The mean spherical approximation and the prediction of the size of the species involved in an ionization equilibrium in saline media
The unrestricted version of the mean spherical approximation is applied to describe the ionization equilibrium of benzoic acid, as a typical simple system for carboxylic acids, in the presence of an excess of background electrolyte and variable ionic strength. When the activity coefficients are theoretically determined by use of the mean spherical approximation (MSA), the thermodynamic properties of the ionization equilibrium in 1:1 electrolytes for ionic strengths up to 2.5 M can be accurately described, provided different sizes are assumed for each species in solution. The results of this study show that the application of MSA allows one to estimate the size of the species involved in an ionization process from experimental data of stoichiometric equilibrium constants, an alternative normally not explored. However, results suggest that it is necessary to include more sophisticated models of MSA in order to develop this approach with more confidence.