Abstract
The enthalpies of formation were determined for Ca4(PO4)2O, α-Ca3(PO4)2, and the hydroxyapatite compositions Ca10(PO4)6(OH)2 and Ca9(HPO4)(PO4)5OH by the analysis of isothermal calorimetric results obtained at 37.4 °C. Both hydroxyapatite compositions were formed by reaction of Ca4(PO4)2O with CaHPO4 or with CaHPO4· 2H2O in water depending on the molar proportions of the reactant used. Ca9(HPO4)(PO4)5OH was also formed by direct hydrolysis of α-Ca3(PO4)2 in water. Enthalpy of formation values were calculated by a Hess's Law analysis using several sets of equations. In summary, the enthalpy of formation values are: for Ca4(PO4)2O, −4762.4 kJ/mol, for α-Ca3(PO4)2, −4096.5 kJ/mol, for Ca9(HPO4)(PO4)5OH, −12719.7 kJ/mol, and for Ca10(PO4)6(OH)2, −13744.0 kJ/mol at 37.4 °C.
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