Page 90 - Analytical Electrochemistry 2d Ed - Jospeh Wang

3-5 STRIPPING ANALYSIS 75

FIGURE 3-11 Potential±time waveform used in alternating current (AC) voltammetry.

current.) For a reversible system, such a response is actually the derivative of the DC
polarographic response. The height of the AC voltammetric peak is proportional to
the concentration of the analyte and, for a reversible reaction, to the square root of
the frequency (o):
2
n F Ao 1=2 D 1=2 C DE
2
i ˆ 3-20†
p
4RT
The term DE is the amplitude. The peak width is independent of the AC frequency,
and is 90.4=n mV (at 25 C).

The detection of the AC component allows one to separate the contributions of

the faradaic and charging currents. The former is phase shifted 45 relative to the

applied sinusoidal potential, while the background component is 90 out of phase.
The charging current is thus rejected using a phase-sensitive lock-in ampli®er (able
to separate the in-phase and out-of-phase current components). As a result,
reversible electrode reactions yield a detection limit around 5 10 7 M.
Substantial loss in sensitivity is expected for analytes with slow electron-transfer
kinetics. This may be advantageous for measurements of species with fast electron-
transfer kinetics in the presence of a species (e.g., dissolved oxygen) that is
irreversible. (For the same reason, the technique is very useful for the study of
electron processes.) Theoretical discussions on AC voltammetry are available in the
literature (16±18).

3-5 STRIPPING ANALYSIS

Stripping analysis is an extremely sensitive electrochemical technique for measuring
trace metals (19,20). Its remarkable sensitivity is attributed to the combination of an
effective preconcentration step with advanced measurement procedures that generate
an extremely favorable signal-to-background ratio. Since the metals are precon-
centrated into the electrode by factors of 100 to 1000, detection limits are lowered
by 2 to 3 orders of magnitude compared to solution-phase voltammetric measure-
ments. Hence, four to six metals can be measured simultaneously in various
matrices at concentration levels down to 10 10 M, utilizing relatively inexpensive