Isopiestic Determination of the Osmotic and Activity Coefficients of the {yNaH2PO4 + (1 − y)KH2PO4}(aq) System at T = 298.15 K

Abstract

Isopiestic measurements have been made for aqueous mixtures of NaH2PO4 and KH2PO4 at T = (298.15 ± 0.01) K, at NaH2PO4 ionic strength fractions y = (0, 0.19108, 0.38306, 0.58192, and 1), assuming that both electrolytes dissociate as 1:1 electrolytes, using KCl(aq) as the reference standard solution. Model parameters for an extended form of Pitzer’s ion-interaction model and also for the Clegg–Pitzer–Brimblecombe equa-tions based on the mole-fraction-composition scale were evaluated at T = 298.15 K for NaH2PO4(aq) using the present isopiestic results (13 values), as were those for KH2PO4(aq) using the present isopiestic results (12 values), together with numerous critically-assessed osmotic coefficients for both electrolytes taken from the published literature. The thermo-dynamic models for KH2PO4(aq) extend to m = 2.187 mol·kg−1, which is slightly above saturation, while those for NaH2PO4(aq) extend to m = 7.5 mol·kg−1, which is below satu-ration. The 39 osmotic coefficients for the ternary mixtures from the present study along with 42 values from a published study were likewise represented with these models, with both the usual Pitzer mixing terms and also Scatchard’s neutral-electrolyte model mix-ing terms being used for the extended ion-interaction model. Two mixing parameters were needed for each of the models, and all three models gave similar quality represen-tations of the experimental results. Maximum differences in calculated values of mean molality-based activity coefficients for these three models are ∆γ±(NaH2PO4) ≤ 0.0080 and ∆γ±(KH2PO4) ≤ 0.0043. The experimental results were also found to nearly conform to Zdanovskii’s rule.