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 constructed. It is of thesimple eutectic type, where the eutectic is found to have the composition X(CsHSO4) = 0.97, ascalculated from the measured thermodynamic properties, and to melt at 179°C. The partialenthalpy and entropy of mixing have been obtained, and the negative entropy points to astructural order of the melt, presumably due to hydrogen bonding, as found earlier for theK2S2O7-KHSO4 system. For each composition measured of the Na2S2O7-NaHSO4 system inthe molten state, the conductivity has been expressed by equations of the form = A(X) +B(X)(T - Tm) + C(X)(T-Tm)^2, where Tm is the intermediate temperature of the measuredtemperature range.