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



present. Deposition potentials on attacked electrodes lie below 

 the discharge potentials on inert electrodes, since they are less 

 by an amount which must correspond in some degree to the 

 energy changes involved in the formation of the compound. 



It is very noteworthy that in the case of silver the substances 

 formed by deposition of anions are the ones which furnish low 

 Ag'-ion concentrations, owing to low solubility or to the 

 formation of complexes. Further, the deposition potentials 

 and ionic concentrations are in the same order. This suggests, 

 to say the least, that the ionic concentration of the silver may 

 be the determining influence in these reaction potentials. Ac- 

 cording to the deposition theory, it must be assumed that, in 

 the case of the thiosulphate and cyanide ions, these anions are 

 first deposited on the silver, and the silver salt thus formed 

 later reacts with the excess of the electrolyte to form the final 

 complexes. 



Significance of the Potential '521 Volts. — The reaction 

 which takes place at the potential "521 volts, -which is the high- 

 est anode potential observed in solutions containing no Ag-ions, 

 amounts in practice to the solution of silver. This action 

 seems to take place irrespective of the anions present or their 

 concentration, provided they do not belong to the group which 

 gives low reaction potentials, as mentioned above. If there is 

 present at this potential a small concentration of the ions of an 

 insoluble silver compound, as OH'-ion or Cl'-ion, a precipita- 

 tion occurs, not only upon the electrode, but in the solution. 

 In view of this behavior, this maximum potential may be 

 called the solution potential of silver. 



The mechanism of this reaction can be explained from three 

 different points of view : 



(a) Anions of the solution are discharged on the silver, a 

 large number of anions having the same deposition potential, 

 •521 volts. 



(b) Hydroxyl ions from the water are discharged, and the 

 silver hydroxide thus formed is instantly neutralized by the 

 acid formed at the same moment. 



(c) Silver sends ions into solution at this potential. 



The first theory seems unlikely, since such a coincidence of 

 discharge potentials for several anions would be remarkable 

 indeed. And the additional fact that the potential is not influ- 

 enced by change of concentration (see the case of sulphuric 

 acid, page 282) is a further argument against it. A third 

 objection is found in the fact that an electrolyte may show two 

 different reaction potentials, and yet contain only a single kind 

 of anion (not counting the OH'-ion from the water.) Such a 

 case was found in 0*5 molar potassium bromate. (See fig. 4.) 

 An anodic reaction potential first appears at '398 volts, and the 



