Chlorine

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Chlorine :: Chlorine_Compounds

Chlorine (from the Greek language χλωρóς 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.

Notable characteristics

The pure chemical element has the physical form of a diatomic yellow-green gas, Cl₂. Chlorine combines readily with nearly all other elements. At 10 °C one litre of water dissolves 3.10 litres of 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 and more commonly, by electrolysis.

As the chloride ion, Cl^-, it is also the most abundant dissolved species in ocean water.

Applications and Uses

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 began developing methods of discharging chlorine gas against enemy trenches.

Purification and Disinfecting

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.

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 Also: 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

It is also used in the production of chlorates, chloroform, carbon tetrachloride, 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. 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 on April 22nd, 1915, and afterwards was used by both sides.

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.

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.

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.

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.

While chlorine itself is not flammable it poses a serious fire risk due to its potential as an oxidizer. Most flammable materials will therefore burn in a chlorine environment making it important that it not be stored in an area where a fire could potentially start. Accidental releases of chlorine can be mitigated with a variety of methods and equipment. Water sprays, neutralization medium, tank offloading, gas scrubbing are only a sample of emergency response tactics developed throughout chlorine’s long history.

Chlorine containers manufactured to The Chlorine Institute specifications are also designed to accommodate emergency containment equipment designed by the Chlorine Institute. The Chlorine Institute Emergency Kits “A” “B” and “C” are large tool boxes of specialized devices and tools designed to contain leaks in each of the three primary chlorine containers, respectively. In addition to the emergency kits, the Chlorine Institute Recovery Vessel is a large hatched tube that totally encapsulates a leaking chlorine cylinder within. This equipment is commonly found at the locations where chlorine is present and with local fire and HazMat response departments.

Chlorine emergency response and mitigation requires comprehensive training and expertise. Chlorine response training is readily available throughout the US and Canada. The Chlorine Institute is also a major source of chlorine safety and response information while also maintaining a network of highly skilled member participants that may respond to major chlorine incidents in the US and Canada: CHLOREP (Chlorine Emergency Plan). See also: Chlorofluorocarbon

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.

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.

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) + 2e^{-^{---> H₂(g)}}

Anode: 2Cl^- ---> Cl₂ (g) + 2e^-

Overall equation: 2NaCl + 2H₂0 ---> Cl₂ + H₂ + 2 NaOH

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

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⁻).

Transportation

In the United States, Chlorine is transported primarily in three types of containers designed to Chlorine Institute specifications and regulated by the Department of Transportation: 100 & 150 lb. Cylinders, One Ton Containers and in Bulk (Railcars /Tank Trucks).

Chlorine Cylinders are of a seamless, steel construction resembling helium or acetylene cylinders in appearance, with capacities of 100 and 150 lb. being the most popular sizes. Each cylinder has a single valve located within a steel protective cap atop of the cylinder. Cylinder valves are manufactured with a pressure relief device (fusible plug) designed to melt, in case of a fire, and release the gas to avoid rupture due to over pressurization.

Ton Containers are steel welded tanks with a chlorine capacity of 2000 lb. Handled horizontally, these containers resemble large tubes (30” OD x 82” lg.) with concave ends containing three fusible plugs in each end and two valves located on one end inside a steel protective covering. The valves are connected internally to eduction tubes which allow for the controlled release of either liquid or vapor chlorine.

Bulk shipments are made in either railcars (55 and 90 ton capacity being the most common) and tank trucks (15-22 ton typical capacity). These large tanks are commonly multi-layered steel shells with identical valve housings atop. Each tank includes a valve housing consisting of two liquid angle valves, two vapor angle valves and one center pressure relief device. Unlike the ton containers and cylinders, the tank car PRD is a pre-set, spring-loaded device to relieve over pressurization.

Internationally, US designed containers are used throughout the world along with a wide variety of chlorine cylinders, ton containers and bulk containers of various designs and configurations, manufactured in many different countries.

Organizations

The Chlorine Institute (www.chlorineinstitute.org) is a trade organization, located in Arlington Va. USA, consisting of chlorine producers, packagers, responders, end users and associated members. The Institute’s mission is “to support the chlor-alkali industry and serve the public by fostering continuous improvements to safety and the protection of human health and the environment connected with the production, distribution and use of chlorine, sodium and potassium hydroxides, and sodium hypochlorite; and the distribution and use of hydrogen chloride. This support extends to giving continued attention to the security of chlorine handling operations.” (Chlorine Institute website). The Institute is a source of technical, regulatory and safety information for the Chlorine industry.

References

Los Alamos National Laboratory – Chlorine

External links

Chemical elements | Halogens

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