THE EXCHANGE OF ATOMS JJETWEEN SOLID AND LIQllD PHASES 



101 



Difi'eiont behaviour is Ibund in studying lead peroxide suiiaces 

 immersed in a lead solution labelled witliTliB. In this ease the exehange 

 is much less ; between a PbOg surface 2 cm- in area and 10 cm^ of a 

 0.001 N Pb(N03).^ solution, containing 0.001 N IINOg and saturated 

 with similarly lalx^llcd PbOa. the following exchange takes place : 



Table 4 



Table 5. — The time of experiment in this ca.se is always 1 min ; the concentra- 

 tion OF the labelled lead nitrate solution varies between 10-1 _^jji5 10-6 X 



Here also an exchange rather than a unilateral dissolution is involved 

 as is proved by the following experiments : This time a labelled Pb02 

 surface 2 cm^ in area is immersed in 10 cm^ of a 0.001 N Pb(N03)2 solu- 

 tion, saturated with PbOa and containing 0.001 N HNO3, and it is found 

 that the following amounts of lead (Ta])le 6) have passed into solution 

 from the solid phase : 



Table 6 



Time 



Amount of lead 

 exchanged 

 (gm) 



Expressed in 



molecular layers 



1 em" in area 



Expressed in fractions of 

 the amount required to 

 impart the PbOj poten- 

 tial to the area of 1 cm' 



32 

 24 

 6 



The experiments just described are made difficult owing to the break- 

 ing off of invisible amounts of lead peroxide which fall into the solution 



