Old Nassau reaction


The Old Nassau reaction or Halloween reaction is a chemical clock reaction in which a clear solution turns orange and then black. This reaction was discovered by two undergraduate students at Princeton University researching the inhibition of the iodine clock reaction by Hg2+, resulting in the formation of orange HgI2. Orange and black are the school colors of Princeton University, and "Old Nassau" is a nickname for Princeton, named for its historic administration building, Nassau Hall.

Chemical equation

The reactions involved are as follows:
  1. Na2S2O5 + H2O → 2 NaHSO3
  2. IO3− + 3 HSO3− → I− + 3 SO42− + 3 H+
This reaction reduces iodate ions to iodide ions.
  1. Hg2+ + 2 I− → HgI2
Orange mercury iodide solid is precipitated until the mercury is used up.
  1. IO3− + 5 I− + 6 H+ → 3 I2 + 3 H2O
The excess I− and IO3− undergo the iodide-iodate reaction
  1. I2 + starch → a blue/black complex
A blue/black starch-iodine complex is formed.