The Iodine Clock

The Iodine Clock reaction, first discovered by Hans Heinrich Landolt in the 19th century, is what is known as a 'chemical clock reaction', and today is one of only few known to science. It is useful as a tool in determining the rate of a chemical reaction due to a marked change in the colour of the reacting solution when a particular reaction has been completed.

In general, two solutions containing iodine, starch, a reducing agent and an oxidising agent are prepared separately. When ready one solution is added to another and after a period of time the reacting solution always changes colour from colourless to a dark blue-black. The time taken for the solution to change colour can be recorded and the rate of reaction thus determined. To get an even more accurate reading colourimetry can be used as the reacting solution is adept at absorbing certain wavelengths of light when it changes colour.

Two reactions occur simultaneously in the iodine clock; in one iodide ions, I- are oxidised to form triiodide ions, (I3)-, and in the other the reverse occurs. This particular version of the iodine clock that will be demonstrated uses sodium thiosulphate (sodium metabisulfite), Na2S2O3 as a reducing agent for iodine whereas hydrogen peroxide, H2O2 is the oxidising agent. Sulphuric acid, H2SO4 is also used to acidify the solution.

In the first reaction iodide ions are oxidised to triiodide ions via the action of hydrogen peroxide:

In the second, triiodide ions are reduced back to iodide ions via the action of the thiosulphate anion:

Once the thiosulphate anions are depleted, an excess of triiodide ions is produced, which binds to the starch in the solution to form the blue-black triiodide-starch complex. At this point, the solution will hence turn blue-black.

Varying the concentrations of the various reactants involved will change the time taken for the colour change of the solution, t. Increasing the concentration of acid, hydrogen peroxide and iodide ions decreases the time taken for the colour change. Increasing the concentration of sodium thiosulphate increases the time taken for the colour change.

Taking rate of reaction to be directly proportional to 1/t, we can thus say that increasing the concentration of acid, hydrogen peroxide and iodide ions increases the rate of reaction, and increasing the concentration of sodium thiosulphate decreases the rate of reaction.

Feel free to watch the video below for the procedure.