Page 222 - Vogel's TEXTBOOK OF QUANTITATIVE CHEMICAL ANALYSIS

7 ION EXCHANGE

Weakly acidic cation exchangers (a.g. polymethylacrylic acid resins). These
resins (Zerolit 226, Amberlite 50, etc.) are usually supplied in the hydrogen
form. They are readily changed into the sodium form by treatment with
1M sodium hydroxide; an increase in volume of 80-100 per cent may be
expected. The swelling is reversible and does not appear to cause any damage
to the bead structure. Below a pH of about 3.5, the hydrogen form exists almost
entirely in the little ionised carboxylic acid form. Exchange with metal ions will
occur in solution only when these are associated in solution with anions of
weak acids, i.e. pH values above about 4.
The exhausted resin is more easily regenerated than the strongly acidic
exchangers; about 1.5 bed volumes of 1 M hydrochloric acid will usually suffice.
Strongly basic anion exchangers (polystyrene quaternary ammonium resins).
These resins (Duolite A113, Amberlite 400, etc.) are usually supplied in the
chloride form. For conversion into the hydroxide form, treatment with 1 M
sodium hydroxide is employed, the volume used depending upon the extent of
conversion desired; two bed volumes are satisfactory for most purposes. The
rinsing of the resin free from alkali should be done with de-ionised water free
from carbon dioxide to avoid converting the resin into the carbonate form;
about 2 litres of such water will suffice for 100 g of resin. An increase in volume
of about 20 per cent occurs in the conversion of the resin from the chloride to
the hydroxide form.
Weakly basic anion exchangers (polystyrene tertiary amine resins). These resins
(Duolite A303, Amberlite 45, etc.) are generally supplied in the free base
(hydroxide) form. The salt form may be prepared by treating the resin with
about four bed volumes of the appropriate acid (e.g. 1 M hydrochloric acid)
and rinsing with water to remove the excess of acid; the final effluent will not
be exactly neutral, since hydrolysis occurs slowly, resulting in slightly acidic
effluents. As with cation exchange, quantitative anion exchange will occur only
if the anion in the resin has a lower affinity for the resin than the anion to be
exchanged in the solution. When the resin is exhausted, regeneration can be
accomplished by treatment with excess of 1 M sodium hydroxide, followed by
washing with de-ionised water until the effluent is neutral. If ammonia solution
is used for regeneration the amount of washing required is reduced.

7.3 ION EXCHANGE CHROMATOGRAPHY

If a mixture of two or more different cations, B, C, etc., is passed through an
ion exchange column, and if the quantities of these ions are small compared
with the total capacity of the column for ions, then it may be possible to recover
the absorbed ions separately and consecutively by using a suitable regenerating
(or eluting) solution. If cation B is held more firmly by the exchange resin than
cation C, al1 the C present will flow out of the bottom of the column before
any of B is liberated, provided that the column is long enough and other
experimental factors are favourable for the particular separation. This separation
technique is sometimes called ion exchange chromatography. Its most spectacular
success has been the separation of complex mixtures of closely related substances
such as amino acids and lanthanides.
The process of removing absorbed ions is known as elution, the solution
employed for elution is termed the eluant, and the solution resulting from elution