Page 229 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

ION EXCHANGE IN ORCANIG AN0 AIIUEOUS-ORCANIC SOLVENTS 7.5

concentration of sample ion. This procedure has contributed to the ease
with which non-absorbing species can be detected, particularly sample
ions with pK, > 7 for which conductance detection is not appropriate.
(b) Fluorescence detectors. Although only a small proportion of inorganic
and organic compounds are naturally fluorescent, the inherent sensitivity
and selectivity of fluorescence detection offers significant advantages. The
development of appropriate pre-column and post-column derivatisation
procedures has furthered the application of fluorescence detection for
the trace analysis of non-fluorescent or weakly fluorescing ~pecies.~'
The reader is recommended to consult the monograph by Scott for
further details of al1 detectors3' used in ion chromatography.
Ion chromatography has been successfully applied to the quantitative analysis
of ions in many diverse types of industrial and environmental samples. The
technique has also been valuable for microelemental analysis, e.g. for the
determination of sulphur, chlorine, bromine, phosphorus and iodine as
heteroatoms in solid samples. Combustion in a Schoniger oxygen flask
(Section 3.31)is a widely used method of degrading such samples, the products
of combustion being absorbed in solution as anionic or cationic forms, and the
solution then directly injected into the ion chromatograph.
A typical application of ion chromatography for the separation and
determination of simple anions is illustrated by the experiment described in
Section 7.15.

7.5 ION EXCHANGE IN ORGANIC AND AQUEOUS-ORGANIC SOLVENTS
Investigations in aqueous systems have established many of the fundamental
principles of ion exchange as well as providing useful applications. The scope
of the ion exchange process has, however, been extended by the use of both
organic and mixed aqueous-organic solvent system~.~~.~~
The organic solvents generally used are 0x0-compounds of the alcohol, ketone
and carboxylic acid types, generally having dielectric constants below 40. Cations
and anions should, therefore, pair more strongly in such solvent systems than
in water and this factor may in itself be expected to alter selectivities for the resin.
In addition to influencing these purely electrostatic forces, the presence of the
organic solvent may enhance the tendency of a cation to complex with anionic
or other ligands, thus modifying its ion exchange behaviour. In mixed aqueous-
organic solvents the magnitude of such effects will clearly be dependent on the
proportion of organic solvent present.
As already indicated, ion exchange resins are osmotic systems which swell
owing to solvent being drawn into the resin. Where mixed solvent systems are
used the possibility of preferential osmosis occurs and it has been shown that
strongly acid cation and strongly basic anion resin phases tend to be
predominantly aqueous with the ambient solution predominantly organic. This
effect (preferential water sorption by the resin) increases as the dielectric constant
of the organic solvent decreases.
An interesting consequence of selective sorption is that conditions for
partition chromatography arise which may enhance the normal ion exchange
separation factors. This aspect has been utilised by K~rkisch~~ separation
for
of inorganic ions by the so-called 'combined ion exchange-solvent extraction
method' (CISE).