84 ADVEXTUKES IN RADIOISOTOPE EESEAKCH 



It is evident that the cells are identical within the limits of experi- 

 mental error. We attach less importance to this than to the fact that 

 the addition of Pb ions to RaD nitrate solution exercises precisely the 

 same effect on the potential difference of the RaD peroxide which, 

 according to Nernst's theory, the Ra D ions (and only they) should have. 



RT , c 



in 



is to be 



This proves that c in the Nernst formula 



^ nF C 



understood as the sum of the concentrations of the isotopic ions present. 

 A special peculiarity of this RaD peroxide electrode deserves to be 

 mentioned. If it is allowed to remain in contact with air for some time 

 it immediately shows, on immersion, a potential which may be one- 

 or two-tenths of a volt higher than the constant electrode potential 

 established after a certain time. This is probably connected with the 

 strong ionization in the vicinity of the wire. 



(d) Experimental Details 



The curves described above for the decomposition potential of RaE 

 were obtained as follows : Two gold electrodes, each 1 cm^ in area, were 

 immersed in 25 cm^ of O.IN nitric acid solution and were polarized for 

 a long time until the desired electrode potential had established a con- 

 stant value. A steady motion of the solution was ensured by passing a 

 current of nitrogen. After the attainment of constant potential a few- 

 tenths of a cubic centimetre of a solution at the same nitric acid con- 

 centration and containing radium-E or RaE and Bi was added^ and the 

 experiment was allowed to run for 24 hr. After this time the electrodes 

 were withdrawn without interrupting the current, w^ashed with distilled 

 water, always in the same way, and measured in an electroscope ; 5 cm^ 

 of the solution were evaporated on a watch glass and likewise measured 

 and hence the percentage deposition of RaE could be calculated. The 

 experiments with ThB also were carried out similarly ; in this case the 

 active solution was added to 100 cm^ of 0.001 N nitric acid and the de- 

 position was made on correspondingly pre-treated platinum electrodes 

 with an area of 4 x 2 cm^. The potential difference was measured hy 

 means of a Siemens compensating apparatus. 



The RaE solution was obtained directly from emanation, and the 

 thorium-B by exposure of a platinum foil to radiothorium. Particular 

 care was used in the latter case to exclude lead completely ; a part of 

 the experiment was carried out in quartz vessels anrl with the use of 

 water purified specifically for this purpose-. 



1 The addition of small amounts of bismut h to make the solution about lO'^'X 

 often caused an initial change of the cathode potential by several millixolts. 



2 The purification of the water was that usually employed for dctei mirations 

 of atomic weicht (of. O. Honigschmid, Mitt. r/. Tn-st. Bndiitiujoy.schinnj. 8, S). 



