Benedict’s Qualitative Reagent Powder

Introduction

Benedict’s Qualitative Solution provides a test to determine the presence of reducing sugars. All monosaccharides and some disaccharides are reducing sugars—that is, they contain a free aldehyde or α-hydroxyketone group that is capable of reducing copper(II) or iron(III) ions. Sucrose, common table sugar and a disaccharide, is a notable exception in that it is not a reducing sugar.

Concepts

• Reducing sugars
• Oxidation–reduction reactions

Materials

Safety Precautions

This activity requires the use of hazardous components and/or has the potential for hazardous reactions. Copper(II) sulfate is a skin and respiratory irritant. It is moderately toxic by ingestion and inhalation. Sodium carbonate is a possible skin irritant. Wear chemical splash goggles, chemical-resistant gloves and a chemical-resistant apron. Use insulated gloves or test tube clamps when handling the heated test tubes during the Benedict’s Qualitative procedure. Please review current Safety Data Sheets for additional safety, handling and disposal information.

Disposal

Please consult your current Flinn Scientific Catalog/Reference Manual for general guidelines and specific procedures, and review all federal, state and local regulations that may apply, before proceeding. All solutions may be flushed down the drain with excess water according to Flinn Suggested Disposal Method #26b.

Prelab Preparation

1. Remove the bag from the bottle. This bag contains 17.3 g of copper(II) sulfate. The bottle contains a mixture of 173 g of sodium citrate and 100 g of sodium carbonate.
2. Dissolve the mixture of 173 g of sodium citrate and 100 g of sodium carbonate in 750–850 mL of distilled or deionized water in a 2-L Pyrex beaker.
3. Warm this solution on a hot plate-magnetic stirrer to about 75–80 °C and stir to aid in dissolving the solids. If all the solids will not go into solution, filter the solution.
4. In a 250-mL beaker, dissolve 17.3 g of copper(II) sulfate in 100 mL of distilled or deionized water.
5. Slowly, while stirring constantly, add the copper(II) sulfate solution to the sodium citrate/carbonate solution.
6. Let the solution cool to room temperature and dilute to 1 liter with distilled or deionized water. This is Benedict’s Qualitative Solution. Store in a labeled plastic 1-L bottle.
7. Dissolve 5 g of dextrose in 100 mL of distilled or deionized water to prepare a 5% dextrose solution.

Procedure

1. Add 5 mL of Benedict’s Qualitative Solution and 8 drops of the 5% dextrose solution to a test tube.
2. Heat a beaker of water to boiling or to near boiling.
3. Using insulated gloves or a test tube clamp, place the test tube in the boiling water bath. Note any changes after three or four minutes.
4. A positive Benedict’s Qualitative test is indicated by the formation of a brownish-red copper(I) oxide precipitate. The color of the precipitate may actually be red, brown, green or yellow depending on the amount of sugar present, although red is most commonly observed. A color change alone is not a positive test—a precipitate must also be present. No precipitation at all indicates a negative test.
5. Students can test for reducing sugars in foods. An example of how to prepare a food sample is the following: Place an apple slice in a mortar along with a few milliliters of distilled water. Crush the apple with a pestle and add water until a mashed apple solution is obtained. Follow steps 1–4, substituting 8 drops of the apple solution for the dextrose solution.
6. Students can test for reducing sugars in a variety of foods using this method.

Teacher Tips

• For effective demonstrations, the volumes of the Benedict’s Qualitative Solution and the sugar solution used in the procedure may be scaled up for better visibility.

Discussion

Benedict’s Qualitative Solution is used to test for the presence of reducing sugars. A reducing sugar contains a free aldehyde or α-hydroxyketone group that is capable of reducing copper(II) or iron(III) ions. In a reaction with copper(II) or iron(III) ions, the free aldehyde is oxidized to a carboxylic acid (see Reaction 1), while the α-hydroxyketone is oxidized to a diketone (see Reaction 2). All monosaccharides and some disaccharides are reducing sugars. Examples of reducing sugars include glucose, fructose, galactose, and lactose. Notably, sucrose is not a reducing sugar.

Initially, the copper(II) ions in the Benedict’s Qualitative Solution impart a characteristic blue color to the solution. However, when Benedict’s Qualitative Solution is added to a solution containing a reducing sugar, the blue copper(II) ions are reduced to copper(I) ions by the reducing sugar to form red copper(I) oxide, Cu₂O, which precipitates out of solution. Therefore, the formation of a precipitate indicates a positive test for reducing sugars.

Benedict’s Qualitative Solution contains not only copper(II) ions, but also sodium citrate and sodium carbonate. Each component serves a specific purpose. The citrate ions form a complex with the copper(II) ions preventing the copper from precipitating out of solution as copper(II) hydroxide. The sodium carbonate provides a basic environment, which is necessary for the reduction to occur.

References

Campbell, B. N.; Ai, M. M. Organic Chemistry Experiments; Brooks/Cole: Pacific Grove, CA, 1994; pp 409–410.

Lehninger, A. L.; Nelson, D. L.; Cox, M. M. Principles of Biochemistry; Worth: New York, 1993; pp 298–307.