The setup: As part of my chemistry course, we're required to complete an in-home lab where we measure the reaction rate of an iodine potassium-iodide (IKI) solution with a hydrogen peroxide solution. We daisy-chain 2 Erlenmeyer flasks together with a system of rigid & flexible tubing, with a tube extending to an open graduated cylinder at the end. The chemical mixture is in the first flask, 200 mL of water is in the second flask, and about 10 mL of water is in the graduated cylinder. Here's how it all looks:

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What's SUPPOSED to happen: once we mix the IKI/hydrogen peroxide solutions in Flask #1, we swirl this flask on and off for something like 20 minutes to start the reaction. The reaction should build up pressure (I think?), causing the water to be forced up a rigid tube in Flask #2 and over to the open cylinder. Note that the longer of the 2 rigid tubes in Flask #2 extends below the surface of the water. The water will then start to drip into the graduated cylinder and we measure the displacement, which will then give us a table of data points to determine the rate constant, rate order, and many cheerful facts (about the square of the hypotenuse...)

What's ACTUALLY happening: nothing. I've been swirling Flask #1 for over an hour, and not a single drop of water has been displaced into the graduated cylinder.

Question: any idea what might be going on? Would there be something wrong with the setup? All the flexible tubing fits pretty tightly over the rigid tubing, and the rubber stoppers on the flasks also fit pretty tightly. I don't know how to troubleshoot this, so any tips would be helpful!

  • $\begingroup$ What are the concentrations of the hydrogen peroxide, of the KI/I2 solution ? Which volume of these solutions have you used ? As far as I can judge from here, I guess the concentrations have been too low. I have done this experience often. The reaction is rapid, and the volume of Oxygen is important. $\endgroup$
    – Maurice
    Sep 19 '20 at 19:34
  • $\begingroup$ For the first trial, we use 10 mL of undiluted (1.0-2.0%) IKI and 5 mL 3% hydrogen peroxide $\endgroup$
    – Will
    Sep 19 '20 at 19:41
  • $\begingroup$ If you tape "Catalytic Decomposition of Hydrogen Peroxide" in Google, you will find an article saying that using 20 mL H2O2 30%, then adding 5 mL KI 2M, the decomposition is nearly immediate. It lasts less than one minute, and it produces a huge amount of Oxygen gas. There is also a second reference by R. Conkin, J. Chem. Educ. 73, 9, 838 (1996), showing that the reaction is rapid with H2O2 3% mixed with an equal volume of a saturated solution of KI. In both cases, the volume of O2 is important. Why don't you have a look in the last publication ? $\endgroup$
    – Maurice
    Sep 19 '20 at 20:47

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