chemically modified microparticulate silicas (p. 124) incorporating cationic or anionic ion-exchange
groups are available. Being rigid and incompressible they are more suited to pressurized systems than
ion-exchange resins although their capacity is lower. Often, however, ion-pair chromatography (p. 147)
on non-polar bonded-phases proves to be superior to the use of ion-exchangers.
Properties of Ion-exchange Resins
The capacity of an ion-exchange resin, i.e. the number of exchangeable ions, is determined by the
degree of cross-linking and is expressed as milli-equivalents per gram of dry resin. Values range
between 2 and 10, depending on the resin, and are generally quoted for the hydrogen or chloride form.
In practice, the full capacity is never available due to non-ideal operating conditions. For column
separation, the practical or 'break-through' capacity is reached when an exchanging ion continuously
introduced at the top of the column leaks through with the column effluent.
Swelling is a phenomenon which accompanies the use of most ion-exchange resins, and is especially
important in column operation. It results from the osmotic pressure set up when a resin bead, which can
be considered to be a concentrated electrolyte solution, is surrounded by a more dilute polar solution.
Solvent flows into the bead and distends the structure in an attempt to reduce the osmotic pressure by
dilution. The change in bed-volume, that is the size of the resin column, can be considerable and may
well alter as different sample or eluting solutions are passed through. This affects pore size and thus the
ease with which exchanging ions can penetrate the beads. In non-polar solvents, microreticular resins
remain almost unswollen so their exchange capacity is small and the rate of exchange slow.
Macroreticular resins, however, still retain an open structure and are therefore particularly useful for
separations in relatively non-polar solvents or for the separation of large organic ions. The effect of the
degree of cross-linking on swelling has already been mentioned.
Selectivity
The affinity between a resin and an exchangeable ion is a function both of the resin and the ion. Ion-
exchange is an equilibrium process which for a cationic resin can be represented by the equation
where R represents the resin matrix.
The equilibrium constant, also known as the selectivity coefficient, is given by
where [Mn+]R and are the concentrations (strictly activities) of the