ion exchange

n.
A reversible chemical reaction between an insoluble solid and a solution during which ions may be interchanged, used in water softening and in the separation of radioactive isotopes.

The reversible exchange of ions of the same charge between a solution and an insoluble solid in contact with it; or between two immiscible solvents, one of which contains a soluble material with immobilized ionic groups. Ions are atoms or molecules containing charge-bearing groups. Their interactions are dominated by the electrostatic forces between charge centers. These interactions are attractive when the ions are of opposite charge, or repulsive when the ions have the same charge. Ions with a net negative charge are called anions, and those with a net positive charge are cations.

A unique property of ions is their capacity to render gases and liquids conducting, and conductivity is a universal method of detecting ions. Ions in solution are in rapid motion and have no distinct partners. Ions in an electric field migrate to the electrode of opposite charge with a velocity roughly proportional to their charge-to-size ratio. This process is known as electrophoresis, and it is one method used to separate and identify ions. See also Electrophoresis.

Ions can also be separated on the basis of their equilibrium with a system containing immobilized ions of opposite charge. Ions can be immobilized by virtue of their location in a rigid matrix. Associated with these fixed ionic sites are mobile counterions of opposite charge. Solution ions with a higher affinity than the counterions for the fixed sites will displace them from the fixed sites and remain localized in the vicinity of the fixed sites. Simultaneously the solution is enriched in the counterions originally localized at the fixed sites. This exchange process for ions of the same charge type is called ion exchange. In a column containing the immobilized ions as part of the stationary phase and the solution of competing ions as the mobile phase, the sample ions can be separated by the repeated equilibrium steps involved as they are transported through the column until they exit it, and are detected. This is an example of ion-exchange chromatography, an important method of separating and identifying ions.

Ion-exchange materials

Ion-exchange polymers are based on styrene and divinylbenzene and, to a lesser extent, polymers prepared from divinylbenzene, or a similar cross-linking agent, and acrylic, methacrylic, or hydroxyalkyl methacrylic acids and esters. These are usually prepared in bead form.

Ion exchangers prepared for the isolation or separation of cations must have negatively charged functional groups incorporated into the polymer backbone. The most common groups are sulfonic and carboxylic acids. Sulfonic acid groups are introduced by reacting the polymer beads with fuming sulfuric acid or a similar reagent. Similarly, carboxylic acid groups can be introduced by a number of common chemical reactions or by hydrolysis of the ester group or oxidation of hydroxyalkyl groups in methyl methacrylate or hydroxyalkyl methacrylate polymers, respectively. Other common functional groups used in cation exchangers include phosphoric acid and phenol and, to a lesser extent, phosphinic, arsonic, and selenonic acids.

A common approach for the preparation of anion exchangers is to react the styrene-divinylbenzene polymer with chloromethylmethyl ether in the presence of a catalyst, which adds the side chain, CH₂Cl; then this chloromethylated product is treated with an amine to introduce the charged functional group. A tertiary amine produces a quaternary ammonium group, while primary and secondary amines give products that are charged only in contact with solutions of low pH. As well as simple alkyl and benzyl amines, hydroxyalkyl amines are used to introduce functional groups of the type [CH₂N(CH₃)₂C₂H₄OH]⁺. See also Quaternary ammonium salts.

Silica-based materials are used primarily in chromatography because of the favorable mechanical and physical properties of the silica (SiO₂) gel support matrix. Ion-exchange groups are introduced by reacting the surface silanol groups of the porous silica particles with silanizing reagents containing the desired functional group (R).

Hydrous oxides of elements of groups 14, 15, and 16 of the periodic table can be used as selective ion exchangers. The most important hydrous oxides used for the separation of organic and inorganic ions are alumina (Al₂O₃·nH₂O), silica (SiO₂·nH₂O), and zirconia (ZrO₂·nH₂O). Silica, by virtue of the presence of surface silanol groups, is used as a cation exchanger at pH > 2. Alumina is amphoteric and can be used as an anion exchanger at low pH and a cation exchanger at high pH. Alumina has the advantage over silica of being chemically stable over a wide pH range. The ion-exchange capacity of silica and alumina is controlled by the pH of the solution in contact with the oxides, since this controls the number of ionized surface functional groups. Alumina is used to isolate nitrogen-containing drugs and biochemically active substances from biological fluids, thus minimizing matrix interferences in their subsequent chromatographic analysis.

Applications

Ion exchange has numerous applications for industry and for laboratory research. By the quantity of materials used, water conditioning is the most important. Ion exchange is one of the primary analytical methods used to identify and quantify the concentration of ions in a wide range of environmental, biological, and industrial samples.

Natural water from rivers and wells is never pure; it is usually hard, that is, it contains calcium and magnesium salts that form curds with soap and leave hard crusts in pipes and boilers. Hard water is softened by passage through a cartridge or bed of cation exchanger in the sodium form (the mobile counterions are sodium in this case).

Many industrial and laboratory processes require a supply of pure water with a very low concentration of salts. This can be achieved by passing water through a bed of mixed strong cation exchanger in the hydrogen form and a strong anion exchanger in the hydroxide form. The cation exchanger removes all the cations from the water by replacing them by hydrogen ions. The anions are removed by the anion exchanger and replaced by hydroxide ions. The hydrogen and hydroxide ions combine to form water.

Toxic ions such as mercury (Hg²⁺), lead (Pb²⁺), chromate (CrO₄²⁻), and ferrocyanide [Fe(CN)₆⁴⁻] are removed by ion exchange from industrial wastewaters prior to their discharge into the environment. Ion exchangers are used to recover precious metals such as gold (Au⁺), platinum (Pt⁺), and silver (Ag⁺) in a useful form from mine workings and metalworking factories. Ion exchange is frequently used to decontaminate waste and concentrate radioactive elements from the nuclear industry.

Ion exchange is used on the laboratory scale for isolation and preconcentration of ions prior to instrumental analysis and to obtain preparative scale quantities of material for use in laboratory studies. Ion exchange is often employed in conjunction with activation analysis to isolate individual elements for quantification by radiochemical detection. Modern chromatographic techniques employ ion exchangers of small particle size and favorable mass-transfer characteristics, and operate at high pressures, providing better resolution of mixtures in a shorter time than with conventional gravity-flow-controlled separations.

Biotechnology requires reliable, efficient methods to purify commercial-scale quantities of proteins, peptides, and nucleic acids for use in the pharmaceutical, agricultural, and food industries. Ion exchange is widely used in the isolation and purification of these materials. Typical applications include the removal of ionic compounds used in the production process, the elimination of endotoxins and viruses, the removal of host-cell proteins and deoxyribonucleic acid (DNA), and the removal of potentially hazardous variants of the main product.

Membranes

Ion-exchange membranes are a class of membranes that bear ionic groups and therefore have the ability to selectively permit the transport of ions through themselves. In biological systems, cell membranes and many other biological membranes contain ionic groups, and the conduction of ions is essential to their function. Synthetic ion-exchange membranes are used in fuel cells, electrochemical processes for chlorine manufacture and desalination, membrane electrodes, and separation processes. Ion-exchange membranes typically consist of a thin-film phase, usually polymeric, to which have been attached ionizable groups. Numerous polymers have been used, including polystyrene, polyethylene, polysulfone, and fluorinated polymers. Ionic groups attached to the polymer include sulfonate (SO₃⁻), carboxylate (COO⁻), tetralkylammonium (N(CH₃)₄⁺), phosphonate (PO₃H⁻), and many others.

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