Concentration and temperature empirical relationships of the electrical conductivity of electrolyte solutions

We have considered the dependences of the specific (κ) and molar (Λ) electrical conductivity (EC) of aqueous electrolyte solutions on the molar concentration and temperature for sulfates of divalent metals (Mn, Co, Ni, Cu, Zn, Cd) in a wide concentration range at 5 – 35°C. To describe such systems we propose a modified cubic equation (MCE): κ = C∙c3k + Q∙c2k + L∙ck, where C, Q, L, k are empirical parameters, fixed parameter k = 0.5 has been considered as well. From the correlation between the calculated parameters we assume that two of them are sufficient. The maximum of specific EC (κm) and the corresponding concentration (cm) have been calculated. We also assume that the systems under study are isomorphic in the normalized coordinates (κ/κm via c/cm). For the dependences like κ = A∙cx + B∙cy it is shown that x = 1 is a good approximation over the generalized sample. Empirical dependences with y = 5/4 and y = 4/3 are also considered. It is shown that they give comparable results to MCE. The proposed approach is tested on EC data of aqueous solutions of some salts. Similar two-parameter κ(κm, cm; c) equations of other authors have been considered. In order to describe the dependence of the specific EC on temperature and concentration we propose an equation κ = (A25 + a∙θ)∙c – (B25 + b∙θ)∙c5/4, where θ is the reduced temperature and A25, a, B25 and b are empirical parameters. Also a generalized equation for the molar EC of concentrated electrolyte solutions is proposed: Λ(Λ*, Λm, cm; c), where Λ* is the effective limiting molar EC, and Λm is the molar EC at c = cm. It was found that Λ* and Λm depend linearly on temperature. The average value of the exponent is close to 1/3, which brings the generalized molar EC equation closer to the equation derived from the quasi-lattice model of electrolyte solutions.