Potassium Hydroxide (KOH): Uses, Grades and Safety

Potassium hydroxide is a strong alkali, commonly called caustic potash. It is the potassium cousin of caustic soda (sodium hydroxide) and behaves much the same way, but soaps made with it are softer and dissolve more easily, which is why it is the lye used for liquid soap.

• Formula: KOH
• CAS number: 1310-58-3
• Molar mass: 56.11 g/mol
• Appearance: white flakes or pellets that pull moisture from the air (deliquescent)
• Melting point: about 410 °C
• Solubility: about 121 g in 100 ml water at 25 °C, and it gets very hot while dissolving

Left open, it soaks up water and carbon dioxide from the air and slowly turns into potassium carbonate, so keep the lid tight.

Azlok lists one potassium hydroxide product, kept in the cosmetic raw ingredients range. It suits soap makers and small workshops making liquid soap, soft soap and cleaning pastes. The pack size and purity are shown on the product page; check the purity figure, because commercial KOH is often around 90% and your lye calculation must allow for it.

It is not sold as food grade or analytical reagent. See today's price per kg.

Soap making

• Liquid hand soap, soft soap and shaving soap bases, made by saponifying oils such as coconut, olive or castor oil

Industry

• Electrolyte in alkaline batteries
• Catalyst in biodiesel production
• Making other potassium salts, such as potassium carbonate and potassium phosphates
• Paint strippers and heavy-duty alkaline cleaners

Lab

• Strong base for titrations and pH adjustment
• Absorbing carbon dioxide in some gas experiments

Food-grade KOH is a permitted food additive (E525), but that needs a food-grade pack, which this product is not.

Making a lye solution. Weigh the KOH and the water separately on a digital scale. Pour the KOH slowly into the water, never water onto KOH, stirring with a stainless steel or heat-proof plastic spoon. The solution can get hot enough to boil at the surface, so do it in a sink or tray with good ventilation and let it cool before use.

Liquid soap. Always run your oil recipe through a soap lye calculator set to KOH and to the purity on the label. Most liquid soap is made as a thick paste first, cooked gently for a few hours until clear, then diluted with hot water to the thickness you want. Do not guess the KOH amount; too much leaves a harsh, alkaline soap.

Lab solutions. A 1 M solution is about 56 g KOH made up to 1 litre with water. Because flakes absorb moisture, standardise it against a known acid if you need an exact strength.

Use stainless steel, glass or HDPE containers. Never use aluminium, tin or zinc; KOH attacks them and gives off hydrogen.

• KOH is corrosive. It burns skin and can cause serious eye damage. Wear chemical splash goggles, nitrile gloves and long sleeves.
• If it touches skin, rinse with plenty of running water for at least 15 minutes. For eyes, rinse immediately and get medical help.
• Keep it away from acids, aluminium, zinc and other reactive metals.
• Store in a tightly closed, labelled plastic container in a dry place. Do not keep it in food jars or bottles.
• Keep away from children and pets. Mix lye solutions when nobody is around to knock them over.
• For disposal of small leftover amounts, dilute heavily with water and neutralise with a weak acid such as vinegar before draining; follow local rules for larger amounts.

For home soap batches, a small pack is enough and is easier to keep dry, since KOH spoils faster once it starts absorbing moisture. If you make liquid soap every week, buy a larger pack and decant into smaller airtight tubs so you are not opening the main pack often.

Check the purity on the label before each new pack and update your lye calculator. For lab work that needs certified purity, buy an LR or AR grade from a lab supplier instead. For 25 kg and above, use the bulk inquiry form.