MECHANISMS OF ION TRANSPORT 



33 



Salt 

 HgO j solution 



-M^ 



(a) 

 Membrane permeable 



to M* and A" 



Salt 



solution 



Salt 



solution 



I 



Kc) 



Membrone permeable 

 to M'^, but not to A" 



Salt 



Salt 



solution j solution 

 I 



M* — \^M* 

 I 



■X" 



A " i > A' 



1(e) 



Membrane permeoble 



to M*and A", but not to X* 



H3O 



Salt 

 solution 



■M" 



■<— A" 



(b) 



Membrane permeable 

 to either M*or A", but 

 not to both 



Salt Salt 



solution solution- 



I 



^--*- 



-^ M2 



i(d) 



Membrane permeable 

 to A", but not to M"^ 



Salt I Salt 

 solution solution 



I 



Membrane permeable 



to M+and A", but not to Y" 



Fig. 7. Ion exchange across membranes 



1949). An equilibrium is established when the ratio of radioactive 

 to non-radioactive ions is equal on the two sides, and the solutions 

 are then said to have the same specific activity. 



E. DoNNAN Equilibria 



A more complex ion-exchange system is one in which a membrane 

 separates a solution containing ions to which the membrane is 

 permeable, from another containing, in addition to mobile ions, 

 either cations or anions to which it is impermeable (Fig. 7e, f.). 

 In the simple situation represented in Fig. 7e, assuming that the 

 concentration of the single anion ([A"]) on the two sides is initially 

 equal, the concentration of diflfusable cations on the right-hand side 

 ([M+]2) will be lower than that on the left-hand side ([M+]i) since 

 part of the anions are balanced by X+. Cations will thus tend to 

 move by diffusion from left to right along a concentration gradient, 



