Introduction

1

INTRODUCTION

(I) BIAMPEROMETRIC TITRATION

The concept of two electrode amperometry was first introduced

by Salomon1 in 1897 who carried out the argentometric titration of

potassium chloride using two polarisable silver electrodes. Revival of

bielectrode amperometry in fact took place after several years when

Foulk and Bawden2 in 1926 conducted a more extensive study of

titrations based on the measurement of current between two identical

electrodes maintained at a constant polarizing emf. These workers,

during iodine - thiosulphate titrimetry observed that when a small emf

(10-15 mV) was impressed between two platinum electrodes placed in

a cell containing iodine solution, the electrodes get depolarized and a

flow of current takes place which decreases gradually with the

progressive addition of thiosulphate solution from burette and finally

comes to a sudden stop with the complete reduction of iodine. This was

termed as the „dead stop‟ end-point method. Similarly, in the reverse

titration (i.e., thiosulphate with iodine), a sudden jump in the flow of

current was observed only after the completion of the oxidation of

thiosulphate which went on increasing regularly with the further

addition of iodine. This way of end-point detection was named as

Introduction

2

„kick-off‟ method. The indication of end-point either by disappearance

or appearance of current, as measured by galvanometer deflection, was

found to be more sensitive than the conventional visual method using

starch. Thus a continuous flow of current becomes possible only if

both the electrodes are maintained in a depolarized state. To explain

the end-point indication Foulk and Bawden2 pleaded that disruption in

the flow of current takes place due to the polarization of either or both

of the electrodes which in turn occurs due to the formation of thin

layers of hydrogen and oxygen on the cathode and anode respectively.

This explanation, however, ceased to be tenable as soon as Bottger and

Forche3 pointed out that the small emf actually applied by Foulk and

Bawden was inadequate for hydrogen formation on the cathode.

A more logical explanation for the electrochemical phenomenon

involved during this type of titration, now called the „biamperometric

titration‟, was given by Delahay4 in 1950 from the study of the

corresponding polarization curves of the systems involved in the

titration. According to him, if a small emf be applied between two

electrodes in a solution containing a reversible couple such as iodine-

iodide, a flow of current is observed because of the oxidation of the

reduced species at the anode and an equal reduction of the oxidized

species at the cathode. If concentration overpotential is not involved,

the flow of current is seen to increase linearly with the magnitude of

Introduction

3

the emf. When the system is an irreversible one (such as thiosulphate

and tetrathionate) the flow takes place only if the applied emf exceeds

the sum of the anodic and cathodic overpotentials. Delahay thus

concluded that with a small emf, redox titrations are possible to be

carried out biamperometrically, when the system to be titrated

contained a reversible couple and the titrant system is irreversible, or

when the situation is reversed. The flow of current will be negligible in

the post-equivalence region in the former and in the pre-equivalence

region in the latter. Accordingly, the titration curve (current – volume

plot) is expected to be of L shape or reverse L-shape (Fig. 2A & B).

Stone and Scholten5 from the study of a large number of

oxidation-reduction titrations reaffirmed that the end-point

phenomenon in a biamperometric titration is controlled by

simultaneous oxidation at the anode and reduction at the cathode.

These workers further established that biamperometric method can also

be applied to those titrations in which the titrant and the titrate both

contain reversible couple. Thus in the cerimetric titration of Fe(II) for

example, initially there is ample ferrous ion available for anodic

oxidation but only an “impurity level” of ferric ions are available for

the necessary simultaneous reduction at the cathode and as such the

current at the beginning of the titration is very small. As the oxidant is

run in, the supply of ferric ion gets increased upon which the current

Introduction

4

Introduction

5

strength rises. The rise continues until the titration is about one half

completed. At the halfway point, if the two electrodes are identical,

they will be equally polarisable having equal cathodic and anodic

currents. Beyond the halfway point the anode becomes more

polarisable than the cathode and oxidation of ferrous ion at anode

determines the indicator current. Thus, because of progressive

decreasing availability of ferrous ion after the mid point of titration, the

current also begins to fall progressively until the equivalence point is

reached. Titrant addition beyond this point introduces ceric ion into the

solution containing enough of its reduced form i.e., cerous ion. The

current, therefore, rises again thus giving V shape to the titration curve

(current-volume plot) near the equivalence point. If the two electrodes

have different areas and/or are subjected to different conditions of

stirring, the maximum current height instead of being exactly at the

halfway point of the titration, will be shifted either to the left or

right6,17

, the extent depending on their relative nature/size

(Fig. 2D and E).

From the above discussion, it is apparent that any titration which

can be so arranged that a reversible couple either appears or disappears

at the equivalence point, can be adapted to a biamperometric end-point

indication. In addition, systems in which both the titrant and the titrate

are irreversible can also be titrated provided the overvoltage for a given

Introduction

6

current density are different4. Chelatometric titrations

7,8 of several

metal ions which involve the oxidation of such complexing agents as

EDTA, TTHA, DCYTA etc., being used as a titrant, have been

reported. The technique may also be extended to many of such cases in

which the usual method of obtaining the end-point is not feasible. This

is achieved by the addition of a small amount of an „electrometric

indicator‟ or „depolariser‟9-11

.

Attempts to explain quantitatively the nature of the titration

curves were made independently by Bradbury12

, Duyckaerts13

,

Ganguin and Charlot14

. Bradbury deduced the conditions when the

current minimum coincides with the equivalence point. In Kolthoff‟s

treatment15

, which appeared a year later (1954), it was shown that the

current variation in the vicinity of the equivalence point should be

linear. A concise and mathematically simple theory was published by

kies16

who apart from presenting a single equation governing the entire

course of the titration curve upto the equivalence point, also showed

that the experimental data fitted the calculated curve exactly. Songina

and Savitskaya17

working with electrodes of different size presented a

modified form of the Kolthoff‟s treatment which enables the titration

curve of a reversible system to be traced for any given ratio of

electrode dimension.

Introduction

7

Apparatus and Experimental set-up

The apparatus consists of a titration cell having a lid with

adequate provision for fixing tightly the electrodes at fixed inter space.

Necessary holes for the inlet and outlet of carbon dioxide/nitrogen (for

maintaining inert atmosphere when needed) and for the insertion of the

burette tip are also provided. A constant polarizing emf is impressed

between the electrodes using a potential divider and the current passing

through the cell is measured either with a microammeter or a sensitive

galvanometer or else with some electronic signalling device.

Automatic titrators have also been constructed18,19

for which strict

control of temperature and stirring conditions are needed. A magnetic

stirrer is normally used for stirring the titrating solution uniformly

during the course of titration. In general, a pair of identical platinum

electrodes constitutes the electrode system; several other pairs such as

of graphite, silver, gold, copper, bismuth, tungsten, mercury have also

been used. Electrodes of the second order (i.e., Ag-Agcl), rotating or

vibrating platinum electrodes and combination of dissimilar electrodes

have in addition been employed on isolated occasions20, 487

.

Advantages and Limitations of the Technique

The notable advantage of the biamperometric technique lies in

its simplicity of experimental set-up and its operation along with

Introduction

8

accuracy of end-point detection. Normally, the sensitivity of the

technique is reasonably high but it can still be improved to certain

extent by suitably adjusting the applied emf, surface area of the

electrodes and the stirring rate of the titrating solution. The change of

current in the close vicinity of the equivalence point is usually so sharp

that for macro titrations even the current-volume plots may be

dispensed with. In the presence of a redox couple, oxidation and

reduction of the respective forms being held simultaneously at the

relevant electrodes, there occurs no loss of the substance due to

electrolysis; this qualifies the technique for being applied to sensitive

analytical work involving low content measurements. Another

significant advantage is that, the use of a reference electrode and salt

bridge being unnecessary, the technique, as such, can be applied for

determinations in non-aqueous and partially non-aqueous media as

well.

Among the limitations of the technique, mention may be made

of the fact that as the indicator current is sensitively affected by a

change in temperature or stirring conditions, the current value, in

general, does not reproducibly represent a particular concentration.

This, apart from presenting difficulties in the automation of the process

also keeps the technique off from being applied to theoretical studies

such as the evaluation of rate and equilibrium constants. However, it

Introduction

9

can be applied for the establishment of the stoichiometery of

precipitates or complexes and for the determination of solubility and

solubility product.

Thus, for routine determinations, both in aqueous and non-

aqueous media, biamperometry is a very simple and convenient

technique and yet is capable of yielding quick and accurate results.

(II) TYPES OF GRAPHITE/CARBON ELECTRODES AND

THEIR APPLICATIONS IN VARIOUS ELECTROMETRIC

TECHNIQUES

The unattackable nature of graphite towards common chemicals

together with its high conducting capacity has rendered its adoption as

a solid electrode for various electrochemical measurements. The ease

with which graphite can be given a desired shape (rod, tubular, plate

etc) and also of the facility of renewal of the exposed surface has

further been advantageous towards its utility not only as an indicator

electrode but also as a reference electrode or else in procuring

generation of a chemical species required sometimes for determination

purposes.

Various types of electrodes derived basically from one form or

the other form of graphite have been prepared from time to time to suit

Introduction

10

the required application and the nature of investigation. A short

account of these has been presented below.

1. Graphite electrode obtained from pyrolytic, nuclear grade or

carbon black materials. These have been used either directly or

after suitable impregnation (with paraffin, ceresin wax, epoxy

resin etc.) or else after subjecting them to pre-treatment with

chemicals (usually by a brief immersion in the solution of a

suitable oxidant).

2. Graphite/Carbon paste electrode – Prepared by mixing graphite

powder with an organic liquid e.g., nujol, mineral oil, carbon

tetrachloride, bromoform, bromophthalene etc.

3. Membrane graphite electrode – Rodelkis type and silicone

rubber based electrodes of low porosity prepared by the method

of Pungor and Szepesvary.

4. Glassy carbon/vitreous carbon electrode – This is a non-

graphitizing carbon which combines glassy and ceramic

properties with those of graphite. The most important properties

are high temperature resistance, harness (7 Mohs), low density,

low electrical resistance (high conductivity), low friction, low

thermal resistance, impermeability to gases and liquids and

extreme resistance to chemical attack. In addition, this can also

Introduction

11

be fabricated in different shapes, sizes and sections. In view of

these favourable properties as desired for an electrode, glassy

carbon is widely used as an electrode material in

electrochemistry.

This Sp2 structure carbon based electrode is made by pyrolyzing

a carbon polymer, under carefully controlled conditions, to a high

temperature like 20000C. An interwinning ribbon-like material results

with retention of high conductivity, hardness and inertness. The

electrochemistry is affected greatly by its surface chemistry of carbon-

oxygen functionalities and its cleanliness i.e., absence of adsorbed

impurities. Glassy carbon electrodes can retain much of their activity if

stored in a solution of alumina or silica.

5. Modified/Coated glassy carbon electrode sensor-Prepared by

suitable treatment of glassy carbon electrode in the laboratory

depending upon the need of determination and the nature of

investigation.

Some other varities of graphite electrodes which have

successfully been employed in a number of electrometric measurement

include carbon (graphite) fibre, carbon glass ceramic, graphite spray,

polythene graphite, apoenzyme-treated graphite, optically transparent

glassy carbon-graphite, procaine graphite impregnated rod,

Introduction

12

fluorographite, graphite-teflon, compressed graphite-Kel-F

fluorocarbon, carbon fiber supported mercury film electrodes etc.

In the following paragraphs a brief account of the applicability

of graphite electrode in various electroanalytical techniques of analysis

mentioning the chemical species studied together with the advantage

obtained or/and the difficulties encounted (if any). However in this,

description no independent section has been devoted for describing its

use in non-aqueous media, instead the relevant information has been

included additionally wherever possible along with those of aqueous

systems. Further, for the sake of brevity, out of a large number of

applications only a selected band has been incorporated here without

giving the details regarding the preparation of the electrode, the nature

of the electrode process and the chemical steps involved, because of

the intention to concentrate mainly to emphasise the widespread utility

of various types of graphite/carbon electrodes in electrometric

investigations using both conventional as well as newer techniques.

Graphite/Carbon Electrode in Potentiometric, Bipotentiometric

and Chronopotentiometric Measurements

Successful application of graphite as an indicator electrode has

been made in the potentiometric titration21-27

of potassium, silver and

thallium (I) with sodium tetraphenyl borate, gold (III) with a derivative

Introduction

13

of dithiocarbamate, mercaptans with ferricyanide and also with

copper (II) chloride, thallium (III) with unithol and its derivatives. In

the potentiometric estimation of titanium (IV) using chromium (II)

Buser and Gynli28

have observed a measurable potential jump when

platinum electrode was replaced by graphite electrode. The behavior

has also been examined by Pastor and coworkers29

in the

potentiometric titration of manganese (II) in presence of complexing

agents.

Selig30

used graphite rod coated with polychloride and dioctyl

phthalate for the potentiometric estimation of tin (II), antimony (III),

thallium, rehenium (VIII) and bismuth (III) using cetyl pyridinium

chloride. Use of graphite and platinum electrodes was also made by

Selig31

in the potentiometric determination of fluoride ion Vs

Lanthanum and Thorium (IV) solution found that in every case, a

partially non-aqueous medium yielded sharpen breaks than in the

corresponding aqueous solution; the sharpest being in the case of

vitreous carbon and pyrolytic graphite sensors. Recently Berestetskii

and Tulyupa32

have reported that a carbon electrode is suitable for used

as an indicator electrode for the potentiometric titration of sulphide,

thiosulphate, thiocyanate and thiourea with silver nitrate, mercuric

nitrate, cadmium nitrate and cupric nitrate solutions. The results are as

Introduction

14

accurate and reproducible as are obtained using a silver or platinum

electrode.

Detailed investigation conducted33-42

on the working of the

graphite electrode after pretreatment (by impregnation) were planned

mainly for the potentiometric acid – base titrations, has shown

significant effect in decreasing the surface porosity and increasing the

sensitivity. Thus, Bercik33

observed that the potential of a wax-

impregnated graphite electrode changes linearly with pH in aqueous

solutions; the sensitivity can be increased by brief pre-immersion in an

acidified solution of an oxidant such as potassium bromate, dichromate

and permanganate.

Pungor and Szepesvary have applied Silicone rubber based

graphite electrodes of low porosity as the indicator electrode for the

acid-base titrations in water as well as in acetone43,44

. These have

further been found useful for the non-zero current potentiometric

determination45,46

of silver (I), mercury (II), palladium (II) and iodide.

Behaviour of some new type of silicone-rubber based membrane

graphite electrode was studied by Pastor and coworkers47

towards

potentiometric titration in acetic acid medium. The Rodelkis membrane

graphite pretreated with oxidants and also the Elektrocarbon (Su 106,

Toplocany) graphite when subjected to similar work were found to

yield unsatisfactory end points in the titration with lead (IV) acetate. In

Introduction

15

the study of potentiometric titration of some reducing substances with

bromine in acetic acid Pastor and coworkers48

observed that the

Rodelkis OP-C-711D graphite electrode showed greater sensitivity

than a platinum or laboratory prepared silicone-rubber-based

membrane graphite electrode. However, the membrane graphite

electrode was found to be more advantageous in respect of rapid

attainment of potential values. Titrations were improved by addition of

potassium acetate to the solutions analysed.

Successful adoption of graphite electrode for similar work in

other non-aqueous media34-41

include methyl ethyl ketone, acetonitrile,

pyridine, dimethyl formamide and ethylene glycol-acetone mixture.

Use of pyrolytic graphite was made by Miller49

for the

potentiometric acid-base studies both in aqueous and in non-aqueous

media and from the observed behavior it was assumed that its function

is like that of an oxygen electrode. Thomason50

also employed the

same electrode in several similar investigations. Selig51

attempted

various electrodes for the potentiometric titration of sulphate with lead

and barium ions in partially non-aqueous medium and from

comparison with titrations obtained with lead-ion selective electrode,

reported that the pyrolytic graphite and high d-graphite conditioned in

neutral potassium permanganate were good alternatives. Pavel and

coworkers52

have studied the reduction of the mononuclear species

Introduction

16

tetranepen toxyphalocyaninato cobalt (II) species at modified pyrolytic

graphite and on comparision found that highly oriented pyrolytic

graphite is the most convenient material to use.

Jennings and Pearson53

reported the use of a single carbon fibre

as an indicator electrode in aqueous solution; further investigation54

of

its response to hydrogen ion concentration in non-aqueous medium by

Jennings has also been fruitful.

Glassy carbon electrode55-57

was employed for acid-base

titrations in potentiometric and also in bipotentiometric techniques. It

was found that the system57

consisting of a glassy carbon and a glassy

carbon graphite oxide give sharp end-points in the titration of both

strong as well as weak acids.

Meullen and Wilcoxon58

used the combination of a graphite rod

with a platinum plate for potentiometric acid-base titration. Although

the breaks in the acid-base titration curves were sharp but these

suffered quite often from the defect of non reproducibility.

Investigation on these lines by other workers59-61

revealed that the

failure was due to ion adsorption on the electrode surface thereby

affecting adversely the potential. In addition to the neutralization

experiments, the combination has been applied for several redox

determinations62-70

in many of which the graphite electrode was

Introduction

17

subjected to pretreatment with molten wax. Other notable examples of

dissimilar electrode bipotentiometry, involving graphite/carbon

electrode, are found in the use of graphite-tantalum (for iodometric

estimation), graphite-tellurium, antimony-carbon, platinum-carbon and

tungeston-carbon (for neutralization titrations) systems.

Extension of dissimilar electrode potentiometry using graphite

with a metallic electrode in non-aqueous media was carried out by

Novak71

who concluded that platinum electrode in combination with

graphite (or carbon) yields satisfactory response in a medium of methyl

alcohol; other notable electrode combination reported are tungeston –

carbon, gold – carbon and antimony – carbon systems which are suited

in acetic acid medium although combination of carbon with either

silver or platinum or tellurium also yields satisfactory results.

Applicability of graphite electrode has also been examined for

chronopotentiometric studies72-74

. Thus, e.g., for the oxidation of

organic substances expected results were obtained by the use of wax-

impregnated graphite75,76

, treatment of freshly exposed surface with

some wetting agents was found to improve the reproducibility.

Chronopotentiometry, employing graphite, was further made in the

study of dissolution of metals77,78

.

Introduction

18

A detailed chronopotentiometric investigation of the pre-

treatment effects on spectrographic grade graphite electrodes were

carried out by Kekedy and Coworkers79

who concluded that adoption

of a suitable chemical or an electrochemical treatment can lead to

significant changes in the response of the electrode.

Pyrolytic graphite electrode was found to be very suitable for the

chronopotentiometric studies80-83

in molten fluorides (of iron) and in

lithium chloride-potassium chloride melt (of vanadium pentoxide),

iodine systems in aqueous media were also studied using the same

electrode. Kitagawa and coworkers84-88

employed carbon paste

electrode for the oxidative study of EDTA complexes of Ni and Co and

also for the determination of some anions. With this electrode,

however, an irreversible behaviour of bromine-bromide couple was

observed by Davis and Everhart89

. Work done by Marek Trojanoweiz

and Wojciech90

on potentiometric stripping determination of Ni(IV)

using carbon paste electrode has shown it to provide sharp and

reporducible analytical signal.

Jennings et al91

have reported the use of a micro area carbon

fibre for chronopotentiometric stripping analysis and through

experiments using Cd(II), it has been shown that this electrode does

not require the use of stirred solutions during plating and stripping.

Glassy carbon electrode was employed for acid-base titrations in

Introduction

19

potentiometric and also in bipotentiometric techniques. Dodson and

Jennings57

observed that the system consisting of glassy carbon and

glassy carbon graphite oxide give sharp end-points in the titration of

both strong as well as weak acids. Shibalko and Stenina92

have tested

this electrode for potentiometric redox titration and have favoured its

use as an universal indicator electrode. Nakashima and coworkers93,94

used a pair of glassy carbon electrode for the determination of cerium

with hydroquinone and cerium in yitrium oxide.

Applicability of Glassy carbon electrode has been examined by

Panicheva and Filanovskii95

, Luong and Vydra96

from their observation

printed out that this electrode was most suitable for being used both as

stationary and rotating disk electrode. Nghi and Vydra97

successfully

applied glassy carbon electrode for the estimation of trace amounts of

gold both in aqueous and non-aqueous media and Kabanova and

coworkers98

used it for the determination of lead(II). The

electroanalytical behaviour in relation to the properties of metal oxides

and the mechanism of oxidation of ascorbic acid and oxalic acid on

glassy carbon electrode surface has been investigated using

chronopotentiometric method by Dong and Kuwana99

. Gunasingham

and Fleet100

observed that the electrochemical response of the glassy

carbon was affected significantly by the state of the carbon surface.

Introduction

20

Recently, Beheshti and Amini101

have proposed a flow injection

method for the determination of iodide based on a potentiometric

sensor prepared by coating a glassy carbon electrode with plasticized

PVC membrance containing Hg(CTP)2 as the active ingredient. The

performance characteristics of the flow injection potentiometry system

and the influence of several operating parameters on its properties have

been investigated. The proposed method has also been used in an assay

to determine iodine in a pharmaceutical product.

Graphite Electrode in Amperometric, Chronoamperometric and

Biamperometric Measurements

Similar to other electroanalytical techniques, graphite electrode

has also been found to respond reliably in various amperometric

estimations76,102

. On some occassions, the working of graphite

electrode when compared with that of platinum has been found to be

more suitable76,102

.

A wax-impregnated graphite was found to be better suited than

platinum by Elving and coworkers76,102

in the determination of

potassium with sodium tetraphenyl borate. Later, Terenteva and

Bernatskaya103

while working with a paraffin impregnated one, during

the estimation of zirconium and sulphate ions, also found it preferable

to rotating platinum electrode. Siska and coworkers46,104

applied

Introduction

21

successfully a silicone rubber-based graphite for the amperometric

titration of mercury(II), palladium (II), silver, potassium and iodide. A

graphite electrode combined with that of a mercury one was used for

the quantitative evaluation of paper chromatograms by amperometric

measurements105

and the method was tested in the estimation of several

metal ions. An apoenzyme-treated graphite electrode was introduced

by Jasaitis and coworkers106

for the amperometric determination of

zinc.

Successful application of Graphite electrode has been reported in

a series of estimations including arsenic (in steels), antimony (in tin-

lead alloys), selenium, tellurium and also some sulphur containing

organic compounds by Usatenko and coworkers107-118

. Several other

workers119-121

have similarly been successful in the study and

estimation of rhenium (VII), vanadium, titanium and uranium. Matrka

and Kroup122

used graphite as a reference electrode in the estimation of

primary aromatic amines with sodium nitrite.

Dayton and coworkers123

have examined the response of a

carbon fibre electrode in solutions containing potassium

hexacyanoferrate(III) for several electrometric measurements including

chronoamperometry.

Introduction

22

The carbon paste electrode124

was selected for cyclic voltage-

sweep chronoamperometry to record the polarographic redox wave and

also in the determination of a number of anions. Studies on the

electrochemically active surface of coal paste electrode125

were carried

out by chronoamperometric measurements. Beilby and coworkers83

observed that the results obtained for ferrocyanide/ferricyanide system

at pyrolytic carbon film electrode to be quite satisfactory and

comparable to those obtained at the platinum electrode; ceresin wax-

impregnated graphite was, however, less reversible.

Stock126

employed an intermittently polarized rotating pyrolytic

graphite for the amperometric determination of corypalline; adsorption

studies of corypalline were also made. The same electrode was further

found suitable for the determination127

of Uranium (IV) with

Cerium (IV).

Dieker128

et al have applied a glassy carbon for the determination

of iron(II) and iron(III) and also for the analysis of standard rocks.

Electrodes of different shapes129,130

(as tubular or disc type) have as

well been in service in isolated occassions as per requirement. In the

determination of nitroprusside using flow injection amperometric

technique with glassy carbon electrode, it was observed by Fogg et

al131

that deoxygenation can be dispensed with.

Introduction

23

In biamperometric works too, graphite electrode has been found

to play significant role. Thus, a pair of graphite electrodes was adopted

by Vorlicek and Vydra132

for the determination of barium, strontium,

calcium and magnesium and the results were found to be in good

agreement with those obtained by using the conventional pair of

platinum electrodes. The same pair was also applied in the

determination133,134

of calcium in limestone and iron in ores. In

chelatometric estimations it was found to be preferable133

to that of

platinum whereas in the titration of chloride ions, in presence of

chromate ions, no special advantage was found over other identical

metal electrode pairs135

. The pair consisting of glassy carbon electrodes

was reported to yield satisfactory results and the technique was

employed not only for acid-base titrations but additionally for the

continuous titration of iron (III) during chemical machining of mild

steel and in the estimation of iron (II) with EDTA.

The platinum-carbon electrode system when used in the

biamperometric estimation139

of peroxy compounds showed that the

results were in agreement with those using two platinum electrodes.

Successful estimation of gold (III) has been reported by Tarayan and

coworkers140

using graphite electrode with a rotating platinum

electrode.

Introduction

24

Dissimilar electrode combination consisting of graphite with a

metal electrode has also been found to yield desired results. Thus,

selenium was determined68

using potassium iodide as the titrant with

platinum-graphite pair. The same combination was successful for the

titration141

of dialkanediols using coulometrically generated bromine.

In the estimation of iodine with sodium thiosulphate, it was found that

graphite used as anode142

produced unmistakable detection of end-

point as compared to that observed by reversing the polarity. The

technique was applied later for several estimation of metal ions and

organic compounds both in aqueous143-147

and non-aqueous

media148-150

.

Biamperometric techniques using two identical glassy carbon

electrodes has been proposed by Milardovic et al151

for selective

determination of antioxidant activity based on 2, 2-diphenyl - 1-picryl

hydrazyl 2, 2-diphenyl-1-1 picryl hydrazine (DPPH/DPPH) redox

couple. The DPPH/DPPH redox couple showed a high degree of

reversibility and the working potential difference was been 50-200

mV. Determination of salbutamol sulphate based on a flow-injection

coupling irreversible biamperometry at poly (aminosulphonic acid) –

modified glassy carbon electrode has been reported recently by Lijun

and coworkers152

.

Introduction

25

Graphite/Carbon Electrode in Conductometry and pH

Measurements

A survey of literature reveals that compared to other

electrometric techniques the application of graphite electrode in

conductometric and pH measurements have been much less,

nevertheless some useful results have been reported. Oehme and

Henkel153

using graphite electrodes for conductivity measurements,

have described the construction of an improved type of carbon

electrode.

Ivanov and coworkers154

have employed graphite electrodes for

the determination of xanthates with an apparatus specially designed for

the purpose. In conductometric continuous analysis155

too, graphite

electrode was found to be advantageous. The applicability of various

carbon membrane electrodes for tensammetric and high frequency

titrations – has been reported by Musha156

.

Gaillochet157

and coworkers have been successful with carbon

paste electrode in the measurement of pH of sulphuric acid solutions.

Such measurements have also been made using a graphite plastic

electrode by Medinskii and coworkers158

who have studied, in addition,

the feasibility of its use as an alternative for the platinum electrode.

The pH response of a reticulated vitreous carbon electrode has also

been discussed in detail by Strohl and Curran159

.

Introduction

26

Jennings and Pearson53,160

have concluded that the potential of a

carbon-fiber-pH sensitive electrode does not depend on the presence of

oxygen in the test solution but is due to the ionization of carboxylic

acid groups formed on the surface by reactions with atmospheric

oxygen.

Graphite/Carbon Electrode in Coulometric Measurements

The applicability of graphite electrode has also been established

in various coulometric measurements. Studies conducted161

on the

electrochemical behavior of iodine on graphite and also on platinum

electrodes have revealed that the former is capable of yielding precise

results; for getting improved results a pretreatment has been suggested

for which suitable conditions have been worked out by Agasayan et

al162,163

. The findings have been applied for the analysis of binary

alloys. Graphite electrode impregnated with paraffin and expoxide

resin in vacuum have been successful particularly in the determination

of molybdenum (IV) and for the successive estimation of gold (III) and

copper (II) in binary mixtures164

.

Comparative studies made on the current efficiency of tin (II)

generation, established the superiority of paraffin impregnated

graphite165

(99.9%) over others.

Introduction

27

Micro quantities of copper, cadmium, palladium and thallium

were estimated with precision using carbon fiber (prepared from

furfural) as the flow - coulometric - column electrode166,167

. Working of

the fibre electrode was additionally compared with that of platinum in

the determination of arsenic (III) with coulometrically generated

iodine. Voorhies and Davis168

have described a useful technique

suitable particularly for the semi-micro analysis of water-soluble

electro-active organic compounds. The technique involves coulometry

with a carbon black electrode by way of quantitative preadsorption of

the oxidisable material.

A carbon glass ceramic electrode for the coulometric

determination of lead has been reported by Goncharov169

.

A specially designed carbon ring disc electrode170

has been

applied for the titration of the solution of phenols and also for the

determination of bromination rate constants.

Yoshimori and coworkers171

used a glassy carbon rod as the

working electrode in anodic stripping coulometry of gold; application

was further extended by Kobanova and Zalogina172

for the

determination of mercury in dilute solutions. Redox titrations of

arsenic (III) and isoniazide were successful with coulometrically

generated bromine using a vitreous carbon electrode173,174

.

Introduction

28

Pastor and Antonijevic175

established conditions for the

generation of iodine in solution of potassium acetate in ethanol with

high current efficiency at glassy carbon electrode in the presence of

tetraethyl ammonium iodide for coulometric titration of various thiols.

These authors176

have also established conditions for the

electrochemical generation of manganese (III) at glassy carbon

electrode in acetic acid and procedures were given for a successful

coulometric titration of reducing substances with this anodically

generated manganese (III). Recently, anodic generation of cerium (IV)

at glassy carbon electrode in acetic acid and coulometric titrations with

the generated reagent has been reported by the same authors177

.

Graphite/Carbon Electrode in Voltammetry and Polarography

Application of graphite/carbon electrode in various voltammetric

and polarographic studies has been quite extensive. Stationary ones

have been found to yield better results over rotating graphite, gold and

even platinum electrodes in the study of the oxidation of some organic

compounds178

through the usefulness of the rotating graphite has also

been noted in other cases179-181

.

Wroblova and Saunders182

have reported that a graphite

electrode can successfully be applied for studying the redox system.

Introduction

29

3 2I I I ; its behavior was found to be similar to that of

platinum. However, azobenzene, azoxybenzene and hydrazobenzene in

50% ethyl alcohol were found to be more reversible at mercury

electrode183

and less reversible at the graphite.

Elving75,186,187

through voltammetric studies has printed out the

importance of impregnation in improving the general working of the

electrode because the unimpregnated ones show high residual current

as also suffers from poor reproducibility186

. The entire practice of

breaking the tip184,187,188

of the impregnated electrode was found to be

untenable as it suffered from the serious drawback of the lack of

guaranteed uniformity. Sanding off of the tip184,185

, holding the

electrode carefully over a rotating sand paper disc or else cutting,

cleaning and polishing preferably using appropriate devices have

yielded very satisfactory results in several voltammetric studies189

.

Epoxy resin impregnated graphite electrode has been applied

with fruitful results in the determination184,190,191

of lead, copper,

cadmium and nickel. Wax-impregnated one75,76,192-198

has similarly

been suitable in a large number of electroanalytical measurements

wherein are included the determinations of gold, molybdenum (VI),

silver and many organic compounds. Similarly, graphite electrodes

impregnated with paraffin and polyethylene have been successful in

the determination199-201

of tellurium and molybdenum using highly

Introduction

30

sensitive and selective inverse voltammetry. Linear sweep

voltammetry with wax-impregnated graphite and glassy carbon was

studied by Gomathi and Prabhakar202

in sulphuric acid solution

containing chloride where a cl- responsive anodic peak were produced

by both type of electrodes.

Investigations on the applicability of pyrolytic graphite electrode

was conducted by Elving186

et al and also by other workers81,203

.

Numerous systems129,204-212

, comprising both organic and inorganic

substances, were studied to establish its utility. In acetonitrile medium,

the usefulness with halogen-halide system has been noteworthy for the

determination of rare earths and transition metals.

Polarographic studies with platinum and with an impregnated

graphite revealed that the electrode processes in both the cases were

essentially similar213

. Elving and Krivis75

worked out the conditions

necessary for the polorographic study of organic compounds with wax-

impregnated graphite electrodes; the findings were further utilized for

obtaining useful correlation between ring substitution and half wave

potential185,214

. Matsunaga and Namba215

measured the concentration of

microbialcells in suspension using graphite electrode modified with

adsorbed 4, 4 biphyridine applying cyclic voltammetry or differential

pulse voltammetry. The electrochemical behaviour493

of cysteine and

Introduction

31

cystine were also studied using the same electrode by cyclic

voltammetry.

Bansal and Anand216

have reported voltametric studies of

iron(II), cobalt (II) and Nickel (II) in molten urea and thiourea at

platinum and graphite electrodes. Likewise Dmitrieva et al217

conducted several experiments with lead, cadmium, tellurium,

antimony and silver using a graphite electrode. Use of graphite

electrode was also made by Kiryushov et al218

for determining thiourea

in solution containing metal ions known to form complexes with it.

Several techniques214,219,220

were devised for the preparation of

carbon paste electrode and theses were tested in different voltammetric

work124,221,222

. An electrode (consisting of a mixture of 3.3g graphite,

1.4g sodium lauryl sulphate and 2.5ml of nujol) was seen to give very

low and reproducible back ground current as compared to that obtained

with platinum223

. The voltammetric behaviour of the carbon-paste

electrodes, prepared from varied compositions, was examined by

Chulkina and coworkers224

who on the basis of their observations,

further suggested the optimum one suitable for the determination of

monogram quantity of substances. Investigation using vanadium

dioxide on a mineral carbon-paste electrode was made by Songina and

coworkers226

which was successful in the estimation of Vanadium (IV)

and Vanadium (V) by Grigoreva and coworkers226

and in the

Introduction

32

voltammetric study of diethyl dithiocarbamate made by Bovenkerk227

.

A modified carbon paste electrode228

prepared by coating the carbon

paste surface with a conducting graphite layer (obtained from the

dispersion of colloidal graphite in a mixture of methyl methacrylate

with butyl acetate) was used in differential pulse anodic-stripping

voltammetry of cd++

in the presence of a mercury film. The important

feature of this electrode is the noticeable improvement in the

reproducibility of peak currents. An electrode composed of compressed

powdered graphite and kcl – F fluorocarbon plastic has been reported

to offer significant advantages over many other materials for general

voltammetric applications in non-aqueous solvents229

.

Gilbert and Curran230

studied the oxidation of dopamine in

acidic solutions at carbon paste electrode and using a sterate modified

carbon paste electrode by A.C. and D.C. cyclic voltammetry. The

utility of modified electrode was studied by Katherine and Hector231

and it was found to give best results in terms of signal, reproducibility

and electrode stability. Troplone modified electrode was used by Wang

et al232

for the study of trace amount of tin by cyclic and differential

pulse voltammetry; the peak current was found to be about 40 fold

larger than the corresponding one at plain carbon – paste electrode. A

comparative study on the anodic behavior of biologically important

molecules (adenine, adenosine and 5-amino monophosphate) in respect

Introduction

33

of peak potential and peak current was conducted by Chang et al233

during differential pulse voltammetry.

Usefulness of carbon fibre electrode in votammedtric

microanalysis has been reported by Muntyanu and Vataman234

, by

MacCallum235

and by Baranski236

in A.C. voltammetry. The analytical

performance of this electrode was investigated in the differential pulse

voltammetry for the reduction of copper(II) by Edmond and

coworkers237

. Voltammetry of a series of metal complexes has been

studied at an electrochemically treated cylindrical carbon fiber

electrode by Kovach238

. Rotating disk electrode has also been tested in

voltammetric study by Premsyl and Jiri239

.

Mercury coated graphite electrode has been used for the

determination of trace amount of lead employing differential anodic

stripping voltammetry by Yuliang et al240

. Composite graphite

electrode prepared with mercury deposition gave very sharp anodic

stripping current peaks allowing high sensitivity and resolution during

separation241

. It was adopted for several voltammetric studies127, 242-247

and in A.C. voltammetry. In a similar way, mercury-plated glassy

carbon electrode was seen to be useful in various determinations250-252

including trace impurities in water and in other compounds. Anderson

and coworkers253

have proposed the use of carbon fiber as a support

Introduction

34

electrode for a mercury film electrode and have evaluated the response

for use in differential pulse anodic stripping voltammetry.

Kauffman and coworkers254

have described the preparation and

characterisation of some graphite-coated metallic electrodes. Such

electrodes (using Al, Cu and Pt) have been found to possess high

electrical conductivity, good mechanical strength, low residual current,

wide operating range and highly reproducible performance. These have

been tested using both anodic stripping voltammetry and cyclic linear

scan voltammetry. Precision of 0.1% has been obtained for peak

currents in the electrochemical oxidation of [Fe(CN)6]4-

and 2.5% for

phenol. The application of a graphite spray electrode in the anodic

stripping voltammetry of Bismuth255

has further been extended.

Silicone-rubber-based graphite electrodes were devised for use

in voltammetric measurements45,183,256-258

and also in a number of

determinations22,46,104,257-261

including thallium (I), potassium, cesium,

chloride ion and drugs. Mascini et al262

described a polythene graphite

electrode and used it for the study of some redox systems.

Glasssy carbon electrodes have also been employed in a number

of studies. From a large number of observations Zittel and Miller80

concluded that the total kinetics of some complex electrochemical

process can be more rapid on a glassy carbon that on a polished

Introduction

35

platinum electrode263

. Differential pulse anodic stripping voltammetry

of copper(II) was carried out at glassy carbon electrode264,265

and the

optimum conditions for its use were investigated; it was also used in

the determination of silver in uranium and plutonium266

and in the

anodic voltammetry of DNA267

and also for the determination268-270

of

thallium, copper and lead in gold chloride and mercury in sea water.

Yann Cheng et al271

evaluated cyclic as well as differential pulse

voltammetric characteristics of the oxidation of ferrocene in

acetonitrile. The electrochemical reduction of oxime272

and nitroso

compounds of heterocyclic series was studied on the same electrode

under acetic acid and neutral pH conditions using cyclic voltammetry.

The voltammetric behaviour of vanillin in aprotic and protic solvents

was investigated by Chandrasekaran and coworkers273

. Gomathi and

Rao274

have reported that the glassy carbon electrode modified with

quinhydrone, was found to catalyse oxidation of ascorbic acid in

glycine. Schwartz and Benzamin275

have reported the voltammetric

determination of morphine in poppy straw concentrate using such an

electrode. A rotating glassy carbon disc electrode employed by Vydra

and coworkers97,276-280

has been shown to be faithful in A.C.

voltammetry and in stripping analysis technique. A new type of

optically transparent electrode281

made from glass carbon and graphite

has also been developed by drilling a small hole (ca 500 m) for which

Introduction

36

the optical and electrochemical responses have been evaluated using

cyclic voltammetry and potential step methods.

A vitreous carbon electrode was reported to be appropriate for

the determination282,283

of copper and lead as also of cadmium and lead

in organo-tin compounds although for the determination of nitrite,

Dian and coworkers284

emphasized the need of defining the conditions

necessary for achieving good sensitivity and reproducibility. A rotating

mercury plated reticulated vitreous carbon electrode285

(with large

surface area) has fruitfully been tested particularly for the

determination of lead and cadmium by square wave anodic stripping

voltammetry.

Jaya and Rao286

investigated the anodic stripping voltammetry of

arsenic (III) at glassy carbon electrode copper coated in situ. Similarly

for the determination of trace amount of lead, Dong and Wang287

have

applied the same electrode coated with a film of nafion. In this, owing

to continuous transfer of lead from the solution to the electrode surface

its sensitivity was found to increase and the method was successfully

applied for such determinations in water samples. Porter and

Kuwana288

have developed a new type of optically transparent

electrode made from glassy carbon and graphite by drilling a small

hole (ca 500µm) for which optical and electrochemical responses have

been evaluated by using cyclic voltammetry and potential step

Introduction

37

methods. Recently Zielinska and Pierozynski289

have conducted cyclic

voltammetric and a.c. impedence spectroscopy studies on adsorption

and electro oxidation of quercetin (3, 3‟, 4‟, 5, 7-pentahydroxy

flavone) compound at glassy carbon electrode surface in 0.1M sodium

acetate-acetic acid buffer in 90% methanol solution.

Determination of adrenaline and dopamine by coulometric

titration and cyclic voltammetry was carried out by Ziyatdinova and

Budnikov290

with electrogenerated halogens on graphite and glassy

carbon electrodes. Relative standard deviation reported was 1-4%.

Preparation of Cu(II) serine schiff-base complex modified glassy

carbon electrode and its electrocatalysis on ascorbic acid has been

reported recently by Wang and coworkers291

. Zare and Nasirizadeh

have reported simultaneous determination of ascorbic acid, adrenaline

and uric acid at a hematoxylin multi-wall carbon nanotube modified

glassy carbon electrode. For the aqueous determination of silver(I) by

anodic stripping voltammetry, a glassy carbon electrode modified with

4-tert-butyl-1 (ethoxy-carbonyl methoxy) thiacalix [4] arene has

recently been used by X. Changzheng and coworkers292

. This modified

electrode has been found to remarkably improve the measuring

sensitivity for Ag. The electro oxidation of d-penicillamine (d-PA) was

studied by J.B. Raoof and coworkers293

in the presence of ferrocyanide

as a homogeneous indicator at the surface of a carbon paste electrode

Introduction

38

in aqueous media using cyclic voltammetry and chronoamperometry.

X. I. Hou et al294

have reported the successful fabrication of a β –

cyclodextrin and multiwalled carbon nanotubes modified glassy carbon

electrode; based on this, a new and rapid electrochemical method was

developed by the same authors for the determination of promethazine

hydrochloride (PTH). The electrochemical properties of PTH at the

prepared electrode were investigated by cyclic voltammetry and the

results indicated that this modified glassy carbon electrode exhibited

efficiently electro catalytic oxidation for PTH. In another recent work a

novel electrochemical sensor was prepared by direct electro-

polymerisation of DL-aspartic acid on the surface of glassy carbon

electrode in aqueous media for the voltammetric determination295

of

ferulic acid. At pH 4.5, using Acetic acid/Sodium acetate buffer

solution, this film modified electrode was found to exhibit excellent

adsorption capacity to ferulic acid, thus improving the electro chemical

response significantly. G. Gopalkrishnan and coworkers296

have used

nano-riboflavin-modified glassy carbon electrode for stripping

voltammetric determination of three analgesics (acetaminophen, acetyl

salicyclic acid and diphrone) in the concentration range of 0.02 – 0.4

µgmL-1

. The suitability of the method for the determination of these

analgesics in pharmaceutical preparation and urine sample was also

ascertained.

Introduction

39

OBJECT AND SCOPE OF THE PRESENT WORK

For biamperometric determinations, as reported in the literature,

two identical electrodes, preferably of platinum, have mostly been

used. Similar electrodes of other materials have also been applied in

isolated cases. Use of dissimilar electrodes, though restricted mostly to

bipotentiometry, has also been reported in some biamperometric

estimations. One such dissimilar electrode pair consisting of a wax-

impregnated graphite and a platinum electrode has been found to yield

successful results in aqueous as well as some non-aqueous media. A

survey of literature revealed that glassy carbon electrode, in recent

years, have widely been used in various electrometric techniques of

analysis and also in other electrochemical studies. Few preliminary

experiments using glassy carbon-platinum combination for

biamperometric indication has been found to show promising result.

Keeping in view the above facts, it appeared of interest to (1) carry out

a systematic study for exploring the suitability of the dissimilar

electrode system consisting of glassy carbon and platinum for

biamperometric determinations, particularly in low concentration

range, in aqueous as well as in commonly used non-aqueous media

employing various current-indicating couples (Part A) and (2) to

undertake some exploratory experiments with the aim of testing the

more general applicability of already reported graphite-platinum

Introduction

40

electrode system for adoption in biamperometry under varied

experimental conditions (Part B).

With this object in mind, glassy carbon-platinum pair was taken

up first for experimentation. In order to work out suitable experimental

conditions particularly in respect of deciding the polarity of electrodes

and the range of polarizing emf, applicable for a given set of titration,

some preliminary experiments had to be arranged. It was observed that

although titrations could be performed irrespective of the choice of

polarity of electrodes but with glassy carbon as positive electrode,

breaks obtained in the titration curves are more well defined. Thus in

all estimations, glassy carbon has uniformly been maintained as the

positive electrode. Firstly, this electrode combination was subjected to

estimations in aqueous medium involving iodine-iodide couple. By

suitable adjustment of polarizing emf, the electrode system was found

to be quite sensitive even for the estimation of iodine content (in ppm)

in fortified salt samples. In order to further examine the response of

this electrode combination towards other common reversible couples

such as Ce(IV) – (III), hexacyanoferrate (III) – (II) and Fe (III) – (II) in

aqueous medium, several experiments were planned by properly

coselecting the substance to be estimated and the corresponding titrant.

The test of applicability was next extended to the commonly

employed (for redox determinations) non-aqueous solvents acetonitrile

and N, N-dimethylformamide. The technique being devoid of the use

Introduction

41

of the salt bridge etc its extension to non-aqueous determinations is of

significant advantage. The reversible couples utilized for current

indication in non-aqueous media were iodine-iodide, Cu(II) - Cu(I) and

Ce(IV) - Ce(III) and the substance estimated were mostly

organosulphur compounds of industrial importance.

The electrode system next applied for experimentation was a

wax-impregnated graphite electrode in combination with platinum.

With this, the series of experiments conducted in aqueous medium

included the determinations of organic acids, mineral acids, and strong

bases involving the use of electrometric indicators (additives), mixtures

of organic compounds and solubility of two organic acids at different

temperatures. In all these determinations the current indicating

reversible couple has been iodine-iodide. In order to examine the

suitability of graphite-platinum electrode system further, some

determinations in non-aqueous mixed solvent media have also been

carried out in which iodine-iodide and Ce(IV)-Ce(III) couples have

served for current indication. Direct determination of some water

insoluble organic compounds and their mixture have been carried out

in non-aqueous media where either 3( )I I or Cu(II) - Cu(I) system

has served for current indication.

As the main object of the present work was to examine the

suitability of two dissimilar bielectrode assemblies viz., glassy carbon-

Introduction

42

platinum and graphite-platinum for biamperometric end-point

indication involving various reversible couples in aqueous and non-

aqueous media, only those substances have been chosen for estimation

which utilize such reagents and reactions that have been reportedly

been employed with success and confirmed for precise determination.

Also for the same reason, no attempt has been made towards

exploration of any new titrant but instead, the entire attention was

concentrated in establishing the conditions suitable for obtaining

reproducible end-point indication as well as in recording the degree of

accuracy attainable thereby, particularly for estimations in low

concentration range. In addition, on some occasions few of the

substance have been selected for analysis more than ones utilising

either different solvent medium or other titrant/reversible couple.

Accurate and reproducible results, as obtained in various

biamperometric determinations in aqueous and non-aqueous media

using the present dissimilar electrode systems, points towards their

scope for adoption as a regular technique of analysis even in low

concentration range. Further, the technique being devoid of using salt

bridge and also as no special arrangement is required in switching over

from aqueous to non-aqueous media, it can also be extended for such

determinations which are accomplished partly in aqueous and partly in

non-aqueous media or else in two different non-aqueous media.

Introduction

43

GENERAL EXPERIMENTAL

The two dissimilar electrode systems, used in the present studies

consisted of (1) a glassy carbon plate (size = 10 x 5 x 1 mm3) and a

micro-platinum electrode and (2) a wax-impregnated cylindrical

graphite rod (diameter ≈ 7.8 mm) and a micro-platinum electrode. The

electrodes of a particular assembly were fitted tightly in a rubber disc

(at a fixed interspace of ~ 3cm) provided with holes for the insertion of

burette tip and for the inlet and outlet of carbon dioxide gas (Fig. 1).

The rest of the apparatus along with its circuitry arrangement was

essentially the same as adopted in the conventional biamperometric

technique.

A pyrex glass vessel of 50 ml capacity was used as the titration

cell. For performing a titration, a measured amount of test solution was

taken in the titration cell and thereafter chemicals (wherever necessary

followed by solvent were added such that the total initial volume of the

titration mixture became 30 ml. In all experiments, initial volume of

titrating solution has uniformly been maintained at 30 ml. The

dissimilar electrode assembly consisting of glassy carbon/wax

impregnated graphite in combination with a micro-platinum was then

put into the cell containing the titrating solution. In order to ensure

uniform stirring of the contents of the titration cell, a magnetic stirrer

actuated with A.C. mains was used. The required polarizing emf,

Introduction

44

Introduction

45

obtained from a battery operated potentiometer was applied between

the electrodes keeping glassy carbon/graphite as positive electrode. For

measuring the current passing through the cell, a sensitive

galvanometer, (with lamp and scale arrangement) connected with

variable resistances, one in series and the other in the parallel for

critical damping, was used.

The titrant was added from a semi-micro burette graduated to

0.01ml divisions. For non-aqueous determinations, however, the

burette used was provided with a guard-tube containing silica gel to

protect the titrant from atmospheric moisture. The galvanometer

readings were recorded at regular intervals after each addition of the

titrant, usually after one or two minutes (except stated otherwise).

Equivalence point was determined graphically by the usual plot of the

galvamometer deflection (representing cell current) against the volume

of the titrant added.

In order to ascertain the polarizing emf, requisite for a particular

determination, the usual method of drawing the polarization curves4,297

(not given here) was followed in aqueous estimations. For non-aqueous

titrations, this was mostly selected by few trial experiments as followed

by Hinsvark and Stone298

, from among the various emf values, by

measuring the sensitivity with each of them (sensitivity = galvanometer

deflection/ml of titrant). Since the determinations undertaken in this

Introduction

46

dissertation do not involve more than one current indicating couple

present simultaneously at any instant, there was no problem in

ascertaining the optimum range of polarizing emf suited for a

particular reversible couple in a given solvent corresponding to a

particular estimation. Titration curves exhibiting the effect of variation

of polarizing emf have been reproduced in many cases.

The Electrodes and their pre-treatment

Glassy carbon electrode used in experiments described in

chapters 1, 2 and 3 comprised of a glassy carbon plate (a product of

BAS Inc. Japan; size 10 x 5 x 1 mm3) having adequate provision for

electrical connection. Before starting a new set of experiment, the

electrode was carefully hand-polished with alumina-water paste using

polishing cloth and then rinsed with double distilled water and finally

with methanol.

Graphite electrode used in all the experiments reported in

chapters 4 and 5, was a cylindrical graphite rod of about 7.8 mm

diameter (a product of Messers Union Carbide Co.) Before starting a

new set of experiment, the graphite rod was impregnated in molten

paraffin wax for about an hour. Prior to actual use, the tip was gently

rubbed with a zero number emery paper to expose a fresh surface. The

micro-platinum electrode was also cleaned thoroughly by dipping in

Introduction

47

chromic-culphuric acid mixture for five minutes followed by repeated

washings with distilled water.

Materials and Reagents

For adding titrant to the titrating solution, a 5 ml burette

(graduated to 0.01 ml) was used. The chemicals used were of

analytical/high purity grade expect when stated otherwise. In the case

of other grade chemicals, there were subjected to further purification in

accordance with the standard methods. Double distilled water was used

for titrations in aqueous medium. A large number of compounds had to

be prepared, purified and their purity checked for which the

recommended methods were followed. Also for the storage and

standardization of solutions (both test and oxidant), reliable procedures

as reported by earlier workers have been followed.

***