Molisch's test
McGraw-Hill Science & Technology Dictionary:
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Molisch test
(′möl·ish ′test)
(biochemistry) A test used for the general detection of carbohydrates; a purple color is produced when the solution containing carbohydrate is treated with strong sulfuric acid in the presence of α-naphthol.
Oxford Dictionary of Biochemistry:
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Molisch test
a qualitative test for free or combined aldohexoses. It depends on the formation of a purple colour when the furfural derivatives, formed when aldohexoses are heated with strong acids (e.g. H2SO4), react with sulfonated 1-naphthol. [After Hans Molisch (1856 — 1937), Austrian chemist.]
| Mohr's salt, Mohr pipette, Moffitt-Yang equation | |
| Molscript, Mon, Monera |
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Molisch's test
Molisch's test (named after Austrian botanist Hans Molisch) is a sensitive chemical test for the presence of carbohydrates, based on the dehydration of the carbohydrate by sulfuric acid to produce an aldehyde, which condenses with two molecules of phenol (usually α-naphthol, though other phenols (e.g. resorcinol, thymol) also give colored products), resulting in a red- or purple-colored compound.
Procedure
The test solution is combined with a small amount of Molisch's reagent (α-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, to form a bottom layer. A positive reaction is indicated by appearance of a purple ring at the interface between the acid and test layers.
Reaction
All carbohydrates – monosaccharides, disaccharides, and polysaccharides – should give a positive reaction, and nucleic acids and glycoproteins also give a positive reaction, as all these compounds are eventually hydrolyzed to monosaccharides by strong mineral acids. Pentoses are then dehydrated to furfural, while hexoses are dehydrated to 5-hydroxymethylfurfural. Either of these aldehydes, if present, will condense with two molecules of naphthol to form a purple-colored product, as illustrated below by the example of glucose:
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McGraw-Hill Science & Technology Dictionary
McGraw-Hill Dictionary of Scientific and Technical Terms. Copyright © 2003, 1994, 1989, 1984, 1978, 1976, 1974 by McGraw-Hill Companies, Inc. All rights reserved.
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Oxford Dictionary of Biochemistry
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