ELECTRODE POTENTIALS 163 



observed and calculated values. This was because of the imperfect 

 understanding of the dissociation of strong electrolytes and because 

 of the incomplete abohtion of the diffusion potential at the contact 

 of the two solutions. It seems that the most appropriate arrange- 

 ment for the demonstration of the absolute exactness of Nernst's 

 equation for concentration chains is the following : 



Here the concentrations Ci and C2 of the HCl must be relatively- 

 small in respect to the KCl concentration (1.0 N), i.e., they should be 

 between 0.0001 and 0.05 N. The excess of KCl accomplishes the 

 following purposes: (1) The degree of dissociation of the HCl 

 (or better, the activity factor of the H+-concentration) is sufficiently 

 near equaUty in both solutions so that the ratio of the HCl concentra- 

 tions is actually equal to that of their H+-activities, and (2) the 

 diffusion potential remains infinitely small. Such chains give 

 E.M.F. values which rarely show a maximum deviation from the 

 calculated figures of 0.0005 volt, and the average of repeated measure- 

 ments usually agrees perfectly with the theory. Thus this verifica- 

 tion of the theory constitutes the convenient method of checking 

 the correctness of the experimental technic for any worker in this 

 field. 



48. Reversible electrodes for concentration chains 



The condition sine qua non, for Nernst's equation is the reversibility 

 of the electrodes. This means that, if the circuit of the chain is 

 closed for a definite period, and then if the exact amount of current 

 produced is sent through the chain in the reversed direction, the 

 original state of the chain must be reestabhshed exactly. Each of 

 the two electrodes must be capable of functioning reversibly. This 

 is alwaj^s the case when a metal is immersed into a solution of its 

 salt and (the circuit being open) no chemical reaction can occur. 

 Thus Zn in dilute H2SO4 is not a reversible electrode, since it goes 

 into solution spontaneously, but Zn in ZnS04 is such an electrode. 

 Zn in CUSO4 is not reversible, for the spontaneous reaction occurs, 

 Zn + CUSO4 -^ ZnS04 + Cu. Hg in water is not a reversible elec- 



