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Potassium hydroxide
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<h2 id="properties-and-structure">Properties and structure</h2> <p>KOH exhibits high <a href="/facts/Thermal_stability/Tbqa12Iq">thermal stability</a>. Because of this high stability and relatively low <a href="/facts/Melting_point/MY0w0h0k">melting point</a>, it is often melt-cast as pellets or rods, forms that have low surface area and convenient handling properties. These pellets become tacky in air because KOH is <a href="/facts/Hygroscopic/1NBGewXv">hygroscopic</a>. Most commercial samples are ca. 90% pure, the remainder being water and carbonates.<a class="footnote-ref" id="fnref:2" href="#fn:2"><sup>2</sup></a> Its <a href="/facts/Solvation/JVM6j8R1">dissolution</a> in water is strongly <a href="/facts/Exothermic/gCuSvpa4">exothermic</a>. Concentrated aqueous solutions are sometimes called potassium <a href="/facts/Lye/bdB2nPFk">lyes</a>. Even at high temperatures, solid KOH does not dehydrate readily.<a class="footnote-ref" id="fnref:3" href="#fn:3"><sup>3</sup></a> </p> <h3>Structure</h3> <p>At higher temperatures, solid KOH <a href="/facts/Crystallization/oCUUG80k">crystallizes</a> in the <a href="/facts/NaCl/MWcXl3Mb">NaCl</a> <a href="/facts/Crystal_structure/bJ4bCeHd">crystal structure</a>. The <a href="/facts/Hydroxide/zW0wxpWD">OH−</a> group is either rapidly or randomly disordered so that it is effectively a spherical <a href="/facts/Anion/wrbwhF3z">anion</a> of radius 1.53 Å (between Cl− and F− in size). At room temperature, the OH− groups are ordered and the environment about the K+ centers is distorted, with K+−OH− distances ranging from 2.69 to 3.15 Å, depending on the orientation of the OH group. KOH forms a series of crystalline <a href="/facts/Hydrate/dev82l69">hydrates</a>, namely the monohydrate KOH · <a href="/facts/Water_of_crystallization/sbaKJNJX">H2O</a>, the dihydrate KOH · 2<a href="/facts/Water_of_crystallization/sbaKJNJX">H2O</a> and the tetrahydrate KOH · 4<a href="/facts/Water_of_crystallization/sbaKJNJX">H2O</a>.<a class="footnote-ref" id="fnref:4" href="#fn:4"><sup>4</sup></a> </p> <h2 id="reactions">Reactions</h2> <h3>Solubility and desiccating properties</h3> <p>About 112 g of KOH <a href="/facts/Solubility/kLE9h3Jb">dissolve</a> in 100 mL water at room temperature, which contrasts with 100 g/100 mL for NaOH.<a class="footnote-ref" id="fnref:5" href="#fn:5"><sup>5</sup></a> Thus on a molar basis, KOH is slightly more soluble than NaOH. Lower molecular-weight <a href="/facts/Ethanol/3lCLfBSP">alcohols</a> such as <a href="/facts/Methanol/EIfpoNCg">methanol</a>, <a href="/facts/Ethanol/3lCLfBSP">ethanol</a>, and <a href="/facts/Propanol/NhqMzEkC">propanols</a> are also excellent <a href="/facts/Solvents/LoMPMz8S">solvents</a>. They participate in an acid-base equilibrium. In the case of methanol the potassium <a href="/facts/Methoxide/fuXWJWFq">methoxide</a> (methylate) forms:<a class="footnote-ref" id="fnref:6" href="#fn:6"><sup>6</sup></a> </p> KOH + CH3OH → CH3OK + H2O <p>Because of its high affinity for water, KOH serves as a <a href="/facts/Desiccant/wD2YIcPc">desiccant</a> in the laboratory. It is often used to dry basic solvents, especially <a href="/facts/Amines/SN7W44fv">amines</a> and <a href="/facts/Pyridines/X817hCch">pyridines</a>. </p> <h3>As a nucleophile in organic chemistry</h3> <p>KOH, like NaOH, serves as a source of OH−, a highly <a href="/facts/Nucleophilic/xYccoaJQ">nucleophilic</a> anion that attacks <a href="/facts/Polar_bonds/KlIJ0zjB">polar bonds</a> in both inorganic and organic materials. Aqueous KOH <a href="/facts/Saponification/xaH52S3q">saponifies</a> <a href="/facts/Ester/FoUAIIw4">esters</a>: </p> KOH + RCOOR' → RCOOK + R'OH <p>When R is a long chain, the product is called a <a href="/facts/Potassium_soap/4ec77I67">potassium soap</a>. This reaction is manifested by the "greasy" feel that KOH gives when touched; <a href="/facts/Fat/87HcP11s">fats</a> on the skin are rapidly converted to soap and <a href="/facts/Glycerol/sRQ6NBQQ">glycerol</a>. </p><p>Molten KOH is used to displace <a href="/facts/Halides/ujYu8kGD">halides</a> and other <a href="/facts/Leaving_group/uowVfqwh">leaving groups</a>. The reaction is especially useful for <a href="/facts/Aromaticity/wK0DzRY6">aromatic</a> reagents to give the corresponding <a href="/facts/Phenol/ACJs5NIU">phenols</a>.<a class="footnote-ref" id="fnref:7" href="#fn:7"><sup>7</sup></a> </p> <h3>Reactions with inorganic compounds</h3> <p>Complementary to its reactivity toward acids, KOH attacks <a href="/facts/Oxide/DXtoVe1C">oxides</a>. Thus, SiO2 is attacked by KOH to give soluble potassium silicates. KOH reacts with <a href="/facts/Carbon_dioxide/xGzpppEp">carbon dioxide</a> to give <a href="/facts/Potassium_bicarbonate/fpdHjk3N">potassium bicarbonate</a>: </p> KOH + CO2 → KHCO3 <h2 id="manufacture">Manufacture</h2> <p>Historically, KOH was made by adding <a href="/facts/Potassium_carbonate/nLvuwSaT">potassium carbonate</a> to a strong solution of <a href="/facts/Calcium_hydroxide/Kz2KzRd8">calcium hydroxide</a> (slaked lime). The <a href="/facts/Salt_metathesis_reaction/YpadY3lq">salt metathesis reaction</a> results in precipitation of solid <a href="/facts/Calcium_carbonate/UQ1wLFna">calcium carbonate</a>, leaving potassium hydroxide in solution: </p> Ca(OH)2 + K2CO3 → CaCO3 + 2 KOH <p>Filtering off the precipitated calcium carbonate and boiling down the solution gives potassium hydroxide ("calcinated or caustic potash"). This method of producing potassium hydroxide remained dominant until the late 19th century, when it was largely replaced by the current method of electrolysis of <a href="/facts/Potassium_chloride/RgT2MdPa">potassium chloride</a> solutions.<a class="footnote-ref" id="fnref:8" href="#fn:8"><sup>8</sup></a> The method is analogous to the manufacture of <a href="/facts/Sodium_hydroxide/xjtdYtq7">sodium hydroxide</a> (see <a href="/facts/Chloralkali_process/5aAamEP9">chloralkali process</a>): </p> 2 KCl + 2 H2O → 2 KOH + Cl2 + H2 <p><a href="/facts/Hydrogen/BoYHjl0J">Hydrogen</a> gas forms as a byproduct on the <a href="/facts/Cathode/tqdlGayU">cathode</a>; concurrently, an anodic oxidation of the <a href="/facts/Chloride/m9NUaUDR">chloride</a> ion takes place, forming <a href="/facts/Chlorine/y27UwYBM">chlorine</a> gas as a byproduct. Separation of the anodic and cathodic spaces in the electrolysis cell is essential for this process.<a class="footnote-ref" id="fnref:9" href="#fn:9"><sup>9</sup></a> </p> <h2 id="uses">Uses</h2> <p>KOH and NaOH can be used interchangeably for a number of applications, although in industry, NaOH is preferred because of its lower cost. </p> <h3>Catalyst for hydrothermal gasification process</h3> <p>In industry, KOH is a good catalyst for hydrothermal gasification process. In this process, it is used to improve the yield of gas and amount of hydrogen in process. For example, production of <a href="/facts/Coke_(fuel)/vD8xjd0m">coke (fuel)</a> from coal often produces much coking wastewater. In order to degrade it, <a href="/facts/Supercritical_fluid/rGkUWb7e">supercritical</a> water is used to convert it to the syngas containing <a href="/facts/Carbon_monoxide/CKuLtEMJ">carbon monoxide</a>, <a href="/facts/Carbon_dioxide/xGzpppEp">carbon dioxide</a>, <a href="/facts/Hydrogen/BoYHjl0J">hydrogen</a> and <a href="/facts/Methane/kBHneYnh">methane</a>. Using <a href="/facts/Pressure_swing_adsorption/lwajr7zV">pressure swing adsorption</a>, we could separate various gases and then use <a href="/facts/Power-to-gas/0GBS85GC">power-to-gas</a> technology to convert them to fuel.<a class="footnote-ref" id="fnref:10" href="#fn:10"><sup>10</sup></a> On the other hand, the hydrothermal gasification process could degrade other waste such as sewage sludge and waste from food factories. </p> <h3>Precursor to other potassium compounds</h3> <p>Many potassium salts are prepared by neutralization reactions involving KOH. The potassium salts of <a href="/facts/Potassium_carbonate/nLvuwSaT">carbonate</a>, <a href="/facts/Potassium_cyanide/xcoXn4I8">cyanide</a>, <a href="/facts/Potassium_permanganate/nyjjuDlo">permanganate</a>, <a href="/facts/Potassium_phosphate/yUNVYzYc">phosphate</a>, and various silicates are prepared by treating either the oxides or the acids with KOH.<a class="footnote-ref" id="fnref:11" href="#fn:11"><sup>11</sup></a> The high solubility of <a href="/facts/Potassium_phosphate/yUNVYzYc">potassium phosphate</a> is desirable in <a href="/facts/Fertilizer/ysh3r6NJ">fertilizers</a>. </p> <h3>Manufacture of soft soaps</h3> <p>The <a href="/facts/Saponification/xaH52S3q">saponification</a> of <a href="/facts/Fat/87HcP11s">fats</a> with KOH is used to prepare the corresponding "potassium <a href="/facts/Soap/S3nJa1oY">soaps</a>", which are softer than the more common <a href="/facts/Sodium_hydroxide/xjtdYtq7">sodium hydroxide</a>-derived soaps. Because of their softness and greater solubility, potassium soaps require less water to liquefy, and can thus contain more cleaning agent than liquefied sodium soaps.<a class="footnote-ref" id="fnref:12" href="#fn:12"><sup>12</sup></a> </p> <h3>As an electrolyte</h3> <p><a href="/facts/Aqueous_solution/eRNr3MH2">Aqueous</a> potassium hydroxide is employed as the <a href="/facts/Electrolyte/4YE8bIVa">electrolyte</a> in <a href="/facts/Alkaline_battery/gL5XUWo4">alkaline batteries</a> based on <a href="/facts/Nickel/KwMCHYRP">nickel</a>-<a href="/facts/Cadmium/wXkV2eIX">cadmium</a>, <a href="/facts/Nickel/KwMCHYRP">nickel</a>-<a href="/facts/Hydrogen/BoYHjl0J">hydrogen</a>, and <a href="/facts/Manganese_dioxide/HRwgXlUe">manganese dioxide</a>-<a href="/facts/Zinc/80b1qgk3">zinc</a>. Potassium hydroxide is preferred over <a href="/facts/Sodium_hydroxide/xjtdYtq7">sodium hydroxide</a> because its solutions are more conductive.<a class="footnote-ref" id="fnref:13" href="#fn:13"><sup>13</sup></a> The <a href="/facts/Nickel%25E2%2580%2593metal_hydride_battery/EvCG8loy">nickel–metal hydride batteries</a> in the <a href="/facts/Toyota_Prius/UBj0Wr3v">Toyota Prius</a> use a mixture of potassium hydroxide and sodium hydroxide.<a class="footnote-ref" id="fnref:14" href="#fn:14"><sup>14</sup></a> <a href="/facts/Nickel%25E2%2580%2593iron_battery/G7L6847l">Nickel–iron batteries</a> also use potassium hydroxide electrolyte. </p> <h3>Food industry</h3> <p>In food products, potassium hydroxide acts as a food thickener, pH control agent and food stabilizer. The <a href="/facts/FDA/fMfdaRAa">FDA</a> considers it generally safe as a direct food ingredient when used in accordance with <a href="/facts/Good_Manufacturing_Practices/s4sETf1y">Good Manufacturing Practices</a>.<a class="footnote-ref" id="fnref:15" href="#fn:15"><sup>15</sup></a> It is known in the <a href="/facts/E_number/n3VSY29I">E number</a> system as E525. </p> <h3>Niche applications</h3> <p>Like sodium hydroxide, potassium hydroxide attracts numerous specialized applications, virtually all of which rely on its properties as a strong chemical base with its consequent ability to degrade many materials. For example, in a process commonly referred to as "chemical cremation" or "<a href="/facts/Resomation/p9U8NGcS">resomation</a>", potassium hydroxide hastens the decomposition of soft tissues, both animal and human, to leave behind only the bones and other hard tissues.<a class="footnote-ref" id="fnref:16" href="#fn:16"><sup>16</sup></a> <a href="/facts/Entomologists/zTtUNTWm">Entomologists</a> wishing to study the fine structure of <a href="/facts/Insect/IgDNvnUL">insect</a> <a href="/facts/Anatomy/p8TCWJuV">anatomy</a> may use a 10% aqueous solution of KOH to apply this process.<a class="footnote-ref" id="fnref:17" href="#fn:17"><sup>17</sup></a> </p><p>In chemical synthesis, the choice between the use of KOH and the use of NaOH is guided by the solubility or keeping quality of the resulting <a href="/facts/Salt_(chemistry)/lsSKwGpn">salt</a>. </p><p>The corrosive properties of potassium hydroxide make it a useful ingredient in agents and preparations that clean and <a href="/facts/Disinfection/33fF2QzG">disinfect</a> surfaces and materials that can themselves resist <a href="/facts/Corrosion/lIC7H4we">corrosion</a> by KOH.<a class="footnote-ref" id="fnref:18" href="#fn:18"><sup>18</sup></a> </p><p>KOH is also used for semiconductor chip fabrication (for example <a href="/facts/Etching_(microfabrication)/kPa01dgL">anisotropic wet etching</a>). </p><p>Potassium hydroxide is often the main active ingredient in chemical "cuticle removers" used in <a href="/facts/Manicure/Q8k6mUje">manicure</a> treatments. </p><p>Because aggressive bases like KOH damage the <a href="/facts/Cuticle_(hair)/HNUtx8H9">cuticle</a> of the <a href="/facts/Hair/L7vt9FL3">hair</a> shaft, potassium hydroxide is used to chemically assist the removal of hair from animal hides. The hides are soaked for several hours in a solution of KOH and water to prepare them for the unhairing stage of the <a href="/facts/Tanning_(leather)/NqL8LFrg">tanning</a> process. This same effect is also used to weaken human hair in preparation for shaving. Preshave products and some shave creams contain potassium hydroxide to force open the hair cuticle and to act as a hygroscopic agent to attract and force water into the hair shaft, causing further damage to the hair. In this weakened state, the hair is more easily cut by a razor blade. </p><p>Potassium hydroxide is used to identify some species of <a href="/facts/Fungi/BHo5DP59">fungi</a>. A 3–5% aqueous solution of KOH is applied to the flesh of a mushroom and the researcher notes whether or not the color of the flesh changes. Certain species of <a href="/facts/Gilled_mushrooms/rF8G0DMG">gilled mushrooms</a>, <a href="/facts/Boletes/X2hjUoo8">boletes</a>, <a href="/facts/Polypores/XddnIw2k">polypores</a>, and <a href="/facts/Lichens/lUrd6Sn7">lichens</a><a class="footnote-ref" id="fnref:19" href="#fn:19"><sup>19</sup></a> are identifiable based on this color-change reaction.<a class="footnote-ref" id="fnref:20" href="#fn:20"><sup>20</sup></a> </p> <h2 id="safety">Safety</h2> <p>Potassium hydroxide is a <a href="/facts/Chemical_burn/GycBJMJo">caustic</a> alkali and its solutions range from irritating to skin and other tissue in low concentrations, to highly corrosive in high concentrations. Eyes are particularly vulnerable, and dust or mist is severely irritating to lungs and can cause <a href="/facts/Pulmonary_edema/sVS5UDxl">pulmonary edema</a>.<a class="footnote-ref" id="fnref:21" href="#fn:21"><sup>21</sup></a> Safety considerations are similar to <a href="/facts/Sodium_hydroxide/xjtdYtq7">those of sodium hydroxide</a>. </p><p>The caustic effects arise from being highly alkaline, but if potassium hydroxide is <a href="/facts/Neutralization_(chemistry)/WC1c5fj1">neutralised</a> with a non-toxic acid then it becomes a non-toxic potassium salt. It is approved as a food additive under the code E525. </p> <h2 id="see-also">See also</h2> <ul><li><a href="/facts/Potash/4e2vBL9L">Potash</a></li> <li><a href="/facts/Soda_lime/gh7cblWS">Soda lime</a></li> <li><a href="/facts/Saltwater_soap/4ec77I67">Saltwater soap</a> – sailors' soap</li></ul> <h2 id="external-links">External links</h2> Wikimedia Commons has media related to Potassium hydroxide. <ul><li><a href="http://www.newscientistspace.com/article/dn10104">Newscientist article dn10104</a></li> <li><a href="http://hazard.com/msds/mf/baker/baker/files/p5884.htm">MSDS from JTBaker</a></li> <li><a href="https://www.cdc.gov/niosh/npg/npgd0523.html">CDC - NIOSH Pocket Guide to Chemical Hazards</a></li></ul> <h2 id="references">References</h2> <ol> <li id="fn:1"><p>Schultz, Heinz; Bauer, Günter; Schachl, Erich; Hagedorn, Fritz; Schmittinger, Peter (2005). "Potassium Compounds". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim, Germany: Wiley-VCH. doi:10.1002/14356007.a22_039. ISBN 978-3-527-30673-2. <a href="978-3-527-30673-2" target="_blank">978-3-527-30673-2</a> <a href="#fnref:1" class="footnote-back-ref">↩</a></p></li> <li id="fn:2"><p>Schultz, Heinz; Bauer, Günter; Schachl, Erich; Hagedorn, Fritz; Schmittinger, Peter (2005). "Potassium Compounds". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim, Germany: Wiley-VCH. doi:10.1002/14356007.a22_039. ISBN 978-3-527-30673-2. <a href="978-3-527-30673-2" target="_blank">978-3-527-30673-2</a> <a href="#fnref:2" class="footnote-back-ref">↩</a></p></li> <li id="fn:3"><p>Holleman, A. F; Wiberg, E. (2001). Inorganic Chemistry. San Diego: Academic Press. ISBN 978-0-12-352651-9. <a href="978-0-12-352651-9" target="_blank">978-0-12-352651-9</a> <a href="#fnref:3" class="footnote-back-ref">↩</a></p></li> <li id="fn:4"><p>Wells, A.F. (1984). Structural Inorganic Chemistry. Oxford: Clarendon Press. ISBN 978-0-19-855370-0. <a href="978-0-19-855370-0" target="_blank">978-0-19-855370-0</a> <a href="#fnref:4" class="footnote-back-ref">↩</a></p></li> <li id="fn:5"><p>Seidell, Atherton; Linke, William F. (1952). Solubilities of Inorganic and Organic Compounds. Van Nostrand. Retrieved 2014-05-29. <a href="https://books.google.com/books?id=k2e5AAAAIAAJ" target="_blank">https://books.google.com/books?id=k2e5AAAAIAAJ</a> <a href="#fnref:5" class="footnote-back-ref">↩</a></p></li> <li id="fn:6"><p>Platonov, Andrew Y.; Kurzin, Alexander V.; Evdokimov, Andrey N. (2009). "Composition of Vapor and Liquid Phases in the Potassium Hydroxide + Methanol Reaction System at 25 °С". J. Solution Chem. 39 (3): 335–342. doi:10.1007/s10953-010-9505-1. S2CID 97177429. <a href="/wiki/Doi_(identifier)" target="_blank">/wiki/Doi_(identifier)</a> <a href="#fnref:6" class="footnote-back-ref">↩</a></p></li> <li id="fn:7"><p>W. W. Hartman (1923). "p-Cresol". Organic Syntheses. 3: 37. doi:10.15227/orgsyn.003.0037; Collected Volumes, vol. 1, p. 175. <a href="http://www.orgsyn.org/demo.aspx?prep=CV1P0175" target="_blank">http://www.orgsyn.org/demo.aspx?prep=CV1P0175</a> <a href="#fnref:7" class="footnote-back-ref">↩</a></p></li> <li id="fn:8"><p>Schultz, Heinz; Bauer, Günter; Schachl, Erich; Hagedorn, Fritz; Schmittinger, Peter (2005). "Potassium Compounds". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim, Germany: Wiley-VCH. doi:10.1002/14356007.a22_039. ISBN 978-3-527-30673-2. <a href="978-3-527-30673-2" target="_blank">978-3-527-30673-2</a> <a href="#fnref:8" class="footnote-back-ref">↩</a></p></li> <li id="fn:9"><p>Römpp Chemie-Lexikon, 9th Ed. (in German) <a href="#fnref:9" class="footnote-back-ref">↩</a></p></li> <li id="fn:10"><p>Chen, Fu; Li, Xiaoxiao; Qu, Junfeng; Ma, Jing; Zhu, Qianlin; Zhang, Shaoliang (2020-01-13). "Gasification of coking wastewater in supercritical water adding alkali catalyst". International Journal of Hydrogen Energy. 45 (3): 1608–1614. Bibcode:2020IJHE...45.1608C. doi:10.1016/j.ijhydene.2019.11.033. ISSN 0360-3199. S2CID 213336330. <a href="https://doi.org/10.1016%2Fj.ijhydene.2019.11.033" target="_blank">https://doi.org/10.1016%2Fj.ijhydene.2019.11.033</a> <a href="#fnref:10" class="footnote-back-ref">↩</a></p></li> <li id="fn:11"><p>Schultz, Heinz; Bauer, Günter; Schachl, Erich; Hagedorn, Fritz; Schmittinger, Peter (2005). "Potassium Compounds". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim, Germany: Wiley-VCH. doi:10.1002/14356007.a22_039. ISBN 978-3-527-30673-2. <a href="978-3-527-30673-2" target="_blank">978-3-527-30673-2</a> <a href="#fnref:11" class="footnote-back-ref">↩</a></p></li> <li id="fn:12"><p>K. Schumann; K. Siekmann (2005). "Soaps". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi:10.1002/14356007.a24_247. 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