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{{Infobox chlorine}}
'''Chlorine''' ({{IPAEng|ˈklɔəriːn}}, from the [[Greek language|Greek]] word 'χλωρóς' (khlôros) meaning 'green'), is the [[chemical element]] with [[atomic number]] 17 and symbol '''Cl'''. It is a [[halogen]], found in the [[periodic table]] in [[periodic table group|group]] VII (formerly VIIa or VIIb). As the [[chloride]] ion, which is part of [[common salt]] and other compounds, it is abundant in nature and necessary to most forms of life, including [[human]]s. In its common elemental form (Cl<sub>2</sub> or "dichlorine") under [[standard conditions]], it is a pale green gas about 2.5 times as dense as air. It has a disagreeable, suffocating odor that is detectable in concentrations as low as 3.5 ppm<ref name="merck">''Merck Index of Chemicals and Drugs'', 9th ed., monograph 2065</ref> and is [[poison]]ous. Chlorine is a powerful [[oxidation|oxidant]] and is used in [[Bleach (chemical)|bleaching]] and disinfectants. As a common disinfectant, chlorine compounds are used in [[swimming pool]]s to keep them clean and sanitary. In the [[upper atmosphere]], chlorine-containing molecules have been implicated in the [[ozone depletion|destruction]] of the [[ozone layer]].

==Characteristics==
[[Image:Chlorine.jpg|thumb|left|160px|Chlorine gas in a transparent plastic container. It is not advisable to store chlorine in this manner.]] Chlorine gas is [[Diatomic molecule|diatomic]], with the formula Cl<sub>2</sub>. It combines readily with all elements except [[Oxygen|O<sub>2</sub>]] and [[Nitrogen|N<sub>2</sub>]]<ref name="webelement">{{cite web|url=http://www.webelements.com/webelements/elements/text/Cl/index.html|title=WebElements.com &ndash; Chlorine|accessdate=2007-03-17|publisher=Mark Winter [The University of Sheffield and WebElements Ltd, UK]}}</ref> and the [[noble gases]]. Compounds with [[oxygen]], [[nitrogen]], and [[xenon]] are known but do not form by direct reaction of the elements.<ref>''Merck Index of Chemicals and Drugs, 9th ed.''</ref> Chlorine, though very reactive, is not as extremely reactive as [[fluorine]]. Pure chlorine gas does, however, support combustion of organic compounds such as [[hydrocarbon]]s, although the carbon component tends to burn incompletely, with much of it remaining as soot.<ref>''Lange's Handbook of Chemistry, 10th ed''</ref> At 10 °[[Celsius|C]] and atmospheric pressure, one [[liter]] of [[water]] dissolves 3.10 L of gaseous chlorine, and at 30°C, 1 L of water dissolves only 1.77 liters of chlorine.<ref name="webelement">{{cite web|url=http://www.webelements.com/webelements/elements/text/Cl/index.html|title=WebElements.com &ndash; Chlorine|accessdate=2007-03-17|publisher=Mark Winter [The University of Sheffield and WebElements Ltd, UK]}}</ref>

This element is a member of the [[salt]]-forming [[halogen]] series and is extracted from chlorides through [[oxidation]] often by [[electrolysis]]. As the chloride ion, Cl<sup>−</sup>, it is also the most abundant dissolved ion in [[ocean]] water.

=== Isotopes ===
{{main|Isotopes of chlorine}}
Chlorine has isotopes with [[mass number]]s ranging from 32 to 40. There are two principal stable [[isotope]]s, <sup>35</sup>Cl (75.77%) and <sup>37</sup>Cl (24.23%), giving chlorine [[atom]]s in bulk an apparent atomic weight of 35.4527&nbsp;g/mol.

Trace amounts of [[radioactive]] [[Chlorine-36|<sup>36</sup>Cl]] exist in the environment, in a ratio of about 7x10<sup>−13</sup> to 1 with stable isotopes. <sup>36</sup>Cl is produced in the atmosphere by [[spallation]] of <sup>36</sup>[[argon|Ar]] by interactions with [[cosmic ray]] [[proton]]s. In the subsurface environment, <sup>36</sup>Cl is generated primarily as a result of [[neutron capture]] by <sup>35</sup>Cl or [[muon capture]] by <sup>40</sup>[[calcium|Ca]]. <sup>36</sup>Cl decays to <sup>36</sup>[[sulfur|S]] and to <sup>36</sup>[[argon|Ar]], with a combined [[half-life]] of 308,000 years. The half-life of this [[hydrophilic]] nonreactive isotope makes it suitable for [[geologic dating]] in the range of 60,000 to 1 million years. Additionally, large amounts of <sup>36</sup>Cl were produced by irradiation of [[seawater]] during atmospheric detonations of [[nuclear weapon]]s between 1952 and 1958. The residence time of <sup>36</sup>Cl in the atmosphere is about 1 week. Thus, as an event marker of 1950s water in [[soil]] and [[ground water]], <sup>36</sup>Cl is also useful for dating waters less than 50 years before the present. <sup>36</sup>Cl has seen use in other areas of the geological sciences, including dating ice and sediments.

=== Occurrence ===
:''See also [[:Category:Halide minerals|Halide minerals]].''
In nature, chlorine is found primarily as the [[chloride ion]], a component of the [[salt]] that is deposited in the earth or dissolved in the [[ocean]]s &mdash; about 1.9% of the mass of seawater is chloride ions. Even higher concentrations of chloride are found in the [[Dead Sea]] and in underground [[brine]] deposits. Most chloride salts are soluble in water, thus, chloride-containing minerals are usually only found in abundance in dry climates or deep underground. Common chloride minerals include ''[[halite]]'' ([[sodium chloride]]), ''[[sylvite]]'' ([[potassium chloride]]), and ''[[carnallite]]'' (potassium magnesium chloride hexahydrate). Over 2000 naturally-occurring organic chlorine compounds are known.<ref name="Euro Chlor">{{cite web|url=http://www.eurochlor.org/upload/documents/document236.pdf|title=Risk assessment and the cycling of natural organochlorines|publisher=Euro Chlor|accessdate=2007-08-12}}</ref>

Industrially, elemental chlorine is usually produced by the [[electrolysis]] of sodium chloride dissolved in water. Along with chlorine, this [[chloralkali process]] yields [[hydrogen]] gas and [[sodium hydroxide]], according to the following [[chemical equation]]:

:2 [[sodium chloride|NaCl]] + 2 [[water|H<sub>2</sub>O]] → Cl<sub>2</sub> + [[hydrogen|H<sub>2</sub>]] + 2 [[sodium hydroxide|NaOH]]

==Production==
=== Gas extraction ===
Chlorine can be manufactured by [[electrolysis]] of a [[sodium chloride]] [[solution]] ([[brine]]). The production of chlorine results in the co-products [[caustic soda]] ([[sodium hydroxide]], [[NaOH]]) and [[hydrogen]] gas (H<sub>2</sub>). These two products, as well as chlorine itself, are highly reactive. Chlorine can also be produced by the [[electrolysis]] of a [[solution]] of [[potassium chloride]], in which case the co-products are [[hydrogen]] and [[caustic potash]] ([[potassium hydroxide]]). There are three industrial methods for the extraction of chlorine by [[electrolysis]] of chloride solutions, all proceeding according to the following equations:

:Cathode: 2 H<sup>+</sup> (aq) + 2 e<sup>−</sup> → H<sub>2</sub> (g)
:Anode: 2 Cl<sup>−</sup> (aq) → Cl<sub>2</sub> (g) + 2 e<sup>−</sup>
Overall process: 2 NaCl (or KCl) + 2 H<sub>2</sub>O → Cl<sub>2</sub> + H<sub>2</sub> + 2 NaOH (or KOH)

* '''Mercury cell electrolysis'''
[[mercury (element)|Mercury]] cell [[electrolysis]], also known as the [[Castner-Kellner process]], was the first method used at the end of the nineteenth century to produce chlorine on an industrial scale.<ref name="pauling">Pauling, Linus, ''General Chemistry'', 1970 ed., Dover publications</ref><ref name="lenn2">{{cite web|accessdate=2007-03-17|title=Electrolytic Processes for Chlorine and Caustic Soda|url=http://www.lenntech.com/Chemistry/electolytic-chlorine-caustic.htm|publisher=Lenntech Water treatment & air purification Holding B.V., Rotterdamseweg 402 M, 2629 HH Delft, The Netherlands}}</ref> The "rocking" cells used have been improved over the years.<ref name="Euro Chlor2">{{cite web|url=http://www.eurochlor.org/animations/mercury-cell.asp|title=Mercury cell|publisher=Euro Chlor|accessdate=2007-08-15}}</ref> Today, in the "primary cell", [[titanium]] anodes (formerly [[graphite]] ones) are placed in a sodium (or potassium) chloride solution flowing over a liquid [[mercury (element)|mercury]] [[cathode]]. When a potential difference is applied and current flows, chlorine is released at the [[titanium]] [[anode]] and [[sodium]] (or [[potassium]]) dissolves in the [[mercury (element)|mercury]] [[cathode]] forming an [[amalgam]].  This flows continuously into a separate reactor ("denuder" or "secondary cell"), where it is usually converted back to [[mercury (element)|mercury]] by reaction with [[water]], producing [[hydrogen]] and [[sodium hydroxide|sodium]] (or [[potassium hydroxide|potassium]]) [[hydroxide]] at a commercially useful concentration (50% by weight). The [[mercury (element)|mercury]] is then recycled to the primary cell.

The mercury process is the least energy-efficient of the three main technologies ([[mercury (element)|mercury]], [[diaphragm]] and [[membrane]]) and there are also concerns about [[mercury (element)|mercury]] [[Pollution|emissions]].

It is estimated that there are still around 100 mercury-cell plants operating worldwide. In [[Japan]], mercury-based chloralkali production was virtually phased out by 1987 (except for the last two [[potassium chloride]] units shut down in 2003). In the [[United States]], there will be only five [[mercury (element)|mercury]] plants remaining in operation by the end of 2008. In [[Europe]], [[mercury (element)|mercury]] cells accounted for 43% of capacity in 2006 and Western European producers have committed to closing or converting all remaining chloralkali [[mercury (element)|mercury]] plants by 2020.<ref name="unep">{{cite web|url=http://www.chem.unep.ch/Mercury/Pretoria-proceedings-finalPDFwriter.pdf|title=Regional Awareness-raising Workshop on Mercury Pollution|publisher=UNEP|accessdate=2007-10-28}}</ref>

*'''Diaphragm cell electrolysis '''
In diaphragm cell [[electrolysis]], an [[asbestos]] (or polymer-fiber) [[diaphragm]] separates a [[cathode]] and an [[anode]], preventing the chlorine forming at the [[anode]] from re-mixing with the [[sodium hydroxide]] and the [[hydrogen]] formed at the [[cathode]].<ref name="Euro Chlor3">{{cite web|url=http://www.eurochlor.org/animations/diaphragm-cell.asp|title=Diaphragm cell|publisher=Euro Chlor|accessdate=2007-08-15}}</ref> This technology was also developed at the end of the nineteenth century. There are several variants of this process: the Le Sueur cell (1893), the Hargreaves-Bird cell (1901), the Gibbs cell (1908), and the Townsend cell (1904).<ref name="saltman">{{cite web|url=http://www.saltsense.co.uk/hist-chem12.htm|title=The Electrolysis of Brine|publisher=Salt Manufacturers' Association|accessdate=2007-03-17}}</ref><ref name="kiefer">{{cite web|url=http://pubs.acs.org/subscribe/journals/tcaw/11/i04/html/04chemistry.html|title=When the Industry Charged Ahead|publisher=Chemistry Chronicles|first=David M.|last=Kiefer|accessdate=2007-03-17}}</ref> The cells vary in construction and placement of the [[diaphragm]], with some having the diaphragm in direct contact with the [[cathode]].

The [[salt]] [[solution]] ([[brine]]) is continuously fed to the anode compartment and flows through the [[diaphragm]] to the [[cathode]] compartment, where the [[caustic]] [[alkali]] is produced and the [[brine]] is partially depleted.

As a result, [[diaphragm]] methods produce [[alkali]] that is quite dilute (about 12%) and of lower purity than do mercury cell methods. But diaphragm cells are not burdened with the problem of preventing [[mercury (element)|mercury]] discharge into the environment. They also operate at a lower [[voltage]], resulting in an energy savings over the mercury cell method<ref name="kiefer"/>, but large amounts of [[steam]] are required if the [[caustic]] has to be [[evaporated]] to the commercial concentration of 50%.

*'''Membrane cell electrolysis '''
Development of this technology began in the 1970s. The [[electrolysis]] cell is divided into two "rooms" by a [[cation]] [[permeable]] [[membrane]] acting as an [[ion exchange|ion exchanger]]. Saturated [[sodium chloride|sodium]] (or [[potassium chloride|potassium]]) chloride [[solution]] is passed through the [[anode]] compartment, leaving at a lower [[concentration]].<ref name="Euro Chlor4">{{cite web|url=http://www.eurochlor.org/animations/membrane-cell.asp|title=Membrane cell|publisher=Euro Chlor|accessdate=2007-08-15}}</ref> [[sodium hydroxide|Sodium]] (or [[potassium hydroxide|potassium]]) [[hydroxide]] [[solution]] is circulated through the [[cathode]] compartment, exiting at a higher [[concentration]]. A portion of the [[concentration|concentrated]] [[sodium hydroxide]] [[solution]] leaving the cell is diverted as product, while the remainder is [[Concentration|diluted]] with [[deionized water]] and passed through the [[electrolysis]] apparatus again.

This method is more efficient than the [[diaphragm]] cell and produces very pure [[sodium hydroxide|sodium]] (or [[potassium hydroxide|potassium]]) [[hydroxide]] at about 32% [[concentration]], but requires very pure [[brine]].

*'''Other electrolytic processes '''
Although a much lower production scale is involved, electrolytic [[diaphragm]] and [[membrane]] technologies are also used industrially to recover [[chlorine]] from [[hydrochloric acid]] solutions, producing [[hydrogen]] (but no [[caustic]] [[alkali]]) as a co-product.

Furthermore, [[electrolysis]] of fused chloride salts ([[Downs Cell|Downs process]]) also enables chlorine to be produced, in this case as a by-product of the manufacture of [[metallic]] [[sodium]] or [[magnesium]].

=== Other methods ===
Before electrolytic methods were used for chlorine production, the direct [[oxidation]] of [[hydrogen chloride]] with [[oxygen]] or [[air]] was exercised in the [[Deacon process]]:

:4 HCl + O<sub>2</sub> → 2 Cl<sub>2</sub> + 2 H<sub>2</sub>O

This reaction is accomplished with the use of [[copper(II) chloride]] (CuCl<sub>2</sub>) as a [[catalyst]] and is performed at high temperature (about 400 °C). The amount of extracted chlorine is approximately 80%. Due to the extremely [[corrosive]] reaction mixture, industrial use of this method is difficult and several pilot trials failed in the past. Nevertheless, recent developments are promising.  Recently [[Sumitomo]] patented a catalyst for the Deacon process using [[ruthenium(IV) oxide]] (RuO<sub>2</sub>).<ref>''J. Catal.'' 255, 29 (2008)</ref>

Another earlier process to produce chlorine was to heat brine with [[acid]] and [[manganese dioxide]].

:2 NaCl + 2H<sub>2</sub>SO<sub>4</sub> + MnO<sub>2</sub> → Na<sub>2</sub>SO<sub>4</sub> + MnSO<sub>4</sub> + 2 H<sub>2</sub>O + Cl<sub>2</sub>

Using this process, chemist [[Carl Wilhelm Scheele]] was the first to isolate chlorine in a laboratory. The [[manganese]] can be recovered by the [[Weldon process]].<ref name="lenn1">{{cite web|url=http://www.lenntech.com/Chemistry/chlorine-industry.htm|title=The Chlorine Industry|publisher=Lenntech Water treatment & air purification Holding B.V., Rotterdamseweg 402 M, 2629 HH Delft, The Netherlands|accessdate=2007-03-17}}</ref> 

Small amounts of chlorine gas can be made in the laboratory by putting concentrated [[hydrochloric acid]] in a flask with a side arm and rubber tubing attached. [[Manganese dioxide]] is then added and the flask stoppered. The reaction is not greatly exothermic. As chlorine is denser than air, it can be easily collected by placing the tube inside a flask where it will displace the air. Once full, the collecting flask can be stoppered.

Another method for producing small amounts of chlorine gas in a lab is by adding concentrated [[hydrochloric acid]] (typically about 5M) to [[sodium hypochlorite]] or [[sodium chlorate]] solution.

=== Industrial production ===
Large-scale production of chlorine involves several steps and many pieces of equipment.  The description below is typical of a membrane plant.  The plant also simultaneously produces [[sodium hydroxide|sodium hydroxide (caustic soda)]] and [[hydrogen]] gas.  A typical plant consists of [[brine]] production/treatment, cell operations, chlorine cooling & drying, chlorine compression & liquefaction, liquid chlorine storage & loading, [[caustic]] handling, evaporation, storage & loading and [[hydrogen]] handling.

*'''Brine '''
Key to the production of chlorine is the operation of the [[brine]] saturation/treatment system.  Maintaining a properly saturated solution with the correct purity is vital, especially for [[membrane]] cells.  Many [[plants]] have a salt pile which is sprayed with recycled [[brine]].  Others have slurry tanks that are fed raw [[salt]].  

The raw [[brine]] is partially or totally treated with [[sodium hydroxide]], [[sodium carbonate]] and a [[flocculant]] to reduce [[calcium]], [[magnesium]] and other [[impurities]].  The [[brine]] proceeds to a large clarifier or a filter where the [[impurities]] are removed.  The total [[brine]] is additionally filtered before entering [[ion exchange|ion exchangers]] to further remove [[impurities]].  At several points in this process, the [[brine]] is tested for [[hardness]] and [[strength]].  

After the [[ion exchange|ion exchangers]], the [[brine]] is considered [[pure]], and is transferred to storage tanks to be pumped into the cell room.  [[Brine]], fed to the cell line, is heated to the correct [[temperature]] to control exit [[brine]] temperatures according to the [[electrical load]].  [[Brine]] exiting the cell room must be treated to remove residual chlorine and control [[pH]] levels before being returned to the [[saturation]] stage.  This can be accomplished via dechlorination towers with [[acid]] and [[sodium bisulfite]] addition.  Failure to remove chlorine can result in damage to the cells. [[Brine]] should be monitored for accumulation of both[[chlorate|chlorate anions]] and [[sulfate|sulfate anions]], and either have a treatment system in place, or purging of the brine loop to maintain safe levels, since [[chlorate|chlorate anions]] can diffuse through the [[membranes]] and contaminate the [[caustic]], while [[sulfate|sulfate anions]] can damage the [[anode]] surface coating.

*'''Cell room '''
The building that houses the many electrolytic cells is usually called a cell room or cell house, although some plants are built outdoors. This building contains support structures for the cells, connections for supplying [[electrical power]] to the cells and piping for the fluids. Monitoring and control of the temperatures of the feed [[caustic]] and [[brine]] is done to control exit temperatures.  Also monitored are the [[voltage|voltages]] of each cell which vary with the [[electrical load]] on the cell room that is used to control the rate of production.  Monitoring and control of the pressures in the chlorine and [[hydrogen]] headers is also done via [[pressure relief valve|pressure control valves]].  

[[direct current|Direct current]] is supplied via a [[rectifier|rectified]] power source.  Plant load is controlled by varying the [[current]] to the cells. As the [[current]] is increased, flow rates for [[brine]] and [[caustic]] and [[deionized water]] are increased, while lowering the feed temperatures.

*'''Cooling and drying '''
Chlorine gas exiting the cell line must be cooled and dried since the exit gas can be over 80°C and contains moisture that allows chlorine gas to be [[corrosive]] to [[iron]] [[piping]].  Cooling the gas allows for a large amount of  moisture from the brine to [[condensation|condense]] out of the gas stream.  Cooling also improves the [[efficiency]] of both the [[compressor|compression]] and the [[liquefaction]] stage that follows. Chlorine exiting is ideally between 18°C and 25°C.  After cooling the gas stream passes through a series of towers with counter flowing [[sulfuric acid]].  These towers progressively remove any remaining [[moisture]] from the chlorine gas.  After exiting the drying towers the chlorine is filtered to remove any remaining [[sulfuric acid]].

*'''Compression and liquefaction '''
Several methods of compression may be used: [[liquid ring]], [[reciprocating compressor|reciprocating]], or [[centrifugal compressor|centrifugal]].  The chlorine gas is compressed at this stage and may be further cooled by inter- and after-coolers.  After compression it flows to the liquefiers, where it is cooled enough to liquefy.  Non condensible gases and remaining chlorine gas are vented off as part of the pressure control of the liquefaction systems.  These gases are routed to a gas scrubber, producing [[sodium hypochlorite]], or used in the production of [[hydrochloric acid]] (by combustion with [[hydrogen]]) or [[ethylene dichloride]] (by reaction with [[ethylene]]).

*'''Storage and loading '''
Liquid chlorine is typically [[gravity|gravity-fed]] to storage tanks. It can be loaded into rail or road tankers via pumps or padded with compressed dry gas.

*'''Caustic handling, evaporation, storage and loading '''
[[Caustic]], fed to the cell room flows in a loop that is simultaneously bled off to storage with a part [[diluted]] with [[deionized water]] and returned to the cell line for strengthening within the cells.  The [[caustic]] exiting the cell line must be monitored for strength, to maintain safe concentrations.  Too strong or too weak a [[solution]] may damage the membranes.  Membrane cells typically produce caustic in the range of 30% to 33% by weight.  The feed [[caustic]] flow is heated at low [[electrical load|electrical loads]] to control its exit temperature.  Higher loads require the [[caustic]] to be cooled, to maintain correct exit temperatures. The [[caustic]] exiting to storage is pulled from a storage tank and may be diluted for sale to customers who require weak [[caustic]] or for use on site.  Another stream may be pumped into a [[multiple effect evaporator]] set to produce commercial 50% [[caustic]].  Rail cars and tanker trucks are loaded at loading stations via pumps.

*'''Hydrogen handling '''
[[Hydrogen]] produced may be vented unprocessed directly to the [[atmosphere]] or cooled, compressed and dried for use in other processes on site or sold to a customer via pipeline, cylinders or trucks.  Some possible uses include the manufacture of [[hydrochloric acid]] or [[hydrogen peroxide]], as well as [[desulfurization]] of [[petroleum|petroleum oils]], or use as a [[fuel]] in [[boiler|boilers]] or [[fuel cell|fuel cells]].
In [[Porsgrunn]] the byproduct is used  for the [[hydrogen station|hydrogen fueling station]] at [[hynor]].

*'''Energy consumption '''

Production of chlorine is extremely energy intensive.<ref name="European Commission1">{{cite web|url=http://www.jrc.es/pub/english.cgi/d733217/05%20Reference%20Document%20on%20Best%20Available%20Techniques%20in%20the%20Chlor-Alkali%20Manufacturing%20industry%20%28adopted%20Dec%202001%29%20-%205.2%20Mb|title=Integrated Pollution Prevention and Control (IPPC) - Reference Document on Best Available Techniques in the Chlor-Alkali Manufacturing Industry|publisher=European Commission|accessdate=2007-09-02}}</ref> Energy consumption per unit weight of product is not far below that for iron and steel manufacture<ref name="European Commission2">{{cite web|url=http://www.jrc.es/pub/english.cgi/d733208/02%20Best%20Available%20Techniques%20Reference%20Document%20on%20the%20Production%20of%20Iron%20and%20Steel%20%28adopted%20Dec%202001%29%20-%209.4Mb|title=Integrated Pollution Prevention and Control (IPPC) - Best Available Techniques Reference Document on the Production of Iron and Steel|publisher=European Commission|accessdate=2007-09-02}}</ref> and greater than for the production of glass<ref name="European Commission3">{{cite web|url=http://www.jrc.es/pub/english.cgi/d733226/08%20Reference%20Document%20on%20Best%20Available%20Techniques%20in%20the%20Glass%20Manufacturing%20Industry%20%28adopted%20Dec%202001%29%20-%202.7%20Mb|title=Integrated Pollution Prevention and Control (IPPC) - Reference Document on Best Available Techniques in the Glass Manufacturing Industry|publisher=European Commission|accessdate=2007-09-02}}</ref> or cement.<ref name="European Commission4">{{cite web|url=http://www.jrc.es/pub/english.cgi/d733211/03%20Reference%20Document%20on%20Best%20Available%20Techniques%20in%20the%20Cement%20and%20Lime%20Manufacturing%20Industries%20%28adopted%20Dec%202001%29%20-%201.3%20Mb|title=Integrated Pollution Prevention and Control (IPPC) - Reference Document on Best Available Techniques in the Cement and Lime Manufacturing Industries|publisher=European Commission|accessdate=2007-09-02}}</ref>

Since [[electricity]] is an indispensable raw material for the production of chlorine, the energy consumption corresponding to the [[electrochemistry|electrochemical]] reaction cannot be reduced. Energy savings arise primarily through applying more efficient technologies and reducing ancillary energy use.

== Compounds ==
:''See also [[:Category:Chlorine compounds|Chlorine compounds]]''
For general references to the chloride ion (Cl<sup>−</sup>), including references to specific chlorides, see [[chloride]]. For other chlorine compounds see [[chlorate]] (ClO<sub>3</sub><sup>−</sup>), [[chlorite]] (ClO<sub>2</sub><sup>−</sup>), [[hypochlorite]](ClO<sup>−</sup>), and [[perchlorate]](ClO<sub>4</sub><sup>−</sup>), and [[chloramine]] (NH<sub>2</sub>Cl).<ref name="Euro Chlor6">{{cite web|url=http://www.eurochlor.org/index.asp?page=678|title=Chlorine compounds of the month|publisher=Euro Chlor|accessdate=2007-08-29}}</ref>

Other chlorine-containing compounds include:

* Fluorides: [[chlorine monofluoride]] (ClF), [[chlorine trifluoride]] (ClF<sub>3</sub>), [[chlorine pentafluoride]] (ClF<sub>5</sub>)
* Oxides: [[chlorine dioxide]] (ClO<sub>2</sub>), [[dichlorine monoxide]] (Cl<sub>2</sub>O), [[dichlorine heptoxide]] (Cl<sub>2</sub>O<sub>7</sub>)
* Acids: [[hydrochloric acid]] (HCl), [[chloric acid]] (HClO<sub>3</sub>), and [[perchloric acid]] (HClO<sub>4</sub>)

=== Oxidation states ===
{| class="wikitable" align="right"
! Oxidation<br>state !! Name !! Formula !! width="200" | Example compounds
|-
| align="center" | −1 || [[chloride]]s || align="center" | Cl<sup>−</sup> || ionic chlorides, organic chlorides, [[hydrochloric acid]]
|-
| align="center" | 0 || chlorine || align="center" | Cl<sub>2</sub> || elemental chlorine
|-
| align="center" | +1 || [[hypochlorite]]s || align="center" | ClO<sup>−</sup> || [[sodium hypochlorite]], [[calcium hypochlorite]]
|-
| align="center" | +3 || [[chlorite]]s || align="center" | ClO<sub>2</sub><sup>−</sup> || [[sodium chlorite]]
|-
| align="center" | +5 || [[chlorate]]s || align="center" | ClO<sub>3</sub><sup>−</sup> || [[sodium chlorate]], [[potassium chlorate]], [[chloric acid]]
|-
| align="center" | +7 || [[perchlorate]]s || align="center" | ClO<sub>4</sub><sup>−</sup> || [[potassium perchlorate]], [[perchloric acid]],[[magnesium perchlorate]]<br> organic perchlorates, [[ammonium perchlorate]]
|}

Chlorine exists in all odd numbered [[oxidation states]] from −1 to +7, as well as the elemental state of zero. Progressing through the states, [[hydrochloric acid]] can be oxidized using [[manganese dioxide]], or [[hydrogen chloride]] gas oxidized catalytically by air to form elemental chlorine gas. The solubility of chlorine in water is increased if the water contains dissolved alkali hydroxide. This is due to [[disproportionation]]:

:Cl<sub>2</sub> + 2OH<sup>−</sup> → Cl<sup>−</sup> + ClO<sup>−</sup> + H<sub>2</sub>O

In hot concentrated alkali solution disproportionation continues:

:2ClO<sup>−</sup> → Cl<sup>−</sup> + ClO<sub>2</sub><sup>−</sup>
:ClO<sup>−</sup> + ClO<sub>2</sub><sup>−</sup> → Cl<sup>−</sup> + ClO<sub>3</sub><sup>−</sup>

[[Sodium chlorate]] and [[potassium chlorate]] can be crystallized from solutions formed by the above reactions. If their crystals are heated, they undergo the final disproportionation step.

:4ClO<sub>3</sub><sup>−</sup> → Cl<sup>−</sup> + 3ClO<sub>4</sub><sup>−</sup>

This same progression from chloride to perchlorate can be accomplished by [[electrolysis]]. The anode reaction progression is:<ref name="c_w">Cotton, F. Albert and Wilkinson, Geoffrey, ''Advanced Inorganic Chemistry'' 2nd ed. John Wiley & sons, p568</ref>

:{| class="wikitable" align="left"
! Reaction !! Electrode<br>potential
|-
| align="center" | Cl<sup>−</sup> + 2OH<sup>−</sup> → ClO<sup>−</sup> + H<sub>2</sub>O + 2e<sup>−</sup> || +0.89 volts
|-
| align="center" | ClO<sup>−</sup> + 2OH<sup>−</sup> → ClO<sub>2</sub><sup>−</sup> + H<sub>2</sub>O + 2e<sup>−</sup> || +0.67 volts
|-
| align="center" | ClO<sub>2</sub><sup>−</sup> + 2OH<sup>−</sup> → ClO<sub>3</sub><sup>−</sup> + H<sub>2</sub>O + 2e<sup>−</sup> || +0.33 volts
|-
| align="center" | ClO<sub>3</sub><sup>−</sup> + 2OH<sup>−</sup> → ClO<sub>4</sub><sup>−</sup> + H<sub>2</sub>O + 2e<sup>−</sup> || +0.35 volts
|}
<br clear="all">
Each step is accompanied at the cathode by

:2H<sub>2</sub>O + 2e<sup>−</sup> → 2OH<sup>−</sup> + H<sub>2</sub> &nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp;&nbsp; −0.83 volts

== Applications and uses ==
=== Production of industrial and consumer products ===
Chlorine's principal applications are in the production of a wide range of industrial and consumer products.<ref name="Euro Chlor5">{{cite web|url=http://www.eurochlor.org/uses|title=Uses|publisher=Euro Chlor|accessdate=2007-08-20}}</ref> <ref name="Chlorine Tree">{{cite web|url=http://www.chlorinetree.org|title=Chlorine Tree|publisher=Chlorine Tree|accessdate=2007-08-20}}</ref> For example, it is used in making plastics, solvents for dry cleaning and metal degreasing, textiles, agrochemicals and pharmaceuticals, insecticides, dyestuffs, etc.
=== Purification and disinfection ===
Chlorine is an important chemical for [[water purification]], in [[disinfectant]]s, and in [[bleach]]. It is used (in the form of [[hypochlorous acid]]) to kill [[bacterium|bacteria]] and other microbes in [[drinking water]] supplies and public [[swimming pool]]s. However, in most private swimming pools chlorine itself is not used, but rather [[sodium hypochlorite]] (household bleach), formed from chlorine and [[sodium hydroxide]], or solid tablets of chlorinated isocyanurates. Even small water supplies are now routinely chlorinated.<ref name="losal">{{cite web|url=http://periodic.lanl.gov/elements/17.html|title=Chlorine|publisher=Los Alamos National Laboratory|accessdate=2007-03-17}}</ref> (''See also'' [[chlorination]])

=== Chemistry ===
Elemental chlorine is an [[oxidizer]]. It undergoes halogen substitution reactions with lower halide salts. For example, chlorine gas bubbled through a solution of bromide or iodide anions oxidizes them to bromine and iodine respectively.

Like the other halogens, chlorine participates in [[free-radical substitution reaction]]s with hydrogen-containing organic compounds. This reaction is often – but not invariably – non-regioselective, and hence may result in a mixture of isomeric products. It is often difficult to control the degree of substitution as well, so multiple substitutions are common. If the different reaction products are easily separated, e.g. by distillation, substitutive free-radical chlorination (in some cases accompanied by concurrent thermal dehydrochlorination) may be a useful synthetic route. Industrial examples of this are the production of [[methyl chloride]], [[methylene chloride]], [[chloroform]] and [[carbon tetrachloride]] from methane, [[allyl chloride]] from propylene, and [[trichloroethylene]] and [[tetrachloroethylene]] from [[ethylene dichloride|1,2-dichloroethane]].

Like the other halides, chlorine  undergoes electrophilic additions reactions, most notably, the chlorination of alkenes and aromatic compounds with a Lewis acid catalyst. Organic chlorine compounds tend to be less reactive in nucleophilic substitution reactions than the corresponding bromine or iodine derivatives, but they tend to be cheaper. They may be activated for reaction by substituting with a tosylate group, or by the use of a catalytic amount of [[sodium iodide]].

Chlorine is used extensively in [[organic chemistry|organic]] and [[inorganic chemistry]] as an oxidizing agent and in [[substitution (chemistry)|substitution]] reactions because chlorine often imparts many desired properties to an [[organic compound]], due to its electronegativity.

Chlorine compounds are used as intermediates in the production of a number of important commercial products that do not contain chlorine. Examples are: [[polycarbonate]]s, [[polyurethane]]s, [[silicone]]s, [[polytetrafluoroethylene]], [[carboxymethyl cellulose]] and [[propylene oxide]].

=== Use as a weapon ===
*'''World War I '''
{{Main|Poison gas in World War I}}

Chlorine gas, also known as '''bertholite''', was first [[chemical warfare|used as a weapon]] in [[World War I]] by Germany on [[April 22]], [[1915]] in the [[Second Battle of Ypres]]. As described by the soldiers it had a distinctive smell of a mixture between pepper and pineapple. It also tasted metallic and stung the back of the throat and chest. Chlorine can react with water in the mucosa of the lungs to form [[hydrochloric acid]], an irritant which can be lethal. The damage done by chlorine gas can be prevented by a gas mask which makes the deaths by chlorine gas much lower then those of other chemical weapons. It was pioneered by a German scientist later to be a Nobel laureate, [[Fritz Haber]] of the Kaiser Wilhelm Institute in Berlin, in collaboration with the German chemical conglomerate [[IG Farben]], who developed methods for discharging chlorine gas against an [[trench|entrenched]] enemy. It is alleged that Haber's role in the use of chlorine as a deadly weapon drove his wife, [[Clara Immerwahr]], to suicide. After its first use, chlorine was utilized by both sides as a chemical weapon, but it was soon replaced by the more deadly gases [[phosgene]] and [[mustard gas]].<ref name="First World War">{{cite web|url=http://www.firstworldwar.com/weaponry/gas.htm|title=Weapons of War: Poison Gas|publisher=First World War.com|accessdate=2007-08-12}}</ref>

*'''Iraq War '''
{{main|2007 chlorine bombings in Iraq}}

Chlorine gas has also been used by insurgents in the [[Iraq War]] as [[Chlorine bomb|a chemical weapon]] to terrorize the local population and coalition forces. On [[March 17]], [[2007]], for example, three chlorine filled trucks were detonated in the Anbar province killing 2 and sickening over 350.<ref name="cnnchlorine">{{cite news|url=http://www.cnn.com/2007/WORLD/meast/03/17/iraq.main/index.html|last=Mahdi|first=Basim|publisher=CNN|date=2007-03-17|accessdate=2007-03-17|title= Iraq gas attack makes hundreds ill}}</ref> Other chlorine bomb attacks resulted in higher death tolls, with more than 30 deaths on two separate occasions.<ref name="Chlorine bomb BBC">{{cite news|url= http://news.bbc.co.uk/2/hi/middle_east/6660585.stm|last = |first= |publisher = BBC News|date=2007-05-17|accessdate = 2007-05-17|title = 'Chlorine bomb' hits Iraq village}}</ref> Most of the deaths were caused by the force of the explosions rather than the effects of chlorine, since the toxic gas is readily dispersed and diluted in the atmosphere by the blast. The Iraqi authorities have tightened up security for chlorine, which is essential for providing safe drinking water for the population.

=== Other uses ===
Chlorine is used in the manufacture of numerous organic chlorine compounds, the most significant of which in terms of production volume are [[1,2-dichloroethane]] and [[vinyl chloride]], intermediates in the production of [[PVC]].  Other particularly important organochlorines are [[methyl chloride]], [[methylene chloride]], [[chloroform]], [[vinylidene chloride]], [[trichloroethylene]], [[perchloroethylene]], [[allyl chloride]], [[epichlorohydrin]], [[chlorobenzene]], dichlorobenzenes and [[trichlorobenzene]]s.

Chlorine is also used in the production of [[chlorate]]s and in [[bromine]] extraction.

== History ==
Chlorine was discovered in 1774 by Swedish chemist [[Carl Wilhelm Scheele]], who called it ''dephlogisticated marine acid'' (see [[phlogiston theory]]) and mistakenly thought it contained [[oxygen]].  Scheele isolated chlorine by reacting [[Manganese dioxide|MnO<sub>2</sub>]] with [[hydrochloric acid|HCl]]. 

:4 HCl + MnO<sub>2</sub> →  MnCl<sub>2</sub> + 2 H<sub>2</sub>O + Cl<sub>2</sub>

Scheele observed several of the properties of chlorine. The bleaching effect on [[litmus]] and the deadly effect on insects additional to the yellow green colour and the smell similar to [[aqua regia]].
Chlorine was given its current name in 1810 by [[Sir Humphry Davy]], who insisted that it was in fact an element.

== Safety ==
[[Image:Skull and crossbones.svg|left|40px]]
Chlorine is a toxic gas that irritates the respiratory system. Because it is heavier than air, it tends to accumulate at the bottom of poorly ventilated spaces. Chlorine gas is a strong oxidizer, which may react with flammable materials.<ref name="msds-cl">"[http://www.westlake.com/datasheets/MSDS_Chlorine.pdf Chlorine]." ''[[MSDS]].'' Issued on [[October 23]], [[1997]]; Revised on [[November 1]], [[1999]]; Retrieved on [[September 8]], [[2007]].</ref>

Chlorine is detectable in concentrations of as low as 3.5 to 4 ppm.  About 1000 ppm can be fatal after a few deep breaths of the gas<ref name="webelement"/>.  Breathing lower concentrations can aggravate the respiratory system, and exposure to the gas can irritate the eyes.

Never use [[ABC Dry Chemical]] to fight a chlorine fire, the resulting chemical reaction with the ammonium phosphate will release toxic gases and/or result in an explosion. Water fogs or [[Compressed Air Foam System|CAFS]] should be used to extinguish the material.<ref name="msds-cl"/>

The number of people allergic to chlorine is very small. People who are allergic to chlorine cannot drink tap water, bathe in tap water or swim in pools. Dechlorinating [[bath salts]] are used to neutralize the chlorine in bath water. Otherwise, fresh water is boiled and cooled.

==Chlorine cracking==
[[Image:Chlorine attack1.jpg|thumb|right|200px|Chlorine attack of an acetal resin plumbing joint.]]
The element is widely used for purifying water owing to its powerful oxidising properties, especially potable water supplies and water used in [[swimming pool]]s. However, some [[polymer]]s are sensitive to attack, including [[acetal resin]] and polybutene. Both materials were used in hot and cold water domestic supplies, and [[stress corrosion cracking]] cause widespread failures in the USA in the 1980's and 90's. One example shows an acetal joint in a water supply system, which when it fractured, caused substantial physical damage to computers in the labs below the supply. The cracks started at [[injection moulding]] defects in the joint and grew slowly until finally triggered. The fracture surface shows iron and calcium salts which were deposited in the leaking joint from the water supply before failure

== See also ==
* [[Chloride]]
* [[Polymer degradation]]

== References ==
{{Reflist|2}}

== External links ==
{{Commons|Chlorine}}
{{wiktionary|chlorine}}
* [http://www.chlorineinstitute.org/ Chlorine Institute] - Trade association and lobby group representing the interests of the chlorine industry
* [http://www.eurochlor.org/ Chlorine Online] - Chlorine Online is an information resource produced by Eurochlor - the business association of the European chlor-alkali industry
* [http://www.compchemwiki.org/index.php?title=Cl2 Computational Chemistry Wiki]
* [http://www.oceana.org/chlorine Chlorine Production Using Mercury, Environmental Considerations and Alternatives]
* [http://www.npi.gov.au/database/substance-info/profiles/20.html National Pollutant Inventory - Chlorine]
* [http://www.cdc.gov/niosh/topics/chlorine/ National Institute for Occupational Safety and Health - Chlorine Page]

{{diatomicelements}}

{{Compact periodic table}}

[[Category:Chemical elements]]
[[Category:Halogens]]
[[Category:Chlorine|*]]
[[Category:Hazardous air pollutants]]
[[Category:Occupational safety and health]]

{{Link FA|sk}}

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[[gl:Cloro]]
[[ko:염소 (원소)]]
[[hy:Քլոր]]
[[hi:क्लोरीन]]
[[hr:Klor]]
[[io:Kloro]]
[[id:Klor]]
[[is:Klór]]
[[it:Cloro]]
[[he:כלור]]
[[sw:Chlorini]]
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[[lt:Chloras]]
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[[hu:Klór]]
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[[ml:ക്ലോറിന്‍]]
[[mi:Hau māota]]
[[ms:Klorin]]
[[nl:Chloor]]
[[ja:塩素]]
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[[simple:Chlorine]]
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[[sl:Klor]]
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[[fi:Kloori]]
[[sv:Klor]]
[[ta:குளோரின்]]
[[th:คลอรีน]]
[[vi:Clo]]
[[tg:Хлор]]
[[tr:Klor]]
[[uk:Хлор]]
[[zh-yue:氯]]
[[bat-smg:Chluors]]
[[zh:氯]]

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