ADSORPTION POTENTIALS AND ELECTROKINETIC PHENOMENA 201 



alkaline reaction, but it is quite clear that this reversal does not in 

 general occur at exact neutrality. 



The investigations by Gyemant^" in the author's laboratory 

 j-ielded results given in table 40 (pp. 262-3). 



77. Application of the acidoid theory to the sign of charge on 



diaphragms 



The profound influence of the H- and OH-ions which Perrin found 

 in his investigations, led him to the assumption that the electric double 

 layer is always formed only by the ions of the water, one layer con- 

 sisting of H-ions and the other of OH-ions. He beheved that other 

 (univalent) ions do not participate in the formation of the double 

 layer. Let us accept Perrin's assumption for the time being, just as 

 in the development of the theory of dissolved electrolytes we started 

 out with the simplifying assumption that no cations competed with 

 H-ions and no anions competed with the OH-ions. Now, by com- 

 bining Perrin's assumption with the theory of ionic adsorption, we 

 may arrive at the following conception: The charge upon any sohd 

 wall depends upon the fact that it adsorbs H- and OH-ions to different 

 extents. A wall which adsorbs only OH-ions can therefore acquire 

 only a negative charge (cellulose, collodion, agar); a wall which is 

 capable of adsorbing only H-ions can have only a positive charge (such 

 are hitherto unknown). A wall which can adsorb either of the two 

 ions will be either positive or negative, depending on the reaction of 

 the adjacent solution. The point of reversal of the charge need not 

 he at the neutral reaction. This can only happen when the wall has, 

 at the neutral reaction, an equal capacity to adsorb H- and OH-ions. 

 We have shown this condition to obtain quite closely in the case of 

 charcoal. In the case of gelatin the relationship is as follows. Its 

 isoelectric point is at [H+] = 2. 10~^ At greater values of [H+] it 

 adsorbs H-ions. Hence it can be said that the gelatin wall consists 

 of electroneutral molecules which acquire an electric charge by 

 adsorbing H-ions; or, that the wall consists of positive gelatin ions, 

 for a molecule which had bound an H-ion in acquiring its charge is 

 according to our earlier definition (page 16) a positive ion. When 

 the [H+] is less than that corresponding to the isoelectric point, 

 hence the [0H~] is greater, the gelatin binds OH-ions and acquires a 

 negative charge; which is the equivalent of saying that the gelatin 



