Question

Define the following concepts and provide an exaple for each: solid-liquid extraction, liquiid-liquid extraction, partition coefficient, miscible solvents, immiscible solvents.

Answer 1

Solid-liquid extraction:

Extraction uses the property of solubility to transfer a solute from one phase to another phase. In order to perform an extraction, the solute must have a higher solubility in the second phase than in the original phase. When the sample is a solid and the analytical phase is a liquid, the extraction is said to be solid-liquid extraction. In solid-liquid extraction, the solute is dispersed in a solid matrix. The solute is extracted from the solid phase to the solvent, and the solid phase is then removed by filtration.

Example: The extraction of organo-chlorine residue from soil. Solid-liquid extraction entails combination of the sample with n-hexanes followed by ultrasonic agitation, filtration, removal of residual water by drying over CaCl₂, and pre-concentration under flowing nitrogen.

Liquid-liquid extraction:

In liquid-liquid extraction, a solute is separated between two liquid phases, typically an aqueous and an organic phase. The extraction separates components based on their relative solubilities in two immiscible liquids. In the simplest case, three components are involved: the solute, the carrier liquid, and the solvent. The initial mixture, containing the solute dissolved in the carrier liquid, is mixed with the solvent. Upon mixing, the solute is transferred from the carrier liquid to the solvent. The solvent that is enriched in solute is called extract. The denser solution settles to the bottom. The location of the solute will depend on the properties of both liquids and the solute. The extraction can be done using separating funnel. Liquid – liquid extraction is also known as partitioning.

Example: Extraction of 1-30% acetic acid from aqeous solution using ethyl acetate or Isopropyl acetate.

An ideal solvent for liquid-liquid extraction will typically have the following properties:

• high solubility for the solute and low solubility for the carrier liquid
• density difference vs. the carrier liquid greater than 150 kg/m³
• mid-level interfacial tension (5–30 dyne/cm)
• high resistance to thermal degradation
• nonreactive with other chemicals involved in the extraction process
• high boiling point (for ease of material handling)
• low viscosity (for ease of handling)
• nontoxic, nonflammable, and not corrosive to process equipment (for ease of handling)

Partition-coefficient or distribution-coefficient is the ratio of concentrations of a compound in a mixture of two immiscible phases at equilibrium.

P = Concentration of Iodine in CCl₄/Concentration of Iodine in water

The octanol-water partition coefficient is given as

K_OW = C_octanol/C_water

Miscibility is the property of substances to mix in all proportions (dissolve in each other at any concentration), forming a homogeneous solution.

Water and ethanol are miscible solvents because they mix in all proportions.

Benzene and water form immiscible pair because they don’t form homogenous solution at any proportion.