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

The osmotic coefficients of aqueous mixtures of KCl and K2HPO4 have been measured at T = (298.15 ± 0.01) K by the isopiestic vapor pressure method over the range of ionic strengths from (2.3700 to 11.250) mol · kg−1 using CaCl2(aq) as the reference solution. Our new experimental results were modeled with an extended form of Pitzer’s ion-interaction model equations, both with the usual mixing terms and with Scatchard’s neutral–electrolyte mixing terms, and with the Clegg–Pitzer–Brimblecombe equations based on the mole-fraction-composition scale. There is a dearth of previously published isopiestic data for mixtures containing salts of HPO42-(aq) and, consequently, no previous measurements are available for comparison with the present results. The present study yields Cl-–HPO42- mixing parameters for these three models that are needed for modeling the thermodynamic activities of solute components of natural waters and other complex aqueous electrolyte mixtures.

► Isopiestic measurements were made for {yKCl + (1 − y)K2HPO4}(aq) at T = 298.15 K.
► The resulting osmotic coefficients were represented by three thermodynamic models.
► Activity coefficients from Pitzer model with Scatchard mixing terms are recommended.