Oxidation-Reduction Titrations:
Indicator electrodes for oxidation-reduction are generally fabricated from platinum, gold mercury or silver. The metal chosen must be un-reactive with respect to the components of the reaction. It is merely a site for electron transfer. The Platinum electrode is most hugely used for oxidation-reduction titrations. It should be recalled in which the curves for many oxidation-reduction titrations are not symmetric. Thus, the equivalence point potential may be significantly different from the potential corresponding to the point of inflection in the curve.
A determining factor within the values of potential is the ratio of the activity or concentration of the oxidised and reduced forms of certain ion species.
Take a general equation
Oxidised form + ne ↔ reduced form ... (2.45)
a potential acquired through the indicator electrode at 25oC is given by
E = E0 + 0.0591/n log(aoxidised form/ areduced form ) ... (2.46)
where E0 is the standard potential of the system. The potential of the immersed electrode is controlled by the ratio of these activities or concentration. In redox reactions, reducing agent is oxidised or oxidising agent is reduced and the ratio and the potential therefore changes more rapidly at the end point of the reaction. Thus titrations involving such reactions like Fe (II) with potassium permanganate or potassium dichromate or ceric sulphate may be followed potentiometrically and equivalence points can be obtained.