Ion Exchange
Ion Exchange
Ion exchange resins
Ion exchange membranes
Ions are electrically charged atoms or groups of atoms. Ion exchange refers to the replacement of one ion with another of similar charge. Ion exchange resins are solids containing strongly bonded charged atom groups. (Positively charged atoms are called cations; negatively charged, anions.) They occur as natural materials and can be synthetically made. An ion of opposite charge is loosely bound to a charged group. When placed in contact with an ionic solution (a solution of an ionic substance), the loosely bound ions are replaced by those in solution and are retained on the solid. Ion exchange membranes are sheet-like films which allow the passage of ions while restricting the crossing of larger, uncharged molecules.
Ion exchange resins
If an ionic solution is brought into contact with a solid having ions that are only weakly joined in its crystalline structure, it is possible for ions from the solution to interchange with those of the same charge in the solid. Electrical neutrality is maintained throughout this exchange; that is, the total number of positive charges equals the total number of negative charges in the solid and the solution at all times. What changes is the type of ion that then resides with the solid and in the solution. A solid that has loosely bound sodium ions, when placed in a solution of potassium chloride, will interchange some of its sodium ions for potassium ions. The result is that the solid and the solution each have sodium and potassium ions in some ratio determined by the inherent capacity of the solid to undergo the exchange process.
There are several naturally occurring materials that function as ion exchangers. Many synthetic ion exchange materials are also available. Many of these synthetic materials are tailor-made to serve a specific purpose and be selective in the type of ions with which they exchange. Zeolites are a naturally occurring class
| Table 1. Selected Charged Ion Exchange Groups. (Thomson Gale.) | |
|---|---|
| Selected charged ion exchange groups | |
| Cation exchange groups joined to an ion exchange framework (1) | |
| (1) R represents the divinylbenzene polymer framework M+ represents and exchangeable cation. X- represents an exchangeable anion. | |
| R–OSO3-M+ | sulfonic acid group |
| R–COO–M+ | carboxylic acid group |
| Anion exchange groups joined to an ion exchange framework | |
| R–CH2NCH3)3+X- | quaternary ammonium group |
| R–NH(CH3)2+X- | ternary ammonium group |
of minerals containing aluminum, silicon, oxygen and a loosely held cation from group 1 or group 2 of the periodic table (e.g., sodium or magnesium). When placed in a solution of an ionic compound, exchange occurs between the loosely held zeolite cation and the dissolved cation in water. Various clay and soil materials also possess ion exchange capabilities. Most often an ion exchange reaction uses a synthetic ion exchange material specifically designed to achieve the desired separation.
Synthetic ion exchangers are composed of a charged group attached to a rigid structural framework. One end of the charged group is permanently fixed to the frame while a positive or negative charged portion loosely held at the other end attracts other ions in solution. Common materials for these ion exchange resins are styrene and divinylbenzene. Molecules of these organic substances can join together forming a divinylbenzene polymer consisting of long rows of styrene cross-linked, that is attached, by divinylbenzene.
The extent to which divinylbenzene is cross-linked affects the ability of the resin to undergo ion exchange with an ion in solution. Resins that are only slightly cross-linked may have sufficient open space to allow solution ions to pass through and avoid contact with the fixed, exchangeable groups. Resins that are too highly cross-linked may not have openings big enough for solution ions to penetrate. This prevents them from contact with the fixed exchangeable groups.
Cation resins often are prepared in their hydrogen ion form. In this state exchange occurs when the resulting product in solution is the acid corresponding to the dissolved solid. A similar exchange between dissolved sodium chloride and a strong anion resin in
the hydroxide (basic) form yields dissolved sodium hydroxide, a strong base.
Complete exchange of solution ions (cation or anion)—that is, complete absorption on the resin— can occur if the sample solution is poured slowly through a packed column of resin material. This allows the sample to come into contact continually with fresh resin; the exchange occurs until none of the original exchangeable ions remains. These ions then can be collected by running another solution through the column, a solution that removes, or elutes, the absorbed ions from the resin.
Applications
Ion exchange and exchange resins have numerous applications. In scientific studies, exchange resins are used to isolate and collect various ionic species, cations on a cation resin, and anions on an anion resin. In industry resins are used to purify water by removing all ions from it. Upon passage of a water sample through both a cation resin (H+ form) and an anion resin (OH- form) the cations and anions in the water are retained. The H+ and OH- ions released from the resin then combine to form additional water. Deionized water also is a source of pure water containing no ionic chemical compounds.
Ion exchange also is used to remove ionic compounds from boiler water used in the steam generation of electric power. Ion exchange resins also are used in the separation and purification of various chemicals. Rare earth elements are separated from their ores and purified in this manner.
Ion exchange membranes
Various membrane materials, both natural and synthetic, have the ability to selectively allow or retard passage of charged and uncharged molecules through their surface. These semipermeable membranes are extremely important in ion transport within living systems and have many industrial applications.
The balance between sodium ion, Na+, and potassium ion, K+, within the cells of living organisms is essential for life. The transport of these ions across the cell membrane allows this proper balance to be maintained.
Semipermeable membranes are used in the purification of large organic molecules. They allow small ionic compounds to pass through, separating them from the larger molecules. This procedure, known as dialysis, is the principle upon which patients with non-functioning kidneys can remove harmful waste products artificially.
KEY TERMS
Anion resin —A solid material with tightly bonded positively charged ions and loosely bonded negative counter ions that will exchange the negative ions for dissimilar negative ions in solution.
Cation resin —A solid material with tightly bonded negatively charged ions and loosely bonded positive counter ions that will exchange the positive ions for dissimilar positive ions in solution.
Ion exchange membrane —A flat sheet-like semipermeable material that allows ions to pass unrestrictedly while serving as a barrier to larger, uncharged molecules.
Commercially, the forced separation of ions from seawater by passing them through a semipermeable membrane is an economical means of transforming seawater into potable water (water safe for drinking). This technique is known as reverse osmosis and is in use by countries bordering oceans or seas to obtain fresh drinking water.
Resources
BOOKS
Gross, M.L., R. Caprioli, and P.B. Armentrout. The Encyclopedia of Mass Spectrometry: Ion Chemistry and Theory. Oxford: Pergamon Press, 2001.
Wachinski, Anthony M. Ion Exchange Treatment for Drinking Water. Denver: American Waterworks Association, 2004.
Zagorodni, Andrei A. Ion Exchange Materials: Properties and Applications. St. Louis: Elsevier Science, 2006.
Gordon A. Parker
Ion Exchange
Ion exchange
Ions are electrically charged atoms or groups of atoms. Ion exchange is the phenomenon of replacing one ion with another of similar charge. Ion exchange resins are solids containing strongly bonded charged atom groups. (Positively charged atoms are called cations; negatively charged, anions.) They occur as natural materials and can be synthetically made. An ion of opposite charge is loosely bound to a charged group. When placed in contact with an ionic solution (a solution of an ionic substance), the loosely bound ions are replaced by those in solution and are retained on the solid. Ion exchange membranes are sheet-like films which allow the passage of ions while restricting the crossing of larger, uncharged molecules.
Ion exchange resins
If an ionic solution is brought into contact with a solid having ions that are only weakly joined in its crystalline structure, it is possible for ions from the solution to interchange with those of the same charge in the solid. Electrical neutrality is maintained throughout this exchange; that is, the total number of positive charges equals the total number of negative charges in the solid and the solution at all times. What changes is the type of ion that then resides with the solid and in the solution. A solid that has loosely bound sodium ions, when placed in a solution of potassium chloride, will interchange some of its sodium ions for potassium ions. The result is that the solid and the solution each have sodium and potassium ions in some ratio determined by the inherent capacity of the solid to undergo the exchange process. Equation 1 illustrates this interchange between cations initially attached to a solid interacting with cations initially in solution.
Water solution is indicated by (aq) for aqueous.
The exchange continues until the ratio of each cation in the solid and the solution remains constant. For this example:
Because the solid in equation 1 exchanges cations with the solution, it is termed a cation exchange solid. Other solids with exchangeable negatively charged ions are called anion exchangers.
There are several naturally occurring materials that function as ion exchangers. Many synthetic ion exchange materials are also available. Many of these synthetic materials are tailor-made to serve a specific purpose and be selective in the type of ions with which they exchange. Zeolites are a naturally occurring class of minerals containing aluminum , silicon, oxygen and a loosely held cation from group 1 or group 2 of the periodic table (e.g., sodium or magnesium ). When placed in a solution of an ionic compound, exchange occurs between the loosely held zeolite cation and the dissolved cation in water. Various clay and soil materials also possess ion exchange capabilities. Most often an ion exchange reaction uses a synthetic ion exchange material specifically designed to achieve the desired separation.
Synthetic ion exchangers are composed of a charged group attached to a rigid structural framework. One end
| Cation exchange groups joined to an ion exchange framework 1 | |
| R-OSO3- M + | sulfonic acid group |
| R-COO- M+ | carboxylic acid group |
| Anion exchange groups joined to an ion exchange framework | |
| R-CH2NCH3)3+ X- | quaternary ammonium group |
| R-NH(CH3)2+ X- | ternary ammonium group |
| 1R represents the divinylbenzene polymer framework | |
| M+ represents an exchangeable cation. | |
| X- represents an exchangeable anion. | |
of the charged group is permanently fixed to the frame while a positive or negative charged portion loosely held at the other end attracts other ions in solution. Common materials for these ion exchange resins are styrene and divinylbenzene. Molecules of these organic substances can join together forming a divinylbenzene polymer consisting of long rows of styrene crosslinked, that is attached, by divinylbenzene.
The extent to which divinylbenzene is crosslinked affects the ability of the resin to undergo ion exchange with an ion in solution. Resins that are only slightly crosslinked may have sufficient open space to allow solution ions to pass through and avoid contact with the fixed, exchangeable groups. Resins that are too highly crosslinked may not have openings big enough for solution ions to penetrate. This prevents them from contact with the fixed exchangeable groups. Table 1 lists various chemical groups that can be joined to the resin framework for attracting ions in solution.
Cation resins often are prepared in their hydrogen ion form. In this state exchange occurs when the resulting product in solution is the acid corresponding to the dissolved solid. An example of this type of exchange is shown in equation 3 where a strong cation resin in the hydrogen form interacts with a sodium chloride salt solution to yield the sodium form of the resin and hydrochloric acid.
A similar exchange between dissolved sodium chloride and a strong anion resin in the hydroxide (basic) form yields dissolved sodium hydroxide , a strong base.
Complete exchange of solution ions (cation or anion)—that is, complete absorption on the resin—can occur if the sample solution is poured slowly through a packed column of resin material. This allows the sample to come into contact continually with fresh resin; the exchange occurs until none of the original exchangeable ions remains. These ions then can be collected by running another solution through the column, a solution that removes, or elutes, the absorbed ions from the resin.
Applications
Ion exchange and exchange resins have numerous applications. In scientific studies, exchange resins are used to isolate and collect various ionic species , cations on a cation resin, and anions on an anion resin. In industry resins are used to purify water by removing all ions from it. Upon passage of a water sample through both a cation resin (H+ form) and an anion resin (OH- form) the cations and anions in the water are retained. The H+ and OH- ions released from the resin then combine to form additional water. Deionized water also is a source of pure water containing no ionic chemical compounds.
Ion exchange also is used to remove ionic compounds from boiler water used in the steam generation of electric power. Ion exchange resins also are used in the separation and purification of various chemicals. Rare earth elements are separated from their ores and purified in this manner.
Ion exchange membranes
Various membrane materials, both natural and synthetic, have the ability to selectively allow or retard passage of charged and uncharged molecules through their surface. These semipermeable membranes are extremely important in ion transport within living systems and have many industrial applications.
The balance between sodium ion, Na+, and potassium ion, K+, within the cells of living organisms is essential for life. The transport of these ions across the cell membrane allows this proper balance to be maintained.
Semipermeable membranes are used in the purification of large organic molecules. They allow small ionic compounds to pass through, separating them from the larger molecules. This procedure, known as dialysis , is the principle upon which patients with nonfunctioning kidneys can remove harmful waste products artificially.
Commercially, the forced separation of ions from seawater by passing them through a semipermeable membrane is an economical means of transforming sea-water into potable water (water safe for drinking). This technique is known as reverse osmosis and is in use by countries bordering oceans or seas to obtain fresh drinking water.
Resources
books
Gross, M.L., R. Caprioli, and P.B. Armentrout. The Encyclopedia of Mass Spectrometry: Ion Chemistry and Theory. Oxford: Pergamon Press, 2001.
Joesten, Melvin D., David O. Johnston, John T. Netterville, and James L. Wood. World of Chemistry. Philadelphia: Saunders College Publishing, 1991.
Simon, George P. Ion Exchange Training Manual. New York: Van Nostrand Reinhold, 1991.
Gordon A. Parker
KEY TERMS
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .- Anion resin
—A solid material with tightly bonded positively charged ions and loosely bonded negative counter ions that will exchange the negative ions for dissimilar negative ions in solution.
- Cation resin
—A solid material with tightly bonded negatively charged ions and loosely bonded positive counter ions that will exchange the positive ions for dissimilar positive ions in solution.
- Ion exchange membrane
—A flat sheet-like semipermeable material that allows ions to pass unrestrictedly while serving as a barrier to larger, uncharged molecules.
ion exchange
Ion Exchange
Ion exchange
The process of replacing one ion that is attached to a charged surface with another. A very important type of ion exchange is the exchange of cations bound to soil particles. Soil clay minerals and organic matter both have negative surface charges that bind cations. In a fertile soil the predominant exchangeable cations are Ca2+, Mg2+ and K+. In acid soils Al3+ and H+ are also important exchangeable ions. When materials containing cations are added to soil, cations leaching through the soil are retarded by cation exchange.