Vapor-Liquid-Solid Pyhase Transitions in Aqueous Sodium Sulfate and Sodium Carbonate from Heat Capacity Measurements near the First Critical End Pont. 2.Phase Boundaries

摘要

The constant volume, constant composition heat capacity data for aqueous sodium sulfate show several interesting features: one peak or step when solid salt first precipitates and, on further heating, a #gamma#-shaped peak when hte vapor-or liquid-phase disappears. These features are interpreted in terms of the solid-liquid-vapor phase diagram, and their consistency is shown with eariler PVTx, phase boundary, and three-phase data from the literature. New data are presented on the density of liquid solutions saturated with respect to the vapor. New liquid compositions or densities supplement the three-phase data available in hte literature, and new fluid densiteis are reported near the critical end point. For aqueous Na_2SO_4, we find T_cep = (648.2 (+-0 0.2)K, and for aqueous Na_2CO_3, T_cep - (649.0 (+-)0.2)K. The present heat capacity data, as well as recent data for aqueous sodium carbonate, have sufficient resolution to distinguish, for the first time. the temperature of the critical end points in aqueous sulfate and aqueous sodium carbonate form the critical temperature of pure water, T_c -= (647.1 (+-)0.1)K.