Unit 9 Ion Exchange Chromatography

UNIT 9 ION EXCHANGE CHROMATOGRAPHY

Ion Exchange Chromatography

Structure 9.1

Introduction Objectives

9.2 9.3

Basic Features of Ion Exchange Mechanism 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 9.6

Trade Names and Nomenclature 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 a lso 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 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 c ertain 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.

5

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 a ccomplishing analytical separations. The ion exchange membranes find quite a good use in physiological chemistry chemistry and biophysics. Ion exchange exchange separation played a major role in the identification of trans-uranium trans-uranium elements by Glen T. Seaborg. Seaborg. The identify of each element of 5 f  series  series was established beyond any doubt by the sequence of their appearance a analogous to the appearance of the corresponding 4 f  elements  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 s ynthesized by following different chemical routes. An idea about it can be had by illustrating the synthesis of some wellknown 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

6

•

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 c ation 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.

9.2

BASIC FEATURES OF ION EXCHANGE MECHANISM

Ion Exchange Chromatography

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 e xchanger. In the first process, a solution containing dissolved CaCl2, say something like hard water, is treated with a solid exchanger, NaX, containing exchangeable exchangeable Na+ ions. The 2+ exchanger removes the Ca  ions from the solution and replaces them with Na +. Thus, a cation exchanger in Na+ form is converted to Ca 2+ 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  by an equivalent amount of another ionic species of the same sign. However, in the c ase 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 f ramework 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? …………………………………………………………………………………………...

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Chromatographic Methods-III

SAQ 2 A sodium phosphate solution is passed through a column of an anion exchanger in the 3− chloride form. The PO4  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 l ike 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 c amerinite groups the exchange of OH – for Cl–, SO 24 and PO 34 has been been obse observe rved. d. Ther Theree are 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. −

8

−

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

Ion Exchange Chromatography

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 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 i on 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 d egradation 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 100ºC. 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 SO 3  remain ionized even at low pH. Similarly, weak base group NH3+ lose a proton, p roton, 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. 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 purposes but with the time, it has attracted

9

Chromatographic Methods-III

the interest of different types of research groups. A detailed discussion on synthetic inorganic ion exchangers 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 an alogy, some authors classify alkylphosphoric 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. chromatography. For this, you may refer to Unit 4, sub-Sec. 4.2.3 where a brief mention has been made about extraction chromatography. chromatography. A variety of metal ion separations are achieved using this technique. In this context, there may be some  justification for having having liquid ion exchangers exchangers as a distinct class of 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 c ite

10

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 e 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 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.

Ion Exchange Chromatography

While synthesizing resinous exchanger, 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 c ation 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 ( SO 3 ) 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 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. monomers. They are more chemically and thermally stable than the condensation polymers. Moreover, Moreover, in addition polymerization, the degree of cross-linking and and particle size are easy to control. A well known cation exchange resin is obtained by the copolymerization copolymerization of

11

Chromatographic Methods-III

styrene and a small proportion proportion of divinylbenzene followed by sulphonation sulphonation by treatment with concentrated sulphuric acid or chlorosulphonic 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 divinylbenzene content.

9.4.2

Anion Exchangers

The earliest anion exchangers synthesized were with we ak base amino groups

Subsequently, Subsequently, resins with strong-base quaternary ammonium groups were prepared It was followed by synthesis of resins with strong-base quaternary phosphonium phosphonium groups and tertiary sulphonium groups.

12

Ion Exchange Chromatography

Like cation exchangers, the earlier known anion exchangers were condensation 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. 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 pr epared by copolymerization 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 ac idic 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.

13

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 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 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 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.

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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

Ion Exchange Chromatography

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

9.6

Type

% DVB

Mesh size

Ionic form

50

X8

20- 50

Na

RESIN PROPERTIES

As a matter of fac t, 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 110º− 115ºC 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.

15

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 r equired 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 b ed 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 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. pronounced. Thus, as the cross linkage increases, the resin has a – swollen volume for essentially the same number of sites and the w et 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

16

exchange resin. In the same light, we can say that if the cross linking is decreased, the permeable selectivity difference is also decreased.

Ion Exchange Chromatography

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 numerical calculations of ion exchange operations 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 The total capacity of an ion ion exchange resin is 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) u tilized in column operation. It depends on operating conditions. capacity which is utilized There is also a capacity known as sorption capacity which is the amount of solute taken up by sorption rather than t han 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 p ick 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 se ries 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.  D =

Conc. of an ion in the re sin Conc. of  the same ion in the solution

17

Chromatographic Methods-III

The conventional units are  D =

Amount / kg of dry re sin Amount / Litre of solution

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 of loading of an ion exchanger exchanger

Sometimes, the distribution ratio is expressed with different values, say  D

=

Amount / L of  wet volume Amount / L of solution

The conversion factor of D to Dv is the bed density,  ρ , where  ρ  is  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.

18

Distribution Coefficient

There is a term synonymous synonymous to distribution ratio which is known as distribution coefficient  (  (K d  a lso used to express the distribution of the ion between the d).   This is also solution and the ion exchange resin. It is more or less the same as distribution ratio. This weight distribution coefficient  of  of ion is given by K d 

Ion Exchange Chromatography

Conc. in 1 g of the re sin

=

Conc. in 1 mL of  the solution

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 a s 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. a nions. 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 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 t o 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 p laced in a solution  A of ion A and allowed to come to equilibrium, the selectivity coefficient (K )  B  for monovalent exchange is given as follows. B ( K ) A

(Conc. of  A in the resin ) × (Conc. of  B in solution ) (Conc. of  B in the resin) × ( Conc. of  A in solution)

=

It can also be written as A + Br B

( K ) A

Ar + B =

[ A] r [B] [A][B] r

… (9.1) … (9.2)

Here, r  in  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

19

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 a re discussed below: i)

Type of functional group

Beyond the primary question of whether the resin is a cation or anion e xchanger, 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+