Electrochemical Methods for Environmental Analysis - Fundamentals of Environmental Sampling and Analysis - page 305

The same microelectrode serves as an anode, where Cd and Cu are oxidized into
their ionic forms (Cd 2þ and Cu 2þ ). The reduction reactions in the deposition step are

Cd 2þ þ2e

!

CdðsÞ ðE 0 ¼0 : 403 VÞ ð11 : 17Þ

Cu 2þ þ2e ! CuðsÞ ðE 0

¼þ0 : 337 VÞ ð11 : 18Þ

For both Cd 2þ and Cu 2þ to be reduced, the potential should be low enough. The
minimum potential to begin reducing Cd 2þ can be calculated from the Nernst
equation. This is called the deposition potential.
After a period of deposition, as shown in Figure 11.7a, the potential is increased
linearly (linear potential scan). As a result, Cd starts tobeoxidized (Cd

!

Cd 2þ þ2e )

at an approximate potential of 0 6 V, causing a sharp increase in current. As the

deposited Cd is consumed, this current drops to a low level (the first peak of Cd).
As potential further increases (sufficiently positive), the second peak for the oxidation
of Cu appears.
Note that the indicator (working) electrodes used in ASV differ from the
electrodes described previously (Section 11.1.3). The hanging mercury drop electrode
(HMDE) and the mercury film electrode (MFE) are the two most commonly used
electrodes for ASV. The larger surface area-to-volume ratio of the MFE compared to
the spherical HMDE enables more metal to be concentrated into a given amount of
mercury during a specified deposition time. Consequently, a given detection limit
can be achieved with a shorter deposition time for the MFE than for the HMDE.
Note also that deposition in stripping voltammetry is not exhaustive. This is in
significant contrast to coulometric techniques in which all analytes are electrolyzed.
Therefore, it is important in ASV to deposit the same fraction of metal for each
stripping voltammogram. To achieve this, the parameters of electrode surface area,
deposition time, and stirring must be carefully duplicated for all standards and
samples. Deposition times vary from 60 s to 30 min, depending on the analyte
concentration, the type of electrode, and the stripping technique. The MFE requires
less time than does the HMDE; differential pulse voltammetry requires less time
than linear sweep voltammetry as the stripping technique.
To conclude, one should bear in mind that only common environmental
applications of electroanalytical techniques are introduced in this chapter. Many
electrochemical sensors have become available and are expected to play an
increasing role in environmental monitoring. Such sensor devices should allow one
to move the measurement of numerous inorganic and organic pollutants from the
central laboratory to the field and to perform them rapidly, inexpensively, and
reliably. Sensors of various types and applications can be found in the references
(Hanrahan et al., 2004; Kellner et al., 2004).

:

REFERENCES

APHA (1998), Standard Methods for the Examination of Water and Wastewater, Washington, DC, 20th
Edition, 1998.
B OCKRIS JO'M, R EDDY AKN (1970), Modern Electrochemistry, Plenum, New York.