The silver-silver chloride electrode: a possible generator of offset voltages and currents

Large input currents can be generated by an asymmetry in the chloride activity seen by Ag-AgCl electrodes used at the input of intracellular amplifiers because offset compensation no longer takes place at the input of the amplifier but rather later in the circuit in many modern amplifiers. This situation occurs when chloride-free solution, such as potassium acetate, potassium citrate or various intracellular dyes are used as pipette and electrode holder filling solutions. Omitting the agar bridge at the reference electrode, or using a platinum electrode will have similar results. Values of cellular membrane potentials can be significantly contaminated under such conditions, particularly where the amplifier input resistance did not exceed 10(10) omega and where small cells were studied. Unexpected iontophoresis of Cl- can occur when the asymmetry is such as to make the recording pipette negative. These offsets can be avoided in all situations by the use of salt bridges which insure that the two Ag/AgCl junctions are in contact with Cl- of the same activity.