Potassium hydroxide 235343 225951379 2008-07-16T04:29:50Z Freestyle-69 6502796 c/e {{Chembox new | Name = Potassium hydroxide | ImageFile = Potassium hydroxide.jpg | ImageSize = 200px | ImageName = Potassium hydroxide | OtherNames = Potash lye, caustic [[potash]], potassia, potassium hydrate | Section1 = {{Chembox Identifiers | CASNo = 1310-58-3 | RTECS = TT2100000 }} | Section2 = {{Chembox Properties | Formula = [[Potassium|K]][[Oxygen|O]][[Hydrogen|H]] | MolarMass = 56.10564 [[gram|g]]/[[Mole (unit)|mol]] | Appearance = white solid,<br />[[deliquescent]] | Density = 2.044 g/cm³, solid | Solubility = 1100 g/L (25 °C) | MeltingPt = 360 °C | BoilingPt = 1320 °C }} | Section3 = {{Chembox Structure | Coordination = octahedral | CrystalStruct = monoclinic }} | Section7 = {{Chembox Hazards | EUClass = Corrosive ('''C''') | {{NFPA 704}} | NFPA-H = 3 | NFPA-F = 0 | NFPA-R = 1 | NFPA-O = | RPhrases = {{R22}} {{R35}} | SPhrases = {{S1/2}} {{S26}} {{S36/37/39}} {{S45}} | FlashPt = non flammable }} | Section8 = {{Chembox Related | OtherAnions = [[Potassium oxide]]; [[Potassium peroxide]]; [[Potassium superoxide]] | OtherCations = [[Lithium hydroxide]]; [[Sodium hydroxide]]; [[Rubidium hydroxide]]; [[Caesium hydroxide]]}} }} '''Potassium hydroxide''' is the [[inorganic compound]] with the formula [[potassium|K]][[hydroxide|OH]]. KOH, as it is familiarly known, along with [[sodium hydroxide]], is a prototypical "strong base". It has many industrial as well as niche applications. Most applications exploit its reactivity toward acids and its corrosive nature. In 2005, an estimated 700 – 800,000 tons were produced. Approximately 100 times more NaOH than KOH is produced annually.<ref name=Ullmann>H. Schultz, G. Bauer, E. Schachl, F. Hagedorn, P. Schmittinger “Potassium Compounds” in Ullmann’s Encyclopedia of Industrial Chemistry, 2005, Wiley-VCH, Weinheim. {{DOI|10.1002/14356007.a22 039}}</ref><ref>"Caustic Potash." [http://www.oxy.com/oxychem/Products/caustic_potash/caustic_potash.htm Oxy.com] Retrieved on [[January 24]] [[2008]].</ref><ref> "Potassium Hydroxide." [http://www.jtbaker.com/msds/englishhtml/P5887.htm MSDS] Retrieved on [[January 24]]</ref> KOH is noteworthy as the precursor to most soft and liquid soaps as well as numerous potassium-containing chemicals. ==Properties and structure== Potassium hydroxide is usually sold as translucent pellets. Samples of KOH become tacky in air because KOH is [[hygroscopic]]. Consequently, KOH characteristically contains varying amounts of water (as well as carbonates, see below). Its dissolution in water is strongly [[exothermic]], leading to a temperature rise, sometimes up to boiling point. Concentrated aqueous solutions are called potassium [[lye]]s. ===Structure=== At higher temperatures, solid KOH crystallizes in the NaCl motif. The OH group is either rapidly or randomly disordered so that the OH<sup>-</sup> group is effectively a spherical anion of radius 1.53 Å (between Cl<sup>-</sup> and F<sup>-</sup> in size). At room temperature the OH<sup>-</sup> groups are ordered and the environment about the K<sup>+</sup> centers is distorted with K<sup>+</sup>---OH<sup>-</sup> distances ranging from 2.69 to 3.15 Å, depending on the orientation of the OH group. KOH forms a series of crystalline [[hydrate]]s, namely the monohydrate KOH·H<sub>2</sub>O, the dihydrate KOH·2H<sub>2</sub>O, and the tetrahydrate KOH·4H<sub>2</sub>O.<ref>Wells, A.F. (1984) Structural Inorganic Chemistry, Oxford: Clarendon Press. ISBN 0-19-855370-6.</ref> ===Solubility and desiccating properties=== Approximately 121 g of KOH will dissolve in 100 mL of water (compared with 100 g of NaOH in the same volume). Lower [[alcohol]]s such as [[methanol]], [[ethanol]], and [[propanol]]s are also excellent solvents, the solubility in [[ethanol]] being about 1 g KOH/2.5 mL. Because of its high affinity for water, KOH is a [[desiccant]]. In the laboratory it is particularly useful for drying basic solvents, especially amines and pyridines. Distillation of these basic liquids from a [[slurry]] of KOH yields the anhydrous reagent. ===Thermal stability=== Like NaOH, KOH exhibits high thermal stability. KOH sublimes unchanged at 400 °C; the gaseous species is [[dimer]]ic. Even at high temperatures, dehydration does not occur.<ref>Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. ISBN 0-12-352651-5.</ref> Because of its high stability and relatively low melting point, it is often melt-casted as pellets or rods, forms that have low surface area and convenient handling properties. ==Reactions== ===As a base=== KOH is highly [[Base (chemistry)|basic]], forming strongly alkali solutions in water and other polar solvents. These solutions are capable of deprotonating many acids, even weak ones. In analytical chemistry, titratons using solutions of KOH are used to assay acids. ===As a nucleophile in organic chemistry=== KOH, like NaOH, serves as a source of OH<sup>-</sup>, a highly nucleophilic anion that attacks polar bonds in both inorganic and organic materials. In perhaps its most famous reaction, aqueous KOH saponifies [[ester]]s: : KOH + RCO<sub>2</sub>R’ → RCO<sub>2</sub>K + R’OH When R is a long chain, the product is called a potassium soap. This reaction is manifested by the ‘’greasy” feel that KOH gives when touched – [[fats]] on the skin are rapidly converted to soap and [[glycerol]]. Molten KOH is used to displace halides and other leaving groups. The reaction is especially useful for [[aromaticity|aromatic]] [[reagent]]s to give the corresponding [[phenol]]s.<ref>{{OrgSynth | author = W. W. Hartman | title = p-Cresol| collvol = 1 | collvolpages = 175 | prep = CV1P0175}}</ref> ===Reactions with inorganic compounds=== Complementary to its reactivity toward acids, KOH attacks anhydrides, defined in the broadest sense. Thus, SiO<sub>2</sub> and CO<sub>2</sub> are attacked by KOH to give the silicates and [[potassium bicarbonate|bicarbonate]], respectively: :KOH + CO<sub>2</sub> → KHCO<sub>3</sub> ==Manufacture== Of historical significance is the old method of boiling a solution of [[potassium carbonate]] (potash) with [[calcium hydroxide]] ([[slaked lime]]). A [[metathesis reaction]] occurs, precipitating [[calcium carbonate]], leaving potassium hydroxide in solution: :Ca(OH)<sub>2 (s), (aq)</sub> + K<sub>2</sub>CO<sub>3 (aq)</sub> → CaCO<sub>3 (s)</sub> + 2 KOH<sub> (aq)</sub> Filtering off the precipitated calcium carbonate, and boiling down the solution gives potassium hydroxide ("calcinated or caustic potash"). This method used potash extracted from wood ashes and slaked lime. Probably known since antiquity, it was the most important method of producing potassium hydroxide until the late 19th century,{{Fact|date=July 2008}} when it was largely replaced by the modern method of electrolysis of [[potassium chloride]] solutions, analogous to the method of manufacturing [[sodium hydroxide]] (see [[chloralkali process]]): :2 K<sup>+</sup><sub> (aq)</sub> + 2H<sub>2</sub>O<sub> (l)</sub> + 2e<sup>−</sup> → H<sub>2 (g)</sub> + 2 KOH<sub> (aq)</sub> [[Hydrogen]] gas forms as a by-product on the [[cathode]]; concurrently, an anodic oxidation of the [[chloride]] ion takes place, forming [[chlorine]] gas as a byproduct: :2 Cl<sup>–</sup> — 2e<sup>−</sup> → Cl<sub>2 (g)</sub> Separation of the anodic and cathodic spaces in the electrolysis cell is essential for this process.<ref>Römpp Chemie-Lexikon, 9th Ed. (in german)</ref> ==Uses== {{see also|Sodium hydroxide}} KOH and NaOH are often used interchangeably, although in industry, NaOH is preferred because of its lower cost. ===Precursor to other potassium compounds=== Many potassium salts are prepared by neutralization reactions involving KOH. The potassium salts of [[potassium carbonate|carbonate]], [[potassium cyanide|cyanide]], [[potassium permanganate|permanganate]], [[potassium phosphate|phosphate]], and various silicates are prepared by treating either the oxides or the acids with KOH.<ref name=Ullmann/> The high solubility of potassium phosphate is desirable in [[fertilizer]]s. ===Manufacture of soft soaps=== The saponification of fats with KOH is used to prepare the corresponding potassium soaps. Such soaps tend to be more soluble and are found in liquid soaps. The more common sodium soaps are more easily solidified.<ref>K. Schumann, K. Siekmann “Soaps” in Ullmann’s Encyclopedia of Industrial Chemistry 2005, Wiley-VCH, Weinheim.</ref> ===As an electrolyte=== Aqueous potassium hydroxide is employed as the [[electrolyte]] in [[Alkaline battery|alkaline batteries]] based on nickel-cadmium and manganese dioxide-zinc. Potassium hydroxide is preferred over sodium hydroxide because its solutions are more conductive.<ref>D. Berndt, D. Spahrbier, “Batteries” in Ullmann’s Encyclopedia of Industrial Chemistry 2005, Wiley-VCH, Weinheim.</ref> ===Niche applications=== KOH attracts numerous specialized applications, but virtually all capitalize on its basic or degradative properties. KOH is widely used in the laboratory for the same purposes. In chemical synthesis, the selection of KOH vs. NaOH is guided by the solubility for the resulting salt. Its corrosivity is sometimes used in cleaning and [[disinfection]] of resistant surfaces and materials.<ref>Römpp Chemie-Lexikon, 9th Ed. (in German)</ref> ==See also== * [[Soda lime]] ==References== <references/> ==External links== * [http://www.newscientistspace.com/article/dn10104 Newscientist article dn10104] * [http://www.jtbaker.com/msds/englishhtml/P5884.htm MSDS from JTBaker] * Reheis [http://www.genchemcorp.com/products/kchloride.shtml] * [http://www.deliberatelife.com/index.php?option=com_content&task=view&id=164&Itemid=70 Making lye from ashes] [[Category:hydroxides]] [[Category:potassium compounds]] [[Category:Deliquescent substances]] [[Category:Bases]] [[ar:هيدروكسيد بوتاسيوم]] [[bs:Kalijum hidroksid]] [[bg:Калиева основа]] [[ca:Hidròxid de potassi]] [[cs:Hydroxid draselný]] [[da:Kaliumhydroxid]] [[de:Kaliumhydroxid]] [[et:Kaaliumhüdroksiid]] [[es:Hidróxido de potasio]] [[fr:Hydroxyde de potassium]] [[gl:Hidróxido de potasio]] [[it:Idrossido di potassio]] [[he:אשלגן הידרוקסידי]] [[lv:Kālija hidroksīds]] [[lt:Kalio šarmas]] [[hu:Kálium-hidroxid]] [[ms:Kalium hidroksida]] [[nl:Kaliumhydroxide]] [[ja:水酸化カリウム]] [[no:Kaliumhydroksid]] [[pl:Wodorotlenek potasu]] [[pt:Hidróxido de potássio]] [[ru:Гидроксид калия]] [[sr:Калијум хидроксид]] [[fi:Kaliumhydroksidi]] [[sv:Kaliumhydroxid]] [[tr:Potasyum hidroksit]] [[zh:氢氧化钾]]