Enthalpie de formation de la whitlockite Ca18Mg2H2(PO4)14

Pure and well crystallised whitlockite Ca₁₈Mg₂H₂(PO₄)₁₄ has been synthesized by precipitation from the magnesium and calcium nitrates and the diammonic phosphate. The product of the reaction has been characterized by X-ray diffraction, IR spectroscopy and chemical analysis. Using differential conduction calorimeters the enthalpies of solution of the whitlockite and of a mixture of the solid reactants - the tricalcium, the trimagnesium and the dicalcium phosphates - have been measured at 25°C for various concentrations of solid in a 46 wt% nitric acid solution. A combination of the enthalpies of solution with the enthalpies of formation of the reactants allows us to determine the standard enthalpy of formation of the whitlockite. The value deduced, -27,93·10³kJ mol^-1, is compared to the standard enthalpies of formation of the trimagnesium and the tricalcium phosphates. This revised version was published online in August 2006 with corrections to the Cover Date.