172 HYDROGEN ION CONCENTRATION 



the divisor of the right hand term because the secondary hydro- 

 quinone ion is bivalent, and, therefore, the transport of F coulombs 

 is equivalent to the osmotic transport of ^ mol. 



The ratio of the concentrations of the secondary hydroquinone 

 ions and of the total hydroquinone is regulated by the mass law in the 

 following manner: 



[CeOsHe] 



where [C6H2O6] means the concentration of the undissociated hydro- 

 quinone molecules. 



When the reaction is not too alkaline, pH < about 8.5, so that the 

 primary as well as the secondary ions of the hydroquinone form but an 

 insignificant portion of the total hydroquinone, the concentration 

 of the undissociated hydroquinone is practically equal to that of the 

 total h3^droquinone. Hence the last equation may be written 



[C.O,H.-, = ypp 



where [C6H2O6] means the total concentration. 

 By substituting (3) into equation (2), we obtain 



E = — In + K 



2F k [CeOoHe] 



which may be further simplified by substituting Ki for K, so as to 

 include k, thus 



^ RT , [CBO2 H4] [H+] 



^ = T ^" [CeO.He] + '"' 



When in a series of such different electrodes the ratio 



[Hydroquinone] . , ^, , 1 ,, rTT_Li • • 1 



— rr— : ^ — is kept constant, = K2, and only the [H+j is varied, 



iQumone] 



then 



E. = 5T ,„ Ell + K. - 51 ,0 IH^I + K. 

 F K2 F 



This means that the potential depends upon the pH, exactly as it 



does with hydrogen-gas electrodes. By setting up a chain consisting 



, , 1 • 1 • , , 1 . • [Quinone] . , , 



of two such electrodes m which the ratio ,^^ ; -■ "i ^^ ^"^ same, 



HydroqumoneJ 



