Fehling’s solution

Fehling's solution distinguishes between aldehydes and ketones in carbonyl-containing compounds. It is also commonly employed as a test for monosaccharides, such as glucose, and can help diagnose diabetes by detecting glucose in urine. 

1.0What is Fehling’s Solution?

Fehling’s solution consists of two chemicals, Fehling A and Fehling B, mixed in equal amounts just before use. It is dark blue, alkaline, and a mild oxidising agent. It is primarily used to test for aldehydes and reduce sugars and glucose.

Components:

• Fehling A: Copper sulfate solution, which gives the characteristic blue colour due to the presence of copper ions.
• Fehling B: Potassium sodium tartrate (Rochelle salt) solution, prepared by dissolving potassium tartrate in sodium hydroxide. It acts as a chelating agent and is colourless.

The two components are stored separately and combined in equal volumes to prepare Fehling's solution, which is unstable over time.

2.0Preparation of Fehling’s Solution

Fehling A (Blue Solution)

• Composition: Aqueous solution of copper sulfate pentahydrate (CuSO₄·5H₂O).
• Preparation: Dissolve copper sulfate pentahydrate in distilled water and add a few drops of dilute sulfuric acid.
• Reaction: CuSO₄+2NaOH→Cu(OH)₂+Na₂SO₄

The copper ions in the solution give it a blue colour.

Fehling B (Colorless Solution)

• Composition: Potassium sodium tartrate (Rochelle salt) in an aqueous sodium hydroxide solution.
• Preparation: Dissolve potassium sodium tartrate in sodium hydroxide.

When Fehling A and B are mixed, Fehling’s solution forms a dark blue copper tartrate complex.

Final Reaction:

CuSO₄⋅5H₂O) +  NaOH +  KNaC₄H₄O₆·4H₂O → Fehling’s Solution

Copper Sulfate Pentahydrate+sodium hydroxide +potassium sodium tartrate  → Fehling’s Solution

3.0Fehling’s Test

Principle

Fehling’s test detects aldehydes based on their ability to reduce copper(II) ions to copper(I) oxide in an alkaline medium. Aldehydes are readily oxidised to carboxylic acids, which reduces copper(II) to a reddish-brown precipitate of copper(I) oxide. Ketones do not generally react with Fehling’s solution, with some exceptions.

Procedure

1. Preparation of Fehling’s Solution: Mix equal volumes of Fehling A and Fehling B to obtain a dark blue solution.
2. Testing for Carbonyl Group: Add the organic compound to be tested (containing an aldehyde or ketone group) to a test tube.
3. Addition of Fehling’s Solution: Add the freshly prepared Fehling’s solution to the compound and heat the mixture in a water bath.
4. Observation:

• If a reddish-brown precipitate forms, the compound contains an aldehyde group.
• If no precipitate forms, the compound likely contains a ketone group.

Redox Reaction

• Without tartrate:

RCHO + 2Cu²⁺ + 5OH⁻ → RCOO⁻ + Cu₂O + 3H₂O

• With tartrate: 

RCHO + 2Cu(C₄H₄O₆)₂²⁻ + 5OH⁻ → RCOO⁻ + Cu₂O + 4C₄H₄O₆²⁻ + 3H₂O 

In this redox reaction, Cu²⁺ is reduced to Cu₂O, forming an insoluble reddish-brown precipitate, indicating the presence of an aldehyde.

4.0Uses and Applications of Fehling’s Solution

• Detection of Monosaccharides: Fehling’s solution tests for reducing sugars such as glucose, which gives a positive result by forming a red precipitate.
• Medical Diagnostics: One significant application is detecting excess glucose in blood and urine, which can indicate diabetes.
• Detection of Aldehydes and Ketones: Fehling’s solution helps differentiate between aldehydes and ketones in organic compounds. Aldehydes react to produce a red precipitate, while ketones typically do not.