Transition from endothermic to exothermic dissolution of hydroxyapatite Ca5(PO4)3OH–johnbaumite Ca5(AsO4)3OH solid solution series at temperatures ranging from 5 to 65 °C

Abstract

Five crystalline members of the hydroxyapatite (HAP; Ca5(PO4)3OH)–johnbaumite (JBM; Ca5(AsO4)3OH) series were crystallized at alkaline pH from aqueous solutions and used in dissolution experiments at 5, 25, 45, and 65°C. Equilibrium was established within three months. Dissolution was slightly incongruent, particularly at the high-P end of the series. For the first time, the Gibbs free energy of formation ∆G0f, enthalpy of formation ∆H0f, entropy of formation S0f, and specific heat of formation Copf were determined for HAP–JBM solid solution series. Based on the dissolution reaction, Ca5(AsO4)m(PO4)3−mOH = 5Ca2+(aq) + mAsO43−(aq) + (3 − m)PO43−(aq) + OH−(aq), their solubility product Ksp,298.15 was determined. Substitution of arsenic (As) for phosphorus (P) in the structure of apatite resulted in a linear increase in the value of Ksp: from HAP logKsp,298.15 = −57.90 ± 1.57 to JBM logKsp,298.15 = −39.22 ± 0.56. The temperature dependence of dissolution in this solid solution series is very specific; in the temperature range of 5°C to 65°C, the enthalpy of dissolution ∆Hr varied around 0. For HAP, the dissolution reaction at 5°C and 25°C was endothermic, which transitioned at around 40°C and became exothermic at 45°C and 65°C.