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

The electromotive force E, of the cell containing the ion-selective electrodes Na-ISE | NaF(m), formamide (Y), H2O(100 − Y) | F-ISE has been measured at 298.15 K as a percentage of the weight of the amide in the mixed solvent. The percentage (Y) was varied from 0 to 90% in 10-unit steps, and the molality of the electrolyte (m) from approximately 0.005 to saturation. The corresponding standard electromotive forces, E0 (molal scale), were determined using traditional methods of extrapolation (Debye–Hückel and Pitzer equations). The results obtained by application of both equations show experimental errors of the same sort as those for this type of measurements. Once the E0 was obtained, we determined the mean ionic activity coefficients for NaF, the transfer free energy from water to the water–formamide mixture, and the NaF primary hydration number. The variation of these parameters, with the composition of this ɛ-increasing solvent, is discussed in comparison with that previously obtained for the ɛ-decreasing methanol–water and ethanol–water systems, in terms of both ion–solvent and ion–ion interactions and of the effect of the medium dielectric constant change.