Iodine (IPAEng|ˈaɪədaɪn, ˈaɪədɪn, or IPA|/ˈaɪədiːn/; from _el. "ιώδης iodes" "violet"), is a
chemical elementthat has the symbol I and atomic number53. Naturally-occurring iodine is a single isotope with 74 neutrons.
Chemically, iodine is the least reactive of the
halogens, and the most electropositive halogen after astatine. However, the element does not occur in the free state in nature. As with all other halogens(members of Group VII in the Periodic Table), when freed from its compounds iodine forms diatomic molecules (I2).
Iodine and its compounds are primarily used in
medicine, photography, and dyes. Although it is rare in the solar systemand Earth's crust, the iodides are very soluble in water, and the element is concentrated in seawater. This mechanism helps to explain how the element came to be required in trace amounts by all animals and some plants, being by far the heaviest element known to be necessary to living organisms.
Iodine under standard conditions is a dark-purple/dark-brown solid. It can be seen apparently sublimating at standard temperatures into a violet-pink gas that has an irritating odor. This halogen forms compounds with many elements, but is less reactive than the other members of its Group VII (halogens) and has some metallic light reflectance.
Elemental iodine dissolves easily in
chloroformand carbon tetrachloride. The solubility of elementary iodine can be vastly increased by the addition of potassium iodide. The molecular iodine reacts reversibly with the negative ion, creating the triiodideanion, I3−, which dissolves well in water. This is also the formulation of some types of medicinal (antiseptic) iodine, although tincture of iodineclassically dissolves the element in alcohol. The deep blue color of starch-iodine complexes is produced only by the free element.
Students who have seen the classroom demonstration in which iodine crystals are gently heated in a test tube to violet vapor may gain the impression that liquid iodine does not exist at atmospheric pressure. This misconception arises because the vapor produced has such a deep colour that the liquid appears not to form. In fact, if iodine crystals are heated carefully to just above their melting point of 113.7 °C, the crystals melt into a liquid which is present under a dense blanket of the vapor.
Iodine was discovered by
Bernard Courtoisin 1811. He was born to a manufacturer of saltpeter (a vital part of gunpowder). At the time of the Napoleonic Wars, Francewas at war and saltpeter was in great demand. Saltpeter produced from French niterbeds required sodium carbonate, which could be isolated from seaweedwashed up on the coasts of Normandyand Brittany. To isolate the sodium carbonate, seaweed was burned and the ash then washed with water. The remaining waste was destroyed by adding sulfuric acid. One day Courtois added too much sulfuric acid and a cloud of purple vapor rose. Courtois noted that the vapor crystallized on cold surfaces making dark crystals. Courtois suspected that this was a new element but lacked the money to pursue his observations.
However he gave samples to his friends,
Charles Bernard Desormes(1777–1862) and Nicolas Clément(1779–1841), to continue research. He also gave some of the substance to Joseph Louis Gay-Lussac(1778–1850), a well-known chemistat that time, and to physicist André-Marie Ampère(1775–1836). On 29 November 1813, Dersormes and Clément made public Courtois’ discovery. They described the substance to a meeting of the Imperial Institute of France. On December 6, Gay-Lussac announced that the new substance was either an element or a compound of oxygen. Ampère had given some of his sample to Humphry Davy(1778–1829). Davy did some experiments on the substance and noted its similarity to chlorine. Davy sent a letter dated December 10 to the Royal Society of Londonstating that he had identified a new element. A large argument erupted between Davy and Gay-Lussac over who identified iodine first but both scientists acknowledged Courtois as the first to isolate the chemical element.
Iodine is used in pharmaceuticals, antiseptics, medicine, food supplements, dyes, catalysts, halogen lights, photography, water purifying, and starch detection.
Tincture of iodine(10% elemental iodine in ethanol base) is an essential component of any emergency survival kit, used both to disinfect wounds and to sanitize surface water for drinking (3 drops per litre, let stand for 30 minutes). Alcohol-free iodine solutions such as Lugol's iodine, as well as other iodophortype antiseptics, are also available as effective elemental iodine sources for this purpose.
*Iodine compounds are important in the field of
*Iodine, as a heavy element, is quite
radio-opaque. Organic compounds of a certain type (typically iodine-substituted benzene derivatives) are thus used in medicineas X-ray radiocontrastagents for intravenous injection. This is often in conjunction with advanced X-ray techniques such as angiographyand CT scanning
Silver iodideis used in photography.
Tungsteniodide is used to stabilize the filaments in light bulbs.
*Iodine crystals are used in the process to make NI3 or
nitrogen triiodide. This compound is a shock-sensitive explosive when dry. It has commonly been used for pranks, but because of its extreme touch sensitivity, is not useful commercially.
Occurrence on earth
Iodine naturally occurs in the environment chiefly as a dissolved
iodidein seawater, although it is also found in some minerals and soils. The element may be prepared in an ultrapure form through the reaction of potassium iodidewith copper(II) sulfate. There are also a few other methods of isolating this element in the laboratory--for example the method used to isolate other halogens: oxidation of the iodide in hydroiodic acid(often made in situ with an iodide and sulfuric acid) by manganese dioxide(see below in "Descriptive chemistry"). Although the element is actually quite rare, kelpand certain plants and other algaehave some ability to concentrate iodine, which helps introduce the element into the food chain.
Iodine is found in the mineral caliche, found in
Chile, between the Andesand the sea. It can also be found in some seaweeds ( algae), as well as extracted from seawater.
Extraction from seawater involves
electrolysis. The brineis first purified and acidified using sulphuric acidand is then reacted with chlorine. An iodine solution is produced but it is yet too dilute and has to be concentrated. To do this airis blown into the solution which causes the iodine to evaporate, then it is passed into an absorbing tower containing acid where sulfur dioxideis added to reduce the iodine. The solution is then added to chlorine again to concentrate the solution more, and the final solution is at a level of about 99%.Fact|date=April 2007
Another source is from
kelp,a kind of brown alga. This source was used in the 18th and 19th centuries but is no longer economically viable.
In 2005, Chile was the top producer of iodine with almost two-thirds world share, followed by
Japanand the USA, reports the British Geological Survey.
Elemental iodine is poorly soluble in water, with one gram dissolving in 3450 ml at 20 °C and 1280 ml at 50 °C. By contrast with
chlorine, the formation of the hypohalite ion (IO–) in neutral aqueous solutions of iodine is negligible.
::I2+ H2O ↔ H+ + I− + HIO ("K" = 2.0×10−13)Advanced Inorganic Chemistry by Cotton and Wilkinson, 2nd ed.] Solubility in water is greatly improved if the solution contains dissolved
iodides such as hydroiodic acid, potassium iodide, or sodium iodide; this extra solubility results from the high solubility of the I3- ion. Dissolved bromides also improve water solubility of iodine. Iodine is soluble in a number of organic solvents, including ethanol(20.5 g/100 ml at 15 °C, 21.43 g/100 ml at 25 °C), diethyl ether(20.6 g/100 ml at 17 °C, 25.20 g/100 ml at 25 °C), chloroform, acetic acid, glycerol, benzene(14.09 g/100 ml at 25 °C), carbon tetrachloride(2.603 g/100 ml at 35 °C), and carbon disulfide(16.47 g/100 ml at 25 °C). [Merck Index of Chemicals and Drugs, 9th ed.] Aqueous and ethanol solutions are brown. Solutions in chloroform, carbon tetrachloride, and carbon disulfide are violet.
Elemental iodine can be prepared by oxidizing
iodides with chlorine:
::2I− + Cl2 → I2 + 2Cl−
::2I− + 4H+ + MnO2 → I2 + 2H2O + Mn2+
Iodine is reduced to
hydroiodic acidby hydrogen sulfide:General Chemistry (volume 2) by N.L. Glinka, Mir Publishing 1981]
::I2 + H2S → 2HI + S↓
hydrazine: ::2I2 + N2H4 → 4HI + N2
Iodine is oxidized to
iodateby nitric acid:General Chemistry by Linus Pauling, 1947 ed.]
::I2 + 10HNO3 → 2HIO3 + 10NO2 + 4H2O
::I2 + 2ClO3− → 2IO3− + Cl2
Notable inorganic iodine compoundsIodides"See also "
table iodine in biology
Iodine is an essential
trace element, the heaviest element known to be needed by living organisms. Its main role in animal biology is as constituents of the thyroid hormones, " thyroxine" (T4) and " triiodothyronine" (T3). These are made from addition condensation products of the amino acid tyrosine, and are stored prior to release in an iodine-containing enzymecalled thyroglobulin. T4 and T3 contain four and three atomsof iodine per molecule, respectively. The thyroid glandactively absorbs iodide from the bloodto make and release these hormones into the blood, actions which are regulated by a second hormone TSHfrom the pituitary. Thyroid hormones are phylogenetically very old molecules which are synthesized by most multicellular organisms, and which even have some effect on unicellularorganisms.
Thyroid hormones play a basic role in biology, acting on gene transcription to regulate the
basal metabolic rate. The total deficiency of thyroid hormones can reduce basal metabolic rate up to 50%, while in excessive production of thyroid hormones the basal metabolic rate can be increased by 100%. T4 acts largely as a precursor to T3, which is (with minor exceptions) the biologically active hormone.
Iodine accounts for 65% of the molecular weight of T4 and 59% of the T3. 15-20 mg of iodine is concentrated in thyroid tissue and hormones, but 70% of the body's iodine is distributed in other tissues, including mammary glands, eyes, gastric mucosa, the cervix, and salivary glands. Its role in mammary tissue is related to fetal and neonatal development, but its role in the other tissues is unknown. It has been shown to act as an antioxidant in these tissues.
Iodine may have a relationship with
selenium, and iodine supplementation in selenium-deficient populations may pose risks for thyroid function.
Human dietary intake
The United States
Recommended Daily Allowance(RDA) is 150 micrograms per day (μg/day) for both men and women, with a Tolerable Upper Intake Level (UL) for adults is 1,100 μg/day (1.1 mg/day). National Research Council. (2000). "Dietary Reference Intakes for Vitamin A, Vitamin K, Arsenic, Boron, Chromium, Copper, Iodine, Iron, Manganese, Molybdenum, Nickel, Silicon, Vanadium, and Zinc", pp. 258-259. National Acadamies Press. [http://books.nap.edu/openbook.php?record_id=10026&page=258 Free full-text available online] .] The tolerable upper limit was assessed by analyzing the effect of supplementation on thyroid-stimulating hormone.
Natural sources of iodine include sea life, such as kelp and certain seafood, as well as plants grown on iodine-rich soil. [International Council for the Control of Iodine Definiciency Disorders [http://www.iccidd.org/pages/iodine-deficiency/sources-of-iodine.php Sources of iodine] ]
Iodized saltis fortified with iodine. [http://www.nlm.nih.gov/medlineplus/ency/article/002421.htm MedlinePlus Medical Encyclopedia: Iodine in diet] ]
As of 2000, the median intake of iodine from food in the United States was 240 to 300 μg/day for men and 190 to 210 μg/day for women. In Japan, consumption is much higher due to the frequent consumption of seaweed or
kombukelp. Estimates range from 5,280 to 13,800 μg/day.cite journal |author=Patrick L |title=Iodine: deficiency and therapeutic considerations |journal=Altern Med Rev |volume=13 |issue=2 |pages=116–27 |year=2008 |month=June |pmid=18590348 |doi= |url=http://www.thorne.com/altmedrev/.fulltext/13/2/116.pdf] Although some Chinese data associates excess iodine with autoimmune thyroiditis and hypothyroidism, these effects have not been observed in Japanese populations, and a protective effect on breast cancer has been hypothesized.
Donald W. Miller believes that the daily FDA intake recommendation may be 100 times too low. [Miller, Donald W. [http://www.donaldmiller.com/JPandS%20Extrathyroidal%20Benefits%20of%20Iodine.pdf Extrathyroidal Benefits of Iodine] . "Journal of American Physicians and Surgeons".]
In areas where there is little iodine in the diet, typically remote inland areas and semi-arid equatorial climates where no marine foods are eaten,
iodine deficiencygives rise to hypothyroidism, symptoms of which are extreme fatigue, goitre, mental slowing, depression, weight gain, and low basal body temperatures.Fact|date=April 2007
Iodine deficiency is the leading cause of preventable
mental retardation, a result which occurs primarily when babies or small children are rendered hypothyroidic by a lack of the element. The addition of iodine to table salt has largely eliminated this problem in the wealthier nations, but as of March 2006, iodine deficiency remained a serious public health problem in the developing world. [ cite web | url = http://www.unicef.org/nutrition/index_iodine.html | title = Micronutrients - Iodine, Iron and Vitamin A | publisher = UNICEF] Iodine deficiency is also a problem in certain areas of Europe. In Germany it has been estimated to cause a billion dollars in healthcare costs per year.
Radioiodine and biology
Radioiodine and the thyroid
Human exposure to radioactive iodine will cause thyroid uptake, as with all iodine, leading to elevated chances of thyroid cancer. Isotopes with shorter half-lives such as I131 present a greater risk than those with longer half-lives since they generate more radiation per unit of time. Taking large amounts of regular iodine will saturate the thyroid and prevent uptake. Iodine pills are sometimes distributed to persons living close to nuclear establishments, for use in case of accidents that could lead to releases of radioactive iodine.
Iodine-123and iodine-125are used in medicine as tracers for imaging and evaluating the function of the thyroid.
*Noncombined (elemental) iodine is mildly toxic to all living things.
radioisotope131I (a beta emitter), has a half-lifeof 8.0207 days. Also known as radioiodine, 131I has been used in treating cancerand other pathologies of the thyroid glands. 123I is the radioisotope most often used in nuclear imaging of the kidneyand thyroid as well as thyroid uptake scans (used for the evaluation of Graves' Disease). The most common compounds of iodine are the iodides of sodiumand potassium(KI) and the iodates (KIO3). Potassium iodide(KI tablets, or "SSKI" = "Saturated Solution of KI" liquid drops) can be given to people in a nuclear disaster area when fission has taken place, to block the uptake of iodine-131 by the thyroid. The protective effect of KI lasts approximately 24 hours, so it should be dosed daily until a risk of significant exposure to radioiodines no longer exists. [ [http://www.fda.gov/Cder/drugprepare/KI_Q&A.htm#KI%20do Frequently Asked Questions on Potassium Iodide (KI) ] ] [ [http://www.fda.gov/cder/guidance/4825fnl.htm Potassium Iodide as a Thyroid Blocking Agent in Radiation Emergencies ] ] The exposure can be reduced by evacuation, sheltering, and by control of the food supply. Iodine-131 also decays rapidly, with a half-life of 8 days, so that 99.95% of the original radioiodine is gone after three months. Iodine-129129I ( half-life15.7 million years) is a product of cosmic ray spallationon various isotopes of xenonin the atmosphere, in cosmic ray muoninteraction with tellurium-130, and also and uraniumand plutoniumfission, both in subsurface rocks and nuclear reactors. Nuclear processes, in particular nuclear fuel reprocessing and atmospheric nuclear weapons tests have now swamped the natural signal for this isotope. 129I was used in rainwater studies following the Chernobyl accident. It also has been used as a groundwater tracer and as an indicator of nuclear waste dispersion into the natural environment.
Radioiodine and the kidney
In the 1970s imaging techniques were developed in
Californiato utilize radioiodine in diagnostics for renal hypertension.
There are 37
isotopes of iodine, but only one, 127I, is stable.
In many ways, 129I is similar to 36Cl. It is a soluble halogen, fairly non-reactive, exists mainly as a non-sorbing
anion, and is produced by cosmogenic, thermonuclear, and in-situ reactions. In hydrologic studies, 129I concentrations are usually reported as the ratio of 129I to total I (which is virtually all 127I). As is the case with 36Cl/Cl, 129I/I ratios in nature are quite small, 10−14 to 10−10 (peak thermonuclear 129I/I during the 1960s and 1970s reached about 10−7). 129I differs from 36Cl in that its halflife is longer (15.7 vs. 0.301 million years), it is highly biophilic, and occurs in multiple ionic forms (commonly, I− and IO3−) which have different chemical behaviors. This makes it fairly easy for 129I to enter the biosphere as it becomes incorporated into vegetation, soil, milk, animal tissue, etc.
Excesses of stable 129Xe in meteorites have been shown to result from decay of "primordial"
iodine-129produced newly by the supernovas which created the dust and gas from which the solar system formed. 129I was the first extinct radionuclideto be identified as present in the early solar system. Its decay is the basis of the I-Xe Iodine-xenon radiometric datingscheme, which covers the first 85 million years of solar systemevolution.
Effects of various radioiodine isotopes in biology are discussed above.
Excess iodine has symptoms similar to those of iodine deficiency. Commonly encountered symptoms are abnormal growth of the thyroid gland and disorders in functioning and growth of the organism as a whole. Elemental iodine, I2, is a deadly poison if taken in larger amounts; if 2–3 grams of it are consumed, it is fatal to humans. Iodides are similar in toxicity to
Direct contact with skin can cause lesions, so it should be handled with care. Iodine vapor is very irritating to the
eyeand to mucous membranes. Concentration of iodine in the air should not exceed 1 mg/m³ (eight-hour time-weighted average). When mixed with ammonia, it can form nitrogen triiodidewhich is extremely sensitive and can explode unexpectedly.
In the United States, the
Drug Enforcement Agency(DEA) regards iodine and compounds containing iodine (ionic iodides, iodoform, ethyl iodide, and so on) as reagents useful for the clandestine manufacture of methamphetamine. Persons who attempt to purchase significant quantities of such chemicals without establishing a legitimate use are likely to find themselves the target of a DEA investigation. Persons selling such compounds without doing due diligenceto establish that the materials are not being diverted to clandestine use may be subject to stiff penalties, such as expensive fines or even imprisonment. [http://www.usdoj.gov/dea/pubs/csa/872.htm 21 USC Sec. 872 01/22/02] ] [http://www.usdoj.gov/dea/pubs/states/newsrel/sanfran032205.html Chemical Supplier Convicted of Diversion of Iodine] ]
Iodide as an antioxidant
Chemical Oxygen Iodine Laser
Nutrition facts label
* [http://lpi.oregonstate.edu/infocenter/minerals/iodine/ "Micronutrient Research for Optimum Health", Linus Pauling Institute, OSU Oregon State University]
* [http://www.atsdr.cdc.gov/csem/iodine/ ATSDR - CSEM: Radiation Exposure from Iodine 131] U.S. Department of Health and Human Services (public domain)
* [http://chemicalelements.com/elements/i.html ChemicalElements.com - Iodine]
* [http://whqlibdoc.who.int/publications/2004/9241592001.pdf who.int, WHO Global Database on Iodine Deficiency]
* [http://www.iodinenetwork.net/ Network for Sustained Elimination of Iodine Deficiency]
* [http://www.organic-chemistry.org/chemicals/oxidations/iodine.shtm Oxidizing Agents > Iodine]
* [http://periodic.lanl.gov/elements/53.html Los Alamos National Laboratory: Iodine]
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