Potassium Hydroxide (KOH)

Overview

Potassium hydroxide (KOH) is a strong inorganic base, closely related to sodium hydroxide (NaOH). It comes as white pellets or flakes that are highly hygroscopic and caustic. The same safety precautions apply as for NaOH — always wear PPE, mix base into water (never water into base), and work in a ventilated space.

KOH has two main cosmetic uses:

• Hot-process and liquid soap making: KOH reacts with fats and oils to produce potassium soaps, which are soft, gel-like, and water-soluble. Liquid hand soap, body wash, and shaving soap traditionally use KOH for saponification.
• pH adjustment: small additions of KOH solution raise the pH of finished formulas.

The key difference from NaOH is that KOH produces soft potassium salts of fatty acids (suitable for liquid and paste soaps), while NaOH produces hard sodium salts (suitable for solid bar soap). The two are not interchangeable in soap recipes — you must use the correct lye calculator for your chosen base.

Shelf life is essentially indefinite if stored sealed against moisture.

What it does in a formula

In soap making, KOH reacts with fatty acids in the oil phase to form potassium soaps + glycerin. The soap paste produced is then diluted with water to create liquid soap. Liquid soap making is a slower process than bar soap — the soap paste needs to fully saponify before dilution (often 24-72 hours).

For pH adjustment, KOH dissolved in water (typically 10-25% solution) raises the pH of finished formulas in small increments. It is functionally similar to NaOH but slightly more soluble and faster to dissolve.

The molecule fully reacts in finished soap and adjusted-pH formulas — no free KOH remains.

How to use

For liquid soap: use a verified lye calculator set to KOH (not NaOH). Mix KOH into cold water slowly, with full PPE. Combine with hot oils, cook with continuous stirring or heat until the paste reaches “clear gel” stage (no separation, translucent), then dilute with water to working concentration.

For pH adjustment: pre-dissolve KOH in distilled water (10-25% solution). Add dropwise to formula while stirring, check pH between additions. Target pH varies by formula.

Best for / Worst for

Best for: liquid soap making, paste soaps, shaving soap (traditional softer formulation), pH adjustment in formulas where potassium is preferred over sodium.

Worst for: solid bar soap (use NaOH instead), beginners without safety training, formulas where simpler pH adjusters (lactic acid, citric acid, TEA) would work.

Common pitfalls

Same safety issues as NaOH. Splash burns, dust inhalation, exothermic mixing. Full PPE required.

Wrong lye for the soap type. KOH for liquid/soft soap, NaOH for hard bar soap. Some recipes use a blend (90% KOH + 10% NaOH) for slightly firmer liquid soap paste.

Storage humidity. KOH pellets absorb water from the air fast. Once opened, store in a sealed container with desiccant.

Substitutes

• Sodium hydroxide (NaOH) — for hard bar soap and most pH adjustment.
• TEA (triethanolamine) — gentler synthetic base for pH.
• Arginine — natural amino acid base for pH neutralization.
• Sodium hydroxide solution (50% commercial) — pre-dissolved version, less safe but ready to use.