Shows positive test for:

All carbohydrates. Monosaccharides give a rapid positive test. Disaccharides and polysaccharides react slower.

Reactions:

The test reagent dehydrates pentoses to form furfural (top reaction) and dehydrates hexoses to form 5-hydroxymethyl furfural (bottom reaction). The furfurals further react with -naphthol present in the test reagent to produce a purple product (reaction not shown).

How to perform the test:

Two ml of a sample solution is placed in a test tube. Two drops of the Molisch reagent (a solution of -napthol in 95% ethanol) is added. The solution is then poured slowly into a tube containing two ml of concentrated sulfuric acid so that two layers form.

A positive test is indicated by:

The formation of a purple product at the interface of the two layers.

 

a negative test (left) and a positive test (right)