Biology Reference
In-Depth Information
(2) Once the redox process begins.
The value of (
I
2
-
I
1
) will
now represent almost wholly the Faradaic current from the
strippingreaction.Bothcurrentswillincreasefirstlinearlyandthen
exponentially with potential, according to the low and high field
approximationstotheButler-Volmerequation[47].Oncetheregion
of exponential increase has been reached the overall value of (
I
2
-
I
1
) will increase, since the potential ramp is linear and therefore
I
2
becomesincreasingly greater than
I
1
.
(3) After the redox peak is reached.
In any potential scan of an
immobilizedredoxmaterial,apeakisobservedduetothedepletion
of that material as the voltage is increased. In the case of DPV, once
the current sampling for
I
2
reaches the peak potential,
I
2
reaches
its maximum value. However, since the sampling of
I
1
lags behind,
I
1
continues to rise. Therefore the value of (
I
2
-
I
1
) goes down.
Eventually
I
1
will reach the peak potential also and then (
I
2
-
I
1
)
will be virtually zero (the difference between them will be the small
difference in the residual capacitive current). In this manner, DPV
provides a lower baseline than LSV, as shown in Fig. 8.7b. The DPV
detectionlimitforaspeciesinbulksolutionisestimatedat5
×
10
−
8
M(c.f.5
×
10
−
6
MbyLSV),andforstrippingthislowersto1
×
10
−
11
M due to the advantage of preconcentration (c.f.
×
10
−
10
MforLSV)
[48]. For a species diffusing from bulk solution the DPV peak height
(
I
2
-
I
1
)
max
is givenby [46]
nFA
√
Dc
√
π
√
t
1
−
σ
1
+
σ
(
I
2
−
I
1
)
max
=
(8.7)
where
exp
nF
E
2
RT
σ
=
(8.8)
The DPV response has also been derived for an HMDE where metal
ions are reduced at the mercury [48], but has not, to the best of
our knowledge been derived for mercury electrodes in conjunction
with ASV. This is probably because such systems are only used for
analytical calibrations and not for the determination of physical
parameters.
