Page 46 - MODERN ASPECTS OF ELECTROCHEMISTRY

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Voltaic Cells in Electrochemistry and Surface Chemistry of Liquids
o
of emersed and UHV-prepared electrodes have suggested the E (abs)
H
89
values to be ca. 4.8 V, i.e., similar to that of the emersed systems.
XI. VOLTAIC CELLS WITH INTERFACES OF IMMISCIBLE
ELECTROLYTE SOLUTIONSA
The interface separating two immiscible electrolyte solutions (e.g.,= one
aqueous and the second based on a polar organic solvent) maybe revers-
ible in respect to one or many ions simultaneously as well as electrons.=
For symmetrical electrolytes of, e.g.,= type 1:1, such a liquid/liquid inter -
face, in an equilibrium state, is characterized by the standard Galvani
potential, usually called the distribution potential. This very important
quantitycan be expressed in three equivalent forms, i.e., (1) using the ionic
standard potentials, (2) using standard Gibbs energies of transfer, (3) and
employing the limiting ionic partition coefficients 15,16,19 :
-
S
0
0
S
+
0
S
∆ ϕ (MX)= 1 _ [∆ ϕ (M ) +∆ Wϕ (X )]
W
W
2
1
= —[∆ S G + ∆ S G ]– W X 0 -
0
W
2F M
+
S,0
RT B W (M )
= ln S,0 - (21)
2F B W (X )
A water-nitrobenzene (NB) system containing tetraethylammonium
picrate in the partition equilibrium state has been proposed by us as a
convenient liquid junction bridge for the liquid galvanic and voltaic cells.=
The distribution potential of this system and the diffusion potential at the
93
contact of nitrobenzene with many organic solvents are close to zero. It
should be stressed that the use of salt bridges, which provide practically
constant or neglible liquid junction potentials, constitutes a nonthermody-
namic procedure that is necessary in many electrochemical experiments.=
In the case of the investigation of interfaces between immiscible electro-
lyte solutions, only the cells with transport provide valuable information.= 15
The interface separating two immiscible electrolyte solutions may
also be made virtually ideally polarized by a suitable choice of the
electrolytes dissolved in the phases.= For most of the water -nitrobenzene
and water–l,2-dichloroethane systems studied, the presence of a
tetrabutylammonium tetraphenylborate in the organic phase and a hydro-