Isopiestic Determination of the Osmotic and Activity Coefficients of the {yNa2HPO4 + (1 − y)K2HPO4}(aq) System at T = 298.15 K

Isopiestic measurements have been made for aqueous mixtures of Na2HPO4 and K2HPO4 at T = (298.15 ± 0.01) K, at Na2HPO4 ionic strength fractions y = (0.2023, 0.4060, 0.6027, 0.8007, and 1), using KCl(aq) as the reference standard solution. The resulting 48 molality-based osmotic coefficients for the ternary mixtures were modeled with an extended form of Pitzer’s ion-interaction model, with the usual Pitzer mixing terms and also with Scatchard’s neutral-electrolyte model mixing terms, along with the Clegg–Pitzer–Brimblecombe equations based on the mole-fraction-composition scale. There are no previously published isopiestic measurements for this ternary system. Model parameters for Na2HPO4(aq) at T = 298.15 K were also evaluated using the present isopiestic results (12 values) along with three other sets of osmotic coefficients from isopiestic measurements taken from the published literature, and all four sets of osmotic coefficients are in good agreement; model parameters for K2HPO4(aq) are already available from our earlier work: (Popović et al. in J Solution Chem 40:907–920, 2011). For this ternary system, only one mixing parameter is needed for each of the three models because the ionic strength dependences of the osmotic coefficients of Na2HPO4(aq) and K2HPO4(aq) are very similar, as expected from their very similar chemistry, with the two variants of the extended Pitzer model giving slightly better representations of the experimental results than the Clegg–Pitzer–Brimblecombe model. Maximum differences in calculated values of mean molality-based activity coefficients for these three models are ∆γ±(Na2HPO4) ≤0.004 and ∆γ±(K2HPO4) ≤0.008 for ionic strengths Im ≤ 2.0 mol·kg−1, with the largest differences occurring for the trace and near-trace activity coefficients of one solute in a solution of the other solute at higher ionic strengths and solutions rich in K2HPO4 at intermediate ionic strengths. These new results complement our earlier investigations of aqueous common-cation mixtures of K2HPO4 with KCl, KBr, KNO3 and K2SO4.