Thermodynamic dissociation constant of the bisulfate ion from Raman and ion interaction modeling studies of aqueous sulfuric acid at low temperatures

The dissociation reaction of the bisulfate ion, HSO₄^- ⇌ SO₄^2- + H⁺, is investigated in aqueous H₂SO₄ solutions with concentrations of 0.54-15.23 mol kg^-1 in the temperature range of 180-326 K using Raman spectroscopy. All investigated H₂SO₄ solutions show a continuous increase in the degree of dissociation of HSO₄^- with decreasing temperature, in contrast to predictions from thermodynamic models of aqueous H₂SO₄ solutions. A Pitzer ion interaction model is used to derive a thermodynamically consistent formulation of the thermodynamic dissociation constant of the bisulfate ion, K_II(T), that is in agreement with the experimental data. The new formulation of K_II(T) is valid from 180 to 473 K. All ion interaction parameters and the corresponding parametrizations of the Pitzer ion interaction model are presented. Calculations with this model reveal significant differences in ion activity coefficients, water activities, water vapor pressure, and HCl solubilities, when compared to existing thermodynamic models of H₂SO₄/H₂O solutions, in particular at lower temperatures.