Anion and cation exchange capacities

Because so much of the behavior of suspensions is determined or modified by charge associated with the solid phases, ZPC may be inferred from a wide variety of experiments involving pH as a master variable. For example, coagulation and sedimentation rates are maximum at the ZPC, and anion and cation exchange capacities (measured with nonspecific, symmetrical electrolytes) are equal and minimum at the ZPC. More direct and less ambiguous are electrophoresis and streaming potential, in any of their modifications. One can estimate the IEP(s) by measuring adsorption of H+ and OH" if one is certain that no specific adsorption of other species occurs. 

Improvements in separations ability offered by the Dionex HPIC-CS5 column, which exhibits both anion and cation exchange capacity, have allowed the determination of ten metal ions in a single injection with a pyridine-2,6-dicarboxylic acid (PCDA) eluent (Fig. 2.3). The post-column reagent used in the detection scheme was 4-(2-pyridylazo) resorcinol. 

Fig. 3-154. Separation of heavy and transition metals on an ion exchanger with both anion and cation exchange capacity. — Separator column IonPac CS5 eluent 0.006 mol/L pyridine-2,6-dicar-boxylic acid, pH 4.8 with LiOH flow rate 1 mL/min detection see Fig. 3-152 injection volume 50 pL solute concentration 1 ppm Fe3+ and Cu2+, 3 ppm Ni2+, 4 ppm Zn2+, 2 ppm Co2+, and 3 ppm Fe2+.

The PZC was determined in [3001] from the difference between anion and cation exchange capacity. 

Reference [3041] is cited in [1] as a source of the PZC of iron oxide. Actually, [3041] reports previously published data on anion- and cation-exchange capacities of various oxides. The results from [3042] are cited in [1] as IEP at pH < 3 obtained by electrophoresis. Actually, the streaming potential was used in [3042], and the pH was not controlled or reported. All samples of quartz, also acid-treated, showed negative potentials in the presence of various electrolytes. 

These reactions generate anion and cation exchange capacity on an iron oxide in this example. X and symbolize the anion and cation of the acid and base, where MX is an indifferent electrolyte. The chemical identity of the acid and base is lost upon reaction with the surface, but the exact chemical nature of the surface-bound ions need not be known to describe thermodynamically the equilibrium state of the oxide-electrolyte system. 

Ion chromatography can also be applied for ultra-trace analysis of transition metals. In comparison to ICP-MS it offers the advantageous ability to specify the oxidation state of metals and, moreover, to carry out multi-element determinations in the lowest ng/L range after pre-concentration. Transition metal separation is performed with ion exchangers that have defined anion and cation exchange capacities detection is carried out photometrically after derivatization... 

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