Page 125 - Analytical Electrochemistry 2d Ed - Jospeh Wang

110 PRACTICAL CONSIDERATIONS

the HMDE to DME by operation of a single switch. When used in the DME mode, it
exhibits a very rapid growth to a given area, which then remains constant (as desired
for minimizing charging-current contributions). The performance of HMDEs can be
improved by siliconizing the interior bore of the capillary.
Several mercury electrodes combine the features of the DME and HMDE. In
particular, one employs a narrow-bore capillary that produces DMEs with drop lives
of 50±70 s (14). Another involves a controlled-growth mercury drop (15). For this
purpose, a fast-response valve offers a wide range of drop sizes and a slowly (step-
by-step) growing drop.
The mercury ®lm electrode, used for stripping analysis or ¯ow amperometry,
consists of a very thin (10±100 mm) layer of mercury covering a conducting support.
Because of the adherent oxide ®lms on metal surfaces, and the interaction of metals
with mercury, glassy carbon is most often used as a substrate for the MFE. The
mercury ®lm formed on a glassy carbon support is actually composed of many
droplets. As a result of not being a pure mercury surface, such ®lm electrodes exhibit
a lower hydrogen overvoltage and higher background currents. Another useful
substrate for the MFE is iridium (because of its very low solubility in mercury and
the excellent adherence of the resulting ®lm). Mercury ®lm electrodes are commonly
preplated by cathodic deposition from a mercuric nitrate solution. An in-situ-plated
MFE is often employed during stripping analysis (16). This electrode is prepared by
simultaneous deposition of the mercury and the measured metals. Most commonly, a
disk-shaped carbon electrode is used to support the mercury ®lm. Mercury ®lm
ultramicroelectrodes, based on coverage of carbon ®ber or carbon microdisk
surfaces, have also received increasing attention in recent years.

4-5.2 Solid Electrodes
The limited anodic potential range of mercury electrodes has precluded their utility
for monitoring oxidizable compounds. Accordingly, solid electrodes with extended
anodic potential windows have attracted considerable analytical interest. Of the
many different solid materials that can be used as working electrodes, the most often
used are carbon, platinum, and gold. Silver, nickel, and copper can also be used for
speci®c applications. A monograph by Adams (17) is highly recommended for a
detailed description of solid-electrode electrochemistry.
An important factor in using solid electrodes is the dependence of the response on
the surface state of the electrode. Accordingly, the use of such electrodes requires
precise electrode pretreatment and polishing to obtain reproducible results. The
nature of these pretreatment steps depends on the materials involved. Mechanical
polishing (to a smooth ®nish) and potential cycling are commonly used for metal
electrodes, while various chemical, electrochemical, or thermal surface procedures
are added for activating carbon-based electrodes. Unlike mercury electrodes, solid
electrodes present a heterogeneous surface with respect to the electrochemical
activity (18). Such surface heterogeneity leads to deviations from the behavior
expected for homogeneous surfaces.