Chapter 20: Electrolyte Solutions: Standard State Partial Molar Enthalpies of Aqueous Solution up to High Temperatures

Integral heat of solution measurements of aqueous electrolytes at very high dilution (10⁻⁴ mol kg⁻¹) are used to establish reliable standard state (infinite dilution) partial molar enthalpy, , for some representative aqueous electrolytes up to 598.15 K at steam saturated pressure, p_sat. The values for obtained from the high dilution calorimetric measurements are compared with the corresponding values calculated from the experimental apparent molar heat capacity and those from heats of dilution measurements of more concentrated aqueous solutions (m≥0.05 mol kg⁻¹). The comparison indicates model dependence of the extrapolation to standard state condition using the apparent molar heat capacity and heats of dilutions data. The comparison also emphasizes the need for more reliable experimental data at higher temperatures (T>473.15 K) at concentrations low enough to allow extrapolations to infinite dilution with an extended Debye–Hückel equation. Furthermore, the limiting law slope for enthalpy, A_H, calculated from the high dilution integral heats of solution measurements at 596.30 K and p_sat (122±6 kJ mol^−3/2kg^1/2) is in excellent agreement with the theoretical value (121 kJ mol^−3/2kg^1/2) from Archer and Wang and the corresponding value of (116 kJ mol^−3/2kg^1/2) from Bradley and Pitzer.