The Bray–Liebhafsky reaction is a chemical clock first described by W. C. Bray in 1921 with the oxidation of iodine to iodate:
:5 HO + I → 2  + 2 H + 4 HO
and the reduction of iodate back to iodine:
:5 HO + 2  + 2 H → I + 5 O + 6 HO