Page 29 - MODERN ASPECTS OF ELECTROCHEMISTRY

16 Zbigniew KoczorowskiA
the dependence of the dipole component of the Galvani potential at the
M
metal/solution interface on the variation in charge [i.e.,= g (ion) potential]
S
is oftenobservedexperimentally. 1,13,22,23
The common example of real potential is the electronic woÀ=function
x
x
of the condensed phase x,φe ,which is a negative value of α e . This term,
which is usuallyused for electrons in metals and semiconductors, is
defined as the woÀ of electron transfer from the condensed phase x to a
point in a vacuum in close proximityto the surface of the phase, but
beyond the action range of purely surface forces, including image inter-
actions.=This point just outside of the phase is about 1 µm in a vacuum.=In
other dielectric media, it is nearer to the phase by ε times, where ε is the
dielectric constant.=
A general discussion and attempts to clarify the definitions of elec-
trochemical potential in light of recent theoretical results, 26 as well as the
27
surface and Volta potentials have been presented recently.
s
The surface potential of a liquid solvent s, χ , is defined as the
difference in electrical potentials across the interface between this solvent
and the gas phase, with the assumption that the outer potential of the
s
solvent is zero. The potential χ arises from a preferred orientation of the
solvent dipoles in the free surface zone.= At the surface of the solution, the
electric field responsible for the surface potential may arise from a
preferred orientation of the solvent and solute dipoles, and from the ionic
s
double layer. The potential χ , as the difference in electrical potential
across the interface between the phase and gas, is not measurable.= How -
ever, the relative changes caused bythe change in the solution's compo-
sition can be determined using the proper voltaic cells (see Sections
XII-XV).

III. VOLTA POTENTIAL AND VOLTAIC CELLSA
Volta potentials are measured by means of voltaic cells, i.e.,= systems
composed of conducting, condensed phases in series, with a gas, liquid
dielectric (e.g.,= decane) or a vacuum (in the case of solid conductors such
as metals) gap situated between two condensed phases. The gap, g, may
contain a gas such as pure air or nitrogen, saturated with vapors of the
liquids present.= Owing to the presence of a dielectric, special methods are
necessaryfor the investigation of voltaic cells (see Section IV).
In the schemes shown in this chapter a single vertical bar (  ) is used
to represent a metal/metal-, metal/gas, metal/solutio/gas phase boundary,