The Old Nassau or Halloween Reaction, a scientific demonstration of a clock reaction in which a solution turns orange and then black (the school colors of Princeton University

Princeton University

Princeton University is a private research university located in Princeton, New Jersey, United States. The school is one of the eight universities of the Ivy League, and is one of the nine Colonial Colleges founded before the American Revolution....

), was discovered by two Princeton undergraduates researching the inhibition of the iodine clock reaction

Iodine clock reaction

The iodine clock reaction is a classical chemical clock demonstration experiment to display chemical kinetics in action; it was discovered by Hans Heinrich Landolt in 1886. Two colorless solutions are mixed and at first there is no visible reaction...

 (or Landolt reaction) by Hg²⁺, resulting in the formation of orange HgI₂. "Old Nassau" is the nickname for Princeton, named for its historic administration building, Nassau Hall

Nassau Hall

Nassau Hall is the oldest building at Princeton University in the borough of Princeton, New Jersey . At the time it was built in 1754, Nassau Hall was the largest building in colonial New Jersey. Designed originally by Robert Smith, the building was subsequently remodeled by notable American...

.

The reactions involved are as follows:
Na₂S₂O₅ + H₂O

> 2 NaHSO₃
IO₃^- + 3 HSO₃^-

> I^- + 3 SO₄^2- + 3 H⁺

This reaction reduces iodate ions to iodide ions.

Hg²⁺ + 2 I^-

> HgI₂

orange mercury iodide solid is precipitated until the mercury is used up.

IO₃^- + 5 I^- + 6 H⁺

> 3 I₂ + 3 H₂O)

the excess I^- and IO₃^- undergo the iodide-iodate reaction

I₂ + starch > a blue/black complex

and blue/black starch-iodine complex is formed.