Mineralization Reactions Involving Calcium Carbonates and Phosphates

Abstract

The calcium carbonates and phosphates may precipitate as different polymorphs and phases under conditions typical of those in vivo. A constant composition method has been used to study the kinetics of crystallization in which the activities of ionic species in the supersaturated solutions are maintained constant by the potentiometrically controlled addition of reagents. For calcite, the rate of reaction is proportional to the square of the supersaturation, expressed in terms of the activities of free calcium and carbonate ions. The constant composition study of the dissolution of this salt indicates the participation of both transport and surface processes over a range of pH. At lower pH values, the reaction is controlled by the rate of formation of carbon dioxide. In the growth experiments, metastable phases may persist for considerable periods and kinetic considerations are of paramount importance in understanding the mechanisms of the reactions. Thus the calcium phosphates which form in solution are markedly dependent on pH, supersaturation, ionic strength, temperature, and the nature of the solid phases already present.