www.elsevier.nl/locate/jelechem Journal of Electroanalytical Chemistry 480 (2000) 9 – 17 Estimation of activity coefficients at different temperatures by using the mean spherical approximation G. Lo ´ pez-Pe ´rez *, D. Gonza ´lez-Arjona, M. Molero Department of Physical Chemistry, Uniersity of Seille, E-41071, Seille, Spain Received 23 July 1999; received in revised form 26 October 1999; accepted 29 October 1999 Abstract A method of estimating activity coefficients for a variety of electrolytes at different temperatures is presented. The MSA approximation is used to calculate the activity coefficients from experimental data available in the literature. This strategy provides suitable results within a wide range of temperatures, electrolyte stoichiometries and concentrations of investigated solutions. © 2000 Elsevier Science S.A. All rights reserved. Keywords: Activity coefficients; Temperature influence; MSA theory; Aqueous electrolytes 1. Introduction Electrolyte solutions exhibit considerable deviations from ideal behaviour. This fact is specially remarkable at high solute concentrations and/or temperatures. It is usual to find many practical applications under these conditions. Thus, the knowledge of the thermodynamic properties of solutions is indispensable for practical and theoretical purposes. Pitzer’s theory has been widely used for the interpo- lation of activity and osmotic coefficients because of its high accuracy, but the functional relationships to con- sider the temperature or mixing effects are complex [1,2]. In fact, the activity coefficients for a single elec- trolyte at a fixed temperature undergo the influence of different kinds of interactions, i.e. electrostatic, short range and even triple ion interactions. To consider the influence of the temperature, Pitzer et al. [1] used a set of equations with a maximum of 19 adjustable parame- ters in order to reproduce the experimental data at different temperatures. On the other hand, the mean spherical approxima- tion (MSA) theory [3–5] is showing up as an excellent tool for the description of electrolyte solutions in the primitive model [6–8]. Unfortunately, the primitive model of electrolytes gives a reasonable representation of real solutions only up to approximately 1 or 2 mol l -1 . Nevertheless, the MSA theory has been also applied to solve an extension of the primitive model considering the change of the ionic sizes and/or solution permittiv- ity with the electrolyte concentration at fixed tempera- ture [9–20]. It is remarkable that only Simonin et al. [13] recognised, in 1996, the existence of extra terms in the formulation of the MSA theory when the ionic sizes and/or solution permittivity show concentration depen- dence. Some important characteristics of the MSA make it a very attractive theory to describe the elec- trolyte solution behaviour. It has an analytical solution and uses a low number of parameters (usually one per anion/cation couple) and, in addition, these parameters have a microscopic meaning. The temperature effect on the electrolyte thermody- namics has rarely been analysed using MSA [21,22]. Moreover, in the applied model the solution permittiv- ity is fixed to the pure solvent value. Unfortunately, the extra contribution to the activity and osmotic coeffi- cients from the concentration dependence of the ionic sizes, pointed out by Simonin et al. [13], was not considered. A new method to estimate activity coefficients of single electrolytes at different temperatures using the MSA theory is presented. In this treatment, all the solutions were considered as strong electrolytes, and * Corresponding author. Fax: +34-95-455-7174. 0022-0728/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved. PII:S0022-0728(99)00438-6