On the Ionization of Calcium Citrate.

Abstract

The hypothesis that citrate ions combine with calcium ions in some way and thus remove them from solution serves to explain a number of observations. Among these may be mentioned the following: non-precipitation of calcium phosphate in alkaline solutions when magnesium citrate is present; the failure of Shipley, Kramer and Howland 1 to obtain calcification in vitro even with a Ca x P product of 60 when the calcium is present as calcium citrate; the unpublished observations of Shelling and Maslow in this laboratory on the production, by intravenous injection of sodium citrate, of convulsions clinically similar to those of tetany, but with a normal total serum calcium and a greatly increased dialyzable calcium; and the results reported very recently in the important contributions of Hastings, Murray and Sendroy 2 on the solubility of calcium carbonate and calcium phosphate.

There are no studies, however, which bear directly on the ionization of calcium citrate. We are, therefore, reporting some experiments which give direct evidence in favor of the above hypothesis.

Conductivity titrations were performed using N/200 calcium chloride and N/200 sodium citrate solutions. The conductivity of each solution was measured, that of the citrate being found the greater. Measured volumes of the citrate solution were added to a measured volume of the calcium solution. The conductivity of the resulting mixture was measured after each addition. The first addition of the citrate produced a decrease in conductivity instead of an increase. Further additions of citrate caused further decreases. It was only after an equivalent quantity of citrate had been added that subsequent additions caused the conductivity to increase. The decrease in conductivity was marked; when an equivalent quantity of citrate was added, the resistance of the mixture was about 50 per cent higher than the value which would have been obtained had there been no ionic reaction.