In the potentiometric titration of $\mathrm{Fe}{}^{2+}$ with $\mathrm{Ce}{}^{4+}$, the indicator electrode normally used is

A redox titration is followed with an inert metal electrode, usually platinum, whose potential responds to the ratio of oxidised and reduced forms present. A glass electrode responds to pH, a zinc rod would itself react with the oxidants, and the calomel electrode serves as the reference, not the indicator.