Redox reactions of neutral red. A pulse radiolysis study

Abstract

One-electron reduction and oxidation reactions of the phenazine dye neutral red (NR) have been studied in homogeneous aqueous medium at different pHs employing various reducing {hydrated electron, H atom, isopropyl ketyl [(CH₃)₂ĊOH]} and oxidizing (Cl₂^–, TI²⁺, Br, Br₂^–, N₃) radicals. The absorption spectra of the product transient species (semireduced and semioxidized neutral red) and their kinetic parameters have been evaluated. By monitoring the absorbance changes as a function of pH pK_a, values of 2.4 ± 0.2 for the semireduced species and 2.9 ± 0.2 and 6.7 ± 0.2 for the semioxidized neutral red have been determined. While all the radicals employed in this investigation react completely by electron transfer, H and OH are found to react to the extent of 42 ± 5% and 100% respectively with neutral red to bring about adduct formation. The one-electron oxidation potential of neutral red has been evaluated.