Electrochemical processes current peak

The theory for cyclic voltammetry was developed by Nicholson and Shain [80]. The mid-peak potential of the anodic and cathodic peak potentials obtained under our experimental conditions defines an electrolyte-dependent formal electrode potential for the [Fe(CN)g] /[Fe(CN)g]" couple E°, whose meaning is close to the genuine thermodynamic, electrolyte-independent, electrode potential E° [79, 80]. For electrochemically reversible systems, the value of7i° (= ( pc- - pa)/2) remains constant upon varying the potential scan rate, while the peak potential separation provides information on the number of electrons involved in the electrochemical process (Epa - pc) = 59/n mV at 298 K [79, 80]. Another interesting relationship is provided by the variation of peak current on the potential scan rate for diffusion-controlled processes, tp becomes proportional to the square root of the potential scan rate, while in the case of reactants confined to the electrode surface, ip is proportional to V [79]. 

If all A, B, and R compounds are electroactive in a suitable electrolyte, voltam-mograms of the mixture must provide peaks corresponding to their respective redox processes. In solid-state voltammetry, in which separate microparticles of each one of the electroactive compounds are mechanically transferred to the surface of an inert electrode, independent electrochemical processes must occur. Accordingly, peak currents, ip j) j =A,B,R), recorded for A-, B-, and R-centered voltammetric processes, can generally be taken as proportional to the amount of each one of the compounds deposited on the electrode [131, 243-245]. If separate peaks are recorded for A, B, and R, the respective peak currents must satisfy the relationship... 

The peak shape and peak current magnitude are strongly influenced by the chemical and electrochemical processes involved. This is true of all step-based techniques since they tend to stress the charge-transfer rate constant. Slow rates lead to lower currents and broader peaks. 

The most popular electroanalytical technique used at solid electrodes is Cyclic Voltammetry (CV). In this technique, the applied potential is linearly cycled between two potentials, one below the standard potential of the species of interest and one above it (Fig. 7.12). In one half of the cycle the oxidized form of the species is reduced in the other half, it is reoxidized to its original form. The resulting current-voltage relationship (cyclic voltammogram) has a characteristic shape that depends on the kinetics of the electrochemical process, on the coupled chemical reactions, and on diffusion. The one shown in Fig. 7.12 corresponds to the reversible reduction of a soluble redox couple taking place at an electrode modified with a thick porous layer (Hurrell and Abruna, 1988). The peak current ip is directly proportional to the concentration of the electroactive species C (mM), to the volume V (pL) of the accumulation layer, and to the sweep rate v (mVs 1). 

Figure 6.12 shows a theoretical linear potential sweep voltammogram for a planar electrode, using the data of Table 6.3. The current drops beyond the peak ip shown in Fig. 6.12 because the species getting oxidized (or reduced) is depleted, in turn because the diffusion of analyte from bulk solution has not kept apace with the electrochemical process at the electrode. 

Cyclic voltammetry was carried out in the presence of penta- and hexacyano-ferrate complexes in order to probe the homogeneity and conductivity of the TRPyPz/CuTSPc films (125), (Fig. 36). When the potentials are scanned from 0.40 to 1.2 V in the presence of [Fe (CN)6] and [Fe CN)5(NH3)] complexes, no electrochemical response was observed at their normal redox potentials (i.e., 0.42 and 0.33 V), respectively. However, a rather sharp and intense anodic peak appears at the onset of the broad oxidation wave, 0.70 V. The current intensity of this electrochemical process is proportional to the square root of the scan rate, as expected for a diffusion-controlled oxidation reaction at the modified electrode surface. The results are consistent with an electrochemical process mediated by the porphyrazine film, which act as a physical barrier for the approach of the cyanoferrate complexes from the glassy carbon electrode surface. 

Differential Pulse Voltammetry (DPV). There are two main differences between differential pulse and NPV. The waveform for DPV, Figure 10(b), involves a pulse of amplitude AEpuise like that of the normal pulse sequence but the step back down is not to the initial potential, instead it is to a specific differential that is used during the measurement. Also, there are two sampling periods for each pulse, once at the end of the potential step up, like in NPV, and an additional sampling period at the end of the step down in potential, after which the difference in the two signals is recorded hence the name DPV. This pulse sequence results in a current signal response different from that of NPV, shown in Figure 10(b). If the electrochemical process is reversible, the peak half width, A p/2, is determined by equation (9), ... 

---