Effect of Calcium Ions on Hydroxyapatite Formation from the Hydrolysis of Anhydrous Dicalcium Phosphate

Effects of solution calcium ions on hydroxyapatite (HAP) formation from the hydrolysis of dicalcium phosphate anhydrous (DCPA) were investigated under controlled solution conditions. The hydrolysis experiments (2.5mmol DCPA/250mL) were carried out at fixed pH in the absence or presence of CaCl2 (0-200mmol/L) by standard pH stat techniques at 37°C. The rate of hydrolysis was much faster, and the yield of HAP approximately doubled in the presence of CaCl2 when compared to no CaCl2, but no significant difference was observed over the range of Ca2+ concentrations examined (25-200mmol/L). At equivalent conversion times, the crystallinity of the HAP formed was also greater in the presence of CaCl2. Calculations showed that much less DCPA dissolved to reach solution equilibrium with respect to HAP in the presence than in the absence of CaCl2. We conclude, therefore, that solution Ca2+ is an important factor in controlling HAP formation from the hydrolysis of acidic calcium phosphates such as DCPA.