Difference Between Benedict’s and Fehling’s Solution

Main Difference – Benedict’s vs Fehling’s Solution

Reducing sugars and aldehydes are chemical compounds that can get oxidized by reducing some other component. This concept can be used to identify the presence of them in a compound mixture. For this identification, Benedict’s test and Fehling’s test can be used. These tests use specific reagents known as Benedict’s solution and Fehling’s solution respectively. The main difference between Benedict’s solution and Fehling’s solution is that Benedict’s solution contains copper(II) citrate whereas Fehling’s solution contains copper(II) tartrate.

Key Areas Covered

1. What is Benedict’s Solution
     – Definition, Chemical Components, Test
2. What is Fehling’s Solution
     – Definition, Chemical Components, Test
3. What are the Similarities Between Benedict’s and Fehling’s Solution
     – Outline of Common Features
4. What is the Difference Between Benedict’s and Fehling’s Solution
     – Comparison of Key Differences

Key Terms: Aldehydes, Benedict’s Solution, Copper, Copper Oxide, Fehling’s Solution, Ketones, Reducing Sugars

What is Benedict’s Solution

Benedict’s solution is a blue solution containing a carbonate, citrate, and sulfate which yields a red, yellow, or orange precipitate upon warming with a sugar (such as glucose) which is a reducing agent. Since it can detect the presence of a reducing sugar, Benedict’s solution can be used to distinguish between aldehydes and ketones. Aldehydes give a positive result, and ketones give a negative result for Benedict’s test.

The end result of Benedict’s test is a brick-red colored precipitate. Any chemical compound that is a reducing agent can give a positive result for Benedict’s test. Benedict’s solution has a dark blue color. This is due to the presence of copper(II) sulfate(CuSO₄.5H₂O). A reducing sugar can reduce the copper ion in this solution into copper oxide, which is a red colored precipitate.

Figure 1: Benedict’s Test

Benedict’s Test

This test uses Benedict’s solution which does not deteriorate quickly. The active component in this solution is copper(II) citrate.

1. Take a suitable amount of Benedict’s solution into an empty test tube.
2. Add a little amount of the sample to be tested.
3. Boil for two minutes.
4. If the reaction mixture gives a red colored precipitate, the sample has a reducing compound.

What is Fehling’s Solution

Fehling’s solution is a blue solution of Rochelle salt and copper sulfate used as an oxidizing agent in a test for sugars and aldehydes. It is a chemical reagent that is useful in identifying reducing sugars. Therefore, it can be used to differentiate between a reducing sugar and a non-reducing sugar. The chemical test used for this differentiation is known as Fehling’s test.

Fehling’s solution is prepared by mixing two solutions together. The two solutions are named as Fehling’s A and Fehling’s B. Fehling’s A has a deep blue color due to the presence of hydrous copper(II) sulfate(CuSO₄.5H₂O). But Fehling’s B is a colorless solution. It is composed of Rochelle salt (potassium sodium tartrate) with sodium hydroxide.

Figure 2: Fehling’s Test

Fehling’s Test

1. The two Fehling’s solutions are first mixed together. The resultant solution is a blue colored solution containing bis(tartrate) complex of Cu²⁺.
2. Then a suitable amount of this solution is taken into an empty test tube. A little amount of the sample is added to the same test tube.
3. The next step is to boil the reaction mixture at 60^oC in a water bath.
4. If it gives a brick-red colored precipitate, then the sample has reducing sugars.

When this test is used to differentiate between aldehydes and ketones, aldehydes give a positive end result because they can get oxidized. While it is oxidizing, the copper(II) complex get reduced to copper oxide insoluble precipitate.

Similarities Between Benedict’s and Fehling’s Solution

• Both solutions are used to identify reducing sugars and aldehydes.
• Both are blue colored solutions.
• The tests done using both solutions give a red precipitate at the end.
• Both tests need to heat the reaction mixture.

Difference Between Benedict’s and Fehling’s Solution

Definition

Benedict’s Solution: Benedict’s solution is a blue solution containing a carbonate, citrate, and sulfate which yields a red, yellow, or orange precipitate upon warming with a sugar (such as glucose) that is a reducing agent.

Fehling’s Solution: Fehling’s solution is a blue solution of Rochelle salt and copper sulfate used as an oxidizing agent in a test for sugars and aldehydes.

Major Component

Benedict’s Solution: The active component in Benedict’s solution is copper(II) citrate.

Fehling’s Solution: The active component in Fehling’s solution is copper(II) tartrate.

Preparation

Benedict’s Solution: Benedict’s solution is available as a ready-to-use reagent.

Fehling’s Solution: Fehling’s solution has to be prepared by mixing two solutions: Fehling’s A and Fehling’s B.

Stability

Benedict’s Solution: Benedict’s solution is stable and does not deteriorate quickly.

Fehling’s Solution: Fehling’s solution deteriorates quickly. Therefore it is prepared only when required.

Test

Benedict’s Solution: In Benedict’s test, the reaction mixture is boiled for about 2 minutes.

Fehling’s Solution: In Fehling’s test, the reaction mixture is heated to 60°C for few minutes.

Conclusion

Benedict’s solution is used for Benedict’s test, and Fehling’s solution is used for Fehling’s test in order to identify a reducing sugar or an aldehyde. The main difference between Benedict’s solution and Fehling’s solution is that Benedict’s solution contains copper(II) citrate whereas Fehling’s solution contains copper(II) tartrate.

References:

1. Lancashire, Robert John. Fehling’s test for reducing sugars. Available here.
2. “Fehling’s solution.” Wikipedia, Wikimedia Foundation, 12 Dec. 2017, Available here.

Image Courtesy:

1. “Próba Benedicta” By Kala Nag – Own work (GFDL) via Commons Wikimedia
2. “Fehling” By FK1954 – Own work (Public Domain) via Commons Wikimedia