New Approach to Electrode Kinetic Measurements in Square-Wave Voltammetry: Amplitude-Based Quasireversible Maximum

Abstract

The influence of the potential pulse height of square-wave voltammetry (SWV) (i.e., the SW amplitude) is studied for a variety of quasireversible electrode mechanisms, including a simple solution-phase electrode reaction at a planar or spherical electrode, a solution phase electrode reaction coupled with a reversible follow-up chemical reaction, and a diffusionless surface confined electrode reaction. The electrode kinetics of all the electrode mechanisms depends critically on the SW amplitude, and the quasireversible kinetic region is a function of both frequency-related electrode kinetic parameters and the SW amplitude. Thus, a novel methodology for electrode kinetics measurements is proposed by altering the SW amplitude only, at a fixed frequency of the SW potential modulation, that is, at a constant scan rate of the voltammetric experiment. Electrode kinetic measurements at a constant SW frequency are of exceptional importance especially when complex electrode mechanisms are studied, which depend on several frequency-related kinetic parameters. The electrode kinetic measurements are based on a novel feature termed the "amplitude-based quasireversible maximum", manifested as a parabolic dependence of the amplitude-normalized net SW peak current versus the SW amplitude. The position of the amplitude-based quasireversible maximum depends on the standard rate constant of the electrode reaction, enabling estimation of this important kinetic parameter in a simple and fast procedure. The novel quasireversible maximum is attributed to all studied electrode mechanisms, implying that it is a general feature of most electrode mechanisms under conditions of SWV.

Measuring the Electrode Kinetics of Surface Confined Electrode Reactions at a Constant Scan Rate

“…An important advantage of the proposed methodology is that the potential separation hardly depends on the electron transfer coefficient, which enables independent estimation of the standard rate constant. However, we have to emphasize that the number of electrons (n) involved in the electrode reactions has to be known, as the overall effect of the amplitude depends on the product nE sw indeed [42][43][44].…”

“…Besides the peak potential separation, the kinetic analysis can be conducted by analyzing the net peak current (DI p ) as a function of the amplitude, based on the recently introduced "amplitude-based quasireversible maximum" [43]. The latter property is associated with a variety of relatively slow quasireversible electrode mechanisms, both of surface confined and dissolved redox couple, being manifested as a parabolic dependency of the ratio DI p /E sw versus the logarithm of the amplitude.…”

“…The latter property is associated with a variety of relatively slow quasireversible electrode mechanisms, both of surface confined and dissolved redox couple, being manifested as a parabolic dependency of the ratio DI p /E sw versus the logarithm of the amplitude. As explained in [43], the position of the maximum depends on the electrode kinetic parameter w, shifting toward higher amplitudes by decreasing w, hence enabling estimation of the standard rate constant. Figure 6 depicts well-developed amplitude-based quasireversible maxima for vitamin K2, measured for three values of the frequency.…”

“…On the contrary, we have developed recently a novel electrokinetic methodology [42,43] by utilizing the power of the amplitude of the potential modulation (E sw ), i.e., the height of SW pulses, at a constant scan rate (constant frequency). Taking into account that the amplitude determines the exact potential of the current measurements one realizes that the amplitude is an intrinsic electrokinetic parameter.…”

“…Hence, it has been theoretically predicted that kinetic measurements are possible by alteration of the SW amplitude only, while keeping constant the scan rate of the experiment, which seems to be rather unique approach in voltammetry in general. In this strategy, several experimental approaches are possible, ranging from the analysis of peak potential separation of the split net SW peak in the case of fast surface confined electrode processes [44], through a more general approach in which the peak potential separation between the forward and backward SW components is analyzed [42], up to the analysis of the net SW peak current as a function of the varying amplitude [43].…”