A kinetic study of precipitation from supersaturated calcium phosphate solutions

The formation of three different crystalline calcium phosphates (DCPD, OCP, HAP) and an amorphous calcium phosphate was studied as a function of pH and supersaturation. Under the experimental conditions the formation of HAP is always found to be preceded by one or more precursors in a sequence that is in agreement with the Ostwald rule of stages. Homogeneous formation of HAP at low concentrations is never observed. A semiquantitative analysis of growth curves and of relaxation times at pH 6.7 and 26°C reveals the growth of OCP to be described best by a mononuclear growth model with fourth-order dependence on the relative supersaturation. The heterogeneous formation of HAP on OCP surfaces is noted to shift to lower α values (reaction extent) with increasing initial supersaturation. It can be described by a polynuclear growth model