A colourful clock reaction ('Old Nassau')

Read our standard health & safety guidance

Lesson organisation

This experiment has a good visual impact and would be one way of stimulating interest in chemistry, perhaps via an Open Day. The reactions involved are ostensibly not too complex and could be introduced during an advanced level course when studying kinetics.

The demonstration lasts about 3 - 5 minutes but about 20 minutes is needed for the preparation beforehand.

Apparatus and chemicals

For each demonstration:

Eye protection: goggles
Protective gloves (preferably nitrile)

Volumetric flasks (1 dm³), 3
Beakers (250 cm³), 3
Measuring cylinders (50 cm³ or 100 cm³), 3
Stopclock or stopwatch (optional)
Diposable gloves (nitrile)

Soluble starch (Low hazard), 4 g
Sodium metabisulfite, Na₂S₂O₅ (Harmful), 13.7 g
Mercury(II) chloride (Very toxic, Dangerous for environment), 3 g
Potassium iodate(V) (Oxidising), 15 g
Deionised or distilled water, 3 dm³

Technical notes

Soluble starch (Low Hazard) Refer to CLEAPSS Hazcard 40C
Sodium metabisulfite, Na₂S₂O₅ (Harmful) Refer to CLEAPSS Hazcard 92
Mercury(II) chloride (Very toxic, Dangerous for environment) Refer to CLEAPSS Hazcard 62
Potassium iodate(V) (Oxidising) Refer to CLEAPSS Hazcard 80

To dispose of reaction mixtures after the demonstration, filter off the insoluble mercury(II) iodide and place the filter paper in a sealed plastic bag or a screw top bottle. Any unused solution B should be treated with excess sodium hydroxide to precipitate insoluble orange mercury hydroxide. This should be filtered off and the filter paper combined with the first residue. The mercury residues should then be retained for professional disposal.

Procedure

Before the demonstration

HEALTH & SAFETY: Wear goggles and protective gloves.

Using deionised or distilled water, make up three solutions, in graduated flasks as follows:

Solution A
a Make a paste of 4 g of soluble starch with a few drops of water.

b Pour 500 cm³ of boiling water on to this, and stir.

c Cool to room temperature

d Add 13.7 g of sodium metabisulfite and stir.

e Make up to 1 dm³ with water in a volumetric flask.

Solution B
a Dissolve 3 g of mercury(II) chloride in some water.

b Make up the solution to 1 dm³ in a volumetric flask. This solution should be labelled Harmful.

Solution C
a Dissolve 15 g of potassium iodate(V) in water.

b Make up the solution to 1 dm³ in a volumetric flask.
.
The demonstration

HEALTH & SAFETY: Wear gloves and eye protection

a Use two measuring cylinders to mix 50 cm³ of solution A with 50 cm³ of solution B in a beaker.

b Use the third measuring cylinder to pour 50 cm³ of solution C into a second beaker.

c Pour the contents of the first beaker into the second.

d After about 5 seconds the mixture turns an opaque orange colour.

e After another few seconds the mixture suddenly turns blue-black.

Teaching notes

A white background helps to make the colour changes more vivid. A white laboratory coat is ideal.

Scale up the volumes if the demonstration is being shown to a large audience.

Diluting all the solutions by a factor of two increases the time taken for the colour changes to occur.

The mechanisms of the various reactions taking place are complex and it is wise initially to keep the volume ratio as 1:1:1. Altering this ratio leads to unpredictable results.

Using a smaller volume of B speeds up the reaction.

If the volume of solution B is doubled, the appearance of the orange colour is delayed and the blue colour fails to appear at all.

The reactions occurring are fairly complex but the following steps explain the observations satisfactorily:

At first the metabisulfite ions react with water to form hydrogensulfite ions:

S₂O₅^2-(aq) + H₂O(l) → 2HSO₃^–(aq).

These hydrogensulfite ions reduce the iodate(V) ions to iodide ions:

IO₃^–(aq) + 3HSO₃^–(aq) → I^–(aq) + 3SO₄^2–(aq) + 3H⁺(aq)

As soon as the concentration of iodide ions is large enough to exceed the solubility product of HgI₂ (4.5 x 10^–29 mol³ dm^{– 9}), orange mercury(II) iodide solid is precipitated until all of the Hg²⁺ ions are used up (provided that there is an excess of I^– ions).

Hg²⁺(aq) + 2I^–(aq) → HgI₂(s)

This precipitate is responsible for the initial orange cloudiness in the mixture.

If at this stage there are still I^– and IO₃^– ions in the mixture, a redox reaction producing molecular iodine takes place:

IO₃^–(aq) + 5I^–(aq) + 6H⁺(aq) → 3I₂(aq) + 3H₂O(l)

and the intense blue-black starch-iodine complex is formed.

Health & Safety checked, August 2008

Web Links

http://www.chem.leeds.ac.uk/delights/texts/expt_10.html

http://chemed.chem.purdue.edu/demos/main_pages/22.5.html

(Websites accessed August 2008)