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Chlorine dioxide

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Chlorine dioxide

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Chlorine dioxide
Structure of the chlorine dioxide molecule3D model of the chlorine dioxide molecule3D electric potential surface of the chlorine dioxide molecule
Molecular formula ClO2
Molar mass 67.45 g/mol
CAS number [10049-04-4]
EINECS number 233-162-8
Density 3.09 g/l (gas)
1.642 g/cm3 (liquid)
Solubility (water) Hydrolysis
Melting point −59C
Boiling point 10C
Thermodynamic data
Standard enthalpy
of formation ΔfHsolid
+104.60 kJ/mol
Standard molar entropy
257.22 J K−1 mol−1
Heat capacity Cp 24.12 J K−1 mol−1
Safety data
EU classification Oxidant (O)
Very toxic (T+)
Dangerous for the
environment (N)
R-phrases R6, R8, R24,
R36, R50
S-phrases S1/2, S23, S26,
S28, S36/37/39,
S38, S45, S61

Chlorine dioxide is a reddish-yellow gas which is one of several known oxides of chlorine. Chlorine dioxide is not stable in the gas state above 15% volume in air at STP (115 mm Hg) and can spontaneously and explosively decompose into chlorine and oxygen. Practically, it is never handled in its pure (concentrated) form. It is almost always used as a dissolved gas in water in a concentration range of 0.5 to 10 grams/liter. Chlorine dioxide is inversely soluble with temperature. In order to keep chlorine dioxide in solution, it is common to used chilled water (5C or 41F) when storing at concentrations above 3 grams/liter. In many countries, such as the USA, chlorine dioxide gas may not be transported at any concentration and is almost always produced at the application site using a chlorine dioxide generator. In some countries, chlorine dioxide solution below 3 grams/liter in concentration may be transported by land, but are relatively unstable and deteriorate quickly.

A number of products are marketed as "stabilized chlorine dioxide" (SCD). These are not actually solutions of chlorine dioxide but solutions of buffered sodium chlorite. A weak acid can be added to SCD to "activate" it and make chlorine dioxide in-situ without a chlorine dioxide generator. The use of SCD is effective when the demand for chlorine dioxide is low and when impurities, such as small amounts of chlorine, can be tolerated. For application requiring above 5 kg day−1 ClO2, chlorine dioxide produced by a generator with either sodium chlorite or sodium chlorate is typically more economical.



Chlorine dioxide is used primarily (>95%) for bleaching of wood pulp, but is also used for the bleaching of flour and for the disinfection of water. The Niagara Falls, New York water treatment plant first used chlorine dioxide for drinking water treatment in 1944 for phenol destruction. Chlorine dioxide was introduced as a drinking water disinfectant on a large scale in 1956, when Brussels, Belgium, changed from chlorine to chlorine dioxide. Its most common use in water treatment is as a pre-oxidant prior to chlorination of drinking water to reduce trihalomethanes which are a carcinogenic disinfection by-product associated with chlorination of naturally occurring organics in the raw water. Chlorine dioxide is also used in conjunction with ozone disinfection of water to reduce the formation of bromates which are regulated carcinogens. Chlorine dioxide is also superior to chlorine when operating above neutral pH, when ammonia is present and for the control of biofilms. Chlorine dioxide is used in many industrial water treatment applications as a biocide including cooling towers, process water and food processing. Chlorine dioxide is less corrosive than chlorine and superior for the control of legionella bacteria.

It is more effective than chlorine against viruses, bacteria and protozoa including the cysts of Giardia and the oocysts of Cryptosporidium.

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. 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.

Chlorine dioxide is used as an oxidant for phenol destruction in waste water streams, control of zebra mussels in water intakes and for odor control in the air scrubbers of animal byproduct (rendering) plants.


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:

2NaClO2 + Cl2   →   2ClO2 + 2NaCl

or the sodium chlorite - hypochlorite method:

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

or the sodium chlorite - hydrochloric acid method:

5NaClO2 + 4HCl   →   5NaCl + 4ClO2

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:

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

High purity chlorine dioxide gas (7.7% in air or nitrogen) can be produced by the Gas:Solid method, which reacts dilute chlorine gas with solid sodium chlorite.

2NaClO2 + Cl2   →   2ClO2 + 2NaCl

These processes and several slight variations are reviewed in Geo. Clifford White's Handbook of Chlorination and Alternative Disinfectants, 4th Edition (Wiley, 1999).


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