KINETICS OF CRYSTAL GROWTH OF DICALCIUM PHOSPHATE DIHYDRATE

The kinetics of crystallization of dicalcium phosphate dihydrate (DCPD) has been studied by following the changes in calcium, phosphate, and hydrogen ion concentrations when stable, supersaturated solutions are inoculated with seed crystals of the salt. After a brief initial surge, the rate of growth of the crystals follows an equation that is second order with respect to concentration, ([Ca2+][HPO42-]), over a wide range of calcium/phosphate ratios suggesting a predominantly surface reaction controlled process. Experiments at 15, 25, and 37° have been made in supersaturated solutions over a range of calcium and phosphate concentrations and the energy of activation corresponding to the crystal growth process is 10.5 kcal mol-1. Addition of sodium pyrophosphate to the supersaturated solutions has a striking inhibitory influence upon the rate of crystal growth of DCPD. The effect is analyzed in terms of adsorption, following the Langmuir isotherm, of pyrophosphate ions at the available crystal growth sites.