This lesson explains how a solute distributes itself between two immiscible solvents, how the distribution coefficient is used, and how this principle is applied in extraction processes.

1. Core Concepts (Short Notes)

19.1 Immiscible Solvents

Two liquids that do not mix (e.g., water and ether). A solute added to the system distributes between both solvents.

19.2 Nernst Distribution Law

At constant temperature, when a solute distributes between two immiscible solvents:

K_D = C₁ / C₂

Where:

• C₁: concentration in solvent 1

• C₂: concentration in solvent 2

• K_D: distribution coefficient

19.3 Conditions for Validity

• Solute must remain in the same molecular state in both solvents.

• Temperature must remain constant.

• Solvents must be immiscible.

• System must reach equilibrium.

19.4 Partition Coefficient (P)

Used commonly in biochemistry: P = [solute]₍organic₎ / [solute]₍aqueous₎

Indicates hydrophobic (high P) or hydrophilic (low P) nature.

19.5 Multiple Extractions

Several small extractions are more efficient than one large extraction. Example: 2 × 50 cm³ extractions > 1 × 100 cm³ extraction.

19.6 Association or Dissociation Effects

If solute associates (dimers) or dissociates (ions), modified distribution equations must be used.

2. Key Formulas to Memorize

Distribution Coefficient

Amount Remaining After One Extraction

W₁ = W₀ × (Vₐ / (Vₐ + K_D × Vₒ))

Where:

• W₀ = initial amount

• Vₐ = volume of aqueous layer

• Vₒ = volume of organic layer

3. Tips & Tricks for Exams

• If K_D > 1 → solute prefers the organic solvent (depending on definition).

• Always use identical units for concentration.

• Multiple extractions drastically increase recovery.

• Check assumptions before applying Nernst Law.

• Mass balance is crucial in all calculations.

4. Important Points to Remember

• Distribution law is essential in solvent extraction techniques.

• Efficiency depends on both the value of K_D and the chosen volumes.

• Deviations occur when solute changes form in one solvent.

Partition coefficient is important in drug design and environmental chemistry.