-
5
Ion Exchange Chromatography UNIT 9 ION EXCHANGE
CHROMATOGRAPHY Structure 9.1 Introduction
Objectives 9.2 Basic Features of Ion Exchange Mechanism 9.3
Classification of Ion Exchangers
Natural Ion Exchangers Synthetic Ion Exchangers Liquid Ion
Exchangers
9.4 Synthesis of Ion Exchange Resins Cation Exchangers Anion
Exchangers Amphoteric Exchangers
9.5 Trade Names and Nomenclature 9.6 Resin Properties
Moisture Content Particle Size Cross Linkages Capacity
Distribution Ratio Equivalency of Exchange Resin Selectivity
9.7 Operating Methods Batch Operation Column Operation Moving
Bed Operation
9.8 Ion Exchange in Mixed Aqueous - Organic Media 9.9 Specific
Cation Exchangers 9.10 Synthetic Inorganic Ion Exchangers
Different Types and Their Characteristics Special Properties and
Applications
9.11 Applications Separation of Metal Ions and Anions Separation
of Organics Separation of Ionized from Nonionized Separation of
Actinide Elements Miscellaneous Applications
9.12 Summary 9.13 Terminal Questions 9.14 Answers
9.1 INTRODUCTION Amongst various separation techniques, ion
exchange is the most popular name because of its use for water
softening. It is also unique in terms of its versatility and
historical developments. Besides the well-known use of ion
exchangers in water treatment, they find use in industry, nuclear
fuel processing, hydrometallurgy, agriculture and biology. The
treatment of water by solid adsorbents is as old as civilization.
There are records available that in the time of Aristotle, sand
filters were used for purification of sea water. Moses used a tree
branch for making bitter water sweet. But the credit of recognizing
the ion exchange phenomenon goes to two agricultural
chemists-Thompson and Way. They observed the exchange of ammonium
ions with calcium ions in soils. The realization of the fact that
certain clay minerals were responsible for the exchange, led to the
attempts to use such materials for water softening. It also
prompted scientists to synthesize materials with similar
properties.
-
6
Chromatographic Methods-III
The first synthetic ion exchanger was prepared in 1903 by two
German chemists-Harm and Rumpler. Another German, Gans, worked on
several pioneering applications of permutits. But the permutits
could not stand in the market because of their poor reproducibility
and chemical stability.
A real breakthrough in the subject came in 1935 when two English
chemists, Adams, and Holmes, observed that crushed phonograph
record exhibited ion exchange properties. This observation led to
the synthesis of several organic ion exchangers which had better
properties. It was illustrated that stable and high capacity cation
exchangers could be prepared as sulphonic acid resins and polyamine
type resins exhibited anion exchange properties. The area of ion
exchange blossomed at a very fast rate. The versatility of ion
exchange resins was readily recognized. Many attempts have been
made to modify and improve the existing materials. It is possible
to tailor make ion exchange resins for specific applications.
Ion exchange is firmly established as a unit operation. All over
the world, numerous 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
reactions to the decontamination of cooling water of nuclear
reactors. In the laboratory, ion exchangers prove themselves as
useful materials for accomplishing analytical separations. The ion
exchange membranes find quite a good use in physiological chemistry
and biophysics. Ion exchange separation played a major role in the
identification of trans-uranium elements by Glen T. Seaborg. The
identify of each element of 5f series was established beyond any
doubt by the sequence of their appearance a analogous to the
appearance of the corresponding 4f elements
The above applications clearly indicate that a variety of ion
exchangers are available and these materials can be used for
different applications. In view of this, it is important to
understand the basic ion exchange mechanism, and a broad
classification of ion exchangers. Ion exchange resins are
synthesized by following different chemical routes. An idea about
it can be had by illustrating the synthesis of some well- known ion
exchange resins. The practical utility of an ion exchanger depends
upon its properties, both chemical and physical. Another point
which is important in this context is as to how the material is
being operated. The discussion on ion exchangers will not be
complete if we do not talk about some special type of ion
exchangers viz. chelating resins and synthetic inorganic ion
exchangers. Finally, a discussion on various types of applications
will be taken up. It may be noted that some of these uses may not
be directly based on separations.
Objectives After studying this Unit, you should be able to
discuss basic ion exchange mechanism,
classify different types of ion exchangers,
describe the synthesis of ion exchange resins,
explain the properties which characterize an ion exchanger,
describe the operating methods for ion exchangers,
explain the behaviour of specific cation exchangers,
present a complete picture about the different types of
synthetic inorganic ion exchangers and their advantages alongwith
applications, and
discuss different types of applications of ion exchangers.
-
7
Ion Exchange Chromatography 9.2 BASIC FEATURES OF ION
EXCHANGE
MECHANISM The term ion exchange generally means exchange of ions
of like sign between a solution and a solid highly insoluble in it.
The solid known as ion exchanger carries 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 of same sign. Carriers of exchangeable cations are known
as cation exchangers and carriers of exchangeable anions as anion
exchangers. Certain materials are capable of both cation and anion
exchange. These are known as amphoteric exchangers. A typical
cation exchange reaction is shown below:
2 NaX + CaCl2(aq) CaX2 + 2 NaCl(aq) Similarly, typical anion
exchange reaction is as follows:
2 XCl + Na2SO4(aq) X2SO4 + 2 NaCl(aq) where, X represents a
structural unit of the ion exchanger.
In the first process, a solution containing dissolved CaCl2, say
something like hard water, is treated with a solid exchanger, NaX,
containing exchangeable Na+ ions. The exchanger removes the Ca2+
ions from the solution and replaces them with Na+. Thus, a cation
exchanger in Na+ form is converted to Ca2+ form.
Ion exchange, with very few exceptions, is a reversible process.
In water softening, a cation exchanger has lost its Na+ ions and
can be regenerated with a solution of a sodium salt such as NaCl.
Ion exchange resembles adsorption in that, in both cases, a
dissolved species is taken up by a solid. The characteristic
difference between the two is that the ion exchange in contrast to
sorption, is a stoichiometric process. Every ion removed from the
solution is replaced by an equivalent amount of another ionic
species of the same sign. However, in the case of sorption a
solute, an electrolyte or non-electrolyte, may be taken up without
any species being replaced.
Ion exchangers owe their characteristics to a particular feature
of their structure. They are built of a framework which is held
together by chemical bonds or lattice energy. The framework carries
a positive or negative surplus charge which is compensated by ions
of opposite charge, called counter ions. The counter ions are free
to move within 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 anion
exchanger as a polycation.
From the above discussion, it emerges out that a useful ion
exchanger must have the following requisites: i) It should have
negligible solubility in the medium to be used. ii) It must contain
sufficient number of accessible ion exchange groups and it must
be chemically stable. iii) It should be sufficiently hydrophilic
to permit diffusion of ions through the
structure at a finite and usable rate. iv) The swollen exchanger
must be denser than water.
SAQ 1 What is the basic difference between adsorption and ion
exchange?
...
-
8
Chromatographic Methods-III
SAQ 2 A sodium phosphate solution is passed through a column of
an anion exchanger in the chloride form. The PO43 ions are taken up
by the ion exchanger. Write down the ion exchange equilibria.
...
...
...
...
9.3 CLASSIFICATION OF ION EXCHANGERS Many different natural and
synthetic products show ion exchange properties. These exchangers
can be either cation or anion exchangers. Therefore, a simple broad
classification can be as i) Natural ii) Synthetic However, within
these two categories the material can be i) Organic ii) Inorganic
For the purposes of simple presentation, we will select the first
classification i.e., natural and synthetic.
9.3.1 Natural Ion Exchangers Most of the natural ion exchange
materials are crystalline aluminosilicates with cation exchange
properties. The typical representative of this group of materials
are zeolites which include among others, the minerals like analcite
Na[SiAlO6]2. H2O, chabazite (CaNa)[SiAlO6]2.6H2O and naturalite
Na2[Si2Al2O10].2H2O. All these minerals have a relatively open
three dimensional framework with channels and interconnecting
cavities in the aluminosilicate lattice. The zeolite lattice
consists of SiO4 and AlO4 tetrahedra. These have their oxygen atoms
in common. Because aluminium is trivalent, the lattice carries a
negative charge. The charge is balanced by alkali and alkaline
earth cations which do not occupy fixed positions and are free to
move in the lattice framework. These ions behave as counter ions
and can exchange with other counter ions.
There are other aluminosilicates with loose layer structure
having cation exchange properties. These materials carry their
counterions in between the layer of the lattice. The typical
mineral of this type is montmorillonite with the approximate
composition Al2[Si4O10(OH)2].nH2O. Such minerals swell in one
direction increasing the interlayer distance.
It may be important to mention here that certain
aluminosilicates can also behave as anion exchangers. In
montmorillonite, kaolinite and feldspar of sodalite and camerinite
groups the exchange of OH for Cl, 24SO and
34PO has been observed. There are
some problems with the use of zeolites as ion exchangers because
of some of their properties. The zeolites are soft minerals and
thus, are not very abrasive resistant. They have poor mechanical
strength. Their frameworks are more rigid hence less open. They
swell very little and the counter ions in their pores do not move
very freely. Above all, they suffer partial decomposition by acids
and alkalis.
-
9
Ion Exchange Chromatography
Another lesser known variety of natural ion exchangers is some
types of coals. They contain carboxylic and possibly other weak
acid groups. They, thus, can be used as cation exchangers. Most of
these materials swell excessively and are decomposed by alkali.
They are, therefore, stabilized before use. Soft and hard coals are
stabilized by metal ion solutions. Most lignites and bituminous
coals and anthracites can be converted into strong cation
exchangers by sulphonation with fuming strong sulphuric acid. These
coals have very limited applications.
9.3.2 Synthetic Ion Exchangers Virtually the field of ion
exchange has been dominated by organic ion exchange resins. An
almost unlimited variety of resins with different compositions and
degrees of cross linking can be prepared. The resins consist of an
elastic three-dimensional network of hydrocarbons which carry fixed
ionic groups. The charge of the group is balanced by mobile counter
ions. As a matter of fact, these resins are cross-linked
polyelectrolytes. In a cation exchanger, the matrix carries ionic
groups like
SO3, COO, PO33 and in an anion exchanger, it carries groups such
as
NH3+, >NH2 , > N+
An ion exchange resin particle is one single macromolecule. The
chemical, thermal and mechanical stability and the ion exchange
behaviour of the resin depend chiefly on the structure and the
degree of cross-linking of the matrix and on the nature and the
number of fixed ionic groups. The degree of cross-linking
determines the mesh width of the matrix which in turn affects the
swelling of the resin and the mobilities of the counter ions. This
finally affects the rate of ion exchange and other processes and
the electrical conductivity. It should be clear that ion exchange
resins do not have unlimited chemical and thermal stability. The
common causes of resin degradation are chemical and thermal
deterioration. A majority of commercial ion exchange resins are
stable in all common solvents except in the presence of strong
oxidizing and reducing agents. They can generally withstand
temperatures slightly higher than 100C.
As pointed out earlier that the ion exchange behaviour of the
resin is mainly determined by the fixed ionic groups. The number of
groups determines the ion exchange capacity. The chemical nature of
groups to a great extent affects the ion exchange equilibria. One
of the important factors is the acid and base strength of the
group. This can be illustrated by taking a few examples. The groups
COO are ionized only at high pH and at low pH, they combine with H+
forming the undissociated COOH. Thus, they no longer act as fixed
charges. On the other hand, strong acid groups like 3SO remain
ionized even at low pH. Similarly, weak base group NH3
+
lose a proton, forming an uncharged NH2 when pH is high and
strong base groups such as N(CH3)3+ remain ionized even at high pH.
Thus, the operative capacity of weak acid and weak base exchanges
is more pH dependent.
In this unit, we will mainly focus on the properties of organic
resins and these will be discussed in more detail in section 9.5.
Inspite of the fact that different types of resins have a variety
of applications, there are some pronounced limitations of these
types of exchangers. They are not very stable at high temperatures
and cannot withstand high dose of ionizing radiations and highly
oxidizing media. From 1950s onwards, interest in the management of
nuclear waste grew at a very fact pace. This led to resurgence of
interest in inorganic ion exchange and a complete subject of
synthetic inorganic exchangers became prominently important. A
variety of amorphous and crystalline inorganic ion exchangers have
been synthesized. The list of these materials is large. Many of
these exchangers show specificity for particular ions and they are
used to separate them. No doubt the area of synthetic inorganic ion
exchangers initially developed for nuclear waste management
purposes but with the time, it has attracted
-
10
Chromatographic Methods-III
the interest of different types of research groups. A detailed
discussion on synthetic inorganic ion exchangers will be taken up
towards the end of this unit.
9.3.3 Liquid Ion Exchangers You can recollect that in Unit 2,
sub-Sec. 2.3.4, it was pointed out that high molecular weight
amines and quaternary ammonium salts behave as liquid anion
exchangers. They extract the anions and anionic metal complexes.
With a similar analogy, some authors classify alkylphosphoric
acids, sulphonic acids and carboxylic acids as liquid cation
exchangers (Unit 3, sub-Sec. 3.2.4). It was also pointed out at the
same time that this analogy should not be extended too far. Besides
other complications, the operation of transfer of solute in solvent
extraction and ion exchange chromatography is different. However,
one situation remains to be considered when these extractants
mainly high molecular weight amines are loaded on inert supports
and the supports are used in columns for separations. This is
classified under the head of extraction chromatography. For this,
you may refer to Unit 4, sub-Sec. 4.2.3 where a brief mention has
been made about extraction chromatography. A variety of metal ion
separations are achieved using this technique. In this context,
there may be some justification for having liquid ion exchangers as
a distinct class of ion exchangers. However, this unit does not
discuss them in detail. An idea about liquid ion exchangers has
already been given in Unit 2 and 3.
SAQ 3 What are the two distinct classes of aluminosilicates
based on their structure?
...
...
...
...
SAQ 4 Under what conditions the organic resinous ion exchangers
deteriorate fast?
...
...
...
...
SAQ 5 Is there any justification of including liquid ion
exchangers as a distinct category of ion exchangers?
...
...
...
9.4 SYNTHESIS OF ION EXCHANGE RESINS It has been made clear
earlier that we will mainly focus on synthesis and properties of
organic resins. If we take synthesis, there are too many types of
resins and different chemical routes are followed to prepare them.
Therefore, it may be difficult to cite
-
11
Ion Exchange Chromatography
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 be accompanied by a few
examples of synthesis. One point which is very clear about
synthesis of ion exchange resin is that it must yield a three
dimensional cross-linked matrix of hydrocarbon chains carrying
fixed ionic groups. This can be achieved in the following ways: i)
Monomeric organic electrolytes can be polymerized in such a way
that a cross
linked network is formed. ii) The matrix can be built from non-
ionic monomers and the fixed ionic groups
are then introduced into the completed network. iii) The fixed
ionic groups are introduced while the polymerization is still
in
progress. While synthesizing resinous exchanger, it should be
kept in mind that it should be sufficiently cross-linked to have
negligible solubility. The cross linking should be such that it
should be able to swell. Polymers which are too highly cross-linked
cannot swell. The mobility of counter ions in such resins is so low
that ion exchange is difficult to take place. The method of
synthesis should be such that the degree of crosslinking can be
controlled. Most of the ion exchange resins are made by either
condensation polymerization or addition polymerization. Now the
addition polymerization processes have more or less replaced the
condensation processes.
9.4.1 Cation Exchangers A broad variety of cation exchangers
with fixed ionic groups of different character and different acid
strength are commercially available. The most common of these are
strong-acid resins with ( 3SO ) and weak acid resins with
carboxylic acid groups (COO). Even if we consider these two types
of resins, the resins of various strength can be made since
dissociation constants are affected by the nature and configuration
of the units to which the groups are attached. The arylsulphonic
acids are stronger than alkylsulphonic acids. Many ion exchangers
contain two or more different types of ionic groups and they are
known as bifunctional or polyfunctional.
a) Condensation polymers The earliest known cation exchange
resin was a condensation product of phenol and formaldehyde. The
list became broader and more extensive. Other monovalent or
polyvalent phenols like resorcinol and naphthol instead of phenol
and other aldehydes instead of formaldehyde can be used. Phenolic
group can act as a fixed ionic group but the resins have a very low
acid strength. Groups with higher acid strength can be introduced
by various methods. The easiest course is sulphonation of phenol
prior to polymerization.
b) Addition polymers The area of synthesis of ion exchange
resins is now dominated by addition copolymers prepared from vinyl
monomers. They are more chemically and thermally stable than the
condensation polymers. Moreover, in addition polymerization, the
degree of cross-linking and particle size are easy to control. A
well known cation exchange resin is obtained by the
copolymerization of
-
12
Chromatographic Methods-III
styrene and a small proportion of divinylbenzene followed by
sulphonation by treatment with concentrated sulphuric acid or
chlorosulphonic acid.
The role of divinylbenzene is as a crosslinking agent. Pure
divinylbenzene is not easily available. The commercial product
consists of different divinylbenzene isomers (around 50%) and
ethylenestyrene (around 50%). Therefore, ethylenestyrene is also
introduced in the matrix. The degree of crosslinking can be
adjusted by varying the divinylbenzene content.
9.4.2 Anion Exchangers The earliest anion exchangers synthesized
were with weak base amino groups
Subsequently, resins with strong-base quaternary ammonium groups
were prepared
It was followed by synthesis of resins with strong-base
quaternary phosphonium groups and tertiary sulphonium groups.
-
13
Ion Exchange Chromatography
Like cation exchangers, the earlier known anion exchangers were
condensation polymers and they are replaced by addition
polymers.
a) Condensation polymers The earliest known anion exchange
resins were prepared from aromatic amines like m- phenylenediamine
by condensation with formaldehyde.
The aldehyde reacts with amino groups. In the process, the
secondary and tertiary amino groups are formed. Thus, the resins
are polyfunctional. Aliphatic polyamines which are not as weakly
basic can also be condensed with aldehydes.
b) Addition polymers Like cation exchangers, a commonly used
anion exchange resin is prepared by copolymerization of styrene and
divinylbenzene followed by chloromethylation (introduction CH2Cl
grouping) say, in the para position and interaction with a base
such as trimethylamine. The polymers containing quaternary ammonium
groups are strong bases and those with amino or substituted amino
groups show weakly basic properties.
9.4.3 Amphoteric Exchangers The ion exchangers which contain
both acidic and basic groups are known as amphoteric exchangers. A
number of exchangers of this type has been synthesized but only a
few have found application.
A well known resin containing both strong base and acid groups
is prepared by copolymerization of styrene, vinylchloride and a
cross-linking agent followed by quaternization and sulphonation of
the product.
-
14
Chromatographic Methods-III
Among the amphoteric resins, the most important are the ones
known as snake- cage polyelectrolytes. They are conventional cation
or anion exchangers within which 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 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
crosslinked matrix that they cannot be displaced by other counter
ions. The situation is something like a snake trapped in a cage.
One significant difference these snake cage polyelectrolytes show
from other amphoteric exchangers is that the poly-counter ions 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 is
not necessary for the resin to have mobile counter ions (counter
ions to the poly-counter ions) to remain electrically neutral
provided the charges of fixed ionic groups and poly-counter ions
are balanced. These exchangers are excellent reversible sorbents
for electrolytes. This will be discussed later when the
applications of ion exchangers are being cited.
At the end of this section on the synthesis of ion exchange
resins, it may be important to point out that the chemical
structures of the polymers shown are hypothetical. It is difficult
to establish the resin structure exactly. Furthermore, the
structures of the polymers do not represent repeating identical
units since the sequence of the monomeric component is essentially
random.
SAQ 6 What are the advantages of addition polymeric resins over
their condensation counterparts?
...
...
SAQ 7 What is the role of divinylbenzene in the synthesis of
styrene-divinylbenzene polymeric resin?
...
...
9.5 TRADE NAMES AND NOMENCLATURE A number of manufacturers of
ion exchange resins sell their products with different trade names.
Some of these are given in Table 9.1.
-
15
Ion Exchange Chromatography
Table 9.1: Some Commercially Available Ion Exchange Resins
Manufacturer
Trade name
Dow Chemical Co., USA Dowex
Rohm & Hass Co., USA Amberlite
Permutit Co., UK Zeo- Karb/ De Acidite
Chemical Process Co., USA Duolite
Bayer-Farben, Germany Lewalit
Wolfen-Farben, Germany Wolfatit Sicso, India Seralite
Nomenclature The trade names of resins are generally so named
that the basic structure is readily apparent. Taking the example of
Dowex resin, it will include i) Type i.e. Dowex 50, 50 W( cation
exchangers); Dowex 1, 2, 4, 21K (anion
exchangers) ii) X- Number or percent divinylbenzene like X8 iii)
Mesh size i.e. 20- 50 ( based on US Standard screen) iv) Ionic form
i.e. Na The label will carry something like
Type % DVB Mesh size Ionic form
50 X8 20- 50 Na
9.6 RESIN PROPERTIES As a matter of fact, the resin is a very
complex material and there are several properties which are to be
known and clearly understood before putting it to any particular
application. Some of the important properties are i) Moisture
content ii) Particle size iii) Crosslinkage iv) Capacity v)
Distribution coefficient vi) Equivalency of exchange vii) Resin
selectivity Let us now study them in detail.
9.6.1 Moisture Content The moisture content of the resins is
determined in the usual manner by heating it at 110 115C overnight
to constant weight. However, several precautionary steps are
necessary in this exercise. For example, some resins are thermally
unstable in the hydrogen and hydroxyl form and therefore, these
should be converted to a stable form before oven drying. Samples
which decompose at these temperatures are occasionally dried at
room temperature over P2O5 for longer periods of time.
-
16
Chromatographic Methods-III
9.6.2 Particle Size The importance of particle size for proper
column performance in an ion exchange unit is quite obvious. Rate
of exchange, pressure drop and back wash expansions are all
dependent on particle size. The resin beads or particles may be
formed with 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 preferred. This corresponds to 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 have fully hydrated diameter. This is the value
that is to be taken into consideration. The size of the water
swollen resin will depend on the type of functional group and the
amount of cross linking of the polymer.
The size of the particle is one of the parameters affecting the
rate of ion exchange reaction. Besides this, the other parameters
affecting rate are size and charge of the ion involved, degree of
cross linking and the temperature. As a matter of fact, decreasing
the size of the particle materially decreases the time required for
the equilibrium to be attained with the contacting solution. Since
the time required to achieve the equilibration is decreased the
efficiency of a given volume of resin increases. In other words,
the volume of the resin required to perform a specific operation
decreases. The physical aspects of operation are also considerably
altered by the change in the particle size. With the decreasing
particle size, the friction loss or pressure drop of a 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.
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 the bed to different
extents depending upon the specific gravity of the resin. The finer
the mesh size and the lower the density, the greater will be the
bed expansion.
Generally, the smaller resin particles (~ 50 mesh) are
physically more stable. This is important when the resin is
mechanically moved or it goes through large volume changes.
9.6.3 Cross Linkages The second variation which can be
introduced into the copolymer bead is that of cross linkage. As
mentioned earlier, the cross linkage in a styrene- divinylbenzene
polymer refers to the fraction of divinylbenzene content. Thus, a
resin of 8% crosslinkage is made with beads containing 8%
divinylbenzene and 2% styrene and other monovinyl monomers.
The cross linkage affects the resin in two ways. As the amount
of cross linkage increases, the dry weight capacity decreases. This
decreased capacity results from the greater difficulty of
substituting active groups on the copolymers probably due to steric
factors. However, as compared to this, the change in water content
is more pronounced. Thus, as the cross linkage increases, the resin
has a swollen volume for essentially the same number of sites and
the wet volume capacity increases.
There are other properties which are affected by the degree of
cross linkage. With the decrease in the cross linkage, the resin
swells more and thus, the diffusion of ions within the resin
becomes faster. This, in turn, gives faster equilibrium rate
particularly, for large ions. On the other hand, if the cross
linkage is increased, the diffusion paths 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 the
separation of sulphate from high molecular weight sulphonic acid by
using highly cross linked anion
-
17
Ion Exchange Chromatography
exchange resin. In the same light, we can say that if the cross
linking is decreased, the permeable selectivity difference is also
decreased.
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 easily deformed.
9.6.4 Capacity If we consider an ion exchanger, it can be taken
as a reservoir of exchangeable ions. In the ion exchange operation,
it is the counter ions which are put to use. The counter ions
content of a given amount of material is equal to the fixed charges
which must be balanced by the counter ions and thus, is essentially
constant. This amounts to the fact that it is independent of
particle size and shape and of the nature of counter ions.
Ion exchangers are characterized in a quantitative manner by
their capacity. In the common usage, it is defined as the number of
ion equivalents in a specified amount of the material. But this
simple definition is not sufficient and will have to be qualified.
The definition becomes acceptable when the conditions are given.
Capacity and related data are primarily used for two purposes, for
characterizing ion exchange materials and for use in numerical
calculations of ion exchange operations. In the second case, it is
more practical to use other definitions or quantitatives which
reflect the effect of operating conditions. The different types of
capacity are given as under.
The total capacity of an ion exchange resin is the number of
ionic (or potentially ionic) sites per unit weight or volume of
resin. The dry weight total capacity is usually expressed in
milliequivalents per gram of anhydrous resin. Scientifically, it is
usually expressed as meq/ g dry H+ or Cl form. The wet volume
capacity is the number of sites per unit volume of the water
swollen resin. The performance of an ion exchange resin is
generally based on volume and the wet volume total capacity is the
theoretical or maximum capacity which the resin can show in any
aqueous ion exchange application. It may be expressed in
milliequivalents per milliliter.
The net number of sites which are utilized in a given volume of
resin in a given cycle in known as the operating capacity of the
resin in that particular cycle. It may be expressed in the same
terms as total capacity or as a percent of total capacity.
There is another term which is known as useful capacity which is
the capacity when equilibrium is not attained. It depends on
experimental conditions viz. ion exchange rates etc. There is
another capacity which is known as breakthrough (dynamic) capacity
which is utilized in column operation. It depends on operating
conditions. There is also a capacity known as sorption capacity
which is the amount of solute taken up by sorption rather than ion
exchange per specified amount of the exchanger.
9.6.5 Distribution Ratio It should be remembered that we should
not speak of a resin to pick up a certain ion without noting that
there is another ion in the resin phase. It is actually the
tendency of an ion exchanger to pick up A+ at the expense of B+.
This tendency of the exchanger to take up A+ will be different if
the resin contains other ions C+ instead of B+. Thus, we can
prepare a resin containing a certain counter ion and then compare a
series of other ions containing this counter ion as a reference.
For the ions in this series, we may simply mention distribution
ratios. Distribution ratio simply expresses the partitioning of ion
between the solution and the resin phases.
solutiontheinionsametheof.Concsinretheinionanof.Conc
=D
-
18
Chromatographic Methods-III
The conventional units are
solutionofLitre/Amountsinredryofkg/Amount
=D
The amount term, in milligram, moles or whatever may be is
proper since the units cancel in calculating the D ratio.
The D values are generally determined by batch method. A known
amount of resin is brought in contact with a known amount of metal
ion in solution until equilibrium is attained. Because isotherms
are non-linear, the D values are taken to be limiting slopes at
very low values (Fig. 9.1). The best solution for this is to
determine D values at low concentrations by taking labeled
solutions using radioisotopes. The D value is determined by simply
counting the solution before and after equilibrium with the
resin.
Fig. 9.1: A typical curve of loading of an ion exchanger
Sometimes, the distribution ratio is expressed with different
values, say
solutionofL/AmountvolumewetofL/Amount
=D
The conversion factor of D to Dv is the bed density, , where is
in kg of dry resin per L of resin bed.
For any ion exchange, the importance is its use for the
separation that means selectivity. For selectivity, the Dvalues
should be different for the ions to be separated. It should be kept
in mind that the Dvalues is conditional. It depends upon the nature
of resin and the composition of the solution in contact with it.
Composition will include pH, ionic strength, type and molarity of
acid and the presence of water miscible organic solvents and other
ions.
-
19
Ion Exchange Chromatography
Distribution Coefficient There is a term synonymous to
distribution ratio which is known as distribution coefficient (Kd).
This is also used to express the distribution of the ion between
the solution and the ion exchange resin. It is more or less the
same as distribution ratio. This weight distribution coefficient of
ion is given by
solutiontheofmL1in.Concsinretheofg1in.Conc
=dK
It is only the difference in terminology and it is determined in
the same manner as distribution ratio. It is expressed as per gram
of the dry resin. It is conditional and dependent on the nature of
resin and the conditions prevailing in the solution.
9.6.6 Equivalency of Exchange It is well known that in the
process, an equivalency of ion exchange is established. It amounts
to the fact that as many ion equivalents of one charge must enter
the resin phase as leave it during a reaction process. But there
are a number of things which occur in an ion exchange to make it
appear otherwise. The simplest case is that of an acid or base
neutralization in which the effluent contains only water. There is
another example where precipitates may form and be filtered out on
the resin. Then too, many substances are physically adsorbed or
occluded in the resin at least temporarily; e.g. organic acids and
amines. Even so, material balances on an equivalent basis are
usually fairly easy to obtain for an ion exchange processes.
9.6.7 Resin Selectivity The strong cation exchanger like Dowex
50 is comparable in acid strength with hydrochloric acid and will
form stable salt like bonds with any cation. Similarly, a strong
anion exchanger like Dowex 1 is comparable to sodium hydroxide and
will form stable bonds with any anions. The only ions which cannot
be held strongly by one or the other of these resins are complex
ions or organic ions which due to their size or configuration are
hindered from entering the interior of resin particle.
The above statement does not mean that all bonds between the
strong resin and the different ions are of equal strength. The ion
exchange resins will have preference for the particular types of
ions they will like to hold if given the choice. It is this
preference which is defined as the selectivity of the resin. In the
resin systems, the typical physical chemistry equilibrium constant
is not strictly applicable. It is substituted by a selectivity
coefficient. For a resin containing B ion placed in a solution of
ion A and allowed to come to equilibrium, the selectivity
coefficient (K) AB for monovalent exchange is given as follows.
( ) ( )( ) ( ) solutionin A of Conc. resin in the B of Conc.
solutionin B of Conc. resin in theA of Conc.)( BA
=K
It can also be written as A + Br Ar + B (9.1)
r
rBA ]B][A[
]B[]A[)( =K (9.2)
Here, r in the subscript represents the resin phase. This
definition ignores the activity coefficient of the ions in the two
phases. There is no fully satisfactory method for determining the
activity coefficient of ions in the resin phase and are thus
omitted. The activity coefficient of ions in solutions can be
obtained from the literature and can be applied in the above
expressions for accurate results when working with other than
-
20
Chromatographic Methods-III
dilute solutions. In the case of concentrated solutions when the
activity coefficient is significantly altered, the selectivity
coefficients values should be applied with caution.
It should be kept in mind that selectivity is dependent upon
many factors. It varies with temperature and pressure. The effect
of pressure has not been investigated due to the nature of the ion
exchange technique. However, there are several factors which are of
more concern and these are discussed below:
i) Type of functional group Beyond the primary question of
whether the resin is a cation or anion exchanger, the effect of
functional groups upon the selectivity of the resin is largely a
matter of acid and base strength. The difference between the weak
and strong exchange resin is rather sharp. But there are shades of
strength in both the categories. These differences are largely
reflected in the position of hydrogen or hydroxyl ion occupies in
the series. To elaborate this point, a typical example can be
cited. In Dowex-1 (a strong anion exchanger) all the three groups
are methyl groups.
Dowex-2 differs only in that one of the methyl groups is
replaced by an ethanol group. This substitution of methyl group
changes the selectivity to give a resin which can be converted to
the free base form much more efficiently.
ii) Valence and nature of exchanging ions a) At low aqueous(
less than 0.1 N) concentrations and ordinary
temperatures the extent of exchange increases with the
increasing valency of the exchanging ion, i.e., Na+ < Ca2+<
Al3+< Th4+ This means that divalent ions are more tightly held
by the resin than monovalent ions and trivalent ions more tightly
than divalent ions.
b) Under similar conditions and constant valence, for univalent
ions the extent of exchange decreases with the size of hydrated
cation. Thus,
Li+< H+ < Na+
-
21
Ion Exchange Chromatography
of ferric ion in concentrated solution of chloride ions. In this
case, ferric ion has a tendency to exist as 4FeCl (a complex ion)
which is strongly held by a quaternary ammonium anion exchange
resin. The iron can be removed from the resin by rinsing with water
or dilute acid since the complex breaks down when chloride ions are
not present in high concentration. Such examples are numerous in
the literature and are successfully exploited for various metal ion
separations.
iv) Ionic forms of resin It should be borne in mind that the
selectivity of an ion exchange resin changes, usually decreases, as
the resin is converted to that particular ionic form. In most
cases, this effect is slight. The selectivity coefficient holds for
one specific composition of the resin. The ionic form is usually
expressed as the mole fraction of the resin which in the A form at
equilibrium and is designated as XA. In general, the greater is the
value of (K) AB , the greater is its change from XA.
v) Total solution ionic strength Mono-monovalent exchanges are
usually little affected by the change in the total ionic strength.
However, it becomes important if the exchanges are taking place in
different valence say mono-divalent exchange.
Let us first consider mono-monovalent exchange NaCl + RSO3H HCl
+ RSO3Na ... (9.3)
(K ++NaH ) = r
r
]H][Na[]H[]Na[
++
++
... (9.4)
By substituting the values as follows,
X rNa+ = Equivalent fraction of Na+ in resin = (Na+)r/ C r
(9.5)
X +Na = Equivalent fraction of Na+ in solution = (Na+) / C r
(9.6)
where C r = total capacity or normality of resin (equiv./ L) and
C = total normality of solution.
Eq. 9.4 takes the following form
r
r
X
X
+
+
Na
Na
1= K
+
+NaH
+
+
Na
Na
1 X
X ( 9.7)
This equation gives the equivalent fraction of Na+ in the resin
as a function of the solution with which the resin is in
equilibrium. It may be noticed that the terms C r and C do not
figure in the Eq. 9.7.
Now consider the exchange of monovalent ion with divalent ion.
CaCl2 + 2 RSO3Na 2 NaCl + (RSO3)2Ca ... (9.8)
K++
+CaNa = 2
r
2r
]Na][Ca[]Na[]Ca[
+++
+++
(9.9)
When Eq. 9.9 is expressed in terms of the equivalent fraction of
Ca++ in the resin as a function of the solution, it becomes
-
22
Chromatographic Methods-III
2rCa
r
Ca
)1( ++++
X
X= K
++
+CaNa 2
Ca
Car
)1( ++++
XX
CC
(9.10)
In Eq. 9.10, the apparent selectivity coefficient is the term
K++
+CaNa )( C
C r.
C r is the total capacity of the resin per unit volume and,
therefore, is fixed for particular resin, the selectivity of the
divalent ion in this exchange is inversely related to the total
concentration of the solution. It can be concluded that the more
dilute the solution, the more selective the resin becomes for the
divalent ions. The normality of the resin phase (C r) will depend
upon the swollen volume of the resin and thus, is a function of
cross linkage. Similarly, the apparent selectivity coefficient has
this same form for exchanges between divalent and trivalent ions.
However, in the case of exchange between
monovalent and trivalent ion, the expression takes the form (K)
++++AB (2)
CC r
.
This means that the selectivity of the resin for the trivalent
ions is inversely related to the square of total solution
concentration.
It can be concluded that this polyvalent ion effect makes
efficient water softening possible. The divalent cations ( Ca++ and
Mg2+) are easily picked up from hard water ( a very dilute
solution) and yet are easily displaced by Na+ ions of a relatively
concentrated (10 15%) salt solution. For the same reason,
rehardening of water i.e., replacement of Ca++ / Mg++ ions by Na+
is difficult by ion exchange. This polyvalent ion effect holds good
for both cation and anion exchange.
SAQ 8 What are the main factors on which the swelling of the
resin bead will depend when immersed in water?
...
SAQ 9 What is the main advantage if the degree of cross linkage
of the resin is decreased?
...
SAQ 10 Name the important variables of the solution on which
D/Kd value of an ion for a particular resin will depend?
...
SAQ 11 State whether the following statements with regard to the
extent of exchange are TRUE/ FALSE. i) The extent of exchange for
Fe3+ is more than for Ce4+. ii) Under similar conditions, the
extent of exchange for Na+ is more than for Li+. iii) For divalent
cations the uptake of ion is only determined by the ionic size of
the
cation.
-
23
Ion Exchange Chromatography 9.7 OPERATING METHODS
So far you have learnt about different properties of the resins
and their effect on the resin behaviour particularly with regard to
their utility for separations. In the context of utility of resins,
a question arises as to how are the resins operated or brought in
contact with the solution. There are two main techniques for this.
They are batch and column methods. The column method may be further
sub-divided as to whether the resin bed is fixed or moving and
whether the feed solution and regenerant solution flow past the
resin in the same relative direction or in opposite direction.
9.7.1 Batch Operation The batch method consists of immersing the
resin in the solution in a container and allowing the equilibrium
to be established. The extent of exchange of ion is limited by the
selectivity of the resin under equilibrium conditions. Therefore,
unless the selectivity is quite favourable, only a relatively small
part of the total capacity of the resin can be utilized. It is
convenient to use a resin batchwise but it is generally impractical
to regenerate the resin for reuse batch-wise.
9.7.2 Column Operation Column operation can be visualized as a
large number of batch operations in series. The extent to which the
exchange takes place in each one of these small batch operations is
limited by the appropriate selectivity coefficient, the overall
effect may be much more favourable. The successive batch operation
in a simulated column may be considered to plates in a distillation
column. In a majority of units, ion exchange containers are used
and they are taken as vertical columns filled with ion exchanger
(Fig. 9.2). The resin is supported on a bed of graded gravel or
some other filter base and the feed and regenerant solution passed
through the column (down-flow operation) or up through the resin
(up-flow operation).
Fig. 9.2: Ion exchange column with fraction collector
Generally, fixed bed units are used with down flow operation. It
gives a maximum of resin solution contact and a minimum of
mechanical problems. A fixed bed ion exchange column may be
operated with counter-current flows. In such a system, the feed is
put through the column down flow and the regenerant is put through
up-flow and vice-versa. A large number of column arrangements have
been designed in which multiple column are piped together to give
semi-continuous operations, maximum resin utilization, regenerant
recovery or some other improvements.
-
24
Chromatographic Methods-III
9.7.3 Moving Bed Operation A somewhat different type of column
operation is that encountered in a moving bed or continuous
counter-current system. In such a system, the resin as well as the
solution, is made to flow through the system. A typical unit
consists of two stages in which the resin is contacted
counter-currently with the exhausting stream and the regenerant
stream. The chemistry of such an operation is similar to fixed bed
operation.
The advantages of moving bed operation are those with continuous
operation say a constant supply of a product of uniform quality and
reduced cost of space, capital and labour. There are some design
problems due to the movement of resin. Maintaining a counter-
current flow of resin and solution depends upon the densities of
two phases.
9.8 ION EXCHANGE IN MIXED AQUEOUS-ORGANIC MEDIA
Up to this point, you may be carrying an impression that ion
exchange takes only when the aqueous solution is brought in contact
with the solid exchanger. There must be ions in both the solution
and the solid. The ions must be free to move and the exchange takes
place. With water as a solvent and with solids which qualify as an
ion exchangers, these conditions are usually met. Water, because of
its high dielectric constant, is an excellent solvent for most
inorganics and quite a number of organic acids, bases and salts.
This is what justifies the choice of water as a phase for ion
exchange. Water, however, is by no means the only solvent which
allows ion exchange to take place. There are other solvents with
high dielectric constant ( ) in which electrolytes can dissolve and
dissociate and in which most of the ion exchangers are stable.
These are ethylene glycol ( = 41), methanol ( =32), ethanol ( = 26)
and acetone ( = 27). The last three solvents, in particular, have
importance in ion exchange. They can be used with or without
addition of water.
This particular section is devoted to ion exchange in mixed
aqueous organic media. It has been observed that some irregular
trends in the distribution of metal ions between mixed aqueous-
organic media and the ion exchanger are exhibited when the
percentage of the organic content is varied. The organic solvents
used are water miscible oxygenated compounds like tetrahydrofuran,
methanol, ethanol and acetone. The behaviour of the metal ions with
varying organic content has been usefully exploited for achieving
some difficult separations. Korkisch pioneered this technique and
named it as combined ion exchange solvent extraction and coined the
abbreviation CIESE for it. In order to explain the behaviour, in
chromatography using these solution two processes are operative:
ion exchange and liquid-liquid extraction. The mechanism can be
explained on the following lines.
i) The addition of organic content reduces the dielectric
constant of the solution promoting thereby ion pair formation
between the ionic species and fixed ions of the ion exchanger.
Consequently, with increasing organic content, the uptake of ionic
species on the ion exchanger may increase.
ii) Sometimes, the addition of organic content of the aqueous
solution causes a decrease in the distribution coefficient of the
ion. This is probably due to the attachment of the anionic complex
to the protonated organic solvent making thereby the anionic
complex less available for the solid anion exchanger. It may be
worthwhile to explain the formation of this ion-pair between the
protonated organic solvent and the anionic metal complex on the
lines similar to that discussed in sub-Sec. 3.2.4 of Unit 3. This
mechanism in this case can be illustrated as follows:
-
25
Ion Exchange Chromatography
(Organic solvent) + H+ ( Protonated organic solvent) (Protonated
organic solvent) + ( Anionic complex of the metal)
complexnassociatioIon)} the metalcomplex of( Anionic solvent) ed
organic{(Protonat
It may be important to point out that the force of attraction
between the two ionic components of the ion association complex
increases with the increasing organic content i.e., decreasing
dielectric constant.
iii) In some rare cases, the organic solvent may form complexes
with the metal ions bringing a change in their distribution
coefficient values.
The ion exchange and solvent extraction may operate
simultaneously in a particular system and compete with each other
resulting in to some irregular trends. Sometimes, the irregularity
of these trends is difficult to explain. Nevertheless, it does not
reduce the potentiality of the technique for separation purposes.
In order to highlight the utility of the technique, the
distribution coefficient of Au (III) and Hg(II) for Amberlite IR
400 (anion exchanger) at 0.6 M HCl with changing percentage of
tetrahydrofuran (THF) are given below in Table 9.2.
Table 9.2: I Values Data THF
Concentration
0 20 40 60 80 90
Au (III)
5232 4308 241 2.6 2.0 0.1
Hg (II)
>104 370 368 282 201 200
In Table 9.2, there is a decreasing trend in the distribution
coefficients of both the metal ions with the increasing percentage
of THF content. Without any THF at 0.6 M HCl, both Au(III) and
Hg(II) are strongly adsorbed on Amberlite IR-400 and it is
difficult to separate them. The best condition for separation is
achieved at 0.6 M HCl with 90% THF.
SAQ 12 From the data given in Table 9.2, comment on the
mechanism operating leading to decrease in the values of
distribution coefficients of the two metal ions.
...
...
9.9 SPECIFIC CATION EXCHANGERS In the beginning of this Unit, it
was mentioned that there have been so much developments in the
field of ion exchange that you can more or less have ion exchangers
tailor made for a specific job. Attempts in this direction led to
what are known as specific cation exchangers. This means these ion
exchangers show unusual selectivity towards a cation or a group of
cations. If we look at the common general purpose cation
exchangers,s they prefer certain counter ions. There is some sort
of specificity. But here we mean the ion exchanger to be
exclusively specific. The basic idea to synthesize such resins come
from the fact that a reagent which either precipitates a cation or
forms a strong complex may be introduced in the matrix of the
-
26
Chromatographic Methods-III
resin. The first attempt in this direction was made by Skogseid
who synthesized a resin containing group with a configuration
similar to that of dipicrylamine.
Dipicrylamine Dipicylamine is a known specific precipitating
agent for K+ ions. The resin is synthesized from polystyrene by
nitration, reduction, condensation with picrylchloride and again
nitration.
Many compounds which form chelates with metal ions have been
incorporated into resins while polycondensation with phenols and
aldehydes. To cite an example, there is one with anthranilic acid.
It is selective for zinc and other transition metal ions.
Other compounds which have been used are o- aminophenol,
anthranilic acid- diacetic acid, m-phenylenediglycine.
There are chelating resins containing groupings similar to those
of the more conventional chelating compounds, e.g., EDTA
(ethylenediamine tetracetic acid) but attached to a cross linked
matrix for gross insolubility. These compounds tightly bond certain
metal species which tend to form highly stable structures. Dowex A-
1, chelating resin, contains iminodiacetate groups attached to a
cross linked polystyrene matrix.
The resin has a very greater affinity for chelate forming di-
and trivalent cations than for cations like Na+ or K+. This resin
is particularly useful where one wishes to overcome the competing
effect of high concentration of one or ions. Thus, it will
effectively remove traces of heavy metal ions such as Fe3+, Cu2+
and Zn2+ from concentrated solutions of alkali and alkaline earth
cations can be removed. Metals can be eluted from the resins with
mineral acids. Selectivity among transition metal ions can be
attained by adjustment of pH.
There is one unattractive feature which is common to all
specific ion exchangers. As a matter of fact, the desired
selectivity for certain ions is attained by introducing certain
-
27
Ion Exchange Chromatography
groups for which the counter ion has the affinity. As a result
of this, the mobility of counter ions is greatly reduced. Thus, the
gain in selectivity is at the cost of rate of ion exchange. There
is another problem which arises due to extreme specificity. It is
difficult to replace the preferred counter ion except when it is
replaced by H+ ions. This will mean that it may be difficult to
regenerate the resin. In light of the above, one should choose a
resin keeping in mind a compromise between selectivity, rate of ion
exchange and ease of regeneration.
SAQ 13 What are the two main limitations of chelating
resins?
...
...
9.10 SYNTHETIC INORGANIC ION EXCHANGERS In sub-Sec. 9.3.2, it
was pointed out that the resinous exchangers cannot withstand high
temperatures and radiation dose. Moreover, they get degenerated in
highly oxidizing media. Towards the end of 1950s, there was a great
deal of activity in nuclear fuel technology, particularly,
reprocessing of nuclear fuel and the need of ion exchangers which
can withstand high temperatures and radiation dose was felt. This
revitalized the interest in inorganic ion exchangers and the
chemists started synthesizing inorganic ion exchangers. In a span
of about three decades, a huge variety of inorganic ion exchangers
were synthesized and put to use for different separations and other
applications. Most of these materials are amorphous in nature,
almost gel like materials which after drying, are ground to the
desired mesh size. Only a few compounds have been synthesized in a
well-defined crystalline structure. Let us now study about
different types and their characteristics.
9.10.1 Different Types and Their Characteristics The different
synthetic inorganic ion exchangers can be broadly classified under
the following categories. i) Hydrous oxides of polyvalent
metals.
ii) Insoluble acidic salts of polyvalent metals.
iii) Salts of heteropolyacids.
iv) Insoluble ferrocyanides.
v) Synthetic aluminosilicates.
vi) Miscellaneous inorganic ion exchangers e.g., synthetic
apatites, end sulphides.
i) Hydrous oxides of polyvalent metals The hydrous oxides are of
particular interest because most of them can function both as
cation and anion exchangers above and below a certain pH value.
These substances are mostly amphoteric in nature and their
behaviour mainly depends upon the basicity of the central atom and
the strength of the M O and O H bonds. The hydrous oxides of
tetravalent elements e.g., Zr (IV), Ti (IV), Mn(IV), Sn(IV) and
Ce(IV) are the most studied compounds of this class. Mixed hydrous
oxides of di- and tetravalent metals, tri- and tetra-valent have
also been investigated. The hydrous oxides of quinquevalent and
sexivalent metals generally show cation exchange properties and are
stable towards most of
-
28
Chromatographic Methods-III
the commonly used reagents. The hydrous oxides are useful for
column operations and can be easily regenerated for use. Apart from
enabling routine separations of various cations and anions, hydrous
oxides have been used for the purification and isolation of
transuranium elements from highly radioactive fission products.
Titanium oxides columns have been used for the recovery of uranium
and plutonium from the spent nuclear fuels.
ii) Insoluble acidic salts of polyvalent metals The acidic salts
of multivalent metals form one of the most extensively studies
class of compounds. A wide range of compounds of this type has been
described as ion exchangers. They include phosphates, arsenates,
antimonates, vanadates, molybdates, tungstates, tellurates etc.
mostly of trivalent and tetravalent metals like Al(III), Cr(III),
Fe(III), Ti(IV), Zr(IV), Sn(IV), Ce(IV) and Th(IV). These salts
mostly act as cation exchangers and their exchange properties
prominently arise from the presence of readily exchangeable
hydrogen ions associated with the anionic group. A bewildering
array of these acidic salts are known, mostly in the form of gels.
Mixed acidic salts like zirconium arsenophosphate, tin (IV)
arsenophosphate etc. have also been explored as ion exchangers.
Gels of these acidic salts have a potential of use for
separation of heavy metal cations by column chromatography. They
have been used for paper chromatography by impregnating the papers
with them. The gels do not have a definite composition and are not
very stable towards the hydrolysis of the acidic group. Because of
uncertainity about the exact composition and structure of gels, it
is very difficult to understand the exact mechanism of ion exchange
reaction. A real breakthrough in these exchangers came when some of
them were prepared in definite crystalline forms. Phosphates and
arsenates of Zr(IV), Ti(IV), Sn(IV), Th(IV) and Ce(IV) have been
obtained in crystalline forms. One of the most studied compounds of
this series is zirconium phosphate which has been obtained with
different degree of crystallinity and in different crystalline
forms.
iii) Salts of heteropoly acids The parent acids of the compounds
are 12-heteropoly acids of the general formula Hm XY12 O40. nH2O (m
= 3, 4, 5) where X may be phosphorus, arsenic, silicon, germanium
and boron and Y different elements such as molybdenum, tungsten and
vanadium. Amongst the exchangers of this category, work has been
mainly reported on 12- molybdophosphates. Ammonium molybdophosphate
and ammonium tungstophosphate are the two widely investigated
compounds for their physicochemical behaviour and practical
applications. Salts of heteropoly acids act mainly as cation
exchangers. They have been mainly used for concentration and
purification of Cs137from fission products. The behaviour of
heteropoly acid salts with quaternary organic cations like
pyridinium, picolinium, collidinium has been studied as ion
exchangers.
iv) Insoluble ferrocyanides The ion exchange properties of a
large number of insoluble ferrocyanides of various metals e.g.,
Ag(I), Zn(II), Cd(II), Cu(II), Ni(II), Co(II), Pb(II), Mn(II),
Fe(III), Ti(IV), Zr(IV), V(V), Mo(VI), W(VI), U(VI), have been
studied. The ferrocyanides act as cation exchangers with a high
affinity for heavy alkali metal ions, specially for Cs+. The ion
exchange mechanism in ferrocyanide is rather complicated and not
yet clear. In order to improve the mechanical properties of
ferrocyanides for use in column operations, the exchanger has been
prepared by precipitation on solid inert supports e.g., bentonite,
silica gel, etc., freezing the
-
29
Ion Exchange Chromatography
gel or bonding the precipitate particles to insoluble polymers
such as polyvinyl acetate.
v) Synthetic aluminosilicates These compounds represent a great
family of inorganic ion exchangers and depending upon their
structure they may be divided into the following three main
groups:
Amorphous
Two dimensional layered aluminosilicates, and
Aluminosilicates with rigid three dimensional structures
(zeolites) Among these groups, synthetic zeolites have attracted
increasing attention because of their molecular and ion-sieving
properties. They have been successfully employed in gas adsorption
and catalysis.
vi) Miscellaneous inorganic ion exchangers Exchangers like
apatites and sulphides are included in this class. The anionic and
cationic components of the apatite structure, M10 (XO4)Y, are
exchangeable, where M = Ca, Sr, Ba, Cd and Pb; X= P, As, V, Cr, Mn,
Si, Ge and Y= F2, Br2 (OH)2, O and CO 23 . The structure,
physicochemical properties, thermal stability and ion exchange
properties of these materials have been reported. The ion exchange
properties of a wide range of sulphides e.g., Ag2S, FeS, CuS, ZnS,
PbS, CdS, NiS, As2S3 and Sb2S3 have been studied. The sulphides are
selective towards cations forming insoluble sulphides. The metal of
the sulphide is displaced by the appropriate ion in solution.
9.10.2 Special Properties and Applications Here, it may be
important to point out that earlier a complete section has been
devoted to the properties of ion exchange resins and the next
section will be presenting a detailed overview of applications of
ion exchange resins. In such a situation, this sub-section for the
properties and applications of synthetic inorganic ion exchanger
may at first sight appear out of place. But in this section, we are
going to discuss about some unusual characteristics of some
inorganic ion exchangers and the applications based upon them. One
important point that has to be kept in mind is that it is difficult
to highlight all the unusual features shown by different classes of
inorganic exchangers and the applications based upon them.
Therefore, only some important characteristics and applications are
being discussed below.
i) Radiation and thermal stability Generally, it is taken as
more or less granted that the synthetic inorganic ion exchangers
are more resistant to radiation damage than their organic
counterparts. However, the studies have shown that it is not
justified to make a generalization about them. Each ion exchanger
needs to be tested for its radiation stability. A large number of
them have been used for processing fission products. The
ferrocyanides and salts of heteropoly acids are used for the
recovery of Cs137. The radiation stability combined with separation
capability yields a system which is known as isotope generator. A
radionuclide generator is a system/ device which makes the repeated
recovery of a short-lived isotope in a pure form from a relatively
long- lived parent isotope. A typical example is a Ba131- Cs131
generator on hydrous zirconium oxide. Short lived isotopes find use
in nuclear medicine for diagnosis and therapy.
-
30
Chromatographic Methods-III
By now a fairly good amount of data is available that the
thermal stability of inorganic ion exchangers particularly of those
amorphous in nature has been over emphasized. A number of amorphous
materials start losing their ion exchange capacity on heating.
However, some of them are quite resistant to heat. Hydrous Ta2O5 is
quite heat resistant upto 300C and is used for decontaminating
nuclear reactor cooling waters. A promising use of these exchangers
could be in fuel cells at high temperatures or for concentrating
the nuclear waste. Thermally stable ion exchanger in the transition
metal forms are used as high temperature catalysts. Layered
zirconium phosphate as such or in other metal form has been used
for catalyzing different organic reactions.
ii) Unusual selectivity Many of these inorganic ion exchangers
show high selectivity for particular ions and therefore, the
separation of these ions can be more conveniently carried out than
on typical organic resins. Such examples are numerous in
literature. Zirconium antimonate exchanger has been used for the
separation of Rb+ and Cs+. Cerric antimonate is more or less
specific for Hg2+ and is used for the separation of Hg2+ from Cd2+/
Pb2+. Zirconium phosphate like ferrocyanides and heteropoly acid
salts shows unusual selectivity for Cs+ and is used for the removal
of Cs+ from the nuclear reprocessing solution. Because of high
capacity of zirconium phosphate for NH +4 , it is used in
artificial kidney machine. The selectivity features are not only
confined to column separations but they have been extended to paper
chromatography in which the papers are impregnated with the
inorganic ion exchangers.
The unusual selectivity and stability of synthetic inorganic ion
exchangers make them suitable for use in ion selective electrodes.
They have been investigated for use as materials for membranes.
Apart from the uses mentioned above, the studies on inorganic
ion exchangers throw light on problems such as sorption of ions by
precipitates, electrophoretic behaviour of suspensions, isotopic
exchange in heterogenous systems and many other areas of solid
state chemistry.
Towards the concluding stage of discussion on synthetic ion
exchangers, it is important to point out that very few inorganic
exchangers have been used on commercial scale. The main reason
seems to be that among the useful ones, the majority are based on
metals which are costly. The other deterent seems to be that their
regeneration power is not as good as that of their organic
counterparts.
SAQ 14 What are the main advantages of majority of inorganic ion
exchangers over their organic counterparts?
...
...
SAQ 15 Which is the most thoroughly studied class of inorganic
ion exchangers? Which particular compound has received the maximum
attention?
...
...
-
31
Ion Exchange Chromatography 9.11 APPLICATIONS
Ion exchange is one of the very powerful tools for separations.
It has a very broad spectrum of applications and the simple
property of exchanging the ions has been very intelligently
exploited for various purposes of separation, enrichment, recovery
and decontamination in various areas of science and technology. In
the limited space of discussion available here, it may not be
possible even to simply list the applications and therefore, it is
difficult to accommodate a detailed explanation about them. But for
the purposes of clarity in presentation in a concise form, the
entire range of applications is being subdivided into the following
heads: i) Separation of metal ions and anions ii) Separation of
organics iii) Separation of ionized from nonionized iv) Separation
of actinide elements v) Miscellaneous applications.
These are briefly presented below:
9.11.1 Separation of Metal Ions and Anions One of the largest
uses of ion exchangers is their capability to separate metal ions.
Ion exchangers show natural selectivity among cations but it can be
enhanced by proper choice of aqueous medium, type of ion exchanger
and eluting agent. The literature is full of examples where
separations of topical interest are conveniently achieved.
The choice of different cation exchangers for the separation of
different cations looks logical but it may be important to point
out that almost equal number of separation of metals are achieved
on anion exchangers. The metals on anion exchangers are not
separated as cations but as anionic metal complexes. In this
context, the best example is the separation of metals as anionic
chlorocomplexes. The ease of formation of these complexes and their
stability determine the selectivity on the anion exchangers. There
is a full periodic table type chart available for the sorption
behaviour of different metal ions on Dowex-1(strong anion
exchanger) in the complete range of acidity of hydrochloric acid.
The data given therein have been very useful in designing metal ion
separations. As a typical example Co(II) and Ni(II) are separated
from hydrochloric medium on a strongly basic anion exchanger
(Dowex-1). The separation is based upon the fact that Co(II) but
not Ni(II) forms an anionic chlorocomplex (probably CoCl 3 ) in 9M
HCl presumably because of instability of the chlorocomplex. The
retained Co(II) is washed from the column by water because the
complex is decomposed and cobalt is recovered as cobaltous
chloride.
There are numerous example like this. The extremely high
selectivity of the quaternary ammonium type anion exchange resins
for the metallic anionic complexes is one of the most amazing
examples of ion exchange chromatography. The industrial application
of cation exchangers for water softening is well-known where mainly
the Ca2+, Mg2+ and other heavy metal ions are removed by the sodium
form of the exchanger. For complete deionization of water, it is
passed through a cation exchanger and then an anion exchanger or a
mixed bed of both types of ion exchangers.
The best illustration of potential of ion exchange to separate
closely similar metal ions is the separation of rare earths. A
cation exchange resin alone can provide separation since the
affinities of the lanthanide ions for the resin vary inversely with
their hydrated radii and these, in turn, vary inversely with the
crystallographic radii. Thus, the order of elution is LuLa. The
similarity of various rare earths necessitates the use of
complexing agents to increase the separation factor. The use of
carboxylic acids
-
32
Chromatographic Methods-III
such as citric, glycolic and tartartic, adjusted to appropriate
pH, as eluting agents markedly enhances the separation. Among the
other useful ligands employed as eluting agents are
-hydroxyisobutyric acid, EDTA, and 2-hydroxy-EDTA.
The rich deposits of the metals are exhausting and the
metallurgists have to depend upon low grade reserves and other
leaner sources like the metal wastes. For the recovery of metal
from low grade ores or the metal wastes, the matrices are first
leached with various reagents. The resulting leach liquors besides
the metal of interest contain several ionic impurities. The ion
exchange technique has proved to be very useful in increasing the
metal values of the leached liquors. Besides this, the resins are
used to recover metal values in the tailings of other
hydrometallurgical operations. Ion exchange resins have also been
employed for the upgrading of impure concentrates. In the
metallurgy of uranium, it is quantitatively recovered from the
leach liquor by means of anion exchange. Ion exchange has been used
for the recovery of gold. Another example of the successful use of
ion exchange is in the recovery of chromium from electroplating
waste.
The separations like molybdenum from rhenium, zirconium from
hafnium and niobium from tantalum have been achieved using ion
exchange chromatography and the conditions developed are used in
metallurgical operations.Several anions interfere in the estimation
of various cations and vice-versa. In such situations, ion exchange
chromatography is confined to inorganics.
9.11.2 Separation of Organics Ion exchange is equally effective
for the separation of organic molecules like amino acids, sugars,
nucleic acid and peptides. The purification of several important
organic molecules have been achieved by ion exchange
chromatography. The use of carboxylic acid cation exchange resin
for recovering and purifying the antibiotic, streptomycin, is one
of the classic examples of industrial use of ion exchange
chromatography. It should be kept in mind that streptomycin is a
cation.
9.11.3 Separation of Ionized from Nonionized The separation of
ionized materials from nonionized or slightly ionized materials
when both are present in water is accomplished by a process known
as ion exclusion. The process utilizes conventional ion exchange
resins. If we look at a column of ion exchange resin, it contains
three phases; the solid network of resin beads, the liquid inside
the beads (resin liquid) and the liquid surrounding the beads
(interstitial liquid). Most low molecular weight solutes diffuse
freely in and out the resin liquid phase. However, the organic
non-ionic solutes tend to exist at the same concentration in both
the resin liquid and the interstitial liquid phases.
The ionic materials, because of Donnan membrane effect, exist at
a considerably lower concentration in the resin liquid than in the
interstitial phase. Thus, if a solution containing ionic and
nonionic substances are fed to the column and the column is rinsed
with water, the ionic solution will reach the bottom first because
it has to essentially displace the interstitial liquid. The
non-ionic solution must displace the interstitial liquid and the
liquid inside the beads. Thus, the nonionic material will emerge
out of the column after the ionic solute has passed out of the
column.
Hence, ion exclusion offers a method of deionizing or removing
majority of ionic constituents from organic products without the
use of heat, electricity and chemical regenerants. Typical examples
of applications of ion exclusion are separation of acids and salts
from glycerine, alcohol and amino acids, the separation of strongly
ionized and weakly ionized materials such as acetic and mineral
acids, mono-, di- and triethanolamine and mono-, di- and
trichloroacetic acid.
-
33
Ion Exchange Chromatography
9.11.4 Separation of Actinide Elements Similar to the separation
of lanthanides ion exchange chromatography has played a major role
in the separation of actinides, especially trans plutonium
elements. Glen T. Seaborg used ion exchange equipment to identify
each element of the 5f series beyond any doubt by the sequence of
their appearance analogues to the sequence of the corresponding 4f
elements. The primary valency of all the actinides is +3 similar to
the lanthanides which also exhibit decreasing ionic radii (cf
lanthanide contraction). In practice, solution containing all the
actinides is sorbed on the top layer of a column containing acidic
cation exchanger. Individual actinides are eluted from the resin
bed by passage of an eluent solution through the column of resin as
shown in Fig. 9.3.
Fig. 9.3: Elution of tripositive actinide and lanthanide
ions
The elution is accomplished by the use of another metal ion
(i.e. M3+) which shifts the equilibrium of the exchange reaction
through competition with M+ for position in the resin.
Alternatively, it is also achieved by adding a complexing anion to
the solution which, by reducing the concentration of free metal
ions, also shifts the equilibrium to the left. In Fig. 9.2 is shown
the elution sequence for the separation of the lanthanide and
actinide cations from a column of cation exchange resin using a
complexing agent. It was considered as the big triumph for ion
exchange chromatography for the separation of various actinides
which was otherwise considered as impossible.
9.11.5 Miscellaneous Applications The mosaic of applications of
ion exchange will be incomplete if we do not mention the
applications from other areas of science. Here too, it is difficult
to enumerate even some of them because they are too many to list.
Thus, only a few of them are being
-
34
Chromatographic Methods-III
mentioned here. Ion exchange resins are polymeric materials that
may be considered insoluble acids and bases. They can be used to
promote reactions which can be catalysed by conventional acids and
bases. Some advantages of solid substantially insoluble ion
exchange catalysts are as follows: i) ease of separation by
filtration or decantation, ii) reduction of cost because the
catalyst can be used repeatedly usually without
regeneration, iii) increased product yield and efficiency, and
iv) elimination of corrosion problem. The major disadvantage of
using ion exchange resins as catalyst appear to be thermal and
chemical stability limitations. In some cases, this particular
problem is resolved by use of inorganic ion exchangers. Some of the
examples of ion exchange catalyzed reactions are acetal formation,
alcohol dehydration, aldol condensation, esterification and ester
hydrolysis.
The utilization of ion exchange in food processing has been
quite successful in both beverage and canning industries. In the
bottling of carbonated beverages, the presence of carbonate and
bicarbonate in water supplies has to be removed. They neutralize
the citric and phosphoric acids added to carbonated beverages. The
treatment of raw wines and whiskeys with ion exchange resins is of
considerable interest. The anion exchange resins remove aldehyde
and catalyze several esterification reactions thereby improving the
taste and bouquet of the product.
Ion exchange resins have been scanned as therapeutic agent or
additives in host of medical disorders or ailments. A purified
highly subdivided weak base anion exchanger has been quite
successfully used as antacid in peptic ulcer therapy and other
gastrointestinal disorders. The use of cation exchangers as means
of removing sodium from the body in the treatment of edemas and
hypertension has been very encouraging.
Ion exchange finds a very useful application in agricultural
science in the form of formulations for plant nutrients. Ion
exchange formulations containing nitrogen, phosphorus and potassium
in addition to the minor nutrients that may be deficient have been
found useful in fortifying a wide variety of potting soils. Such a
fortified soil may retain a large supply of nutrient without injury
to plant. The need for frequent fertilization is eliminated,
nutrition is continuous and self-regulatory.
Before we conclude this section, it may be mentioned that
preceding text simply acts as pointer to the great scope of ion
exchangers in different areas of science and technology.
SAQ 16 Give two important examples of use of ion exchange from
organic chemical technology.
...
...
SAQ 17 How does ion exchange help in improving the quality of
alcoholic drinks?
...
...
-
35
Ion Exchange Chromatography 9.12 SUMMARY
The unit begins with a historical background of the process of
ion exchange and focuses on the point that the most important
breakthrough in the field was the discovery of ion exchange resins.
The cation, anion and amphoteric exchangers are known. The basic
features of ion exchange mechanism are discussed. There are organic
and inorganic ion exchangers but the former dominate the field.
Some important features of synthesis of ion exchange resins are
discussed.
There are two main routes of synthesis-condensation
polymerization, and addition polymerization. Because of certain
advantages, the addition polymerization has taken an edge over the
condensation polymerization route. An idea is given about the trade
names of different types of ion exchangers assigned by the
manufacturers and the information generally provided on the
labels.
The ion exchange polymers are very complex materials and some of
their characteristics have to be properly understood before they
are put to use. The different properties discussed in detail are
moisture content, particle size, cross-linkage, capacity,
distribution ratio, equivalency of exchange and resin
selectivity.
The different methods used for operating ion exchangers are
elaborated. The mechanism operating during the uptake of cations in
mixed aqueous organic media is explained. This is followed by
discussion on chelating resin and synthetic inorganic ion
exchangers. The important features, classes, advantages and
drawbacks of these types of exchangers are discussed.
The unit concludes with a discussion on applications. There are
too many applications. But for the purposes of clarity of
presentation, only a few representative ones are cited and they are
discussed under different groups.
9.13 TERMINAL QUESTIONS 1. What are the important
characteristics of a useful ion exchanger?
2. What are the broad parameters on which the chemical, thermal,
and mechanical stability and ion exchange behaviour of the resins
depend?
3. What are the advantages if the particle size of the ion
exchange resin bead is decreased? Is there any serious drawback in
decreasing the particle size?
4. Name the factors on which the selectivity of an ion exchanger
for an ion depends.
5. What are the apparent selectivity coefficients for a
mono-divalent and mono- trivalent exchange? What are the
implications of the term?
6. Suggest the best possible ion- exchange based method for the
following: i) Separation of K+ from Na+
ii) Separation of Co2+ from Mg2+ iii) Recovery of Cs137 from
highly radioactive fission product waste. iv) Separation of HCl
from CH3COOH. State only the broad outlines.
-
36
Chromatographic Methods-III
7. What are the broad categories of synthetic inorganic ion
exchangers? Which one of these shows anion exchange properties?
8. What particular property is responsible for the separation of
lanthanides by ion exchange chromatography? How is the separation
potential of this technique enhanced?
9. What is the principle involved in ion-exclusion method?
10. What are the advantages of using ion-exchange resins as
catalysts?
9.14 ANSWERS Self Assessment Questions 1. The characteristic
difference between adsorption and ion exchange is that the
later is a stoichiometric process. Every ion removed from the
solution is replaced by an equivalent amount of another ionic
species of same sign. However, in adsorption solute may be taken up
without any species being replaced.
2. 3 XCl + Na3PO4(aq) X3PO4 + 3 NaCl(aq)
Here, X represents the structural unit of the ion exchanger and
PO 34 is present in the solution.
3. The two distinct classes of aluminosilicates are one with
three-dimensional network and the other with loose layered
structure.
4. The organic resinous exchangers deteriorate fast at high
temperature, under high dose of ionizing radiations and in highly
oxidizing media.
5. Yes, there is some justification because of the following two
reasons: i) The mechanism of transfer in both cases is ion exchange
ii) The column loaded with liquid ion exchanger ( extraction
chromatography) behaves more or less in a way similar to that
shown by a column packed with solid ion exchanger.
6. The addition polymeric resins are chemically and thermally
more stable than the condensation polymers. Moreover, in addition
polymerization, the degree of cross linking and particle size is
easy to control.
7. The role of divinylbenzene in the synthesis of
styrene-divinylbenzene polymeric resin is as a cross-linking agent.
The degree of cross-linking can be adjusted by varying the amount
of divinylbenzene content.
8. The size of the swollen bead will depend on the type of
functional group and the degree of cross linking of the
polymer.
9. If the degree of cross-linking is decreased, the resin swells
more and thus, the diffusion of ions within the resin becomes
faster. This, in turn, gives faster equilibrium rate particularly
for large ions.
10. The D/Kd value depends on the composition of the solution.
This includes pH, ionic strength, type and molarity acid and
presence of water miscible organic solvents and other ions.
-
37
Ion Exchange Chromatography
11. i) FALSE ii) TRUE
iii) FALSE
12. From the decreasing trend in the Kd value data with
increasing THF content, it can be proposed that the anionic
chlorocomplexes of both the metal ions are not being available to
the solid anion exchanger. They are being held as ion association
complex by the protonated organic solvent. The prominent mechanism
is
(Organic solvent) + H+ ( Protonated organic solvent) (Protonated
organic solvent) + ( Anionic complex of the metal)
lexation compIon associ)lexmetal comp( Anionic solvent) d
organic (Protonate
The force of attraction between the two is increasing with
increasing organic solvent, i.e. lowering of dielectric constant of
the medium.
13. The two main limitations of chelating resins are as given
below: i) By introduction of certain groups, the mobility of
counter ions is reduced;
hence, the kinetics of exchange is slowed down. ii) It is
difficult to regenerate the resin.
14. The main advantages of synthetic inorganic ion exchanger
over their organic counterparts are
i) higher thermal and radiation stability ii) higher chemical
stability in highly oxidizing media, and iii) unusual selectivity
for particular ions.
15. The most studied class of inorganic ion exchanger is acidic
salts of polyvalent metal ions and zirconium phosphate has received
maximum attention.
16. The two examples are as follows: i) separation of amino
acids, and ii) recovery and purification of streptomycin.
17. The quality of alcoholic drinks is improved by anion e