Originally publishod in Phijs. Z. 56, 790 (l<)2!)) 



14. THE APPLICATION OF RADIOACTIVE RECOIL 

 IN DIFFUSION MEASUREMENTS 



G. Hevesy iuul \\'. Seith 

 From lli(> Institulo of Physical Chemistry, Universil_\ of Froiburg 



A LAYER ol" thorium-B chloride placed on the surface of PblJlg show s a 

 decrease in a-recoil yield after heating. The velocity of diffusion of Ihc 

 thorium-B ion in lead chloride and thus the velocity of self-diffusion 

 of the lead ions can be determined from this effect. This extraordinarily 

 sensitive method by means of which diffusion constants down to 10 ^"^ 

 cm2 day"^ can be determined permits the measurement of the velocity of 

 diffusion in Pb( l, and Pblg in the vicinity of 100°C or at a higher 

 temperature. 



Two different cases must be distinguished in diffusion in crystalline 

 substances, heterogeneous diffusion and self-diffusion. The difference 

 between these two cases exists also in other states of aggregation but is 

 only slightly perceptible in the liquid and gaseous forms. During hetero- 

 geneous diffusion in crystalline substances individual lattice components 

 are replaced by foreign particles or the foreign ions (atoms) intrude into 

 the interstices of the lattice. In self-diffusion the lattice components are 

 replaced by identical particles, (.'onsiderable affinities between the 

 diffusing and lattice-element substances often operate during heteroge- 

 neous diffusion and we are confronted with a process which is a combi- 

 nation of a chemical reaction, often proceeding with a significant decrease 

 in entropy, and a true diffusion process. Self-diffusion produces merely a 

 positional mixing of the lattice components without any practicable change 

 of entropy. The phenomenon of self-diffusion is employed when informa- 

 tion is required on the strength of binding of the individual ions (atoms) 

 in the crystalline compound. In such measurement the method often used 

 is to study the diffusion of an ion, e.g. a cation, in the crystalline compound 

 whose cations are closely related to the diffusing one. For example, the 

 diffusion rate of cuprous ions in silver salts may be measured or that 

 of the cuprous ions in silver salts the cuprous and silver ions ])eing 

 considered to be nearly identical from the standpoint of diffu- 

 sion. The binding strength of silver and cuprous ions in various 

 compounds can be determined to a fair approximation from such 

 measurements. On the other hand i1 is not possible to determine the 



