Iodine Clock Experiment

The classic iodine clock reaction demonstrates the properties of chemical kinetics through its mesmerizing change in color, and it is sure to fascinate you and perhaps your audience.

Key Takeaways

  • The blue-black color observed in Parts I and II occurs as a result of two separate
    reactions
  • In the presence of starch, iodine forms the intense blue-black color as a result of the
    iodine molecules being trapped in the long starch molecules

What you will need

The preparation for this experiment needs some time, but the results are definitely worth it! You’ll need to prepare three solutions which are then mixed to yield the reaction.

Solution A

Solution B

Solution C

How to do the iodine clock experiment

  1. Prepare all the solutions mentioned above. If you having trouble dissolving the starch, try to heat using a microwave. 
  2. Filter all solutions so they become as clear as possible
  3. Add a few drops of Solution C in a test tube
  4. When ready, simultaneously add solutions A and B in a 1:2 ration (you can experiment with the ratio)

How does the iodine clock reaction work?

We call this reaction a clock-reaction because it takes a predictable amount of time for the color change to occur. 

In this protocol, iodide ion is generated by the following slow reaction between the iodate and bisulfite:

 IO3 + 3 HSO3 → I + 3 HSO4

This first step is the rate determining step. Next, the iodate in excess will oxidize the iodide generated above to form iodine:

IO3 + 5 I + 6 H+ → 3 I2 + 3 H2O

However, the iodine is reduced immediately back to iodide by the bisulfite:

I2 + HSO3 + H2O → 2 I + HSO4 + 2 H+

When the bisulfite is fully consumed, the iodine will survive (i.e., no reduction by the bisulfite) to form the dark blue complex with starch.

After some time the solution always changes color to a very dark blue, almost black.

When the solutions are mixed, the second reaction causes the iodide ion to be consumed much faster than it is generated, and only a small amount of iodide is present in the dynamic equilibrium. Once the thiosulfate ion has been exhausted, this reaction stops and the blue colour caused by the iodide – starch complex appears.

Anything that accelerates the first reaction will shorten the time until the solution changes color. Decreasing the pH (increasing H+ concentration), or increasing the concentration of iodide or hydrogen peroxide will shorten the time. Adding more thiosulfate will have the opposite effect; it will take longer for the blue colour to appear

Iodine Clock Disposal

The hydrogen peroxide and iodate are consumed and the iodine is complexed with starch so there is no oxidizer hazard. Therefore the solution can be rinsed down the drain.

The use of this instruction and the information provided therein takes place at the user’s own risk.

Experiment Video