This experiment continues the theme of `clock' reactions. The demonstration is known as the `Old Nassau Reaction', a clock reaction which turns orange and then black (and has therefore also been named the `Halloween Reaction') . As Alyea describes  `the formation of orange HgI2 was discovered accidentally by two Princeton undergraduates ... when they were carrying out original research on the inhibition, by Hg2+, of the Landolt reaction'. From this, by reducing the Hg2+ concentration, the present demonstration was subsequently developed .
The name `Old Nassau', comes from Nassau Hall which was named after William III, King of England, Prince of Orange and Nassau. Nassau Hall can boast of a colourful history . `At the time it was built it was the largest college building in North America. On January 3, 1777 General Washington crossed the Delaware to sieze the British ammunition stored there: his victorious Battle of Princeton followed. In 1796 it was perhaps the earliest undergraduate chemistry laboratory in the world,* where Dr John Maclean, Professor of Chemistry, had the students, themselves, carrying out chemical experiments. At that time Europe still practised apprenticeship: colleges in the New World gave only lecture demonstrations. In the late 1830s, several years before Samuel Morse sent his first telegraph message, Dean Joseph Henry, using an electromagnet, sent `clicks' from his office in Nassau Hall to his home nearby to alert his servants that he was coming home shortly, and to start heating water for his tea'.
The reaction in this experiment takes place in several steps . First, sodium metabisulphite reacts with water to form sodium hydrogen sulphite:
Na2S2O5 + H2O ==> 2 NaHSO3 (10.1)
Hydrogen sulphite ions reduce iodate(V) ions to iodide ions:
IO3- + 3 HSO3-==> I- + 3 SO42- + 3 H+ (10.2)
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-==> HgI2 (orange or yellow) (10.3)
If there are still I- and IO3- ions in the mixture, the iodide-iodate reaction
IO3- + 5 I- + 6 H+==> 3 I2 + 3 H2O (10.4)
takes place and the blue starch-iodine complex is formed,
I2 + starch ==> complex (blue or black) (10.5)
A full account of the reaction can be found in Shakhashiri’s book .
Preparation. The following three solutions need to be prepared.
A. Make a paste of 4 g of soluble starch with a few mils of water. Pour onto this 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.
B. Dissolve 3 g of mercury(II) chloride in water and make the solution up to 1 l with water.
C. Dissolve 15 g of potassium iodate (KIO3) in water and make the solution up to 1 l with water.
Demonstration. 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 further 5 seconds the mixture suddenly turns blue-black as a starch-iodine complex is formed. The second colour change (orange to black) is not normally expected by the audience and comes as a real surprise.
This experiment can be extended in several ways . Diluting all the solutions by a factor of two increases the time taken for the colour changes to occur. Using a smaller volume of solution B speeds up the reaction. The effect of changing the amounts and concentrations of the various reactants cannot always be predicted simply because of the complexity of the system. For example, 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.
If using mercury salts is not desirable, a somewhat simpler clock reaction can be performed. This is known as iodine clock reaction or 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. There are several extensions to this reaction as well, which can be found, for example in Ref. .
Safety. All soluble mercury salts are poisonous and should be treated accordingly.
To see a PowerPoint demonstration of this reaction, click on thumbnail
1. Tested Demonstrations in Chemistry, ed. G.L. Gilbert, et al., Denison University, Granville, OH, 1994, vol. 1, p. I-49.
2. H.N. Alyea, J. Chem. Educ., 1955, 32, 9.
3. B.N. Menschutkin, `A Russian physical chemist of the eighteenth century', J. Chem. Educ., 1927, 4, 1079.
4. H.S. Van Klooster, `The beginnings of laboratory instruction in chemistry in the USA', in Chymia: Annual Studies in the History of Chemistry, ed. T.L. Davis, vol. 2, Philadelphia, University of Pennsylvania Press, 1949, pp. 1-15.
5. T. Lister, Classic Chemistry Demonstrations, ed. C. O'Driscoll and N. Reed, London, Royal Society of Chemistry, 1995, p. 50.
6. B.Z. Shakhashiri, Chemical Demonstrations: A Handbook for Teachers of Chemistry, Volume 4, Wisconsin, US, The University of Wisconsin Press, 1992.
7. Ref. 1, p. I-46
* In fact it was M.V. Lomonosov (1711-1765) in Russia who established (1749) the first chemical laboratory where students were regularly taught practical chemistry [3,4].