Standard enthalpy of formation of disodium hydrogen phosphate hexahydrate and sodium diphosphate

Commercial disodium hydrogen phosphate dodecahydrate (Na₂HPO₄·12H₂O) was used as a precursor for synthesizing disodium hydrogen phosphate hexahydrate (Na₂HPO₄·6H₂O) and sodium diphosphate (Na₄P₂O₇). The purity of the synthesized products was checked up by IR spectroscopy and X-ray diffraction. The heat of dissolution of these compounds, in acid solutions of several concentrations (w/w) of H₃PO₄ was measured in a C-80 SETARAM calorimeter. Many dilution and mixing processes were also realized in the calorimeter in order to get the standard enthalpy of formation of these products. The values obtained for the enthalpies of formation are: (?3210.5) and (?3516.5) kJ · mol^?1 for sodium diphosphate (Na₄P₂O₇) and disodium hydrogen phosphate hexahydrate (Na₂HPO₄·6H₂O), respectively.