288 HYDROGEN ION CONCENTRATION 



Although this problem is of great importance in colloidal chemistry 

 and in biology, it does not appear desirable to dwell upon it at 

 greater length, as long as the experimental material pertaining to it 

 remains as scant as it is. The fundamentals involved are still un- 

 certain, and yet the great importance of this problem, together with 

 that promise of its early solution which has been given of late, especi- 

 ally by FreundKch's researches, induced the author to present the 

 above provisional outUne. 



86. Coehn's rule 



All of the considerations of electrokinetic phenomena in the pre- 

 ced'ing pages were made with the assumption that one of the two 

 phases was water or an aqueous solution. We shall now briefly touch 

 upon the available facts and theory concerning these phenomena, 

 when a fluid other than water is the displaceable phase. The theoretic 

 treatment of this case is all the more difficult, since too Httle is known 

 about the dissociation of most solvents or of the electrolytes dis- 

 solved in them. An empirical rule has been worked out by Coehn. 

 This investigator determined*^^ a large number of soHd wall-potentials 

 by means of electroosmotic experiments with glass or quartz capil- 

 laries, and also in electrophoresis experiments on the migration of 

 liquid drops in an immiscible fluid. He first determined chiefly the 

 sign of the charge, and later he also attempted to estimate quanti- 

 tatively the potential by means of endosmotic measurements and 

 from Hehnholtz's equation.''*^ While Smoluchowski'^^ showed that 

 these numerical values could not be utilized without further ampli- 

 fication, still the qualitiative results are at least partially acceptable. 

 Coehn proposed the following rule : Every substance becomes negatively 

 charged with respect to another substance of a higher dielectric constant, 

 and it becomes positively charged with respect to substances of lower 

 dielectric constants. When the sohd wall is a glass capillary, then all 

 substance whose dielectric constant (D.K.) > 5 are positively 

 charged, and those whose constant < 5 negatively charged; see, 

 for example, table 43. 



The same rule appUes to the sign of the electric charge resulting 

 from the friction of two solid substances. For it is generally ap- 



«» A. Coehn, Wiedem. Ann. 64, 217 (1898). 



" A. Coehn and U. Raydt, Ann. d. Phys. 30, 777 (1909). 



