Chapter 3: Builders and water softeners · Part 1: Phosphates
Tetrasodium and tetrapotassium pyrophosphate
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The worst fact: direct contact causes "severe irritation and corneal injury" in rabbit eyes. About half the company reports to Europe's chemicals agency class tetrasodium pyrophosphate as causing serious eye damage.
Why brands use it: it softens water, stops scale forming and keeps dirt suspended.
Take care if: you get it in your eyes. Rinse straight away and get medical advice.
Comes from: phosphoric acid and soda ash make disodium phosphate, which is heated to about 500°C.
Myth check: nil.
Contaminants: nil.
Fair to say: reported cases in people show mild to moderate skin and eye effects. A 1% solution of the sodium salt is pH 10.2, below Australia's poison trigger.
Correct as of November 2026. What the badges mean
Sources
- Safe Work Australia. Hazardous Chemical Information System (HCIS): classifications for each chemical, and for formaldehyde.
- HSDB (US National Library of Medicine), via PubChem: production, pH and impurity data for each chemical.
The full source list is at the end of Chapter 3.