Activity Coefficients of the System {yNaCl + (1 − y)NaH2PO4}(aq) AT T = 298.15 K Determined by Electromotive Force Measurements

The mean ionic activity coefficients of NaCl in the system {yNaCl + (1 − y) NaH₂PO₄}(aq) were determined by electromotive force measurements (EMF) in two series in which the NaCl ionic strength fraction was as follows: I series, y = (0.2368; 0.3101; 0.4101; 0.5051; 0.6090; 0.7775; 0.9039) and II series, y = (0.1998; 0.4005; 0.5993; 0.8105) in the range of total ionic strength of the solution I_m = (0.0887–1.0081) mol·kg⁻¹ at a temperature T = 298.15 K. A cell of the Na–ISE∣\({\text{NaCl}(m}_{\text{NaCl}})\), \({\text{Na}}{\text{H}_{2}\text{PO}}_{4}{(m}_{{\text{Na}}{\text{H}_{2}\text{PO}}_{4}})\)∣Ag∣AgCl type was utilized for the EMF measurements. The standard electrode potential of the electrode pair was estimated as E⁰ = 23.2288 mV. The values of the mean ionic activity coefficient of NaCl in the mixed electrolyte solution, \({\gamma }_{\pm \text{NaCl}}\), were determined using the Nerst equation. The experimental results from this study were treated with the models proposed by Pitzer, Clegg and Scatchard to estimate the mixture parameters. A high degree of agreement was found between the experimental and calculated values of the mean ionic activity coefficients of NaCl with an average standard deviation of fit being \(\text{s}.\text{d}.\left({\gamma }_{\pm }\right)\sim\) 2.5·10^–3 for each of the three models. The values of the osmotic coefficients of the system {yNaCl + (1 − y)NaH₂PO₄}(aq) were estimated based on the determined model parameters and compared with literature data. Negligible differences were found between the estimated and experimental values of the osmotic coefficients.