CHAPTER IX 



Membrane Potentials 



summary of contents 



When two aqueous electrolyte solutions are separated by a membrane 

 which is impermeable for one of the ionic species present, a potential 

 difference arises between the two solutions, and also an unequal distri- 

 bution of the ions, including even the freely diffusible ions, ensues. 

 This effect diminishes with increasing concentrations of the electrolytes. 

 The relation of these potentials to the phase boundary potentials described 

 in the preceding chapter is discussed. 



62. The membrane as the cause of a potential 



The phase boundary potentials which we have just considered are 

 closely related to certain potential differences which we shall desig- 

 nate as membrane potentials. 



As was stated in a preceding chapter (see page 210), Wilhelm Ost- 

 wald suggested the idea that the impermeabihty of membranes for 

 any ionic species may give rise to potential differences. This theory 

 was first fully developed by Donnan^ whose investigations have 

 proved to be exceedingly fruitful. Donnan's derivation will be 

 first shown for the simplest possible case. Let us take a solution 

 containing only a Na-salt of an acid, the anions of this acid as well as 

 its undissociated molecules being unable to penetrate the membrane, 

 and let R~ represent this anion. Let us take a second solution con- 

 taining only NaCl, and interpose between the solutions a membrane 

 shown by a vertical fine in the diagram: 



Na+ 

 R- 



Na+ 



ci- 



(1) (2) 



1 F. G. Donnan, Zeitsehr. f. Elektrochemie, 17, 572 (1911). For the most 

 recent account in English see F. G. Donnan. The Theory of Membrane Equi- 

 libria. Chemical Reviews, 1, 73 (1924). 



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