Page 91 - Analytical Electrochemistry 2d Ed - Jospeh Wang

76 CONTROLLED-POTENTIAL TECHNIQUES

instrumentation. The ability to obtain such low detection limits depends strongly on
the degree to which contamination can be minimized. Expertise in ultratrace
chemistry is required.
Essentially, stripping analysis is a two-step technique. The ®rst, or deposition,
step involves the electrolytic deposition of a small portion of the metal ions in
solution into the mercury electrode to preconcentrate the metals. This is followed by
the stripping step (the measurement step), which involves the dissolution (stripping)
of the deposit. Different versions of stripping analysis can be employed, depending
upon the nature of the deposition and measurement steps.

3-5.1 Anodic Stripping Voltammetry
Anodic stripping voltammetry (ASV) is the most widely used form of stripping
analysis. In this case, the metals are preconcentrated by electrodeposition into a
small-volume mercury electrode (a thin mercury ®lm or a hanging mercury drop).
The preconcentration is done by cathodic deposition at a controlled time and

potential. The deposition potential is usually 0.3±0.5 V more negative than E for
the least easily reduced metal ion to be determined. The metal ions reach the
mercury electrode by diffusion and convection, where they are reduced and
concentrated as amalgams:

M n‡ ‡ ne ‡ Hg ! M Hg† 3-21†
The convective transport is achieved by electrode rotation or stirring of the
solution (in conjunction with the mercury ®lm electrode) or by stirring of the
solution (when using the hanging mercury drop electrode). Quiescent solutions can
be used when using mercury ultramicroelectrodes. The duration of the deposition
step is selected according to the concentration level of the metal ions in question,
from less than 0.5 min at the 10 7 M level to about 20 min at the 10 10 M level. The
concentration of the metal in the amalgam, C , is given by Faraday's law:
Hg
i t
l d
C Hg ˆ 3-22†
nFV Hg
where i is the limiting current for the deposition of the metal, t is the length of the
d
l
deposition period, and V Hg is the volume of the mercury electrode. The deposition
current is related to the ¯ux of the metal ion at the surface. The total amount of metal
plated represents a small (but reproducible) fraction of the metal present in the bulk
solution.
Following the preselected time of the deposition, the forced convection is
stopped, and the potential is scanned anodically, either linearly or in a more sensitive
potential±time (pulse) waveform that discriminates against the charging background
current (usually square-wave or differential-pulse ramps). Such pulse excitations also
offer reduced oxygen interferences and analyte replating, respectively. During this