Continuous vapour pressure measurements of salt-water mixtures: thermodynamics of the system NaNO3-LiNO3-H2O at 179 °C

Vapour pressures of water over the three-component system NaNO₃-LiNO₃-H₂O are reported for five different compositions (100, 60, 40, 20, and 0 cationic % Na) at 179 °C. Water-activities calculated from these data are fitted to the equations of Pitzer [1], of Pitzer and Simonson [2], and Horsák and Sláma [3]. Excess thermodynamic properties are calculated from these models. A new vapour pressure apparatus is presented, which automatically records the water-activity of a salt-water mixture over the concentration range from a dilute solution to the fused salt during one experiment. The concentration of the liquid solution in the measuring cell can be changed by stepwise distillation of small portions of water, which are collected in a cup placed on a balance. The computer-based data acquisition makes it possible to collect the experimental data three times faster than on a discontinuous setup. The new distillation apparatus is tested against earlier data on NaNO₃-H₃O at 179 °C [4].