3. Voltametric techniques
Chapter 2
Prof. Rezvani

4. Redox behavior of a species
Electron transfer
Redox behavior of a species
Thermodynamic aspects
Kinetic aspects

5. Cyclic voltammetry 1

6. What happen? Overview of the redox aptitude
Varying applied potential to an electrode with time in a solution of the species
Recording the relevant current-potential curves

7. What reveal? Potential of redox processes
Shape of the response as a function of the potential scan rate (coupled chemical complications)
Generated current by the faradic processes
(proportional to concentration of the electro active species)
What reveal?

8. The most popular voltammetric techniques in the inorganic chemistry
Cyclic voltammetry
The most popular voltammetric techniques in the inorganic chemistry
(always coupled with complementary techniques)
bases on a linear potential sweep chronoamperometric technique

9. What does chronoamperometry mean?
Potential step Voltammetry (Chronoamperometry)
Electroanalysis by measuring at a working electrode the rate of change of current versus time; the potential is
controlled.
time i
i ∝ 1 / √t

10. 1-Cyclic voltammetry Fig 1 The meaning of cyclic
Carrying out under stationary conditions(dominant diffusive mass transport)
Varying applied potential from Ei to Ef at a constant rate(single sweep voltammetry - a in fig1)
Reversing scan rate at Ef in same scan rate, back to initial value(cyclic voltametry – b in fig1)
Fig 1
V = 0.02 – 100 Vs-1

11. Linear Sweep Voltammetry
Concept similar to Chronocoulometry * but a voltage sweep applied instead of a pulse.
time V2 V1
* involves measurement of the charge vs. time response to an applied potential step waveform. The shape of the resulting chronocoulogram can be understood by considering the concentration gradients in the solution adjacent to the electrode surface. It is useful for measuring electrode surface areas, diffusion coefficients, the time window of an electrochemical cell, adsorption of electroactive species, and the mechanisms and rate constants for chemical reactions coupled to electron transfer reactions

12. 1-1- reversible (Nernstian) processes
A simple reversible reduction
Fig2 - Trace ABC

13. Maximum value of increasing
Initial concentration gradient
fig3
Rising concentration gradient
Maximum value of increasing
decreasing concentration gradient

14. When the direction of scan is reversed
Fig2 - Trace CDE
Same phenomenon for the newly generated species Red

15. discussion Quantitative
By solving fick's law:
Boundary conditions:
Initial (assumption: Dox=Dred=D)
Semi-infinite
Electrode surface
If reversible
Jox(0,t)= -Jred(0,t)
Then:

16. Potential – time relationship
Initial potential
In which the potential scan is reversed
Potential waveform: E
A New Waveform Ei
 t

17. Laplace’s transform and fick’s second law:
25oC
Randles – sevcik equation:
Forward peak current
(experimental)

18. 1-1-1- Diagnostic criteria to identify a reversible process
ipr/ipf
The parameter which allows one to judge the chemical reversibility of an electrode reaction
Calculation
Computerized instrumentation
Graphical method(base line)
or empirical equation
ipr/ipf =1
then Red is stable
reversible

19. What is the baseline? Graphical method (ipr)0 : (if)0 : Fig 4b
Reverse peak current not corrected for the baseline
(if)0 :
Forward peak current at Ef
Fig 4b
Graphical method

20. Properties of the potential
Reversible process
Properties of the potential
Epf is independent of scan rate
ΔEp(250C)=59/n mV ; constant with scan rate and dependent on T
ΔEp=2.3RT/nF
Properties of the
current
ipf/v1/2 is independent of scan rate
ipr/ipf=1 and constant

21. 1-1-2-The chemical meaning of electrochemically reversible process
Activation barrier of a redox process
The extent of geometrical reorganization(accompanied with e.t.)
geometrical reorganization
reversibility

22. Ferrocene/ferrocenium oxidation process
Fig 5
Electrochemical reversibility
Maintenance of the original molecular geometry( fc fc+)
Ferrocene/ferrocenium oxidation process

23. 1-1-1-1-Cyclic voltammetry at spherical electrodes
Fig 6
Radial diffusion
Sigmoidal profile
amount of Ox at the electrode surface in the unit time for unit area with respect to a planar electrode
Micron size radius of electrode
Long time or slow scan rate
Domination over the linear diffusion