Steady voltammogram

Similarly to the response at hydrodynamic electrodes, linear and cyclic potential sweeps for simple electrode reactions will yield steady-state voltammograms with forward and reverse scans retracing one another, provided the scan rate is slow enough to maintain the steady state [28, 35, 36, 37 and 38]. The limiting current will be detemiined by the slowest step in the overall process, but if the kinetics are fast, then the current will be under diffusion control and hence obey the above equation for a disc. The slope of the wave in the absence of IR drop will, once again, depend on the degree of reversibility of the electrode process. 

LCEC is a special case of hydrodynamic chronoamperometry (measuring current as a function of time at a fixed electrode potential in a flowing or stirred solution). In order to fully understand the operation of electrochemical detectors, it is necessary to also appreciate hydrodynamic voltammetry. Hydrodynamic voltammetry, from which amperometry is derived, is a steady-state technique in which the electrode potential is scanned while the solution is stirred and the current is plotted as a function of the potential. Idealized hydrodynamic voltammograms (HDVs) for the case of electrolyte solution (mobile phase) alone and with an oxidizable species added are shown in Fig. 9. The HDV of a compound begins at a potential where the compound is not electroactive and therefore no faradaic current occurs, goes through a region... 

FIG. 19 Dependence of the half-wave potentials for Fc (curve 1) and ZnPor (curve 2) oxidation in benzene on CIO7 concentration in the aqueous phase. In these measurements, half-wave potentials were extracted from reversible steady-state voltammograms obtained at a 25 pm diameter Pt UME. The benzene phase contained 0.25 M tetra-w-hexylammonium perchlorate (THAP) and either 5 mM Fc or 1 mM ZnPor. All potentials were measured with respect to an Ag/AgCl reference electrode in the aqueous phase. (Reprinted from Ref. 48. Copyright 1996 American Chemical Society.)... 

The shape of steady-state voltammograms depends strongly on the geometry of the microhole [13,14], Wilke and Zerihun presented a model to describe diffusion-controlled IT through a microhole [15], In that model, a cylindrical microhole is assumed to be filled with the organic phase, so that a planar liquid-liquid interface is located at the aqueous phase side of the membrane. Assuming that the diffusion is linear inside the cylindrical pore and spherical outside [Fig. 2(a)], the expression for the steady-state IT voltammo-gram is... 

The mathematical formulations of the diffusion problems for a micropippette and metal microdisk electrodes are quite similar when the CT process is governed by essentially spherical diffusion in the outer solution. The voltammograms in this case follow the well-known equation of the reversible steady-state wave [Eq. (2)]. However, the peakshaped, non-steady-state voltammograms are obtained when the overall CT rate is controlled by linear diffusion inside the pipette (Fig. 4) [3]. 

Quinn et al. studied ET at micro-ITIES supported by micropipettes or microholes [16]. The studied systems involved ferri/ferrocyanide redox couple in aqueous phase and ferrocene, dimethylferrocene, or TCNQ in either DCE or o-nitrophenyl octyl ether. Sigmoidal, steady-state voltammograms were obtained for ET at the water-DCE interface supported at a micropipette. The semilogarithmic plot of E versus log[(/(j — /)//] was... 

This equation describes the cathodic current-potential curve (polarization curve or voltammogram) at steady state when the rate of the process is simultaneously controlled by the rate of the transport and of the electrode reaction. This equation leads to the following conclusions ... 

Figure 6 Steady state rotating ring-disk voltammograms of (A) compound (42) (B) compound (43) and (C) a Ru-bridged polymer of (43) each adsorbed to a graphite working electrode. Disk current shows reduction of 02 while ring current reveals the presence of H202 simultaneously reoxidised at the ring anode poised at +1.0 V (reproduced with permission of the American Chemical Society from Acc. Chem. Res., 1997, 30, 437-444).

Figure 3.98 Comparison of a reversible conventional cyclic voltammogram (linear diffusion) and reversible steady-state voltammogram obtained at a single microelectrode disc where mass transport is solely by radial diffusion. Current axis not drawn to scale. From A.M. Bond and H.A.O. Hill, Metal Inns in Biological Systems, 27 (1991) 431. Reprinted by courtesy of Marcel...

Fig. 18b.4. Steady-state voltammograms when initially (a) O is present and (b) both O and R are present.

Explain the conditions under which a steady-state voltammogram is obtained. 

After the electrode reaction starts at a potential close to E°, the concentrations of both O and R in a thin layer of solution next to the electrode become different from those in the bulk, cQ and cR. This layer is known as the diffusion layer. Beyond the diffusion layer, the solution is maintained uniform by natural or forced convection. When the reaction continues, the diffusion layer s thickness, /, increases with time until it reaches a steady-state value. This behaviour is also known as the relaxation process and accounts for many features of a voltammogram. Besides the electrode potential, equations (A.3) and (A.4) show that the electrode current output is proportional to the concentration gradient dcourfa /dx or dcRrface/dx. If the concentration distribution in the diffusion layer is almost linear, which is true under a steady state, these gradients can be qualitatively approximated by equation (A.5). 

A CV voltammogram can be recorded under either a dynamic or a steady state depending on the electrode design and solution convection mode. In a stationary solution with a conventional disk electrode, if the scan rate is sufficiently high to ensure a non-steady state, the current will respond differently to the forward and backward potential scan. Figure 63 shows a typical CV for a reversible reduction.1... 

In most of the works referred to in this section, catalytic activities were observed through cyclic voltammograms, which exhibit only primary trends for electrocatalysts. The observation of steady current at certain potentials is quite important because the degree of catalytic activity under the steady-state condition must be known in order to develop... 

Figure 2. Steady-state cyclic voltammograms of Cu electrodes, both bare and coated with thick films (approximately 15 pm) of BTA, PVI-1, or UDI in 0.1 M HCIO, in air, v = 50 mV/s, w = 15 Hz, 25 "C. ...

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