Activity Coefficients of the System {yNaCl + (1 – y)Na₂HPO₄}(aq) at T = 298.15 K Determined by Electromotive Force Measurements

Abstract

The mean ionic activity coefficients of NaCl in a ternary system {yNaCl + (1 – y)Na₂HPO₄}(aq) were determined from measurements of the electromotive force (EMF) where the NaCl ionic strength fraction was y = (0.3012; 0.4015; 0.5011; 0.5988; 0.7706; 0.8997) in the range of total ionic strength of the solution I_m = 0.0701–1.0161 mol kg^–1 at temperature T = 298.15 K. A cell of the Na–ISE|${\text{NaCl}}({{m}_{{{\text{NaCl}}}}})$, ${\text{N}}{{{\text{a}}}_{2}}{\text{HP}}{{{\text{O}}}_{4}}({{m}_{{{\text{N}}{{{\text{a}}}_{2}}{\text{HP}}{{{\text{O}}}_{4}}}}})~$|AgCl|Ag, type was employed for the EMF measurements. The experimental results from this work and the model of Pitzer’s were used to determine the mixing parameters. Good agreement was found between the experimental and calculated values of the mean ionic activity coefficients of NaCl with a standard deviation of fit 1.5 × 10^–3. The values of the osmotic coefficients of the system {yNaCl + (1 – y)Na₂HPO₄}(aq) were estimated on the basis of the determined parameters and compared with literature data. The differences in the osmotic coefficients between the estimated and experimental values were negligible.