36 



OTHER ELECTRODES 



I. A. SILVER 



Electrodes that are used to stimulate or record electrical activity in bio- 

 logical fluids may become polarized unless special precautions are taken. 

 The larger and more prolonged the current that passes between electrodes 

 the greater the likelihood of polarization. When only small currents of 

 short duration are being considered simple metal electrodes are adequate, 

 but special modification of the electrode or stimulating apparatus or both 

 may be necessary with higher current values. 



Polarization appears as a partial blockage of current flow in conjunction 

 with the presence of a back electromotive force. This back e.m.f. is associated 

 with electrolysis of the medium around the electrode but the exact nature 

 of over- voltage is not yet understood (see Butler^; Kortiim and Bockris^). 

 Although a study of polarization of electrodes in biological systems may 

 be used in the investigation of the chemical state of the systems, most 

 physiologists are chiefly concerned in avoiding or overcoming polarization 

 phenomena. 



NON-POLARIZABLE ELECTRODES 



In experiments that require the passage of constant current or prolonged 

 stimulation with monophasic shocks simple metal electrodes will polarize 

 and the current passing will become progressively smaller, thereby altering 

 the conditions of the experiment. 



The general principle of 'depolarizers' is to avoid the electrolysis of the 

 medium and in particular to suppress the production of bubbles of gas near 

 the surface of the electrodes. This is most simply accomplished by keeping 

 an electrode in contact with a saturated solution of one of its own salts. 



The most generally useful non-polarizable electrode is the silver/silver 

 chloride system, provided that only small currents are being employed. 

 The electrode consists of a silver wire or plate on which silver chloride has 

 been deposited electrolytically. The great advantage of the Ag/AgCl 

 electrode is that it can be placed directly in contact with living cells, it is 

 sufficiently strong to be easy to handle and insert deeply into tissues, and it 

 is relatively non-toxic owing to the insolubility of silver chloride. A high 

 concentration of silver ions is maintained around the electrode but there 

 is very little diff"usion and most tissues will tolerate the presence of silver 

 electrodes for several weeks. Spermatozoa, however, are particularly 

 susceptible to poisoning by silver ions so that investigations on electrical 

 properties of semen should be conducted with electrodes of platinum or 

 gold. Silver may also react with sulphides if it is left embedded in tissues 

 for long periods. 



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