10-21
10.3
Pulse Techniques (SCP, NPV/P,
DPV/P)
The basis of all pulse techniques is the difference in the rate of decay of background
and faradaic currents following a potential step. The background current decays
exponentially, whereas the faradaic current decays as a function of 1/(time)
1/2
; that is,
the rate of decay of the background current is considerably faster than the decay of
the faradaic current. The background current is negligible at a time of 5R
u
C
dl
after
the potential step (R
u
C
dl
is the time constant for a given electrochemical cell, and
ranges from
µ
s to ms). Therefore, after this time, the measured current consists solely
of the faradaic current; that is, measuring the current at the end of the potential pulse
allows discrimination between the faradaic and charging current.
The important parameters for Pulse techniques are as follow:
a) Pulse Amplitude is the height of the potential pulse in mV.
b) Pulse Width is the duration of the potential pulse in msec.
c) Sample Width is the time (in msec) at the end of the pulse during which the
current is measured. It must be at least 2 msec shorter than the Pulse Width. The
current is sampled and averaged 16 times per msec. The default Sample Width is
the time required for 1 line cycle.
d) Pulse Period/Dropping Time - This is the time required for one potential cycle
(in msec) and must be at least twice the Pulse Width. Pulse Period is used for
voltammetry experiments, whereas Dropping Time applies to polarography
experiments, in which the potential pulse, current sampling and renewal of the
mercury drop are coordinated (this is controlled by the BAS 100B/W). As
discussed above, the current is sampled at the end of the pulse, and the mercury
drop is knocked off at the end of the Sample Width (which coincides with the
end of the pulse). For polarography experiments using a dropping mercury
electrode (DME), the advantage of sampling the current at the end of the drop
lifetime is that the rate of change of the surface area (which contributes to the
charging current) is at a minimum at this point.
Three different pulse techniques are available on the BAS 100B/W Electrochemical
Workstation. These differ in the potential pulse waveforms and the number of
sampling times. The discrimination against the charging current that is inherent in
these techniques leads to lower detection limits (compared to linear sweep
techniques), which make these techniques suitable for quantitative analysis.
