What is "sulfurous acid"

alt. (context American spelling inorganic compound English) The weak acid, H₂SO₃, present in a solution of sulfur dioxide in water; it is a reducing agent and mild bleach. n. (context American spelling inorganic compound English) The weak acid, H₂SO₃, present in a solution of sulfur dioxide in water; it is a reducing agent and mild bleach.

Sulfurous acid (also sulphurous acid) is the chemical compound with the formula HSO. 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 hydrogen sulfite) and sulfite. Sulfurous acid is an intermediate species in the formation of acid rain from sulfur dioxide.

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

SO + HO + HK = 1.54; pK = 1.81.

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

[H–OSO] ⇌ [H–SO]

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.