ADSORPTION 177 



We are forced at the outset to admit that all attempts to dis- 

 tinguish between types of adsorption are, in the present state 

 of knowledge, futile. We cannot distinguish between electronic 

 adsorption and primary valence, or polar and nonpolar adsorp- 

 tion; but that differences exist is evident; for example, the adsorp- 

 tion of an electrolyte by a difficultly soluble salt such as kaolin 

 is probably different from that of a nonelectrolyte adsorbed by 

 what we may call a "nonsalt," for want of a better term, such 

 as charcoal. The adsorption of an ionized salt, like the chloride 

 of methylene blue, by kaolin, involves an exchange of ions (the 



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yVVVYVYV 



/yVVyyyy^ 



vAyVvVVVV 

 VVV\A 



XXX>(X 



Fig. 99. — Diagrammatic representation of the free valence bonds of atoms at the 



surface of a block of carbon. 



cation of the dye methylene is adsorbed by the kaolin; the anion, 

 chlorine, stays in solution; in order to keep the solutions elec- 

 trically balanced, the kaolin gives up calcium ions), but when 

 charcoal takes carmine out of its solution, there is no exchange of 

 ions. 



While it is best not to distinguish too sharply as yet and to 

 regard adsorption as any form of surface concentration, yet 

 differences exist, and we may, therefore, consider the types of 

 bonds that could be responsible for adsorption phenomena. 



The union between atoms established by the sharing of an 

 electron is primary valence. Adsorption may be of this type and 

 is then said to be electronic. It is distinguished from primary 

 valence in that it is strictly a surface reaction, involving only 

 those molecules which are exposed at the surface of the adsorbent. 

 The distinction is, however, purely arbitrary. It is illustrated 

 in Fig. 99, which is an imaginary cross section of a block of carbon. 

 All four of the valence bonds of each carbon atom within the 



