Skip to content
Home » Molisch Test for Fructose

Molisch Test for Fructose

Molisch test for fructose post we briefly summarises about: principle, reagents requirements, Molisch test for carbohydrates procedure, result, application and limitations of Molisch test.

Molisch Test for Fructose

Molisch test for carbohydrates is a chemical method for determining whether or not a sample contains carbohydrates. Hans Molisch, a Czech-Austrian botanist, is credited with the discovery of this test.

Molisch test for carbohydrates is mixing the samples with Molisch’s reagent (an ethanol solution of α-naphthol) and then adding a few drops of concentrated H2SO4 (sulphuric acid) to the mixture.

The presence of carbohydrates in the samples is confirmed by the creation of a purple or purplish-red ring at the point of contact between the H2SO4 and the sample + Molisch’s reagent mixture.

Principle

Molisch test for carbohydrates reaction depends on the concentrated acid catalysing the dehydration of carbohydrates to produce furfural (from pentoses) or hydroxymethylfurfural (from hexoses). At the acid-test layer contact, either of these aldehydes condenses with two molecules of naphthol to form a purple or violet coloured complex.

If the carbohydrate is a poly- or disaccharide, a glycoprotein, or a glycolipid, the acid first hydrolyzes it into monosaccharides, which are then dehydrated to create furfural or derivatives.

If any impurities are present in the reagent, they may combine with α -naphthol and the acid, resulting in a green ring. A rind ring is seen if a concentrated sugar solution is used. This might be due to the charring of the sugar due to the acid.

Image Source: AlexNB.

Requirements

Reagents

1. Test solution: 5 % Glucose, 5 % Sucrose, 5 % Starch

2. Molisch’s reagent (5 % α naphthol in ethanol)

3. H2SO4

4. Distilled water

Equipment’s

1. Test tubes

2. Test tube stand

3. Pipette

4. Distilled water

Procedure

1. Separately, take 2 mL of distilled water and test sugar solutions in four test tubes. To each tube, add two drops of Molisch reagent.

2. Gently pour 1 ml concentrated H2SO4 along the test tube’s wall while holding the test tube in an angled position. The acid should not be mixed with the solution.

3. If concentrated acid is not introduced slowly enough, the heat generated by the reaction might burn the carbohydrates, resulting in a black ring.

4. Look for the creation of a purple-colored ring at the interface between the solution and the acid in the test tube.

Result

Molisch Test for Fructose

The development of a purple ring at the layer formed by the concentrated acid is a positive indicator for Molisch’s test. If no purple or reddish-purple colour arises, the given sample does not contain any carbohydrate.

Applications

1. Molisch test for carbohydrates t is used to determine whether or not a sample contains carbohydrates. Molisch test for carbohydrates can be used to identify carbohydrates as a by-product of various processes and distinguish them from other biomolecules.

Limitations

1. Trioses and tetroses do not have the necessary five carbon atoms for furfural formation, so Molisch test for carbohydrates do not give a positive result for this reaction.

2. Molisch test for carbohydrates is not a specific test for carbohydrates. Furfurals as such or furfural yielding substance, some organic acids like citric acids, lactic acid, oxalic acid, formic acid, etc. can give a positive result.

Further Readings

Reference