Paper Chromatography: Identifying Substances

Introduction

Objective: Describe how paper chromatography is used to identify substances.
Context: Previously covered in atomic structure and periodic table.
Application: Determine colors in food coloring.

Sample: Food coloring (unknown labeled as U).
Known Colorings: Labeled A to D.
Tools: Ruler, pencil, chromatography paper, capillary tube, beaker, water (solvent), glass rod, tape.

Prepare Chromatography Paper
- Draw a horizontal pencil line 2 cm from the bottom.
- Mark five pencil spots evenly across the line, leaving 1 cm clear on each side.
Apply Samples
- Use a capillary tube to place small spots of known and unknown colors on the pencil spots.
Set Up Chromatography
- Pour water into a beaker to a 1 cm depth.
- Attach paper to a glass rod with tape and lower it into the beaker, ensuring:
  - Pencil line is above water surface.
  - Paper sides do not touch the beaker walls.
- Place a lid on the beaker to minimize solvent evaporation.
Develop Chromatogram
- Allow water to move up the paper, carrying the colors.
- Remove paper when water reaches 3/4 of the way up.
- Mark the final water level with a pencil and hang to dry.

Observations: Unknown color separates into three spots, matching colors A, C, and D.
Conclusion: Unknown is a mixture of colors A, C, and D.

Measure Distances
- From pencil line to center of each spot.
- From pencil line to water front.
- Example: Color A - Spot distance = 14 mm, Water distance = 70 mm.
Calculate RF Value
- Formula: RF = (Distance moved by chemical) / (Distance moved by solvent)
- Example Calculation: RF = 14 mm / 70 mm = 0.2
- Note: RF values have no units.
Identifying Chemicals
- Look up RF in a database to identify the chemical.
- Multiple chemicals may share RF values; use different solvents if needed.
- If RF is unknown, further analysis is required.

Practice: Vision Workbook with practical questions, available via provided link.