Sulfurous acid

Sulfurous acid is the conventional formula H₂SO₃ used for aqueous sulfur dioxide chemistry. In water, sulfur dioxide exists mainly as hydrated SO₂ and its ions rather than as a stable population of discrete H₂SO₃ molecules.

Sulfur dioxide: Sulfur dioxide is a colorless gas that dissolves in water and forms acidic sulfur-containing species. Dissolved SO₂ is the main chemical source conventionally represented as sulfurous acid.
Oxyacid: An oxyacid is an acid containing hydrogen, oxygen, and at least one other element. Sulfurous acid belongs to the oxygen-containing acids of sulfur.
Acid rain: Acid rain is precipitation made unusually acidic by atmospheric pollutants, especially sulfur and nitrogen oxides. Dissolved sulfur dioxide forms acidic species that contribute to sulfur-driven atmospheric acidity.
Thiosulfurous acid: Thiosulfurous acid is a proposed sulfur oxyacid with formula H₂S₂O₂ and limited stability. Its uncommon structure contrasts with sulfurous acid's familiar but non-isolable molecular formula.
Sulfur dioxide hydration: Sulfur dioxide hydration is the reversible addition of water to SO₂ to form hydrated sulfur dioxide. This equilibrium is often simplified as formation of H₂SO₃, though hydration is limited.
Conjugate acid–base pair: A conjugate acid–base pair consists of two species differing by one proton. H₂SO₃, HSO₃⁻, and SO₃²⁻ are conventionally arranged in successive pairs.
Sulfur dioxide in winemaking: Sulfur dioxide in winemaking is used to limit oxidation and inhibit unwanted microbes in wine. Bisulfite and related dissolved sulfur dioxide species provide much of its preservative action.
Sulfurous acid disproportionation: Disproportionation is a reaction in which one chemical species is simultaneously oxidized and reduced. Sulfur(IV) species can disproportionate under suitable conditions into sulfur at different oxidation states.
Bisulfite: Bisulfite is the hydrogen sulfite ion, HSO₃⁻, formed when dissolved sulfur dioxide loses a proton. It is the principal singly deprotonated form in many mildly acidic solutions.
Sulfur oxidation state: An oxidation state is a formal charge assignment used to track electron distribution in compounds. Sulfur has oxidation state +4 in sulfurous acid and its sulfite species.

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