A colourful oscillating reaction

A colourful oscillating reaction

Demonstration 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.

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.

Chemicals

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

Apparatus

For one demonstration or each pair of students:

Eye protection (Goggles when preparing Solution A - Note 2)

Beakers (100 cm³), 4

Petri dish

Measuring cylinder (10 cm³)

Plastic syringes (1 or 2 cm³), 3

Dropping pipette

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, HOOCCH₂COOH(s), (HARMFUL) - see CLEAPSS Hazcard.

Ferroin indicator - see CLEAPSS Hazcard and CLEAPSS Recipe Book.

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: 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.

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.