The impedance of a spherical monopolar electrode

The impedance of a monopolar electrode immersed in an environmental volume conductor consists of two parts; the impedance of the active electrode-electrolyte interface, and the resistance of the environmental conductor. Two studies were carried out to quantitate these components. First, impedance-frequency data were collected for five spherical stainless-steel electrodes (ranging from 0.473 to 1.11 cm in diameter) immersed in 0.9% saline (p = 70 omega-cm). Impedance measurements were made from 100 Hz to 100 kHz and two sets of data were obtained; one before and one after each electrode was polished with fine emery paper. At low frequency, the measured impedances were high and varied with electrode surface preparation. However, above a transition frequency, the impedances were resistive, independent of the electrode surface preparation, and equal to rho/2 pi d as predicted from the theory. This study indicates that the low frequency impedance of a monopolar electrode is dominated by the impedance of the electrode-electrolyte interface. Above a transition frequency, the resistance of the environmental conductor dominates, the value of this resistance depending on the electrode geometry and the resistivity (rho) of the environmental conductor. A second study was conducted, to examine the effect of the distance to the indifferent electrode. A frequency (100 kHz) above the transition frequency was used and impedance data were collected for various distances between the monopolar and indifferent electrodes. The measured resistance increased asymptotically as the distance between the electrodes was increased. When the indifferent electrode diameter was at least 10 times the diameter of the spherical monopolar electrode, the measured resistance was within 5% of the value predicted for an indifferent electrode at infinity.