Redox titration
577846
220183894
2008-06-18T17:45:58Z
Kumorifox
3295299
/* Example */
'''Redox titration''' (also called '''oxidation-reduction titration''') is a type of [[titration]] based on a [[redox reaction]] between the [[analyte]] and [[titrant]].
Redox titration may involve the use of a [[redox indicator]] and/or a [[potentiometer]].
==Example==
An example of a redox titration is treating a solution of [[iodine]] with a reducing agent and using [[starch]] as indicator. Iodine forms an intensely blue [[complex (chemistry)|complex]] with starch. Iodine (I<small><sub>2</sub></small>) can be reduced to [[iodide]] (I<small><sup>−</sup></small>) by e.g. [[thiosulphate]] (S<small><sub>2</sub></small>O<small><sub>3</sub><sup>2−</sup></small>), and when all iodine is spent the blue colour disappears. This is called an [[iodometry|iodometric]] titration.
Most often the reduction of iodine to iodide is the last step in a series of reactions where the initial reactions are used to convert an unknown amount of the [[analyte]] (the substance you want to analyse) to an equivalent amount of iodine, which may then be titrated. Sometimes other [[halogen]]s than iodine are used in the intermediate reactions because they are available in better measurable [[standard solution]]s and/or react more readily with the analyte. The extra steps in iodometric titration may be worth while because the [[equivalence point]], where the blue turns colourless, is more distinct than some other analytical methods.
Such a series of reactions in [[water|aqueous]] solution with [[resorcinol]] as analyte, [[bromate]] as standard, [[thiosulfate]] as titrant and [[starch]] as indicator, can be as follows. Oxidation numbers are given in parentheses.
<ol type="i">
<li><math> BrO_3^-(5) + 5Br^-(-1) + 6H^+ \ \to \ 3Br_2(0) + 3H_2O </math>
<br/> Bromate + surplus of [[bromide]] in acidic solution → [[Bromine]] + water. Bromine continues to ii. </li>
<li><math> 3Br_2(0) + resorcinol \ \to \ 3HBr(-1) + tribromo(+1)resorcinol </math>
<br/> Bromine + resorcinol → [[Hydrogen bromide]] + tribromoresorcinol. Unreacted bromine continues to iii. </li>
<li><math> Br_2(0) + 2I^-(-1) \ \to \ 2Br^-(-1) + I_2(0) </math>
<br/> Bromine + surplus of [[iodide]] → Bromide + [[iodine]]. Iodine continues to iv. </li>
<li><math> I_2(0) + 2SSO_3^{2-}(-1,5) \ \to \ 2I^-(-1) + (SSO_3)_2^{2-}(0,5) </math>
<br/> Iodine + thiosulfate → Iodide + [[tetrathionate]]. </li>
</ol>
Let us call the initial amounts of resorcinol and bromate ''R'' and ''B'', and the amount of thiosulfate ''T'', where ''B'' or ''T'' are known and ''R'' is unknown – but we have to make sure that ''R'' is less than ''B''. (All amounts are in [[mole (unit)|mole]].) Then '''''R'' = ''B'' - ''T''/6'''
==See also==
*[[Titration]]
*[[Oxidizing agent]]
*[[Reducing agent]]
*[[Redox indicator]]
[[Category:Titration]]
[[de:Redox-Titration]]
[[nn:Redokstitrering]]
[[pt:Titulação redox]]