Q. Which of the following carbohydrates gives a dark blue colour with iodine?
Solution:
↵ The iodine test is used for the detection of carbohydrates in the sample solution.
Around two drops of iodine solution is added to 2ml of the carbohydrate-containing test solution and observed for the color change.
The blue-black color is due to the formation of a starch-iodine complex. Starch is a polymer of α-amylose and amylopectin that forms a complex with iodine to produce a blue-black color. On the addition of iodine to amylose, the helical shape of the unbranched polysaccharide traps iodine to produce a deep blue-black complex.
Cellulose, amylopectin and glycogen react with iodine to produce red-brown color. Glycogen produces a reddish-purple color.
Monosaccharides are too small to trap iodine molecules and do not form dark colors with iodine.
Around two drops of iodine solution is added to 2ml of the carbohydrate-containing test solution and observed for the color change.
The blue-black color is due to the formation of a starch-iodine complex. Starch is a polymer of α-amylose and amylopectin that forms a complex with iodine to produce a blue-black color. On the addition of iodine to amylose, the helical shape of the unbranched polysaccharide traps iodine to produce a deep blue-black complex.
Cellulose, amylopectin and glycogen react with iodine to produce red-brown color. Glycogen produces a reddish-purple color.
Monosaccharides are too small to trap iodine molecules and do not form dark colors with iodine.