ENTHALPIES OF FORMATION OF COMPOUNDS IN THE P2O5CAO-H2O SYSTEM

The enthalpies of formation were determined for Ca-4(PO4)(2)O, alpha-C a-3(PO4)(2), and the hydroxyapatite compositions Ca-10(PO4)(6)(OH)(2) and Ca-9(HPO4)(PO4)(5)OH by the analysis of isothermal calorimetric re sults obtained at 37.4 degrees C. Both hydroxyapatite compositions wer e formed by reaction of Ca4PO4)(2)O with CaHPO4 or with CaHPO4 . 2H2O in water depending on the molar proportions of the reactant used. Ca-9 (HPO4)PO4)(5)OH was also formed by direct hydrolysis of alpha-Ca-3(PO4 )(2) in water. Enthalpy of formation values were calculated by a Hess' s Law analysis using several sets of equations. In summary, the enthal py of formation values are: for Ca-4(PO4)(2)O, -476(2.4) kJ/mol, for a lpha-Ca-3(PO4)(2), -409(6.5) kJ/mol, for Ca-9(HPO4)(PO4)(5)OH, -127(19 .7) kJ/mol, and for Ca-10(PO4)(6)(OH)(2), -137(44.0) kJ/mol at 37.4 de grees C.