Variations in solution chemistry during calcium-deficient and stoichiometric hydroxyapatite formation from CaHPO4.2H2O and Ca4(PO4)2O.

This study explores the mechanistic paths taken when calcium-deficient hydroxyapatite, CDHAp (Ca/P = 1.50), and stoichiometric hydroxyapatite, SHAp (Ca/P = 1.67), form by reaction between particulate calcium phosphate salts. The acidic reactant was CaHPO4.2H2O (DCPD) and the basic reactant was Ca4(PO4)2O (TetCP). Variations in pH, calcium and phosphate concentrations, and the solids present during apatite formation, were determined as functions of reaction temperature (25.0 degrees, 37.4 degrees, and 50.0 degrees C) and time. It was found that the dissolution of TetCP was rate limiting for both hydroxyapatite (HAp) compositions at all three temperatures. However, the retrograde solubility and incongruent dissolution of DCPD became increasingly important in influencing the kinetics as the reaction temperature was increased. An amorphous intermediate phase was observed regardless of the HAp stoichiometry. The solutions from which the SHAp formed approached equilibrium at much shorter reaction times (1-2 days) than those from which the CDHAp formed. The latter continued to display changes in pH and in calcium and phosphate concentrations for 6 months. CDHAp was shown to be a thermodynamically stable phase. The dissolution of CDHAp is incongruent, showing a Ca/P molar ratio in solution less than 0.5.