Ion Exchange Chromatography | Types of ion exchangers | Factors controlling the selectivity of resin and application of Ion Exchange chromatography.

Ion exchange chromatography

Chromatography is a separation technique. Ion exchange chromatography is used for the separation of ions of similar properties which are very difficult to separate by another method.

Ion exchange chromatography involves a reversible exchange of ions present on the exchange resin and those present in the solution. The ion exchange resin which is a solid material act as a stationary phase and the solution will be a mobile phase. The ion exchanger can be for cations or the anions of the solution

Classification of Ion Exchange Chromatography based on functional groups

The ion exchange resin which is a solid material act as a stationary phase. The ion exchanger can be for cations and anions of the solution. It can be natural or synthetic in origin.

Natural ion exchangers lack reproducibility. Hence synthetic ion exchangers are preferred to natural ores.

Synthetic ion exchangers are high molecular weight, polymeric ionic materials containing a large number of exchangeable ionic functional groups per single molecule. Cation and anion exchangers are the two major groups of ion exchangers. The cation exchanger should have a replicable positive i.e. H⁺ ion and anion exchangers should have a replaceable negative i.e. OH^-­, Cl^– or NO₃^– ion.

Types of Ion-exchangers in ion exchange chromatography

On the basis of active functional groups present, ion-exchangers are classified under two categories :

(1) CationExchangers:-A cation exchanger resin may be defined as a high molecular weight, cross-linked polymer containing sulphonic (-SO3H), carboxylic (-COOH), or phenolic (-OH) groups, as an integral part of the resin and an equivalent amount of cations. A cation exchanger consists of a polymeric anion (R-) and active cations. In these resins, the H+ ions are mobile and exchangeable with other cations. The anions (-COO-, SO₃^–, O^– ) remain attached to the resin network. When such a resin is placed in a solution of salt, some of the H⁺ions of the resin enter the solution and an equivalent quantity of the cations of salt get attached to the resin. The simplified reaction may be represented as

The resin containing sodium ions can exchange these ions with other cations. The reaction with Ca²⁺ ions may be represented as,

(2) Anion Exchangers:-

 An anion exchange resin is a polymer containing amino or quaternary ammonium groups (-N+ R3 group). They contain the equivalent amount of anions generally Cl^–, OH^–, SO₄^2- etc. These anions are mobile and exchangeable. The anion exchange behavior of these materials may be represented as :

For the cation exchanger, the functional group attached to the resin should be an acidic group that will readily release protons. When –SO3H group is present on the resin the cation exchanger is called a strong cation exchanger and when –COOH group is present is called a weak cation exchanger.

When the tertiary amino group is present the anion exchanger is a strong anion exchanger and with a primary or a secondary amino group, the exchanger is the weak anion.

Ion Exchange Capacity in ion exchange chromatography

Ion exchange capacity is defined as the milli-equivalents of H⁺ ions that are exchanged from the surface of one gm of dry resin.

Capacity determination of cation exchanger

Take W gms of dry resins. Now it is taken in a long glass tube which acts as the column on passing NaCl Soln. through it. The solution coming out is HCl

R SO₃H   +   NaCl ———-> R SO₃^– Na⁺  +HCl

Resin                          Mobile

Let the volume of HCl collected be V ml.

Take some definite volume of this acid solution and titrate it against NaOH Soln. by using a suitable indicator. From this titration capacity of the cation exchanger is calculated as follows:-

1000 ml of 1N NaOH=      1000 milliequivalent of H⁺

   1 ml of 1N NaOH =          1 milliequivalent of H⁺

ml of   N NaOH       =        milliequivalent of H⁺

These many milliequivalents will be present in W gms of the resin.

Hence, the capacity of the resin column is:-

The capacity of determination of anion exchanger in Ion exchange chromatography

Take W gms of dry resins. Now it is taken in a long glass tube which acts as a column on passing Na₂SO₄ Solution through it. The solution coming out is of Cl^– ions.

R-N(CH₃)⁺ Cl ^–     +  SO₄^{2 –}  R-N (CH₃)⁺   ½ SO₄^2- + Cl ^–

The released chloride ions can then be estimated by titration with a standard solution of silver nitrate.

The capacity of the resin is then calculated as follows:-

1000 ml of 1 N AgNO₃       =          1000 milliequivalent of Cl^–

1 ml of 1 N AgNO₃       =          1 milliequivalent of Cl^–

x ml of  N AgNO_{3 =}              xy milliequivalent of Cl^–

The weight of the resin in W gm,

Hence,   

Requirements or properties of Ion-Exchange resin

The best resins must satisfy the following conditions:-

• The resin must be a three-dimensional cross-linked open structure

• It possesses an ionizable functional group.

• They must be water-insoluble and inert.

• It must be porous so that the flow of the mobile phase must be easier through porosity.

• It must have sufficient affinity for H₂O.

• The resin must be chemically stable and must contain a sufficient no. of exchange sites.

• The swollen resins must be denser than H₂O. So it must settle at the bottom of the container

• The particle size must be as small as possible.

Ion–Exchange Equilibria Ion exchange chromatography

• The process of ion – exchange involves the exchange of mobile ions of the ion – exchange with the ions of the same charge from a solution.
• Let us consider the resin bead present in the hydrogen form.
• When the bead is immersed in water, the protons, although more or less free to wander within the resin matrix, cannot escape from it because it demands electron neutrality.
• When another cation in solution comes close to the bead, exchange between the resin-bound proton and the cation in solution becomes possible.
• The equilibrium established can be written as,

• Where M⁺ is the free cation and R – H symbolizes the resin-bound proton.
• Application of the law of mass action gives:-

• Where K_d is known as the ‘equilibrium distribution coefficient‘ or ‘selectivity coefficient ‘ and refers to the activity coefficients of the species concerned.
• There is no satisfactory way of measuring the activities or activity coefficients of ions on the resin and for dilute solutions, activities may be replaced by concentrations.
• Hence, the expression becomes, 

• The term ‘Q’ is called ‘concentration quotient ’ or ‘ practical selectivity coefficient ’.
• If the value of ‘Q’ is large, the band will be narrow and concentrated.
• If the two ions, having different affinities for the resin, are passed through the column, they will move at different rates down the column and separation will be achieved.
• If the elution curves are sufficiently apart, a quantitative separation is possible.
• But if the elution curves overlap, the separation will be incomplete.

Factors controlling ‘selectivity  on Resin’

The ability of the resin to separate the ions involved i.e. selectivity depends upon a number of factors such as :

Nature of Resin  

The affinity of resin towards ions is greatly influenced by the nature of the resin. This requires consideration of the nature of the functional group (weak or strong), degree of cross-linking, etc. The degree of cross-linking decides porosity, while the size of the resin decides permeability. It affects the flow of solvent through the column. Porosity controls the total or partial exclusion of larger ions. The acid or base strength of resin is also an important factor.

Nature of exchanging ions

Selectivity of resin depends upon the nature of exchanging ions as follows:-

(a) At low concentrations and at ordinary temperature the extent of exchange   increases with increasing valency of the exchanging ion i.e.

                               Na⁺ < Ca²⁺ < Al ³⁺ < Th⁴⁺

(b) Under similar conditions and constant valence for univalent ions, the extent of exchange increases with an increase in the size of the hydrated cation.

                        Li⁺ < H⁺ < Na⁺ < NH₄⁺ < K⁺ <Rb⁺ < Cs⁺

Amongst the doubly charged cations, the capacity increases in the order

         Mg²⁺ < Zn²⁺ < Co²⁺ < Cu²⁺ < Ni²⁺ < Ca²⁺ < Sr²⁺ < Pb²⁺ < Ba²⁺

(c) With strongly basic anion exchange resin, the extent of exchange for univalent anion varies with the size of the hydrated ion in a similar manner to that indicated in cations.

                         F^–< OH^–< CN^–< Br^–< NO₃^–< I^–

(d) When a cation in solution is being exchanged for an ion of different valency, the relative affinity of the ion having higher valency increases in direct proportion to the dilution. Thus, the exchange will be favored by high dilutions if the lower valent ion is present in the exchanger and the higher valent is in the solution and vice versa.

Complex formation

The extent of exchange is also affected by complex formation. By forming suitable complexes it is possible to change the tendency of a particular metal ion to remain on the exchange column. As the tendency of unhydrated ions to form complexes increases, their ability to remain on resin decreases. Complex formation can even alter the charge carried by ions, making separation possible. For E.g. Zn2+ and Mg2+ can be separated into column using 6M HCl which forms complex [ZnCl₄]²⁺.

Concentration of solution

From the expression of the selectivity coefficient, it is evident, that the more the concentration of an exchangeable ion in solution, the more will be the concentration of an ion in the resin phase. Hence, the higher the concentration of ions in the solution, the greater the exchange.

Effect of pH

The affinity of resin towards ions depends upon the PH of the solution. This is especially important when the resin is weakly acidic or weakly basic. Weakly acidic resins containing (-COOH or –OH) groups bind protons more strongly than other ions. Consequently, their ion-exchange capacity increases with an increase in pH

Application of Ion Exchange Chromatography

Following are the applications of Ion Exchange Chromatography

(i)   The Softening of Water:-

The hardness of the water is due to the pressure of Ca+2 ions and Mg+2  ions. Thus, by exchanging Ca+2 ions and Mg+2  ions with Na⁺¹of resin the water which comes out from the column becomes soft.

ii) Preparation of de-ionized, demineralized, or conductivity water:- The water which contains only H+ ions as cation and OH- ions as anions are known as conductivity water or demineralized water. It is obtained by passing tap water over-acidic cationic exchanger. So that H+ ions of resins will be exchanged with the cation of water.

The above water with H⁺ions is then passed through an anionic exchanger, where anions as impurities of water (x) are exchanged with OH^‑ions of resin.

This water will be free from all impurities and possess only H⁺and OH^– ions. So it is known as conductivity water.

iii)  Traces amount of substance can be isolated or the concentration of the metal ion can be increased by passing repeatedly of Soln. of metal ion over the resin. The traces of the solute present in the solution will be retained on the surface of the resin. This retained solute can be eluted out with a little volume of solvent.

iv)  The phenomenon is used in inorganic qualitative analysis.

v) The separation of Cl^–, Br^‑, I^‑ ions can be done by means of this chromatography by using 0.5 N NaNO₃ as an eluting agent. Cl^‑ ion is first eluted out then Br ^– ions & finally I^– ions.

vi) Preparation of standard reagent:-

 It is not possible to prepare a standard solution of strong acid and strong base because they are not available in their pure form. But KCl and NaCl are available in their pure form.

We prepare an exact normal solution of either NaCl or KCl. This solution is then passed over the resin which H⁺ion or OH^– ions form. The solution thus obtained from the column will be standard acid or standard base.

vii) Separation of Amino acids:-

By controlling P^H it is possible to separate the mixture of amino acids.

viii) Separation of uranium from Th, Zr, and other interfering ions can be done by ion-exchange chromatography.

ix) Separation of Lanthanides:-

            When the mixture of the lanthanides is passed through suitable resin, the lanthanides retain on the surface of the resin. Then passing the ammonium citrate, we can elute them out.

Factors affecting ion exchange chromatography

The following are the factors with effects the separation,

a. Nature and property of ion exchange resin.

b. Nature of exchanging ion.

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