Iodine clock reaction

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The Iodine clock reaction or Landolt reaction is a classical chemical clock demonstration experiment to display chemical kinetics in action; it was discovered in 1886 ^[1]. Two clear solutions are mixed and at first there is no visible reaction, but after a short time delay, the liquid suddenly turns dark. The iodine clock reaction exists in several variations.

Contents 1 Hydrogen peroxide variation 2 Iodate variation 3 Chlorate variation 4 See also 5 External links 6 References

[edit] Hydrogen peroxide variation

This reaction starts from a solution of hydrogen peroxide with sulfuric acid. To this is added a solution containing potassium iodide, sodium thiosulfate, and starch. There are two reactions occurring in the solution.

In the first, slow reaction, the triiodide ion is produced .

H₂O₂(aq) + 3 I^-(aq) + 2 H⁺ → I₃^- + 2 H₂O.

In the second, fast reaction, triiodide is reconverted to iodide by the thiosulfate.

I₃^-(aq) + 2 S₂O₃^2-(aq) → 3 I^-(aq) + S₄O₆^2-(aq)

After some time the solution will rapidly change color to a very dark blue, almost black.

When the solutions are mixed, the second reaction causes the triiodide ion to be consumed much faster than it is generated, and only a small amount of triiodide is present in the dynamic equilibrium. Once the thiosulfate ion has been exhausted, this reaction stops and the blue color caused by the triiodide - starch complex appears.

Anything that accelerates the first reaction will shorten the time until the solution changes color. Increasing the pH, or the concentration of iodide or hydrogen peroxide will shorten the time. Adding more thiosulfate will have the opposite effect; it will take longer for the blue color to appear.

[edit] Iodate variation

An alternative protocol uses a solution of iodate ion (for instance potassium iodate) to which an acidified solution (again with sulfuric acid) of sodium thiosulfate is added.

In this protocol, iodine and the hydrogen sulfate ion are generated by the reaction:

2 IO₃^- + 5 HSO₃^- + 2 H⁺ → I₂ + 5 HSO₄^-(aq) + H₂O

This is the rate determining step of the reaction.

and iodine is again consumed by the reaction:

I₂ + HSO₃^- + H₂O → 2 I^- + SO₄^2-(aq) + 3 H⁺

When the bisulfite is consumed, the iodine is again able to form triiodide and the blue complex with starch.

[edit] Chlorate variation

An experimental iodine clock sequence has also been established for a system consisting of iodine potassium-iodide, sodium chlorate and perchloric acid that takes place through the following reactions ^[2].

iodide anions present in equilibrium with triiodide ion and iodine:

I₃^- → I^- + I₂

chlorate ion oxidizes iodide ion to hypoiodous acid and Chlorous acid in slow and rate-determining step:

ClO₃^- + I^- + 2H⁺ → HOI + HClO₂

chlorate consumption is accelerated by reaction of hypoiodous acid to iodous acid and more chlorous acid.

ClO₃^- + HOI + H⁺ → HIO₂ + HClO₂

more autocatalysis when newly generated iodous acid also converts chlorate in the fastest reaction step.

ClO₃^- + HIO₂ → IO₃^- + HClO₂

In this clock the induction period is the time it takes for autocatalytic process to start after which the concentration of free iodine falls rapidly as observed by UV/VIS spectroscopy.

[edit] See also

• clock reaction
• Old Nassau reaction

[edit] External links

• Hydrogen peroxide variation
• Potassium iodate variation
• Potassium iodate variation II

[edit] References

1. ↑  Landolt, H. Ber. Dtsch. Chem. Ges. 1886, 19, 1317-1365.
2. ↑  The Chlorate-Iodine Clock Reaction André P. Oliveira and Roberto B. Faria J. Am. Chem. Soc.; 2005; 127(51) pp 18022 - 18023; (Communication) DOI: 10.1021/ja0570537 Abstract