Chlorine dioxide 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 explode. Practically, it is never handled in its pure form. The explosion factor also depends on how chlorine dioxide is generated. The two stage process is more combustable than a three stage process. This is why three stage generators are used at oil production and refining sites.

Uses

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 in 1956, when Brussels, Belgium, changed from chlorine to chlorine dioxide.

Recently, after the disaster of Hurricane Katrina in New Orleans, Louisiana and the surrounding Gulf Coast, Chlorine Dioxide has been used to eradicate dangerous mold from houses inundated by water from massive flooding.

Preparation

Chlorine dioxide can be produced with high efficiency 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

Over 95% of the chlorine dioxide produced in the world today is made via the sodium chlorate method for pulp bleaching.

A much smaller but important market for chlorine dioxide is for use as a disinfectant. Since 1999 a growing proportion of the chlorine dioxide made globally for water treatment and other small scale applications has been made using the chlorate, hydrogen peroxide and sulfuric acid method which can produce a chlorine free product at high efficiency.

Traditionally, chlorine dioxide for disinfection applications has been made by one of three methods using sodium chlorite:

sodium chlorite - chlorine gas method:

2 NaClO₂ + Cl₂   →   2 ClO₂ + 2 NaCl

or the sodium chlorite - hypochlorite method:

2 NaClO₂ + 2 HCl + NaOCl   →   2 ClO₂ + 3 NaCl + H₂O

or the sodium chlorite - hydrochloric acid method:

5 NaClO₂ + 4 HCl   →   5 NaCl + 4 ClO₂

All three sodium chlorite chemistries can produce chlorine dioxide with high chlorite conversion yield, but the chlorite-HCl method suffers from the requirement of 25% more chlorite to produce an equivlent amount of chlorine dioxide.

Chlorine dioxide can also be produced by electrolysis of a chlorite solution:

NaClO₂ + H₂O   →   ClO₂ + NaOH + 1/2 H₂

References

• National Library of Medicine (US)
• WebElements
• NIST Standard Reference Database
• European Chemicals Bureau

External links

• National Pollutant Inventory - Chlorine dioxide

Oxides | Chlorine compounds | Bleaches | Disinfectants

Chloroxide | Dioxyde de chlore | 二酸化塩素 | Klordioxid | 二氧化氯