The ionization of sulfurous acid in Na-Mg-Cl solutions at 25°C

The stoichiometric pK₁^* and pK₂^* for the ionization of sulfurous acid has been determined from emf measurements in NaCl solutions with varying concentrations of added MgCl₂ (m=0.1, 0.2 and 0.3) from I=0.5 to 6.0 molal at 25°C. These experimental results have been treated using both the ion pairing and Pitzer's specific ion-interaction models. The Pitzer parameters for the interaction of Mg²⁺ with SO₂ and HSO₃^- yielded λ=0.085±0.004, β⁽⁰⁾ = 0.35±0.02, β⁽¹⁾ = 1.2±0.04, and C^φ = -0.072±0.007. The Pitzer parameters β⁽⁰) = -2.8±0.4, β⁽¹⁾ = 12.9±2.9 and β⁽²⁾ = -2071±57 have been determined for the interactions of Mg²⁺ with SO₃^2-. The calculated values of pK₁^* and pK₂^* using Pitzer's equations reproduce the measured values to within ±0.04 pK units. The ion pairing model with log K_MgSO3=2.36±0.02 and logγ_MgSO3 = 0.1021, reproduces the experimental values of pK₂^* to ±0.01. These results demonstrate that treating the data by considering the formation of MgSO₃ yields a better fit of the experimental measurements with fewer adjustable parameters. With these derived coefficients obtained from the Pitzer equations and the ion pairing model, it is possible to make reliable estimates of the activity coefficients of HSO₃^- and SO₃^2- in seawater, brines and marine aerosols containing Mg²⁺ ions.