Define the Iodine Test - Carbohydrates?
This test is given by polysaccharides and is not given by monosaccharides or disaccharides.
Principle
Iodine forms a coloured adsorption complex with polysaccharides. When starch is mixed with iodine, an intensely colored starch/iodine complex is formed. The minute details of the reaction are still not clear.
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Test
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Reagents Required
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Methodology
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Observation
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Iodine Test
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- Solutions of glucose, fructose, galactose, lactose, maltose, sucroseand polysac- charide.
- Iodine solution in potassium iodide
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For mono and disaccharides: To 5 ml of glucose solution taken in a 10 ml test tube, add 3-4 drops of iodine solution. Observe the change in colour. Repeat the test with fructose, galactose, lactose, maltose, and sucrose.
For polysaccharides: To about 5 ml of polysacch-aride solution add drop by drop a dilute solution of iodine in potassium iodide. See the change in colour. (in Performing this test the solution must always be neutral or acid in reaction).
Divide the coloured solution into three parts:
(i) Heat one part and cool it.
(ii) To the second part add alcohol.
(iii) To the third part add NaOH solution.
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No change in the colour of the sugar solution containing monosaccharide on addition of iodine.
Disaccharides do not form adsorption complex with iodine solution.
The polyasaccharide molecules form adsorption complexes with iodine. The composition of the coloured complex is however, not well defined.
Polysaccharides are large colloidal molecules which form aggregated particles called micelles
These micelles combine with iodine to form coloured adsorption complexes.
The starch-iodine or dextrin- iodine adsorption complex dissociates on heating and is reformed on cooling. It is also broken up by alcohol and NaOH.
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