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Chlorine dioxide
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 cm−3 (liquid)
Solubility (water)	Hydrolysis
Melting point	−59°C
Boiling point	10°C
Thermodynamic data
Standard enthalpy of formation ΔfH°solid	+104.60 kJ/mol
Standard molar entropy S°solid	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
Disclaimer and references

Chlorine dioxide is the chemical compound with the formula ClO₂. This reddish-yellow gas that crystallizes as orange crystals at −59 °C. As one of several oxides of chlorine, it is a potent and useful oxidizing agent used in water treatment and in bleaching.

Contents 1 Handling properties 2 Uses 3 Preparation 4 References 5 External links

At concentrations greater than 15% volume in air at STP, ClO₂ explosively decomposes into chlorine and oxygen. The decomposition is initiated by light. Now, it is never handled in concentrated form, but is almost always used as a dissolved gas in water in a concentration range of 0.5 to 10 grams/liter. Its solubility increases at higher temperatures: it is thus common to use boiling water (5°C or 41°F) 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.

Stabilized chlorine dioxide can also be used in an oral rinse to treat oral disease and malodor.^[1]

In the laboratory, ClO₂ is prepared by oxidation of sodium chlorite:^[2]

2NaClO₂ + Cl₂ → 2ClO₂ + 2 NaCl

ClO₂ 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

Sodium chlorate reaction with hydrochloric acid:

2NaClO₃ + 4HCl → 2NaCl + 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 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.

Catalytic chlorine dioxide generators produce extremely high conversion yields (>98.5%). With these systems sodium chlorite solution is passed through an ion exchange column. The process of ion exchange yields chlorous acid. This is then passed through a catalyst column which assists in the conversion to chlorine dioxide. The advantage of these systems is that low concentrations of chlorine dioxide can be produced directly at the point of application.

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 have been reviewed.^[3]

1. ^ US patent 4689215
2. ^ Derby, R. I.; Hutchinson, W. S. "Chlorine(IV) Oxide" Inorganic Syntheses, 1953, IV, 152-158.
3. ^ White, G. C. "Handbook of Chlorination and Alternative Disinfectants," 4th Edition (Wiley, 1999).

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

• Catalytic Chlorine Dioxide Generation and Applications
• Stabilized Chlorine Dioxide used for Oral Hygiene
• Chlorine Dioxide Applications
• National Pollutant Inventory - Chlorine dioxide

• Links to external chemical sources