Thermal, Conductivity, NMR, and Raman Spectroscopic Measurements and Phase Diagram of the Cs2S2O7−CsHSO4 System

Abstract

The conductivity of the binary system Cs2S2O7−CsHSO4 has been measured at 20 different molten compositions in the full composition range and in the temperature range 430−750 K. From the obtained liquidus−solidus phase transition temperatures, the phase diagram has been constructed. It is of the simple eutectic type with the composition X(CsHSO4) = 0.86 and temperature of fusion of 470 K for the eutectic. The previously unpublished temperature of fusion of CsHSO4 was found to be 491.7 K. The experimental phase diagram is in good accordance with a calculated diagram based on measured thermodynamic properties of the pure compounds and the heat of liquid−liquid mixing measured for X(CsHSO4) = 0.5 and 1.0. 133Cs and 17O NMR spectra and Raman spectra of the liquid Cs2S2O7−CsHSO4 system indicate the presence of a temperature sensitive equilibrium 2HSO4- ⇌ S2O72- + H2O where the water molecules are strongly associated in the melt. Fast exchange reactions take place between the species present, leading to the observed 17O NMR single line with an averaged chemical shift. Super-ionic and thermodynamic stable phases and the temperatures of phase transitions have been identified from the NMR measurements on CsHSO4, Cs2S2O7, and Cs2S2O7−CsHSO4 mixtures. For 11 selected compositions covering the entire composition range of the Cs2S2O7−CsHSO4 binary system, the conductivity of the molten state has been expressed by equations of the form κ(X) = A(X) + B(X)(T − Tm) + C(X)(T − Tm)2 where Tm is the intermediate temperature of the measured temperature range and X is the initial mol fraction of CsHSO4.