Department of Chemistry and Biochemistry

Old Nassau Clock Reaction

Solutions

• Solution A: 4gm soluble starch, 15gm NaHSO₃ per liter H₂O

  Dissolve 4gm soluble starch in 500ml boiling water, then add 15gm NaHSO₃ and 500ml water.

• Solution B: 3gm HgCl₂ per liter H₂O
• Solution C: 15gm KIO₃ per liter H₂O

Procedure

Mix equal amounts of solutions A, B, and C in that order with stirring. Solution turns bright orange, then suddenly turns dark blue.

Or you can pour half of solution C into the mixture and wait until a nice orange color develops before adding the remainder of solution C.

Reactions

IO3^- + 3HSO₃ --> I^- + 3SO₄^2- + 3H⁺

Hg²⁺ + 2I^- --> HgI₂(orange)

6H⁺ + IO₃^- + 5I^- --> 3I₂+ 3H₂O

I₂ + starch --> (blue)

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Last Updated: 5/23/02