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A new method is used to measure the direct-current (Faradic) resistance of a single electrode/electrolyte interface. The method employs a constant-current pulse and a potential-sensing electrode. By choosing a sufficiently long pulse duration, the voltage between the test and potential-sensing electrode exhibits a three-phase response. In the steady-state phase, the voltage measured is equal to the current flowing through the electrode Faradic resistance and the resistance of the electrolyte between the test and potential-sensing electrode. By measuring this latter resistance with a high-frequency sinusoidal alternating current, the voltage drop in the electrolyte is calculated and subtracted from the voltage measured between the test and potential-sensing electrode, thereby allowing calculation of the Faradic resistance. By plotting the reciprocal of the Faradic resistance against current density and fitting the data points to a third-order polynomial, it is possible to determine the zero-current density (Faradic) resistance. This technique was used to determine the Faradic resistance of electrodes (0.1 cm2) of stainless-steel, platinum, platinum-iridium and rhodium in 0.9 per cent NaCl at 25 degrees. The zero current Faradic resistance is lowest for platinum (30.3 k omega), slightly higher for platinum-iridium (47.6k omega), much higher for rhodium (111k omega) and highest for type 316 stainless-steel (345k omega). In all cases, the Faradic resistance decreases dramatically with increasing current density.

In some instances the same electrodes are used for stimulation and then for recording a bioelectric event immediately after the stimulus. However, after the current pulse there remains an electrode potential that decays quasiexponentially. We have designated this falling potential the electrode-recovery potential. This study investigated the recovery potentials of single electrodes of rhodium, stainless steel, platinum and platinum-iridium in contact with 0.9% saline at room temperature (25 degrees C) over a current density ranging from 0.1 to 100 mA/cm2 using a constant-current pulse. In all cases, with increasing current density, there was a decrease in the time for the electrode potential to fall to one half of the immediate post-stimulus value. Above about 20 mA/cm2 the decrease in recovery time was smooth with increasing current density. Below 20 mA/cm2, the recovery time was slightly irregular. The shortest recovery times were for platinum and platinum-iridium. The largest decrease in recovery time with increasing current density was for stainless steel, which decreased 10 fold from 0.1 to 100 mA/cm2. The recovery time for rhodium decreased about three-and-one half fold over the same current density range. It was found that the waveform of the recovery potential is not a simple exponential because the Warburg and Faradic components of the electrode-electrolyte interface are current-density dependent. In general, for all current densities studied (0.1-100 mA/cm2), there was a sudden initial fall in electrode potential with cessation of current flow, followed by a very gradual nonexponential decrease in potential.

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