Notes on Separating Mixtures

Chromatography

• Definition: Analytical technique to separate a mixture into its individual components by differences in partition coefficients between two or more phases.
• History: Mikhail S. Tsvet ($1872-1919$), Russian-Italian botanist; coined chromatography; Chroma = color, graphein = to write.
• Common applications: detect substances in samples; pigments, dyes, amino acids, vitamins, polymers; examples include cocaine in urine, alcohol in blood, PCBs in fish, lead in water.
• Main types:
  • Liquid chromatography: uses liquids; analyzes metal ions and organic compounds in solutions; separates hydrophilic and insoluble molecules.
  • Gas chromatography: analyzes volatile gases; uses a carrier gas (e.g., helium) to move the mixture through a column; applications in forensics, drugs/alcohol, bomb detection.
  • Thin-layer chromatography: uses an absorbent on flat glass plates; simple, rapid purity check for organic compounds; detects residues like pesticides.
  • Paper chromatography: paper as stationary phase; relies on capillary action to separate solutes; used for amino acids, anions, RNA fingerprinting, histamines, antibiotics.

Distillation

• Definition: Method to separate two liquids with different boiling points; vaporize and condense to purify.
• Boiling point concept: $P<em>{ ext{vap}} = P</em>{ ext{atm}}$; higher boiling liquids require more energy.
• Simple distillation:
  • Principle: most volatile component vaporizes at the lowest temperature and is collected as distillate.
  • Setup (essential components): distilling flask, condenser, receiver, heat source, and a thermometer for monitoring boiling points.
• Fractional distillation:
  • Principle: vapors contact a changing liquid in a fractionating column, enabling separation of components with closer boiling points.
• Steam distillation:
  • Used for heat-sensitive components; steam lowers the effective boiling point; Dalton’s law applies: $P<em>{ ext{total}} = \sum</em>i P_i.$ The boiling point of the mixture can be below that of the more volatile component; typically separation can occur below $100^{\circ}C$ at atmospheric pressure.
• Vacuum distillation:
  • Distillation under reduced pressure; prevents decomposition of components; lowers boiling points to safer temperatures.

Evaporation

• Purpose: Separate soluble solids from solutions; recover the solute, not the solvent.
• Examples: sea salt from seawater; solar evaporation uses heat from the Sun.

Filtration

• Purpose: Separate undissolved/insoluble solids from a liquid.
• Key terms:
  • Residue: solid left behind.
  • Filtrate: liquid that passes through the filter.
  • Filter paper: medium for separation.

Other separation techniques

• Sedimentation: settling of sediments by gravity.
• Decantation: pouring off a clear liquid to separate solids.
• Centrifugation: spinning to apply centrifugal force to settle solids; liquid (centrifugate) can be decanted.
• Magnetic separation: removal using magnets for magnetic metals (e.g., iron, nickel, cobalt).
• Manual separation: picking apart larger components by hand.
• Sifting: using a sieve to separate by particle size.
• Sublimation: solid → gas with subsequent deposition on a cool surface (iodine with sand as classic example).
• Separatory funnel: separates immiscible liquids by density; denser liquid forms the bottom layer; used to separate oil and water.

Separation by phase: homogeneous vs heterogeneous

• Homogeneous: Distillation, Evaporation, Chromatography.
• Heterogeneous: Sieving, Filtration, Decantation, Centrifugation, Magnetic separation, Separatory funnel.