Page 235 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

OETERMINATION OF THE CAPACITï OF AN ION EXCHANCE RESIN (COLUMN METHOO) 7.9

hydroxide alternately through the column, with distilled-water rinsings in
between, and then washing with water until the effluent is neutral and salt-free.
'Analytical grade' and/or 'chromatographic grade' ion exchange resins that
have undergone this preliminary washing are available commercially.
For analytical work the exchange resin of 'analytical' grade (Amberlite) or
of 'chromatographic' grade (Duolite; Amberlite, etc.) of a particle size of
100-200 mesh is preferred. However, for student work, the 'standard' grade of
resin of 50-100 or 15-50 mesh, which is less expensive, is generally satisfactory.
The 'standard' grade of resin must, however, be conditioned before use. Cation
exchange resins must be soaked in a beaker in about twice the volume of
2M hydrochloric acid for 30-60 minutes with occasional stirring; the fine
partictes are removed by decantation or by backwashing a column with distilled
or de-ionised water until the supernatant liquid is clear. Anion exchange resins
may be washed with water in a beaker until the colour of the decanted wash
liquid reaches a minimum intensity; they may then be transferred to a wide
glass column and cycled between 1 M hydrochloric acid and 1 M alkali. Sodium
hydroxide is used for strongly basic resins, and ammonia (preferably) or sodium
carbonate for weakly basic resins. For al1 resins the final treatment must be
with a solution leading to the resin in the desired ionic form.
A 50 mL or 100 mL burette, with Pyrex glass-wool plug or sintered-glass
disc at the lower end, can generally be used for the determinations described
below: alternatively, the column with side arm (Fig. 7.4~) is equally convenient
in practice for student use. Reference will be made to the Duolite resins; the
equivalent Amberlite or other resin (see Table 7.1 in Section 7.1) may of course
be used.

7.9 DETERMINATION OF THE CAPACITY OF AN ION EXCHANGE RESIN (COLUMN
METHOD)
Cation exchange resin. Dry the purified resin (e.g. Duolite C225 in the hydrogen
form) by placing it in an evaporating dish, covering with a clockglass supported
on two glass rods to provide protection from dust while giving access to the
air, and leaving in a warm place (25-35 OC) until the resin is completely
free-running (2-3 days). The capacity of the resulting resin remains constant
over a long period if kept in a closed bottle. Drying at higher temperatures (say,
100 OC) is not recommended, owing to possible fracture of the resin beads.
Partly fil1 a small column, 15 cm x 1 cm (Fig. 7.4~) with distilled water, taking
care to displace any trapped air from beneath the sintered-glass disc. Weigh
out accurately about 0.5 g of the air-dried resin in a glass scoop and transfer it
with the aid of a small camel-hair brush through a dry funnel into the column.
Add sufficient distilled water to cover the resin. Dislodge any air bubbles that
stick to the resin beads by applying intermittent pressure to the rubber
tubing, thus causing the level of the liquid in the column to rise and fa11 slightly.
Adjust the level of the outlet tube so that the liquid in the column will drain
to a level about 1 cm above the resin beads.
Fil1 a 250 mL separatory funnel with ca 0.25M sodium sulphate solution.
Allow this solution to drip into the column at a rate of about 2 mL per minute,
and collect the effluent in a 500 mL conical flask. When al1 the solution has
passed through the column, titrate the effluent with standard 0.1 M sodium
hydroxide using phenolphthalein as indicator.