Iodine is a chemical element in the periodic table that has the symbol I and atomic number 53. This is an insoluble element that is required as a trace element for living organisms. Chemically, iodine is the least reactive of the halogens, and the most electropositive metallic halogen. Iodine is primarily used in medicine, photography and in dyes.
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Iodine is a bluish-black, lustrous solid that sublimes at standard temperatures into a blue-violet gas that has an irritating odor. This halogen also forms compounds with many elements, but is less active than the other member of its series and has some metallic-like properties. Iodine dissolves easily in chloroform, carbon tetrachloride, or carbon disulfide to form purple solutions (It is only slightly soluble in water).
The deep blue color with starch solution is characteristic of the free element.
In areas where there is little iodine in the diet - typically remote inland
areas where no marine foods are eaten - iodine deficiency gives rise to
goitre, so called endemic goitre. In many (but not all) such areas, this is now prevented by the addition of small amounts of sodium iodide to table salt - this product is
known as iodised salt Other uses:
- One of the halogens, it is an essential trace element; the thyroid hormones, thyroxine and triiodotyronine contain atoms of iodine.
- 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/L, let stand for 30 minutes.)
- Iodine compounds are important in the field of organic chemistry and are very useful in medicine.
- Iodides and thyroxine which contains iodine, are both used in internal medicine and, in combination with alcohol (as tincture of iodine) are used externally to disinfect wounds.
- Potassium iodide is used in photography.
- Tungsten iodide is used to stabilise the filaments in light bulbs.
- Nitrogen triiodide is an explosive, too unstable to be used commercially, but is a common ingredient in college pranks.
HistoryIodine (Gr iodes meaning violet) was discovered by Barnard Courtois in 1811.
Iodine can be prepared in an ultrapure form through the reaction of potassium iodide with copper sulfate. There are also several other methods of isolating this element.
There are thirty isotopes of iodine and only one, I-127, is stable. The artificial radioisotope I-131 (a beta emitter) which has a half-life of 8 days, has been used in treating cancer and other pathologies of the thyroid glands. The most common compounds of iodine are the iodides of sodium and potassium (KI) and the iodates (KIO3).
Iodine has only one stable isotope, I-127. However, radioactive isotopes of iodine have been used extensively. I-129 (half-life 17 million years) is a product of Xe-129 spallation in the atmosphere, but is also the result of U-238 decay. As U-238 is produced during a number of nuclear power- related activities, its presence (as an I-129/I ratio) can indicate the type of activity going on at any one site. For this reason, I-129 was used in rainwater studies following the Chernobyl accident. It also has been used as a ground-water tracer and as an indicator of waste dispersion into the natural environment. Other applications may be hampered by the production of I-129 in the lithosphere through a number of decay mechanisms.
In many ways, I-129 is similar to Cl-36. 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, I-129 concentrations are usually reported as the ratio of I-129 to total I (which is virtually all I-127). As is the case with Cl-36/Cl, I-129/I ratios in nature are quite small, 10-14 to 10-10 (peak thermonuclear I-129/I during the 1960s and 1970s reached about 10-7). I-129 differs from Cl-36 in that its half-life is longer (1.6 vs 0.3 million years), it is highly biophilic, and occurs in multiple ionic forms (commonly, I- and iodate) which have different chemical behaviors.
Direct contact with skin can cause lesions so care needs to be taken in handling iodine. Iodine vapor is very irritating to eyes and mucous membranes. The maximum allowable concentration of iodine in air should not exceed 1 m/m³ (8-hour time-weighted average - 40-hour).