Sulfurous Acid

Sulfurous acid (British English: sulphurous acid) is the chemical compound with the formula H₂SO₃. There is no evidence that sulfurous acid exists in solution, but the molecule has been detected in the gas phase. The conjugate bases of this elusive acid are, however, common anions, bisulfite (or hydrogensulfite) and sulfite.

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 = 1.54×10−2; pK_a = 1.81.

17O NMR spectroscopy provided evidence that solutions of sulfurous acid and protonated sulfites contains a mixture of isomers, which is in equilibrium:

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Aqueous solutions of sulfur dioxide, which sometimes are referred to as sulfurous acid are used as reducing agents and as disinfectants, as are solutions of bisulfite and sulfite salts. They are also mild bleaches, and are used for materials which may be damaged by chlorine-containing bleaches.