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.
Read more about this topic: Iodine Clock Reaction
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