The Iodine Clock Reaction

Initializing live version
Download to Desktop

Requires a Wolfram Notebook System

Interact on desktop, mobile and cloud with the free Wolfram Player or other Wolfram Language products.

The iodine clock reaction (also known as the Harcourt–Esson reaction) is a classic chemical clock experiment for displaying chemical kinetics in action; it was discovered by Hans Heinrich Landolt in 1886. Two colorless solutions are mixed and at first there is no visible reaction. After a short, predictable time delay, the liquid suddenly turns a shade of dark blue.


Adjustment of the relative amounts of the iodate and the bisulfite determines the time interval (shown on the graph) before the blue color appears.


Contributed by: Benson R. Sundheim (December 2011)
Open content licensed under CC BY-NC-SA



The iodide ion is generated by the slow reaction between the iodate and bisulfite: .

The iodate also oxidizes the generated iodide to form iodine: .

The iodine is reduced immediately back to iodide by the bisulfite: .

When the bisulfite is fully consumed, the iodine will accumulate (i.e., not be reduced by the bisulfite) and form the dark blue complex with starch.

Feedback (field required)
Email (field required) Name
Occupation Organization
Note: Your message & contact information may be shared with the author of any specific Demonstration for which you give feedback.