Iodine Clock Reaction - Hydrogen Peroxide Variation

Hydrogen Peroxide Variation

This reaction starts from a solution of hydrogen peroxide with sulfuric acid. To this is added a solution containing potassium iodide, sodium thiosulfate, and starch. There are two reactions occurring in the solution.

In the first, slow reaction, the triiodide ion is produced:

H₂O₂ + 3 I− + 2 H+ → I₃− + 2 H₂O

In the second, fast reaction, triiodide is reconverted to iodide by the thiosulfate:

I₃− + 2 S₂O₃2− → 3 I− + S₄O₆2−

After some time the solution always changes colour to a very dark blue, almost black.

When the solutions are mixed, the second reaction causes the triiodide ion to be consumed much faster than it is generated, and only a small amount of triiodide is present in the dynamic equilibrium. Once the thiosulfate ion has been exhausted, this reaction stops and the blue colour caused by the triiodide – starch complex appears.

Anything that accelerates the first reaction will shorten the time until the solution changes color. Decreasing the pH (increasing H+ 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.