•102 J. II. Reedy — Anodic Potentials of Silver. 



complete the silver began to dissolve. The weight of the 

 chlorine found was less than the theory by amounts varying 

 from 1 to 6 milligrams. 



To ascertain if the solution of the silver could be due to 

 oxidation at the anode, reducing agents like urea, formic acid 

 and acetaldehyde were added to the electrolyte. Such expedi- 

 ents did not prevent the loss of silver from the anode, as 

 minute flecks of the metal or its salts always appeared in the 

 solution sooner or later. 



Determinations in neutral solutions were also tried, and the 

 results obtained in this case were likewise always too low. 



Isolation of the Anode Potential. — In all the above experi- 

 ments the voltage measured was the potential fall across the 

 cell. Since, however, the fixation of the halogens is strictly 

 an anodic process, it is obviously better to determine the anode 

 potential apart from the cathode potential, which, as shown 

 by Specketer,* is strongly influenced by conditions of acidity 

 and contact with air. The third electrode meets this require- 

 ment satisfactorily and the anodic potentials in the following 

 experiments were all determined in this Avay. 



Nature of the Electrolytic Medium. — In addition to the 

 halide to be determined, especially when its amount is small, 

 it was found best to have some other electrolyte present in the 

 solution in order to facilitate the passage of the current. As 

 has been shown by Foerster f and others, alkaline solutions 

 favor the formation of oxidized halogen compounds, such as 

 iodates. This would introduce an error into the determination, 

 making the results too low, since the silver salts of the oxygen- 

 halogen acids are appreciably soluble. In neutral solutions, 

 the solution will become basic unless an amount of metal ions 

 equivalent to the anions discharged is also separated on the 

 cathode, as would be the case when zinc or cadmium salts 

 constitute the electrolyte. This would increase the difficulty 

 of estimating any silver transported to the cathode, since it 

 would necessitate in every case an analysis of the deposit on 

 this electrode. It is much better to be able to estimate the 

 silver directly by gain in weight of the cathode, without having 

 to make a chemical separation. Consequently acid solution 

 appears to be the most favorable medium for the electrolysis. 



Since the deposition potentials of the halogens increase with 

 the dilution, we would seek as a solvent that solution which 

 has the highest reaction potential with silver ; for in this way 

 the largest margin for the complete separation of the halogens 

 would be available. But most acids show the same reaction 

 potential,— -521 volts4 (See Table I.) 



*Zeitschr. Elecktroch., iv, 542. \ Elektrochemie, 1905, p. 348 et seq. 



t Referred to the hydrogen normal electrode as volts. All potential 

 measurements in this investigation have been referred to this standard. 



