Studies on the incongruent solubility of hydroxyapatite

The solubility of hydroxyapatite was examined in calcium phosphate mixtures produced by combining Ca(OH)2 and H3PO4 in varying proportions under a wide variety of conditions. Analyses of supernatants for calcium, phosphorus, and pH provided the data for solubility calculations. Alteration of the solid/liquid ratio by removing supernatant or by evacuating water from equilibrated mixtures had little effect on the solubility product. However, when precipitates separated from some mixtures were reequilibrated at various solid/solution ratios in either water, acetate buffer, or supernatants obtained from previous equilibrations, incongruent solubility was demonstrated and was attributed to the formation of other calcium phosphate phases which coated the apatite. The experiments indicated that hydroxyapatite has a thermodynamic solubility product between 10(-57) and 10(-60), but the exact value is difficult to determine because of the formation of surface coats.