The Science House - It's About Time

Iodine-Starch Clock Reaction
from It's About Time Colloquy funded by the Burroughs Wellcome Fund
Another way to measure time is too observe how long it takes a chemical reaction to occur.

In this experiment two solutions (A and B) are mixed by pouring from one beaker into the other. After many seconds, the clear, mixed solution turns dark blue, the color of the starch-iodine complex. Changing the concentration or the temperature of the solutions changes the time required for the blue color to be produced.

Solution A

In a 200 mL beaker, combine:

20 ml of 0.10M KIO₃
10 ml of 1% starch solution
20 ml distilled water

Solution B

In a 100 ml beaker, combine:

5 ml of 0.25M NaHSO₃
25 ml of distilled water

At room temperature, the reaction time ideally is 10-15 s. To change the time of the reaction, adjust the amount of sodium metabisulfite in solution B or dilute solution A.

You can also adjust the temperature to see its effect on the reaction rate by placing your beakers in a water bath of hot or cold water. **Be sure you don't raise the temperature above 35^o because the starch-iodine solution becomes unstable at higher temperatures.

Investigation

Investigate the affect of temperature and/or concentration on the time for the reaction.

Choose a variable and collect data. Be sure to conduct multiple trials.

Make observations as you go.

What conclusion can you draw from your experiments?

Be prepared to share your results with the group.

Where/when/why would a chemical clock be useful? How could it be used?

***WEAR GOGGLES!

The reaction mechanism can be summarized as follows:

Iodate reacts with bisulfite to form iodide (I^-)

Iodide reacts with Iodate to form Iodine (I₂)

Iodine is immediately consumed by reaction with bisulfite, giving back iodide.

When all bisulfite has been used up, iodine interacts with starch to form a blue complex

Species such as I_₅^- (equivalent to two iodines plus one iodide) and I₃^- (equivalent to one iodine plus one iodide) fitted inside the coiled amylose structure are responsible for the blue starch-iodine complex. Metabisulfite hydrolyzes to bisulfite (HSO₃^-) in water solution.

Reference

L.R. Summerlin and J.L. Ealy, Jr., Chemical Demonstrations, A Sourcebook for Teachers, Vol.1, 2nd Edition, American Chemical Society, Washington (1988).