Page 221 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

ACTION OF ION EXCHANCE RESINS 7.2

It should be noted that large-size ions rnay not be absorbed by a medium
cross-linked resin so that its effective capacity is seriously reduced. A resin with
larger pores should be used for such ions.

Changing the ionic form: some widely used resins. It is frequently necessary to
convert a resin completely from one ionic form to another. This should be done
after regeneration, if this is being practised to 'clean' the resin (for example, if
the 'standard' grade of ion exchanger is used). An excess of a suitable salt
solution should be run through a column of the resin. Ready conversion will
occur if the ion to be introduced into the resin has a higher, or only a slightly
lower, affinity than that actually on the resin. When replacing an ion of lower
charge number on the exchanger by one of higher charge number, the conversion
is assisted by using a dilute solution of replacing salt (preferably as low as
0.01 M), while to substitute a more highly charged ion in the exchanger by one
of lower charge number, a comparatively concentrated solution should be used
(say, a 1 M solution).
Strongly acidic cation exchangers are usually supplied in the hydrogen or
sodium forms, and strongly basic anion exchangers in the chloride or hydroxide
forms; the chloride form is preferred to the free base form, since the latter readily
absorbs carbon dioxide from the atmosphere and becomes partly converted
into the carbonate form. Weakly acidic cation exchangers are generally supplied
in the hydrogen form, while weakly basic anion exchange resins are available
in the hydroxide or chloride forms.
The resins are available in 'standard' grade, in a purer 'chromatographic'
grade, and in some cases in a more highly purified 'analytical' grade: 'standard'
grade materials should be subjected to a preliminary 'cleaning' by a regeneration
procedure (see Section 7.8).

Strongly acidic cation exchangers (polystyrene sulphonic acid resins). These
resins (Duolite C225, Amberlite 120, etc.) are usually marketed in the sodium
form* and to convert them into the hydrogen form (which, it rnay be noted,
are also available commercially) the following procedure rnay be used.
The cleaned standard grade resin (which rnay of course be replaced by one
of the purer grades) is treated with 2M or with 10 per cent hydrochloric acid;
one bed volume of the acid is passed through the column in 10-15 minutes.
The effluent should then be strongly acid to methyl orange indicator; if it is
not, further acid must be used (about three bed volumes rnay be required). The
excess of acid is drained to almost bed level and the remaining acid washed
away with distilled or de-ionised water, the volume required being about six
times that of the bed. This operation occupies about 20 minutes: it is complete
when the final 100mL of effluent reauires less than 1 mL of 0.2M sodium
hydroxide to neutralise its acidity using methyl orange as indicator. The resin
can now be employed for the exchange of its hydrogen ions for cations present
in a given solution. Tests on the effluent show that its acidity, due to the exchange,
rises to a maximum, which is maintained until the capacity is exhausted when
the acidity of the treated solution falls. Regeneration is then necessary and is
performed, after backwashing, with 2M hydrochloric acid as before.

*The resin is supplied in moist condition, and should not be allowed to dry out; particulate fracture
rnay occur after repeated drying and re-wetting.