Calcium Phosphate Solubility: The Need for Re-Evaluation

Abstract

The determination of the solubility of calcium phosphates by the conventional large excess of solid method has been demonstrated to be inappropriate. The problem lies in incongruent dissolution, leading to phase transformations, and lack of detailed solution equilibria: all calculations have been based on simplifications, which are only crudely approximate. The absolute solid-titration approach shows excellent reliability and reproducibility. Using solid titration, the true solubility isotherm of hydroxyapatite (HAp) has been found to lie substantially lower than previously reported. In addition, contrary to wide belief, dicalcium phosphate dihydrate (DCPD) is not the most stable phase below pH ∼4.2, where calcium-deficient HAp is less soluble. The misunderstanding here arises from the metastability of DCPD, which nucleates much more easily than HAp at low pH. Such results indicate that the Ca−P system is in need of complete reappraisal. The solid-titration method can be extended to other complex systems.