Page 218 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

7 ION EXCHANCE

The reaction is reversible; by passing a solution containing sodium ions through
the product, the calcium ions may be removed from the resin and the original
sodium form regenerated. Similarly, by passing a solution of a neutral salt
through the hydrogen form of a sulphonic resin, an equivalent quantity of the
corresponding acid is produced by the following typical reaction:
(Res.SO;)H+ + Na'Cl- (soln) e (Res.SO;)Na+ + H + Cl- (soln)
For the strongly acidic cation exchange resins, such as the cross-linked
polystyrene sulphonic acid resins, the exchange capacity is virtually independent
of the pH of the solution. For weak acid cation exchangers, such as those
containing the carboxylate group, ionisation occurs to an appreciable extent
only in alkaline solution, i.e. in their salt form; consequently the carboxylic
resins have very little action in solutions below pH 7. These carboxylic
exchangers in the hydrogen form will absorb strong bases from solution:
(Res.CO0-)H+ + Na+ OH - (soln) e (Res.CO0-)Na+ + H20
but will have little action upon, Say, sodium chloride; hydrolysis of the salt form
of the resin occurs so that the base may not be completely absorbed even if an
excess of resin is present.
Strongly basic anion exchange resins, e.g. a cross-linked polystyrene containing
quaternary ammonium groups, are largely ionised in both the hydroxide and
the salt forms. Some of their typical reactions may be represented as:
2(Res.NMe:)Cl- + Soi- (soln) e (Res.NMe:),SOi- + 2C1- (soln)
(Res.NMe:)Cl- + OH - (soln) e (Res.NMe:)OH - + Cl- (soln)
(Res.NMe:)OH - + H + Cl- (soln) e (Res.NMe:)Cl- + H20
These resins are similar to the sulphonate cation exchange resins in their activity,
and their action is largely independent of pH. Weakly basic ion exchange resins
contain little of the hydroxide form in basic solution. The equilibrium of, Say,

is mainly to the left and the resin is largely in the amine form. This may also
be expressed by stating that in alkaline solutions the free base Res.NHMe2.0H
is very little ionised. In acidic solutions, however, they behave like the strongly
basic ion exchange resins, yielding the highly ionised salt form:

They can be used in acid solution for the exchange of anions, for example:
(Res.NHMe;)Cl- + NO; (soln) e (Res.NHMe;)NO; + Cl- (soln)

Basic resins in the salt form are readily regenerated with alkali.
Ion exchange equilibria. The ion exchange process, involving the replacement
of the exchangeable ions Ar of the resin by ions of like charge B, from a solution,
may be written:

The process is a reversible one and for ions of like charge the selectivity
coefficient, K, is defined by: