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

The potential of the cell Na-ISE | NaF (m), N-methylformamide (Y), H₂O (100-Y) | F-ISE, which contains two selective electrodes (bi-ISE cell), has been measured at 298.15 K as a function of the weight percentage, Y, of N-methylformamide in a mixed solvent and the molality of NaF. Y was varied between 0 and 80% in ten-unit steps, and the molality of the electrolyte (m) was between approximately 10⁻⁴ mol kg⁻¹ and the saturation point. The values of the average apparent standard potential, E^0∗ (molal scale), were determined using habitual methods of extrapolation together with the extended Debye-Hückel (DH), Scatchard (S), and Pitzer (P) equations. The obtained results provided good internal consistency within the normal limits of experimental error encountered in these types of measurements. Once E^0∗ was determined, the mean ionic activity coefficients for NaF were calculated and subsequently modeled according to the TCPC (modified three-characteristic-parameter-correlation) method. Additionally, the values of the total standard Gibbs energy of transfer from the water to the N-methylformamide (NMF) – water mixture, their ionic contributions, the standard solubility product, and the NaF primary hydration number were also determined. The variations of these magnitudes with the composition of the aqueous mixture are discussed in terms of ion-solvent and ion-ion interactions as a function of the properties of the medium.