10-49
direction and time. Steady state voltammograms have a characteristic sigmoidal
shape. The faster mass transport can also increase the sensitivity for quantitative
analysis, and the Rotating Disk Electrode is often used for the deposition step of
stripping experiments.
The limiting current (mass transport controlled current) for a reversible process is
given by the Levich equation:
i
nFACD
l
=
0 62
2 3
1 6
1 2
.
/
/
/
υ ω
where:
n = number of electron transferred/mole
F = Faraday constant, 96500 C/mole
A = electrode area (cm
2
)
C = Concentration (mol/cm
3
)
D = diffusion coefficient (cm
2
/s)
υ
= kinematic viscosity (cm
2
/s)
ω
= 2
π
f (frequency)/rps
Therefore, a plot of i
l
vs.
ω
1/2
for a reversible process is a straight line (Levich plot).
Reversibility in this instance requires electron transfer to be rapid relative to the rate
of mass transport; hence, a redox process may change from reversible to quasi-
reversible as the rotation rate is increased. This is reflected in a deviation from
linearity in the Levich plot. The rate of electron transfer can be calculated from the
kinetic current, which is the current at infinite rotation rate. The kinetic current is best
measured from the intercept of the reciprocal Levich plot (1/i
l
vs. 1/
ω
1/2
). This method
is often used in studies on corrosion and batteries; it has also been used to measure
the rate of electron transfer through a polymer film on the surface of the electrode.
The rotation rate can also be used to vary the effect of any chemical reaction that
follow electron transfer. In particular, the Rotating Disk Electrode has been used to
study ECE processes (i.e., two Electron transfers with an intervening Chemical
reaction). In order for the second electron transfer to occur, the chemical reaction
must occur in the interfacial region (the region adjacent to the electrode surface). As
the rotation rate increases, a molecule spends less time in the interfacial region, so
there is less opportunity for the chemical reaction to occur. The apparent number of
electron transferred therefore decreases from a limit of 2 (slow rotation limit) to 1
(fast rotation limit).
The basic Rotating Disk Electrode experiment discussed above is carried out on the
BAS 100W using the RDE mode. Typical potential and rotation rate waveform are
shown in Figure 10-28, and a typical current response is shown in Figure 10-29. The
limits for the rotation rate are 0 rpm to 10000 rpm (although in practice the lowest
reasonable rotation rate is about 100 rpm).
