Fehling’s Solution

Fehling’s Solution is a chemical reagent that is used to check the presence of aldehyde functional groups in organic compounds. It is also used to differentiate between water-soluble carbohydrates (like glucose, fructose, etc.) and ketone(-C=O-) functional groups.

In this article, we will learn more about Fehling’s solution, its formation, its test, and some of its uses.

What is Fehling’s Solution?

In organic chemistry, Fehling’s solution is defined as a chemical reagent that is used to check the presence of aldehyde (-CHO functional group). Fehling’s solution acts as a mild oxidizing agent, it is dark blue in colour and basic in nature.

Learn more about, Oxidation of Aldehydes

It is a chemical mixture made up of two chemicals: Copper Sulphate and Rochelle salt (Potassium Sodium Tartrate solution), which are known as Fehling A and Fehling B solutions respectively. This solution can be used to differentiate between ketone and water-soluble carbohydrates. It is used as a supplement to the Tollens’ Reagent Test.

The most important use of this solution is to carry out Fehling’s test, this test is used to check the presence of reducing sugar (for example, glucose). This solution was named after a German chemist Hermann von Fehling, who developed it in 1849.

Preparation of Fehling’s Solution

The preparation of Fehling’s solution involves the mixture of two chemicals named Fehling A solution and Fehling B solution. These solutions are prepared separately in laboratories and later mixed together in equal amounts to form a solution called Fehling’s solution.

Preparation of Fehling A Solution

Fehling A is made of copper sulphate. When 7g of pentahydrated copper sulphate is dissolved in 100 ml distilled water with the addition of a few drops of dilute sulphuric acid, Fehling A solution is formed. It is a blue-coloured aqueous solution of copper sulphate. The image added below shows structure of copper sulphate.

Structure of Copper Sulphate

Preparation of Fehling B Solution

Fehling B is made of Rochelle salt (potassium sodium tartrate solution). The image for the same is added below,

Structure of potassium sodium tartrate

When 35g of potassium tartrate is dissolved in a 100 ml aqueous solution of sodium hydroxide (which is 12g), Fehling B solution is formed. It is a colorless solution.

Fehling’s solution is always prepared Fresh. These two solutions (Fehling A Solution and Fehling B Solution) are generally stored in glass or rubber bottles, which are mixed together in equal volume to form a fresh Fehling’s solution whenever required. The following reaction is involved in the formation of Fehling’s solution:

CuSO₄ + 2NaOH → Cu(OH)₂ + Na₂SO₄

Fehling’s solution formed is deep blue in colour which is due to the presence of complex copper ions (Cu²⁺) formed due to Fehling A solution and Fehling B solution is used as a chelating agent in the solution.

What is Fehling’s Test?

Felhing’s test is used for the detection of aldehydes in the solution. In this test, the freshly prepared Fehling’s solution is added to the compound containing an aldehyde group. This will result in the formation of a reddish-brown colour precipitate confirming the presence of aldehyde in the compound.

In the case of Ketones and aromatic aldehydes, no reaction occurs during Fehling’s Test. This test is mainly performed in laboratories to distinguish between ketones and aldehydes functional groups.

Fehling’s Test Principle

Fehling’s Test is based on the principle that aldehyde undergoes a redox reaction and gets oxidized to form carboxylic acids. In this test, the compound containing carbonyl groups (aldehyde or ketone) can act as reducing sugars.

Fehling’s Test Procedure

Step 1: In a test tube, mix Fehling A solution and Fehling B solution in equal volumes to obtain a freshly prepared Fehling’s solution.

Step 2: Take another test tube, and add the compound to be tested.

Step 3: Add the freshly prepared Fehling’s solution to the compound under observation.

Step 4: Heat the test tube in a water bath for a few minutes.

Step 5: Observe and record the changes (if any) that occurred in the solution.

Result of Fehling’s Test

If a reddish-brown precipitate is formed, it confirms the presence of aldehyde in the compound. If there is no change/reaction in the solution then ketone is present in the compound.

Reaction of Fehling’s Test

In Fehling’s test, the redox reaction takes place. In this reaction, copper(II) ions dissociate into copper(I) ions and aldehyde oxides to form carboxylic acids.

Reaction of Aldehyde in Fehling’s solution

The reaction of aldehyde in Fehling’s solution is given below,

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

The overall reaction after the addition of sodium tartrate is as follows:

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

Limitations of Fehling’s Test

Some Limitations of Fehling’s Test are,

• Contact with Fehling’s solution can irritate or burn the skin as it is corrosive and toxic in nature.
• This test fails to detect the aromatic aldehydes functional group.
• This test fails to take place in an acidic environment and can take place only in an alkaline environment.

Uses of Fehling’s Reagent

Various uses of Fehling’s Solutions are,

• It is commonly used in laboratories, for the detection of aldehydes and ketones functional groups in a compound.
• It is used to distinguish between reducing and non-reducing sugar present in carbohydrates.
• It is used to determine the amount of sugar content in food and beverages.
• It can be used to detect diabetes by determining the glucose level in blood and urine.
• It can also be used in forensics investigations, to detect the presence of blood stains.

Fehling’s Solution-FAQs

1. What is Difference between Fehling’s and Tollen’s solution?

• Tollen’s solution is a stronger oxidizing solution and can oxidize both aliphatic and aromatic aldehydes.
• Fehling’s solution is moderately strong and can oxidize aliphatic aldehydes only.

However, both shows a negative result for ketones.

2. Why is Fehling’s Solution always Freshly Prepared?

• When Fehling A and Fehling B solutions react, it forms an unstable compound Bistartrato Cuprate (II) complex which decomposes quickly and results into inactivity of Fehling’s solution in the test.
• This is the reason Fehling A and Fehling B solutions are kept separately and only mixed when required.

3. Which Sugar Does not React with Fehling’s solution?

• Sugars, that does not contain a free aldehyde group doesnot react with Fehling’s solution/reagent.
• Sucrose is one such example for the same.

4. Why does Ketones does not reacts with Fehling’s and Tollen’s Reagent?

Ketones are only oxidised in the presence of strong oxidising agents. Fehling’s solution and Tollen’s reagent being mild oxidizing agent fails to oxidize ketones and thus no reaction takes place.

5. Which Compounds gives, Positive Result in Fehling’s Test?

Aldehydic compounds having an alpha hydrogen (For example: Formaldehyde and Acetaldehyde) will always reacts positively with Fehling’s test.

6. What is the Reaction of aldehyde in Fehling’s solution.

The reaction of aldehyde in Fehling’s solution is represented as,

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

7. What is Fehling’s Solution A and B?

• Fehling’s A, is aqueous solution of copper(II) sulfate which is deep blue in color.
• Fehling’s B, is an aqueous potassium sodium tartrate which is colorless in nature.

8. What is Another Name for Fehling’s Solution?

Fehling’s B solution is also called Rochelle Salt (Potassium Sodium Tartrate Tetrahydrate).