SPECIAL FOR STUDENTS APPEARING IN ENTRY TEST FOR MEDICAL ENGINEERING OR OTHER PROFESSIONAL COLLEGES IMPORTANT POINTS ON 1. PURIFICATION BY SUBLIMATION 2. PURIFICATION BY SOLVENT EXTRACTION SUBLIMATION DEFINITION:- The process in which a solid is converted into the gaseous state by heating without passing through the liquid state is called sublimation. Solid --------------- Gas The energy required for this conversion is called heat of sublimation. The reverse process is called deposition in which vapours are condensed to crystals solid by cooling. The solid substance being sublimed is called sublimand. The pure substance obtained after sublimation is called sublimate. Only those substances can sublime which has high V.P at lower temperature (lower than M.P). These substances has weaker inter molecular forces of attraction. For substances which decompose on heating, then sublimation is carried out at reduced pressure. By sublimation volatile substances are separated from non volatile substances. Naphthalene, Camphor, Benzoic acid, Iodine, Ammonium Chloride and Dry Ice can purify by sublimation. It is of little importance because very little substance undergoes sublimation. SOLVENT EXTRACTION Definition:- The process of separating a substance from its aqueous solution by shaking it with a suitable and immiscible solvent is called solvent extraction. This is a separation technique and useful when the product is volatile or thermally unstable. Solvent extraction is an equilibrium process based upon the distribution law or partition law. DISTRIBUTION LAW; When a substance is dissolved in two immiscible liquid, the solute (substance) distribute itself between these immiscible liquid in such a way that the ratio of its concentration in each liquid is constant irrespective of the amount of solute added. Only separating funnel is required for this techniques. Types:- (1) Solid: Liquid extraction. (2) Liquid: Liquid extraction. (1) Solid – Liquid Extraction:- In this process a substance is removed from solid by dissolving them in solvent. The instrument used is called “Soxhlet Extraction Apparatus”. 2. Liquid – Liquid Extraction:- (1) Separating funnel is used: - It is usually applied to separate organic solute from their aqueous solution. In this method aqueous solution containing organic solute is shaking up with ether in a separating funnel. The organic solute will move from water layer to the ether layer. The inorganic impurities remain in the aqueous layer. The separator is shaken vigorously for some time until equilibrium is established. The separator is fixed in a stand until the liquid and the solvent phases is separated. The lower layer is running out by turning on the outlet tape and the extract is collected in a beaker. In this way many extractions are made. It is preferable to take many extractions with small fraction of the solvent as compared with single extraction using large volume of the solvents because the quantity of the extracted product depends on distribution constant. Distribution Law:- According to this law solute distribute itself between the two immiscible liquids in such a way that the ratio of its concentration in each liquid become constant irrespective of the amount of solute added. This ratio of constant concentration is called Distribution Coefficient. It is symbolized as (KD) e.g. for an aqueous/organic system. Kd = Concentration of solute in organic phase ( divided by) Concentration of solute in aqueous phase
Posted on: Tue, 16 Dec 2014 10:05:05 +0000
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