Molisch’s Test

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What is Molisch’s Test?

Molisch test is a colorimetric method for the analysis of presence of carbohydrate in a given analyte. This test is named after Austrian botanist Hans Molisch. Molisch’s test is done by using Molisch reagent. A solution of - naphthol in ethanol (95%) is known as Molisch reagent. Its also known as purple ring test. Other than Molisch reagent concentrated sulfuric acid is also used in the test.

 

Molisch test gives positive result for all carbohydrates. Although tetrose and triose are exceptions. In Molisch’s test monosaccharides give a positive test faster. While disaccharides and polysaccharides react slowly with Molisch reagent and give positive test in delay. Thus, Molisch’s test for carbohydrates is very useful in detection of presence of carbohydrates in a substance.  

 

Molisch’s Test Principle 

Carbohydrates reacting with mineral acids such as sulfuric acid get dehydrated and form furfural derivatives. These compounds react with naphthol and get condensed to form purple colored product as illustrated below by taking example of glucose – 

 

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Molisch’s Test Procedure 

Molisch’s test procedure can be explained by following points – 

1. 2ml of Sample is taken in a test tube. 

2. Now 2 drops of Molisch’s reagent is added in the sample and mix it. 

Molisch Reagent preparation - Molisch’s reagent can be prepared by adding naphthol in 95% ethanol. 

3. Now add slowly concentrated sulphuric acid in sloping the test tube by its sides without mixing vigorously so that it will get time to react and will form a layer. 

 

Observation 

A purple ring is appeared at the interface between the acid and test layers which confirms the presence of carbohydrate. 

 

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 A positive test

 

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  A negative test 

 

Applications of Molisch’s Test 

Molisch’s test is used to determine the presence of carbohydrates or sugars in the substance. Most practical use of this test is that if some food product is labelled as sugar free then you can easily test the product by Molisch’s test that if the product contains carbohydrate (or sugar) or not. Presence of some glycoproteins and nucleic acids can also be determined by Molisch’s test. 

 

Limitations of Molisch’s Test 

Although almost all carbohydrates and even some glycoproteins and nucleic acids can be detected in a substance by Molisch’s test, but a few carbohydrates are exceptions. Such as tetrose and triose sugars don’t give Molisch’s test. 

 

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