10-41
10.6
Stripping Techniques (BSWSV,
DPSV, LSSV, OSWSV)
Stripping voltammetry is a very sensitive method for analysis of trace amounts of
electroactive species in solution. Detection limits for metal ions at sub-ppb levels
have been reported.
There are 3 important parts in a stripping experiment. These are:
a) Deposition
b) Quiet Time
c) Stripping
These can best be explained by discussing the stripping experiment for detection of
lead. In this experiment, a mercury electrode is used; this is either the Hanging
Mercury Drop Electrode (HMDE) or the Thin Mercury Film Electrode (TMFE). The
latter electrode is made by depositing a mercury film on the surface of a glassy
carbon electrode. This deposition can be done either prior to the stripping experiment
or during the deposition step.
During the deposition step, the potential is held at a value (Deposition Potential) at
which the lead ions are reduced to lead metal for a pre-determined time period
(Deposit Time). The metallic lead then amalgamates with the mercury electrode
(when the TMFE is being used, mercuric ions are added to the solution, and mercury
metal is co-deposited with the lead during the deposition step). The effect of the
deposition step is to concentrate the lead in the mercury electrode; that is, the
concentration of lead in the mercury electrode is significantly greater than the
concentration of lead in solution (this step is often called the preconcentration step).
The efficiency of the deposition step is increased by stirring the solution or using a
Rotating Disk Electrode. The time elapsed during the deposition step is shown on the
PC screen by a software generated clock.
During the Quiet Time, the stirring is stopped and lead distributes within the
mercury electrode to produce a uniform concentration. Typically, 5 to 10 sec are
required for this step.
During the stripping step, the potential is scanned in a positive direction, and the lead
in the mercury electrode is oxidized back to lead ions in solution at a specific
potential; that is, the lead is 'stripped' from the electrode. The stripping potential can
be used for qualitative identification of the metal, and the stripping current can be
used for quantitation of the concentration of metal ions in solution. There are a
number of different waveforms available for this potential scan: Differential Pulse
(DPSV), Linear Sweep (LSSV), Osteryoung and Barker Square Wave (OSWSV and
