The Molisch test is a chemical method for detecting the presence of carbohydrates, whether they exist freely or are bound with larger biomolecules such as proteins or lipids.
The Molisch test was discovered by the Czech-Austrian botanist Hans Molisch. The Molisch test involves the addition of Molisch’s regent (a solution of α-naphthol in ethanol) to the sample and a few drops of concentrated H2SO4 (sulphuric acid).
Positive Molisch’s test can be detected by the formation of a purple or reddish-purple ring at the interface (where the two layers meet). This confirms the presence of carbohydrates.
The Molisch test test is based on the ability of concentrated sulphuric acid to catalyze the dehydration of sugars, forming furfural from pentoses and hydroxymethylfurfural (HMF) from hexoses.
Aldehyde formed then undergoes condensation with two phenol-type molecules (α-naphthol). This condensation reaction produces a strong purple colour or reddish-purple color.
When a carbohydrate-containing sample is added, the sulphuric acid first hydrolyses it into component monosaccharides, this further dehydrates to form furfural or its derivatives.
Molisch Reagent: Molisch reagent is prepared by dissolving 3.75 g of α-naphthol in 25 ml of Ethanol 99%. This reagent should be prepared fresh.
Take 2 ml of distilled water in a clean test tube and add a test sample to the test tube and shake it well and add 2 to 3 drops of the Molisch reagent.
Hold the test tube in an slightly inclined position and slowly add 1 mL of concentrated H₂SO₄ along the wall of the test tube. After adding H₂SO₄ do not shake the test tube. If the H₂SO₄ added too fastly it might forms a black layer or ring because of the heat generated by the dehydration reaction where acid which can char the carbohydrates.
Observe the test tube for the formation of purple ring at the layer between the solution and the H₂SO₄.
Molisch Test Positive Result: Appearance of purple ring at the interface indicates the presence of carbohydrates.
Molisch Test Negative Result: No ring or any color changes indicates the negative test result. This suggests the absence of carbohydrates.
Several factors can affect the test outcomes; a green ring may occur due to impurities in the sample.
The Molisch test is used to detect the presence of carbohydrates in different laboratory samples. It is used as a preliminary screening test in biochemistry.
It can be used to detect the formation of carbohydrates as a by-product in different reactions. It is also used for identification of sugars in food samples, biological research and medical research.
Trioses (3 carbons) and tetroses (4 carbons) sugars do not have enough carbon atoms to form furfural. So they do not give a positive molisch test.
Molisch’s test does not react only to carbohydrates. Furfural yielding substances such as citric acid, lactic acid, oxalic acid and formic acids might give a positive result.
The Molisch test was discovered by the Czech-Austrian botanist Hans Molisch. The Molisch test involves the addition of Molisch’s regent (a solution of α-naphthol in ethanol) to the sample and a few drops of concentrated H2SO4 (sulphuric acid).
Positive Molisch’s test can be detected by the formation of a purple or reddish-purple ring at the interface (where the two layers meet). This confirms the presence of carbohydrates.
Principle of Molisch Test
The Molisch test test is based on the ability of concentrated sulphuric acid to catalyze the dehydration of sugars, forming furfural from pentoses and hydroxymethylfurfural (HMF) from hexoses.
Aldehyde formed then undergoes condensation with two phenol-type molecules (α-naphthol). This condensation reaction produces a strong purple colour or reddish-purple color.
When a carbohydrate-containing sample is added, the sulphuric acid first hydrolyses it into component monosaccharides, this further dehydrates to form furfural or its derivatives.
Requirements of Molisch Test
Molisch Reagent: Molisch reagent is prepared by dissolving 3.75 g of α-naphthol in 25 ml of Ethanol 99%. This reagent should be prepared fresh.
Procedure of Molisch Test
Take 2 ml of distilled water in a clean test tube and add a test sample to the test tube and shake it well and add 2 to 3 drops of the Molisch reagent.
Hold the test tube in an slightly inclined position and slowly add 1 mL of concentrated H₂SO₄ along the wall of the test tube. After adding H₂SO₄ do not shake the test tube. If the H₂SO₄ added too fastly it might forms a black layer or ring because of the heat generated by the dehydration reaction where acid which can char the carbohydrates.
Observe the test tube for the formation of purple ring at the layer between the solution and the H₂SO₄.
Interpretation of Test Results
Molisch Test Positive Result: Appearance of purple ring at the interface indicates the presence of carbohydrates.
Molisch Test Negative Result: No ring or any color changes indicates the negative test result. This suggests the absence of carbohydrates.
Several factors can affect the test outcomes; a green ring may occur due to impurities in the sample.
Applications of Molisch’s Test
The Molisch test is used to detect the presence of carbohydrates in different laboratory samples. It is used as a preliminary screening test in biochemistry.
It can be used to detect the formation of carbohydrates as a by-product in different reactions. It is also used for identification of sugars in food samples, biological research and medical research.
Limitations of Molisch’s Test
Trioses (3 carbons) and tetroses (4 carbons) sugars do not have enough carbon atoms to form furfural. So they do not give a positive molisch test.
Molisch’s test does not react only to carbohydrates. Furfural yielding substances such as citric acid, lactic acid, oxalic acid and formic acids might give a positive result.
