Chemical clock

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In an Iodine clock reaction, colour changes after a time delay.

A chemical clock (or clock reaction) is a complex mixture of reacting chemical compounds in which the onset of an observable property occurs after a predictable induction time.[1] In cases where one of the reagents has a visible color, crossing a concentration threshold can lead to an abrupt color change after a reproducible time lapse.

One class of example is the iodine clock reactions, in which an iodine species is mixed with redox reagents in the presence of starch. After a delay, a dark blue color suddenly appears due to the formation of a triiodide-starch complex.

Additional reagents can be added to some chemical clocks to build a chemical oscillator. For example, the Briggs-Rauscher reaction is derived from an iodine clock reaction by adding perchloric acid, malonic acid and manganese sulfate.[2].


  1. ^ Wright, Stephen W. (2002-01-01). "Tick Tock, a Vitamin C Clock". Journal of Chemical Education. 79 (1): 40A. doi:10.1021/ed079p40.
  2. ^ Briggs, Thomas S.; Rauscher, Warren C. (1973-07-01). "An oscillating iodine clock". Journal of Chemical Education. 50 (7): 496. Bibcode:1973JChEd..50..496B. doi:10.1021/ed050p496. ISSN 0021-9584.