Electrode polarization immittance

The term electrode polarization immittance is sometimes related to the total immittanee of the equivalent circuit of Figure 7.20, but it is sometimes useful to exclude the series resistance Rsoi if it has the character of being an access resistance to the electrode processes. Without electrochemical reaction, measured currents are due to double layer components and sorption. The term electrode polarization immittance should more often either be avoided or defined. The processes involved are of a very different nature, and in a measuring set-up different variables must be controlled dependent on what effects are to be studied. The electrode immittance of an electronic conductor in contact with 0.9% NaCl is of special interest to us. Data for such interfaces are found in Section 7.4. 

With zero DC current in the electrode wire, the electrode is at equilibrium. A polarization still existing must therefore be endogenic. Even with zero current flow in the electrode, wire local currents may flow caused by inhomogeneities of the metal surface or contact with a nonisoelectric tissue area. Endogenic polarization is important and manifests itself by a polarization immittance measured with low AC currents so that the system is linear (see Section 8.4). 

Tlie series resistance in these models may be regarded as an access resistance to the electrode interface, dependent on electrolyte conductivity and geometry, but not linked to the polarization processes. As such it belongs to the chapter on contact media, and should be subtracted from measured electrode impedance when analyzing polarization immittance. 

Figure 7.17 showed that platinum black is a preferred electrode metal element with respect to low polarization impedance. However, it may easily be polarized by a DC eurrent, and for such cases an Ag/AgCl electrode is usually preferred. Typieal immittance plots for one pregelled Ag/AgCl ECG electrode obtained from two electrodes in front to front contact (no tissue in between) is shown in Figure 7.17. The data were obtained with a constant amplitude current of 1 pA/cm rms, corresponding to a single electrode maximum (1 mHz) voltage of 2.5 mV rms.

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