Halous acid

One method is to add oxygen to the corresponding hypohalous acid. This can be done with another hypohalous acid—or even the same one.

HBrO + HClO → HBrO₂ + HCl

2 HBrO → HBrO₂ + HBr

The latter is a disproportionation reaction.

Another method of oxidizing hypobromous acid can be used:

HBrO + H₂O - 2e⁻ → HBrO₂ + 2H⁺

The oxidized bromine-containing acid need not contain oxygen originally, as in this comproportionation reaction:

2 HBrO₃ + HBr → 3 HBrO₂

Bromous acid was originally prepared using another method of adding oxygen to hypobromous acid, where the free oxygen was freed from water when the free hydrogen was taken by NO₃ (to form nitric acid) that was freed from silver nitrate when the silver was taken by elemental bromine to form silver bromide.

2 AgNO₃ + HBrO + Br₂ + H₂O → HBrO₂ + 2 AgBr[2]

Chlorous acid may be generated from salts of the conjugate base, chlorite, which is more stable than chlorous acid.[3]

Ba(ClO₂)₂ + H₂SO₄ → BaSO₄ + 2 HClO₂[4]

Chlorous acid, bromous acid, and iodous acid are all unstable. Chlorous acid is the only isolatable halous acid,[5] and while it has stable salts, they tend to decompose rapidly, some even explosively, upon heating.[3] Neither bromous acid nor iodous acid has ever been isolated, and while a few salts of bromous acid have been isolated, the same is not true for iodous acid.[5]

One method of decomposition of halous acids is disproportionating to the corresponding hypohalous acid and halic acid:

2 HClO₂ → HClO + HClO₃

2 HBrO₂ → HBrO₃ + HOBr

In acidic solutions, chlorous acid tends to decompose via another disproportionation reaction:

5 HClO₂ → 4 ClO₂ + HCl + 2 H₂O[4]