at the Electrodes in a Solution. 



47 



The decomposition of mixtures has recently been examined 

 theoretically by Nernst*. We shall consider the same subject 

 from a slightly different point of view. 



For this purpose we adopt the view, probably best adapted 

 for the treatment of chemical problems, that the energy ex- 

 pended by the current in passing through the drop of potential 

 at either electrode is the equivalent of the free energy required 

 to effect the change from ionic to free state, or vice versa, 

 taking place there, plus the energy expended on any other 

 processes which may accompany the passage of the current, 

 and which finally result in an irreversible heating-effect. For 

 our purposes we are justified in considering each electrode 

 separately, as we can always suppose the electrode not under 

 consideration so large that the nature of the processes taking 

 place there and the energy expended there per g.-ion do not 

 vary appreciably with the current-strength. We shall sup- 

 pose a mixture of two salts given, and consider the depo- 

 sition of «! g.-equivalents of the cation most easily deposited. 

 For the present we shall assume that if processes occur which 

 irreversibly cause the production of heat, this quantity of 

 heat shall be proportional to the number of g.-equivalents 

 liberated, so that it can be represented by /m 1? h being a 

 constant. Under these conditions, if w x be the quantity of 

 free energy required to liberate one g.-equivalent, the amount 

 of work done in the deposition of the n } g.-equivalents will be 

 independent of the current-density and equal to n^Wx-f-h). 

 Now the quantity of electricity which causes this will, 

 according to Faraday's law, have the value fn^ f being 



Faraday's constant of 96540 — — — \ — : . Therefore, if the 



g.-equiv. 



drop of potential from the electrode to the liquid be E, the 

 work done by the current in passing through it will be 

 Ett lt /*, and this according to our assumptions is equal to 

 n 1 (w l -\-h), or 'Ef=io 1 + h. 



This means that the difference of potential between the 

 electrode and the liquid is independent of the current-density 

 employed, and is always the same as the minimum E.M.F. 

 necessary to deposit those ions which are most easily set free. 

 If therefore there be ions of a second substance present, which 

 require for their liberation a minimum E.M.F. , E 2 >E, none 

 of them will be liberated so long as any of the ions first con- 

 sidered are left in contact with the electrode. It is only 

 when E 2 = E that simultaneous deposition of both occurs. Our 

 suppositions have thus proved equivalent to Le Blanc's view. 



* Zeitschr. phys. Chem. xxii. p. 541. 



