Page 45 - MODERN ASPECTS OF ELECTROCHEMISTRY

Zhigniew KoczorowskiA
32
the metal and the potential across the emerging double layer of the bulk
electrode) transferred into an emersed state evidently remain intact. 81–84
The 1: 1 correlation between the Volta and emersion potentials i.e.,= the
potential supplied prior to emersion) presents a strong argument in favor
of the above conclusion.= However, the removal of the double layer
becomes less perfect with decreasing ionic strength of the electrolyte
(< 0.1 M) because of the increasing extension of the diffuse part of the
double layer.= A 1 : 1 correlation of the Volta potential with the emersion
potential has been found also for electrodes coated with exchange polymer
membranes. 85–86
The emersed electrode, in principle, may be treated as the experimen-
tal realization of a single electrode.= However, it is doubtful whether its
liquid layer has the same bulk properties.= This is probably=the main reason
o
for the different results of E H (abs) found for emersed electrodes, e.g.,=
87
–4.85 V.83 Samec et al. have found that emersion of electrodes in a
nitrogen atmosphere decreases the Volta potential and therefore the abso-
lute electrode potential by ca.=0.32 V relative to the value in solution.=They
have attributed this mainly to the reorientation of the water molecules at
the free surface.=
88
The non situ experiment pioneered by Sass uses a preparation of an
electrode in an ultrahigh vacuum through cryogenic coadsorption of
known quantities of electrolyte species (i.e., solvent, ions, and neutral
molecules) on a metal surface. 88–91 Such experiments serve as a simula-
tion, or better, as a synthetic model of electrodes.= The use of surface
spectroscopic techniques m¸es it possiblp to determine the coverage and
structure of a synthesized electrolyte.= The interfacial potential (i.e.,= the
electrode woÀ=function) is measured using the voltaic cell technique.= Of
course, there are reasonable objections to the UHV technique, such as too
little water, too low a temperature, too small interfacial potentials, and lack
of control of ionic activities. 89,91
Non-situ and ex sit studies can provide important information for
understanding the properties of metal/electrolyte interfaces. The applica-
bility of these methods for fundamental studies of electrochemistry seems
to be firmly established. The main differences between common electro-
chemical and UHV experiments are the temperature gap (ca.= 300 vs.= 150
K) and the difference in electrolyte concentration (very high concentra-
tions in UHV experiments). In this respect, experimental research on
double-layer properties in frozen electrolytes can be treated as a liØ=
92
between in situ experiments. The measurements of the woÀ= functions