Overview
This lecture introduces paper chromatography, explains its process, discusses the theory behind it (mobile and stationary phases), and demonstrates how to calculate Rf values.
Introduction to Paper Chromatography
- Chromatography is a technique used to separate and identify substances in a mixture.
- Paper chromatography separates dyes in an ink mixture to analyze individual components.
Paper Chromatography Procedure
- Draw a pencil baseline near the bottom of filter paper.
- Apply a sample of ink to the baseline.
- Place the paper in a beaker with a shallow solvent (water or ethanol), ensuring the baseline stays above the solvent.
- Optionally, cover the beaker to prevent solvent evaporation.
- As the solvent moves up the paper, it carries the dye components, separating them.
- Dyes that are insoluble remain on the baseline.
- Once the solvent nears the top, remove and dry the paper; the resulting pattern of spots is called a chromatogram.
Theory: Mobile and Stationary Phases
- The mobile phase is the liquid solvent where molecules can move (in this case, water/ethanol).
- The stationary phase is the solid material where molecules cannot move (the paper).
- Substances alternate between dissolving in the solvent (moving) and attaching to the paper (stopping).
- Molecules more soluble in the solvent travel faster and farther up the paper.
- Molecules more attracted to the paper (less soluble) travel slower and stay lower on the paper.
Rf Values and Identification
- Rf (retention factor) = distance travelled by the substance ÷ distance travelled by the solvent.
- Example: If a dye travels 6 cm and the solvent travels 10 cm, Rf = 0.6.
- Pure substances produce a single spot; mixtures produce multiple spots.
- Rf values depend on the solvent and type of paper used.
- Chemists must match Rf values using the same solvent and paper type to identify substances accurately.
Key Terms & Definitions
- Chromatography — Analytical technique for separating and identifying mixture components.
- Paper Chromatography — Type of chromatography using filter paper to separate dyes.
- Mobile Phase — The solvent that moves through the paper (water or ethanol).
- Stationary Phase — The solid phase that doesn't move (the filter paper).
- Chromatogram — The final pattern of separated spots on the paper.
- Rf Value — The ratio of distance travelled by substance to distance travelled by solvent.
Action Items / Next Steps
- Practice drawing and labeling a paper chromatography setup.
- Calculate Rf values using sample measurements.
- Review how changing solvents or paper types affects chromatogram results.