Halloween Reaction or Old Nassau Reaction

The Old Nassau or Halloween reaction is a clock reaction in which the color of a chemical solution changes from orange to black. Here's how you can do this reaction as a chemistry demonstration and a look at the chemical reactions that are involved.

Halloween Reaction Materials

• water
• soluble starch
• sodium metabisulphite (Na₂S₂O₅)
• mercury(II) chloride
• potassium iodate (KIO₃)

Prepare the Solutions

• Solution A: Mix 4 g soluble starch in a couple milliliters of water. Stir the starch paste into 500 ml boiling water. Allow the mixture to cool to room temperature. Add 13.7 g of sodium metabisulphite. Add water to make 1 liter of solution.

• Solution B: Dissolve 3 g mercury(II) chloride in water. Add water to make 1 liter of solution.

• Solution C: Dissolve 15 g potassium iodate in water. Add water to make 1 liter of solution.

Perform the Demonstration

1. Mix 50 ml solution A with 50 ml of solution B.

2. Pour this mixture into 50 ml of solution C.

The color of the mixture will change to an opaque orange color after a few seconds as the mercury iodide precipitates. After another few seconds the mixture will turn blue-black as the starch-iodine complex forms.

If you dilute the solutions by a factor of two then it takes longer for the color changes to occur. If you use a smaller volume of solution B the reaction will proceed more rapidly.

Chemical Reactions

1. Sodium metabisulfite and water react to form sodium hydrogen sulfite:
   
   Na₂S₂O₅ + H₂O --> 2 NaHSO₃

2. Iodate(V) ions are reduced to iodide ions by the hydrogen sulfite ions:
   
   IO₃^- + 3 HSO₃^- --> I^- + 3 SO₄^2- + 3 H⁺

3. When the concentration of iodide ions becomes sufficent for the solubility product of the HgI₂ to exceed 4.5 x 10^-29 mol³ dm^-9, then orange mercury(II) iodide precipitates until the Hg²⁺ ions are consumed (assuming an excess of I^- ions):
   
   Hg²⁺ + 2 I^- --> HgI₂ (orange or yellow)

4. If I^- and IO₃^- ions remain, then an iodide-iodate reaction takes place:
   
   IO₃^- + 5 I^- + 6 H⁺ --> 3 I₂ + 3 H₂O

5. The resulting statch-iodine complex is black to blue-black:
   
   I₂ + starch --> a blue/black complex