Chemical clock

Recreating the Briggs-Rauscher oscillating reaction

Chemists are so punc­tu­al – they’ve even de­signed a chem­i­cal clock!

Safe­ty pre­cau­tions

At­ten­tion! All ex­per­i­ments are per­formed by pro­fes­sion­als. Do not at­tempt.

Process de­scrip­tion

Hy­dro­gen per­ox­ide is a very ac­tive com­pound and can be both an ox­i­diz­er and a re­duc­ing agent. In this process, it re­duces iod­ic acid to form molec­u­lar io­dine and a so-called tri­io­dide com­plex. This process can be sim­pli­fied to the fol­low­ing chem­i­cal re­ac­tions:

HIO₃ + 3H₂O₂ → HI + 3O₂↑ + 3H₂O

5HI + HIO₃ → 3I₂ + 3H₂O

HI + I₂ → H[I₃]

Since oxy­gen is formed dur­ing the first re­ac­tion, we can ob­serve the emer­gence of the gas. The io­dine and tri­io­dide com­plex turn the so­lu­tion am­ber. But the so­lu­tion im­me­di­ate­ly turns blue, since the starch mol­e­cules, which are very long and look like spi­rals, trap io­dine mol­e­cules like a fish­net, form­ing a deep blue io­dine-starch com­plex. Un­der the ac­tion of mal­onic acid, this com­plex is de­stroyed, since it re­duces io­dine mol­e­cules, while the so­lu­tion it­self be­comes col­or­less:

C₃H₄O₄ + I₂ → C₃H₃O₄I + HI

The process re­peats ev­ery few sec­onds, and the re­ac­tion pe­ri­od de­pends on the con­cen­tra­tions of the ini­tial so­lu­tions. What is this if not a chem­i­cal clock?

You can choose won­der­ful chem­i­cal sets and get them on a month­ly ba­sis for your kids.