Analysis of carbohydrates

Carbohydrate analysis focuses on mono– and –disaccharides which are colorless solids or viscous liquids that dissolve readily in water. In the IR spectrum they exhibit strong hydroxyl absorptions and usually no carbonyl absorption.

A main feature of carbohydrate analysis is that the “reductant” sugars are oxidized by the Tollens’ reagent and the Fehling reagent (see Fehling’s test).

Molisch’s test

It is a carbohydrate analysis specific to pentoses and hexoses.

Procedure: Place 5 mg of substance in 0.5 ml of water. To add two drops of a dissolution of α-naphthol to 10 % in EtOH. With the help of an eyedropper pour 1 ml of concentrated H2SO4 so that it slides down the wall of the tube tilting it so that the H2SO4, being denser, is deposited at the bottom without mixing with the water.

Molisch's test pentoses hexoses
Molisch’s test for pentoses and hexoses

If a carbohydrate is present a red ring will form on the separation surface of the two liquids. If we let the solution rest some minutes and we dilute with 5 ml of water a dark violet precipitate will appear.

Return to the functional group analysis page.