ADSORPTION POTENTIALS AND ELECTROKINETIC PHENOMENA 287 



upon the nature of the solution. Really pure metal surfaces do not 

 last for any length of time in solutions, and most metals always 

 become coated, at least with a layer of oxide. 



It is furthermore clear that the layer interposed between A and C 

 need not be a third phase. The interposition of a layer in which, 

 because of interface effects, the ions would have a different distri- 

 bution from that in the interior of the phase will suffice to resolve 

 the potential between A and C into two stages. Since the composi- 

 tion of these interface layers changes but gradually in the direction 

 of the interior of the fluid, it may be also assumed that the potential 

 drop caused by such an adsorption layer will be altered in a gradual 

 way represented by a smooth potential-gradient curve.*'^ It may 

 even happen that this curve will show a maximum and minimum, 

 and that a portion of this potential curve will have a sign different 

 from that of the total poten- 

 tial.f'3 Thus Freundhch and 

 Gyemanf^^^ described cases in 

 which they found on one and 

 the same experimental mate- 

 rial different signs before the 

 values of e and f . In figm-e 32 

 is shown Smoluchowski's sche- 

 matized graphic representation 

 of the gradual fall of potential 

 at an interphase layer. 



If this interpretation is correct, we should expect a discrepancy 

 between the electromotive and the electrokinetic potentials in all 

 those cases in which an adsorption layer is present at the interface, 

 and this should always be the case with neghgible exceptions. 



In figure 32, A represents the interface of water-glass, and AB is the 

 thickness of the adhering stationarj^ water layer. The curve repre- 

 sents schematically the change of potential; in the interior of the 

 water phase it is at the level c. The electromotive potential drop, 

 which is manifested in the case of the glass chain, is e = a — c. The 

 electrokinetic potential drop corresponds to f = a — b. 



a — c 



a-6 = f 



Water 



6/ass 



Fig. 32 



62 



V. Smoluchowski, in L. Graetz Handbuch der Elektrizitat und des Mag- 

 netismus. 1912 and 1921. Bd. II. 



«2 H. Freundlich, KoUoid-Zeitschr. 28, 240 (1921). 



'* Freundlich and Gyemant, Zeitschr. f. physikal. Chem. 100, 182 (1922). 



