139
DRU10002 (REV 004 / JAN 2017)
voltammograms with facile kinetics (see Figure 10.7) with a set of voltammograms
with sluggish kinetics (see Figure 10.9), the mass transport limited current plateau
(marked by red circles in each figure) is shifted further away from the standard
electrode potential (
Eº
) when there are slow kinetics. Stated another way, when
a sluggish redox half reaction is studied with a rotating disk electrode, a larger
overpotential must be applied to the electrode to overcome the sluggish kinetics
and reach the mass transport limited current.
This distortion of the ideal sigmoidal shape of the voltammogram can be
exploited as a way to measure the standard rate constant (
kº
). The general
approach is to acquire a set of voltammograms at different rotation rates (i.e.,
perform a Levich study) and then plot the reciprocal current (sampled at
particular locations along the rising portion of each voltammogram) on a
Koutecky-Levich Plot. In the example provided (see Figure 10.9, left), the current
was sampled at two locations along the rising portion of the voltammograms (at
0 and 50 mV vs.
Eº,
marked with blue triangles and purple squares) and at one
location on the limiting current plateau (at 350 mV vs.
Eº
, marked with red circles).
A linear relationship is evident (see Figure 10.9, right) when these sampled currents
are plotted on a Koutecky-Levich Plot.
For the set of currents sampled on the limiting current plateau (red circles), an
extrapolation back to the vertical axis (i.e., to infinite rotation rate) yields a zero
intercept. This is the identical result obtained for a facile half-reaction (see Figure
10.8, right) because these currents are sampled at a high enough overpotential
that there are no kinetic limitations. Only mass transport limits the current, and the
usual Levich behavior applies.
However, for the two sets of currents sampled on the rising portion of the
voltammogram (see Figure 10.9, blue triangles and purple squares), the
extrapolation back to the vertical axis yields non-zero intercepts. This non-zero
intercept indicates a kinetic limitation, meaning that even if mass transport were
infinite (i.e., infinite rotation rate), the rate of the half-reaction would still be limited
by the slow kinetics at the electrode surface.
The linear portion of the data on a Koutecky-Levich plot is described by the
Koutecky-Levich equation.
2
/
1
6
/
1
3
/
2
620
.
0
1
1
1
C
D
A
F
n
i
i
K
Plotting the reciprocal current (
1 /
i
) against the reciprocal angular rotation rate
(
ω
-1/2
) yields a straight line with an intercept equal to the reciprocal kinetic current
(
i
K
). The kinetic current is the current that would be observed in the absence of
any mass transport limitations. By measuring the kinetic current at a variety of
Summary of Contents for MSR 636A
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