Page 124 - Analytical Electrochemistry 2d Ed - Jospeh Wang

4-5 WORKING ELECTRODES 109

solution creeping, and accumulation of dirt). More elaborate DMEs, based on a
mechanical drop detachment after reproducible time intervals, are used for pulse
polarography.
The hanging mercury drop electrode is a popular working electrode for stripping
analysis and cyclic voltammetry. In this con®guration, stationary mercury drops are
displaced from a reservoir through a vertical capillary. Early (Kemula-type) HMDE
designs relied on mechanical extrusion (by a micrometer-driven syringe) from a
reservoir through a capillary (12). The mercury reservoir should be completely ®lled
with mercury; air must be fully eliminated. Modern HMDEs (particularly with the
Model 303 of EG&G PAR, shown in Figure 4-7) employ electronic control of the
drop formation, which offers improved reproducibility and stability (13). For this
purpose, a solenoid-activated valve dispenses the mercury rapidly, with the drop size
being controlled by the time during which the valve is opened. A wide-bore capillary
allows a mercury drop to be grown very rapidly when the valve is opened. Three
valve opening times produce drops that are described as small, medium, or large.
Since the potential scan is accomplished after the valve has been closed (i.e., with a
stationary electrode), charging-current contributions due to drop growth are elimi-
nated. All the components of this electrode, including the mercury reservoir, are
contained in a compact unit. Such a commercial probe allows the conversion from

FIGURE 4-7 The static mercury drop electrode and its cell stand.