An investigation of the theoretical background for the stability of the calcium-phosphate salts and their mutual conversion in aqueous solutions.

A computer program for calculation of ion-activity products, in aqueous calcium-phosphate solutions and suspensions, was used to construct solubility diagrams which demonstrate the stability and instability fields of the calcium phosphates relative to various parameters. With increasing pH, the solubility of all calcium-phosphate salts decreased (with a solubility increase above pH 12); the most important factors in apatite solubility were pH, the fluoride concentration and the ion strength. It was inferred that apatite owes its instability below pH 5 to its increasing solubility relative to that of brushite. Calculation on amorphous tricalcium phosphate showed that this salt in the pH range above 5 was so soluble that the concurrent supersaturation with respect to either brushite or apatite would induce their formation causing the instability of the amorphous salt.