Improved Ag/AgCl pressure electrodes
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When electrodes are used to measure slowly varying signals such as the slow wave in the gastrointestinal tract (0·01 to 10 Hz) it is necessary that the impedances of the electrodes are low at these frequencies, and the drift must also be low. We have carried out a programme of (a) electrode surface pretreatment, (b) electrolytic AgCl deposition, (c) impedance measurements, (d) electrode storage methods and (e) noise and drift measurements. Optimal preplating treatment was found to be roughening of the cleaned Ag surface. The electrode impedance could be lowered by about an order of magnitude using 50 μm Al2O3 abrasive powder. The impedance measured at fixed frequencies (0·01 to 0·05 Hz etc) plotted against the plating product (mAs cm−2) showed a sharp minimum at 3 mAs cm−2. This mimimum was very sharp at the very low frequencies. This indicates a critical dependence of impedance on the plating product. Storage of the electrodes in saturated AgCl solution was found to be better than any other method tried. Electrodes pretreated as above and optimally plated showed drift voltages of less than 1 μV min−1 and noise voltages of less than 1 μV. The low drift allowed the use of d.c. coupling during slow wave measurement on intestinal smooth muscle. The waveform in this case approximated the intracellular one closely. The electrode impedance could be described accurately in the frequency range from 0·01 Hz to 1 kHz by the relationships R=1851×f−0·347Ω and |X|=1190×f−0·356Ω where f=frequency in Hertz.
KeywordsAg/AgCl electrodes Biological electrodes
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