Activity coefficients of NaCl in aqueous mixtures with ϵ -increasing co-solvent: N-methylformamide–water mixtures at 298.15 K

The electromotive force of the cell containing two ion-selective electrodes (ISE), Na–ISE|NaCl(m), N-methylformamide (Y), H2O(100-Y) Cl–ISE has been measured at 298.15K as a function of the weight percentage Y of N-methylformamide in the mixed solvent. Y was varied between 0 and 100% in ten-unit steps and the molality of the electrolyte (m) was between ca. 0.04 and saturation. The values of the apparent standard electromotive force, E0* (molal scale), were determined using routine methods of extrapolation together with extended Debye–Hückel (DH), Scatchard (S), Pitzer (P), and modified three-characteristic-parameter-correlation (TCPC) models. 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 standard Gibbs energy of transfer from the water to the N-methylformamide–water mixture, the standard solubility product and the primary NaCl hydration number were calculated. The variation of these magnitudes with the composition of this aqueous mixture with ɛ-increasing co-solvent is discussed in comparison with those containing formamide (ɛ-increasing co-solvent) and N,N-dimethylformamide (ɛ-decreasing co-solvent) in terms of the ion-solvent and ion-ion interactions and their changes with the properties of the medium.