Analytical technique Voltametry Instrumentation, working and theory. Cyclic voltamety, polarography, stripping, pulse, square wave voltamety, Animated cyclic voltamery polarogram

1. VOLTAMMETRY
Presented By
Waqas Ilyas

2. Contents
▪ Introduction
▪ Classification
▪ Definition
▪ Working Principle
▪ Instrumentation
▪ Working
▪ VoltammetricTechniques
▪ Limitations
▪ Applications
▪ References

3. Electrochemistry
▪ The branch of chemistry that deals with the relation between
electrical and chemical phenomena.
▪ Electrochemistry deals with analytical techniques that use a
measurement of potential, charge, or current to determine an
analyte’s concentration or to characterize an analyte’s
chemical reactivity.
▪ Electrochemistry it a vast field.The classification of
electrochemical techniques is based on:
▪ Do we control the potential or the current?
▪ How do we change the applied potential or applied current?
▪ Do we stir the solution?

4. Classification tree
highlighting the
similarities and
differences between a
number of interfacial
electrochemical
techniques.
The specific
techniques are shown
in red, the
experimental
conditions are shown
in blue, and the
analytical signals are
shown in green.

5. Definition
▪ Voltammetry is an electrochemical method in which information
about an analyte is obtained by measuring current (i) as a function
of applied potential (E).
▪ The resulting plot of current versus applied potential is called
voltammogram.
▪ It is the electrochemical equivalent of a spectrum in spectroscopy,
providing quantitative and qualitative information about the species
involved in the oxidation or reduction reaction [Maloy, J.T.J. Chem. Educ.
1983, 60, 285–289]
▪ Voltammetry (polarography) first reported in 1922 by Czech
Chemist Heyrovsky. Later given Nobel Prize for method.
VOLTAMMETRY

6. ▪ Differences from Other Electrochemical Methods
▪ Potentiometry: measures potential of sample or system at or near
zero current.
VS
voltammetry – measures the current as a change in potential.
▪ Coulometry: uses all of analyte in process of measurement at fixed
current or potential.
VS
voltammetry – use only small amount of analyte while varying
the potential.
VOLTAMMETRY

7. Working Principle:
▪ In voltammetry we apply a time-dependent potential excitation
signal to the working electrode—changing its potential relative to
the fixed potential of the reference electrode—and measure the
current that flows between the working and auxiliary electrodes.
(Modern Analytical Chemistry 2.0 by David Harvey)
▪ Early voltammetric methods used
only two electrodes, a modern
voltammeter makes use of a three
electrode potentiostat, as shown in
Figure.
VOLTAMMETRY

8. Instrumentation
Three electrodes in solution containing
analyte
Working electrode: Electrode whose
potential is varied with time.
Reference electrode: potential remains
constant (Ag/AgCl electrode or
calomel).
Auxiliary electrode: Hg or Pt that
completes circuit, conducts e- from
signal source through solution to the
working electrode.
Supporting electrolyte: excess of
nonreactive electrolyte (alkali metal)
to conduct current.
VOLTAMMETRY
Typical electrochemical cell for voltammetry.

9. Working:
• Voltage is applied on working electrode.
• Analyte selectivity is provided by the applied potential on
the working electrode.
• Current flows between working and counter electrodes.
• Potential controlled by potentiostat between working and
reference electrodes.
• Electroactive species in the sample solution are drawn
towards the working electrode where a half-cell redox
reaction takes place.
• Another corresponding half-cell redox reaction will also
take place at the counter electrode to complete the
electron flow.
VOLTAMMETRY

10. Working:
• The resultant current flowing through the electrochemical cell reflects
the activity (i.e.  concentration) of the electroactive species involved.
Pt working
electrode
SCE
Ag Auxiliary
electrode
X M of PbCl2
0.1M KCl
Pb2+ + 2e- Pb
K+ + e- K
VOLTAMMETRY
AgCl Ag + Cl
-

11. Pb2+
Pb2+
Pb2+
Pb2+
Pb2+
Pb2+
Pb2+
Pb2+
Pb2+
Pb2+
Pb2+
Pb2+
Pb2+
Pb2+
Pb2+
Pb2+
Pb2+
Pb2+
Pb2+
Pb2+ Pb2+
K+
K+
K+
K+
K+
K+
K+
K+
K+
K+
K+ K+
K+
K+
K+
K+
K+
K+
K+
K+
-1.0 V vs SCE
Pb2+ + 2e- Pb
K+
K+
K+
K+
K+ K+
Layers of K+ build up around the electrode stop the
migration of Pb2+ via coulombic attraction
Concentration gradient created
between the surrounding of the
electrode and the bulk solution
Pb2+ migrate to
the electrode
via diffusion

12. Movement of ions is by three transport mechanisms:
(i) migration – movement of ions through solution by
electrostatic attraction to charged electrode
(ii) convection – mechanical motion of the solution as
a result of stirring or flow
(iii) diffusion – motion of a species caused by a
concentration gradient
VOLTAMMETRY

13. Theory of Voltammetry
• As the potential set by instrument on working electrode is increased, the
more ionized species come to electrode surface for electron exchange.
• These species get deposited on and around electrode and hinder the
transport of furthers ions from bulk solution to electrode.
• Concentration of Reduced and Oxidized Species at electrode is based
on Nernst Equation:
Eelectrode = E0 - log
0.0592
n
(Con. Of Oxidized)
(Con. Of Reduced)
VOLTAMMETRY

14. At Electrodes Surface:
Eappl = Eo - log
0.0592
n
[Mred]s
[Mox]s
at surface of electrode
Applied potential
If Eappl = Eo:
0 = log
so, [Mox]s = [Mred]s
0.0592
n
[Mred]s
[Mox]s
Mox + e- Mred

15. -0.2 -0.4 -0.6 -0.8 -1.0 -1.2 -1.4
i (A)
Potential applied on the working electrode is usually swept over (i.e. scan)
a pre-defined range of applied potential
0.001 M Cd2+ in 0.1 M KNO3 supporting electrolyte
V vs SCE
Working electrode is
no yet capable of
reducing Cd2+ 
only small residual
current flow through
the electrode
Electrode become more and more
reducing and capable of reducing Cd2+
Cd2+ + 2e- Cd
Current starts to be registered at the
electrode
Current at the working
electrode continue to rise as
the electrode become more
reducing and more Cd2+
around the electrode are being
reduced. Diffusion of Cd2+
does not limit the current yet
All Cd2+ around the electrode has
already been reduced. Current at
the electrode becomes limited by
the diffusion rate of Cd2+ from the
bulk solution to the electrode.
Thus, current stops rising and
levels off at a plateau
Id is
proportional to
concentration of
analyte
E½ is standard
electrode potential
and used for
identification
Base line
of residual
current

16. ▪ Faradaic Current:The current from the redox reactions
at the working electrode and the auxiliary electrodes is
called a faradaic current.
▪ Non-Faradaic Current: A small, short-lived current
produced due to movement of cation and anions in
solution near the electrode on change of potential of
electrode. Also called charging current.
▪ Residual current: A small, measurable current that flows
through an electrochemical cell due to Faradaic
(oxidation and reduction of impurities) and non-
Faradaic current in the absence of analyte.
VOLTAMMETRY

17. VoltametricTechniques
▪ Polarography
▪ HydrodynamicVoltammetry
▪ PulseVoltammetry
– Normal PulseVoltammetry
– Differential PulseVoltammetry
– Square-waveVoltammetry
▪ StrippingVoltammetry
– Anodic StrippingVoltammetry
– Cathodic StrippingVoltammetry
▪ CyclicVoltammetry
VOLTAMMETRY

18. Polarography
▪ It is an electromechanical technique of analyzing
solutions that measures the current flowing between two
electrodes in the solution as well as the gradually
increasing applied voltage to determine respectively the
concentration of a solute and its nature.
▪ It takes place in an unstirred solution
▪ Most common electrode is Dropping Mercury Electrode
VOLTAMMETRY

19. Normal Pulse
▪ This technique uses a series of potential pulses of increasing
amplitude.The current measurement is made near the end of each
pulse, which allows time for the charging current to decay. It is
usually carried out in an unstirred solution.
▪ Normal pulse polarography (Figure a), uses a series of potential
pulses characterized by a cycle of time τ , a pulse-time of tp , a
pulse potential of ΔEp , and a change in potential per cycle of ΔEs
VOLTAMMETRY
▪ The duration of the
pulse, t, is usually 1
to 100 msec and the
interval between
pulses typically 0.1
to 5 sec.
Fig (a)

20. Differential Pulse
▪ Similar to normal pulse voltammetry, potential is also scanned
with a series of pulses.
▪ However, each potential pulse is fixed, of small amplitude (10 to
100 mV), and is superimposed on a slowly changing base potential.
▪ Current is measured at two points for each pulse,
– Just before the application of the pulse
– At the end of the pulse.
VOLTAMMETRY
Differential pulse Voltammetry

21. Square-Wave Voltammetry
▪ The excitation signal consists of a symmetrical square-
wave pulse of amplitude E superimposed on a staircase
waveform of step height ΔE, where the forward pulse of
the square wave coincides with the staircase step.
▪ The net current, inet, is obtained by taking the difference
between the forward and reverse currents (ifor – irev) and is
centered on the redox potential
VOLTAMMETRY
Square-wave Voltammetry

22. Hydrodynamic Voltammetry
▪ It is type of voltammetry in which analyte solution is
continuously kept in motion.
▪ There are two main approaches
▪ First the electrode can be held in a fixed position and
solution is flowed over the surface by an applied force
(usually a pressure).
▪ Second the electrode can be designed to move which helps
to mix the solution via convection.
VOLTAMMETRY

23. Hydrodynamic Voltammetry
▪ Hydrodynamic techniques
– Solid disc electrode in stirred solution
– The Rotating Disc and Ring Disc Electrode
– The Wall Jet Electrode
– The Channel Electrode
VOLTAMMETRY

24. Stripping Voltammetry
▪ Stripping method involve deposition of analyte on one of the
electrode followed by its removal by reversing the polarity of
electrode.
▪ In first step analyte is deposited on the electrode by applying
voltage. It is called electrode deposition step.
▪ In next step, polarity of electrode is reversed to removed
deposited analyte, It is called stripping.
▪ Two types of StrippingVoltammetry
– Anodic stripping voltammetry (ASV)
– Cathodic stripping voltammetry (CSV)
VOLTAMMETRY

25. Cyclic Voltammetry
▪ This technique is based on varying the
applied potential at a working electrode in
both forward and reverse directions (at
some scan rate) while monitoring the
current. For example, the initial scan could
be in the negative direction to the switching
potential. At that point the scan would be
reversed and run in the positive direction.
▪ Depending on the analysis, one full cycle, a
partial cycle, or a series of cycles can be
performed.
▪ It is used to look at mechanisms of redox
reactions in solution.
▪ It gives i vs E response of small, stationary
electrode in unstirred solution using
triangular waveform for excitation
Cyclic voltammogram
VOLTAMMETRY

26. A Initial negative current due to oxidation of H2O to give O2
K3Fe(CN)6 in KNO3 electrolyte solution
Working Electrode is Pt & Reference electrode is SCE
No current between A & B. (+0.7 to +0.4V) No reducible or
oxidizable species present in this potential range.
B At 0.4V, current begins because of the following reduction
at the cathode:
Fe(CN)6
3- +e- > Fe(CN)6
4-
B - D Rapid increase in current as the surface concentration
of Fe(CN)6
3- decreases
D Cathodic peak potential (Epc) and peak current (ipc)
D - F Current decays rapidly as the diffusion layer is extended
further from electrode surface
F Scan direction switched (-0.15V), potential still negative
enough to cause reduction of Fe(CN)6
3-
F - J Eventually reduction of Fe(CN)6
3- no longer occurs and
anodic current results from the re-oxidation of Fe(CN)6
4-
J Anodic peak potential (Epa) and peak current (ipa)
K Anodic current decreases as the accumulated Fe(CN)6
4- is
used up at the anodic reaction

27. Limitations
▪ General
– Substance must be oxidizable or reducible in the range were
the solvent and electrode are electrochemically inert.
– It provides very little or no information on species identity.
– Sample must be dissolved.
▪ Accuracy
– Accuracy varies with technique from 1 to 10%.
▪ Sensitivity and Detection Limits
– Detection limit varies with technique from parts per thousand
to parts per trillion.
VOLTAMMETRY

28. Applications
▪ Quantitative and qualitative determination of pharmaceutical
compounds.
▪ To detect quantities of ingredients and their contaminants in
pharmaceutical preparations, rotating mercury film electrodes
on glassy carbon coupled with anodic stripping voltammetry
seem to provide the best results. (Wang & Dewald, 1983).
▪ Elemental levels in seawater are now widely determined as a
check on contaminant. By using different electrolytes some
twenty different elements may be detected at levels greater
than 10 to 30ng / litre. (O'Halloran, 1982; Batley, 1983; Drabaek et al., 1983).
▪ Anodic stripping voltammetry provides a ready means for
determining several metal contaminants in plants and food; for
example, Pb, Cd and Zn. (Satzger et al., 1982; Gajan et al., 1982).
▪ A differential pulse polarographic method is now available for
the determination of aflatoxins in food to supplement the
previously used fluorescence technique (Table 1: Smyth et al., 1979).
VOLTAMMETRY

29. Applications
▪ Quantitative determination of organic and inorganic
compounds in aqueous and non-aqueous solutions.
▪ Anodic stripping voltammetry has been accepted by the US
National Institute of Occupational Safety and Health as a
second reference method for detection of lead in petrol
(Taylor, 1977).
▪ Measurement of kinetic rates and constants.
▪ Determination adsorption processes on surfaces
▪ Determination electron transfer and reaction mechanisms.
▪ Fundamental studies of oxidation and reduction processes in
various media.
VOLTAMMETRY

30. Applications
▪ Quantitative determination of organic and inorganic
compounds in aqueous and non-aqueous solutions.
▪ Measurement of kinetic rates and constants.
▪ Determination of redox potentials.
▪ Detection of eluted analytes in high-performance liquid
chromatography (HPLC) and flow injection analysis.
▪ Determination of thermodynamic properties of solvated
species.
VOLTAMMETRY

31. References
▪ Modern AnalyticalChemistry 2.1 by David Harlay
▪ Fundamentals ofAnalytical Chemisry by F. James Holler
▪ CyclicVoltammetry and Its Applications by Pipat Chooto.
▪ VoltammetricTechniques, Samuel P. Kounaves,Tufts
University Department of Chemistry
▪ AnalyticalApplication of SquareWaveVoltammetry, Matthew
S. Krause, Jr.,l and Louis Ramaley, Department of Chemistry,
University of Arizona,Tucson, Ariz. 85721
▪ University of Cambridge:
https://www.ceb.cam.ac.uk/research/groups/rg-
eme/Edu/hydrodynamic-voltammetry