Voltammetry techniques - CV- Analysis

1. Cyclic
Voltammetry
Dr.J.Anuradha

2. ELECTRO ANALYTICAL
TECHNIQUES
Electro analytical chemistry, as the name implies, involves the analysis of
chemical species through the use of electrochemical methods. Generally, we
monitor alterations in the concentration of a chemical species by measuring
changes in current in response to an applied voltage with respect to time.
According to Faraday's law, the charge is directly proportional to the
amount of species undergoing a loss (oxidation) or gain (reduction) of electrons.
Q = n F e
Current is the change in charge as a function of time.
I = dQ / dt

3. VOLTAMMETRY
Voltammetry gives information about an analyte is obtained by measuring the
current as the potential is varied
• Linear sweep voltammetry
• Staircase voltammetry
• Squarewave voltammetry
• Anodic stripping voltammetry
• Cathodic stripping voltammetry
• Adsorptive stripping voltammetry
• Alternating current voltammetry
• Normal pulse voltammetry
• Differential pulse voltammetry
• Chronoamperometry
• Cyclic voltammetry (CV)
Types

4. WHY CV ?
 A wide range of current over potentials
 Fast technique (single scans typically recorded in every 100 ms)
 Low signal to noise ratio.
 the product of the electron transfer reaction that occurred in the forward scan can
be probed again in the reverse scan
 Implication:
A single CV scan doesn’t tell you much, don’t over interpret it !

5. CYCLIC
VOLTAMMETRY
 Cyclic Voltammetry is a type of potentiodynamic electrochemical measurement.
 In a cyclic voltammetry experiment the working electrode potential is ramped linearly
versus time.
 Cyclic voltammetry takes the three electrode setup; when working electrode reaches
a set potential, the ramp is inverted. This inversion can happen multiple times during
a single experiment.
 The current at the working electrode is plotted versus the applied voltage to give the
cyclic voltammogram trace.

6.  Cyclic voltammetry is a very versatile electrochemical technique which allows
to probe the mechanics of redox and transport properties of a system in
solution.
 The potential is applied between the reference electrode and the working
electrode and the current is measured between the working electrode and
the counter electrode.
 In cyclic voltammetry, the direction of the potential is reversed at the end of
the first scan. Thus, the waveform is usually of the form of an isosceles
triangle.
Gives
The potential is scanned back and forth
linearly with time between two extreme
values – the switching potentials using
triangular potential waveform

7. The
black
box
Reference
Electrode:
Defines “0”
potential for the
cell.
Use: Ag/AgCl
Working Electrode:
Where the redox
reaction action occurs
Use: glassy carbon,
platinum and gold.
Counter /
Auxiliary
Electrode:
Needed to
complete circuit.
Use a Pt wire

8. For an redox reaction induced at a working electrode, the rate determining step may
be any one of the following individual step depending on the system: rate of mass
transport of the electroactive species, rate of adsorption or desorption at the electrode
surface, rate of the electron transfer between the electroactive species and the
electrode, or rates of the individual chemical reactions which are part of the overall
reaction scheme. For the oxidation reaction involving n electrons,
The Nernst Equation gives the relationship between the potential and the
concentrations of the oxidized and reduced form of the redox couple at equilibrium (at
298 K):
where E is the applied potential and E0' the formal potential; [Ox] and [Red] represent
surface concentrations at the electrode / solution interface, not bulk solution
concentrations.

9. During the reducing scan, the surface concentration of species Ox
progressively decreases, resulting in an increased concentration gradient and a larger
current. As reduction continues, the concentration of Ox at the electrode surface is
depleted, and the current peak will decay if fresh Ox from the bulk solution does not
have enough time to diffuse to the electrode surface. When the direction of the
potential scan is reversed, a peak resulting from the re-oxidation of reduced Red is
observed.
When a redox reaction is 100% reversible, the oxidation and reduction
peak currents are equivalent, and the peak current is given by the relation:
ip = 0.4463nF (nF/RT)1/2 (Da)1/2 (V)1/2 Ca A
where
V is the potential scan rate in V/s and Ca is analyte concentration in mol/cm3.

23. Cycle could be run
again, many times.

24. 1. Number of charge (Q)
The integrated area under each wave represents the charge Q associated with
the reduction or oxidation of the adsorbed layer
Q = n F A Γ
n: number of electrons
F: Faraday constant
A: the electrode surface area
Γ: the surface coverage in moles of adsorbed molecules per surface area
2. Capacitance (C)
I = vC
The peak current is proportional to
scan rate v,
Icap: current
v: scan rate
Cd: capacitance
Cyclic voltammograms _ quantitative information

25. 3. Number of electrons (n)
DEp = │Epa - Epc│ = 90.6 / n
4. Surface coverage (Γ)
Ipeak = n2F2vAΓ(4RT )-
For a reversible electrode reaction at 25 °C, the difference in peak potentials,
DEp is expected to be
When the number of electrons is known, the surface coverage can be
calculated by the equation:

26. The Baseline Issue
• Record i-t over i-V

27. 1. Study of Reaction Mechanisms
APPLICATIONS
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
- Oxidation of dopamine in the presence of ascorbic acid
- Electrochemical oxidation of aniline
2. Quantitative Determination
To evaluate electrochemical reversibility by looking at the difference between the
peak potentials for the anodic and the cathodic scans.
Ep= Ep,a− Ep,c = 0.05916 V / n
For example, for a two-electron reduction, we expect a ∆Ep of approximately 29.6
mV. For an electrochemically irreversible reaction the value of ∆Ep will be larger
than expected.
3. Electrochemical Reversibility and Determination of n
Concentration of analyte, Number of electrons per molecule in analyte and
Diffusion coefficient

28. 4. Adsorption in cyclic voltammetry
- Repetitive voltammograms for micromolar
riboflavin at a HMDE
• note: peak separation is smaller than for
solution phase couple
•Peak area also gives coverage
• Q =nFA , can be used to determine area
occupied by molecule - can give
orientational information
- Ferrocene was irreversibly adsorbed in the form of a self-assembled monolayer to
a Au surface using a long-chain alkylthiol.

29. Cyclic sweeps are used to measure corrosion that proceeds at about the
same rate all over the metals surface (uniform) and corrosion at discrete sites on
the surface e.g. pitting crevice and stress corrosion cracking (localised).
- To determine the degree of localised corrosion the amount of hysteresis between
the positive going sweep and the negative going sweep is calculated.
5. Corrosion studies in CV
1 mM K3Fe(CN)6 in 0.1 M KCl
solution
red - 20 mV/s, gold - 50 mV/s,
green - 100 mV/s, blue - 250
mV/s, black - 500 mV/s

30. CV can provide information about the chemistry of redox couples. This
is perhaps the most attractive feature of this technique. For example, if a redox
couple undergoes two sequential electron-transfer reactions, you will see two
peaks in the voltammogram. If either the reduced or oxidized species is unstable,
you might see a voltammogram such as that shown in Figure. Such qualitative
information is very useful.
Cyclic voltammogram of a system
where the reduced species is unstable
6. Study of Reactions Mechanisms