Tautomeric equilibria and pKa values for 'sulfurous acid' in aqueous solution: a thermodynamic analysis

The free energy of formation of dimethyl sulfite in aqueous solution can be calculated as −91.45 ± 0.79 kcal/mol; this calculation required measurement of the solubility of dimethyl sulfite. From this value and the pK a of SO(OH) 2 , using previously reported methods, the free energy of formation of SO(OH) 2 can be calculated to be −129.26 ± 0.89 kcal/mol. Comparison of this value with the value obtained from the free energy of formation of 'sulfurous acid' solutions, calculated from the free energy of formation of sulfite ion and the apparent pK a , values, permits evaluation of the free energy of covalent hydration of SO 2 as 1.6 + 1.0 kcal/mol, in agreement with earlier qualitative spectroscopic observations. From the apparent pK a and the anticipated pK a values for the tautomers (SO(OH) 2 , pK 1  = 2.3; HSO 2 (OH), pK 1  = −2.6) it is possible to calculate the free energy change for tautomerization of SO(OH) 2 to H—SO 2 (OH) as +4.5 ± 1.2 kcal/mol. All equilibrium constants required for Scheme 1, describing the species present in dilute aqueous solutions of SO 2 , have been calculated. In agreement with previous Raman studies the major tautomer of 'bisulfite ion' is calculated to be H—SO 3 − .