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3. Voltammetry
Voltammetry: is a technique which concerned with the study of voltagecurrent-time relationships during electrolysis in a cell. The technique
commonly involves the influence of the change in applied potential
(voltage) on the current flowing in the cell, but in some cases, the variation
of current with time may be investigated.
Electrolysis: is the occurrence of chemical reaction under the influence of
an electromotive force at electrode immersed in solution.
As many of other instrumental methods, the substance to be determined is
exposed to source of excitation (potential) to obtain a response (the
resulting variation of current with potential which called voltammogram).
Voltammetry is based upon the measurement of a current that develops in
an electrochemical cell under conditions of complete concentration
polarization.
(Potentiometric measurements are made at currents that approach
zero and where polarization is absent.)
Voltammetry
Furthermore, in voltammetry a minimal consumption of analyte takes
place, whereas in electrogravimetry and coulometry essentially all of
the analyte is converted to another state
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Potential applied to the electrochemical cell is supplied by an apparatus
called potentiostat. ·
Electrochemical
cell
for
voltammetric study.
It is usually composed of three
electrodes as shown in figure
1-Working (indicator) electrode:
at
which
substance
the
electrolysis
of
under investigation
takes place, this electrode is
usually small in size and polarized
i.e.
it
adopts
the
potential
externally imposed on it e.g.
dropping mercuric electrode (DME), solid electrodes like gold, platinum
and carbon.
2-Reference electrode: Which is used to measure the potential of working
electrode. This electrode is larger in size relative to working electrode and
is none polarized (depolarized) i.e. it retains a constant potential e.g.
saturated calomel electrode (SCE), Ag/AgCI.
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Dr. Hisham E Abdellatef
Voltammetry
3- Counter (auxiliary) electrode: which together with the working
electrode carries the electrolysis current. It can be made from any metal,
but its presence must not affect on the oxidation or reduction of the
reactive substance, so it usually put in separate compartment inside the
cell (put in sintered glass tube). It made from gold or platinum or other
metals.
In some cases only two electrodes are used (working and reference) e.g. in
complete aqueous medium where the resistance for the current flow is
very small, but usually three electrode cell is used
Solutions: redox couple + solvent + supporting electrolyte
Supporting electrolyte: it’s a solution of indifferent electrolyte, which by
itself don't oxidized or reduced at working electrode at the selected
potential range. It has several rules:
1- it raises the conductivity of the solution,
2- It carries the bulk of the current so prevent the migration of
electroactive materials to the working electrode.
3- It may control or (buffer) the level of hydrogen ion activity in
solution.
4- Also it may associate with the electroactive solute in the complexing
of the metal ions by legends.
eg. KNO3, NaCl, Na3PO4, buffer or mixture of buffer andorganic solvents. If
the medium is completely non aqueous example when the experiment is
done in dimethyl formamide DMF, tetralkylammonium salts are used e.g.
tetramethyl ammonium perchlorate (CH3).·,NCIO4, The concentration of
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Dr. Hisham E Abdellatef
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Voltammetry
supporting electrolyte must be at least 100 times larger than the
concentration of the electro-active substance. Before carrying electrolysis
this supporting electrolyte must be purged by a steam of inert gas e.g.
nitrogen or argon to remove oxygen.
Voltammetric Techniques:
• Polarography
• Differential Pulse
• Square Wave Voltammetry
• Normal Pulse
• Cyclic Voltammetry
• Sampled DC
• LSV
• Stripping Analysis
POLAROGRAPHY
Polarography: is the exception case of voltammetry, where the working
electrode is the dropping mercuric electrode (DME) (as cathode) and the
mercuric pool is the reference electrode (as anode). It is usually used for
determination of the reduction side. The plot of the applied potential vs
corresponding current is called polarogram and the apparatus is called a
polarograph.
When an external electromotive force (e.m.f.) is applied to a cell that
containing solution of substance that can be reduced e.g.
CdCl2, the following reaction will occur.
Cd+2 + 2e + Hg ⇄ Cd(Hg)
From A to B, practically no current will pass through the cell. At B, when
the potential of the working electrode (DME.) is equal to the deposition
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Dr. Hisham E Abdellatef
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Voltammetry
potential of the metallic cadmium, the current suddenly increases and the
working electrode becomes depolarized by the cadmium ion.
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At point C, the current no longer increase linearly with the applied
potential but approaches a
steady limiting value.
At point D no increase in
current is observed at higher
cathodic potential unless a
second compound able to
depolarize
the
electrode,
present
working
in
the
solution.
The
polarogram
characterized
by
is
different
parameters:
1- Residual current: at small
applied potential only a residual current flow in the cell which caused by: _
a- The presence of impurities in the supporting electrolyte or sample.
b- The charging current, which is the current that required for the
charging of the so called electrical double layer at the surface of
mercuric electrode.
2- Limiting current: is the region on the polarogram in which the current
after increased sharply, becomes essentially independent on the applied
potential.
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Voltammetry
3- Diffusion current: it is the difference between limiting and residual
currents. It is directly proportional to the concentration of the - reactive
substance.
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Half wave potential
(E½): is the potential at
which the current is
equal to one half the
diffusion current. It is a
characteristic
of
the
nature of the reactive
material, independent
on concentration, depend on pH of the medium, type of supporting
electrolyte and type of electrode.
Decomposition potential: is the potential at which the substance begins
to oxidize or reduce. `
Factors affecting electrode reaction rate and current
The following factors affect on the electrode
reaction rate:
1- Mass transfer.
2- Electron transfer at the electrode surface.
3- Chemical reactions such as protonations or dimerizations, or
catalytic decomposition on the electrode surface.
4- Other surface reactions, such as adsorption, desorption, or
crystallization.
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Voltammetry
Mass transfer: which is the movement of material from one location in
solution to another arises from the following different sources:
a- Migration; which is the movement of a charged bodies under the
influence of difference inthe electrical field.
b- Diffusion: which is the movement of species under the influence of
difference in concentration (concentration gradient).
c- Convection: which is the movement of species under the influence of
stirring or hydrodynamic transport.
In most cases of voltammetry and polarography, the measurements are
carried out in quiet solution at fixed temperature, so there is no effect of
convection. The use of high concentration of supporting electrolyte
neglect the effect of migration. So diffusion is the only mode of mass
transfer that controlled the electrode reaction rate.
Diffusion current
When the potential applied to a polarographic cell exceeds the
decomposition potential of an electrolyte species, its concentration at the
surface of the electrode (d.m.e.) is immediately diminished. A
concentration gradient is therefore established and more ofthat species
diffused from the bulk solution to the electrode surface (Fick's low of
diffusion). The resulting current flow is proportional to the rats of diffusion.
I α (C-Co)
or
i=K(C—Co)
where C and Co are the concentration of the electroactive species in the
bulk solution and at the surface of electrode , respectively. By increasing
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Dr. Hisham E Abdellatef
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Voltammetry
the applied voltage, reduction occurs more rapidly and (Co) becomes zero
and the concentration gradient reaches a maximum. At this point, the rate
of the diffusion and the current flowing in the cell reaches a limiting value.
llkovic Equation
It is the equation that relates the parameters that determine the
magnitude of the diffusion current obtained with a dropping rnercuric
electrode.
Id (average) = 607 n D1/2m2/3t1/6C where
id = is the average diffusion current in microamper.
n = is the number of electrons consumed in the reduction of one molecule
of the electroactive species.
D = is the diffusion coefficient of the reducible or oxidizible substance
express as Cm2/s.
M = is the rate of flow of mercury from the dropping mercuric electrode
express in mg/s.
T = is the drop time in s.
C = is the concentration in m mol/1.
The constant 607 is a combination of natural constants including the
Faraday's constant, it is slightly temperature-dependent and this value
(607) at 25°C.
Advantages of dropping mercuric electrode (DME)
1- Its surface is reproducible, smooth and continuously renewed, this
eliminate the poisoning effect.
2- Mercury forms amalgams (solid solution) with many metals.
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Voltammetry
3- The diffusion current assumed a steady value immediately after each
change of applied potential and is reproducible.
4- The large hydrogen over-potential of mercury renders possible
deposition of substance that difficult to reduce e.g. alkaline metal
e.g. Al+3 and M+2. .
5- The surface area can be calculated from the weight of the drop.
Disadvantage of dropping mercuric electrode:
1- At potential more positive than +0.4V vs SCE, mercury e dissolve and
producing polarographic wave which masks the other waves of other
oxidizable species e in the solution, so d.m.e can be used only for the
analysis of reducible or easily oxidizeble substance.
2- The capillary is very small in size so easy to block and so all process
stopped.
3- The presence of impurities in the mercury cause blockage of the
capillary results in malfunction of the electrode.
Substances determined by polarography
i- Polarographic analysis of Inorganic compounds
Most metal ions are reducible at DME, and multicomponent mixtures can
often be analysed by selecting an appropriate supporting electrolyte so
that the half-wave potentials of two ions are differed by about -0.2 V vs
SCE or by using complexing agents by taking the advantage of complexing
ability of the metal ions.
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Voltammetry
Based on this, polarography is used predominantly for trace metal analysis
of alloys, ultra-pure metals, minerals/metallurgy, environmental analysis
(air, water, soil and sea water contaminants), foodstuffs, beverages and
body-fluids, toxicology and clinical analysis. Reducible anions such as
BrO3, IO3-, Cr2O72-and NO2- can also be determined using well buffered
solutions.
ii. Polarographic analysis of organic compounds
This technique is used in organic chemistry for qualitative and quantitative
analysis and structure determinations. Most of the organic compounds are
insoluble in pure aqueous medium and also in mercury to form amalgam.
Therefore, the solvent in which the organic compound and its electrode
product is soluble is added to the supporting electrolyte. These solvents
include various alcohols or ketones, dimethyl formamide, acetonitrile,
ethylene diamine and others. The commonly used supporting electrolytes
which are easily mixed with organic solvents are various quaternary
ammonium salts such as tetrabutyl ammonium iodide.
Some of the organic functional groups that are reducible at DME :
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Voltammetry
Application of polarography (voltammetry)
A- Qualitative application
1- This technique can be used for the determination of equilibrium
constant and reaction rate.
2- Elucidation of
and reaction mechanism coupled with the actual
electrode process.
3- Study the reversibility of the reaction
4- This technique is useful for stability studies and study the
connection between polarographic properties and physiological
activities of drugs.
5- Polarography (volmmmetry) can be used in connection with
spectroelectrochemistry (SEC)to show the change in the spectrum
of the substance after the
application of potential. Also this
technique is used to study some p application phenomena like
adsorption.
B- Quantitative application ,
The polarographic technique can be used in the determination of
concentration range from 10-5M to 10-2M and employ volumes Less than 1
ml. two or more electroactive ions may be determined successively if their
half-wave potentials differ by at least 0.4 volt for single charged ions and
0.2volt for doubly charged ions ’ provided that the ions are present in
approximately equal concentration if the concentration differ considerably,
the difference between the half-wave potential must be larger
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Voltammetry
1. Polrography (voltametry) can be
concentration
of
the
used for determination of the
electreactive
compounds
by
direct
measurements of the diffusion current.
2. It can be used in combination with other
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technique e.gt high
performance liquid chromatography (HPLC) to determine the
concentration of substances presents in minute amounts especially
in fluids.
3. Also it can be used for determination of the concentration at certain
potential and this called amperometry. Amperometric titration is
the determination of the relationship between the volume of titrant
and the corresponding current of substance at constant potential
provided that both reactant and reagent are electrcochemically
active or only one is active.
Amperometric titration
Polarography can be used for determination of the concentration at
certain potential and this called amperometry. Amperometric titration is
the determination of the relationship between the volume of titrant and
the corresponding current of substance at constant potential provided that
both reactant and reagent are electrochemically active or only one is
active, so there are several types of curves appear in the figure
1. If the reactant is active while the titrant is inactive e.g. titration of
Pb+2 with SO-4
2. If the reactant is inactive while the titrant is active e.g. titration of
Mg2+ with 8-hydroxyquinoline.
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Voltammetry
3. If both the reactant and the titrant are active e. g the titrant of Pb+2
and CrO42-.
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Evaluation of quantitative results:
a- Wave height – concentration plots.
The wave heights are measured for a series of different of the substance
under investigation. The polarogram of the unknown is measured exactly
as the standard, and the concentration is read from the graph.
Measurement of wave-heights:
With a well defined polarographic wave where the limiting current plateau
is parallel to the residual current curve, the measurement of the diffusion
current is relatively simple. A reliable procedure is illustrated in figure.
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Voltammetry
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b- Internal standards or Pilot ion method:
This method is based on the fact that the relative diffusion current
constants I are independent of the particular capillary used, provided the
nature and concentration
Of supporting electrolyte and the temperature are kept constant. Hence ,
upon determining the relative wave height or diffusion current of the
unknown ion and with some standard or pilot ion added to the solution in
known amount and comparing these with the ratio for known amounts of
the same two ions. The main requirement for such an ion (singly charged)
is that its half wave potential should differ by at least 0.# Volts from the
unknown or any other ion in the solution with which it might interfere.
From the Ilkovic equation ,
I d = 607 n D 1/2 m 2/3 t 1/6 c ;
Now , for the pilot ion :
I
d1=
607 n1D11/2 m 2/3 t 1/6 c1 ;
Now for test ion :
I
d2=
607 n2 D21/2m2/3 t 1/6 c2 ;
now,
I d1 = 607 n1 D1½ 𝑚2/3 𝑡 1/6 𝑐1
2/3 𝑡 1/6 𝑐
I d2 = 607 n2 D½
2
2 𝑚
If I = 607 n D ½ then
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Voltammetry
I d1 = I1 c 1
I d2 I2 c 2
The ratio I1 /I2 is known as the pilot ion ratio (R) and is independent of the
capillary characteristics so that:
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I d1 = c 1 .R
I d2 c 2
c- Methods of standard addition:
The Polarogram of the test solution of known volume is initially
recorded then an accurately measured quantity of a standard solution of
the substance is added and a second polarogram is obtained. The original
concentration of the substance can be calculated from the increase in the
diffusion current from the following formula:
C1 =
Advantages
c2I2v2
v 1(I 2- I )+I 2 v 2
c= concentration,
I = diffusion current
v=volume
Where,
c 1 = test solution
c 2 = standard solution
of the polarographic technique for analysis
of
pharmaceutical compounds
1- Only small volumes of samples are necessary.
2- Turbid and colored solutions, which make other methods
Impossible, can be analyzed.
3- The usefulness of the method is not restricted to electrochemically
active compounds but the field of application is extended by indirect
method.
4- For the analysis of pharmaceutical products, it is sufficient simply to
dilute the liquid sample e.g. injection solution-with the supporting
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Voltammetry
electrolyte or to dissolve tables in such a solution. The insoluble
parts of the tablet or additives do not usually influence the
polarographic curves. When present, they should be left to settle
after which aliquot simple may be taken from the supernatant liquid;
The separation of the exicipients in many cases is not necessary and
this simplifies the preparation of the samples.
5- The sensitivity of voltammetiy (polarography) is sufficient high to
enable the determination of; small amount of substance or traces of
perhaps toxic impurities.
Stripping voltammetry
In this method, a Hanging Mercury Drop Electrode (HMDE) is used, the
potentiostat is set a fixed value (0.2-0.4 V) more negative than the highest
reduction potential encountered among the reducible ions, then
electrolysis will occur, deposition of metals will take place on the HMDE
cathode and amalgam formation will take place. The reducible ion may be
transferred to the mercury cathode, this operation is referred as a
concentration step, and the metals become concentrated into the
relatively small volume of the mercury drop. The cell is allowed to stand for
30 seconds. The potentiostat is then caused to make a voltage sweep in
reverse. This means that a gradually increasing positive potential is applied
to the HMDE, which is now the anode of the cell. As the potential
approaches the oxidation potential of one of the metals dissolved in the
mercury, then ions of that metal pass into solution from the amalgam and
the current increases rapidly and attains a maximum value when the
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Voltammetry
potential has a value approximating to the appropriate oxidation potential.
The metal is said to be stripped from the amalgam and the curve shows a
peak! The curve is termed Stripping voltammogram The technique can be
-8
-9
used to measure concentrations in the range 10 - 10 M that is suitable for
the determination of trace metal
impurities,
analysis
semiconductor
investigation
of
materials,
of
pollution
problems
Anodic Stripping Voltammogram of 0.3
uM Pb2+ in 0.1 M KNO3 with 20 uM Hg2+,
Cyclic voltammetry
Cyclic voltammetry is the most widely used technique for acquiring
qualitative information about electrochemical reactions.
In typical cyclic voltammetry, a solution component is electrolyzed
(oxidized or reduced) by placing the solution in contact with an electrode
surface and then making that surface sufficiently positive or negative in
voltage to force electron transfer. In simple cases, the surface is started at
a particular voltage with respect to a reference half-cell such as calomel or
Ag/AgCl, the electrode voltage is changed to a higher or lower voltage at a
linear rate and finally, the voltage is changed back to the original value at
the same linear rate. When the surface becomes sufficiently negative or
positive, a solution species may gain electrons from the surface or transfer
electrons to the surface.
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Voltammetry
This results in a measurable current in the electrode circuitry. The result of
cyclic voltammetry obtained in form of cycle between current and
potential, potential on X axis and voltage on Y axis.
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Figure shows a generic voltammogram. A voltammogram explains the
reversibility of the redox couple.
Epc= Peak cathodic potential; Ipc= Peak cathodic current; Epa= Peak
anodic potential; Ipa=Peak anodic current
The peak potential of the anodic sweep, Epa and the peak potential for
cathodic peak, Epc, can be directly read from the program, and the
difference between them, ∆Epeak, can be calculated. If redox couple is
reversible, then the relationship,
n ∆ Epeak= 59mV
(1.1)
In addition, the ratio of the anodic peak current to the cathodic peak
current is given by:
ipa / ipc = 1
(1.2)
The formal potential Eo, for a reversible redox couple is easily determined
as the average of the two peak potentials as follows.
Eo=(Epa+Epc) / 2
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Dr. Hisham E Abdellatef
Voltammetry
Quantitative information regarding analyte concentration can be obtained
from the voltammogram using Randles- Sevcik equation. This equation
specifies the peak current, ip (anodic and cathodic), in terms of analyte
concentration ,C.
ip= 0.4463 n FAC (n F v D/ R T)1/2
(1.4)
where, n=no. of electrons appearing in the half-reaction for the redox
couple v=rate at which potential is swept
F=Faradays constant (96485 C/mol),
A=electrode area (cm2)
R=universal gas constant (8.314 J/mol K)
T=absolute temperature (K)
D= analyte’s diffusion coefficient (cm2/sec)
If temperature is assumed to be 250C (298.15 K), the eqn. can be written as:
Ip = (2.687 x 105) n3/2 v1/2 D1/2 A C
Where the constant is understood to have units (2.687 x 105 C mol-1 V-1/2).
During the forward scan the Fe(III) get reduced as :
Fe (III) + e-
Fe(II)
Similarly during the reverse potential the oxidation of the Fe (II) takes place
to (III) which can be shown as:
Fe (III) -
Fe(II) + e-
The redox reaction therefore can be used as an indication of major
analytical tool for the determination of trace elements which are electro
active in nature.
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