Ion exchange may be defined as " a reversible exchange of like ions between solid phase and liquid phase in which there is no permanent change in structure ". This technique is sometimes called as ion exchange chromatography.
Principle
Ion-exchange technique is carried out using special material called as ion-exchange resin.
These resins are insoluble organic polymers into which charged groups are introduced. The base polymer is generally styrene-divinyl-benzene copolymer. The resin contains some ionic sites where exchange of ions can take place. These sites may be cationic or anionic. When ions in the solution come in contact with these ions, they displace these ions from their positions and the solution coming out of the column contain ions of the resin.
After complete separation of ions on resin, one of the ion is eluted out with the help of suitable solvent. Other ions remain intact on the resin. Changing the solvents one by one , each ion in the mixture can be eluted out selectively.
Types of ion-exchange resins
Ion-exchange resins are the insoluble organic polymers with charged groups attached at suitable positions. The charged group determines the type of resin. Ion-exchange resins are two types:
Cation exchanger :
Cation exchanger is a high molecular weight, cross linked polymer having sulphonic, carboxylic, phenolic etc groups as main groups and equivalent number of cations are attached to them. These cations are active ions and can be exchanged with the cations in solution. Generally, they are used in H+ or Na+ form.
For cation exchanger in H+ form, the ion exchange equilibrium can be shown as,
HnR + nNa+ ⇌ NanR + nH+
(Resin) (Solution) (Resin) (Solution)
For cation exchange in Na+ form the ion exchange equilibrium can be shown as
2NanR + nCa++ ⇌ CanR₂ + 2nNa+
(Resin) (Solution) ( Resin) (Solution)
Cation exchangers may be further classified into strongly acidic and weakly acidic exchanger. In strongly acidic cation exchanger like cross linked polystyrene sulphonic acid (-SO₃H), the ion exchange capacity is independent of pH. However, in case of weakly acidic cation exchangers like those containing -COOH group, basic conditions have to be adjusted so as to enhance ion exchange capacity.
Anion exchanger:
Anion exchangers are polymers having amine or quaternary ammonium groups as integral parts of the resin and equivalent number of anions like chloride, sulphate,Anion exchange equilibrium can be shown as
RCln + nOH- ⇌ R(OH)n + nCl-
(Resin) (Solution) (Resin) (Solution)
This can also be classified into strongly basic and weakly basic exchangers. Strongly basic resins containing quaternary ammonium groups act independent of pH while activity of weakly basic resins is high in acidic conditions.
Ion exchange capacity
The total ion exchange capacity of a resin is dependent upon the total number of ion-active groups per unit weight of material. Greater is the number of active groups greater is the exchange capacity.
Ion-exchange capacity is defined as the number of millimoles of exchangeable groups per gram exchanger. For acidic resin, it is determined in laboratory by measuring the number of milligram moles of sodium ions absorbed by 1 g of dry resin in hydrogen form. For basic resins, it is determined by measuring the number of milligram moles of chloride ions absorbed by 1 g of dry resin in hydroxide form.
Determination of ion exchange capacity
In actual practice, about 0.5 g of dried resin (in hydrogen form) is accurately weighed and transferred to a column partly filled with distilled water. Some more distilled water is added so that the resin is completely covered. Care is taken to remove any air bubble on the resin.
With the help of separatory funnel, about 200 ml of 0.25 M sodium sulphate solution is dripped into the column at a flow rate of about 2 ml per minute.
During this process, hydrogen ions of resin get exchanged with sodium ions of solution and get eluted out.
HnR + nNa+ ⇌ NanR + nH+
(Resin) (Solution) (Resin) (Solution)
The eluent is titrated with 0.1 M sodium hydroxide solution using phenolphthalein indicator. From the concentration of hydrogen ions and weight of resin taken, one can calculate ion exchange capacity as follows.
Ion-exchange capacity=NV
W
N is the normality of NaOH solution, V is volume of NaOH solution and W is weight of resin.
For anion exchange resin in chloride form, 0.25 M sodium nitrate is used to exchange chloride ions with nitrate ions as
RCln + nNO₃− ⇌ R(NO₃)n + nCl-
It is titrated with 0.1 M silver nitrate solution using potassium chromate indicator.
Factors affecting ion-exchange equilibrium
The extent to which one ion is absorbed in preference to other is of fundamental importance. It will determine the readiness with which two or more substances, which form ions of like charge, can be separated by the resin . The factors determining the distribution of inorganic ions between the ion exchange resin and solution are-
1. Nature of exchanging ions
A) Charge on ion:
At low concentration of solution, the extent of exchange increases with in charge on ion. That is, the extent of exchange follows the order
Na+ < Ca2+ < Al3+ < Th4+
B) Size of hydrated ion:
When charge on ions is same, the extent of exchange decreases with increase in size hydrated ion. For singly charged cations, the extent of exchange follows the order
Li+ < H+ < Na+ < NH₄+ < K+ < Rb+ < Cs+
It should be noted that the size of hydrated ion of lithium is largest and so exchange ability is minimum.
C) When charge on ion in the solution is higher than that in the resin, dilution of the solution increases the extent of exchange. When charge on ion in the solution is lower than that in the resin, concentration of the solution increases the extent of exchange.
2. Nature of resin
Absorption of ions depends upon the nature of functional groups in the resin. It also depends upon the degree of cross linking. As the degree
of cross linking in the resin increases, the resin
becomes more selective towards the ion whose
effective size is smaller (Effective size includes the increase in size due to hydration). A good resin should posses following properties-.
It must be sufficiently cross-linked and should have negligible solubility.
It should be sufficiently hydrophilic to permit diffusion of ions at finite rate.
It should contain sufficient number of exchangeable groups.
Swollen resin should have higher density than water.
Some of the commercially available resins are listed in table
Applications
Being a separation technique, ion-exchange chromatography is used in separation of multicomponent mixtures. Some applications involving separation of binary mixtures are discussed in details.
Separation of chloride and bromide:
An anion exchange column is converted into nitrate form by passing concentrated NaNO₃ solution. Concentration solution oy mixture of chloride and bromide is added from the top of the column. The halide ions exchange with nitrate ions in the resin forming a band at the top of the column. These ions are eluted by adding NaNO₃ solution from top. Chloride ions are eluted first and then bromide ions come out. The fractions are titrated with standard silver nitrate solution using adsorption indicators. Thus, it is possible to calculate concentration of Cl- and Br- ions in the mixture.
Separation of zinc and magnesium:
For separation of zinc and magnesium, they are converted into their chloro Complex [ZnCl₄]²−. The mixture of these Complexes is poured from top of an anion exchanger column. Magnesium is eluted using 2M HCl. Zinc remains in the column when HCl is added. After complete collection of magnesium, zinc is eluted using 0.25 M HNO₃ . Further analysis of the separate solutions is possible using complexometric titration using EDTA.
Separation of cadmium and zinc:
Cadmium and zinc from strong chloro Complexes with HCl. They are negatively charged and can be separated by using anion exchanger. The mixture is added to anion exchange column zinc is eluted first with 2M NaOH solution containing NaCl. It comes out of the column in the form of sodium zincate (Na₂ZnO₂). After complete elution of zinc, cadmium is eluted with 1M HNO₃ solution in the form of cadmium nitrate. The zinc and cadmium in their respective effluents may be determined by titration with standard EDTA.
Separation of Lanthanides:
Lanthanides series elements occur in nature in various ores. These ores contain almost all lanthanides together. Separation of these lanthanides is difficult task, as they resemble in most of their properties. They have almost similar size and all of them exist in +3 oxidation state. So, they have equal charge density.
Ion exchange technique is the most rapaid and effective general method for the separation and purification of lanthanides. The mixture of lanthanides is poured into a cation exchanger column of Dowex -50 in hydrogen form. They get exchanged with hydrogen ions as-
Ln³+ + 3HR ⇌ LnR₃ + 3H+
They are eluted using ammonium citrate-citric acid buffer. The smaller lanthanide ions like La³+,Ce³+ etc form strong complex with citrate ion and eluted out first while larger lanthanide ions like Lu³+ are eluted latter.
LnR₃ + 3H+ + (Citrate)³- ⇌3HR + Ln(Citrate)
De-ionization of water:
De-ionization of water is carried out by passing natural water through two columns. First column is of acidic cation exchanger in hydrogen form. This column replaces all cation with H+ ions. Second column is of basic anion exchanger in hydroxide form. This column replaces all anions with OH-. These H+ and OH- ions combine to give pure water.
Separation of Na+ and K+:
Separation of sodium and potassium ions can be achieved by passing their solution through cation exchanger in acidic form. Elution is carried out using 0.7 M hydrochloric acid solution. Sodium, being less firmly held , move down the column more rapidly while potassium elutes out afterwards. From these solutions, solvent is evaporated and the chloride salts are redissolved in water. Mohr's titration with AgNO₃ is used for analysis.
Separation of Ni²+ and Co²+:
This separation can be achieved using strong base anion exchange resin. Solution of the ions is prepared in 9M HCl solution in which Co²+ form a chloro Complex [CoCl₄]²− but nickel does not. This Complex is retained by the resin while Ni²+ passes unaffected. After complete elution of Ni²+, Co²+ is eluted using 3M HCl. The separated solution can be analyzed colorimetrically. Nickel is estimated using DMG while cobalt is estimated using 1-nitroso-2-napthol.
0 comments:
Post a Comment