N-(4-Methoxyphenyl)-1,4-Phenylenediamine Radical Cation: Formation, Stabilization, and Application for Assessing the Antioxidant Capacity of Commercial Infusion

N-(4-methoxyphenyl)-1,4-phenylenediamine (MPPD), a p-phenylenediamine also known as Variamine Blue B basic dye, has been employed as a visual indicator in the complexometric titration of Fe(III) with EDTA at pH 2.0. In a buffered acidic medium (pH 4.0), MPPD (blue) can be oxidized to its radical cation MPPD●+ (purple), which is consumed in the presence of phenolic acids, flavonoids, and certain vitamins, leading to solution decolorization. The physicochemical parameters influencing MPPD●+ formation and stabilization were first investigated, followed by evaluation of its reactivity toward phenolic acids, flavonoids, and vitamins at known concentrations, enabling the establishment of a structure–reactivity relationship for antioxidant consumption of the cation radical. The stabilized MPPD●+ solution was then applied to determine the antioxidant capacity of polyphenol-rich samples, such as teas and plant infusions, and the results were compared with those obtained using the ABTS●+ radical cation assay, as well as with total polyphenol content determined by the Folin-Denis Reagent, as recommended in the Brazilian Pharmacopoeia.