DISSOLUTION DECELERATION OF CALCIUM-PHOSPHATE CRYSTALS AT CONSTANT UNDERSATURATION

The dissolution of dicalcium phosphate dihydrate (CaHPO4.2H2O) and octacalcium phosphate (Ca8H2(PO4)6.5H2O) has been followed as a function of time at constant undersaturations. The rate, after correction for changes in crystal surface area, decreases with time in spite of the sustained driving force, suggesting a decrease in the density of active sites on the crystal surface. This deceleration becomes more pronounced as the undersaturation decreases, leading to an increase in the effective dissolution order. The results of experiments in both Ultrapure and Reagent grade electrolyte solutions suggest that gradual contamination of the crystal surface is unlikely to account for the rate deceleration which may be interpreted by a decrease in the dislocation density during dissolution.