4 instr an. volt.zek ek kopya
TRANSCRIPT
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Instrumental Analysis
2012-2013
1
VOLTAMMETRY
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Unit 4Voltammetry
2
Fundamentals of Analytical Chemistry SKOOG WEST HOLLER CROUCH
ANALYTICAL ELECTROCHEMISTRY Joseph Wang
Electroanalytical Methods Fritz Scholz
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qi
qE
L
2
Interfacial methods
Static methods (i=0)
PotansiyometryControlled
potential Current controlled
coulometryVarying
potential
Dynamic methods (i0)
Voltammetry
Constant
potential
Bulk analysis
Controlled
current
AmperometryPotential controlled
coulometry
Conductometry
ELECTROANALYTICAL METHODS
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Voltammetry is the general name given to a group ofelectroanalytical methods in which the current is measured as a functionof applied potential wherein the polarization of the indicator or workingelectrode is enhanced. The working electrodes used in voltammetry aregenerally microelectrodes having surface area of few square micrometersto square millimeters.
Polarography which is a particular type of voltammetry has beendeveloped in this field at first time by Heyrovsky in the early 1920sanachievement for which he was awarded the Nobel Prize in Chemistry in1959. A dropping mercury electrode was used as the working micro-
electrode.The electrochemical cell, where the voltammetric experiment is
carried out, consists of a working electrode, a reference electrode, andusually a counter (auxiliary) electrode.
Description of voltammtric system and How It Works
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In general, an electrode provides the interface acrosswhich a charge can be transferred or its effects felt.Because the working electrode is where the reaction or
transfer of interest is taking place, whenever we refer to theelectrode, we always mean the working electrode.
The reduction or oxidation of a substance at thesurface of a working electrode, at the appropriate applied
potential, results in the mass transport of new material tothe electrode surface and the generation of a current. Thus,we can say that the electrochemical method in which thecurrent is measured as a function of the potential applied to
a microelectrode is known as voltammetry.
These electrodes may be solid like gold, platinumand glassy carbon or mercury. If the electrode is formed bya drop of mercury dropping from a fine glass capillary, thetechnique is called Polarography
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Other forms of working electrodes used are hanging mercury
dropping electrode (HMDE), static mercury drop electrode (SMDE),
thin mercury film electrode (TMFE), glassy carbon electrode (GCE),
carbon paste electrode (CPE), etc. Working electrodes made of noblemetals are used less frequently.
Processes
Various methods are assigned to the terms voltammetry and
polarography which differ in the measuring techniques and the type
of electric potential used to excite the determination process.
Voltammetry is widely used for the quantitative analysis of trace
metal ions and electroactive organic compounds at micro level.
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Voltammetry is widely used for the quantitative analysis of tracemetal ions at micro level. This study of redox processes lead to the
qualitative and quantitative analysis of heavy metals and some
organic substances in solution. In addition to this aspect this
technique is quite versatile for research purpose in the fields of
i) redox processes
ii) electron transfer mechanisms at chemically modified electrode
surfaces
iii) adsorption processes
iv) corrosion proof materials
v) new electronic processes for chemical industries
vi) production of new types of batteries that can store large
quantities of energy
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COMMON VOLTAMMETRIC METHODS
The most widely used voltammetric methods are
A) Direct current methods
i) D.C. Polarographyii) Linear sweep voltammetryiii) Sampled D.C. Polarographyiv) Hydrodynamic voltammetryB) Pulse methods
v) Normal Pulse voltammetryvi) Differential Pulse voltammetryvii) Square wave voltammetryviii) Cyclic voltammetry
C) Stripping voltammetryix) Anodic stripping voltammetryx) Cathodic stripping voltammetryxi) Adsorptive stripping voltammetry
D) Alternating current methods
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In voltammetry, the effects of the applied potential and thebehavior of the redox current are described by several well-known laws. The applied potential controls theconcentrations of the redox species at the electrode surface(CO0and CR0) and the rate of the reaction (k0), as describedby the Nernst or Butler Volmer equations, respectively.
In the cases where diffusion plays a controlling part, thecurrent resulting from the redox process (known as thefaradaic current) is related to the material flux at the
electrodesolution interface and is described by Ficks law.The interplay between these processes is responsible for thecharacteristic features observed in the voltammograms of thevarious techniques.
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The advantages of
controlled-potential techniques10
These include high sensitivity, selectivity towardelectroactive species, a wide linear range, portable andlow-cost instrumentation, speciation capability, and a widerange of electrodes that allow assays of unusualenvironments.
Extremely low (nanomolar) detection limits can be achievedwith very small (520-L) sample volumes, thus allowing
the determination of analyte amounts ranging from 1013to1015mol on a routine basis. Improved selectivity may beachieved via the coupling of controlled-potential schemeswith chromatographic or optical procedures.
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Voltammetry11
In voltammetry a time-dependent potential is applied to anelectrochemical cell, and the current flowing through the cell is
measured as a function of that potential.
A plot of current as a function of applied potential is called avoltammogram and is the electrochemical equivalent of a spectrum
in spectroscopy, providing quantitative and qualitative information
about the species involved in the oxidation or reduction reaction.
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Jaroslav Heyrovsk was the inventor of the
polarographic method, and the father ofelectroanalytical chemistry, for which he was the
recipient of the Nobel Prize. His contribution to
electroanalytical chemistry can not be
overestimated. All modern voltammetric methods
used now in electroanalytical chemistry originate
from polarography.
On February 10, 1922, the "polarograph" was born as Heyrovsk recorded the current-voltage curve
for a solution of 1 M NaOH. Heyrovsk correctly interpreted the current increase between -1.9 and -2.0 V as being due to deposition of Na+ions, forming an amalgam.
History of Polarography
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Jaroslav Heyrovsky
The earliest voltammetric technique to be introduced was
polarography, which was developed by Jaroslav Heyrovsky(18901967) in the early 1920s, for which he was awardedthe Nobel Prize in chemistry in 1959.
Since then, many different forms of voltammetry have been
developed.
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Volt
Stripping voltammetry Unstirred soln
Polarography and voltammetrywith quiescent electrodes
Pulse polarographyand voltammetry
Stirred soln.
Hydrodynamicvoltammetry
Cyclicvoltammetry
amper metriVOLTAMMETRY
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Electroanalytical methods that depend on the measurement ofcurrent as a function of applied potential are called voltammetric
methods. They employ conditions that encourage polarization of
the indicator or working electrode. Generally, to enhance
polarization, the working electrodes in voltammetry arerelatively small, with surface areas of a few square millimeters
at the most and, in some applications, only a few square
micrometers.
Voltammetry is based on the measurement of current in anelectrochemical cell under conditions of complete concentration
polarization in which the rate of oxidation or reduction of the
analyte is limited by the rate of mass transfer of the analyte to
the electrode surface.
Voltammetric Methods
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In voltammetry, the voltage of the working electrode isvaried systematically while the current response ismeasured. Several different voltage-time functions,called excitation signals, can be applied to theelectrode.
The simplest of these is a linear scan, in which thepotential of the working electrode is changed linearly
with time.
Other waveforms that can be applied are pulsedwaveforms, square wave and triangular waveforms.
These technique will be discussed soon.
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Voltammetric Measurements
Although early voltammetric methods relied on the use of only
two electrodes, modern voltammetry makes use of a three-
electrode potentiostat.
A time-dependent potential excitation signal is applied to the
working electrode, changing its potential relative to the fixedpotential of the reference electrode.
The resulting current between the working and auxiliary
electrodes is measured. The auxiliary electrode is generally a
platinum wire, and the SCE and Ag/AgCl electrode are commonreference electrodes.
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Two special electrodes
Supporting electrolyte : Usually relatively higher concentra
tion of strong electrolytes (alkali metal salts) serves as sup
porting electrolyte
Dissolved oxygen is usually removed by bubbling nitrogen
through the solution
Voltage scanning Under unstirred state, recording voltage -
current curve
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REFERENCE ELECTRODES
VEClHgeClHg
VEClHgeClHg
24.0KCl)(doy222
24.0222
'0
22
0
22
+=++
+=++
Saturated calomel electrode,SCE
Silver/silver chloride electrodeAg/AgCl
VEClAgeAgCl
VEClAgeAgCl
222.0KCl)(doy
197.0
'0
0
+=++
+=++
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Reference
electrode
Indicator(working)electrode
Potentiometric measurements2 electrode-system
Reference
electrode
Workingelectrode
Auxiliaryelectrode
Voltammetric measurements3 electrode-system
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elektrotlu voltammetrik lm sistemi
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Auxiliary electrodes
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Working electrodes
Several different materials have
been used as working electrodes,
including mercury, platinum, gold,
silver, and carbon.
The earliest voltammetric
techniques, including polarography,
used mercury for the workingelectrode. Since mercury is a liquid,
the working electrode often consists
of a drop suspended from the end
of a capillary tube.
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INDICATOR ELECTRODES
ALIMA ELEKTROTLARI
Mercury based electrodes
Carbon based electrodes
Metallic electrodes
Modified electrodes
Compozite electrodes
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dropping mercury electrode, DME
DME is an electrode in which successive
drops of Hg form at the end of a capillary
tube as a result of gravity, with each drop
providing a fresh electrode surface.
In the DME, mercury drops form at the endof the capillary tube as a result of gravity.
Unlike the HMDE, the mercury drop of a
DME grows continuously and has a finite
lifetime of several seconds.At the end of its lifetime the mercury drop is
dislodged, either manually or by gravity,
and replaced by a new drop.
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Dropping mercury electrode (DME)
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Mercury electrode dropping
with gravity effect
Mercury electrode dropping
with a magnetic hammer
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Hanging mercury drop electrode30
HMDE is an electrode in which a drop of
Hg is suspended from a capillary tube.
In the HMDE, a drop of the desired size
is formed by the action of a micrometer
screw that pushes the mercury through a
narrow capillary tube.
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Static mercury drop electrode, SMDE
The static mercury drop electrode, or SMDE, is anelectrode in which successive drops of Hg form at
the end of a capillary tube as the result of a
mechanical plunger, with each drop providing a
fresh electrode surface.SMDE uses a solenoid-driven plunger to control
the flow of mercury. The SMDE can be used as
either a hanging mercury drop electrode or as a
dropping mercury electrode. A single activation
of the solenoid momentarily lifts the plunger,
allowing enough mercury to flow through the
capillary to form a single drop. To obtain a
dropping mercury electrode the solenoid is
activated repeatedly.
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Advantages of mercury electrodes
Mercury has several advantages as a working electrode.
High overpotential for the reduction of H3O+ to H2, which
allows for the application of potentials as negative as -1 V
versus the SCE in acidic solutions, and 2 V versus the SCE in
basic solutions. A species such as Zn2+, which is difficult toreduce at other electrodes without simultaneously reducing
H3O+ , is easily reduced at a mercury working electrode.
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http://www.google.com.tr/url?sa=i&rct=j&q=zinc+ion+polarogram&source=images&cd=&cad=rja&docid=MO-UoE1E1FU8XM&tbnid=lfhx42UfF0KXRM:&ved=0CAUQjRw&url=http%3A%2F%2Fwww.sciencedirect.com%2Fscience%2Farticle%2Fpii%2FS0263876212002511&ei=GZhyUYTHJInVPKrqgXA&bvm=bv.45512109,d.ZWU&psig=AFQjCNH6F5sgtfVH892nY3klIjhekq3vag&ust=1366550792088634http://www.google.com.tr/url?sa=i&rct=j&q=zinc+ion+polarogram&source=images&cd=&cad=rja&docid=Vfp8OLUBoIk9SM&tbnid=-Tdwe-jyj_bvSM:&ved=0CAUQjRw&url=http%3A%2F%2Fwww.sciencedirect.com%2Fscience%2Farticle%2Fpii%2FS0022072800000176&ei=j5dyUYmjK4nVPKrqgXA&bvm=bv.45512109,d.ZWU&psig=AFQjCNH6F5sgtfVH892nY3klIjhekq3vag&ust=1366550792088634 -
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Advantages of mercury electrodes
Other advantages include the ability of metals to dissolve in the
mercury, resulting in the formation of an amalgam, and the ability
to easily renew the surface of the electrode by extruding a new
drop. Amalgam is a metallic solution of mercury with another
metal.
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One limitation to its use as a working electrode is the ease with
which Hg is oxidized.
For this reason, mercury electrodes cannot be used as at potentials
more positive than 0.3 V to +0.4 V versus the SCE, depending on
the composition of the solution.
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ALIMA ELEKTRODUNUN TRNE GREALIMA POTANSYEL PENCERES
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-0.2 -0.4 -0.6 -0.8 -1.0 -1.2 -1.4
i (A)
0.001 M Cd2+in 0.1 M KNO3supporting electrolyte
V vs SCE
Working electrode is
no yet capable of
reducing Cd
2+
only small residual
currentflow 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 workingelectrode 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 plateauid
E
Base line
of residual
current
R id l C Th id l i l h i h
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Residual Currents: The residual current in polarography is the
small current observed in the absence of an electroactive species.
Residual Currentcontains both double layer or capacitive current
and faradaic current which is formed by the impurities of
solutions.
The faradaic current part can be easily eliminated with pre-
electrolysis process.
But the double layer or capacitive current deterimined the
sensitivty of the voltammetric technique. It can also be eliminatedby new developed time techniques.
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At any electrode immersed in an electrolyte solution, a
specific interfacial region is formed. This region is called the
double layer. The electrical properties of such a layer are
important, since they significantly affect the electrochemical
measurements.
In an electrical circuit used to measure the current that flows
at a particular working electrode, the double layer can be
viewed as a capacitor.
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To obtain a desired potential at the working electrodes, the
double-layer capacitor must be first appropriately charged,
which means that a capacitive current, not related to the
reduction or oxidation of the substrates, flows in the electrical
circuit.
While this capacitive current carries some information
concerning the double layer and its structure, and in some cases
can be used for analytical purposes, in general, it interferes withelectrochemical investigations.
A variety of methods are used in electrochemistry to depress,
isolate, or filter the capacitive current.
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Although many models for the double layer have been publishedin the literature, there is no general model that can be used in all
experimental situations.
This is because the double-layer structure and its capacity dependon several parameters such as electrode material (metals, types of
carbon, semiconductors, material porosity, the presence of layers
of either oxides or polymeric films or other solid materials at the
surface), type of solvent, type of supporting electrolyte, extent of
specific adsorption of ions and molecules, and temperature.
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The composition of the double layer influences the electrontransfer rate. Some ions and molecules specifically adsorbed at the
electrode surface enhance the rate of the electrode process. In
such a situation, we talk about heterogeneous electrocatalysis.
On the other hand, there are numerous compounds that, after
adsorption, decrease the electron transfer rate and therefore are
simply inhibitors. Some surface-active compounds can be very
strongly adsorbed.
This may lead to the total isolation of the electrode surface and,
finally, to the disappearance, or substantial decrease, of the
voltammetric peaks or waves.
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The concept of the existence of the double layer at the surface ofa metal being in contact with an electrolyte appeared in 1879
(Helmholtz).
That first theoretical model assumed the presence of a compactlayer of ions in contact with the charged metal surface.
The next model, of Gouy and Chapman, involves a diffuse
double layer in which the accumulated ions, due to the
Boltzmann distribution, extend to some distance from the solid
surface.
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A classic, simplified model
of the double layer formed
at the metal electrode
surface is presented in Fig.
There is a layer of adsorbed
water molecules on the
electrode surface.
Since it has been assumed
that there is excess of
negative charge at the
metal phase, the hydrogenatoms of adsorbed water
molecules are oriented
toward the metal surface.
A specifically adsorbed largeneutral molecule is also shownin Fig.
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Bulk solution
Two planes are usually associated with the double layer. The first one, the
inner Helmholtz plane (IHP), passes through the centers of specifically
adsorbed ions (compact layer in the Helmholtz model), or is simply located
just behind the layer of adsorbed water. The second plane is called the outer
Helmholtz plane (OHP) and passes through the centers of the hydrated ions
that are in contact with the metal surface.
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Current in Voltammetry
When an analyte is oxidized at the working electrode, a current
passes electrons through the external electric circuitry to the
auxiliary electrode, where reduction of the solvent or other
components of the solution matrix occurs.
Reducing an analyte at the working electrode requires a source
of electrons, generating a current that flows from the auxiliary
electrode to the cathode. In either case, a current resulting from
redox reactions at the working and auxiliary electrodes is calleda faradaic current.
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Currents
Faradaic current: Any current in an electrochemical cell due to an
oxidation or reduction reaction.
cathodic current: A faradaic current due to a reduction reaction.
anodic current: A faradaic current due to an oxidation
Sign Conventions
Since the reaction of interest occurs at the working electrode,
the classification of current is based on this reaction. A current
due to the analytes reduction is called acathodic current and,
by convention, is considered positive.
Anodic currents are due to oxidation reactions and carry a
negative value.
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Eat i
Limiting currentRelated to concentrati
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The potential at which the current is equal to one half the
limiting current is called the half-wave potential and given
the symbolE1/2.
Half-wave potential polarographic qualitative analysis
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How it works?
The applied voltage is gradually increased, typically bygoing to a more positive( more negative decomposing potential)
A small residual current is observed.
When the voltage becomes great enough, reduction occurs atthe analytical electrode causing a current.
The electrode is rapidly saturated so current production is
limited based on diffusion of the analyte to the smallelectrode.
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How it works ?
The reduced species alters the surface of the mercury electrode.
To prevent problems, the mercury surface is renewed by
knocking off a drop providing a fresh surface.
This results in an oscillation of the data as it is collected.
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The diffusion current theory and polarographic wave
equation
In above equations, K is called Ilkovic constant, it is expressedas follows:
id= KC
We have already known:
K = 607n D1/2m2/3t1/6
Thus,id= 607nD
1/2m2/3t1/6C
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id= 607nD1/2m2/3t1/6C
Average limiting diffusioncurrent denoting average
current on mercury drop fromdrop forming to falling (mA)
Number of transferringelectrons in electrode
reaction(e/mol)
Diffusion coefficientof electroactiveanalyte in
solution(cm2.sec-1)
Mercury mass flowrate(mg.sec-1)
Drop time(sec)
Concentration ofelectro-active
analyte(mmol.L-1)
From above equation, we can find thatwhen temperature,
matrix solution and capillary characteristic are kept
constant, idis proportional to C
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Limiting diffusion current -- A basis of polarographically
quantitative analysis
When the applied voltage exceeds the decompositionvoltage, diffusion-controlled current is expressed as:
i= K(C-C0)
When the applied voltage gets more negative, C0 0,
current becomes only diffusion limited, then
id= KC
Idreaches a limiting value proportional to ion concentrationC in bulk solution, and do not changes with applied voltage
longer
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polarographic wave equation
ii
i
nF
RTEE
d
ln2/1
When i = id, log term in above equation is equal to zero,
corresponding potential is calledhalfwave potential E1/2
E1/2independent on the concentration
basis of qualitative analysis
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FARADAIC PROCESSES57
The objective of control led-potential electroanalytical
experiments is to obtain a current response that is
related to the concentration of the target analyte.
Such an objective is accomplished by monitoring the
transfer of electron(s) during the redox process of
the analyte:
where O and R are the oxidized and reduced forms,respectively, of the redox couple.
Such a reaction will occur in a potential region that
makes the electron transfer thermodynamically or
kinetically favorable.
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For systems controlled by the laws of thermodynamics,
the potential of the electrode can be used to establish
the concentration of the electroactive species at the
surface [CO(0,t) and CR(0,t)] according to the Nernst
equation:
where E is the standard potential for the redox reaction,
R is the universal gas constant (8.314 J/mol K), T is theKelvin temperature, n is the number of electrons
transferred in the reaction, and F is the Faraday constant
[96,487C(coulombs)].
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Faradaic Current59
The current resulting from a change in oxidation stateof the electroactive species is termed the faradaic
currentbecause it obeys Faradays law.
The faradaic current is a direct measure of the rate of
the redox reaction. The resulting currentpotential plot,known as the voltammogram, is a display of current
signal [vertical axis (ordinate)] versus the excitation
potential [horizontal axis (abscissa)].
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60 The pathway of the electrode reaction can be quite
complicated, and takes place in a sequence thatinvolves several steps. The rate of such reactions is
determined by the slowest step in the sequence.
Simple reactions involve only mass transport of the
electroactive species to the electrode surface,electron transfer across the interface, and transport
of the product back to the bulk solution.
More complex reactions include additional chemical
and surface processes that either precede or follow
the actual electron transfer. The net rate of the
reaction, and hence the measured current, may be
limited by either mass transport of the reactant or the
rate of electron transfer.
Influence of Applied Potential on the
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Influence of Applied Potential on the
Faradaic Current
As an example, lets consider the faradaic current when a solutionof Fe(CN)6
3is reduced to Fe(CN)64at the working electrode.
The relationship between the concentrations of Fe(CN)63and
Fe(CN)64and the potential of the working electrode is given by theNernst equation; thus
where +0.356 is the standard-state potential for the
Fe(CN)63/Fe(CN)6
4redox couple,
andx = 0 indicates the surface concentrations.
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Lets assume that we have a solution for which the concentration
of Fe(CN)63 is 1.0 mM and in which Fe(CN)6
4is absent.
A ladder diagram for this redox example is shown in the Figure.
62
If a potential of +0.530 V is applied to
the working electrode, the
concentrations of Fe(CN)63and
Fe(CN)64
at the surface of theelectrode are unaffected, and no
faradaic current is observed.
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Switching the potential to +0.356 V, however, requires that
which is only possible if a portion of the Fe(CN)63at the
electrode surface is reduced to Fe(CN)64. If this was all that
occurred after the potential was applied, the result would be a
brief surge of faradaic current that would quickly return to zero.
However, although the concentration of Fe(CN)64at the electrode
surface is 0.50 mM, its concentration in the bulk of solution is
zero. As a result, a concentration gradient exists between the
solution at the electrode surface and the bulk solution. This
concentration gradient creates a driving force that transports
Fe(CN)64away from the electrode surface.
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The subsequent decrease in the
concentration of Fe(CN)64at the
electrode surface requires the further
reduction of Fe(CN)63, as well as its
transport from bulk solution to theelectrode surface.
Thus, a faradaic current continues to
flow until there is no difference
between the concentrations ofFe(CN)6
3and Fe(CN)64, at the
electrode surface and their
concentrations in the bulk of solution.
64
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Although the applied potential at the working electrode
determines if a faradaic current flows, the magnitude of the
current is determined by the rate of the resulting oxidation or
reduction reaction at the electrode surface.
Two factors contribute to the rate of the electrochemicalreaction:
the rate at which the reactants and products are transported
to and from the surface of the electrode, and
the rate at which electrons pass between the electrode and
the reactants and products in solution.
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15
ANA ZELT
Influence of Mass Transport on
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68
There are three modes of mass transport that influence
the rate at which reactants and products are transported
to and from the electrode surface:
diffusion, migration, and convection.
Diffusionfrom a region of high concentration to a regionof low concentration occurs whenever the concentration
of an ion or molecule at the surface of the electrode is
different from that in bulk solution.
When the potential applied to the working electrode issuff icient to reduce or oxidize the analyte at the electrode
surface, a concentration gradient similar to that shown in
Figure is established.
Influence of Mass Transport on
the Faradaic Current
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69
The volume of solution in which
the concentration gradient exists
is called the diffusion layer.
Without other modes of mass
transport, the width of thediffusion layer, d, increases with
time as the concentration of
reactants near the electrode
surface decreases.The contribution of diffusion to
the rate of mass transport,
therefore, is t ime-dependent.
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70
Elektriksel ift katmanda potansiyelin
elektrottan uzakla gre deiimi
ELEKTRKSEL FT KATMAN
M T C ll d R i
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Mass-Transport-Controlled Reactions71
Mass transport occurs by three different modes: Diffusion : the spontaneous movement under the
influence of concentration gradient, from regions
of high concentrations to regions of lower ones,
aimed at minimizing concentration differences.
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72
Convectionoccurs when a mechanical means is usedto carry reactants toward the electrode and to remove
products from the electrode. The most common
means of convection is to stir the solution using a stir
bar. Other methods include rotating the electrode andincorporating the electrode into a flow cell.
Convection: transport to the electrode by a gross
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73
Convection: transport to the electrode by a gross
physical movement; the major driving force for
convection is an external mechanical energy
associated with stirring or flowing the solution orrotating or vibrating the electrode (i.e., forced
convection). Convection can also occur naturally
as a result of density gradients.
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74
Migration occurs when charged particles in solution
are attracted or repelled from an electrode that has a
positive or negative surface charge.
Thus, when the electrode is positively charged,negatively charged particles move toward the
electrode, while positively charged particles move
toward the bulk solution.
Unlike diffusion and convection, migration only affects
the mass transport of charged particles.
M T t C t ll d R ti
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Mass-Transport-Controlled Reactions75
Migration: movement of charged particles along
an electrical field (i.e., where the charge is
carried through the solution by ions according
to their transference number).
The pathway of the electrode reaction can be quite
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76
complicated, and takes place in a sequence that
involves several steps. The rate of such reactions is
determined by the slowest step in the sequence.
Simple reactions involve only mass transport of the
electroactive species to the electrode surface,
electron transfer across the interface, and transport of
the product back to the bulk solution.
More complex reactions include additional chemical
and surface processes that either precede or follow
the actual electron transfer. The net rate of the
reaction, and hence the measured current, may be
limited by either mass transport of the reactant or the
rate of electron transfer.
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77
For a given system, the rate-determining step maythus depend on the potential range under
investigation.
When the overall reaction is controlled solely by
the rate at which the electroactive species reachthe surface (i.e., a facile electron transfer), the
current is said to be mass-transport-limited.
Such reactions are called nernstian or reversible,
because they obey thermodynamic relationships.
Several important techniques rely on such mass-
transportlimited conditions.
Fl
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Flux78
The flux (J), a common measure of the rate of masstransport at a fixed point, is defined as the number of
molecules penetrating a unit area of an imaginary
plane in a unit of time and is expressed in units of
mol/cm2
s.
Fl
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Flux79
The flux to the electrode is described mathematically
by a differential equation, known as the Nernst
Planck equation, given here for one dimension
D is the diffusion coefficient (cm2/s); (in aq. about 105and 106cm2/s)
[C(x,t)]/xis the concentration gradient (at distance x and time t);[(x,t)]/x is the potential gradient;
z and C are the charge and concentration, respectively, of the
electroactive species;
and V(x,t) is the hydrodynamic velocity (in the x direction).
C t
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Current80
The current (i) is directly proportional to the flux and
the surface area (A)
The situation is quite complex when the three modes
of mass transport occur simultaneously. This
complication makes it difficult to relate the current
to the analyte concentration.
Diffusion Migration Convection
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81
The situation can be greatly simplified by suppressingthe electromigrationthrough the addition of inert salt.
This addition of a high concentration of the supporting
electrolyte helps reduce the electrical field by
increasing the solution conductivity.
Convection effects can be eliminated by using a
quiescent solution.
In the absence of migration and convection effects,
movement of the electroactive species is limited by
diffusion.The reaction occurring at the surface of the electrode
generates a concentration gradient adjacent to the
surface, which in turn gives rise to a diffusional flux.
Ficks laws
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Ficks laws82
According to Ficks first law, the rate of diffusion (i .e.,
the flux) is directly proportional to the slope of the
concentration gradient:
Hence, the current (at any time) is proportional to
the concentration gradient of the electroactive
species.
Ficks second law
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Fick s second law
83
The diffusional flux is time-dependent. Such dependenceis described by Ficks second law (for l inear diffusion):
This equation reflects the rate of change with time of
the concentration between parallel planes at points x
and (x + dx) (which is equal to the difference in flux at
the two planes).Ficks second law is valid for the conditions assumed,
namely, planes parallel to one another and
perpendicular to the direction of diffusion, specifically,
conditions of linear diffusion.
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84
Concentration profiles for different t imes after the
start of a potential-step experiment.
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85
The flux of material to and from the electrode surface isa complex function of all three modes of mass transport.
In the limit in which diffusion is the only significant
means for the mass transport of the reactants and
products, the current in a voltammetric cell is given by
where n is the number of electrons transferred in the
redox reaction, F is Faradays constant, A is the area ofthe electrode, D is the diffusion coefficient for the
reactant or product, Cbulkand Cx=0are the concentration
of the analyte in bulk solution and at the electrode
surface, and d is the thickness of the diffusion layer.
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86For this equation to be valid, convection and migration
must not interfere with the formation of a diffusion layerbetween the electrode and the bulk of solution.
Migration is eliminated by adding a high concentration of
an inert supporting electrolyte to the analytical solut ion.
Ions of simi lar charge are equally attracted or repelledfrom the surface of the electrode and, therefore, have an
equal probability of undergoing migration.
The large excess of inert ions, however, ensures that few
reactant and product ions will move as a result of
migration. Although convection may be easily eliminated
by not physically agitating the solution, in some
situations it is desirable either to stir the solution or to
push the solution through an electrochemical
flow cell.
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87
Fortunately, the
dynamics of a fluid
moving past an
electrode results in a
small diffusion layer,typically of 0.001
0.01-cm thickness, in
which the rate of mass
transport byconvection drops to
zero.
Influence of the Kinetics of Electron
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Transfer on the Faradaic Current88
The rate of mass transport is one factor influencing thecurrent in a voltammetric experiment.
The ease with which electrons are transferred between
the electrode and the reactants and products in solut ion
also affects the current. When electron transfer kineticsare fast, the redox reaction is at equilibrium, and the
concentrations of reactants and products at the electrode
are those specif ied by the Nernst equation. Such systems
are considered electrochemically reversible. In other
systems, when electron transfer kinetics are suff iciently
slow, the concentration of reactants and products at
the electrode surface, and thus the current, differ from
that predicted by the Nernst equation. In this case the
system is electrochemically irreversible.
Nonfaradaic Currents
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Nonfaradaic Currents89
Faradaic currents result from a redox reaction at the
electrode surface. Other currents may also exist in an
electrochemical cell that are unrelated
to any redox reaction.
These currents are called nonfaradaic currents andmust be accounted for i f the faradaic component of the
measured current is to be determined.
The most important example of a nonfaradaic current
occurs whenever the electrodes potential is changed.
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90
In discussing migration as a means of mass transport,we noted that negatively charged particles in solution
migrate toward a positively charged electrode, and
positively charged particles move away from the same
electrode.When an inert electrolyte is responsible for migration, the
result is a structured electrodesurface interface called
the electrical double layer, or EDL, the exact structure of
which is of no concern in the context of this text.
The movement of charged particles in solution, however,gives rise to a short-lived, nonfaradaic charging current.
Changing the potential of an electrode causes a change in
the structure of the EDL, producing a small charging
current.
Residual Current
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Residual Current91
Even in the absence of analyte, a small current
inevitably flows through an electrochemical cell. This
current, which is called the residual current, consists
of two components:
a faradaic current due to the oxidation or reductionof trace impurit ies,
the charging current.
Methods for discriminating between the faradaiccurrent due to the analyte and the residual current
are discussed later.
Shape of Voltammograms
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Shape of Voltammograms92
The shape of a vol tammogram is determined by severalexperimental factors, the most important of which are how
the current is measured and whether convection is
included as a means of mass transport. Despite an
abundance of different voltammetric techniques, onlythree shapes are common for voltammograms.
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93
This voltammogram is characterized by a current thatincreases from the background residual current to a
limiting current at potentials at which the analyte is
oxidized or reduced. Since the magnitude of a faradaic
current is inversely proportional to , a limiting current
impl ies that the thickness of the diffusion layer
remains constant.
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94
The simplest method for obtaining a limiting current isto stir the solution, which can be accomplished with a
magnetic stir bar, or by rotating the electrode.
Voltammetric techniques that include convection by
stirring are called hydrodynamic voltammetry.When convection is absent, the thickness of the
diffusion layer increases with t ime, resulting in a peak
current in place of a limiting current.
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95
In the voltammograms given above, the current ismonitored as a function of the applied potential.
Alternatively, the change in current following a change in
potential may be measured.
The result ing voltammogram also is characterized by apeak current.
3. Interference current
i l i l DC l h
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Residual current(1) redox reactions of impurities in solution
(2) charging of Hg drop(non-faradaic current / non-redox current)
Migration current
The current produced by static attraction of the
electrode to sought-for ions
in classical DC polarography
Polarographic Maximum (or malformed peak )
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Complex arti factualphenomenon
Less likely at low drop
rates, in concentrated
electrolyte, or lowconcentration of
electroactive species
Lessened by inclusion of
surfactants in medium
Polarographic Maximum (or malformed peak )
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Oxygen waveDissolved oxygen is easily reduced at many working
electrodes. Thus an aqueous solution saturated with airexhibits two distinct oxygen waves.
The first results from the reduction of oxygen to
hydrogen peroxide:O2 + 2H+ + 2e- H2O2
The second wave corresponds to the further reduction ofhydrogen peroxide:
H2O2 + 2H+ + 2e- 2H2O
Sparge solutions with high purity N2or Ar for 5-20 min
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F h ff li i i diff i
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Factors that affect limiting diffusion current
Characteristics of capillary hight of Hg
Potential of dropping Hg electrode
Composition of solution
Temperature
Factors that affect
half-wave potentialType and concentration ofsupporting electrolyte
TemperatureForming complex
Acidic of solution
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101
The polarographic technique is also useful in the analysis of biologicalsystems to determine vitamins, alkaloids, hormones, terpenoid substances
and natural colouring substance, analysis of drugs and pharmaceutical
preparations, determination of pesticide or herbicide residues in foods, in
the structure determination of many organic compounds etc.
Polarographic analysis of organic compounds
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102This technique is used in organic chemistry for qualitative
and quantitative analysis and structure determinations.
Most of the organic compounds are insoluble in pureaqueous medium and also in mercury to form amalgam.
Therefore, the solvent in which the organic compound andits electrode product is soluble, is added to the supportingelectrolyte. These solvents include various alcohols orketones, dimethyl formamide, acetonitrile, ethylene diamine
and others.
The commonly used supporting electrolytes which areeasily mixed with organic solvents are various quaternary
ammonium salts such as tetrabutyl ammonium iodide.
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103
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104
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105
Alternating Current Polarography
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The a.c. Polarography technique involves the application ofpotential wave form of the type shown in Fig. 1 a for a DME.
This may be envisaged as linearly increasing d.c. potentail
ramp upon which is superimposed a small alternating voltage
with an amplitude af about 10-20 mV.
The current flow through the cell is containing both a.c and
d.c. components, but when the former is ploted as a function
of the applied d.c potential a symmetrical peak shape is
obtained Fig.1.b
106
Fig.1 a) Exitation wave
form and b) response
obtained in a.c.
Polaraography.
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A.c. voltammetry is more sensitive then d.c. voltammetry,because it is possible to use electronic technique to
discriminate between faradaic and capacity current. It can be
achieved through the use of phase-sensitive detectors, which
are based on the diffreneces in phase angle between faradaic
and capacity current.
The technique can be used only for completely reversible
electrochemical process and the peak potential corresponds to
the E1/2 value..
The current can be flowed from the system after the Ox/Red
species formed on the electrode surface. The detection limit of
the a.c polarography/voltammetry is about 1.10-6 M for
reversible electrode reaction process.
107
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108
Fig. Polarograms obtained d.c and a.c polarography at DME.
A.C voltammograms can be obtained in the presence of
small amount of oxygen. Because, first oxygen reduction is
quasireversible therefore, it is not interfere to electroactive
species current.
PULSE METHODS
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The sensitivity of linear scan voltammetry or d.c polarography
is limited by charging current which is large initially in the
drop life. The slowness, inconvenient apparatus and poor
detection limits of linear scan voltammetry or polarography
were the reasons for its decline.
The sensitivity of these two methods is improved by pulse
methods by about 6.5 times by eliminating charging current.
The current in these methods is measured at a time when thedifference between the Faradaic current and the interfering
charging current is large.
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There are four important pulse methods namely i) Normal pulse voltammetry/polarography
ii) Differential pulse voltammetry/polarography
iii) Square wave voltammetry iv) Cyclic voltammetry
110
Normal Pulse Polarography (NPP)
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Pulse polarographic techniques were developed by Barker and
Gardner in order to improve the polarographic performance and
lower the detection limits for the electroactive species.
The capacity current is the major source of interference inpolarography and the pulse wave forms were designed to
discriminate against this and measure the required faradaic
current.
111
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112
This is achieved by applying apotential pulse with a short duration
near the end of the mercury drop
life. This is then repeated at exactly
the same time in the life of thefollowing drops. Since capacity
currents decrease more rapidly to
zero than farradaic current and it is
possible to discriminate against the
former by making a currentmeasurement at the end of a fixed
pulse period.
In normal pulse polarography, the applied wave form employedin Following Fig. A pulse applied by stepping from an initial
potential E to another potential E and then returning to E at
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113
Sampling scheme for normal pulse polarography. (a) Potential
program, (b) Current and (c) potential during a single drop's
lifetime.
potential E1 to another potential E2 and then returning to E1 at
afixed time drop life.
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This process is continued by appllying further pulses from E1,the amplitude of subsequent pulses is increased by constant
amount of to produce a linear potentail scan.
The pulse witdh is normaly set at about 50ms, incerases inpulse amplitude are function of the scan rate and drop time are
around 5 mV.
The sampled current for a periot of 15-20 s at the end of theeach pulse. The response obtained is an S shape as given in
Fig.
114
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115
Differential Pulse Polarography (DPP)
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A still greater improvement is reached by using theprogramming wave form. In this technique a 50 mV pulse is
applied during the last 50 ms of the life time of the mercury
drop.
The current is measured twice during the life time of each
drop, firstly just before the pulse and second one just before
the drop falls.
The difference in the current is plotted resulting a peak shaped
curve. The top of the peak corresponds toE and the height of
the peak depends on the concentration.
116
The peak current in differential pulse voltammetry
increases linearl with the concentration of the
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increases linearly with the concentration of the
analyte. It also increases with the pulse amplitude but
in a complicated linear fashion. In practice amplitudesgreater than 100 mV are not used, since the peaks
broaden and the resolution is significantly inhibited.
The higher detection limit in this technique is due to
superior discrimination against charging current and
impurity Faradaic current. The peak current in
differential pulse polarography is given by
117
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where, ip = Peak current, n = Number of exchanged electrons, F = Faraday constant,
R = Gas constant, T = Temp. in Abs. Scale,A = Electrode surface
area, Ca = Concentration of the analyte, EA = Pulse amplitude,
D = Diffusion coefficient of the analyte, and tp = Pulse duration.
The detection limit for the determination by this technique is
similar to square wave polarography at 107
108
mol/dm3
. Butfor irreversible reaction it is lowered.
118
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119
Fig. a) Excitation wave form and response obtained indifferential-pulse polarography /voltammetry.
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121
For reversible reactions, a larger pulse is used so that its size is
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, g p
great enough for the oxidation of the product formed in the
forward reaction occurs during the reverse phase.
The forward pulse gives the cathodic current i1, while the
reverse phase gives the anodic current i2 and the difference
between these two i = i2 i1 is plotted to give thevoltammograms. This difference is directly proportional to the
concentration, the potential at the peak corresponds to the E.
It is possible to increase the precision ofthe analysis by signal
averaging data from several voltammetric scans.
Detectionlimits for square wave voltammetry are reported tobe 107 to 108 M.
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123
Cyclic Voltammetry
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Cyclic Voltammetry (CV) is an attractive method for theteaching of a number of concepts in electrochemistry.
In brief, it is perhaps the most versatile electrochemical
technique for the study of electroactive species.
The effectiveness of CV results from its capability for rapidlyobserving the redox behaviour over a wide range of potential.
The resulting voltammogram is similar to a conventional
spectrum in that it conveys information as a function of an
energy scan. In spite of its wide applications, this technique is not generally
well understood as compared to other instrumental techniques
such as spectroscopy and chromatography.
124
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125
Fundamentals of Cyclic Voltammetry: It consists of cycling the
potential of an electrode that is immersed in an unstirred solution of
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the electroactive species and then measuring the resulting current.
The potential of this working electrode is controlled versus a
reference electrode such as a saturated calomel electrode (SCE) or
asilver/silver chloride electrode (Ag/AgCl).
The controlling potential which is applied across these two
electrodes can be considered as excitation signal. The excitation
signal for CV is a linear potential scan with a triangular waveform
as given in Fig.
126
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This triangular potential excitation signal sweeps the potentialof the electrode between two values, sometimes called the
switching potentials. The excitation signal in Fig. causes the
potential first to scan negatively from +0.80 (initial potential)
to 0.20 V (E
F final potential) versus SCE at which point thescan direction is reversed, resulting a positive scan back to the
original potential of +0.80 V (Ei).
The scan rate as reflected by the slope is 50 mV per second asshown in Fig. A second cycle is indicated by the darked line.
Single or multiple cycles can be used.
127
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128
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In some cases, the anodic current is low as compared to thecathodic current.
This is because throughout most of the experiment there is a
concentration difference which drives back the reduced
species away from the electrode, most of the reduced speciesproduced in forward reduction process, therefore, diffuse into
the bulk solution and cannot be re-oxidized on the time scale
of a cyclic voltammetric experiment during reverse direction,
i.e. anodic process. Hence, it gives reduced anodic peak current as compared to
cathodic peak current.
129
Types of indicator electrodes
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Metal solid
Pt, Au, Ag, C
liquiddropping mercury electrode (DME)
Semiconductors
Si, GaAs
In-SnO2/glass (optically transparent)
Solid Electrodes
http://moneynewsworld.net/wp-content/plugins/home-page-sentences/indium-tin-oxide-electrodehttp://www.alibaba.com/product-gs/299200189/ITO_conductive_glass.html -
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131
Solid electrodes constructed using platinum, gold, silver, or carbonmay be used over a range of potentials.
For example, the potential range for aPt electrode extends from
approximately +1.2 V to 0.2 V versus the SCE in acidic solut ions and
from +0.7 V to -1 V versus the SCE in basic solutions.
Solid electrodes, therefore, can be used in place of mercury for manyvoltammetric analyses requir ing negative potentials and for
voltammetric analyses at posit ive potentials at which mercury
electrodes cannot be used.
POTENTIAL WINDOW FOR
WORKING ELECTRODES
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37
WORKING ELECTRODES
Acidic
Neutral
Basic
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2
Carbon
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Paste With nujol (mineral oil)
Glassy carbon (GC)
Amorphous Pyrolytic graphite - more ordered than GC
Basal Plane
Edge Plane (more conductive)
Solid Electrodes
http://www.autolabj.com/construction.files/electrode.files/basal%20plane.htmhttp://www.intechopen.com/books/metal-ceramic-and-polymeric-composites-for-various-uses/graphite-composites-alternatives-for-electrochemical-biosensorhttp://electrochemistry.co.kr/mall1/m_mall_detail.php?ps_goid=251http://www.autolabj.com/construction.files/electrode.files/basal%20plane.htmhttp://blog.innomats.de/2010/05/some-notes-on-pyrolytic-graphite.htmlhttp://electrochemistry.co.kr/mall1/m_mall_detail.php?ps_goid=395 -
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135
Except for the carbon paste electrode, solid electrodes are fashionedinto disks that are sealed into the end of an inert support and are in
contact with an electrical lead.
TheCPE is made by filling the cavity at the end of the inert support
with a paste consisting of carbon particles
and a viscous oil.
Solid electrodes are not without problems, the most important of
which is the ease with which the electrodes surface may be altered by
the adsorption of solution species or theformation of oxide layers.
For this reason solid electrodes need
frequent reconditioning, either by
applying an appropriate potential orby polishing.
Fiber used in in-vivo studies
http://www.intechopen.com/books/metal-ceramic-and-polymeric-composites-for-various-uses/graphite-composites-alternatives-for-electrochemical-biosensorhttp://www.intechopen.com/books/metal-ceramic-and-polymeric-composites-for-various-uses/graphite-composites-alternatives-for-electrochemical-biosensor -
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6
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Mula niv.
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140
The first is the reduction of trace impurities that arealmost inevitably present in the blank solution. Thecontributors here include small amounts of dissolvedoxygen, heavy metal ions from the distilled water, and
impurities present in the salt used as the supportingelectrolyte.
The second component of the residual current is the so-called charging or capacitive current resulting from aflow of electrons that charge the mercury droplets withrespect to the solution; this current may be eithernegative or positive.
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141