Quantitative study of the reaction between quinones and iron(II) in phosphoric acid medium.

A simple method is described for the determination of some quinones with ammonium iron(II) sulphate, by titration potentiometrically or with cacotheline, Methylene Blue, Methylene Green, thionine, Toluidine Blue, Azure A, Azure C or Azure-2-eosin as redox indicators. The reverse titrations can be used for estimation of iron(II). Alternatively, excess of iron (II) can be added and the iron(III) produced determined colorimetrically with thiocyanate.