This experiment provides a visual illustration of a series of interconnected redox equilibria. This repeating cycle of reactions oscillates between red and blue over a period of several minutes.
Demonstration and/or class practical
Acidified sodium (or potassium) bromate and bromide solutions are mixed, then propanedioic (malonic) acid, ferroin indicator and a surface active agent added. The mixture exhibits patterns of oscillation from red to blue over a period of several minutes. It provides an illustration of how a series of interconnected redox equilibria and their rates can lead to a repeating cycle of reactions, made visible by the inclusion of a suitable redox indicator.
The experiment provides an interesting and colourful demonstration, but the reactions involved and the mechanism are complex – see Teaching notes. The reaction can be carried out as a demonstration (especially for open days or chemistry clubs), or as a class exercise.
Time required should be about 10 min.
For one demonstration or each pair of students:
Eye protection (Goggles when preparing Solution A - Note 2)
Beakers (100 cm3), 4
Measuring cylinder (10 cm3)
Plastic syringes (1 or 2 cm3), 3
Sodium bromate(V) (TOXIC, OXIDISING), 5 g (enough for 10 experiments) or potassium bromate(V) (TOXIC, OXIDISING), 5.5 g
Sodium bromide, 1 g or potassium bromide, 1.2 g
Concentrated sulfuric acid (CORROSIVE), 2 cm3
Propanedioic (malonic) acid (HARMFUL), 1 g
Ferroin indicator solution, 1 cm3
‘Photoflo’ solution, 1 drop (Note 1)
Distilled or deionised water, about 100 cm3
Refer to Health & Safety and Technical notes section below for additional information.
Health & Safety and Technical notes
Sodium bromate(V), NaBrO3(s) or potassium bromate(V), KBrO3(s), (TOXIC, OXIDISING) - see CLEAPSS Hazcard.
Concentrated sulfuric acid, H2SO4(l), (CORROSIVE) - see CLEAPSS Hazcard.
Sodium bromide, NaBr(s), or potassium bromide, KBr(s) - see CLEAPSS Hazcard.
Propanedioic (malonic) acid, HOOCCH2 COOH(s), (HARMFUL) - see CLEAPSS Hazcard .
Ferroin indicator - see CLEAPSS Hazcard and CLEAPSS Recipe Book.
1 ’Photoflo’ solution - can be purchased via online retailers, photography shops or many supermarkets. It is a surface-active agent used in photographic developing and printing.
2 In advance of the demonstration, prepare the following solutions in small beakers:
Solution A: dissolve 5 g of sodium bromate(V) (or 5.5 g of potassium bromate(V)) and 2 cm3 of concentrated sulfuric acid in 67 cm3 of distilled or deionised water.
Solution B: dissolve 1 g of sodium bromide (or 1.2 g of potassium bromide) in 10 cm3 of distilled or deionised water.
Solution C: dissolve 1 g of propanedioic acid in 10 cm3 of distilled or deionised water.
a Place 6 cm3 of Solution A in a small beaker, using a measuring cylinder.
b Add 0.5 cm3 of Solution B, using a syringe.
c Add 1.0 cm3 of Solution C, using a syringe. A brown colour appears. When this disappears, add 1.0 cm3 of ferroin solution, using a syringe.
d Add 1 drop of ‘Photoflo’ (or equivalent) solution, using a dropping pipette.
e Swirl to mix well and pour enough of the mixture in to the Petri dish to half-fill it. Wait for the oscillations to begin.
This is a version of the classic Belousov-Zhabotinksy oscillating reaction. Its detailed mechanism is very complicated – see references or weblink below. Suffice it to say here that bromate and bromide ions first react with propanedioic acid to produce a bromopropanedioate. Bromate also oxidises the iron(II) in the red ferroin indicator to produce a blue iron(III) species. The bromopropanedioate and the blue species then react to form bromide. Bromide inhibits the reaction of the red iron(II) species to form the blue iron(III) species and so a red colour re-appears.
References: Further details can be obtained in:
New Scientist Guide to Chaos, p. 111, N.Hall (Ed), London: Penguin, 1991 or in Chemical Demonstrations – Volume 2, B.Shakhashiri, University of Wisconsin Press, 1986.
Health & Safety checked, 2016
This Practical Chemistry resource was developed by the Nuffield Foundation and the Royal Society of Chemistry.
© Nuffield Foundation and the Royal Society of Chemistry
Lycèe Faidherbe de Lille - An extensive article by G. Dupuis and N. Berland, on oscillating reactions, with some good pictures, and detailed mechanisms.
Page last updated October 2015
This is a resource from the Practical Chemistry project, developed by the Nuffield Foundation and the Royal Society of Chemistry. This collection of over 200 practical activities demonstrates a wide range of chemical concepts and processes. Each activity contains comprehensive information for teachers and technicians, including full technical notes and step-by-step procedures. Practical Chemistry activities accompany Practical Physics and Practical Biology.