Dual constant composition studies of phase transformation of dicalcium phosphate dihydrate into octacalcium phosphate

Abstract

The nucleation kinetics of octacalcium phosphate (OCP) in a suspension of dissolving dicalcium phosphate dihydrate (DCPD) crystals has been investigated using the dual constant composition (DCC) technique at pH 7.00–7.40 and 37°C. The rate of DCPD dissolution is directly proportional to the relative undersaturation and the small magnitude of the rate constant suggests that desorption of the lattice ions from the surface is the primary rate-determining step. The rate of heterogeneous OCP nucleation on DCPD crystal surfaces is strongly dependent upon the solution pH and the nature of the supporting electrolytes. Magnesium ion markedly prolongs the induction period for OCP nucleation at concentrations above 1 × 10 −4 mol/liter, while a comparable inhibition is achieved with a zinc ion concentration as low as 1 × 10 −6 mol/liter. At pH 7, DCPD appears to be the first precipitating crystalline phase in the slightly supersaturated solutions.