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.

Preparation of Fehling B Solution

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

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.

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.