Detection of the end point in oxidation-reduction titrations

Internal oxidation-reduction indicators. As discussed in Sections 10.10-10.16, acid-base indicators are employed to mark the sudden change in pH during acid-base titrations. Similarly an oxidation-reduction indicator should mark the sudden change in the oxidation potential in the neighbourhood of the equivalence point in an oxidation-reduction titration. The ideal oxidation-reduction indicator will be one with an oxidation potential intermediate between 

An oxidation-reduction indicator (redox indicator) is a compound which exhibits different colours in the oxidised and reduced forms  

The oxidation and reduction should be reversible. At a potential E the ratio of the concentrations of the two forms is given by the Nernst equation  

If the colour intensities of the two forms differ considerably the intermediate colour is attained at potential somewhat removed from Efn, but the error is unlikely to exceed 0.06 volt. For a sharp colour change at the end point, Efn should differ by about at least 0.15 volt from the standard (formal) potentials of the other systems involved in the reaction. 

One of the best oxidation-reduction indicators is the 1,10-phenanthroline-iron(II) complex. The base 1,10-phenanthroline combines readily in solution with iron(II) salts in the molecular ratio 3 base l iron(II) ion forming the intensely red l,10-phenanthroline-iron(II) complex ion with strong oxidising agents the iron(III) complex ion is formed, which has a pale blue colour. The colour change is a very striking one  

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