1. Electrochemical Analysis
Potentiometry,
Voltammetry,
Amperometry,

2. Applications Study Redox Chemistry Electrochemical analysis
electron transfer reactions, oxidation, reduction, organics & inorganics, proteins
Adsorption of species at interfaces
Electrochemical analysis
Measure the Potential of reaction or process
E = const + k log C (potentiometry)
Measure the Rate of a redox reaction; Current (I) = k C (voltammetry)
• Electrochemical Synthesis
Organics, inorganics, materials, polymers

3. Electrochemical Cells
Galvanic Cells and Electrolytic Cells
• Galvanic Cells – power output; batteries
• Potentiometric cells (I=0) read Chapter 2
– measure potential for analyte to react
current = 0 (reaction is not allowed to occur)
Equil. Voltage is measured (Eeq)
Electrolytic cells, power applied, output meas.
The Nernst Equation
For a reversible process: Ox + ne- → Red
E = Eo – (2.303RT/nF) Log (ared/aox)
a (activity), related directly to concentration

4. Voltammetry is a dynamic method
Related to rate of reaction at an electrode
O + ne = R, Eo in Volts
I = kA[O] k = const. A = area
Faradaic current, caused by electron transfer
Also a non-faradaic current forms
part of background current

5. Electrical Double layer at Electrode
Heterogeneous system: electrode/solution interface
The Electrical Double Layer, e’s in electrode; ions in solution – important for voltammetry:
Compact inner layer: do to d1, E decreases linearly.
Diffuse layer: d1 to d2, E decreases exponentially.

6. Electrolysis: Faradaic and Non-Faradaic Currents
Two types of processes at electrode/solution interface that produce current
Direct transfer of electrons, oxidation or reduction
Faradaic Processes. Chemical reaction rate at electrode proportional to the Faradaic current.
Nonfaradaic current: due to change in double layer when E is changed; not useful for analysis
Mass Transport: continuously brings reactant from the bulk of solution to electrode surface to be oxidized or reduced (Faradaic)
Convection: stirring or flowing solution
Migration: electrostatic attraction of ion to electrode
Diffusion: due to concentration gradient.

7. Reduction at electrode Causes current flow in External circuit O e-
Typical 3-electrode
Voltammetry cell
Reference electrode
Counter
electrode
Working electrode O
Reduction at electrode
Causes current flow in
External circuit O e-
Mass transport R R
End of Working electrode
Bulk solution

8. Analytical Electrolytic Cells
Use external potential (voltage) to drive reaction
Applied potential controls electron energy
As Eo gets more negative, need more energetic electrons in order to cause reduction. For a reversible reaction:
 Eapplied is more negative than Eo, reduction will occur
if Eapplied is more positive than Eo, oxidation will occur
O + ne- = R Eo,V electrode reaction

9. Current Flows in electrolytic cells
Due to Oxidation or reduction
Electrons transferred
Measured current (proportional to reaction rate, concentration)
Where does the reaction take place?
On electrode surface, soln. interface
NOT in bulk solution

10. Analytical Applications of Electrolytic Cells
Amperometry
Set Eapplied so that desired reaction occurs
Stir solution
Measure Current
Voltammetry
Quiet or stirred solution
Vary (“scan”) Eapplied
Indicates reaction rate
Reaction at electrode surface produces concentration gradient with bulk solution
Mass transport brings unreacted species to electrode surface

11. Cell for voltammetry, measures I vs. E
wire
potentiostat
insulator
electrode
material
reference N2
inlet
counter
working electrode
Electrochemical cell
Output, I vs. E, quiet solution
Input: E-t waveform
reduction
E, V
time
Figure1

12. Polarization - theoretical
Ideal Non-Polarized Electrode
Ideally Polarized Electrode
reduction
No oxidation or reduction
oxidation

13. Possible STEPS in electron transfer processes
Charge-transfer may be rate limiting
Rate limiting step may be mass transfer
Rate limiting step may be chemical reaction
Adsorption, desorption or crystallization polarization

14. Overvoltage or Overpotential η
η = E – Eeq; can be zero or finite
E < Eeq  η < 0
Amt. of potential in excess of Eeq needed to make
a non-reversible reaction happen, for example
reduction
Eeq

15. NERNST Equation: Fundamental Equation for reversible electron transfer at electrodes
O + ne- = R, Eo in Volts
E.g., Fe e- = Fe2+
If in a cell, I = 0, then E = Eeq
All equilibrium electrochemical reactions obey the
Nernst Equation
Reversibility means that O and R are at equilibrium at all times, not all
Electrochemical reactions are reversible
E = Eo - [RT/nF] ln (aR/aO) ; a = activity
aR = fRCR ao = foCo f = activity coefficient, depends on ionic strength
Then E = Eo - [RT/nF] ln (fR/fO) - [RT/nF] ln (CR/CO)
F = Faraday const., 96,500 coul/e, R = gas const.
T = absolute temperature

16. E = Eo’ - [0.0592/n] log (CR/CO); equil. systems
Ionic strength I = Σ zi2mi,
Z = charge on ion, m = concentration of ion
Debye Huckel theory says log fR = 0.5 zi2 I1/2
So fR/fOwill be constant at constant I.
And so, below are more usable forms of Nernst Eqn.
E = Eo - const. - [RT/nF] ln (CR/CO) Or
E = Eo’ - [RT/nF] ln (CR/CO); Eo’ = formal potential of O/R
At 25 oC using base 10 logs
E = Eo’ - [0.0592/n] log (CR/CO); equil. systems

17. A Three electrode cell used for anodic stripping voltammetry.
Voltammetric Methods
Voltammetry are based on measurement of current as a function of the potential applied to a small electrode. Unlike potentiometry measurements, which employ only two electrodes, voltammetric measurements utilize a three electrode electrochemical cell. The use of the three electrodes (working, auxiliary, and reference) along with the potentiostat instrument allow accurate application of potential functions and the measurement of the resultant current.
A Three electrode cell used for anodic stripping voltammetry.
The working electrode is a glassy carbon electrode on which a thin mercury film has been deposited.
An electrolysis step is used to deposit lead into the mercury film as an amalgam.
After the electrolysis step, the potential is scanned anodically toward positive values to oxidize (strip) the metal from the film.

18. CYCLIC VOLTAMMETRY
- Involves linear scanning of potential of a stationary
electrode using a triangular waveform
- Solution is unstirred
- The most widely used technique for quantitative analysis
of redox reactions
Provides information on
- the thermodynamics of redox processes
- the kinetics of heterogeneous electron transfer reactions
- the kinetics of coupled reactions

19. CYCLIC VOLTAMMETRY
- The current resulting from an applied potential is
measured during a potential sweep
- Current-potential plot results and is known as
cyclic voltammogram (CV)

20. CYCLIC VOLTAMMOGRAM (CV)
Triangular waveform (left) and CV (right) of ferricyanide

21. Voltammogram A + ne P → ← iI = kCA
Linear sweep voltammogram for the reduction of a hypothetical species A to give a product P. The limiting current il is proportional to the analyte concentration and is used for quantitative analysis. The half-wave potential E1/2 is related to the standard potential for the half reaction and is often used for qualitative identification of species.
A + ne P → ←
iI = kCA

22. Cyclic Voltammetry t0 → t1 : cathodic wave t1 → t2 : anodic wave
Instead of leaving off at the top of the wave, current decreases at more negative potential
← diffusion is too slow to replenish analyte near the electrode
t1 → t2 : anodic wave
The potential is reversed and, reduced product near the electrode is oxidized
Cyclic voltammograms are recorded either with an osciloscope or with a fast X-Y recorder.
The current decreases after the cathodic peak because of concentration polarization.
For a reversible reaction, half-wave potential lies midway between the cathodic and anodic peaks. 22

24. Potential vs time waveform and
cyclic voltammogram for a solution that is 6.0 mM in K3Fe(CN)6 and 1.0M in KNO3.

25. 5.375mM (left) and 0.5375 (right) mM Ferrocene in Acetonitrile
Fe(C5H5)2
5.375mM (left) and (right) mM Ferrocene in Acetonitrile

26. a) Structure of C60 (buckminsterfullerene), b) Cyclic voltammetry
Cyclic voltammogram of the insecticide parathion in 0.5 M pH 5 sodium acetate buffer in 50% ethanol.
a) Structure of C60 (buckminsterfullerene),
b) Cyclic voltammetry
c) Differential pulse polarography

27. CYCLIC VOLTAMMETRY - Assume only O is present initially
- A negative potential sweep results in the reduction of O to R
(starting from a value where no reduction of O initially occurs)
- As potential approaches Eo for the redox process, a cathodic
current is observed until a peak is reached
- The direction of potential sweep is reversed after going
beyond the region where reduction is observed
- This region is at least 90/n mV beyond the peak

28. CYCLIC VOLTAMMETRY
- R molecules generated and near the electrode surface
are reoxidized to O during the reverse (positive) scan
- Results in an anodic peak current
- The characteristic peak is a result of the formation of a
diffusion layer near the electrode surface
- The forward and reverse currents have the same shape

29. CYCLIC VOLTAMMETRY
- Increase in peak current corresponds to achievement
of diffusion control
- Decrease in current (beyond the peak) does not depend
on the applied potential but on t-1/2
Characteristic Parameters
- Anodic peak current (ipa)
- Cathodic peak current (ipc)
- Anodic peak potential (Epa)
- Cathodic peak potential (Epc)

30. CYCLIC VOLTAMMETRY Reversible Systems
- Peak current for a reversible couple is given by the
Randles-Sevcik equation (at 25 oC)
n = number of electrons
A = electrode area (cm2)
C = concentration (mol/cm3)
D = diffusion coefficient (cm2/s)
ν = potential scan rate (V/s)

31. CYCLIC VOLTAMMETRY Reversible Systems ip is proportional to C
- Implies electrode reaction is controlled by mass transport
ipa/ipc ≈ 1 for simple reversible couple
- For a redox couple

32. CYCLIC VOLTAMMETRY Reversible Systems
- The separation between peak potentials
- Used to determine the number of electrons transferred
- For a fast one electron transfer ∆Ep = 59 mV
- Epa and Epc are independent of the scan rate

33. CYCLIC VOLTAMMETRY Reversible Systems - The half peak potential
- E1/2 is called the polarographic half-wave potential
Multielectron Reversible Systems
- The CV consists of several distinct peaks if the Eo values for
the individual steps are well separated
(reduction of fullerenes)

34. CYCLIC VOLTAMMETRY Irreversible Systems
- Systems with sluggish electron transfer
- Individual peaks are reduced in size and are widely separated
- Characterized by shift of the peak potential with scan rate

35. CYCLIC VOLTAMMETRY Irreversible Systems α = transfer coefficient
na = number of electrons involved in a charge transfer step
ko = standard heterogeneous rate constant (cm/s)
- ip is proportional to C but lower depending on the value of α
For α = 0.5
ip,reversible/ip,irreversible = 1.27
- That is irreversible peak current is ~ 80% of reversible ip

36. Quasi-reversible Systems
CYCLIC VOLTAMMETRY
Quasi-reversible Systems
- Current is controlled by both charge transfer and mass transport
- Voltammograms are more drawn out
- Exhibit larger separation in peak potentials compared
to reversible systems
- Shape depends on heterogeneous rate constant and scan rate
- Exhibits irreversible behavior at very fast scan rates

38. CYCLIC VOLTAMMETRY Applications 1. Study of Reaction Mechanisms
E = redox step and C = chemical step E
- Only redox step
O + ne- ↔ R

39. CYCLIC VOLTAMMETRY Applications E = redox step and C = chemical stepEC
- Redox step followed by chemical step
O + ne- ↔ R + A → Z
- R reacts chemically to produce Z
- Z is electroinactive
- Reverse peak is smaller since R is chemically removed
ipa/ipc < 1
- All of R can be converted to Z for very fast chemical reactions

40. CYCLIC VOLTAMMETRY Applications E = redox step and C = chemical stepEC
- Redox step followed by chemical step
O + ne- ↔ R + A → Z
Examples
- Ligand exchange reactions as in iron porphyrin complexes
- Oxidation of chlorpromazine to produce a radical cation and
subsequent reaction with water to produce sulfoxide

41. CYCLIC VOLTAMMETRY Applications E = redox step and C = chemical stepEC
- Catalytic regeneration of O during a chemical step
O + ne- ↔ R + A ↔ O
- Peak ratio is unity
Example
- Oxidation of dopamine in the presence of ascorbic acid

42. CYCLIC VOLTAMMETRY Applications E = redox step and C = chemical stepCE
- Slow chemical reaction precedes the electron transfer step
Z → O + ne- ↔ R
ipa/ipc > 1 (approaches 1 as scan rate decreases)
ipa is affected by the chemical step
ipc is not proportional to ν1/2

43. CYCLIC VOLTAMMETRY Applications E = redox step and C = chemical step
ECE
- Chemical step interposed between redox steps
O1 + ne- ↔ R1 → O2 + ne- → R2
- The two redox couples are observed separately
- The system behaves as EE mechanism for very fast
chemical reactions
- Electrochemical oxidation of aniline

44. CYCLIC VOLTAMMETRY Applications 2. Study of Adsorption Processes
- For studying the interfacial behavior of electroactive compounds
Symmetric CV
∆Ep = 0
- Observed for surface-confined nonreacting species
- Ideal Nernstian behavior

45. CYCLIC VOLTAMMETRY Applications Symmetric CV
- Peak current is directly proportional to surface coverage (Γ)
and scan rate (ν)
Holds for relatively
- slow scan rates
- slow electron transfer
- no intermolecular attractions within the adsorbed layer

46. CYCLIC VOLTAMMETRY Applications Symmetric CV Q (area under peak)
current
∆Ep,1/2
volts

47. CYCLIC VOLTAMMETRY Applications Symmetric CV
- The surface coverage can be determined from the
area under the peak (Q)
Q = quantity of charge consumed or

48. CYCLIC VOLTAMMETRY Applications 3. Quantitative Determination
- Based on the measurement of peak current

49. Additional methods

50. SPECTROELECTROCHEMISTRY
- Simultaneous measurement of spectral and electrochemical signals
- Coupling of optical and electrochemical methods
- Employs optically transparent electrode (OTE) that allows light to
pass through the surface and adjacent solution
Examples
Indium tin oxide (ITO), platinum, gold, silver, nickel
deposited on optically transparent glass or quartz substrate

51. SPECTROELECTROCHEMISTRY
ipa = anodic peak current
ipc = cathodic peak current
Modulated Absorbance
Am = -log(I/Io)

52. SPECTROELECTROCHEMISTRY
Applications
- Useful for elucidation of reaction kinetics and mechanisms
(for probing adsorption and desorption processes)
- Thin layer SE methods for measuring Eo and n (Nernst equation)
- Infrared SE methods for providing structural information
- UV-Vis spectroscopic procedures
- Vibration spectroscopic investigations
- Luminescence reflectance and scattering studies

53. ELECTROCHEMILUMINESCENCE (ECL)
- Technique for studying electrogenerated radicals that emit light
- Involves electrochemical generation of light-emitting
excited-state species
- Usually carried out in deoxygenated nonaqueous media
Examples of Species
Ru(bpy)32+
Nitro compounds
Polycyclic hydrocarbons
Luminol

54. SCANNING PROBE MICROSCOPY
- Used to acquire high resolution data of surface properties
- Achieved by sensing the interactions between a probe tip and
the target surface as the tip scans across the surface
Examples
- Scanning Tunneling Microscopy (STM)
- Atomic Force Microscopy (AFM)
- Scanning Electrochemical Microscopy (SECM)

55. SCANNING TUNNELING MICROSCOPY (STM)
- Direct imaging of surfaces on the atomic scale
- Very sharp atomic tip moves over the sample surface with a
ceramic piezoelectric translator
- The basic operation is the electron tunneling between the
metal tip and the sample surface
- Tunneling current is measured as potential is applied between
the tip and the sample
- Measured current at each point is based on sample-tip separation

56. ATOMIC FORCE MICROSCOPY (AFM)
- High resolution imaging of the topography of surfaces
(surface structure)
- Allows for nanoscopic surface features while the electrode
is under potential control
- Measures the force between the probe and the sample
- The probe has a sharp tip made from silicon or silicon nitride
attached to a force-sensitive cantilever
- Useful for exploring both insulating and conducting regions

57. SCANNING ELECTROCHEMICAL
MICROSCOPY (SECM)
- Faradaic currents at a microelectrode tip are measured while the
tip is moved close to the substrate surface immersed in a
solution containing an electroactive species
- The tip currents are a function of the conductivity and chemical
nature of the substrate as well as the tip-substrate distance
Images obtained give information on
- electrochemical activity
- chemical activity
- surface topography

58. SCANNING ELECTROCHEMICAL
MICROSCOPY (SECM)
- Cannot be used for obtaining atomic resolution
Used to investigate
- Ionic flux through the skin or membranes
- Localized biological activity (biosensors)
- Heterogeneous reaction kinetics

59. ELECTROCHEMICAL QUARTZ CRYSTAL
MICROBALANCE (EQCM)
- For elucidating interfacial reactions based on simultaneous
measurement of electrochemical parameters and mass
changes at the electrode surface
- Uses a quartz crystal wafer sandwiched between two electrodes
which induces electric field
- The electric field produces a mechanical oscillation in the
bulk of the wafer

60. ELECTROCHEMICAL QUARTZ CRYSTAL
MICROBALANCE (EQCM)
- The frequency change (∆f) relates to the mass change (∆m)
according to the Sauerbrey equation
n = overtone number
fo = base resonant frequency of the crystal (prior to mass change)
A = area (cm2)
μ = shear modulus of quartz (2.95 x 1011 gcm-1s-1)
ρ = density of quartz (2.65 g/cm3)

61. ELECTROCHEMICAL QUARTZ CRYSTAL
MICROBALANCE (EQCM)
- Decrease in mass corresponds to increase in frequency
Useful for probing
- processes that occur uniformly across the surface
- deposition or dissolution of surface layers
- ion-exchange reactions at polymer films
- study of polymeric films
- Cannot be used for molecular level characterization of surfaces

62. IMPEDANCE SPECTROSCOPY
- For probing the features of chemically-modified electrodes
- For understanding electrochemical reactions
- For electron transfer kinetics and diffusional
characteristic studies
Impedance
- Complex resistance encountered when a current flows
through a circuit made of combinations of
resistors, capacitors, or inductors

63. IMPEDANCE SPECTROSCOPY
- Plots of faradaic impedance spectrum is known as Nyquist plot
Consists of
- a semicircle portion at high frequencies
(corresponds to the electron-transfer-limited process)
and
- a straight line portion at low frequencies
(coreesponds to the diffusion-limited process)

64. IMPEDANCE SPECTROSCOPY
- The impedance spectrum has only the linear portion
for very fast electron transfer processes
- Very slow electron transfer processes are characterized
by a large semicircle region
- Diameter of the semicircle equals the electron
transfer resistance