410 J. II. Reedy — Anodic Potentials of Silver. 



nometer still showed a minute current ; and although no silver 

 had dissolved, a considerahle amount of chlorine remained in 

 the solution. By reference to fig. 1 it will be seen that just 

 such errors as these found in the chlorine estimations should 

 he expected. Upon progressive dilution the deposition poten- 

 tial of chlorine approximates "521 volts — the solution potential 

 of silver, — and at that potential both the deposition of chlo- 

 rine and the solution of silver will proceed simultaneously. 

 That is, the anode potential of silver can not be raised high 

 enough to deposit all of the chlorine without silver beginning 

 to dissolve. For this reason, the estimation of chlorine on sil- 

 ver anodes can not be an accurate and trustworthy method. 



Separation of the Halogens. — By reference to fig. 1 it will 

 be seen that the halogens can not be separated electrolytically 

 regardless of their concentrations. For example : For a 

 volume of 200 ec , an anodic potential of *190 volts is necessary 

 to completely precipitate iodine on silver anodes. The same 

 potential corresponds to a concentration of about - 0115 molar 

 bromine. That is, if the bromine ions are present in greater 

 concentration than this, there is a certain range of potential 

 for which both iodine and bromine will be deposited simul- 

 taneously. For similar reasons chlorine must not be present in 

 greater concentration than '0014 molar if bromine is to be pre- 

 cipitated free from chlorine. In view of this small concentra- 

 tion, it is not surprising that Boettger and Kelly,* working 

 more or less empirically, reported that bromine could not be 

 separated from chlorine electrolytically. In the separation of 

 iodine from chlorine, however, the concentration of the chlorine 

 may be even higher than molar without its deposition potential 

 overlapping the maximum potential for iodine. 



Evidently, the approximate bromine and chlorine concentra- 

 tions must be known before a separation may be attempted. 

 If the} 7 exceed the above limits, proper dilution must be made. 

 It should be remembered, however, that dilution is always 

 made at the expense of accuracy in the determination of the 

 halogen of low concentration. 



Analytical Separations. — The conclusions drawn above as 

 to the necessary conditions for the separation of the halogens 

 were applied experimentally in a number of determinations, 

 with the results indicated in Table III. 



In Exp. No. 23 the anode potential rose too high and some 

 bromine was deposited, which accounts for the positive error. 



In Exp. No. 32, the chlorine concentration was -0024 molar 

 — considerably higher than "0014 molar, which was taken to be 

 the maximum chlorine concentration allowable for a complete 



* Verh. Ges. deutsch. Aertze, 1913 (1914), ii, 361. 



