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It is the only physical theory of universal content concerning which

I am convinced that, within the framework of the applicability of

its basic concepts, it will never be overthrown.

-Albert Einstein

CHAPTER-I

GENERAL INTRODUCTION

Chapter – I

1

1.1. KINETIC STUDIES

Chemical kinetics, also known as reaction kinetics, is the study of rates

of chemical processes. Chemical kinetics deals with the experimental

determination of reaction rates from which the rate laws and rate constants are

derived. Kinetic studies are receiving much importance in the recent years

since they provide us most powerful methods of investigating the detailed

reaction mechanisms. It is one of the most intriguing and challenging areas of

chemistry, which deals mechanisms of reactions. To many chemists the real

heart of chemistry is the study of mechanisms. Thus, chemical kinetics can be

defined as the branch of chemistry concerned with the study and prediction of

time dependent systems. To understand the mechanism of any reaction we must

know a reaction as a function of time, the exact positions of all the atoms as the

reactants are converted into product molecules. Virtually all information

regarding reaction mechanism comes by inference of indirect evidence. Hence,

it is the important job of chemists to device the proper experiments to generate

most conclusive evidences.

At a more fundamental level we want to understand what happens to

the molecules in a chemical reaction – that is what happens in a single reactive

encounter between two reagent molecules. By understanding this we may be

able to develop theories that can be used to predict the outcome and rate of

reactions.

One more reason for the importance of this subject is that it provides the

Chapter – I

2

information which is mandatory to arrive at the mechanism of a reaction. The

order of a reaction can be used to interpret the reaction on molecular level.

Chemists propose reaction mechanism by predicting the sequence in which

bonds break and atoms rearrange during the reaction by considering the order

of a reaction with respect to different reactive species. Virtually all information

regarding reaction mechanism comes by implication of indirect evidence.

Hence, it is the important job of chemists to device the proper experiments to

generate most conclusive substantiation. The crucial steps in any kinetic

investigation1 are: (1) product and intermediate detection, (2) concentration

determination of all species present, (3) deciding on a method of following the

rate, (4) the kinetic analysis and (5) determination of the mechanism.

Electron transfer reactions play a significant role in physical, chemical

and biological processes. Because of the ubiquity of electron transfer processes,

the study of electron transfer reactions, perhaps more so than that of any other

area of chemistry is characterized by a strong interplay of theory and

experiment2. The importance of electron transfer in transition metal redox

chemistry and in organic chemistry are well documented3,4

.

The award of Nobel prize for the year 1992 to Prof. R. A. Marcus on

“Electron Transfer Reactions”, Nobel prize for the year 1999 to Prof. Ahmed

Zewail for his discovery in “Femtochemistry”, Nobel prize for the year 2001 to

Profs. William Knowles, K. Barry Sharpless and Royji Noyori for their work

on “Chirally Catalysed Hydrogenation Reactions” and Nobel Prize for the year

2005 to Profs. Robert Grubbs, Richard Schrock, and Yves Chauvin on their

Chapter – I

3

research achievements on “Metathesis Catalyst Technology” emphasize the

importance of field of reaction kinetics.

The work of Henry Taube5 in redox systems unequivocally

demonstrated the transport of electron from reductant to oxidant. This

discovery certainly added many important features in the syntheses of

coordination complexes and organometallics. An oxidation process is always

accompanied by a reduction process6; such reactions are called redox reactions.

Therefore, oxidation-reduction reaction needs at least two reactants, one

capable of gaining electrons (oxidant) and the other capable of losing electrons

(reductant), i.e., a reducing agent (reductant) by losing electrons, gets oxidised

and an oxidising agent (oxidant) by gaining the electrons, gets reduced. Redox

reactions are the basis for numerous biochemical pathways and cellular

chemistry, biosynthesis, and regulation7.

1.1.1. Oxidation-reduction in organic reactions

The oxidation-reduction concepts, however, are not so clearly applicable

in organic chemistry, when carbon compounds are oxidized their component

atoms are very seldom derived of their surrounding complete electron shells.

Covalent bond fission is an essential feature of organic reactions and it can be

affected by two different pathways8, viz., “Homolytic reactions” in which

electron pairs are symmetrically disrupted and “Heterolytic reactions” in

which electron pairs are transferred from one molecule to another as an

Chapter – I

4

undivided entity. Electron removal by these two pathways has clearly

distinguishable characteristics.

1.1.2. Oxidation-reduction in inorganic reactions

Two general classes of transition states emerge for redox reactions

involving metal complexes, the so called outer-sphere and inner-sphere

types. In outer-sphere electron transfer, the bimolecular transition state is

traversed with the separate coordination spheres of both the electron donor and

the electron acceptor essentially intact, whereas, in the inner-sphere, the

unimolecular (collapsed) transition state typically results from the mutual

interpenetration of coordination spheres via a critical bridging ligand. From

Franck-Condon principle, it follows that before electron transfer between two

ions is possible, the energy of the electron must be the same in two sites. There

must also be sufficient orbital overlap between the two sites to provide for a

reasonable probability of a transfer.

1.1.3. Probable ways of electron transfer reactions

Oxidation-reduction reaction may involve one or more electron transfer.

Depending upon the number of electrons transferred between oxidant and

reductant, the reaction may proceed in one or more steps. Transition metals

such as iron and cobalt and several others usually exhibit stable oxidation states

differing by one electron and react with each other through one equivalent

steps. However, the stable oxidation states in post transition elements such as

arsenic, antimony etc differ by two electrons. Thus, on the basis of their pattern

Chapter – I

5

of reactivity, the reactions of these elements are classified into two main

categories5,9

.

Two main types of electron-transfer reactions10,11

are “Complementary

reactions”12

in which the oxidant and reductant change their oxidation state by

an equal number of units, and “Non-complementary reactions”13

in which the

oxidant and the reductant change their oxidation states by a different number of

units.

Electron transfer reactions are governed by two classical principles:

a) Michaelis principle of compulsory univalent oxidation steps14

b) Shaffer’s principle of equivalent change15

Atom or group transfer10

is possible in aqueous solutions, rather than

electron transfer occurring in a redox reaction. In general, transfer of a positive

group or atom is equivalent to the transfer of electrons and transfer of a

negative group or atoms is equivalent to the taking up of electrons. The

problem, then, in studying the mechanism of an oxidation–reduction reaction is

to find out whether atom transfer or electron transfer occurs, which atoms are

transferred, and what intermediate species are formed. A complete study would

include a detailed picture of the transition state for all steps involved. Not only

the composition, but also the geometry of the transition state is desired.

1.1.4. Unstable oxidation states

The formation of unstable oxidation state during the course of non –

Chapter – I

6

complimentary reactions has been now anticipated in a number of such

reactions with sufficient proofs. For example, the reductions of thallium(III) by

iron(II)16

, vanadium(III) or vanadium(IV)17,18

and chromium(VI) by

thallium(I)19

can only be explained through the formation of unstable

thallium(II) species. Similar unstable oxidation states have been observed in

other studies20,21

. The interconversions between chromium(III) and

chromium(VI) always appear to involve the unstable states, chromium(IV) and

chromium(V). A number of studies of the catalysis by platinum metals of

oxidation reactions have been made22

. The catalysis by Ag(I)23

, Cu(II)24

,

Mn(III)25

and Cr(III)26

in oxidation- reduction reactions are also found to occur

through formation of unstable oxidation states.

1.1.5. Active species

If a particular substance (oxidant, reductant or catalyst) is capable of

existence in several forms in aqueous solution, all the species existing may not

be active. Those species, which are involved in a slow step, will influence the

reaction. The reaction conditions will determine the nature of the active

species27

.

1.1.6. Catalysis

Any substance, other than reactants which influences the rate of

chemical reaction and itself remains unchanged chemically as the end, is called

a catalyst. The phenomenon of rate alteration is designated as catalysis.

Chapter – I

7

Catalysts influence the reactions by changing the reaction path. Such catalytic

influences arise as consequences of lowering of the energy of activation.

A catalyst can be used over and over with no apparent loss to the

catalyst; although in reality there is some loss due to secondary reactions.

Though, the mechanism of catalysis depends on the nature of the substrate,

oxidant and other experimental conditions, it has been shown that metal ion

acts as catalyst by different pathways28

. It is known that Os(VIII)29

, Pd(II)30

,

Cr(III)31

, Ru(III)32

, V(V)33

etc., can cause appreciable rate accelerations in

various reactions.

Catalysts are widely used in nature, in industry, and in the laboratory,

and it is estimated that they contribute to one-sixth of the value of all

manufactured goods in industrialized countries. Many of the synthetic

chemicals are produced directly or indirectly by catalysis. Catalysts play a

steadily increasing role in achieving a cleaner environment. In addition to their

economic importance and contribution to the quality of life, catalysts are

interesting in their own right: the subtle influence a catalyst as reagents can

completely change outcome of the reaction.

1.1.7. Applications of kinetics

The applied chemist uses kinetics to devise new and/or better ways of

achieving desired chemical reactions. This may involve improving the yield of

desired products or developing a better catalyst. The mathematical models that

describe chemical reaction kinetics provide chemists and chemical engineers

Chapter – I

8

with tools to better understand and describe chemical processes such as food

decomposition, microorganism growth, stratospheric ozone decomposition, and

the complex chemistry of biological systems. These models can also be used in

the design or modification of chemical reactors to optimize product yield, more

efficiently separate products, and eliminate environmentally harmful by-

products.

1.2. DEVELOPMENT OF ANALYTICAL METHODS

Analytical chemistry is the science of making quantitative

measurements. In practice, quantifying an analyte in a complex sample

becomes an exercise in problem solving. To be efficient and effective, an

analytical chemist must know the tools that are available to tackle a wide

variety of problems. For this reason, analytical chemistry courses are often

structured along the lines of the analytical methods (the tools-of-the-trade).

Understanding the analytical toolbox requires a scientist to understand

the basic principles of the analytical techniques. With a fundamental

understanding of analytical methods, a scientist faced with a difficult analytical

problem can apply the most appropriate technique(s). A fundamental

understanding also makes it easier to identify when a particular problem cannot

be solved by traditional methods, and gives an analyst the knowledge that is

needed to develop creative approaches or new analytical methods.

Chapter – I

9

1.2.1. HIGH PERFORMANCE LIQUID CHROMATOGRAPHY

STUDIES,

High Performance Liquid Chromatography (HPLC) is one mode of

chromatography, one of the most used analytical techniques. Chromatographic

process can be defined as separation technique involving mass-transfer

between stationary and mobile phase. HPLC utilises a liquid mobile phase to

separate the components of a mixture. The stationary phase can be a liquid or a

solid phase. These components are first dissolved in a solvent, and then forced

to flow through a chromatographic column under a high pressure. In the

column, the mixture separates into its components. The amount of resolution is

important, and is dependent upon the extent of interaction between the solute

components and the stationary phase. The stationary phase is defined as the

immobile packing material in the column. The interaction of the solute with

mobile and stationary phases can be manipulated through different choices of

both solvents and stationary phases. As a result, HPLC acquires a high degree

of versatility not found in other chromatographic systems and it has the ability

to easily separate a wide variety of chemical mixtures (Fig.I (i)).

Figure I. (i) HPLC chromatogram showing ability to easily separate a wide

Chapter – I

10

variety of chemical mixtures

1.2.2. Theory

HPLC is a dynamic adsorption process. Analyte molecules, while

moving through the porous packing beads, tend to interact with the surface

adsorption sites. Depending on the HPLC mode, the different types of the

adsorption forces may be included in the retention process: Hydrophobic (non-

specific) interactions are the main ones in reversed-phase (RP) separations.

Dipole-dipole (polar) interactions are dominant in normal phase (NP)

Ionic interactions are responsible for the retention in ion-exchange

chromatography.

All these interactions are competitive. Analyte molecules are competing with

the eluent molecules for the adsorption sites. So, the stronger analyte molecules

interact with the surface. The weaker the eluent interaction, the longer the

analyte will be retained on the surface.

SEC (Size-Exclusion Chromatography) is another case. It is the separation of

the mixture by the molecular size of its components.

The basic principle of SEC separation is that the bigger the molecule, the less

possibility there is for it to penetrate into the adsorbent pore space. So, the

bigger the molecule the less it will be retained.

Chapter – I

11

1.2.3. Importance of HPLC in pharmaceutical analysis

HPLC provides reliable quantitative precision and accuracy, along with

a linear dynamic range sufficient to allow for the determination of the active

pharmaceutical ingredients and related substances in the same run using a

verity of detectors, and can be performed on fully automated instrumentation.

HPLC provides excellent reproducibility and is applicable to a vide array of

compound types by judicious choice of HPLC column chemistry. Separation

of chiral molecules into their respective enantiomers is possible by HPLC.

This involves precolumn derivatisation to form diasteriomers or addition of

the derivatization reagents to the chromatographic mobile phase to form

dynamic diastereomers during the separation proceses.

Alternatively, special columns prepared with cyclo dextrins or specific

chiral moieties as stationary phase may be used.

1.2.4. Advantages of HPLC

For accurate quantative measurements.

Repetitive and reproducible analysis using the same column.

Automation of the analytical procedure and data handling.

Adsorption, partition, ion-exchange and exclusion column separations

are excellently made.

Both aqueous and non-aqueous samples can be analyzed with little or

no sample pretreatment.

A variety of solvents and column packing are available, providing a

Chapter – I

12

high degree of sensitivity for specific analysis.

Provides a means for determination of multiple components in a single

analysis.

1.2.5. VOLTAMMETRIC STUDIES

Electrochemistry is a well established discipline that has encompassed

both applied and fundamental aspects of chemistry for nearly a century.

Voltammetric technique is concerned mainly with the measurement of

electrical quantities, such as current, potential, or charge, and their relationship

to the chemical parameters. In contrast to many chemical measurements that

involve homogeneous bulk solutions, electrochemical processes take place at

the electrode-solution interface. The distinction between various voltammetric

techniques reflects the type of electrical signal used for the quantification.

Historically, the branch of electrochemistry we now call voltammetry

developed from the discovery of polarography34

in 1922 by the Czech Chemist

Jaroslav Heyrovsky, for which he has been awarded by the 1959 Nobel Prize in

Chemistry. Voltammetry refers to the measurement of current that result from

the application of potential. However, in the 1960s and 1970s significant

advances were made in all areas of voltammetry (theory, methodology, and

instrumentation), which enhanced the sensitivity and expanded the repertoire of

analytical methods. The coincidence of these advances with the advent of low-

cost operational amplifiers also facilitated the rapid commercial development

of relatively inexpensive instrumentation. Unlike potentiometric measurements,

Chapter – I

13

which employ only two electrodes, voltammetric measurements utilize a three

electrode electrochemical cell.

1.2.6. How it works?

The electrochemical cell, where the voltammetric experiment is carried

out, consists of a working (indicator) electrode, a reference electrode, and

usually a counter (auxiliary) electrode. In general, an electrode provides the

interface across which a charge can be transferred or its effects felt. Because

the working electrode is where the reaction or transfer of electrons takes place,

whenever we refer to the electrode, we always mean the working electrode.

The reduction or oxidation of a substance at the surface of a working electrode,

at the appropriate applied potential, results in the mass transport of new

material to the electrode surface and the generation of a current. Even though

the various types of voltammetric techniques may appear to be very different at

first glance, their fundamental principles and applications derive from the same

electrochemical theory.

1.2.7. Voltammetric techniques and their theoretical aspects

The different voltammetric techniques that are used are distinguished

from each other primarily by the potential function that is applied to the

working electrode to drive the reaction and by the material used as the working

electrode. These can be described as follows;

Chapter – I

14

1.2.8. Linear sweep voltammetry

Linear sweep voltammetry (LSV) involves applying a linear potential

sweep to the working electrode while monitoring simultaneously the current

flowing in the circuit. This technique is based on a potential being ramped from

one potential to another. The potential sweep rate may vary from a few

millivolts per second through to several hundred volts per second. The current

is monitored throughout this potential sweeping - and if plotted with respect to

the potential, the current - potential profile is known as a voltammogram. If

the potential is swept from one potential to another and stopped, the technique

is known as linear sweep voltammetry.

The limiting current derived from a redox process in solution during LSV

may be used to quantitatively determine the concentration of electroactive

species in solution.

1.2.9. Square wave voltammetry

Among the various voltammetric techniques, exceptionally versatility is

found in a method called square wave voltammetry (SWV), which was

invented by Ramaley and Krause, and developed extensively by the

Osteryoungs and their co-workers. It’s a differential technique in which

potential waveform composed of a symmetrical square wave of constant

amplitude is superimposed on a base staircase potential35

. It is the plot of the

difference in the current measured in forward (if) and reverse cycle (ir), plotted

against the average potential of each waveform cycle. In this technique, the

Chapter – I

15

peak potential occurs at the E1/2 of the redox couple because the current

function is symmetrical around the potential36

. Main advantages of SWV are

excellent peak separation and high sensitivity.

1.2.10. Normal Pulse Voltammetry (NPV)

Pulse polarographic techniques are variants of the polarographic

measurement which try to minimize the background capacitive contribution of

current by eliminating the continuous varying potential ramp and replacing it

with a series of potential steps of short duration. In NPV, each potential step

begins at the same value and the amplitude of each subsequent steps increases

in small increments. The current measurement is made near the end of each

pulse, which allows time for the charging current to decay. The potential is

pulsed from an initial potential Ei. The duration of the pulse, t, is usually 1 to

100 m sec and the interval between pulses typically 0.1 to 5 sec. The resulting

voltammogram displays the sampled current on the vertical axis and the

potential to which the pulse is stepped on the horizontal axis.

1.2.11. Differential pulse voltammetry

Differential pulse voltammetry (DPV) technique was proposed by

Barker and Gardner37

. DPV can provide greater sensitivity and more efficient

resolution and differentiation of various species. This technique differs from

NPV because each potential pulse is fixed, of small amplitude (0.01 to 0.1).

Current is measured at two points from each pulse, just before the application

of the pulse and at the end of the pulse. The difference between the current

Chapter – I

16

measurements at these points for each pulse is determined and plotted against

the base potential. At potentials around the redox potential, the difference is

current reaches a maximum and decreases to zero as the current becomes

diffusion controlled. The current response is therefore a symmetric peak.

1.2.12. Cyclic voltammetry

Cyclic voltammetry (CV) is a dynamic method used for investigating

the electrochemical behavior of a system. It is most widely used for acquiring

quantitative information about electrochemical reactions. The power of cyclic

voltammetry results from its ability to rapidly provide considerable information

on the thermodynamics of redox processes, on the kinetics of heterogeneous

electron-transfer reactions, and on coupled chemical reactions or adsorption

processes. CV is often the first experimental approach performed in an

electroanalytical study, since it offers rapid location of redox potentials of the

electroactive species and convenient evaluation of the effect of media upon the

redox process38,39

.

CV is a potential sweep technique. Usually the potential is scanned back

and forth linearly with time between two extreme values – the switching

potentials using triangular potential waveform (Fig. I (ii)). To carry out an

oxidation process, a positive potential ramp is applied and the electroactive

species loses an electron at the electrode giving rise to an anodic peak current

(ipa) which usually gives an oxidation peak at a given potential (Epa). Similarly,

cathodic currents (ipc) are observed when the potential is applied in the negative

Chapter – I

17

direction leading to a reduction process, typically giving a reduction peak at a

given potential (Epc). The CV is usually initiated at a potential where species

are not electroactive. A typical voltammogram is shown in Fig I (ii).

Figure I (ii) (a) A cyclic voltammetry potential waveform with switching

potentials; (b) The expected response of a reversible redox couple during a

single potential cycle

One of the most important applications of cyclic voltammetry is for

qualitative diagnosis of chemical reactions that proceed or succeed the redox

process40

. The occurrence of such chemical reactions, which directly affect the

available surface concentration of the electroactive species, is common to

redox processes of many important organic and inorganic compounds. Changes

in the shape of the cyclic voltammogram, resulting from the chemical

competition for the electrochemical reactant or product, can be extremely

useful for elucidating these reaction pathways and for providing reliable

chemical information about reactive intermediates.

(a) (b)

Chapter – I

18

1.2.13. Electron transfer (ET) or charge transfer process

The electron transfer at the interface between the electrode and

electrolyte is central to an electrode reaction. Electroactive species having

moved from the bulk of the solution by either diffusion or under forced

convection enters in the electrical double layer, which is under direct influence

of the electrode. On entering the double layer the species undergoes a structural

orientation so that it can take up or give up electrons from or to the electrode

surface respectively with the least activation energy when a suitable potential is

applied and macroscopically, we observe current. This state of the reactant

species is known as transition state. Being unstable the species in transit state,

converts itself to final product by release of activation energy and gets reduced

or oxidized. This final product after undergoing suitable reorientation either

gets deposited on the electrode surface or moves away from the electrode

surface into the bulk solution. The transfer of electrons to or from the substrate

is an activated process. The electron transfer process can be “Reversible

process” in which the electron transfer is much faster than the mass transfer

(transport control), “Irreversible process” in which electron transfer is much

slower than the mass transfer (electron transfer control) and “Quasi-reversible

process” in which electron transfer and mass transport has comparable rates.

1.2.14. Electrodes

The advent of modern electrochemistry created the need for new

electrodes and electrode set-ups. The most common arrangement today is the

electrochemical cell with three electrodes:

Chapter – I

19

Working electrode

Reference electrode

Counter electrode

immersed in a solvent containing the analyte and also an excess of supporting

electrolyte. The choice of the solvent is dictated primarily by the solubility of

the analyte and its redox activity and by solvent properties41,42

. Supporting

electrolytes are required in controlled-potential experiments to decrease the

resistance of the solution and also to maintain constant ionic strength.

1.2.15. Working electrode

The ideal working electrode is very clean metal surface with a well-

defined geometry that is in direct contact with an electrochemical test solution.

Working electrodes intended for general purpose work are usually made from a

metal that is electrochemically inert over a wide range of potentials. The most

widely used metals are mercury, platinum, gold and various forms of carbon.

Solid metals are typically fashioned into disks surrounded by a chemically inert

shroud made from Teflon, glass or epoxy. Mercury, being a liquid, tends to be

used as a spherical droplet in contact with the solution. The working electrode

should provide high signal-to-noise characteristics, as well as a reproducible

response. Thus, its selection depends primarily on factors such as the redox

behavior of the target analyte, geometry of the electrode, potential window,

electrical conductivity, surface reproducibility, mechanical properties, cost,

Chapter – I

20

availability and the background current over the potential region required for

the measurement.

Mercury electrode

This is the most popular electrode makes use of liquid mercury as a

working electrode. In most common incarnation, the dropping mercury

electrode, a reservoir of mercury is allowed to slowly drain through a vertical

capillary tube immersed in the electrochemical test solution. As the mercury

slowly exits from the capillary; it forms a small drop with a nearly spherical

shape that is in contact with the test solution. Electroactive analytes in the test

solution undergo oxidation or reduction reactions at the surface of the drop.

Platinum electrode

Despite the expense associated with these precious metal, platinum is

one of the most widely used materials for fabricating working electrodes.

Platinum has the advantage of being an easily machined metal that is

electrochemically inert. In aqueous solvent systems, the platinum working

electrode is a good choice when working with positive potentials. Its primary

disadvantage is that it has limited use at negative potentials in aqueous

solutions. In rigorously anhydrous organic solvent systems, platinum is the best

and more popular choice for the working electrode material due to its wide

potential window in both the positive and negative directions.

Gold electrodes

Chapter – I

21

Gold working electrodes are designed along the same lines as platinum

working electrodes. Gold is usually less expensive than platinum, but it is not

as electrochemically inert. The surface of a gold electrode is subjected to

oxidation and such electrodes offer favorable electron-transfer kinetics and a

large anodic potential range43

.

Carbon electrodes

Various forms of carbon are used as working electrode materials44,45

.

Carbon electrodes are useful over a wide potential window in both the positive

and negative directions, and their principle advantage over platinum electrodes

is the ability to work at more negative potentials in aqueous solutions. Solid

carbon electrodes are usually made from glassy carbon or pyrolytic graphite,

both of which are fairly expensive materials that are more difficult to machine

than platinum or gold. The surface of a carbon electrode usually needs to be

polished quite frequently, and the surface sometimes has to be activated by

various empirical methods in order to obtain maximal performance from the

electrode. A less expensive carbon electrode can be fashioned using carbon

paste46

. A cylindrical recess is drilled into a Teflon shroud and an electrical

contact is placed in the back of the recess. Each time the electrode to be used,

the recess is packed with the paste that contains carbon particles and then the

paste is carefully polished to a smooth disk shaped surface.

Carbon electrodes are widely used because of their broad potential

window, low background current, rich surface chemistry, low cost, chemical

Chapter – I

22

inertness, and suitability for various sensing and detection applications.

Working with paste electrodes is more demanding as the paste can be gouged

inadvertently after being used.

Chemically modified electrodes

Chemically modified electrodes represent a modern approach to

electrode systems. These rely on the placement of a reagent onto the surface, to

impart the behavior of that reagent to the modified surface. Such deliberate

alteration of electrode surfaces can thus meet the needs of many

electroanalytical problems, and may form the basis for new analytical

applications and different sensing devices. These analytical applications and

improvements have been extensively reviewed47

.

1.2.16. Reference electrode

The potential of a working electrode in a voltammetry experiment is

always controlled with respect to some standard and that standard is the

reference electrode. The calomel electrode and the silver/silver chloride

electrode are most commonly used reference electrodes.

1.2.17. Counter electrode

In tradition two electrode cells that have only a working electrode and a

reference electrode, current is necessarily forced to flow through the reference

electrode whenever a measurement is made. If enough current flows through a

reference electrode, its internal chemical composition may be significantly

Chapter – I

23

altered, causing its potential to drift away from the expected standard value.

For this reason it is desirable to make electrochemical measurements without

current flowing through the reference electrode. The auxiliary (or counter)

electrode provides an alternate route for the current to follow, so that only a

very small current flows through the reference electrode. In most of the cases, a

coil of platinum wire is used as counter electrode.

Because current flows at auxiliary electrode, electrochemical processes

will occur there also. If the working electrode is reducing something, then the

auxiliary electrode must oxidize something, and vice versa. The products

generated at the auxiliary electrode, if allowed to diffuse to the working

electrode, may interfere with the experimental measurement. When this is a

problem, the auxiliary electrode is placed in a separate compartment containing

an electrolyte solution that is in ionic contact with the main test solution via a

glass frit. In most cases, however, the auxiliary can be placed right in the test

solution along with the reference and working electrodes.

1.2.18. Applications of voltammetry

Analytical chemists routinely use voltammetric techniques for the

quantitative determination of a variety of dissolved inorganic and organic

substances. Inorganic, physical, and biological chemists widely use

voltammetric techniques for a variety of purposes, including fundamental

studies of oxidation and reduction processes in various media, adsorption

processes on surfaces, electron transfer and reaction mechanisms, kinetics of

Chapter – I

24

electron transfer processes, and transport, speciation, and thermodynamic

properties of solvated species. Voltammetric methods are also applied to the

determination of compounds of pharmaceutical interest and, when coupled with

HPLC, they are effective tools for the analysis of complex mixtures. A number

of excellent voltammetric techniques are well described in the literature48,49

.

Recent advances in instrumental techniques, however, promise access to

molecular-level information about electrochemical systems. This exciting

development opens up essential new opportunities in fundamental and applied

science.

1.3. SYNTHESIS OF NANO-PARTICLES

Nanomedicine promises to use nanotechnology to treat diseases at the

cellular level using drug therapies that are targeted to malignant tissues. Over

the past decade, new nanoparticles have been developed for drug delivery,

imaging, the detection of biomarkers, and other diagnostic and therapeutic

applications. It has been shown that nanomaterial properties such as size and

surface charge can affect the dose of nanoparticles that is delivered into a

biological system.

There are number of techniques available50,51

to synthesize different

types of nonmaterial in the form of colloids, clusters, powders etc. In those

important methods are physical, chemical, biological, and hybrid method but

we have selected biological method.

Chapter – I

25

1.4. REFERENCES

1. M. R. Wright,

"An Introduction to Chemical Kinetics", John Wiley and Sons, Inc.,

New York, (2004)

2. J. J. Zuckerman,

“Inorganic Reactions and Methods”, Vol. 15, VCH Publishers, Florida,

(1986)

3. Sir. G. Wilkinson,

“Comprehensive Coordination Chemistry”, Vol. 1, Pergamon Press,

(1987) p.332

4. R. A. Sheldon and J. K. Kochi,

“Metal Catalysed Oxidation of Organic Compounds”, Academic Press,

New York, (1981)

5. H. Taube,

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i

SUMMARY OF THE PRESENT WORK

In the present thesis, some redox reactions in alkaline medium have been

studied. Reactions were followed conveniently by spectrophotometer in the UV-

visible region. Some analytical methods like cyclic voltammetry, HPLC method

and biosynthesis of Ag nano-particles were also done. The details of such studies

are given below.

The thesis is divided in to three parts and is presented in seven main

chapters including the general introduction.

I. General introduction

This chapter introduces about the kinetics, mechanism, analytical methods

and biosynthesis of nano-particles of reactions in general.

Part A: KINETICS STUDIES

II. Oxidation of D-Glucose by silver(III) periodate complex in presence of

Ru(III)/Os(VIII) as a homogeneous catalyst: a comparative mechanistic

Study

The kinetics of the oxidation of ruthenium(III) (Ru(III)) and osmium(VIII)

(Os(VIII)) catalysed oxidation of D-Glucose (D-Glu) by silver(III) periodate

complex (DPA) in aqueous alkaline medium at 298K and constant ionic strength

0.003 mol dm-3

was studied by spectrophotometrically. The reaction between DPA

and D-glucose in alkaline medium exhibits 1:2 Stoichiometry in both catalysed

reactions (D-Glu: DPA). The main products were identified as D-arabinonic acid

and formic acid by spot test and chromatoghraphic techniques. Probable

ii

mechanisms were proposed and discussed. The activation parameters with respect

to slow step of the mechanism were computed and discussed and thermodynamic

quantities were also calculated. It has been observed that the catalytic efficiency

for the present reaction is in the order of Os(VIII)>Ru(III). The active species of

catalyst and oxidant have been identified

III. Mechanistic investigations of uncatalysed and ruthenium(III) catalyzed

oxidation of D-mannitol by diperiodatoargentate(III) complex in

aqueous alkaline medium

The kinetics of oxidation of D-Mannitol(D-Manni) by

diperiodatoargentate(III) (DPA) both in the absence and presence of

ruthenium(III) catalyst in alkaline medium at 298 K and at a constant ionic

strength of 0.10 mol dm-3

was studied spectrophotometrically. The reaction

exhibits a 1:2 stoichiometry ([D-Manni]:[DPA] and first order in [DPA], less than

unit order in [D-mannitol] in both the cases. The alkali had retarding effect and

added periodate had no effect on the rate of the reaction in both the cases. The

order with respect to [Ru(III)] was unity. The main products were identified by

spot tests, FT-IR, LC-MS spectral studies. Based on the experimental results

possible mechanisms were proposed. The reaction constants involved in the

different steps of the mechanisms were evaluated. The catalytic constant (KC) was

also calculated for Ru(III) catalysis at different temperatures. The values of

activation parameters with respect to the catalyst have been evaluated. The

activation parameters with respect to slow step of the mechanisms were computed

iii

and discussed and thermodynamic quantities were also determined. Kinetic studies

suggest that the active species of DPA and ruthenium(III) were found to be

[Ag(H3IO6)2]- and [Ru(H2O)5OH]

2+ respectively.

Part B: DEVELOPMENT OF ANALYTICAL METHODS

IV). RP-HPLC Method for the determination of 5-Flurouracil in

pharmaceutical formulations and spiked human plasma

A simple, rapid and accurate reverse phase high-performance liquid

chromatographic method for the determination of 5-Flurouracil (5-FU) in

pharmaceutical formulations and human plasma samples has been developed and

validated. The assay of the drug was performed on a CLC C18 (5 μm, 25 cm ×

4.6 mm i.d.) with UV detection at 266 nm. The mobile phase consisted of

methanol-water mixture in the ratio of 98:2, and a flow rate of 1 ml/min was

maintained. The standard curve was linear over the range of 0.9-18.4 µg/ml

(r2=0.9966). Analytic parameters have been evaluated. Within-day and between-

day precision as expressed by relative standard deviation was found to be less than

2%. The method has been applied successfully for the determination of 5-FU in

spiked human plasma samples and pharmaceutical formulations. The method will

be useful for routine quality control analysis

V. Voltammetric oxidation and determination of 5-flurouracil and its analysis

in pharmaceuticals and biological fluids at glassy carbon electrode mediated

by surfactant cetyltrimethyl ammonium bromide

iv

The voltammetric oxidation and determination of 5-Fluorouracil 5-(FU)

was studied at a glassy carbon electrode (GCE) in the presence of cetyltrimethyl

ammonium bromide (CTAB) by cyclic and differential pulse voltammetry at pH-

7. The results indicated that the voltammetric responses of 5-Flurouracil are

drastically increased in the low concentration of CTAB, suggesting that CTAB

exhibits observable enhancement effect to the determination of 5-Flurouracil.

Under the optimal conditions the peak current was proportional to 5-Flurouracil

concentration in the range of 2.0 x 10-8

to 6.0 x 10-7

M with detection limit of

20.13nM by differential pulse voltammetry. The proposed method was applied to

the determination of 5-Fluorouracil in pharmaceuticals. The analytical

performance of this sensor has been evaluated for detection of analyte in human

serum and urine as real samples

VI. Studies based on the electrochemical oxidation of orphendrine

hydrochloride at gold electrode and its analytical applications

This work describes a novel type of working electrode for use in

voltammetric methods. The electrochemical oxidation of orphendrine

hydrochloride has been investigated for the first time by cyclic, linear sweep and

differential-pulse voltammetry at different pH at gold electrode. Cyclic

voltammetric studies were performed in a wide range of sweep rates and various

concentrations of orphendrine hydrochloride. The effects of surfactants were

studied. The anodic peak was characterized and process was adsorption-

controlled. The probable oxidation mechanism was proposed. According to the

v

linear relation between the peak current and the orphendrine hydrochloride

concentration, differential-pulse voltammetric method for the quantitative

determination in pharmaceuticals was developed. The linear response was

obtained in the range 0.1-20µM with a detection limit (LOD) of 1.27nM and limit

of quantification (LOQ) of 4.24nM under the physiological condition i.e. pH 7.0

The proposed method was successfully applied to orphendrine hydrochloride

determination in pharmaceutical samples and for the detection of orphendrine

hydrochloride in urine as a real sample.

Part C: SYNTHESIS OF Ag NANO-PARTICLES

VII. Biosynthesis, characterization and activity studies of Ag nano particles

by (Costus Ingneus) Insulin plant extract

Nanotechnology is receiving much importance in the present century due to

its capability of modulating metals in to their nanoparticles. Research in

nanotechnology highlights the possibility of their green chemistry pathways to

produce important nanomaterials. We report in this chapter on the biological

synthesis of silver nano-particles using Costus Ingneus extract and its activity

studies on antidiabatic, antibacterial and antifungal activities. Characterization of

newly synthesized silver nanoparticles was made using TEM and XRD studies.

The results showed that silver nanoparticles from Costus Ingneus extract showed

good antidiabatic activity than plant extract themselves.