Page 39 - MODERN ASPECTS OF ELECTROCHEMISTRY

26
0
S
W
S
E =E – E =l/F∆ W α Ag + RT/F ln(a ± /a± ) Zbigniew Koczorowski
(10)
3
2
at the same concentrations in both solvents and assumes that the surface
potentials of the solvents are unmodified by the presence of low amounts
of AgClO i.e., <10 mol dm (see Section XII). The method was applied
–3
–2
4
to many solvents, among others, to methanol, ethanol, propanol, butanol,
ethylene glycol, acetonitrile, acetone and γ-butyrolactane, as well as to
mixtures of some organic solvents with waters 56–63
According to Eq. (2), the real and chemical energies of transfer differ
by the term containing the difference in surface potentials of a given
solvent and water (see Section XIV).
VII. REAL ION ACTIVITY
Rabinovich et al. have shown that it is possible to propose an extrather-
*
modynamic definition of single-ion activity, a , as a function of the real
potentials of those particles. 64–66 By carrying out the measurements of
voltaic cells containing electrodes reversible to the same ionic species in
solutions of different concentrations in the same solvent,

(IV)

it is possible to find the ratio of the real activities of the M+ ions in both
solutions:
S
a
E = RT/F In a /a + ∆ a χ = RT/F In a /a 2 * (11)
*
1
2
1
1
2
Therefore the real and chemical ionic activity coefficients are related by
the formula:
a
.
* 1
γi /γ i = γ /γ exp z F∆ a χ/RT (12)
* .2
1
i
i
i
2
If the concentration change in the surface potential, ∆ a χ, is close to zero
a 1
2
(see Section XII), the real and chemical activities are the same.
On the basis of this definition, one can determine, for instance, the
activity of electrolytic solutions in terms of the “real” hydrogen ion
65
activity. Rybkin et al. found that the ∆χ effect may be stabilized by
66

adding surface-active substances in small quantities to the solution. Ac-