Halous acid

Synthesis

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

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

Ba(ClO₂)₂ + H₂SO₄ → BaSO₄ + 2 HClO₂