Liquid-Liquid Extraction: Theory and Concepts

Overview

• Utilizes differences in solubility of compounds in immiscible solvents.
• Common, simple, and effective for separating complex mixtures.

Kitchen Example

• Scenario: Sugar mixed with vegetable oil.
• Observation: Sugar particles too small to remove; oil tastes sweet.
• Solution: Use water (more sugar-soluble) to extract sugar from oil.

Key Principles

• Immiscible Solvents: Water and oil are immiscible, allowing separation based on solubility differences.
• Agitation: Increases contact area between phases, aiding in solute transfer.
• Outcome: Sugary water and non-sweet oil after extraction.

Extraction Theory

• Solute Transfer: Solute moves from one liquid phase to another based on solubility.
• Distribution Coefficient (KD): Ratio of concentrations of solute in each solvent; constant at a given temperature.
• Partition Coefficient: Describes solute distribution between two immiscible solvents.

Solvent Selection

• Common Solvents: Water (aqueous phase) and an organic solvent.
• Immiscibility Requirement: Solvents must not dissolve in each other.
• KD Values: Typically greater than 1 for organic solutes in organic solvents.

Efficiency of Extraction

• Volume and Number of Extractions: More efficient to perform multiple small volume extractions.
• Example: Two extractions with half volume more efficient than one with full volume.
• Practical Application: Three extractions with a third of the volume often optimal.

Acid-Base Extraction

• Chemically Active Extraction: Use acidity/basicity to alter solubility.
• Organic Acids and Bases: Can be converted to water-soluble forms.
  • Acids: Carboxylic acids and phenols (proton removal with bases).
  • Bases: Amines (protonation with acids).

Separation Strategies

• Selective Deprotonation: Use weaker bases for selective separation.
• Sequential Extraction: Separate mixtures by altering solubility through acid-base reactions.

Practical Aspects

• Separatory Funnel: Essential lab equipment for performing liquid-liquid extractions.
• Shaking and Venting: Crucial for mixing and pressure management.
• Layer Separation and Emulsions: Allow distinct separation before draining.

Practical Tips

• Check Equipment: Ensure separatory funnel components are in good condition.
• Careful Shaking: Always support stopper; vent frequently.
• Efficient Extraction: Multiple extractions yield better separation.

Conclusion

• Solvent Efficiency: Smaller, repeated extractions more effective.
• Acid-Base Knowledge: Essential for complex mixture separations.
• Lab Practice: Proper equipment use and technique crucial for successful extraction.