Rotating-disc electrodes. ECE and DISP1 processes

Abstract

Theory is presented for the shape of current-voltage curves measured at a rotating-disc electrode for the cases where the substrate reacts via either an ECE or a DISPl process in which the kinetic step is sufficiently fast to allow the use of the reaction-layer concept in the theoretical treatment. When the electrode potential for the electrode reaction can be found under conditions where there are no complications of coupled kinetics, analysis of the shift in half-wave potential with disc rotation speed allows the deduction of the rate constant for the kinetics of the homogeneous chemical process involved in each mechanism. If an independent measurement of this rate constant is available (e.g. from spectroelectrochemistry) then discrimination between ECE and DISPl pathways is possible. Experiments involving the reduction of fluorescein in aqueous carbonate/bicarbonate buffers were found to be in good agreement with theory. Furthermore, comparison of the kinetic data with transient electrochemical e.s.r. data revealed the reaction mechanism to be DISPl, in agreement with previous observations.

Voltammetry in the absence of excess supporting electrolyte – ECE-DISP1 reactions: The electrochemical reduction of 2-nitrobromobenzene in acetonitrile solvent

“…Using classical voltammetric techniques, the DISP2 mechanism can be readily assigned [6]; however, the voltammetric discrepancy between ECE, DISP1 and ECEE is negligible when studied using cyclic voltammetry under linear diffusion conditions, and the unambiguous resolution of these mechanisms has been a 1572-6657/$ -see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jelechem.2011.04.017 long standing problem [1][2][3][4] in electrochemistry, leading researchers to often turn to non-voltammetric techniques, such as ESR [3], to complement the voltammetry.…”

“…In particular, the resolution of ECE and DISP1 mechanisms is most difficult [7][8][9]4,10]. A range of voltammetric strategies have been used to make this distinction.…”

“…The reduction of aryl halides such as 2-nitrobromobenzene in acetonitrile has been widely studied and the difficulty in distinguishing between ECE and DISP1 processes, both generally and in this specific case, noted [1][2][3][4]. In particular a large variety of voltammetric techniques have been applied to this topic with many authors resorting to spectro-electrochemical methods to support their claims.…”

A Consideration of the Application of Koutecký-Levich Plots in the Diagnoses of Charge-Transfer Mechanisms at Rotated Disk Electrodes

“…Subsequent work by Amatore and coworkers [22,23] and Compton et al [24] further developed the theoretical model for the ECE mechanism. In a cathodic ECE mechanism, the reactant (A) is reduced to the electroinactive species B via electron-transfer.…”

Electrochemical reactions where the variation of supporting electrolyte concentration is mechanistically revealing: ECE-DISP1 processes in which the chemical step is an isomerisation

“…The origin of the second electron transfer can be either heterogeneous (step 7, ECE) or a homogeneous disproportionation (step 6, DISP1). The difficulty in using voltammetry to differentiate between these mechanistic limits is well known, and discussed elsewhere [17][18][19][20]. The weakly supported study, while concluding, on the basis of other factors, a DISP1 mechanism, showed that in this case migration effects could not be used as a means to differentiate the two cases.…”