Effects of carbonate on hydroxyapatite formed from CaHPO4 and Ca4(PO4)2O

Carbonated hydroxyapatites were formed via reactions in NaHCO₃/NaH₂PO₄ solutions from a mixture of particulate tetracalcium phosphate (TetCP) and anhydrous dicalcium phosphate (DCPA). Reactions were followed by determinations of pH and ion concentrations. The solids formed were analyzed by XRD and FTIR. Rates of heat evolution were established by isothermal calorimetry. Reactions in the absence of NaH₂PO₄ did not reach completion within 24 h. Constitution of reactants to achieve a DCPA-to-NaHCO₃ ratio of 1, in conjunction with the presence of NaH₂PO₄ as a buffer, was found to be optimal for formation of apatite with no remaining reactant. The amount of carbonate incorporated in this apatite was 4–5 wt%. Calorimetry indicated the reaction mechanism to depend on the bicarbonate concentration in solution. The presence of NaH₂PO₄ was found to increase the reaction rate but decrease the extent of carbonate uptake.