Many diphenylamine derivatives are used as redox indicators that are particularly useful in alkaline redox titrations. The diphenylaminesulfonic acid is a simple prototype redox indicator, owing to its improved aqueous solubility compared with diphenylamine. Attempts have been made to explain the color changes associated with the oxidation of diphenylamine.
In a related application, diphenylamine is oxidized by nitrate to give a similar blue coloration in the diphenylamine test for nitrates.