The equivalent electric conductivity of hydrochloric acid in solvents of lower permittivity than water, and its interrelation with the acid's activity coefficient: A theoretical analysis

Theory-experiment fit for electrolytes in solvents other that pure water has always been challenging. In ionic activity, solvent's permittivity (epsilon) plays an important role; at ambient pressure, epsilon is only a function of temperature, but in different solvents, it is also influenced by the nature of the solvent used, being smaller in less polar solvents. Employing the extended Debye-Huckel ion activity theory DH-SiS, HCl in nonaqueous or mixed solvents provided, up to moderate concentration, an excellent theory-experiment agreement even at very low epsilon, < 10 (Fraenkel, J. Phys. Chem. B 115 (2011) 14634). A parallel analysis is now reported for the ionic electric conductance of HCl in such solvents; specifically, in pure methanol, methanol-water mixtures, and dioxane-water mixtures. The analysis is done using DHO-SiS - a recently presented refined and extended Debye-Huckel-Onsager theory (Fraenkel, Phys. Chem. Chem. Phys. 20 (2018) 29896). In conductance, theory is further complicated by the effect of solvent's viscosity (eta) that - like epsilon - in addition to varying with temperature, varies also with the kind of solvent. However, good-to-excellent theory-experiment fit is achieved with different eta values at epsilon > 30; and the equivalent conductivity proves, as before, to be theoretically interrelated with the activity coefficient. At epsilon < 30, a fit occurs only with an adjusted epsilon (epsilon(adj)) postulated to reflect the influence of the exerted external electric field on solvent's molecular arrangement.