engleski

Theory of square-wave voltammetry of two-step electrode reaction using an inverse scan direction

In square-wave voltammetry (SWV) it is usual that only the reactant is initially present in the solution and that at the starting potential no electrode reaction occurs. However, there is a variation of SWV in which the measurement starts at the potential at which the electrode reaction is controlled by the diffusion of reactant and the scan direction is reversed comparing to the classical SWV. This inverse mode was used for the determination of kinetic parameters of electrode reactions. In the present communication it is demonstrated that the inverse SWV can be used for the identification of quasireversible two-step electrode reaction with thermodynamically unstable intermediate. It is well known that the response of this type of mechanism depends on standard rate constants and transfer coefficients of both electrons. If the second charge transfer is slower than the first one, the response may consist of a single two-electron voltammetric peak at the lowest square-wave frequency and of two peaks at the highest frequency. Nevertheless, there are only a limited number of reactions that can undergo such transformation within the available range of frequencies. It is shown here that in many of other cases the separation of the response into two peaks can be obtained by inverting the scan direction in SWV. The separation of these peaks increases with frequency. Fig. 1 Theoretical square-wave voltammograms of two-step electrode reaction . E20 – E10 = 0.1 V, COx*/Ctot* = 1, CInt* = 0, CRed* = 0, λ1 = 10, λ2 = 0.01, α1 = 0.5, α2 = 0.5 , Esw = 50 mV, ׀dE ׀ = 5 mV and Est - E10 = 0.4 V (A) and -0.4 V (B). A dimensionless net current (ΔΦ) and its forward (Φf) and backward (Φb) components are shown.

square-wave voltammetry; two-step electrode reaction

nije evidentirano