After a short time delay, the liquid suddenly turns to a shade of dark blue due to the formation of a triiodide–starch complex.
Once the thiosulfate ion has been exhausted, this reaction stops and the blue colour caused by the iodine – starch complex appears.
Adding more thiosulfate will have the opposite effect; it will take longer for the blue colour to appear.
This clock reaction uses sodium, potassium or ammonium persulfate to oxidize iodide ions to iodine.
Salters website) while ammonium persulfate has a higher solubility and is used instead in the reaction described in examples from Oxford University.