282 J. R. Reedy — Anodic Potentials of Silver. 



Reference Electrodes. — As a standard electrode for most of 

 the measurements, a mercury-mercurous sulphate electrode was 

 used, the electrolyte being - 5 molar H 2 S0 4 . The potential of 

 this electrode was found by trial to be +'679 volts, referred to 

 the hydrogen normal electrode as 0.* This value for the 

 mercurous sulphate electrode is the same as that found by 

 Wilsmore.f It was found to be quite constant, no sensible 

 change in its value appearing after standing several months. 



With alkaline solutions a mercuric oxide electrode was used. 

 This consisted of mercury covered with a layer of yellow 

 mercuric oxide in contact with a normal solution of sodium 

 hydroxide. Its potential was found by experiment to be +'112 

 volts. WilsmoreJ assigns it the value +'110 volts. The 

 potential was found to change slowly, § so the electrode was 

 always freshly prepared and compared with a standard calomel 

 electrode before using. 



Apparatus. — Figure 1 is a diagram of the apparatus used. 

 The electrolytic cell B was a glass cylinder about 14 cm high 

 and 7 cm in diameter. It was closed by a rubber stopper 

 through which the electrode connections were introduced. 

 These consisted of heavy platinum wires, fixed in glass tubing 

 where they passed through the stopper, and terminated in 

 hooks from which the electrodes were suspended. 



The cathode C was a piece of bright sheet platinum, 

 21 X 50 mm . The anode A was of sheet silver, 35 X 38 mm . 

 This was purchased for " tine silver," and test showed it to be 

 free from copper. Each electrode had a wire stem ending in 

 a loop for ready attachment to the hooks mentioned above. 



The main circuit was operated by the lead storage cell L, as 

 indicated in the diagram. B was a sliding resistance, by means 

 of which the voltage of the electrolyzing current could be 

 altered. A sensitive Hartmann & Braun galvanometer G of 

 the type furnished for use with a platinum thermocouple was 

 placed in the main circuit to serve as a current indicator, one 

 scale division (about l om ) representing "009 milliamperes. 

 When larger currents were to be measured, the Weston milliam- 

 meter M could be substituted for the galvanometer by shifting 

 the switches S and S'. 



F was the third electrode, and D an intermediate vessel con- 

 taining a solution to eliminate diffusion potentials. For this 

 purpose saturated potassium chloride solution was used in all 

 cases except those in which it was necessary to guard against 

 the introduction of traces of chlorides into the solution. In 

 such cases saturated ammonium nitrate solution was used in 



* Throughout this paper, all potentials are referred to this standard. 



\ Zeitschr. phys. Chem., xxxvi, 94. 



J Ibid., xxxv, 325. 



gCf. Ostwald-Luther, "Messungen," 1910, p. 445. 



