Several mercury electrodes combine the features of the DME and HMDE.
In particular, one employs a narrow-bore capillary that produces DMEs with
drop lifetimes of 50–70 s (15). The other involves a controlled growth mercury
drop (16). For this purpose, a fast-response valve offers a wide range of drop
sizes and a slowly (step-by-step) growing drop.
The mercury film electrode (MFE), used for stripping analysis or flow
amperometry, consists of a very thin (10–100-µm) layer of mercury covering
a conducting and inert support. Because of the adherent oxide films on metal
surfaces, and the interaction of metals with mercury, glassy carbon is most
often used as a substrate for the MFE. The mercury film formed on a glassy
carbon support is actually composed of many droplets. Because they do not
126 PRACTICAL CONSIDERATIONS
Figure 4.7 The static mercury drop electrode and its cell stand.
have a pure mercury surface, such film 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 film). Mercury film electrodes are commonly
preplated by cathodic deposition from a mercuric nitrate solution. An in
situplated MFE is often employed during stripping analysis (17). 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 film. Mercury film ultramicroelectrodes, based on coverage of carbon
fiber or carbon microdisk surfaces, have also received a growing attention in
recent years.