Question

An abstract report on test for carbohydrates

Answer 1

1. Molisch’s Test: Molisch’s reagent is 5% solution of alpha naphthol in alcohol.
Alcoholic alpha naphthol forms furfural and furfural derivatives, such as hydroxymethylfurfural, by the concentrated sulphuric acid acting on the sugar. This compound forms a reddish-violet coloured ring at the junction of the two liquids.

2.

Lugol's reagent (iodine solution) changes from yellowish-brown to dark purple/black.

3.

3. Bial’s Test for Pentoses: Bial’s reagent is prepared by dissolving 3 gm of orcinol and 0.1 gm of ferric chloride in 100 ml of ethanol.

4.

Seliwanoff’s Test: Seliwanoff s reagent is 0.5% resorcinol in conc. HCl diluted 1:1 with water.

5.

Reduction Tests:

A. Fehling’s Test: Fehling’s reagent is prepared freshly by mixing equal volumes of two stock solutions Fehling A and Fehling B. Solution Fehling A is 6.93 grams of CuSO4.5H2O per 100 mL of water and Solution Fehling B is 20 grams of KOH and 34.6 grams of sodium potassium tartarate (Rochelle salt) per 100 ml solution.
Carbohydrates with free aldehyde or ketone groups reduce copper sulphate to cuprous oxide forming a yellow or brownish red coloured precipitate.

B. Benedict’s Test: Bennedict’s solution is prepared by dissolving sodium citrate, sodium carbonate and water, and heated.
In case of reducing carbohydrate (or sugars) there will be an appearance of red precipitate.

C. Barfoed’s Test: Barfoed’s reagent is prepared by dissolving neutral copper acetate in water and then adding glacial acetic acid.
Appearance of a red precipitate of cupric oxide (Cu₂O) indicates the presence of reducing sugar.