Activity coefficients of NaCl in PEG 4000 + water mixtures at 288.15, 298.15 and 308.15 K

The electromotive force of the cell containing two ion-selective electrodes (ISE),Na-ISE|NaCl (m), PEG   4000 (Y), H2O (100−Y)|Cl-ISENa-ISE|NaCl (m), PEG   4000 (Y), H2O (100−Y)|Cl-ISEhas been measured at temperatures of 288.15, 298.15, and 308.15 K as a function of the weight percentage Y of PEG 4000 in a mixed solvent. Y was varied between 0 and 25% in five-unit steps and the molality of the electrolyte (m) was between ca. 0.003 and 5 mol kg−1. The values of the standard electromotive force were determined using routine methods of extrapolation, together with Debye–Hückel and Pitzer equations. The results obtained produced good internal consistency for all the temperatures studied. Once the standard electromotive force was determined, the mean ionic activity coefficients for NaCl, the Gibbs energy of transfer from the water to the PEG 4000–water mixture and the primary NaCl hydration number were calculated.