78 ADVEXTUEES IN RADIOISOTOPE RESEARCH 



several cases for separation. On the other hand, the theory mentioned 

 above considers the chemical properties as essentially independent of 

 the mass, and this applies also to the spectrum and radius of the atom. 

 Differentiation between gravitational and electronic properties is 

 naturally only clear-cut in limiting cases ; for example, the velocity 

 of diffusion in liquids, which is primarily governed by the radius, is 

 not independent of the mass^ and, according to Bohr, the same 

 should also apply in respect of the Rydberg constant of the spectrum 

 series^ ; a difference in atomic weight of 1 per cent affects the latter 

 quantity by about 0.05 per cent. The characteristic vibrations of the 

 molecules in the space lattice, and consequently the specific heats, also 

 are probably noticeably different in isotopes^. 



2. CAN ISOTOPIC ELEMENTS REPLACE EACH OTHER CHEMICALLY? 



From the above discussions it is evident that isotopic elements are 

 certainly not truly identical ; the question now is whether they can be 

 denoted as chemically identical, i. e. whether they can replace each other 

 in their chemical mass action. It is well known that the concentration 

 of substances taking part in all chemical reactions is important (law of 

 mass action of Guldberg and Waage) ; if isotopes are chemically 

 identical the concentration must be represented by the sum of the 

 isotopic elements present. For example, the solubility product of ba- 

 rium-free radium-mesothorium chloride would be written in the form 



[Ra** -f mesothorium**] [Cl]2 = K 



Now there is a particularly clear method of testing for replaceability. 

 In electrochemical processes a jump in potential is determined by the 

 concentration of ions of the metal involved ; now when two elements 

 (A and B) are replaceable, the addition of ions of the element B to those 

 of A should exercise the same effect on the potential jump as if the ele- 

 ment A had been raised to the ionic concentration A + B. For example, 

 the potential difference RaD metal/RaD nitrate solution should be 

 changed to the same extent by the addition of lead nitrate solution, 

 within the meaning of Nernst's theory of the galvanic production of 

 current, as if the ionic concentration of RaD had been increased, and 

 vice versa. 



Instead of the electrode potential of a metal, the so-called decomposi- 

 tion potential, which, according to Le Blanc, is of the same magnitude 



iG. Hevesy, Phys. Z. 14, 1209 (1913). 



2N. Bohr, Phil. Mag. 27, 512 (1914). 



3 K. Fajans, Naturwissenschaften 2, 544 (1914). 



