Potassium is a soft, silver-white metal. Physically and chemically it resembles the other alkali metals in Group 1 of the periodic table. It is extremely reactive, more so than sodium. It combines so readily with oxygen that it is usually stored submerged in kerosene or some other hydrocarbon, out of contact with air. It reacts violently with water to form potassium hydroxide, KOH, releasing hydrogen, which usually ignites. It combines directly with the halogens, sulfur, and other nonmetallic elements (except nitrogen). It reacts with many organic compounds.
The metal has limited use since it so closely resembles sodium, which is readily available at lower cost. Nonetheless, potassium compounds are widely used in industry, although they are usually more expensive than the similar sodium compound. Potassium carbonate, or potash, K2CO3, is used principally in soap and glass manufacture. The chloride, KCl, is used in fertilizers and in the production of other potassium compounds. The chlorate, KClO3, and perchlorate, KClO4, are used in explosives and fireworks. The hydroxide, or caustic potash, KOH, is used in soaps. The nitrate, saltpeter (or niter), KNO3, is used in matches and explosives. Other commercially useful compounds include the bromide, KBr, the cyanide, KCN, the chromate, K2CrO4, the dichromate, K2Cr2O7, and the iodide, KI.
Javelle water contains potassium hypochlorite, KClO, a compound found only in solution. The metasilicate, K2SiO3, is used in water glass. Potassium has several useful tartaric acid salts, e.g., Rochelle salt (sodium potassium tartrate), tartar (argol) and cream of tartar (potassium hydrogen tartrate), and tartar emetic (potassium antimony tartrate). Potassium aluminum sulfate, KAl(SO4)2·12H2O, is a compound used in tanning, in water purification, and in baking powder; usually called alum, it is also called potash alum to distinguish it from other alkali aluminum sulfates. Potassium permanganate, KMnO4, a purple-black, crystalline compound that forms deep purple, aqueous solutions, is used in the chemical laboratory as a powerful oxidizing agent and in medicine as an antiseptic and disinfectant.
With sodium the metal forms alloys that are liquid at room temperature; these alloys are sometimes used in chemical reactions. Substances containing potassium impart a purple color to a flame. Potassium does not occur uncombined in nature but is found widely distributed in sylvite (KCl), carnallite (MgCl2·KCl), feldspar, mica, and other minerals. It is the seventh most abundant element in the earth's crust and the sixth most abundant of the elements in solution in the oceans. It is found in mineral waters, brines, and salt deposits. Potassium is an essential nutrient for plants and animals.
Potassium metal is produced commercially by a thermochemical process in which molten potassium chloride is reacted with sodium vapor; this method is also used to produce liquid sodium-potassium alloys. The metal may be produced electrolytically from fused potassium hydroxide, but, unlike sodium and lithium, it reacts with carbon electrodes and may form explosive compounds. Potassium was discovered in 1807 by Humphry Davy, who decomposed potash with an electric current. Potassium was the first metal so discovered; Davy discovered sodium a few days later by a similar experiment.
Method for determining the age of igneous rocks based on the amount of argon-40 in the rock. Radioactive potassium-40 decays to argon-40 with a half-life of about 1.3 billion years, making this method useful for dating rocks that are billions of years old. A more sophisticated method, called argon-argon dating, provides a more accurate estimate of the original potassium-40 content by means of the ratio of argon-40 to argon-39 in the rock, thus yielding a more accurate age determination. Seealso dating.
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Chemical element, one of the alkali metals, chemical symbol K, atomic number 19. It is a soft, silvery white metal, not found free in nature and rarely used as the metal (except as a chemical reagent) because of its extreme reactivity. Potassium is essential for life and is present in all soils. Potassium ions (K+) and sodium ions act at cell membranes in electrochemical impulse transmission and in transport. Potassium in compounds has valence 1. The chloride is used as a fertilizer and a raw material for producing other compounds, and the hydroxide for making liquid soaps and detergents and in preparing various salts. The iodide is added to table salt to protect against iodine deficiency. The nitrate is also called saltpetre, and the carbonate is called potash.
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