Chlorine

History of chlorine

Properties and uses of chlorine

Compounds of chlorine

Resources

Chlorine is the non-metallic chemical element of atomic number 17, symbol Cl, atomic weight 35.45, melting point–149.8°F (–101°C), and boiling point–29.02°F(–33.9°C). It consists of two stable isotopes, of mass numbers 35 and 37. Ordinary chlorine is a mixture of 75.77% chlorine-35 atoms and 24.23% chlorine-37 atoms. The average human body contains about 3.5 ounces (95 grams) of chlorine, primarily in the form of hydrochloric acid (HCl; stomach acid), sodium chloride (NaCl), and potassium chloride (KCl).

Chlorine is a highly poisonous, greenish yellow gas, about two and a half times as dense as air, with a strong, sharp, choking odor. It was, in fact, one of the first poisonous gases used in warfare—in 1915 during World War I (1914–1918). In spite of its disagreeable nature, there are so many everyday products that contain chlorine or are manufactured through the use of chlorine that it is among the top ten chemicals produced in the United States each year. In 2004, according to the Chlorine Institute, more than 26 billion pounds (12 billion kilograms) of chlorine were produced in the U.S. an increase of almost 7% over 2003 figures.

In nature, chlorine is widely distributed over the Earth in the form of the salt (sodium chloride) in sea water. At an average concentration of 0.67 oz (19 g) of chlorine in each liter of sea water, it is estimated that there are some 10¹⁶ tons of chlorine in the world’s oceans. Other compounds of chlorine occur as minerals in the Earth’s crust, including huge underground deposits of solid sodium chloride.

Along with fluorine, bromine, iodine, and astatine, chlorine is a member of the halogen family of elements in group 17 of the periodic table—the most non-metallic (least metallic) and most highly reactive group of elements. Chlorine reacts directly with nearly all other elements; with metals, it forms salts called chlorides. In fact, the name halogen, meaning salt producer, was originally defined for chlorine (in 1811 by J. S. C. Schweigger), and it was later applied to the rest of the elements in this family.

History of chlorine

The most common compound of chlorine, sodium chloride, has been known since ancient times; archaeologists have found evidence that rock salt was used as early as 3000 BC. The first compound of chlorine ever made by humans was probably hydrochloric acid (hydrogen chloride gas dissolved in water), which was prepared by the Arabian alchemist Rhazes around AD 900. Around AD 1200, aqua regia (a mixture of nitric and hydrochloric acids) began to be used to dissolve gold; it is still the only liquid that will dissolve gold. When gold dissolves in aqua regia, chlorine is released along with other nauseating and irritating gases. Other than getting as far away from them as possible, the gases themselves were ignored.

The credit for first preparing and studying gaseous chlorine went to Swedish chemist Carl Wilhelm Scheele (1742–1786)in 1774. Scheele discovered several other important elements and compounds, including barium, manganese, oxygen, ammonia, and glycerin. Scheele thought that chlorine was a compound, which he called dephlogisticated marine acid air. All gases were called airs at that time, and what is now known as hydrochloric acid was called marine acid because it was made from sea salt. The word dephlogisticated came from a completely false theory that slowed the progress of chemistry for decades.

It was not until 1811 that English chemist Sir Humphry Davy (1778–1829) announced to the Royal Society of London that chlorine gas was an element. He suggested the name chlorine because it is the same pale, yellowish green color that sick plants sometimes develop, a color that is known as chloros in Greek. (The sick plants are said to have chlorosis.)

Properties and uses of chlorine

Because it is so reactive, chlorine is never found alone—chemically uncombined—in nature. It is prepared commercially by passing electricity through a water solution of sodium chloride or through molten sodium chloride.

When released as the free element, chlorine gas consists of diatomic (two-atom) molecules, as expressed by the formula Cl₂ . The gas is very irritating to the mucous membranes of the nose, mouth, and lungs. It can be smelled in the air at a concentration of only 3 parts per million (ppm); it causes throat irritation at 15 ppm, coughing at 30 ppm, and is very likely to be fatal after a few deep breaths at 1, 000 ppm.

Chlorine gas dissolves readily in water, reacting chemically with it to produce a mixture of hydro-chloric acid (HCl) and hypochlorous acid (HOCl), plus some unreacted Cl₂ . This solution, called chlorine water, is a strong oxidizing agent that can be used to kill germs or to bleach paper and fabrics. It is used to obtain bromine (another member of its halogen family) from sea water by oxidizing bromide ions to elemental bromine.

In organic chemistry, chlorine is widely used, not only as an oxidizing agent, but as a way of making many useful compounds. For example, chlorine atoms can easily replace hydrogen atoms in organic molecules. The new molecules, with their chlorine atoms sticking out, are much more reactive and can react with various chemicals to produce a wide variety of other compounds. Among the products that are manufactured by the use of chlorine somewhere along the way are antiseptics, dyes, explosives, foods, insecticides, medicines, metals, paints, paper, plastics, refrigerants, solvents, and textiles.

Probably the most important use of chlorine is as a water purifier. Water supplies in the United States and in much of the rest of the world are rendered safe for drinking by the addition of chlorine. Several chlorine-releasing compounds are also used as general disinfectants.

Bleaching is another very practical use of chlorine. Until it was put to use as a bleach around 1785, bright sunlight was the only way people could bleach out stains and undesired colors in textiles and paper. Today, in the form of a variety of compounds, chlorine is used almost exclusively. Here is how it works: many compounds are colored because their molecules contain loose electrons that can absorb specific colors of light, leaving the other colors unabsorbed and therefore visible. An oxidizing agent such as chlorine water or a compound containing the hypochlorite ion OCl^- removes those electrons (an oxidizing agent is an electron remover), which effectively removes the substance’s light-absorbing power and therefore its color. Ordinary laundry bleach is a 5.25% solution of sodium hypochlorite in water.

Among the important organic compounds containing chlorine are the chlorinated hydrocarbons—hydro-carbons that have had some of their hydrogen atoms replaced by chlorine atoms. A variety of chlorinated hydrocarbons have been used as insecticides. One of the earliest to be used was DDT, dichlorodiphenyltrichloroethane. Because it caused serious environmental problems, its use has largely been banned in the United States, although it is still used in many foreign countries. Other chlorinated hydrocarbons that are used as pesticides include dieldrin, aldrin, endrin, lindane, chlordane, and heptachlor. Because all of these compounds are very stable and do not degrade easily, they also have serious environmental drawbacks.

Compounds of chlorine

The following are a few of the important compounds of chlorine.

Calcium hypochlorite, CaOCl, is a white powder known as bleaching powder and used for bleaching and as a swimming pool disinfectant. Both its bleaching and its disinfectant qualities come from its chemical instability. It decomposes to release chlorine gas.

Chlorates are compounds of metals with the anion ClO^– ₃ . An example is potassium chlorate, KClO₃. Chlorates can cause explosions when mixed with flammable materials, because the chlorate ion decomposes under heat to release oxygen, and the oxygen speeds up the combustion process to explosive levels. Potassium chlorate is used in fireworks.

Chlorides are the salts of hydrochloric acid, HCl. They are compounds of a metal with chlorine and nothing else. Some common examples are sodium chloride (NaCl), ammonium chloride (NH₄ Cl), calcium chloride (CaCl₂), and magnesium chloride (MgCl₂ ). When dissolved in water, these salts produce chloride ions, Cl^-. Polyvinyl chloride, the widely used plastic known as PVC, is a polymer of the organic chloride, vinyl chloride.

Freons are hydrocarbons with fluorine and chlorine atoms substituted for some of the hydrogen atoms in their molecules. They have been widely used as the liquids in refrigerating machines and as propellants in aerosol spray cans. They have been implicated in destroying the ozone layer in the upper atmosphere, however, and their use is now severely restricted.

See also Chlorination; Chloroform; Dioxin; Halogenated hydrocarbons; Halogens; Hydrochlorofluorocarbons; Mercurous chloride.

Resources

BOOKS

Ede, Andrew. The Chemical Element: A Historical Perspective. Westport, CT: Greenwood Press, 2006.

Emsley, John. Nature’s Building Blocks: An A-Z Guide to the Elements. Oxford: Oxford University Press, 2002.

Siekierski, Slawomir. Concise Chemistry of the Elements. Chichester, UK: Horwood Publishing, 2002.

Sconce, J. S. Chlorine, Its Manufacture, Properties and Uses. New York: Reinhold, 1962.

Robert L. Wolke

CHLORINE (REVISED)

Note: This article, originally published in 1998, was updated in 2006 for the eBook edition.

Overview

Chlorine ranks among the top 10 chemicals produced in the United States. In 1992, about 10.5 billion kilograms (23 billion pounds) of the element were produced in the United States. Chlorine, in one form or another, is added to most swimming pools, spas, and public water supplies because it kills bacteria that cause disease. Many people also use chlorine to bleach their clothes. Large paper and pulp mills use chlorine to bleach their products.

Chlorine is a greenish-yellow poisonous gas. It was discovered in 1774 by Swedish chemist Carl Wilhelm Scheele (1742-86). Scheele knew that chlorine was a new element, but thought it contained oxygen as well.

SYMBOL
Cl

ATOMIC NUMBER
17

ATOMIC MASS
35.453

FAMILY
Group 17 (VIIA)
Halogen

PRONUNCIATION
CLOR-een

Chlorine is a member of the halogen family. Halogens are the elements that make up Group 17 (VIIA) of the periodic table, a chart that shows how elements are related to one another. They include fluorine, bromine, iodine, and astatine. Chlorine is highly reactive, ranking only below fluorine in its chemical activity.

Discovery and naming

Chlorine compounds have been important to humans for thousands of years. Ordinary table salt, for example, is sodium chloride (NaCl). Still, chlorine was not recognized as an element until 1774, when Scheele was studying the mineral pyrolusite. Pyrolusite consists primarily of manganese dioxide (MnO₂). Scheele mixed pyrolusite with hydrochloric acid (HCl), then called spiritus salis. He found that a greenish-yellow gas with a suffocating odor "most oppressive to the lungs" was released. The gas was chlorine.

Scheele found that the new gas reacted with metals, dissolved slightly in water, and bleached flowers and leaves. He gave the gas the rather complex name of dephlogisticated marine acid.

Chlorine occurs commonly both in the Earth's crust and in seawater.

The true nature of Scheele's discovery was not completely understood for many years. Some chemists argued that his dephlogisticated marine acid was really a compound of a new element and oxygen. This confusion was finally cleared up in 1807. English chemist Sir Humphry Davy (1778-1829) proved that Scheele's substance was a pure element. He suggested the name chlorine for the element, from the Greek word chloros,meaning "greenish-yellow." (See sidebar on Davy in the calcium entry.)

Physical properties

Chlorine is a dense gas with a density of 3.21 grams per liter. By comparison, the density of air is 1.29 grams per liter. Chlorine changes from a gas into a liquid at a temperature of -34.05°C (-29.29°F) and from a liquid to a solid at -101.00°C (-149.80°F). The gas is soluble (dissolvable) in water. It also reacts chemically with water as it dissolves to form hydrochloric acid (HCl) and hypochlorous acid (HOCl).

Chemical properties

Chlorine is a very active element. It combines with all elements except the noble gases. The noble gases are the elements that make up Group 18 (VIIIA) of the periodic table. The reaction between chlorine and other elements can often be vigorous. For example, chlorine reacts explosively with hydrogen to form hydrogen chloride:
Chlorine does not burn but, like oxygen, it helps other substances bum. Chlorine is a strong oxidizing agent (a chemical substance that gives up or takes on electrons from another substance).

Occurrence in nature

Chlorine occurs commonly both in the Earth's crust and in seawater. Its abundance in the earth is about 100 to 300 parts per million. It ranks 20th among the elements in abundance in the earth. Its abundance in seawater is about 2 percent. The most common compound of chlorine in seawater is sodium chloride. Smaller amounts of potassium chloride also occur in seawater.

The most common minerals of chlorine are halite, or rock salt (NaCl), sylvite (KCl), and camallite (KCl MgCl₂). Large amounts of these minerals are mined from underground salt beds that were formed when ancient oceans dried up. Over millions of years, the salts that remained behind were buried underground. They were also compacted (packed together) to form huge salt "domes." A salt dome is a large mass of salt found underground.

Isotopes

Two naturally occurring isotopes of chlorine exist, chlorine-35 and chlorine-36. Isotopes are two or more forms of an element. Isotopes differ from each other according to their mass number. The number written to the right of the element's name is the mass number. The mass number represents the number of protons plus neutrons in the nucleus of an atom of the element. The number of protons determines the element, but the number of neutrons in the atom of any one element can vary. Each variation is an isotope.

Seven radioactive isotopes of chlorine are known also. A radioactive isotope is one that breaks apart and gives off some form of radiation. Radioactive isotopes are produced when very small particles are fired at atoms. These particles stick in the atoms and make them radioactive.

One radioactive isotope of chlorine is used in research. That isotope is chlorine-36. This isotope is used because compounds of chlorine occur so commonly in everyday life. The behavior of these compounds can be studied if chlorine-36 is used as a tracer. A tracer is an isotope whose presence in a material can be traced (followed) easily.

For example, engineers are interested in knowing how seawater damages metals. This information is important in determining the best techniques to use in building ships. An experiment can be done by adding pieces of metal to seawater that contains radioactive chlorine-36. The sodium chloride in the seawater is changed slightly so that it contains radioactive chlorine instead of normal chlorine. As the sodium chloride attacks the metal, its actions can be followed easily. The radioactive chlorine, chlorine-36, gives off radiation. That radiation can be detected by holding an instrument near the experiment. A scientist can find out exactly what happens when the sodium chloride attacks the metal.

Extraction

Chlorine is produced by passing an electric current through a water solution of sodium chloride or through molten (melted) sodium chloride. This process is one of the most important commercial processes in industry. The products formed include two of the most widely used materials: sodium hydroxide (NaOH) and chlorine (Cl₂). With a water solution, the reaction that occurs is:

Hydrogen gas (H₂) is also formed in the reaction.

Uses and compounds

Chlorine is widely used throughout the world to purify water. In the United States, only about 6 percent of the chlorine manufactured is used in water purification. About three times as much is used in the paper and pulp industry as a bleach.

The most important use of chlorine is to make other chemicals. For example, chlorine can be combined with ethene, or ethylene, gas (C₂)H₂), to make ethylene dichloride (C₂)H₂)Cl₂):

Large amounts of chlorine minerals are mined from underground salt beds that were formed when ancient oceans dried up.

About one-third of the chlorine produced in the United States goes to making ethylene dichloride. About 90 percent of ethylene dichloride goes to the manufacture of polyvinyl chloride (PVC). PVC is used to make piping, tubing, flooring, siding, film, coatings, and many other products. Ethylene dichloride has become one of the most popular products in American industry. About 4.5 billion kilograms (10 billion pounds) of the material are made each year.

Who is getting poisoned?

O ne of the most troubling uses of chlorine has been in making pesticides. A pesticide is a chemical used to kill pests. Pesticides have special names depending on the kind of pests they are designed to kill. Insecticides kill insects, rodenticides kill rodents (rats and mice), fungicides kill fungi, and nematicides kill worms.

Certain chlorine compounds have become very popular as pesticides. These compounds are called chlorinated hydrocarbons. They contain carbon, hydrogen, and chlorine.

Probably the most famous chlorinated hydrocarbon is dichlorodiphenyltrichloroethane, or DDT. DDT was first prepared in 1873, but was not used as a pesticide until World War II (1939-45). Public health officials were at first delighted to learn that DDT kills disease-carrying insects very efficiently. There was great hope that DDT could be used to wipe out certain diseases in some parts of the world.

Farmers were also excited about DDT. They found it could kill many of the pests that attacked crops. By the end of the 1950s, many farmers were spraying huge amounts of DDT on their land to get rid of pests.

But problems began to appear. Many fish and birds in sprayed areas began to die or become deformed. Soon, these problems were traced to the use of DDT. The fish and birds ate insects that had been sprayed with DDT or drank water that contained DDT. It had a toxic effect on the fish and birds, just as it did on insects. Bird populations declined drastically as DDT caused eggs to be so thin-shelled that young birds did not survive.

Eventually, many governments began to ban the use of DDT. Since 1973, the United States has not allowed the compound to be used. It is still used in other nations of the world, however. These nations feel that the benefits of using DDT outweigh the harm it may cause. They feel that DDT can save lives by killing disease-causing pests. They know they can increase their food supplies by using DDT on crops.

DDT is not the only chlorinated hydrocarbon used as a pesticide. Other compounds in this class include dieldrin, aldrin, heptachlor, and chlordan. The use of these compounds has also been banned or restricted in the United States. The U.S. government has decided the harm they cause to the environment is more important than the benefits they provide to farmers and other users.

Another compound made using chlorine is propylene oxide (CH₃CH0CH₂). There is no chlorine in propylene oxide, but chlorine is used in the process by which the compound is made. Propylene oxide is used to make a group of plastics known as polyesters. Polyesters are found in a wide range of materials, including car and boat bodies, bowling balls, fabrics for clothing, and rugs.

At one time, a large amount of chlorine was used to make a group of compounds known as chlorofluorocarbons (CFCs). CFCs are a family of chemical compounds containing carbon, fluorine, and chlorine. CFCs were once used in a wide variety of applications, such as air conditioning and refrigeration, aerosol spray products, and cleaning materials. They are now known to have serious environmental effects and have been banned from use in the United States and many other countries in the world.

The reason for this ban is the damage caused by CFCs on the Earth's ozone layer. Ozone (O₃) is a form of oxygen that filters out harmful radiation from the sun. When CFCs escape into the atmosphere, they attack and destroy ozone molecules. They reduce the protection against radiation provided by ozone.

Health effects

Chlorine gas is extremely toxic. In small doses, it irritates the nose and throat. A person exposed to chlorine may experience sneezing, running nose, and red eyes.

In larger doses, chlorine can be fatal. In fact, chlorine gas was used during World War I (1914-18) by German armies as a weapon. Thousands of soldiers were killed or seriously wounded by breathing it. Those who survived gas attacks often were crippled for life. They were unable to breathe normally as a result of the damage to their throats and lungs.

On the other hand, chlorine compounds are essential to plants. They become sick or die without it. In plants, chlorine is regarded as a micronutrient, which is a substance needed in very small amounts to maintain good health. Leaves turn yellow and die when plants get too little chlorine from the soil.

Chlorine is produced by passing an electric current through a water solution of sodium chloride or through molten (melted) sodium chloride.

Compounds of chlorine are important in maintaining good health in humans and animals. The average human body contains about 95 grams (about 3.5 ounces) of chlorine. Hydrochloric acid (HCl) in the stomach, for example, helps in the digestion of foods. Sodium chloride (NaCl) and potassium chloride (KCl) play an important role in the way nerve messages are sent throughout the body. Because humans eat so much salt, a lack of chlorine compounds is seldom a health problem.

Chlorine gas is extremely toxic. In large doses, it can be fatal. On the other hand, chlorine compounds are essential to plants.

Chlorine

Chlorine is an element of atomic number 17 and atomic mass of 35.45 atomic mass units. It belongs to Group 17 of the periodic table and is thus a halogen. Halogens are a highly reactive group of elements that, in addition to chlorine, include fluorine, iodine, bromine and another element that does not occur in nature , astatine, but is produced by bombarding bismuth with alpha particles. Halogens are extremely reactive because they have an unpaired electron in their outermost electron shell. Thus so highly reactive, chlorine is usually not found in a pure form in nature, but is rather typically bound to other elements such as sodium, calcium, or potassium. In its pure form, chlorine exists as a diatomic molecule, meaning a molecule containing two of the same atoms. This form of chlorine is a yellow-green gas at room temperature. Chlorine gas is more dense than air, condenses to form a liquid at -29°F(-34°C), and freezes into a solid at -153°F(-103°C). Because of its reactivity, desirable properties, and abundance, chlorine is an exceptionally useful element. Since it readily combines with other elements and molecules, chlorine is a main component and vital reactant in the manufacture of thousands of useful products. In addition, almost all municipal water treatment systems in the United States depend on chlorine chemicals to provide clean and safe drinking water. It is estimated that chlorine is used in the production of 85% of all pharmaceuticals, and in 96% of all crop protection chemicals (pesticides and herbicides). Also, chlorine chemicals are powerful bleaching agents used in paper processing and inexpensive but highly effective disinfectants.

Chlorine also has an important impact on the economy. The chlorine industry provides almost 2 million jobs having a combined yearly payroll of over $52 billion. In the United States, 212 industries are direct users of chlorine or related products. The Chlorine Chemistry Council reports that nearly 40% of U.S. jobs and income are directly or indirectly dependent on chlorine. The chlorine-dependent industries are mostly found in Texas, Michigan, and New Jersey. The automobile industry relies heavily on chlorine because cars contain many components that use chlorine in their manufacture. Three chlorine products, PVC, pickled steel, and paint create more than 345,000 jobs in the United States. Polyvinylchloride (or PVC) is a chlorinated hydrocarbon polymer that is used to make dashboards, air bags, wire covers, and sidings. Chlorine is also used in the manufacture of car exterior paints. Automobile coatings typically use titanium dioxide which requires chlorine for synthesis. In addition, chlorine is used in the production of pickled steel for automobile frames and undercarriages. The pickling process provides an impurity-free steel surface for rust resistance treatment and painting. Hydrochloric acid ,a chlorine chemical, is a main component in pickling steel. Roughly 25% of the hydrochloric acid produced in the United States is used in steel pickling for the auto industry. Overall, the auto industry consumes 3.1% of all chlorine produced in the form of hydrochloric acid, 30% of all chlorine produced in the form of PVC, and 4.1% of all chlorine produced involved in the manufacture of titanium dioxide for paints.

Industrially, chlorine is also used in massive quantities for sanitation purposes. An important advancement in public health has been the widespread chlorination of drinking water. Chlorine-based water purifying chemicals were first introduced in 1908. Chlorine water purification has practically eradicated diseases that were once devastating, such as cholera and dysentery. In 1992, the Public Health Advisory Board was created to guide the chlorine industry in issues pertaining to drinking water safety. Because it is so highly reactive, chlorine is one of the most effective germicides available. Chlorine chemicals kill bacteria, algae, fungi , and protozoans and inactivate viruses. Simple household bleach, sodium hypochlorite, can even inactivate and sterilize equipment from such deadly viruses as the Ebola virus . Chlorine is also used to sanitize pools and spas. Chlorine disinfectants are used to prevent institutional disease transmission in food preparation, day care centers, nursing homes, and hospitals.

Chlorine is a vital chemical in the pharmaceutical and medical industries. Approximately 85% of all drugs either contain chlorine or are manufactured using chlorine. The chemical addition of chlorine to a drug molecule can enhance its absorption and delay its elimination from the body, increasing the duration of its action. Also, chlorine can be used to create soluble forms of drugs that dissolve easily into solutions. When hydrochloric acid is reacted with poorly soluble drugs that are weak bases, the result is a chloride salt that is more soluble in water. Greater solubility enhances absorption and allows some drugs to be incorporated into syrups or elixirs. This is very useful since many people prefer oral liquid drug intake. Soluble drugs also have the advantage that they can be inhjected. Finally, about 25% of all medical equipment is made from chlorinated hydrocarbons , vinyl or PVC.

Chlorinated hydrocarbons are specific hydrocarbon molecules that also have atoms of the element chlorine chemically bonded to them. The number of carbon atoms and how they are arranged in three-dimensions determines the chemical and physical properties of these compounds. Because there is a wide variety of possible chlorinated hydrocarbons, this class of useful chemicals has a wide range of applications that are of great economic and practical importance. Chlorinated hydrocarbons include products such as the synthetic rubbers used in car tires and tennis shoes. They are also used in packaging plastics , and a variety of products such as fluid pipes, furniture, home siding, credit cards, fences, and toys. Chlorinated hydrocarbons also can be used as precursors in the production of non-stick coatings such as Teflon. Chlorine is also used in the manufacture of solvents such as carbon tetrachloride and trichloroethylene used in dry cleaning .

In addition to their use in the manufacture of polymers, rubbers, plastics, solvents, and cleaners, chlorinated hydrocarbons also are powerful pesticides. They rank among the most potent and environmentally persistent insecticides, and when combined with fluorine, they yield the refrigerants called chlorofluorocarbons , or CFCs. Because of their wide array of uses, chlorinated hydrocarbons are among the most important industrial organic compounds. Perhaps the best known chlorinated hydrocarbon insecticide is Dichloro-DiphenylTrichloroethane, or DDT. DDT was first used as an insecticide in 1939. After its effectiveness and relative safety to humans was established, the use of DDT burgeoned around the world in the war against disease-carrying and agricultural insect pests. However, the very success of DDT in the fight against insect-transmitted diseases (especially malaria ) subsequently led to its massive overuse. Widespread excessive use led to the emergence of DDT-resistant insects. Additionally, evidence started to show that toxic levels of DDT could accumulate in the fatty tissues of mammals, including humans. Because of such harmful effects, the use of DDT has now been banned in many countries despite its effectiveness. Another important issue with chlorinated hydrocarbons is their environmental persistence. Many, such as DDT, refrigerants, and solvents are not easily broken-down in the environent. Also, because other, less persistent, insecticide alternatives have been developed, the use of chlorinated insecticides has been drastically reduced.

Other toxic chemicals resistant to degradation also contain chlorine such as chlorinated and polychlorinated biphenyls (PCBs) and polyaromatic hydrocarbons (PAHs). These substances are known environmental pollutants, with PAHs found for example in diesel exhaust emissions and PCBs in industrial effluents, contaminating rivers as well as the Oceans. They have been shown to accumulate in whales and a variety of other marine mammals.

Another environmentally important chlorine compound class are the chemicals known as ChloroFluoroCarbons, or CFCs. Chlorofluorocarbons are single carbon atoms chemically bound to both chlorine and fluorine. The compounds trichlorofluoromethane (Freon-11) and dichlorodifluoromethane (Freon-12) are widely used CFCs. They are odorless, nonflammable, very stable compounds that are used as refrigerants in commercial refrigerators and air conditioners. CFCs are also used as aerosol propellants . Because CFCs are detrimental to the ozone layer, the portion of the atmosphere that blocks out the harmful wavelengths of ultraviolet light associated with skin cancer , they are being phased out as propellants and refrigerants. An ozone hole detected above Antarctica has been attributed by scientists to the release of CFCs into the atmosphere.

The Environmental Protection Agency (EPA) is concerned with toxic and persistent chlorine chemicals that are the byproducts of industrial activity. Two such byproducts are dioxins and furans , produced when organic (carboncontaining) compounds are heated to high temperatures in the presence of chlorine. Together, dioxins and furans represent a group of over two hundred chemicals. Some dioxins are extremely toxic and are a significant cause for concern to environmental agencies. Dioxin compounds are highly persistent in the environment . The most toxic dioxin is 2,3,7,8-tetrachlorodibenzo-p-dioxin or TCDD. Dioxins are also by-products of industrial processes involving chlorine such as waste incineration , chemical and pesticide manufacturing and pulp and paper bleaching. Dioxin is a known carcinogen and was the principal toxic component of Agent Orange . The EPA reports that incinerators are the largest source of dioxins and furans released into the environment in the United States. Other industrial sources of these toxic compounds include smelters, cement kilns, and fossil fuel burning power plants . Because of new standards for incineration, the levels of dioxins and furans have been steadily declining.

[Terry Watkins ]

RESOURCES

BOOKS

Khanna, N. "Chlorine Dioxide in Food Applications." In: Proceedings of the Fourth International Symposium, Chlorine dioxide: The state of science, regulatory, environmental issues, and case histories. Denver: American Water Works Association, 2002.

Stringer, Ruth, and Paul Johnston. Chlorine and the Environment: An Overview of the Chlorine Industry. New York: Kluwer Academic Publishers, 2001.

Watts, Susan. Chlorine (Elements). Tarrytown: Benchmark Books, 2001.

White, G.C. "Chlorination of Potable Water." In: The Handbook of Chlorination, 2nd Ed. New York: Nostrand Reinhold, 1986.

PERIODICALS

Bal'a, M.F.A. et al. "Moderate heat or chlorine destroys Aeromonas hydrophila biofilms on stainless steel." Dairy, Food, and Environmental Sanitation 19, (1999): 29-34.

McFarland, M. "Investigations of the environmental acceptability of fluorocarbon alternatives to chlorofluorocarbons." Proc. Natl. Acad. Sci. U.S.A 89, no. 3 (February, 1992): 807-811.

OTHER

Winter, Mark. Chlorine. WebElements Periodic Table. <http://www.webelements.com/webelements/scholar/index.html>

ORGANIZATIONS

Chlorine Chemistry Council (CCC), 1300 Wilson Boulevard, Arlington, VA USA 22209 703) 741-5000, , http://c3.org/index.html

Euro Chlor: Chlorine Online Information Ressource, Avenue E Van Nieuwenhuyse 4, box 2, Brussels, Belgium B-1160 + 32 2 676 7211, Fax: + 32 2 676 7241, Email: eurochlor@cefic.be , <http://www.eurochlor.org>

Chlorine

Chlorine is the non-metallic chemical element of atomic number 17, symbol Cl, atomic weight 35.45, melting point -149.8°F (-101°C), and boiling point -29.02°F (-33.9°C). It consists of two stable isotopes, of mass numbers 35 and 37. Ordinary chlorine is a mixture of 75.77% chlorine-35 atoms and 24.23% chlorine-37 atoms.

Chlorine is a highly poisonous, greenish yellow gas, about two and a half times as dense as air, and with a strong, sharp, choking odor. It was, in fact, one of the first poisonous gases used in warfare—in 1915 during World War I. In spite of its disagreeable nature, there are so many everyday products that contain chlorine or are manufactured through the use of chlorine that it is among the top ten chemicals produced in the United States each year. In 1994, more than 24 billion lb (11 billion kg) of chlorine were produced.

In nature, chlorine is widely distributed over the earth in the form of the salt (sodium chloride ) in sea water . At an average concentration of 0.67 oz (19 g) of chlorine in each liter of sea water, it is estimated that there are some 1016 tons of chlorine in the world's oceans. Other compounds of chlorine occur as minerals in the earth's crust, including huge underground deposits of solid sodium chloride.

Along with fluorine, bromine, iodine and astatine, chlorine is a member of the halogen family of elements in group 17 of the periodic table-the most non-metallic (least metallic) and most highly reactive group of elements. Chlorine reacts directly with nearly all other elements; with metals, it forms salts called chlorides. In fact, the name halogen, meaning salt producer, was originally defined for chlorine (in 1811 by J. S. C. Schweigger), and it was later applied to the rest of the elements in this family.

History of chlorine

The most common compound of chlorine, sodium chloride, has been known since ancient times; archaeologists have found evidence that rock salt was used as early as 3000 b.c. The first compound of chlorine ever made by humans was probably hydrochloric acid (hydrogen chloride gas dissolved in water), which was prepared by the Arabian alchemist Rhazes around a.d.900. Around a.d.1200, aqua regia (a mixture of nitric and hydrochloric acids) began to be used to dissolve gold; it is still the only liquid that will dissolve gold. When gold dissolves in aqua regia, chlorine is released along with other nauseating and irritating gases, but probably nobody in the thirteenth century paid much attention to them except to get as far away from them as possible.

The credit for first preparing and studying gaseous chlorine went to Carl W. Scheele (1742-1786) in 1774. Scheele was a Swedish chemist who discovered several other important elements and compounds, including barium , manganese, oxygen , ammonia, and glycerin. Scheele thought that chlorine was a compound, which he called dephlogisticated marine acid air. All gases were called airs at that time, and what we now know as hydrochloric acid was called marine acid because it was made from sea salt. The word dephlogisticated came from a completely false theory that slowed the progress of chemistry for decades and which is best left unexplained.

It was not until 1811 that Sir Humphry Davy (1778-1829) announced to the Royal Society of London that chlorine gas was an element. He suggested the name chlorine because it is the same pale, yellowish green color that sick plants sometimes develop, a color that is known as chloros in Greek. (The sick plants are said to have chlorosis.)

Properties and uses of chlorine

Because it is so reactive, chlorine is never found alone-chemically uncombined—in nature. It is prepared commercially by passing electricity through a water solution of sodium chloride or through molten sodium chloride.

When released as the free element, chlorine gas consists of diatomic (two-atom) molecules, as expressed by the formula Cl2. The gas is very irritating to the mucous membranes of the nose, mouth and lungs. It can be smelled in the air at a concentration of only 3 parts per million (ppm); it causes throat irritation at 15 ppm, coughing at 30 ppm, and is very likely to be fatal after a few deep breaths at 1,000 ppm.

Chlorine gas dissolves readily in water, reacting chemically with it to produce a mixture of hydrochloric acid (HCl) and hypochlorous acid (HOCl), plus some unreacted Cl2. This solution, called chlorine water, is a strong oxidizing agent that can be used to kill germs or to bleach paper and fabrics. It is used to obtain bromine (another member of its halogen family) from sea water by oxidizing bromide ions to elemental bromine.

In organic chemistry, chlorine is widely used, not only as an oxidizing agent, but as a way of making many useful compounds. For example, chlorine atoms can easily replace hydrogen atoms in organic molecules. The new molecules, with their chlorine atoms sticking out, are much more reactive and can react with various chemicals to produce a wide variety of other compounds. Among the products that are manufactured by the use of chlorine somewhere along the way are antiseptics, dyes, explosives , foods, insecticides , medicines, metals, paints, paper , plastics , refrigerants, solvents, and textiles .

Probably the most important use of chlorine is as a water purifier. Every water supply in the United States and in much of the rest of the world is rendered safe for drinking by the addition of chlorine. Several chlorine-releasing compounds are also used as general disinfectants.

Bleaching is another very practical use of chlorine. Until it was put to use as a bleach around 1785, bright sunlight was the only way people could bleach out stains and undesired colors in textiles and paper. Today, in the form of a variety of compounds, chlorine is used almost exclusively. Here's how it works: many compounds are colored because their molecules contain loose electrons that can absorb specific colors of light , leaving the other colors unabsorbed and therefore visible. An oxidizing agent such as chlorine water or a compound containing the hypochlorite ion OCl- removes those electrons (an oxidizing agent is an electron remover), which effectively removes the substance's light-absorbing power and therefore its color. Ordinary laundry bleach is a 5.25% solution of sodium hypochlorite in water.

Among the important organic compounds containing chlorine are the chlorinated hydrocarbons-hydrocarbons that have had some of their hydrogen atoms replaced by chlorine atoms. A variety of chlorinated hydrocarbons have been used as insecticides. One of the earliest to be used was DDT, dichlorodiphenyl-trichloroethane. Because it caused serious environmental problems, its use has largely been banned in the United States. Other chlorinated hydrocarbons that are used as pesticides include dieldrin, aldrin, endrin, lindane, chlordane , and heptachlor. Because all of these compounds are very stable and do not degrade easily, they also have serious environmental drawbacks.

Compounds of chlorine

Following are a few of the important compounds of chlorine.

Calcium hypochlorite, CaOCl: A white powder known as bleaching powder and used for bleaching and as a swimming pool disinfectant. Both its bleaching and its disinfectant qualities come from its chemical instability: it decomposes to release chlorine gas.

Chlorates: Chlorates are compounds of metals with the anion ClO−3. An example is potassium chlorate, KClO3. Chlorates can cause explosions when mixed with flammable materials, because the chlorate ion decomposes under heat to release oxygen, and the oxygen speeds up the combustion process to explosive levels. Potassium chlorate is used in fireworks.

Chlorides: Chlorides are the salts of hydrochloric acid, HCl. They are compounds of a metal with chlorine and nothing else. Some common examples are sodium chloride (NaCl), ammonium chloride (NH4Cl), calcium chloride (CaCl2), and magnesium chloride (MgCl2). When dissolved in water, these salts produce chloride ions, Cl-. Polyvinyl chloride, the widely used plastic known as PVC, is a polymer of the organic chloride, vinyl chloride.

Freons are hydrocarbons with fluorine and chlorine atoms substituted for some of the hydrogen atoms in their molecules. They have been widely used as the liquids in refrigerating machines and as propellants in aerosol spray cans. They have been implicated in destroying the ozone layer in the upper atmosphere, however, and their use is now severely restricted.

See also Chlorination; Chloroform; Dioxin; Halogenated hydrocarbons; Halogens; Hydrochlorofluorocarbons; Mercurous chloride.

Resources

books

Emsley, John. Nature's Building Blocks: An A-Z Guide to theElements. Oxford: Oxford University Press, 2002.

Greenwood, N.N., and A. Earnshaw. Chemistry of the Elements. Oxford: Butterworth-Heinneman Press, 1997.

Sconce, J.S. Chlorine, Its Manufacture, Properties and Uses. New York: Reinhold, 1962.

periodicals

"Chlorine Industry Running Flat Out Despite Persistent Health Fears." Chemical & Engineering News (November 21, 1994).

Robert L. Wolke

Chlorine

melting point: −101.5°C
boiling point: −34°C
density: 3.214 g/cm³ (liq. at 0°C)
most common ions: Cl¹⁻, Cl¹⁺, Cl³⁺, Cl⁵⁺, Cl⁷⁺

Chlorine is one of the halogen family of elements and the first of that family to be discovered. Swedish chemist Carl Wilhelm Scheele produced chlorine in 1774 by the reaction of manganese dioxide (MnO₂) with a solution of hydrochloric acid (HCl). In 1810 English chemist Sir Humphry Davy determined chlorine to be an element and named it for the color of the gas (the Greek word chloros means "pale green").

Elemental chlorine is a diatomic molecule and is toxic. It has a high electronegativity that is about equal to that of oxygen. The most common oxidation numbers are 7, 5, 3, 1, and −1.

Chlorine is the most abundant of the halogens having a concentration of 19.87 parts per thousand (ppt) by weight in seawater and an average of 0.17 ppt in Earth's crust. It is produced commercially by electrolysis of seawater and brines. Sodium chloride (NaCl), or common table salt, can be obtained from seaside evaporation pools or mined from underground deposits.

Chlorine and its compounds have a large number of everyday uses. Chlorine is used in water purification as well in the production of safe plumbing components constructed of polyvinyl chloride (PVC). Chlorinated dyes, medicines, pesticides, disinfectants, and solvents have widespread applications. Chlorofluorocarbon compounds (CFCs) were once widely used as refrigerants, solvents, foaming agents, and spray-can propellants, but are now banned by international agreement due to the role of those compounds in the depletion of Earth's protective ozone layer. Chlorine was used during World War I (1914–1918) as a chemical warfare agent.

see also Bleaches; Davy, Humphry; Scheele, Carl.

John Michael Nicovich

Bibliography

Lide, David R., ed. (2003). The CRC Handbook of Chemistry and Physics, 84th edition. Boca Raton, FL: CRC Press.

Internet Resources

Winter, Mark. "Chlorine." The University of Sheffield and WebElements Ltd., UK. Available from <http://www.webelements.com>.