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Potassium iodide is a white crystalline salt with chemical formula , used in photography and radiation treatment. It finds widespread application as an iodide source because it is less hygroscopic than sodium iodide, making it easier to work with. KI can turn yellow upon heating in air or upon standing in moist air for long periods, because of oxidation of the iodide to iodine.
Chemical properties Potassium iodide behaves as a simple ionic salt, K+I−. Since the iodide ion is a mild reducing agent, I− is easily oxidised to I2 by powerful oxidising agents such as chlorine: 2 KI(aq) + Cl2(aq) → 2 KCl + I2(aq) Even air will oxidize iodide as evidenced by the observation of a purple extract when KI is rinsed with dichloromethane. Under acidic conditions, KI is oxidised even more easily, due to the formation of hydroiodic acid (HI), which is a powerful reducing agent. KI forms I3− when combined with elemental iodine. KI(aq) + I2(s) → KI3(aq) Unlike I2, I3− salts can be highly water-soluble. I2 and I3− have virtually identical redox potentials (0.535 and 0.536 V vs NHE, respectively), i.e. they are both mild oxidants relative to H2. Therefore, this reaction allows the iodine to be used in aqueous solutions for redox titrations. Potassium iodide also serves in some organic reactions as a source of iodide ion (see "uses" below). Preparation Potassium iodide may be prepared by the reaction of a potassium base with hydroiodic acid, for example: HI + KHCO3 → KI + H2O(l) + CO2(g) Alternatively iron(II) iodide, prepared using scrap iron and iodine (made from iodide rich brines or from Chile saltpeter, can be treated with potassium carbonate: FeI2 + K2CO3 → 2 KI + FeCO3 Uses
Role of potassium iodide in radiological emergency preparedness Potassium iodide may also be used to protect the thyroid from radioactive iodine in the event of an accident or attack at a nuclear power plant, or other nuclear attack, especially where a nuclear reactor is breached and the volatile radionuclides, which contain significant amount of 131I, are released into the environment. Radioiodine is a particularly dangerous radionuclide because the body concentrates it in the thyroid gland. Potassium iodide cannot protect against other causes of radiation poisoning, however, nor can it provide any degree of protection against a dirty bomb unless the bomb happens to contain a significant amount of radioactive iodine. In case of a nuclear emergency, iodine used for the cleaning of wounds should not be ingested. It is a poison. See fission products and the external links for more details. Precautions Mild irritant, wear gloves. Chronic overexposure can have adverse effects on the thyroid. | |||||||||||||||||
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