gelatin can be observed in figure 1 of the former article (5). The results obtained from two such experiments are given in table 1. I n the first column are the distances of each ring from the point of beginning, and in the second column the time of the beginning of the formation of each band. h,/h,..l represents the distance of each band divided by the distance of the preceding band. h/.\/i represents the distance of each ring divided by the square root of the time of its formation. TABLE 1 Measurements made on T 1s of manganese sulfide 1 A! MnClr; 0.15 M (NH+S h in om.
Timein hours
-___
1.45 1.85 2.35 2.95 3.70 4.45 5.25 6.20 7.35 8.80
14 24 38 54 82 116 162 217 280 400
a. h - 1
1.276 1.270 1.255 1.254 1.203 1.180 1.181 1.184
1.197
0.26 M MnClr; 0.15M ( N H ~ Z S
h
3
-1-1-1-
0.3875 0.3776 0.3812 0.4014 0.4084 0.4132 0.4125 0.4209 0.4393 0.4400
Thickness of initial precipitate, 3 cm.
1.06 1.35 1.72 2.20 2.75 3.40 4.15 5.00 6.00 7.25
I
11 18
27.5 39 62 Bo
128 193 269 391
1.276 1.274 1.279 1.250 1.236 1.220 1.205 1.200 1.208
0.3915 0!3182 0.3280 0.3523 0.3492 0.3584 0.3668 0.3599 0.3659 0.3667
Thickness of initial precipitate, 1.9cm.
According to Hughes' theory the numbers in the last two columns should be constant. This is seen to be the case approximately. The numbers in the third column being approximately constant show that the rings are in geometrical progression, which has been found to be the case in most experiments of this kind. Many determinations of these values were made in other experiments, and the average found for rings of manganese sulfide of varying distances apart was 1.21. Hughes has derived an equation' giving the value of this constant in 1 Equation (ii), Kolloid-Z. 72, 213 (1935). To illustrate his theory he uses the results obtained by Morse and Pierce (Z. physik. Chem. 46, 589 (1903)) with bands of silver chromate in gelatin. These bands were all complete in from one to one and three-quarters hours after the beginning of the experiment, and consequently results are to be expected differing considerably from those described here, where the time taken for the formation of a series was from ten to twenty days.
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OLIN F. TOWER
an independent manner, which requires for its solution the value of the solubility product of the substance forming the bands. The solubility product of manganese sulfide in water is 1.4 X 10-15. Its solubility product in gelatin would have to be about 3.6 X in order that Hughes' equation ii should have a value for K equal to 1.21. This of course is highly improbable. However, from the appearance of the bands when they begin to form, it is evident that the solution is highly supersaturated, for a band appears rapidly in voluminous quantity and then gradually increases in density and thickness. This seems to show that the sulfide ions build up a concentration considerably in excess of that required for the solubility product of manganese sulfide before a band begins to form. Furthermore, the strong adsorption of manganese2 ions by the bands previously formed (2) delays the formation of a new band. For these reasons it is somewhat remarkable that the numbers in the fourth column in the table are as constant as they are. SUMMARY
1. A method for obtaining Liesegang rings in silica gel is described. 2. Manganese sulfide bands in sillica and other gels are spaced approximately in geometrical progression, the multiplier having the average value of 1.21. 3. Although in general the bands of manganese sulfide are formed by a process of diffusion essentially in accord with Hughes' theory, the time of formation of the bands is considerably influenced by supersaturation and adsorption. REFERENCES (1) CHATTERJI AND D H A R :Kolloid-Z. 40,97 (1926). (2) DAWSAND TOWER:J. Phys. Chem. 93, 608 (1929). (3) HIJQHES:Biochem. J. 28, 1036 (1934); see also Kolloid-Z. 71, 100 (1935). (4) TOWERAND CHAPMAN: J. Phys. Chem. 36, 1474 (1931). (5) Reference 4, p. 1475.
* Analysis of the manganese sulfide precipitate, which composes the rings, shows an excess of manganese over sulfur of 25 per cent above that required for the formula MnS.
