THE EXCHANGE OF ATOMS BETWEEN SOMD AND LIQIII) I'HASES 99 



a potential difference of zero prevails between the lead nitrate solution 

 and the metallic lead (absolute zero point of the electrolytic potential). 

 Thus it might be thought that the amount of lead deposited in the above 

 experiments does not represent the exchange between the two phases 

 but is the result of such an unbalanced electrolytic process. The calcu- 

 lation of the amount which can be expected to be deposited at an isolated 

 electrode shows, however, that the value is much smaller than that 

 actually observed. 



The capacity of the douljle layer at 1h(^ metal-electrolyte boundary, 

 from the measurements of Krijger and Krumreich^, amounts to 27 

 /.iF ; the potential difference of this condenser in the case of Pb/Pb(N03)2 

 is always less than -^ 0.2 V ; thus the calculated charge of the condenser 

 is 5.4 X 10~^ C. This quantity of electricity corresponds to 5.6 x 10-^ gm 

 Pb , which is considerably less than the deposit of lead found by experiment . 



In order also to confirm experimentally that an exchange of atoms 

 between the two phases, and not a one-sided deposition, is involved, the 

 following experiments were performed : 



A lead rod similar to those used in the experiments already mentioned 

 was coated electrolytically with a layer of metallic lead labelled with 

 ThB, immersed in 10 cm^ of lead nitrate solution for 1 min and then, 

 by determining the ThB content of the solution, the number of lead 

 atoms transferred from the metallic phase into the lead nitrate solution 

 was determined. 



Thus it was possible to establish that while 1.7 X 10^^ gj^-^ j^^j depos- 

 ited from 10 cm^ of a 0.1 M lead nitrate solution on an area of 2 cm^ 

 in 1 min., 1.6 X 10"* g Pb had correspondingly entered the solution in 

 identical conditions in the experiment just mentioned. 



In our experiments, therefore, there is indeed an exchange of atoms 

 between the metallic phase and the lead nitrate solution. Because of 

 the magnitude of the amounts exchanged, which in certain conditions 

 amount to one hundred times the unimolecular layer, the process cannot 

 be a pure "kinetic" exchange (exchange at complete thermodynamic 

 equilibrium) but involves nonuniformity of the lead surface and pre- 

 cipitation of the lead atoms at particular points from the solution. 

 The observed exchange is essentially a result of "local currents". 



The velocity with which the exchange of the lead atoms takes 

 place in the interior of the solid metallic phase can be computed approx- 

 imately since it is equal to the velocity of diffusion of lead in solid lead. 



The diffusion velocity of lead in mercury according to M. v. Wogau^ 

 amounts to 0.6 cm^ hr"i at 18°C ; in solid lead the value is many times 



1 Kruger and Krumreich, Z. Elektrochem. 19, 020 (1913). 



2 M. V. WoGAr, Ann. Pluj-s. 23, 34."", (IflOT). 



