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, meaning "violet"), is a chemical element that has the symbol I and atomic number 53. Chemically, iodine is the least reactive of the halogens, and the most electropositive halogen after astatine. Iodine is primarily used in medicine, photography and dyes. It is required in trace amounts by most living organisms.
As with all other halogens (members of Group VII in the Periodic Table), iodine forms diatomic molecules, and hence, has the molecular formula of I2.
Occurrence on earth
Iodine naturally occurs in the environment chiefly as dissolved iodide in 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 iodide with copper(II) sulfate. There are also a few other methods of isolating this element. Although the element is actually quite rare, kelp and certain other plants have the ability to concentrate iodine, which helps introduce the element into the food chain as well as keeping its cost down.
Iodine is used in pharmaceuticals, antiseptics, medicine, food supplements, dyes, catalysts, halogen lights and photography.
There are 37 isotopes of iodine and 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 half-life 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" 129I produced newly by the supernovas which created the dust and gas from which the solar system formed. 129I was the first extinct radionuclide to be identified as present in the early solar system. Its decay is the basis of the I-Xe radiometric dating scheme, which covers the first 83 million years of solar system evolution.
Effects of various radioiodine isotopes in biology are discussed below.
Iodine is a dark-gray/purple-black solid that sublimes at standard temperatures into a purple-pink gas that has an irritating odor. This halogen forms compounds with many elements, but is less active than the other members of its Group VII (halogens) and has some metallic-like properties. Iodine dissolves easily in chloroform, carbon tetrachloride, or carbon disulphide to form purple solutions (It is only slightly soluble in water, giving a yellow solution). The deep blue color of starch-iodine complexes is produced only by the free element.
Many students who have seen the classroom demonstration where iodine crystals are gently heated in a test tube come away with the impression that liquid iodine cannot exist at atmospheric pressure. This misconception arises because sublimation occurs without the intermediacy of liquid. The truth is that if iodine crystals are heated carefully to their melting point of 113.7 °C, the crystals will fuse into a liquid, which will be present under a dense blanket of the vapour.
Iodine can be found in the Mineral Caliche, found in Chile, between the Andes and the sea. It can also be found in some seaweeds as well as extracted from seawater, however extracting Iodine from the mineral is the only economical way to extract the substance.[How to reference and link to summary or text]
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) 
Solubility in water is greatly improved if the solution contains dissolved iodides such as hydroiodic acid, potassium iodide, or sodium iodide. 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). 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–
or with manganese dioxide in acid solution:
- 2I– + 4H+ + MnO2 → I2 + 2H2O + Mn2+
Iodine is reduced to hydroiodic acid by hydrogen sulfide:
- I2 + H2S → 2HI + S↓
or by hydrazine:
- 2I2 + N2H4 → 4HI + N2
Iodine is oxidized to iodate by nitric acid:
- I2 + 10HNO3 → 2HIO3 + 10NO2 + 4H2O
or by chlorates:
- I2 + 2ClO3– → 2IO3– + Cl2
Iodine is converted in a two stage reaction to iodide and iodate in solutions of alkali hydroxides (such as sodium hydroxide):
I2 + 2OH– → I– + IO– + H2O (K = 30) 3IO– → 2I– + IO3– (K = 1020)
Iodine was discovered by Bernard Courtois in 1811. He was born to a manufacturer of saltpeter (potassium nitrate, a vital part of gunpowder). At the time France was at war, saltpeter, a component of gunpowder, was in great demand. Saltpeter produced from French niter beds required sodium carbonate, which could be isolated from seaweed washed up on the coasts of Normandy and 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 chemist at that time, and to 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 London stating 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 Barnard Courtois as the first to isolate the chemical element.
Notable inorganic iodine compounds
- Ammonium iodide (NH4I)
- Caesium iodide (CsI)
- Copper(I) iodide (CuI)
- Hydroiodic acid (HI)
- Iodic acid (HIO3)
- Iodine cyanide (ICN)
- Iodine heptafluoride (IF7)
- Iodine pentafluoride (IF5)
- Lead(II) iodide (PbI2)
- Lithium iodide (LiI)
- Nitrogen triiodide (NI3)
- Potassium iodide (KI)
- Silver iodide (AgI)
- Sodium iodide (NaI)
See also iodine compounds
Stable iodine in biology
Iodine is an essential trace element; its only known roles in biology are 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 a protein-like molecule called thryroglobulin. T4 and T3 contain four and three atoms of iodine per molecule, respectively. The thyroid gland actively absorbs iodide ion from the blood to make and release these hormones into the blood, actions which are regulated by a second hormone TSH from the pituitary. Thyroid hormones are phylogenetically very old molecules which are synthesized by most multicellular organisms, and which even have some effect on unicellular organisms.
Thyroid hormones play a very 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 some minor exceptions) the biologically active hormone.
The United States Food and Drug Administration recommends (21 CFR 101.9 (c)(8)(iv)) 150 micrograms of iodine per day for both men and women. This is necessary for proper production of thyroid hormone. Natural sources of iodine include sea life, such as kelp and certain seafood.  Salt for human consumption is often enriched with iodine and is referred to as iodized salt.
- Main article: Iodine deficiency
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 deficiency gives rise to hypothyroidism, symptoms of which are extreme fatigue, goiter, mental slowing, depression, weight gain, and low basal body temperatures.
Iodine deficiency is also the leading cause of preventable mental retardation, an effect which happens primarily when babies and small children are made hypothyroid by lack of the element. The addition of iodine to table salt has largely eliminated this problem in the wealthier nations, but iodine deficiency remains a serious public health problem in the developing world.
Toxicity of iodine
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 deadly poison if taken in larger amounts; if 2-3 grams of it is consumed, it is fatal to humans. Iodides are similar in toxicity to bromides.
Radioiodine and biology
Radioiodine and the thyroid
The artificial radioisotope 131I (a beta emitter), also known as radioiodine which has a half-life of 8.0207 days, has been used in treating cancer and other pathologies of the thyroid glands. 123I is the radioisotope most often used in nuclear imaging of the kidney and thyroid as well as thyroid uptake scans (used for the evaluation of Grave's disease). The most common compounds of iodine are the iodides of sodium and potassium (KI) and the iodates (KIO3).
129I (half-life 15.7 million years) is a product of 130Xe spallation in the atmosphere and uranium and plutonium fission, 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 ground-water tracer and as an indicator of nuclear waste dispersion into the natural environment.
If humans are exposed to radioactive iodine, the thyroid gland will absorb it as if it were non-radioactive iodine, leading to elevated chances of thyroid cancer. Isotopes with shorter half-lifes such as 131I 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-123 and iodine-125 are used in medicine as tracers for imaging and evaluating the function of the thyroid.
- Iodine-131 is used in medicine for treatment of thyroid cancer and Grave's disease.
- Uncombined (elemental) iodine is mildly toxic to all living things.
- Potassium iodide (KI tablets, or "SSKI" = "Super-Saturated KI" liquid drops) can be given to people in a nuclear disaster area when fission has taken place, to flush out the radioactive iodine-131 fission product. The half-life of iodine-131 is only eight days, so the treatment would need to continue only a couple of weeks. In cases of leakage of certain nuclear materials without fission, or certain types of dirty bomb made with other than radioiodine, this precaution would be of no avail.
Radioiodine and the kidney
In the 1970s imaging techniques were developed in California to utilize radioiodine in diagnostics for renal hypertension.
- Tincture of iodine (3% elemental iodine in water/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 iodophor type antiseptics are also available, as effective elemental iodine sources for this purpose.
- Iodine compounds are important in the field of organic chemistry
- Iodine, as a heavy element, is quite radio-opaque. Organic compounds of a certain type (typically iodine-substituted benzene derivatives) are thus used in medicine as X-ray radiocontrast agents for intravenous injection. This is often in conjuction with advanced X-ray techniques such as angiography and CT scanning
- Silver iodide is used in photography.
- Tungsten iodide is used to stabilize the filaments in light bulbs.
Precautions for stable iodine
Direct contact with skin can cause lesions, so it should be handled with care. Iodine vapor is very irritating to the eye and 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 triiodide which 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 diligence to establish that the materials are not being diverted to clandestine use may be subject to stiff fines 
- Advanced Inorganic Chemistry by Cotton and Wilkinson, 2nd ed.
- Merck Index of Chemicals and Drugs, 9th ed.
- General Chemistry by Linus Pauling, 1947 ed.
- 21 USC Sec. 872 01/22/02
- Chemical Supplier Convicted of Diversion of Iodine
- Los Alamos National Laboratory - Iodine
- 21 CFR 101.9 (c)(8)(iv) (Text PDF) — FDA nutritional facts label information for vitamins and minerals
- Iodised salt
- Chemical Oxygen Iodine Laser
- WebElements.com - Iodine
- ChemicalElements.com - Iodine
- who.int - WHO Global Database on Iodine Deficiency
- Network for Sustained Elimination of Iodine Deficiency
- Oxidizing Agents > Iodine
- Link page to external chemical sources.
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