CHAPTER VII 



Diffusion Potentials (Potential Differences at Liquid 



Junctions) 



SUMMARY of CONTENTS 



Potential differences arise at the junction of two aqueous solutions 

 which differ in kind or in the concentration of the dissolved electrolytes. 

 These can he explained and calculated on the basis of Nernsfs diffusion 

 theory. The calculation is simple in a few special cases, but in general 

 it is difficult. Since these potentials are of little significance in the 

 realm of physiology, and since they can, in experimental technic, be 

 readily avoided, their calculation may well be omitted in this work. 

 The most suitable method of preventing diffusion potentials is through 

 the use of a salt bridge consisting of a saturated KCl solution. 



51. When a solution has in different places a var5dng content of 

 electrolytes a potential diJfference arises between the places of different 

 composition. The difference in composition may either be one of 

 concentration of the same electrolyte or one of the kind of electrolyte 

 involved; or it may consist of both of these variations at once. But 

 the solvent is necessarily the same throughout the solution, and, as 

 a rule, it is water. Cases in which this last condition is not fulfilled 

 will be dealt with in another chapter. Nernst furnished an explana- 

 tion of these potentials on the basis of the ionic theory in the follow- 

 ing manner. Let us assume that we have in the left part of a horizon- 

 tal tube an HCl solution of the concentration Ci and in the right hand 

 part an HCl solution whose concentration is C2. At the junction of 

 these two solutions diffusion occurs, and then a concentration gradient 

 is formed. The relative incline of such a gradient depends upon the 

 difference of the concentrations and also upon the rate of diffusion 

 (or diffusion coefficient) of the diffusing substance. The more 

 rapidly the substance diffuses the smaller the gradient. If two 

 different substances are present in solution, then each develops its 

 own gradient independent of the other. Since HCl in solution is 

 really a mixture of H- and Cl-ions, the same would apply in this 



174 



