Unique information from EHD impedance

Use of the chronoamperometric response following a step change in rotation rate, as a general method for determination of the diffusion 

Albery et al. [15] derived an accurate solution and then gave a particularly simple method, accurate to a few percent, for evaluation of D from experimental data if the rotation rate is increased in a step to a final value of WflHz, then the time tq for the current to attain a fraction q of the final, steady-state value is given by the following, where the values of the proportionality constant Bq are tabulated below  

Blaedel and Engstrom [48] noted that for a quasi-reversible process the current could be simply expressed in terms of the rate constant and mass-transport coefficient. Application of a square wave step in the rotation rate of a RDE (i.e., PRV, see Section 10.4.1.3) resulted in modulation of the diffusion-limited current and hence modulation of the mass-transfer coefficient. By solving the appropriate quadratic equation it was possible to derive a value for the heterogeneous rate constant for the electrochemical cathodic, kf, or anodic, kb, process of interest. Values for the standard heterogeneous rate constant and transfer coefficient were subsequently 

As discussed in Section 10.3.6.1, the characteristic frequency, cap, which determines the roll-off frequency for the current response, increases with 

On a RDE, if the electrode is not uniformly active, then the individual sites can behave as if they were isolated UMEs embedded in the wall of a channel. Except, in this case, the solution velocity parallel to the wall varies with radial position of the electrode. For an electrode with an array of active sites, sufficiently separated, two relaxations are expected, as illustrated in Fig. 10.18. At sufficiently low modulation frequencies, the distance scale, SHm, of the perturbation of the hydrodynamic boundary layer is much larger than the size and spacing of the sites, so the electrode behaves as a uniform surface. A deviation is observed when hm becomes comparable to the spacing between the sites. Under these conditions, the surface responds to the perturbation as a set of isolated electrodes. 

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