Directions for Benedict's Solution

Corn syrup contains glucose, a reducing sugar.

Sugars represent a subcategory of a broad class of compounds called carbohydrates. The simplest sugars, monosaccharides, typically consist of three to six carbon-linked atoms with hydrogen or oxygen attached. Chemists refer to monosaccharides as "reducing" sugars because they sometimes donate electrons to other compounds. More complex sugars, called disaccharides, oligosaccharides or polysaccharides, consist of two ore more monosaccharides linked together. The more complex sugars tend to not act as reducing sugars. Biochemists use Benedict's solution to test for the presence of monosaccharides. Any color change in the test solution represents a positive test for monosaccharides.

Things You'll Need

• 10 grams sodium carbonate
• 17.3 grams sodium citrate dihydrate
• Balance capable of measuring to 0.1 grams
• 2 glass Erlenmeyer flasks
• Distilled water
• 100-ml graduated cylinder
• 1.7 grams copper sulfate pentahydrate
• Dropper bottle
• 250-mL beaker
• Hot plate
• Small test tube
• Corn syrup
• Test-tube clamp or tongs

Instructions

Preparing the Reagent

1. Weigh out 10 grams of sodium carbonate, Na2CO3, and 17.3 grams of sodium citrate dihydrate separately on a balance and add them to a glass container, such as an Erleymeyer flask. Add about 50 ml of distilled water to the flask and swirl until the solids have completely dissolved.

2. Transfer the sodium carbonate and sodium citrate solution to a 100-ml graduated cylinder, and bring the volume to 85 ml with distilled water. Transfer the solution back to the flask. Rinse the graduated cylinder with distilled water.

3. Weigh out 1.7 grams of copper sulfate pentahydrate, CuSO4-5H2O, on a balance and place it in a separate flask. Add 100 ml of distilled water to the flask and swirl until the copper sulfate has completely dissolved.

4. Add the copper sulfate solution to the sodium carbonate/sodium citrate solution slowly, pausing every few milliliters to swirl the flask. When you have added all of the copper solution, transfer the solution to a dropper bottle and clearly label it "Benedict's Solution."

Using the Reagent

1. Fill a 250-ml beaker about half full with tap water and place it on a hot plate under medium heat. This will serve as a hot-water bath for the Benedict test.

2. Add 1 ml, or about 20 drops, of Benedict's solution to a small test tube, then add 5 drops of the sample being tested. To test the Benedict's solution, first test a sample known to contain monosaccharides, such as corn syrup.

3. Place the test tube in the 250-ml beaker serving as the hot-water bath and heat the solution in the test tube to almost boiling. Remove the test tube using a test-tube clamp or tongs and examine the solution. A change to any color besides blue indicates a positive test for a reducing sugar. In general, the progression from green to orange to red to brown reflects increasing amounts of reducing sugars in the sample.

Tips & Warnings

• The use of safety glasses and rubber gloves is strongly advised whenever working with chemicals.

References

• Rensselaer Polytechnic Institute, Department of Biochemistry & Biophysics: Carbohydrates - Sugars and Polysaccharides
• Clackamas University: Reducing Sugars
• Mansfield University: Reagents
• Tufts University: Use of Benedict's Solution (PDF)
• Ohio State University, Departent of Biological Sciences: Benedict's Test for Reducing Sugars

Resources

• Journal of Chemical Education: Benedict's Test for Reducing Sugars