204 



HYDROGEN ION CONCENTRATION 



when its ends are led off by means of two similar metal electrodes 

 (such as mercury-calomel-electrodes in 0.1 N KCl) may be easily 

 calculated. All electrode potentials are removed, with the excep- 

 tion of those on the two sides of the silver plate. These two sides, 



Ci 



according to Nernst, have in fact the potential difference of RT In—, 



C2 



which means that this chain is in reality one form of arrangement 

 of the usual (Nernst's) concentration chain. It can be imagined 

 that the metal silver strip is split into two plates which are con- 

 nected by a wire, and one of which is in contact only with the left 

 and the other only with the right solution. The whole of this ar- 

 rangement represents the usual concentration chain into whose 

 circuit are introduced two calomel electrodes, the potentials at 

 which are mutually aboHshed. It is hence permissible to neglect 

 these two calomel electrodes, and then the above arrangement be- 

 comes a typical concentration chain for Ag-ions. This demonstrates 

 that Nernst's concentration chain ma}^ be conceived of as a special 

 case of the phase boundary chain. 



The following is the simplest case in which the concentration 

 effect = 0. Two aqueous solutions of different concentrations of 

 the same electrolyte are separated by an oil. The oil is chemically 

 completely inert, is neither acid nor basic in character and does not 

 enter into any chemical reaction with the electrolyte. In such a 

 chain the E.M.F. is zero, in spite of the concentration differences, 

 as in the chain. 



The essential point involved in this case is that K+ concentration 

 is everywhere in the chain equal to the Cl~ concentration. In this 

 chain neither K+ nor CI" may be regarded as the common ion. 

 In the case of the K+ we should have as was shown on page 193. 



E = RT In ^' X ^^ 



C2 Cl 



