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

Abstract

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.