This method starts with a solution of hydrogen peroxide and sulfuric acid.  To this  a solution containing potassium iodide, sodium thiosulfate, and starch is added. There are two reactions occurring simultaneously in the solution.
In the first, slow reaction, iodine is produced:
In the second, fast reaction, iodine is reconverted to two iodide ions by the thiosulfate:
After some time the solution always changes color to a very dark blue, almost black.
When the solutions are mixed, the second reaction causes the iodine to be consumed much faster than it is generated, and only a small amount of iodine is present in the dynamic equilibrium. Once the thiosulfate ion has been exhausted, this reaction stops and the blue colour caused by the iodine &ndash; starch complex appears.
Anything that accelerates the first reaction will shorten the time until the solution changes color. Decreasing the pH (increasing  concentration), or increasing the concentration of iodide or hydrogen peroxide will shorten the time. Adding more thiosulfate will have the opposite effect; it will take longer for the blue colour to appear.
Aside from using sodium thiosulfate as a substrate, cysteine can also be used.
Iodide from potassium iodide is converted to iodine in the first reaction:
2  + 2 H + HO → I + 2 HO
The iodine produced in the first reaction is reduced back to iodide by the reducing agent, cysteine. At the same time, cysteine is oxidized into cystine.
2 CHNOS + I → CHNOS + 2  + 2 H
Similar to thiosulfate case, when cysteine is exhausted, the blue color appears.