Polarization resistance measurements, complications

E. Complications with Polarization Resistance Measurement by the Linear Polarization Method... 

Error-producing complications related to the polarization resistance method and possible remedies are reported in the literature (14,15,22-27). The most common errors involve (1) invalidation of the results through oxidation of some other electroactive species besides the corroding metal in question, (2) a change in the open-circuit or corrosion potential during the time taken to perform the measurement, (3) use of AE that is too large, invalidating the assumption of a linear relationship between im and E required by Eq. (2) (i.e., AE/p < 0.1), (4) too... 

As far as conductometry is concerned, there remain a few complications caused by processes at the electrodes, e.g., electrolysis above the decomposition voltage of the electrolyte with some liberation of decomposition products at the electrode, or apparent capacitance and resistance effects as a consequence of polarization of the electrode and exchange of electrons at its surface. In order to reduce these complications the following measures are taken ... 

Variations of resistance with frequency can also be caused by electrode polarization. A conductance cell can be represented in a simplified way as resistance and capacitance in series, the latter being the double layer capacitance at the electrodes. Only if this capacitance is sufficiently large will the measured resistance be independent of frequency. To accomplish this, electrodes are often covered with platinum black 2>. This is generally unsuitable in nonaqueous solvent studies because of possible catalysis of chemical reactions and because of adsorption problems encountered with dilute solutions required for useful data. The equivalent circuit for a conductance cell is also complicated by impedances due to faradaic processes and the geometric capacity of the cell 2>3( . 

It was not until alternating voltages (AC) were applied to the electrodes that the polarization was eliminated and consistent results were obtained. Theoretically, it is the impedance not the resistance of the electrode system that should be measured. When an AC voltage is applied across a conductor, there can be an inductive current controlled by the inductance of the conductor, a capacity current controlled by its capacity and a resistive current controlled by the conductor s resistance. The situation is further complicated by the fact that there is a phase difference between the three currents, the inductive current leads the resistive current and the capacity current lags behind the resistive current and thus the measurement of the electrode impedance would appear to be rather difficult. In practice, however, due to the geometry of the electrode system, the inductive and capacity components of the current are made extremely small compared with the resistive current and thus the measurement of resistance can be made using the total current that is in phase with the applied voltage. 

Whenever electricity flows across a circuit, there is a resistance to flow encountered by the electrons. For pacing systems, the resistance is determined by the complex interaction of multiple components. Because some of these components are also characterized by the ability to retain charge or capacitance, the term impedance is preferred. At the time of lead implantation, it is this complicated series of resistance and capacitance factors that are measured and are referred to as system impedance. For a pacing circuit, the system impedance has five basic components a low, purely resistive conductor impedance, a high cathode electrode impedance, complex polarization effects at the electrode-tissue interface, a low tissue impedance, and the anode electrode impedance (Fig. 1.3). 

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