Halloween Clock Reaction – Old Nassau Reaction

Old Nassau or Halloween Clock Reaction
The Old Nassau reaction is called the Halloween reaction because it changes color from orange to black. It is a clock reaction, so the concentration of the chemicals causes change at a predictable and reproducible time.

The Halloween clock reaction or Nassau reaction is a classic chemistry Halloween demonstration. Mix three solutions to make a clear liquid that changes from bright orange to black. Because it’s a clock reaction, the timing of the color change depends on the concentration of the solutions and is predictable. The demonstration raises interest in chemistry and illustrates redox reactions and factors that affect rate of reaction. Here’s how to do the Halloween clock reaction and a look at the chemical reactions involved.

“Old Nassau” is a nickname for Princeton University. Its administration building is called Nassau Hall. Princeton’s school colors are orange and black.


  • Water
  • 1% soluble starch solution
  • 0.25 M sodium metabisulphite (Na2S2O5)
  • 0.01 M mercury(II) chloride
  • 0.10 M potassium iodate (KIO3)
  • Timer

Prepare the solutions prior to performing the demonstration. To make the starch solution, stir starch powder into a small amount of water and then stir it into boiling water. Let the solution cool before use. The other solutions simply involve dissolving the solid into water.

  • Add 13.7 g of sodium metabisulphite. Add water to make 1 liter of solution.
  • Dissolve 3 g mercury(II) chloride in water. Add water to make 1 liter of solution.
  • Dissolve 15 g potassium iodate in water. Add water to make 1 liter of solution.

Set up the following:

  1. In a 250-ml beaker, mix 65 ml of the 0.25 M sodium metabisulfate solution and 85 ml of the starch solution.
  2. In a 400-ml beaker, add 150 ml of the 0.01 M mercury chloride solution.
  3. In a 600-ml beaker, mix 110 ml potassium iodate and 40 ml deionized water.

Perform the Halloween Clock Reaction Demo

To perform the Halloween clock reaction demonstration:

  1. Pour the sodium metabisulfate + starch solution into the beaker with the mercury chloride solution.
  2. Quickly pour the resulting mixture into the 600 ml beaker containing the potassium iodate solution.
  3. It only takes a few seconds for the clear solution to turn bright opaque orange. After another few seconds, the mixture turns blue-black from the formation of starch-iodine complex. Time the color changes.

The concentration of the chemicals will be:

  • 0.036 M NaHSO3
  • 0.0033 M HgCl2
  • 0.024 M KClO3
See the Halloween clock reaction in action.

Chemical Reactions

The reaction is a variation of the Landolt iodine clock. The chemical reactions of the Halloween clock are as follows:

  1. Sodium metabisulfite and water react to form sodium hydrogen sulfite:
    Na2S2O5 + H2O → 2 NaHSO3 (clear)
  2. Iodate(V) ions are reduced to iodide ions by the hydrogen sulfite ions:
    IO3 + 3 HSO3 → I + 3 SO42− + 3 H+ (clear)
  3. Orange mercury iodide (HgI2) solid precipitates until the mercury is used up. HgI2 is the limiting factor (assuming excess iodine ions).
    Hg2+ + 2 I → HgI2
  4. The excess I and IO3 undergo the iodide-iodate reaction.
    IO3 + 5 I + 6 H+ → 3 I2 + 3 H2O
  5. A blue/black starch-iodine complex is formed as the I3 becomes I5. At high concentration (as in this demo), the liquid appears black. If you add enough water, the color is indigo.
    I2 + starch → a blue/black complex

Playing With Reaction Kinetics

Since this is a clock reaction, if you repeat the demonstration then the color changes occur after the same amount of time. You can play with two factors that affect the rate of the reaction: temperature and reactant concentration. Decreasing the temperature of the solutions (i.e., refrigerating them) increases the time required for the color change, while increasing the temperature speeds the reaction. Diluting the starting solutions with water decreases their concentration and slows the reactions. For example, dilute each solution by a factor of two. Another option is to play with the concentration of one component. Using a smaller volume of mercury chloride solution causes the reaction to proceed more slowly. Potassium chlorate is a strong oxidizer, while sodium metabisulfate is a strong reducing agent. So, while diluting the solutions is fine, working with more concentrated solutions is not advised.

Safety and Clean-Up

Because of the chemicals involved, this reaction is suitable for a high school or college chemistry lab (not for an untrained instructor). If you’re uncomfortable working with mercury, another excellent Halloween color change demonstration is the “water into blood” reaction. Gloves and safety goggles should be worn when mixing the solutions and performing the demonstration. Collect the mercury by filtering the insoluble mercury(II) iodide. Place the filter paper and precipitate in a collection bag or bottle. Precipitate mercury from any leftover mercury(II) chloride solution by reacting it with excess sodium hydroxide. This forms insoluble orange mercury hydroxide. Add the filter paper and solid to the collection container. Follow local regulations for the disposal of the mercury.


  • Alyea, Hubert N. (1977). “The Old Nassau reaction”. Journal of Chemical Education. 54: 167. doi:10.1021/ed054p167.2
  • Fortman, John J. (1992). “The old Nassau demonstration: Educational and entertaining variations”. Journal of Chemical Education. 69: 236. doi:10.1021/ed069p236
  • Huber, Hanspeter (1979). “Simulation of the Old Nassau Reaction”. Journal of Chemical Education. 56: 320. doi:10.1021/ed056p320
  • Lambert, Jack L.; Fina, Gary T. (1984). “Iodine clock reaction mechanisms”. Journal of Chemical Education. 61: 1037. doi:10.1021/ed061p1037
  • Shakhashiri, B. Z. (1992). Chemical Demonstrations: A Handbook for Teachers of Chemistry. The University of Wisconsin Press. 4:29-36.

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