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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