Activity coefficients of NaCl in aqueous mixtures with ɛ-increasing co-solvent: Formamide–water mixtures at 298.15 K

The electromotive force of the cell containing two ion-selective electrodes (ISE),Na-ISE|NaCl(m), formamide (Y), H2O(100 − Y)|Cl-ISEhas been measured at a temperature of 298.15 K as a function of the weight percentage Y of formamide in a mixed solvent. Y was varied between 0 and 100% in ten-unit steps and the molality of the electrolyte (m) was between ca. 0.01 and saturation. The values of the standard electromotive force, E0 (molal scale), were determined using routine methods of extrapolation together with Debye-Hückel, Pitzer and Scatchard equations. The results obtained produced good internal consistency, within the normal limits of experimental error encountered in these types of measurement. Once E0 was determined, the mean ionic activity coefficients for NaCl, the Gibbs energy of transfer from the water to the formamide–water mixture, the standard solubility product and the primary NaCl hydration number were calculated. The variation of these magnitudes with the composition of this mixture with ɛ-increasing co-solvent is discussed in comparison with those previously obtained with ɛ-decreasing methanol–water and ethanol–water systems in terms of the ion–solvent and ion–ion interactions and their changes with the properties of the medium.