Chlorous acid is an inorganic compound with the formula HClO2. It is a weak acid. Chlorine has oxidation state +3 in this acid. The pure substance is unstable, disproportionating to hypochlorous acid (Cl oxidation state +1) and chloric acid (Cl oxidation state +5):
2 HClO2 → HClO + HClO3
Although the acid is difficult to obtain in pure substance, the conjugate base, chlorite, derived from this acid is stable. One example of a salt of this anion is the well-known sodium chlorite. This and related salts are sometimes used in the production of chlorine dioxide.
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HClO2 can be prepared through reaction of barium or lead chlorite and dilute sulfuric acid:
Ba(ClO2)2 + H2SO4 → BaSO4 + 2 HClO2
Pb(ClO2)2 + H2SO4 → PbSO4 + 2 HClO2
Chlorous acid is a powerful oxidizing agent, although its tendency to undergo disproportionation counteracts its oxidizing potential.
Chlorine is the only halogen to form an isolable acid of formula HXO2.[1] Fluorine resists oxidation above the level of hypofluorous acid. Neither bromous acid nor iodous acid has ever been isolated. A few salts of bromous acid, bromites, are known, but no iodites.[1]