VARIATIONS IN SOLUTION CHEMISTRY DURING CALCIUM-DEFICIENT AND STOICHIOMETRIC HYDROXYAPATITE FORMATION FROM CAHPO4-CENTER-DOT-2H(2)O AND CA-4(PO4)(2)O

This study explores the mechanistic paths taken when calcium-deficient hydroxyapatite, CDHAp (Ca/P = 1.50), and stoichiometric hydroxyapatit e, SHAp (Ca/P = 1.67), form by reaction between particulate calcium ph osphate salts. The acidic reactant was CaHPO4 . 2H(2)O (DCPD) and the basic reactant was Ca4(PO4)(2)O (TetCP). Variations in pH, calcium and phosphate concentrations, and the solids present during apatite forma tion, were determined as functions of reaction temperature (25.0 degre es, 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) c ompositions at all three temperatures. However, the retrograde solubil ity 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 st oichiometry. The solutions from which the SHAp formed approached equil ibrium at much shorter reaction times (1-2 days) than those from which the CDHAp formed. The latter continued to display changes in pH and i n calcium and phosphate concentrations for 6 months. CD-HAp was shown to be a thermodynamically stable phase. The dissolution of CDHAp is in congruent, showing a Ca/P molar ratio in solution less than 0.5. (C) 1 997 John Wiley & Sons, Inc.