Starch iodine test for polysaccharides

Starch iodine test is a specific test for polysaccharides (starch positive). Iodine test indicates the presence of polysaccharides in any solution. When iodine solution reacts with starch, a bluish colour is obtained indicating the presence of starch.

REAGENTS:

i. 1% Iodine solution ( which contain potassium iodide and water).

ii. Original solution (O.S) containing a carbohydrate

PRINCIPLE:

Starch, glycogen and higher dextrins on reaction with dilute iodine solution form coloured compounds.

PROCEDURE:

• Take 2 ml of the starch solution (original solution) in a test tube.
• Add 1-2 drops of the diluted iodine solution.
• Observe the production of a blue colour.
• The blue colour produced disappears on heating, and it reappears on cooling the solution.

OBSERVATION AND RESULT:

Starch, glycogen and higher dextrins on reaction with dilute iodine solution form coloured compounds as shown in the table below:

Starch	Blue colour
Amylodextrin	Purple colour
Erythrodextrin	Red colour
Glycogen	Red colour
Achrodextrin (Lower dextrins), Disaccharides (Maltose) and Monosaccharides	No change in colour

Precautions:

• Wash the apparatus before and after the experiment.
• Carefully handle all the chemicals.

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