60 Gooch and Beyer — Electrolytic Estimation of Lead. 



Fig. i. 



deposits with a current N.D. 100 — 10 amp. and 4*5 volts, acting 

 upon 125 cmS of solution containing 20 cm3 of concentrated nitric 

 acid. 



In some of our preliminary trials of electrolysis in the closed 

 cell with subsequent filtration it was found that when the con- 

 centration of nitric acid amounted to 10 cm3 in 60 cm3 of liquid, 

 with a current of 4 amperes (JST.D. 100 =10 amp.) and 6 volts, 

 two sources of error appeared. In the first place, the depo- 

 sition of metallic lead upon the cathode was often noticeable ; 

 and secondly, it appeared to be impossible to make the precipi- 

 tation of lead dioxide complete so long as that substance was 

 allowed to float in the liquid. Similar results were obtained 



in experiments in which urea 

 was added to the liquid for the 

 purpose of obviating the sol- 

 vent action of dissolved oxides 

 of nitrogen upon lead dioxide. 

 In the experiments with this 

 form of apparatus the stirring 

 of the asbestos felt by gas 

 evolved upon the bottom of the 

 crucible used as an anode, as 

 well as the deposition of oxide 

 on the outer surface of the 

 crucible, was prevented by tak- 

 ing the precaution to moisten 

 the asbestos, from the outside, 

 with a drop of nitrobenzene 

 which, being insoluble in 

 water, prevents the contact of 

 the aqueous electrolyte with 

 the electrode surface under- 

 neath the asbestos. An increase 

 of nitric acid to the proportion 

 of 30 cm3 in 100 cm3 of solution 

 served to prevent the deposi- 

 tion of lead upon the cathode, 

 but to prevent the re-solution of lead dioxide it was found 

 to be necessary to use the process of continuous filtration, 

 so that the deposit might be compacted upon the felt, and 

 after deposition was complete to replace the acid liquid by 

 a solution of ammonium nitrate without interruption of the 

 current. After washing out the nitric acid with the solution 

 of ammonium nitrate the final washing was completed with 

 water. The form of apparatus employed, shown in figure 1, 

 and the manner of using, were fully described in the former 

 article to which reference has been made."'' In Table I are 

 * This Journal, xxv, 249, 1908. 



