ION-EXCHANGE CHROMATOGRAPHY

1. THEORETICAL PRINCIPLES

A) Types:

There are two types of ion-exchangers: cation and anion, in the form of spherical beads called resins. The first exchange cations by cations and the second anions by anions:

R^--H⁺ + NaCl R^--Na⁺ + HCl,

R⁺-OH^- + NaCl R⁺-Cl^- + NaOH.

The widely used ion-exchange resins are composed of crosslinked sulphonated polystyrene with a sulphonic acid group (-SO₃H) as the functional cation-exchange group, or a tetraalkylammonium hydroxide (-CH₂-N(CH₃)₃.OH) as the functional anion-exchange group (Fig. 19).

Fig. 19: Crosslinked sulphonated polystyrene.

Since these two groups are strong electrolytes, their resins are known as strong cation or strong anion-exchangers respectively. Examples of the different types of resins are given in the following table:

Type	Functional group	Working pH	Trade name
Strong cation	Sulphonic acid	0 - 14	Amberlite IR-120, Dowex 50
Weak cation	Carboxylic on polymethacrylate	5 - 14	Amberlite IRC-50
Strong anion	Quaternary ammonium	0 - 12	Amberlite IRA-400, Dowex 1.
Weak anion	Polyamine (2ry or 3ry) on polystyrene or phenylformaldehyde.	0 - 9	Amberlite IR-45, Dowex 3

The useful pH ranges are significant. Below pH 5, the weak acid resins are so slightly dissociated so that cation exchange becomes negligible. The converse is true for weakly basic types above pH 9.

B) Selectivity coefficient:

Since ion-exchange reactions are reversible, the law of mass action holds:

2 (RSO₃)^--K⁺ + Ca²⁺ [(RSO₃)^-]₂-Ca²⁺ + K⁺ ;

and:

E (Ca/K) = [(RSO₃)₂Ca] . [K⁺]² / [RSO₃K]² . [Ca²⁺]

Where E (Ca/K) is the selectivity coefficient.

In spite of the variable nature of selectivity coefficients, it is possible to arrange a series of ions in the order of their increasing selectivity coefficients for a given resin, all with respect to a common standard ion. For example, for univalent cations on sulphonate resins, the order is:

Li⁺, H⁺, Na⁺, NH₄⁺, K⁺, Rb⁺, Cs⁺, Ag⁺, Tl⁺ ;

That is Li⁺ is held least strongly on the resin.

C) Equivalence:

The ion-exchange process occurs in equivalent amounts. When the potassium form of Dowex 50 (RSO₃)^-K⁺ is treated with a dilute solution of calcium nitrate, the quantity of potassium ions released from the resin is equivalent to the quantity of calcium ions absorbed.

D) Swelling:

Because the concentration of the internal solution exceeds that of the external solution, osmotic forces tend to drive water into the resin, thus causing it to swell. Therefore, the resin must be made fully swollen by immersing in water before packing to avoid shattering of the column.

E) Regeneration:

To reuse the resin, the absorbed ions have to be removed by regeneration whereby it is converted to an exchangeable form. This is achieved by treating the resin with a corresponding acid or alkali (strong acid for strong cation-exchanger and so on). The form produced is the hydrogen or hydroxyl respectively. For conversion of the resin from a univalent ion form to a polyvalent ion form, a more concentrated solution of the former is required than for the reverse.

2. ANALYSIS

It is common practice in ion-exchange chromatography to collect and analyse large number of small fractions, generally of equal volumes. The effective time of analysis can be decreased with the aid of an automatic fraction collector which is rotated intermittently by the action of a timing, drop-counting, or weight-actuated device. The most widely used technique for continuously recording the process of chromatographic separations have been based on conductivity, pH, radioactivity, refractive index, light absorption and polarographic measurements.

3. CHROMATOGRAPHIC APPLICATIONS

A) Separation of ion:

Ion-exchange resins are used in the separation of simple ions or mixtures of them. Fig. 20 shows the complete separation of Na⁺ and K⁺ ions as an example. Quantitative analysis was made by titration of the corresponding Cl^- ion by the Mohre method.

Fig. 20: Ion-exchange separation of sodium and potassium on a cation-exchange resin, Dowex-50, eluted with 0.7 F-HCl.

B) Removal of interfering ions:

Cations such as Na⁺, (NH₄)⁺ and Fe³+ co-precipitate with BaSO₄ in gravimetry, and can be removed by passing the solution through a H⁺ form cation exchange resin instead of the other laborious methods.

C) Ion-exclusion:

A mixture of an electrolyte and non-electrolyte is resolved by passing it over an ion-exchange resin, the electrolyte emerges first.

D) Salting out:

Mixtures of organic compounds that cannot be separated by elution with water can often be easily separated by elution with a fairly concentrated salt solution, hence the nomenclature “salting out”. For example, no separation is achieved by eluting a mixture of diethylene glycol and dipropylene glycol through a 70 cm column of Dowex 1-x8 (sulphate form). However, a quantitative separation is accomplished through a 10 cm column of the same resin by elution with 3 M-ammonium sulphate.

4. NON-CHROMATOGRAPHIC APPLICATIONS

A) Deionized water:

Removal of ions from water is accomplished by passing it through a hydrogen form cation-exchanger then a hydroxyl type, or through a mixed bed of cation-anion resins. Cations are exchanged by hydrogen ions and anions by hydroxyls, producing water instead of salts. The principle is utilized in the production of very pure water (conductivity water) for research work, medical and pharmaceutical purposes, or on the industrial scale. Such water purification systems may incorporate pre-distillation units to get rid of the greatest part of salts before inter into the ion-exchangers, whereby the exhaustion time of resins is increased. Fig. 21 shows a semi-automatic water purification system involving distillation and deionisation.

Fig. 21: Apparatus for preparation of highly purified water for trace analysis.

B) Preparations:

Carbonate free solutions of sodium or potassium hydroxide are prepared by ion-exchange with the advantage of yielding directly a standard solution.

C) Concentration of traces:

The ions to be concentrated from large volumes are passed through a resin and eluted by a small volume of a proper eluent.