Page 129 - Analytical Electrochemistry 2d Ed - Jospeh Wang

114 PRACTICAL CONSIDERATIONS

and history of the carbon surface (20,21). While all common carbon electrode
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materials share the basic structure of a six-membered aromatic ring and sp bonding,
they differ in the relative density of the edge and basal planes at their surfaces. The
edge orientation is more reactive than the graphite basal plane toward electron
transfer and adsorption. Materials with different edge-to-basal plane ratios thus
display different electron-transfer kinetics for a given redox analyte. The edge
orientation also displays undesirably high background contributions. A variety of
electrode pretreatment procedures have been proposed to increase the electron-
transfer rates. The type of carbon, as well as the pretreatment method, thus has a
profound effect upon the analytical performance. The most popular carbon electrode
materials are those involving glassy carbon, carbon paste, carbon ®ber, screen-
printed carbon strips, carbon ®lms, or other carbon composites (e.g., graphite epoxy,
wax-impregnated graphite, Kelgraf). The properties of different types of carbon
electrodes are discussed below.

4-5.2.2.1 Glassy-Carbon Electrodes Glassy (or ``vitreous'') carbon has been
very popular because of its excellent mechanical and electrical properties, wide
potential window, chemical inertness (solvent resistance), and relatively reproducible
performance. The material is prepared by means of a carefully controlled heating
program of a premodeled polymeric (phenol±formaldehyde) resin body in an inert
atmosphere (22). The carbonization process is carried out very slowly over the 300±

1200 C temperature range to insure the elimination of oxygen, nitrogen, and
hydrogen. The structure of glassy carbon involves thin, tangled ribbons of cross-
linked graphite-like sheets. Because of its high density and small pore size, no
impregnating procedure is required. However, a surface pretreatment is usually
employed to create active and reproducible glassy-carbon electrodes and to enhance
their analytical performance (18,23). Such pretreatment is usually achieved by
polishing (to a shiny ``mirror-like'' appearance) with successively smaller alumina
particles (down to 0.05 mm) on a polishing cloth. The electrode should then be rinsed
with deionized water before use. Additional activation steps, such as electrochemi-
cal, chemical, heat, or laser treatments, have also been used to enhance the
performance. The improved electron-transfer reactivity has been attributed to the
removal of surface contaminants, exposure of fresh carbon edges, and an increase in
the density of surface oxygen groups (which act as interfacial surface mediators).
Several reviews provide more information on the physical and electrochemical
properties of glassy-carbon electrodes (20,24).
A similar, but highly porous, vitreous carbon materialÐreticulated vitreous
carbon (RVC)Ðhas found widespread application for ¯ow analysis and spectro-
electrochemistry (25). As shown in Figure 4-10, RVC is an open-pore (``sponge-
like'') material; such a network combines the electrochemical properties of glassy
carbon with many structural and hydrodynamic advantages. These include a very
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high surface area ( 66 cm cm 3 for the 100-ppi grade), 90±97% void volume, and
a low resistance to ¯uid ¯ow.