Objective
Molisch’s test is a sensitive chemical test, named after Australian Botanist Hans Molisch whose main objective is to test the presence of carbohydrates in a given analyte.
Principle
Molisch’s test entails the addition of Molisch’s reagent (a solution of α-naphthol in ethanol) to the analyte and the subsequent addition of a few drops of concentrated sulphuric acid (H2SO4) to the mixture.
Molisch’s test is based on the principle that when monosaccharides are treated with concentrated sulphuric acid (H2SO4) or concentrated hydrochloric acid (HCl), -OH group of sugar are removed in the form of water resulting in the formation of an aldehyde. These products react with sulphonated α-naphthol to give a purple (reddish-purple) colored complex.
A positive reaction for Molisch’s test is given by almost all carbohydrates except tetroses and trioses. It is also important to note that some glycoproteins and nucleic acids give positive results for this test (since they tend to undergo hydrolysis when exposed to strong mineral acids and form monosaccharides).
Reagents And Apparatus
- Test solution: 5% glucose, 5% Sucrose and 5% Starch
- Molisch’s reagent (5% α naphthol in ethanol)
- Concentrated Sulphuric Acid (H2SO4)
- Dry test tubes
- Pipettes
Procedure of Molisch’s Test
- Take 2ml of sample in dry test tube
- Take 2ml of distilled water in another test tube as a control
- Add 2-3 drops of Molisch’s reagent to the solution
- Gently pipette concentrated sulphuric acid (H2SO4) drop-wise along the walls of the test tube into the test tube so as to facilitate the formation of a layer and avoid mixing.
- Observe color change at the junction of two layers.
- Appearance of purple color indicates the presence of carbohydrates.
Results Interpretation
Positive Molisch’s Test: The formation of a purple ring at the layer formed by the concentrated acid is a positive indicator for Molisch’s test.
Negative Molisch’s Test: If no purple or reddish-purple color forms, the given analyte does not contain any carbohydrate.