RHYTHMIC PRECIPITATION WITH ORGANIC PRECIPITANTS‘ J. E. HECK
AND
M. G. MELLON
Department of Chemistry, Purdue University, Lafayette, Indiana Received August 11, 1963
As a result of many investigations ( 6 ) since Liesegang’s discovery (8) of rhythmic precipitation there is considerable belief (1, 2, 7, 9) that it is a general phenomenon and may occur under suitable conditions with any pair of substances that form a fairly insoluble precipitate. When precipitation occurs in gels Hedges ( 5 ) distinguishes four types of structures that may appear: (a) precipitates of continuous structure very finely divided, as with silver chloride; (b) discrete structures frequently containing well-formed crystals of significant size; (e) cellular structures; and (d) periodic structures with definite bands or rings. In view of these different possibilities it seemed of interest to determine the type of structure produced in a gel by precipitates formed with certain organic compounds that are finding increasing use in analytical chemistry. Precipitates such as nickel dimethylglyoxime are very voluminous in character. EXPERIMENTAL IVORK
Materials Gels of silicic acid or agar were used, the former being prepared from sodium silicate and the latter from an American product. Salts were of “reagent” quality and organic compounds were used as purchased. Procedure As preliminary experiments showed that solutions of the organic precipitants did not diffuse well into the gels, in most cases these compounds were put in the gels and the solutions of the salts on top. Agar gels were prepared by dissolving 1g. of agar in about 80 ml. of hot water, adding 1020 ml. of a solution of the organic compound, diluting the mixture to 100 g., and filtering through glass wool into test tubes which had been previously coated with gelatin (3) to assist the gel in sticking to the tube. In preparing gels of silicic acid, sodium silicate was dissolved in water to give a solution of approximately 1.06 specific gravity. The alkalinity of the 1 Taken from Part I of a dissertation presented by J. E. Heck to the Graduate School of Purdue University in partial fulfillment of the requirements for the degree of Doctor of Philosophy. 85
86
J. E. HECK AND M. G . MELLON
TABLE 1 Data .for silica g e l s GEL
Sodium
Precipitant
11
8 j
.-
DIFFUSING SOLUTION
silicate
STRUCTURE A
Concentration
8 4.3 , 9-c
Lc3
pb --_
-ml.
.$
8
8.0 1.06 b. 8.01.06 8.0 1.06 8.01.06 0 . 0 1 08 8.01.06
a b a a a
14.8
I(
8.01.06 a
3 3
10.0 13.6
LL
0.01.08 a 8.0 1.06 b
3
13.6
3 jlO.O!
3
I I
10.0
4 , s 10.6 4,8 14.8
5 5
5 5
5
6
Concentration
ml.
Satd. soln. 1* 13.0{ 0.96 N AcOH LL 1 14.0 LL 1 13.0 I< 1 14.5 LL 2,4 10.6 2,414.8 2
Salt
F
((
L
