Bio-Rad Chelex® 100 Resin - Manual (Page 3)

scale clean-up, for example metals from waste waters,
where analytical purity is not a major concern.
Chelex 100 resin and Chelex 20 resin are styrene
divinylbenzene copolymers containing paired imin-
odiacetate ions which act as chelating groups in bind-
ing polyvalent metal ions. Chelex chelating resin is
classed with the weakly acidic cation exchange resins
by virtue of its carboxylic acid groups, but it differs
from ordinary exchangers because of its high selectiv-
ity for metal ions and its much higher bond strength.
Chelex chelating resin is efficiently regenerated
in dilute acid and operates in basic, neutral, and
weakly acidic solutions of pH 4 or higher. At very
low pH, the resin acts as an anion exchanger. Figure 1
shows the zwitterionic forms of the Chelex resin as a
function of pH.
Fig. 1. Change in structure of Chelex resin with
increasing pH.
Selectivity for Heavy Metal Ions
The selectivity of Chelex resin for metal cations
corresponds to that of iminodiacetic acid. A list of
selectivity factors for several divalent cations is given
in Table 1. The selectivity factor is a quantitative mea-
sure of the affinity that Chelex resin displays for a par-
ticular cation compared to its affinity for a reference
cation, in this case Zn
+2 .
CH2COOH CH2COOH CH2COO- CH2COO-
Ø–CH2–NH + Ø–CH2NH + Ø–CH2–NH + Ø–CH2–N
CH2COOH CH2COO- CH2COO- CH2COO-
pH 2.21 pH 3.99 pH 7.41 pH 12.30
2 3
LIT200B 6/17/98 11:49 AM Page 2