Thermodynamics of electrolytes in mixed solvent media — Application of the Pitzer ion interaction approach

Although for a long time considerable attention has been paid to the application of the Pitzer ion interaction approach to describe the thermodynamic properties of a variety of aqueous electrolyte solutions, such studies in mixed solvent media are still scarce. The present paper discusses the application of the Pitzer ion interaction approach in the case of electrolyte solutions in mixed solvent media. In particular, a comprehensive equation for the thermodynamic properties of hydrochloric acid in dioxane–water mixtures within the temperature range 273.15 to 323.15 K has been generated in the ion interaction (Pitzer) equation form on the basis of electrochemical cell measurements of the hydrochloric acid available in the literature. It was necessary to set varying values of the exponential coefficients α1 and α2 of the standard Pitzer model in different mixed solvent media to achieve a satisfactory fit. The present model quantitatively reproduces the experimental cell electromotive forces over the entire concentration and temperature range used for the experiments. The Pitzer ion interaction approach is, thus, proved to be a very valuable method for the correlation and prediction of thermodynamic properties of electrolytes in mixed solvent media. The implications of the Pitzer ion interaction parameters (β(0), β(1) β(2), and Cϕ) have also been discussed in terms of interionic forces. The present study also stresses the need for additional emf measurements to help interpolation through the dioxane composition. Key words: Pitzer's equations, Pitzer ion interaction parameter, activity coefficients, emf, hydrochloric acid.