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(ī`ədīn, –dĭn) [Gr.,=violet], nonmetallic chemical element; symbol I; at. no. 53; at. wt. 126.90447; m.p. 113.5°C;; b.p. 184.35°C;; sp. gr. 4.93 at 20°C;; valence −1, +1, +3, +5, or +7. Iodine is a dark-gray to purple-black, lustrous, solid element with a rhombic crystalline structure. It is the least active of the halogenshalogen
[Gr.,=salt-bearing], any of the chemically active elements found in Group 17 of the periodic table; the name applies especially to fluorine (symbol F), chlorine (Cl), bromine (Br), and iodine (I).
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, which are found in Group 17 of the periodic tableperiodic table,
chart of the elements arranged according to the periodic law discovered by Dmitri I. Mendeleev and revised by Henry G. J. Moseley. In the periodic table the elements are arranged in columns and rows according to increasing atomic number (see the table entitled
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. It is normally diatomic, i.e., it has 2 iodine atoms in each molecule, in the solid, liquid, and vapor states. When heated it passes directly from the solid to the vapor state (sublimation), the vapor having an intense violet color and a characteristic irritating odor. Iodine is only slightly soluble in water but dissolves readily in a solution of sodium or potassium iodide. Tincture of iodine is a solution of iodine and potassium iodide in alcohol. Iodine also dissolves in carbon disulfide, carbon tetrachloride, and chloroform, giving a deep violet solution. Iodine forms many compounds. With hydrogen it forms hydrogen iodide, which in water solution becomes hydriodic acid. It forms compounds with certain nonmetals (e.g., carbon, nitrogen, phosphorus, and oxygen) and with most metals. Iodine is displaced from its compounds by the other halogens. The element is obtained from salt deposits, as from the saltpeter beds in Chile, where it occurs in small quantities as an iodate, and from the salt brines associated with some oil wells in California and Louisiana. It is also found as an iodide in the ash of certain seaweeds. Iodine may be prepared by displacement from its compounds with chlorine. Treating an iodide with manganese dioxide and sulfuric acid sublimes the iodine. Iodine is important in medical treatment; tincture of iodine and iodoformiodoform
, CHI3, yellow crystalline solid that has a penetrating odor. It melts at 119°C; and is insoluble in water but soluble in ether or ethanol. Iodoform was formerly used as an antiseptic.
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 are widely used. Iodine is employed in the preparation of certain drugs and in the manufacture of some dyes. Silver iodide, a yellow salt, is used in photography; it is water insoluble and turns black when exposed to light. Starchstarch,
white, odorless, tasteless, carbohydrate powder. It plays a vital role in the biochemistry of both plants and animals and has important commercial uses. In green plants starch is produced by photosynthesis; it is one of the chief forms in which plants store food.
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 turns deep blue (almost black) in the presence of a small amount of iodine; this reaction serves as a test for either starch or iodine. Iodine in small amounts is essential to human nutrition; in the thyroid glandthyroid gland,
endocrine gland, situated in the neck, that secretes hormones necessary for growth and proper metabolism. It consists of two lobes connected by a narrow segment called the isthmus. The lobes lie on either side of the trachea, the isthmus in front of it.
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 it becomes a part of the iodine-containing hormones. Goiter, a swelling of the thyroid, is often a symptom of inadequate iodine in the diet. Iodine has only one stable isotope, iodine–127; it is the only isotope of iodine occurring in nature, although 24 iodine isotopes are known. Iodine–131 is a radioactive isotope with a half-life of 8 days. It is used medically to diagnose abnormalities of the thyroid gland. It is also a component of fallout produced by nuclear explosions. Iodine was discovered in 1811 by Bernard Courtois.



(Latin iodum) I, a chemical element of Group VII in the Mendeleev periodic system; one of the halogens (the symbol J also appears in the literature). Atomic number, 53; atomic weight, 126.9045. Grayish-black crystals with a metallic luster. Natural iodine is composed of one stable isotope with atomic number 127. It was discovered by the French chemist B. Cour-tois in 1811. Upon heating a mother liquor from seaweed ash with concentrated sulfuric acid, he observed the liberation of a violet vapor (hence the name “iodine,” from the Greek iodes or ioeides, “violet”), which condensed to brilliant dark lamellar crystals. In 1813–14, the French chemist J. L. Gay-Lussac and the English chemist H. Davy demonstrated the elemental nature of iodine.

Occurrence in nature. The average iodine content in the earth’s crust is 4 x 10 −5 percent by weight. Iodine compounds are dispersed in the mantle and magma and in the rocks that form from them (granite, basalt, and others); plutonic iodine minerals are unknown.

The history of iodine in the lithosphere is closely related to living matter and biogenic migration. Processes of iodine concentration are observed in the biosphere, particularly in marine organisms (seaweed and sponges). There are eight known supergene iodine minerals that form in the biosphere; however, they are extremely rare. The world’s oceans are the main reservoir of iodine for the biosphere (the average iodine content is 5 X 10−5 g/l). Iodine compounds dissolved in drops of seawater enter the atmosphere from the ocean and are carried by wind to the continents. (Areas located far from the ocean or isolated from marine winds by mountains are depleted of iodine.) Iodine is readily adsorbed by organic substances in the soil and marine sediment. Upon compaction of the sediment and subsequent formation of sedimentary rock, desorption occurs, and part of the iodine compounds passes into subterranean waters, forming iodobromine waters, which are used for extraction of iodine and are particularly characteristic of petroleum deposits (in places, the iodine content of such waters is more than 100/mg/l).

Physical and chemical properties. The density of iodine is 4.94 g/cm3; melting point, 113.5°C; and boiling point, 184.35°C. A molecule of liquid and gaseous iodine is composed, of tbwo atoms (I2). A high degree of dissociation (I2s⇆ 2l) is observed above 700°C, as well as upon exposure to light. Iodine evaporates even at room temperature, forming a violet vapor with a sharp odor. Upon slight heating it sublimates, precipitating in the form of fine lustrous flakes; this process is used in the laboratory and in industry for the purification of iodine. Iodine is poorly soluble in water (0.33 g/l at 25°C) and readily soluble in carbon disulfide and organic solvents (benzene, alcohol, and others) and aqueous iodide solutions.

The configuration of the outer electrons in an iodine atom is 5s25p5. As a result, iodine compounds exhibit variable valence (degree of oxidation):—1 (in HI and KI), +1 (in HIO and KIO), +3 (in ICI3), +5 (in HIO3 and KIO3), and + 7 (in HIO4 and KIO4). Iodine is chemically active, although to a lesser degree than chlorine and bromine. When heated to low temperatures, it interacts vigorously with metals to form iodides (Hg + I2 = HgI2). Iodine reacts with hydrogen only upon heating, and then not fully, forming hydrogen iodide; it does not combine directly with carbon, nitrogen, or oxygen. Elementary iodine is a less powerful oxidizing agent than chlorine and bromine. Hydrogen sulfide H2S, sodium thiosulfate Na2S2O3, and other reducing agents reduce iodine to I (I2 + H2S = S + 2HI). Chlorine and other strong oxidizing agents in aqueous solutions convert iodine into IO3 (5Cl2 + I2 + 6H2O = 2HIO3 + 10HCl). When dissolved in water, iodine reacts partially with it (I2 + H2O ⇆ HI + HIO); iodide and iodate are formed in hot aqueous solutions of alkalies (3I2 + 6NaOH = 5NaI + NaIO3 + 3H2O). Iodine is adsorbed by starch, staining it dark blue; this reaction is used in iodometry and in qualitative analysis for iodine detection.

Iodine vapor is toxic and irritates the mucous membranes. It has a cauterizing and disinfecting effect on skin. Iodine stains can be removed with soda or sodium thiosulfate solutions.

Preparation and use. Oil-well drilling waters are a source of raw material for commercial preparation of iodine in the USSR. In other countries, seaweed and mother liquors of Chile saltpeter (sodium nitrate), which contain up to 0.4 percent iodine in the form of sodium iodate, are used for this purpose. To extract iodine from oil-well waters, with an average iodine content of 20–40 mg/l in the form of iodiles, they are first treated with chlorine (2NaI + Cl2 = 2NaCl + I2) or nitrous acid (2NaI + 2NaNO2 + 2H2SO4 = 2Na2SO4 + 2NO + I2 + 2H2O). The extracted iodine is either adsorbed by active carbon or is blown out by air. The iodine adsorbed by carbon is treated with caustic alkali or sodium sulfite (I2 + Na2SO3 + H2O = Na2SO4 + 2HI). Free iodine is extracted from the reaction products with chlorine or sulfuric acid and an oxidizing agent, such as potassium dichromate (K2Cr2O7 + 7H2SO4 + 6NaI = K2SO4 + 3Na2SO„ + Cr2(SO4)3 + 312). If the blow-off method is used, iodine is absorbed by a mixture of sulfur dioxide and water vapor (2H2O + SO2 + I2 = H2SO4 + 2HI), and the iodine is subsequently displaced by chlorine (2HI + Cl2= 2HCL + I2). Raw crystalline iodine is purified by sublimation.

Iodine and its compounds are used primarily in medicine and analytical chemistry, as well as organic synthesis and photography. The use of iodine in industry is still negligible in terms of volume but is highly promising. The preparation of high-purity metals is based on the thermal dissociation of iodides.


Ksenzenko, V. I., and D. S. Stasinevich. Tekhnologiia broma i ioda. Moscow, 1960.
Pozin, M. E. Tekhnologiia mineral’nykh solei, 3rd ed. Leningrad, 1970. Chapter 8.
Rolsten, R. F. Iodidnye metally i iodidy metallov. Moscow, 1968.(Translated from English.)


Iodine in the organism. Iodine is an essential trace element for animals and humans. In the soil and plants of taiga-forest, non-chernozem, dry-steppe, desert, and mountain biogeochemical zones, the iodine content is either insufficient or is not balanced with certain other trace elements (cobalt, manganese, and copper); as a result, goiter is endemic in these zones. The average iodine content in soil is approximately 3 X 10−4 percent; in plants, 2 X 10−5 percent. There is little iodine in surface drinking water (10−7—10−9 percent). In coastal areas the iodine content in air may be as high as 50 micrograms (Mg) per cu m; in continental and mountainous regions, however, it is 1 or even 0.2 mg/m3.

Iodine absorption by plants depends on the content of iodine compounds in the soil and on the species of plant. Certain organisms (called iodine concentrators), such as fucuses, laminarias, and phyllophoras, accumulate up to 1 percent iodine; and certain sponges concentrate up to 8.5 percent in the skeletal substance spongin. Algae that concentrate iodine are used for commercial preparation of the element.

Iodine enters animal organisms through food, water, and air. Vegetable products and fodder are the primary sources of iodine. Iodine absorption occurs in the anterior sections of the small intestine. In the human body, 20–50 mg of iodine are accumulated, including approximately 10–25 mg in muscle tissue and 6–15 mg in the thyroid gland. Radioactive iodine (l31I and125I) is used to demonstrate that iodine accumulates in the mitochondria of epithelial cells in the thyroid, and is included in the diiodo- and moniodotyrosines (formed in the mitochondria), which are concentrated in the hormone tetraiodo-thyronine (thyroxine). Iodine is excreted from the organism primarily through the kidney (up to 70–80 percent) and the mammary, salivary, and sweat glands, and partially from the bile.

In different biogeochemical provinces, the daily iodine ration varies (for humans, 20–240 μg; for sheep, 20–400 μg). The iodine requirements of animals depend on the physiological condition of the animal, the season, the temperature, and the adaptation of the organism to the iodine content in the environment. The daily iodine requirement for humans and animals is approximately 3 μg per kilogram of weight (it increases during pregnancy and growth and under conditions of cooling). The introduction of iodine into an organism increases its basal metabolism, intensifies oxidation processes, tones the muscles, and stimulates sexual activity.

Iodized table salt, which generally contains 10–25 g of potassium iodide per ton, is used because of the usual iodine deficiency in food and water. The application of fertilizers containing iodine can double and triple the iodine content in farm crops.


Guthbertson, D. P. “Mikroelementy.” In Novoe v fiziologii domashnikh zhivotnykh, vol. 1. Moscow-Leningrad, 1958. (Translated from English.)
Turakulov, la. Kh. Biokhimiia ipatokhimiia shchitovidnoizhelezy. Tashkent, 1963.
Bersin, T. Biokhimiia gormonov. Moscow, 1964. (Translated from German.)
Rapoport, S. M. Meditsinskaia biokhimiia. Moscow, 1966. (Translatedfrom German.)


Iodine in medicine. Preparations containing iodine have antibacterial and fungicidal properties, as well as antiphlogistic and revulsive effects. They are applied externally to disinfect wounds and to prepare the area to be operated on. When taken internally, iodine preparations affect the metabolism and intensify the activity of the thyroid gland. Reduced doses of iodine (microiodine) inhibit thyroid activity, affecting the formation of tyrotropic hormones in anterior hypophyseal lobes. Since iodine has an effect on protein and fat (lipid) metabolism, it has come to be used in the treatment of atherosclerosis (it lowers the cholesterol level and increases fibrinolyic activity in the blood).

X-ray contrast substances containing iodine are used for diagnostic purposes.

In cases of prolonged use of iodine preparations and increased sensitivity to such treatment, symptoms of iodism may develop—rhinitis, hives, angioneurotic edema, salivation and lacrima-tion, and pustular eruptions on the skin. Iodine preparations should not be prescribed in cases of pulmonary tuberculosis, pregnancy, kidney disease, chronic pyoderma, hemorrhagic diathesis, and hives.

Radioactive iodine. Artificially radioactive isotopes of iodine (such as12SI,131I, and132I) are widely used in biology, and particularly in medicine, to determine the functional condition of the thyroid gland and to treat a number of related diseases. The use of radioactive iodine in diagnostics is associated with the selective accumulation of iodine in the thyroid. Its therapeutic purpose is based on the ability of the beta radiation of iodine radioisotopes to destroy the secretory cells of the gland. In case of contamination of the environment by products of nuclear fission, radioactive iodine isotopes rapidly become part of the biological cycle, eventually entering milk and, consequently, the human body. Their effect on children is particularly harmful, since a child’s thyroid is one-tenth the size of an adult’s and therefore has a greater radiosensitivity. Preparations of stable iodine (100–200 mg per dose) are recommended to reduce the deposition of radioactive iodine isotopes in the thyroid gland. Radioactive iodine is rapidly and fully absorbed into the gastrointestinal tract and is selectively deposited in the thyroid. Its absorption depends on the functional condition of the gland. Relatively high concentrations of radioactive iodine are also detected in salivary and mammary glands and in the mucous glands of the gastrointestinal tract. Radioactive iodine that has not been absorbed by the thyroid is comparatively fully and rapidly excreted with the urine.



A nonmetallic halogen element, symbol I, atomic number 53, atomic weight 126.9045; melts at 114°C, boils at 184°C; the poisonous, corrosive, dark plates or granules are readily sublimed; insoluble in water, soluble in common solvents; used as germicide and antiseptic, in dyes, tinctures, and pharmaceuticals, in engraving lithography, and as a catalyst and analytical reagent.


a bluish-black element of the halogen group that sublimates into a violet irritating gas. Its compounds are used in medicine and photography and in dyes. The radioisotope iodine-131 (radioiodine), with a half-life of 8 days, is used in the diagnosis and treatment of thyroid disease. Symbol: I; atomic no.: 53; atomic wt.: 126.90447; valency: 1, 3, 5, or 7; relative density: 4.93; melting pt.: 113.5?C; boiling pt.: 184.35?C
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