Which of the following represents Fehling solution B?

The correct answer is Option (1) → Alkaline sodium potassium tartarate

Fehling’s reagent is used to detect aldehydes, especially reducing sugars. It consists of two separate solutions that are mixed just before use:

• Fehling’s solution A
• Fehling’s solution B

Fehling’s Solution A

Fehling’s solution A contains:

Aqueous copper(II) sulphate (CuSO₄)

This provides the Cu²⁺ ions required for the oxidation–reduction reaction.

Fehling’s Solution B

Fehling’s solution B contains:

Alkaline sodium potassium tartrate (Rochelle salt) in sodium hydroxide solution

Functions of this component:

• Provides an alkaline medium
• Forms a complex with Cu²⁺ ions to keep them in solution

Working of Fehling’s test

When Fehling’s solution reacts with an aldehyde, the aldehyde is oxidized to a carboxylate ion, and Cu²⁺ ions are reduced to Cu₂O (brick-red precipitate).

General reaction:

R–CHO + Cu²⁺ → R–COO⁻ + Cu₂O (red precipitate)

Option-wise Explanation

Option 1: Alkaline sodium potassium tartrate

This compound forms Fehling’s solution B, which provides the alkaline tartrate complex. Therefore this option is correct.

Option 2: Aqueous copper sulphate

This represents Fehling’s solution A, not Fehling’s solution B. Hence this option is incorrect.

Option 3: Ammoniacal silver nitrate solution

This is Tollens’ reagent, used for the silver mirror test for aldehydes. Therefore this option is incorrect.

Option 4: Zn – Hg / HCl

This reagent is used in Clemmensen reduction to convert carbonyl groups to hydrocarbons. Hence this option is incorrect.