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Module 5 : Electrochemistry

Lecture 24 : Applications of Electrode Potentials

24.4

Potentiometric Titrations
One of the widely used applications of emf measurements is to detect the end point of a titration by measuring the emf of a cell consisting of an indicator electrode (electrode, whose potential depends on the concentration of the reactant ions) and a reference electrode (e.g., SCE) as the titration progresses. This is called a potentiometric titration (varying emf measured during the progress of titration). Since the electrode potential of the reference electrode is constant, the observed change in emf as the reaction progresses, is due to the change in the electrode potential of the indicator electrode.

During acid-base titrations, the pH of the solution changes. By monitoring the change in pH as the titration progresses, it is possible to detect the end point in acid-base titrations using a pH meter. Near the equivalence point, the change in pH is quite large, and at the end point the variation is the largest.

In a redox titration between Fe²⁺and Ce⁴⁺, the overall reaction is
Ce⁴⁺ + Fe²⁺ Ce³⁺ + Fe³⁺
The overall reaction can be described in terms of the two half cell reactions
Ce⁴⁺ + e ^- Ce³⁺ ; E^o(Ce⁴⁺, Ce³⁺) = + 1.610 V and
Fe³⁺ + e ^- Fe²⁺ ; E^o( Fe³⁺, Fe²⁺) = + 0.771 V

Initially when no Ce⁴⁺ is added to the system, the potential of the electrode, is determined by the Fe³⁺, Fe²⁺ couple, it is given by Eqn. (24.11 )

E_Fe = E^o_Fe - 0.059 log [Fe²⁺ ] / [Fe³⁺ ]	(24.11)

When all the Fe²⁺ has reacted, if a slight excess of Ce⁴⁺is added, the potential is characteristic of the Ce⁴⁺, Ce³⁺ system and is given by

E_Ce = E^o_Ce - 0.059 log [ Ce³⁺ ] / [Ce⁴⁺]	(24.12)