CHAPTER X 



Adsorption Potentials and Electrokinetic Phenomena 



summary of contents 



Adsorption is defined as that phenomenon in which a substance dis- 

 solved in one phase becomes concentrated at the boundary surface of an 

 adjacent phase, irrespective of the forces causing this concentration. If 

 such adsorption involves dissolved electrolytes, then the unequal ad- 

 sorption of positive and negative ions may give rise to a potential differ- 

 ence at the boundary surface. By studying the adsorption of electrolytes 

 on charcoal one may obtain an idea of the adsorbability of the different 

 ions. That of the H- and OH-ions appears to be especially great. The 

 adsorbing surfaces may be grouped, according to their capacity to bind H- 

 or OH-ions, as acidoids, basoids or ampholytoids. This classification 

 corresponds to that derived on the basis of chemical constitution for the 

 water-soluble acids, bases and ampholytes. In the process of adsorption 

 all cations compete with the H-ions, while all anions compete with the 

 OH-ions. Because of an opposite exchange of ionogenic radicals between 

 the adsorbent and the dissolved substances exchange-adsorptions arise, 

 which may be considered almost entirely in terms of purely chemical 

 phenomena. The adsorption of ions is the cause of the electric charge on 

 the boundary surface. These potential differences become manifest in 

 the phenomena of endosmosis or cataphoresis, whose history and theory 

 will be reviewed. The tangential displacements of the double layers 

 against each other are chiefiy involved. The electrically charged or ionic 

 layer moving in water carries along with it water particles. According 

 to whether the water or the solid particles are fixed by mechanical means, 

 cataphoresis or endosmosis ensues. The effect of the dissolved electro- 

 lytes upon the magnitude and direction of the endosmosis or the cata- 

 phoresis permits one to predict the magnitude and direction of the 

 adsorption potentials, and the results correspond to those to be expected 

 from the determination of the actual adsorption of ions. In this con- 

 nection we find again thai the adsorbents may be grouped into: always 

 negative (acidoids), always positive (basoids) and those with a change- 



231 



