title: Old Nassau reaction
text: # 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 (or Landolt reaction) by $Hg^{2+},$ resulting in the formation of orange $HgI_2.$ 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:
  
  * $Na_2S_2O_5$ + $H_2O$ → 2 $NaHSO_3$
  * $IO_3^−$ + 3 $HSO_3^−$ → $I^−$ + 3 $SO_4^{2−}$ + 3 $H^+$ This reaction reduces iodate ions to iodide ions.
  * $Hg^{2+}$ + 2 $I^−$ → $HgI_2$ Orange mercury iodide solid is precipitated until the mercury is used up.
  * $IO_3^−$ + 5 $I^−$ + 6 $H^+$ → 3 $I_2$ + 3 $H_2O$ The excess $I^−$ and $IO_3^−$ undergo the iodide-iodate reaction
  * $I_2$ + starch → a blue/black complex A blue/black starch-iodine complex is formed.
  