Enthalpies of Formation of Compounds in the P₂O₅−CaO−H₂O System

The enthalpies of formation were determined for Ca 4 (PO 4 ) 2 O, α-Ca 3 (PO 4 ) 2, and the hydroxyapatite compositions Ca 10 (PO 4 ) 6 (OH) 2 and Ca 9 (HPO 4 )(PO 4 ) 5 OH by the analysis of isothermal calorimetric results obtained at 37.4 °C. Both hydroxyapatite compositions were formed by reaction of Ca 4 (PO 4 ) 2 O with CaHPO 4 or with CaHPO 4 · 2H 2 O in water depending on the molar proportions of the reactant used. Ca 9 (HPO 4 )(PO 4 ) 5 OH was also formed by direct hydrolysis of α-Ca 3 (PO 4 ) 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 Ca 4 (PO 4 ) 2 O, −476 2.4 kJ/mol, for α-Ca 3 (PO 4 ) 2, −409 6.5 kJ/mol, for Ca 9 (HPO 4 )(PO 4 ) 5 OH, −127 19.7 kJ/mol, and for Ca 10 (PO 4 ) 6 (OH) 2, −137 44.0 kJ/mol at 37.4 °C.