12.3 Chromatography

2026 Syllabus Objectives

Core:

  1. Describe how paper chromatography is used to separate mixtures of soluble coloured substances, using a suitable solvent
  2. Interpret simple chromatograms to identify:
    • (a) unknown substances by comparison with known substances
    • (b) pure and impure substances

Supplement: 3. Describe how paper chromatography is used to separate mixtures of soluble colourless substances, using a suitable solvent and a locating agent. Knowledge of specific locating agents is not required 4. State and use the equation for RfR_f: Rf=distance travelled by substancedistance travelled by solventR_f = \frac{\text{distance travelled by substance}}{\text{distance travelled by solvent}}

What is Chromatography? 🔬

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Chromatography is a separation technique used to separate two or more dissolved solids in solution. While there are several types of chromatography, they all follow the same basic principles.

Paper chromatography was originally developed as a method for separating soluble pigments (coloured substances such as dyes and inks) using filter paper.

Principles of Paper Chromatography

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Key Factors for Separation ⚡

The coloured substances separate based on two important factors:

  1. Different solubilities in the solvent - substances with higher solubility move faster
  2. Different degrees of attraction for the filter paper - substances with less attraction to the paper move faster

These two factors determine how fast the pigments move across the filter paper.

Key Terms 🔑

  • Paper chromatography: A simple type of chromatography used to separate the components of soluble substances based on their rate of migration in a solvent (mobile phase) on sheets of filter paper (stationary phase)

  • Solvent front: The moving boundary of the liquid solvent that moves up the paper during chromatography

  • Chromatogram: The result of a paper chromatography run, showing where the spots of the samples have moved to

  • Baseline (origin): The pencil line drawn near the bottom of the chromatography paper where the sample is placed

Procedure for Paper Chromatography

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Step-by-Step Method 📌

Stage 1: Preparing the paper

  • Draw a baseline (origin) using a pencil near the bottom edge of a strip of chromatography paper
  • A pencil is used because it does not dissolve and interfere with the separation
  • Place a drop of concentrated solution on the baseline using a capillary tube or dropping pipette
  • Allow the spot to dry

Stage 2: Setting up the chromatography

  • Dip the paper in the solvent
  • Critical: The level of the solvent must start below the sample on the baseline
  • The solvent moves up the paper by capillary action

Stage 3: Separation occurs

  • As the solvent moves up the paper, the substances are carried with it and begin to separate
  • The substance that is most soluble moves fastest up the paper
  • An insoluble substance would remain at the baseline

Stage 4: Stopping the run

  • The chromatography run is stopped just before the solvent front reaches the top of the paper
  • Mark the position of the solvent front with a pencil

Stage 5: Drying

  • The final product can be dried using a hairdryer and is known as a chromatogram

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