Sulfurous acid

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Sulfurous acid
Image:Sulfurous acid.jpg
General
Systematic name	Sulfurous acid
Molecular formula	H₂SO₃
Molar mass	82.07 g/mol
Properties
Acid dissociation constant pK_a	1.81 (18°C) 1 6.91 (18°C) 2
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) Infobox disclaimer and references

Sulfurous acid (or sulphurous acid in British spelling) is a name given to aqueous solutions of sulfur dioxide. There is no evidence that the sulfurous acid molecule, H₂SO₃, exists in these solutions. They cannot be isolated as a pure substance, because boiling the sulphurous acid will drive away sulphur dioxide, leaving water. They react with alkalis to form bisulfite (or hydrogensulfite) and sulfite salts.

Raman spectra of solutions of sulfur dioxide in water show only signals due to the SO₂ molecule and the bisulfite ion, HSO₃⁻. The intensities of the signals are consistent with the following equilibrium:

SO₂ + H₂O ⇌ HSO₃⁻ + H⁺

K_a = 1.54x10⁻² L/mol; pK_a = 1.81.

The hydrogen atom in the bisulfite ion is bonded to the sulfur atom and not to an oxygen atom as is more usual in the case of oxoanions. This has been shown in the solid state by X-ray crystallography and in aqueous solution by Raman spectroscopy (ν(S–H) = 2500 cm⁻¹). It is, however, acidic according to the following equilibrium:

HSO₃⁻ ⇌ SO₃²⁻ + H⁺

K_a = 1.02x10⁻⁷ L/mol; pK_a = 6.97.

Solutions of sulfur dioxide ("sulfurous acid") and of bisulfite and sulfite salts are used as reducing agents and as disinfectants. They are also mild bleaches, and are used for materials which may be damaged by chlorine-containing bleaches.