Abstract

The electromotive force of the cell containing two ion-selective electrodes (ISE),Na-ISE|NaCl(m), formamide (Y), H 2O(100 - Y)|Cl-ISE. has been measured at a temperature of 298.15 K as a function of the weight percentage Y of formamide in a mixed solvent. Y was varied between 0 and 100% in ten-unit steps and the molality of the electrolyte (m) was between ca. 0.01 and saturation. The values of the standard electromotive force, E 0 (molal scale), were determined using routine methods of extrapolation together with Debye-Hückel, Pitzer and Scatchard equations. The results obtained produced good internal consistency, within the normal limits of experimental error encountered in these types of measurement. Once E 0 was determined, the mean ionic activity coefficients for NaCl, the Gibbs energy of transfer from the water to the formamide-water mixture, the standard solubility product and the primary NaCl hydration number were calculated. The variation of these magnitudes with the composition of this mixture with ?-increasing co-solvent is discussed in comparison with those previously obtained with ?-decreasing methanol-water and ethanol-water systems in terms of the ion-solvent and ion-ion interactions and their changes with the properties of the medium. © 2008 Elsevier B.V. All rights reserved.