J. H. Reedy — Anodic Potentials of Silver. 287 



Coated Electrodes. — When a reaction takes place with the 

 formation of an insoluble compound on the anode, it was found 

 that the reaction potential (as will be discussed in full later) 

 depends on whether the anode is bright, or whether it is 

 already coated with the insoluble compound in question.* 

 Bright electrodes give potentials which are not definite and 



Fig. 4. 



SO 



40 



S 



3 30 



s 



<° 20 



3 

 O 



10 



.400 



.450 



.500 



£50 



.600 



Potential in Volts. 



> 



Fig. 4. Eeaction potentials of silver with - 5 molar KBr0 3 . 



change rapidly, but with coated electrodes the potential is con- 

 stant and always reproducible. For this reason reaction poten- 

 tials for reactions which involve the formation of insoluble 

 products have been determined on coated electrodes. 



These coatings were usually formed electrolytically by mak- 

 ing the electrode the anode in a cell containing the potassium 

 compound of the anion required. It was then thoroughly 

 washed and dried at 150°. In the case of the halides, anodes 

 which were coated chemically by standing in solutions of the 

 * Verh. Ges. deutsch. Aertze, 1913 (1914), ii, 361. 



Am. Jour. Sci. — Fourth Series, Vol. XL, No. 237. 

 19 



-September, 1915. 



