272 HYDROGEN ION CONCENTRATION 



then we must consider that the amount of water carried along by 

 an ion depends upon the so-called hydration of the ionic species 

 involved (see page 123). Hence we must also consider that the 

 amount of water transported in endosmosis at a given wall potential 

 depends at least as much upon the water binding capacity of the ionic 

 species passing along the wall as upon the internal friction of the 

 water. Furthermore, Helmholtz's theory regards the water as a 

 whole, and it does not admit the possibihty that the various con- 

 stituents of the solution, the water and the individual ionic species 

 in solution, may be differently displaced in the process of endos- 

 mosis. The superiority of the newer conception could be proven 

 if we succeded in demonstrating local changes in concentration of the 

 dissolved ions through endosmosis. Although these changes in con- 

 centration had been previously described, ^^ the more exact and con- 

 sistent study of this aspect of the problem was furnished in the im- 

 portant contributions by Bethe.^^ The fundamental observation 

 made by Bethe was the following. During the process of electroen- 

 dosmosis through a collodion, parchment or gelatin membrane, or 

 through a diaphragm of powdered material, the originally neutral 

 solution on the anode side of the diaphragm became acidified while 

 that at the cathode side it became alkahne (or the reverse of what 

 happens in the course of ordinary electrolysis of a neutral solution 

 between two platinum electrodes). A change in the concentrations 

 of the other ions was also observed, but by far the greatest and the 

 most easily demonstrable was the change in the concentrations of 

 the H- and OH-ions. This is but a natural consequence of the present 

 day theorj^ of endosmosis. Bethe and Toropoff developed this 

 theory in the following way: 



When, for example, the diaphragm wall is negatively charged, the 

 am'ons are held in the pores in an adhering layer, and to these is due 

 the negative charge on the wall. Above all others are the OH-ions so 

 held, then the polyvalent or the strongly adsorbable organic anions, 

 while the common univalent anions are bound but to a very slight 

 degree. Now, if the capillary spaces be only sufficiently small, then, 

 under certain conditions, almost all OH-ions, and possibly almost all 



<i Hittorf, Zeitschr. f. physikal. Chem. 39, 9, 613 (1901). 



«2 A. Bethe, Zentralbl. f. Physiol. 23, 1909, No. 9; Internat. Physiol. Kongr. 

 Wien, 1910; Munch, med. Wochnschr. 1911, No. 23; A. Bethe and Th. Toropoff, 

 Zeitschr. f . physikal. Chem. 88, 686 (1914) and 89, 597 (1915). 



