QUANTITATIVE ANALYSES OF LIESEGANG RINGS A. T. LINCOLN
AND
JOHN C. HILLYER
Department of Chemistry, Carleton College, Northfield, Minnesota Received January 11, 1986
As part of a program of careful quantitative study of the Liesegang phenomenon, we have performed analyses for the ions present in both bands and clear spaces in a copper chromate structure in silicic acid gel. By removing the banded structure from the test tube and subjecting the sections to analyses, we have shown both copper and chromate ions to be present together in the lower sections of this gel without the formation of a precipitate. Very few studies of this type have been made. Earlier work by Hedges and Henley (1) on magnesium hydroxide made no attempt to distinguish between the soluble portion and the widely dispersed precipitate. Hughes (2) has recently made a very careful and complete survey of the distribution of ions in a large number of different systems.' Our present work extends and corroborates his recent findings. Banded structures of copper chromate were prepared by a technique previously described (3). After formation, these were removed from the tubes, cut into sections, the soluble salts extracted, and the copper, sulfate, and chromate ions determined. In all cases the copper and sulfate ions were distributed throughout the whole tube in a manner compatible with diffusion from the top, while the chromate ion was exhausted in all the upper sections. However, in most tubes both copper and chromate ions were found together in the lowest clear space, below which a band was just forming. The concentrations were such as to cause a precipitate in the absence of the gel. Since no definite information could be found on the solubility product of this salt, tests were made to determine whether the actual concentrations found did cause a precipitate in the absence of a gel. Solutions were made which upon mixing yielded exactly the same concentrations as found. Heavy precipitates formed in all cases. Additional solutions were made containing the exact amount of free acetic acid present in the gels, and also, both the acid and the large amount of sodium acetate formed in precipitating the silicic acid. I n these cases also a precipitate formed, although somewhat slowly. For the preparation and analysis of the gels we are indebted to Mr. John Main. 645
646
A. T. LINCOLN AND JOHN C. HILLYER EXPERIMENTAL
The gels were formed from equal volumes of 1.06 sp. gr. sodium silicate solution and 0.65 N acetic acid solution by a technique previously described (3). The gels were made 0.1 M and 0.2 M with respect to potassium chromate. Concentratiohs of 0.3 M and 0.4 &I copper sulfate were used on top. After the completion of the reaction the bottoms of the tubes were filed and cracked off. The gels were then pushed out with a rod, Ioosening first
1
REACTION
TABLE 1 Results of typical analyses __ IBTANC TOP
space
I. KZCrOd, 0.08 M ; CUSOI,0.3 M
Ist band
~
FROM
SPACE
{
A B
space space 11. K2CrOd, 0.2 M ; CuSOa, 0.3 M
'
1st band
i
space space
CUSO~,0.4 M
A
B
WEIQRT
OR CEN'
OF
so*--
-
OF
OR CEN
OF
cut-
#EXCENT OF
grams
067
cent
Der cent per
13169 1 4948 1 2948 1 4598 4 7322 3 1824
0 0 0 0 0 0
86 87 78 79 68 68
0 0 0 0 0 0
39 40 19 19
Kone None Kone Kone 0 10 0.12
1 0689 1 2486 1 3846 13519 3 8695 3 4502
0 0 0 0 0 0
69 69 58 57 28 29
0.28 0.27 0 25 0 24 0 09 0 09
None None None None 0 19 0 18
1 7517 1 6385 1 3690 1 4006 4 7593 3 8891
083 0 83 0 71 0 73 0 58 0 58 -
0 0 0 0 0 0
None None None None 0.10 0.08
SANPLE
-
43 46
39 39 35 35 18 20
(304-
cent
-
by momentary immersion in hot water if necessary. The gel was then cut into sections by means of a razor blade. Usually two clear spaces and one ring were analyzed. Analytical procedure The portions selected for analysis were placed in weighing bottles and carefully weighed. The soluble salts were extracted with boiling water and filtered. The extraction was repeated five or six times. Chromate ions were first reduced by boiling with dilute hydrochloric
ANALYSES OF LIESEGANG RINGS
647
acid and ethyl alcohol (4). After expelling the excess alcohol, the sulfate was precipitated as barium sulfate and weighed as such, according to the standard procedure. The filtrate from this precipitation was then treated with hydrogen sulfide to precipitate the copper, which was redissolved, and determined iodometrically. Finally, chromium was precipitated as the hydroxide, taking the usual precautions to eliminate excess ammonia, ignited to the oxide, and weighed. The results of typical analyses are shown in table 1. Precipitation i n the absence of gel
Two solutions were made containing respectively 1.416 g. and 0.756 g. of CuSOa.5Hz0 in 100 ml., and two solutions containing respectively 0.402 g. and 0.666 g. of KZCi-04. These were twice the amounts found in the corresponding sections of the same gels. When equal volumes of the corresponding solutions were mixed, the final solutions should contain exactly the same concentrations as found in the gel. Heavy precipitates formed in both cases. The copper sulfate solutions of the same concentration were prepared containing (1) 3.0 ml. of acetic acid per 100 ml. and (2) 7.7558 g. of NaCzH302.3 H z 0 and the acid as in (1). Adding these to the corresponding chromate solutions produced a heavy precipitate of copper chromate. SUMMARY
By quantitative analyses of rhythmically banded copper chromate structures in silicic acid gel it has been shown that, while the copper and the sulfate ions are distributed throughout the tube in a manner to be expected from simple diffusion, there are present in the lower clear space both copper and chromate ions in moderate concentration. Copper chromate has been found to precipitate when the ions are present in the concentrations found in these spaces, even in the presence of the same amount of free acetic acid and of sodium acetate present in these gels. It appears that the presence of silicic acid inhibits the precipitation of this compound until higher concentrations are reached, and so aids in the formation of banded structures. REFERENCES (1) HEDGES AND HENLEY: J. Chem. soo. 1928, 2714.
(2) HUGHES:Bioohem. J. 28, 1086 (1934). (3) LINCOLN AND HILLYER: J. Phys. Chem. 88, 907 (1934). (4) SCOTT: Standard Methods of Chemical Analysis. D. Van Nostrand Co., New York (1925).
