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Potassium hydroxide
View on Wikipediafrom Wikipedia
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| Names | |
|---|---|
| IUPAC name
Potassium hydroxide
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Other names
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| Identifiers | |
3D model (JSmol)
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| ChEBI | |
| ChemSpider | |
| ECHA InfoCard | 100.013.802 |
| EC Number |
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| E number | E525 (acidity regulators, ...) |
PubChem CID
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| RTECS number |
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| UNII | |
| UN number | 1813 |
CompTox Dashboard (EPA)
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|
| |
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| Properties | |
| KOH | |
| Molar mass | 56.105 g·mol−1 |
| Appearance | white solid, deliquescent |
| Odor | odorless |
| Density | 2.044 g/cm3 (20 °C)[1] 2.12 g/cm3 (25 °C)[2] |
| Melting point | 410[3][4] °C (770 °F; 683 K) |
| Boiling point | 1,327 °C (2,421 °F; 1,600 K) |
| 85 g/100 mL (−23.2 °C) 97 g/100 mL (0 °C) 121 g/100 mL (25 °C) 138.3 g/100 mL (50 °C) 162.9 g/100 mL (100 °C)[1][5] | |
| Solubility | soluble in alcohol, glycerol insoluble in ether, liquid ammonia |
| Solubility in methanol | 55 g/100 g (28 °C)[2] |
| Solubility in isopropanol | ~14 g / 100 g (28 °C) |
| Acidity (pKa) | 14.7[6] |
| −22.0·10−6 cm3/mol | |
Refractive index (nD)
|
1.409 (20 °C) |
| Thermochemistry | |
Heat capacity (C)
|
65.87 J/mol·K[2] |
Std molar
entropy (S⦵298) |
79.32 J/mol·K[2][7] |
Std enthalpy of
formation (ΔfH⦵298) |
−425.8 kJ/mol[2][7] |
Gibbs free energy (ΔfG⦵)
|
−380.2 kJ/mol[2] |
| Hazards | |
| GHS labelling: | |
  [8]
| |
| Danger | |
| H290, H302, H314[8] | |
| P280, P305+P351+P338, P310[8] | |
| NFPA 704 (fire diamond) | |
| Flash point | nonflammable |
| Lethal dose or concentration (LD, LC): | |
LD50 (median dose)
|
273 mg/kg (oral, rat)[10] |
| NIOSH (US health exposure limits): | |
PEL (Permissible)
|
none[9] |
REL (Recommended)
|
C 2 mg/m3[9] |
IDLH (Immediate danger)
|
N.D.[9] |
| Safety data sheet (SDS) | ICSC 0357 |
| Related compounds | |
Other anions
|
Potassium hydrosulfide Potassium amide |
Other cations
|
Lithium hydroxide Sodium hydroxide Rubidium hydroxide Caesium hydroxide |
Related compounds
|
Potassium oxide |
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
| |
Potassium hydroxide is an inorganic compound with the formula KOH, 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 utilize its caustic nature and its reactivity toward acids. About 2.5 million tonnes were produced in 2023.[11] KOH is noteworthy as the precursor to most soft and liquid soaps, as well as numerous potassium-containing chemicals. It is a white solid that is dangerously corrosive.[12]
Properties and structure
[edit]KOH exhibits high thermal stability. Because of this 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. These pellets become tacky in air because KOH is hygroscopic. Most commercial samples are ca. 90% pure, the remainder being water and carbonates.[12] Its dissolution in water is strongly exothermic. Concentrated aqueous solutions are sometimes called potassium lyes. Even at high temperatures, solid KOH does not dehydrate readily.[13]
Structure
[edit]At higher temperatures, solid KOH crystallizes in the NaCl crystal structure. The OH− group is either rapidly or randomly disordered so that it is effectively a spherical anion 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 hydrates, namely the monohydrate KOH · H2O, the dihydrate KOH · 2H2O and the tetrahydrate KOH · 4H2O.[14]
Reactions
[edit]Solubility and desiccating properties
[edit]About 112 g of KOH dissolve in 100 mL water at room temperature, which contrasts with 100 g/100 mL for NaOH.[15] Thus on a molar basis, KOH is slightly more soluble than NaOH. Lower molecular-weight alcohols such as methanol, ethanol, and propanols are also excellent solvents. They participate in an acid-base equilibrium. In the case of methanol the potassium methoxide (methylate) forms:[16]
- KOH + CH3OH → CH3OK + H2O
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.
As a nucleophile in organic chemistry
[edit]KOH, like NaOH, serves as a source of OH−, a highly nucleophilic anion that attacks polar bonds in both inorganic and organic materials. Aqueous KOH saponifies esters:
- KOH + RCOOR' → RCOOK + 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.[17]
Reactions with inorganic compounds
[edit]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 potassium bicarbonate:
- KOH + CO2 → KHCO3
Manufacture
[edit]Historically, KOH was made by adding potassium carbonate to a strong solution of calcium hydroxide (slaked lime). The salt metathesis reaction results in precipitation of solid calcium carbonate, 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 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.[12] The method is analogous to the manufacture of sodium hydroxide (see chloralkali process):
- 2 KCl + 2 H2O → 2 KOH + Cl2 + H2
Hydrogen gas forms as a byproduct 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.[18]
Uses
[edit]KOH and NaOH can be used interchangeably for a number of applications, although in industry, NaOH is preferred because of its lower cost.
Purity requirements
[edit]Manufactured KOH are used in various applications that need various purities. For industrial uses, like cleaning metals or treating waste gases, only 90% purity, minimal, is required. Food grade ones also require 90%, except it must have arsenic less than 3 ppm and lead less than 5 ppm. KOH at this purity is used to debitterate olive and the manufacture of cocoa. Electronic grade ones which are used to etch semiconductors and treat silicon wafers in photovoltaic cells require at least 99.9% purity and less than 1 ppm for total metal ion concentration. Reagent level ones that are used in pharmaceutical applications and professional analytical labs require purity higher than 99.99% and heavy metal less than 0.1 ppm for safety to use.[11]
Catalyst for hydrothermal gasification process
[edit]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 from coal often produces much coking wastewater. In order to degrade it, supercritical water is used to convert it to the syngas containing carbon monoxide, carbon dioxide, hydrogen and methane. Using pressure swing adsorption, various gases could be separated, and then power-to-gas technology is used to convert them to fuel.[19] On the other hand, the hydrothermal gasification process could degrade other waste such as sewage sludge and waste from food factories.
Precursor to other potassium compounds
[edit]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.[12] The high solubility of potassium phosphate is desirable in fertilizers.
Manufacture of soft soaps
[edit]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.[20]
As an electrolyte
[edit]
Aqueous potassium hydroxide is employed as the electrolyte in alkaline batteries based on nickel-cadmium, nickel-hydrogen, and manganese dioxide-zinc. Potassium hydroxide is preferred over sodium hydroxide because its solutions are more conductive.[21] The nickel–metal hydride batteries in the Toyota Prius use a mixture of potassium hydroxide and sodium hydroxide.[22] Nickel–iron batteries also use potassium hydroxide electrolyte.
Food industry
[edit]In food products, potassium hydroxide acts as a food thickener, pH control agent and food stabilizer. The FDA considers it generally safe as a direct food ingredient when used in accordance with Good Manufacturing Practices.[23] It is known in the E number system as E525.
Niche applications
[edit]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.[24] Entomologists wishing to study the fine structure of insect anatomy may use a 10% aqueous solution of KOH to apply this process.[25]
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.
The corrosive properties of potassium hydroxide make it a useful ingredient in agents and preparations that clean and disinfect surfaces and materials that can themselves resist corrosion by KOH.[18]
KOH is also used for semiconductor chip fabrication (for example anisotropic wet etching).
Potassium hydroxide is often the main active ingredient in chemical "cuticle removers" used in manicure 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 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 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, boletes, polypores, and lichens[26] are identifiable based on this color-change reaction.[27]
Safety
[edit]Potassium hydroxide is a caustic 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 pulmonary edema.[28] Safety considerations are similar to those of sodium hydroxide.
The caustic effects arise from being highly alkaline, but if potassium hydroxide is neutralised with a non-toxic acid then it becomes a non-toxic potassium salt. It is approved as a food additive under the code E525.
See also
[edit]- Potash
- Soda lime
- Saltwater soap – sailors' soap
References
[edit]- ^ a b Lide, D. R., ed. (2005). CRC Handbook of Chemistry and Physics (86th ed.). Boca Raton, Florida: CRC Press. p. 4-80. ISBN 0-8493-0486-5.
- ^ a b c d e f "potassium hydroxide". chemister.ru. Archived from the original on 18 May 2014. Retrieved 8 May 2018.
- ^ Otto, H.W; Seward, R.P. (1964). "Phase equilibria in the potassium hydroxide-sodium hydroxide system". J. Chem. Eng. Data. 9 (4): 507–508. doi:10.1021/je60023a009.
- ^ Seward, R.P; Martin, K.E. (1949). "The melting point of potassium hydroxide". J. Am. Chem. Soc. 71 (10): 3564–3565. Bibcode:1949JAChS..71R3564S. doi:10.1021/ja01178a530.
- ^ Seidell, Atherton; Linke, William F. (1952). Solubilities of Inorganic and Organic Compounds. Van Nostrand. Retrieved 2014-05-29.
- ^ Popov, K.; et al. (2002). "7Li, 23Na, 39K and 133Cs NMR comparative equilibrium study of alkali metal cation hydroxide complexes in aqueous solutions. First numerical value for CsOH formation". Inorganic Chemistry Communications. 3 (5): 223–225. doi:10.1016/S1387-7003(02)00335-0. ISSN 1387-7003. Retrieved October 20, 2018.
- ^ a b Zumdahl, Steven S. (2009). Chemical Principles 6th Ed. Houghton Mifflin Company. p. A22. ISBN 978-0-618-94690-7.
- ^ a b c Sigma-Aldrich Co., Potassium hydroxide. Retrieved on 2014-05-18.
- ^ a b c NIOSH Pocket Guide to Chemical Hazards. "#0523". National Institute for Occupational Safety and Health (NIOSH).
- ^ Chambers, Michael. "ChemIDplus - 1310-58-3 - KWYUFKZDYYNOTN-UHFFFAOYSA-M - Potassium hydroxide [JAN:NF] - Similar structures search, synonyms, formulas, resource links, and other chemical information". chem.sis.nlm.nih.gov. Archived from the original on 12 August 2014. Retrieved 8 May 2018.
- ^ a b Chen, Fernando (2025-03-21). "Potassium Hydroxide (KOH): The "Alkaline Engine" Driving Modern Industry". Kelewell Trading. Retrieved 2025-08-11.
- ^ a b c d 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.
- ^ Holleman, A. F; Wiberg, E. (2001). Inorganic Chemistry. San Diego: Academic Press. ISBN 978-0-12-352651-9.
- ^ Wells, A.F. (1984). Structural Inorganic Chemistry. Oxford: Clarendon Press. ISBN 978-0-19-855370-0.
- ^ Seidell, Atherton; Linke, William F. (1952). Solubilities of Inorganic and Organic Compounds. Van Nostrand. Retrieved 2014-05-29.
- ^ 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.
- ^ W. W. Hartman (1923). "p-Cresol". Organic Syntheses. 3: 37. doi:10.15227/orgsyn.003.0037; Collected Volumes, vol. 1, p. 175.
- ^ a b Römpp Chemie-Lexikon, 9th Ed. (in German)
- ^ 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.
- ^ K. Schumann; K. Siekmann (2005). "Soaps". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi:10.1002/14356007.a24_247. ISBN 978-3527306732.
- ^ D. Berndt; D. Spahrbier (2005). "Batteries". Ullmann's Encyclopedia of Industrial Chemistry. Weinheim: Wiley-VCH. doi:10.1002/14356007.a03_343. ISBN 978-3527306732.
- ^ "Toyota Prius Hybrid 2010 Model Emergency Response Guide" (PDF). Toyota Motor Corporation. 2009. Archived from the original (PDF) on 2012-03-20.
- ^ "Compound Summary for CID 14797 - Potassium Hydroxide". PubChem.
- ^ Green, Margaret (January 1952). "A RAPID METHOD FOR CLEARING AND STAINING SPECIMENS FOR THE DEMONSTRATION OF BONE". The Ohio Journal of Science. 52 (1): 31–33. hdl:1811/3896.
- ^ Thomas Eisner (2003). For the Love of Insects. Harvard University Press. p. 71.
- ^ Elix, J.A.; Stocker-Wörgötter, Elfie (2008). "Chapter 7: Biochemistry and secondary metabolites". In Nash III, Thomas H. (ed.). Lichen Biology (2nd ed.). New York: Cambridge University Press. pp. 118–119. ISBN 978-0-521-69216-8.
- ^ Testing Chemical Reactions Archived 2009-10-15 at the Wayback Machine at MushroomExpert.com
- ^ "Hazardous Substance Fact Sheet: Potassium Hydroxide" (PDF). New Jersey Department of Health. Retrieved 7 October 2024.
External links
[edit]
Wikimedia Commons has media related to Potassium hydroxide.
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| National | |
| Other | |
Potassium hydroxide
View on Grokipediafrom Grokipedia
History
Discovery and early production
Potassium hydroxide, known historically as caustic potash, has roots in the extraction of potash from wood ashes, a practice dating back to medieval times when wood was burned to produce alkaline residues for cleaning and manufacturing. Potash, primarily potassium carbonate (K₂CO₃), was obtained by leaching these ashes with water, yielding a solution used in rudimentary chemical processes across Europe and later in colonial America.[4][5] In the early 18th century, a key advancement allowed for the conversion of this potash into the more reactive caustic potash through a simple chemical reaction with slaked lime, or calcium hydroxide (Ca(OH)₂). The process involved mixing a solution of potassium carbonate with calcium hydroxide, resulting in the precipitation of calcium carbonate (CaCO₃) and the formation of potassium hydroxide solution, as described by the equation: This method produced a stronger alkali suitable for various applications, though the resulting product was often impure due to contaminants from the ashes.[6][7] By the late 1700s, chemists such as Antoine Lavoisier began recognizing caustic potash as a distinct compound in their systematic studies of alkalis, distinguishing it from milder forms like potash carbonate and exploring its role in acid-base reactions and gas absorption experiments. Lavoisier's work on respiration and combustion highlighted its ability to react with carbon dioxide, solidifying its identity as a powerful base.[8][9] Prior to industrial scaling, production remained small-scale and labor-intensive, primarily for artisanal uses in soap-making, where it saponified fats into soft soaps, and in textile dyeing to fix colors on fabrics. The limited purity and yield constrained its availability to local workshops and households, relying on seasonal wood burning and manual leaching. This early approach persisted until the 19th century, when electrolytic methods began to emerge for larger-scale production.[5][4]Industrial development
The isolation of potassium metal in 1807 by Humphry Davy through electrolysis of molten potassium hydroxide marked a foundational milestone in understanding electrolytic processes for alkali metals, laying the groundwork for future industrial applications.[10] This laboratory achievement highlighted the reactivity of potassium hydroxide under electrical decomposition, inspiring subsequent efforts to scale such methods for commercial production.[11] Industrial electrolytic production of potassium hydroxide emerged in the 1890s, adapted from the chlor-alkali process developed for sodium hydroxide by substituting potassium chloride (KCl) brine as the feedstock. This adaptation enabled the simultaneous generation of potassium hydroxide, chlorine gas, and hydrogen, addressing the growing demand for these chemicals in the expanding dye and soap industries. A key milestone occurred in 1890 when the German firm Chemische Fabrik Griesheim-Elektron initiated commercial-scale production using a diaphragm cell design, representing one of the first large-scale electrolytic facilities for the compound.[12] Post-1900, the technology gained widespread adoption through refinements in cell designs, particularly the mercury cell and diaphragm cell processes, which enhanced energy efficiency and product concentration. The mercury cell, originally patented by Hamilton Castner and Karl Kellner in 1892 for sodium hydroxide, was adapted for potassium hydroxide to yield higher-purity solutions suitable for specialty applications.[13] Diaphragm cells, evolving from the Griesheim design, facilitated cost-effective separation of anode and cathode products, promoting broader industrial integration. These advancements shifted production from batch to continuous operations, aligning with the rapid growth of the global chemical sector. By the mid-20th century, electrolytic methods had supplanted historical lime-based processes—where potassium carbonate reacted with calcium hydroxide to form potassium hydroxide and calcium carbonate precipitate—due to superior efficiency, scalability, and the economic value of chlorine byproducts.[14] Early 20th-century production expanded in tandem with chemical industry booms, particularly in Europe and North America, though global annual output remained modest, initially under 100,000 tonnes as facilities scaled up from pilot levels.[15]Properties
Physical properties
Potassium hydroxide appears as a white, deliquescent crystalline solid in its anhydrous form.[1] It is commercially available in various forms, including 90% pure flakes or pellets for solid applications and 45-50% aqueous solutions for liquid uses.[16][17] The compound has a melting point of 406 °C and a boiling point of 1327 °C, at which point it decomposes.[1] Its density is 2.044 g/cm³ at 20 °C.[1] Potassium hydroxide is odorless and has a bitter taste. It exhibits high solubility in water, dissolving up to 121 g per 100 mL at 25 °C.[1] Dissolution in water is highly exothermic, releasing approximately 57 kJ/mol of heat.[18] Due to its hygroscopic nature, potassium hydroxide rapidly absorbs atmospheric moisture, often leading to deliquescence.[1] This behavior stems from its ionic structure, which facilitates strong interactions with water molecules.[1]Chemical structure
Potassium hydroxide (KOH) is an ionic compound composed of potassium cations (K⁺) and hydroxide anions (OH⁻) in a 1:1 stoichiometric ratio.[1] This structure arises from the transfer of an electron from a potassium atom to an oxygen atom in the hydroxide group, forming the stable K⁺ ion and the polyatomic OH⁻ ion, which features a covalent bond between oxygen and hydrogen but interacts ionically with K⁺.[19] In the solid state, KOH adopts a monoclinic crystal lattice (space group P₂₁), where the potassium cations are coordinated by multiple hydroxide anions through strong electrostatic ionic bonds, resulting in a highly stable crystalline network.[1][20] Upon dissolution in aqueous solution, the compound fully dissociates into free K⁺ and OH⁻ ions, enhancing its reactivity as a strong base.[1] The bonding in KOH is purely ionic, with no significant covalent character, distinguishing it from more polar compounds.[21] KOH exhibits a similar ionic nature to sodium hydroxide (NaOH), both being alkali metal hydroxides with comparable dissociation behavior.[21] However, the larger ionic radius of K⁺ compared to Na⁺ leads to weaker lattice energy in KOH, which accounts for its slightly higher solubility in water.[22] Infrared spectroscopy provides insight into the hydroxide moiety, revealing a broad absorption band for the O-H stretching vibration at approximately 3600 cm⁻¹ in the solid form, reflecting the vibrational mode of the OH⁻ ion within the lattice.[23] At lower temperatures, this band may split into a doublet due to coupling between OH⁻ ions in the unit cell.[23]Reactions and reactivity
Solubility and hygroscopic properties
Potassium hydroxide exhibits high solubility in water, with its dissolution capacity increasing significantly with temperature. The solubility is approximately 97 g of KOH per 100 g of water at 0 °C and rises to 178 g per 100 g at 100 °C. This temperature dependence is detailed in the following table, showing values at selected temperatures:| Temperature (°C) | Solubility (g KOH / 100 g H₂O) |
|---|---|
| 0 | 97 |
| 10 | 103 |
| 20 | 112 |
| 25 | 121 |
| 30 | 126 |
| 100 | 178 |
Nucleophilic reactions in organic chemistry
Potassium hydroxide serves as a source of the hydroxide ion (OH⁻), a strong nucleophile and base due to its high basicity (pKₐ of conjugate acid H₂O is 15.7), facilitating nucleophilic attacks and deprotonations in protic solvents like water or ethanol.[27] In nucleophilic substitution reactions, particularly SN2 processes, OH⁻ from KOH displaces leaving groups on primary or methyl alkyl halides, converting them to alcohols. This backside attack mechanism involves concerted bond formation and breaking, resulting in inversion of stereochemistry at the carbon center. The reaction is typically conducted in aqueous ethanol to balance solubility and minimize elimination side products; for example, bromoethane reacts with KOH to yield ethanol via the pathway: where R is an alkyl group and X is a halide. The rate follows second-order kinetics, depending on both substrate and nucleophile concentrations, and is favored for unhindered substrates. Potassium hydroxide also promotes elimination reactions, notably the Hofmann elimination of quaternary ammonium salts, where OH⁻ acts as a base to abstract a β-hydrogen, leading to E2 elimination and formation of the less substituted alkene (Hofmann product) due to the bulky leaving group (e.g., NMe₃). This contrasts with Zaitsev elimination by favoring the less stable alkene because of steric control in the transition state. The process involves heating the quaternary ammonium hydroxide, often generated in situ from the halide salt and a hydroxide source like KOH, as illustrated for ethyltrimethylammonium: (CH₃)₃N⁺CH₂CH₃ OH⁻ → CH₂=CH₂ + (CH₃)₃N + H₂O The mechanism proceeds through anti-periplanar geometry in the E2 step, with the leaving amine departing as a neutral species. This reaction is valuable for synthesizing terminal alkenes from amines.[28] A key nucleophilic reaction of KOH is the saponification of esters, a base-catalyzed hydrolysis where OH⁻ adds to the carbonyl carbon, forming a tetrahedral intermediate that collapses to expel the alkoxide and generate the potassium carboxylate salt. The overall transformation is: This irreversible process (due to the stable carboxylate product) follows BAC₂ mechanism kinetics, second-order overall, with the rate-determining step being OH⁻ addition. The reaction rate varies with ester structure: electron-withdrawing groups on the acyl portion accelerate it by enhancing carbonyl electrophilicity, while steric bulk around the carbonyl (e.g., in ortho-substituted benzoates) hinders nucleophilic approach, slowing the rate by factors up to 10³ compared to methyl acetate.[29][30] As a strong base, KOH facilitates deprotonation of weak acids in organic synthesis, such as active methylene compounds (pKₐ ~13–20) to generate carbanions for further reactions. In the Claisen condensation, KOH can deprotonate the α-hydrogen of esters under phase-transfer conditions to form enolates, which then attack another ester's carbonyl, yielding β-keto esters after protonation; however, aqueous KOH risks competing hydrolysis, so it is less common than alkoxide bases. For instance, in dichloromethane-water systems with a phase-transfer catalyst, KOH enables the condensation of ethyl acetate to ethyl acetoacetate. This highlights KOH's role in generating nucleophilic enolates from weakly acidic C–H bonds.[31]Reactions with inorganic compounds
Potassium hydroxide (KOH) serves as a strong base in neutralization reactions with inorganic acids, fully dissociating in water to yield hydroxide ions that react completely with protons from the acid, producing water and the corresponding potassium salt. A representative example is its reaction with hydrochloric acid: This complete dissociation, where KOH ionizes as , ensures the reaction goes to completion without equilibrium constraints typical of weak bases.[1] The pronounced basicity of KOH manifests in highly alkaline solutions; for instance, a 0.1 M aqueous solution exhibits a pH of 13.0, arising from M and the relation . KOH further illustrates its reactivity through amphoteric interactions with oxides like aluminum oxide (), which behaves as a Lewis acid toward the base, forming soluble potassium aluminate: This dissolution underscores the amphoteric nature of , enabling it to react with both acids and strong bases.[32][1][33] KOH also forms soluble inorganic salts via reactions with non-metal oxides, such as carbon dioxide, yielding potassium carbonate: This process, which absorbs from the atmosphere or solutions, exemplifies acid-base chemistry where acts as an acidic anhydride. Additionally, in the presence of water, KOH promotes the redox reaction of aluminum metal, generating hydrogen gas and potassium tetrahydroxoaluminate: Here, the hydroxide ions facilitate the breakdown of aluminum's protective oxide layer, enabling oxidation to aluminate and reduction of water to hydrogen.[34][1][35]Production
Industrial manufacture
Potassium hydroxide is predominantly manufactured on an industrial scale via the chlor-alkali electrolysis process, utilizing potassium chloride (KCl) brine as the feedstock in membrane cell technology. This method electrolytically decomposes the brine to produce potassium hydroxide (KOH), alongside valuable co-products hydrogen (H₂) and chlorine (Cl₂). Membrane cells, which employ ion-exchange membranes to separate the anode and cathode compartments, have become the standard due to their superior environmental profile compared to legacy mercury cell processes, which have been fully phased out globally by 2025 to prevent mercury contamination under the Minamata Convention. The overall reaction for the process is: This electrochemical approach originated from early 20th-century electrolytic innovations but has evolved significantly for modern scalability.[36][37][38] Global production of potassium hydroxide reached approximately 2.3 million tonnes as of 2024, with key manufacturing hubs concentrated in China (the largest producer), the United States, and Europe. The market value stood at around USD 3.4 billion as of 2024 and is projected to reach USD 3.5 billion in 2025, driven by a compound annual growth rate (CAGR) of 3-4%. Major producers include Occidental Chemical Corporation and Olin Corporation in the US, INEOS and Solvay in Europe, and various state-owned enterprises in China, benefiting from integrated chlor-alkali facilities that optimize co-product utilization. The economic viability of the process is enhanced by the revenue from H₂ and Cl₂, which together can account for a significant portion of overall output value.[39][40][41] The electrolysis process is energy-intensive, typically requiring 2,200-2,500 kWh per tonne of KOH produced in advanced membrane cells, though newer generations of technology have reduced this to as low as 2,000 kWh per tonne. Efforts to lower energy demands include innovations in electrode materials and membrane efficiency, which not only cut operational costs but also align with rising electricity prices and sustainability mandates. Recent developments emphasize a transition to low-energy, zero-mercury membrane systems to comply with regulations such as the EU's Industrial Emissions Directive updates and global Minamata Convention enforcement on mercury pollution. These advancements support broader decarbonization goals in the chemical sector by integrating renewable energy sources for electrolysis.[42][43][40]Laboratory preparation
In laboratory settings, one common method for preparing potassium hydroxide involves the reaction of potassium metal with water, which proceeds according to the balanced equation: This reaction generates hydrogen gas and releases significant heat, making it highly exothermic and necessitating precautions such as controlled addition of the metal to water under inert atmosphere to mitigate risks of ignition or explosion. An alternative approach is the electrolysis of an aqueous potassium chloride (KCl) solution using a divided cell, which separates the anode and cathode compartments to prevent mixing of the gaseous products and the alkaline solution. At the cathode, reduction of water produces hydroxide ions that combine with potassium cations to form KOH, alongside hydrogen gas evolution; at the anode, oxidation yields chlorine gas. This technique mirrors industrial electrolysis principles on a small scale, enabling isolation of pure KOH solution.[44] Purification of the resulting KOH typically involves recrystallization from ethanol, where the compound's solubility allows selective precipitation of impurities, or vacuum distillation of aqueous solutions to remove water and volatile contaminants. These methods yield KOH with purity exceeding 95%, free from industrial-scale impurities like carbonates, suitable for precise research applications.[45]Uses
Precursor to other compounds
Potassium hydroxide (KOH) is widely utilized as a key precursor in the synthesis of various potassium salts through neutralization and oxidation reactions, enabling the production of compounds essential for industrial and agricultural applications. Its strong basicity facilitates straightforward reactions with acids or oxidizable species, yielding high-value derivatives with controlled stoichiometry. One primary application is the production of potassium carbonate (K₂CO₃), achieved by reacting KOH with carbon dioxide gas. The balanced equation for this process is: This reaction is typically conducted in aqueous solution under controlled conditions to ensure efficient absorption of CO₂. Potassium carbonate derived this way serves as a flux in glass manufacturing, where it lowers the melting point of silica and enhances the transparency and refractive index of the final product. Additionally, it is employed in fertilizers to supply potassium nutrients to crops, improving soil fertility and plant growth.[46][47][48] Another important derivative is monopotassium phosphate (KH₂PO₄), formed via the neutralization of phosphoric acid with KOH. The reaction proceeds as: This method is a commercial route for producing KH₂PO₄, often carried out by careful addition of KOH to phosphoric acid to achieve the desired pH and avoid over-neutralization to dipotassium or tripotassium phosphates. The resulting salt functions as a buffering agent in pharmaceutical and laboratory preparations, maintaining stable pH in solutions. In agriculture, KH₂PO₄ acts as a fertilizer providing both phosphorus and potassium, promoting root development and fruit quality in crops.[49][50][51] KOH also serves in the synthesis of other specialized potassium compounds, such as potassium permanganate (KMnO₄), a powerful oxidizing agent. This involves fusing manganese dioxide with KOH to form a potassium manganate intermediate, followed by oxidation with chlorine gas.[52] To ensure the purity of these derived salts, high-purity KOH (typically 99% or greater) is essential, as impurities in the starting material can carry over and compromise the quality of the final product in sensitive applications.[53]Soap and detergent production
Potassium hydroxide plays a crucial role in the saponification process for producing liquid and soft soaps, where it reacts with triglycerides from fats or oils, such as those in coconut oil, to form potassium soaps and glycerol.[54] The general reaction is represented by the equation: where denotes the fatty acid chains.[55] This nucleophilic acyl substitution yields potassium carboxylates that are highly soluble in water.[54] The resulting potassium soaps are softer and more water-soluble than their sodium counterparts due to the larger potassium ion, which promotes better hydration and disrupts crystal lattice formation, enabling easier dissolution in aqueous solutions.[56] This property makes them ideal for formulating liquid detergents, shampoos, and other soft soap products that require high solubility and mildness.[57] On an industrial scale, soap and detergent production accounts for approximately 35% of global potassium hydroxide consumption as of 2023, driven by demand for liquid formulations in personal care and cleaning products.[58] Specific formulations often incorporate antimicrobial agents, such as carica fruit extracts, alongside potassium hydroxide and coconut oil to enhance antibacterial efficacy in liquid hand soaps.[59] Historically, potassium hydroxide-derived "soft soap" was produced using potash lye extracted from wood ashes, a traditional method dating back to colonial times for creating paste-like soaps.[60]Electrolyte in batteries
Potassium hydroxide (KOH) is widely used as the electrolyte in zinc-manganese dioxide alkaline batteries, which are primary cells designed for high energy density and long shelf life. Typically, a 30-40% aqueous solution of KOH is employed, offering high ionic conductivity attributable to the high mobility of K⁺ ions in the electrolyte.[61] This concentration ensures efficient ion transport between the zinc anode and manganese dioxide cathode, enabling sustained power output for consumer electronics such as remote controls and flashlights.[62] The electrochemical processes in these batteries involve the following half-cell reactions. At the anode, zinc is oxidized:At the cathode, manganese dioxide is reduced in a regenerative manner:
These reactions facilitate electron flow through an external circuit while regenerating KOH at the cathode, maintaining electrolyte balance during discharge. KOH is preferred over sodium hydroxide (NaOH) as an alternative electrolyte due to its superior ionic conductivity and reduced corrosivity toward the zinc anode, which minimizes self-discharge and extends battery life.[63] The higher mobility of K⁺ ions compared to Na⁺—stemming from a smaller hydrated radius—contributes to lower internal resistance and better performance under varying drain rates.[64] This application constitutes a significant share of KOH consumption, with approximately 15% of global production as of 2019 directed toward primary battery electrolytes.[65]