Cyclic voltammetry electro-reversibility with

Reversibility. The first aspect we analyse with cyclic voltammetry is electrochemical reversibility . Table 6.3 above lists the simplest voltammetrically determined tests of reversibility. A system that fulfills each of these criteria is probably electro-reversible, while a system that does not fulfill one or more of the criteria is certainly not fully electro-reversible. The CV shown in Figure 6.13 is that of a fully electro-reversible couple in a single electron-transfer ( E ) reaction. 

Since the Co(II) to Co(III) complexes were redox active, an electrochemical method of analysis seemed viable for the quantification of the two species in the reaction. The specific electrochemical technique developed to monitor the activation reaction allowed the simultaneous quantitative measurement of (salen)Co(II) and (salen)Co(III) species in the medium. The principle of the method is based on the electro-oxidation of both species on a platinum-rotating electrode linearly polarized with respect to a standard electrode [7]. The electrochemical reactions operative with this cyclic voltammetry technique involve the single electron oxidation of each species and occur at the revolving surface of the electrode. With this salen ligand system, the Co(II) to Co(III) transformation was determined as being fully reversible, while the Co(III) to Co(IV) reaction was irreversible. 

This equation is often used to determine the formal potential of a given redox system with the help of cyclic voltammetry. However, the assumption that mid-peak potential is equal to formal potential holds only for a reversible electrode reaction. The diagnostic criteria and characteristics of cyclic voltammetric responses for solution systems undergoing reversible, quasi-reversible, or irreversible heterogeneous electron-transfer process are discussed, for example in Ref [9c]. An electro-chemically reversible process implies that the anodic to cathodic peak current ratio, lpa/- pc equal to 1 and fipc — pa is 2.218RT/nF, which at 298 K is equal to 57/n mV and is independent of the scan rate. For a diffusion-controlled reduction process, Ip should be proportional to the square root of the scan rate v, according to the Randles-Sevcik equation [10] ... 

To understand the various phenomena take place during electro-oxidation of methanol and ethanol on Pt-black surface, Verma et al. (2005e) studied half-cell using cyclic voltammetry. Fig. 8 shows the cyclic voltammograms with and without the fuel (methanol and ethanol) in 1 M KOH solution at the scan rate of 50 mV s". In the forward scan, ethanol electro-oxidation shows a prominent peak at 0.03 V as compared to electro-oxidation of methanol, which shows a broad plateau from -0.4 V to 0.6 V. It implies that Pt-black electode catalyst is more active in the case of ethanol than methanol for electro-oxidation. During reverse... 

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