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. 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.
Properties and structure 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. 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.
Structure 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.
Reactions
Solubility and desiccating properties About 112 g of KOH dissolve in 100 mL water at room temperature, which contrasts with 100 g/100 mL for NaOH. Thus, KOH is slightly more soluble than NaOH. However, it is slightly less soluble on a molar basis due to its higher molar mass. 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:
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 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.
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 to give potassium bicarbonate:
KOH + CO2 → KHCO3
Manufacture 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. 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.
Uses 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 Manufactured KOH is used in various applications that need various purity levels. For industrial uses, like cleaning metals or treating waste gases, at least 90% purity is required. Food grade applications also require 90% or more purity, and must have arsenic less than 3 ppm and lead less than 5 ppm. KOH at this purity is used to debitterate olives and in the manufacture of cocoa. Electronic grade applications which are used to etch semiconductors and treat silicon wafers in photovoltaic cells require at least 99.9% purity and less than 1 ppm total metal ion concentration. Reagent level uses in pharmaceutical applications and professional analytical labs require purity higher than 99.99% and heavy metals less than 0.1 ppm for safe use.
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 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. 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 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. The high solubility of potassium phosphate is desirable in fertilizers.
Manufacture of soft soaps 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.
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