Old Nassau reaction

The Old Nassau or Halloween Reaction, is a scientific demonstration of a clock reaction in which a solution turns orange and then black (the school colors of Princeton University), was discovered by two Princeton undergraduates researching the inhibition of the iodine clock reaction (or Landolt reaction) by Hg²⁺, resulting in the formation of orange HgI₂. "Old Nassau" is the alma mater for Princeton, named for its historic administration building, Nassau Hall.

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The reactions involved are as follows:

(1) Na₂S₂O₅ + H₂O ==> 2 NaHSO₃

(2) IO₃^- + 3 HSO₃^- ==> I^- + 3 SO₄^2- + 3 H⁺

This reaction reduces iodate ions to iodide ions.

(3) Hg²⁺ + 2 I^- ==> HgI₂

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

(4) IO₃^- + 5 I^- + 6 H⁺ ==> 3 I₂ + 3 H₂)

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

(5) I₂ + starch ==> a blue/black complex

and blue/black starch-iodine complex is formed.

See also

• iodine clock reaction