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Potassium hydroxide is an inorganic compound with the formula KOH, and is commonly called caustic potash.
Along with sodium hydroxide (NaOH), this colorlesssolid is a prototypical strong base. It has many industrial and nicheapplications; most applications exploit its reactivity toward acids and itscorrosive nature. An estimated 700,000 to 800,000 tonnes were produced in 2005.Approximately 100 times more NaOH than KOH is produced annually. KOH isnoteworthy as the precursor to most soft and liquid soaps as well as numerouspotassium-containing chemicals.
Potassium hydroxide can be found in pureform by reacting sodium hydroxide with impure potassium. Potassium hydroxide isusually sold as translucent pellets, which will become tacky in air because KOHis hygroscopic. Consequently, KOH typically contains varying amounts of water(as well as carbonates, see below). Its dissolution in water is stronglyexothermic. Concentrated aqueous solutions are sometimes called potassium lyes.Even at high temperatures, solid KOH does not dehydrate readily.
Potassium hydroxide solutions withconcentrations of approximately 0.5 to 2.0% are irritating when coming incontact with the skin, while concentrations higher than 2% are corrosive.
Solubilityand desiccating properties
Approximately 121 g of KOH will dissolve in100 mL of water at room temperature compared with 100 g of NaOH in the samevolume (on a molar basis, KOH is slightly less soluble than NaOH). Lowermolecular weight alcohols such as methanol, ethanol, and propanols are alsoexcellent solvents.
Because of its high affinity for water, KOHserves as a desiccant in the laboratory. It is often used to dry basicsolvents, especially amines and pyridines: distillation of these basic liquidsfrom a slurry of KOH yields the anhydrous reagent.
Like NaOH, KOH exhibits high thermalstability. The gaseous species is dimeric. Because of its high stability andrelatively low melting point, it is often melt-cast as pellets or rods, formsthat have low surface area and convenient handling properties.
KOH is highly basic, forming stronglyalkaline solutions in water and other polar solvents. These solutions arecapable of deprotonating many acids, even weak ones. In analytical chemistry,titrations using solutions of KOH are used to assay acids.
As anucleophile in organic chemistry
KOH, like NaOH, serves as a source of OH−
, a highly nucleophilic anion that attackspolar bonds in both inorganic and organic materials. In perhaps its mostwell-known reaction, aqueous KOH saponifies esters:
KOH+ RCO2R' → RCO2K + R'OH
When R is a long chain, the product iscalled a potassium soap. This reaction is manifested by the "greasy"feel that KOH gives when touched — fats on the skin are rapidly converted tosoap and glycerol.
Molten KOH is used to displace halides andother leaving groups. The reaction is especially useful for aromatic reagentsto give the corresponding phenols.
Reactionswith inorganic compounds
Complementary to its reactivity towardacids, KOH attacks oxides. Thus, SiO2 is attacked by KOH to give solublepotassium silicates. KOH reacts with carbon dioxide to give bicarbonate:
KOH+ CO2 → KHCO3
Historically KOH was made by addingpotassium carbonate (potash) to a strong solution of calcium hydroxide (slakedlime), leading to a metathesis reaction which caused calcium carbonate toprecipitate, leaving potassium hydroxide in solution:
Ca(OH)2 + K2CO3 → CaCO3 + 2 KOH
Filtering off the precipitated calciumcarbonate and boiling down the solution gives potassium hydroxide("calcinated or caustic potash"). It was the most important method ofproducing potassium hydroxide until the late 19th century, when it was largelyreplaced by the current method of electrolysis of potassium chloridesolutions.The method is analogous to the manufacture of sodium hydroxide (seechloralkali process):
2KCl + 2 H2O → 2 KOH + Cl2 + H2
Hydrogen gas forms as a by-product on thecathode; concurrently, an anodic oxidation of the chloride ion takes place,forming chlorine gas as a byproduct. Separation of the anodic and cathodicspaces in the electrolysis cell is essential for this process.
KOH and NaOH can be used interchangeablyfor a number of applications, although in industry, NaOH is preferred becauseof its lower cost.
Precursorto other potassium compounds
Many potassium salts are prepared byneutralization reactions involving KOH. The potassium salts of carbonate,cyanide, permanganate, phosphate, and various silicates are prepared bytreating either the oxides or the acids with KOH.The high solubility ofpotassium phosphate is desirable in fertilizers.
Although more expensive than using sodiumhydroxide, KOH works well in the manufacture of biodiesel bytransesterification of the triglycerides in vegetable oil. Glycerin frompotassium hydroxide-processed biodiesel is useful as an inexpensive foodsupplement for livestock, once the toxic methanol is removed.
Manufactureof soft soaps
The saponification of fats with KOH is usedto prepare the corresponding "potassium soaps," which are softer thanthe more common sodium hydroxide-derived soaps. Because of their softness andgreater solubility, potassium soaps require less water to liquefy, and can thuscontain more cleaning agent than liquefied sodium soaps.
Potassium carbonate, formed from thehydroxide solution leaking from an alkaline battery
Aqueous potassium hydroxide is employed asthe electrolyte in alkaline batteries based on nickel-cadmium, nickel-hydrogen,and manganese dioxide-zinc. Potassium hydroxide is preferred over sodiumhydroxide because its solutions are more conductive. The Nickel Metal Hydridebatteries in the Toyota Prius use a mixture of potassium hydroxide and sodiumhydroxide.Nickel–iron batteries also use potassium hydroxide electrolyte.
Like sodium hydroxide, potassium hydroxideattracts numerous specialized applications, virtually all of which rely on itsproperties as a strong chemical base with its consequent ability to degrademany materials. For example, in a process commonly referred to as"chemical cremation" or "resomation," potassium hydroxidehastens the decomposition of soft tissues, both animal and human, to leavebehind only the bones and other hard tissues. Entomologists wishing to studythe fine structure of insect anatomy may use a 10% aqueous solution of KOH toapply this process.
In chemical synthesis, the choice betweenthe use of KOH and the use of sodium hydroxide, NaOH, is guided by thesolubility of the resulting salt.
The corrosive properties of potassiumhydroxide make it a useful ingredient in agents and preparations that clean anddisinfect surfaces and materials that can themselves resist corrosion by KOH.
KOH is also used for semiconductor chipfabrication. See also: anisotropic wet etching.
Potassium hydroxide is often the mainactive ingredient in chemical "cuticle removers" used in manicuretreatments.
Because aggressive bases like KOH damagethe cuticle of the hair shaft, potassium hydroxide is used to chemically assistthe removal of hair from animal hides. The hides are soaked for several hoursin a solution of KOH and water to prepare them for the unhairing stage of thetanning process. This same effect is also used to weaken human hair inpreparation for shaving. Pre-shave products and some shave creams containpotassium hydroxide to force open the hair cuticle and to act as a hygroscopicagent to attract and force water into the hair shaft, causing further damage tothe hair. In this weakened state, the hair is more easily cut by a razor blade.
Potassium hydroxide is used to identifysome species of fungi. A 3–5% aqueous solution of KOH is applied to the fleshof a mushroom and the researcher notes whether or not the color of the fleshchanges. Certain species of gilled mushrooms, boletes, polypores, and lichensare identified based on this color-change reaction.
Potassium hydroxide is also used inpetroleum and natural gas refining for removal of organic acids and sulfurcompounds.