Standard enthalpy, entropy and Gibbs free energy of formation of “B” type carbonate fluorapatites

• The “B” type carbonate fluorapatites were dissolved in 26.22 wt% H3PO4 acid solution.
• Dissolution processes were realized in a C80 SETARAM calorimeter.
• The standard enthalpy of formation was determined by thermochemical cycle.
• Estimation standard entropy of formation a value of the Gibbs free energy can deduce.
• Increasing the amount of carbonate generates a decrease of the stability.

The “B” type carbonate fluorapatites Ca10−x+u(PO4)6−x(CO3)xF2−x+2u, with 0 ⩽ x ⩽ 2 and u ⩽ x/2, were synthesized by a double decomposition method. The samples were characterized by X-ray diffraction and infrared spectroscopy. The heat of dissolution of these products in a phosphoric acid solution was measured by a C-80 SETARAM microcalorimeter. A thermochemical cycle was proposed and complementary processes were carried out in order to get the standard enthalpies of formation of these apatites. Estimation of the values of entropy of formation allowed the determination of standard Gibbs free energies of formation of these compounds. The results showed that incorporation of carbonate ions results in a decrease of the stability of the apatite structure.