Detection and measurement of impedance

In this chapter, after a description of specialized methods for impedance measurement, the procedure for deducing the kinetics and mechanism from the impedance spectra is demonstrated. 

The principles of a.c. circuits necessary for the comprehension of some of the ideas and concepts presented here are given in Appendix 2. The impedance is the proportionality factor between potential and current if these have different phases then we can divide the impedance into a resistive part, R, where the voltage and current are in phase, and a reactive part, Xc = l (oCy where the phase difference between current and voltage is 90°. As shown in Appendix 2, it is often easier for posterior calculation and analysis to display the impedance vectorially in complex-plane diagrams. 

There are three types of technique for the detection and measurement of impedance6,13. 

These bridges, as shown in Fig. 11.1, use the principle of balance between the electrochemical cell under study and a variable impedance, Zs, which in research on electrode processes normally consists of a resistance Rs in series with a capacitance Cs. Given that 

The capacitance is due only to the working electrode, whilst the resistance includes the resistive components of the electrode process, of the solution, etc. In some cases a combination of resistance and capacitance in parallel has also been used. In these conditions the analysis is more easily carried out in terms of admittance Y = 1/Z see Appendix 2. 

---