S IV Aqueous Equilibria

Sulfur dioxide gas dissolves in water to set up equilibria similar to those of C02  

As a result, dissolved S02 really includes three chemical species hydrated S02 (S02 H20), the bisulfite ion (HSOj), and the sulfite ion (SO2-). While HSOJ is commonly referred to as bisulfite, it should be noted that it can exist in two possible structures HOS02- or 

Recent calculations indicate that the sulfonate form, HSOj, is the most stable form (Brown and Barber, 1995). However, Hoffmann (1986) argues that the chemical reactivity of bisulfite in aqueous solutions is consistent with HO-SOJ, being the major reactive automer, which is in rapid equilibrium with HSOj. While the formation of an S02 H zO complex similar to that shown in (11, —11) can, in principle, occur in the gas phase as well, the binding energy is low (—1.8—3.5 kcal mol-1 e.g., see Li and McKee, 1997 and Bishenden and Donaldson, 1998) and hence this complex is not important in the atmosphere. 

Because of the different forms in which dissolved S02 exists in solution, the oxidation state (i.e., +4) is often used to denote all these forms of S02 taken together, that is, 

The oxidized form of sulfur (i.e., sulfuric acid and sulfate) is in the +6 oxidation state and hence is commonly referred to as S(VI). 

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