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Introduction
1
CHAPTER-1
INTRODUCTION
Heteropoly compounds are the ionic solids having high molecular weight
belonging to the family of polyoxometalates and composed of a compact skeleton of
metal-oxygen octahedral anions as the basic structural unit surrounding a central atom.
The octahedra are connected together giving an extremely stable and firm framework of
the heteropolyanions. The cations may be hydrogen or any other metal ions. HPA’s
represent a class of acid made up of a particular combination of hydrogen and oxygen
with specific metals and non-metals.
A large number of structures, incorporating several metal atoms are known for
molybdenum, tungsten, vanadium and niobium heteropolyanions1-2
. Numerous elements
are playing active role as addenda atom or central metal atom as compiled in the table 1.1
A heteropoly compound must be having:
Metal atom such as tungsten, molybdenum, chromium or vanadium, generally
from d-block termed the addenda atom;
Oxygen;
Element generally from the p-block of the periodic table such as silicon,
phosphorus, aluminium or arsenic etc. termed the hetero atom.
The metal addenda atoms and oxygen atoms form a cluster with the hetero-atom
inside bonded via oxygen atoms.
1.1 HISTORY
Berzelius3 in 1826, first time introduced the ammonium 12-molybdophophate
[(NH4)3(PMo12O40]. Marginac in 1862, gave the precise analytical composition of
tungstosilicic acid and their salts. Then, Werner’s coordination theory was introduced to
derive the composition of heteropolyacid anions. After that Miolati and Pizzighelii
hypothesis was adopted and a major break was given by Rosenheim. According to MR
theory, heteropolyacids are having a central hetero metal atom which coordinates with
MO42-
and M2O72-
units. Pauling proposed the structure for the 12:1 complex and
proposed formula for tungs tosilicic acid as H4[SiO4W12(OH)36 in accordance with its
basicity. Then, Keggins used X-ray technique and resolved the structure of
Introduction
2
H3[PW12O40].5H2O and the structure was confirmed by Bradley and Illiny Worths
investigations based on powder photographs1. Although voluminous research work has
been done in this field but still a large number of concepts are to be discovered, like
detailed mechanism of numerous syntheses etc, thus giving a broad spectrum to attract the
researchers in this direction.
Table 1.1: Elements acting as central atoms or heteroatoms in heteropoly
compounds
Atoms participating in the formation of Heteropoly compounds Periodic
Group
H+ I
Be+2
, B+3
, N+5
II
Al+3
, Si+4
, P+5
, P+3
, S+4
III
Ti+4
, V+4
, V
+5, Cr
+3, Mn
+2, Mn
+4, Fe
+3, Co
+2, Co
+3, Ni
+2, Ni
+4, Cu
+2, Zn
+2,
Ga+3
, Ge+4
, As+5
, Se+4
IV
Zr+4
, Mo7+
, Rh+3
, Ag+, Sn
+4, Sb
+3, Sb
+4, Sb
+5, I
+7 V
Cs2+
, Hf+4
, Ce+4
, Ce+3
, W+5
, Pt3+
, Pt+4
,Hg2+
, Bi+3
, Te+4
, Te+6
, Tl3+
, Th+4
, VI & VII
1.2 NOMENCLATURE
Traditional method consists of suffixing the names of the central atoms to the
words -date or -dic acid, for example; tungstomolybdate or tungstophophate. The number
of atoms of the central element denoted by Greek prefixes eg. dodecatungstophosphate.
According to International Union of Pure and Applied Chemistry nomenclature
names of heteropolyanions begins with Arabic numerals designing the simplest ratio of
molybdenum or phosphoros atoms to the central atom and then followed by the prefix
molybdo or phospho by the name of the simple anion which contains the central atom in
the corresponding oxidation state. Roman numerals may also be used to represent
oxidation state of the central atom. Heteropoly compounds need a more adequate system
of nomenclature which should include the structure, degree of polymerization and
oxidation state of the central atom. Keeping in view of above properties, an advanced
system of nomenclature extended IUPAC nomenclature, where the oxidation state of the
central atoms is expressed by a Roman numeral in parentheses. The prefix molybdo or
Introduction
3
vanado designates the peripheral atoms and the italicized prefix pent, hex, oct etc.,
allocated to the stereochemistry concerning the peripheral and the central atoms. Arabic
numerals designate the repective ratio of the number of peripheral and the central atoms.
The charge of the anion is indicated by a superscript Arabic numeral at the end of the
name. The Greek letter is used to assign bridging between central atoms as shown in table
1.2.
Table 1.2: IUPAC and trivial Nomenclature in HPA
S. No. Formula Proposed Names IUPAC Names
1. Na3[PMo12O40] Sodium12-oct- molybdotet-
phosphate(V)
Trisodium
dodecamolybdophosphate(V)
2. Zr[PW12O40] Zirconium12-oct-tungstotet-
phosphate (V)
Zirconium
dodecatungstophosphate (V)
1.3 METHODS OF SYNTHESIS
The properties of inorganic ion exchanger are greatly affected by the preparatory
conditions such as concentration of the metal, ageing time, temperature and pH etc.
Keeping in view of the above aspects, different conditions yield the product of different
characteristics. Following are the methods used for the synthesis of heteropolyacid salts:
Electrodialysis
Ion exchange
Etherate
Precipitation.
But the most widely used methods are ion exchange and precipitation whereas all
other methods are having certain limitaions.
Etherate method needs a strongly acidified aqueous solution of the
heteropolyanion with diethyl ether. In spite of too many efforts, this method does not
yield pure hereopolyacid salts, however we get a mixture of various structures of HPA’s.
Moreover, this technique is expensive, time consuming, require dangerous chemicals,
produces large amount of harmful wastes and quantitatively also not suitable.
Introduction
4
The synthesis of the heteropoly compounds via electrodialysis is rather more
complicated since it requires the electrodialysis of a mixture of its ingradients and then
the mixture is subjected to thermal treatment for many hours and then further followed by
dialysis to obtain the desired product. Inspite of plentiful of dilemmas, Maksimov et al
applied electrodialysis to synthesise iso and heteropoly acids of various structures in
highly concentrated aqueous solutions4.
Matkovic et al investigation demonstrates that the “ion exchange” methodology is
suitable for the synthesis of phosphotungstic and molybdic acids in a high yield5. In
general, ion exchange methodology is superior method as it yields pure and non-degraded
acids. Matkovic experiments indicated that the use of an organic media greatly favors the
ion exchange since a lower amount of resin is required than in aqueous media. Wijesekera
et al patented the synthesis of phospho-molybdic Wells-Dawson-type polyoxometallates
through an ion exchange methodology in aqueous media6.
1.4 STRUCTURE ELUCIDATION OF HETEROPOLY COMPOUNDS
Structural characterization of ion exchangers can be completed by spectroscopic
studies, XRD and thermal analysis.
1.4.1 SPECTROSCOPIC STUDIES
Infrared spectroscopy: Infrared spectroscopy (vibrational spectroscopy) is a simple and
reliable technique widely used in inorganic/organic chemistry in research and industry. It
is used in quality control, dynamic measurement and monitoring applications. Vibrational
spectroscopy (IR spectra) is an extensively used tool in polyoxometalate chemistry
mainly for structure elucidation and to identify M-O bonds in the polymeric backbone of
the ion exchanger. There are four types of metal-oxygen linkages present in the most
elaborated structure (XM12O40):
X-O-M, long and weak bond ( 4 internal oxygens connecting X-M),
M-O-M (12 edge sharing oxygen connecting M’s),
M-O-M (12 corner sharing oxygen) and
M-O, having almost double bond character (12 terminal oxygen bonding to M atom).
Introduction
5
The region of interest for Keggin type heteropoly compounds is from 1100-600
cm-1
designating the absorptions due to metal-oxygen stretching vibrations. The
stretching frequencies in lacunary heteropolyanions are different than those of Keggin
structure7. Lacunary compounds (XM11O39)
n- have a defected structure in which one
metal atom and its terminal oxygen atoms are mislaid. These anions have a central cavity
surrounded by five oxygen atoms and thus, they perform as pentadentate ligands.
Scanning Electron Microscopy & Energy Dispersive Spectroscopy (SEM &EDS): SEM
and EDS are the analytical techniques used for the elemental analysis or chemical
characterization of a sample with a high magnification images at high resolution
combined with the ability to generate localised chemical information. It is based on the
principle of interaction of the electrons of the atoms with a beam of electrons in a raster
scan pattern that make up the sample producing signals containing information about the
sample's surface topography, composition and other properties such as electrical
conductivity. A typical EDS spectrum is portrayed as a plot of x-ray counts vs. energy
peaks representing the various elements present in the sample. Its characterization
capability is due to the fundamental principle that each element has a unique atomic
structure allowing unique set of peaks on its X-ray spectrum8.
1.4.2 X-RAY DIFFRACTION TECHNIQUE
X-ray diffraction is the most reliable and advanced technique used to determine
the crystallinity of the ion exchange materials and the composition on an atomic scale.
The technique involves nondestructive methodology and yields crystal structure information
relatively easy in an atmospheric environment. XRD analysis involves the methodology by
which multiple beams of X-ray create a three dimensional picture of the density of
electrons of any crystalline structure. The wavelength of X-rays is of approximately same
order of magnitude as equivalent to the interatomic or intermolecular distance between
atoms in crystalline materials and therefore, crystals act as diffraction gratings for X-rays.
In X-ray diffraction work, we normally distinguish between single crystal and
polycrystalline or powder applications.
1.4.3 THERMAL ANALYSIS
TGA, DTA and DTG measurements give an idea about
Introduction
6
(i) Quantitative measurement of mass change in materials associated with transition
and thermal degradation.
(ii) Number and nature of water molecules present in the HPA.
(iii) Thermal stability characteristics of the exchanger material.
TGA is a technique in which the mass of a substance is monitored as a function of
temperature or time as the sample specimen is subjected to a controlled temperature
variation in a controlled atmosphere. TGA records the change in mass due to dehydration,
decomposition and oxidation of a sample with time and temperature. The results of
thermogravimetric analysis gave the evidence for the presence of two types of water
molecules in heteropoly compounds i.e. “water of crystallisation” and “constitutional
water molecules”. The total loss of crystallisation9 of water usually occurs at temperatures
170-200oC.
1.5 UNIQUE FEATURES OF HETEROPOLY COMPOUNDS
1.5.1 STRUCTURE AND MOLECULAR WEIGHT
Heteropolyacid salts have a complex cage like structure and very high molecular
weight. They are multifunctional and their chemical composition can be altered very
easily.
1.5.2 SOLUBILITY
They are insoluble in non polar solvent. But the heteropoly salts of small cations
are soluble in water. Solubility of the heteropoly compounds in water must be attributed
to very low lattice energy and solvation of cations. Solubility is governed by packing
considerations in the crystals.
1.5.3 HYDRATION
The crystalline as well as amorphous both salts confirmed the presence of great
number of water molecules forming hydrated complex. They melt in their own water of
hydration between 400C to 100
0C. In dry air, they begin to lose water molecules.
Introduction
7
1.5.4 ACIDIC BEHAVIOUR
They are strong bronsted acids. Therefore, act as strong oxidizing agents and can
be very readily changed to fairly stable reduced form and these reduced species are called
“heteropoly blues”. Structure, composition of heteropolyanions, extent of hydration, type
of support, thermal treatment, etc. govern the acidic and related properties of the HPA’s.
Solid heteropoly compounds are pure bronsted acids.
1.5.5 COLOUR
Most of the HPA’s are white. But existence of coloured compounds can not be
denied.
1.5.6 STABILITY
They exhibit the structural mobility. All heteropolyacid and their salts
decomposed in strongly basic solutions. The final degradation products of these
compounds are simple oxometalates ions. Thermal stability is also quite high. The
decomposition at high temperatures causes loss of water molecules, consequently
modification in acidity also. Heteropolyacids are generally used as solid acid catalysts for
vapour phase reactions at high temperatures.
1.5.7 MULTIFUNCTIONALITY
Partial exchange of protons which are loosely bonded with other cations can alter
both the structural features as well as catalytic properties. They behave as cation
exchanger due to the presence of exchangeable protons contained in the structural
hydroxyl groups. HPAs possess appropriate redox properties, which can be redesigned by
varying the chemical composition of heteropolyanion.
1.5.8 CATALYSIS
They can function as homogeneous as well as heterogeneous catalysts for the
organic syntheses. Their use gives environmentally benign results.
1.6 INORGANIC ION-EXCHANGERS
The key component of a sensor is the ionophore or electroactive component which
imparts the selectivity and enables the sensor to respond selectively to a particular
Introduction
8
analyte. These inorganic compounds in the form of ion exchange materials have found
extensive applications in analytical chemistry as well as in industrial chemistry in view of
their elegant characteristics such as insoluble matrix, stoichiometric exchange, good
selectivity, specificity and applicability to column operations. Synthetic inorganic ion
exchangers are useful in the separation of radioactive materials and other analytical
applications can also be achieved even at elevated temperatures. Organic resins are not
suitable for such applications, as they will exhibit severe decline in ion exchange capacity
as well as selectivity on exposure to radiations, owing to physical degradation at both the
molecular and macroscopic leval. Besides, the above limitations, degradation also
accounted at high temperatures. Inorganic ion exchangers can be implanted in an inert
binder material like PVC or epoxy resin for membrane fabrication and the glass transition
temperature (Tg) of the polymer defines the compatibility as a sensing membrane10
. If a
polymer is having high Tg value, then inorganic ion exchangers exhibit high selectivity
for specific ions resulting much larger separation than those exhibited by organic resins.
Rigid structure and no appreciable dimensional change during ion exchange reactions,
leads to specific and unusual selectivities11
. Hydrous oxides incorporated with anions
such as phosphates, vanadates, molybdates and antimonates produced excellent ion
exchangers12-15
.
Inorganic ion exchangers were first classified by Vesely and Pekarek16
in 1972. In
1988, Clearfield17
classified ion exchangers with significant ion exchange capacity into
14 categories. Again in 1995, Clearfield18
classified ion exchangers into three categories
based on their structures:
1.6.1 HYDROUS OXIDES
This class principally contains hydrous oxides of zirconium and tin. These
hydrous oxides have also been referred to as framework hydrates as expalined by England
et al19
.
1.6.2 LAYERED ION-EXCHANGERS
These ion exchangers are either oxides or phosphates of group 4 and 14 or layered
double hydroxides. In group 4 and 14, phosphates are the most extensively studied
exchangers of this class.
Introduction
9
1.6.3 EXCHANGERS WITH TUNNEL STRUCTURES
Exchangers with tunnel structure were first reported by Clearfield20
. Clearfield
and Stynes did the pioneer work in the field of inorganic ion exchanger by crystallizing
zirconium phosphate21
. Ammonium phosphotungstate and ammonium molybdophosphate
were probably the foremost heteropoly compounds, which have been studied as ion
exchangers22
.
1.7 CHARACTERIZATION OF ION-EXCHANGER
An ion exchanger is characterized by the parameters like:
1.7.1 ION-EXCHANGE CAPACITY
The ion exchange capacity conveys about the number of ionogenic groups
contained per gram of the exchanger material, when the material is completely
converted to H+
form, which is devoid of sorbed solutes and solvents. It can be expressed
as the quantity of ions that can be swapped by a specific volume of the resin. The
characteristic constant obtained in this way is called scientific weight capacity and is
expressed in units of milli-equivalents per gram (meq/g). Equivalents refer to the
equivalent weight (EW) of the substance expressed in grams or meq in milligrams (mg).
Ion exchange capacity of a material is determined for its characterization as an ion
exchanger. The ion exchange capacity can be estimated very easily by means of column
operation methodology. However, most of the synthetic inorganic ion exchangers behave
as weak ion exchangers and therefore, their direct titration is not practicable, where H+
ions of the exchanger are replaced by cations of a neutral salt and then, the equilibrium
ion exchange capacity is determined by pH titration.
1.7.2 SORPTION STUDIES
Distribution of an ion between exchanger phase and the solvent gives the measure
of the selectivity of the exchanger for that particular ion. Distribution coefficients and
consequential selectivity, helps to predict the separation of different counter ions. Batch
adsorption procedure was followed to carry out the sorption studies. The selectivity may
depend upon following factors:
Donnan potential
Sieve action
Introduction
10
Complex formation
The Distribution coefficient (Kd) value were calculated after attainment of the equilibrium
from the equation
Kd = I
FI X
2.0
20
Where I is the initial volume of EDTA used to neutralise the metal ion, initially and F is
the final volume of EDTA (after the attainment of equillibrium) used to neutralise the
metal ion.
1.8 ELECTROCHEMICAL PROPERTIES OF ION-EXCHANGE MEMBRANES
According to the theory of permselectivity, properties of certain natural and
artificial membranes can be explained by the existence of the easily dissociated ions that
are loosely bounded to the fixed groups of the opposite charge. Above properties help
inorganic membranes for their use in cells for electrolytic desalting of brackish water23
,
fuel cells and electrical storage batteries24
. The basic theory of permselective membranes
and their electrochemical properties was first developed by Teorell and then by Meyer
and Sievers25-26
. An ideal permselective membrane is difficult to produce but can be
approximated by careful choice of such membrane variables like the composition and
physicochemical properties.
These ion-exchange materials have the following analytical applications:
Ion-selective electrodes.
To locate the end point in indicator titrations.
Ion-exchange paper chromatographic separations.
Micro determination by ion exchange colorimetry.
Electrophoresis.
Purification of substances.
Separations of metal ions, catalysis and use of ion exchange membranes as ion
selective electrodes are the most widely accepted applications. As inorganic ion
exchangers are solid and insoluble. They can be easily incorporated into some inert
polymeric matrix so as to give heterogeneous membranes.
A sensor material should posses the following qualities:
Compatible with the matrix,
Introduction
11
Right proportion,
Appropriate grain size (1-15nm),
Rapid ion exchange at the membrane-solution interface with low solubility
product.
1.9 APPLICATION AS ION-SELECTIVE ELECTRODE
More than thousands of ion exchangers have been synthesised during the last few
decades. Majority of these are amorphous in nature but crystalline form can also be
synthesised after digestion and then their structure elucidation was done to elaborate their
nature, composition and structure. Ion exchanger today forms one of the most important
groups of chemical sensors. However, for many analytes especially for anions and heavy
metal ions, considerable efforts have led to the development of selective sensors for
alkali, alkaline earth and rare earth metal ions27-30
. Ruzicka et al31
were the first
investigators to introduce liquid state electrode based on carbon.
Potentiometric sensors have found the most widespread practical applicability
since the early 1930’s, due to their simplicity, familiarity and cost.
Types of Potentiometric Sensors
(a) Ion-selective electrodes (ISE),
(b) Field-effect transistors and
(c) Potentiometric solid state sensors with new sensing systems, namely solid contact
electrodes32
(SCE).
Synthetic ion exchangers are mixed with inert binders, like SBR, epoxy
resin, polystyrene or polyvinyl chloride etc. in an optimum proportion so as to provide
working strength to the membrane. A large number of such electrodes have been prepared
by Mittal et al33-39
and applied for the quantitative analysis of analyte samples.
1.9.1 THEORY OF POTENTIOMETRIC ION SENSORS
Ion sensors work on the principle of chemical sensing in which information is
obtained about the chemical composition of a system with the help of an amplified
electric signal produced by it. It involves two sequential steps: recognition and
intensification. Interaction of an ion with the electrode is highly precise. The signal
Introduction
12
(electrode) is amplified (pH meter) in order to obtain the information in an applied and
authentic form. The recognition (selectivity) is based on assorted chemical interaction,
while a transducer translates the magnitude of the signal into a measure of the amount of
the analyte in different perceivable ways. Two types of interactions are there: (i) surface
interaction where the interested species is adsorbed at the surface and (ii) bulk interaction
in which the species under consideration partitions between the sample phase and the
sensor.
Potentiometric ion sensors are based on the principle of sensing the metal ion by
ion selective membrane. These sensors make use of the development of an electrical
potential at the surface of a solid material when it is placed in a analyte solution. Ions
penetrate the boundary between two phases generating an electrochemical equilibrium, in
which potentials difference between two phases are formed. The magnitude of the
potential difference generated between the phases is governed by activities of the target
ion in these phases and the measurement of the cell potential is made under a 'zero
current' condition. Equilibrium means the transference of analyte ions from the membrane
into solution is equivalent to the transfer of the same from the solution to the membrane.
If the ion selective membrane separates two solutions of different ionic activities (a1 and
a2) and mounted membrane is permeable to only specific ion, then the potential difference
(E) across the membrane is described by the Nernst equation:
E = E0+ RT/ z F . ln ( a2 / a1 ) --------------------------------------- 1.1
If activity of the target ion (a1) in its respective phase is kept constant, the unknown
activity (ax) in phase 2 is related to EMF generated:
E = constant + RT/ zx F . log (ax / a1) = constant + S. log10 (ax) ------ 1.2
Where, S = 59.16 / zx mV at 298 K
and zx = charge of the analyte.
The potential difference can be measured between two identical reference
electrodes placed in two phases. Practically, the potential difference (electromotive
Introduction
13
force) is measured between an ion selective electrode and a reference electrode,
placed in the sample solution and the observed signal is a collective measurement of
potentials difference created at all solid-solid, solid-liquid and liquid-liquid interfaces.
A series of analyte ion solutions can be used to draw the response
curve/calibration curve of an ion selective electrode. Plot of electromotive force versus
the activity of analyte is used to find the linear range which in turn explains the activity of
the target ion in any unknown solution. However, it should be specified that only at
constant ionic strength, a linear relationship between the signal measured and the
concentration of the analyte is maintained as the activity is directly proportional to the
concentration.
Practically, there is not even a single membrane which is truly selective for a
single type of ions and completely non-selective for other ions. The potential of such a
membrane is not only governed by the activity of the primary (target) ion and but also by
the activity of other interfering ions. The consequence of the presence of interfering
species in a sample solution on the measured potential difference can be taken into
consideration in the Nikolski-Eisenman formalism:
E = E0 + S. [ log (ax ) + (zx / zy) . log (Kxy.ay) ----------------------- 1.3
where, ay is the activity of an interfering ion, zy is its charge and Kxy represents the
selectivity coefficient (determined experimentally).
1.9.2 CHARACTERIZATION AS ION SELECTIVE ELECTRODE
Characterization as ion selective electrode can be done with the help of following
properties:
Selectivity Coefficient: Selectivity coefficient (KA,Bpot) is an important parameter of ISEs
as it often determines whether a reliable measurement in the sample is possible or not.
Selectivity means the extent to which it can determine particular analyte in a complex
mixture without any interference of any other components present in the mixture40
. Then
the term “selectivity” promoted and definition was revised as Selectivity is the
Introduction
14
recommended term in analytical chemistry to express the extent to which a particular
method can be used to determine analytes under given conditions in the presence of other
components of similar behavior41
. Different methods of the selectivity determination
can be found in the literature.
The IUPAC suggests three methods: Separate Solution Method (SSM), Fixed
Interference Method (FIM) and Matched Potential Method (MPM). If the electrode
exhibits non-Nernstian response, the values of KA,Bpot obtained by the SSM, FIM or MPM
method depends up on the activity of the interfering ion. Gadzepko et al determined the
selectivity coefficient of the membrane electrode against a number of interfering ions by
using Matched Potentioal Method42
(MPM).
Ion exchangers prefer one species over another due to several causes:
Electrostatic interaction between the charged framework and the counter ions
Size, valency and nature of the counter ion.
Interactions between ions and their environment.
Sterically exclusion from the narrow pores of the ion exchanger.
All these effects may lead to preferential uptake of a species by the ion exchanger.
This precise ability of the ion exchanger to distinguish between the various counter ion
species is called selectivity.
Slope: According to the Nernst equation, slope of the linear part of the calibration curve
of the electrode is
59.16 mV/decade at 298 K for a single charged analyte ion,
59.16/2 = 29.58 mV per decade for a double charged analyte ion,
59.16/3= 19.72 mV per decade for a triple charged analyte ion.
However, in certain applications the value of the electrode slope is not in
accordance with Nernstian equation but this deviation from Nernstian slop does not
exclude its usefulness.
Linear Range and Detection Limit: According to IUPAC recommendations, the
detection limit is defined by the cross-section of the two extrapolated linear parts of the
Introduction
15
ion selective calibration curve. The observed detection limit is often governed by the
presence of other interfering ions or impurities. Electrode calibration curve generally
exhibits a linear response range between 10-6
and 10-1
M, although at a high and very low
target ion activities, there are deviations from linearity. It is possible to enhance the
detection limit down to 10-10
M with the help of suitable metal buffers.
Response Time: According to IUPAC recommendations, it was defined as the time at
which the ion concentration in a solution is changed on contact with ISE and a
reference electrode and it is the first instant at which the potential of the cell becomes
equal to its steady-state value within 1 mV or has reached 90% of the final equilibrium
value. This definition can be extended to consider the drift of the system. In this case, the
second time instant is defined as the time at which the e.m.f vs. time slope becomes equal
to a limiting value. Practically, it is calculated by measuring the time required to achieve
90% of the equilibrium potential from the moment of addition of metal ion solution.
Nature of Solution: Varying conditions such as pH and the non aqueous medium are
taken into consideration.
Lifetime: Lifetime of a sensor refers to the time period during which the sensor can be
used effectively for the determination of the analyte and it is determined by the stability
of the ion exchanger material.
1.9.3 RARE EARTH METAL IONS
Rare earth elements have been traditionally defined as 4f block elements with
atomic numbers 57-71 as well as the elements yttrium and scandium which behave
chemically similar to the lanthanide elements. The electronic configuration43
of free
atoms of most of the lanthanide series is generally accepted to be [Xe] 4fn5d
06s
2.
Coordination: The lanthanide ions have a high charge, which favour the formation of
complexes. They are strong reducing agents. Lanthanides have high coordination number
varying from 6-9 and give a variety of stereochemistry. Coordination numbers 10 and 12
occur in the larger lanthanides and small chelating ligands NO3- and SO4
2-. The complex
Introduction
16
[Ce(NO3)6]3-
has one of the largest, 12-coordinated geometry known among
lanthanides44
.
Many techniques have been applied to study the coordination chemistry of rare
earth elements45
. However, the selective determination of these metal ions is complicated
by virtue of similarity in their chemical properties and the fact that they all exist mostly as
trivalent cations in solution46
.
Toxicity: Rare earth elements are extremely toxic if they tend to remain in the lungs,
lever, spleen and kidneys47
. The toxicity of rare earth elements causes loss of muscle
coordination, labored respiration and cardiovascular collapse. Rare earths can degrade
DNA molecules and have been reported to produce tumors at the site of interaction48-49
.
They can also bond with plasma proteins and accumulate in bones50
.
Uses: lanthanides are generally used as catalysts in the production of glasses,
superconductors, in optoelectronics applications, cathode ray tube technology and
television sets. These are used in making colour TV's tubes and LED and also used for
ulcer therapy and edema therapy studies51-53.
Uses of resins as artificial kidneys, bacterial
adsorbents, catalysts etc. testify to the widespread applications of these materials. In
laboratories, ion-exchangers are used as an aid in analytical and preparative chemistry.
Ion exchangers have the distinction of being the only successful technique for their
separation and selective determination from mixtures.
1.10 APPLICATION OF HETEROPOLY COMPOUNDS AS CATALYST
Catalysis by heteropolyacids (HPA) and related compounds is a field of growing
importance, attracting attention worldwide in which many novel and exciting
developments are taking place both in the area of research and technology and their
compounds act as consistent candidates as green, eco-friendly and effective catalysts55-57
.
Although, there are many structural categories of HPAs, but the majority of the
catalytic applications constitute the most extensively studied and important class of
polyoxometalates, Keggin-type HPAs because of easy availability and chemical
stability58
. The Keggin structures possess qualities such as good thermal stability, high
acidity, undergo fast reversible multi-electron transfers, mobility of protons at the
Introduction
17
electrocatalytic interfaces and high oxidising ability. Other catalysts such as Wells-
Dawson and Preyssler heteropolyacids have begun to be used2,59
. The use of conventional
liquid and Lewis acids such as sulphuric acid, hydrochloric acid, hydrofluoric acid, zinc
chloride, aluminum trichloride and boron trifluoride are no longer considered as a reliable
means for synthesis because of numerous shortcomings and therefore, there is an urgent
need to eliminate the aggressive mineral acid from the acid catalysed processes60
. Global
efforts are going on to replace the conventional liquid acid catalysts by the solid acid
catalyst.
1.10.1 LIMITATIONS OF CONVENTIONAL ACID CATALYST
Though, the mineral and Lewis acids are widely used in different industries for the
production of quantitative yield of the products, they are having certain limitaions:
Poses threat in handling,
Risky disposal,
Difficulty in regeneration due to their corrosive nature,
Sophisticated storage,
Highly volatile,
Toxicity,
Contamination of products,
Purification by costly methods,
Liquid acids also involve large investment for treating the effluents.
1.10.2 ADVANTAGES OF USING HETEROPOLYACIDS SALTS AS CATALYST
FOR VARIOUS SYNTHESES
Activity: Molar catalytic activity of heteropolyacid salts is thousands times greater than
that of mineral acids.
Separation: Being heterogenous in nature, they can be easily recovered from the reaction
mixture by simple filtration. Moreover, the exhausted catalyst can be regenerated and
reused after activation or without activation, thereby making the process economically
viable.
Introduction
18
Reuse: In many cases, heterogeneous catalysts can be recovered with only minor change
in activity and selectivity so that they can be conveniently used in continuous flow
reactions reducing the generation of waste byproducts61-63
.
Prevention from corrosion: Use of solid acids eliminates the corrosive action of liquid
acids.
Ecofriendly: Use of solid (heterogeneous) catalysts in organic synthesis and in the
industrial manufacturing of chemicals is important since they provide green alternatives
to the homogeneous catalysts64
.
Numerous developments are being carried out in basic research as well as in fine
chemistry processes65
. In recent times, inorganic solid acid catalysed organic
transformations are gaining much more importance due to the proven advantage of
heterogeneous catalysts. Heterogeneous catalysts are used extensively in protection or
deprotection procesess in organic synthesis66
. They may find wide applications in fine
chemical industry such as fragrance, pharmaceutical and food67-68
etc.
1.10.3 REACTIONS PROMOTED BY HETEROPOLYACID SALTS
Cyclisation & Isomerisation
Alkylation & Dealkylation
Hydration & Dehydration
Esterification & Trans-esterification
Condensation & Hydrolysis
Oxidation & deoxidation
Acylation
Etherification
Polymerisation
Protection reactions
Reduction and various other miscellaneous reactions involve heteropolyacid
salts as catalyst.
Introduction
19
Salts of heteropoly acids have been studied in different organic transformations
under heterogeneous conditions69-70
. They play a vital role in the treatment of
environmental pollutants. Keggin-type heteropolyacids show remarkable acid catalytic
properties71-74
that are of practical importance as illustrated in numerous reviews75-78
. The
fact that heteropolyanions undergo spontaneous adsorption (from aqueous solution) on
various substrates provides a simple mechanism for modification of electrode surfaces.
The applications of heterogeneous acid catalysts in place of homogeneous acid catalysts
in organic synthesis is the interesting area of research in the laboratory as well as in the
industrial context. Because of exclusive physicochemical properties, they have found
various industrial applications, in numerous processes79-81
. The use of HPAs salt in non-
polar solvents improves product selectivity and moreover, separation of HPA's salt from
the reaction mixture is effortless7.
1.10.4 ACID CATALYSIS IN HETEROPOLYACIDS SALTS
The catalytic property of the heteropoly compounds is associated with the strong
bronsted acidity, which is comparatively superior to the mineral acid catalysts. They are
highly efficient and selective in catalytic reactions. Moreover, due to the inert nature of
the anions, they do not pop in side reactions like chlorination, sulphonation and nitration
as observed in mineral acids. The negative charge of the heteropolyanion spread over a
much larger surface area resulting in less interaction but easy dissociation of the proton.
In aqueous and non-aqueous solutions, the heteropolyacid salts consists of solvate ion-
pairs H+(H2O)m-HPA
n- in which the protons are hydrated and linked to the anion as a
whole and not to a specific centre in heteropolyanion. Higher the negative charge on the
central sphere, larger will be its acid strength. That’s why; generally tungstophosphoric
acids and its salts are most widely employed as compared to other HPA containing other
metal ions. They are easy to use and no side reactions are observed with the
heteropolyacid salts.
Introduction
20
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Introduction
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