Iodine clock

The iodine clock is a chemical clock experiment devised by Hans Landolt, a Swiss chemist, in 1886. It illustrates the theories of chemical kinetics and redox chemistry via reactions that generate aqueous iodine as a product, and manifests the concept of a chemical clock through the time taken by one or more parallel reactions to consume the iodine produced.

An example is the oxidation of iodide by hydrogen peroxide to form aqueous iodine:

H_2O_2+2H^++2I^-\rightarrow 2H_2O+I_2\; \; \; \; \; \; \; \; 46

The iodine produced immediately reacts with a known amount of reducing agent, e.g. sodium thiosulphate, which is added to the peroxide-iodide mixture together with some starch solution at the start of the reaction.

2S_2O_3^{\; \: 2-}+I_2\rightarrow S_4O_6^{\; \: 2-}+2I^-\; \; \; \; \; \; \; \; 47

As the thiosulphate-iodine reaction occurs at a much faster rate than the peroxide-iodide reaction and it regenerates the exact stoichiometric amount of iodide ions that is present at the start of the reaction, the presence of iodine is only detected when all of the thiosulphate has been reacted.

When there is no more thiosulphate to remove the iodine, iodine combines with the unreacted iodide to form the triiodide ion, which in turn forms an intense blue-black charge-transfer complex with starch:

I_2+I^-\rightleftharpoons I_3^{\; \; -}\; \; \begin{matrix} starch\\\rightarrow \end{matrix}\; \; coloured\, complex

The different times needed for the mixtures to turn blue-black are recorded for a series of experiments using different concentrations of hydrogen peroxide, acid and iodide. For a constant amount of thiosulphate, the longer the time taken for the mixture to turn blue-black, the slower the peroxide-iodide reaction. In other words, the rate of the peroxide-iodide reaction is inversely proportional to the time taken for the mixture to turn blue-black.

Another version of the iodine clock experiment is the reaction between ammonium persulphate and potassium iodide:

S_2O_8^{\; \; 2-}+2I^-\rightarrow 2SO_4^{\; \; 2-}+I_2

I_2+2S_2O_3^{\; \; 2-}\rightarrow2I^-+S_4O_6^{\; \; 2-}


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