ADSORPTION POTENTIALS AND ELECTROKINETIC PHENOMENA 277 



bines these two properties: (1) neither of its ions must have a marked 

 degree of adsorbabiHty as compared to H- and OH-ions, (2) 

 the mobihties of its two ions must be equal to each other. Of 

 all the electrolytes KCl fulfills these conditions best, other binary 

 univalent electrolytes, such as NaCl, only to a certain extent, but 

 NaoSOi, for example, not at all. All of the authors mentioned before 

 considered the indifference point of water mobihty as the isoelectric 

 point of the diaphragm, but according to Bethe and Toropoff this 

 is not quite correct. 



81. History of the theory of adsorption potentials 



The theory of adsorption potentials, as it is developed in this 

 section, is not to be found in this form in the literature. The fact 

 that ions may become adsorbed is quite obvious and was stated by a 

 number of authors, without there being any evident historic sequence 

 on this point. The present day theory dates back chiefly to two 

 sources, and may be thought of as the result of the blending of the 

 two, as it were. One is the earlier description by the present author^* 

 and the other was that given by Freundlich.^* Beginning with the 

 first, Michaelis, on the basis of a theoretical research of Nernst's,^^ 

 conceived a colloidal particle suspended in water, for example of a 

 resin acid (mastic), as of a "binary electrode," which, in distinction 

 to a simple electrode, tends to send into solution not one ion species 

 only, but H-ions and acid anions simultaneously. From the different 

 solution tensions of these two kinds of ions taken in conjunction 

 with the differing concentrations in which these two ions are present 

 in a state of true solution in the aqueous phase, the author derived 

 the potential of the double layer at the interface. Should there be 

 present in the solution any species of ion which is capable of forming 

 a salt of this acid, an ionic exchange occurs in the double layer 

 which alters its potential. Already at that time the author explained 

 this ionic exchange by stating that it is identical with ionic adsorption. 



In a theoretical derivation of the ionic equilibrium the author"**^ 



" L. Michaelis, Physik. Chemie der KoUoide, in Richter-Koranyis Hand- 

 buch: Physikalische Chemie und Medizin. Leipzig 1908. L. Michaelis, 

 Dynamik der Oberfliichen. Dresden 1909. P. 49 ff. 



** H. Freundlich, Kapillarchemie. Leipzig 1909. p. 213 ff. 



« W. Nernst, Zeitschr. f. physikal. Chem. 9, 137 (1892). 



" L. Michaelis, Zeitschr. f. Elektrochemie 14, 353 (1908). 



