Iodine Clock Reaction Experiment

First discovered in 1886 by Hans Heinrich Landolt, the iodine clock reaction is one of the best classical chemical kinetics experiments. Here’s what to expect: Two clear
solutions are mixed. At first there is no visible reaction, but after a short time, the liquid suddenly turns dark blue.

 

There are actually two reactions going on at the same time. When you combined the two solutions, the hydrogen peroxide (H₂O₂) combines with the iodide ions (I) to create triiodide (I₃) and water (H₂O). The sodium thiosulfate (S₂O₃²) grabs the triiodide to form iodine, which is clear. But the sodium thiosulfate eventually runs out, allowing the triiodide to accumulate (indicated by the solution changing color). The time you measure is actually the time it takes to produce slightly more iodide ions than the sodium thiosulfate can wipe out.