Chlorine
From Encyclopædia
The chemical element chlorine is a greenish yellow gas with a sharp, disagreeable odor. It belongs to the HALOGENS, a group that includes the elements bromine,
fluorine,
iodine, and astatine. Its chemical symbol is Cl,
atomic number 17, and
atomic weight 35.453. Chlorine is about 2.5 times as dense as air and moderately soluble in water, forming a
Pale yellowish green solution. The name "chlorine," from a Greek word meaning "greenish yellow," was given to the gas by Sir Humphry DAVY, who proved (1810) it was an element. The gas had been discovered some 30 years earlier by Swedish chemist Karl Wilhelm SCHEELE.Because chlorine is very reactive, it never occurs free in nature. Chemically bound as an ion, the
chloride ion constitutes 0.15% of the Earth's crust and 1.9% of
seawater. In the human body,
chloride and
sodium ions are the main ions in extracellular materials. They are involved in maintaining osmotic equilibrium.
chloride ion is needed for the
stomach's digestive juices. Lost in great quantities through perspiration, the ion is readily resupplied by the salt content of the normal diet. Persons on low-salt diets, such as vegetarians, may be advised to take salt supplements.Toxicity and PrecautionsChlorine was used in
world War I as a poison gas. It is very corrosive to moist tissue and has a very irritating effect on the lungs and mucous membranes of the
nose and throat. Inhalation of chlorine gas can cause
edema of the lungs and respiratory stoppage.Chlorine is easily liquefied and is usually transported in its liquid state in pressurized drums. Great care must be taken, however, to prevent the containers from bursting and liberating large amounts of the gas. In the
United States and most European countries, large quantities of chlorine may only be transported by train.The present trend is to limit the transport of chlorine as much as possible by producing and using the element in the same location. In the production of PVC (polyvinyl
chloride) plastic, for example, the raw material ethylene is shipped to the chlorine
plant and the chlorinated product returned. Although this procedure is more expensive, the dangerous transport of chlorine is avoided.Chemical PropertiesLike all members of the halogen group, chlorine has a strong tendency to gain one
electron and become a
chloride ion. Chlorine is less active than
fluorine but more so than bromine and
iodine. It readily reacts with
metals to form
chlorides, most of which are soluble in water. Chlorine also reacts directly with many nonmetals such as
sulfur, phosphorus, and other halogens. When hydrogen and chlorine gases are mixed together, the mixture is stable if kept in a cool, dark place. If heated or exposed to sunlight, the mixture explodes violently.Chlorine can support
combustion; a candle thrust into a vessel of chlorine continues to burn, releasing dense, black clouds of smoke. The chlorine combines with the hydrogen of the paraffin, forming hydrogen
chloride, and uncombined
carbon is left in the form of soot.Chlorine forms five different oxides: dichlorine monoxide; chlorine monoxide; chlorine dioxide; chlorine hexoxide; and chlorine heptoxide.PreparationBy far the most important method for the preparation of chlorine is the electrolysis of a solution of common salt,
sodium chloride. The chlorine gas is liberated at the positive anode, which is made of graphite since a
metal anode would react with chlorine. At the
iron cathode,
sodium ions are reduced to
sodium metal, which reacts immediately with water to form
sodium hydroxide.Another method of preparing chlorine is by the electrolysis of molten salt. This process is used specifically to produce
sodium, and the chlorine is a commercial by-product. When large quantities of waste hydrochloric acid are available--for example, from the chlorination of
carbon compounds--chlorine may be recovered by oxidation of the acid. Although the oxidation proceeds only at high temperatures and
pressures and is not economical, it has the advantage of converting great quantities of waste acid to useful substances.No matter what process is used to prepare chlorine, the gas must be well dried. Dry chlorine is much less corrosive than moist chlorine gas. In the laboratory chlorine may be prepared by heating
manganese oxide with hydrochloric acid.Chlorine CompoundsChlorine is one of the most important basic chemicals. The development of the
chemical industry in a given country is sometimes measured by the amount of chlorine used. The following are the most important products made from chlorine gas.Bleaching
agents: The bleaching action of chlorine in aqueous solution is due to the formation of hypochlorous acid, a powerful oxidizing
agent (see OXIDATION AND REDUCTION). If a colored, oxidizable material is present, hypochlorous acid releases its
oxygen to oxidize the material to a colorless compound. Liquid bleach is usually an aqueous solution of
sodium hypochlorite, and dry powdered bleaches contain
chloride of lime (calcium hypochlorite). Because chlorine destroys
silk and wool, commercial hypochlorite bleaches should never be used on these fibers.Disinfectant: The oxidizing ability of
chloride of lime enables it to destroy bacteria; therefore large quantities are used to treat municipal water systems. This chemical is also used in
swimming pools and for treating sewage.
monomers: Many chlorinated organic compounds can be polymerized into useful synthetics (see
plastics). Phosgene is used, among other compounds, for the preparation of polyurethanes; VINYL
chloride is used for PVC; and chloroprene is used for a type of synthetic rubber.
solvents: Large quantities of
chloroform,
carbon TETRACHLORIDE, trichloroethylene, and perchloroethylene are used as industrial
solvents for dissolving and diluting other organic compounds.Pesticides: Although DDT (see DDT) has come into disrepute because of its adverse ecological impact, it is still used in parts of the
world where the emphasis is placed on highest possible crop yields. Dieldrin and pentachlorophenol are other chlorinated compounds are effective in controlling insect pests (see
PESTICIDE).Starting Materials: Numerous chlorine-containing compounds are used as reactants to produce other important chemicals. Some of the major starting compounds are chlorobenzene, chloroacetone, ethyl
chloride, and epichlorohydrin.In addition to these products, for which the preparation of chlorine gas is needed, various other chlorine compounds play an important part in chemistry and the
chemical industry. These include common salt and a series of other important
chlorides. The
chlorides of most
metals are easily soluble in water, which widens their applicability. Also important are the chlorates, the perchlorates, and the hypochlorites. HYDROCHLORIC ACID is one of the most frequently used acids.Stephen FleishmanBibliography: [[Asimov, Isaac|
Isaac Asimov]], Building Blocks of the Universe, rev. ed. (1974);
Downs, A. J., The Chemistry of Chlorine, Bromine,
iodine and Astatine (1975); Hamilton, E. I., The Chemical Elements and Man (1978); Nechamkin, Howard, Chemistry of the Elements (1968); Ruben, Samuel, Handbook of the Elements, 2d ed. (1967; repr. 1985); Trifon?v, D. N., Chemical Elements: How They Were Discovered (1985).
