Molisch Test for Carbohydrates

The Molisch test is a general test for the presence of carbohydrates. Molisch reagent is a solution of alpha-naphthol in 95% ethanol. This test is useful for identifying any compound which can be dehydrated to furfural or hydroxymethylfurfural in the presence of H2SO4. Furfural is derived from the dehydration of pentoses and pentosans, while hydroxymethylfurfural is produced from hexoses and hexosans. Oligosaccharides and polysaccharides are hydrolyzed to yield their repeating monomers by the acid. The alpha-naphthol reacts with the cyclic aldehydes to form purple colored condensation products. Although this test will detect compounds other than carbohydrates (i.e. glycoproteins), a negative result indicates the ABSENCE of carbohydrates.

Method

Add 2 drops of Molisch reagent to 2 ml of the sugar solution and mix thoroughly. Incline the tube, and GENTLY pour 5 ml of concentrated H2SO4 down the side of the test tube. A purple color at the interface of the sugar and acid indicates a positive test. Disregard a green color if it appears.