232 HYDROGEN ION CONCENTRATION 



able sign of charge {ampholytoids) . Among the acidoids there are some 

 whose sign of charge can be reversed, if not by H-ions, then by trivalent 

 cations. In the electroendosmosis of water, the changes in the concen- 

 trations of dissolved electrolytes on both sides of the diaphragm are 

 interrelated, especially the changes in H-ion concentrations. 



Another manifestation of adsorption potentials are the hydrodynamic 

 potentials. 



The adsorption potentials are electromotively inactive. When they 

 are led off to electric measuring instruments, in the same way as the 

 previously described phase boundary potentials, then only such phase 

 boundary potentials are manifested. The adsorption potential is only 

 a stage in that potential difference whose totality represents the phase 

 boundary potential. It is best, therefore, to represent the phase boundary 

 potential not as an unstable potential difference, but rather as a gradual 

 change of potential within a very thin but measurable layer. 



Finally, it will be attempted to correlate Coehn's law with these phe- 

 nomena, and the relation of boundary potentials to colloid chemistry 

 will be discussed. 



64. Definition of adsorption 



The concept of adsorption potentials as developed below includes a 

 whole series of electric phenomena which are of great importance in 

 biology and are of no less interest to the pure physicist. The prob- 

 lems of this field are as yet even further removed from their ultimate 

 solution than those presented above. It is possible that the method of 

 presentation chosen will contribute towards further elucidation, 

 insofar as it is unified in character and at the same time points out 

 the existing gaps in our knowledge. Therefore, the whole method of 

 presentation employed may be regarded as an attempt to fuse to- 

 gether facts and theories of varied origin into a single coherent 

 structure. The future is to show of what developments such a 

 structure is capable. The point of departure will be the phenomonon 

 of adsorption, especially the adsorption of ions. 



For this purpose it will be necessary to define adsorption in the 

 sense in which we shall use this term. Different authors have given 

 it different interpretations. For our purposes we shall define it 

 as that process by which a substance accumulates at a boundary 

 surface of two contiguous phases in a concentration higher than it 

 exists in the interior of these two phases. The question of the forces 



