Study and thermodynamics of the dissolution of the fluorapatite in the phosphoric acid

A modification on the reversal cells of a C-80 SETARAM calorimeter allows a Joule effect in the reaction cells to determine a transfer function in experimental conditions close to those of dissolution of a solid in a liquid. The calorimeter was then used to determine in isothermal conditions both thermodynamic and kinetic parameters associated with dissolution of synthetic fluorapatite in phosphoric acid solutions. Due to the rapidity of reaction, the heat flow recordings were deconvoluted and the resulting curves were analyzed iteratively to obtain the rates of digestion and the enthalpy of dissolution at 298K of fluorapatite in phosphoric acid solution containing 10, 18 or 30wt% P2O5. The results agree with a two-step dissolution mechanism. The first step is the dissolution of the solid in phosphoric acid, and the second is the formation of the complex Ca(H2PO4)+.