Fehling’s Test for Reducing Sugars
Background:
Carbohydrates are a major food source. Rice, potatoes, bread, corn, candy, and
fruits are rich in carbohydrates. A carbohydrate can be classified as a
monosaccharide (for example, glucose or fructose), a disaccharide (sucrose or
lactose), which consists of two joined monosaccharides, or a polysaccharide
(starch or cellulose), which consists of thousands of monosaccharide units linked
together.
Sugars can be categorized as reducing and non-reducing sugars. A sugar
reducing sugar tests positive in the Fehling's test. The two solutions used in the
Fehling's test are Fehling's A and Fehling's B. Fehling's A is turquoise and is a
solution of copper sulfate. Fehling's B is clear and contains sodium hydroxide
and Rochelle salt. A positive result is indicated by a change in color once the
sugar is added to the test solution.
Purpose:
To identify which carbohydrates give a positive result for the Fehling’s test
Materials:
Fehling’s solution A, Fehling’s solution B, glucose, fructose, lactose, sucrose,
honey, corn starch, cellulose, piece of onion, hot plate, beaker for water bath, 10
test tubes
Safety:
Fehling’s solution B contains sodium hydroxide, which is a strong base. Wear
goggles and gloves. Hot plates stay hot long after they are turned off or
unplugged.
Procedure:
1. Label the test tubes. (Except for the control test tube, each test tube will
contain one of the carbohydrate samples used.)
2. Add two pipettes of Fehling’s A to each test tube.
3. Add two pipettes of Fehling’s B to each test tube.
4. Gently mix the solutions by tapping the test tube against your hand.
5. Add a pea sized amount of the appropriate carbohydrate sample to each
test tube.
6. Place the test tubes into the hot water bath.
7. Record observations.
8. Allow test tubes to stand overnight, and again record observations.