J. H. Reedy — Anodic Potentials of Silver. 291 



flexure at - 521 volts indicates that at that potential a second 

 reaction has been superposed upon the first. The lower poten- 

 tial may in all probability be taken to mean the deposition of 

 Br0 3 / -ions, exactly analogous to the deposition of the halides 

 and other anions which form silver compounds of low solubility. 

 The second potential, "521 volts, therefore must mean either 

 the discharge of the hydroxyl ions of the water (hypothesis b), 

 or the direct formation of silver ions (hypothesis c). 



Hypothesis b assumes that hydroxyl ions from the water are 

 discharged on silver at "521 volts, forming AgOH (or Ag,0), 

 and that this reacts with the hydrogen ion, also formed by the 

 dissociation of water, to form water and silver ions, — 



AgOH -f H >H a O + Ag\ 



This hypothesis is open to the following objections : (i) The 

 reaction is uninfluenced by acidity since acids and their neu- 

 tral salts give the same reaction potentials. (See Table I, page 

 288.) If this hypothesis were correct, the theory of equi- 

 librium between the ions and the electrode would require that 

 the potential for acids should be much higher than for neutral 

 salts, since in the acids the concentration of the hydroxyl ion 

 must be very low. This should mean a difference of "4 volts 

 between the potential in an acid of normal concentration and that 

 of one of its neutral salts, (ii) Neutral solutions were electro- 

 lyzed on silver anodes, and no acidity was developed at the 

 anode, as would be expected if hydroxyl ions were precipitated. 

 Nor was there any evidence of the formation of silver oxide, 

 either by its dark color or by loss in weight on ignition. 



The following considerations appear to favor the theory that 

 the silver goes directly into solution in the form of silver ions 

 (hypothesis c) : (i) If a very dilute solution of hydrochloric 

 acid is electrolyzed at an anode potential of less than "521 volts, 

 silver chloride is formed on the anode, and no turbidity appears 

 in the solution. But if the potential is raised above "521 volts 

 the solution at once shows the characteristic opalescence. This 

 effect is made all the more striking by avoiding agitation of 

 the electrolyte. In this case a white cloud soon envelops the 

 anode and slowly diffuses outward into the liquid. Within 

 this region of precipitation Ag-ions seem to be in excess, and 

 as they diffuse outward are precipitated by the Cl'-ions. A 

 similar phenomenon was produced with alkaline solutions, 

 (ii) The theory of the direct formation of Ag-ions is made 

 more probable by thermal data. According to the theory of 

 the primary electrolysis of water (see b, page 290), the solution 

 of silver would take place in stages, which, with their heats of 

 reaction, would be represented by the following equations : 



