Bio-Rad AG® MP-50 Cation Exchange Resins - Manual (Page 5)

ion with equivalent or lower affinity. Table 3 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. Finally, the order of
selectivity can be used to estimate the difficulty of
converting the resin from one form to another.
Conversion from a highly selected to a less highly
selected form requires an excess of the new ion.
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Table 2. Summary of the Properties of AG 50
and AG MP 50 Resins
Active Resistance Resistance
Group Thermal Solvent to Oxidizing to
(X8 Resin) Stability Stability Agents Reducing
R-SO
3
- Good to Very good Slowly oxidizes in Very good
150 °C hot 15% HN0
3
Section 3
Mechanism
In an ion exchange procedure, the counterions on the
resin are replaced by sample ions that have the same
charge. In applications involving a cation exchange
resin, such as AG 50 resin, neutral molecules and anions
do not interact with the resin. AG 50 resin is available
with H
+ , Na + , or NH
3
+ counterions. 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 when
introduced, and can be eluted by introducing an ion with
higher affinity for the resin or a high concentration of an
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LIT203B 6/17/98 12:00 PM Page 4