Thermochemical and Ionic Speciation Modeling of the Aqueous Sulfuric Acid System Up to 6 Molal and 0–100 °C

Sulfuric acid (\({\text{H}}_{2}{{\text{SO}}}_{4}\)) is one of the most widely used chemicals, acting as a reagent in several industries and metallurgy. The chemical behavior in aqueous solutions can be as strong or weak if the acid concentration is low or high, respectively. The aim of this work is the estimation of the thermodynamic properties and to predict the speciation, density and ionic conductivity of aqueous sulfuric acid solutions up to 6 molal and 0–100 °C, using the Pitzer model adapted to include the interaction parameters of sulfate complexes as \({\text{HSO}}_{4}^{-}\) and \({\text{H}}_{2}{\text{S}}{\text{O}}_{4}^{0}\). A thermodynamic model that includes a set of aqueous species, components, equilibrium reactions, activity coefficients, and mass balances was defined as a function of temperature. The parameters of the equilibrium constants for \({\text{HSO}}_{4}^{-}\) and \({\text{H}}_{2}{\text{S}}{\text{O}}_{4}^{0}\), the Equation of State (EOS) HKFmoRR for solution density, the Casteel–Amis relationship for ionic conductivity, and the Pitzer model for water activity were combined by coupling of the optimization software PEST with the hydro-geochemical code PHREEQC. The Pitzer model was calibrated and resulting in a standard deviation of water activity adjustment of 0.7%. Sulfuric acid distributes in water forming common anions, cations, and neutral species as \({\text{SO}}_{4}^{{2}-}\), \({\text{HSO}}_{4}^{-}\), \({\text{H}}^{+}\), and \({\text{H}}_{2}{\text{S}}{\text{O}}_{4}^{0}\), where the association of sulfate increase with both electrolyte concentration and temperature. The solution density and ionic conductivity calculations were in good agreement with experimental data, presenting a standard deviation of adjustment of 0.2 and 4.8%, respectively, over the temperature and concentration ranges studied.