Ion Exchange Ion Exchange Chromatography Chromatography
UNIT
UNIT 9
9 ION
ION EXCHANGE
EXCHANGE
CHROMATOGRAPHY
CHROMATOGRAPHY
Structure Structure 9.1 9.1 IntroductionIntroduction Objectives Objectives 9.29.2 Basic Features of Ion Exchange MechanismBasic Features of Ion Exchange Mechanism 9.3
9.3 Classification of Ion ExchangersClassification of Ion Exchangers Natural Ion Exchangers
Natural Ion Exchangers Synthetic Ion Exchangers Synthetic Ion Exchangers Liquid Ion Exchangers Liquid Ion Exchangers 9.4
9.4 Synthesis of Ion Exchange ResinsSynthesis of Ion Exchange Resins Cation Exchangers Cation Exchangers Anion Exchangers Anion Exchangers Amphoteric Exchangers Amphoteric Exchangers 9.5
9.5 Trade Names and NomenclatureTrade Names and Nomenclature 9.6
9.6 Resin PropertiesResin Properties Moisture Content Moisture Content Particle Size Particle Size Cross Linkages Cross Linkages Capacity Capacity Distribution Ratio Distribution Ratio Equivalency of Exchange Equivalency of Exchange Resin Selectivity Resin Selectivity 9.7
9.7 Operating MethodsOperating Methods Batch Operation Batch Operation Column Operation Column Operation Moving Bed Operation Moving Bed Operation 9.8
9.8 Ion Exchange in Mixed Aqueous - Organic MediaIon Exchange in Mixed Aqueous - Organic Media 9.9
9.9 Specific Cation ExchangersSpecific Cation Exchangers 9.10
9.10 Synthetic Inorganic Ion Synthetic Inorganic Ion ExchangersExchangers Different Types and Their Characteristics Different Types and Their Characteristics Special Properties and Applications Special Properties and Applications 9.11
9.11 ApplicationsApplications
Separation of Metal Ions and Anions Separation of Metal Ions and Anions Separation of Organics
Separation of Organics
Separation of Ionized from Nonionized Separation of Ionized from Nonionized Separation of Actinide Elements Separation of Actinide Elements Miscellaneous Applications Miscellaneous Applications 9.12
9.12 SummarySummary 9.13
9.13 Terminal QuestionsTerminal Questions 9.14
9.14 AnswersAnswers
9.1
9.1
INTRODUCTION
INTRODUCTION
Amongst various separation techniques, ion exchange is the
Amongst various separation techniques, ion exchange is the most popular namemost popular name because of its use for water softening. It is a
because of its use for water softening. It is a lso unique in terms of its versatility andlso unique in terms of its versatility and historical developments. Besides the well-known use of ion
historical developments. Besides the well-known use of ion exchangers in waterexchangers in water treatment, they find use
treatment, they find use in industry, nuclear fuel processing, hydrometallurgy,in industry, nuclear fuel processing, hydrometallurgy, agriculture and biology. The treatment of water by solid adsorbents is as old as agriculture and biology. The treatment of water by solid adsorbents is as old as civilization. There are records available that in
civilization. There are records available that in the time of Aristotle, sand filters werethe time of Aristotle, sand filters were used for purification of sea water. Moses used a tree branch for making bitter water used for purification of sea water. Moses used a tree branch for making bitter water sweet. But the credit of
sweet. But the credit of recognizing the ion exchange phenomrecognizing the ion exchange phenomenon goes to twoenon goes to two agricultural chemists-Thompson and Way. They observed the
agricultural chemists-Thompson and Way. They observed the exchange of ammoniumexchange of ammonium ions with calcium ions in soils. The
ions with calcium ions in soils. The realization of the fact that crealization of the fact that certain clay mineralsertain clay minerals were responsible for the exchange, led to the attempts to use
were responsible for the exchange, led to the attempts to use such materials for watersuch materials for water softening. It also prompted scientists to synthesize materials with similar properties. softening. It also prompted scientists to synthesize materials with similar properties.
Chromatographic Chromatographic Methods-III
Methods-III The first synthetic ion exchanger was prepared in 1903 by two GermanThe first synthetic ion chemists-Harm and Rumpler. Another German, Gans, chemists-Harm and Rumpler. Another German, Gans, worked on several pioneeringexchanger was prepared in 1903 by two Germanworked on several pioneering applications of permutits. But the permutits could not stand in the market because of applications of permutits. But the permutits could not stand in the market because of their poor reproducibility and chemical stability.
their poor reproducibility and chemical stability.
A real breakthrough in the subject came in 1935 when two English chemists, Adams, A real breakthrough in the subject came in 1935 when two English chemists, Adams, and Holmes, observed that
and Holmes, observed that crushed phonograph record exhibited ion exchangecrushed phonograph record exhibited ion exchange properties. This observation led to the
properties. This observation led to the synthesis of several organic ion synthesis of several organic ion exchangersexchangers which had better properties. It was illustrated that stable and high capacity cation which had better properties. It was illustrated that stable and high capacity cation exchangers could be prepared as sulphonic acid resins and
exchangers could be prepared as sulphonic acid resins and polyamine type resinspolyamine type resins exhibited anion exchange properties. The area of
exhibited anion exchange properties. The area of ion exchange blossomed at a ion exchange blossomed at a veryvery fast rate. The versatility of
fast rate. The versatility of ion exchange resins was readily recognized. Many attemptsion exchange resins was readily recognized. Many attempts have been made to modify and improve the existing materials. It is possible to
have been made to modify and improve the existing materials. It is possible to tailortailor make ion exchange resins for specific
make ion exchange resins for specific applications.applications. Ion exchange
Ion exchange is firmly established as is firmly established as a unit operation. All over the world, numerousa unit operation. All over the world, numerous plants are in operation accomplishing the tasks that range from the recovery of metals plants are in operation accomplishing the tasks that range from the recovery of metals from industrial wastes to the separation of rare earths and from catalysis of organic from industrial wastes to the separation of rare earths and from catalysis of organic reactions to the decontamination of cooling water of nuclear reactors. In the
reactions to the decontamination of cooling water of nuclear reactors. In the laboratory, ion exchangers prove themselves as useful materials for a
laboratory, ion exchangers prove themselves as useful materials for a ccomplishingccomplishing analytical separations. The ion exchange membranes find quite a good use in analytical separations. The ion exchange membranes find quite a good use in physiological chemistr
physiological chemistry and biophysics. y and biophysics. Ion exchangIon exchange separation played a major rolee separation played a major role in the identification of trans-urani
in the identification of trans-uranium elements by Glen T. Seaborgum elements by Glen T. Seaborg. . The identify ofThe identify of each element of 5
each element of 5 f f series was series was established beyond any doubt by the sequence of established beyond any doubt by the sequence of theirtheir appearance a analogous to the
appearance a analogous to the appearance of the corresponding 4appearance of the corresponding 4 f f elements elements The above applications clearly indicate that a variety of
The above applications clearly indicate that a variety of ion exchangers are availableion exchangers are available and these materials can be used for different applications. In view of this,
and these materials can be used for different applications. In view of this, it isit is important to understand the basic ion
important to understand the basic ion exchange mechanism, and a broad classificationexchange mechanism, and a broad classification of ion exchangers. Ion exchange resins are s
of ion exchangers. Ion exchange resins are s ynthesized by following differentynthesized by following different chemical routes. An idea about it can be
chemical routes. An idea about it can be had by illustrating the synthesis of some well-had by illustrating the synthesis of some well-known ion exchange resins. The practical utility
known ion exchange resins. The practical utility of an ion of an ion exchanger depends upon itsexchanger depends upon its properties, both chemical and physical. Another point which is important in this properties, both chemical and physical. Another point which is important in this context is as to how the
context is as to how the material is being operated. The discussion on ion exchangersmaterial is being operated. The discussion on ion exchangers will not be complete if we
will not be complete if we do not talk about some special type of do not talk about some special type of ion exchangersion exchangers vizviz.. chelating resins and synthetic inorganic ion
chelating resins and synthetic inorganic ion exchangers. Finally, a discussion onexchangers. Finally, a discussion on various types of applications will be taken up. It may be
various types of applications will be taken up. It may be noted that some of these usesnoted that some of these uses may not be directly based on separations.
may not be directly based on separations. Objectives
Objectives
After studying this Unit, you should be able to After studying this Unit, you should be able to
•
• discuss basic ion exchange mechanism,discuss basic ion exchange mechanism, •
• classify different types of ion classify different types of ion exchangers,exchangers, •
• describe the synthesis of describe the synthesis of ion exchange resins,ion exchange resins, •
• explain the properties which characterize an explain the properties which characterize an ion exchanger,ion exchanger, •
• describe the operating methods for ion describe the operating methods for ion exchangers,exchangers, •
• explain the behaviour of specific cexplain the behaviour of specific c ation exchangers,ation exchangers, •
• present a complete picture about the different types of synthetic inorganic ionpresent a complete picture about the different types of synthetic inorganic ion
exchangers and their advantages alongwith
exchangers and their advantages alongwith applications, andapplications, and
•
Ion Exchange Ion Exchange Chromatography Chromatography
9.2
9.2
BASIC FEATURES OF ION EXCHANGE
BASIC FEATURES OF ION EXCHANGE
MECHANISM
MECHANISM
The term
The term ion exchangeion exchange generally means exchange of ions of like sign between a generally means exchange of ions of like sign between a solution and a solid highly insoluble in it. The
solution and a solid highly insoluble in it. The solid known as ion exchanger carriessolid known as ion exchanger carries exchangeable cations and anions. When the exchanger is in contact with an electrolyte, exchangeable cations and anions. When the exchanger is in contact with an electrolyte, these ions can be exchanged for a stoichiometrically equivalent amount of other ions these ions can be exchanged for a stoichiometrically equivalent amount of other ions of same sign. Carriers of exchangeable cations are known as
of same sign. Carriers of exchangeable cations are known as cation exchangerscation exchangers and and carriers of exchangeable anions as
carriers of exchangeable anions as anion exchangersanion exchangers. Certain materials are . Certain materials are capable ofcapable of both cation and anion exchange.
both cation and anion exchange. These are known asThese are known as amphoteric exchangersamphoteric exchangers.. A typical cation exchange reaction is shown below:
A typical cation exchange reaction is shown below: 2 NaX + CaCl
2 NaX + CaCl2(aq)2(aq) CaXCaX22 + 2 NaCl + 2 NaCl(aq)(aq)
Similarly, typical anion exchange reaction is
Similarly, typical anion exchange reaction is as follows:as follows: 2 XCl + Na
2 XCl + Na22SOSO4(aq)4(aq) XX22SOSO44 + 2 NaCl + 2 NaCl(aq)(aq)
where, X represents a structural unit of the ion e
where, X represents a structural unit of the ion e xchanger.xchanger. In the first process, a solution containing dissolved CaCl
In the first process, a solution containing dissolved CaCl22, say something like hard, say something like hard
water, is treated with a
water, is treated with a solid exchanger, NaX, containing exchansolid exchanger, NaX, containing exchangeable Nageable Na++ ions. The ions. The exchanger removes the Ca
exchanger removes the Ca2+2+ ions from the solution and replaces them with Na ions from the solution and replaces them with Na++. Thus,. Thus, a cation exchanger in Na
a cation exchanger in Na++ form is converted to Ca form is converted to Ca2+2+ form. form. Ion exchange, with very few exceptions, is a
Ion exchange, with very few exceptions, is a reversiblereversible process. In water softening, a process. In water softening, a cation exchanger has lost its Na
cation exchanger has lost its Na++ ions and can be regenerated with a ions and can be regenerated with a solution of asolution of a sodium salt such as NaCl. Ion exchange resembles adsorption in that, in both cases, a sodium salt such as NaCl. Ion exchange resembles adsorption in that, in both cases, a dissolved species is taken up by a
dissolved species is taken up by a solid. The characteristic difference between the twosolid. The characteristic difference between the two is that the
is that the ion exchange in contrast to sorption, is a stoichiometric process. Every ionion exchange in contrast to sorption, is a stoichiometric process. Every ion removed from the solution is
removed from the solution is replaced replaced by an equivalent amount of another ionic by an equivalent amount of another ionic species of the same sign. However, in the c
species of the same sign. However, in the c ase of sorption a solute, an electrolyte orase of sorption a solute, an electrolyte or non-electrolyte, may be taken up without
non-electrolyte, may be taken up without any species being replaced.any species being replaced.
Ion exchangers owe their characteristics to a particular feature of their structure. They Ion exchangers owe their characteristics to a particular feature of their structure. They are built of a f
are built of a framework which is held together by chemical bonds or lattice energy.ramework which is held together by chemical bonds or lattice energy. The framework carries a positive or
The framework carries a positive or negative surplus charge which is compensated bynegative surplus charge which is compensated by ions of opposite charge, called
ions of opposite charge, called counter ionscounter ions. The counter ions are free to . The counter ions are free to move withinmove within the framework and be replaced by other ions of same sign. The framework of cation the framework and be replaced by other ions of same sign. The framework of cation exchanger may be regarded as a macromolecule or a crystalline polyanion, that of an exchanger may be regarded as a macromolecule or a crystalline polyanion, that of an anion exchanger as a polycation.
anion exchanger as a polycation.
From the above discussion, it emerges out that a useful ion
From the above discussion, it emerges out that a useful ion exchanger must have theexchanger must have the following requisites:
following requisites: i)
i) It should have negligible solubility in the medium to be used.It should have negligible solubility in the medium to be used. ii)
ii) It must contain sufficient number of It must contain sufficient number of accessible ion exchange groups and it accessible ion exchange groups and it mustmust be chemically stable.
be chemically stable. iii)
iii) It should be sufficiently hydrophilic to permit It should be sufficiently hydrophilic to permit diffusion of ions through thediffusion of ions through the structure at a finite and usable rate.
structure at a finite and usable rate. iv)
iv) The swollen exchanger must be denser than The swollen exchanger must be denser than water.water. SAQ 1
SAQ 1 What is the
What is the basic difference between adsorption and ion exchange?basic difference between adsorption and ion exchange?
………... ………...
Chromatographic Chromatographic Methods-III
Methods-III SAQ 2SAQ 2
A sodium phosphate solution is passed through a
A sodium phosphate solution is passed through a column of an anion exchanger in column of an anion exchanger in thethe chloride form. The PO
chloride form. The PO443−3− ions are taken up by the ion ions are taken up by the ion exchanger. Write down the ionexchanger. Write down the ion
exchange equilibria. exchange equilibria. ………... ………... ………... ………... ………... ………... ………... ………...
9.3
9.3
CLASSIFICATION OF ION EXCHANGERS
CLASSIFICATION OF ION EXCHANGERS
Many different natural and synthetic products show ion exchange properties. These Many different natural and synthetic products show ion exchange properties. These exchangers can be either cation
exchangers can be either cation or anion exchangers. Therefore, a simple broador anion exchangers. Therefore, a simple broad classification can be as
classification can be as i)
i) NaturalNatural ii)
ii) SyntheticSynthetic
However, within these two categories the material can be However, within these two categories the material can be i)
i) OrganicOrganic ii)
ii) InorganicInorganic
For the purposes of simple presentation, we will select the first
For the purposes of simple presentation, we will select the first classificationclassification i.e.i.e.,, natural and synthetic.
natural and synthetic. 9.3.1
9.3.1 Natural Ion ExchangersNatural Ion Exchangers
Most of the natural ion exchange materials are crystalline aluminosilicates with cation Most of the natural ion exchange materials are crystalline aluminosilicates with cation exchange properties. The typical representative of this
exchange properties. The typical representative of this group of materials aregroup of materials are zeolites zeolites which include among others, the minerals l
which include among others, the minerals l ikeike analciteanalcite Na[SiAlO Na[SiAlO66]]22. H. H22O,O, chabazitechabazite
(CaNa)[SiAlO
(CaNa)[SiAlO66]]22.6H.6H22O andO and naturalitenaturalite Na Na22[Si[Si22AlAl22OO1010].2H].2H22O. All these minerals have aO. All these minerals have a
relatively open three
relatively open three dimensional framework with channels and dimensional framework with channels and interconnectinginterconnecting cavities in the aluminosilicate lattice. The zeolite lattice consists of SiO
cavities in the aluminosilicate lattice. The zeolite lattice consists of SiO44 and AlO and AlO44
tetrahedra. These have their oxygen atoms
tetrahedra. These have their oxygen atoms in common. Because aluminium isin common. Because aluminium is trivalent, the lattice carries a
trivalent, the lattice carries a negative charge. The charge is balanced by alkali andnegative charge. The charge is balanced by alkali and alkaline earth cations which do not occupy fixed positions and are free to move in the alkaline earth cations which do not occupy fixed positions and are free to move in the lattice framework. These ions behave as
lattice framework. These ions behave as counter ions and can exchange with othercounter ions and can exchange with other counter ions.
counter ions.
There are other aluminosilicates with
There are other aluminosilicates with loose layer structure having cation exchangeloose layer structure having cation exchange properties. These materials carry their counterions in between the layer of the lattice. properties. These materials carry their counterions in between the layer of the lattice. The typical mineral of this type is
The typical mineral of this type is montmorillonitemontmorillonite with with the approximate compositionthe approximate composition Al
Al22[Si[Si44OO1010(OH)(OH)22].].nnHH22O. Such minerals swell in one direction increasing the interlayerO. Such minerals swell in one direction increasing the interlayer
distance. distance.
It may be important to mention here that
It may be important to mention here that certain aluminosilicates can also behave ascertain aluminosilicates can also behave as anion exchangers. In montmorillonite, kaolinite and feldspar of sodalite and c
anion exchangers. In montmorillonite, kaolinite and feldspar of sodalite and c ameriniteamerinite groups the exchange of OH
groups the exchange of OH–– for Cl for Cl––,, 22−− 4 4 S SO O aanndd 33−− 4 4 PO
PO has has beebeen on obsebserverved. Td. Therhere are aree some problems with the use of zeolites as ion
some problems with the use of zeolites as ion exchangers because of some of theirexchangers because of some of their properties. The zeolites are soft minerals and thus, are not
properties. The zeolites are soft minerals and thus, are not very abrasive resistant.very abrasive resistant. They have poor mechanical strength. Their frameworks are more
They have poor mechanical strength. Their frameworks are more rigid hence lessrigid hence less open. They swell very little and the
open. They swell very little and the counter ions in their pores do not move verycounter ions in their pores do not move very freely. Above all, they suffer partial decomposition by acids and alkalis.
Ion Exchange Ion Exchange Chromatography Chromatography Another lesser known variety of natural ion
Another lesser known variety of natural ion exchangers is some types of coals. exchangers is some types of coals. TheyThey contain carboxylic and possibly other weak acid
contain carboxylic and possibly other weak acid groups. They, thus, can be used groups. They, thus, can be used asas cation exchangers. Most of these
cation exchangers. Most of these materials swell excessively and materials swell excessively and are decomposed byare decomposed by alkali. They are, therefore, stabilized before use. Soft and hard coals are
alkali. They are, therefore, stabilized before use. Soft and hard coals are stabilized bystabilized by metal ion solutions. Most lignites and bituminous coals and anthracites can be
metal ion solutions. Most lignites and bituminous coals and anthracites can be converted into strong cation
converted into strong cation exchangers by sulphonation with fuming strong sulphuricexchangers by sulphonation with fuming strong sulphuric acid. These coals have very limited applications.
acid. These coals have very limited applications. 9.3.2
9.3.2 Synthetic Ion ExchangersSynthetic Ion Exchangers Virtually the field of
Virtually the field of ion exchange has been dominated by organic ion ion exchange has been dominated by organic ion exchangeexchange resins. An almost unlimited
resins. An almost unlimited variety of resins with different compositions and degreesvariety of resins with different compositions and degrees of cross linking can be prepared. The resins consist of an elastic
of cross linking can be prepared. The resins consist of an elastic three-dimensionthree-dimensionalal network of hydrocarbons which carry fixed ionic groups. The charge of the
network of hydrocarbons which carry fixed ionic groups. The charge of the group isgroup is balanced by mobile counter ions. As a matter of fact,
balanced by mobile counter ions. As a matter of fact, these resins are cross-linkedthese resins are cross-linked polyelectrolytes. In a cation exchanger, the matrix carries ionic groups like
polyelectrolytes. In a cation exchanger, the matrix carries ionic groups like − SO − SO33 − − , − COO , − COO−−, − PO, − PO33 3− 3−
and in an anion exchanger, it carries groups such as and in an anion exchanger, it carries groups such as
−NH −NH33 + + , >NH , >NH22 , > N , > N + +
An ion exchange resin particle is one single macromolecule. The chemical, thermal An ion exchange resin particle is one single macromolecule. The chemical, thermal and mechanical stability and
and mechanical stability and the ion exchange behaviour of the resin the ion exchange behaviour of the resin depend chieflydepend chiefly on the structure and the degree of cross-linking of the matrix and on the nature and on the structure and the degree of cross-linking of the matrix and on the nature and thethe number of fixed ionic groups. The degree
number of fixed ionic groups. The degree of cross-linking determines the mesh widthof cross-linking determines the mesh width of the matrix which in turn affects the
of the matrix which in turn affects the swelling of the resin and the mobilities of theswelling of the resin and the mobilities of the counter ions. This finally affects the rate of i
counter ions. This finally affects the rate of i on exchange and other processes and theon exchange and other processes and the electrical conductivity. It should be clear that ion exchange resins do not have
electrical conductivity. It should be clear that ion exchange resins do not have unlimited chemical and thermal stability.
unlimited chemical and thermal stability. The common causes of resin dThe common causes of resin degradation areegradation are chemical and thermal deterioration. A majority of commercial ion exchange resins are chemical and thermal deterioration. A majority of commercial ion exchange resins are stable in all
stable in all common solvents except in the presence of strong oxidizing and reducingcommon solvents except in the presence of strong oxidizing and reducing agents. They can generally withstand temperatures slightly higher
agents. They can generally withstand temperatures slightly higher than 100ºC.than 100ºC. As pointed out earlier that the
As pointed out earlier that the ion exchange behaviour of the resin is mainlyion exchange behaviour of the resin is mainly determined by the fixed
determined by the fixed ionic groups. The number of groups determines the ionionic groups. The number of groups determines the ion exchange capacity. The chemical nature of groups to a great extent affects the exchange capacity. The chemical nature of groups to a great extent affects the ionion exchange equilibria. One of the important factors is the acid and base strength of exchange equilibria. One of the important factors is the acid and base strength of thethe group. This can be illustrated by taking a few
group. This can be illustrated by taking a few examples. The groups COOexamples. The groups COO–– are ionized are ionized only at high
only at high p pH and at lowH and at low p pH, they combine with HH, they combine with H++ forming the undissociated forming the undissociated COOH. Thus, they no longer act as fixed charges. On the other hand, strong acid COOH. Thus, they no longer act as fixed charges. On the other hand, strong acid groups like
groups like −− 3 3
SO
SO remain ionized even at low remain ionized even at low p pH. Similarly, weak base group NHH. Similarly, weak base group NH33++
lose a p
lose a proton, forming an uncharged NHroton, forming an uncharged NH22 when when p pH is high H is high and strong base groupsand strong base groups
such as –N(CH such as –N(CH33))33
+ +
remain ionized even at high
remain ionized even at high p pH. Thus, the operative capacity ofH. Thus, the operative capacity of weak acid and weak base exchanges is more pH dependent.
weak acid and weak base exchanges is more pH dependent. In this unit, we will
In this unit, we will mainly focus on the properties of organic resins and these will bemainly focus on the properties of organic resins and these will be discussed in more detail in section 9.5.
discussed in more detail in section 9.5. Inspite of the fact that different types of resinsInspite of the fact that different types of resins have a variety of applications, there are some pronounced limitations of these types of have a variety of applications, there are some pronounced limitations of these types of exchangers. They are not very stable
exchangers. They are not very stable at high temperatures and cannot withstand highat high temperatures and cannot withstand high dose of ionizing radiations and highly oxidizing media.
dose of ionizing radiations and highly oxidizing media. From 1950s onwards, interestFrom 1950s onwards, interest in the management of nuclear waste grew at a
in the management of nuclear waste grew at a very fact pace. This led to very fact pace. This led to resurgence ofresurgence of interest in inorganic ion
interest in inorganic ion exchange and a complete subject of exchange and a complete subject of synthetic inorganicsynthetic inorganic exchangers became prominently important. A variety of amorphous and crystalline exchangers became prominently important. A variety of amorphous and crystalline inorganic ion exchangers have been synth
inorganic ion exchangers have been synthesized. The list of esized. The list of these materials is large.these materials is large. Many of these exchangers show specificity for particular
Many of these exchangers show specificity for particular ions and they are used ions and they are used toto separate them. No doubt the
separate them. No doubt the area of synthetic inorganic ion area of synthetic inorganic ion exchangers initiallyexchangers initially developed for nuclear waste management purp
Chromatographic Chromatographic Methods-III
Methods-III the interest of different types of research groups. A detailed discussion on the interest of inorganic ion exchangerinorganic ion exchangers will be different types of research groups. A detailed discussion on synthetics will be taken up towards the end of this unit.taken up towards the end of this unit. synthetic 9.3.3
9.3.3 Liquid Ion ExchangersLiquid Ion Exchangers You can recollect that in Unit 2,
You can recollect that in Unit 2, sub-Sec. 2.3.4, it was pointed out that high molecularsub-Sec. 2.3.4, it was pointed out that high molecular weight amines and quaternary ammonium salts behave as
weight amines and quaternary ammonium salts behave as liquid anion exchangers.liquid anion exchangers. They extract the anions and anionic metal complexes. With a similar an
They extract the anions and anionic metal complexes. With a similar an alogy, somealogy, some authors classify alkylphospho
authors classify alkylphosphoric acids, ric acids, sulphonic acids and carboxylic acids as sulphonic acids and carboxylic acids as liquidliquid cation exchangers (Unit 3, sub-Sec. 3.2.4). It was also pointed out at the same
cation exchangers (Unit 3, sub-Sec. 3.2.4). It was also pointed out at the same timetime that this analogy should not be extended too far. Besides other complications, the that this analogy should not be extended too far. Besides other complications, the operation of transfer of solute in
operation of transfer of solute in solvent extraction and ion exchange chromatographysolvent extraction and ion exchange chromatography is different. However, one situation remains to
is different. However, one situation remains to be considered when these extractantsbe considered when these extractants mainly high molecular weight amines are
mainly high molecular weight amines are loaded on inert supports and the loaded on inert supports and the supports aresupports are used in columns for separations. This is classified under the head of extraction
used in columns for separations. This is classified under the head of extraction chromatograp
chromatography. For this, hy. For this, you may refer to Unit you may refer to Unit 4, sub-Sec. 4.2.3 where a 4, sub-Sec. 4.2.3 where a briefbrief mention has been made about
mention has been made about extraction chromatograextraction chromatography. A variety of phy. A variety of metal ionmetal ion separations are achieved using this technique. In this context, there may be some separations are achieved using this technique. In this context, there may be some justification for havi
justification for having liquid ion exchng liquid ion exchangers as a distinct class oangers as a distinct class of ion exchangers.f ion exchangers. However, this unit does not discuss them in detail. An
However, this unit does not discuss them in detail. An idea about liquid ionidea about liquid ion exchangers has already been given in Unit 2 and 3.
exchangers has already been given in Unit 2 and 3. SAQ 3
SAQ 3
What are the two distinct classes
What are the two distinct classes of aluminosilicates based on their structure?of aluminosilicates based on their structure?
………... ………... ………... ………... ………... ………... ………... ………... SAQ 4 SAQ 4
Under what conditions the organic resinous ion exchangers deteriorate fast? Under what conditions the organic resinous ion exchangers deteriorate fast?
………... ………... ………... ………... ………... ………... ………... ………... SAQ 5 SAQ 5
Is there any justification of including liquid ion exchangers as a distinct category of Is there any justification of including liquid ion exchangers as a distinct category of ion
ion exchangers?exchangers?
………... ………... ………... ………... ………... ………...
9.4
9.4
SYNTHESIS OF ION EXCHANGE RESINS
SYNTHESIS OF ION EXCHANGE RESINS
It has been made clear earlier
It has been made clear earlier that we will mainly focus on synthesis and properties ofthat we will mainly focus on synthesis and properties of organic resins. If we take synthesis, there are too many types of
organic resins. If we take synthesis, there are too many types of resins and differentresins and different chemical routes are followed to prepare them. Therefore, it may be difficult to c chemical routes are followed to prepare them. Therefore, it may be difficult to c iteite
Ion Exchange Ion Exchange Chromatography Chromatography here even the few important ones. Hence, to highlight the synthetic chemistry of ion
here even the few important ones. Hence, to highlight the synthetic chemistry of ion exchange resins, some discussion will be taken up on general terms and that will b exchange resins, some discussion will be taken up on general terms and that will b ee accompanied by a few examples of synthesis. One point which is very clear about accompanied by a few examples of synthesis. One point which is very clear about synthesis of ion exchange resin is that it must
synthesis of ion exchange resin is that it must yield a three dimensional cross-linkedyield a three dimensional cross-linked matrix of hydrocarbon chains carrying fixed ionic groups. This can be
matrix of hydrocarbon chains carrying fixed ionic groups. This can be achieved in theachieved in the following ways:
following ways: i)
i) Monomeric organic electrolytes can be polymerized in such a Monomeric organic electrolytes can be polymerized in such a way that a crossway that a cross linked network is
linked network is formed.formed. ii)
ii) The matrix can be built from non- ionic monomers and the fixed ionic groupsThe matrix can be built from non- ionic monomers and the fixed ionic groups are then introduced into the
are then introduced into the completed network.completed network. iii)
iii) The fixed ionic groups are introduced while the polymerization is still inThe fixed ionic groups are introduced while the polymerization is still in progress.
progress.
While synthesizing resinous exchang
While synthesizing resinous exchanger, it should be kept in mind er, it should be kept in mind that it should bethat it should be sufficiently cross-linked to have negligible solubility. The
sufficiently cross-linked to have negligible solubility. The cross linking should be suchcross linking should be such that it should be able
that it should be able to swell. Polymers which are too highly cross-linked cannotto swell. Polymers which are too highly cross-linked cannot swell. The mobility of counter ions in such resins is
swell. The mobility of counter ions in such resins is so low that ion exchange isso low that ion exchange is difficult to take place. The method of
difficult to take place. The method of synthesis should be such that the degree ofsynthesis should be such that the degree of crosslinking can be controlled. Most of the ion exchange resins are made by either crosslinking can be controlled. Most of the ion exchange resins are made by either condensation polymerization or addition polymerization. Now the
condensation polymerization or addition polymerization. Now the additionaddition polymerization processes have more or less replaced
polymerization processes have more or less replaced the condensation processes.the condensation processes. 9.4.1
9.4.1 Cation ExchangersCation Exchangers A broad variety of c
A broad variety of c ation exchangers with fixed ionic groups of different character ation exchangers with fixed ionic groups of different character andand different acid strength are commercially available. The most common of these are different acid strength are commercially available. The most common of these are strong-acid resins with (
strong-acid resins with ( −− 3 3
SO
SO ) and weak acid ) and weak acid resins with carboxylic acid groupsresins with carboxylic acid groups (−COO
(−COO––). Even if we consider these two ). Even if we consider these two types of resins, the resins of various strengthtypes of resins, the resins of various strength can be made since dissociation constants are affected by the nature and
can be made since dissociation constants are affected by the nature and configuratioconfigurationn of the units to which
of the units to which the groups are attached. The arylsulphonic acids are stronger thanthe groups are attached. The arylsulphonic acids are stronger than alkylsulphonic acids. Many ion exchangers contain two or
alkylsulphonic acids. Many ion exchangers contain two or more different types ofmore different types of ionic groups and they are
ionic groups and they are known as bifunctional or polyfunctional.known as bifunctional or polyfunctional. a)
a) Condensation polymersCondensation polymers
The earliest known cation exchange resin
The earliest known cation exchange resin was a condensation product of phenolwas a condensation product of phenol and formaldehyde. The list became broader and
and formaldehyde. The list became broader and more extensive. Othermore extensive. Other
monovalent or polyvalent phenols like resorcinol and naphthol instead of phenol monovalent or polyvalent phenols like resorcinol and naphthol instead of phenol and other aldehydes instead of formaldehyde can be
and other aldehydes instead of formaldehyde can be used. Phenolic group canused. Phenolic group can act as a
act as a fixed ionic group but the resins have a very low acid strength. Groupsfixed ionic group but the resins have a very low acid strength. Groups with higher acid strength can
with higher acid strength can be introduced by various methods. The easiestbe introduced by various methods. The easiest course is sulphonation of phenol prior to polymerization.
course is sulphonation of phenol prior to polymerization.
b)
b) Addition polymersAddition polymers The area of
The area of synthesis of ion exchange resins is synthesis of ion exchange resins is now dominated by additionnow dominated by addition copolymers prepared from vinyl monomer
copolymers prepared from vinyl monomers. They are s. They are more chemically andmore chemically and thermally stable than
thermally stable than the condensation polymers. Moreovthe condensation polymers. Moreover, in additioner, in addition polymerization, the degree of cross-linking an
polymerization, the degree of cross-linking and particle size are d particle size are easy to control.easy to control. A well known cation
Chromatographic Chromatographic Methods-III
Methods-III styrene and a small proportiostyrene and a small proportion of divinylbenzene treatment with concentrated sulphuric acid or treatment with concentrated sulphuric acid or chlorosulphn of divinylbenzene followed by sulphochlorosulphonic acid.followed by sulphonation byonic acid. nation by
The role of divinylbenzene is as a crosslinking agent. Pure divinylbenzene is not The role of divinylbenzene is as a crosslinking agent. Pure divinylbenzene is not easily available. The
easily available. The commercial product consists of different divinylbenzenecommercial product consists of different divinylbenzene isomers (around 50%) and ethylenestyrene (around 50%).
isomers (around 50%) and ethylenestyrene (around 50%). Therefore,Therefore, ethylenestyrene is also introduced in the
ethylenestyrene is also introduced in the matrix. The degree of crosslinking canmatrix. The degree of crosslinking can be adjusted by varying the
be adjusted by varying the divinylbenzedivinylbenzene content.ne content.
9.4.2
9.4.2 Anion ExchangersAnion Exchangers
The earliest anion exchangers synthesized were with we
The earliest anion exchangers synthesized were with we ak base amino groupsak base amino groups
Subsequently
Subsequently, resins , resins with strong-base quaternary ammonium groups were preparedwith strong-base quaternary ammonium groups were prepared It was followed by
It was followed by synthesis of resins with strong-base quaternary phosphoniusynthesis of resins with strong-base quaternary phosphoniumm groups and tertiary sulphonium groups.
Ion Exchange Ion Exchange Chromatography Chromatography
Like cation exchangers, the earlier
Like cation exchangers, the earlier known anion exchangers were condensatioknown anion exchangers were condensationn polymers and they are replaced by
polymers and they are replaced by addition polymers.addition polymers. a)
a) Condensation polymersCondensation polymers
The earliest known anion exchange resins were prepared from aromatic
The earliest known anion exchange resins were prepared from aromatic aminesamines like
like mm- phenylenediamine by condensation with - phenylenediamine by condensation with formaldehyformaldehyde.de.
The aldehyde reacts with amino
The aldehyde reacts with amino groups. In the process, the groups. In the process, the secondary andsecondary and tertiary amino groups are formed. Thus, the
tertiary amino groups are formed. Thus, the resins are polyfunctional. Aliphaticresins are polyfunctional. Aliphatic polyamines which are not as weakly basic can also be condensed with
polyamines which are not as weakly basic can also be condensed with aldehydes.
aldehydes. b)
b) Addition polymersAddition polymers Like cation
Like cation exchangers,exchangers, a commonly used anion exchange resin is pra commonly used anion exchange resin is pr epared byepared by copolymerizatio
copolymerization of n of styrene and divinylbenzene followed by chloromethylationstyrene and divinylbenzene followed by chloromethylation (introduction –CH
(introduction –CH22Cl grouping) say, in theCl grouping) say, in the para para position and interaction with a position and interaction with a
base such as
base such as trimethylamine. The polymers containing quaternary ammoniumtrimethylamine. The polymers containing quaternary ammonium groups are strong bases and those with
groups are strong bases and those with amino or substituted amino groups showamino or substituted amino groups show weakly basic properties.
weakly basic properties.
9.4.3
9.4.3 Amphoteric ExchangersAmphoteric Exchangers The ion exchangers which contain both ac
The ion exchangers which contain both ac idic and basic groups are idic and basic groups are known asknown as amphoteric exchangers
amphoteric exchangers. A number of . A number of exchangers of this type has been exchangers of this type has been synthesizedsynthesized but only a
but only a few have found application.few have found application. A well known resin
A well known resin containing both strong base and acid containing both strong base and acid groups is prepared bygroups is prepared by copolymerization of styrene, vinylchloride and a cross-linking agent followed by copolymerization of styrene, vinylchloride and a cross-linking agent followed by quaternization and sulphonation of the
Chromatographic Chromatographic Methods-III Methods-III
Among the amphoteric resins, the most
Among the amphoteric resins, the most important are the ones known asimportant are the ones known as snake- cagesnake- cage polyelectrolytes
polyelectrolytes. They are . They are conventional cation or anion exchangers within whichconventional cation or anion exchangers within which polycation or polyanions, respectively have been formed by polymerization. A typical polycation or polyanions, respectively have been formed by polymerization. A typical example is that a snake- cage polyelectrolyte can be prepared by converting a strong example is that a snake- cage polyelectrolyte can be prepared by converting a strong base anion exchanger to acrylate form and then acrylate anion is polymerized in the base anion exchanger to acrylate form and then acrylate anion is polymerized in the resin. The linear chains of the poly-counter ions are so intricately interwined with the resin. The linear chains of the poly-counter ions are so intricately interwined with the crosslinked matrix that they cannot be displaced by other counter ions. The situation is crosslinked matrix that they cannot be displaced by other counter ions. The situation is something
something like a snake trapped in a cage. One significant differelike a snake trapped in a cage. One significant difference these snake cagence these snake cage polyelectrolytes show from other amphoteric exchangers is that the
polyelectrolytes show from other amphoteric exchangers is that the poly-counter ionspoly-counter ions are not attached to the matrix. Therefore, the charges of poly-counter ions of the are not attached to the matrix. Therefore, the charges of poly-counter ions of the matrix have more freedom to move. As a result, it
matrix have more freedom to move. As a result, it is not necessary for the resin to haveis not necessary for the resin to have mobile counter ions (counter ions to
mobile counter ions (counter ions to the poly-counter ions) to remain electricallythe poly-counter ions) to remain electrically neutral provided the charges of fixed ionic
neutral provided the charges of fixed ionic groups and poly-countegroups and poly-counter ions are r ions are balanced.balanced. These exchangers are excellent reversible sorbents for electrolytes. This
These exchangers are excellent reversible sorbents for electrolytes. This will bewill be discussed later when the applications of ion exchangers are being cited.
discussed later when the applications of ion exchangers are being cited. At the end of this section on
At the end of this section on the synthesis of ion exchange resins, it may be importantthe synthesis of ion exchange resins, it may be important to point out that the
to point out that the chemical structures of the polymers shown are hypothetical. It ischemical structures of the polymers shown are hypothetical. It is difficult to establish the resin structure exactly. Furthermore, the structures of the difficult to establish the resin structure exactly. Furthermore, the structures of the polymers do not represent repeating identical units since the sequence of the polymers do not represent repeating identical units since the sequence of the monomeric component is essentially random.
monomeric component is essentially random. SAQ 6
SAQ 6 What are the
What are the advantages of addition polymeric resins over their advantages of addition polymeric resins over their condensationcondensation counterparts? counterparts? ………... ………... ………... ………... SAQ 7 SAQ 7 What is the
What is the role of divinylbenzene in the role of divinylbenzene in the synthesis of styrene-divinylbenzsynthesis of styrene-divinylbenzeneene polymeric resin? polymeric resin? ………... ………... ………... ………...
9.5
9.5
TRADE NAMES AND NOMENCLATURE
TRADE NAMES AND NOMENCLATURE
A number of manufacturers of ion exchange resins
A number of manufacturers of ion exchange resins sell their products with differentsell their products with different trade names. Some of these are given in
Ion Exchange Ion Exchange Chromatography Chromatography Table 9.1: Some Commercially Available Ion Exchange Resins
Table 9.1: Some Commercially Available Ion Exchange Resins Manufacturer
Manufacturer Trade Trade namename
Dow
Dow Chemical Chemical Co., Co., USA USA DowexDowex Rohm
Rohm & & Hass Hass Co., Co., USA USA AmberliteAmberlite Permutit
Permutit Co., Co., UK UK Zeo- Zeo- Karb/ Karb/ De De AciditeAcidite Chemical
Chemical Process Process Co., Co., USA USA DuoliteDuolite Bayer-Farben,
Bayer-Farben, Germany Germany LewalitLewalit Wolfen-Farben,
Wolfen-Farben, Germany Germany WolfatitWolfatit Sicso,
Sicso, India India SeraliteSeralite
Nomenclature Nomenclature
The trade names of resins are generally so named that
The trade names of resins are generally so named that the basic structure is readilythe basic structure is readily apparent. Taking the example of Dowex resin, it will include
apparent. Taking the example of Dowex resin, it will include i)
i) TypeType i.e.i.e. Dowex 50, 50 W( Dowex 50, 50 W( cation exchangers); Dowex 1, 2, 4, cation exchangers); Dowex 1, 2, 4, 21K (anion21K (anion exchangers)
exchangers) ii)
ii) “X- Number” or percent divinylbenzene like X8“X- Number” or percent divinylbenzene like X8 iii)
iii) Mesh sizeMesh size i.e.i.e. 20- 50 ( based on 20- 50 ( based on US Standard screen)US Standard screen) iv)
iv) Ionic formIonic form i.e.i.e. Na Na
The label will carry something like The label will carry something like
Type
Type % % DVB DVB Mesh Mesh size size Ionic Ionic formform 50
50 X8 X8 20- 20- 50 50 NaNa
9.6
9.6
RESIN
RESIN PROPERTIES
PROPERTIES
As a matter of fac
As a matter of fac t, the resin is a t, the resin is a very complex material and there are severalvery complex material and there are several properties which are to be
properties which are to be known and clearly understood before putting it to known and clearly understood before putting it to anyany particular application. Some of the
particular application. Some of the important properties areimportant properties are i)
i) Moisture contentMoisture content ii)
ii) Particle sizeParticle size iii)
iii) CrosslinkageCrosslinkage iv)
iv) CapacityCapacity v)
v) Distribution coefficientDistribution coefficient vi)
vi) Equivalency of exchangeEquivalency of exchange vii)
vii) Resin selectivityResin selectivity
Let us now study them in detail. Let us now study them in detail. 9.6.1
9.6.1 Moisture ContentMoisture Content
The moisture content of the resins is determined in the usual
The moisture content of the resins is determined in the usual manner by heating it atmanner by heating it at 110º− 115ºC overnight to constant weight. However, several precautionary steps are 110º− 115ºC overnight to constant weight. However, several precautionary steps are necessary in this exercise. For example, some resins are thermally unstable in necessary in this exercise. For example, some resins are thermally unstable in thethe hydrogen and hydroxyl form and therefore, these should be converted to a stable form hydrogen and hydroxyl form and therefore, these should be converted to a stable form before oven drying. Samples which decompose at these temperatures are
before oven drying. Samples which decompose at these temperatures are occasionallyoccasionally dried at room temperature over P
Chromatographic Chromatographic Methods-III
Methods-III 9.6.29.6.2 Particle SizeParticle Size
The importance of particle size
The importance of particle size for proper column performance in an ion for proper column performance in an ion exchangeexchange unit is quite
unit is quite obvious. Rate of exchange, pressure drop and back obvious. Rate of exchange, pressure drop and back wash expansions arewash expansions are all dependent on particle size. The resin beads or particles
all dependent on particle size. The resin beads or particles may be formed withmay be formed with diameters ranging from 1mm to less than 0.04 mm. For most of the ion exchange diameters ranging from 1mm to less than 0.04 mm. For most of the ion exchange operations, an effective size of 0.4 – 0.6 mm diameter is
operations, an effective size of 0.4 – 0.6 mm diameter is preferred. This corresponds topreferred. This corresponds to particle size distribution falling between the 20- and 50-mesh screens. The ion
particle size distribution falling between the 20- and 50-mesh screens. The ion
exchange reactions are mostly conducted in the aqueous media in which the particles exchange reactions are mostly conducted in the aqueous media in which the particles have fully hydrated diameter. This is the value that is
have fully hydrated diameter. This is the value that is to be taken into consideration.to be taken into consideration. The size of the water
The size of the water swollen resin will depend on the type of functional group and theswollen resin will depend on the type of functional group and the amount of cross linking of the
amount of cross linking of the polymer.polymer. The size of the particle
The size of the particle is one of the parameters affecting the rate of is one of the parameters affecting the rate of ion exchangeion exchange reaction. Besides this, the other parameters affecting rate are
reaction. Besides this, the other parameters affecting rate are sizesize and and charge of the ioncharge of the ion involved
involved ,, degree of cross linkingdegree of cross linking and the and the temperaturetemperature. As a matter of . As a matter of fact, decreasingfact, decreasing the size of the
the size of the particle materially decreases the time required for the equilibrium to beparticle materially decreases the time required for the equilibrium to be attained with the contacting solution. Since the time r
attained with the contacting solution. Since the time r equired to achieve theequired to achieve the
equilibration is decreased the efficiency of a given volume of resin increases. In other equilibration is decreased the efficiency of a given volume of resin increases. In other words, the volume of the
words, the volume of the resin required to perform a resin required to perform a specific operation decreases.specific operation decreases. The physical aspects of operation are also considerably altered by the change in the The physical aspects of operation are also considerably altered by the change in the particle size. With the decreasing particle size,
particle size. With the decreasing particle size, the friction loss or pressure drop of athe friction loss or pressure drop of a liquid flowing through the column increases. This means that for a given flow rate, liquid flowing through the column increases. This means that for a given flow rate, with decreasing particle size, the pressure drop in a column increases.
with decreasing particle size, the pressure drop in a column increases.
An ion exchange column is usually backwashed at the end of an operating cycle to An ion exchange column is usually backwashed at the end of an operating cycle to remove the foreign material and reclassify the particles. The back washing step expand remove the foreign material and reclassify the particles. The back washing step expand the bed to different extents depending upon the specific gravity of the resin. The finer the bed to different extents depending upon the specific gravity of the resin. The finer the mesh size and the
the mesh size and the lower the density, the greater will be the blower the density, the greater will be the b ed expansion.ed expansion. Generally, the smaller resin particles (~ 50 mesh) are physically more stable.
Generally, the smaller resin particles (~ 50 mesh) are physically more stable. This isThis is important when the resin is
important when the resin is mechanically moved or it goes mechanically moved or it goes through large volumethrough large volume changes.
changes. 9.6.3
9.6.3 Cross LinkagesCross Linkages
The second variation which can be introduced into the copolymer bead is that of cross The second variation which can be introduced into the copolymer bead is that of cross linkage. As mentioned earlier, the
linkage. As mentioned earlier, the cross linkage in a cross linkage in a styrene- divinylbenzene polystyrene- divinylbenzene polymermer refers to the fraction of divinylbenzene content. Thus, a resin of 8% crosslinkage is refers to the fraction of divinylbenzene content. Thus, a resin of 8% crosslinkage is made with beads containing 8% divinylbenzene and 2%
made with beads containing 8% divinylbenzene and 2% styrene and other monovinylstyrene and other monovinyl monomers.
monomers.
The cross linkage affects the resin in two ways. As
The cross linkage affects the resin in two ways. As the amount of cross linkagethe amount of cross linkage increases, the dry weight capacity decreases. This decreased capacity results from the increases, the dry weight capacity decreases. This decreased capacity results from the greater difficulty of substituting active groups on
greater difficulty of substituting active groups on the copolymers probably due tothe copolymers probably due to steric factors. However, as compared to this, the change in water content is steric factors. However, as compared to this, the change in water content is moremore pronounce
pronounced. Thus, as the cross linkage increases, the resin d. Thus, as the cross linkage increases, the resin has a – swollen volume forhas a – swollen volume for essentially the same number of sites and the w
essentially the same number of sites and the w et volume capacity increases.et volume capacity increases.
There are other properties which are affected by the degree of cross linkage. With the There are other properties which are affected by the degree of cross linkage. With the decrease in the cross linkage, the resin swells
decrease in the cross linkage, the resin swells more and thus, the diffusion of ionsmore and thus, the diffusion of ions within the resin becomes faster. This, in turn, gives faster equilibrium rate particularly, within the resin becomes faster. This, in turn, gives faster equilibrium rate particularly, for large ions. On the other hand, if the
for large ions. On the other hand, if the cross linkage is increased, the diffusion pathscross linkage is increased, the diffusion paths may become small enough for the entrance of large ions. This offers a possibility of may become small enough for the entrance of large ions. This offers a possibility of separation of ions based on ionic sizes. A typical example is
separation of ions based on ionic sizes. A typical example is the separation of sulphatethe separation of sulphate from high molecular weight sulphonic acid by using highly cross
Ion Exchange Ion Exchange Chromatography Chromatography exchange resin. In the same light, we can
exchange resin. In the same light, we can say that if the cross say that if the cross linking is decreased, thelinking is decreased, the permeable selectivity difference is also decreased.
permeable selectivity difference is also decreased.
Cross linkage affects the physical properties also. Highly crosslinked resin is brittle. Cross linkage affects the physical properties also. Highly crosslinked resin is brittle. On the other hand, low cross linked resins are highly swollen; therefore, soft and On the other hand, low cross linked resins are highly swollen; therefore, soft and easily deformed.
easily deformed. 9.6.4
9.6.4 CapacityCapacity
If we consider an ion exchanger, it can be
If we consider an ion exchanger, it can be taken as a reservoir of exchangeable ions. Intaken as a reservoir of exchangeable ions. In the ion exchange operation, it is the counter ions which are put
the ion exchange operation, it is the counter ions which are put to use. The counterto use. The counter ions content of a given amount of material is
ions content of a given amount of material is equal to the fixed charges which must beequal to the fixed charges which must be balanced by the counter ions and thus, is essentially constant. This amounts to
balanced by the counter ions and thus, is essentially constant. This amounts to the factthe fact that it is
that it is independent of particle size and shape and of the nature of counter ions.independent of particle size and shape and of the nature of counter ions. Ion exchangers are characterized in a quantitative manner by their
Ion exchangers are characterized in a quantitative manner by their capacitycapacity. In the. In the common usage, it is defined as
common usage, it is defined as the number of ion equivalents in a specified amount ofthe number of ion equivalents in a specified amount of the material
the material. But this simple definition is . But this simple definition is not sufficient and will have to be qualified.not sufficient and will have to be qualified. The definition becomes acceptable when the
The definition becomes acceptable when the conditionsconditions are given. Capacity and are given. Capacity and related data are primarily used for two purposes, for
related data are primarily used for two purposes, for characterizing ion exchangecharacterizing ion exchange materials
materials andand for use in nume for use in numerical calculations of ion rical calculations of ion exchange operexchange operationsations. In the. In the second case, it is
second case, it is more practical to use other definitions or quantitatives which reflectmore practical to use other definitions or quantitatives which reflect the effect of operating conditions. The different types of capacity are given as under. the effect of operating conditions. The different types of capacity are given as under. The
The total capacity of an i total capacity of an ion exchange resinon exchange resin is is the number of ionic the number of ionic (or potentially(or potentially ionic) sites per unit weight or volume of
ionic) sites per unit weight or volume of resinresin. The. The dry weight total dry weight total capacitycapacity is usually is usually expressed in milliequivalents per gram of
expressed in milliequivalents per gram of anhydrous resin. Scientifically, it is anhydrous resin. Scientifically, it is usuallyusually expressed as meq/ g dry H
expressed as meq/ g dry H++ or Cl or Cl–– form. The form. The wet volume capacitywet volume capacity is the number of is the number of sites per unit volume of the water swollen resin.
sites per unit volume of the water swollen resin. The performance of an ion exchangeThe performance of an ion exchange resin is generally based on volume and the wet volume total capacity is
resin is generally based on volume and the wet volume total capacity is the theoreticalthe theoretical or maximum capacity which the
or maximum capacity which the resin can show in resin can show in any aqueous ion exchangeany aqueous ion exchange application. It may be expressed in milliequivalents per milliliter.
application. It may be expressed in milliequivalents per milliliter. The net number of sites which are utilized
The net number of sites which are utilized in a given volume of resin in a in a given volume of resin in a given cyclegiven cycle in known as the
in known as the operating capacityoperating capacity of the resin in that of the resin in that particular cycle. It may beparticular cycle. It may be expressed in the same terms as
expressed in the same terms as total capacity or as a percent of total capacity or as a percent of total capacity.total capacity. There is another term which is known as
There is another term which is known as useful capacityuseful capacity which is the capacity when which is the capacity when equilibrium is not attained.
equilibrium is not attained. It depends on experimental conditionsIt depends on experimental conditions vizviz. ion exchange. ion exchange rates
rates etcetc. There is another capacity which is known as. There is another capacity which is known as breakthrough (dynamic)breakthrough (dynamic) capacity
capacity which is u which is utilized in column operation. It tilized in column operation. It depends on operating conditions.depends on operating conditions. There is also a
There is also a capacity known ascapacity known as sorption capacitysorption capacity which is the amount of which is the amount of solutesolute taken up by sorption rather t
taken up by sorption rather than ion exchange per specified amount of han ion exchange per specified amount of the exchanger.the exchanger. 9.6.5
9.6.5 Distribution RatioDistribution Ratio
It should be remembered that we should not speak of a resin to p
It should be remembered that we should not speak of a resin to p ick up a certain ionick up a certain ion without noting that there is another ion in the
without noting that there is another ion in the resin phase. It is actually the tendency ofresin phase. It is actually the tendency of an ion exchanger to pick up A
an ion exchanger to pick up A++ at the expense of B at the expense of B++. This tendency of the exchanger to. This tendency of the exchanger to take up A
take up A++ will be different if will be different if the resin contains other ions Cthe resin contains other ions C++ instead of B instead of B++. Thus, we. Thus, we can prepare a resin containing a certain counter ion and then compare a se
can prepare a resin containing a certain counter ion and then compare a se ries of otherries of other ions containing this counter ion as a reference. For the ions in
ions containing this counter ion as a reference. For the ions in this series, we maythis series, we may simply mention distribution ratios. Distribution ratio simply expresses
simply mention distribution ratios. Distribution ratio simply expresses the partitioningthe partitioning of ion between the solution and the resin phases.
of ion between the solution and the resin phases.
solution solution the the in in ion ion same same the the of of .. Conc Conc sin sin re re the the in in ion ion an an of of .. Conc Conc = = D D
Chromatographic Chromatographic Methods-III
Methods-III The conventional units areThe conventional units are
solution solution of of Litre Litre / / Amount Amount sin sin re re dry dry of of kg kg / / Amount Amount = = D D
The amount term, in milligram, moles or whatever may be is
The amount term, in milligram, moles or whatever may be is proper since the unitsproper since the units cancel in calculating the
cancel in calculating the D D ratio. ratio. The
The D D values are values are generally determined bygenerally determined by batch method batch method . A known amount of resin is. A known amount of resin is brought in contact with a known amount of metal ion in solution until equilibrium is brought in contact with a known amount of metal ion in solution until equilibrium is attained. Because isotherms are
attained. Because isotherms are non-linear, thenon-linear, the D D values are taken to be values are taken to be limiting slopeslimiting slopes at very low values (Fig. 9.1). The best solution for this
at very low values (Fig. 9.1). The best solution for this is to determineis to determine D D values at low values at low concentrations by taking labeled
concentrations by taking labeled solutions using radioisotopes. Thesolutions using radioisotopes. The D D value is value is determined by simply counting the solution before and after
determined by simply counting the solution before and after equilibrium with theequilibrium with the resin.
resin.
Fig. 9.1:
Fig. 9.1: A typical curve oA typical curve of loading of an ion exf loading of an ion exchangerchanger Sometimes, the distribution ratio is
Sometimes, the distribution ratio is expressed with different values, sayexpressed with different values, say
solution solution of of L L / / Amount Amount volume volume wet wet of of L L / / Amount Amount = = D D
The conversion factor of
The conversion factor of D D to to D Dvv is the is the bed density,bed density, ρ ρ , where, where ρ ρ is in is in kg of dry resin perkg of dry resin per
L of resin bed. L of resin bed.
For any ion exchange, the importance is its use for the
For any ion exchange, the importance is its use for the separation that meansseparation that means selectivity
selectivity. For selectivity, the. For selectivity, the D Dvaluesvalues should be different for the ions to be should be different for the ions to be separated.separated. ItIt
should be kept in mind that the
should be kept in mind that the D Dvaluesvalues is conditional is conditional. It depends upon the nature of. It depends upon the nature of
resin and the composition of the solution in contact with it.
resin and the composition of the solution in contact with it. Composition will includeComposition will include p
pH, ionic strength, type and molarity of acid and the H, ionic strength, type and molarity of acid and the presence of water misciblepresence of water miscible organic solvents and other ions.
