Chlorine
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.
The pure chemical element has the physical form of a
diatomic yellow-green gas, Cl
2. It has a pungent odor, even in low concentrations. Chlorine combines readily with nearly all other elements, although is not so extremely reactive as
fluorine. 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.
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.
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.
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 equation2
NaCl + 2
H2O → Cl
2 +
H2 + 2
NaOHSee also Halide minerals.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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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:
ChlorofluorocarbonChlorine 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 NaOHOther 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 + Cl2For 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),
*Fluorides: chlorine monofluoride (ClF), chlorine trifluoride (ClF3), chlorine pentafluoride (ClF5)
*Oxides: chlorine dioxide (ClO2), dichlorine monoxide (Cl2O), dichlorine heptoxide (Cl2O7)
*Acids: hydrochloric acid (HCl), chloric acid (HClO3), and perchloric acid (HClO4)
See also Chlorine compounds.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.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.*Los Alamos National Laboratory – Chlorine*Chloride
*Computational Chemistry Wiki
*National Pollutant Inventory - Chlorine
*WebElements.com – Chlorine
*Chlorine Tree
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