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Chapter 10: Acids and alkalis · Part 1: Alkalis

Potassium hydroxide

  • Poison schedule (Australia)
  • Contaminant risk
Also called
caustic potash
Found in
liquid soaps, drain cleaners, paint and varnish strippers

The worst fact: it's as corrosive as caustic soda and sits under the same poison rules. Strong caustic damage to the gullet can narrow it weeks, months or even years later, and raises the risk of cancer there later in life. A related product shows how strong alkalis hide in kitchens: a "lye water" cooking product, made with potassium carbonate (48.8%) rather than potassium hydroxide, had a pH of about 14, was labelled with "nutrition information", and burned a child's oesophagus and stomach.

Why brands use it: it turns oils into soft, liquid soap, and it cuts grease in drain cleaners.

Take care if: you have children, or for your eyes. In Australia it's a Schedule 5 Poison at 5% or less when the pH is above 11.5, and a Schedule 6 Poison above 5%.

Comes from: potassium chloride (potash salt) split by electricity.

Myth check: nil.

Contaminants: mercury. Mercury is a possible impurity from older factories. Food-grade potassium hydroxide in the EU must have no more than 1 mg/kg. It's unlikely in a well-made product, but it's a factor you can't see on the label.

Fair to say: Australia uses far less of it than caustic soda: 1,000 to 10,000 tonnes a year in 2001–02, against over a million tonnes of caustic soda.

We use this ingredient to make our soap. It turns olive and coconut oil into castile soap, and it's used up in that reaction. It is not added to the finished product.

Correct as of November 2026. What the badges mean

Sources

  • Australian Government. Poisons Standard October 2026 (SUSMP), F2026L01327: sodium hydroxide, potassium hydroxide, alkaline salts, monoethanolamine, sulfamic acid, hydrochloric acid, acetic acid; Appendix B (sodium bicarbonate).
  • TGA. Notice of interim decisions (18 Dec 2025) and final decisions (25 May 2026): ACCS #40, sodium and potassium hydroxide.
  • Safe Work Australia. Hazardous Chemical Information System (HCIS): classifications for each chemical, and for ethylene oxide, carbon monoxide and acetaldehyde.
  • European Union. Regulation (EU) 2017/852 on mercury; Regulation (EU) 231/2012 food additive specifications. Minamata Convention on Mercury.
  • HSDB (US National Library of Medicine), via PubChem: production and impurity data for each chemical.

The full source list is at the end of Chapter 10.