Radio analytical separation techniques, Chemistry tutorial

Introduction:

Identical analytical separation method can be employed for radioactive nucleotides as they have properties identical to their stable, inactive, counterparts. Here, we will study the radio-analytical separation methods. Such separation methods comprise precipitation, ion exchange, liquid extraction, solid phase extraction, distillation and electrodeposition.

Precipitation:

Solids can be made up in a solution or inside the other solid throughout a chemical reaction. This method is termed as precipitation and the solid made up in a liquid is termed as precipitate. The liquid remaining above the solid is termed as the supernate or supernatant. Precipitation takes place in chemical reaction whenever an insoluble substance is introduced to a solution.

In soluble substances, precipitation is improved as the solution becomes supersaturated. The relative supersaturation is proportional to (Q-S)/S. Here, 'Q' is the total concentration of the solute at any instant, 'S' is the equilibrium solubility. It must be noted as well that that the particle size of a precipitate differs inversely as the relative supersaturation. Precipitates are made by nucleation and by particle growth. Nucleation is a method in which a minimum number of atoms, ions or molecules join altogether to provide a stable solid. Nucleation rises with increase with in relative supersaturation. Whenever nucleation predominates, a large number of fine particles results. If particles growth predominates, a smaller number of bigger particles are obtained. 

As a practical matter, precipitation is generally carried out in hot, dilute aqueous solutions to let the slow formation of large crystals. The pH of the solution is selected to minimize the colloid formation. After precipitation, the precipitate is washed carefully to eliminate impurities, dissolved and re-precipitated to cause further purification. The precipitate is collected via filtration.

340_Filtration apparatus used in radiochemistry.jpg

Fig:  Filtration apparatus used in radiochemistry

The filter paper is supported via a glass frit clamped between the two glass tubes. The precipitate is washed lastly by acetone or alcohol to dry it. The precipitate is selected to encompass a known Stoichiometry to allow computation of the result of the separation and must not absorb water or CO2 in such a way that an accurate weight can be obtained. (The filter paper employed in the filtration should be treated by all the reagents beforehand, dried and weighed in such a way that any material loss in filtration is minimized).

Distillation:

Distillation is a method of separating mixtures based on the differences in volatilities of components in a boiling liquid mixture. Distillation is a physical separation method and not a chemical reaction. Distillation lets the separation of components in a mixture whose partition coefficients between solution and vapor phases differ considerably. The separation method is simple whenever one species consists of a partition coefficient which is large compared by the other components of the mixture. A fractional distillation method is needed for those systems in which the difference in partition coefficient is small.

Solvent Extraction:

A solute can be separated from the solution via shaking a solution with solvent in which the solute is more soluble. The extent to which solutes distribute themselves between the two immiscible liquids differs greatly and such difference has been employed to accomplish separations of chemical species. The distribution ratio or else known as the partition coefficient which is equivalent to the concentration of a solution in the organic phase divided via its concentration in the aqueous phase. The distribution ratio mainly depends on temperature, the concentration of chemical species in the system.

Separation via liquid-liquid extraction (that is, solvent extraction) has played a significant role in the radiochemical separations.

Multistage countercurrent continuous methods are utilized for metals like the lanthanides, mostly as the separation factors between the lanthanides are so infinitesimal and most of the extraction stages are needed. This is possible to extract uranium, plutonium or thorium from acid solutions. Organophosphate tri-n-butyl-phosphate is an extremely common solvent that could be employed this extraction.

Ether extraction of uranium was employed in early weapons development, and the use of tri-butyl phosphate (TBP) as an extractant for U and Pu was recognized in the year 1946, resultant in the commercial PUREX method for reprocessing spent reactor fuel. In recent years, there has been a good deal of growth.

Ion Exchange:

Ion exchange is one of the radiochemical separation methods, which consists of high selectivity and better capability to perform separations rapidly. Ion exchange is as well a method of purification and decontamination of ion having solutions utilizing solid polymers or mineralic ion exchangers. In ion exchange, a solution having the ions to be separated is brought into contact by a synthetic organic resin having particular functional groups which selectively bind the ions in question. Organic ion exchangers encompass reactive groups like -OH, -COOH, or -SO3H, and are insoluble in water and also organic solvents. These exchangers must have an open, permeable molecular structure in such a way that ions and solvent molecules can move freely in and out of the molecular structure.

The ions in question can be eliminated from the resin through elution with appropriate solution which differs from the initial solution. Characteristically the solution having the ions is run via a column packed with resin beads. The resins are generally cross--linked polystyrenes with attached functional groups. Most of the cation exchangers (like Dowex 50) contain free sulphonic acid groups, SO3H, where the cation moves the hydrogen ion. The anion exchangers (like Dowex 1) have quaternary amine groups, like CH2N(CH3)3Cl

A group of ions can be absorbed on the column material and then selectively eluted. Eluants are complexing agents that form complexes of varying solubility by the absorbed ions. Competition generally exists between the complexing agent and the resin for each ion and each ion will be exchanged between the resin and the complexing agent in succession as it moves down the column. Spatial separation between the 'bands' of different ions takes place due the different rates at different ions migrate down the column The ions can be collected separately in successive eluant fractions (see figure below).

1839_Elution of tripositive lanthanide and actinide ions on Dowex-50.jpg

Fig: Elution of tripositive lanthanide and actinide ions on Dowex-50

The most broadly cited application of ion exchange methods is the separation of the rare earths or actinides from one other. This is done by cation exchange by employing a complexing agent of hydroxyisobutyric acid ('but'). The order of elution of ions from a cation exchange column is usually in order of the radii of the hydrated ions by the largest hydrated ions leaving first; therefore lawrencium elutes first and americium last among the tri-positive actinide ions (as shown in figure). In case of data of figure, the separation between adjacent cations and the order of elution is derived from the comparative stability of the aqueous actinide or lanthanide complexes by hydroxyisobutyrate. As illustrated in the figure, there is a strikingly analogous behavior in the elution of the actinides and lanthanides which allowed chemists to prove the identity of new elements in the discovery of elements 97 - 102 (Bk - No). For cation exchange, the strength of absorption goes as M4+ > M3+ > MO22+ > M2+ > MO2+. A mixture of Mn (II), Co (II), Cu (II), Fe (III), and Zn (II) can be separated by being put on a Dowex 1 column by using 12 M HCl, followed by elutions by 6M HCl (Mn), some of the inorganic ion exchangers, like the zeolites, have been very helpful and are employed in situations where heat and radiation might prevent the utilization of organic resins though the establishment of equilibria might be slow.  

More selective resins have been developed; among such are the chelating resins (like Chelex-100) which have functional groups which chelate metal ions. Most of the common functional groups comprise iminodiacetic acids, 8-hydroxyquinoline or macrocyclic units like the crown ethers, calixarenes or cryptands. The bifunctional chelating ion exchange material, Diphonix resin-a substituted diphosphonic acid resin, exhibits promise in treating radioactive waste. Significant newer resins comprise those by immobilized phosphorus ligands.

Electrochemical deposition:

This is a method via which a film of solid metal is deposited from a solution of ions to an electrically conducting surface. The deposited film generally consists of dimensions in the nano-scale and therefore the resultant product has gone via a method of nano-manufacturing. Electrolysis or electrochemical deposition can be employed to plate out active material of interest as the case might be or plate out other substances thus leaving active material in solution.

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