Test for glucose

Class: 3ème

Lesson: Les ailments

Rationale: Test for the presence of glucose in food samples.

Materials:

Sodium carbonate (Na₂CO₃) (science kit)

Copper sulfate (CuSO₄) (science kit)

Sodium citrate (I had this sent from the States.)

test tubes (Iycée) (or glasses)

gaz stove

cooking pot

needle and 5 ml syringe (pharmacies)

matches

balance (Iycée) (or make one with the science kit manual.)

graduated cylinder (science kit)

Suggestions for food sample:

Positive: milk, banana, sweet potatoe

Negative: manioc flour, peanut butter, oil

Procedure:

To prepare solutions:

1. Fehling's solution:

This is actually Benedict's solution but since the French use Fehling's, I tell the student's that it is Fehling's. The two solutions react the same but Benedict's isn't caustic and keeps longer.

a. Dissolve 17.3 g of sodium citrate and 10 g of Na₂CO₃ in 80 mls of warm water.

b. Dissolve 1.7 g of CuSO₄ in 20 ml of cold water.

c. Add the CuSO₄ solution to the sodium citrate solution while stirring.

In class:

2. Start a water bath.

3. For liquids (milk, oil) add I ml to a labeled test tube. For solids (flour, peanut butter, banana) mix or mash in I ml of water.

4. Add I ml of Fehling's (Benedict's) to each tube.

5. Put the tubes in a boiling water bath for ~ 3 minutes.

6. A red-orange precipitate indicates the presence of glucose.

Note: Fehling's (Benedict's) does not give a red precipitate with complex sugars such as sucrose. Starches will give a cloudy green color.

Source: Mackean Class Experiments in Biology, p. 13; Teacher's Book, p. 4, 19.