Amorphous to crystalline calcium phosphate phase transformation at elevated pH

The spontaneous precipitation of calcium phosphate was studied in the pH range 9.25–12.80. The first-formed solid phase was an amorphous calcium phosphate (ACP) which transformed after a reproducible induction period into a crystalline apatitic phase. The induction period for this transformation was shown to first increase and then decrease with increasing pH. The maximum stability of ACP occurred at a pH of about 10.25. At lower pH, ACP had a well-defined solubility-determining molecular unit, but in the pH range of this study, a material with a variable solubility was formed suggesting a breakdown in the short-range order of the amorphous phase. Although the amorphous-to-crystalline transformation occurs via an octacalcium phosphate (OCP)-like phase in the more-nearly physiological pH range 7–9, nucleation kinetics suggested that no such intermediate phase formed at high pH. In fact, thermodynamic considerations rule out the participation of OCP in the amorphous to crystalline transformation above pH 10.7. It is possible that either a different mechanism for the amorphous-crystalline transformation takes precedence at high pH or the variable thermodynamic properties of the ACP formed at high pH result in a substrate with varying ability to nucleate the first-formed crystalline form of calcium phosphate.