Lugol / Fehling

OBJECTIVES:

1. Identify different sugars
2. Understand the relation between structures

MATERIAL:

• 10 test tubs
• Beaker
• Spatula
• Bunsen burner
  
• 3 pipets
• Wire brush
• Distilled water
• Fehling A
• Fehling B
• Lugol
• Glucose
• Maltose
• Sucrose
• Lactose
• Starch

PROCEDURE: We put 5 test tubs in a test tube-rack, each one with each glucid, and we do the same with the other five test tubs in another test tube-rack. In one grup we will put Fehling and in the other Lugol.

Lugol: 

1. First put a little bit of each glucid in each test tube
2. Add 5ml of distilled water in each test tub
3. Observe what happens
4. Add 2 drops of Lugol
5. Observe the colour 

1. 
   

Fehling:

1.  Put a little bit of each glucid in each test tube
2. Add 2ml of distilled water
3. Add 2ml of Fehling A and 2ml of Fehling B
4. Put the test tube in a boiling water bath (beaker)
5. Observe the colour

RESULTS:

                     Glucose      Maltose      Sucrose      Lactose      Starch

Sweet            sweet          sweet           sweet         sweet          no sweet

Solubility      soluble        soluble        soluble       soluble        no soluble

Lugol            negative      negative       negative     negative       positive
Fehling         positive       positiva       negative     positive        negative

• In Lugol experiment: The Lugol reacts when in the substance there is starch, so when we do the experiment, only the solution with starch it's positive. The others are negative because don't have starch.

• In Fehling experiment: The Fehling reacts with the aldehydes, not with the ketones. So glucose, maltose, and lactose are positive in this experiments becouse are aldehyds.