THE PKOBLEM OF TlIK ISOTOPIC ELEMENTS 71)1 



and is the voltage at which the element can be deposited elect lolytically, 

 can be considered. This was the first method which we adopted to solve 

 the above problem, namely, to determine whether the decomposition 

 potential of an element is displaced when an isotopic element is added 

 to it. The sensitivity of radioactive methods permits the quantitative 

 determination of even the unweighable amounts which always deposit 

 l)elow the decomposition potential, and this opened up a second method 

 of testing the problem ; we studied the variation in these amounts on 

 adding isotopic elements. The third method depended directly on mea- 

 suring the potential difference shown by a RaD peroxide electrode. 

 More details will be discussed below concerning the method of depositing 

 RaD peroxide which we have succeeded in preparing from radium ema- 

 nation in visible amounts. 



3. STUDIES ON THE REPLACEABILITY OF ISOTOPES 



(a) The Decomposition Potential of Radioelements 



When determining the curve of the decomposition potential it is usual 

 to measure the current passed by the cell as a function of the electrode 

 potential. In plotting these curves it is always postulated that the 

 current is carried essentially by the ion whose decomposition potential 

 is to be determined and that current can pass continuously only when 

 the potential difference attained at the cathode is equal to that which 

 would be registered when the metal in question is immersed in the so- 

 lution. This method of determining decomposition potentials is not 

 applicable in radio-electrochemistry since the concentration of the radio- 

 ions is not sufficient to carry the current exclusively. Therefore, we 

 studied the amounts of the radioelements deposited during a time of 

 24 hr, under precisely the same conditions, as a function of the cathode 

 potential. In the first method a sudden increase in the current strength 

 occurred at the value of the decomposition potential ; in the second 

 method there was a sudden increase in the amount deposited ; a further 

 difference between the two types of decomposition-potential curves 

 consists in that the deposition in the second type can be investigated at 

 potentials even higher than the decomposition potential whereas in 

 the the first type the cathode potential does not rise even when the 

 current is increased. 



We have plotted in Fig. I a decomposition-potential curve of 

 the second type for radium-E ; the solution was about 10"^N in RaE 

 (isotopic with bismuth). 



It is evident from the curve that some RaE is deposited at any potential 

 and that because of the sensitivity of the method this amount can be 



