lecture6 - university of guelph 6.pdf · lecture6.pdf author: administrator created date: 5/14/2003...
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Section 6-5 :Experimental Difficulties
1. Avoiding convection
- any vibrations and the build-up of densitygradients will cause convective disruption ofthe diffusion layer. The measured current atlong times following a potential step is oftenlarger than that predicted by the Cottrellequation.
T > 100 s are often problematic.
2. RuC Effects.
- Whenever the potential is stepped, current mustflow to charge the electrode. This currentcontains no analytical information about theFaradaic processes.
- As a rule of thumb the time for full DLcharging is 5RuC . The initial 5RuC secondsdefine the time scale of kinetic events you canstudy.
3. Potentiostat limitations.
- A potentiostat can not change the potential itapplies infinitely fast.
- Rise time : tR = (1/∆f)
- Noise versus speed.
4. Recording device limitations
-The initial current is very large but very quicklydecays toward zero. It is extremely difficult toprecisely measure both the long and short timecurrent (or charge) signals.
Worked Example : Adsorption of Dodecyl Sulfate ona gold electrode.
Differential Capacity
Allows for the selection of E0 = -600 mV
-600 -400 -200 0 200 400 6000
10
20
30
40
50
60
C /
µF
cm
-2
E / V vs SCE
0.3 mM SDS electrolyte only
1 6 0 . 0
- 1 0 . 0
0 . 0
1 0 . 0
2 0 . 0
3 0 . 0
4 0 . 0
5 0 . 0
6 0 . 0
7 0 . 0
8 0 . 0
9 0 . 0
1 0 0 . 0
1 1 0 . 0
1 2 0 . 0
1 3 0 . 0
1 4 0 . 0
1 5 0 . 0
d a t a p o i n t
4 5 0 0 . 00 . 0 5 0 0 . 0 1 0 0 0 . 0 1 5 0 0 . 0 2 0 0 0 . 0 2 5 0 0 . 0 3 0 0 0 . 0 3 5 0 0 . 0 4 0 0 0 . 0
-620
-600
-580
-560
-540
-520
-500
-480
-460
-440
E /
mV
vs
SC
E
time
-600 -400 -200 0 200 400
-20
-10
0
10
Cha
rge
(µC
m-2)
Potential (mV vs SCE)
-600 -400 -200 0 200 400
0
5
10
15
20
25
30
35
40
45
0.006 mM 0.01 mM 0.02 mM 0.06 mM 0.09 mM 0.12 mM 0.16 mM 0.19 mM 0.23 mM 0.26 mM 0.29 mM 0.33 mMπ
(mN
/m)
Potential (mV)
0.04 0.08 0.12 0.16 0.20 0.24
0
4
8
12
16
20
24
28
32
36
-300 mV
-200 mV
-100 mV
0 mV
100 mV
200 mV
π / m
N m
-1
ln cS D S
-600 -400 -200 0 200 400
0
1
2
3
4
5
(Γ x
1010
) / m
oles
cm
-2
E / mV vs SCE