Abstract

Antioxidant compounds present in plant-based foods and beverages play a crucial role in preventing oxidative stress by neutralizing reactive oxygen species. This study reports the formation and stabilization of the N,N,N′,N′-tetramethyl-p-phenylenediamine radical cation (TMPD●+) in aqueous medium and its application as a spectrophotometric probe for total antioxidant capacity (TAC) determination. Unlike widely used ABTS●+ and DPPH●+ assays, which often require organic solvents, the proposed method operates entirely in water, representing a more environmentally friendly alternative.
The chemical parameters affecting TMPD●+ stability were systematically optimized, including pH, buffer system, solvent effects, and oxidizing agents. The optimal conditions (λmax = 563 nm) were established at pH 5.0 using Fe(III) 1.0×10⁻⁴ M and TMPD 2.0×10⁻⁴ M. Standard antioxidants were ranked according to their reducing capacity based on TMPD●+ consumption, with tannic acid and ascorbic acid showing the highest responses. When applied to herbal teas, TAC values expressed as ascorbic acid equivalents showed strong correlation with the ABTS●+ assay (r = 0.900). These findings demonstrate that stabilized TMPD●+ provides a simple, low-cost, sensitive, and greener approach for TAC determination in plant-based beverages

Key words: N,N,N′,N′-Tetramethyl-p-phenylenediamine; Antioxidant; herbal teas; Polyphenolic Compounds; Vegetables Beverages