192 



HYDROGEN ION CONCENTRATION 



different boundary potentials produces under certain conditions 

 (not always) a curtent-jdelding chain. 

 One such arrangement would be: 



+ 



Na-oleate, 

 aqueous solu- 

 tion 



II 



Na-oleate in oil 



(oil layer rich 



in salt) 



TTl 



7r2 



The lack of equilibrium between two contiguous layers will be 

 found in this system at 2. But this is not a phase boundary and it 

 can only be the seat of an ordinary diffusion potential. As was 

 stated above (page 176), such a potential for the common salts 

 may amount at most to a few millivolts, while this chain produces 

 several centivolts. We have here two phase boundaries, both 

 behaving as metal electrodes; and the difference between their 

 potentials constitutes the E.M.F. of this metal-free chain, which 

 could be closed by means of a saturated KCl solution and thus 

 rendered free of diffusion potentials as well as metal-free. 



In order to obtain large E.M.F. values it is important to choose 

 an inorganic salt and an organic salt both of which are easily soluble 

 in the oU phase. This is comparable to a metal-containing chain 

 in which metals as far apart as possible in the electromotive series 

 are chosen as electrodes. 



The calculation of the E.M.F. of a chain of this type is begun by 

 finding that ion-species which is common to all the phases. In this 

 case it is the Na-ion. This chain can be designated as "a reversible 

 chain with respect to Na+." If now we designate the concentra- 

 tions of Na+ in the four vessels I, II, III and IV of the chain as 

 Ci, cii. Cm and Civ respectively, we shall have at boundary sur- 

 face 1: 



XI = RT In — + K 



and at the boundary 3: 



■"■2 



= RT In 



ai 



-IV 



^m 



+ K 



