This indicator functions well giving a sharp colour change and is also resistant to oxidative
decomposition. The transition potential may be modified by ring substitution with nitro or methyl
groups (Table 5.5).
A second important group includes the diphenylamine indicators. In the presence of a strong oxidizing
agent, diphenylamine is irreversibly converted to diphenylbenzidine. This latter compound undergoes a
reversible redox reaction accompanied by a colour change,
The use of a sulphonic acid derivative of diphenylamine overcomes the problem of low indicator
solubility.
Selection of a Visual Indicator for a Redox Titration
Because of the relatively small number of indicators available and their pH dependence, selection is not
as straightforward as in the case of acid-base titrations. For example, iron(II) may be titrated with
cerium(IV) or chromium(VI) (Table 5.4), whilst equation (5.6) in conjunction with Table 5.5 will
suggest suitable indicators. p-Ethoxychrysodine, diphenylamine and diphenylamine sulphonic acid will
all suffice in dilute acid whilst both erioglaucin A and 1,10-phenanthroline-iron(II) have been used in
stronger acid. A further problem arises, however, as a result of the complexing action of many anions.
Where one oxidation state in a redox equilibrium is preferentially complexed the reaction will be
displaced and the potential shifted accordingly. Notable in this complexing action is phosphoric acid, a
reagent which can be used to advantage in some instances. The titration of iron(II) in a phosphoric acid
medium is discussed in a later section. As a result of the uncertainties outlined above, selecting an
indicator which will change at exactly the right point can be difficult, and large indicator blanks may
have to be tolerated. Experimentally determined titration curves will generally be of considerable help
in the selection of an indicator.