Effects of Na2HPO4 and NaH2PO4 on hydroxyapatite formation.

Particulate solids CaHPO4 (DCP) and Ca4(PO4)2O (TetCP) can react to form hydroxyapatite (HAp) under conditions which may be compatible with those in vivo. Effects of sodium phosphate solutions on the formation of calcium deficient HAp at 25 degrees C by this reaction in dilute suspensions (liquid-to-solids ratio of 100) and at 1/s = 1 were studied. An 1/s ratio of one is not greatly different from that at which monolithic HAp forms. Solution chemistry results showed that the sodium phosphate used and its concentration strongly influenced the kinetics of HAp formation and the HAp composition. HAp formation occurred by the same mechanism in water and in the Na2HPO4 solutions investigated; however, HAp formation occurred more rapidly in Na2HPO4 solutions. DCP dissolution was congruent in these solutions and this delayed the overgrowth of DCP by HAp. Reaction in NaH2PO4 solutions resulted in the formation of CaHPO4.2H2O as an intermediate product. The final product formed in the NaH2PO4 solutions was affected by the l/s ratio used. CaHPO4.2H2O, not HAp, was the product formed from dilute suspension but it was not observed during reaction at l/s = 1. Variations in solution were correlated with the formation of HAp and intermediate products, with the consumption of reactants, and with the heat evolution characteristics. The latter were determined by isothermal calorimetry. Calorimetric analysis of the reactions indicated increased heat evolution in sodium phosphate solutions and acceleration in HAp formation. This appears to be related to accelerated dissolution of CaHPO4, which was rate limiting.