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Separation Techniques

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This is a revision guide for the GCE A-level Chemistry - Separation Techniques topic
  TECHNIQUES OF SEPARATING ORGANIC MOLECULES The topic deals with different techniques of isolation/separation of a desired component from a mixture in its pure form. Three techniques are discussed here; Solvent extraction, Chromatography and Electrophoresis. 1.   SOLVENT EXTRACTION Is the transfer of a dissolved compound (a solute) from one solvent into another second solvent, in which it is more soluble. Most impurities will be left behind in the first solvent. The two solvents must be mutually immiscible. They are temporarily mixed together in a separating/separatory funnel (and shaken) so that the solute can pass from one to the other. e.g. a solution of isobutyric acid is more soluble in water (aqueous layer) than in ether (organic layer), and so when the two solvents are placed together, isobutyric acid transfers to the water layer. Some isobutyric acid will remain dissolved in the ether phase, so the extraction should be repeated several times with fresh solvent (water), this ensures that more product is obtained. This is called  successive extractions.   N/B: Once the compound has been isolated in its purified form in a solvent, it can then be obtained by evaporation of the solvent.   Also, if a solute such as iodine is soluble in two immiscible solvents (e.g. water and hexane), its solubility (usually measured in g/cm 3  or mol/dm 3 ) is very unlikely to the same in both solvents. One solvent will be better at dissolving it than the other.   When some iodine crystals are shaken with a mixture of hexane and water until no further change takes place, and the two layers allowed to separate we find that the ratio of the concentrations of  iodine in each layer is a constant, no matter how much iodine we start with. This constant is the equilibrium constant for the change:  K  c  is called the  partition coefficient,  (  K   pc ) of iodine between hexane and water. Partition coefficient:  Is the equilibrium constant representing the distribution of a solute between two solvents or the ratio of the concentration of a solute between two immiscible solvents when an equilibrium has  been established. Like all equilibrium constants, the value of  K   pc   changes with temperature and in general they do not have units.  Application of K     pc : Useful in working out how much solvent is needed in order to extract a minimum amount of solute from one solvent into another.   The amount extracted steadily increases as we split the amount of extracting solvent into smaller and smaller portions. However, it is impossible to extract all of a solute, no matter how many  portions of solvent we use, since it is never possible to move any equilibrium completely to one side or the other.  2.   CHROMATOGRAPHY Is the separation of components of a mixture by their different speeds of movement through/over the stationary phase.   Four techniques/types of chromatography are described here: Paper Chromatography (PC) Thin Layer Chromatography (TLC) Gas/Liquid Chromatography (GLC) and High Performance Liquid Chromatography (HPLC).  The basic principles and techniques The two types of chromatographic separation are; Partition ( the division/separation/distribution of a solute between two phases)  i.e. The solute molecules partition themselves between the stationary phase and the moving phase.  Adsorbtion ( the attraction/bonding of a molecule to the surface of a solid) . All chromatographic methods use the same principle of a mobile/moving phase  (a liquid or a gas) moving past a stationary phase . The different components of the mixture become  partitioned   or adsorbed   to different extends between the two phases. a)   Paper Chromatography (PC) Procedure;    The mixture to be separated and analysed is dissolved in a solvent, such as water or ethanol, and a small spot of the solution is placed about 1 cm from the edge of a rectangular sheet of chromatography paper. Other spots of “reference” compounds may also be applied at the same distance from the edge.    The solvent is allowed to evaporate to allow the solutes to become adsorbed/bonded into the cellulose fibres of the paper (often by means of hydrogen bonding to the -OH groups of the glucose molecules in the cellulose fibres, or more likely to the H 2 O molecules that are still associated with cellulos e; a sheet of „dry‟ paper contains about 10% by mass of water  ). This water acts as the stationary phase  in PC.    The edge of the sheet is then immersed in the chromatography solvent (is the mobile phase)  with the spots kept above the surface of the solvent. The solvent then moves up the paper  by capillary action.    Once the solvent has reached the top end of the sheet of paper, the paper is removed and the solvent is allowed to evaporate. All solutes are re-adsorbed onto the area of the paper where they had reached.    Presence of the separated components can be directly visible or made visible either by staining with a reagent that forms colour with the separated components like ninhydrin (used for amino acids and small peptides) or detected by fluorescing the components under a UV light.
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