Activity coefficients for NaF in methanol-water and ethanol-water mixtures at 25°C

Abstract

The electromotive force, E, of the cell: Na-ISE | NaF (m), H 2O (100−Y), alcohol (Y) | F−ISE where (ISE) stands for ion-selective electrode and Y for the weight percent of alcohol (either methanol or ethanol), was measured at 298.15 K. Y was ranged between 0 and 100% for methanol-water system and between 0 and 80% for the ethanol-water one, in both cases by steps of 10 units. The electrolyte molatity ( m) was ranged from 0.01 to near saturation. The values for the standard electromotive force, E 0 (molal scale), were determined from both the Debye-Hückel extended equation and the Pitzer equation by using extrapolation iterative methods. Electromotive force data were also analysed by using a chemical model to take into account possible ion pair formation (Na +F −) 0 and E 0∗ and K A (equilibrium constant of the ion pairs) were obtained. In all cases, the values found for E 0 agree with each other within the estimated experimental error for this type of measurements and, consequently, average values were obtained and afterwards used to calculate the stoichiometric mean ionic activity coefficients at each alcohol-water mixture. In water, these last values thus encountered differ from those reported in the literature by less than ±0.005 units. Values for the standard free energy of transference, ΔG t 0 and solubility product of NaF, K sp, s 0, were calculated, comparatively discussed in terms of both ion-solvent and ion-ion interactions and the effect of the change in certain physical properties of the solvent and also compared with the sodium halide series. Finally, hydration number of NaF, n hydr , was estimated for the different alcohol-water mixtures studied and compared with other literature data reported for other sodium halides.