THE EFFECT OF MAGNESIUM-IONS ON THE PRECIPITATION OF CALCIUM PHOSPHATES

The effect of magnesium ions on the precipitation of calcium phosphates has been studied at 30°C by measuring pH, calcium ion activity and the calorimetric heat power as a function of time. The results can be interpreted by magnesium mainly inhibiting the formation of the recently reported new amorphous phase called ACP2 and the growth of OCP. Magnesium does not enter the solid phases to any great extent, nor are the apparent solubility constants significantly affected by the presence of magnesium. The enthalpy of transformation of ACP1 to ACP2 is found to be an endothermic reaction. The enthalpy of the mixing process and the formation of ACP1 or ACP2, calculated per mol both Ca and Mg bound, is comparable to the enthalpy of formation of the CaHPO4 ion pair. This could indicate that an important contribution to the change in enthalpy is related to the dehydration of calcium and magnesium ions. © 1990.