Molar Volumes and Heat Capacities of Aqueous Solutions of Potassium Hydroxide and for Water Ionization up to 573 K at 10 MPa

Abstract

Densities (ρ) and isobaric volumetric heat capacities (σ p ) of carefully purified aqueous solutions of potassium hydroxide have been measured at concentrations up to 6 mol•kg −1 at a pressure of 10 MPa over the temperature range 323 ≤ T/K ≤ 573 using a vibrating-tube densimeter and a Tian−Calvet differential calorimeter, respectively. Apparent molar volumes (V ϕ ) and isobaric heat capacities (C pϕ ) calculated from these data were fitted with modified Redlich−Meyer type equations to derive the corresponding standard molar volumes and heat capacities at infinite dilution. Exact comparisons with literature data were not possible because of pressure differences but the present values, which greatly extend the database for KOH(aq) in terms of precision, temperature and concentration, are broadly compatible with most of the earlier results. Comparison of the present values of V ϕ and C pϕ for KOH(aq) with corresponding data for solutions of the other alkali metal hydroxides indicated that all of these systems show similar behavior except for LiOH(aq), probably because of greater ion pairing in the latter. The present results for KOH(aq) were combined with reliable literature data for HCl(aq) and KCl(aq) to estimate the standard molar volume and heat capacity changes for the ionization of water up to 573 K.