Theory Exercises

Introduction to Separation Techniques

Chromatography Techniques

What are Separation Techniques?

Separation techniques are methods used to separate mixtures into their individual components based on differences in physical or chemical properties. These techniques are fundamental in chemistry and are essential for purifying substances, analyzing compositions, and preparing materials.

Classification of Mixtures

Before choosing a separation technique, we must understand the types of mixtures:

  • Homogeneous mixtures: Components cannot be distinguished visually (solutions)
  • Heterogeneous mixtures: Components can be distinguished visually (suspensions, emulsions)

Properties Used for Separation

Separation techniques exploit differences in physical properties:

  • Particle size: Filtration, sieving
  • Boiling point: Distillation
  • Solubility: Crystallization, extraction
  • Density: Decantation, centrifugation
  • Magnetic properties: Magnetic separation
  • Adsorption affinity: Chromatography

Physical Separation Techniques

Filtration

Filtration separates solids from liquids using a porous barrier that allows the liquid to pass through but retains solid particles.

Types of Filtration:
  • Gravity filtration: Uses gravity to pull liquid through filter
  • Vacuum filtration: Uses suction to speed up the process
  • Hot filtration: Performed at elevated temperature
[Equipment needed:]
  • Filter paper
  • Funnel
  • Beaker or flask
  • Glass rod (for pouring)
[Applications:]
  • Separating precipitates from solutions
  • Removing insoluble impurities
  • Water purification

Decantation

Decantation separates components based on density differences by allowing denser components to settle and carefully pouring off the lighter layer.

When to use:
  • Separating oil and water
  • Removing settled particles from liquid
  • Separating immiscible liquids
[Procedure:]
  1. Allow mixture to settle completely
  2. Tilt container carefully
  3. Pour off the top layer slowly
  4. Stop when approaching the interface

Distillation

Distillation separates components based on differences in boiling points through vaporization and condensation.

Simple Distillation

Used when components have significantly different boiling points (>25°C difference). [Equipment:]

  • Distillation flask
  • Thermometer
  • Condenser
  • Receiving flask
  • Heating mantle
[Fractional Distillation] Used for components with similar boiling points. Uses a fractionating column for better separation. [Applications:]
  • Purifying liquids
  • Separating alcohol from water
  • Petroleum refining
  • Producing distilled water

Evaporation and Crystallization

These techniques separate dissolved solids from solvents by removing the solvent.

Evaporation:
  • Simple heating to remove solvent
  • Used when solid is not damaged by heat
  • Quick but may produce impure crystals
[Crystallization:]
  • Controlled cooling of saturated solution
  • Produces pure, well-formed crystals
  • Can be used for purification

Centrifugation

Centrifugation uses centrifugal force to separate components based on density differences, much faster than gravity settling.

Principle:
  • Spinning creates artificial gravity
  • Denser particles move outward
  • Creates distinct layers
[Applications:]
  • Blood component separation
  • Separating cells from liquid
  • Purifying precipitates

Advanced Separation Techniques

Chromatography

Chromatography separates components based on their different affinities for a stationary phase and a mobile phase.

Paper Chromatography
Principle: Different substances travel at different rates through paper due to varying solubilities. [Equipment needed:]
  • Chromatography paper
  • Solvent (mobile phase)
  • Beaker or chamber
  • Pencil for origin line
[Procedure:]
  1. Draw pencil line near bottom of paper
  2. Place sample spot on origin line
  3. Place paper in solvent (below sample spot)
  4. Cover chamber to prevent evaporation
  5. Allow solvent to rise up paper
  6. Remove when solvent front nears top
  7. Mark solvent front and analyze spots
[Applications:]
  • Separating dyes and pigments
  • Analyzing amino acids
  • Food coloring analysis
  • Drug testing
Thin Layer Chromatography (TLC)

Similar to paper chromatography but uses a thin layer of adsorbent material on a glass or plastic plate.

Column Chromatography

Uses a column packed with stationary phase material. Components separate as they travel down the column at different rates.

Extraction

Extraction transfers a substance from one phase to another based on relative solubilities.

Liquid-Liquid Extraction
  • Uses immiscible solvents
  • Substance dissolves preferentially in one solvent
  • Performed in separatory funnel
[Solid-Liquid Extraction]
  • Extracts substances from solid matrix
  • Examples: brewing tea, making coffee
  • Can use heat or pressure to improve efficiency

Specialized Separation Techniques

Magnetic Separation

Principle:

Exploits magnetic properties to separate ferromagnetic materials from non-magnetic substances. [Equipment:]

  • Strong magnet
  • Non-magnetic container
  • Protective barrier (paper/plastic)
[Procedure:]
  1. Spread mixture on flat surface
  2. Cover magnet with paper
  3. Move magnet over mixture
  4. Magnetic particles are attracted
  5. Remove magnet with attracted material
  6. Remove paper to release magnetic material
[Applications:]
  • Separating iron filings from sand
  • Recycling metal from waste
  • Removing iron contaminants
  • Mining operations

Sublimation

Sublimation separates substances that can transition directly from solid to gas from those that cannot.

Principle:
  • Some substances sublime (solid → gas)
  • Others remain solid
  • Sublimed material can be collected by condensation
[Examples of substances that sublime:]
  • Iodine
  • Dry ice (solid CO₂)
  • Caffeine
  • Naphthalene (mothballs)

Choosing the Right Technique

Decision Matrix

Mixture TypeProperty DifferenceRecommended TechniqueExample
Solid in liquidParticle sizeFiltrationSand in water
Two liquidsBoiling pointDistillationWater and ethanol
Dissolved solidSolubilityEvaporation/CrystallizationSalt in water
Immiscible liquidsDensityDecantationOil and water
Mixed colorsPolarity/SizeChromatographyFood dyes
Metals mixedMagnetic propertyMagnetic separationIron and aluminum

Factors to Consider

  • Purity required: Some techniques give better purification
  • Quantity: Scale affects technique choice
  • Time available: Some methods are faster than others
  • Equipment available: Simple vs. complex apparatus
  • Safety concerns: Heat, solvents, pressure considerations
  • Cost: Solvent and energy costs

Laboratory Procedures and Safety

General Safety Guidelines

  • Always wear appropriate PPE (goggles, lab coat, gloves)
  • Work in well-ventilated areas or fume hoods when using solvents
  • Never heat closed systems
  • Be aware of fire hazards with flammable solvents
  • Properly dispose of waste according to regulations
  • Know location of safety equipment

Common Mistakes to Avoid

  • Using wrong filter paper porosity
  • Adding boiling chips to hot liquid
  • Overfilling distillation flask
  • Not marking solvent front in chromatography
  • Mixing incompatible solvents
  • Heating crystals too rapidly

Practical Examples

Example 1: Separating a mixture of sand, salt, and iron filings
Problem: You have a mixture containing sand (SiO₂), salt (NaCl), and iron filings (Fe). Separate and recover each component. Solution strategy:
  1. Magnetic separation: Use magnet to remove iron filings
  2. Dissolution: Add water to dissolve salt, leaving sand
  3. Filtration: Filter to separate dissolved salt solution from sand
  4. Evaporation: Evaporate water to recover solid salt
  5. Drying: Dry sand to remove residual moisture
Result: Three separate, pure components recovered.
Example 2: Purifying impure salt
Problem: You have salt contaminated with sand and other insoluble impurities. Purification process:
  1. Dissolution: Dissolve mixture in minimum amount of hot water
  2. Hot filtration: Filter while hot to remove insoluble impurities
  3. Cooling: Allow filtrate to cool slowly
  4. Crystallization: Pure salt crystals form as solution cools
  5. Collection: Filter and wash crystals with cold water
  6. Drying: Dry crystals to remove moisture
Result: Pure salt crystals with improved purity.
Example 3: Analyzing food coloring by chromatography
Problem: Determine which dyes are present in a green food coloring. Chromatography procedure:
  1. Preparation: Cut chromatography paper to fit chamber
  2. Origin line: Draw pencil line 2 cm from bottom
  3. Sample application: Place small spot of green dye on line
  4. Solvent setup: Add water/alcohol mixture to chamber
  5. Development: Place paper in solvent, cover chamber
  6. Observation: Watch as dyes separate and travel up paper
  7. Analysis: Mark final positions and calculate Rf values
Expected result: Green dye separates into blue and yellow components.
Example 4: Extracting caffeine from tea
Problem: Extract and isolate caffeine from tea leaves. Extraction process:
  1. Hot water extraction: Brew strong tea to extract caffeine
  2. Basification: Add sodium carbonate to make solution basic
  3. Liquid-liquid extraction: Extract with dichloromethane
  4. Separation: Use separatory funnel to separate layers
  5. Drying: Remove water traces with anhydrous salt
  6. Evaporation: Evaporate solvent to obtain crude caffeine
  7. Sublimation: Purify by sublimation
Result: Pure caffeine crystals isolated from tea.