"Iodine clock" experiment (the Briggs–Rauscher reaction)
How to do it at home
Sometimes science can be like magic, and the little example that we will look at today demonstrates this wonderfully. In this experiment you’ll see an absolutely transparent liquid turn yellow, dark blue, and then colorless again.
To demonstrate the reaction, prepare three solutions.
The first solution contains 100 ml of 30% hydrogen peroxide and 250 ml of distilled water.
In solution №2 mix 1.1 ml of concentrated sulfuric acid (H₂SO₄), 300 ml of water and 3.52 g of iodic acid (HIO₃).
We make the third solution from 0.08 g of starch, 3.9 g of malonic acid CH₂(COOH)₂, 0.85 g of manganese sulfate (II) and 200 ml of distilled water.
Pour the three colorless solutions into a large glass and watch the color change. The solution turns yellow, then blue, then colorless again.
In the interaction of hydrogen peroxide, iodic acid, manganese sulfate (II), sulfuric and malonic acid and starch, the solution constantly changes from colorless to gold to blue. These reactions are called oscillating. The reason for this is that in the first reaction, molecular iodine forms, which gives the solution an amber color. The iodine then turns into triiodide ions, which bond with starch and form a complex compound of a dark blue color. Parallel with this process, a reaction takes place between molecular iodine and malonic acid, with the formation of iodine malonic acid. The concentration of iodine decreases, and so the dark-blue solution with starch stops forming. The solution becomes transparent. The hydrogen peroxide causes the iodine malonic acid to break down, with the formation of carbon dioxide, water and iodide ions. Then molecular iodine forms, which gives the solution an amber color. This repeats many times. The color of the solution constantly changes. After each cycle, the reaction slows down, and the next cycle takes somewhat longer than the previous one. Over time, the malonic acid in the solution ends, the reaction stops and the solution becomes dark blue, as there is no more malonic acid in the solution which could reduce the iodide that forms.
Safety precautions: In working with concentrated hydrogen peroxide and concentrated sulfuric acid, wear rubber gloves and protective glasses, as they cause burns if they come into contact with the eyes or skin.
Warning! Substances of this experiment are toxic and highly dangerous for your health. Do not try this at home. Only under professional supervision.