J. ff. Reedy — Anodic Potentials of Silver. 297 



tial is to be attributed to the halogen ions, produced by the 

 hydrolysis of the free halogen, according to a reaction of the 

 type, Br,+ H,0±I£H-+JBr'' + HOBr. 



The same assumptions explain very satisfactorily the rise in 

 potential which was found to take place with silver electrodes 

 in acidified iodide solutions upon standing in contact with the 

 air. Fart of the hydriodic acid in the solution is oxidized to 

 tree iodine, which combines with silver to form silver iodide. 

 Besides the reduction of the iodine content of the solution in 

 this way, the F-ion concentration is further diminished by the 

 fact that some of the iodine passes into the form of free halo- 

 gen and hypoiodous acid, both of which have no effect on the 

 potential. In brief, oxidation amounts to a decrease in F-ion 

 concentration. 



Influence of Cations on Reaction Potentials. — As is seen in 

 the table of reaction potentials on page 288, all sulphates have 

 the same reaction potential of "521 volts except those of cop- 

 per and mercury. With solutions of these metals the results 

 were unexpectedly high. Moreover, silver anodes in these 

 solutions behaved abnormally in that their potentials were in- 

 definite, and showed a tendency to rise, even during the course 

 of a determination. Upon examination of the silver electrodes 

 after such experiments it was found by means of chemical tests 

 that some of the less noble metal had been deposited on the 

 silver. This points to the following reactions taking place, at 

 least to a small degree : 



2Ag 

 Ag 



+ 



+ 



Cu- 

 Hg- 



<- 

 <- 



-> 



2Ag- 

 Ag- 



+ 

 + 



Cu 

 Hg 



The progressive increase in the reaction potential may be 

 accounted for in some degree by the appearance of silver ions 

 in the electrolyte. That this small increment of the Ag"-ion 

 concentration should cause such large changes in the reaction 

 potential seems very surprising. It is much more likely that 

 this elevation is mainly due to the formation of solid solutions 

 of copper and of mercury, respectively, in silver. As has been 

 pointed out by Foerster,* the solution of a small amount of a 

 less noble metal (here copper and mercury) in a more noble 

 metal may diminish the electrolytic solution pressure of the 

 latter, just as the presence of a small amount of dissolved sub- 

 stance reduces the vapor pressure of the solvent, even though 

 the solute in the pure state may have the higher vapor pressure. 

 For example, a small amount of ether dissolved in a large 

 amount of water may raise the boiling point of the latter. In 

 a perfectly analogous way, the potential of silver may be raised 



* Elecktrochemie, 1905, p. 208. 



