Dissolution of hydroxyapatite and beta-tricalcium phosphate in nitric acid

Using an isoperibol calorimeter for rapid reactions and a Calsol type micro calorimeter for slow processes, are applied to determine the enthalpies of solution of two synthetic phosphate products in nitric acid. Namely, beta t ricalcium phosphate Ca-3(PO4)(2) and the calcium hydroxyapatite Ca-10(PO4)( 6)(OH)(2) are measured by varying pH value of the solvent. Some dissolution mechanisms are proposed for various pH values. They are ensured by complem entary reactions of solution of Ca(NO3)(2), Ca(H2PO4)(2) and H3PO4 in the s ame solvents. An extrapolation of solution enthalpies to pH=7 leads to the enthalpy of solution of these products in the pure water. These values are Delta H-sol degrees = -138.3 kJ mol(-1) for Ca-3(PO4)(2) and -393.6 kJ mol( -1) for Ca-10(PO4)(6)(OH)(2).