Molisch's Test for Carbohydrate

Molisch test is used to indicate the presence of carbohydrates in a solution. Molish test applies to all classes of carbohydrates. Neither monosaccharide, disaccharide, oligosaccharides, or polysaccharides can react with molish's reagent. A positive reaction in the molish's test indicated by purple ring color on the substance being tested. Concentrated acid will dehydrate carbohydrate to form furfural. With alpha naphthol, furfural will form a purple-colored compound.

Molisch's test reaction

Material and reagents:

• Molisch reagent: 10 g of alpha naphthol was dissolved in 100 ml ethylalcohol 96%.
• Concentrated acids: H2SO4 (sulfuric acid)
• Test solution: 1% solution of glucose, 1% starch etc.

Work steps:

• Take 2 ml of the test solution into a test tube
• Drop 2 drops of molisch's reagent
• Shake the test tube until well blended
• Tilt the tube and stream carefully 2 ml of concentrated sulfuric acid through the tube wall
• Note the presence or absence of a purple ring at the boundary between two layers of fluid

Note:

If the sulfuric acid is not contaminated will appear purple rings in the boundary of two layers of fluid. If the sulfuric acid has been contaminated with another substance, around the purple rings will appear other colors such as greenish and brownish. If H2SO4 is not available, can be replaced with HCl (hydrochloric acid), although the result will be less maximum. The use of HNO3 (nitric acid) does not give positive result.

Molisch test was discovered by Hans molisch, a plant expert from Austria. He was born on December 6, 1856 and became a professor at the University of Prague, Vienna University, Tohoku University, and the University of India.