Determination of the Limiting Conductances and the Ion-Association Constants of Calcium and Manganese Sulfates in Water from Electrical Conductivity Measurements

Marija Bešter-Rogač*

Faculty of Chemistry and Chemical Technology, University of Ljubljana, SI-1000 Ljubljana, Slovenia.
* Corresponding author: E-mail: marija.bester@fkkt.uni-lj.si
Tel: +386 1 2419 410; Fax: +386 1 2419 425

Abstract
Electric conductivities of diluted calcium and manganese sulfate solutions aqueous solutions were measured from 5 to 35 °C (steps of 5 °C) in the concentration range 2 × 10^–4 < c/mol dm^–3 < 5 × 10^–3. Evaluation of the limiting molar conductivity Λ^∞ and the association constant K_A is based on the chemical model of electrolyte solutions, including shortrange forces. From the temperature dependence of the limiting molar conductivities Eyring’s enthalpy of activation of charge transport was estimated. The standard Gibbs energy, enthalpy and entropy of the ion-pairing process were calculated from the temperature dependence of the ion-association constants. The comparison with some divalent metal sulfates aqueous solutions was made.

Keywords: Divalent metal sulfates, calcium sulfate, manganese sulfate, aqueous solutions, ion association, electric conductivity