cm 



J. H. Reedy — Anodic Potentials of Silver. 405 



more or less uncertain, owing to the difficult}' of obtaining 

 concordant results at the high dilutions involved. 



The method used in this investigation for fixing the end 

 point in a precipitation was to arbitrarily choose those points 

 on the reaction potential curves (extrapolating when necessary) 

 which correspond to 0"1 milligram— the ordinary limit of accu- 

 racy in analytical work. (See fig. 1.) This method promises 

 greater accuracy than those mentioned above, since it depends 

 only upon the probable assumption that the relation which is 

 found to exist at low dilutions also exists at high dilutions. 

 Only in the case of chlorine is the limit value of the anode 

 potential doubtful, owing to the fact that its graph is not a 

 straight line, as in the case with the other two halogens. This 

 method gives the following results for volumes of 200 cc : 



Maximum potential for ehlorine '505 volts 



Maximum potential for bromine *400 volts 



Maximum potential for iodine — *190 volts 



Ajpparattis. — Excepting the electrolytic cell, the apparatus 

 was the same as that used in the determination of reaction 

 potentials, as described in the preceding paper. The electro 

 lytic cell (see fig. 2) was a glass cylinder ll cm high and 8 - 7 

 in diameter. It was closed with a lid of wood fiber, through 

 which the various connections were introduced. All electrical 

 connections were insulated by glass tubing. 



The anode (A in the figure) consisted of a frame of heavy 

 silver wire whose cylindrical outer surface was covered with 

 fine silver gauze. The diameter of this anode was 47 cm , and 

 the height (of the cylindrical portion) was 2*7 C1D . It was 

 rotated with a speed of about 200 revolutions per minute. The 

 anode before using was heavily coated with silver by making 

 it the cathode in an ammoniacal silver solution to which am- 

 monium oxalate had been added, as recommended by Gooch 

 and Feiser.* It was then thoroughly washed and heated to 

 about 500° in an electric oven, in order to make the deposit 

 firm and adherent. 



The cathode C was a piece of bright sheet platinum, 21 X 50 ram , 

 provided with a stem of platinum wire ending in a loop, for 

 convenient attachment to the cathode connection. To prevent 

 swinging of the cathode by the rotating electrolyte, it was held 

 in place by a glass rod whose lower end was drawn out into a 

 fiber which was bent so as to form a hook, as shown in the 

 figure. The cathode was weighed before and after each deter- 

 mination and an)' silver transported was estimated directly by 

 difference. 



* This Jcrarnal, xxxi, 109. 



