Biology Reference
In-Depth Information
baseline represents the capacitive (i.e., non-Faradaic) current
I
c
during the potential sweep. The capacitive current arises because
of the rearrangement of ions at the double layer in response to the
changing electrode potential. Itis related to the scan rate by [46]
I
c
=
−
−
t
/
R
S
C
d
)] (8.5)
where
t
is the time and
R
S
is the solution resistance, taken as being
in series to the double-layer capacitance
C
d
.
I
c
increases to reach a
constantvalueduringthescan.Increasingthescanratewillincrease
I
p
, but will increase
I
c
by the same amount. In contrast to the
potential sweep, when a potential step to a value
E
is applied to
the same series resistor-capacitor combination,
I
c
can be shown to
decay exponentially withtime according to [46]
vC
d
[1
exp (
E
R
S
exp(
−
t
/
R
S
C
d
)
=
I
c
(8.6)
based on the equation for the charging of a capacitor. However, the
Faradaic current from the same potential step, as
ex
pressed by the
Cottrell equation [46], decays in proportion to 1
/
√
t
. Therefore, the
capacitive current falls more quickly. This fact may be utilized in
pulse voltammetry to lower the baseline of the voltammogram, and
thus improve the sensitivity.
8.4.3.2 Differential pulse voltammetry
The potential waveform for differential pulse voltammetry (DPV) is
shown in Fig. 8.6b. The pulse height (
E
in Fig. 8.6b) is typically
a few tens of mV, and the pulse width (
t
in Fig. 8.6b) is typically
50 to 60 ms. The current is sampled immediately before the pulse is
applied(
I
1
)andthenattheendofthepulse(
I
2
).Thevoltammogram
output is the difference
I
2
-
I
1
plotted as a function of potential,
as shown in Fig. 8.7b. To understand the principle of DPV we can
consider the valueof (
I
2
-
I
1
) at three different stages:
(1) Before the redox process begins.
Here (
I
2
-
I
1
) represents the
difference in the capacitive currents at each set of the potentials
where
I
1
and
I
2
are measured. Because recording occurs after the
pauses shown in Fig. 8.6b, the currents will have decayed with time
according to Eq. (8.6). Therefore, the value of (
I
2
-
I
1
) will be very
small.
