Bromine
Bromine is an element that belongs to Group 17 on the periodic table of elements, the group that contains substances called halogens. Other halogens include fluorine, chlorine, and iodine. Halogens are important elements that are used in heavily in industry. Bromine is used in the manufacture of dyes, fumigants, fire-retardant materials, medicines, pesticides, and photographic emulsions. Bromine is also used for water purification. While bromine and bromine-containing products are very useful, some concern surrounds the use of some bromine compounds because of their impact on the environment, particularly the ozone layer.
The many uses of bromine are due to its chemical properties. The halogens are the most reactive non-metal elements in the periodic table. Halogens characteristically react very rapidly and readily with almost all metals to form salts. The high reactivity of the halogens is due to the presence of seven (rather than the stable noble-gas configuration of eight) electrons in their outer shell. Thus, they have an unpaired electron that is readily available for chemical bonding and reactions with other elements. Unlike chlorine and iodine, bromine is a liquid at room temperature. It is the only liquid nonmetallic element.
Bromine is a fairly heavy inorganic element. First isolated in 1826, the atomic number of bromine is 35, which means it has 35 electrons and 35 protons. Bromine also contains 45 neutrons in its nucleus, making its average atomic mass 79.9 atomic mass units. At room temperature, it is a reddish-brown liquid that emits pungent, noxious fumes that irritate the eyes. Appropriately, the word bromine is derived from the Greek word bromos, which means "stench." Bromine readily dissolves in water and carbon disulfide. It has a boiling point of 138.6°F(59.2°C) and is more reactive than iodine, but less reactive than chlorine. When bromine chemically reacts with other elements or compounds, it has a bleaching effect. It is a fairly strong oxidizing agent. Relative to the other halogens, bromine is a stronger oxidizing agent than iodine, but a weaker oxidizer than both chlorine and fluorine. Even so, liquid bromine is very caustic and can seriously damage skin on contact. Bromine, like the other halogens, has a very high affinity for itself, and therefore forms diatomic molecules, which are molecules that contain only two atoms. Molecular bromine exists as two bromine atoms bonded together.
Bromine is obtained from natural salt deposits. Two areas within the United States that have historically been associated with bromine extraction are Arkansas and Michigan. Bromine may also be extracted from seawater. Aqueous bromine may also be produced from the oxidation of bromides during chlorination of water, especially when seawater is used as a coolant. Since it contains a very slight concentration of bromine salts (about 85 parts per million), seawater is not a major source of industrial bromine. The most common form of mineralized bromine is silver bromide (bromargyrite), found chiefly in Mexico and Chile. Most of the supply of bromine used for industrial purposes, however, comes from the United States and Israel, with smaller amounts produced in France, Japan, and Russia.
Bromine-containing substances have been used for centuries. The first use of a material containing bromine was in ancient Rome. During the Roman Empire, a highly prized purple dye was painstakingly extracted from marine mussels. The dye, a bromine compound, was very expensive because it was so difficult to obtain and only the very wealthy could afford clothing dyed with the hue of this dye, which resulted in the term "royal purple." Nowadays, bromine is used not only the production of dyes, but also in the production of chemicals that improve safety, agriculture, and sanitation.
Bromine is a very effective agent in controlling the growth of aquatic microorganisms. As such, and like chlorine, it is used for water treatment because it can kill microorganisms and keep water clear and free from foul odors. For example, bromine salts, like sodium bromide, are used to control the growth of algae and bacteria in recreational spaces like hot tubs and swimming pools. Another common bromine aquatic biocide is bromochlorodimethylhydantoin. Occasionally, bromine is used by municipalities to control disease-causing microbes in drinking water because some of these microorganisms are more susceptible to bromine than chlorine.
Bromine compounds are also used as pesticides. Like other halogenated hydrocarbons such as DDT, some brominated hydrocarbons are powerful insecticides. A very effective and important bromine-containing hydrocarbon pesticide is methyl bromide. This agent, also known as bromomethane, is used to fumigate stored grain and produce to free them from pest infestations. Methyl bromide is also used to fumigate soil for valuable crops like strawberries, peppers, eggplants, tomatoes, and tobacco. Since the 1940s, methyl bromide has been used as a soil pretreatment to kill insect, roundworm, and weed species that decrease the productivity of economically important crops. In 1992, approximately 73,000 tons of methyl bromide were used. However it was discovered that methyl bromide, like chlorofluorocarbons, also contributes to the depletion of the ozone layer. As a result, the Environmental Protection Agency (EPA) made recommendations that its use be phased-out based upon the expert assessments of atmospheric scientists from the World Meteorological Organization and the National Oceanic and Atmospheric Administration. Under the Clean Air Act, the EPA is enforcing reductions in the use of methyl bromide. A mandatory reduction of 25% was achieved in 1999. By 2001, 50% reduction had been enforced. By the year 2003, a 70% reduction in the use of methyl bromide must be attained with a goal for a complete ban of the compound scheduled for the year 2005.
Despite the ban on methyl bromide, bromine will continue to be a valued chemical. The pharmaceutical industry relies heavily on bromine both in the manufacturing process and as constituent substance in pharmaceutical agents. The general anesthetic, halothane, contains bromine. The production of naproxen sodium, an over-the-counter non-steroidal analgesic, uses bromine in intermediate reactions. Brompheniramine, the widely available antihistamine cold and allergy medication, contains bromine. The synthetic addition of bromine, or other halogens, to medications such as these facilitates their uptake into tissues. Other drugs use bromine to create easily absorbed dosage forms. For example, dextromethorphan hydrobromide, a cough medication, is a soluble bromine salt form of the poorly soluble active drug dextromethorphan, and dissolves easily as a clear solution.
Bromine is also used to create fire-resistant plastics. Plastic is made of very flammable hydrocarbon polymers. The addition of bromine compounds creates fire-retardant plastic products. Brominated flame-retardants are used in televisions, stereos, computers, and electrical wiring to reduce fire hazard when these common electronic appliances generate excessive heat. Brommine-containing fire-retardant chemicals are also used in carpeting, draperies, and furniture foam padding. While bromine compounds make products more fire-resistant, they do not make them fire-proof. Rather, they reduce the likelihood that a plastic item will ignite and delay the spread of fire. As bromine-treated plastic products burn, they release brominated hydrocarbons that threaten the ozone layer not unlike chorofluorocarbons (CFCs). For this reason, research is now directed at finding alternatives to bromine flame-retardant chemicals. For example, promising new fire-resistant compounds use silicon.
Aside from its use in dyes, pesticides, water treatment, pharmaceuticals, and fire retardants, bromine compounds are also used in photographic film and print paper emulsions, hydraulic fluids, refrigeration fluids, inks, and hair products. As useful as bromine is, however, concern for the ozone layer has resulted in heightened vigilance concerning the overuse of bromine-containing chemicals.
Resources
Books
Nilsson, Annika. Ultraviolet Reflections: Life Under a Thinning Ozone Layer. New York: John Wiley & Son Ltd, 1996.
Price, D. et al. Bromine Compounds: Chemistry and Applications. New York: Elsevier Science Ltd., 1988.
Periodicals
Alaee, M. and Wenning, R. J. "The significance of brominated flame retardants in the environment: current understanding, issues and challenges." Chemosphere. 46, no. 5 (February, 2000): 579-582.
Anbar, A.D. et al. "Methyl bromide: ocean sources, ocean sinks, and climate sensitivity." Global Biogeochemistry Cycles 10, no. 1 (March, 1996): 175-190.
Rhew, R. C. et al. "Natural methyl bromide and methyl chloride emissions from coastal salt marshes." Nature 403, no. 6767 (January, 2000): 292-295.
Other
Winter, Mark. Bromine. WebElements Periodic Table. <http://www.webelements.com/webe lements/scholar/index.html>
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