J. H. Reedy — Anodic Potentials of Silver. 407 



rent was turned on, the voltage at all times being adjusted so 

 that the anode potential would conform to the limits mentioned 

 before. In order to expedite the precipitation, the anode poten- 

 tial was brought at the start to a value just below the maxi- 

 mum. This potential requires constant attention and must be 

 adjusted very frequently since it tends to rise very rapidly 

 with the decrease of the halogen ions of the solution. The 

 completion of the precipitation is indicated by the lack of 

 deflection of the needle of a sensitive galvanometer. Con- 

 ducted in this way, twenty to forty minutes are required for 

 an electrolysis. The electrodes were then removed from the 

 cell, carefully washed, and finally dried in an electric oven at 

 about 500°. The gain in weight of the anode represents the 

 halogen deposited. 



Before making another determination the silver halide was 

 reduced to metallic silver by making it the cathode in a dilute 

 sodium hydroxide solution, using a tension of about two volts, 

 and allowing the current to pass for a period of 50 to 60 min- 

 utes. The anode was then washed in dilute sulphuric acid, 

 carefully rinsed in distilled water, and finally heated to about 

 500° in the electric oven for an hour or more. The silver 

 after reduction is black in color and not very adherent, so that 

 the anode must be handled very carefully to prevent loss. 

 After heating it is white and firmly sintered together, so that 

 there is no risk of mechanical loss in the subsequent manipula- 

 tions. 



Nature of the Haloid Deposit. — As long as the solution 

 potential of silver is not exceeded, the haloid deposits on gauze 

 anodes are quite adherent, and may be washed without fear of 

 loss. If, however, this critical potential is exceeded, the silver 

 halide seems to be formed in part out of actual contact with 

 the metal, and for this reason does not adhere firmly. No 

 such expedient as the alkaline tartrates used by Vortmann is 

 necessary to make the deposit more adherent, if the proper 

 attention is given to the anode potential. The precipitate is 

 free from silver oxide, and hence does not require to be heated 

 to a high temperature to effect the decomposition of this com- 

 pound, as Vortmann, Grooch and Read, and Peters found nec- 

 essary in their researches. 



Influence of Current Strength. — Under the description of 

 the procedure in an electrolysis, reference was made to the 

 fact that the anodic potential must be watched continually, 

 since it rises rapidly as the electrolysis progresses. To keep 

 this potential from exceeding its critical value, the voltage of 

 the electrolyzing current must be reduced from time to time, 

 and this entails a corresponding diminution of current strength. 

 Moreover, the current strength must be reduced, not in an 



