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.