13.7 Chromatography
209
13.7.3 Ion-Exchange Method
An ion-exchange resin consists of a polymeric framework of an insoluble hydrocarbon to which ionizable groups are attached. The resin may be visualized as an
insoluble polyelectrolyte. These polymers are obtained by polymerization of phenol
and formaldehyde or by copolymerization of styrene and divinylbenzene. Specific
ionizable groups are introduced to provide the desired exchange properties. In simpler
words, ion-exchange substances are insoluble solids with acidic or basic properties,
capable of forming loosely bound compounds with cations or anions respectively.
These ions exchange for an equivalent amount of other ions of similar charge i.e.,
when the ion-exchanger comes into contact with electrolyte solutions.
Ion-exchange resin can be classified as cation-exchange resin or anionexchange resin, depending upon the nature of the ionizable or exchangeable group.
These may be further classified as a strong or weak acid or base, according to the
characteristic of the ionizable group. The strong functional groups are well suited
for the inorganic separation and the weak groups for organic separation e.g., mixture
of amino acids.
Through the process of adsorption and diffusion, an ion in solution in contact with
an ion-exchange resin can exchange with or replace an ion supplied by the functional
or “ion-active” group of the resin. Thus, for a cation-exchange resin, the functional
group in the acid form, such as the sulfuric acid (-SO 3 H) and carboxylic acid group
(-COOH) can ionize and reversibly exchange for H
+ for cations such as Na
+ , Ca
++ ,
La
3+ , Th
4+ etc., in an appropriate stoichiometric ratio. The same functional group
in the salt form, e.g., -SO 3 Na and -COONa, can undergo exchange with H
+ or other
metal cations.
For anion exchange resin, quaternary ammonium bases (=N—OH,) and amines
(−NH 2 > NH, = N correct this) are typical strong and weak functional groups,
respectively. The strong quaternary ammonium base type resin exchanges rapidly
with anions, e.g., Cl
− , FeCl
−
4 in the base or the salt form.
An ion-exchange column can be used to separate not only one constituent of a
sample, but many constituents, provided they have different affinities for the ionexchanger. The exchange capacity depends upon the type of polymer and the type
of functional group. The capacity of a weak acid or weak base type is dependent
upon the pH of the contacting solution. For strong functional groups, the capacity
is independent of pH except for effect a resin from weak groups which may also
be present in the resin to a minor extent. At very high acidity the capacity of a
cation-exchange resin is decreased for ions of low affinity. Ion-exchange capacity
may be expressed in several ways, milliequivalent/g of dry resin or milliequivalent
per milliliter for wet resin.
Ion-exchange resins may be used in the separation of ionic species present in a
sample. The separation is carried out either by contacting the solution containing the
ionic material with the resin in a batchwise mixing process or by passing the solution
through a bed of resin contained in an ion-exchange column (Fig. 13.2A). The former
technique is used for equilibrium distribution studies; but the non-equilibrium column
209
13.7.3 Ion-Exchange Method
An ion-exchange resin consists of a polymeric framework of an insoluble hydrocarbon to which ionizable groups are attached. The resin may be visualized as an
insoluble polyelectrolyte. These polymers are obtained by polymerization of phenol
and formaldehyde or by copolymerization of styrene and divinylbenzene. Specific
ionizable groups are introduced to provide the desired exchange properties. In simpler
words, ion-exchange substances are insoluble solids with acidic or basic properties,
capable of forming loosely bound compounds with cations or anions respectively.
These ions exchange for an equivalent amount of other ions of similar charge i.e.,
when the ion-exchanger comes into contact with electrolyte solutions.
Ion-exchange resin can be classified as cation-exchange resin or anionexchange resin, depending upon the nature of the ionizable or exchangeable group.
These may be further classified as a strong or weak acid or base, according to the
characteristic of the ionizable group. The strong functional groups are well suited
for the inorganic separation and the weak groups for organic separation e.g., mixture
of amino acids.
Through the process of adsorption and diffusion, an ion in solution in contact with
an ion-exchange resin can exchange with or replace an ion supplied by the functional
or “ion-active” group of the resin. Thus, for a cation-exchange resin, the functional
group in the acid form, such as the sulfuric acid (-SO 3 H) and carboxylic acid group
(-COOH) can ionize and reversibly exchange for H
+ for cations such as Na
+ , Ca
++ ,
La
3+ , Th
4+ etc., in an appropriate stoichiometric ratio. The same functional group
in the salt form, e.g., -SO 3 Na and -COONa, can undergo exchange with H
+ or other
metal cations.
For anion exchange resin, quaternary ammonium bases (=N—OH,) and amines
(−NH 2 > NH, = N correct this) are typical strong and weak functional groups,
respectively. The strong quaternary ammonium base type resin exchanges rapidly
with anions, e.g., Cl
− , FeCl
−
4 in the base or the salt form.
An ion-exchange column can be used to separate not only one constituent of a
sample, but many constituents, provided they have different affinities for the ionexchanger. The exchange capacity depends upon the type of polymer and the type
of functional group. The capacity of a weak acid or weak base type is dependent
upon the pH of the contacting solution. For strong functional groups, the capacity
is independent of pH except for effect a resin from weak groups which may also
be present in the resin to a minor extent. At very high acidity the capacity of a
cation-exchange resin is decreased for ions of low affinity. Ion-exchange capacity
may be expressed in several ways, milliequivalent/g of dry resin or milliequivalent
per milliliter for wet resin.
Ion-exchange resins may be used in the separation of ionic species present in a
sample. The separation is carried out either by contacting the solution containing the
ionic material with the resin in a batchwise mixing process or by passing the solution
through a bed of resin contained in an ion-exchange column (Fig. 13.2A). The former
technique is used for equilibrium distribution studies; but the non-equilibrium column