The Dissociation Constants of Organic Calcium Complexes

Cations of the alkali earth group have strong tendencies to form partially dissociated complexes in solution with the anions of carboxylic acids and other weak acids. The affinity of this association tends to increase with the number of carboxylic acid groups of the acid anion. In a considerable number of cases the effect has been studied quantitatively and expressed in terms of well defined dissociation or association constants. The interaction of calcium ions and strontium ions with citrate ions has been studied by means of the frog heart preparation, and the equilibrium expressed according to the mass action law (6). The equilibrium constants for the interaction of alkali earths and other bivalent cations with a number of carboxylic acids have been determined by the effects of the cations on the pH titration curves of the acids (2). This method has been applied also to the study of solutions containing carbonates and phosphates (3,9). Few of the results reported in the literature have been checked by independent methods of determining the equilibria. For this reason, it is of interest to study the interaction of calcium, barium, and strontium ions with weak organic acids by means of electrodes of the third kind. In an earlier paper (7) the interactions of the alkali earth chlorides with glycine, alanine, and egg albumin were studied by this method. In this paper, it is applied to carboxylic acids with one, two, or three carboxyl groups per molecule.