ADSORPTION POTENTIALS AND ELECTROKINETIC PHENOMENA 285 



the wall: this is electroendodmosis. When the sohd wall is displace- 

 able (suspended particles) but the fluid is not displaceable (either 

 because it is in an enclosed chamber, or, w^hen in a U-tube, because 

 hydrostatic differences in pressure can not be maintained), then the 

 solid wall is moved along the fluid: — this is electrophoresis. 



Conversely, when the solid wall is mechanically displaced along 

 the fluid, a potential is formed in the direction of this displacement. 

 If the wall is fixed in place (a capillary or a diaphragm), and the fluid 

 flows along the wall, then a hydrodynamic current is formed. The 

 fluid being stationary and the sohd wall movable (falling particles), 

 a potential of falling particles arises. 



85. The splitting of phase boundary potentials 



The relation of phase boundary potentials to adsorption potentials 



In comparing the chapters on phase boundary potentials (pages 

 183 ff.) and adsorption potentials it appears rather remarkable that 

 the potential difference occurring at an interface maj^ be considered 

 in two entirely distinct ways. Taking, for example, the phase bound- 

 ary" glass-water and considering the potential as a phase boundary 

 potential, then the latter depends exclusively upon the [H+] in the 

 solution. But if the potential is considered as an adsorption poten- 

 tial, then it depends upon all the ions, and to an especially great 

 extent upon any trivalent cation present in the solution, which would 

 be of no significance for the phase boundary potential. Thus the 

 potential difference at the interface glass-aqueous solution, for 

 example, does not appear to be rigidly defined, but may differ accord- 

 ing to the nature of its manifestation which is taken into account. 

 We regarded it as a phase boundary potential when it mani- 

 fested itself in the form of an electromotive force in a Haber glass- 

 chain. We regard it as an adsorption potential when it manifests 

 itself in the form of electrokinetic phenomena. This discord w^as for 

 a long time a source of an ambiguity, which however is lately being 

 slowly dispelled, through the explanations which were developed by 

 Freundlich. For a better understanding of this problem we shall 

 present the following considerations offered by Haber.^° 



Let us take the chain shown in figure 31. A silver electrode on the 



»» F. Haber, Ann. d. Physik. [4], 927 (1918), cf. p. 949. R. Beutner, Dissert., 

 Karlsruhe (1908). 



