Potassium hydroxide is an inorganic compound with the formula K OH, and is commonly called caustic potash. Along with sodium hydroxide (NaOH), KOH is a prototypical strong base. It has many industrial and niche applications, most of which exploit its

caustic Caustic most commonly refers to: * Causticity, a property of various corrosive substances ** Sodium hydroxide, sometimes called ''caustic soda'' ** Potassium hydroxide, sometimes called ''caustic potash'' ** Calcium oxide, sometimes called ''caus ...

nature and its reactivity toward

acids In computer science, ACID ( atomicity, consistency, isolation, durability) is a set of properties of database transactions intended to guarantee data validity despite errors, power failures, and other mishaps. In the context of databases, a ...

. An estimated 700,000 to 800,000

tonne The tonne ( or ; symbol: t) is a unit of mass equal to 1000 kilograms. It is a non-SI unit accepted for use with SI. It is also referred to as a metric ton to distinguish it from the non-metric units of the short ton ( United State ...

s were produced in 2005. KOH is noteworthy as the precursor to most soft and liquid

soap Soap is a salt of a fatty acid used in a variety of cleansing and lubricating products. In a domestic setting, soaps are surfactants usually used for washing, bathing, and other types of housekeeping. In industrial settings, soaps are use ...

s, as well as numerous potassium-containing chemicals. It is a white solid that is dangerously corrosive.

Properties and structure

KOH exhibits high

thermal stability In thermodynamics, thermal stability describes the stability of a water body and its resistance to mixing.Schmidt, W. 1928. Über Temperatur und Stabilitätsverhältnisse von Seen. Geogr. Ann 10: 145 - 177. It is the amount of work needed to t ...

. Because of this high stability and relatively low

melting point The melting point (or, rarely, liquefaction point) of a substance is the temperature at which it changes state from solid to liquid. At the melting point the solid and liquid phase exist in equilibrium. The melting point of a substance depen ...

, 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

hygroscopic Hygroscopy is the phenomenon of attracting and holding water molecules via either absorption or adsorption from the surrounding environment, which is usually at normal or room temperature. If water molecules become suspended among the substan ...

. Most commercial samples are ca. 90% pure, the remainder being water and carbonates. Its

dissolution Dissolution may refer to: Arts and entertainment Books * ''Dissolution'' (''Forgotten Realms'' novel), a 2002 fantasy novel by Richard Lee Byers * ''Dissolution'' (Sansom novel), a 2003 historical novel by C. J. Sansom Music * Dissolution, in mu ...

in water is strongly exothermic. Concentrated aqueous solutions are sometimes called potassium

lye A lye is a metal hydroxide traditionally obtained by leaching wood ashes, or a strong alkali which is highly soluble in water producing caustic basic solutions. "Lye" most commonly refers to sodium hydroxide (NaOH), but historically has been u ...

s. Even at high temperatures, solid KOH does not dehydrate readily.

Structure

At higher temperatures, solid KOH

crystallizes Crystallization is the process by which solid forms, where the atoms or molecules are highly organized into a structure known as a crystal. Some ways by which crystals form are precipitating from a solution, freezing, or more rarely dep ...

in the

NaCl Sodium chloride , commonly known as salt (although sea salt also contains other chemical salts), is an ionic compound with the chemical formula NaCl, representing a 1:1 ratio of sodium and chloride ions. With molar masses of 22.99 and 35.45 g/ ...

crystal structure In crystallography, crystal structure is a description of the ordered arrangement of atoms, ions or molecules in a crystalline material. Ordered structures occur from the intrinsic nature of the constituent particles to form symmetric patterns ...

. The OH group is either rapidly or randomly disordered so that the group is effectively a spherical

anion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conve ...

of radius 1.53 Å (between and in size). At room temperature, the groups are ordered and the environment about the centers is distorted, with distances ranging from 2.69 to 3.15 Å, depending on the orientation of the OH group. KOH forms a series of crystalline hydrates, namely the monohydrate , the dihydrate and the tetrahydrate .

Reactions

Solubility and desiccating properties

About 121 g of KOH dissolve in 100 mL water at room temperature, which contrasts with 100 g/100 mL for NaOH. Thus on a molar basis, NaOH is slightly more soluble than KOH. Lower molecular-weight alcohols such as methanol,

ethanol Ethanol (abbr. EtOH; also called ethyl alcohol, grain alcohol, drinking alcohol, or simply alcohol) is an organic compound. It is an alcohol with the chemical formula . Its formula can be also written as or (an ethyl group linked to a ...

, and

propanol There are two isomers of propanol. *1-Propanol, ''n''-propanol, or propan-1-ol : CH3CH2CH2OH, the most common meaning *2-Propanol, Isopropyl alcohol Isopropyl alcohol (IUPAC name propan-2-ol and also called isopropanol or 2-propanol) is a c ...

s are also excellent solvents. They participate in an acid-base equilibrium. In the case of methanol the potassium

methoxide In organic chemistry, methoxides are organic salts with a anion. They are the simplest alkoxides. Sodium methoxide and potassium methoxide have widespread use, though other metal-cation variants such as lithium methoxide, rubidium methoxide, ...

(methylate) forms: : 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 In chemistry, amines (, ) are compounds and functional groups that contain a basic nitrogen atom with a lone pair. Amines are formally derivatives of ammonia (), wherein one or more hydrogen atoms have been replaced by a substituent such ...

and

pyridines Pyridine is a basic heterocyclic organic compound with the chemical formula . It is structurally related to benzene, with one methine group replaced by a nitrogen atom. It is a highly flammable, weakly alkaline, water-miscible liquid with a ...

As a nucleophile in organic chemistry

KOH, like NaOH, serves as a source of , a highly

nucleophilic In chemistry, a nucleophile is a chemical species that forms bonds by donating an electron pair. All molecules and ions with a free pair of electrons or at least one pi bond can act as nucleophiles. Because nucleophiles donate electrons, they are ...

anion that attacks polar bonds in both inorganic and organic materials. Aqueous KOH saponifies

ester In chemistry, an ester is a compound derived from an oxoacid (organic or inorganic) in which at least one hydroxyl group () is replaced by an alkoxy group (), as in the substitution reaction of a carboxylic acid and an alcohol. Glycerides a ...

s: : 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 —

fat In nutrition, biology, and chemistry, fat usually means any ester of fatty acids, or a mixture of such compounds, most commonly those that occur in living beings or in food. The term often refers specifically to triglycerides (triple est ...

s on the skin are rapidly converted to soap and

glycerol Glycerol (), also called glycerine in British English and glycerin in American English, is a simple triol compound. It is a colorless, odorless, viscous liquid that is sweet-tasting and non-toxic. The glycerol backbone is found in lipids known ...

. Molten KOH is used to displace

halides In chemistry, a halide (rarely halogenide) is a binary chemical compound, of which one part is a halogen atom and the other part is an element or radical that is less electronegative (or more electropositive) than the halogen, to make a fluor ...

and other

leaving group In chemistry, a leaving group is defined by the IUPAC as an atom or group of atoms that detaches from the main or residual part of a substrate during a reaction or elementary step of a reaction. However, in common usage, the term is often limited ...

s. The reaction is especially useful for

aromatic In chemistry, aromaticity is a chemical property of cyclic ( ring-shaped), ''typically'' planar (flat) molecular structures with pi bonds in resonance (those containing delocalized electrons) that gives increased stability compared to satur ...

reagents to give the corresponding

phenol Phenol (also called carbolic acid) is an aromatic organic compound with the molecular formula . It is a white crystalline solid that is volatile. The molecule consists of a phenyl group () bonded to a hydroxy group (). Mildly acidic, it ...

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 Carbon dioxide ( chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is trans ...

to give

potassium bicarbonate Potassium bicarbonate (IUPAC name: potassium hydrogencarbonate, also known as potassium acid carbonate) is the inorganic compound with the chemical formula KHCO3. It is a white solid. Production and reactivity It is manufactured by treating an ...

: :

Manufacture

Historically, KOH was made by adding

potassium carbonate Potassium carbonate is the inorganic compound with the formula K2 CO3. It is a white salt, which is soluble in water. It is deliquescent, often appearing as a damp or wet solid. Potassium carbonate is mainly used in the production of soap and gl ...

to a strong solution of calcium hydroxide (slaked lime). The

salt metathesis reaction A salt metathesis reaction, sometimes called a double displacement reaction, is a chemical process involving the exchange of bonds between two reacting chemical species which results in the creation of products with similar or identical bonding a ...

results in precipitation of solid calcium carbonate, leaving potassium hydroxide in solution: : 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 potassium chloride solutions. The method is analogous to the manufacture of sodium hydroxide (see

chloralkali process The chloralkali process (also chlor-alkali and chlor alkali) is an industrial process for the electrolysis of sodium chloride (NaCl) solutions. It is the technology used to produce chlorine and sodium hydroxide (caustic soda), which are commodit ...

): :

Hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic ...

gas forms as a byproduct on the

cathode A cathode is the electrode from which a conventional current leaves a polarized electrical device. This definition can be recalled by using the mnemonic ''CCD'' for ''Cathode Current Departs''. A conventional current describes the direction in wh ...

; concurrently, an anodic oxidation of the

chloride The chloride ion is the anion (negatively charged ion) Cl−. It is formed when the element chlorine (a halogen) gains an electron or when a compound such as hydrogen chloride is dissolved in water or other polar solvents. Chloride sa ...

ion takes place, forming

chlorine Chlorine is a chemical element with the symbol Cl and atomic number 17. The second-lightest of the halogens, it appears between fluorine and bromine in the periodic table and its properties are mostly intermediate between them. Chlorine i ...

gas as a byproduct. Separation of the anodic and cathodic spaces in the electrolysis cell is essential for this process.Römpp Chemie-Lexikon, 9th Ed. (in German)

Uses

KOH and NaOH can be used interchangeably for a number of applications, although in industry, NaOH is preferred because of its lower cost.

Catalyst for hydrothermal gasification process

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 coke (fuel) from coal often produce much of coking wastewater. In order to degrade it, supercritical water is used to convert it to the syngas containing

carbon monoxide Carbon monoxide (chemical formula CO) is a colorless, poisonous, odorless, tasteless, flammable gas that is slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom connected by a triple bond. It is the simple ...

;

hydrogen Hydrogen is the chemical element with the symbol H and atomic number 1. Hydrogen is the lightest element. At standard conditions hydrogen is a gas of diatomic molecules having the formula . It is colorless, odorless, tasteless, non-toxic ...

and

methane Methane ( , ) is a chemical compound with the chemical formula (one carbon atom bonded to four hydrogen atoms). It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The relative abundance of methane on Ea ...

. Using the

pressure swing adsorption Pressure swing adsorption (PSA) is a technique used to separate some gas species from a mixture of gases (typically air) under pressure according to the species' molecular characteristics and affinity for an adsorbent material. It operates at ne ...

, we could separate various gas and then,

power-to-gas Power-to-gas (often abbreviated P2G) is a technology that uses electric power to produce a gaseous fuel. When using surplus power from wind generation, the concept is sometimes called windgas. Most P2G systems use electrolysis to produce hydrogen ...

is used to convert it to fuel. On the other hand, the hydrothermal gasification process could degrade other waste such as sewage sludge and waste from food factory.

Precursor to other potassium compounds

Many potassium salts are prepared by neutralization reactions involving KOH. The potassium salts of

carbonate A carbonate is a salt of carbonic acid (H2CO3), characterized by the presence of the carbonate ion, a polyatomic ion with the formula . The word ''carbonate'' may also refer to a carbonate ester, an organic compound containing the carbonate ...

cyanide Cyanide is a naturally occurring, rapidly acting, toxic chemical that can exist in many different forms. In chemistry, a cyanide () is a chemical compound that contains a functional group. This group, known as the cyano group, consists of ...

permanganate A permanganate () is a chemical compound containing the manganate(VII) ion, , the conjugate base of permanganic acid. Because the manganese atom is in the +7 oxidation state, the permanganate(VII) ion is a strong oxidizing agent. The ion is a tr ...

phosphate In chemistry, a phosphate is an anion, salt, functional group or ester derived from a phosphoric acid. It most commonly means orthophosphate, a derivative of orthophosphoric acid . The phosphate or orthophosphate ion is derived from phosph ...

, and various silicates are prepared by treating either the oxides or the acids with KOH. The high solubility of potassium phosphate is desirable in

fertilizer A fertilizer (American English) or fertiliser (British English; see spelling differences) is any material of natural or synthetic origin that is applied to soil or to plant tissues to supply plant nutrients. Fertilizers may be distinct from ...

Manufacture of soft soaps

The

saponification Saponification is a process of converting esters into soaps and alcohols by the action of aqueous alkali (for example, aqueous sodium hydroxide solutions). Soaps are salts of fatty acids, which in turn are carboxylic acids with long carbon chains. ...

s with KOH is used to prepare the corresponding "potassium

soaps Soap is a surfactant cleaning compound used for personal or other cleaning. Soap may also refer to: Education * Supplemental Offer and Acceptance Program, for medical students who were not initially matched with U.S. residencies by the National ...

", 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 An alkaline battery (IEC code: L) is a type of primary battery where the electrolyte (most commonly potassium hydroxide) has a pH value above 7. Typically these batteries derive energy from the reaction between zinc metal and manganese dioxi ...

based on

nickel Nickel is a chemical element with symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel is a hard and ductile transition metal. Pure nickel is chemically reactive but large pieces are slow ...

cadmium Cadmium is a chemical element with the symbol Cd and atomic number 48. This soft, silvery-white metal is chemically similar to the two other stable metals in group 12, zinc and mercury. Like zinc, it demonstrates oxidation state +2 in most of ...

, and manganese dioxide-

zinc Zinc is a chemical element with the symbol Zn and atomic number 30. Zinc is a slightly brittle metal at room temperature and has a shiny-greyish appearance when oxidation is removed. It is the first element in group 12 (IIB) of the periodi ...

. Potassium hydroxide is preferred over sodium hydroxide because its solutions are more conductive. The nickel–metal hydride batteries in the

Toyota Prius The is a car built by Toyota which has a hybrid drivetrain, combining an internal combustion engine with an electric motor. Initially offered as a four-door sedan, it has been produced only as a five-door liftback since 2003. In 2007, ...

use a mixture of potassium hydroxide and sodium hydroxide. Nickel–iron batteries also use potassium hydroxide electrolyte.

Food industry

In food products, potassium hydroxide acts as a food thickener, pH control agent and food stabilizer. The

FDA The United States Food and Drug Administration (FDA or US FDA) is a federal agency of the Department of Health and Human Services. The FDA is responsible for protecting and promoting public health through the control and supervision of food ...

considers it generally safe as a direct food ingredient when used in accordance with

Good Manufacturing Practices Current good manufacturing practices (cGMP) are those conforming to the guidelines recommended by relevant agencies. Those agencies control the authorization and licensing of the manufacture and sale of food and beverages, cosmetics, pharmaceutica ...

. It is known in the E number system as E525.

Niche applications

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 " resomation", potassium hydroxide hastens the decomposition of soft tissues, both animal and human, to leave behind only the bones and other hard tissues.

Entomologists Entomology () is the scientific study of insects, a branch of zoology. In the past the term "insect" was less specific, and historically the definition of entomology would also include the study of animals in other arthropod groups, such as arach ...

wishing to study the fine structure of

insect Insects (from Latin ') are pancrustacean hexapod invertebrates of the class Insecta. They are the largest group within the arthropod phylum. Insects have a chitinous exoskeleton, a three-part body ( head, thorax and abdomen), three ...

anatomy Anatomy () is the branch of biology concerned with the study of the structure of organisms and their parts. Anatomy is a branch of natural science that deals with the structural organization of living things. It is an old science, having it ...

may use a 10% aqueous solution of KOH to apply this process. 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

salt Salt is a mineral composed primarily of sodium chloride (NaCl), a chemical compound belonging to the larger class of salts; salt in the form of a natural crystalline mineral is known as rock salt or halite. Salt is present in vast quant ...

. The corrosive properties of potassium hydroxide make it a useful ingredient in agents and preparations that clean and

disinfect A disinfectant is a chemical substance or compound used to inactivate or destroy microorganisms on inert surfaces. Disinfection does not necessarily kill all microorganisms, especially resistant bacterial spores; it is less effective than ste ...

surfaces and materials that can themselves resist

corrosion Corrosion is a natural process that converts a refined metal into a more chemically stable oxide. It is the gradual deterioration of materials (usually a metal) by chemical or electrochemical reaction with their environment. Corrosion engi ...

by KOH. KOH is also used for semiconductor chip fabrication (for example

anisotropic wet etching Anisotropy () is the property of a material which allows it to change or assume different properties in different directions, as opposed to isotropy. It can be defined as a difference, when measured along different axes, in a material's physic ...

). Potassium hydroxide is often the main active ingredient in chemical "cuticle removers" used in

manicure A manicure is a mostly cosmetic beauty treatment for the fingernails and hands performed at home or in a nail salon. A manicure usually consists of filing and shaping the ''free edge'' of nails, pushing and clipping (with a cuticle pusher ...

treatments. Because aggressive bases like KOH damage the cuticle of the hair 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

tanning Tanning may refer to: *Tanning (leather), treating animal skins to produce leather *Sun tanning, using the sun to darken pale skin **Indoor tanning, the use of artificial light in place of the sun **Sunless tanning, application of a stain or dye t ...

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. Potassium hydroxide is used to identify some species of

fungi A fungus ( : fungi or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, separately from ...

. 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

gilled mushrooms The fungal order Agaricales, also known as gilled mushrooms (for their distinctive gills) or euagarics, contains some of the most familiar types of mushrooms. The order has 33 extant families, 413 genera, and over 13,000 described species, along ...

boletes {{refimprove, date=July 2020 A bolete is a type of mushroom, or fungal fruiting body. It can be identified thanks to a unique mushroom cap. The cap is clearly different from the stem. On the underside of the cap there is usually a spongy surfa ...

polypores Polypores are a group of fungi that form large fruiting bodies with pores or tubes on the underside (see Delimitation for exceptions). They are a morphological group of basidiomycetes-like gilled mushrooms and hydnoid fungi, and not all polypo ...