Chemistry Demonstration: Iodine Clock Reaction

Introduction

The iodine clock reaction is a classical chemical clock demonstration that displays chemical kinetics in action. It was first discovered by Hans Heinrich Landolt in 1886, so many texts may refer to it as the Landolt Reaction. In this experiment, two colorless solutions are mixed. At first, there is no reaction, and after a given period of time, the solution turns dark blue.

This time period is termed the, “clock period”, and it is the amount of time that the mixture remains colorless until the change to dark blue. This can be calculated according to the following equation:

0.003 sM² / [IO₃^–]₀ [HSO₃^–]₀The procedure provided here should result in a clock period of approximately 9 seconds. Note that if the [HSO₃^–]₀ is three times greater than the [IO₃^–]₀, the reaction will not produce a color change. The overall experiment can be described by the following series of reactions:

IO₃^– + 3 HSO₃^– –> I^– + 3 SO₄^-2 + 3 H⁺

IO₃^– = 8 I^– + 6 H⁺ –> 3 I₃^– + 3 H₂O

I₃^– + HSO₃^– + H₂O –> 3 I^– + SO₄^-2 + 3 H⁺

2 I₃^– + starch –> starch-I₅^– complex (blue)The generation in of I₃^– in the presence of starch generates the blue starch color change. The triiodide ion is consumed by any remaining bisulfite ion, which prevents starch complexation. So the color change occurs when the bisulfite ion is consumed.

Materials

• 0.1 M KIO₃
• 1% starch solution
• 0.25 M NaHSO₃
• deionized water
• ice bath
• graduated cylinder, 100 mL
• beakers, 400 mL and 600 mL
• Syringes
• stopwatch

Procedure

1. Add the KIO₃^–-starch solution to a 400 mL beaker containing 100 mL 0.1 M KIO₃, 50 mL 1% starch solution, and 100 mL deionized water (labeled SOLUTION A). Add 20 mL of 0.25 M NaHSO₃ and 130 mL of deionized water to a 600 mL beaker (labeled SOLUTION B).
2. During the demonstration, add SOLUTION A to SOLUTION B. You may use syringes to mix the two solutions together. Note the amount of time required to observe the change in color to dark blue. When these solutions are mixed, the [KIO₃] = 0.025 M and the [NaHCO₃] = 0.013 M.
3. The effect of concentration on the rate of the reaction can be measured by varying the concentrations of the starting solutions. Prepare some variants of the KIO₃ mixture at 0.04 M and 0.02 M and repeat the experiment, noting the difference in the amount of time required for the color change to take place.

Safety

KIO₃ is a strong oxidizing agent and NaHSO₃ is a strong reducing agent. Avoid mixing solid or concentrated solutions of NaHSO₃ with KIO₃. Eye protection and latex gloves should be worn in this experiment.

Disposal of Waste Products

For disposal of waste products, combined all of the solutions used in this experiment with solid sodium thiosulfate until the mixture is no longer blue. The resulting clear mixture can then be discarded by flushing down the drain upon the addition of water.

References

1. Mitchell, R.S. Iodine Clock Reaction. J. Chem. Educ., 1996, 73 (8), 783.