Potassium Hydroxide (KOH): The Complete Guide to Uses, Grades, Safety and How It Differs From Sodium Hydroxide
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📋 What You'll Learn
This guide walks you through potassium hydroxide (koh): the complete guide to uses, grades, safety and how it differs from sodium hydroxide with detailed instructions.
In October 1807 Humphry Davy melted a lump of caustic potash, touched it with the current from a large voltaic battery, and watched small bright globules of metal appear on the surface. It was the first metal ever isolated by electricity, and it burned lavender when it touched water. The metal was potassium. The compound Davy tore it out of — potassium hydroxide — turned out to be far more useful than the metal itself, and it is still in your alkaline batteries and your hand soap today.
What is potassium hydroxide?
Potassium hydroxide is a strong inorganic base with the formula KOH and a molar mass of 56.106 g/mol. It is a white, intensely hygroscopic solid that dissolves readily in water and releases a large amount of heat as it does. Commercially it is produced by the electrolysis of potassium chloride brine, and it reaches the market as flakes, pellets, sticks, lumps, powders and as pre-made solutions, most commonly 45%.
You will see it under several names, all of which mean the same compound: caustic potash, potash lye, potassa, and simply KOH. The CAS number is the same regardless of grade, concentration or supplier.
CAS Number: 1310-58-3 · Formula: KOH · Molar mass: 56.106 g/mol · Melting point: 360 °C · Appearance: white deliquescent solid
Two physical properties drive almost every practical handling decision. First, KOH is deliquescent — it does not merely absorb moisture, it pulls enough water out of ordinary room air to dissolve itself into a puddle. Second, it reacts with carbon dioxide in air to form potassium carbonate. A drum left open does not just get damp; it gets weaker and less pure, which is why analytical users standardise KOH solutions before relying on their concentration.
Is potassium hydroxide the same as lye?
Both potassium hydroxide and sodium hydroxide are correctly called lye — “lye” is a category, not a single chemical. When a soap-making recipe or a drain-cleaning guide says “lye” without qualification it almost always means sodium hydroxide (caustic soda), but potash lye is the older meaning of the word and the two are not interchangeable by weight.
| Potassium hydroxide | Sodium hydroxide | |
|---|---|---|
| Formula | KOH | NaOH |
| Molar mass | 56.106 g/mol | 39.997 g/mol |
| Common name | Caustic potash, potash lye | Caustic soda, lye |
| CAS | 1310-58-3 | 1310-73-2 |
| Soap it makes | Soft, liquid, water-soluble | Hard bar soap |
| Typical strengths | Biodiesel, liquid soap, alkaline cells, electrolysers | Bar soap, pulp & paper, drain openers, chemical manufacturing |
| Relative cost | Higher per pound | Lower per pound |
The number people actually need: KOH is heavier per mole than NaOH, so swapping one for the other in a recipe requires a mass conversion, not a straight substitution. 56.106 ÷ 39.997 = 1.403, so a formulation calling for 100 g of NaOH needs roughly 140 g of KOH to deliver the same number of moles of hydroxide. Many KOH products intended for soap-making are also sold at about 90% assay rather than 100%, in which case recipes divide again by 0.90. Check the assay on your certificate of analysis rather than assuming.
For a full treatment of the sodium side of the family, see our caustic soda guide and how sodium hydroxide shaped modern industry.
What is potassium hydroxide used for?
Potassium hydroxide is used wherever a strong base is needed and the potassium ion is either an advantage or harmless. That covers six major industries.
1. Liquid soap and Castile soap
KOH is the reason liquid soap is liquid. It is the standard saponifying base for Castile soaps, shaving soaps, soft soaps and agricultural spray adjuvants.
2. Alkaline batteries
The “alkaline” in an alkaline battery is potassium hydroxide. Alkaline cells use zinc and manganese dioxide as the electrode materials with a concentrated KOH electrolyte, which is precisely what distinguishes them from the older zinc–carbon cells that used acidic ammonium chloride or zinc chloride. The chemistry gave the battery type its name.
3. Biodiesel production
KOH is a transesterification catalyst for converting vegetable oils and animal fats into fatty acid methyl esters. Producers often prefer it over sodium hydroxide because it dissolves more readily in methanol and because the potassium salts left in the glycerol by-product have value as a potash fertiliser rather than being purely a disposal problem.
4. Food processing
Potassium hydroxide is affirmed as generally recognised as safe as a direct human food ingredient under 21 CFR 184.1631, where it is permitted as a formulation aid, pH control agent, processing aid, and stabiliser and thickener, used in accordance with good manufacturing practice. Material used this way must meet Food Chemicals Codex specifications.
Grade note: the existence of a GRAS listing does not make every drum of KOH food-suitable. Alliance Chemical supplies Technical Grade and ACS Reagent Grade, neither of which is sold as a food-grade product. If your application is a direct food contact use, you need material supplied to FCC specification — tell us the application and we will tell you plainly whether what we stock fits it.
5. Agriculture
KOH is used to adjust pH and to supply potassium, and as the base in potassium-salt formulations. A related potassium compound, potassium bicarbonate, is used very differently and should not be confused with it.
6. Alkaline water electrolysis and hydrogen
Alkaline electrolysers run on concentrated KOH solution as the conducting medium, which is one of the oldest and most mature routes to hydrogen at scale. We cover that application in depth in potassium hydroxide in electrolysis.
How is potassium hydroxide made?
Potassium hydroxide is made by the electrolysis of potassium chloride brine, which is the same basic process used to make sodium hydroxide from common salt. Direct current is passed through a concentrated solution of potassium chloride; chlorine is liberated at the anode, hydrogen at the cathode, and potassium hydroxide accumulates in the remaining solution, which is then concentrated and, for solid product, dried to flake, pellet or powder.
The separator between the two electrode compartments is what distinguishes the plant types. Older diaphragm cells use a porous barrier, which is the configuration named in the federal food-ingredient listing for potassium hydroxide. Modern plants generally use ion-exchange membrane cells, which keep the chloride out of the product more effectively and therefore yield a purer caustic with lower energy use. Mercury cells, once common, have been phased out.
Two consequences follow for buyers. First, chloride is the characteristic residual impurity in caustic potash, which is why chloride limits appear on reagent-grade specifications. Second, KOH and chlorine are co-products of the same reaction, so the availability and price of caustic potash are tied to chlor-alkali economics rather than to potassium demand alone.
Historically it was made the other way round. Before electrolysis, potash was leached from wood ashes and then causticised by boiling it with slaked lime, which converts potassium carbonate into potassium hydroxide. That is the process Davy’s caustic potash came from — and it is why potash and caustic potash are two different compounds that are often carelessly treated as one.
How strong is potassium hydroxide?
Potassium hydroxide is a strong base, meaning it dissociates essentially completely in water, so the hydroxide concentration is set by how much you dissolved rather than by an equilibrium. A 0.1 molar solution sits near pH 13, and concentrated solutions exceed pH 14 on the nominal scale.
| Solution | Approximate concentration | Approximate pH |
|---|---|---|
| 0.001 M | 0.006% w/w | 11 |
| 0.01 M | 0.06% w/w | 12 |
| 0.1 M | 0.56% w/w | 13 |
| 1 M | 5.6% w/w | 14 |
| 45% commercial solution | about 10 M | beyond the useful pH scale |
Two cautions about those numbers. The pH scale becomes a poor description of behaviour above roughly 14, where activity rather than concentration governs, so a “pH 15” claim for a concentrated caustic is not meaningful in the way it appears. And pH says nothing about how much acid a solution can absorb: a dilute strong base and a concentrated weak base can read similar pH values while differing enormously in neutralising capacity, which is the number that matters when sizing a spill response.
Why analytical users standardise: because potassium hydroxide absorbs atmospheric carbon dioxide, a solution made up by weight starts drifting away from its nominal concentration as soon as it is exposed to air. Laboratories titrate KOH solutions against a primary standard rather than trusting the label, and store them protected from air.
Why does Castile soap have potassium hydroxide?
Castile and other liquid soaps use potassium hydroxide because potassium salts of fatty acids are soft and water-soluble, while the equivalent sodium salts are hard. The saponification reaction is otherwise the same: a triglyceride plus a strong base yields a fatty-acid salt (the soap) plus glycerol. The base you pick decides the physical form of the product, not whether the reaction happens.
Stearic acid saponified with sodium hydroxide gives sodium stearate, which is a hard solid at room temperature — a bar. The same acid saponified with potassium hydroxide gives potassium stearate, which is soft and dissolves readily, so it can be thinned with water to a pourable consistency. That is the whole difference between a bar and a bottle.
A point that reassures a lot of first-time buyers: there is no potassium hydroxide left in a correctly made, fully saponified soap. The KOH is consumed by the reaction. Seeing it on an ingredient list describes how the soap was made, not what you are washing with.
See also our note on sodium hydroxide in soap creation for the bar-soap side of the same chemistry.
Is potassium hydroxide harmful to humans?
Yes. Potassium hydroxide is a Category 1 skin corrosive, and its danger comes from corrosivity rather than from toxicity in the usual sense. It destroys tissue on contact by saponifying the fats in skin, which is the same reaction it performs on cooking oil.
That mechanism is why caustic burns behave differently from acid burns. A strong acid tends to coagulate surface protein and form a barrier that partially limits its own penetration. A strong alkali does not — it keeps dissolving tissue and keeps going deeper for as long as it remains in contact. Alkali splashes to the eye are a genuine ophthalmic emergency and can cause permanent damage very quickly.
Exposure limits, stated correctly: NIOSH sets a recommended exposure limit (REL) of 2 mg/m³ as a ceiling for potassium hydroxide. There is no OSHA permissible exposure limit currently in force — the 1989 limit was vacated when the U.S. Court of Appeals remanded OSHA’s updated PEL rule. Suppliers routinely cite the 2 mg/m³ figure as an “OSHA PEL”. It is not one.
Potassium hydroxide exposure limits, stated correctly
| Commonly published claim | Verdict |
|---|---|
| “OSHA PEL for potassium hydroxide = 2 mg/m³” | Incorrect. There is no OSHA permissible exposure limit for potassium hydroxide in force. |
| “NIOSH REL for potassium hydroxide = 2 mg/m³ ceiling” | Correct. A recommended (advisory) limit, not an enforceable one. |
Why the confusion exists: OSHA set a 2 mg/m³ ceiling for potassium hydroxide in its 1989 Air Contaminants rule. That rule was vacated in its entirety by the Eleventh Circuit in AFL-CIO v. OSHA, 965 F.2d 962 (11th Cir. 1992), decided 7 July 1992, and enforcement of the decision began 30 June 1993. The limits reverted to the 1971 values — and because potassium hydroxide had no 1971 PEL, it was left with none. No substitute has been adopted since. NIOSH lists the affected values in its “1989 Air Contaminants Update Project — Exposure Limits NOT in Effect” appendix.
Practical consequence: an SDS or safety plan citing an “OSHA PEL” for KOH is citing a limit that has not existed for over thirty years. Control the exposure to the NIOSH ceiling of 2 mg/m³ regardless — but do not represent it as an OSHA requirement.
Primary sources: NIOSH Pocket Guide — Potassium hydroxide · NIOSH Appendix G — Exposure Limits NOT in Effect · NIOSH 1988 PEL Project — potassium hydroxide (CAS 1310-58-3)
What does potassium hydroxide do to skin?
At any working concentration it causes deep chemical burns, and dilute solutions can feel slippery and deceptively mild at first because the sensation is your own skin lipids being converted to soap. KOH does appear on some cosmetic ingredient lists, where it is present at trace level purely to adjust pH and is fully neutralised in the finished product. That is a manufacturing input, not a skin treatment, and it says nothing about the safety of handling the raw material.
First response
- Skin: flush with running water for at least 15 minutes, removing contaminated clothing while flushing. Do not attempt to neutralise a caustic burn with an acid.
- Eyes: flush immediately for at least 15 minutes holding the eyelids open, and get emergency medical attention. Treat every eye exposure as serious.
- Inhalation of dust or mist: move to fresh air; seek medical attention if irritation persists.
- Always work from the current Safety Data Sheet for the specific product and grade in hand.
What not to mix with potassium hydroxide
The most dangerous incompatibility for potassium hydroxide is not another chemical you add to it on purpose — it is the metal you store it in.
| Do not combine with | What happens |
|---|---|
| Aluminium, zinc, tin, galvanised metal | KOH attacks these metals and releases hydrogen gas, which is flammable and can build to an explosive concentration in a closed container. Never store or transfer KOH in aluminium. |
| Acids of any strength | Violent neutralisation with rapid heat release; the mixture can boil and spatter caustic liquid. |
| Ammonium salts | Liberates ammonia gas. |
| Water added to the solid | Localised boiling and spattering. Always add the caustic to a large volume of cool water, never water to caustic. |
| Nitro compounds and organic halogens | Can react vigorously or unpredictably. |
One myth worth clearing up: potassium hydroxide is not the bleach-and-ammonia hazard people are usually thinking of. Mixing KOH with household bleach does not generate chlorine gas — alkali is what bleach is already stabilised with. The KOH hazards to plan around are hydrogen generation from light metals, the exotherm on dilution, and direct contact burns.
For spent-bath handling, the neutralisation principles in our guide to neutralising spent caustic baths apply to potassium hydroxide as well.
Is baking soda potassium hydroxide?
No. Baking soda is sodium bicarbonate, NaHCO₃, a weak base that reaches roughly pH 8.3 in solution and is safe to handle bare-handed. Potassium hydroxide is a strong base that can reach pH 13 to 14 and is corrosive to skin on contact. The two are not related in strength, hazard class or function, and they share neither an ion nor a use case.
The confusion usually comes from the word “alkaline” being applied loosely to both. It is worth being precise: sodium bicarbonate is a mild buffering agent, and potassium hydroxide is an industrial caustic.
Which grade of potassium hydroxide do you need?
Buy the grade your application actually requires — over-specifying costs money for purity you will never use, and under-specifying can invalidate an analytical result. For potassium hydroxide the decision is usually straightforward.
| Grade | What it means | Choose it when |
|---|---|---|
| Technical Grade | Industrial-purity material. Trace metal content is not certified against ACS limits. | The KOH is consumed or neutralised by your process: biodiesel catalysis, soap-making, cleaning and degreasing formulations, pH adjustment, general process chemistry. |
| ACS Reagent Grade | Meets American Chemical Society reagent specifications, with defined limits on specific impurities. | The impurities themselves would change your answer: titration and standard preparation, analytical chemistry, QC laboratories, research, and any method that cites ACS reagents. |
A useful test: ask whether a trace metal in the KOH could end up in your result or your product. In a biodiesel reactor or a soap kettle it cannot — the base is consumed and the impurity is either washed out or present at a level nothing downstream can detect. In a titration it absolutely can, because you are measuring small differences and an unknown contaminant becomes an unknown error.
Tell us the application and we will spec the grade. That is a faster route to the right answer than reading two specification sheets side by side, and it stops you paying for reagent purity in a process that will neutralise it in the first ten minutes. Every drum and tote ships with a certificate of analysis.
How should you store and handle potassium hydroxide?
Store potassium hydroxide sealed, dry, and away from acids and light metals. Because it is both deliquescent and a carbon dioxide absorber, an imperfectly closed container degrades the product in two ways at once: it takes on water, and it converts at the surface to potassium carbonate, which lowers the effective assay.
- Containers: high-density polyethylene or appropriate steel. Never aluminium, zinc, tin or galvanised metal.
- PPE: chemical splash goggles, and a face shield when pouring or dissolving; nitrile or butyl gloves; a chemical-resistant apron; sleeves.
- Ventilation: handle flake and powder so as to minimise airborne dust.
- Emergency provision: an eyewash and safety shower within immediate reach of the work.
- Segregation: store away from acids, ammonium salts and oxidisers.
Dissolving KOH releases a great deal of heat. Adding it too quickly, or adding water to the solid instead of the reverse, can drive the solution to boiling and spatter caustic liquid out of the vessel. Add the solid slowly to a large volume of cool water, stirring, and let the temperature settle between additions.
Key numbers and sources
| Fact | Value | Source |
|---|---|---|
| CAS Registry Number | 1310-58-3 | PubChem CID 14797 |
| Molar mass | 56.106 g/mol | PubChem |
| NIOSH REL | 2 mg/m³ (ceiling) | NIOSH Pocket Guide |
| OSHA PEL | None currently in force | NIOSH 1988 PEL Project |
| Food use status | GRAS, direct human food ingredient (FCC spec) | 21 CFR 184.1631 |
| NaOH→KOH mass factor | 1.403× | 56.106 ÷ 39.997 (molar masses) |
| Isolated by | Humphry Davy, October 1807 | WebElements — potassium history |
Potassium hydroxide from Alliance Chemical
Two grades, stocked from 2 lb packs up to 2,200 lb pallet quantities, with a certificate of analysis on every order. Not sure which one your process needs? Tell us the application and we will spec it with you rather than sell you the most expensive option on the shelf. Orders typically ship in 1–2 business days.
Related: How to Buy Potassium Hydroxide in the United States — the sourcing side — supply channels, pack sizes, the documents to demand, shipping class, and the seven questions to ask before ordering.
Frequently Asked Questions
Is potassium hydroxide the same as lye?
Both potassium hydroxide and sodium hydroxide are correctly called lye. "Lye" is a category rather than one chemical. Unqualified references to lye in soap or drain-cleaning contexts usually mean sodium hydroxide, while potassium hydroxide is specifically potash lye. They are not interchangeable by weight: KOH has a molar mass of 56.106 g/mol against 39.997 for NaOH, a factor of about 1.403.
What is potassium hydroxide used for?
Potassium hydroxide is used to saponify liquid and Castile soaps, as the electrolyte in alkaline batteries, as a transesterification catalyst in biodiesel production, as a pH control agent and processing aid in food manufacturing under 21 CFR 184.1631, to supply potassium and adjust pH in agriculture, and as the conducting medium in alkaline water electrolysis for hydrogen.
Is potassium hydroxide harmful to humans?
Yes. Potassium hydroxide is a Category 1 skin corrosive. It burns tissue by saponifying skin lipids, and because alkalis do not self-limit the way acids do, the burn keeps penetrating while contact continues. Eye exposure is an emergency. NIOSH sets a recommended exposure limit of 2 mg/m3 as a ceiling; there is no OSHA permissible exposure limit currently in force.
What should you not mix with potassium hydroxide?
Never store or mix potassium hydroxide with aluminium, zinc, tin or galvanised metal, which it attacks to release flammable hydrogen gas. Keep it away from acids, which neutralise violently and can boil and spatter, and from ammonium salts, which release ammonia. Always add the caustic to water rather than water to the caustic.
Why does Castile soap contain potassium hydroxide?
Potassium hydroxide is used because the potassium salts of fatty acids are soft and water-soluble, so the finished soap can be thinned to a pourable liquid. Sodium hydroxide produces hard sodium salts and therefore bar soap. In a correctly and fully saponified soap no potassium hydroxide remains; it is consumed by the reaction.
Is baking soda potassium hydroxide?
No. Baking soda is sodium bicarbonate, NaHCO3, a weak base of about pH 8.3 in solution that is safe to handle. Potassium hydroxide is a strong base reaching pH 13 to 14 and is corrosive on contact. They share neither an ion, a hazard class, nor a use.
What is the difference between Technical Grade and ACS Reagent Grade potassium hydroxide?
Technical Grade is industrial-purity material whose trace metal content is not certified against ACS limits, and it is appropriate wherever the KOH is consumed or neutralised by the process, such as biodiesel, soap-making, cleaning formulations and pH adjustment. ACS Reagent Grade meets American Chemical Society specifications with defined impurity limits and is required for titration, analytical chemistry, QC and research where a trace contaminant would bias the result.
How should potassium hydroxide be stored?
Store it sealed, dry and segregated from acids and light metals, in high-density polyethylene or appropriate steel, never aluminium or galvanised containers. Potassium hydroxide is deliquescent and also absorbs carbon dioxide from air to form potassium carbonate, so a poorly sealed container both takes on water and loses effective assay.
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