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 nickname for Princeton, named for its historic administration building, Nassau Hall.
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.
(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 H2O)
the excess I- and IO3- undergo the iodide-iodate reaction
