Thermodynamic Study of Binary Sodium Chloride and Rubidium Chloride in Mixed Methanol–Water Employing the EMF Method

This article reports the results of thermodynamic modeling of activity and osmotic coefficients of the quaternary system of mixed NaCl + RbCl in mixed CH₃OH + H₂O solvent based on the potentiometric method at 298.15 ± 0.05 K. By employing the EMF method without the liquid junction comprising chloride-selective and sodium-selective electrodes, the measurements were accomplished over the concentration range from 0.0005 up to 2.4941 mol·kg^–1 for different series of salt molal ratios (r = m_NaCl/m_RbCl = 100, 150, 200, and 250) with various alcohol mass percentages in water (w = 0.0, 0.10, 0.20, 0.30, and 0.40). The experimental results were modeled and interpreted in terms of the Pitzer ion-interaction approach. Exploiting the Pitzer model permitted the optimization and determination of the unknown Pitzer mixing parameters (θ_NaRb and ψ_NaRbCl) for each series of the investigated system. Eventually, having evaluated these parameters of the Pitzer model, it was possible to compute the activity coefficients of the constituent’s electrolyte, the excess Gibbs energy, and osmotic coefficients for various fractions of methanol in water for mixtures of sodium and rubidium electrolytes with common anion. The results of modeling for the system under consideration were surprisingly in good agreement with an empirical rule by Harned for the second electrolyte.