Potassium hydroxide

3 atoms in one formula unit of an ionic lattice.

The drawing shows one formula unit of an ionic lattice, not a molecule; potassium hydroxide has no molecular shape.

IUPAC name: potassium hydroxide
Molar mass: 56.1050(22) g/mol
Atoms: 3
SMILES: [OH-].[K+]
InChIKey: KWYUFKZDYYNOTN-UHFFFAOYSA-M
PubChem CID: 14797

Made of

In the classification of ChEBI, the database of Chemical Entities of Biological Interest, potassium hydroxide is an alkali metal hydroxide, the potassium member of that family. Its formula unit is one potassium cation, K⁺, and one hydroxide anion, OH⁻, in which oxygen carries a single hydrogen. The molar mass is 56.105 g/mol.

NIOSH, the US federal institute for workplace safety, describes an odourless solid, white or slightly yellow. Buyers receive it as pellets, flakes, sticks, lumps or powder, the forms its US food-additive regulation, 21 CFR 184.1631, lists. Left exposed, the solid pulls both moisture and carbon dioxide out of the air, according to HSDB, the Hazardous Substances Data Bank. The International Chemical Safety Card for the compound calls its solution in water a strong base.

Structure and bonding

The drawing holds one potassium ion with a charge of +1 and one hydroxide ion with a charge of −1. Its single line is the O–H bond inside the hydroxide ion, and no line reaches the potassium. The pair is one formula unit of an ionic lattice and does not exist as a separate particle. Commercial potassium hydroxide also holds water, which a three-atom drawing has no place for. HSDB records that the 5 to 10 per cent of water left in the solid is bound as a monohydrate, one water molecule held for each KOH formula unit.

HSDB describes the solid as orthorhombic crystals, a form whose three axes meet at right angles but differ in length. The sodium hydroxide page sets out how the ions of a Group 1 hydroxide are arranged in such a crystal. What holds an ionic solid together is the subject of the article on chemical bonding.

In water the lattice comes apart completely: dissolved potassium hydroxide splits into separate K⁺ and OH⁻ ions, HSDB states, and a 0.1 molar solution reaches pH 13.5. Dissolving the solid, in water or in alcohol, releases a great deal of heat.

Where it turns up

Soaps, drain cleaners and alkaline batteries are described with the element itself, on the potassium page. Biodiesel is made by reacting fats or oils with an alcohol. Its catalyst is usually sodium or potassium hydroxide, according to the Alternative Fuels Data Center of the US Department of Energy (DOE). In chip making the compound serves as a wet etchant, a liquid that dissolves away unwanted material, Haz-Map records.

Water treatment uses it to adjust pH, according to the US Environmental Protection Agency (EPA). HSDB lists it as an absorbent for carbon dioxide and for hydrogen sulfide. It has a place in cocoa as well. The US standard for cacao nibs, 21 CFR 163.110, permits treating them with an alkali, potassium hydroxide among the choices beside ammonium and sodium compounds. Nibs treated this way are labelled “processed with alkali”.

Why it matters

Chlor-alkali production is the main route: an electric current passed through a brine, a solution of potassium chloride in water, yields chlorine alongside the hydroxide, a process called electrolysis. Demand for the chemicals made from the hydroxide tends to decide how much of that electrolysis runs, the EPA finds. Potassium carbonate is the chemical most often made from it, according to the EPA, and potassium permanganate is another. In HSDB’s breakdown of consumption for 1984, potassium carbonate took 25 per cent and liquid fertilisers 15 per cent.

Spacecraft gave the compound a different job. Alkaline fuel cells, the first type the US space programme used widely, generated electricity and water on board spacecraft, according to the DOE. Their electrolyte is potassium hydroxide dissolved in water. Carbon dioxide is their weakness: even the small amount in air can badly harm the cells’ performance and working life, because carbonate forms. Airborne particles of potassium hydroxide meet the same fate, turning quickly into potassium bicarbonate and carbonate as they take up carbon dioxide, HSDB notes.

Properties

Properties of potassium hydroxide Molar mass summed from IUPAC/CIAAW 2024 standard atomic weights; formula, atom and bond counts and PubChem's molecular weight from PubChem CID 14797.
Property	Value	Unit	Source
Formula (Hill order)	HKO
Molar mass	56.1050(22) g/mol	g/mol	IUPAC/CIAAW 2024
Potassium atoms	1
Oxygen atoms	1
Hydrogen atoms	1
PubChem molecular weight	56.106 g/mol	g/mol	PubChem

Safety

Potassium hydroxide, solid or dissolved, is corrosive to eyes, skin and the airways, and corrosive if swallowed. In the European Union’s classification it carries the hazard statements “Causes severe skin burns and eye damage” and “Harmful if swallowed”. The PubChem Laboratory Chemical Safety Summary for potassium hydroxide gathers its full hazard data. Its NIOSH Pocket Guide card gives a workplace exposure limit.

Questions

How is potassium hydroxide made?

Mainly by electrolysis: an electric current through a solution of potassium chloride gives potassium hydroxide and chlorine together. The hydroxide is then separated by the membrane method or the mercury method, according to the US Environmental Protection Agency. Evaporating the solution leaves a largely water-free solid of roughly 90 to 95 per cent KOH, HSDB states. Refineries also recover used potassium hydroxide from their wastewater and reuse it, mostly on site, to neutralise hydrofluoric acid.

Is potassium hydroxide used in food?

Yes. The US Food and Drug Administration (FDA) affirms potassium hydroxide as generally recognised as safe (GRAS) for adding directly to food. Under its regulation, 21 CFR 184.1631, the compound may serve as a formulation aid, a pH control agent, a processing aid, or a stabiliser and thickener. Cocoa makers may also use it as one of the alkalis permitted for treating cacao nibs.

Why is potassium hydroxide so corrosive?

The hydroxide ion is the likely cause. HSDB judges that its acute harm probably comes from the corrosive OH⁻ ion, a matter of pH, and less from any effect of K⁺ spread through the body. On contact, HSDB explains, potassium hydroxide breaks down the fats of skin, and by dissolving its proteins it lets hydroxide ions reach deeper into the tissue.