Thermochemistry of the mixed calcium phosphate Ca8P2O7(PO4)4

The calcium mixed phosphate Ca8P2O7(PO4)4 has been synthesized by thermal decomposition of octacalcium phosphate previously prepared by precipitation in ammoniacal phosphate solution. The enthalpy of formation at 298.15 K referenced to β-tricalcium phosphate and calcium pyrophosphate is determined. β-Tricalcium phosphate was prepared by two methods: precipitation in ammoniacal aqueous medium and high temperature solid-state reaction. Calcium pyrophosphate was prepared by high temperature solid-state reaction. All the compounds are characterized by chemical analysis, X-rays diffraction and IR spectroscopy. The enthalpy of formation +10.83 ± 0.63 kJ mol−1 is obtained by solution calorimetry at 298.15 K in nitric acid.