Impedance Format

The impedance can be expressed as the complex ratio of potential and current contributions, i.e., 

As discussed in Section 4.1.2, the impedance for a series arrangement of passive elements is additive. The impedance for the parallel arrangement of impedances Zi and Zi is given by equation (4.24). 

For a resistor Re and capacitor C in series, shown in Table 16.1(a), the impedance is given by 

The real part of the impedance is independent of frequency, and the imaginary part of the impedance tends to — cx3 as frequency tends toward zero according to 1/ct . In fact. 

The system comprising the resistor Re and capacitor C in series provides an example of a class of systems for which, at the zero-frequency or dc limit, current cannot pass. Such systems are considered to have a blocking or ideally polarizable electrode. Depending on the specific conditions, batteries, liquid mercury electrodes, semiconductor devices, passive electrodes, and electroactive polymers provide examples of systems that exhibit such blocking behavior. 

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Fig. 5.26. Metabolism of the piperidine ring according to the mechanism in Fig. 5.23. Diphen-idol (5.97) and DN-9893 (5.73) yield both amino acid and lactam metabolites [177], Phencyclidine (5.98) yields only the amino acid derivative steric hindrance at the N-atom appears to impede formation of the lactam metabolite [190].

Another problem in mimicking the oxygen uptake by heme proteins is the preference of iron(II) for forming low-spin, six-coordinate Fe complexes. Nitrogen donor ligands such as pyridines and imidazoles normally coordinate readily to the iron to form complexes of the general type PFe B2, as discussed above in Section 4.2. While formation of these six-coordinate Fe complexes impedes irreversible oxidation to the /x-oxo dimer, it also impedes formation of the... 

Impedance data are presented in different formats to emphasize specific classes of behavior. The impedance format emphasizes the values at low frequency, which t5rpically are of greatest importance for electrochemical systems that are influenced by mass transfer and reaction kinetics. The admittance format, which emphasizes the capacitive behavior at high frequencies, is often employed for solid-state systems. The complex capacity format is used for dielectric systems in which the capacity is often the feature of greatest interest. 

The admittance format is not particularly well suited for analysis of electrochemical and other systems for which identification of Faradaic processes parallel to the capacitance represents the aim of the impedance experiments. When plotted in impedance format, the characteristic time constant is that corresponding to the Faradaic reaction. When plotted in admittance format, the characteristic time constant is that corresponding to the electrol5rte resistance, and that is obtained only approximately when Faradaic reactions are present. 

Develop plots similar to Figures 17.2-17.6 for the impedance data of the circuits given in Figure 17.1(a) with data presented in admittance rather than impedance format. 

Figure 20.4 Comparison in impedance format of the model presented in Figure 20.1 to impedance data obtained for reduction of ferricyanide on a Pt rotating disk electrode a) real and b) imaginary.

One should note that the real and the imaginary parts of the Warburg impedance in Eq. (16.22) depend on frequency in the same way. Therefore the phase shift generated by the Warburg impedance is independent of frequency. Plotted in the complex-plane impedance format, this leads to a straight line with a slope of unity, as shown in Figure 16.6. 

In consequence, it is likely that CCCP acts as an uncoupler by avoiding protonation of the cytochrome and thus impeding formation of a proton gradient across the thylakoid membrane. In contrast, ammonium would have a different mechanism of action, presumably exerting its uncoupling effect on the FoF- -ATPase itself. 

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