Originally piiblishod in Phu-s. Z. 15, 797 (1915) 



8. THE EXCHANGE OF ATOMS BETWEEN SOLID 



AND LIQUID PHASES 



G. Hevesy 

 From the Institute for Jiadium Research of the Academy of Sciences of Vienna 



When a liquid is in contact with its saturated vapour there will 

 take place, in accordance with kinetic ideas, a constant exchange bet- 

 ween the molecules in the two phases. Correspondingly, it is to be ex- 

 |)ected that a kinetic exchange of the molecules will likewise occur when 

 a solid phase is in contact with its saturated solution. 



Radiochemical methods permit an experimental study of this ex- 

 change. For example, if the exchange between the molecules of a solid 

 layer of lead chloride and a saturated solution of lead chloride is to be 

 determined the following procedure is adopted : A known amount, in 

 relative (electroscopic) units, of ThB is added to a solution of known 

 Pb(N03)2 content and the whole is precipitated with hydrochloric acid. 

 In accordance with all previous experience the ThB can no longer be 

 removed chemically from such a mixture of PbClg and ThBClg ; if there 

 is, for example, one atom of ThB mixed with lO^*^ lead atoms on the 

 average in this mixture, this ratio will remain the same after any che- 

 mical operation and if a ThB atom can be detected electroscopically in 

 the lead chloride phase which was previously free from ThB the con- 

 clusion can be drawn that 10^° of the lead atoms originally mixed with 

 the ThB have also entered this phase. Thus the ThB or another isotope 

 of lead serves as an "indicator" for lead. 



If solid lead chloride labelled with ThB is shaken with a saturated 

 (unlabelled) solution of lead chloride for 36 hr at 20°C it is found 

 that less than 14 pcr cent has been transferred from one phase into tlie 

 other. 



The determination of the velocity of dissolution of lead chloride having 

 the same grain size showed that 44 per cent of the amount of PbCU 

 corresponding to saturation passed into solution within 1 hr ; since 

 the number of exchanged molecules (expressed as a percentage of all 

 those present) is extremely small it appears that the velocity of exchange 

 of lead chloride molecules between solid lead chloride and the saturated 

 solution of PbC'lg is vanishingly small compared with the velocity of 

 dissolution of solid lead chloride. 



/ Ilovesy 



