Bio-Rad Bio-Rex® 5 Anion Exchange Resins User Manual
Page 4
exchanged with the anion in the sample. Neutral
compounds and cations will not interact with the resin,
and should pass through in the void volume. A resin can
be converted from one ionic form to another. Usually the
resin is used in an ionic form with a lower selectivity for
the functional group than the sample ions to be
exchanged. The sample ions are then exchanged onto the
resin when introduced, and can be eluted by introducing
an ion with higher affinity for the resin or a high
concentration of an ion with equivalent or lower affinity.
Table 1 shows the relative selectivity of various
counterions. In general, the lower the selectivity of the
counterion, the more readily it exchanges for another ion
of like charge. The order of selectivity can also be used
to estimate the effectiveness for different ions as eluants,
with the most highly selective being the most efficient.
To convert from a highly selected to a less highly
selected form requires an excess of the new ion. Table 2
outlines common techniques for converting ion
exchange resins from one ionic form to another. Resin
conversion is most efficiently carried out in the column
mode.
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the resin, the nitrogen in the functional group becomes
protonated, allowing the adsorption of the mineral acid.
When either of these two resins is washed with base, the
resin goes back into the free base form. Figure 1 shows
the adsorption of hydrochloric acid onto the resin.
R – CH
2
– N
+ H
+
CI
-
R – N – HCI
Resin in free-base form
Mineral Acid
Adsorption of acid
Fig. 1. Adsorption of hydrochloric acid.
Weakly basic anion exchangers, such as AG 3-X4
and AG 4-X4 resins, will act as anion exchangers when
equilibrated and used at or below pH 7. In an ion
exchange procedure, the counterions on the resin are
replaced by sample ions that have the same charge. The
functional group on these resins is a tertiary amine,
which will be positively charged at or below pH 7. The
corresponding anion is the counterion which will be
4
CH
3
CH
3
CH
3
CH
3
←
→
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