Molisch's Test- Definition, Principle, Procedure, Result, Uses

Molisch Test Definition

 Molisch's Test is a general test for all carbohydrates.    

Objectives of Molisch Test

  • To identify the presence of carbohydrates in a given sample.
  • To differentiate carbohydrates from other biomolecules.

Principle of Molisch Test


Diagrammatically representation of the principle of Molisch's Test

Image Source: AlexNB, Public domain, via Wikimedia Commons

  • Concentrated sulphuric acid dehydrates carbohydrates to give furfural (in the case of pentoses) and furfural derivatives (in the case of hexoses and heptoses). These furfural derivatives condense with alpha naphthol to form a violet-coloured compound. 
  • Oligosaccharides and polysaccharides are first hydrolysed to monosaccharides by concentrated acid. They undergo dehydration to form furfural or its derivatives.

Requirements

Reagents: 

  1. Molisch’s Reagent: Dissolve 1 gm α-naphthol in 95% ethyl alcohol and make the volume 100 ml.
  2. Concentrated H2SO4
  3. Sugar Sample

1.      Materials required

·       Test tubes

·       Test tube stand

·       Pipette

·       Distilled water


Procedure of Molisch Test

  • Take 4-5 ml of sugar sample (carbohydrate solution) in a test tube.
  • Add 1-2 drops of Molisch’s Reagent. Mix well.
  • Incline the test tube and add slowly 3 ml of concentrated H2SO4 from the side of the test tube.
  • Now hold the test tube vertical position and gently rotate it between your hands.
  • Note: Green or Brown ring is appeared due to the presence of impurities in the H2SO4.

Result and Interpretation of Molisch Test



Observation: A violet coloured ring obtained at the junction of two liquids.

Inference: Carbohydrates present in the given samples.

Why violet colour is produced in the Molisch’s Test?

The violet colour is produced because condensation of the aldehyde group with a hydroxyl group of phenolic compounds.

We can use anthrone instead of α-naphthol.

Limitations of Molisch Test

All carbohydrates with more than 4 carbon atoms will give positive Molisch’s test.

The following molecules gives the negative Molisch’s Test because they do not have the requisite 5 carbon atoms for the formation of furfural derivatives.

1.      Trioses

2.      Tetroses

3.      Amino Sugar

Furfural, aldehyde and some organic acids, e.g., oxalic acid, lactic acid, citric acid, formic acid etc. also give a positive Molisch’s Test.

Molisch test is not a definite test for carbohydrates.

Reference and Sources

Online Plagarism checker: https://www.quetext.com/

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