Which among the following purification methods is based on the principle of "Solubility" in two different solvents?

Step 1: Understand "Solubility in Two Different Solvents"

This principle exploits the fact that a compound has different solubilities in two immiscible solvents. The compound migrates preferentially into the solvent in which it is more soluble. This is governed by the distribution (partition) coefficient:

$K_D = \frac{\text{Concentration in organic solvent}}{\text{Concentration in aqueous phase}}$

Step 2: Evaluate Each Option

| Method | Underlying Principle | |---|---| | Column Chromatography | Differential adsorption on solid stationary phase | | Sublimation | Vapour pressure (solid → vapour direct conversion) | | Distillation | Boiling point differences between liquid components | | Differential Extraction ✅ | Differential solubility in two immiscible solvents |

Step 3: Mechanism of Differential Extraction

1. Impure compound dissolved in water is placed in a separating funnel
2. An immiscible organic solvent (diethyl ether, CH₂Cl₂, etc.) is added
3. The funnel is shaken — compound migrates toward the solvent where $K_D$ is higher
4. Layers are allowed to separate; the organic layer is drained off
5. Organic solvent is evaporated to recover the purified compound
6. Repeated extractions with small volumes of fresh solvent are more efficient

Step 4: Conclusion

Answer: (d) Differential Extraction

Key Points to Remember:

• Uses two immiscible solvents (water + organic solvent)
• Distribution coefficient $K_D$ determines how effectively the compound is extracted
• Multiple small-volume extractions > one large-volume extraction (efficiency)
• Equipment used: separating funnel