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The Hallowe’en Clock Reaction (Old Nassau)


Prepare the following three mixtures:

  1. Make a paste of 4 g of soluble starch with a few mls of water.  Add 500 ml of boiling water and stir.  Cool to room temperature, add 13.7 g of sodium metabisulphite (Na2S2O5) and make up to 1 l with water.
  2. Dissolve 3 g of mercury (II) chloride in water and make the solution up to 1L with water.
  3. Dissolve 15 g of potassium iodate (KIO3) in water and make the solution up to 1L with water.


Mix 50 ml of solution A with 50 ml of solution B.  Then pour into this mixture 50 ml of solution C.  After about 5 seconds the mixture will turn an opaque orange colour as insoluble mercury iodide precipitates.  After another 5 seconds the mixture suddenly turns blue-black as a starch-iodine complex is formed. 

All soluble mercury salts are poisonous and should be treated accordingly

Another famous reaction is the iodine clock (Landolt reaction).  The experiment is performed by mixing equal volumes of two solutions, one containing 2 g dm-3 KIO3 and H2SO4 0.03 M; the second - 0.4 g dm-3 of NaHSO3 in starch (2 g dm-3) previously dissolved in boiling water.  The initially colourless mixture suddenly turns dark blue. This is commoner in schools and avoids the use of mercury salts.

The Chemistry

Sodium metabisulphite reacts with water to form sodium hydrogen sulphite: Na2S2O5 + H2O  à  2 NaHSO3

Hydrogen sulphite ions reduce iodate(V) ions to iodide ions: IO3- + 3 HSO3-  à  I- + 3 SO42- + 3 H+

Once the concentration of iodide ions is large enough that the solubility product of HgI2 (4.5 x 10-29 mol3 dm-9) is exceeded, 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+ + 2 I-  à  HgI2 (orange)

If there are still I- and IO3- ions in the mixture, the iodide-iodate reaction IO3- + 5 I- + 6 H+  à  3 I2 + 3 H2O takes place and the blue starch-iodine complex is formed, I2 + starch ==> complex (blue or black)

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