Fehling’s Test: Procedure, Applications, Fehling’s Solution

Fehling’s test is one of the most frequently used tests to determine, estimate and identify the sugars which can be both reducing or non-reducing in nature. The formulation of this test was made by the eminent German chemist Hermann von Fehling. The main utilization of this test is for the differentiation between functional groups of ketone and water-soluble carbohydrates. 

Keyterms: Ketone, Carbohydrates, Copper II Sulphate, Sodium Hydroxide, Alkali, Fehling’s A, Fehling’s B, Aldehyde

---

Fehling’s Solution

[Click Here for Sample Questions]

Fehling’s test is carried out with a solution that is freshly prepared in the laboratories. The solution is initially present in the form of two solutions known as Fehling’s A and Fehling’s B. 

Fehling’s A Solution contains copper (II) sulphate. It is a deep blue liquid in nature.

Fehling’s B solution contains potassium sodium tartrate (Rochelle salt) along with a strong alkali, most commonly sodium hydroxide. It is a clear liquid in nature. These solutions are separately prepared and subsequently stored.

Later on, both Fehling’s A solution and Fehling’s B solution are mixed so that the final Fehling’s solution can be derived. The equal volumes of these solutions need to be taken into consideration while mixing. The solution is deep blue in colour. The deep blue ingredient that is being taken is a tartrate complex of Cu²⁺. The tartrate tetra-anions serve as a chelating agent in the solution.

Fehling’s Test

If the outcome of the test is negative, it results in the formation of Ketone. If the outcome of the test is positive, it results in the formation of Aldehyde. Therefore, Fehling’s test is important to distinguish between aldehydes and ketones. 

Also Read:

---

Fehling’s Test Procedure

[Click Here for Sample Questions]

The process for the conducting of the Fehling’s test is as follows:

• Take a dry test tube and add the sample to it.
• Keep distilled water in another test tube as a control.
• Add the Fehling’s solution in both these test tubes
• The tubes are recommended to be kept or held in water baths
• Observations are to be made and any growth or development of red precipitate needs to be noted

If there is a production or formation of a reddish-brown precipitate, the result is positive. If there is no such kind of transition, the result is negative.

Once the mixture of both the solutions is heated, Fehling’s B solution prevalent in the reagent carries out the task of chelation. If the solution of the test presents a sugar molecule or an aldehyde or chelated compound which is reddish-brown in colour along with Fehling’s reagent. If such reddish-brown precipitate is formed, it indicates that a reduced sugar or an aldehyde group is prevalent. The determination of reduced sugar is the primary aim of performing Fehling’s test. To ensure the efficiency and effectiveness of the test, it is required to prepare new reagents. As far as Fehling’s reagent test is concerned, lactose, fructose and glucose being sugars give positive results. 

---

Reactions of Fehling’s Test

[Click Here for Sample Questions]

The reaction between the copper (II) ions and an aldehyde is presented as:

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

Once there is an addition of tartrate, it is presented as:

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

---

Applications of Fehling’s Test 

[Click Here for Sample Questions]

Fehling’s test can be taken into consideration for various kinds of purposes. 

1. The most popular application of Fehling’s test is to identify and determine whether the carbonyl group concerned is a ketone or an aldehyde. Aldehydes tend to be oxidised and display positive results. Ketones, except the alpha-hydroxy-ketones, do not display any kind of reaction. 
2.  Fehling’s test is also generally used for monosaccharides, commonly known as simple sugars, along with other reducing sugars. In terms of monosaccharides, it displays positive results not only for aldose monosaccharides but also for ketose monosaccharides, since ketoses are transformed into aldoses through the base contained in the reagent. 
3. It is used to determine the presence of glucose in the urine. It is applied to identify diabetes as being prevalent in the person. 
4. Fehling’s test is also applied for the breaking down of starch to convert it into glucose syrup along with maltodextrins. 
5. To determine the Formic acid. The formic acid (HCO₂H) gives a positive result for Fehling’s Test.

---

Things to Remember

• To carry out the Fehling’s Test, the substance is heated in the presence of Fehling’s Solution. 
• Fehling’s Solution is prepared freshly in the laboratory. 
• It is prepared by mixing an equal amount of Fehling’s Solution A and Fehling’s Solution B. 
• If a reddish-brown precipitate is formed then the result is positive. If there is no such kind of changes, the result is negative. 
• Fehling’s test is most commonly used to determine whether the carbonyl group is a ketone or an aldehyde. 

Also Read: