PREPARATION OF NON-POLARIZABLE ELECTRODES 



Platinum plated with platinum chloride is also commonly used and as 

 platinum wire is stiffer than silver wire it is better for inserting deeply into 

 resistant tissue. Platinum is however most frequently used without being 

 plated with chloride since it has a very low gas over-voltage and is therefore 

 virtually a non-polarizable electrode where very small currents are being 

 considered. 



The passage of heavier currents requires a metal electrode in contact 

 with a very soluble salt if polarization is to be avoided. The zinc/zinc 

 sulphate system is very commonly used in biological work, but as zinc is 

 highly toxic it cannot be allowed to come into contact with the tissues 

 directly. The greatest care is therefore necessary to confine the zinc ions 

 behind a porous barrier, electrical contact being made through an agar/chlor- 

 ide bridge, and a cotton wick. 



Mercury/mercuric chloride electrodes are also useful but, like zinc, 

 mercuric ions are highly toxic to living tissues. 



PREPARATION OF NON-POLARIZABLE ELECTRODES 



Silver jsilver chloride 



For accurate and critical work it is necessary to plate silver electrodes very 

 carefully. The following method produces stable electrodes with a large 

 surface area. 



Clean the silver wire with emery paper and degrease with Trilene or 

 petroether. With the cleaned wire as the anode in N/10 HCl electrolyte 

 and another silver wire as the cathode in circuit with a 2 V accumulator 

 and a reversing switch, pass current through the circuit for 30 sec and then 

 reverse the switch for 30 sec: repeat this three times. The anode-cathode 

 distance should be very short and the plating dish should be in a dark box. 

 After plating, the electrodes should be kept in Ringer's solution in the dark. 



For very critical work the electrode can be imbedded in agar and connected 

 to the tissues by a saline soaked wick. 



Platinum I platinum chloride 



Wash the platinum wire in distilled water followed by concentrated 

 sulphuric acid. Rinse thoroughly in distilled water and place in platinum 

 chloride solution. Connect the chosen electrode to the negative terminal 

 of a 2 V accumulator and make a circuit through a dummy platinum 

 electrode and a reversing switch. Pass current for 15 sec and then reverse it: 

 repeat this 4 to 6 times ending with the chosen electrode positive, which 

 will give a final even coat of platinum chloride. Wash the electrode and 

 keep it in distilled water. 



Zincjzinc sulphate electrode 



Ideally this consists of a pure zinc rod immersed in a saturated solution 

 of zinc sulphate, but in practice zinc usually contains some impurities 

 which are liable to produce small galvanic cells on the surface of the metal 

 which will give rise to back e.m.f.s. and therefore such an electrode will 

 appear to be polarized. This can be overcome by coating the surface of the 

 zinc with a thin layer of mercury. The resulting amalgam behaves as a 

 pure zinc surface. 



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