Page 43 - MODERN ASPECTS OF ELECTROCHEMISTRY

Zbigniew KoczorowskiA
¨
Haber–Fajans cycle, e.g., for the standard hydrogen electrode in the
form 22,73–75.
o ,W
o
o
E H (abs) = (∆G at + G ion + α +)/F (18)
W
H
where the right side of the equation contains the energies of atomization
(dissociation), ionization of hydrogen, and real hydration of the hydrogen
ion.
On the basis of the above equations, the recommended absolute
potential of the standard hydrogen electrode is 22,73–75 :
o ,W
E H (abs)= 4.44 ± 0.02 eV (19)
o W,
In the calculation of EH the standard Volta potential of the calomel
48
electrode found by Randles has been used (see Section V). The other
0 ,W 76-78 76
estimators of E H (abs), for instance, -4.73 V byGomer andTryson
seem to be doubtful. A detailed discussion of this topic maybe found in
thepapersbyTrasatti. 22,73–75
Knowledge of the value of E H o , W (abs) m¸es it possible to convert all
relative values of electrode potential to an absolute scale.= For instance, the
standard electrode potentials of the oxygen electrode, the zero charge of
mercury, and the hydrated electron, in the absolute scale are equal to -5.67,
–4.25, and 1.57 V, recpectively. 73
Owing to the existence of relation (3), it is possible to define three
other single (i.e.,= absolute) electrode potentials.= However, only the poten -
tial defined by Eq. (16) has the reference state at a point close to the surface
where the Volta potential is measured. This case corresponds to a truly
universal absolute state (“electrons at rest in a vacuum”) adopted by
physicists to measure electronic woÀ= functions.=
The determination of the real energies of solvation from measure-
ments of the voltaic cells (Section VI) m¸es it possible to find the absolute
electrode potentials in nonaqueous solvents owing to the relation
0 ,W
0 , S
0 ,W
E H 0 ,S (abs) - EH (abs) = (α H -α H )/F
(20)
Some results of α 0.5 0 ,W are presented in Tablp l. 79
H and E H
The physical concept of a single electrode potential has been also
discussed in terms of the energy levels of ions in electrode systems.= This
concept maybe useful in the cases where the system has no electronic
energy levels in a range of practical interest, such as in ionic solid
crystalline and electronically nonconductive membrane electrodes. 80