Page 219 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

ACTION OF ION EXCHANGE RESINS 7.2

where the terms in brackets represent the concentrations of ions A and B in
either the resin or solution phase. The values of selectivity coefficients are
obtained experimentally and provide a guide to the relative affinities of ions for
a particular resin. Thus if KB, > 1 the resin shows a preference for ion B, whereas
if KB, < 1 its preference is for ion A; this applies to both anion and cation
exchanges.
The relative selectivities of strongly acid and strongly basic polystyrene resins,
with about 8 percent DVB, for singly charged ions are summarised in Table 7.2.
It should be noted that the relative selectivities for certain ions may Vary with
a change in the extent of cross-linking of the resin; for example, with a 10 per
cent DVB resin the relative selectivity values for Li+ and Cs+ ions are 1.00 and
4.15, respectively.

Table 7.2
Cation Relative selectivity Anion Relative selectivity

F - 0.09
OH- 0.09
Cl - 1 .O0
Br- 2.80
NO; 3.80
1 - 8.70
ci04 10.0

The extent to which one ion is absorbed in preference to another is of
fundamental importance: it will determine the readiness with which two or more
substances, which form ions of like charge, can be separated by ion exchange
and also the ease with which the ions can subsequently be removed from the
resin. The factors determining the distribution of inorganic ions between an ion
exchange resin and a solution include:
1. Nature of exchanging ions. (a) At low aqueous concentrations and at ordinary
temperatures the extent of exchange increases with increasing charge of
the exchanging ion, i.e.

(b) Under similar conditions and constant charge, for singly charged ions the
extent of exchange increases with decrease in size of the hydrated cation

while for doubly charged ions the ionic size is an important factor but the
incomplete dissociation of salts of such cations also plays a part

(c) With strongly basic anion exchange resins, the extent of exchange for
singly charged anions varies with the size of the hydrated ion in a similar