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



Effect of Deposits of Silver Compounds on the Anode. — In 

 general, the presence of a coating of a silver compound on a 

 silver anode raises the reaction potential for solutions contain- 

 ing the corresponding anion. This is shown in the following 

 table of reaction potentials for the halogen ions: 





Table V. Efft 



ict of Coating 



on Anodes. 







Solution 



Bright silver 

 anode 



Coated 

 silver 

 anode 



Elevation 



M. 



KC1 in -5M. H 2 S0 4 



•234 



■222 



,•012 volts 



M. 



KBr in -5M. H,SO 



•028 



•077 



•049 



M. 



KI in -5M. HSO, 



-•175 



— •152 



•023 



Evidently the effect of the coating of silver halide is to oppose 

 the discharge of that particular halogen ion — a behavior iden- 

 tical in effect with the so-called electrolytic solution pressure of 

 the anion of the insoluble compound in electrodes of the 

 second class. 



Boettger,* in a recent paper, reports that he found the same 

 elevation, though he does not appear to have isolated and 

 measured it. 



The amount of the deposit — provided it exceeds an exceed- 

 ingly thin layer — seems to have no influence on the potential. 

 The approximate thickness of the layer necessary to make the 

 electrode function normally as an electrode of the second class 

 was determined in the following way : A silver electrode, 

 35x38""", was made the anode in a molar solution of potassium 

 chloride until it gave the same potential as a coated one. From 

 the gain in weight (assuming that the density of silver chloride 

 is 5 - 56) the average thickness of the layer was calculated to be 

 2*4 X 10 _3mm . Below this limit the potential is indefinite, and 

 lies between the values for bright and coated electrodes. 



Influence of Free Halogens. — Free halogens exert a strong 

 depressing action of the reaction potential of silver, as is shown 

 in the following way : Solutions of bromine and iodine were 

 made by shaking the free halogens with 0*5 molar sulphuric 

 acid. These may be considered approximately saturated. 

 Using in these solutions the coated silver anodes as in the pre- 

 vious experiments, reaction potentials of '259 volts and "357 

 volts, respectively, were obtained. Compared with the reac- 

 tion potential of 0'5 molar sulphuric acid ("521 volts), these 

 represent lowerings of *262 and "164 volts. Undoubtedly the 

 reaction in such cases is to a large degree molecular, as for 



example, 2Ag + Br 2 >>2AgBr ; and as such may be assumed 



to have no effect on the potential. The lowering of the poten- 



* Verh. Ges. deutsch. Aertze, 1913 (19] 4), II, 361. 



