Electrode potentials, ring

The selective oxidation of the activated aromatic ring, substituted with electron-donating hydroxy or methoxy groups, can be perfomed at relatively low electrode potential (Ep = 0.3-1.2 V vs SCE) and ring closure is the result of the intramolecular nucleophilic attack of an amino group on the oxidized aromatic ring. 

Carbon-oxygen bonds adjacent to an aromatic ring or an alkene function can be cleaved by reduction at very negative potentials [1]. The process is often followed by reduction of the activating group as in 1. In these processes, the reduction potential of the activating group controls the electrode potential required. Thus an... 

The disk electrode potential is controlled to bring about the reaction indicated for A. The ring is held at a potential to cause B to react to C. Then the fraction of B obtained on the ring is N. Knowledge of this helps in understanding the formation of C, hence the mechanism of the overall reaction. There are many reactions of this type (called E. C.E. reactions). Some proteins undergo bromination in this sequence, the bromina-tion step reaction being at B. 

Figure 2. Ring current and disk current as a function of potential of the ZnO disk electrode. Potential of Pt ring 0 V vs. SCE rotation rate 1000 rpm solution 10 2MKIin0.5MK ,SOl.

The ion trap generally consists of three electrodes, one ring electrode with a hyperbolic inner surface and two hyperbolic endcap electrodes at either end (a cross section of an ion trap is found in Figure 1.11). The ring electrode is operated with a sinusoidal radio frequency field while the endcap electrodes are operated in one of three modes. The endcap may be operated at ground potential, or with either a DC or an AC voltage. 

Values of A are approximate B and B are functions of r C is a function of radius and ring potential C is a function of electrode length and detector electrode potential. 

In the non-steady state, changes of stoichiometry in the bulk or at the oxide surface can be detected by comparison of transient total and partial ionic currents [32], Because of the stability of the surface charge at oxide electrodes at a given pH, oxidation of oxide surface cations under applied potential would produce simultaneous injection of protons into the solution or uptake of hydroxide ions by the surface, resulting in ionic transient currents [10]. It has also been observed that, after the applied potential is removed from the oxide electrode, the surface composition equilibrates slowly with the electrolyte, and proton (or hydroxide ion) fluxes across the Helmholtz layer can be detected with the rotating ring disk electrode in the potentiometric-pH mode [47]. This pseudo-capacitive process would also result in a drift of the electrode potential, but its interpretation may be difficult if the relative relaxation of the potential distribution in the oxide space charge and across the Helmholtz double layer is not known [48]. 

The direct four-electron pathway involves no hydrogen peroxide formation in the solution. This fact, however, does not preclude the participation of an adsorbed peroxide intermediate in the course of the reaction. The distinction between both reaction pathways is usually investigated by the rotating ring-disc electrode technique [55]. From the rotation speed and potential dependence of the disc electrode to ring electrode current ratio, it is possible to determine the relative contribution of each reaction pathway to the overall reaction [56]. 

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