4 Instr an. Volt.zek Ek Kopya

8/12/2019 4 Instr an. Volt.zek Ek Kopya

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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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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.

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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.

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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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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
http://upload.wikimedia.org/wikipedia/commons/e/e0/RC_Series_Filter_(with_V&I_Labels).svg


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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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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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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.

109


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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.

122


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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
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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
http://electrochemistry.co.kr/mall1/m_mall_detail.php?ps_goid=395


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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
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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
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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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4 Instr an. Volt.zek Ek Kopya

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