Heat capacity of Rb2SO4(aq) and Cs2SO4(aq) solutions and thermodynamic modelling of (Rb2SO4 + H2O) and (Cs2SO4 + H2O) systems

Specific heat capacity of Rb 2 SO 4 (aq) and Cs 2 SO 4 (aq) solutions was determined using a Calvet-type calorimeter from 288.15 K to 338.15 K and from about 0.25 mol·kg −1 to concentration near room temperature solubility limits. For both Rb 2 SO 4 (aq) and Cs 2 SO 4 (aq) solutions, the specific heat capacity decreases with the increase of molality at constant temperature. Combining the heat capacity data determined in this study and other thermodynamic properties ( i.e. water activity, osmotic coefficient, mean ionic activity coefficient, enthalpy of dilution and solubility) reported in literature, thermodynamic and phase equilibria properties of the binary Rb 2 SO 4  + H 2 O and Cs 2 SO 4  + H 2 O systems were modelled using a comprehensive thermodynamic framework, in which a Pitzer-Simonson-Clegg model was applied to represent the non-ideality of aqueous phases. The models can represent all the properties over wide temperature and concentration ranges. This study forms a basis for solubility prediction in multi-component systems containing Rb and Cs sulfates.