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(pətăs`ēəm), a metallic chemical element; symbol K [Lat. kalium=alkali]; at. no. 19; at. wt. 39.0983; m.p. 63.25°C;; b.p. 760°C;; sp. gr. .862 at 20°C;; valence +1.

Potassium is a soft, silver-white metal. Physically and chemically it resembles the other alkali metalsalkali metals,
metals found in Group 1 of the periodic table. Compared to other metals they are soft and have low melting points and densities. Alkali metals are powerful reducing agents and form univalent compounds.
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 in Group 1 of the periodic tableperiodic table,
chart of the elements arranged according to the periodic law discovered by Dmitri I. Mendeleev and revised by Henry G. J. Moseley. In the periodic table the elements are arranged in columns and rows according to increasing atomic number (see the table entitled
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. 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 carbonatepotassium carbonate,
chemical compound, K2CO3, white, crystalline, deliquescent substance that forms a strongly alkaline water solution. It is available commercially as a white, granular powder commonly called potash, or pearl ash.
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, 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 waterJavelle water
or Javel water
, Fr. eau de Javelle, aqueous solution of sodium or potassium hypochlorite. It was originally made near the French town of Javelle (now part of Paris) and was the first chemical bleach, a use first demonstrated by C. L.
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 contains potassium hypochlorite, KClO, a compound found only in solution. The metasilicate, K2SiO3, is used in water glasswater glass
or soluble glass,
colorless, transparent, glasslike substance available commercially as a powder or as a transparent, viscous solution in water. Chemically it is sodium silicate, potassium silicate, or a mixture of these.
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. Potassium has several useful tartaric acid salts, e.g., Rochelle saltRochelle salt,
colorless to blue-white orthorhombic crystalline salt with a saline, cooling taste. It is also called Seignette salt after Pierre Seignette, an apothecary of La Rochelle, France, who was the first to make it (c.1675).
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 (sodium potassium tartrate), tartartartar
or argol
, impure potassium hydrogen tartrate deposited as a crust in fermenting vessels during wine making. When purified, it yields cream of tartar. Tartar is the chief natural source of tartaric acid.
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 (argol) and cream of tartarcream of tartar,
white crystalline powder. Chemically it is potassium hydrogen tartrate, KC4H5O6, the acidic potassium salt of tartaric acid. It is used as the leavening agent in baking powders.
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 (potassium hydrogen tartrate), and tartar emetictartar emetic,
poisonous, odorless, transparent rhombic crystals or white powder with a metallic, sweetish taste. Chemically, it is potassium antimony tartrate, KSbC4H4O7· 1-2H2O. It is used as a mordant in dyeing.
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 (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 alumalum
, any one of a series of isomorphous double salts that are hydrated sulfates of a univalent cation (e.g., potassium, sodium, ammonium, cesium, or thallium) and a trivalent cation (e.g., aluminum, chromium, iron, manganese, cobalt, or titanium).
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, 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 DavyDavy, Sir Humphry,
1778–1829, English chemist and physicist. The son of a woodcarver, he received his early education at Truro and was apprenticed (1795) to a surgeon-apothecary at Penzance.
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, 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.

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A chemical element, symbol K, atomic number 19, atomic weight 39.0983; an alkali metal. Also known as kalium.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.


a light silvery element of the alkali metal group that is highly reactive and rapidly oxidizes in air; occurs principally in carnallite and sylvite. It is used when alloyed with sodium as a cooling medium in nuclear reactors and its compounds are widely used, esp in fertilizers. Symbol: K; atomic no.: 19; atomic wt.: 39.0983; valency: 1; relative density: 0.862; melting pt.: 63.71°C; boiling pt.: 759°C
Collins Discovery Encyclopedia, 1st edition © HarperCollins Publishers 2005