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



place of the potassium chloride. A comparison of the results 

 obtained by the use of these two solutions showed that their 

 effects were practically the same, anodic potentials with 

 saturated ammonium nitrate being found to be higher than 

 those with the saturated potassium chloride by about - 002 

 volts. No change in this intermediate solution was ever made 

 during any series of measurements, so that any error due to 

 this difference could be disregarded. 



The arrangement of bulbs and stopcocks between the elec- 

 trolytic cell J3 and the third electrode F was devised to provide 

 for convenient filling of the connecting tubes, and to prevent 

 mixing of the solutions by diffusion and gravity. Potentials 

 were measured with the stopcocks closed. 



In the subsidiary circuit, E was a capillary electrometer 

 which was used as a null instrument. K was a lead storage 

 cell, connected with the bridge as indicated. In order to 

 obviate calculations as well as errors due to changes in the 

 storage cell operating the electrometer circuit, a Weston volt- 

 meter Y with a range of 3 volts was placed in a shunt between 

 the positive end of the bridge and the sliding contact, the 

 point of balance being found as usual by the electrometer, and 

 the potential then read directly on the voltmeter. This com- 

 bination was carefully calibrated against a cadmium cell, and 

 the corrections so determined have been applied to all volt- 

 meter readings recorded below. 



Experimental Procedure. — The general method for deter- 

 mining reaction potentials is to subject the cell to a gradually 

 increasing voltage, and to plot on coordinate paper the elec- 

 trode potentials as abscissae and the corresponding current 

 strengths as ordinates. A graph is drawn, and the potential 

 corresponding to the appearance of an appreciable current is 

 taken as the reaction potential. 



When the reaction is reversible, the curve has the general 

 form typified by the graph for 0*1 molar KC1 (see fig. 2), for 

 which the reaction potential is seen to be *285 volts. The 

 portion of the graph below the horizontal axis represents the 

 behavior of the electrode as a cathode ; that above, the behavior 

 as an anode. There is no discontinuity upon passing from one 

 region to the other. On the other hand, the line is quite 

 straight and makes an angle of nearly 90° with the potential 

 axis, indicating that the electrode is almost perfectly reversible. 



When the reaction is not reversible, the shape of the curve 

 is different, since for a greater or less range of potential the 

 current is practically zero ; that is, the anodic behavior is not 

 continuous with the behavior as a cathode. The curve for 0*5 

 molar sulphuric acid (see fig. 2) is typical of this class. 

 No cathode current is found — at least within the range 



