Page 230 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

7 ION EXCHANCE

7.6 CHELATING ION EXCHANGE RESINS
The use of complexing agents in solution in order to enhance the efficiency of
separation of cation mixtures (e.g. lanthanides) using conventional cation or
anion exchange resins is well established. An alternative mode of application
of complex formation is, however, the use of chelating resins which are ion
exchangers in which various chelating groups (e.g. dimethylglyoxime and
iminodiacetic acid) have been incorporated and are attached to the resin matrix.
An important feature of chelating ion exchangers is the greater selectivity
which they offer compared with the conventional type of ion exchanger. The
affinity of a particular metal ion for a certain chelating resin depends mainly
on the nature of the chelating group, and the selective behaviour of the resin
is largely based on the different stabilities of the metal complexes formed on
the resin under various pH conditions. It may be noted that the binding energy
in these resins is of the order of 60-105 kJ mol-', whereas in ordinary ion
exchangers the strength of the electrostatic binding is only about 8-13 kJ mol - '.
The exchange process in a chelating resin is generally slower than in the
ordinary type of exchanger, the rate apparently being controlled by a particle
diffusion mechanism.
According to Gregor et the following properties are required for a
chelating agent which is to be incorporated as a functional group into an
ion exchange resin:
1. the chelating agent should yield, either alone or with a cross-linking substance,
a resin gel of sufficient stability to be capable of incorporation into a polymer
matrix;
2. the chelating group must have sufficient chemical stability, so that during
the synthesis of the resin its functional structure is not changed by polymerisation
or any other reaction;
3. the steric structure of the chelating group should be compact so that the
formation of the chelate rings with cations will not be hindered by the resin
matrix.
4. the specific arrangements of the ligand groups should be preserved in the
resin. This is particularly necessary since the complexing agents forming
sufficiently stable complexes are usually at least tridentate.
These considerations indicate that many chelating agents could not be
incorporated into a resin without loss of their selective complexing abilities.
Ligands which do not form 1: 1 complexes (e.g. 8-quinolinol) would be
unsuitable, as also would molecules such as EDTA, which are insufficiently
compact. In the latter case, it is improbable that the chelate configurations
occurring in aqueous solution could be maintained in a cross-linked polymer.
The closely related iminodiacetic acid group does, however, meet the requirements
described, being compact and forming 1 : 1 complexes with metal cations.