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Data for the partial molar volume at infinite dilution for several gaseous solutes and H3BO3 in water have been correlated using a generalized Krichevskii parameter. The result is a simple function for in terms of the water density and compressibility for conditions from ambient to 725 K and 40 MPa. The accuracy is much better with only two parameters than with a previous five-parameter model.

The electrical conductivities of aqueous solutions of NaCl have been measured at 651 and 670 K at 28 MPa for molalities up to 1.0 mol kg -1 . These conductivities as well as the results of Hwang et al. (High Temp. High Pres. 1970, 2, 651-669), Ritzert and Franck (Ber. Bunsen-Ges. Phys. Chem. 1968, 72, 798-808), andMangold and Franck (Ber. Bunsen-Ges. Phys. Chem. 1969, 73, 21-27) for aqueous KCl have been fit to the conductance equation of Turq et al. (J. Phys.Chem. 1995, 99, 822-827) with a consensus mixing rule and either mean spherical approximation or Debye-Hückel activity coefficients. Except at one state point (NaCl at 670 K and 28 MPa), where the interactions are the strongest ( * ) 17.8), the simplest model that fits the experimental results reasonably well (( 2-3%) at molalities up to 4.5 mol kg -1 is one with only the limiting equivalent conductance and a pair association constant adjusted. Activity coefficients calculated with either the MSA and ionic diameters or with the Debye-Hückel equation of Oelkers and Helgeson (Geochim. Cosmochim. Acta 1990, 54, 727-738) (with no salting out) can be used with similar accuracy. At high concentrations this model predicts strong redissociation of the ion pairs that form at low concentrations. Oelkers and Helgeson (Geochim. Cosmochim. Acta 1993a, 57, 2673-2697) proposed a model with substantial multi-ion association (triplets and quartets, etc.). This model has two more adjustable parameters and does not fit the data without the physically unrealistic salting out coefficient used by Oelkers and Helgeson (Geochim. Cosmochim. Acta 1991, 55, 1235-1251), so this model is not recommended. For the point with the highest *, more complex models are needed at concentrations above 0.05 mol kg -1 . Good fits to the data were found for multi-ion association models (five adjustable parameters) and reasonable fits for more complex activity models (four adjustable parameters) with only pair association, so that the models are about equally accurate at equal complexity. The cluster model of Laria et al. (J. Chem. Soc., Faraday Trans. 1990, 86, 1051-1056 for the restricted primitive model is consistent with the qualitative predictions of our preferred model with only pair association.

A review of requirements for equations to calculate the conductivity of a mixture of ions in low dielectric constant solvents (i.e., water at high temperatures) shows that there are conceptual difficulties with all current equations. To explore whether these difficulties limit our ability to predict mixtures, four models for the activity coefficients, two models for the conductivity of a single strong electrolyte, and an equation for the change in equivalent conductivity on mixing single strong electrolytes were chosen. These equations were then tested on the theoretical equation of Turq et al. (J. Phys. Chem. 1995, 99, 822−827) for three ion mixtures. Next the equations were tested on a single 1−1 electrolyte, NaCl (aq) at 652.6 K and 22.75 MPa measured by Gruskiewicz and Wood (J. Phys. Chem. B 1997, 101, 61549−6559) and new measurements at 623.9 K and 19.79 MPa. Then it was tested with new measurements on Na2SO4 (aq) from 300 to 574 K because, in water at high temperatures, this salt produces a solution containing six different ions (Na+, SO4 2-, NaSO4 -, HSO4 -, H+, OH-). The equations were able to reproduce the experimental data. Values of equilibrium constants, K, for the dissociation of NaCl and NaSO4 - and equivalent conductances Λo derived by a least-squares fit agreed with reported data determined by other methods, showing that conductivity measurements can yield accurate equilibrium constants in complex mixtures of ions. The values of K and Λo were not very sensitive to changes in (1) the single electrolyte conductance equation, (2) assumed values of Λo for minor species, or (3) equilibrium constants for minor reactions. Uncertainty in the activity coefficient model was the largest contributor to uncertainty in K and Λo. This method should allow rapid and accurate measurements of the equilibrium constant for any reaction, which changes the number of ions in solution. The equilibrium constants for many reactions of this type are unknown in water at high temperatures.

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