102 ADVEXTUKES IX EADIOISOTOPE RESEARCH 



and are co-determined when the solution is evaporated, thus producing 

 an erroneously high exchange. 



In the experiments discussed here it was merely assumed that lead 

 and ThB cannot be separated by chemical and electrochemical reactions, 

 as was first proved by Fleck and later confirmed by many authors. 

 If one phase contains on the average lO^o atoms per atom of ThB and 

 if we can detect a ThB atom in the other phase which was originally 

 free from ThB then, as already mentioned in the introduction, the con- 

 clusion can be drawn that lO^o lead atoms also have been transferred 

 from the first to the second phase. Our experiments do not indicate how 

 many atoms have changed places more than once between the two phases. 



It should be mentioned that when diffusion processes are involved the 

 presence of one ThB atom cannot strictly be taken to imply the accom- 

 paniment by 10^0 atoms of lead, since the diffusion velocities of ThB and 

 Pb are not equal. As far as solid and liquid phases are concerned, however, 

 in which the diffusion velocity is very little dependent on the mass, it 

 is practicable to draw the above-mentioned conclusion and, for example, 

 to equate the velocity of diffusion of lead isotopes in lead to that of lead 

 in lead. 



Summary 



The exchange of atoms between two phases, for example, between metalhc 

 lead and a lead nitrate solution, can be followed by labelling the lead in one 

 phase with one of its isotopes, for example, with ThB; the amount of labelled 

 lead transferred in a given time into the other phase can then be determined. 



In the case of Pb/Pb(N03)2 the exchange is very rapid and depends mainly 

 on the local currents. At particular points in the metal some lead goes into solution 

 and at other places lead is deposited from the solution. 



The exchange between a surface of lead peroxide and a lead nitrate solution 

 is much less ; in the experimental conditions described in the paper it amounts 

 to only one-third of a molecular layer of lead peroxide in a 0.001 N solution 

 during the course of 1 min. The whole molecular surface layer is replaced only 

 after 1 hr has passed. 



In using stable lead peroxide the ideal case of kinetic exchange is much more 

 nearly approached — exchange with complete thermodynamic equilibrium 

 between the two phases — than when metallic lead is used. 



