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Molisch Test of Carbohydrates

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

Molisch Test of Carbohydrates

Molisch test procedure is a sensitive chemical test for all carbohydrates, as well as some compounds containing carbohydrates in a combined form, that is based on the dehydration of the carbohydrate by sulfuric acid to produce an aldehyde (either furfural or a derivative), which then condenses with the phenolic structure to produce a red or purple-coloured compound. Hans Molisch, an Austrian botanist, provided the inspiration for this test.

Principle

Molisch test procedure is a carbohydrate test that can be used for any type of carbohydrate. Carbohydrates are dehydrated in this test when they are treated with conc. H2SO4 to create furfural and its derivatives.

The OH group of sugar is eliminated in the form of water when monosaccharides are treated with concentrated H2SO4 or concentrated HCl, and furfural is generated from pentose sugar while hydroxymethyl furfural is formed from hexose sugar. Sulphonated-naphthol interacts with these compounds to produce a purple (violet red) tinted complex.

Requirements

Reagent

1. Molisch reagent: Dissolve 3.75 g of α-naphthol in 25 ml of Ethanol 99%. This reagent should be prepared fresh.

2. Concentrated sulphuric acid

3. Test sample

Materials

1. Test tubes

2. Test tube stand

3. Pipette

4. Distilled water

Molisch Test 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.

Results

Molisch Test of Carbohydrates

At the contact between the sulphuric acid and the test solution, the purple coloured ring forms. Because sulphuric acid is denser than the test solution, it remains above the test solution. A lack of colour denotes a negative outcome.

Applications

1. Molisch test procedure is performed to check if a sample contains carbohydrates or not. Molisch test procedure can be used to distinguish carbohydrates from other biomolecules as a by-product of several activities.

Limitations

1. Due to the lack of the five carbon atoms required for furfural synthesis, trioses and tetroses do not yield a good result in this reaction.

2. Molisch test procedure does not distinguish between carbohydrates and non-carbohydrates. Some organic acids, such as citric acids, lactic acid, oxalic acid, formic acid, and others, can produce furfurals or furfural-like substances.

Further Readings

Reference