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

Research highlights: → Na2HPO4.6H2O and Na4P2O7 were synthesized from commercial Na2HPO4.12H2O. → Dissolution, dilution and mixing processes were realized in a C80 SETARAM calorimeter. → The solids were dissolved in H3PO4 aqueous solutions of several concentrations (w/w). → The enthalpy of formation of Na2HPO4.6H2O was determined as -3516.5 kJ mol-1. → The enthalpy of formation of Na4P2O7 was determined as -3210.5 kJ mol-1. - Abstract: 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.