Page 37 - MODERN ASPECTS OF ELECTROCHEMISTRY

Zbigniew KoczorowskiA
24
molecules with the metal is rather we¸. The water dipoles are preferen -
tiallyoriented with their protons toward the mercury. 23
-
0
H
The standard Volta potential of the calomel electroded ∆ Ψ (Hg,Cl )
W
was determined for the first time by Klein and Lange, but the accepted
and usually used value of this potential (equal to 0.207 V) was found by
Randles.48 Later measurements byFarrel and McTigue et al.21 and Antro-
53
pov et al. gave 0.212 V and 0.220 V, respectively. In the methanol a value
equal to 0.25 has been found. 54
VI. REAL POTENTIALS OF IONS IN ELECTROLYTE
SOLUTIONS
S
The real energyof ion solvation, α i , defined byEq. (2), expresses the
change in ion energyupon its transfer from a gas phase into a solution.
Unløp the chemical energyof solvation, µsi, the value of the real energy
of solvation, also in the standard state, can be calculated from experimen-
tal data using the formula, e.g., for the hydration of the cation 48,55 :
0 0 0 ,M 0 0
H g
H g
0 ,W =F(E HgCl- -E Me, ⊥ Me α e + ∆G subl +∆G ,M - F∆ W Ψ (Hg,Cl-)
+
-α M ion
(8)
o
where E denotes the standard electrode's potential relative to a given and
H g
calomel electrode, α e is the real potential of electron in mercury(i.e.,
o
Hg
-φ e ), and ∆G gives the standard GibbÈ energies of sublimation and
ionization of Hg. The latter three terms, according to the thermodynamic
cycle, replace the real potential of ion in metal:
M + Hg 0 0 ,M (9)
-α M = α e +◊ G subl +G ion
Equation (8) may be easily found from the standard emf of the cell:
MCL (I)
M Hg Hg Cl , M
2
2
s
expressed as the difference in relative standard electrode potentials and
the sum of the Volta potentials of the three interfaces, and using definition
M
(I). It is worth noting that ∆ S ϕ or g (ion) potential differences fulfill the
M
S
Nernst logarithmic depedencies.=
Expression (8) can be used to calculate the real energy of ions in any
solvent, provided the standard potential of the calomel electrode, and the
-
0
H g,0
standard potentials of the elements and the ∆ s Ψ (Hg, Cl ) under study