Oxidation-reduction electrodes titrations, with indicators

In addition, potentiometric titration methods exist in which an electrode other than an ion-selective electrode is used. A simple platinum wire surface can be used as the indicator electrode when an oxidation-reduction reaction occurs in the titration vessel. An example is the reaction of Ce(IV) with Fe(II) ... 

Ferroin With the introduction of Ce(IV) as an oxidant and the evaluation of the formal potential of the Ce(rV)-Ce(III) couple, the need for indicators with higher electrode potentials became evident. The indicator ferroin, tris(l,10-phenanthroline)-iron(II), was discovered by Walden, Hammett, and Chapman, and its standard potential was evaluated at 1.14 V. Hume and KolthofiF found that the formal potential was 1.06 V in 1 M hydrochloric or sulfuric acid. The color change, however, occurs at about 1.12 V, because the color of the reduced form (orange-red) is so much more intense than that of the oxidized form (pale blue). From Figure 15-1 it can be seen that ferroin should be ideally suited to titrations of Fe(II) and other reductants with Ce(lV), particularly when sulfuric acid is the titration medium. It has the further advantages of undergoing a reversible oxidation-reduction reaction and of being relatively stable even in the presence of oxidant. 

In oxidation-reduction titrations, an electrode potential related to the concentration ratio between the oxidized and reduced forms of either of the reactants is determined as a function of the titrant volume. The indicator electrode must be responsive to at least one of the couples involved in the reaction. Indicator electrodes for oxidation-reduction titrations are generally constructed from platinum, gold, mercury, or palladium. The metal chosen must be unreactive with respect to the components of the reaction. The indicator metal is merely a medium for electron transfer. 

The potentiometric technique involves the use of glass, ISE and platinum electrodes, the latter used in connection with nearly all oxidation-reduction titrations. These electrodes use external or internal reference electrodes. In the main, the reference is an Ag/AgCl (3M KCl) unit with an outer compartment capable of being filled with an electrolyte of choice and changeable. For chloride titrations, for example, the indicator electrode is often a silver billet coated with AgCl, with a Ag/AgCl reference 3M KNO3 filled. 

The variation of E with the ratio [Ox]/[Red] is represented graphically in Fig.2.6. As discussed in Sec.2.2.3, a titration of an oxidant against a reductant is possible if their E values are sufficiently different and if a suitable redox indicator is available. However, if the latter is not available or if the colour of the solutions prohibit the use of an indicator, potentiometric redox titration can be carried out by connecting the metal electrode and a reference electrode (dipping in the titration vessel through its salt bridge) to a pH meter. The plot of E against the volume of titrant will show an inflection point which indicates the end-point where the equivalence point of reductant and oxidant is reached. 

---