Thermodynamic Study of the NaNO3–Cd(NO3)2–H2O Ternary System at 298.15 K by the Potential Difference Method

In this work, we have made some thermodynamic investigations about the NaNO3–Cd(NO3)2–H2O ternary system. The potential difference method of the battery cell, Na–ISE|NaNO3 (m1), Cd(NO3)2 (m2)|NO3–ISE, was used to study the activity coefficients in this mixed system at 298.15 K in the total ionic strengths range 0.0100–1.0000 mol·kg–1 with different ionic strength fractions yb of Cd(NO3)2 with yb = (0, 0.2, 0.4, 0.6, and 0.8). There was a good Nernst response between Na–ISE and NO3–ISE in this work, and the mean activity coefficients of NaNO3 can be interpreted well by the Pitzer models. Furthermore, the mixing Pitzer ion interaction parameters of θNa,Cd and ψNa,Cd,NO3 were fitted from Pitzer’s equation, and the two parameters were applied to compute the values of the mean activity coefficients of Cd(NO3)2, the osmotic coefficients, the solvent activity, and the excess Gibbs free energy in this mixed system.