290 HYDROGEN ION CONCENTRATION 



that he, in spite of considerable errors, has also brought forward 

 some useful suggestions, which, however still await experimental 

 verification. 



The following interpretation of Coehn's rule appears to the author 

 the most plausible. When one of the phases is of a distinctly iono- 

 genic character (silicic acid, ferric oxide), then Coehn's rule does not 

 applj^ at all; the laws covering this case were stated before. For 

 here the dielectric constant plays a subordinate part, and, according 

 to the nature and the concentration of the dissolved electrolytes, 

 the water may be either positively or negatively charged with respect 

 to the other phase. When the non-aqueous phase possesses a great 

 adsorption capacity (charcoal), Coehn's rule does not hold again; for 

 the charge on charcoal may be either positive or negative, again 

 depending upon the electrolytes in the contiguous aqueous phase. 

 On the other hand, the rule does seem to be valid in those cases where 

 at least one of the substances involved is of a chemically indifferent 

 nature, and where, at the same time, no great adsorption capacity, 

 such as that of charcoal, is present, and where, therefore, the adsorp- 

 tion of ions at the interphase boundary is of the "apparent" type 

 (see page 234) . In these latter cases the following might be a general 

 interpretation of Coehn's rule, which is offered with reservations only 

 as a provisional working hypothesis. TJ ,cause of the charge on the 

 interface hes chiefly in the traces of wat»„i .: ^ of the two ions com- 

 posing water the OH-ion is the more si active one. Since 

 water ionizes to a greater extent in a medium possessing the higher 

 dielectric constant, there occurs at the surface of such a medium an 

 accumulation of OH-ions, and hence this surface appears to be nega- 

 tively charged with respect to its own medium. 



87. The significance of adsorption potentials in physiology and 



colloidal chemistry 



The first question which occurs to us is whether adsorption po- 

 tentials bear any relation to the electric potentials and currents 

 which we observe with the help of electrical measuring instruments 

 in the Hving organism. This question had to be apparently answered 

 in the negative. For it was shown above that the adsorption poten- 

 tials manifest themselves only through electrokinetic phenomena 

 (electroendosmosis, etc.) and that they can not be led off by means of 

 metalHc electrodes. As soon as we attempt to lead off any potential 



