Chlorine dioxide (ClO₂) is a reddish-yellow gas, which is one of several known oxides of chlorine. Chlorine dioxide is relatively stable in the gas and liquid states, but can be explode easily. Practically, it is never handled in its pure form. Its boiling point is approximately 11 °C.

Chlorine dioxide is used in the disinfection of water and bleaching of flour and wood pulp. It can also be used for air disinfection, and was the principal agent used in the decontamination of buildings in the United States after the 2001 anthrax attacks. It is effective against viruses, bacteria and protozoa – including the cysts of Giardia and the oocysts of Cryptosporidium. This distinguishes it from chlorine, which has been used for more than a century as the standard water disinfectant. Chlorine dioxide was introduced as a drinking water disinfectant on a large scale when in 1956 Brussels, Belgium, changed from chlorine to chlorine dioxide.

Chlorine dioxide, ClO₂ can be produced by reducing sodium chlorate in a strong acid solution with a suitable reducing agent (for example, hydrogen peroxide, sulfur dioxide, or hydrochloric acid):

2ClO₃^- + 2Cl^- + 4H⁺   →   2ClO₂ + Cl₂ + 2H₂O

However, this reaction is of little practical value for disinfection applications. As a disinfectant, chlorine dioxide is almost exclusively produced by either the chlorite - chlorine method:

2 NaClO₂ + Cl₂   →   2 NaCl + 2 ClO₂

or the chlorite - hydrochloric acid method:

5 NaClO₂ + 4 HCl   →   5 NaCl + 4 ClO₂ + 2 H₂O

The latter has gained the reputation of generating an almost chlorine-free product in high yields (nominally > 95% with modern generators).

References

• National Library of Medicine (US)