Reversible reaction steady-state voltammetry

FIGURE 1.16 Scheme of nanogap-based SECM measurements of a fast electron-transfer reaction at a macroscopic snbstrate in the (a) steady-state feedback mode and in quasi-steady-state (b) feedback and (c) SG/TC modes, (d) Quasi-steady-state i-j- — Eg voltammograms of TCNQ in acetonitrile (solid cnrves). The tip was held at —0.235 or 0 V versus an Ag quasireference electrode for feedback or SG/TC modes, respectively. Snbstrate potential was cycled at 50 mV/s. Closed circles and dotted lines are theoretical cnrves for quasi-reversible (k = 7 cm/s and a = 0.5) and reversible snbstrate reactions, respectively, with EP = -88 mV. The inset shows a reversible voltammogram with a peak separation of 61 mV simultaneously measured at the substrate. (Reprinted with permission from Nioradze, N. et al.. Quasi-steady-state voltammetry of rapid electron transfer reactions at the macroscopic substrate of the scanning electrochemical microscope. Anal. Chem., Vol. 83, 2011 pp. 828-835. Copyright 2011, American Chemical Society.)... 

This section deals with the solution corresponding to an EC mechanism (see reaction scheme 4.IVc) in Reverse Pulse Voltammetry technique under conditions of kinetic steady state (i.e., the perturbation of the chemical equilibrium is independent of time see Sect. 3.4.3). In this technique, the product is electrogenerated under diffusion-limited conditions in the first period (0 < t < ) and then exam-... 

In the type of linear-sweep voltammetry discussed thus far, the potential is changed slowly enough and mass transfer is rapid enough that a steady state is reached at the electrode surface. Hence, the mass transport rate of analyte A to the electrode just balances its reduction rate at the electrode. Likewise, the mass transport of P away from the electrode is just equal to its production rate at the electrode surface. There is another type of linear-sweep voltammetry in which fast scan rates (1 V/s or greater) are used with unstirred solutions. In this type of voltammetry, a peak-shaped current-time signal is obtained because of depletion of the analyte in the solution near the electrode. Cyclic voltammetry (see Section 23D) is an example of a process in which forward and reverse linear scans are applied. With cyclic voltammetry, products formed on the forward scan can be detected on the reverse scan if they have not moved away from the electrode or been altered by a chemical reaction. 

Figure 4-3. Electrochemical techniques and the redox-linked chemistries of an enzyme film on an electrode. Cyclic voltammetry provides an intuitive map of enzyme activities. A. The non-turnover signal at low scan rates (solid lines) provides thermodynamic information, while raising the scan rate leads to a peak separation (broken lines) the analysis of which gives the rate of interfacial electron exchange and additional information on how this is coupled to chemical reactions. B. Catalysis leads to a continual flow of electrons that amphfles the response and correlates activity with driving force under steady-state conditions here the catalytic current reports on the reduction of an enzyme substrate (sohd hne). Chronoamperometry ahows deconvolution of the potenhal and hme domains here an oxidoreductase is reversibly inactivated by apphcation of the most positive potential, an example is NiFe]-hydrogenase, and inhibition by agent X is shown to be essentially instantaneous.

The plateau current of a simple reversible wave is controlled by mass transfer and can be used to determine any single system parameter that affects the limiting flux of electroreactant at the electrode surface. For waves based on either the sampling of early transients or steady-state currents, the accessible parameters are the fi-value of the electrode reaction, the area of the electrode, and the diffusion coefficient and bulk concentration of the electroactive species. Certainly the most common application is to employ wave heights to determine concentrations, typically either by calibration or standard addition. The analytical application of sampled-current voltammetry is discussed more fully in Sections 7.1.3 and 7.3.6. 

In this section, we will treat the one-step, one-electron reaction O + R using the general (quasireversible) i-E characteristic. In contrast with the reversible cases just examined, the interfacial electron-transfer kinetics in the systems considered here are not so fast as to be transparent. Thus kinetic parameters such as kf, and a influence the responses to potential steps and, as a consequence, can often be evaluated from those responses. The focus in this section is on ways to determine such kinetic information from step experiments, including sampled-current voltammetry. As in the treatment of reversible cases, the discussion will be developed first for early transients, then it will be redeveloped for the steady-state. 

Cyclic voltammetry is a method frequently used to measure 7s,i ni. Mediated bioelectrocatalysis yields cyclic voltammograms (CVs) of different shapes as illustrated in Fig. 2, depending on the measuring conditions [11]. Curve (a) is the wave for a reversible electrode reaction of an Mox/Mred redox couple. Bioelectrocatalysis mediated with the Mqx/ Mred redox couple produces a sigmoidal catalytic wave as curve (c) under the conditions [Mred] - M and [S] Ks. When [Mred] is increased to higher concentrations, an anodic peak of the diffusion current of Mred rnay be overlapped on the catalytic current as depicted by curve (d) the current, however, becomes steady state after appropriate periods... 

Very promising is the use of microelectrodes in pulse voltammetry. The theory describing the reversible electrode reaction in RPV at a disc microelectrode was derived and verified in [77]. Due to the steady-state... 

The specific geometry of the micropipet produces an asymmetric diffusion field, i.e., ion transfer from inside the pipet to ontside is confined to a linear diffusion field which can produce a peak-shaped wave in cyclic voltammetry. In the reverse process, the diffusion field is hemispherical, which produces a steady-state wave in cyclic voltammetry (see the inset in Figure 17.3.10). This unique characteristic of micropipets has been used to identify species responsible for limiting the potential window as well as in the development of mechanisms for FIT reactions (47, 65). Dual-pipets have been used in the generation/ collection mode for ionic processes, and these have employed to study complicated ET-IT and IT-IT coupling reactions (96). 

Noticeably, the advantages of a pair of the steady-state voltammograms based on forward and reverse processes of a nearly reversible reaction were originally demonstrated by voltammetry of ion transfer at nanopipette-supported ITIES [100,110],... 

Another issue complicating kinetic analysis of rapid CT reactions is a weak dependence of the shape of an almost reversible steady-state voltammogram on kinetic parameters and, consequently, the lack of the unique fit between the theoretical and experimental curves. The possibility to fit the same experimental curve using different combinations of k and a leads to significant uncertainties in extracted parameter values. This problem was addressed by using common ion voltammetry (see Seetion 1.3.4). 

Fig. 4.3 Simulated CVs showing the effects of increasing the voltammetric scan rate during cyclic voltammetry using a 10- am-radius UME. The responses were simulated for a reversible (Nernstian) reaction with = 1, Dox=T>Red= 10 cmVs, Cqx= 1.0 mM, and T=25 °C. Steady-state responses are observed at u = 0.01 V/s but at 0.1 V/s and above peaks begin to appear in the CVs. Reprinted with permission from Bard, A J Faulkner, L R (2001) Electrochemical Methods Fundamentals and Applications 2nd ed. Copyright 2001 John Wiley and Sons...

When the sweep rate is very low, in the range of v = (0.1—5) mVs , measurement is conducted under quasi-steady-state conditions. The sweep rate plays no role in this case, except that it must be slow enough to ensure that the reaction is effectively at steady state along the course of the sweep. This type of measurement is widely used in corrosion and passivation studies, as we shall see, and also in the study of some fuel cell reactions in stirred solutions. Reversing the direction of the sweep should have no effect on the current-potential relationship, if the sweep is slow enough. Deviations occur sometimes as a result of slow formation and/or reduction of surface oxides or passive layers. Because the sweep rate is slow, the potential is often swept only in one direction, and the experiment is then referred to as linear sweep voltammetry (LSV). 

---