Chapter-l

Introduction, Review of Cyclic Voltammetry and Theoretical

Considerations

^(^pfi 9^. <Biizz'^dams—Inventorof CafSon (Paste electrode

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1.1. Introduction

Electrochemistry may simply be defined as the study of chemical reactions used

to produce electric power or alternatively, the use of electricity to effect chemical

processes or systems [1,2] Hence, electrochemistry can be seen as the relationship

between electricity and chemistry, namely the measurements of electric quantities, such

as current, potential and charge and their relationship to chemical parameters. These

chemical reactions involving the transfer of electrons to and from a molecule or ion are

often referred to as redox (reduction/oxidation) reactions. The use of electrochemistry for

analytical purposes has found a wide range of applications in industrial quality control,

metallurgy, geology, pharmacy, medicinal chemistry, biomedical analysis and

environmental monitoring [2].

Unlike many chemical measurements, which involve homogenous bulk solutions,

the fundamental electrochemical reactions are heterogeneous in nature as they take place

at interfaces, usually electrode-solution boundaries. The electrode creates a phase

boundary that differentiates otherwise identical solute molecules, those at a distance from

the electrode and those close enough to the surface of the electrode to participate in the

electron transfer process [1,2]. This section takes a closer look at some of the

electroanalytical techniques and electrode processes employed in this thesis.

In the first quarter of 20* century, electrochemistry was dealing with the common

subjects like electroanalysis, potentiometry and conductometry [3]. No attention was,

however, given towards the interpretation of mass transfer process by diffusion or

convection. In 1922 Heyrovskey [4] published the first paper dealing with polarography.

This thesis gave an in depth knowledge to electrochemistry pertaining to

analytical approach. Polarography, the interpretation of current-potential characteristics

of an electrolytic system exhibited by the dropping mercury electrode, proved to be the

first analytical tool in electrochemistry. This discovery extended the domain of

electrochemistry to physicochemical studies like kinetics of rapid reactions and reversible

or irreversible electron transfer processes [5].

In 1941 Laitinen and Kolthoff [6] used stationary electrodes instead of dropping

mercury electrode, for the determination of current- potential curves in the analysis of

various chemical systems. This replacement of test electrode is called voltammetry. This

technique proved to be more sensitive and faster than polarography [7], because the

sensitivity is decreased due to the presence of charging current in polarography. Various

test electrodes have been used, however, hanging mercury drop electrode received great

importance due to its better performance [8-10]. Matheson and Nicholas [11] contributed

numerous investigations in the theory of stationary electrode polarography while Randies

[12] and Seveik [13] reported the single scan method for a reversible reaction taking

place at a planar electrode. This approach was extended to totally irreversible charge

transfer reactions by Delahay [14]. Matsuda and Ayabe [15, 16] further extended this

work to intermediate quasi-reversible cases.

Voltammetry, which is similar to polarography, deals with the measurements of

the current which flows at a stationary electrode as a function of the applied potential

[17]. However, the surface of the indicator electrode is not renewed compared to

dropping mercury electrode which is used in polarography. The effect of voltage scan

(linearly) on the flow of current as a result of redox reaction of an electroactive species at

the test electrode results in a current potential curve which is the electrochemical

equivalent of a spectrum obtained in spectroscopy [17, 18]. Information extracted from

these current potential curves help analytical chemists to deal with quantitative and

qualitative studies of a chemical reaction [17]. Similarly, a physical chemist may infer

results pertinent to thermodynamics and kinetics of a chemical reaction [18].

Cyclic voltammetry is a modified form of the rapid scan technique. In cyclic

voltammetry voltage is linearly scanned beyond peak potential. After traversing the

potential region, the direction of the linear scan is reversed, registering the

voltammogram of both cathodic and anodic electrode processes occurring at the test

electrode [19, 20]. Cyclic voltammetry has become a very popular technique for the

initial electrochemical studies of new systems [21]. A great deal of useful information

can be obtained from cyclic voltammetry such as quantitative and qualitative studies of

intermediates and products formed during the forward scan and the mechanism of the

electrode reaction [21, 22]. Today cyclic voltammetry has become the most versatile

electroanalytical method due to its extensive use in the fields of electrochemistry,

inorganic chemistry, organic chemistry and biochemistry [20]. For example, the

selections of a proper oxidizing agent by means of its cyclic voltammetric study [23]. The

study of reaction mechanism of the biosynthetic reactions [24] and in the study of

electrochemically generated free radicals [25]. In solar energy, it gives information

regarding effects of ligends or the redox potential of the central metal in complexes of

single or multi nuclear cluster [26,27]. Such information also contributes to the studies of

enzymatic catalysis [28].

1.2. Fundamentals of Cyclic Voltammetry

1.2.1. Circuit

Voltammetric analysis consists of two circuits one of which is a polarizing circuit

that applies the potential to the cell and the other is a measuring circuit that monitors the

cell current. The working electrode is potentiostatically controlled. The potential is varied

in some systematic manner and resulting current vs. potential plot is known as

voltammogram.

1.2.2. Scan rate

A simple potential waveform that is used often in electrochemical experiments is

the linear waveform i.e., the potential is continuously changed as a linear function of

time. The rate of change of potential with time is called scan rate.

1.2.3. Switching potentials and the excitation signal

Cyclic voltammetry involves the cycling of potential of an electrode between two

designated values called the Switching potentials in an unstirred solution and measuring

the resulting current. The controlling potential applied across the working electrode (WE)

and the reference electrode (RE) is called the excitation signal which is a linear potential

scan with a triangular waveform as shown in Fig, 1.1. The excitation signal causes the

potential to scan negatively from +0.8V to -0.2V vs SCE, at which point the scan

direction is reversed causing a positive scan back to the original potential of +0.8V.

Single or multiple cycles can be used.

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1.2.4. Potential control

The potential control of the external point is done using a potentiostat and a three

electrode system in which the potential of the WE is controlled relative to the RE,

saturated calomel electrode (SCE) or Silver-Silver chloride (Ag/AgCl) electrode. The

current passes between WE and the auxiliary electrode (AE).

Because of its greater experimental simplicity, CV has became a very popular

technique for electrochemical studies of new systems and has proved as a sensitive tool

for obtaining information about fairly complicated electrode reactions.

CV is a technique, where in a species that undergoes a reduction during a cathodic

polarization of the WE in an unstirred solution is reoxidized by applying a reverse

(i.e., anodic) scan. The correlation of the cathodic and the anodic peak currents and

differences in cathodic and anodic potentials with the voltage scan rates has been studied

mathematically for different electrochemical reaction [29, 30]. The sweep rates in the CV

can be about the same as in single sweep voltammetry.

1.2.5. CV- an active electrochemical method

CV can describe as 'active' electrochemical method because the experiment

drives an electrochemical reaction by incorporating the chemistry in to a circuit and then

controlling the reaction by circuit parameter such as voltage.

1.2.6. Characteristic parameters of a cyclic voltammogram

The parameters of a cyclic voltammogram are peak potential and peak current.

There are two peaks associated with the redox reaction and accordingly we have the

anodic peak potential {E^ and cathodic peak potential (£pc) and the corresponding

^ ^ ^ = ^ = = ^ ^ = = = ^ = ^ ^ = = = ^ ^ = ^ ^ = ^ = ^ ^ ^ = ^ = ^ ^ ^ = ^ = ^ chapter-1 =^=

current associated are anodic peak current (/pa) and cathodic peak current (/pc)

respectively. Fig.l depicts a typical voltammogram for a reversible process with current

(vertical) vs. potential. Since the potential varies linearly with time, the horizontal axis

can also be thought of as a time axis. More positive potentials will speed up all oxidations

and more negative potential will speed up all reductions.

1.3. General Theory

The current response obtained in controlled-potential experiments is as a result of

the analyte species that is oxidized or reduced at the electrode-solution interface. This

current response is deduced from the transfer of electrons during the redox process of the

target analyte as shown in Equation 1.1.

Where there is oxidation, there is reduction

Substance oxidized Substance reduced loses electron(s) gains electron(s)

Ox -Hie- — Red

(1.1)

Where Ox and Red represent the oxidized and reduced forms of the analyte respectively,

and n is the number of electrons transferred. The current that arises from the oxidation or

reduction of the analyte species is called the Faradaic current. For a thermodynamically

^ ^ ^ = = CHapter-1 ^ ^ = ^ =

controlled reversible process the applied potential (E) of the electrode is given by the well

known Nemst equation, Equation 1.2.

^ n, 2.303RT , ^ox E=E + log-

^^ Cred (1.2)

where E° - Standard potential of the red ox couple, R = Universal gas Constant

T = Temperature (K) , n = number of electrons transferred, F= Faraday's constant,

Cox = Concentration of the oxidized species, C Red= Concentration of the reduced species.

Non Faradic currents are a result of those processes that do not involve the

transfer of electrons across the electrode-solution interface and they stem from the

electrical capacitance present at the interface. The capacitance (C) of the electrical double

layer can be calculated using Equation 1.3.

E (1.3)

where, q and E represent charge and potential respectively.

1.4. Solvent and Supporting Electrolyte

Electrochemical measurements are commonly carried out in a medium which

consists of solvent containing a supporting electrolyte. Sometimes in most cases,

supporting electrolyte has to be added to the dissolved sample in an attempt to achieve

the following [Heyrovsky and Zuman (31)]

^===========^===^=^=^====== chapter-1 = = ^ = ^

a) To make solution conductive

b) To control the pH value so that organic substances are reduced in a given

potential range and inorganic substances are not hydrolyzed

c) To ensure the formation of such complexes that give well developed and well

separated waves

d) To shift the hydrogen evaluation towards more negative potentials and to

eliminate catalytic effects on hydrogen evolution

e) To suppress unwanted maxima by addition of surface-active substances to the

supporting electrolyte.

The choice of the solvent is primarily by the solubility of the analyte, its redox

activity and also by solvent properties such as electrical conductivity, electrochemical

activity and chemical reactivity. The solvent should not react with the analyte and should

not undergo electrochemical reaction over a wide potential range. In aqueous solution the

cathodic potential is limited by the reduction of hydrogen ions.

2 H^ (aq) + 2 e-^H2 (g) (1.4)

resulting hydrogen evolution current. The more acidic the solution the more positive

is the potential of this current due to the reaction expressed by,

E = E° H /̂H*-0.059 pH (1.5)

The composition of the electrolyte may affect the selectivity of voltammetric

measurements. The ideal electrolyte should give well-separated and well-shaped peaks

for all the analytes sought, so that they can be determined simultaneously. For example

Kontoyannis et al., [32] used tris buffered saline (TBS) at pH 7.4 as the supporting

electrolyte for simultaneous determination of diazepam and liposome using DPP

= = ^ =s ctiapter-1 = ^ ^ = = s

technique. Inam and Somer [33] have determined selenium (Se) and lead (Pb)

simultaneously in whole blood sample by the same technique using 0.1 M HCl as the

supporting electrolyte. They observed that there were three peaks at -0.33 V, -0.54 V and

-0.41 V which belonged to an intermetallic compound (PbSe), Se and Pb respectively.

Barbeira et al, [34] have developed anodic stripping voltammetric technique for

simultaneous determination of trace amounts of zinc, lead and copper in rum without pre-

treatment and in the absence of supporting electrolyte. They observed that there were

three peaks at -0.92 V, -0.42 V and 0.05 V which belong to Zn, Pb and Cu respectively.

Because of the sensitivity of the voltammetric method, certain impurities in supporting

electrolyte can affect the accuracy of the procedures. It is thus necessary to prepare the

supporting electrolyte from highly purified reagents and should not easily oxidized and

reduced. To obtain acceptable ionic strength of supporting electrolyte, certain

concentration should be prepared which is usually about 0.1 M. This level is a

compromise between high conductivity and minimum contamination. The low ionic

strength which is 0.01 M of supporting electrolyte (HCIO4 - NaC104) was very effective

for the adsorptive accumulation of analyte on the electrode as found by Berzas et al, [35]

when they developed adsorptive stripping square wave technique for determination of

sildenafil citrate (Viagra) in pharmaceutical tablet. Dissolved oxygen must be removed

from supporting electrolyte first since the reduction of dissolved oxygen will cause two

cathodic peaks at -0.05 V and -0.9 V (versus SCE) as reported by Reinke and Simon [36].

With increasing pH, the waves due to reduction of oxygen are shifted to more negative

potential. The oxygen reduction generates a large background current, greater than that of

the trace analyte, and dissolved oxygen therefore tends to interfere with voltammefric

= ^ ^ = ^ ^ = = ^ CHapter-l ^ = ^ = =

analysis [37]. The common method for the removal of dissolved oxygen is by purging

with an inert gas such as nitrogen or argon where longer time may be required for large

sample volume or for trace measurements. To prevent oxygen from reentering, the cell

should be blanketed with the gas while the voltammogram is being recorded. However,

this conventional procedure is time consuming and not suitable for flow analysis. Due to

this reason, Colombo and van den Berg [37] have introduced in-line deoxygenating for

flow analysis with voltammetric detection. They have used an apparatus which is based

on the permeation of oxygen through semi-permeable silicone tubing into an oxygen free

chamber and enables the determination of trace metals by flow analysis with

voltammetric determination.

1.5. Electrodes

In the present work three electrode system is used i.e. WE / AE / REs. The RE

used is standard calomel electrode (SCE) which is often isolated from the solution by a

salt bridge to prevent contamination by leakage from the RE. The platinum foil as AE

and WEs are carbon paste electrode, or Modified carbon paste electrode.

1.5.1. Working Electrode (WE)

The vast majority of commercially available electrochemical detectors use three

electrodes. The working, the counter (auxiliary) and the reference. A fixed potential

difference is applied between the working electrode and the reference electrode. This

potential drives the electrochemical reaction at the working electrode's surface (as shown

in below figure).

10

B + e"

CHapter-l

Working Electrode

Flow [The 'Flit ' Design flow -through |K>ioasgraphite •worldwg

electrode.]

The current produced from the electrochemical reaction at the working electrode

is balanced by a current flowing in the opposite direction at the counter electrode. The

reference electrode acts as a reference point for the redox couple. The current resulting

from the electrochemical reaction is amplified and when plotted as a function of time,

appears as a peak on the recording device. Current measured at the working electrode

surface results from not only the redox reaction of interest (faradic current) but also from

unwanted redox reactions coming from the mobile phase (faradic noise) and from other

sources of noise such as the working electrode material itself, the solvent delivery system

and the potentiostat (non-faradic noise). Some of the faradic and non-faradic noise can be

minimized by careful mobile phase production, HPLC system cleanliness, a quit pump

and the correct choice of working electrode material.

A wide variety of working electrodes are now available. The most common

working electrode materials utilize carbon. Originally the carbon paste electrode was

11

developed but this was soon replaced by more 'convenient' and stable carbon-based

working electrodes including those made from glassy carbon, carbon paste, pyrolytic

carbon and porous graphite. Metals such as platinum, gold, silver, nickel, mercury, gold-

amalgam and a variety of alloys arc now also commonly used as working electrode

materials.

The optimal working electrode choice is dependent upon many factors, including

the usable applied potential range, involvement of the electrode in the redox reaction and

the kinetics of the electron transfer reaction. Other factors, such as compatibility with and

the composition of the mobile phase, will also play a role. For example, carbon paste

electrodes cannot be used with mobile phases containing high amounts of organic

modifier because the electrode will dissolve unless a polymeric binder is used.

Remember, for an electrochemical detector to function the mobile phase must contain an

electrolyte to permit the flow of current. Too little electrolyte may prevent electrolysis

from occurring at the working electrode resulting in diminished response. Further more,

in some electrochemical detectors the inability to monitor current may the potentiostat to

apply a considerable potential to the working electrode which may destroy it. Too much

electrolyte can result in considerable background current (noise) limiting the sensitivity

of the system and damaging the working electrode.

Solid electrodes covered by membranes or modified with polymers, gels and

various composite materials cannot be treated by polishing. The only way to make them

work reproducibly is to apply an appropriate conditioning potential before the

voltammetric experiments.

12

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1.5.2. Reference Electrode (RE)

Choosing a reference electrode the purpose of the reference electrode is to provide

a stable, well-known half-reaction on which to reference the redox process occurring at

the working electrode. In addition, the potentiostat uses the voltage output of the

reference electrode to stabilize against IR drops and polarization effects that would be

unavoidable in a two-electrode system. The reference electrode should be chosen

judiciously. The saturated calomel electrode (SCE) is probably the most popular for

aqueous electroanalytical chemistry. Another stable and reliable reference electrode is the

Ag/AgCl/KCl electrode. It can be built very compact and has a low temperature

coefficient. However, neither can be used in solutions where metal ions may precipitate

with chloride, or where leakage of chloride ion may interfere with the analyte. Also, both

electrodes may be problematic when used in nonaqueous analyte solution, especially

where contamination by water is to be avoided. In these cases the Ag/AgNOsCCHsCN)

electrode may be a good choice. Another possibility is to use a pseudoreference

electrode. These are the easiest and most compact of all reference electrodes, but are not

as stable or well-defined. A pseudo reference electrode can be as simple as a length of

silver wire, and can be used in either aqueous or nonaqueous media. Pseudo reference

electrodes should be calibrated by spiking the solution with a small amount of a well-

behaved reversible redox species like ferrocene (for nonaqueous solutions) or

ferricyanide (for aqueous solutions).

1.5.3. Counter Electrode (CE)

The counter electrode, alternatively referred to as the auxiliary electrode, acts as

source or sink electrons in the electrochemical circuit formed with the working electrode.

13

g = ^ = CHapter-l =

By the addition of tiiis third electrode, the potentiostat is able to pass current through the

analyte solution without passing current into or out of the reference electrode. Thus,

reference voltage variations due to electrode polarization or IR drops are eliminated. In

principle, the nature of the counter electrode should have little or no effect on an

electroanalytical measurement. However, if the surface area of the counter electrode is

small relative to the working electrode area inaccuracies may arise due to the additional

resistance imposed by the counter electrode. For this reason, it is wise to keep the surface

area of the counter electrode relatively large. Also, the counter electrode should be made

of an electrochemically inert material such as platinum or graphite. An excellent

inexpensive counter electrode is a simple HB pencil lead (actually graphite-impregnated

clay). The lead can be inserted through a rubber septum cap for use in cells requiring an

air-tight seal . Occasionally it is necessary to separate the counter electrode from the

analyte solution by a frit to circumvent interference by redox active contaminants

generated at the counter electrode. The counter electrode can be placed in a fritted glass

tube and inserted through the septum cap.

1.6. Discharge process at a constant potential electrode (Mass Transport

Processes)

The fundamental movement of charged or neutral species in an electrochemical

cell to the electrode surface is facilitated by three processes namely, diffusion, migration

or convection [2, 38] as illustrated in following Figures.

1.6.1. Diffusion is mass transport resulting from the spontaneous movement of analyte

species from regions of high concentrations to lower ones, with the aim of minimizing

14

concentration differences. A concentration gradient develops if an electrochemical

reaction depletes (or produces) some species at the electrode surface. To minimize the

concentration difference an electroactive species will diffuse from the bulk solution to the

electrode surface (or from the electrode surface into the bulk solution.

<K>

•*-o •*-o © o •*—O *-0 Ze% rP J^ •*-QO fs

*-o "—0"*~^_0 o ^ _0 oo

Schematic representation of the diffusion mass transport mode

1.6.2. Migration refers to movement of a charged particle in a potential field. In most

voltammetric experiments, migration is undesirable but can be eliminated by the addition

of a large excess of supporting electrolyte. Inert anions and cations (i.e.,

electrochemically inert - not oxidized or reduced) that are formed from dissociation of the

supporting electrolyte now function as the migration current carriers and also increase the

conductivity of the solution [1].

/ -M O

u n "t 3 M in ^ o

x> ~ 2 ~ ^ <• m -v -

•€) o-©-

a -—©

-© - — ^

Migration 0-*

Schematic representation of the migration mass transport mode

15

1.6.3. Finally convection is a mass transport achieved by some form of external

mechanical energy acting on the solution or the electrode such as stirring the solution,

solution flow or rotation and/or vibration of the electrode.

UJ

Schematic representation of the convection mass transport mode

1.6.4. Faradic current and capacidve current

The electric current flowing through the working electrode has two components.

The first, the faradic current, fallows the faraday laws and is due to the discharge of the

electro active compound (Aox).

The second, the capacitive current, is produced by the growth of a double

electrical layer on the interface between the electrode and the solution. This double layer

is due to the high concentration of the supporting electrolyte in the solution and acts as a

condenser with high capacity. The total current flowing through the electrode is finally

due to the sum of the charging current (capacitive current) of this condenser and the

faradic current.

The capacitive current acts as a non specific background interference of the

faradic current and sometimes can be higher than the latter, when the depolarizer is

present at low concentration in solution. In this case the measure of the faradic current is

difficult and some electronic adjustment has to be used. Therefore polarography and

16

= = = = = = ^ = ^ = = = = = ^ = = = = ^ = ^ CRapter-l = = = = =

voltammetry is growth, as analytical technique, only after the progress in the electronic

field, so affirm that the development of this technique is strictly linked to the tentative to

electronically overcome problems due to capacitive current.

1.6.5. Reversible reactions

A reversible process is one in which the electron transfer process is rapid, and the

electroactive oxidized (or reduced) species in the forward scan is in equilibrium with the

electroactive reduced (oxidized) species in the reverse scan (Eq. 1.6).

Re«l ^ ^ Ox-bie-

(1.6)

Figure 1 shows a typical CV for a reversible process. The electroactive species are

stable and so the magnitudes of Ipc and Ipa are equal and proportional to the

concentrations of the active species. AEp (Epa-Epc) should be independent of the scan rate

(u) but in practice AEp increases slightly with increasing u, this is due to the solution

resistance (Rs) between the reference and working electrodes [39,40] Theoretically, the

potential difference between the oxidation and reduction peaks is 59 mv for one-electron

reversible redox reactions. However, in practice, AEp is sometimes found in the 60-100

mv range.

Reversibility is a direct and straight forward means of probing the stability of an

electroactive species. An unstable species reacts as it is formed and hence produces no

current wave in the reverse scan whereas a stable species remains in the vicinity of the

electrodes surface and produces a current wave of opposite polarity to the forward scan.

Larger differences or asymmetric reduction and oxidation peaks are an indication of

17

^ = ^ ^ = = ^ ^ = ^ = = = ^ = ^ = ^ = = = = = ^ = = = Cdapter-l = = = ^ =

irreversible reactions. Irreversibility is a result of slow exchange between the redox

species and the working electrode [41]. At 25° C, the peak current is given by the

Randles-Sevcik equation [2,38].

ip = 2.69X10V'^AD''^Cv"^ (1.7)

where, ip = peak current (A); n = number of electrons transferred; A = electrode area

(cm^); C = concentration (mol cm^); D = diffusion coefficient (cm^ s"') and v = scan rate

(Vs"'). These parameters make CV most suitable for characterization and mechanistic

studies of redox reactions at electrodes.

A linear plot of ip vs. v"^ indicates that the currents are controlled by planar

diffusion to the electrode surface [39]. The ratio of anodic to cathodic currents ipa/ipc is

equal for a totally reversible process and deviation from this is indicative of a chemical

reaction involving either one or both of the redox species. The potential where the current

is half of its limiting value is known as the half-wave potential E1/2 (also called formal

potential or equilibrium potential, E° which is the average of the two peak potentials,

represented by Equation 1.7.

(orE*^) ^P^+^P*^ (1-8)

where Epa and Epc are the anodic and cathodic peak potentials, respectively. The

separation between two peak potentials, for a reversible couple is given by Equation 1.8

and can be used to obtain the number of electrons transferred.

AE= E„-E^=2303^^ (1.9)

18

= = ^ = = ^ = ^ = = ^ = = ^ = = = ^ = ^ = ' ^ = = = = Cfiapter-1 = = = ^ ^

AE is independent of the scan rate, and at 25°C Equation 1.9 can be simplified to

Equation 1.10.

RT 0 Q59V A£,= 2.303 ^ = 2£££K , , „ ,

At appropriate conditions (i.e. at 25 °C, first cycle voltammogram) the standard

rate constant (k) for the heterogeneous electron transfer process can be estimated [1,42].

1.6.6. Irreversible systems

For an irreversible process, only forward oxidation (reduction) peak is observed

but at times with a weak reverse reduction (oxidation) peak as a result of slow electron

exchange or slow chemical reactions at the electrode surface [43] the peak current, ip for

irreversible process is given by Equation 1.11.

ip= (2.99x10^^ )u [(l-a)ft 1^^AC(DV)1/2 (l-^^)

where a is the coefficient of electron transfer, the rest of the symbols are defined above

in equation 1.7. For a totally irreversible system, AEp is calculated from Equation 1.12.

AE^ E" -^^ ^ onF

0 . 7 8 - l n 4 ^ 1 n r ^ (1.12)

where all symbols are defined above. At 25 °C, Ep and E1/2 differ by 0.048/an.

1.6.7. Quasi reversible systems

Unlike the reversible process in which the current is purely mass- transport

controlled, currents due to quasi-reversible process are controlled by a mixture of mass

19

transport and charge transfer kinetics [2, 44]. The process occurs when the relative rate of

electron transfer with respect to that of mass transport is insufficient to maintain Nemst

equilibrium at the electrode surface. For quasi-reversible process, ip increases with v"^

but not in a linear relationship and AEp > 0.059/n [38].

1.7. Applications of cyclic voltammetry

Cyclic voltammetry (CV) is the most effective and versatile electro analytical

technique available for the mechanistic study of redox systems [45-49]. It enables the

electrode potential to be rapidly scanned in search of redox couples once located, a

couple can then be characterized from the potential of peaks on the cyclic voltammogram

and from changes caused by variation of the scan rate.

CV has become increasingly popular in all fields of chemistry as a means of

studying redox states [47]. The method enables a wide potential range to be rapidly

scanned for reducible or oxidizable species. This capability together with its variable time

scale and good sensitivity make this the most versatile electro analytical technique. It

must however be emphasized that its merits are largely in the realm of qualitative or

"diagnostic" experiments. CV has its ability to generate a species during one scan and

then probe its fate with subsequent scans.

1.8. Introduction to Literature Survey

Electron transfer plays a fundamental role in governing the pathway of chemical

reactions. Measurement of speed of electron transfer process and the number of electrons

involved are difficult in traditional experimental method spectroscopy. Consequently our

knowledge of the driving force for many reactions remains exclusive. Electrochemical

20

methods offer the potential to investigate this process directly by the determination of the

number of electrons involved.

Research interests involve the study of different modified carbon paste

electrodes and the behavior of modifier on the analyte that are taken in the system, it also

involve electropolymerization, its application in simultaneous determination.

K.H. Lubert et al, [50] studied the cyclic voltammetric characterization of an

insoluble tetra thiaflilvalene (TIF) derivative in acetonitrile in 0.3 M TBAP as supporting

electrolyte by means of modified carbon paste electrode. S.B. Khoo and S.X. Guo [51]

proposed a novel method of generating a rapidly renewable and reproducible

electropolymerised surface at a monomer modified carbon paste electrode. S. Majid

et ah, [52] studied the carbon paste electrode bulk modified with the conducting polymer

poly (1,8 Diamino naphthalene) and its application to lead determination. The

applications of chemically modified electrodes (CMES) to the determination of trace

amounts of metal and organic analytes are studied by Damien and W.M. Arrigan [53].

Sha-yuan Shi et ah, [54] studied the electrochemical behavior of marmatite at carbon

paste electrode in the absence and presence of bacterial strains. A. Radi [55] studied the

electro oxidation of nifiirooxazide by cyclic and differential pulse voltammetry at carbon

paste and sephadex modified carbon paste electrodes. A. Radi [56] studied the

electrochemical oxidation of nicergoline in Britton Robinson Buffer by using cyclic and

differential pulse voltammetry at carbon paste electrode. M.S.P. Francisco et al, [57]

studied a carbon paste electrodes of Si02/Nb205 material were used as the electrode in the

development of a dissolved dioxygen sensor in 1 M KCl solution at pH 6.2. The material

was prepared by sol-gel method. Its electrochemical properties were investigated by

linear, cyclic voltammetric and chronoamperometric techniques. S.A. John and R.

21

===^=^===^===^^^=^^^=^=^=^=^= Cfiapter-1 ^ = ^ = = ^

Ramaraj [58] studied the polarity of microenvironments with in a nafion film by

electrochemical and photo electrochemical techniques using a phenothiazine dye,

thiamine as a probe. D. Ekinic et al, [59] studied the anodic oxidation of 2-amino-3-

cyano-4 naphthyl thiophine by cyclic voltammetry and UV-Vis-NIR absorption

spectroscopy at platinum disc electrode. B.Piro et al, [60] studied the DNA hybridization

transduction behavior of a quinone containing electro active polymer by cyclic

voltammetry and electrochemical impedance spectroscopy at glassy carbon electrode. F.

Gokmese et al, [61] studied the electrochemical reduction mechanism of l-[N-(2-

pyridyl) aminomethylidene]-2(lH)-naphthalenone (PN) was investigated by using

various electrochemical techniques in O.IM tetra butyl ammonium tetrafluoroborate in

acetonitrile at glassy carbon electrode.

A.R. Fakhari et al, [62] studied the electrochemical oxidation of catechol, 3-

methyl cathechol and 3-methoxy catechol in the presence of 4,6-dihydroxy-2-methyl

pyrimidine as a neucleophile in aqueous solution using cyclic voltammetry and controlled

potential coulometry at glassy carbon disc electrode. X. Lui et al, [63] investigated an

electrochemical interaction of surfactant with a supported bilayer lipid membrane on a

glassy carbon electrode using 0.2M NaHCOs as the supporting electrolyte. J.Obirai

et al, [64] studied the electrochemical oxidation of phenol and its derivatives using poly-

nickel hydroxyl tetraphenoxy pyrrole phthalocyanine modified vitreous carbon

electrodes. R.Ojani et al, [65] studied the electro catalytic oxidation of some

carbohydrates by usmg poly (l-naphthylamine)/nickel modified carbon paste electrode.

M.L. Calvo-Munoz et al, [66] studied the post-polymerization functionalization of a poly

(N-substituted pyrrole) film, the P-ferrocene ethylamine used as redox probe which was

immobilized via a chemical coupling on the surface of a preformed poly pyrrole film. P.

22

^ = = ^ = ^ = ^ chapter-1 = = ^ = ^

Manishankar et al, [67] studied the electro catalytic reduction of molecular dioxygen by

1,4-naphthoquionens at glassy carbon electrode using riboflavin as the electron transfer

mediator.C.L. Forryan et al, [68] studied the electrochemical reductions of 4-nitrophenol,

2-cyanophenol and 4-cyanophenol in N,N-dimethyl formamide at gold electrode using

0.2M tetra butyl ammonium per chlorate using voltammetry. Z. Zheing et al, [69] studied

the poly (3-phenylthiophene) films electrochemically synthesized with oxidation of 3-

phenylthiophene using boron tri fluoride diethyl etherate as a supporting electrolyte at

platinum electrode. M.S. Ureta-Zanartu et al, [70] studied the electrochemical behavior

of 1-octanol-P-cyclodextrin in 0.5 M HCIO4 at platinum electrodeposited on an Au/quartz

crystal (pt/Au/q) by cyclic voltammetry, the electrochemical quartz crystal microbalance

(EQCM) and impedance measurements. D. Nematollahi and M. Hesari [71] studied the

electro oxidation of 3- substituted catechols in the presence of dibenzyl amine in water

and acetonitrile (90/10) solution, using electrochemical and spectroelectrochemical

methods in 0.2M phosphate buffer (pH 7) at glassy carbon disc electrode. J.A.P. Piedade

et al, [72] studied the voltammetric behavior of oligonucleotide lipoplexes adsorbed on

to glassy carbon electrode. G.D. Allen [73] studied the electrooxidation of bromide in

acetonitrile and the room temperature ionic liquid, l-butyl-3-methylimidazolium bis

(trifluoromethyl sulfonyl) imide at platinum electrodes. M.A.La-Scalea et al, [74] studied

the voltammetric behavior of metronidazole at mercury electrode. C.R. Raj and S.Behera

[75] studied the voltammetric and Faradic impedance analysis using [KsFe (CN) 6 ] redox

marker with O.IM KCl as a supporting electrolyte. H.C. Kosheiry et al, [76] studied the

electrochemical spectroscopic properties of a new family of stable radical cations based

on 2(3H)-Thiazolone Azine at platinum electrode using O.IM lithium

bis(trifluoromethane) sulfonamide as a supporting electrolyte. Polymer modified

23

electrodes (PMEs) have received great attention in recent years, as the polymer film have

good stability, reproducibility, more active sites, homogeneity in electrochemical

deposition and strong adherence to the electrode surface. Chuneya Li [77] worked on

voltammetric determination of tyrosine based on chemically electropolymerisation of L-

serine. Rui Zhang et al, [78] published work on poly (acid chrome blue K) modified

GCE by electropolymerisation and achieved selective separation of dopamine, ascorbic

acid and uric acid in real sample of human urine. Yong Xin Li et al, [79] worked on

simultaneous electro analysis of dopamine ascorbic acid by poly (vinyl alcohol) modified.

Yuzhong Zhang et al, [80] determined dopamine in presence of ascorbic acid by poly

(amidosulfonic acid) modified GCE. Xing-Yuan Liu et al, [81] electropolymersied/7o/>'

(carmine) on GCE for detection of parathion. Tae-Hun et al, [82] worked on

electrochemical preparation oipoly (p-phenyl vinylene) in aceto nitrile. A.M. Yu et al,

[83] worked on catalytic oxidation of uric acid at poly (glycine) modified electrode and

its trace determination. Y. Zhang et al, [84] determined dopamine in presence of ascorbic

acid using poly (acridine-red) on modified glassy carbon electrode. M. Zhao et al, [85]

determine by electropolymerisation oipoly (2-picolinic acid) on modified. L. Zhang et

al, [86] studied for simultaneous determination of uric acid and ascorbic acid with

modified poly (glutamic) acid. T. Selvaraju et al, [87] worked on simultaneous detection

of dopamine and serotonin in presence of ascorbic acid and uric acid at poly (o-

phenyldiamine) electrode.

1.9. Objective and Scope of the Thesis

The present thesis is aimed at investigating the electrochemical studies of

bioactive molecules, dopamine, ascorbic acid, uric acid, Epinephrine, clozapine and also

potassium ferrocynide as standard.

24

Dopamine is the important neurotransmitter catecholamine which is responsible

for the central nervous system of the human brain. In order to determine the

concentration of the dopamine, here an investigation is carried out and fabricated a

chemically modified carbon paste electrode for the detection of DA in the AA, EP and

UA mixture.

Electropolymerisation of some selected active substrate, such as Vanillin, Aniline

blue, Rosaniline, Tannic acid, Maleic acid. Nicotinic acid and its application in

simultaneous determination of dopamine in ascorbic acid, epinephrine and uric acid

mixture solution. The chemicals like Lamotrigine, Eperisone, Phthalic acid, Mannitol,

Salicylic acid, Carbon nanotube, CTAB, SDS, TX-lOO Surfactants are also used as

component in the carbon paste electrode as modifiers.

1.9.1 Objectives of the investigation

1. Preparation of carbon paste electrode.

2. To calibrate the bare carbon paste electrode for the determination of electro active

species.

3. Effect of modifier and its concentration.

4. Effect of electro polymerization to determine electro active species.

5. To optimize the effect of scan rate, concentration, pH, number of cycles, modifiers,

surfactants at bare and modified carbon paste electrode.

6. Effect of interferants.

7. Simultaneous determination of dopamine, ascorbic acid, Epinephrine and uric acid

at bare and modified carbon paste electrode.

25

Cfiapter-1

—ve

^switch E p a / E p c ^ ^

_ 3 ' y f ^ ^ ^_>i "

Ei

_ 3 ' y > L-' i

f

Ei

7 L-' i

f

Ei Ei

-+-ve -ve

Fig. 1. Typical cyclic voltammogram

26

1.10. References

[I] A. E. Kaifer, M. Gamez-Kaifer, Supramolecular Electrochemistry, Wiley-VCH,

New York, 1999.

[2] J. Wang, Analytical Electrochemistry, VCH Publishers Inc. New York, 1994.

[3] P. Delahay, New Instrumental Methods in Electrochemistry, Interscience New

York, 1954.

[4] J. Heyrovskey, Chem. Listy. 16 (1922) 256.

[5] A.J. Bard and H. Lund, Encyclopedia of the Electrochemistry of the Elements,

Marcel Dekker, New York, 16 (1973).

[6] H.A. Laitinen and I.M. Kolthoff, J. Phys. Chem. 45 (1941) 1979.

[7] J.W. Ross, R.D. Demars and I. Shain, Analyt. Chem. 28 (1956) 1768.

[8] R.S. Nicholson and Sham, Anal. Chem. 36 (1964) 706.

[9] W. Kemula, in Advances in Polarography, I- Langmuir. Pergamon Press, New

yorit. 1(1960)105.

[10] J. Riha, in Progress in Polarography, P. Zuman, Interseience, New York. 2 (1962)

17.

[II] L.A. Matheson and N. Nicholas, Trans. Electrochem. Soc. 73 (1938) 193.

[12] J.E.B. Randies, Trans. Faraday Soc. 44 (1948) 327.

27

^==^ CAapter-l = = ^ =

[13] A. Seveik, Collect. Czech. Chem. Commm. 13 (1948) 349.

[14] P. Delahay, J. Am. Chem. Soc. 75 (1953) 1190.

[15] H. Matsuda and Y. Ayabe, Z. Elektrochem. 59 (1955) 494.

[16] H.Matsuda, Z. Elektrochem. 61 (1957) 489.

[17] J.T. Maloy, J. Chem. Educ. 60 (1983) 285.

[18] P.T. Kissinger and W.R. Hemneman, J. Chem. Ed. 60 (1983) 702.

[19] D.H. Evans, K.M. O'Connell, R.A. Petersen and M.J. Kelly. J. Chem. Educ. 60

(1983)290.

[20] G.A. Mabbott. J. Chem. Educ. 60 (1983) 697.

[21] R. Greef, R. Peat, L.M. Peter, D. Pletcher and J. Robinson, Instrumental Methods

in Electrochemistry, John Wiley. New York 1985.

[22] A.J. Bard and L.R. Faulkner, In Electrocheinical Methods Fundamentals and

Applications, Wiley New York 1980.

[23] D.R. Henton, R.L. McCreery and J.S. Swenton, J. Org. Chem. 45 (1980) 369.

[24] J.M. Bobbit and J.P. Wills, J. Org. Chem. 45 (1980) 1978.

[25] S.F. Nelsen, C.R. Kessal, R. Carl, D.J. Brien and F. Weinliok . J. Org Chem. 45

(1980)2116.

[26] M.J. Powers and T.J. Meyer, J. Am. Chem. Soc. 102 (1980) 1289.

28

[27] Kalyanasundaram, K. Kiwi, J. & Gratzel, M Helv. Chim. Acta . 61 (1978) 2720.

[28] C.A. Rice and J.T. Spence, Inorg. Chem. 19 (1980) 2845.

[29] R.S.Nicholson,^«a/.CAe/w. 37(1965)1351.

[30] R.S. Nicholson and I. Shain, Anal. Chem. 37 (1965) 190.

[31] Heyrovsky and Zuman, Practical Polarography, Academic press, London, New

York, 1968.

[32] Christos, G. Kontoyannis, G. S. Antimisiaris and D. Douroumis . Anal. Chim.

Acta. 391 (1999) S3.

[33] R. Inam , G. Somer, Talanta. 46 (1998) 1347.

[34] P. J. S. Barbeira and N. R. Stradiotto, Talanta. 44 (1997) 185.

[35] J. J. Berzas, J. Rodriguez, G. Castaneda and M. J. Villasefior, Anal. Chim. Acta.

417(2000)143.

[36] R. Reinke , J. Simon, Anal. Bioanal. chem. 374 (2002) 1256.

[37] C. Colombo, C. M.G. Van den Berg, Anal. Chim. Acta. Zll (1998) 229.

[38] A.J. Bard, L. R. Faulkner, Electrochemical Methods, Fundamentals and

Applications, T^ Ed., John Wiley & Sons, Hoboken, NJ. (2001) 376

[39] J. E. B. Randies, Trans. Faraday Soc. 44 (1948) 327.

29

- CHapter-l =

[40] P.A. Christenson, A. Hamnet, Techniques and Mechanisms in Electrochemistry,

P'Ed., Blackie Academic and Professional, London, 1994.

[41] R.G. Compton, C.E. Banks, Understanding Voltammetry, World Scientific

Publishing Co., Singapore, 2007.

[42] R. S. Nicholson, Anal. Chem. 37 (1965) 1351.

[43] P.M. Hawkridge in: P.T. Kissinger and W.R. Heineman (Eds.), Laboratory

Techniques in Electroanalytical Chemistry, Marcel Dekker Inc., New York, 1996.

[44] E. R. Brown, R. F. Large in: A. Weissberger and B. Rossiter (Eds.), Physical

Methods of Chemistry. Electrochemical Methods, Wiley-Interscience, New York,

1(1971).

[45] R.S. Nicholson and L Shain, Anal. Chem. 36 (1977) 706.

[46] D.H. Evans, Ace. Chem. Res. 10 (1997) 313.

[47] W.R. Heineman and P.T. Kissinger, Am. Lab. 14 (1982) 29.

[48] D.H. Evans, K.M.O. Connell, R.A. Peterson and M.J. Kelly, J. Chem. Edu., 60

(1983) 290.

[49] P.T. Kissinger and W.R. Heinman, J. Chem. Educ. 60 (1983) 702.

[50] K.H. Lubert, M. Wagner and R.M. 01k, Anal. Chim. Acta. 336 (1996) 77.

[51] S.B. Khoo and S.X. Guo, J. Electroanal. Chem. 465 (1999) 102.

30

- CHapter-l ^

[52] S. Majid, M.E. Rhazi, A. Amine, A. Curalli and G. Palleschi, Michrochim. Acta.

143 (2003) 195.

[53] Damien and W.M. Arrigan, Analyst. 119 (1994) 1933.

[54] Shao-Yuan Shi, Zhao-Heng Fang and Jin-Ren Ni, Bioelectrochem.68 (2006) 113.

[55] A. Radi, Fresenium, J. Anal. Chem. 364 (1999) 590.

[56] A. Radi, Michrochim. Acta. 132 (1999) 49.

[57] M.S.P. Fancisco, W.S. Cardoso and Y. Gushikem, J. Electroanal. Chem.SlA

(2005)291.

[58] S.A. John and R. Ramaraj, J. Electroanal. Chem. 561 (2004) 119.

[59] D. Ekinic, F. Turner and U. Demir, J. Electroanal. Chem. 562 (2004) 167.

[60] B. Piro, J. Haccoun, M.C. Pham, L.D. Iran, A. Rubin, H. Perrot and C. Gabrielli,

J. Electroanal. Chem. 511 (2005) 155.

[61] F. Gokmese, E. Gokmese, A.O. Solak, M. Isiklan and Z. Kilic, J. Electroanal.

Chem. 581 (2005) 46.

[62] R. Fakhari, D. Nematollahi, A.B. Moghaddam, J. Electroanal. Chem. 577 (2005)

205.

[63] X. Liu, W. Huang, E. Wang, J. Electroanal. Chem. 577 (2005) 349.

[64] J. Obirai, F. Bedioui and T. Nyokong, J. Electroanal. Chem. 576 (2005) 323.

31

= ^ = ^ = = ^ = ^ = = = ^ = = ^ = = ^ = = ^ = ^ = ^ = ^ = ^ = ^ Cfiapter-1 ^^^==^

[65] R. Ojani, J. Raoof and P.S. Afagh, J. Electroaml Chem. 571 (2004) 1.

[66] M.L. Calvo-Munoz, B. Ehui-Avo Bile, M. Billon and G. Bidan, J. Electroanal.

CAew. 578(2005)301.

[67] P. Manisankar, A.M. Pushpalatha, S. Vasanthkumar, A. Gomathi and S.

Vishwanathan, J. Electroanal. Chem. 571 (2004) 43.

[68] C.L. Forryan, N.S. Lawrence, N.V. Rees and R.G. Compton, J. Electroanal.

Chem. 561 (2004) 53.

[69] Zhang and G. Shi, J. Electroanal. Chem. 569 (2004) 197.

[70] M.S. Ureta-Zanartu, C. Yanez and C. Gutierrez, J. Electroanal. Chem. 569 (2004)

275.

[71] D. Nematollahi and M. Hesari, J. Electroanal. Chem. 577 (2005) 197.

[72] J.A.P. Piedade, M. Mano, M.C. Pedroso deLima, T.S. Oretskaya and A.M.

Oliveire-Brett, J. Electroanal. Chem. 564 (2004) 25.

[73] G.D. Allen, M.C. Buzzeo, C. Villargran, C. Hardacre and R.G. Compton, J.

Electroanal. Chem. 575 (2005) 311.

[74] M.A. La-Scalea, S.H.P. Serrano and I.G.R. Gutz, J. Braz. Chem. Soc. 2 (1999)

127.

[75] C.R. Raj and S. Behera, J. Electroanal. Chem. 581 (2005) 61.

32

=1==^===^=^=^=^=^=======^=^ chapter-1 ^ =

[76] H.C. Kosheiry, A. Hammami, B. Garcia, Y.A. Lebdeh, P.J. Alarco, A.

Abowmrane and M. Armand, Bull. Chem. Soc. Jpn. 11 (2004) 1843.

[77] Chunya Li, Coll and Surf. B: Biointerface. 50 (2006) 147.

[78] Rui Zhang, Gen-DinJin, Da Chin and Xiao-Ya Hu, Sensor and Actuator B. 138

(2009) 174.

[79] Yong Xin Li and Xiangqin Lin, Sens and Actuator B. 115 (2006) 134.

[80] Yuzhong Zhang, Guiying Jin, Zhoushing Yang and Hong Zhao, Microchem. Acta.

147 (2004) 225.

[81] Xing-Yuan Liu, Chun-Ya Li and Shengshui Hu, Microchem. Acta. 154 (2006)

275.

[82] Tae-Hun Kim and Su-Moon Park, Electrochim. Acta. 50 (2005) 1461.

[83] A.M Yu, H. Zhang and H.Y. Chin, Analyst. Vll (1997) 839.

[84] Y. Zhang, G. Jin, Y.Wang and Z.Yang, Sensors. 3 (2003) 443.

[85] M. Zhao, Y.Z. Zhang and Z.B. Yuan, Analyst. 126 (2001) 358.

[86] L. Zhang and X. Lin, Analyst. 126 (2001) 367.

[87] T. Selvaraju and R. Ramaraj, J. Appl. Electrochem. 33 (2003) 759.

33