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

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Chlorous acid

Names
IUPAC name Chlorous acid, Chloric (III) acid
Identifiers
CAS Number	13898-47-0 Y
3D model (JSmol)	Interactive image
ChEBI	CHEBI:29219 Y
ChemSpider	22861 Y
KEGG	C01486 Y
PubChem CID	24453
UNII	7JRT833T5M Y
CompTox Dashboard (EPA)	DTXSID80160853
InChI InChI=1S/ClHO2/c2-1-3/h(H,2,3) Y Key: QBWCMBCROVPCKQ-UHFFFAOYSA-N Y InChI=1/ClHO2/c2-1-3/h(H,2,3) Key: QBWCMBCROVPCKQ-UHFFFAOYAO
SMILES O[Cl+] [O-]
Properties
Chemical formula	H Cl O2
Molar mass	68.46 g/mol
Acidity (pKa)	1.96
Conjugate base	Chlorite
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). N verify (what is YN ?) Infobox references

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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.

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.^{[citation needed]}

Chlorine is the only halogen to form an isolable acid of formula HXO₂.^[1] Neither bromous acid nor iodous acid has ever been isolated. A few salts of bromous acid, bromites, are known, but no iodites.^[1]