Page 32 - MODERN ASPECTS OF ELECTROCHEMISTRY

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Voltaic Cells in Electrochemistry and Surface Chemistry of Liquids
reversible galvanic cells.= The use of a twophase salt bridge containing
-
tetraethylammonium picrate (TEAPi), [e.g.,= water/nitrobenzene (w/nb)],=
in the galvanic cell shown in Fig. 3 is explained in Section XI.=
Generally, the electrochemical information obtained from an investi-
gation of voltaic cells depends on the composition, structure, and experi-
mental conditions of the systems.= It is easy to distinguish three basic
groups of such information.= The first (main) deals with the possibility of
measuring the Volta potentials and the differences in real potentials of ions
[Eq. (l)] for such two-phase systems as metal/metal, metal/liquid solu-
tions of electrolytes (Sections V, VI, IX, X), metal/solid electrolytes
(Section VIII), and electrolyte solutions in two solvents (Sections VI,
XIII) or in the same solvent.= The latter systems provide the basis for
measuring so-called real activities (Section VII). The second group of
information originates from the Volta potential measurements in systems
where conditions are created to keep the bulk and surface properties of
one phase consistent; the phase is treated as the reference. If the bulk
properties of the second (i.e.,= investigated phase are constant (second
group), the changes in the surface potential caused, e.g.,=by adsorption can
be measured (Section XII). This is the most popular application of voltaic
cells.= If the surface potential of the investigated phase is constant (third
group), one can measure, for instance, partition (Section XIII), diffusion,
and Donnan potentials.=
In a discussion of liquid voltaic cells it is necessary to distinguish two
groups of immiscible 1iquid/liquid interfaces: water/polar organic solvent
(e.g.,= nitrobenzene) and wated/nonpolar organic solvent, such as water/oil
or water/hydrocarbon (e.g.,= decane - or octane-type systems). As is sche-
matically shown in Fig. 4, the main difference is the presence of a
dissociated electrolyte in the organic phase of the first group in contradic-
4
tion to the second. As was clearly shown by Davies and Rideal, this
controls the character of the potential differences to be measured (see Fig.
4). If the organic phase constitutes the solution of the dissociated electro-
lyte, the ionic double layer is created there, and the Galvani potential
changes may be measured. In the opposite case, the voltaic cell allows the
surface potential changes to be measured. This behavior is fulfilled well
for systems in an equilibrium state, which contain a nonpolar oil phase.=
Between these extremes lie the intermediate cases of very slightly polar
solvents, to which belongs, for instance, the benzene phase saturated with
4
water. For these cases the results are dependent on the time and the
measurement method.