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IODINE TEST

IODINE TEST. Iodine test is a qualitative test for detection of polysaccharides. By this test we can also differentiate between starch,dextrin and glycogen. PRINCIPLE:.

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IODINE TEST

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  1. IODINE TEST

  2. Iodine test is a qualitative test for detection of polysaccharides. By this test we can also differentiate between starch,dextrin and glycogen.

  3. PRINCIPLE: • This test depends upon the property of adsorption possessed by the large polysaccharide molecules. Helically coiled polysaccharides adsorbs smaller molecules of iodine on their surfaces to form a complex of ill defined chemical nature. Atleast eight monosachharide units should be joined in linear chain to give the coloured complex. The starch,iodine and glycogen iodine complexes are blue,reddish purple and reddish brown respectively.

  4. The alkalis disrupt the secondary valencies of polysaccharides,thus iodine is released from their surface,therefore the colour disappears. The strong acid restores the secondary valencies and helps in adsorption of iodine thus solution again gains its respective colour. • The property of adsorption decreases on heating. On heating the iodine and polysaccharide complex dissociates and

  5. therefore colour disappears. On cooling the complex is reformed and the colour reappears except in case of dextrin. It has been suggested that heating produces a change on the surface of dextrin molecules as a result of which they loose the property of adsorption,therefore, colour does not reappear on cooling.

  6. REAGENTS: • 2% Lugol’s iodine solution: which consist of 2.5g iodine and 7.5g of potassium iodide dissolved in 500ml distilled water. Iodine is added slowly untill a deep yellow colour is produced. • 10% NaOH • Conc. HCl • Conc.H2SO4

  7. PROCEDURE: • Take 3ml of carbohydrate solution in a test tube. Acidify it with one drop of conc. HCl. • Now add one small drop of iodine solution in this test tube and mix it. Excess of iodine may give false result so iodine should be added carefully.

  8. INTERPRETATION: • Starch gives a blue colour on addition of iodine. • Dextrin gives a reddish purple colour on addition of iodine. • Glycogen gives a reddish brown colour with iodine. • No change in colour indicates the absence of polysaccharide in the given solution.

  9. Effect Of pH: • Perform iodine test and add 10% NaOH drop by drop till the colour disappears. • Now add conc. Sulphuric acid drop by drop till colour reappears. • INTERPRETATION: • Iodine test is positive only in acidic medium.

  10. Effect Of Temperature: • Perform iodine test and heat the test tube till the colour disappears. • Now cool the test tube till colour reappears. • INTERPRETATION: • Iodine test is negative at higher temperaturesand positive at lower temperatures. It is better to perform it at or near room temperature.

  11. PRECAUTION: • This test should not be carried out in an alkaline medium. • Iodine should not be added in excess.

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