PRACTICALS CARBOHYDRATES

Benedict’s test Add 8 drops of glucose solution into a test tube containing 5mL of Benedict’s solution and boil for 2 minutes over a Bunsen flame/boiling water bath. Allow the test to cool at room temperature and observe the precipitate.

Identification of reducing property of sugars Repeat the first experiment for glucose, maltose & Sucrose at the same time. Observe the color change/precipitate & explain your observation

Fehling’s test Take 1mL of Fehling’s solution into a test tube and 6 drops of glucose solution and keep it in a hot water bath for five minutes. Observe the brick red precipitate

Molisch’s test Principle All carbohydrates (monosaccharides, disaccharides and polysaccharides) give a positive reaction for Molisch test. It is based on the dehydration of the carbohydrate by Sulphuric acid to produce an aldehyde, Aldehyde condenses with two molecules of α-naphthol, resulting in appearance of a purple ring at the interface

Test To a 2mL of glucose solution add 4 drops of 1% solution of α- naphthol in alcohol and tilt the tube carefully without mixing. Add 2mL of concentrated sulfuric acid by pouring it down the side of the tube.

Barfoed’s test Principle Reducing monosaccharides are oxidized by the copper ion in solution to form a carboxylic acid and a reddish precipitate of copper (I) oxide within three minutes. Reducing disaccharides undergo the same reaction, but do so at a slower rate.

Test Add 1mL of sugar solution to a 3mL of freshly prepared Barfoed’s reagent and mix well Place in a boiling water bath for 1 minute Allow to stand at room temperature for 5 minutes Perform this test for both glucose and maltose and compare the time takes for the formation of red precipitate

Seliwanoff’s Test Principle Seliwanoff’s reagent contains a non-oxidizing acid (HCl) and resorcinol. When a ketose is reacted with this reagent, it becomes dehydrated and a cherry-red complex forms (not a precipitate).

Add 10 drops of seliwanoff reagent solution into a test tube containing 2 mL of Fructose solution. Mix well and place the test tube in the boiling water bath for 10 minutes. Observe the cherry red colour solution..

Test for polysaccharides Add 1 drop of iodine solution into 1mL of starch solution. Observe the blue-black colour solution. Repeat the same test for glycogen and observe the colour change Explain your observation

Hydrolysis of Starch/sucrose Place 3mL of 2% starch in two test tubes and 3mL of 2% sucrose solution in two more test tubes. To one sample each of sucrose and starch, add 20 drops Of 10% HCl. To the other samples of sucrose and starch, add 20 drops of H2O.

Label the test tubes and place in a boiling water bath for 10 minutes. Remove the test tubes from water bath and let them cool. To the samples containing HCl add 10% NaOH (about 20 drops) until one drop of the mixture turns litmus paper blue, indicating the HCl has been neutralized. Perform the iodine test and Benedict’s test all four samples