Isopiestic determination of the osmotic and activity coefficients of the {yKNO3 + (1 − y)K2HPO4}(aq) system at T = 298.15 K

► Isopiestic measurements were made for aqueous (KNO3 + KH2PO4) mixtures at T = 298.15 K. ► Results were analyzed with Pitzer's model and a mole-fraction-composition based model. ► Activity and osmotic coefficients are reported at selected ionic strengths. ► Model parameters are also reported for KNO3(aq) from the present measurements and published reports. ► The thermodynamic solubility product of KNO3(cr) was evaluated at T = 298.15 K.