ADSORPTION POTENTIALS AND ELECTROKINETIC PHENOMENA 271 



The absolute amount of adsorbed H- or OH-ions depends upon 

 their concentration. That hydrion concentration at which H- and 

 OH-ions are equally well adsorbed, in the absence of other strongly 

 adsorbable ions, is designated as the isoelectric point of the adsorbent. 

 If it is at some finite [H+] then we are deahng with an ampholyte or 

 ampholjioid. If it is at [H+] = ^, then the substance is an acid 

 or an acidoid, and, if it is at [0H~] = o= , the substance is a base or a 

 basoid. 



As soon as, in addition to the H- and OH-ions, still other ions par- 

 ticipate in the adsorption (i.e., poorly adsorbable ions in high con- 

 centrations, or strongly adsorbable ions in any concentration) the 

 relations become more or less different. The presence of well ad- 

 sorbable cations had the same effect as an increase in the [H+], 

 while the presence of well adsorbable anions, analogously, as an 

 increase in the [0H+]. 



The reduction of these laws to the Hmiting case, when H- and 

 OH-ions only are present as actively participating ions, is surely but 

 an arbitrary or optional method. Theoretically one could start with 

 anj^ other pair of ions. But this representation has the advantage 

 that the ideal limiting case — absence of other active ions — can be 

 readily realized, which would not be possible for any other single pair 

 of ions. This arbitrariness is no greater than that involved in ascrib- 

 ing to the acids and bases alone of all the electrolytes an exceptional 

 position. 



80. Changes in concentration, especially of H-ions, in electro- 



endosmosis 



In comparing Helmholtz's theory of endosmosis with the modern 

 conceptions developed later, certain incongruities become apparent. 

 Helmholtz calculated from the potential difference between the 

 porous diaphragm and the water, which he assumed as given, the 

 amount of water transported by a given external electrical force. 

 His internal friction factor enters into his equation only from the 

 intrinsic properties of the water. Hence it appears as if any elect ro- 

 Ij^tes dissolved in the water, which scarcely affect its viscositj^, can 

 influence the process of endosmosis only insofar as they ma}'- alter the 

 potential of the diaphragm. But if we accept our above developed 

 view that the current of the external forces acts primarily upon the 

 movable ionic layer, and that the latter only carries the water along, 



