Iodine clock reaction
2907170
221430793
2008-06-24T13:40:56Z
Ewen
3610
/* Hydrogen peroxide variation */
The '''Iodine clock reaction''' is a classical [[chemical clock]] demonstration experiment to display [[chemical kinetics]] in action; it was discovered by [[Hans Heinrich Landolt]] in 1886 <ref>[[Hans Heinrich Landolt|Landolt, H.]] Ber. Dtsch. Chem. Ges. '''1886''', 19, 1317-1365.</ref>. Two clear solutions are mixed and at first there is no visible reaction, but after a short time delay, the liquid suddenly turns to a shade of dark blue. The iodine clock reaction exists in several variations.
== 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 .
:H<sub>2</sub>O<sub>2</sub>(aq) + 3 I<sup>-</sup>(aq) + 2 H<sup>+</sup> → I<sub>3</sub><sup>-</sup> + 2 H<sub>2</sub>O.
:In the second, fast reaction, triiodide is reconverted to iodide by the thiosulfate.
:I<sub>3</sub><sup>-</sup>(aq) + 2 S<sub>2</sub>O<sub>3</sub><sup>2-</sup>(aq) → 3 I<sup>-</sup>(aq) + S<sub>4</sub>O<sub>6</sub><sup>2-</sup>(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. Decreasing the pH (increasing H+ concentration), or increasing 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.
== 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 bisulfite]] is added.
In this protocol, [[iodide]] ion is generated by the following slow reaction between the iodate and bisulfite:
:IO<sub>3</sub><sup>-</sup> (aq) + 3HSO<sub>3</sub><sup>-</sup> (aq) → I<sup>-</sup> (aq) + 3HSO<sub>4</sub><sup>-</sup>(aq)
This is the rate determining step. The iodate in excess will oxidize the iodide generated above to form iodine:
:IO<sub>3</sub><sup>-</sup> (aq) + 5I<sup>-</sup> (aq) + 6H<sup>+</sup> (aq) → 3I<sub>2</sub> + 3H<sub>2</sub>O (l)
However, the iodine is reduced immediately back to iodide by the bisulfite:
:I<sub>2</sub> (aq) + HSO<sub>3</sub><sup>-</sup> (aq) + H<sub>2</sub>O (l) → 2I<sup>-</sup> (aq) + HSO<sub>4</sub><sup>-</sup>(aq) + 2H<sup>+</sup> (aq)
When the bisulfite is fully consumed, the iodine will survive (i.e., no reduction by the bisulfite) to form the dark blue complex with starch.
==Persulfate variation==
This clock reaction uses [[Sodium persulfate|sodium]], [[Potassium persulfate|potassium]] or [[ammonium persulfate]] to [[redox|oxidise]] [[iodide]] ions to [[iodine]]. [[Sodium thiosulfate]] is used to reduce Iodine back to Iodide before the Iodine can complex with the [[starch]] to form the characteristic blue-black colour.
:Iodine is generated:
:2I<sup>-</sup><sub>(aq)</sub> + S<sub>2</sub>O<sub>8</sub><sup>2-</sup><sub>(aq)</sub> → I<sub>2 (aq)</sub> + 2SO<sub>4</sub><sup>2-</sup><sub>(aq)</sub>
:And is then removed:
:I<sub>2 (aq)</sub> + 2S<sub>2</sub>O<sub>3</sub><sup>2-</sup><sub>(aq)</sub> → 2I<sup>-</sup><sub>(aq)</sub> + S<sub>4</sub>O<sub>6</sub><sup>2-</sup><sub>(aq)</sub>
Once all the thiosulfate is consumed the Iodine may form a complex with the starch.<ref>[http://ptcl.chem.ox.ac.uk/~hmc/tlab/experiments/502.html 5.02 Kinetics of the persulfate-iodide clock reaction<!-- Bot generated title -->]</ref> Potassium persulfate has a low solute potential, according to documents on the Salters website. Ammonium persulfate has a higher solubility and is used instead in reaction document examples from Oxford University.
== 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.<ref>{{cite journal | title = The Chlorate-Iodine Clock Reaction | author = André P. Oliveira and Roberto B. Faria | journal = [[J. Am. Chem. Soc.]] | year = 2005 | volume = 127 | issue = 51 | pages = 18022–18023 | doi = 10.1021/ja0570537}}</ref>
: iodide anions present in equilibrium with [[triiodide]] ion and [[iodine]]:
:I<sub>3</sub><sup>-</sup> → I<sup>-</sup> + I<sub>2</sub>
: chlorate ion oxidizes iodide ion to [[hypoiodous acid]] and [[Chlorous acid]] in slow and [[rate-determining step]]:
:ClO<sub>3</sub><sup>-</sup> + I<sup>-</sup> + 2H<sup>+</sup> → HOI + HClO<sub>2</sub>
: chlorate consumption is accelerated by reaction of hypoiodous acid to [[iodous acid]] and more chlorous acid.
:ClO<sub>3</sub><sup>-</sup> + HOI + H<sup>+</sup> → HIO<sub>2</sub> + HClO<sub>2</sub>
: more [[autocatalysis]] when newly generated iodous acid also converts chlorate in the fastest reaction step.
:ClO<sub>3</sub><sup>-</sup> + HIO<sub>2</sub> → IO<sub>3</sub><sup>-</sup> + HClO<sub>2</sub>
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]].
==See also==
*[[Clock reaction]]
*[[Old Nassau reaction]]
==External links==
* [http://jchemed.chem.wisc.edu/JCESoft/CCA/CCA3/MAIN/CLOCKRX/PAGE1.HTM Hydrogen peroxide variation]
* [http://www.ucdsb.on.ca/tiss/stretton/CHEM2/ratelab1.htm Potassium iodate variation]
* [http://www.chemie.uni-regensburg.de/Organische_Chemie/Didaktik/Keusch/D-Landolt-e.htm Potassium iodate variation II]
== References ==
<div class="references-small"><references /></div>
[[Category:chemical kinetics]]
[[Category:Iodine|Clock reaction]]