Critical curves of aqueous electrolytes related to ionization behaviour: new temperatures for sodium chloride solutions

Abstract

New liquid–vapour critical temperatures for aqueous sodium chloride solutions from pure water (374.0 °C) to a molality of 0.3 mol kg^–1 have been obtained with an accuracy within 0.2–0.5 °C. These data, combined with published values to 4.3 mol kg^–1 and ionization constants (K) for NaCl, were used to obtain critical temperature elevation (CTE) constants that describe ionic (A_i) and neutral molecule (A_n) effects on elevating the critical temperature. Values for K(NaCl) were obtained also from the critical curve. By assuming that the ionic effect is closely similar for different monovalent ions, ionization constants for 11 additional electrolytes along their critical curves were estimated. With the interpretation, these critical curves appear to behave ideally. A water density close to the critical density of pure water (0.322 g cm^–3) is necessary throughout for independently calculating values of K(NaCl) that provide the best fits. Good agreement is observed with predictions of theoretical models (Pitzer and Tanger; Gallagher and Levelt Sengers) that closely describe the initial experimental curvature of t_cvs. molality and with recent data of Knight and Bodnar and compilations of Bischoff and Pitzer at high molalities.