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
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
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.
***