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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24 Responses to “Iodine Clock Reaction”

  1. Aurora Lipper says:

    The amount of time that passes in the video is about 24 seconds, but we trimmed the video…so it could take longer when you try it.

  2. suzanne_mangeri says:

    I counted 25 seconds for the video. Am I correct?

  6. Malcolm Smith says:

    Where can you get corn starch from? We tried the supermarket, but they didn’t have any. So sad!
    Thanks!

  7. Malcolm Smith says:

    Hi Aurora,
    Thanks for getting back to me!
    You see, that’s just the problem. The link you posted goes to “Home Science Tools,” (they don’t post over seas,) and with us all the way over here in Australia, we can’t order anything from them.
    Did you know you can use a Vitamin C tablet (Sodium Ascorbate,) instead of Sodium Thiosulfate? You just need one tablet.
    It has the same outcome!
    Juliette (12)
    ps. It’s the one before my cornflour question.
    Thanks!

  8. No bother at all! The KI is a requirement for this experiment. You can get it here or it looks like this company has put everything you need all in one smaller kit here.

    P.S. Which question did I miss? (Sorry if I missed one! I try to catch them all.)

  9. Malcolm Smith says:

    Hi Aurora,
    Sorry to bother you again, but is there a alternative for Potassium Iodide? We’ve found some Sodium Thiosulfate, but not any Potassium Iodide. Is there something else we can use instead? Thanks!
    Juliette (12)
    (Not sure if you saw my question below.)

  10. I am not sure if the corn flour has enough of the starch in it to work, so you might try a teaspoon of cornstarch.

  11. Malcolm Smith says:

    Can you use corn flour instead of a packing peanut?
    Juliette (12)

  12. Malcolm Smith says:

    Hi Aurora,

    We are having a bit of trouble finding the chemicals for all of the experiments. We live all the way over here in Australia, so we can’t order chemicals from the website where you get them from, (Home Science Tools,) because they only do shipping in America. We’re trying very hard to find a site that sells chemicals here, but we’ve had no luck so far. We’ve found a few sites that sell them, but in large amounts. (We don’t really want kilos of chemicals hanging around our house!) Dad tried a local high school, but the principal said he couldn’t sell us anything, because he said if we blew ourselves up, they would go to jail. (Though we are not planning to blow ourselves up, he has a good point.) Do you think you could help us? Thanks!
    Juliette (Age 12)

  13. The greater the peroxide concentration, the less time it takes for the solution to turn blue.

  14. Julie Kuehler says:

    How does increasing the hydrogen peroxide effect the rate of reaction.

    Thanks

  15. The video is real-time. Yours will take between 2 seconds and a full minute, depending on the concentrations you use.

  16. Dominique Poli says:

    how long did it take?

  17. Cool – it sounds like you figured out how to adjust the reaction speed! There are notes int he written part about how and why this works, as well as other things to try, but it sounds like you’ve got it! 🙂

  18. Anita Mullins says:

    My son and I worked on the experiment some more this evening. So yesterday our solution changed instantly when we poured our second solution into the first solution a large beaker.

    This time, our first run was ~70 seconds and our second run was over 3 minutes. In both runs we used the plastic tubes with caps for the individual solutions and then combined the solutions into the taller glass tubes.

    For the first run:
    1st solution: 3 parts starch, 2 parts distilled vinegar, and 3 “itty bitty” spoonfuls of the Potassium Iodide. The spoon was the super tiny spoon that comes with the chemistry equipment.
    2nd solution: 3 parts starch, 1 (instead of 2) parts of Hydrogen Peroxide, and 2 parts sodium thiosulfate.

    For the second run (we reduced the Potassium Iodide by one part)
    1st solution: 3 parts starch, 2 parts distilled vinegar, and 2 spoonfuls of the Potassium Iodide.
    2nd solution: 3 parts starch, 1 part of Hydrogen Peroxide, and 2 parts sodium thiosulfate.

  19. Great job in keeping track of what happened in your experiment! Okay, so if you add the chemicals to one test tube and it’s already turning a color, you have either cross-contamination (like when you use the same dropper for all the chemicals or your test tubes aren’t clean enough), or your KI solution is too strong.

    You can change the time delay by changing the amount of hydrogen peroxide you use – feel free to experiment with the amounts (half it, double it) – you’re not going to harm anything.

    If your test tube is too full, simply shake it up and dump half the contents into another test tube so now you have two sets you can experiment with. You don’t have to use test tubes – you can use disposable plastic cups, too.

    It sounds like your test tubes are on the small side (not surprising, as they’ve changed everything else about this kit, why not change the test tube side too?) The videos for this unit was taken of myself myself using the entire kit contents, so you can see how things were done with your exact kit, but it sounds like this idea backfired! 🙂

    Let me know how it goes! And you’re right – science is like this… in real life, too!

  20. The KI solution should be slightly thicker than water and pale yellow or clear. It takes a lot of KI crystals to get a saturated solution, so only add a little bit of water when you mix it in. If you added too much, simply heat it up and boil off the excess. If you have alcohol burner, hold the glass test tube in your clamps and heat over the burner slowly with gloves protecting your hands.

  21. Anita Mullins says:

    We realized that we forgot to add a comment to the earlier posting. We had continued to add Potassium Iodide to our tube because we never saw the iodide accumulate at the bottom of the tube. So, eventually, the tube turned the (very) light brown.

  22. Anita Mullins says:

    Hi,

    On previous experiments you said to add the elements, mix, and look at the bottom of the test tube to see if any element was not dissolved. When this occurred, you said we had a saturated solution. So, for the first tube, we added three parts starch solution (incidentally, the packing peanut never 100% dissolved – more like 80%), and then we added two parts distilled vinegar. Next, we kept adding crystallized Potassium Iodide in order to make it a saturated solution. The solution started to turn light brown (or a tinge of brown) – so we stopped adding the element. The plastic test tube was 90% full.

    On the second solution, when we added the three parts starch solution, two parts hydrogen peroxide, and two parts sodium thiosulfate solution, the test tube was 100% full. (We tried to reduce the “parts” size to the tube [how much we squeezed with the disposable droppers], but still ended up being around 98-100% full in the test tube.)

    Since we were unable to pour one tube into another to make the final solution, we poured the solutions into a beaker. When the solutions touched each other, they turned black in approximately one second.

    We put the four primary main liquids on a shelf to save and retry.

    How should we alter the experiment? I am guessing that we do not add as much Potassium Iodide – but how much or how little?

    Thanks.

  23. Ah, they’ve changed it recently. Simply make a saturated solution (add enough water into the crystals until they completely dissolve) and PRESTO! You have the KI solution. Does that help?

  24. Anita Mullins says:

    Hi,

    I think we are a little stuck. In your video and written instructions, you state we should add a certain number of parts of Potassium Iodide. The Potassium Iodide that came with the Chemistry kit from Montana comes as a crystal form. What should we do?

    Thanks..