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Molisch’s Test: Qualitative Detection of Carbohydrates in Solutions

Molisch's Test is a qualitative method for detecting carbohydrates in a given solution, revealing a positive result for all carbohydrates, whether free or bound to proteins (glycoproteins) or lipids (glycolipids). Named after Austrian botanist Hans Molisch, it employs Molisch's reagent, made by dissolving alpha-naphthol in ethyl alcohol, and concentrated sulfuric acid. The reaction leads to the formation of a reddish-violet ring in the presence of carbohydrates, while certain organic compounds may cause false positives. This simple test is essential in carbohydrate chemistry.

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Molisch’s Test: Qualitative Detection of Carbohydrates in Solutions

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  1. Experiment no.4 MOLISCH’S TEST

  2. It is a qualitative group test for detection of Carbohydrates in the given solution, and is positive for all carbohydrates whether free or bound to other substances such as proteins (e.gglyco or muco proteins) or lipids (e.g glycolipids). • It is named after Austrian Botanist Hans Molisch

  3. Reagents: • Molisch’s reagent: this is prepared by dissolving 1gm of alpha naphthol in 95% ethyl alcohol and making the volume upto 100ml. • Concentrated sulphuric acid.

  4. Principle: • When carbohydrates are treated with strong acids.the polysaccharides and disaccharides are hydrolyzed into monosaccharides. • The monosaccharides are dehydrated by acid to form furfural or one of its derivatives. • Furfural or its derivatives condense with alpha naphthol to form a violet coloured compound.

  5. Furfural: • When sugars are treated with strong acids,on heating, they are hydrolyzed into monosaccharides. These monosaccharides undergo dehydration( removal of water molecules) by acids. Depending on type of monosaccharides these dehydrated monosaccharides are called furfural or furfural derivatives.

  6. Procedure: • Take two test tubes and label them S for sample and B for blank. • Take 3 ml of distilled water in test tube B and 3 ml carbohydrate solution in test tube S. • Add 2 to 3 drops of molisch’s reagent in both test tubes. • Mix it and incline the test tubes and run 3ml of conc. Sulphuric acid along the wall of both test tubes. Make the test tubes upright and rotate them between your palms slowly and gently.

  7. Interpretation: • Appearance of a reddish violet ring at the junction of two liquids indicate the presence of carbohydrate in test tube S. no such ring will appear in test tube B. in presence of excessive alpha naphthol a green ring may also be seen this should be ignored. • Repeat the test with different carbohydrate solutions.

  8. Precaution: • After formation of the ring, do not shake tube contents it may distort the ring shape. • FALLACIES: • Furfural,aldehydes and some organic acids e.g formic acid, oxalic acid, lactic acid, citric acid etc can also give a positive molisch’s test.

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