Chlorine

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Chlorine (from the Greek language Chloros, meaning "pale green"), is the chemical element with atomic number 17 and symbol Cl. It is a halogen, found in the periodic table in group 17. 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. As chlorine gas, it is greenish yellow, is two and one half times as heavy as air, has an intensely disagreeable suffocating odor, and is exceedingly poisonous. In its liquid and solid form it is a powerful oxidizing, bleaching, and disinfecting agent.

1 Notable characteristics
2 Applications
3 History
4 Occurrence
5 Isotopes
6 Precautions
7 Chlorine gas extraction
8 Compounds
9 References
10 External links

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Notable characteristics

The pure chemical element, has the physical form of a diatomic yellow-green gas, Cl₂.

This element is a member of the salt-forming halogen series and is extracted from chlorides through oxidation and more commonly, by electrolysis. Chlorine is a greenish-yellow gas that combines readily with nearly all other elements. At 10°C one liter of water dissolves 3.10 liters of chlorine and at 30 °C only 1.77 liters.

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Applications

Chlorine is an important chemical for some processes of water purification, in disinfectants, and in bleach. Ozone can also be used for killing bacteria, and is preferred by many municipal drinking water systems because ozone does not form organochlorine compounds and does not remain in the water after treatment.

Chlorine is also used widely in the manufacture of many everyday items.

Used (in the form of hypochlorous acid) to kill bacteria and other microbes from drinking water supplies and swimming pools. Even small water supplies are now routinely chlorinated. See chlorination.
Used widely in paper product production, antiseptic, dyestuffs, food, insecticides, paints, petroleum products, plastics, medicines, textiles, solvents, and many other consumer products.

This element is used extensively in organic chemistry as an oxidizing agent and in substitution reactions because chlorine often imparts many desired properties in an organic compound when it is substituted for hydrogen (as in synthetic rubber production).It has the highest electron affinity among halides.

Other uses are in the production of chlorates, chloroform, carbon tetrachloride, and in the bromine extraction.

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History

Chlorine (Gr. χλωρος, greenish yellow) was discovered in 1774 by German chemist Carl Wilhelm Scheele, who mistakenly thought it contained oxygen. Chlorine was given its name in 1810 by Sir Humphry Davy, who insisted that it was in fact an element.

Chlorine gas, also known as bertholite, was first used as a weapon against human beings in WWI on April 22nd, 1915.

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Occurrence

In nature chlorine is found only as the chloride ion. Chlorides make up much of the salt dissolved in the Earth's oceans—about 1.9% of the mass of seawater is chloride ions. Even higher concentrations of chloride are dissolved in the Dead Sea and in underground brine deposits.

Most chlorides are soluble in water, so solid chlorides 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).

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 chemical equation

2 NaCl + 2 H₂O → Cl₂ + H₂ + 2 NaOH

See also Halide minerals.

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Isotopes

There are two principal stable isotopes of chlorine, of mass 35 and 37, found in the relative proportions of 3:1 respectively, giving chlorine atoms in bulk an apparent atomic weight of 35.5. Chlorine has 9 isotopes with mass numbers ranging from 32 to 40. Only three of these isotopes occur naturally: stable ³⁵Cl (75.77%)and ³⁷Cl (24.23%), and radioactive ³⁶Cl. The ratio of ³⁶Cl to stable Cl in the environment is about 700*10^-15 to 1. ³⁶Cl is produced in the atmosphere by spallation of ³⁶Ar by interactions with cosmic ray protons. In the subsurface environment, ³⁶Cl is generated primarily as a result of neutron capture by ³⁵Cl or muon capture by ⁴⁰Ca. ³⁶Cl decays to ³⁶S and to ³⁶Ar, 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 ³⁶Cl were produced by irradiation of seawater during atmospheric detonations of nuclear weapons between 1952 and 1958. The residence time of ³⁶Cl in the atmosphere is about 1 week. Thus, as an event marker of 1950s water in soil and ground water, ³⁶Cl is also useful for dating waters less than 50 years before the present. ³⁶Cl has seen use in other areas of the geological sciences, including dating ice and sediments.

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Precautions

Chlorine irritates respiratory systems especially in children and the elderly. In its gaseous state it irritates mucous membranes and in its liquid state it burns skin. It takes as little as 3.5 ppm to be detected as a distinct odor, but it takes 1000 ppm or more to be fatal. Because of this, chlorine was one of the gases used during World War I as a war gas. (See: Use of poison gas in World War I)

Exposure to this gas should therefore not exceed 0.5 ppm (8-hour time-weighted average - 40 hour week).

Acute exposure to high but non-lethal concentrations of chlorine can result in pulmonary edema, or fluid in the lungs, an extremely unpleasant condition. Chronic low-level exposure weakens the lungs, increasing susceptibility to other lung disorders.

Toxic fumes may be produced when bleach is mixed with urine, ammonia, hydrochloric acid, or another cleaning product. These fumes consist of a mixture of chlorine gas, chloramine and nitrogen trichloride; therefore these combinations should be avoided.

Chlorine gas extraction

Chlorine can be manufactured by electrolysis of a sodium chloride solution (brine). There are three industrial methods for the extraction of chlorine by electrolysis.

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Mercury cell electrolysis

Mercury cell electrolysis was the first method used to produce chlorine on an industrial scale. Titanium anodes are located above a liquid mercury cathode and a solution of sodium chloride is positioned between the electrodes. When an electrical current is applied, chloride is released at the titanium anodes and sodium dissolves into the mercury cathode forming an amalgam.

The amalgam can be regenerated into mercury by reacting it with water, producing hydrogen and sodium hydroxide. These are useful byproducts.

This method consumes vast amounts of energy and there are also concerns about mercury emissions.

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Diaphragm cell electrolysis

An asbestos diaphragm is deposited on an iron grid cathode preventing the chlorine forming at the anode and the sodium hydroxide forming at the cathode from re-mixing.

This method uses less energy than the mercury cell, but the sodium hydroxide is not as easily concentrated and precipitated into a useful substance.

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Membrane cell electrolysis

The electrolysis cell is divided into two by a membrane acting as an ion exchanger. Saturated sodium chloride solution is passed through the anode compartment leaving a lower concentration. Sodium hydroxide solution is circulated through the cathode compartment exiting at a higher concentration. A portion of this concentrated sodium hydroxide solution is diverted as product while the remainder is diluted with deionized water and passed through the electrolyzer again.

This method is nearly as efficient as the diaphragm cell and produces very pure sodium hydroxide but requires very pure sodium chloride solution.

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Other methods

Before arising the electrolysis procedures for chlorine production also the direct oxidation of hydrogen chloride with oxygen or air was exercised in the Deacon procedure:

2HCl + O₂ → Cl₂ + H₂O

The execution of this non-complete reaction was accomplished at catalysts on basis by CuCl₂. Due to the extremely corrosively working reaction mixture technical execution is however connected with large difficulties.

Another earlier process to produce chlorine is to heat brine with acid and manganese dioxide. The manganese is recovery by Weldon process.

In a laboratory, small amounts of chlorine gas can be created by adding concentrated hydrochloric acid (typically about 5M) to sodium chlorate solution.

Chemist Carl Wilhelm Scheele was the first to isolate Chlorine in a laboratory, with the following extremely complicated method:

2NaCl + 2H₂SO₄ + MnO₂ → Na₂SO₄ + MnSO₄ + 2H₂O + Cl₂

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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⁻). See also chloramine (NH₂Cl), chlorine dioxide (ClO₂), chloric acid (HClO₃), chlorine monofluoride (ClF), chlorine trifluoride (ClF₃), chlorine pentafluoride (ClF₅) dichlorine monoxide (Cl₂O), dichlorine heptoxide (Cl2O₇), hydrochloric acid (HCl), perchloric acid (HClO₄), and potassium chloride (KCl).