ELECTRODE POTENTIALS 171 



from c'2 to c'l the work amounting to RTln —j-. But since the electric 



C2 



work of the current and the osmotic work are maximal, they must 

 be equal to each other, hence 



Fa- = RTln - + RTln ^ 

 Ci c 2 



or, 



^,^^ RT, C2c'i 



IT = E.M.F. = -— - In — - 



F cic'2 



when -T- = Co, then 

 c'2 



E.M.F. = ^ In ^^ + ^ In Co = ^ In ^^ + K (1) 



F ci F F ci 



where K is a constant characterizing the given chemical reaction. 



Therefore equation (1) represents also the potential of a single Fe+++ 



— Fe++ electrode, in which equation an additive constant is left 



undetermined. 



B. The Quinhydrone Electrode — An Application. 



When an incompletely dissociated electrolyte participates in such 

 a reaction as outlined above, some special aspects arise which may 

 be developed into a very useful practical application, as in the case 

 of the quinonehydroquinone electrode. 



Hydroquinone, C6O2H6, is a very weak dibasic acid. Quinone, 

 C6O2H4, on the other hand, is practically a non-electrolyte. When- 

 ever a mixture of both of those substances is placed in contact with 

 a platinum electrode in a closed circuit the following process takes 

 place : 



C602Hr ;^ C6O2H4 + 20 



Secondary hydroquinone ion Quinone 



In analogy with equation (1) above, the potential of such a single 

 electrode is 



RT o' 

 E.f,n|+K (2) 



where c' is the concentration of the quinone and c is the concentra- 

 tion of the secondary hydroquinone ions. The value 2 appears in 



