Page 126 - Analytical Electrochemistry 2d Ed - Jospeh Wang

4-5 WORKING ELECTRODES 111

Solid electrodes can be stationary or rotating, usually in a planar disk con®g-
uration. Such electrodes consist of a short cylindrical rod of the electrode material
embedded in a tightly ®tting tube of an insulating material (Te¯on, Kel-F, etc.). The
construction of a typical disk electrode is illustrated in Figure 4-8. It is essential to
avoid crevices between the sleeve and the electrode materials, and thus to prevent
solution creeping (and an increased background response). Electrical contact is made
at the rear face. Disk solid electrodes are also widely employed in ¯ow analysis in
connection with thin-layer or wall-jet detectors (see Section 3-6). Growing attention
is being given to other con®gurations of solid electrodes, including various
ultramicroelectrodes (Section 4-5.4) and microfabricated screen-printed strips or
silicon-based thin-®lm chips (Section 6-5).

4-5.2.1 Rotating Disk and Ring-Disk Electrodes The rotating disk elec-
trode (RDE) is vertically mounted in the shaft of a synchronous controllable-speed
motor and rotated with constant angular velocity (o) about an axis perpendicular to
the plain disk surface (Figure 4-9a). (o ˆ 2pf , where f is the rotation speed in rps.)
As a result of this motion, the ¯uid in an adjacent layer develops a radial velocity
that moves it away from the disk center. This ¯uid is replenished by a ¯ow normal to
the surface. Hence, the RDE can be viewed as a pump that draws a fresh solution up
from the bulk solution. Under laminar ¯ow conditions (usually up to about
4000 rpm), the thickness of the diffusion layer decreases with increasing electrode
angular velocity according to

n
d ˆ 1:61D 1=3 o 1=2 1=6 4-4†
where n is the kinematic viscosity (de®ned as the viscosity divided by the density in
1
2
cm s ). Rotation speeds of 100±4000 rpm thus correspond to d values in the
5±50 mm range. Equation (4-4) suggests that the thickness of the diffusion layer is

FIGURE 4-8 Construction of a typical disk electrode.