Tests of equations for the electrical conductance of electrolyte mixtures:Measurements of association of NaCl (Aq) and Na2SO4 (Aq) at high temperatures

A review of requirements for equations to calculate the conductivity of a m ixture of ions in low dielectric constant solvents (i.e., water at high tem peratures) shows that there are conceptual difficulties with all current eq uations. To explore whether these difficulties limit our ability to predict mixtures, four models for the activity coefficients, two models for the co nductivity of a single strong electrolyte, and an equation for the change i n 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 equat ions were tested on a single 1-1 electrolyte, NaCl (aq) at 652.6 K and 22.7 5 MPa measured by Gruskiewicz and Wood (J. Phys. Chem. B 1997, 101, 61549-6 559) and new measurements at 623.9 K and 19.79 MPa. Then it was tested with new measurements on Na2SO4 tag) from 300 to 574 K because, in water at hig h temperatures, this salt produces a solution containing six different ions (Na+, SO42-, NaSO4-, HSO4-, H+, OH-). The equations were able to reproduce the experimental data. Values of equilibrium constants, K, for the dissoci ation of NaCl and NaSO4- and equivalent conductances A, 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 A, were not very sensitive to changes in (Ij the single electrolyte conductance equation, (2) assumed va lues of A, for minor species, or (3) equilibrium constants for minor reacti ons. Uncertainty in the activity coefficient model was the largest contribu tor to uncertainty in K and A,. This method should allow rapid and accurate measurements of the equilibrium constant for any reaction, which changes t he number of ions in solution. The equilibrium constants for many reactions of this type are unknown in water at high temperatures.