An isopiestic study of aqueous solutions of the alkali metal hydrogensulfates at elevated temperatures

Isopiestic molalities of LiHSO 4(aq), KHSO 4(aq), and CsHSO 4(aq) have been measured over the temperature range 383.46 K to 498.19 K with NaCl(aq) serving as the reference electrolyte for the calculation of excess thermodynamic quantities. Molalities of the reference electrolyte ranged from about 0.5 mol ·kg −1. The ion-interaction model of Pitzer gave an excellent description of the experimental results when the solutions were treated as mixed-electrolyte systems resulting from partial ionization of HSO 4 -(aq). A simple polynomial model for the temperature dependence of the ion-interaction parameters is sufficient to reproduce the experimental results. When calculated as stoichiometric activity coefficients for 1 –2 charge types, the resulting quantities are quite small, especially when compared with activity coefficients based on the assumption of a 1 –1 charge type. The activity coefficients are consistent with limited dissociation of the hydrogensulfate anion which decreases with increasing temperature.