Molecules or ions in solution can exchange with molecules or ions bound to polymer resin, at constant pH and salt concentration, as solution flows past resin {ion-exchange chromatography}. After solution passes through resin, solvent at different pH and salt concentration, or different solvent, passes through resin to remove bound molecules or ions.
ion types
Polystyrene or CM-cellulose sulfonated groups bind hydrogen ions or sodium ions {cation exchange}. DEAE-cellulose amine groups bind anions {anion exchange}. Anion-exchange resins, such as DEAE Sephadex, can have surface depressions for size separation.
clay
Besides polymer resins, ion-exchange chromatography can use clays, as in water softening, or sodium aluminum silicate zeolites.
chemical activity
Debye-Hückel theory relates potential-energy and chemical-potential lowering to ion solubility. Low chemical potential means solution is more random and more soluble. Ions in solution have more potential energy than uncharged molecules. Ion chemical potential depends on molecule concentration, size, and charge. Other ions go faster or slower as they pass charge. Opposite-charge counterions surround ions, making ions closer together than in random arrangements and lowering potential energy and chemical potential. Counterions shield ions and reduce effective charge, lowering potential energy and chemical potential.
chemical activity: solvent polarization
Solvent has dielectric constant and polarization. Polarization decreases attraction between ions, lowering potential energy and chemical potential. Water lowers chemical potential most, because it has highest dielectric constant.
chemical activity: factors
Lower concentration, higher temperature, higher solvent dielectric constant, lower ion charge, and larger ion size cause lower chemical potential, because potential energy is lower.
chemical activity: field
External electric field increases chemical potential. Negative voltage decreases positive-ion chemical potential, because ions have less-directed motion. Low chemical potential makes low current. Electric field can come from two different ion concentrations or two different phases. At equilibrium, potentials are equal in all phases.
Physical Sciences>Chemistry>Analytical Chemistry>Chromatography>Kinds
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Date Modified: 2022.0224