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Chlorine (IPA: , , 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. In its elemental form under standard conditions, it is a pale green gas about 2.5 times as dense as air. It has a disagreeable suffocating odor and is poisonous. Chlorine is a powerful oxidant and is used in bleaching and disinfectants. Notable characteristics Chlorine gas is diatomic with the formula Cl2. It combines readily with nearly all other elements, although it is not as extremely reactive as fluorine. At 10 °C one litre of water dissolves 3.10 litres of gaseous chlorine and at 30 °C only 1.77 litres. 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–, it is also the most abundant dissolved species in ocean water. 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. Chlorine was given its current 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 by Germany on April 22nd, 1915. Afterwards, it was used by both sides as a chemical weapon. Occurrence In nature, chlorine is found mainly as the chloride ion, a component of the salt that is deposited in the earth or dissolved in the oceans—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). 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 . Isotopes Chlorine has 9 isotopes with mass numbers ranging from 32 to 40. There are two principal stable isotopes, 35Cl (75.77%) and 37Cl (24.23%), found in the relative proportions of 3:1 respectively, giving chlorine atoms in bulk an apparent atomic weight of 35.5. Trace amounts of radioactive 36Cl exist in the environment, in a ratio of about 700 Chlorine 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 are highly reactive. There are three industrial methods for the extraction of chlorine by electrolysis. 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. However, this method consumes vast amounts of energy and there are also concerns about mercury emissions. Diaphragm cell electrolysis In 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. 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. Cathode: 2 H+(aq) + 2 e– → H2 (g) Anode: 2 Cl– → Cl2 (g) + 2 e– Overall equation: 2 NaCl + 2H20 → Cl2 + H2 + 2 NaOH 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 was accomplished with the use of CuCl2 as a catalyst. Due to the extremely corrosive reaction mixture, industrial use of this method is difficult. 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. In a laboratory, small amounts of chlorine gas can be created by adding concentrated hydrochloric acid (typically about 5M) to sodium chlorate solution. Purification and Disinfection 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 every-day items, or to purify water in various forms. Oxidizing agent 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 when it is substituted for hydrogen (as in synthetic rubber production) because of its high electron affinity. World War I Chlorine became the first killing agent to be employed during World War I. German chemical conglomerate IG Farben had been producing chlorine as a by-product of their dye manufacturing. In cooperation with Fritz Haber of the Kaiser Wilhelm Institute for Chemistry in Berlin, they developed methods of discharging chlorine gas against an entrenched enemy. 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). See also: See also . Other Uses It is also used in the production of chlorates, chloroform, carbon tetrachloride, and in bromine extraction. Safety 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. For more information see an MSDS. See also | |||||||
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