The kinetics of neutral methyl viologen in acidic H2O+DMF mixed solutions studied by cyclic voltammetry

The chemistry of the two-electron reduction product of viologen (1,1′-dialkyl-4,4′-bipyridinium, V2+) neutral species, is important in understanding the electrochemical behavior of viologens and their utilization. The kinetics for the reactions of neutral methyl viologen (V0) in the presence of H+ (from HCl), CH3COOH (pKa=4.75), ClCH2CH2COOH (pKa=4.00), HCOOH (pKa=3.75) in aqueous media was examined by cyclic voltammetry according to the EECi mechanism. To avoid the electrodeposition of V0, we used a 9:1 (v/v%) H2O+DMF mixture as the solvent medium. To evaluate the rate constants for the chemical reaction followed by the second electron transfer step of V2+, the ratio of the anodic and cathodic peak current (Ipa2/Ipc2) corresponding to V0–e−⇄V+ was plotted against log τ, where τ is the time between E1/2 and the switching potential, at various scan rates of 0.02–3.5 V s−1. The chemical reaction was found to be a parallel reaction consisting of H+-catalyzed and general-acid (HA) catalyzed reactions. The second-order rate constants are determined as kH+=3.5×103 M−1 s−1, kCH3COOH=5.7 M−1 s−1, kHCOOH=4.6×101 M−1 s−1, and kClCH2CH2COOH=3.2×101 M−1 s−1 using the Nicholson–Shain method and kH2O was estimated as <3×10−6 M−1 s−1. The CVs were digitally simulated under the assumption of a two-step reaction of V0 following the two-step electrode reactions of V2+ to V0. The simulated CVs show good agreement with those obtained experimentally, when the first-step reaction of V0 is a relatively fast reversible reaction and the second-step reaction is a slow irreversible one. Based on these results, we propose that V0 is in pseudo-equilibrium with H+ or HA to produce VH+ which undergoes a reaction with H2O.