109 



Comment on papeks 7 — 9 



In paper 7 it was shown that a kinetic interchange takes place between the loud 

 atoms of solid lead chloride and the lead ions of a surrounding satnialed lead 

 chloride solution. This problem was later studied in detail by Paneth. lie found 

 that the uppermost molecular layer of lead sulphate powder participates only 

 in an interchange process. When investigating the behaviour of natural crystals 

 of lead compounds in several cases he found just a fraction of the lead atoms 

 of the uppermost molecular layer participated in a kinetic interchange, piesumably 

 positioned at the edges of the crystal. The other extreme case, an interchange 

 of almost all atoms of a precipitate with those of the surrounding solution, was 

 observed in the case of freshly prepared silver bromide b_\^ Langer and by 

 Zimmer. 



In paper 7 the velocity of dissolution and that of intert^iange of massive and 

 molecular layers was compared ; among other things, it was demonstrated that 

 the velocity of dissolution of ThB is diminished in the presence of lead ions in 

 the surrounding solution : however, the velocity of dissolution of the bismuth 

 isotope The was not diminished. This investigation, carried out between 1913 and 

 1914, aimed at the demonstration of the identity of the behaviour of isotopes. 

 \\'hen extending these studies to an interchange between the lead atoms of a 

 lead foil and the surrounding lead ions, several hundred atomic layers were found 

 1 o be involved in an interchange process presumably due to a dissolution of more 

 elec^tropositive parts of the lead foil followed by a precipitation of lead atoms on 

 moi"e electronegative parts of the foil. An adsorption of lead ions on the metallic 

 sur"face takes place as well, but its role is insignificant compared with the inter- 

 change of lead atoms. An investigation of the behaviour of colloidal load particles 

 c^arried out by the author and M. Biltz in 1929 brought out a marked adsorption 

 of lead ions by the colloidal lead particles and a slow interchange only betweeir 

 the lead atoms of the colloidal particles and the lead ions of the liquid phase. 

 In a simultaneous investigation of a system composed of colloidal copper and silver 

 ions (i.e. two metals showing a marked difference in their electrochemical potential) 

 besides some adsorption of silver ions on copper colloids an intense replacement 

 of copper atoms by silver atoms was observed. 



In papers 8 and 9 interchange of atoms between heterogenous phast^s \\as 

 studied. Paper 9 contains a report on experiment aimed at the elucidation if 

 and to what extent interchange of atoms takes place in a homogeneous phase. 

 When dissolving in the same solute non-radioactive tetraphen^l lead and labelled 

 lead chloride, or vice versa, no interchange of lead atoms was observed; this is 

 in contrast to a solution containing 1 mole of non-radioactive lead nitrate and 

 1 mole of labelled lead chloride ; after subsequent separation by crystallization 

 an equipartition of the radioactive lead atoms was found to lake place between 

 tlie chloride and nitrate of lead. The last mentioned result can be consideied 

 to be the most direct proof of the theory of electrolytic dissociation. 



Reference 



A. Langer (1943) J Chem. Phy.s. 11. 11 



K. Zimmer (1946) Arhiv f. Kemi, A 21, No 17. 



G. Hev-esy and M. Biltz (1929) Z. Phij-s. Chem. B 3, 271. 



