Polysaccharide iodine test

Polysaccharide iodine test is performed to differentiate polysaccharides from mono and disaccharides.

Principle

With various polysaccharides, iodine forms coloured adsorption complexes. Because of iodine adsorption on the polysaccharide chains, these complexes are formed. The colour intensity depends on the length of the available unbranched or linear chain for the complex formation.

Therefore, amylose gives a deep blue color and amylopectin, the unbranched helical component of starch, the branched component gives red color because the chains do not effectively coil. Glycogen, which is also highly branched, adds iodine to the red colour. This experiment is performed in acidic or neutral solutions.

Reagents

• Iodine solution: prepare a 2% (w/v) KI solution in water to which a few iodine crystals are added until the solution is deep yellow.
• Starch solution: dissolve 1g of starch in about 10-20mL of boiling water and add saturated sodium chloride solution to a volume of 100mL.

Procedure

• In a test tube, take 3 mL of the test solution and add 1-2 drops of dil.HCl. 
• Mix and then add 1-2 drops of the solution of iodine. 
• Mix and observe the change in colour. 
• Heat the tube and again observe the colour.
• When heated, the blue colour disappears and reappears when cooled.