184 HYDROGEN ION CONCENTRATION 



It will be useful to recall clearly the following points. 



A current producing chain can only be represented by a system 

 which, when considered as a whole, is not in a state of chemical 

 equilibrium. 



The E.M.F. of such a chain is the algebraic sum of all the single 

 potential differences present between the poles of the chain. Such a 

 single potential is always found between two contiguous laj^ers. 

 These two contiguous layers may coexist in chemical equihbrium 

 (metal electrode potentials, phase boundary potentials, and see below 

 for Donnan's membrane potentials), i.e., the electric force is com- 

 pensated by an opposing force. Or they may not coexist in a state 

 of equihbrium (diffusion potentials). Two contiguous layers, con- 

 sidered as an isolated chemical system, retain their potential differ- 

 ence as long as they remain in chemical equilibrium (e.g., a single 

 silver electrode immersed into a AgNOa solution) ; and they gradually 

 lose their potential difference when the latter depends only upon the 

 lack of equilibrium (e.g., the boundary layers between a concentrated 

 and a dilute HCl solution which are in contact; when these two 

 solutions become homogeneous through diffusion, the potential 

 difference disappears, without any electric current having been 

 generated) . 



Two true aqueous solutions never form distinct phases in relation 

 to each other. For this purpose there are required two solutions 

 having immiscible or poorly miscible solvents, e.g., water and oil. 

 As a rule in the following discussion we shall assume an aqueous 

 phase in contact with an ''oil" phase as it was designated by Beutner. 

 A better way of stating this would be: two phases of different 

 dielectric constants. 



The first fundamental principles for a rational examination of 

 boundary potentials are to be found in a paper by Nernst^ who also 

 in this field must be regarded as the founder of the theoiy; and then 

 also in the equally important investigations of Haber.- The signifi- 

 cance of these investigations of Nernst and of Haber for physiologi- 

 cal processes had not been recognized for a long time by physiologists, 

 although these authors were fully aware of the far reaching physio- 



1 W. Nernst, Zeitschr. f. physikal. Chem. 9, 140 (1892); Riesenfeld, tTber 

 elektrolytische Erscheinungen und elektromotorische Krafte an der Grenze 

 zweier Losungsmittel. Diss. Gottingen 1901 ; Nernst and Riesenfeld, Ann. d. 

 Physik. [4] 8, 600 (1902). 



2 F. Haber, Ann. d. Physik. [4] 26, 927 (1908); Haber and Klemensiewicz, 

 Zeitschr. f. physikal. Chem. 67, 385 (1909). 



