The Iodine Clock or Landolt Reaction

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Chemical Concepts Demonstrated

Kinetics
Concentration effects on reaction rate

Demonstration

The 5 beakers contain 4 solutions in varying amounts:

The solutions are poured into the beakers simultaneously.

H₂O
Starch solution
KIO₃ solution
Na₂SO₃

Beaker 5 contains the most amount of water and the least amount of KIO₃ solution; Beaker 1 is the opposite. The remaining beakers are in a series relative to these two.

Observations

The beakers turn blue-black in a specific order.

Explanation (including important chemical equations)

This demonstration is based on the following mechanism:

1. IO₃^{^-}(aq) + 3 HSO₃^{^-}(aq) --> I^{^-}(aq) + 3 SO₄^{^2-}(aq) + 3 H^⁺(aq)

2. IO₃^{^-}(aq) + 5 I^{^-}(aq) + 6 H^⁺(aq) --> 3 I₂ (aq) + 3 H₂O (l)

3. I₂ (aq) + HSO₃^{^-}(aq) + H₂O (l) --> 2 I^{^-}(aq) + SO₄^{^2-}(aq) + 3 H^⁺(aq)

4. I₂ (aq) + I^{^-}(aq) + starch (aq) --> starch-I^{^3-}(aq)

Because steps 1 and 2 are much slower than step 3, no I₂ accumulates as long as HSO₃^{^-} remains in solution. When all of the HSO₃^{^-} is consumed, I₂ accumulates and the blue-black starch-triiodide complex is formed in step 4.

Contributors

Dr. George Bodner (Perdue University)