Paper Chromatography

Introduction

• Objective: Learn how paper chromatography can identify substances in a mixture.
• Previously studied separation techniques: filtration, crystallization, simple and fractional distillation.

What is Paper Chromatography?

• A technique that separates substances based on different solubilities.
• Used to determine if a pen contains a single color or a mixture of colors.

How Paper Chromatography Works

1. Preparation:
   • Use chromatography paper and draw a pencil line near the bottom.
   • Apply a dot of color from each pen on the line.
2. Solvent Application:
   • Place the bottom of the paper in a solvent.
   • Solvent travels up the paper, dissolving the ink, carrying it upwards.

Important Concepts

• Stationary Phase: The chromatography paper (does not move).
• Mobile Phase: The solvent (moves up the paper).

Identifying Pure vs. Mixed Compounds

• Pure Compound: Produces a single spot in all solvents.
• Mixture: May produce different spots depending on the solvent.
• Example:
  • Red ink forms a single spot (pure).
  • Color B separates into two spots (mixture).

Why Use Pencil for the Baseline?

• Pencil is used because it won't move with the solvent, unlike pen.

Using Paper Chromatography to Identify Unknown Substances

1. Experiment Setup:
   • Place a dot of the unknown chemical on the pencil line.
   • Place the paper into a solvent.
2. Observation:
   • If a single spot appears, the substance is pure.
3. Measurement:
   • Measure the distance from the pencil line to the center of the spot (e.g., 44 mm).
   • Measure the distance moved by the solvent (e.g., 75 mm).

Calculating RF Value

• Formula: RF = (Distance moved by substance) / (Distance moved by solvent).
• Example Calculation: 44 mm / 75 mm = 0.57.
• Note: RF values have no units.

Identifying Substances

• Look up RF value in a database to identify the substance.
• If multiple substances have the same RF value, repeat with different solvents.
• If the substance is not in the database, perform further analysis.

Conclusion

• Paper chromatography is a powerful tool for separating and identifying substances in mixtures.
• For further practice, refer to revision workbooks available online.