Counterintuitive Crystallization: Rate Effects in Calcium Phosphate Nucleation at Near-Physiological pH

Calcium phosphates are widely present in geological and industrial settings and make up the majority of our bone's inorganic content; however, their formation from solution is not fully understood. The nucleation of calcium phosphate phases was studied using a state-of-the-art titration setup. The effect of varied calcium addition rate was studied at a range of pH values between pH 7 and pH 8; the precipitated crystals were isolated and analyzed. Dicalcium phosphate dihydrate (DCPD) was formed at lower pH and a slow addition rate. Intermediate addition rates yielded a mix of DCPD and poorly crystalline hydroxyapatite (PC-HA). At fast addition rates and above pH 7.5, poorly crystalline hydroxyapatite was precipitated exclusively. The results indicate that counterintuitive kinetic effects play a substantial role in the nucleation of calcium phosphates.