preface to fifteenth edition

11.32 SECTION 11
11.4.3 Ion-Exchange (Normal Pressure, Columnar)
Ion-exchange methods are based essentially on a reversible exchange of ions between an external
liquid phase and an ionic solid phase. The solid phase consists of a polymeric matrix, insoluble, but
permeable, which contains fixed charge groups and mobile counter ions of opposite charge. These
counter ions can be exchanged for other ions in the external liquid phase. Enrichment of one or
several of the components is obtained if selective exchange forces are operative. The method is
limited to substances at least partially in ionized form.
11.4.3.1 Chemical Structure of Ion-Exchange Resins. An ion-exchange resin usually consists
of polystyrene copolymerized with divinylbenzene to build up an inert three-dimensional, crosslinked
matrix of hydrocarbon chains. Protruding from the polymer chains are the ion-exchange sites
distributed statistically throughout the entire resin particle. The ionic sites are balanced by an equivalent
number of mobile counter ions. The type and strength of the exchanger is determined by these
active groups. Ion-exchangers are designated anionic or cationic, according to whether they have an
affinity for negative or positive counter ions. Each main group is further subdivided into strongly
or weakly ionized groups. A selection of commercially available ion-exchange resins is given in
Table 11.16.
The cross-linking of a polystyrene resin is expressed as the proportion by weight percent of
divinylbenzene in the reaction mixture; for example, “8” for 8 percent cross-linking. As the percentage
is increased, the ionic groups come into effectively closer proximity, resulting in increased
selectivity. Intermediate cross-linking, in the range of 4 to 8 percent, is usually used. An increase
in cross-linking decreases the diffusion rate in the resin particles; the diffusion rate is the ratecontrolling
step in column operations. Decreasing the particle size reduces the time required for
attaining equilibrium, but at the same time decreases the flow rate until it is prohibitively slow unless
pressure is applied.
In most inorganic chromatography, resins of 100 to 200 mesh size are suitable; difficult separations
may require 200 to 400 mesh resins. A flow rate of 1 mL · cm 2 · min 1 is often satisfactory.
With HPLC columns, the flow rate in long columns of fine adsorbent can be increased by applying
pressure.
11.4.3.1.1 Macroreticular Resins. Macroreticular resins are an agglomerate of randomly
packed microspheres which extend through the agglomerate in a continuous non-gel pore structure.
The channels throughout the rigid pore structure render the bead centers accessible even in nonaqueous
solvents, in which microreticular resins do not swell sufficiently. Because of their high
porosity and large pore diameters, these resins can handle large organic molecules.
11.4.3.1.2 Microreticular Resins. Microreticular resins, by contrast, are elastic gels that, in
the dry state, avidly absorb water and other polar solvents in which they are immersed. While taking
up solvent, the gel structure expands until the retractile stresses of the distended polymer network
balance the osmotic effect. In nonpolar solvents, little or no swelling occurs and diffusion is impaired.
11.4.3.1.3 Ion-Exchange Membranes. Ion-exchange membranes are extremely flexible, strong
membranes, composed of analytical grade ion-exchange resin beads (90%) permanently enmeshed
in a poly(tetrafluoroethylene) membrane (10%). The membranes offer an alternative to column and
batch methods, and can be used in many of the same applications as traditional ion exchange resins.
Three ion-exchange resin types have been incorporated into membranes: AG 1-X8, AG 50W-X8,
and Chelex 100.
11.4.3.2 Functional Groups
Sulfonate exchangers contain the group 9SO , which is strongly acidic and completely disso-
3
ciated whether in the H form or the cation form. These exchangers are used for cation exchange.