Chemi Pure
Potassium hydroxide
Properties and structure
Potassium hydroxide can be found in pure form by reacting sodium hydroxide with impure potassium. Potassium hydroxide is usually sold as translucent pellets, which will become tacky in air because KOH is hygroscopic. Consequently, KOH typically contains varying amounts of water (as well as carbonates, see below). Its dissolution in water is strongly exothermic, meaning the process gives off significant heat. Concentrated aqueous solutions are sometimes called potassium lyes. Even at high temperatures, solid KOH does not dehydrate readily.
Structure
At higher temperatures, solid KOH crystallizes in the NaCl motif. The OH group is either rapidly or randomly disordered so that the OH group is effectively a spherical anion of radius 1.53 (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+H distances ranging from 2.69 to 3.15 , depending on the orientation of the OH group. KOH forms a series of crystalline hydrates, namely the monohydrate KOHH2O, the dihydrate KOH2H2O, and the tetrahydrate KOH4H2O.
Solubility and desiccating properties
Approximately 121 g of KOH will dissolve in 100 mL of water at room temperature (compared with 100 g of NaOH in the same volume). Lower alcohols such as methanol, ethanol, and propanols are also excellent solvents. The solubility in ethanol is about 40 g KOH/100 mL.
Because of its high affinity for water, KOH serves as a desiccant in the laboratory. It is often used to dry 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. The gaseous species is dimeric. Because of its high stability and relatively low melting point, it is often melt-cast as pellets or rods, forms that have low surface area and convenient handling properties.
Reactions
As a base
KOH is highly basic, forming strongly alkaline solutions in water and other polar solvents. These solutions are capable of deprotonating many acids, even weak ones. In analytical chemistry, titrations using solutions of KOH are used to assay acids.
As a nucleophile in organic chemistry
KOH, like NaOH, serves as a source of OH, a highly nucleophilic anion that attacks polar bonds in both inorganic and organic materials. In perhaps its most well-known reaction, aqueous KOH saponifies esters:
KOH + RCO2R’ RCO2K + 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 aromatic reagents to give the corresponding phenols.
Reactions with inorganic compounds
Complementary to its reactivity toward acids, KOH attacks oxides. Thus, SiO2 is attacked by KOH to give soluble potassium silicates . KOH reacts with carbon dioxide to give bicarbonate:
KOH + CO2 KHCO3
Manufacture
Historically KOH was made by boiling a solution of potassium carbonate (potash) with calcium hydroxide (slaked lime), leading to a metathesis reaction which caused calcium carbonate to precipitate, leaving potassium hydroxide in solution:
Ca(OH)2 + K2CO3 CaCO3 + 2 KOH
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 using slaked lime. It was the most important method of producing potassium hydroxide until the late 19th century, when it was largely replaced by the current method of electrolysis of potassium chloride solutions, analogous to the method of manufacturing sodium hydroxide (see chloralkali process):
2 KCl + 2 H2O 2 KOH + Cl2 + H2
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. Separation of the anodic and cathodic spaces in the electrolysis cell is essential for this process.
Uses
See also: Sodium hydroxide
KOH and NaOH can be used interchangeably for a number of applications, 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 carbonate, cyanide, permanganate, phosphate, and various silicates are prepared by treating either the oxides or the acids with KOH. The high solubility of potassium phosphate is desirable in fertilizers. or a high solubility of a potassium -hydroxide .0
Manufacture of biodiesel
Although more expensive than using sodium hydroxide, KOH works well in the manufacture of biodiesel by saponification of the fats in vegetable oil. Glycerin from potassium hydroxide-processed biodiesel is useful as an inexpensive food supplement for livestock, once the toxic methanol is removed.
Manufacture of soft soaps
The saponification of fats with KOH is used to prepare the corresponding “potassium soaps,” which are softer than the more common sodium hydroxide-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.
As an electrolyte
Aqueous potassium hydroxide is employed as the electrolyte in alkaline batteries based on nickel-cadmium and manganese dioxide-zinc. Potassium hydroxide is preferred over sodium hydroxide because its solutions are more conductive.
Niche applications
KOH attracts numerous specialized applications, which virtually all rely 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 corrosive properties make it useful as an ingredient in cleaning and disinfection of resistant surfaces and materials.
It is often the main active ingredient in chemical “cuticle removers.”
KOH is also widely used as a way to remove hair from animal hides, leaving the hides in a solution of KOH and water for a few hours. It is used in resomation to dissolve human remains.
Aggressive bases will damage the cuticle of the hair shaft, and thus is useful for weakening the hair in preparation for shaving. Pre-shave products and shave creams such as Proraso contain Potassium Hydroxide in order to force the cuticle open and act as a hygroscopic agent to attract and force water into the shaft, causing further damage to the hair. In this state, the hair is more easily cut by razor blade.
See also
Soda lime
References
^ a b H. Schultz, G. Bauer, E. Schachl, F. Hagedorn, P. Schmittinger otassium Compounds in Ullmann Encyclopedia of Industrial Chemistry, 2005, Wiley-VCH, Weinheim. doi:10.1002/14356007.a22_039
^ “Caustic Potash.” Oxy.com Retrieved on January 24, 2008.
^ “Potassium Hydroxide.” MSDS Retrieved on January 24
^ Holleman, A. F.; Wiberg, E. “Inorganic Chemistry” Academic Press: San Diego, 2001. ISBN 0-12-352651-5.
^ Wells, A.F. (1984) Structural Inorganic Chemistry, Oxford: Clarendon Press. ISBN 0-19-855370-6.
^ W. W. Hartman, “p-Cresol”, Org. Synth., http://www.orgsyn.org/orgsyn/orgsyn/prepContent.asp?prep=CV1P0175 ; Coll. Vol. 1: 175
^ Rmpp Chemie-Lexikon, 9th Ed. (in german)
^ http://www.utahbiodieselsupply.com/chemicals.php#KOH
^ http://www.livestocktrail.uiuc.edu/uploads/dairynet/papers/2007 dd Glycerin.pdf
^ http://animal.cals.arizona.edu/swnmc/2008/08proceedings/14 Donkin Glycerol from Biodiesel Production.pdf
^ K. Schumann, K. Siekmann oaps in Ullmann Encyclopedia of Industrial Chemistry 2005, Wiley-VCH, Weinheim. doi:10.1002/14356007.a24_247
^ D. Berndt, D. Spahrbier, atteries in Ullmann Encyclopedia of Industrial Chemistry 2005, Wiley-VCH, Weinheim. doi:10.1002/14356007.a03_343
^ Rmpp Chemie-Lexikon, 9th Ed. (in German)
External links
Newscientist article dn10104
MSDS from JTBaker
v d e
Potassium compounds
KBr KBrO3 KCN KCNO KCl KClO3 KClO4 KF KH KHCO2 KHCO3 KHF2 KHSO3 KHSO4 KH2AsO4 KI KIO3 KIO4 KMnO4 KNO2 KNO3 KOCN KOH KO2 KPF6 KSCN K2CO3 K2CrO4 K2Cr2O7 K2FeO4 K2MnO4 K2O K2O2 K2PtCl4 K2PtCl6 K2S K2SO3 K2SO4 K2SO5 K2S2O5 K2S2O7 K2S2O8 K2SiO3 K3[Fe(CN)6] K3[Fe(C2O4)3] K4[Fe(CN)6] K3PO4 K4MnO4 K4Mo2Cl8
Categories: Hydroxides | Potassium compounds | Deliquescent substances | Bases | Photographic chemicalsHidden categories: Chemboxes which contain changes to watched fields
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