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

The molality-based osmotic coefficients of aqueous mixtures of KBr and K2HPO4 have been measured at T = (298.15 ± 0.01) K by the isopiestic method at KBr ionic strength fractions y = (0.18328, 0.38241, 0.58031, 0.79186, and 1) and over the ionic strength range (2.5452 to 10.0418) mol · kg−1, using KCl(aq) and CaCl2(aq) as the reference standard solutions. These experiments were performed using glass sample dishes because preliminary experiments showed that some corrosion of our usual gold-plated silver dishes occurred with dissolution of silver, presumably by “pinhole” corrosion at imperfections in the gold layer. These osmotic coefficients were modeled with an extended form of Pitzer's ion-interaction model equations, both with the usual Pitzer 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 are no previously published isopiestic measurements for this ternary system. The present assessment yields mixing parameters for these models. Model parameters for KBr(aq) at T = (298.15 ± 0.01) K were also evaluated using the present results along with four sets of osmotic coefficients taken from the published literature. For the {yKBr + (1 − y)K2HPO4}(aq) system, the model with five Scatchard mixing terms gave a significantly better representation of the osmotic coefficients than any of the other models and is therefore recommended for calculation of osmotic and activity coefficients.