Isopiestic determination of osmotic coefficients in the ionic strength range Im = (0.9670–2.2160) mol⋅kg−1 and activity coefficients determined by electromotive force measurements in the range Im = (0.0897–1.0054) mol⋅kg−1 of the {yKCl + (1 – y) K2HPO4}(aq) system at T = 298.15 K

The mean activity coefficients of KCl in the ternary system {y KCl + (1 – y) K2HPO4}(aq) were experimentally determined at T = (298.15 ± 0.01) K by the electromotive force measurement (EMF method) using the cell of the type, K–ISE∣KCl(mKCl), K2HPO4(mK)∣Ag∣AgCl (ISE = ion-selective electrode), in the range of total ionic strength of the solution Im = (0.10 to 1.00) mol·kg−1 at different ionic strength fractions of KCl being y = (0.1011; 0.1997; 0.3016; 0.4027; 0.5006; 0.6018; 0.7018; 0.8051; 0.8988; 1). In addition, the osmotic coefficients of the system have been measured at the same temperature by the isopiestic vapor pressure method over the range of ionic strengths from (0.96698 to 2.21602) mol⋅kg−1 using KCl(aq) as the reference solution, at ionic strength fractions of KCl being y = (0.2063; 0.3849; 0.6099; 0.8011; 1). Values of the mean ionic activity coefficients of KCl and osmotic coefficients in the system {yKCl+(1–y)K2HPO4}(aq) were treated by an extended form of Pitzer's ion-interaction model, Scatchard's neutral-electrolyte model and Clegg–Pitzer–Brimblecombe's equations based on the mole-fraction-composition scale. All three models gave similar quality representations of both sets of experimental results. There are two sets of previously published isopiestic data that were taken into consideration during treatment. The model parameters were fitted and optimized to all available thermodynamic data from EMF and isopiestic measurements to obtain an accurate and self-consistent description of properties and to form a set of parameters that can be used in a wide range of ionic strength of the system. The standard deviation of the fit for the ionic strength range from Im = (0.0897 to 10.4350) mol·kg−1 with two optimized mixing parameters of Pitzer's model without higher–order electrostatic terms was for the mean ionic activity coefficients of KCl and osmotic coefficients, s.d. (γ±) = 3.5⋅10–3 and s.d. (ϕ) = 4.7⋅10–3 respectively, with two optimized mixing parameters of the Scatchard model standard deviations were s.d. (γ±) = 4.9⋅10–3 and s.d. (ϕ) = 5.0⋅10–3 and with Clegg–Pitzer–Brimblecombe's model higher–order electrostatic terms not included, standard deviations were s.d. (γ±) = 4.2⋅10–3 and s.d. (ϕ) = 4.8⋅10–3