FEHLING'S TEST: REDUCING SUGARS

Introduction:

Fehling's solution is a chemical test used to differentitate between reducing and non-reducing sugars. This test is based on the reaction of a functional group of sugar molecules with Fehling's reagent. Fehling's have two separate solutions:

Fehling's reagent has to separate solutions: Fehling's A and Fehling's B.

Fehling's A: is a blue aqueous solution of copper (II) sulphate.

Fehling's B: clear and colourless solution of potassium sodium tartrate and sodium hydroxide.

Materials:

-Test tube rack

-10mL Pipet

-Distilled water

-5 test tubes

-5 spatula

-Lactose

-Maltose

-Glucose

-Sucrose

-Starch

-Fehling's A and B solutions

-HCl

Objectives:

- Identify reducing sugars.

- Comprehend redox reactions.

- Understand the relation between structure and reducing ability of some sugars.

Procedure:

1. Take 5 test tubes and label: G, M, S, L, ST

2. Put 2mL of distilled water inside each tube.

3. With different spatulas put a small amount of each sugar. Dissolve the sugar.

4. Add 2mL of Fehling's A solution and then Fehling's B.

5. Place each test-tube in a boiling water bath (250 mL beaker on a hotplate stirrer).

6. Observe what is happening.

Starch Hydrolysis: 

7. Place 2mL of 1% starch in a test tube and add 0,5 mL of 3M HCL. Mix and place this mixture in a boiling water bath for 10 minutes.

8. After 10 minutes, remove the tube from the water bath and let it cool. Neutralize this solution with 1M NaOH and mix well.

9. Transfer 10 drops of this solution to a small test tube.

10. Add 1mL of Fehling's A solution and 1mL of Feghling's B.

11. Heat for a few minutes in a boiling water bath.

12. Record your observations. Compare the results of this test with your results for unhydrolyzed starch in the step 1 of this experiment.

Observations: 

-Lactose: has reducing power

-Maltose: has reducing power

-Glucose: has reducing power

-Sucrose: doesn't have reducing power

-Starch: doesn't have reducing power