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 (pK_a = 4.75), ClCH₂CH₂COOH (pK_a = 4.00), HCOOH (pK_a = 3.75) in aqueous media was examined by cyclic voltammetry according to the EEC_i 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 (I_pa2/I_pc2) corresponding to V⁰-e^-⇄V^·+ was plotted against log τ, where τ is the time between E_1/2 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 k_H+ = 3.5 × 10³ M^-1 s^-1, k_CH3COOH = 5.7 M^-1 s^-1, k_HCOOH = 4.6 × 10¹ M^-1 s^-1, and k_ClCH2CH2COOH = 3.2 × 10¹ M^-1 s^-1 using the Nicholson-Shain method and k_H2O 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.