Standard enthalpies of formation of "A" type carbonate phosphobaryum hydroxyapatites

Phosphobaryum carbonate hydroxyapatites Ba-10(PO4)(6)(OH)((2-2x))(CO3) (x) with 0 a parts per thousand currency sign x a parts per thousand currency sign 1, were synthesized in this study by solid gas reaction at a high temperature. Carbonate content was determined by thermogravimetric analysis and X-ray diffraction. The heat of the solution in a 3 wt% nitric acid solution was measured at 298 K using an isoperibol calorimeter. The combination of the enthalpies of solution with the enthalpies of formation of the reactants allowed us to determine the standard enthalpies of formation of the studied apatites. The result showed a decrease in formation enthalpy with the carbonate amount introduced in the lattice, suggesting an increase in stability of these compounds as the ratio of the substitution increases.