3. 2.4. Chronoamperometry measurement of currents as a function of time
a kind of ‘controlled-potential voltammetry’ or ‘controlled-potential micro electrolysis (unstirred solution)’
Use of the same equipment of cyclic voltammetry
When cyclic voltammetry does not succeed in identifying the electrode mechanisms

4. Single potential step Chronoamperometry
Ox + ne Red
applying an appropriate potential (under stationary conditions similar to those of cyclic voltammetry), which allows the electron transfer to run instantaneously to completion (COx (0,t) → 0)
decay of the generated current is monitored

5. More negative by about 0.2 V with respect to the peak potential of the cathodic process
The fundamental law of Chronoamperometry is the Cottrell equation
current

6. Double potential step Chronoamperometry
- reminiscent of the reversal electrolysis
a first cathodic potential step (COx(0,t) → 0)
at a time t by the application of a second anodic potential step (CRed(0,t)→0)
Total time = 10 sec
Fig 48

7. possible presence of coupled chemical reactions
diagram represents the chronoamperometric single potential step response (Cottrell equation)
reoxidation of Red, diagram represents the double potential step response
t<t
t>t
Fig 49
possible presence of coupled chemical reactions

8. 2.4.1.Coupled Chemical Reactions

9. 2.4.1.1 ‘Preceding’ chemical reactions
the current-time response is governed initial concentration of Ox; (Cottrell equation)
slow
conversion Y/Ox is significant and assuming DOx = D
intermediate
Rapid conversion of Y/Ox, concentration of Ox can be assumed constant
fast kf kr
Y Ox
Ox + ne Red
single potential step experiment

10. Fig 50

11. ErCi 2.4.1.2. ‘Following’ chemical reactions Ox + ne- Red Red Z kf
First-order chemical reaction
ErCi
Chronoamperometry cannot distinguish between reversible and irreversible processes
use of double potential step technique (sensitive to the chemical fate of Red)
The cathodic response before the inversion of the applied potential (t < t)
The anodic response after the inversion of the potential (t > t)

12. Calculation of kf is a complex function of kf , t and t
=1 if not complicated
Fig 51

13. 2.4.1.2.2. Catalytic regeneration of the reagent
use of single potential step technique
The responses of the forward and reverse steps are both affected by the chemical complication
the reduction current will be greater than that predicted by the Cottrell equation
the reoxidation current will be lower than that predicted by the Cottrell equation
Ox + ne Red
Z + Red Ox + Y kf

14. kf current under catalytic conditions
diffusive current observed in the absence of Z
current ratio kf

15. 2.4.1.2.3. The ECE mechanism Ox + n1e Red Red Ox ‘ Ox’ +n2e Red’
Eo’1
Eo’2 Kf kr
The simplest and most useful case that one can study by single potential step Chronoamperometry is that in which Eo’1 >> Eo’2
for kf t → 0 (or slow chemical kinetics), the first electron transfer
for kf t→ (or fast chemical kinetics), both the electron transfers
for intermediate values of kf t

16. at short times after the potential step (corresponding to kf= 0) the current depends only on the process Ox - Red, thus the slope of the function depends on the n1
At longer times (corresponding to kf= ) the current is a function of the complete reduction of Ox to Red’, thus the slope is determined by the n1 + n2
At intermediate times the current varies between these limits. The time is a function of the rate constant of the interposed chemical reaction

17. 2.5. Determination of the Number of Electrons Involved in an Electron Transfer Process from the Correlation Between Cyclic Voltammetry and Chronoamperometry
in cyclic voltammetry the peak current is given by the Randles-Sevcik equation
For the same process, in Chronoamperometry, the Cottrell equation holds
Example:
one electron, R = 4.93
two electrons, R = 6.97

18. ..That ′s all