Covalent Reactions in the Toxicity of SO2 and Sulfite

Sulfur dioxide (SO₂) is a major urban air pollutant, resulting from combustion of sulfur containing fossil fuels. It readily dissolves in water forming sulfurous acid, which dissociates to form bisulfite and sulfite ions (collectively referred to as sulfite), in a ratio depending on the pH of the solution (Petering and Shih, 1975): $$S{{O}_{2}}+{{H}_{2}}O\rightleftharpoons {{H}^{+}}+HS{{O}_{3}}^{-}\rightleftharpoons 2{{H}^{+}}+S{{O}_{3}}{{^{2}}^{-}}$$ Sulfur dioxide and sulfite are also commonly used as antimicrobial and antioxidant agents in the preservation of foods and beverages (Chichester and Tanner, 1972).