244 HOWARD J. CURTIS 



Silver-Silver Chloride Electrodes. The calomel electrode is 

 rather bulky and inconvenient, so silver electrodes in the presence 

 of a saturated solution of silver chloride are often used. Silver is 

 usually electroplated on a gold electrode of any convenient shape 

 to insure having a pure silver surface. This electrode is then made 

 the anode in an electroplating cell using a pure solution of sodium 

 or potassium chloride. Current is passed slowly over a consider- 

 able period of time until a thick adherent coating of silver chloride 

 covers the entire electrode. The electrode is then ready for use, 

 and it may be placed directly in the liquid in question or against the 

 tissue, or may be placed in a potassium chloride solution and con- 

 nected to the solution in question by means of a salt bridge. The 

 electrode should never be allowed to dry out, and if accurate abso- 

 lute values are desired the electrodes should never be exposed to the 

 light. It will be clear that in use the silver electrode will be sur- 

 rounded by a saturated solution of silver chloride, since the silver 

 chloride coat will very slowly dissolve and it takes only a minute 

 amount to form a saturated solution. This then fulfills all the re- 

 quirements for a reversible and reproducible electrode. 



It is possible to make these electrodes so that they will give very ac- 

 curate results. However, the requirements that must be met are more 

 stringent than in the case of calomel electrodes. Therefore it is not 

 recommended that these electrodes be used as reference electrodes 

 except in special cases. Their greatest usefulness is as working 

 electrodes. 



Working Electrodes. In many biological investigations there 

 is no interest in the absolute values of electrode potentials, but the 

 electrodes are used in pairs of identical electrodes so the electrode 

 potentials, being equal and opposite, cancel out. It is only neces- 

 sary then to make sure that the two electrodes are identical. It is 

 customary to make electrodes in pairs some days before they are 

 needed. They are then stored in approximately the same solution 

 in which they will be used, and short-circuited. Usually shght dif- 

 ferences of potential are equahzed in this way. These electrodes 

 will remain quite constant even during the passage of current. 

 However, the larger the current to be passed, the larger the surface 

 area of the electrode should be. 



A very convenient form for these electrodes is shown in Figure 4. 

 The electrode itself is in the form of a helix of silver wire (silver plat- 

 ing is unnecessary here) immersed in a tube completely filled with a 



