Iodine Clock Reaction

First discovered in 1886 by Hans Heinrich Landolt, the iodine clock reaction is one of the best classical chemical kinetics experiments. Here’s what to expect:  Two clear solutions are mixed. At first there is no visible reaction, but after a short time, the liquid suddenly turns dark blue.

Usually, this reaction uses a solution of hydrogen peroxide with sulfuric acid, but you can substitute a weaker (and safer) acid that works just as well:  acetic acid (distilled white vinegar). The second solution contains potassium iodide, sodium thiosulfate (crystals), and starch (we’re using a starch packing peanut, but you can also use plain old cornstarch). Combine one with the other to get the overall reaction, but note that there are actually two reactions happening simultaneously.

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Materials:

• sodium thiosulfate
• potassium iodide
• two plastic test tubes
• packing peanut
• disposable droppers
• hydrogen peroxide
• distilled white vinegar
• distilled water
• four disposable cups
• popsicle sticks
• clock
• measuring spoons and cups
• goggles and gloves

Download Student Worksheet & Exercises

In the first (slow) reaction, the triiodide ion is produced:

H₂O₂ + 3 I^– + 2 H⁺ ? I₃^– + 2 H₂O

In the second (fast) reaction, triiodide is reconverted to iodide by the thiosulfate.

I₃^– + 2 S₂O₃^2- ? 3 I^– + S₄O₆^2-

After some time the solution always changes color to a very dark blue, almost black (the solution changes color due to the triiodide-starch complex).

Let’s get started! Rinse everything out very thoroughly with water three times, to ensure that nothing is contaminated before the experiment so you can get a clean start.  You can use droppers or measuring spoons (dedicated just to chemistry, not used for cooking) to measure your chemicals.  For droppers, make sure you’re using one dropper per chemical, and leave the dropper in the chemical when not in use to decrease the chances of cross-contamination.

Measure out 1 cup of distilled water and pour it into your first cup. Add ½ teaspoon sodium thiosulfate and stir until all the crystals are dissolved.  Touch the cup to feel the temperature change.  Is it hotter or colder?

Measure out 1 cup of distilled water into a new container.  Drop in the starch packing peanut and stir it around until it dissolves.  Packing peanuts can be made of cornstarch (as yours is, which is why it “melts” in water) or polystyrene (which melts in acetone, not water).

Into a third cup, measure out 1 cup of hydrogen peroxide.

Into the fourth cup, measure out 1 cup of distilled white vinegar.

Fill your plastic test tube with three parts starch (packing peanut) solution.  Add two parts distilled vinegar and two parts potassium iodide.  (Make sure you don’t cross-contaminate your chemicals — use clean measuring equipment each time.)  Your solution should be clear.

Into another plastic test tube, measure out three parts starch solution. Add two parts hydrogen peroxide and two parts sodium thiosulfate solution.  If the solution in the test tube is clear, you’re ready to move on to the next step.

Your next step is to pour one solution into the other and cap it, rocking it gently to mix the solution.  While you’re doing this, have someone clock the time from when the two solutions touch to when you see a major change.

What’s going on? There are actually two reactions going on at the same time.  When you combined the two solutions, the hydrogen peroxide (H₂O₂) combines with the iodide ions (I) to create triiodide (I₃) and water (H₂O). The sodium thiosulfate (S₂O₃²) grabs the triiodide to form iodine, which is clear.  But the sodium thiosulfate eventually runs out, allowing the triiodide to accumulate (indicated by the solution changing color).  The time you measure is actually the time it takes to produce slightly more iodide ions than the sodium thiosulfate can wipe out.

By accelerating the first reaction, you can shorten the time it takes the solution to change color. There are a few ways to do this: You can decrease the pH (increasing H+ concentration), or increase the iodide or hydrogen peroxide. To lengthen the time delay, add more sodium thiosulfate.

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