86 ADVENTURES IN RADIOISOTOPE RESEARCH 



The determination of the decomposition potential by means of the 

 methods mentioned above is based on the assumption that the current 

 strength is large enough to permit deposition of the whole quantity 

 of the radioelement within the duration of the experiment. It is easily 

 seen, e. g. in the electrolysis of a 0.001 N lead nitrate solution, that the 

 above condition is far from being satisfied, since the electrode potential 

 is attained in our apparatus at a current strength of about 3 x 10"^ A 

 which, in the course of 24 hr, is capable of depositing only a very minute 

 fraction of the lead ions present. This is particularly emphasized since, 

 if this point is not taken into consideration, there will be found too 

 high a value in determining the decomposition potential by the methods 

 mentioned (sudden increase in the amount deposited). For example, 

 Fig. 4 shows the apparent decomposition potential of ThB using 0.001 N 

 solution ; it is considerably higher than the calculated value, and the 

 explanation is probably to be found in the reason mentioned above. 



We hope to be able to revert to several of the points which have been 

 discussed, particularly to the deposition below the decomposition 

 potential. 



4. DISCUSSION 



It has already been mentioned above that the difference in the atomic 

 weights of individual isotopic elements exists without any doubt. Hence 

 it follows that, in so far as gravitational properties are concerned, the 

 isotopes are not identical and that by centrifuging, for example, meso- 

 thorium should be easier to separate than its isotope radium from 

 barium. On the other hand, a similar differential in the chemical pro- 

 perties of isotopic elements is not observed, we have found replace- 

 ability in the electrochemical behaviour. It is concluded that the electrode 

 potential may be written in the form : 



_ RT Ec 



— ~z, In 



where Zr denotes the total concentration of all the isotopes present, 

 and correS])()ndingly the mass action law may be written in th(> form : 



[Zisotope A]"' [i:isotope B]"^ ... 



— = A 



[Zisotope A']"' [Zisotope B']"^ . . 



The proposition that two atoms with different weights can replace 

 each other in their mass action seems at first glance to contradict the 

 second law of thermodynamics. The contradiction disappears, however, 

 when the concept of chemical individually, to which the mutual replace- 

 ability is related, is considered more closely and is defined appropriately. 



