Cyclic Voltammetry in the Absence of Excess Supporting Electrolyte Offers Extra Kinetic and Mechanistic Insights: Comproportionation of Anthraquinone and the Anthraquinone Dianion in Acetonitrile

Abstract

Voltammetry is normally performed in the presence of excess supporting electrolyte, which is deliberately added to screen electric fields and to compensate solution resistance. Voltammetric theory is then a simple "diffusion-only" problem. Several situations exist, however, in which kinetic and mechanistic information can be unobtainable or ambiguous under diffusion-only conditions. For example, stepwise twoelectron reactions are commonly encountered in electrochemistry [Eqs.(1)- (3)]. [1,2]

Investigating Comproportionation in Multielectron Transfers via UV–Visible Spectroelectrochemistry: The Electroreduction of Anthraquinone-2-sulfonate in Aqueous Media

“…In all cases, a satisfactory agreement is reached between the experimental and simulated (red line) curves with the values indicated in Table . The values obtained for k comp point out that the comproportionation of AQ2S – and AQ2S 3– is significant and fast in all the aqueous media investigated, as also reported in acetonitrile . Moreover, the COMP kinetics is found to be dependent on the cationic species in solution, following the same order as that found for the ion pairing with the electrolyte cation, which would enhance the second-order kinetics between like-charged species (AQ2S – and AQ2S 3– ) somehow in analogy to a kinetic salt effect.…”

“…Alternatively, in this work the use of UV–vis spectroelectrochemistry (SEC) is investigated. SEC can provide in situ and real-time direct evidence of the formation or consumption of chemical species and so assist the unambiguous elucidation of intricate electrochemical mechanisms. − Within the present context, as the concentration profiles of the redox forms are affected by the DISP-COMP reactions (see below and refs and ), the species-sensitive absorbance signal can enable us to reveal and characterize COMP-DISP processes under diffusion-only conditions and with direct access to the species concentrations, which can make the modeling, interpretation of results, and data analysis more simple, conclusive, and accurate. This is examined theoretically in the first part of the manuscript for a generic two-electron transfer under different mass transport conditions and SEC arrangements; thus, the parallel and normal light beam modes are examined (see Figure S1), where the former is generally more advantageous since it enables the use of opaque electrodes as well as higher absorbance signals.…”

“…Within the present context, it is well-known that the voltammetry of diffusion-only multielectron transfers is not or scarcely , sensitive to COMP-DISP processes since the “average oxidation state” in solution is not or slightly altered . In order to overcome this limitation, voltammetry in weakly supported media has been applied on the basis of the different mass transport by migration of the distinct redox forms in eqs and as long as they are differently charged. Alternatively, in this work the use of UV–vis spectroelectrochemistry (SEC) is investigated.…”

New Chemical Insights Using Weakly Supported Voltammetry: Ion Pairing in the EC2 Reduction of 2,6‐Diphenylpyrylium in Acetonitrile

“…This eliminates migration effects and "diffusion-only" voltammetry is observed. However, very recently it has been suggested [1] that performing cyclic voltammetry in the presence of varying concentrations of the supporting electrolyte, from a large excess to very little, can reveal quantitative mechanistic information not observable under diffusion-only conditions. This communication builds on the empirical and/or qualitative work done early by others, [2][3][4][5] and reports new insights arising from such experiments.…”

“…[1,9,10] The model makes use of the Nernst-Planck-Poisson (NPP) system of equations. The Nernst-Planck equation describes the mass transport of the species due to diffusion and migration, the latter of which arises when the supporting electrolyte is not present in large excess.…”