278 HYDROGEN ION CONCENTRATION 



made the error of correlating the solution tensions of the ions with 

 their mobiUty, which error was pointed out by F. Haber.'*'' The 

 solution tension bears no relation to the mobiHty of the ion. 



Freundlich's conception is based on the idea that the ions, as well as 

 the ordinary molecules, may be adsorbed on boundary surfaces, and 

 that the adsorbabilities of the anion and of the cation of an elec- 

 trolyte are unequal. He also ascribed a particularly high degree of 

 adsorbabiHty to the H- and OH-ions. He derived the adsorption 

 potential from the different adsorption tendencies of the anions and 

 cations. To the adsorbents, according to their chemical (acidic or 

 basic) character, he ascribed the tendency to dissociate off H- or 

 OH-ions respectively, which ions would then lead to the formation 

 of the double layer. At this juncture Freundlich fell into the same 

 error as Michaelis did, in correlating the spHtting off of these ions with 

 their high mobiUties. But Freundlich showed at the same time that 

 the adsorption potential need not be identical with the potential 

 which the same adsorbent (when it is a metal, for example) would 

 have towards the same solution, when it is used as a pole of a galvanic 

 chain. This point must be recognized as being of very great im- 

 portance, and this phenomenon will become more comprehensible 

 in section 85 on "The SpHtting of Potentials." 



In general there does not appear to be any great difference between 

 FreundUch's conception and that of the author. However, one 

 difference between the two does appear in the following respect: 

 Freundlich takes pain to maintain as far as possible a Hne of demarca- 

 tion between chemical reaction and adsorption on boundary surfaces. 

 He regards the adsorbent chiefi}^ as a means of increasing the surface 

 of the solution, and he does not attribute any particular value to the 

 chemical nature of the adsorbent. This idea appears more distinctly 

 in his chapters on the ordinary adsorption of non-electrolytes than 

 it does in the above cited pages dealing with adsorption potentials. 

 In this latter chapter Freundlich cannot help attributing a special 

 importance to the acidic or basic nature of the adsorbent. And yet it 

 does not appear from this chapter that he thereby relinquishes his 

 sharp line of demarcation between adsorption and chemical reaction. 

 The present author, on the contrary, took pains at the outset to re- 

 move the contrast between adsorption and chemical reaction, at 

 least in the case of chemically reactive (acidic or basic) adsorbents. 



" F. Haber, Ann. d. Physik. [4], 927 (1908) ; s. p. 948. 



