Rotating disk electrode concentration profile

Volt mmetiy. Diffusional effects, as embodied in equation 1, can be avoided by simply stirring the solution or rotating the electrode, eg, using the rotating disk electrode (RDE) at high rpm (3,7). The resultant concentration profiles then appear as shown in Figure 5. A time-independent Nernst diffusion layer having a thickness dictated by the laws of hydrodynamics is estabUshed. For the RDE,... 

Fig. 3. Steady state concentration profiles of catalyst and substrate species in the film and diffusion layer for for various cases of redox catalysis at polymer-modified electrodes. Explanation of layers see bottom case (S + E) f film d diffusion layer b bulk solution i, limiting current at the rotating disk electrode other symbols have the same meaning as in Fig. 2 (from ref.

FIGURE 1.10. Rotating disk electrode voltammetry. A + e B, with a concentration of A equal to C° and no B in the solution a Linearized concentration profiles —, at the plateau (vertical arrow in b), , at a less negative potential (horizontal arrow in b). b Current potential curve, c Concentrations of A and B at the electrode surface, d Logarithmic analysis of the current potential curve. 

Assume that the reaction ox + c <=> red at the planar electrode is diffusion controlled. Sketch and correlate the concentration profiles Cox =f(x), where x is the distance from the electrode surface to the bulk of the solution, with the shape of the current-potential curve for electrolysis carried out at (a) a stationary disk electrode and (b) a rotating disk electrode. Support your explanation by the equations. (Skompska)... 

Figure 17-12 (a) Rotating disk electrode. Only the polished bottom surface of the electrode, which is typically 5 mm in diameter, contacts the solution, (b) Schematic concentration profile of analyte near the surface of the rotating disk electrode when the potential is great enough to reduce the concentration of analyte to 0 at the electrode surface. 

The study of rotating disk electrode behavior provides a unique opportunity to develop a model that predicts the effect of diffusion and convection on the current. This is one of the few convective systems that have simple hydrodynamic equations that may be combined with the diffusion model developed herein to produce meaningful results. The effect of diffusion is modeled exactly as it has been done previously. The effect of convection is treated by integrating an approximate velocity equation to determine the extent of convective flow during a given At interval. Matter, then, is simply transferred from volume element to volume element in accord with this result to simulate convection. The whole process repeated results in a steady-state concentration profile and a steady-state representation of the current (the Levich equation). 

The third example is the reflection measurement at a rotating disk electrode (RDE). Scherson and his coworkers have developed near-normal incidence UV-visible reflection-absorption spectroscopy at RDEs [50-52]. Both (AR/R)dc and (AR/R)er have been measured under hydrodynamic conditions. The use of an RDE enables them to quantitatively control the diffusion layer concentration profile of the solution phase species, especially the species generated electro-... 

Analytical solutions of (49) have been presented for special configurations where the velocity profile is well established, e.g., the rotating disk electrode or laminar flow in a chaimel. An important simplification has been proposed by Levich, who recognized that electrochemical systems are typically characterized by a concentration boundary layer that is much thinner than the corresponding velocity boundary layer. Levich recognized that in such systems it is... 

In the absence of the following reaction, we think of the concentration profile for R as decreasing linearly from a value Cr(x — 0) at the surface to the point where Cr = 0 at 5, the outer boundary of the Nemst diffusion layer. The coupled reaction adds a channel for disappearance of R, so the R profile in the presence of the reaction does not extend as far into the solution as 5. Thus, the added reaction steepens the profile and augments mass transfer away from the electrode surface. For steady-state behavior, such as at a rotating disk, we assume the rate at which R disappears from the surface to be the rate of diffusion in the absence of the reaction [(mRCR(x = 0) see (1.4.8)] plus an increment proportional to the rate of reaction [/jikCj (x = 0)]. Since the rate of formation of R, given by (1.4.6), equals its total rate of disappearance, we have... 

Concentration profiles in laminar fiow regime (b) concentration profiles of the electroactive species at different rotational speed stagnant is intended with respect to the rotating electrode disk... 

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