Molisch's test

Molisch's test is a sensitive chemical test for the presence of carbohydrates, based on the dehydration of the carbohydrate by sulfuric acid or hydrochloric acid to produce an aldehyde, which reacts with two molecules of phenol, resulting in a purple-colored compound. This test is named after the Austrian botanist Hans Molisch, who developed the test in 1913. It is a general test that can detect virtually all carbohydrates, including monosaccharides, disaccharides, and polysaccharides, as well as carbohydrate-containing substances such as glycoproteins and glycolipids.

Procedure

The Molisch's test involves adding a few drops of Molisch's reagent (a solution of alpha-naphthol in 95% ethanol) to a test solution of the carbohydrate. After mixing, concentrated sulfuric acid or hydrochloric acid is carefully added down the side of the test tube without mixing, forming a layer. A positive result is indicated by the appearance of a violet or purple ring at the interface between the acid and test layers. The formation of the purple ring is due to the formation of a furfural or related compound, which then reacts with the alpha-naphthol to produce a purple-colored compound.

Interpretation

A positive Molisch's test indicates the presence of carbohydrates in the test sample. However, it is a qualitative test and does not provide information about the quantity or specific type of carbohydrate present. Further tests, such as the Benedict's test, Barfoed's test, or Seliwanoff's test, are required to identify the specific type of carbohydrate.

Applications

Molisch's test is widely used in biochemistry and organic chemistry laboratories to screen for the presence of carbohydrates in various samples. It is also used in food industry quality control and in the identification of unknown substances in forensic analysis.

Limitations

While Molisch's test is highly sensitive and can detect minute amounts of carbohydrates, it is not specific to any particular type of carbohydrate. Additionally, certain substances, such as uric acid and sulfonamides, can interfere with the test and produce false positive results. Therefore, it is important to use Molisch's test in conjunction with other carbohydrate-specific tests for accurate identification.

See Also

• Carbohydrate chemistry
• Chemical tests
• Fehling's test
• Benedict's test