686 RICE ART. L 



where c is the concentration of the ion in the solution, km. a 

 quantity "representing the concentration and environment in 

 the metal" and ke "represents the environment in the electro- 

 lyte". The solution is supposed to be dilute; in stronger 

 solutions log c would be replaced by the logarithm of the activity. 

 This is formally somewhat like Nernst's formula, km — ke replac- 

 ing the term containing the logarithm of the solution pressure. 

 Butler has derived from a statistical argument the result 



y. _ u + kt{\og r + log g) 

 ve 



where u is the energy change for the transference of one ion 

 from metal to solution, a the activity of the ion in solution 

 and r a small constant characteristic of the metal and depending 

 on the number of metal ions per sq. cm. of the metal surface. 

 (See Trans. Faraday Soc, 19, 729 (1924)). 



All these formulae for electrode potentials exhibit one 

 common feature. They attempt to express the P.D. as the 

 difference of two quantities, one related to the metal and one to 

 the electrolyte, and in that respect they resemble the theoretical 

 result obtained above for a contact potential between metals; 

 but the quantity related to the metal can scarcely be said to be 

 "characteristic" of the metal in the sense that it depends only 

 on the metal. Thus consider the formula of Butler; it appears 

 in the proof that uisw2 — wi, where Wi is a loss of energy by the 

 ion in travelling from the surface to a certain point in the liquid 

 against the ordinary attractive forces of the solid and adjacent 

 liquid, and w^ is a similar quantity for a movement from the 

 interior of the Hquid to the point. A careful examination of 

 the proof shows, however, that the position of this point would 

 alter with the concentration of the electrolyte, so that Wi would 

 change with this concentration; and so the quantity related to 

 the metal depends as regards its value on the nature of the 

 electrolyte. But, of course, the simpler state of affairs which 

 holds for metals in a chain could not be true for metals and 

 electrolytes; for if it were, no current would flow in any complete 

 circuit made up of metals and electrolytes, as is true in the 

 case of a complete chain of metals. 



