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

Abstract

Theory–experiment fit for electrolytes in solvents other that pure water has always been challenging. In ionic activity, solvent’s permittivity (ε) plays an important role; at ambient pressure, ε 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–Hückel 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 ε, < 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–Hückel–Onsager theory (Fraenkel, Phys. Chem. Chem. Phys. 20 (2018) 29896). In conductance, theory is further complicated by the effect of solvent’s viscosity (η) that – like ε – in addition to varying with temperature, varies also with the kind of solvent. However, good-to-excellent theory–experiment fit is achieved with different η values at ε > 30; and the equivalent conductivity proves, as before, to be theoretically interrelated with the activity coefficient. At ε < 30, a fit occurs only with an adjusted ε (εadj) postulated to reflect the influence of the exerted external electric field on solvent’s molecular arrangement.