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Fehling’s test Principle, Procedure, Result and Uses

The Fehling’s test is one of the most common methods for estimating or detecting reducing and non-reducing sugars. The test devised by German chemist H.C. Von Fehling can also distinguish between ketone functional groups and water-soluble carbohydrates.

Fehling test Solution

Fehling’s Test uses a mixture called Fehling’s Solution. It is made fresh in labs and is a mixture of two other solutions called Fehling’s A and Fehling’s B. Fehling’s A is a blue liquid that contains copper sulfate.

Fehling’s B is a clear liquid that contains a mixture of potassium sodium tartrate (Rochelle salt) and a strong chemical called sodium hydroxide. These two solutions are made separately during the test. When doing the Fehling’s test, Fehling’s A and Fehling’s B solutions are mixed together in equal amounts to make the final Fehling solution. It is deep blue in color.

Fehling’s Test solution

Figure 1: Fehling test solution

Fehling’s Test Principle

Carbohydrates with free or potentially free carbonyl groups (aldehyde or ketone) can act as reducing sugars. Fehling’s solution is a dark blue solution composed of copper sulphate, potassium sodium tartrate, and a strong alkali, typically sodium hydroxide.

When heated, the sample containing the Fehling’s solution, bistartarocuprate (II) complex oxidises the aldoses to the corresponding aldonic acids. During the process, the complex’s copper (II) ions are reduced to an insoluble yellow or red precipitate or cuprous (I) oxide (Cu2O) ions.

In contrast, ketones are oxidised to form shorter acid chains. By creating a bistartarocuprate (II) complex, tartrate ions prevent the development of insoluble Cu(OH)2 as a result of the interaction between copper sulphate and sodium hydroxide in a solution.

This combination releases cupric ions for reduction gradually, preventing the production of cupric oxide. A black cupric oxide precipitate occurs when Fehling’s solution is heated without reducing sugars present.

Fehling test Reaction

2Cu(OH)2 + reducing sugar    →    2Cu2O  +  Aldonic acid

The net reaction between an aldehyde and the copper(II) ions in Fehling’s solution may be written as:

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

or with the tartrate included:

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

Fehling test Materials

  1. Fehling’s solution A, which is made by dissolving 7 g of CuSO47H2O in 100 ml of water.
  2. Fehling’s solution B is made by combining 24 g of KOH and 34.6 g of potassium sodium tartrate in 100 mL of water.
  3. Fehling’s solution is made by combining equal parts of both solutions just before use.
  4. Sample for Testing (5 percent Glucose, 5 percent Sucrose, 5 percent Fructose, 5 percent Starch, 5 percent lactose)
  5. Pipettes, test tubes, a test tube stand, and a water bath are among the additional materials.

Fehling's test Procedure

  1. Get a clean test tube and put 1 ml of the sample in it. The sample concentration should be 5% (w/v).
  2. Get a clean test tube and put 1 ml of distilled water in it. Then, add 2-3 drops of Fehling’s reagent to the test tube with distilled water and mix well. 
  3. Put both test tubes in a water bath for 1-2 minutes. Watch for any color changes in the test tubes and make a note of the color you see.

Fehling's test Result

Positive Result: The presence of reducing sugars is indicated by the appearance of reddish-brown precipitate in test tubes (test samples).

Negative Result: The lack of reddish precipitate or the appearance of a deep blue colour indicates a negative result and a lack of reducing sugars.

Result of Fehling’s Test

Figure 2: Fehling Test Positive and Negative Result

Application of Fehling’s Test

  1. Fehling’s test is used to find out if there are aldehydes and ketones in carbs, but it doesn’t work for certain types of ketones. 
  2. The test can be used to check for glucose in urine, which helps detect diabetes. 
  3. It can also be used to measure the amount of reducing sugar when breaking down starch to make glucose syrup and maltodextrins, this is called Dextrose Equivalent (DE).

Limitations of Fehling’s Test

  1. Aromatic aldehydes cannot be detected using the Fehling test.
  2. This reaction can only occur in an alkaline environment. If the mixture is acidic, the copper (II) ions will be stabilised and will be difficult to oxidise. The reaction would then fail. Alcohols and aldehydes would also produce a positive result because they have a similar reducing group.
  3. Store below 30°C in a tightly sealed container away from direct sunlight.
  4. After opening, the product should be stored in a dry, ventilated area away from extreme temperatures and ignition sources. After use, tightly close the container.

FAQ

FAQs on Fehling’s test

Benzaldehyde is an aromatic aldehyde, which means it has a ring structure with an aldehyde group. Aromatic aldehydes, such as benzaldehyde, typically do not react in the Fehling test.

Ketones do not give a positive result in the Fehling test as it is specifically designed to detect the presence of aliphatic aldehydes.

The Fehling’s test is one of the most common methods for estimating or detecting reducing and non-reducing sugars.

Aromatic aldehydes such as benzaldehyde, typically do not give a positive result in the Fehling test.

Fehling test is a chemical test used to detect the presence of reducing sugars in a sample.

Reducing sugars such as glucose, fructose, and lactose have a free aldehyde or ketone group that can reduce copper ions in Fehling’s reagent, and therefore will give a positive result.