Page 94 - Analytical Electrochemistry 2d Ed - Jospeh Wang

3-5 STRIPPING ANALYSIS 79

The mercury ®lm electrode has a higher surface-to-volume ratio than the hanging
mercury drop electrode and consequently offers a more ef®cient preconcentration
and higher sensitivity (equations 3-22 through 3-25). In addition, the total exhaus-
tion of thin mercury ®lms results in sharper peaks and hence improved peak
resolution in multicomponent analysis (Figure 3-14).
The major types of interferences in ASV procedures are overlapping stripping
peaks caused by a similarity in the oxidation potentials (e.g., of the Pb, Tl, Cd, Sn or
Bi, Cu, Sb groups), the presence of surface-active organic compounds that adsorb on
the mercury electrode and inhibit the metal deposition, and the formation of
intermetallic compounds (e.g., Cu-Zn) which affects the peak size and position.
Knowledge of these interferences can allow prevention through adequate attention to
key operations.
Improved signal-to-background characteristics can be achieved using dual-work-
ing-electrode techniques, such as ASV with collection or subtractive ASV (but at the
expense of more complex instrumentation).
Other versions of stripping analysis, including potentiometric stripping, adsorp-
tive stripping, and cathodic stripping schemes, have been developed to further
expand its scope and power.

3-5.2 Potentiometric Stripping Analysis
Potentiometric stripping analysis (PSA), known also as stripping potentiometry,
differs from ASV in the method used for stripping the amalgamated metals (22). In
this case, the potentiostatic control is disconnected following the preconcentration,
and the concentrated metals are reoxidized by an oxidizing agent [such as O 2 or
Hg(II)] that is present in the solution:

M Hg†‡ oxidant ! M n‡ 3-26†

A stirred solution is also used during the stripping step to facilitate the transport of
the oxidant. Alternately, the oxidation can be carried out by passing a constant
anodic current through the electrode. During the oxidation step, the variation of the
working electrode potential is recorded, and a stripping curve, like the one shown in
Figure 3-15a, is obtained. When the oxidation potential of a given metal is reached,
the potential scan is slowed down as the oxidant (or current) is used for its stripping.
A sharp potential step thus accompanies the depletion of each metal from the
electrode. The resulting potentiogram thus consists of stripping plateaus, as in a
redox titration curve. The transition time needed for the oxidation of a given metal,
t , is a quantitative measure of the sample concentration of the metal:
M

t / C n‡t =C 3-27†
M M d ox
where C ox is the concentration of the oxidant. Hence, the signal may be increased
by decreasing the oxidant concentration. The qualitative identi®cation relies on