Potassium hydroxide
Potassium hydroxide (KOH) is an inorganic compound, also known as caustic potash or potassium hydrate, that serves as a strong alkali and versatile industrial chemical with the molecular formula KOH and a molar mass of 56.11 g/mol.[1][2] It typically appears as a white, odorless, hygroscopic solid in forms such as flakes, pellets, or granules, which readily absorbs moisture from the air to form a deliquescent solution.[1][2] Highly soluble in water (up to 121 g/100 g water at 25°C) and ethanol, it reacts exothermically to produce a clear, corrosive aqueous solution with a density of approximately 1.45–1.50 g/cm³ for common concentrations.[1][2] Produced primarily through the electrolytic decomposition of potassium chloride brine in a process similar to the chloralkali method, potassium hydroxide is commercially available as a 45–50% aqueous solution or in solid form, with key physical properties including a melting point of 360–406°C and a boiling point of 1324–1327°C.[1][3] As a strong base, it exhibits high reactivity, neutralizing acids to form salts and water, and reacting with metals like aluminum and zinc to generate hydrogen gas, which underscores its role in chemical synthesis and its potential hazards.[1][2] Potassium hydroxide finds extensive applications across industries, including the production of soaps and detergents through saponification of fats, as an electrolyte in alkaline batteries, in petroleum refining for removing impurities, and in agriculture for formulating fertilizers and de-icing fluids.[3][2] It is also used in water treatment for pH adjustment, in cosmetics and bleach manufacturing, and as a food additive (generally recognized as safe in limited quantities) for regulating acidity in products like chocolate and soft drinks.[1][3] Additionally, it serves in the synthesis of other potassium compounds and in biodiesel production as a catalyst.[1] Despite its utility, potassium hydroxide is highly corrosive and non-combustible but poses severe health risks, causing chemical burns, irritation, or permanent damage to skin, eyes, and respiratory tissues upon contact, inhalation, or ingestion; it is classified as a hazardous substance requiring protective handling, storage in airtight containers, and emergency response protocols.[1][2] Its reactivity with water and certain metals can lead to exothermic reactions or flammable gas evolution, necessitating careful transport via pipelines, tank cars, or sealed containers to mitigate risks.[3][2]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: \mathrm{K_2CO_3 + Ca(OH)_2 \rightarrow 2 KOH + CaCO_3} 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: \text{R–X} + \text{KOH} \rightarrow \text{R–OH} + \text{KX} 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: \text{RCOO R'} + \text{KOH} \rightarrow \text{RCOO K} + \text{R'OH} 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: \ce{KOH + HCl -> KCl + H2O} This complete dissociation, where KOH ionizes as \ce{KOH -> K+ + OH-}, 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 [\ce{OH-}] = 0.1 M and the relation \mathrm{pH} = 14 - \mathrm{pOH}. KOH further illustrates its reactivity through amphoteric interactions with oxides like aluminum oxide (\ce{Al2O3}), which behaves as a Lewis acid toward the base, forming soluble potassium aluminate: \ce{Al2O3 + 2KOH -> 2KAlO2 + H2O} This dissolution underscores the amphoteric nature of \ce{Al2O3}, 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: \ce{2KOH + CO2 -> K2CO3 + H2O} This process, which absorbs \ce{CO2} from the atmosphere or solutions, exemplifies acid-base chemistry where \ce{CO2} 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: \ce{2Al + 2KOH + 6H2O -> 2K[Al(OH)4] + 3H2} 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: $2 \mathrm{KCl} + 2 \mathrm{H_2O} \rightarrow 2 \mathrm{KOH} + \mathrm{H_2} + \mathrm{Cl_2} 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: $2 \mathrm{K} + 2 \mathrm{H_2O} \rightarrow 2 \mathrm{KOH} + \mathrm{H_2} 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: $2 \ce{KOH} + \ce{CO2} \rightarrow \ce{K2CO3} + \ce{H2O} 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: \ce{KOH} + \ce{H3PO4} \rightarrow \ce{KH2PO4} + \ce{H2O} 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: (\ce{RCOO})_3\ce{C3H5} + 3\ce{KOH} \rightarrow 3\ce{RCOOK} + \ce{C3H5(OH)3} where \ce{RCOO} 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:\ce{Zn + 2 KOH -> K2ZnO2 + H2O + 2 e^-}
At the cathode, manganese dioxide is reduced in a regenerative manner:
\ce{2 MnO2 + H2O + 2 e^- -> Mn2O3 + 2 KOH}
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]