Molisch's test

Molisch's Test is a chemical test for the presence of carbohydrates, based on the dehydration of the carbohydrate by sulfuric acid to produce an aldehyde.

The test solution is combined with a small amount of Molisch's reagent (&alpha;-naphthol dissolved in ethanol) in a test tube. After mixing, a small amount of concentrated sulfuric acid is slowly added down the sides of the sloping test-tube, without mixing. A positive reaction is indicated by a purple ring forming at the interface between the acid and test layers.

All carbohydrates -- monosaccharides, disaccharides, and polysaccharides -- should give a positive reaction. Glycoproteins also give a positive reaction. Pentoses are dehydrated to furfural; hexoses are dehydrated to hydroxymethylfurfural. Either of these aldehydes, if present, will condense with two molecules of naphthol to form a purple-colored product. >O2 (furfural) + Reactions:


 * 1. C5H10O5 (pentose) + (conc.) H2SO4 &rarr; C5H4O2 + 3 H2O


 * 2. C5H4 + 2 C10H8OH (&alpha;-naphthol) &rarr; colored product