Reaction between iodide ions and hyrogen peroxide takes place in acidic medium as follows:
• In this reaction mixture if calculated amount of sodium thiosulphate (Na2S2O3) is added in the presence of starch solution as an indicator.
• Liberated Iodine reacts with thiosulphate ions as fast as it is formed and is reduced back to iodide ions till all the thiosulphate are oxidised to tetrathionate ions.
• Time required to consume a fixed amount of the thiosulphate ions is reproducible.
• This reaction is also called clock reaction because time for the appearance of colour shows the accuracy of clock.
