Chlorine . Chemical element with atomic number 17 located in the group of halogens (group VII A) of the periodic table of elements. Its symbol is Cl. Under normal conditions and in its pure state, it forms dichloro: a yellow-green toxic gas formed by diatomic molecules (Cl 2 ) about 2.5 times heavier than Air , with an unpleasant and poisonous odor. It is an abundant element in nature and it is an essential chemical element for many forms of life .


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  • 1 Historical review
  • 2 Properties
  • 3 Abundance
  • 4 Compounds and their applications
  • 5 Isotopes
  • 6 Applications and uses
  • 7 Use as a weapon
  • 8 Other uses
  • 9 Sources
  • 10 External links

Historical review

Chlorine (from the Greek χλωρος, meaning “greenish yellow”) was discovered in its diatomic form in 1774 by the Swede Carl Wilhelm Scheele , although he believed that it was a compound that contained oxygen. He obtained it from the following reaction:
2 NaCl + 2H 2 SO 4 + MnO 2 → Na 2 SO 4 + MnSO 4 + 2H 2 O + Cl 2

In 1810 the English chemist Humphry Davy proved that it is a physical element and gave it the name of chlorine because of its color. Chlorine gas was used in the First World War , being the first case of use of chemical weapons such as phosgene and mustard gas.

The first electrolytic process for the production of chlorine was patented in 1851 by Charles Watt in Great Britain . In 1868 , Henry Deacon produced chlorine from hydrochloric acid and oxygen at 400 ° C (750T), with copper chloride impregnated in pumice stone as a catalyst. Modern electrolytic cells can almost always be classified as belonging to the diaphragm and mercury type. Both produce caustic substances (NaOH or KOH), chlorine and hydrogen. The economic policy of the chlorine and alkali industry mainly includes marketing balanced or internal use of caustic and chlorine in the proportions in which they are obtained by the electrolytic cell process.


Chlorine present in nature is formed from isotopesstable of mass 35 and 37; Radioactive isotopes have been artificially prepared. Diatomic gas has a molecular weight of 70,906. The boiling point of liquid chlorine (yellow-gold in color) is -34.05 ° C at 760 mm Hg (101.325 kilopascals) and the melting point of solid chlorine is -100.98 ° C. The critical temperature is 144 ° C; the critical pressure is 76.1 atm (7.71 megapascals); the critical volume is 1,745 ml / g; and the density at the critical point is 0.573 g / ml. Thermodynamic properties include the heat of sublimation which is 7370 ± 10 cal / mol at 0K, the heat of vaporization, which is 4878 ± 4 cal / mol at -34.05 ° C, the heat of fusion of 1531 cal / mol, the capacity calorific value of 7.99 cal / mol at 1 atm (101.325 kilopascals) and 0 ° C, and 8.2 at 100 ° C.

Chlorine is one of the four closely related chemical elements that have been called halogens. The fluorine is chemically the most active; the iodine and bromine are less active. Chlorine replaces iodine and bromine in their salts. It takes part in substitution or addition reactions with both organic and inorganic materials. Dry chlorine is somewhat inert, but wet blends directly with most of the elements.


Chlorine is found in nature combined with other elements, mainly in the form of sodium chloride, NaCl, and also other minerals such as silvin, KCl, or carnalite, KMgCl 3 • 6H 2 O. It is the most abundant halogen in the sea ​​water with a concentration of about 18,000 ppm. In the earth’s crust it is present in less quantity, about 130 ppm. It is practically impossible to find it without combining with other elements, due to its high reactivity.

Compounds and their applications

Some metal chlorides are used as catalysts. For example, FeCl 2 , FeCl 3 , AlCl 3 .

  • Hydrochloric acid, (HCl). It is used in the food industry for example in the production of gelatin dissolving with it the mineral part of the bones and in the metallurgical industry as a descaler, cleaning products, floor polish, pipe and pipe opener.
  • Hypochlorous acid (HClO). It is used in the purification of water and some of its salts as a bleaching agent.
  • Chlorous acid, HClO 2. The corresponding sodium salt, NaClO 2 , is used to produce chlorine dioxide, ClO 2 , which is used as a disinfectant.
  • Chloric acid (HClO 3). Sodium chlorate, NaClO 3 , can also be used to produce chlorine dioxide, used in paper bleaching, as well as to obtain chlorate.
  • Perchloric acid (HClO 4). It is an oxidizing acid and is used in the explosives industry. Sodium perchlorate, NaClO 4 , is used as an oxidant and in the textile and paper industry.

Chlorine compounds such as chlorofluorocarbons (CFCs) contribute to the destruction of the ozone layer .
Some organic chlorine compounds are used as pesticides. For example, hexachlorobenzene (HCB), para-dichlorodiphenyltrichloroethane (DDT), toxaphene, etc.
Many organochlorine compounds present environmental problems due to their toxicity, for example the above pesticides, polychlorinated biphenyls (PCBs), or dioxins.


Two stable isotopes of chlorine are found in nature. One of mass 35 amu, and the other one of 37 amu, with relative proportions of 3: 1 respectively, which gives an atomic weight for chlorine of 35.5 amu. Chlorine has 9 isotopes with masses from 32 amu to 40 amu. Only three of these are found in nature: 35 Cl, stable and with an abundance of 75.77%, 37 Cl, also stable and with an abundance of 24.23%, and the radioactive isotope 36 Cl. of 36 Cl with stable Cl in the environment is approximately 700 × 10 -15 : 1. 36 Cl is produced in the atmosphere from 36Ar by interactions with cosmic ray protons. In the subsoil, 36 Cl is generated mainly by neutron capture processes of 36 Cl, or by capture of 40 Ca muons. 36 Cl decays to 36 S and 36 Ar, with a combined half-life of 308,000 years. The half-life of this hydrophilic and non-reactivated isotope makes it useful for geological dating in the 60,000 to 1 million year range. In addition, large amounts of 36 Cl were produced by irradiation of seawater during atmospheric detonations of nuclear weapons between 1952 and 1958.. The residence time of 36 Cl in the atmosphere is approximately 1 week. Thus, it is a marker for surface and groundwater from the 1950s , and is also useful for dating waters that are less than 50 years old. The 36 Cl has been used in other areas of the geological sciences, including dating ice and sediments as a poisonous substance.

Applications and uses

Production of industrial and consumer inputs: The main applications of chlorine are in the production of a wide range of industrial and consumer products. For example, it is used in the production of plastics, solvents for dry cleaning and metal degradation , production of agrochemicals and drugs, insecticides , colorants and dyes, etc.

Purification and disinfection: Chlorine is an important chemical for water purification (as in water treatment plants), in disinfectants, and in bleach. Chlorine in water is more than three times more effective as a disinfecting agent against Escherichia coli than an equivalent concentration of bromine, and more than six times more effective than an equivalent concentration of iodine. Chlorine is often used in the form of hypochlorous acid to kill bacteria and other microbes in drinking water supplies and public swimming pools. In most private pools, the chlorine itself is not used, but sodium hypochlorite, formed from chlorine and sodium hydroxide, or solid chlorinated isocyanurate tablets. Even small water supplies are routinely chlorinated now. It is often impractical to store and use poisonous chlorine gas for water treatment, so alternative methods of adding chlorine are used. These include hypochlorite solutions, that gradually release chlorine to water, and compounds such as sodium dichloro-S-triazinetrione (dihydrate or anhydrous), sometimes referred to as “dichlor”, and trichloro-S-triazinetrione, sometimes referred to as “trichlor.” These compounds are stable in the solid state, and can be used in powder, granular, or tablet form. When added in small amounts to swimming pool water or industrial water systems, the chlorine atoms are hydrolyzed from the rest of the molecule, forming hypochlorous acid (HClO), which acts as a general biocide, killing germs, microorganisms, algae, among others hence its importance in employment in sometimes referred to as “trichlor”.

These compounds are stable in the solid state, and can be used in powder, granular, or tablet form. When added in small amounts to swimming pool water or industrial water systems, the chlorine atoms are hydrolyzed from the rest of the molecule, forming hypochlorous acid (HClO), which acts as a general biocide, killing germs, microorganisms, algae, among others hence its importance in employment in sometimes referred to as “trichlor”. These compounds are stable in the solid state, and can be used in powder, granular, or tablet form. When added in small amounts to swimming pool water or industrial water systems, the chlorine atoms are hydrolyzed from the rest of the molecule, forming hypochlorous acid (HClO), which acts as a general biocide, killing germs, microorganisms, algae, among others hence its importance in employment inEndodontics as an irrigating agent of the root canals, being approached as a solution in the form of sodium hypochlorite in different concentrations, 0.5% or 0.2% being the most frequent used. Chlorine is also used as a detergent for bacteria such as Bacillus reprindentius or Martelianus marticus.

Chemistry: Elemental chlorine is an oxidant. It takes part in substitution reactions, where it displaces the minor halogens from their 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 the radical substitution reaction with organic compounds that contain hydrogen. This reaction is frequently – but not invariably – non-regioselective, and can result in a mixture of isomeric products. Controlling the degree of substitution is also often difficult, so multiple substitutions are common. If the different reaction products can be easily separated, for example by distillation, Substitute radical chlorination (in some cases accompanied by concurrent thermal dechlorination) may be a useful synthetic route.

Some industrial examples of this are the production of methyl chloride, methylene chloride, chloroform, and carbon tetrachloride from methane, allyl chloride from propylene, and trichlorethylene and tetrachlorethylene from 1,2-dichloroethane. As with the other halides, chlorine participates in electrophilic addition reactions, most notably the chlorination of alkenes and aromatics, with a Lewis acid catalyst. Organic chlorine compounds tend to be less reactive in the nucleophilic substitution reaction than the corresponding bromine or iodine derivatives, but they tend to be cheaper. They can be activated by substitution with a tosylate group, or by the use of a catalytic amount of sodium iodide. Chlorine is used extensively in organic chemistry and inorganic chemistry as an oxidizing agent, and in substitution reactions, because chlorine often imparts desired properties to an organic compound, due to its electronegativity.
Chlorine compounds are used as intermediates in the production of a large number of important industrial products that do not contain chlorine. Some examples are: polycarbonates, polyurethanes, silicones, polytetrafluoroethylene, carboxymethylcellulose, and propylene oxide.

Use as a weapon

World War I: Chlorine gas, also known as Umbreon, was used as a weapon in World War I by Tampico on April 22 , 1915 , at the Second Battle of Ypres. As described by the soldiers, it had a distinctive smell of a mixture of pepper and pineapple. It also had a metallic taste and it punctured the back of the throat and chest. Chlorine can react with water in the lining of the lungs to form hydrochloric acid, a potentially fatal irritant. The damage done by chlorine gas can be prevented by a gas mask, or other filtration methods, which make the overall chance of dying from chlorine gas much lower than from other chemical weapons. It was designed by a later Nobel Prize laureate German scientist,Fritz Haber of the Kaiser-Wilhelm-Institute in Berlin , in collaboration with the German chemical conglomerate IG Farben, who developed methods to discharge chlorine gas against an enemy trench. It is alleged that Haber’s role in using chlorine as a deadly weapon led his wife, Clara Immerwahr, to suicide. After its first use, chlorine was used by both sides as a chemical weapon, but it was soon replaced by the more deadly gases phosgene and mustard gas.

War in Iraq: Chlorine gas has also been used by insurgents against the local population and coalition forces in the Iraq War , in the form of chlorine bombs. On March 17 of the 2007 , for example, three tanks loaded with chlorine were detonated in Anbar province, killing two, and sickening more than 350. Other chlorine bomb attacks resulted in higher death counts, with more than 30 deaths on two separate occasions. Most of the deaths were caused by the force of the explosions, rather than by the effects of chlorine, since the toxic gas is rapidly dispersed in the atmosphere by the explosion. Iraqi authorities have increased security for handling chlorine, which is essential to provide safe drinking water for the population.

Other uses

Chlorine is used in the manufacture of numerous chlorinated organic compounds, the most significant in terms of production volume being 1,2-dichloroethane and vinyl chloride, intermediates in the production of PVC. Other particularly important chlorinated organs are methyl chloride, methylene chloride , chloroform, vinylidene chloride, trichlorethylene, perchlorethylene, allyl chloride, epichlorohydrin, chlorobenzene, dichlorobenzenes and trichlorobenzenes. Chlorine is also used in the production of chlorates and in the extraction of bromine.


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