The iodine clock reaction is a type of clock reaction, or a reaction that has a delayed visual response due to the kinetics of the reactions taking place. There are several different types of iodine clock reactions, but they all rely on the fact that the iodide anion (I-) is colorless while elemental iodine (I2) is highly colored. The most common type of iodine clock reaction involves hydrogen peroxide, potassium iodide, and sodium thiosulfate.
In the reaction, a solution of hydrogen peroxide and sulfuric acid is added to a solution of potassium iodide, sodium thiosulfate, and starch. The first reaction that occurs is the oxidation of the iodide anion to the triiodide anion via hydrogen peroxide (an oxidizing agent).
H2O2 + 3KI + H2SO4 --> KI3 + 2H2O + K2SO4
But as soon as the triiodide anion is produced, it is consumed in the second reaction below where the triiodide is reduced back to the iodide anion by the sodium thiosulfate (a reducing agent).
KI3 + 2Na2S2O3 --> KI + 2NaI + Na2S4O6
The first reaction is slow but the second reaction is fast. So the colored triiodide is consumed as fast as it is produced, thus leaving the solution colorless. The reactions continuously cycle back and forth until the sodium thiosulfate is completely consumed. At this point the KI3 has nothing else to do but react with the starch to produce an iodinated starch that is highly colored in solution. This is how the iodine clock reaction works.