Hans Jenny 



125 



EQUILIBRIUM STUDIES ON ION EXCHANGE BETWEEN ADSORBENTS 



Symbolically, we may formulate the relationship of contact intake 

 and contact depletion between plant roots (R) and clay particles: 



Vh 



+ K 



clay ^ 



V 



K + H clay | 



The question whether the reaction will tend to the left (contact deple- 

 tion) or to the right (contact uptake) depends on the amounts of the 

 two adsorbents and on the forces with which the various ions are held 

 to the two surfaces. 



In approaching a quantitative elucidation of the above process, it 

 appears advisable to first study in detail the exchange reaction between 

 two adsorbents in vitro. Let us consider the exchange of the ammonium 

 and sodium ions, between two different adsorbents, either by solution 

 or by contact, as represented by the following system: 



Ion-X NH 4 + Na Amberlite 



Ion-X Na + NH 4 Amberlite 



Amberlite and Ion-X are artificial cation exchange resins. It is pos- 

 sible to study the reaction quantitatively by mixing coarse Amberlite 

 particles (> 0.5 mm.) with fine Ion-X particles (< 0.1 mm.). After 

 a shaking period of several days the two adsorbents are separated by 

 wet sieving and analyzed individually. In the writer's laboratory, Dr. 

 K. Sengupta has shown that the above reaction gives a characteristic 

 exchange constant according to the equation: 



Na Ion-X • NH 4 Amberlite 



.46 (experimental) 



NH 4 Ion-X 



Na Amberlite 



It is possible to calculate this exchange constant without actually mix- 

 ing the adsorbents, provided the ordinary base exchange reactions be- 

 tween the adsorbents and neutral salt solutions are known. 



Consider the two connecting vessels of Figure 8. Initially the stopcock 

 is closed. The left hand vessel (system X) contains coarse ammonium- 

 Ion-X and sodium chloride. At equilibrium we have: 



