Activity Coefficients of NaBr in Aqueous Mixtures with High Relative Permittivity Cosolvent: Formamide + Water at 298.15 K

Abstract

The mean ionic activity coefficients of NaBr were experimentally determined in formamide + water mixtures at 298.15 K from potential difference measurements of the following electrochemical cell containing two ion selective electrodes (ISEs): Na-ISE|NaBr (m), formamide (w), H2O (1–w)|Br-ISE. The molality (m) varied between 0.02 mol·kg–1 and almost saturation, whereas the mass fraction of formamide in the mixture (w) was varied between 0 and 1 in 0.1-unit steps. The determination of E0* (molal scale) was carried out following a method similar to that proposed by Hitchcock and using the classical extended Debye–Hückel and Scatchard equations. We also use for this purpose a modification of the Pitzer equation proposed by Rard and Archer and the most recent modified three-characteristic-parameter-correlation model. The results obtained produced good internal consistency, within the normal limits of experimental error in these types of measurement. Once E0* was determined, the mean ionic activity coefficients of NaBr (γ), the Gibbs energy of transfer of the NaBr from the water to the formamide + water mixture (ΔGt0), standard solubility product of NaBr in formamide + water (Ksp0), and NaBr primary hydration number (nhyd) were estimated. The results were comparatively analyzed with those of NaF and NaCl previously obtained in similar mixtures.