Page 216 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

7 ION EXCHANCE

Numerous types of both cation and anion exchange resins have been
prepared, but only a few can be mentioned here. Cation exchange resins include
that prepared by the copolymerisation of methacrylic acid (E) with glycol

bismethacrylate (F) (as the cross-linking agent); this contains free -COOH
groups and has weak acidic properties. Weak cation exchange resins containing
free -COOH and -OH groups have also been synthesised. Anion exchange
resins containing primary, secondary, or tertiary amino groups possess weakly
basic properties. We may define a cation exchange resin as a high-molecular-
weight, cross-linked polymer containing sulphonic, carboxyiic, phenolic, etc.,
groups as an integral part of the resin and an equivalent number of cations: an
anion exchange resin is a polymer containing amine (or quaternary ammonium)
groups as integral parts of the polymer lattice and an equivalent number of
anions, such as chloride, hydroxyl, or sulphate ions.
The fundamental requirements of a useful resin are:
1. the resin must be sufficiently cross-linked to have only a negligible solubility;
2. the resin must be sufficiently hydrophilic to permit diffusion of ions through
the structure at a finite and usable rate;
3. the resin must contain a sufficient number of accessible ionic exchange groups
and it must be chemically stable;
4. the swollen resin must be denser than water.
A new polymerisation technique yields a cross-linked ion exchange resin
having a truly macroporous structure quite different from that of the conventional
homogeneous gels already described. An average pore diameter of 130 nm is
not unusual and the introduction of these macroreticular resins (e.g. the
Amberlyst resins developed by the Rohm and Haas Co.) has extended the scope
of the ion exchange technique. Thus, the large pore size allows the more complete
removal of high-molecular-weight ions than is the case with the gel-type resins.
Macroporous resins are also well suited for non-aqueous ion exchange
applications. '
New types of ion exchange resins have also been developed to meet the
specific needs of high-performance liquid chromatography (HPLC) (Chapter 8).
These include pellicular resins and microparticle packings (e.g. the Aminex-type
resins produced by Bio-Rad). A review of the care, use and application of the
various ion exchange packings available for HPLC is given in Ref. 19.
Some of the commercially available ion exchange resins are collected in
Table 7.1. These resins, produced by different manufacturers, are often
interchangeable and similar types will generally behave in a similar manner.
For a more comprehensive list of ion exchange resins and their properties,
reference may be made to the booklet published by BDH Ltd (see the
Bibliography, Section 9.10).
Finally, mention should be made to the development of silica-based ion
exchange packings for HPLC. Their preparation is similar to that for the