Thermochemistry and Relative Stability of Apatite Phosphates

Abstract

Fluor-hyroxy-and chlor-apatites of Ca, Ba, Sr, Cd and Pb have been synthesized and characterized by X ray diffraction, IR spectroscopy and chemical analysis. They are dissolved in a nitric solution using an isoperibol calorimeter. Combining the dissolution results with other results obtained in the same device or picked from literature leads to the enthalpies of formation of these products. The relative stability of the products is first discussed by considering the enthalpies of formation. However, one can access to a more realistic stability scale by calculating the Gibbs energies of formation. This calculation is based on the involvement of the apatites in reactions for which the entropies of the reactants are tabulated. Ca fluor-and hydroxy-apatites are taken as examples, and the results show that P 4 O 10 reactional schemes could be considered to calculate f G°(T 0) for all the products. This calculation leads to more realistic stability scale for the apatites.