|Jmol-3D images||Image 1
|Molar mass||67.45 g mol−1|
|Appearance||Yellow to reddish gas|
|Density||2.757 g dm−3|
-59 °C, 214 K, -74 °F
11 °C, 284 K, 52 °F
|Solubility in water||8 g dm-3 (at 20 °C)|
|Solubility||soluble in alkaline and sulfuric acid solutions|
|Std enthalpy of
|257.22 J K-1 mol-1|
|EU classification||O T+ C N|
|R-phrases||R6, R8, R26, R34, R50|
|S-phrases||(S1/2), S23, S26, S28, S36/37/39, S38, S45, S61|
|LD50||292 mg/kg (oral, rat)|
| (what is: / ?)
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Chlorine dioxide is a chemical compound with the formula ClO2. This yellowish-green gas crystallizes as bright 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.
Structure and bonding
The molecule ClO2 has an odd number of valence electrons and therefore, it is a paramagnetic radical. Its electronic structure has long baffled chemists because none of the possible Lewis structures is very satisfactory. In 1933 L.O. Brockway proposed a structure that involved a three-electron bond. Chemist Linus Pauling further developed this idea and arrived at two resonance structures involving a double bond on one side and a single bond plus three-electron bond on the other. In Pauling's view the latter combination should represent a bond that is slightly 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 HOMO is indeed an incompletely-filled orbital.
Chlorine dioxide is a compound that can decompose extremely 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. Arranging handling in a safe manner is essential.
- 2 NaClO2 + Cl2 → 2 ClO2 + 2 NaCl
Over 95% of the chlorine dioxide produced in the world today is made from sodium chlorate and is used for pulp bleaching. It is produced with high efficiency by reducing sodium chlorate 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;
Chlorate + Acid + reducing agent → Chlorine Dioxide + By-products
- HClO3 + HCl → HClO2 + HOCl
- HClO3 + HClO2 → 2 ClO2 + Cl2 + 2 H2O
- HOCl + HCl → Cl2 + H2O
The commercially more important production route uses methanol as the reducing agent and sulfuric acid for the acidity. Two advantages by 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.
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:
- 2 NaClO2 + 2 HCl + NaOCl → 2 ClO2 + 3 NaCl + H2O
or the sodium chlorite - hydrochloric acid method:
- 5 NaClO2 + 4 HCl → 5 NaCl + 4 ClO2 + 2 H2O
All three sodium chlorite chemistries can produce chlorine dioxide with high chlorite conversion yield, but unlike the other processes the chlorite-HCl method produces completely chlorine-free chlorine dioxide but suffers from the requirement of 25% more chlorite to produce an equivalent amount of chlorine dioxide. Alternatively, hydrogen peroxide may efficiently be used also in small scale applications.
Very pure chlorine dioxide can also be produced by electrolysis of a chlorite solution:
- 2 NaClO2 + 2 H2O → 2 ClO2 + 2 NaOH + 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.
- 2 NaClO2 + Cl2 → 2 ClO2 + 2 NaCl
These processes and several slight variations have been reviewed.
At gas phase concentrations greater than 30% volume in air at STP (more correctly: at partial pressures above 10 kPa ), ClO2 may explosively decompose into chlorine and oxygen. The decomposition can be initiated by, for example, light, hot spots, chemical reaction, or pressure shock. Thus, chlorine dioxide gas is never handled in concentrated form, but is almost always handled as a dissolved gas in water in a concentration range of 0.5 to 10 grams per liter. Its solubility increases at lower temperatures: it is thus common to use chilled water (5 °C or 41 °F) when storing at concentrations above 3 grams per 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 per liter in concentration may be transported by land, but are relatively unstable and deteriorate quickly. The use of chlorine dioxide generators is steadily falling out of fashion in industry, as these systems generally require the use of strong acids to work and can take several hours to reach their full yield with poor efficiency. The requirement to store the hazardous gas in a pressurized chamber poses a risk some sites prefer not to take. Newer dosing systems have proven to provide a safer alternative producing ClO2 in solution and offering a huge leap in efficiency converting around 95% instantly, only requiring the use of weak FDA-approved acids, offering a much safer method of producing ClO2, as there is no storage, which allows a more effective chlorine dioxide water treatment.
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 world wide. About 95% of all bleached Kraft pulp is made using chlorine dioxide in ECF bleaching sequences.
The Niagara Falls, New York water treatment plant first used chlorine dioxide for drinking water treatment in 1944 for phenol destruction.: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 produce trihalomethanes on exposure to free chlorine. Trihalomethanes are suspect carcinogenic disinfection by-products associated with chlorination of naturally occurring organics in the raw water. Chlorine dioxide is also superior to chlorine when operating above pH 7,:4-33 in the presence of ammonia and amines and/or 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: 1) is an EPA registered biocide, 2) is not negatively impacted by pH 3) does not lose efficacy over time (the bacteria will not grow resistant to it) and 4) is not negatively impacted by silica and phosphate, which are commonly used potable water corrosion inhibitors. Some unscrupulous biocide manufacturers will state that their product is EPA registered as a biocide. All EPA registered biocides must have a product label that is supplied with the product. This label will contain specifications as far as the product's EPA registration. EPA will register certain products as a general biocide, but others will have specifications for what bacteria the product can protect against. For instance, although chlorine dioxide is a registered biocide, it is not registered to protect against Legionella. If a biocide is sold without an EPA approved biocide label that is because the product is not registered as an EPA approved biocide.
It is more effective as a disinfectant than chlorine in most circumstances against water borne pathogenic microbes 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 ppm in drinking water in the USA.:4-33 This EPA standard limits the use of chlorine dioxide in the USA to relatively high quality water or water, which is to be treated with iron based coagulants (Iron can reduce chlorite to chloride).
Other disinfection uses
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. 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 the flood-water. Sometimes it is used as a fumigant treatment to 'sanitize' fruits such as blueberries, raspberries, and strawberries that develop molds and yeast.
Chlorine dioxide is 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 deionised water and low-level antioxidant.
Chlorine dioxide is used as an oxidant for phenol destruction in waste water streams and for odor control in the air scrubbers of animal byproduct (rendering) plants.:4-34
Safety issues in water and supplements
Chlorine dioxide is toxic, hence limits on exposure to it are needed 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. Occupational Safety and Health Administration, OSHA, an agency of the United States Department of Labor has set a 8 hour permissible exposure limit of 0.1 ppm in air (0.3 milligrams per cubic meter (mg/m(3))) for people working with chlorine dioxide.
On July 30, 2010 and again on October 1, 2010, the United States Food and Drug Administration, FDA, warned against the use of the product "Miracle Mineral Supplement" or "MMS", which when made up according to instructions produces chlorine dioxide. MMS has been marketed as a treatment for a variety of conditions, including HIV, cancer, and acne. The FDA warnings informed consumers that MMS can cause serious harm to health, and stated that it has received numerous reports of nausea, severe vomiting, and life-threatening low blood pressure caused by dehydration, among other symptoms, such as diarrhea.
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