Rotated Disk Electrode Voltammetry RDEV

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Rotated Disk Electrode Voltammetry RDEV
insulator
r1
w in s-1, so f in rps revolutions
per second
dead or diffusion layer
Conductor electrode
Laminar flow occurs up to a point, at too high w,
we find that turbulent flow occurs. This is when
the value
exceeds the
Reynolds number for that particular fluid with a
given kinematic viscosity, n, in cm2 s-1.
Re2x105
(Poise)
Look in Table 9.2.1
So, w should be 2 105 n/r2, but other
limitations actually mean w lt 1000 s-1 or f
10,000 rpm. On the low w side, must rotate fast
enough to establish constant, homogeneous supply
of material to electrode surface. w gt 10 s-1
2
If one applies a potential which is that needed
to obtain mass-transport limited conditions,
then what is i ? consider -
hydrodynamics - diffusion Why?
O n e R only O present
C 0 x
d
Real profile
Must scan E slowly, n lt 100 mV s-1
Recall that d is F(1/w). So,
How solve? As we did before except incorporate
hydrodynamics. Also
Two Cases 1. Reversible use
q expression (Nernst) 2. Before reach MT
limit and -
irrev. - quasi-rev. ET
rxns.
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no iDL effects. Case 1 Levich
Equation Know Know
Levich Layer
Levich plot
If reaction is DC, then ilim vs. w1/2 is linear
with zero intercept. Also if ET reversible
No dependence of wave shape on w!
E1/2
Then plot of Eapp vs. will be
straight with slope
Case 2A Totally irreversible O
only But, kf is F(E) , so we denote this
kf(E). We call this current iK and it is
This is the Kinetic current. So, at high
enough -h, we should get kf ?? NO.
Zero
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We have no ET effects at -h, Irrev.
Rev. for i, so we merely get ilim
B-V / No MT
ic
- ia
MT effects
E vs Ref
ET effects
So, if we could vary E and measure iK, we could
get ??
Yes! How? Turns out we have
Koutecký Levich or Inverse Levich plot
So make plot of vs. at a
given Eapp.
E1 E2 E3
E4(on i lim)
Same slopes In each.
E1
Slope is
turbulence
vary Eapp
intercept is
w -1/2
w -1/2
i
iK Levich line
E1gtE2gtE3gtE4
(for EgtE4)
More -
w1/2
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Case 2B Quasi Reversible for O
and R
MT?
Do this on your own.
Fnc(E) ET
Now we have both kf and kb a function of n.
Thus, the Koutecký Levich plots do not have
same slope for various Potentials (h).
Problems! Minimize errors by using small
potential range near the foot of the wave where i
is not changing so drastically.
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RRDE
dR
r1
(R) Ring (D) Disk
r3
dD
r2
dR dD
r1 disk radius r2 r1 gap r3 r2 width
of ring
The collection Efficiency, N, is defined as
It is a Function of electrode geometry but
is independent of etc. if
R is stable. kchem If R Z
occurs, then Nexptl lt Ntheo and N F(w).
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For RRDE Collection Experiments 1. ERing is
held positive enough so as to oxidize any R. 2.
No bulk 3. EDisk
is scanned. 4. iDisk is measured. 5. iRing
is measured.
iD,C O ne
R ERing iD,lim
- EDisk vs. Ref
iR,lim R O ne
iR,a
Review