3D model (JSmol)
CompTox Dashboard (EPA)
|Molar mass||67.45 g·mol−1|
|Appearance||Yellow to reddish gas|
|Density||2.757 g dm−3|
|Melting point||−59 °C (−74 °F; 214 K)|
|Boiling point||11 °C (52 °F; 284 K)|
|8 g/L at 20 °C|
|Solubility||Soluble in alkaline solutions and sulfuric acid|
|Vapor pressure||>1 atm|
|4.01×10−2 atm m3 mol−1|
|257.22 J K−1 mol−1|
Std enthalpy of
|Occupational safety and health (OHS/OSH):|
|Highly toxic, corrosive, unstable, powerful oxidizer|
|H271, H300+H310+H330, H314, H372|
|P210, P220, P260, P264, P271, P280, P283, P284, P301+P310, P304+P340, P305+P351+P338, P306+P360, P371+P380+P375, P403+P233, P405, P501|
|NFPA 704 (fire diamond)|
|Lethal dose or concentration (LD, LC):|
LD50 (median dose)
|94 mg/kg (oral, rat)|
LCLo (lowest published)
|260 ppm (rat, 2 hr)|
|NIOSH (US health exposure limits):|
|TWA 0.1 ppm (0.3 mg/m3)|
|TWA 0.1 ppm (0.3 mg/m3) ST 0.3 ppm (0.9 mg/m3)|
IDLH (Immediate danger)
|Safety data sheet (SDS)||Safety Data Sheet Archive.|
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
(what is ?)
Chlorine dioxide is a chemical compound with the formula ClO2 that exists as yellowish-green gas above 11 °C, a reddish-brown liquid between 11 °C and −59 °C, and as bright orange crystals below −59 °C. It is usually handled as an aqueous solution. It is commonly used as a bleach. More recent developments have extended its applications in food processing and as a disinfectant.
Structure and bonding
The molecule ClO2 has an odd number of valence electrons, and therefore, it is a paramagnetic radical. It is an unusual "example of an odd-electron molecule which is stable towards dimerization" (nitric oxide being another example).
In 1933, Lawrence O. Brockway, a graduate student of Linus Pauling, proposed a structure that involved a three-electron bond and two single bonds. However, Pauling in his General Chemistry shows a double bond to one oxygen and a single bond plus a three-electron bond to the other. The valence bond structure would be represented as the resonance hybrid depicted by Pauling. The three-electron bond represents a bond that is weaker than the double bond. In molecular orbital theory this idea is commonplace if the third electron is placed in an anti-bonding orbital. Later work has confirmed that the highest occupied molecular orbital is indeed an incompletely-filled antibonding orbital.
Chlorine dioxide can decompose violently when separated from diluting substances. As a result, preparation methods that involve producing solutions of it without going through a gas-phase stage are often preferred.
Oxidation of chlorite
or the sodium chlorite–hydrochloric acid method:
or the chlorite–sulfuric acid method:
All three methods can produce chlorine dioxide with high chlorite conversion yield. Unlike the other processes, the chlorite–sulfuric acid method is completely chlorine-free, although it suffers from the requirement of 25% more chlorite to produce an equivalent amount of chlorine dioxide. Alternatively, hydrogen peroxide may be efficiently used in small-scale applications.
Reduction of chlorate
or with oxalic and sulfuric acid:
Over 95% of the chlorine dioxide produced in the world today is made by reduction of sodium chlorate, for use in pulp bleaching. It is produced with high efficiency in a strong acid solution with a suitable reducing agent such as methanol, hydrogen peroxide, hydrochloric acid or sulfur dioxide. Modern technologies are based on methanol or hydrogen peroxide, as these chemistries allow the best economy and do not co-produce elemental chlorine. The overall reaction can be written as:
which gives the overall reaction
The commercially more important production route uses methanol as the reducing agent and sulfuric acid for the acidity. Two advantages of not using the chloride-based processes are that there is no formation of elemental chlorine, and that sodium sulfate, a valuable chemical for the pulp mill, is a side-product. These methanol-based processes provide high efficiency and can be made very safe.
The variant process using sodium chlorate, hydrogen peroxide and sulfuric acid has been increasingly used since 1999 for water treatment and other small-scale disinfection applications, since it produce a chlorine-free product at high efficiency, over 95%.
Very pure chlorine dioxide can also be produced by electrolysis of a chlorite solution:
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:
At partial pressures above 10 kPa (1.5 psi) (or gas-phase concentrations greater than 10% volume in air at STP) of ClO2 may explosively decompose into chlorine and oxygen. The decomposition can be initiated by light, hot spots, chemical reaction, or pressure shock. Thus, chlorine dioxide is never handled as a pure gas, but is almost always handled in an aqueous solution in concentrations between 0.5 to 10 grams per liter. Its solubility increases at lower temperatures, so it is common to use chilled water (5 °C, 41 °F) when storing at concentrations above 3 grams per liter. In many countries, such as the United States, chlorine dioxide may not be transported at any concentration and is instead almost always produced on-site. In some countries,[which?] chlorine dioxide solutions below 3 grams per liter in concentration may be transported by land, but they are relatively unstable and deteriorate quickly.
Chlorine dioxide is used for bleaching of wood pulp and for the disinfection (called chlorination) of municipal drinking water,: 4–1  treatment of water in oil and gas applications, disinfection in the food industry, microbiological control in cooling towers, and textile bleaching. As a disinfectant, it is effective even at low concentrations because of its unique qualities.
Chlorine dioxide is sometimes used for bleaching of wood pulp in combination with chlorine, but it is used alone in ECF (elemental chlorine-free) bleaching sequences. It is used at moderately acidic pH (3.5 to 6). The use of chlorine dioxide minimizes the amount of organochlorine compounds produced. Chlorine dioxide (ECF technology) currently is the most important bleaching method worldwide. About 95% of all bleached kraft pulp is made using chlorine dioxide in ECF bleaching sequences.
The water treatment plant at Niagara Falls, New York first used chlorine dioxide for drinking water treatment in 1944 for destroying "taste and odor producing phenolic compounds.": 4–17  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 destroy natural water impurities that would otherwise produce trihalomethanes upon exposure to free chlorine. Trihalomethanes are suspected carcinogenic disinfection by-products associated with chlorination of naturally occurring organics in raw water. Chlorine dioxide also produces 70% fewer halomethanes in the presence of natural organic matter compared to when elemental chlorine or bleach is used.
Chlorine dioxide is also superior to chlorine when operating above pH 7,: 4–33 in the presence of ammonia and amines, and for the control of biofilms in water distribution systems. 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. Chlorine dioxide is superior to some other secondary water disinfection methods, in that chlorine dioxide is not negatively impacted by pH, does not lose efficacy over time, because the bacteria will not grow resistant to it), and is not negatively impacted by silica and phosphates, which are commonly used potable water corrosion inhibitors. In the United States, it is an EPA-registered biocide.
It is more effective as a disinfectant than chlorine in most circumstances against waterborne pathogenic agents such as viruses, bacteria, and protozoa – including the cysts of Giardia and the oocysts of Cryptosporidium.: 4-20–4-21
The use of chlorine dioxide in water treatment leads to the formation of the by-product chlorite, which is currently limited to a maximum of 1 part per million in drinking water in the USA.: 4–33 This EPA standard limits the use of chlorine dioxide in the US to relatively high-quality water, because this minimizes chlorite concentration, or water that is to be treated with iron-based coagulants, because iron can reduce chlorite to chloride. The World Health Organization also advises a 1ppm dosification.
Use in public crises
Chlorine dioxide has many applications as an oxidizer or disinfectant. Chlorine dioxide can 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. After the disaster of Hurricane Katrina in New Orleans, Louisiana, and the surrounding Gulf Coast, chlorine dioxide was used to eradicate dangerous mold from houses inundated by the flood water.
In addressing the COVID-19 pandemic, the U.S. Environmental Protection Agency has posted a list of many disinfectants that meet its criteria for use in environmental measures against the causative coronavirus. Some are based on sodium chlorite that is activated into chlorine dioxide, though differing formulations are used in each product. Many other products on the EPA list contain sodium hypochlorite, which is similar in name but should not be confused with sodium chlorite because they have very different modes of chemical action.
Other disinfection uses
Chlorine dioxide may be used as a fumigant treatment to "sanitize" fruits such as blueberries, raspberries, and strawberries that develop molds and yeast.
Chlorine dioxide may be used to disinfect poultry by spraying or immersing it after slaughtering.
Chlorine dioxide may be used for the disinfection of endoscopes, such as under the trade name Tristel. It is also available in a trio consisting of a preceding pre-clean with surfactant and a succeeding rinse with deionized water and a low-level antioxidant.
Chlorine dioxide is used as an oxidant for destroying phenols in wastewater streams and for odor control in the air scrubbers of animal byproduct (rendering) plants.: 4–34 It is also available for use as a deodorant for cars and boats, in chlorine dioxide-generating packages that are activated by water and left in the boat or car overnight.
Safety issues in water and supplements
Chlorine dioxide is toxic, and limits on human exposure are required to ensure its safe use. The United States Environmental Protection Agency has set a maximum level of 0.8 mg/L for chlorine dioxide in drinking water. The Occupational Safety and Health Administration (OSHA), an agency of the United States Department of Labor, has set an 8-hour permissible exposure limit of 0.1 ppm in air (0.3 mg/m3) for people working with chlorine dioxide.
Chlorine dioxide has been fraudulently and illegally marketed as an ingestible cure for a wide range of diseases, including childhood autism and coronavirus. Children who have been given enemas of chlorine dioxide as a supposed cure for childhood autism have suffered life-threatening ailments. The U.S. Food and Drug Administration (FDA) has stated that ingestion or other internal use of chlorine dioxide, outside of supervised oral rinsing using dilute concentrations, has no health benefits of any kind, and it should not be used internally for any reason.
On 30 July and 1 October 2010, the United States Food and Drug Administration warned against the use of the product "Miracle Mineral Supplement", or "MMS", which when prepared according to the instructions produces chlorine dioxide. MMS has been marketed as a treatment for a variety of conditions, including HIV, cancer, autism, acne, and, more recently, COVID-19. Many have complained to the FDA, reporting life-threatening reactions, and even death. The FDA has warned consumers that MMS can cause serious harm to health, and stated that it has received numerous reports of nausea, diarrhea, severe vomiting, and life-threatening low blood pressure caused by dehydration. This warning was repeated for a third time on 12 August 2019, and a fourth on 8 April 2020, stating that ingesting MMS is just as hazardous as ingesting bleach, and urging consumers not to use them or give these products to their children for any reason, as there is no scientific evidence showing that chlorine dioxide has any beneficial medical properties.
- Haynes, William M. (2010). Handbook of Chemistry and Physics (91 ed.). Boca Raton, Florida, USA: CRC Press. p. 4–58. ISBN 978-1-43982077-3.
- NIOSH Pocket Guide to Chemical Hazards. "#0116". National Institute for Occupational Safety and Health (NIOSH).
- Dobson, Stuart; Cary, Richard; International Programme on Chemical Safety (2002). Chlorine dioxide (gas). World Health Organization. p. 4. hdl:10665/42421. Retrieved August 17, 2020.
- "Chlorine dioxide". Immediately Dangerous to Life or Health Concentrations (IDLH). National Institute for Occupational Safety and Health (NIOSH).
- Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. p. 845. ISBN 978-0-08-037941-8.
- Brockway, L. O. (March 1933). "The Three-Electron Bond in Chlorine Dioxide" (PDF). Proceedings of the National Academy of Sciences. 19 (3): 303–307. Bibcode:1933PNAS...19..303B. doi:10.1073/pnas.19.3.303. PMC 1085967. PMID 16577512.
- Linus Pauling (1988). General chemistry. Mineola, New York: Dover Publications. p. 264. ISBN 0-486-65622-5.
- Flesch, R.; Plenge, J.; Rühl, E. (2006). "Core-level excitation and fragmentation of chlorine dioxide". International Journal of Mass Spectrometry. 249–250: 68–76. Bibcode:2006IJMSp.249...68F. doi:10.1016/j.ijms.2005.12.046.
- "mp-23207: ClO2 (Orthorhombic, Pbca, 61)". Materials Project. Retrieved November 3, 2022.
- Aieta, E. Marco, and James D. Berg. "A Review of Chlorine Dioxide in Drinking Water Treatment." Journal (American Water Works Association) 78, no. 6 (1986): 62-72. Accessed April 24, 2021. http://www.jstor.org/stable/41273622
- Derby, R. I.; Hutchinson, W. S. (1953). Chlorine(IV) Oxide. Inorganic Syntheses. Vol. 4. pp. 152–158. doi:10.1002/9780470132357.ch51. ISBN 978-0-470-13235-7.
- Vogt, H.; Balej, J.; Bennett, J. E.; Wintzer, P.; Sheikh, S. A.; Gallone, P.; Vasudevan, S.; Pelin, K. "Chlorine Oxides and Chlorine Oxygen Acids". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH.
- Ni, Y.; Wang, X. (1996). "Mechanism of the Methanol Based ClO2 Generation Process". International Pulp Bleaching Conference. TAPPI. pp. 454–462.[permanent dead link]
- White, George W.; White, Geo Clifford (1999). The handbook of chlorination and alternative disinfectants (4th ed.). New York: John Wiley. ISBN 0-471-29207-9.
- Swaddle, Thomas Wilson (1997). Inorganic Chemistry: An Industrial and Environmental Perspective. Academic Press. pp. 198–199. ISBN 0-12-678550-3.
- Alternative Disinfectants and Oxidants Manual, chapter 4: Chlorine Dioxide (PDF), US Environmental Protection Agency: Office of Water, April 1999, archived from the original (PDF) on September 5, 2015, retrieved November 27, 2009
- Block, Seymour Stanton (2001). Disinfection, Sterilization, and Preservation (5th ed.). Lippincott, Williams & Wilkins. p. 215. ISBN 0-683-30740-1.
- Simpson, Gregory Deward (2005). Practical Chlorine Dioxide (Volume 1 ed.). Colleyville, Texas: Greg D. Simpson & Associates. ISBN 0-9771985-0-2.
- Sjöström, E. (1993). Wood Chemistry: Fundamentals and Applications. Academic Press. ISBN 0-12-647480-X. OCLC 58509724.
- "AET – Reports – Science – Trends in World Bleached Chemical Pulp Production: 1990–2005". Archived from the original on July 30, 2017. Retrieved February 26, 2016.
- Harrel, C. G. (1952). "Maturing and Bleaching Agents in Producing Flour". Industrial & Engineering Chemistry. 44 (1): 95–100. doi:10.1021/ie50505a030.
- Sorlini, S.; Collivignarelli, C. (2005). "Trihalomethane formation during chemical oxidation with chlorine, chlorine dioxide and ozone of ten Italian natural waters". Desalination. 176 (1–3): 103–111. doi:10.1016/j.desal.2004.10.022.
- Li, J.; Yu, Z.; Gao, M. (1996). "A pilot study on trihalomethane formation in water treated by chlorine dioxide". Zhonghua Yufang Yixue Zazhi (Chinese Journal of Preventive Medicine) (in Chinese). 30 (1): 10–13. PMID 8758861.
- Volk, C. J.; Hofmann, R.; Chauret, C.; Gagnon, G. A.; Ranger, G.; Andrews, R. C. (2002). "Implementation of chlorine dioxide disinfection: Effects of the treatment change on drinking water quality in a full-scale distribution system". Journal of Environmental Engineering and Science. 1 (5): 323–330. doi:10.1139/s02-026.
- Pereira, M. A.; Lin, L. H.; Lippitt, J. M.; Herren, S. L. (1982). "Trihalomethanes as initiators and promoters of carcinogenesis". Environmental Health Perspectives. 46: 151–156. doi:10.2307/3429432. JSTOR 3429432. PMC 1569022. PMID 7151756.
- "Guidelines for drinking-water quality, 4th edition, incorporating the 1st addendum". World Health Organization. Retrieved November 29, 2021.
- "Chlorine dioxide as a disinfectant". Lenntech. Retrieved November 25, 2021.
- Andrews, L.; Key, A.; Martin, R.; Grodner, R.; Park, D. (2002). "Chlorine dioxide wash of shrimp and crawfish an alternative to aqueous chlorine". Food Microbiology. 19 (4): 261–267. doi:10.1006/fmic.2002.0493.
- Zhang, Zhe; McCann, Carole; Stout, Janet E.; Piesczynski, Steve; Hawks, Robert; Vidic, Radisav; Yu, Victor L. (2007). "Safety and Efficacy of Chlorine Dioxide for Legionella control in a Hospital Water System" (PDF). Infection Control and Hospital Epidemiology. 28 (8): 1009–1012. doi:10.1086/518847. PMID 17620253. S2CID 40554616. Archived from the original (PDF) on July 19, 2011. Retrieved November 27, 2009.
- Ogata, N.; Shibata, T. (January 2008). "Protective effect of low-concentration chlorine dioxide gas against influenza A virus infection". Journal of General Virology. 89 (pt 1): 60–67. doi:10.1099/vir.0.83393-0. PMID 18089729.
- "Chlorine Dioxide & Chlorite | Public Health Statement | ATSDR". United States: Centers for Disease Control and Prevention. Retrieved November 25, 2021.
- Zhang, Y.-L.; Zheng, S.-Y.; Zhi, Q. (2007). "Air Disinfection with Chlorine Dioxide in Saps". Journal of Environment and Health. 24 (4): 245–246.
- "Anthrax spore decontamination using chlorine dioxide". United States: Environmental Protection Agency. 2007. Retrieved November 27, 2009.
- Sy, Kaye V.; McWatters, Kay H.; Beuchat, Larry R. (2005). "Efficacy of Gaseous Chlorine Dioxide as a Sanitizer for Killing Salmonella, Yeasts, and Molds on Blueberries, Strawberries, and Raspberries". Journal of Food Protection. International Association for Food Protection. 68 (6): 1165–1175. doi:10.4315/0362-028x-68.6.1165. PMID 15954703.
- "How we know disinfectants should kill the COVID-19 coronavirus". Chemical & Engineering News. Retrieved March 28, 2020.
- "List N: Disinfectants for Use Against SARS-CoV-2". Environmental Protection Agency. United States. March 13, 2020. Retrieved March 28, 2020.
- O'Brian, D. (2017). "Chlorine Dioxide Pouches Can Make Produce Safer and Reduce Spoilage". AgResearch Magazine. USDA Agricultural Research Service (July). Retrieved June 21, 2018.
- "The truth behind the chlorinated chicken panic". The Big Issue. May 29, 2019. Retrieved February 5, 2020.
- Coates, D. (2001). "An evaluation of the use of chlorine dioxide (Tristel One-Shot) in an automated washer/disinfector (Medivator) fitted with a chlorine dioxide generator for decontamination of flexible endoscopes". Journal of Hospital Infection. 48 (1): 55–65. doi:10.1053/jhin.2001.0956. PMID 11358471.
- "Tristel Wipes System Product Information" (PDF). Ethical Agents. Archived from the original (PDF) on April 15, 2016. Retrieved November 1, 2012.
- Gibbs, S. G.; Lowe, J. J.; Smith, P. W.; Hewlett, A. L. (2012). "Gaseous chlorine dioxide as an alternative for bedbug control". Infection Control & Hospital Epidemiology. 33 (5): 495–9. doi:10.1086/665320. PMID 22476276. S2CID 14105046.
- Langlois, Krista (March 13, 2018). "How to Treat Backcountry Water on the Cheap". Sierra. Sierra Club. Retrieved February 10, 2021.
- "A Guide to Drinking Water Treatment and Sanitation for Backcountry & Travel Use". United States: Centers for Disease Control and Prevention. April 10, 2009. Retrieved February 10, 2021.
- "Toxicological Profile for Chlorine Dioxide and Chlorite" (PDF). Agency for Toxic Substances and Disease Registry, US HHS. Archived from the original (PDF) on June 14, 2019.
- López, María I.; Croce, Adela E.; Sicre, Juan E. (1994). "Explosive decomposition of gaseous chlorine dioxide". J. Chem. Soc., Faraday Trans. 90 (22): 3391–3396. doi:10.1039/FT9949003391. ISSN 0956-5000.
- "ATSDR: ToxFAQs™ for Chlorine Dioxide and Chlorite".
- "Occupational Safety and Health Guideline for Chlorine Dioxide". Archived from the original on December 4, 2012. Retrieved December 8, 2012.
- "Parents are poisoning their children with bleach to 'cure' autism. These moms are trying to stop it". NBC News. Retrieved May 21, 2019.
- "Fake news: Chlorine dioxide won't stop coronavirus". Detroit News. Retrieved April 3, 2020.
- Friedman, Lisa (April 3, 2020). "E.P.A. Threatens Legal Action Against Sellers of Fake Coronavirus Cleaners". The New York Times. ISSN 0362-4331. Retrieved April 3, 2020.
- Spencer, Sarnac Hale. "Those coronavirus 'cures' you're hearing about? They're fake. Don't drink chlorine dioxide". USA TODAY. Retrieved April 3, 2020.
- "Drinking bleach will not cure cancer or autism, FDA warns". NBC News. Retrieved August 13, 2019.
- "FDA warns consumers about the dangerous and potentially life threatening side effects of Miracle Mineral Solution". United States: Food and Drug Administration. August 12, 2019. Archived from the original on August 14, 2019. Retrieved August 16, 2019.
- Bartley, Lisa (October 29, 2016). "Group of SoCal parents secretly try to cure kids with autism using bleach". ABC 7 News. ABC. Retrieved March 24, 2019.
- Ryan, Frances (July 13, 2016). "The fake cures for autism that can prove deadly". The Guardian. Retrieved March 24, 2019.
- "Press Announcements – FDA Warns Consumers of Serious Harm from Drinking Miracle Mineral Solution (MMS)". Food and Drug Administration. Archived from the original on January 12, 2017.
- "'Miracle' Treatment Turns into Potent Bleach". U.S. Food and Drug Administration. November 20, 2015. Archived from the original on November 1, 2017. Retrieved December 6, 2017.
- "FDA Warns Consumers of Serious Harm from Drinking Miracle Mineral Solution (MMS)". United States: Food and Drug Administration. February 3, 2011. Archived from the original on February 3, 2011. Retrieved April 5, 2018.
- Media related to chlorine dioxide at Wikimedia Commons