Isopiestic determination of the osmotic and activity coefficients of {yK2HPO4 + (1 − y)KH2PO4}(aq) at T = 298.15 K

•Isopiestic measurements were made for K2HPO4(aq), KH2PO4(aq) and their mixtures at T = 298.15 K.•Derived osmotic coefficients are represented by an extended form of Pitzer’s model and Clegg, Pitzer and Brimblecombe model.•Mean activity coefficients of both solutes were calculated for both their binary solutions and the ternary mixtures.•The thermodynamic solubility product and standard Gibbs energy of solution of K2HPO4·3H2O(cr) are evaluated at T = 298.15 K.•The CODATA-compatible value of the standard Gibbs energy of formation of K2HPO4·3H2O(cr) was calculated. Isopiestic measurements have been made at 55 compositions of the {yK2HPO4 + (1 − y)KH2PO4}(aq) system at T = (298.15 ± 0.01) K, 11 for each of the limiting binary solutions and 33 for mixture compositions at K2HPO4 stoichiometric ionic strength fractions y = (0.23330, 0.47671, and 0.73177), using KCl(aq) as the reference standard. Model parameters for the binary subsystems were evaluated at this temperature for an extended form of Pitzer’s ion-interaction model and also for the Clegg, Pitzer and Brimblecombe model based on the mole-fraction-composition scale, using the present isopiestic results along with critically-assessed osmotic coefficients for both of these aqueous electrolytes as extracted from the published literature. The thermodynamic models for KH2PO4(aq) extend to slightly above the saturated solution molality at T = (298.15 ± 0.01) K, whereas those for K2HPO4(aq) extend to m = 9.7429 mol·kg−1, which is the molality of the saturated solution, also at T = (298.15 ± 0.01) K. These results yield the CODATA-compatible standard Gibbs energy of formation ΔfGmo(K2HPO4·3H2O,cr,298.15K)=-2367.70±1.60kJ·mol-1. The 33 osmotic coefficients for the ternary mixtures were likewise represented with these models, using both the usual Pitzer mixing terms and also Scatchard’s neutral-electrolyte model mixing terms for the extended ion-interaction model. Two mixing parameters are needed for each of the three models for {yK2HPO4 + (1 − y)KH2PO4}(aq), and both of these ion-interaction models give similar high-quality representations of the experimental results. However, the Clegg, Pitzer and Brimblecombe model had more difficulty in representing the osmotic coefficients of K2HPO4(aq), especially below 3 mol·kg−1, and consequently the corresponding mixture model with two mixing parameters is slightly less accurate for representing the osmotic coefficients. The maximum difference in calculated values of the