244 HYDROGEN ION CONCENTRATION 



tendency to form ions or to exchange ions with the solution about it. 

 The simplest case is that of an adsorbent of the type of an insoluble 

 acid (silicic acid, mastic-resin acid), or of an insoluble base (metal 

 oxides). It was found that in such cases the adsorption proceeds 

 along the lines of ordinary chemical laws, and here it would be quite 

 futile to attempt to demonstrate any contrast between chemical 

 reactions and adsorptions. Thus, for example, ferric oxide (colloidal 

 ferric hydroxide) adsorbs picric acid and eosin, because ferric picrate 

 and ferric eosinate are insoluble. Likewise, silicic acid adsorbs 

 methylene blue, because methylene blue-siHcate is insoluble. To be 

 sure, the experimental study of this state is beset with unexpected 

 difficulties. For it is almost impossible to obtain such "insoluble 

 acids or bases" free of ions adsorbed on their surfaces with which they 

 form insoluble salts. Thus there is no siHcic acid preparation free of 

 Ca++,^^ and there is no colloidal iron preparation free of Cl~ (or of 

 some other anion, depending upon the method of preparation) with 

 which it forms an insoluble basic salt. For this reason those iono- 

 genic adsorbents which may be conceived of as insoluble "salts" are 

 more convenient for experimental work. These are represented by 

 such substances as kaohn (silicates of alkahne earths) and the so- 

 called ferric oxide (basic ferric chloride). Such adsorbents behave 

 entirely according to the well known laws of chemical reactions and 

 react with the electrolytes in aqueous solution by means of an ex- 

 change of ions. A solution of methylene blue chloride reacts with 

 kaohn according to the following scheme : 



Alkaline earth silicate + methylene blue chloride :^ Methylene blue silicate + 



alkaline earth chloride 



Since methylene blue silicate is insoluble, and the alkaline earth 

 chloride is soluble, the kaohn exchanges its alkaline earth cation 

 (especially the ever present Ca++) for the methylene blue cation, 

 while the Cl-ion of the methylene blue remains in solution, not as 

 HCl, however, but as CaCl2. An exchange of ions between kaolin 

 and NaCl cannot be demonstrated by analytical methods, although 

 doubtlessly according to general chemical laws such an exchange 

 can take place. Not a trace of eosin or picric acid is adsorbed by 

 kaohn, for an exchange of ions in these instances would not result 

 in the formation of insoluble salts. 



" Michaelis and Rona, Bioch. Zeitschr. 97, 57 (1919), cf. p. 71. 



