COMMUNICATION TO THE EDITOR THE SOLUBILITY OF CALCIUM BICARBONATE. I1 PARTIAL PEPTIZATION^ One of my colleagues noted that in my report on the solubility of calcium bicarbonate (J. Phys. Chem. 42,971 (1938)) I did not mention the possibility that part of the salt might be peptized, and part might be in true solution. This should be considered, since it is well known that gelatin and certain soaps fall iiito this class, although in these cases the major porhion is peptized, not dissolved, by the water. Experiments with collodion membranes similar to the previous ones were carried out, except that the original solutions were filtered free from visibly suspended calcium carbonate, and unit concentrations were determined for the diffusand and the diffusate. If part of the calcium bicarbonate were merely peptized, the total calcium concentration a t equilibrium would be higher in the diffusand than in the diffusate, which fact can be seen by examining the general expression for a Donnan equilibrium. A solution with a concentration of 68.8 mg. per 100 cc. (calculated as calcium carbonate and discounting the ordinary solubility of this salt) was used. The final concentration inside the membrane was 9.64 mg. per 100 cc.; that outside was 9.63 mg. per 100 cc. A second run with a different membrane and an original concentration of 70.5 mg. per 100 cc. gave values of 8.89 and S.91 mg. per 100 cc., respectively. Thus, within the error of analysis, all of the calcium bicarbonate seems to be in true solution. Since the green of the streams referred to in the first paper becms not to be due to colloidal calcium bicarbonate, we are planning to obtain samples of the water to determine the source of the color, if possible. STEPHENS. HUBARD Department of Chemistry Cornel1 University Ithaca, New York 'Received November 1, 1938.
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