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Isothermal ternary mutual diffusion coefficients (interdiffusion coefficients) have been measured for the system { z,NaCl + (1 -z,)Na,SO,}(aq) at a constant total molarity of 0.5000 mol dm-, and 298.15 K using either Rayleigh or Gouy interferometry. Measurements were performed at NaCl molarity fractions of z1 = 1, 0.90, 0.75, 0.50, 0.25 and 0. Densities of all solutions used in the diffusion experiments were measured using pycnometers and/or a vibrating densimeter. Trace diffusion coefficients have been evaluated from these results for the Cl-(aq) ion in 0.5 mol dm-, Na,SO,(aq) and for the S0,2-(aq) ion in 0.5 mol dmP3 NaCl(aq). The resulting values are D*(Cl-) = (1.681 f 0.002) x m2 s -l and D*(S042-) = (0.900 f 0.006) x lo-' m2 s-', respectively. At all compositions, (D2,)", the cross-term diffusion coefficient of Na2S0, due to a concentration gradient of NaCl, was found to be negative, whereas (DI2)" , the cross-term diffusion coefficient of NaCl due to a concentration gradient of Na2S0, , was found to be positive.

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Isopiestic vapor-pressure measurements were performed at 298.15 K for CaCl 2 (aq) solutions at 66 molalities from (4.3235 to 10.253) mol‚kg -1 , using H 2 SO 4 (aq) as the reference standard, with emphasis given to the accurate characterization of the osmotic coefficients near and above saturation. Published isopiestic molalities, direct vapor pressures, and emf results for CaCl 2 (aq) have been critically reevaluated and recalculated in an internally consistent manner. This critically-assessed database was used to evaluate the parameters of Pitzer's equations and various extended forms at 298.15 K and 0.1 MPa. Neither the standard equations nor empirically extended versions were able to represent the thermodynamic results over more than part of the molality range without large cyclic systematic deviations. It was possible, however, to obtain essentially quantitative agreement between experiment and model over the full molality range if (1) the presence of CaCl + (aq) ion pairs was included explicitly and (2) higherorder virial terms were included. One such quantitative model is presented here in detail. Without these higher-order virial terms the same model is able to represent the φ(CaCl 2 ) fairly well only to about 8 mol‚kg -1 . The osmotic coefficient of CaCl 2 (aq) goes through a minimum around 0.11 mol‚kg -1 , which is followed by a regular increase with molality to a broad maximum in the supersaturated molality region where φ(CaCl 2 ) is nearly constant at 3.169-3.173 from about (8.5 to 9.5) mol‚kg -1 . It then decreases slightly by about 0.3% at higher molalities.