COMMUNICATION TO T H E EDITOR ATTEMPTS TO PREPARE GREEN GOLD SOLS' In attempting to prepare red gold sols by the method suggested by Ostwald,*namely, reduction of gold chloride solution by tannic acid in the presence of potassium carbonate, repeated trials resulted in a sol of a definitely green color. Suspicion pointed to the tannic acid solution which had been on the side-shelf for a long period of time.3 In order to check this point, the experiments were repeated with freshly made solutions of tannic acid, and again with samples of this substance which were ( I ) allowed to age naturally exposed to air and (9) artificially a,ged by TABLE 1 Precipitation experiments o n sols Sample dialyzed 24 hr.; 10 re. of gold sol taken in each case; time elapsed after addition of electrolyte = 15 min. A D D E D ELECTROLYTE
(1 CC.)
-
0 01 N sodium nitrate 0.1 N potassium chloride 1 . 0 N sulfuric arid
1
I
,
REDSOL
Positive Positive Slight
1
QREENBOL
Negative (slight) Yegative Segative
0.1 N sodium hydroxide caused immediate precipitation in both cases.
being bubbled with air for periods of 48 hr. Uniformly the fresh solutions gave the red sol, but both the naturally and artificially aged samples gave the green sols. These green sols were relatively stable, some of them being preserved in scalcd tubes for eight months without coagulation. A number of precipitation experiments on samples of the red and green sols of comparable strengths was tried. A Tyndall light cone from a brilliant source of illumination was used as the criterion of precipitation. The results of t h w experiments are summarized in table 1. If the Tyndall effect is listed as positive, the sol had not completely precipitated, Fhile a negative effect signifies that it was optically clear. These results are Received .4ugust 27, 1938. See Holmes, Laboratory Manual of Colloid Clieinistiy, p 32. John Wiley and Sons, Inc., S r m T o r k (1928) a The reference cited in note 2 points out t h a t tannin solutions have a great tendency to develop mold, and suggests the addition of rhloroform to prevent I t . 1149 2
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typical of many different experiments with vwying times arid concentrations of added electrolytes, and were essentially the saine whether the samples were previously dialyzed or not. In a!l caseb the green gold sol precipitated more completely in a given time than the red Attempts were made t o separate portions of the aged t n i i i k acid by fractional dialysis. Howcver, !+-hen each of thcse fractions a as separately used for reducing gold chloride in a similar manner. green sols invariably resulted. In other words. our attempts to isolate the different possible oxidation products which might be responsible for the two kinds of sols were unsuccesqful. Severtheless, we feel that thr experiirients were of sufficient interest to justify their being recorded. HAROLD B. F R I C D M A K . J. JOED ~ V I SJR. , Department of Chemistry Georgia School of Technology Atlanta, Georgia
