Chlorine dioxide

The molecule has an odd number of valence electrons, and therefore it is a paramagnetic radical. It is an unusual "example of an odd-electron molecule stable toward dimerization" (nitric oxide being another example). crystallizes in the orthorhombic Pbca space group. ==History==

Chlorine dioxide was first prepared in 1811 by Sir Humphry Davy. 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 occupies an anti-bonding orbital. Later work has confirmed that the highest occupied molecular orbital is indeed an incompletely filled antibonding orbital. == Preparation ==

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. : 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 In the laboratory, can be prepared by oxidation of sodium chlorite with chlorine: Traditionally, chlorine dioxide for disinfection applications has been made from sodium chlorite 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. 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: As a typical example, the reaction of sodium chlorate with hydrochloric acid in a single reactor is believed to proceed through the following pathway: 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. 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: == Handling properties ==

Chlorine dioxide is very different from elemental chlorine. One of the most important qualities of chlorine dioxide is its high water solubility, especially in cold water. Chlorine dioxide does not react 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. At partial pressures above (or gas-phase concentrations greater than 10% volume in air at STP) of 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 . Its solubility increases at lower temperatures, so it is common to use chilled water () 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, chlorine dioxide solutions below in concentration may be transported by land, but they are relatively unstable and deteriorate quickly. ==Uses==

Chlorine dioxide is used for bleaching of wood pulp and for the disinfection (called chlorination) 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. As a disinfectant, it is effective even at low concentrations because of its unique qualities. Water treatment 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." Trihalomethanes are suspected carcinogenic disinfection by-products associated with chlorination of naturally occurring organics in raw water. Chlorine dioxide is also superior to chlorine when operating above pH 7, and for the control of biofilms in water distribution systems. 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. The World Health Organization also advises a 1ppm dosification. 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 may be used for control of zebra and quagga mussels in water intakes. For water purification during camping, disinfecting tablets containing chlorine dioxide are more effective against pathogens than those using household bleach, but typically cost more. Other uses 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. == Safety issues in water and supplements ==

Potential hazards with chlorine dioxide include poisoning and the risk of spontaneous ignition or explosion on contact with flammable materials. 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 conditions, including autism and coronavirus. Children who have been given enemas of chlorine dioxide as a supposed cure for autism have experienced life-threatening complications. == Further reading ==