page_title: Iodine
section_title: Chemical analysis
hierarchical_section_title: Iodine / Applications / Chemical analysis
caption_attribution_description: English: Testing a seed for starch with a solution of iodine. Română: Testarea cu o soluţie de iod a unei seminţe pentru depistarea amidonului.
context_page_description: Iodine is a chemical element with the symbol I and atomic number 53. The heaviest of the stable halogens, it exists as a lustrous, purple-black non-metallic solid at standard conditions that melts to form a deep violet liquid at 114 degrees Celsius, and boils to a violet gas at 184 degrees Celsius. However, it sublimes easily with gentle heat, resulting in a widespread misconception even taught in some science textbooks that it does not melt. The element was discovered by the French chemist Bernard Courtois in 1811, and was named two years later by Joseph Louis Gay-Lussac, after the Greek ἰώδης "violet-coloured".
  Iodine occurs in many oxidation states, including iodide, iodate, and the various periodate anions. It is the least abundant of the stable halogens, being the sixty-first most abundant element. It is the heaviest essential mineral nutrient. Iodine is essential in the synthesis of thyroid hormones. Iodine deficiency affects about two billion people and is the leading preventable cause of intellectual disabilities.
  The dominant producers of iodine today are Chile and Japan. Iodine and its compounds are primarily used in nutrition.
context_section_description: The iodide and iodate anions are often used for quantitative volumetric analysis, for example in iodometry. Iodine and starch form a blue complex, and this reaction is often used to test for either starch or iodine and as an indicator in iodometry.The iodine test for starch is still used to detect counterfeit banknotes printed on starch-containing paper.
  The iodine value is the mass of iodine in grams that is consumed by 100 grams of a chemical substance typically fats or oils. Iodine numbers are often used to determine the amount of unsaturation in fatty acids. This unsaturation is in the form of double bonds, which react with iodine compounds. In biology, linoleic acid (C18:2 n-6), omega-6 and alpha-linolenic (C18:3 n-3) omega-3, arachidonic acid (AA) – omega-6 (C20: 4n-6), and docosahexaenoic acid (DHA) – omega-3 (C22:6n-3) synthesized  with iodine iodolipids developed among cell membranes during the evolution of life,  important in the mechanism of apoptosis, carcinogenesis and degenerative diseases.
  Potassium tetraiodomercurate(II), K₂HgI₄, is also known as Nessler's reagent. It is often used as a sensitive spot test for ammonia. Similarly, Cu₂HgI₄ is used as a precipitating reagent to test for alkaloids. Aqueous alkaline iodine solution is used in the iodoform test for methyl ketones.
caption: Testing a seed for starch with a solution of iodine
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