Manufacture
The free acid, chlorous acid, HClO2, is only stable at low concentrations. Since it cannot be concentrated, it is not a commercial product. However, the corresponding sodium salt, sodium chlorite, NaClO2 is stable and inexpensive enough to be commercially available. The corresponding salts of heavy metals (Ag+, Hg+, Tl+, Pb2+, and also Cu2+ and NH4+) decompose explosively with heat or shock.
Sodium chlorite is derived indirectly from sodium chlorate, NaClO3. First, the explosively unstable gas chlorine dioxide, ClO2 is produced by reducing sodium chlorate in a strong acid solution with a suitable reducing agent (for example, sodium chloride, sulfur dioxide, or hydrochloric acid).
H2SO4(aq) + NaClO3(s) → NaHSO4(aq) + HClO3(aq)
3HClO3(aq) → 2ClO2(g) + HClO4(aq) + H2O(l)
(Other routes for the preparation of chlorine dioxide are available depending on the initial salt).
The chlorine dioxide is then absorbed into an alkaline solution and reduced with hydrogen peroxide, H2O2 yielding sodium chlorite (NaClO2). (Sodium, Na+, spectator ions are not shown in the following equations).
2ClO2(g) + 2OH- → ClO2-(aq) + ClO3-(aq) + H2O(l)
ClO3-(aq) + H2O2(l) → ClO2-(aq) + H2O(l) + O2(g)
Read more about this topic: Chlorite
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