Friday, 14 December 2012

Iodine Clock Reaction Kinetics

The following is a quick description of a reaction I developed and refined over the period of three years. It is an adaptation of this experiment.
The Berocca makes it quicker to accurately prepare solutions of Vitamin C, and the distilled water removes any variability from the hardness of the tap water.


  • A Berocca tablet
  • Tincture of iodine (2%)
  • Hydrogen peroxide (3%)
  • Starch solution (Laundry starch solution works well).
  • Distilled water.


  1. Dissolve the Berocca into 50ml distilled water and label the solution “Stock Solution”.
  2. Put 5 ml of Stock Solution into a beaker and add 50 ml of distilled water and 5 ml of iodine. The solution should turn clear. Label this "Solution A".
  3. In another beaker, mix 50 ml of distilled water, 15 ml of hydrogen peroxide and 2.5 ml of the liquid starch. Label this "Solution B"
  4. Mix Solution A with Solution B, either by pouring backwards and forwards between the beakers, using a stirring rod, or a mechanical stirrer.
Once you mix the two solutions together, you get a spectacular ... lack of any visual change at all ... at least for a couple of minutes. Once the time is up, the entire reaction will turn a deep blue/black colour.

What you can do with it (Kinetics)

The reaction is a near perfect experiment to investigate and demonstrate reaction rates for two (broad) reasons.

As far as my students and I have been able to determine, the basic mechanism of the reaction is as follows:

1. Peroxide oxidizes the iodine to iodide slowly by a multistep reaction. The overall equation is:
H2O2 + 3I2 → O2 + 2H+ + 2I3-

2. The vitamin C reduces the iodine back to iodide rapidly.
Vit. C + 2H++ 2I3- → reduced Vit. C + 3I2
(No idea if the hydrogens go onto the vitamin C, or produce a water molecule. However, it's pretty clear that they're needed to balance up the charges.)

3. When the vitamin C is finally consumed by the above cycle, the remaining peroxide oxidizes the iodine permanently.

4. The iodide ions can now bind to the starch, turning the reaction bright blue.

I won't go further in my suspected reaction mechanism as it's too fun to find out for yourself.
However, I will state that experimenting with acid, peroxide and iodide concentrations will quickly identify the two chemicals involved in the rate-limiting step, and that each of the two chemicals show first-order kinetics. This is the first reason why this experiment is such a convenient experiment.

The second group of reasons have to do with the following:
  • The acid and iodine are both regenerated/absorbed by the cyclic reactions 1 and 2. Thus, both these chemicals stay constant throughout the reaction, until the final colour change.
  • The levels of peroxide are in considerable excess of the vitamin C and the iodine. Thus, while it's concentration will drop, it stays pretty constant.
Thus the time it takes for the reaction to change colour is inversely proportional to the concentrations of the rate-limiting chemicals. No rate estimation using graphical techniques, no power or exponential curve fitting (with all the drama of estimating steady state levels). Just take the time (t) to change and 1/t, and plot against each of the chemical concentrations.

Be warned that two of my students did an experiment with 1/10th the peroxide - the graph took a distinctly non-linear appearance due to the last point above. Sadly these students were unable to realize why this occurred.

What you can do with it (Chemical Reaction Demo)

Finally, the experiment is great when you teach young students how to recognize a chemical reaction (change in colour, production of solid or gas, release of energy in the form of heat, light, sound, etc.) and leads to some deeper understandings of chemistry.

Put the clock reaction into a series of demonstrations for each of the indicators above. Teach a prep. lesson on the indicators of a chemical reaction. Emphasis the importance of observation in science. Then run through about half of the demos, and go:
"And as mentioned last lesson, we can identify a chemical reaction if a colour change occurs."
Mix the chemicals, and pretend to be upset that the reaction hasn't "occurred", then pretend to cover. "Oh well, let's go onto the next demo, and I'll see if I can get that one to work later."

Plan your timing for the reaction to finish it's countdown in the middle of your final demonstrations. You've got a good science class if some of the students point out the colour change before you finish. You've got a GREAT science class if the students recognize that a chemical reaction was occurring all along, despite the lack of a visual change.

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