Chlorine

Chlorine (from Gr. 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. Chlorine gas is greenish yellow, is two and one half times as heavy as air, has an intensely disagreeable suffocating odor, and is exceedingly poisonous. It is a powerful oxidizing, bleachinging, and disinfecting agent. As part of common salt and other compounds, it is abundant in nature and necessary to most forms of life, including the human body.

Notable characteristics

The pure chemical element has the physical form of a diatomic green gas. The name chlorine is derived from chloros, meaning green, referring to the color of the gas.

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.

Applications

Chlorine is an important chemical in water purification, in disinfectants in bleach and in mustard gas.

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.

Used widely in paper product production, antiseptic, dyestuffs, food, insecticides, paints, petroleum products, plastics, medicines, textiles, solvents, and many other consumer products.

Organic chemistry uses this element extensively as an oxidizing agent and in substitution because chlorine often imparts many desired properties in an organic compound when it is substituted for hydrogen (as in synthetic rubber production).

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

History

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

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

Compounds

Compounds of chlorine include chlorides, chlorites, chlorates, perchlorates, chloramines.

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 Cl-35 (75.77%)and Cl-37 (24.23%), and radioactive Cl-36. The ratio of Cl-36 to stable Cl in the environment is about 700 E -15 : 1. Cl-36 is produced in the atmosphere by spallation of Ar-36 by interactions with cosmic ray protons. In the subsurface environment, Cl-36 is generated primarily as a result of neutron capture by Cl-35 or muon capture by Ca-40. Cl-36 decays to S-36 and to Ar-36, 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 Cl-36 were produced by irradiation of seawater during atmospheric detonations of nuclear weapons between 1952 and 1958. The residence time of Cl-36 in the atmosphere is about 1 week. Thus, as an event marker of 1950s water in soil and ground water, Cl-36 is also useful for dating waters less than 50 years before the present. Cl-36 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 when a liquid it burns skin. It takes as little as 3.5 ppm to be detected as 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 or another cleaning product. These fumes consist of a mixture of chlorine gas and nitrogen trichloride; therefore these combinations should be avoided.

The chemical processes for extraction of chlorine gas

Chlorine can be manufactured via the electrolysis of a sodium chloride solution, ie. brine. There are three methods for the extraction of chlorine by electrolysis used industrially.

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, a solution of sodium chloride is positioned between the electrodes. When an electrical current is applied chloride is released at the titanium anodes, whilst the 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 placed in the anode compartment whilst distilled water is placed in the cathodes compartment.

This method is nearly as efficient as the diaphragm cell and yet produces very pure sodium hydroxide.

References

Los Alamos National Laboratory – Chlorine

External links

WebElements.com – Chlorine

EnvironmentalChemistry.com – Chlorine

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