Conductivity, thermal measurements, and phase diagram of the Na2S2O7-NaHSO4 system

The conductivity of the Na2S2O7-NaHSO4 binary system has been measured for 15 different compositions in the full composition range, and in the temperature range 400-700 K. Phase transition temperatures were obtained, and the phase diagram was constructed. It is of the simple eutectic type, where the eutectic is found to have the composition X-NaHSO4 = 0.97, as calculated from the measured thermodynamic properties, and to melt at 179 degrees C. The partial enthalpy and entropy of mixing have been obtained, and the negative entropy points to a structural order of the melt, presumably due to hydrogen bonding, as found earlier for the K2S2O7-KHSO4 system. For each composition measured of the Na2S2O7-NaHSO4 system in the molten state, the conductivity has been expressed by equations of the form kappa = A(X) + B(X)(T - T-m) + C(X)(T - T-m)(2), where T,is the intermediate temperature of the measured temperature range.