Oct.18, 2010

In the following text, our group found the demonstration for the Halloween reaction or the "Old Nassau Reaction."

Old Nassau
            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') [1].  As Alyea describes [1] `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 [2].
            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 [1].  `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 [5].  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 SO₄^2- + 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 [6].

            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 [5].  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. [7].

            Safety.  All soluble mercury salts are poisonous and should be treated accordingly.

References.
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


Next, our group found the demonstration to the Volcano reaction or the Ammonium Dichromate Reaction.

Ammonium Dichromate Reaction

By Anne Marie Helmenstine, Ph.D., About.com Guide

Introduction
The eruption of an ammonium dichromate [(NH₄)₂Cr₂O₇] volcano is a classic chemistry demonstration. The ammonium dichromate glows and emits sparks as it decomposes and produces copious amounts of green chromium (III) oxide ash. This demonstration is simple to prepare and perform. The decomposition of ammonium dichromate commences at 180°C, becoming self-sustaining at ~225°C. The oxidant (Cr⁶⁺) and the reductant (N^3-) are present in the same molecule.
(NH₄)₂Cr₂O₇ --> Cr₂O₃ + 4 H₂O + N₂
The procedure works well in both a lighted or darkened room.
Materials

• ~20 grams of ammonium dichromate
• sand tray or ceramic tile, for use in ventilation hood OR
• 5-liter round bottom flask and porcelain filtering funnel
• gas burner (e.g., Bunsen) OR
• butane lighter or match, for use with flammable liquid (e.g., ethanol, acetone)

Procedure
If you are using a hood:

1. Make a pile (volcanic cone) or ammonium dichromate on a tile or tray of sand.
2. Use a gas burner to heat the tip of the pile until the reaction begins or dampen the tip of the cone with a flammable liquid and light it with a lighter or match.

If you are not using a ventilation hood:

1. Pour the ammonium dichromate into a large flask.
2. Cap the flask with a filtration funnel, which will prevent the majority of the chromium (III) oxide from escaping.
3. Apply heat to the bottom of the flask until the reaction begins.

Notes
Chromium III and chromium VI, as well at its compounds, including ammonium dichromate, are known carcinogens. Chromium will irritate the mucous membranes. Therefore, take care to perform this demonstration in a well-ventilated area (preferably a ventilation hood) and avoid skin contact or inhalation of the materials. Wear gloves and safety goggles when handling ammonium dichromate.
References
B.Z. Shakhashiri, Chemical Demonstrations: A Handbook for Teachers of Chemistry, Vol. 1, University of Wisconsin Press, 1986, pp. 81-82.
Delights of Chemistry, University of Leeds.