"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.

Step-by-step in­struc­tions

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.

Pro­cess­es de­scrip­tion

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

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.