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In this study, the enthalpies of dissolution of the calcium fluorapatite Ca10(PO4)6F2 were measured in nitric acid solutions at various pH at 298.15 K with the aim of attaining the dissolution enthalpy in pure water which cannot be directly measured. Mechanisms of the reactions of dissolution involving Ca(NO3)2, Ca(H2PO4)2, CaHPO4, H3PO4,and HF entities were proposed at different pH ranges. The solution enthalpies of the products reactions were measured in the same solvent and temperature but at various pH values. When supported by the dissolution model of the calcium fluorapatite at various pH, the extrapolation of the solution enthalpies at pH=7 leads to the solution enthalpy of this apatite in pure water -275.1 kJ mol-1.

Solution enthalpies of calcium fluorapatite in nitric acid and in pure water