For several years it has been known that clays comprise metal ions in their composition and that these ions can change places with other metal ions in their region, this investigation was important to the understanding of soil chemistry. Zeolites (sodium aluminum silicates) have some features alike to those of clays. Using this phenomenon, natural and artificial zeolites became useful in removing Ca2+ and Mg2+ from water.
The Ca2+ and Mg2+ ions dissolved in the water are changed for Na2+ ions from the zeolite, thereby giving soft water. Ion exchangers have been created by mixing a polymer and a functional group. Such ion exchanges have features very similar to those zeolites.
Two types are available, cationic and anionic exchangers. Both use a resin as the insoluble inert support, but they use different functional groups to provide the type of exchanger necessary. The phenomenon of ion exchange was first reported by two British agricultural chemists in 1858 they proved that soil can remove potassium or ammonium salts from water with the release of an equivalent amount of Calcium salt.
After the work of Gans in 1913, natural and synthetic inorganic cation exchangers were used for softening hard water. Advanced ion exchange resins were first used in 1935 by Adams and Holms. These resins consist of three-dimensional networks of polymeric chains cross-linked with short chains including ionizable functional groups. The fundamental work on chromatographic separations by the use of ion-exchange resins is mainly derived from the investigations carried out in connection with the work on atomic energy in the USA.
The principle feature carrying this form of chromatography is the attraction between oppositely charged particles. Many biological materials, for example, amino acids and proteins, have ionizable groups, and the fact that they may carry a net positive or negative charge can be utilized in classifying compounds. The net charge presented by such compounds is dependent on their pKa and the pH of the solution following the Henderson-Hasselbalch equation.
A cation exchanger is a high molecular weight, a cross-linked polymer group as an essential part of the resin, and an equal amount of cations. Thus, a cation exchanger is nothing but a polymeric anion to which active cations are connected. In these cation exchangers, the hydrogen ions are mobile and interchangeable with other cations. The anions (-COO-, SO2- 3 and –O) remain connected to the resin network.
An anion exchanger is a polymer having an amine or quaternary ammonium groups as essential parts of the resin and an equal amount of anions such as Cl-, SO4 2-, Ohioans, etc. These anions are mobile and interchangeable.
Ion exchange Column used in Chromatographic Separations The principle of ion-exchange chromatography is based upon the simple fact that different cations (or anions) have different capacities to undergo exchange reactions on the surface of a given exchanger. The capacity of an ion to undergo an exchange reaction has been found to depend upon the charge and the size of the hydrated ion in the solution.
Under similar conditions, the capacity has been found to increase in the charge on the ion. If we compare other hydrated ions of the same size, it is observed that the ionic charge plays an important role to determine their ability to support exchange reaction.
Ion Exchange Methods
Two techniques are generally used to ring solutions in contact with ion exchange resins.
(a) Batch Method- This involves single-step equilibrium. The resin and the solution are mixed in a vessel until equilibrium is attained. The solution is then filtered off. The extent to which ions from the solution are exchanged for those on the resins depends upon the selectivity coefficient. In the single-step equilibrium of this type, only a small portion of the exchange capacity of the resin is utilized. The batch method is used for softening hard water and the creation of deionized or demineralized water.
(b) Column Method- Ion exchange chromatography. Typically, ion exchange chromatography is a technique in which the separation of the components of a mixture is brought about by taking advantage of the different selectivity coefficients for the resin. The differences in the selectivity coefficient lead to different migration rates on an ion-exchange column.
Applications of Ion Exchange
Because of the particular selectivity forces of material, separation with ion exchange is used. Organic ions which form salts with the oppositely charged ions in another phase can be separated too.
Almost all ion exchanger reactions require aqueous solvents. Some applications of ion exchangers are as follows:
• Separation of Similar ions from one another
Ion exchange chromatography is being used to separate similar ions from one another because the different ions undergo exchange reactions to different extents.
• Purification of Organic compounds
Extracted in water- many natural products extracted in water are contaminated with ions originally present in water. Those ions can be removed by using ion-exchange processes.
• Separation of Sugars
This method was developed by Khym and Zill (1951). These sugars are first of all converted into borate complexes.
• Removal of Interfering Radicals
In the estimation of Ca2+ or Ba2+ ions by the oxalate or sulfate method, phosphate ion is found to interfere. Therefore, its removal becomes necessary which is achieved by passing a solution of Ca2+ or Ba2+ ions having phosphate ions through a sulphonic acid cation exchanger.
• Softening of Hard water
We are aware of the fact that the hardness of water is due to the presence of Ca2+, Mg2+, and other divalent ions. These ions may be removed by passing hard water through cation exchangers charged with Na+.
Many metals are recovered and purified commercially by ion exchange. Examples include uranium, thorium, lanthanides, and actinides, gold, silver, and platinum. In some cases, the scale of operation is relatively small e.g., lanthanides, but the intrinsic value of these metals is very high. In addition, the separation and recovery of trace amounts of toxic metals from effluent and waste streams is an important application from environmental considerations.