Isopiestic Determination of Osmotic and Activity Coefficients of the {yNaH2PO4 + (1 – y)Na2HPO4}(aq) System at T = 298.15 K

Isopiestic measurements have been made for aqueous solutions of the common sodium cation mixtures of NaH 2 PO 4 and Na 2 HPO 4 at T = 298.15 ± 0.01 K, at NaH 2 PO 4 ionic strength fractions y = (0, 0.24851, 0.49862, 0.74544, and 1), where the ionic strength fractions were calculated by assuming complete electrolytic dissociation of NaH 2 PO 4 as 1:1 and Na 2 HPO 4 as 2:1 electrolytes; CaCl 2 (aq) was used as the reference standard solution. Model parameters for an extended form of Pitzer’s ion-interaction model and also for the Clegg–Pitzer–Brimblecombe equations based on the mole-fraction-composition scale were evaluated at T = 298.15 K for both NaH 2 PO 4 (aq) and Na 2 HPO 4 (aq) using the isopiestic results from this study (17 values each) together with numerous critically assessed osmotic coefficients for both electrolytes taken from the published literature. The thermodynamic models for NaH 2 PO 4 (aq) extend to m = 7.5 mol·kg –1, whereas those for Na 2 HPO 4 (aq) extend to m = 2.6050 mol·kg –1, which is well above the solubility limit for the thermodynamically stable phase Na 2 HPO 4 ·12H 2 O(cr). The 51 osmotic coefficients for the ternary mixtures were treated with these two models together with Scatchard’s neutral-electrolyte model; one previous set of osmotic coefficient values for { y NaH 2 PO 4 + (1 – y )Na 2 HPO 4 }(aq) mixtures was found in the literature [ Scharge, T.; J. Chem. Thermodyn. 2015, 80, 172−183], and hence an analysis and comparison were made of our results with theirs.