When starch reacts with iodine, why does it create the
characteristic blue-black colour?
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Answer Posted / mallikarjun
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(aq) + 3 I−(aq) + 2 H+ → I3 + 2 H2O.
In the second, fast reaction, triiodide is reconverted to iodide by the thiosulfate.
I3(aq) + 2 S2O32−(aq) → 3 I−(aq) + S4O62−(aq)
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 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 color caused by the triiodide – starch complex appears.
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