Aqueous formation of hydroxyapatite

The kinetics of stoichiometric (Ca/P = 1.67) and calcium-deficient (Ca/P = 1.5) hydroxyapatite formed in aqueous solution by acid-base reactions involving CaHPO4 and Ca4(PO4)2O were determined. Complete reaction occurs within 6 h at 37.4 degrees C regardless of composition with stoichiometric hydroxyapatite forming more rapidly. Stoichiometric hydroxyapatite formed more rapidly because the particle sizes of its precursors were smaller. Hydroxyapatite formation is characterized by an initial period of surface hydration of the precursors, an induction period, and a period during which the bulk of the conversion to hydroxyapatite occurs. During the first 3 h of reaction at 37.4 degrees C, the pH is about 8.25 and 7.6, respectively, as the stoichiometric and calcium-deficient hydroxyapatite are being formed. Subsequently the pH values move toward those of the related invariant points: Ca10(PO4)6(OH)2-Ca(OH)2 solution, and CaHPO4-Ca9HPO4(PO4)5OH solution. The concentrations of calcium and phosphate in solution never exceed those in serum. Seeding with 5 wt% hydroxyapatite eliminates induction regardless of composition. The kinetics are first-order and follow the Arrhenius relationship regardless of composition. The total heats of reaction (delta Hr) were determined at constant temperatures between 25 degrees C and 70 degrees C. delta Hr values of 261.3 and 320 kJ/mol were determined for the formation of calcium-deficient hydroxyapatite and stoichiometric hydroxyapatite, respectively. Activation energies of 84.5 and 87.4 kJ/mol were calculated for the formation of calcium-deficient and stoichiometric hydroxyapatite, respectively. Heats of formation for Ca4(PO4)2O and Ca9HPO4(PO4)5OH were calculated to be -4764.1 and -12707.7 kJ/mol, respectively.