Chemistry Reference
In-Depth Information
E
1/2
E
1/2
(a)
(b)
2.13
Electrical-current signal in differential pulse voltammetry with (a)
the DC-current signal due to applied and linearly increasing DC
ramp and (b) the signal due to the application of potential pulses.
Potential (V)
Potential (V)
and a maximum in the difference between
I
2
and
I
1
will be obtained at the
inflection point of the voltammetric wave. This point corresponds to the
half-wave potential; therefore, the peak potential in DPV is identical to
the half-wave potential of the envisaged oxidation or reduction reaction.
Typical detection limits obtained in differential pulse voltammetry are 1 ¥
10
-8
mol l
-1
, with a resolving capacity of 40 mV and a separating capacity of
1 ¥ 10
5
/1.
Square-wave voltammetry
In square-wave voltammetry
74-82
,a symmetrical square-wave pulse (Fig.
2.14a) is superponated to a staircase wave (Fig. 2.14b) resulting in the
square wave (Fig. 2.14c). The duration of the pulse, t, is equal to the length
of the staircase, and the superponation is obtained in such a way that the
forwards pulse of the square wave coincides with the first half of that stair-
case. Two other important parameters are
E
sw
, the pulse height of the square
wave, and
E
sc
, the increase of the staircase for each step.
In square-wave voltammetry, the current signal is the result of a differ-
ence between the experimentally measured currents. The first current is
measured at the end of the forward square-wave pulse (
I
1
,Fig. 2.14c), and
the second one is measured at the end of the return square-wave pulse (
I
2
,
Fig. 2.14c); the difference results in a larger peak because both individual
signals are opposite in sign (Fig. 2.15). This can also be explained as follows,
supposing that the following reversible reaction occurs at the electrode
surface:
