This is the Briggs Rauscher reaction, the coolest chemistry demonstration on the planet. This is an oscillating reaction, which means the reactants will form products, which will reform the reactants several times, until one of the reactants is extinguished and the reaction stops.
The blue color is an iodine-starch compound. Well technically, it is a compound with the triiodide ion, I3–, which is formed from elemental iodine and the iodine ion. Iodine is formed from iodate ions reacting with hydrogen peroxide. Iodine and iodine ions react with the starch, a carbohydrate, to form a blue compound. The iodine goes away when it is consumed by malonic acid. The amber color is caused by the presence of elemental iodine.
Manganese (II) ions also play a role as a catalyst. After a couple of minutes, the oscillations cease, and the solution is dark-blue.
Briggs Rauscher Ingredients / Recipe
This will make enough for at least 3 runs of the reaction, or even more, depending on how much you mix from each beaker. Of course, these quantities can be scaled up or down. Special thanks goes to Bassam Shakhashiri, who’s book “Chemical Demonstrations Volume 2” was an invaluable source of knowledge for this reaction.
295 ml distilled water
205 ml 30% hydrogen peroxide H2O2. Use gloves when handling this concentration of peroxide.
21.5 grams potassium iodate
2.27 ml 93% sulfuric acid (4.15 grams). For safety reasons, add the acid to the water – not the other way around. Watch how the sulfuric acid travels through the water when you pour it in, it’s really cool!
498 ml distilled water
8 grams malonic acid
1.7 grams manganese (II) sulfate monohydrate MnSO4
450 ml distilled water
Take 0.15 grams starch, corn starch works fine, and add it to 50ml of water. Boil the water for a few seconds, then let it sit for five minutes to dissolve it, then let is cool down a little. It may look a little cloudy – because technically you have formed a colloidal dispersion of the amylose in the starch, not a solution. Then, pour all 50ml of the starch solution to beaker C. Learn more about the starch-iodine compound here.
Briggs Rauscher Procedure & Instructions
You should now have 500ml of 3 different solutions labeled A, B and C. Performing the demonstration is actually quite simple. You mix together equal amounts of liquid from solution A and solution B, and then add in the same amount of solution C. Try to mix A and B together right before you add in solution C. I used 167ml of each, which meant I had enough of the solutions for three runs of the reaction.
The solution will oscillate between amber and blue-black every 5-15 seconds. After a couple minutes, the oscillations will stop. Oxygen and carbon dioxide gas are produced
What reactions are taking place?
Long story – it’s complicated. Slightly shorter story:
The hydrogen peroxide reduces iodate ions IO3– to elemental iodine in an acidic environment. But hydrogen peroxide can also oxidize iodine back to iodate. While these reactions are occurring, lots of oxygen is being produced. The iodide ion I– is produced as an intermediary, and the iodine and iodide ion combines to form the triiodide ion, I3–, which forms a blue compound with starch. The manganese (II) ions Mn+2 act as a catalyst, producing hypoiodous acid HOI, which reacts with the iodide ion to produce iodine, which reacts with the malonic acid to give us back more iodide ions. Got it? Oh, and some IO2 and HO2 radicals also help out when needed. Key to all this is the fact that the malonic acid reaction is slower than the other reactions, allowing various species to build up in concentration.
Cleanup and Hazards
This reaction will produce copious amount of elemental iodine, and iodine vapor. After the reaction is complete, do not leave the resultant blue solution indoors for more than a couple of minutes, unless you want the room to smell like iodine. And iodine vapor is toxic and corrosive, so you don’t want to breathe it in for any length of time. That said, I do feel like this is overall quite a safe reaction if basic precautions are followed.
Iodine stains and spills are easily cleaned up with a sodium thiosulfate solution. Thiosulfate can also be used to neutralize the final product of the reaction, but note that it can also restart the reaction, resulting in heat, lots of bubbling and some interesting colors – we saw the solution turn green.
Thirty percent hydrogen peroxide can burn the skin and needs to be treated with care, as does sulfuric acid which is highly corrosive. Malonic acid is a strong irritant.