Heat Capacities of Aqueous Solutions of HNO3, Mg(NO3)2, Ni(NO3)2, and Co(NO3)2, and Some of Their Mixtures, at 298.15 K and 0.1 MPa

Heat capacities of binary aqueous solutions of HNO₃, Mg(NO₃)₂, Ni(NO₃)₂, and Co(NO₃)₂ have been measured up to high concentrations using a Picker-type flow calorimeter at 298.15 K and 0.1 MPa. Where comparisons were possible, the present results were mostly in good agreement with literature data. Greater differences in Ni(NO₃)₂(aq) and Co(NO₃)₂(aq) may be due to cation hydrolysis. Heat capacities were well fitted with an extended Redlich–Rosenfeld–Meyer-type equation for HNO₃(aq), and Pitzer-type equations for the three salts. Ternary solutions HNO₃ + M(NO₃)₂ (M = Mg, Ni, Co) were measured as functions of solution composition at constant ionic strengths of (6.0–12.0, 12.0, and 10.44) mol·kg^–1, respectively. In addition, data were obtained at constant molality fractions for Mg(NO₃)₂ + HNO₃ at x(Mg²⁺) = 0.3331, and for Ni(NO₃)₂ + HNO₃ at x(Ni²⁺) = 0.2523. It was established that ternary solution heat capacities could be predicted from binary component properties alone, either using Young’s rule (based on molar quantities) or an empirical mixing rule based on massic (“specific”) heat capacities; neither requires information beyond the relevant binary solution quantities, i.e., no additional mixing parameters are needed.