The molal equilibrium quotients for the protolytic dissociation of bisulfate ion (HS04-) were measured potentiometrically in aqueous sodium chloride media by using a hydrogen-electrode concentration cell with liquid junction. The ionic strength was varied from 0.1 to 5.0 mol kg-l at temperatures ranging from 50 to 250 O C . Equilibrium quotients obtained from the measured potentials were combined with published values of the corresponding equilibrium constants, enthalpy, and heat capacity changes at infinite dilution and ambient temperatures, and fitted to an empirical equation containing the Pitzer form of the extended Debye-Hiickel term and nine adjustable parameters. IntroductionThe second dissociation constant of sulfuric acid in aqueous solutions near room temperature has been the subject of intense investigation over the past 60 years because of the importance of sulfur chemistry in industry, oceanography, and geochemisHowever, measurements at elevated temperatures are sparse and considerable discrepancies exist among the available literatureSeveral a~t h o r s '~. '~,~~* *~ have discussed the difficulties in computing precise and accurate values of the dis-

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Molecular dynamics simulations of supercritical electrolyte solutions with three different ion–water models are performed to study the anion–cation potential of mean force of an infinitely dilute aqueous NaCl solution in the vicinity of the solvent’s critical point. The association constant for the ion pair Na+/Cl− and the constant of equilibrium between the solvent-separated and the contact ion pairs are determined for three models at the solvent critical density and 5% above its critical temperature. The realism of the aqueous electrolyte models is assessed by comparing the association constants obtained by simulation with those based on high temperature conductance measurements. Some remarks are given concerning the calculation of the mean-force potential from simulation and the impact of the assumptions involved.