Potassium hydroxide is aninorganic compound with the formulaKOH, and is commonly calledcaustic potash.
Along withsodium hydroxide (NaOH), KOH is a prototypicalstrong base. It has many industrial and niche applications, most of which utilize itscaustic nature and its reactivity towardacids. About 2.5 milliontonnes were produced in 2023.[11] KOH is noteworthy as the precursor to most soft and liquidsoaps, as well as numerouspotassium-containing chemicals. It is a white solid that is dangerously corrosive.[12]
KOH exhibits highthermal stability. Because of this high stability and relatively lowmelting 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 ishygroscopic. Most commercial samples are ca. 90% pure, the remainder being water and carbonates.[12] Itsdissolution in water is stronglyexothermic. Concentrated aqueous solutions are sometimes called potassiumlyes. Even at high temperatures, solid KOH does not dehydrate readily.[13]
At higher temperatures, solid KOHcrystallizes in theNaClcrystal structure. TheOH− group is either rapidly or randomly disordered so that it is effectively a sphericalanion of radius 1.53 Å (betweenCl− andF− in size). At room temperature, theOH− groups are ordered and the environment about theK+ centers is distorted, withK+−OH− distances ranging from 2.69 to 3.15 Å, depending on the orientation of the OH group. KOH forms a series of crystallinehydrates, namely the monohydrateKOH ·H2O, the dihydrateKOH · 2H2O and the tetrahydrateKOH · 4H2O.[14]
About 112 g of KOHdissolve 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-weightalcohols such asmethanol,ethanol, andpropanols are also excellentsolvents. They participate in an acid-base equilibrium. In the case of methanol the potassiummethoxide (methylate) forms:[16]
KOH + CH3OH → CH3OK + H2O
Because of its high affinity for water, KOH serves as adesiccant in the laboratory. It is often used to dry basic solvents, especiallyamines andpyridines.
KOH, like NaOH, serves as a source ofOH−, a highlynucleophilic anion that attackspolar bonds in both inorganic and organic materials. Aqueous KOHsaponifiesesters:
KOH + RCOOR' → RCOOK + R'OH
When R is a long chain, the product is called apotassium soap. This reaction is manifested by the "greasy" feel that KOH gives when touched;fats on the skin are rapidly converted to soap andglycerol.
Complementary to its reactivity toward acids, KOH attacksoxides. Thus, SiO2 is attacked by KOH to give soluble potassium silicates. KOH reacts withcarbon dioxide to givepotassium bicarbonate:
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 ofpotassium chloride solutions.[12] The method is analogous to the manufacture ofsodium hydroxide (seechloralkali process):
2 KCl + 2 H2O → 2 KOH + Cl2 + H2
Hydrogen gas forms as a byproduct on thecathode; concurrently, an anodic oxidation of thechloride ion takes place, formingchlorine gas as a byproduct. Separation of the anodic and cathodic spaces in the electrolysis cell is essential for this process.[18]
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 havearsenic less than 3 ppm andlead less than 5 ppm. KOH at this purity is used to debitterateolive and the manufacture of cocoa. Electronic grade ones which are used to etch semiconductors and treat silicon wafers inphotovoltaic 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 professionalanalytical labs require purity higher than 99.99% andheavy metal less than 0.1 ppm for safety to use.[11]
In industry, KOH is a good catalyst forhydrothermal gasification process. In this process, it is used to improve the yield of gas and amount of hydrogen in process. For example, production ofcoke from coal often produces much coking wastewater. In order to degrade it,supercritical water is used to convert it to the syngas containingcarbon monoxide,carbon dioxide,hydrogen andmethane. Usingpressure swing adsorption, various gases could be separated, and thenpower-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.
Many potassium salts are prepared by neutralization reactions involving KOH. The potassium salts ofcarbonate,cyanide,permanganate,phosphate, and various silicates are prepared by treating either the oxides or the acids with KOH.[12] The high solubility ofpotassium phosphate is desirable infertilizers.
Thesaponification offats with KOH is used to prepare the corresponding "potassiumsoaps", which are softer than the more commonsodium 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]
In food products, potassium hydroxide acts as a food thickener, pH control agent and food stabilizer. TheFDA considers it generally safe as a direct food ingredient when used in accordance withGood Manufacturing Practices.[23] It is known in theE number system asE525.
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 ofinsectanatomy 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 resultingsalt.
The corrosive properties of potassium hydroxide make it a useful ingredient in agents and preparations that clean anddisinfect surfaces and materials that can themselves resistcorrosion by KOH.[18]
Potassium hydroxide is often the main active ingredient in chemical "cuticle removers" used inmanicure treatments.
Because aggressive bases like KOH damage thecuticle of thehair 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 thetanning 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 offungi. 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 ofgilled mushrooms,boletes,polypores, andlichens[26] are identifiable based on this color-change reaction.[27]
Potassium hydroxide is acaustic 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 causepulmonary edema.[28] Safety considerations are similar tothose of sodium hydroxide.
The caustic effects arise from being highly alkaline, but if potassium hydroxide isneutralised with a non-toxic acid then it becomes a non-toxic potassium salt. It is approved as a food additive under the code E525.
^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.S2CID97177429.
^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.
