Use of Ag-AgCl Electrodes

There are several considerations in the use of Ag-AgCl electrodes. In making the electrical connection to the electrode one must be careful. If solder is used, it should be coated with a waterproof insulating material to prevent it from coming into contact with the electrolyte in which the electrode is immersed. This is also true of the connecting wire otherwise, contamination of the electrode may occur because of chemical reactions between the solder or wire and the electrolyte. 

Silver-silver chloride electrodes require Cl ions for proper operation. When used in biological electrolytes, they have a sufficient supply of Cl ions available. If they are used as skin-surface electrodes in such applications as EEG or EKG recording, it is necessary to use a wetting solution or paste which contains Cl ions. 

Because Ag-AgCl electrodes are thermodynamically reversible, they exhibit (after stabilization) low noise and theoretically zero electrode polarization impedance effects. They do produce a steady electrode potential, however, which produces a dc offset in direct-coupled systems. This requires compensation and is discussed in Chapter 7. This situation may cause problems in the sensing of low-level dc potentials. Because of the electrochemical nature of these electrodes, each one assumes an absolute potential. When two such electrodes are used as a sensing pair, a dc potential difference exists (frequently of the order of a few millivolts). If this potential difference remains constant, any measurement of a bioelectric potential is unaffected, except for a steady baseline elevation. In the usual case, however, the resting potentials of the two electrodes change unequally with time and environmental temperature. This results in objectionable baseline drift in experimental determinations. 

Recently a new, and reportedly much more stable, solid silver-silver chloride pellet electrode has been introduced by Beckman Instruments, which does not appear to exhibit the undesirable qualities to the same extent as its fluid-bathed progenitors. 

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