ADSORPTION POTENTIALS AND ELECTROKINETIC PHENOMENA 239 



way it was possible to estimate the true adsorption equilibrium of 

 OH-ions by the addition of NaCl to a NaOH solution. The striking 

 result was in the finding that the adsorhahilily of H-ions and the 

 OH-ions is practically the same. 



The full value of the experiment in demonstrating quantitatively 

 the above relations is obtained only, when it is possible to assume 

 that the K-ions are so poorly adsorbed in comparison with the H-ions, 

 that they do not increase the adsorption of the H-ions, in spite of 

 their greater concentration. This assumption is practically quite 

 justifiable, for the very slight degree of adsorption of K-ions in com- 

 parison with H-ions is easily demonstrable by separate adsorption 

 tests with pure HCl and KCl solutions. The above does not hold true 

 for acids with strongly adsorbable organic anions. Thus, for in- 

 stance, saHcylic acid is adsorbed to a much greater extent than 

 HCl,^ apparently because its organic anion is more strongly ad- 

 sorbed than its H-ion. 



In this way the following figures were obtained. The limiting 

 value of the adsorption of HCl, HNO3, H2SO4, KOH and NaOH, in 

 the presence of an excess of the corresponding salts, from a 0.01 A^ 

 solution by one per cent by weight of the same kind of charcoal 

 (Merck's blood charcoal) was 40-45 per cent. (From a 0.01 N NaCl 

 solution, under the same conditions, the limiting values were 2 to 5 

 per cent). 



67. Charcoal as an insoluble ampholyte: acidoid, basoid, ampholy- 



toid, saloid 



An acid has been defined above (see p. 18) as a molecular species 

 which, while maintaining its negative electric charge, binds OH-ions, 

 and a base as one binding H-ions. This definition could also be ex- 

 tended to substances which are not molecularly dispersed. Thus we 

 could say that charcoal is an absolutely insoluble ampholyte, not molec- 

 ularly dispersed, which binds H- and OH-ions to the same extent. Its 

 isoelectric point is, therefore, in the vicinity of the true neutral 

 reaction. In the section on electroendosmosis it will be shown how 

 closely this theory agrees with the observations made upon the elec- 

 tric charge on charcoal. 



An attempt to apply the concepts of acid, base and ampholyte to 

 an absolutely insoluble substance may, however, lead to a certain 



^ Unpublished results. 



