A colourful oscillating reaction
Acidified sodium bromate and bromide solution 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.
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Lesson organisation
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.
Apparatus and chemicals
For one demonstration or each pair of students:
Eye protection (Goggles when preparing Solution A - see note 2)
Beakers (100 cm3), 4
Petri dish
Measuring cylinder (10 cm3)
Plastic syringes (1 or 2 cm3), 3
Dropping pipette
Sodium bromate(V) (Oxidising, Toxic), 5 g (enough for 10 experiments)
Concentrated sulfuric acid (Corrosive), 2 cm3
Sodium bromide, NaBr (Low hazard), 1 g
Propanedioic (malonic) acid (Harmful), 1 g
Ferroin indicator solution, 1 cm3
‘Photoflo’ solution, 1 drop (see note 1)
Distilled or deionised water, about 100 cm3
Technical notes
Sodium bromate(V) (Oxidising, Toxic). Refer to CLEAPSS Hazcard 80.
Concentrated sulfuric acid (Corrosive). Refer to CLEAPSS Hazcard 98A.
Sodium bromide (Low hazard) Refer to CLEAPSS Hazcard 47B.
Propanedioic (malonic) acid (Harmful). Refer CLEAPSS Hazcard 36B.
Ferroin indicator. Refer to CLEAPSS Recipe Card 37
1 'Photoflo’ solution - can be found at any photography shop. 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: 5 g of sodium bromate and 2 cm3 of concentrated sulfuric acid in 67 cm3 of distilled or deionised water.
Solution B: 1 g of sodium bromide in 10 cm3 of distilled or deionised water.
Solution C: 1 g of propanedioic acid in 10 cm3 of distilled or deionised water.
Procedure
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.
Teaching notes
This 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 and safety checked, February 2008
Web Links
http://www.faidherbe.org/site/cours/dupuis/oscil.htm
An extensive article on oscillating reactions, with some good pictures, and detailed mechanisms.
(Websites accessed September 2007)
Updated 17 Mar 2008
4 out of 5
Please put videos of the experiments. They will attract non science viewers too. Your efforts are good.
Submitted by: Dr Aamarpali on 26 June 2008