Potentiometry with two indicator electrodes

When two indicator electrodes are used, the potential difference between them is plotted as a function of titrant concentration. The shapes of the individual titration curves vary with the degree of irreversibility of the couples as shown in Fig. 6.9. The titration of ferrous ion by ceric ion gives a curve of the type shown in Fig. 6.9a and the reactions occurring at various points on the curve will be briefly considered. 

The most important stages of the titration occur at the points on the curve labelled 1-4. 

Fe - Fe + e at the anode Since these two reactions occur at well-separated potentials, A is large. 

Potentiometric titration curves using two indicator electrodes. 

Since the second suggested anode reaction only occurs at a very positive potential it is an unlikely contribution. LE is now small since the cathode and anode reactions occur at almost the same potential (the ferric/ ferrous couple is fairly reversible). 

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