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Fehling’s Solution Test for Aldehydes and Ketones

Fehling’s solution test for aldehydes and ketones post we briefly summarises about: principle, reagents requirements, fehling test for aldehyde and ketone procedure, result, application and limitations of fehling test for aldehyde and ketone.

Fehling’s Test

Fehling test for aldehyde and ketone, is a chemical reagent used to differentiate between aldehydes and ketones other than α-hydroxy ketone. In practice, it is used to determine the presence of reducing and non-reducing sugars in carbs. For aromatic aldehydes, Fehling’s solution is ineffective.

A solution is used in this fehling test for aldehyde and ketone, which is normally made fresh in laboratories. Initially, the solution is in the form of two independent solutions, Fehling’s A and Fehling’s B, which are labelled as such. Fehling’s A is a blue solution that contains copper (II) sulphate. Fehling’s B is a clear liquid that contains potassium sodium tartrate (Rochelle salt) and a strong alkali, most commonly sodium hydroxide. During the test, solutions A and B are each made and stored separately.

Principle

Fehling test for aldehyde and ketone is based on the same idea as Benedict’s test. When aldehydes are introduced to Fehling’s solution, the bistartratocuprate (II) complex quickly oxidises them. Copper (II) ions are reduced to copper (I) ions during this process, resulting in a red copper (I) oxide precipitate (Cu2O). A good result is indicated by the presence of red precipitate.

Fehling’s Solution Test for Aldehydes and Ketones

Fehling Test for Aldehyde and Ketone

Requirements

Reagent

1. Fehling’s solution A : Dissolve 6.92 grams of cupric sulphate in distilled water and make up the volume to 100 mL in a volumetric flask. Store this reagent in a reagent bottle.

2. Fehling’s solution B: Dissolve 34.6 grams of sodium potassium tartrate and 25 grams of potassium hydroxide in distilled water and make up the volume to 100 mL with distilled water and store in a reagent bottle.

3. Fehling’s solution: Mix equal volumes of both the solution just before use.

4. Sample Solution: (5% Glucose, 5% Sucrose, 5% Fructose, 5% Starch, 5% lactose).

Materials

1. Pipettes

2. Test tubes

3. Test tube stand

Equipment

1. Water bath

Procedure

1. Take 1ml of sample in dry test tube. Take 1ml of distilled water in another tube as control. Add 1ml of Fehling’s reagent (A and B) to all the tubes.

2. Keep in boiling water bath for 1-2 minutes. Make observations and record if there is any development of red precipitate.

Results

Fehling's Solution Test for Aldehydes and Ketones

The appearance of a reddish-brown precipitate indicates a positive result and the presence of reducing sugars (glucose, fructose, lactose).

The absence of the reddish precipitate or the appearance of deep blue color indicates a negative result and lack of reducing sugars (sucrose, starch).

Applications

1. Fehling test for aldehyde and ketone is used to differentiate between aldehydes and ketones in carbohydrates because ketone sugars other than alpha-hydroxy-ketone do not react in this test.

2. In medical institutions, fehling test for aldehyde and ketone is used to detect the presence of glucose in urine. Fehling test for aldehyde and ketone aids in determining whether or not the patient has diabetes.

Limitations

1. Only in an alkaline atmosphere does his response occur. The copper (II) ions would be stable and difficult to oxidise in an acidic environment, causing the fehling test for aldehyde and ketone process to fail.

Further Readings

Reference