Crystal growth and phase transformation in the precipitation of calcium phosphates

Abstract

A survey of calcium phosphate precipitation in the neutral and slightly acidic pH range is presented. The concentration regions, within which precipitates are formed by direct crystallization and via precursor phases are defined. As examples, the kinetics of the precipitation and growth of calcium monohydrogen phosphate dihydrate (CaHPO₄·2H₂O, DCPD) and of amorphous calcium phosphate (ACP) were followed at 25 °C. DCPD was precipitated from 0.15 M NaCl solutions at constant pH 5. Precipitates were formed by heterogeneous nucleation. The crystal growth process is best described by the rate equation: dX/dt= 3.6 × 10⁹X^2/3([CaHPO₄^o]_t–[CaHPO₄^o]_s)³ where X is the amount of DCPD precipitated per dm³ of solution and [CaHPO₄^o]_t and [CaHPO₄⁰]_s are the concentrations of the neutral soluble complex at time t and at equilibrium. ACP was prepared at zero time pH 7.2 ± 0.05 and subsequent changes of the pH, the solution calcium concentration and the particle diameter were followed as a function of time. Precipitates were formed by homogeneous nucleation and consisted of spherical, approximately monodispersed particles. Agglomeration occurred in the early stages of precipitation. Later, during the period of metastability of ACP, the number of individual spherules approached a constant value but their volume increased with time. The rate of growth was a first order function of the supersaturation.