AskDefine | Define chlorine

Dictionary Definition

chlorine n : a common nonmetallic element belonging to the halogens; best known as a heavy yellow irritating toxic gas; used to purify water and as a bleaching agent and disinfectant; occurs naturally only as a salt (as in sea water) [syn: Cl, atomic number 17]

User Contributed Dictionary




  1. a toxic, green, gaseous chemical element (symbol Cl) with an atomic number of 17.


External links

For etymology and more information refer to: (A lot of the translations were taken from that site with permission from the author)


Extensive Definition

Chlorine (, from the 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 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 humans. In its common elemental form (Cl2 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 and is poisonous. Chlorine is a powerful oxidant and is used in bleaching and disinfectants. As a common disinfectant, chlorine compounds are used in swimming pools to keep them clean and sanitary. In the upper atmosphere, chlorine-containing molecules have been implicated in the destruction of the ozone layer.



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 (H2). 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+ (aq) + 2 e− → H2 (g)
Anode: 2 Cl− (aq) → Cl2 (g) + 2 e−
Overall process: 2 NaCl (or KCl) + 2 H2O → Cl2 + H2 + 2 NaOH (or KOH)
  • Mercury cell electrolysis
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. The "rocking" cells used have been improved over the years. Today, in the "primary cell", titanium anodes (formerly graphite ones) are placed in a sodium (or potassium) chloride solution flowing over a liquid 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 cathode forming an amalgam. This flows continuously into a separate reactor ("denuder" or "secondary cell"), where it is usually converted back to mercury by reaction with water, producing hydrogen and sodium (or potassium) hydroxide at a commercially useful concentration (50% by weight). The mercury is then recycled to the primary cell.
The mercury process is the least energy-efficient of the three main technologies (mercury, diaphragm and membrane) and there are also concerns about mercury 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 plants remaining in operation by the end of 2008. In Europe, mercury cells accounted for 43% of capacity in 2006 and Western European producers have committed to closing or converting all remaining chloralkali mercury plants by 2020.
  • 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. 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). 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 discharge into the environment. They also operate at a lower voltage, resulting in an energy savings over the mercury cell method Sodium (or potassium) hydroxide solution is circulated through the cathode compartment, exiting at a higher concentration. A portion of the concentrated sodium hydroxide solution leaving the cell is diverted as product, while the remainder is 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 (or 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 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 + O2 → 2 Cl2 + 2 H2O
This reaction is accomplished with the use of copper(II) chloride (CuCl2) 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 (RuO2).
Another earlier process to produce chlorine was to heat brine with acid and manganese dioxide.
2 NaCl + 2H2SO4 + MnO2 → Na2SO4 + MnSO4 + 2 H2O + Cl2
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.
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 (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 exchangers to further remove impurities. At several points in this process, the brine is tested for hardness and strength.
After the 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 bothchlorate anions and sulfate anions, and either have a treatment system in place, or purging of the brine loop to maintain safe levels, since chlorate anions can diffuse through the membranes and contaminate the caustic, while 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 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 control valves.
Direct current is supplied via a 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 condense out of the gas stream. Cooling also improves the efficiency of both the 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, or 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-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 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 oils, or use as a fuel in boilers or fuel cells. In Porsgrunn the byproduct is used for the hydrogen fueling station at hynor.
  • Energy consumption
Production of chlorine is extremely energy intensive. Energy consumption per unit weight of product is not far below that for iron and steel manufacture and greater than for the production of glass or cement.
Since electricity is an indispensable raw material for the production of chlorine, the energy consumption corresponding to the electrochemical reaction cannot be reduced. Energy savings arise primarily through applying more efficient technologies and reducing ancillary energy use.


See also Chlorine compounds
For general references to the chloride ion (Cl−), including references to specific chlorides, see chloride. For other chlorine compounds see chlorate (ClO3−), chlorite (ClO2−), hypochlorite(ClO−), and perchlorate(ClO4−), and chloramine (NH2Cl).
Other chlorine-containing compounds include:

Oxidation states

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:
Cl2 + 2OH− → Cl− + ClO− + H2O
In hot concentrated alkali solution disproportionation continues:
2ClO− → Cl− + ClO2−
ClO− + ClO2− → Cl− + ClO3−
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.
4ClO3− → Cl− + 3ClO4−
This same progression from chloride to perchlorate can be accomplished by electrolysis. The anode reaction progression is:
Each step is accompanied at the cathode by
2H2O + 2e− → 2OH− + H2          −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. 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 disinfectants, and in bleach. It is used (in the form of hypochlorous acid) to kill bacteria and other microbes in drinking water supplies and public swimming pools. 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. (See also chlorination)


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 reactions 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 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 and inorganic chemistry as an oxidizing agent and in 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: polycarbonates, polyurethanes, silicones, polytetrafluoroethylene, carboxymethyl cellulose and propylene oxide.

Use as a weapon

  • World War I
Chlorine gas, also known as bertholite, was first 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 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.
  • Iraq War
Chlorine gas has also been used by insurgents in the Iraq War as 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. Other chlorine bomb attacks resulted in higher death tolls, with more than 30 deaths on two separate occasions. 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 trichlorobenzenes.
Chlorine is also used in the production of chlorates and in bromine extraction.


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 MnO2 with HCl.
4 HCl + MnO2 → MnCl2 + 2 H2O + Cl2
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.


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.
Chlorine is detectable in concentrations of as low as 1 ppm. Coughing and vomiting may occur at 30 ppm and lung damage at 60 ppm. About 1000 ppm can be fatal after a few deep breaths of the gas. 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 CAFS should be used to extinguish the material.
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

The element is widely used for purifying water owing to its powerful oxidising properties, especially potable water supplies and water used in swimming pools. However, some polymers 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


External links

chlorine in Afrikaans: Chloor
chlorine in Arabic: كلور
chlorine in Asturian: Cloru
chlorine in Azerbaijani: Xlor
chlorine in Bengali: ক্লোরিন
chlorine in Belarusian: Хлор
chlorine in Bosnian: Hlor
chlorine in Bulgarian: Хлор
chlorine in Catalan: Clor
chlorine in Chuvash: Хлор
chlorine in Czech: Chlór
chlorine in Corsican: Cloru
chlorine in Welsh: Clorin
chlorine in Danish: Klor
chlorine in German: Chlor
chlorine in Estonian: Kloor
chlorine in Modern Greek (1453-): Χλώριο
chlorine in Spanish: Cloro
chlorine in Esperanto: Kloro
chlorine in Basque: Kloro
chlorine in Persian: کلر
chlorine in French: Chlore
chlorine in Friulian: Clôr
chlorine in Irish: Clóirín
chlorine in Manx: Cloreen
chlorine in Galician: Cloro
chlorine in Korean: 염소 (원소)
chlorine in Armenian: Քլոր
chlorine in Hindi: क्लोरीन
chlorine in Croatian: Klor
chlorine in Ido: Kloro
chlorine in Indonesian: Klor
chlorine in Icelandic: Klór
chlorine in Italian: Cloro
chlorine in Hebrew: כלור
chlorine in Swahili (macrolanguage): Chlorini
chlorine in Haitian: Klò
chlorine in Latin: Chlorum
chlorine in Latvian: Hlors
chlorine in Luxembourgish: Chlor
chlorine in Lithuanian: Chloras
chlorine in Lingala: Koloki
chlorine in Lojban: kliru
chlorine in Hungarian: Klór
chlorine in Macedonian: Хлор
chlorine in Malayalam: ക്ലോറിന്‍
chlorine in Maori: Hau māota
chlorine in Malay (macrolanguage): Klorin
chlorine in Dutch: Chloor
chlorine in Japanese: 塩素
chlorine in Norwegian: Klor
chlorine in Norwegian Nynorsk: Klor
chlorine in Novial: Klore
chlorine in Occitan (post 1500): Clòr
chlorine in Uzbek: Xlor
chlorine in Low German: Chlor
chlorine in Polish: Chlor
chlorine in Portuguese: Cloro
chlorine in Romanian: Clor
chlorine in Quechua: Kluru
chlorine in Russian: Хлор
chlorine in Albanian: Klori
chlorine in Simple English: Chlorine
chlorine in Slovak: Chlór
chlorine in Slovenian: Klor
chlorine in Serbian: Хлор
chlorine in Serbo-Croatian: Hlor
chlorine in Saterfriesisch: Chlor
chlorine in Finnish: Kloori
chlorine in Swedish: Klor
chlorine in Tamil: குளோரின்
chlorine in Thai: คลอรีน
chlorine in Vietnamese: Clo
chlorine in Tajik: Хлор
chlorine in Turkish: Klor
chlorine in Ukrainian: Хлор
chlorine in Contenese: 氯
chlorine in Samogitian: Chluors
chlorine in Chinese: 氯
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