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Unit 1: Solvent based Separation & Purification Techniques

Solvent Extraction: Principles

Solvent extraction, or liquid-liquid extraction, is a technique used to separate a solute from a liquid (Phase 1) by transferring it into a second, immiscible liquid (Phase 2).

Principle

The underlying principle is the Nernst Distribution Law (or Partition Law). It states that at equilibrium, a solute will distribute itself between two immiscible solvents in such a way that the ratio of its concentrations in the two solvents is constant (at a constant temperature).

Partition Coefficient (KD): KD = (Corg) / (Caq)
Where Corg = Concentration of solute in the organic solvent, Caq = Concentration of solute in the aqueous solvent.

A high KD (e.g., KD > 10) means the solute is much more soluble in the organic solvent, and extraction will be effective.

Classification

Solvent extraction can be classified based on the nature of the extraction:

  • Batch Extraction: The simplest method, where the two solvents are mixed in a separatory funnel and then separated.
  • Continuous Extraction: Used when the solute's KD is not very high. The extracting solvent is continuously recycled (by boiling and condensing) and passed through the solution to be extracted (e.g., in a Soxhlet apparatus).
  • Counter Current Extraction: A highly efficient industrial method where the two solvents flow in opposite directions, maximizing the concentration gradient and separation.

    • Efficiency of the Technique

    The efficiency of extraction is measured by the amount of solute remaining in the original (aqueous) phase after one or more extractions.

    • The formula for the amount remaining, Wn, after *n* extractions is:
      Wn = W ( (Vaq) / (KD Vorg + Vaq) )n
      Where:
      • W = Initial amount of solute
      • Wn = Amount of solute remaining in aqueous phase after *n* extractions
      • Vaq = Volume of the aqueous phase
      • Vorg = Volume of the organic solvent used *in each extraction*
      • n = Number of extractions
    Key takeaway: It is mathematically more efficient to perform multiple extractions with small portions of solvent (e.g., 3 extractions with 20mL each) than one extraction with a large portion (e.g., 1 extraction with 60mL).

    Solvent Extraction: Mechanisms and Techniques

    Mechanism of Extraction

    The solute must be transferred from the aqueous phase to the organic phase. For this to happen, the solute must be electrically neutral and more "like" the organic solvent (like dissolves like).

  • Extraction by Solvation:
    • What: Used for neutral molecules or ion pairs.
    • How: The neutral solute molecule is solvated by (forms weak bonds with) the organic solvent molecules, which "pulls" it into the organic layer. This is common for extracting organic compounds (like caffeine) from water into a solvent like dichloromethane.
  • Extraction by Chelation:
    • What: Used to extract metal ions from an aqueous solution.
    • How: Metal ions are positive (Mn+) and water-soluble (hydrophilic), so they won't enter the organic phase. A chelating agent (an organic ligand, HL) is added.
    • Step 1: The chelating agent reacts with the metal ion to form a stable, neutral metal chelate (MLn).
    • Step 2: This new MLn complex is neutral, and its exterior is "organic-like" (hydrophobic).
    • Step 3: The neutral, hydrophobic metal chelate is now readily extracted into the organic solvent.

    • Technique of Extraction

    • Batch Extraction: Done in a separatory funnel. The two immiscible liquids are added, the funnel is stoppered, shaken (to maximize surface area and reach equilibrium), and vented. The layers are then allowed to separate, and the denser layer is drained from the bottom.
    • Continuous Extraction: Done in specialized glassware (e.g., Soxhlet extractor). The extracting solvent is heated, its vapor travels to a condenser, and the liquid condensate drips onto the sample (often a solid), leaching the solute. The solvent (now with solute) siphons back into the boiling flask. This repeats, concentrating the solute in the flask.
    • Counter Current Extraction: An industrial-scale process where the solute-containing liquid and the extracting solvent move in opposite directions to maximize extraction efficiency.

    Purification by Sublimation and Crystallization

    Sublimation

  • Principle: The process where a solid transitions directly into a gas upon heating, without passing through the liquid phase. The gas then re-solidifies (deposits) upon cooling.
  • Requirement: This technique only works for substances that have a high vapor pressure at their melting point. It is excellent for separating a volatile solid from a non-volatile impurity.
  • Technique: The impure solid is gently heated (e.g., in an evaporating dish with a watch glass on top). The pure solid sublimes, and the gas deposits as pure crystals on the cold watch glass.
  • Examples: Phthalic acid, Camphor, Naphthalene, Iodine.

    • Crystallization

  • Principle: Based on the difference in solubility of a solid in a given solvent at different temperatures. Most solids are much more soluble in a hot solvent than in a cold one. Impurities are often present in smaller amounts and have different solubility.
  • Technique (Steps):
    1. Choose a Solvent: The ideal solvent dissolves the solid *poorly* when cold but *very well* when hot.
    2. Dissolution: Dissolve the impure solid in the *minimum* amount of *boiling* solvent to create a saturated solution.
    3. Hot Filtration (if needed): If there are insoluble impurities, filter the hot solution quickly to remove them.
    4. Cooling: Allow the hot, clear solution to cool slowly and undisturbed. As it cools, the solution becomes supersaturated, and the pure solid crystallizes out.
    5. Isolation: The impurities remain dissolved in the cold solvent (the "mother liquor"). The pure crystals are collected by filtration (e.g., Büchner funnel) and washed with a small amount of cold solvent.
    6. Drying: The crystals are dried in an oven or desiccator.
  • Example: Purification of benzoic acid from hot water.

    • Purification by Distillation

    Distillation is a process for separating components of a liquid mixture based on differences in their boiling points (volatility).

    Distillation (Simple Distillation)

    • Principle: Used to separate a liquid from a non-volatile solid impurity (e.g., salt water) or from another liquid with a large difference in boiling points (e.g., > 100°C).
    • Technique: The mixture is heated in a flask. The more volatile liquid boils, its vapor travels to a condenser, where it is cooled by water and turns back into a pure liquid (the distillate), which is collected.

    Fractional Distillation

    • Principle: Used to separate two or more miscible liquids with close boiling points (e.g., < 25-30°C difference).
    • Technique: A fractionating column (packed with glass beads, rings, or structured packing) is placed between the boiling flask and the condenser.
    • How it works: The column provides a large surface area for repeated vaporization-condensation cycles. With each cycle, the vapor becomes progressively more enriched in the *more volatile* (lower-boiling) component. By the time the vapor reaches the top, it is nearly pure, and it condenses and is collected.

    Vacuum Distillation

    • Principle: Used to purify liquids that have very high boiling points or that decompose at their normal boiling point.
    • Technique: The apparatus is connected to a vacuum pump, which reduces the pressure inside. A liquid boils when its vapor pressure equals the *external* pressure. By lowering the external pressure, the liquid can be made to boil at a much *lower, safer* temperature.

    Steam Distillation

  • Principle: Used to purify compounds that are immiscible with water and have a high boiling point (but are volatile in steam).
  • Technique: Steam is bubbled *directly* into the impure mixture. The mixture boils when the sum of the partial pressures of water and the organic compound equals the atmospheric pressure (Patm = Pwater + Porg).
  • Result: The mixture boils at a temperature *below* the boiling point of water (100°C), allowing the compound to co-distill with the steam, preventing its decomposition.
  • Example: Purification of aniline.
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