Potential sweep experiments with ultramicroelectrodes

In the above discussion, semi-infinite linear diffusion has been assumed. In practical work, this means that the diameter of the working electrode, d, is much larger than the thickness of the diffusion layer, S. The question that then arises is what happens when the diameter of the electrode is allowed to decrease and finally reaches the thickness of the diffusion layer  

Obviously, as illustrated in Fig. 6.18, the deviations from linear diffusion always observed at the edges of the working electrode become more and more significant until, in the limit, the diffusion is hemi-spherical as shown in Fig. 6.18c. 

An important feature of the voltammogram shown in Fig. 6.19d is the absence of a peak. Instead, the current reaches a plateau indicating that a steady state has been obtained. In this case, the diffusion of R away from the electrode is so effective that reverse current for the re-oxidation of R to O cannot be observed, even when R is non-reactive. 

The limiting current, zfAj, observed at the plateau (Fig. 6.19d) is given by Equation 6.50, where rQ is the radius of the conducting part of the electrode  

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Already in the mid-1960s, there was rich potential of applying such experiments to the determination of concentrations but even more to the elucidation of reaction mechanisms and kinetics coupled to electron transfer at an electrode was recognized. Today the resulting Knear sweep or cyclic voltammetries are employed as simple, flexible routine techniques in particular as sophisticated means to solve chemical and mechanistic problems. The combination with computer control, ultramicroelectrodes, and digital simulation has further contributed to their success. 

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