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| Names | |||
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| IUPAC name Dioxygen chloride | |||
Other names
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| Identifiers | |||
3D model (JSmol) | |||
| ChEBI | |||
| ChemSpider |
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| ECHA InfoCard | 100.030.135 | ||
| EC Number |
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| 1265 | |||
| MeSH | Chlorine+dioxide | ||
| RTECS number |
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| UNII | |||
| UN number | 9191 | ||
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| Properties | |||
| ClO2 | |||
| Molar mass | 67.45 g·mol−1 | ||
| Appearance | Yellow to reddish gas | ||
| Odor | Acrid, somewhat chlorine-like[1] | ||
| Density | 2.757 g/dm3[2] | ||
| Melting point | −59 °C (−74 °F; 214 K) | ||
| Boiling point | 11 °C (52 °F; 284 K) | ||
| 8 g/L (20 °C (68 °F; 293 K)) | |||
| Solubility | Soluble in alkaline solutions andsulfuric acid | ||
| Vapor pressure | >1 atm[3] | ||
Henry's law constant (kH) | 4.01×10−2 (atm·m3)/mol | ||
| Acidity (pKa) | 3.0(5) | ||
| Thermochemistry | |||
Std molar entropy(S⦵298) | 257.22 J/K·mol | ||
Std enthalpy of formation(ΔfH⦵298) | 104.60 kJ/mol | ||
| Hazards | |||
| Occupational safety and health (OHS/OSH): | |||
Main hazards | Highly toxic, corrosive, unstable, powerful oxidizer | ||
| GHS labelling:[citation needed] | |||
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| Danger | |||
| 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)[4] | ||
LCLo (lowest published) | 260 ppm (rat, 2 hr)[5] | ||
| NIOSH (US health exposure limits): | |||
PEL (Permissible) | TWA0.1 ppm (0.3 mg/m3)[3] | ||
REL (Recommended) |
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IDLH (Immediate danger) | 5 ppm[3] | ||
| Safety data sheet (SDS) | Safety Data SheetArchive. | ||
Except where otherwise noted, data are given for materials in theirstandard state (at 25 °C [77 °F], 100 kPa). | |||
Chlorine dioxide is achemical compound with the formulaClO2 that exists as yellowish-greengas above 11 °C (52 °F), a reddish-brown liquid between 11 and −59 °C (52 and −74 °F), and as bright orange crystals below −59 °C (−74 °F). It is usually handled as an aqueous solution. It is commonly used as ableach. Applications are infood processing and as adisinfectant.
The moleculeClO2 has an odd number ofvalence electrons, and therefore it is aparamagneticradical. It is an unusual "example of an odd-electron molecule stable toward dimerization" (nitric oxide being another example).[6]
ClO2 crystallizes in the orthorhombicPbca space group.[7]
Chlorine dioxide was first prepared in 1811 bySir Humphry Davy.[8]
In 1933,Lawrence O. Brockway, a graduate student ofLinus Pauling, proposed a structure that involved athree-electron bond and two single bonds.[9] However, Pauling in hisGeneral 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.[10] The three-electron bond represents a bond that isweaker than the double bond. Inmolecular orbital theory this idea is commonplace if the third electron occupies an anti-bonding orbital. Later work has confirmed that thehighest occupied molecular orbital is indeed an incompletely filled antibonding orbital.[11]
The reaction of chlorine with oxygen under conditions of flash photolysis in the presence of ultraviolet light results in trace amounts of chlorine dioxide formation.[12]
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.
In the laboratory,ClO2 can be prepared by oxidation ofsodium chlorite with chlorine:[13]
Traditionally, chlorine dioxide fordisinfection applications has been made from sodiumchlorite or the sodium chlorite–hypochlorite method:
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.[1]
Addition of sulfuric acid or any strong acid tochlorate salts produces chlorine dioxide.[10]
In the laboratory, chlorine dioxide can also be prepared by reaction ofpotassium chlorate withoxalic acid:
or with oxalic and sulfuric acid:
Over 95% of the chlorine dioxide produced in the world today is made by reduction ofsodium chlorate, for use inpulp bleaching. It is produced with high efficiency in a strong acid solution with a suitablereducing agent such asmethanol,hydrogen peroxide,hydrochloric acid orsulfur dioxide.[1] 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:[14]
As a typical example, the reaction ofsodium chlorate withhydrochloric acid in a single reactor is believed to proceed through the following pathway:
which gives the overall reaction
The commercially more important production route usesmethanol as the reducing agent andsulfuric acid for the acidity. Two advantages of not using the chloride-based processes are that there is no formation of elemental chlorine, and thatsodium 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.[1]
The variant process using sodium chlorate, hydrogen peroxide and sulfuric acid has been increasingly used since 1999 for water treatment and other small-scaledisinfection applications, since it produce a chlorine-free product at high efficiency, over 95%.[citation needed]
Very pure chlorine dioxide can also be produced by electrolysis of a chlorite solution:[15]
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:[15]
Chlorine dioxide is very different from elemental chlorine.[1] One of the most important qualities of chlorine dioxide is its high water solubility, especially in cold water. Chlorine dioxide does notreact with water; it remains a dissolved gas in solution. Chlorine dioxide is approximately 10 times more soluble in water than elemental chlorine but its solubility is very temperature-dependent.[1]
At partial pressures above 10 kPa (1.5 psi) (or gas-phase concentrations greater than 10% volume in air atSTP) ofClO2 may explosivelydecompose intochlorine andoxygen.[1] 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 between0.5 and 10 g/L. 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.[1] In some countries,[which?] chlorine dioxide solutions below3 g/L in concentration may be transported by land, but they are relatively unstable and deteriorate quickly.
Chlorine dioxide is used forbleaching of wood pulp and for thedisinfection (calledchlorination) of municipal drinking water, treatment of water in oil and gas applications, disinfection in the food industry, microbiological control in cooling towers, and textile bleaching.[16][17]: 4–1 [18][19] As a disinfectant, it is effective even at low concentrations because of its unique qualities.[1][17][19]
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 acidicpH (3.5 to 6). The use of chlorine dioxide minimizes the amount oforganochlorine compounds produced.[20] Chlorine dioxide (ECF technology) currently is the most importantbleaching method worldwide. About 95% of all bleachedkraft pulp is made using chlorine dioxide in ECF bleaching sequences.[21]
Chlorine dioxide has been used to bleachflour.[22]
The water treatment plant atNiagara Falls, New York first used chlorine dioxide fordrinking water treatment in 1944 for destroying "taste and odor producingphenolic compounds."[17]: 4–17 [18] Chlorine dioxide was introduced as a drinking water disinfectant on a large scale in 1956, whenBrussels, Belgium, changed from chlorine to chlorine dioxide.[18] 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 producetrihalomethanes upon exposure to free chlorine.[23][24][25] Trihalomethanes are suspected carcinogenic disinfection by-products[26] associated with chlorination of naturally occurring organics in raw water.[25] Chlorine dioxide also produces 70% fewer halomethanes in the presence of natural organic matter compared to when elemental chlorine or bleach is used.[27]
Chlorine dioxide is also superior to chlorine when operating abovepH 7,[17]: 4–33 in the presence of ammonia and amines,[28] and for the control of biofilms in water distribution systems.[25] Chlorine dioxide is used in many industrial water treatment applications as abiocide, includingcooling towers, process water, and food processing.[29]
Chlorine dioxide is less corrosive than chlorine and superior for the control ofLegionella bacteria.[18][30]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 bysilica andphosphates, which are commonly used potable water corrosion inhibitors. In the United States, it is anEPA-registered biocide.
It is more effective as a disinfectant than chlorine in most circumstances against waterborne pathogenic agents such asviruses,bacteria, andprotozoa – including thecysts ofGiardia and theoocysts ofCryptosporidium.[31][17]: 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.[17]: 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.[32] The World Health Organization also advises a 1ppm dosification.[27]
Chlorine dioxide has many applications as an oxidizer or disinfectant.[1] Chlorine dioxide can be used for air disinfection and was the principal agent used in the decontamination of buildings in the United States after the2001 anthrax attacks.[33][34] After the disaster ofHurricane Katrina in New Orleans, Louisiana, and the surrounding Gulf Coast, chlorine dioxide was used to eradicate dangerousmold from houses inundated by the flood water.[35]
In addressing the COVID-19 pandemic, theU.S. Environmental Protection Agency has posted a list of manydisinfectants that meet its criteria for use in environmental measures against the causativecoronavirus.[36][37] Some are based onsodium chlorite that is activated into chlorine dioxide, though differing formulations are used in each product. Many other products on the EPA list containsodium hypochlorite, which is similar in name but should not be confused with sodium chlorite because they have very different modes of chemical action.
Chlorine dioxide may be used as a fumigant treatment to "sanitize" fruits such as blueberries, raspberries, and strawberries that develop molds and yeast.[38]
Chlorine dioxide may be used to disinfect poultry by spraying or immersing it after slaughtering.[39]
Chlorine dioxide may be used for the disinfection ofendoscopes, such as under the trade name Tristel.[40] It is also available in a trio consisting of a preceding pre-clean withsurfactant and a succeeding rinse withdeionized water and a low-level antioxidant.[41]
Chlorine dioxide may be used for control ofzebra andquagga mussels in water intakes.[17]: 4–34
Chlorine dioxide was shown to be effective inbedbug eradication.[42]
For water purification duringcamping, disinfecting tablets containing chlorine dioxide are more effective against pathogens than those using household bleach, but typically cost more.[43][44]
Chlorine dioxide is used as an oxidant for destroyingphenols inwastewater streams and for odor control in the air scrubbers of animal byproduct (rendering) plants.[17]: 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.
In dilute concentrations, chlorine dioxide is an ingredient that acts as an antiseptic agent in somemouthwashes.[45][46]
Potential hazards with chlorine dioxide include poisoning and the risk of spontaneous ignition or explosion on contact with flammable materials.[47][48]
Chlorine dioxide is toxic, and limits on human exposure are required to ensure its safe use. TheUnited States Environmental Protection Agency has set a maximum level of 0.8 mg/L for chlorine dioxide in drinking water.[49] TheOccupational Safety and Health Administration (OSHA), an agency of theUnited States Department of Labor, has set an 8-hourpermissible exposure limit of 0.1 ppm in air (0.3mg/m3) for people working with chlorine dioxide.[50]
Chlorine dioxide has been fraudulently and illegally marketed as an ingestible cure for a wide range of conditions, including autism[51] andcoronavirus.[52][53][54] Children who have been givenenemas of chlorine dioxide as a supposed cure for autism have experienced life-threatening complications.[51] TheU.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.[55][56]
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