Iodine (I)
Iodine (I) is a chemical element with the atomic number 53 and is categorized as a halogen in Group VII of the periodic table. It appears as a purple-black solid with metallic sheen and has unique properties, such as easy sublimation and a melting point of 113.7° Celsius. Iodine is primarily sourced from brines and caliche ores, with significant production occurring in countries like Chile, Japan, and China. It plays a critical role in various applications, including animal feed supplements, photographic materials, disinfectants, and notably in iodized salt, which is essential for preventing goiter and other iodine deficiency-related health issues.
This trace element is crucial for thyroid hormone production in humans, and while deficiency can lead to serious health problems, excessive iodine intake may also pose risks. Historical usage of iodine began in the early 19th century when it was discovered from algae ash. Today, iodine's applications continue to evolve, with uses in pharmaceuticals, sanitation, and potential environmental solutions, such as alternatives to ozone-depleting substances. Global awareness of iodine’s health benefits and its roles in various industries highlights its importance in modern society.
Iodine (I)
Where Found
Iodine is widely distributed at a low concentration. However, only in brines and caliche ores is the concentration sufficient to make separation practical. The largest producers of iodine are Chile, followed by Japan, China, Turkmenistan, Russia, Azerbaijan, Indonesia, and Uzbekistan.
Primary Uses
Iodine is used primarily in animal feed supplements, catalysts, inks, colorants, photographic equipment, and disinfectants. An important use is in iodized salt, which prevents goiter.
Technical Definition
Iodine (abbreviated I), atomic number 53, belongs to Group VII (the halogens) of the periodic table of the elements and resembles chlorine in its chemical properties. One stable isotope exists with an atomic weight of 126.9045. At room temperature, iodine is a purple-black color with a metallic sheen. Its elemental form is diatomic (two atoms of iodine bonded together). The solid has a density of 4.942 grams per cubic centimeter and sublimes easily. The melting point of iodine is 113.7° Celsius, and the boiling point is 184.5° Celsius.
Description, Distribution, and Forms
Iodine is the sixtieth element in order of abundance, at 0.46 part per million in the Earth’s crust. Commercial deposits are usually iodates such as lautarite Ca(IO3)2 and dietzeite 7Ca(IO3)2·8CaCrO4. Some brines in Louisiana, California, and Michigan contain 30 to 40 parts per million iodide ion, while some Japanese brines contain 100 parts per million. Iodine is only 0.05 part per million in seawater, but some sea plants concentrate iodine up to 0.45 percent (4,500 parts per million) of their dry weight.
Iodine is a necessary trace element in animals. An iodine deficiency may cause a range of problems, including goiter, mental retardation, increased stillbirths and miscarriages, and the severe mental and physical handicaps of cretinism. Common table salt (“iodized” salt) contains iodine at a 0.01 percent level, which is enough to safely prevent these ailments. Iodine is used in the body to produce the growth-regulating hormone thyroxine. An excess of iodine may lead to thyroid cancer or interfere with hormone production. Although throughout history, iodine shortage has normally been the problem, the use of iodine in animal feed, sanitizers, and food processing causes Americans to consume many times the recommended daily allowance of iodine. The effects of this are not truly known, but it may prove to be unhealthy. Iodine is highly toxic to plants and does not appear to be necessary for plant life.
History
In 1811, Bernard Courtois, the son of a saltpeter manufacturer, first noticed iodine while extracting compounds from the ash of algae gathered along the seashore. He observed a cloud of violet vapor and an irritating odor. Courtois tested the dark crystals that formed on cold objects as well as he could in his simple laboratory. Because he suspected that this was a new element, he provided samples to two of his friends, Charles-Bernard Desormes and Nicolas Clément at the Conservatoire des Arts et des Métiers. With better equipment, they continued the investigation of this new substance and announced the discovery of iodine in 1813. The name comes from the Greek word iodes, for “violetlike.” The first iodine-containing mineral was found in Mexico in 1825. The discovery of iodate as a contaminant of the Chile saltpeter beds was an even more important discovery.
Obtaining Iodine
The method of iodine production depends on the source of the iodine. From the Chilean saltpeter beds, the sodium iodate is dissolved by an alkaline solution, converted to iodide ion by reaction with sodium hydrogen sulfite, and iodine is then precipitated by adding iodate solution. From brines, the iodide ion is converted to iodine by reaction with chlorine. Air blowing through the solution collects the iodine, which then precipitates. Purification is by resublimation. In an alternate method the iodide ion is precipitated with silver ion, reacted with iron to make iron iodide, and reacted with chlorine to produce iodine. Another method uses an ion-exchange resin to collect the iodine after it has reacted with chlorine. The annual production of iodine is about 25,000 metric tons.
Uses of Iodine
Iodine has a multitude of small-percentage usages. It is difficult to track percentages of iodine devoted to specific consumer end uses, because many intermediate iodine compounds—such as ethyl and methyl iodide, crude iodine, potassium iodide, sodium iodide, povidine-iodine, and ethylenediamine dihydroiodide—are marketed to manufacturers before end-use patterns can be established.
Iodine is used in catalysts for synthetic rubber manufacture, stabilizers, dyestuffs, pigments, sanitizers, pharmaceuticals, lithium-iodine batteries, high-purity metals, motor fuels, lubricants, and photographic chemicals for high-speed negatives (a declining use with the advent of digital cameras and other digital-imaging systems). An alcohol solution of iodine called tincture of iodine is a well-known antiseptic. A possible use may be in trifluoromethyl iodide (CF3I) as a replacement for chlorofluorocarbons (CFCs) as refrigerants. The trifluoromethyl iodide does not cause the damage to the ozone layer that the CFCs do.
Radioactive iodine, either I-123 or I-131, can be used to treat thyroid disease, including cancer, or as a contrast agent in generating medical images, particularly of the thyroid. Iodine can also be used as a contrast agent in producing X rays of soft tissue such as the gallbladder. Uses of iodine will continue to develop, as it is a reactive element that forms compounds with every group of elements except the noble gases.
Global consumption for health and sanitation—to combat diseases caused by iodine deficiencies and to treat water, for example—is on the rise, as is the use of iodine in compounds designed to take the place of ozone-depleting CFCs.
Bibliography
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Greenwood, N. N., and A. Earnshaw. “The Halogens: Fluorine, Chlorine, Bromine, Iodine, and Astatine.” In Chemistry of the Elements. 2d ed. Boston: Butterworth-Heinemann, 1997.
Hetzel, Basil S. The Story of Iodine Deficiency: An International Challenge in Nutrition. New York: Oxford University Press, 1989.
Kogel, Jessica Elzea, et al., eds. “Iodine.” In Industrial Minerals and Rocks: Commodities, Markets, and Uses. 7th ed. Littleton, Colo.: Society for Mining, Metallurgy, and Exploration, 2006.
Massey, A. G. “Group 17: The Halogens: Fluorine, Chlorine, Bromine, Iodine, and Astatine.” In Main Group Chemistry. 2d ed. New York: Wiley, 2000.
Mertz, Walter, ed. Trace Elements in Human and Animal Nutrition. 5th ed. 2 vols. Orlando, Fla.: Academic Press, 1986-1987.
U.S. Geological Survey.