Potassium hydroxide

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.[13]

About 112 g of KOH dissolve in 100 mL water at room temperature, which contrasts with 100 g/100 mL for NaOH.[14] Thus on a molar basis, NaOH is slightly more soluble than KOH. 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: [15]

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.

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.[16]

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

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 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, we could separate various gases and then use power-to-gas technology to convert them to fuel.[18] 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 of carbonate, cyanide, permanganate, phosphate, and various silicates are prepared by treating either the oxides or the acids with KOH.[11] The high solubility of potassium phosphate is desirable in fertilizers.

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.[19]

Potassium carbonate, formed from the hydroxide solution leaking from an alkaline battery

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.[20] The nickel–metal hydride batteries in the Toyota Prius use a mixture of potassium hydroxide and sodium hydroxide.[21] Nickel–iron batteries also use potassium hydroxide electrolyte.

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.[22] It is known in the E number system as E525.

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.[23] Entomologists wishing to study the fine structure of insect anatomy may use a 10% aqueous solution of KOH to apply this process.[24]

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.[17]

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[25] are identifiable based on this color-change reaction.[26]