Separation & Purification Techniques: Paper Chromatography (Rf Values, Interpretation) (Copy)
Separation & Purification Techniques
Paper Chromatography (Rf Values, Interpretation)
Purpose of Paper Chromatography in ATP
- Paper chromatography is used to:
- Separate mixtures of soluble substances
- Identify components qualitatively
- ATP questions test:
- Understanding of Rf values
- Interpretation of chromatograms
- Correct calculation
- Correct description of trends or separations
Core Examiner Rule
- Chromatogram answers must include:
- Origin and solvent front labelled
- Correct calculation of Rf value
- Correct trend interpretation
- Observation descriptions
Step 1: Setting Up Paper Chromatography
- Draw pencil baseline on chromatography paper
- Spot mixture at baseline using capillary tube
- Place paper in solvent without touching baseline
- Allow solvent to rise (develop chromatogram)
- Remove paper before solvent reaches top
- Mark solvent front immediately
Step 2: Observing Spots
- Spots may be:
- Visible naturally (coloured compounds)
- Developed using UV light or chemical spray
- Measure:
- Distance from baseline to centre of spot
- Distance from baseline to solvent front
Step 3: Calculating Rf Values
- Formula:
Rf = Distance travelled by substance ÷ Distance travelled by solvent
- Example:
- Distance spot moved = 4.5 cm
- Solvent front moved = 6.0 cm
- Rf = 4.5 ÷ 6.0 = 0.75
Examiner Notes on Rf
- Rf values are unitless
- Must always be between 0 and 1
- Round to 2 decimal places if necessary
- Used to identify substances by comparing known Rf values
Written and Compiled By Sir Hunain Zia, World Record Holder With 154 Total A Grades, 7 Distinctions and 11 World Records For Educate A Change O Level And IGCSE Chemistry Full Scale Course
Step 4: Interpreting Chromatograms
- Different compounds separate based on:
- Polarity
- Solubility in solvent
- Closer to solvent front = more soluble
- Closer to baseline = less soluble
Step 5: Comparing Unknown with Known Substances
- Spot positions or Rf values compared to standards
- If Rf matches known value → likely same compound
- Differences in Rf → different substance
Example Table of Observed Rf Values
| Substance | Distance travelled (cm) | Solvent front (cm) | Rf value |
|---|---|---|---|
| A | 3.2 | 5.0 | 0.64 |
| B | 2.5 | 5.0 | 0.50 |
| C | 4.0 | 5.0 | 0.80 |
Step 6: Labelling Chromatogram
- Baseline marked in pencil
- Solvent front marked immediately after removal
- Spot locations labelled clearly (A, B, C)
Common Examiner Traps
- Using pen for baseline → ink runs
- Forgetting to mark solvent front
- Miscalculating Rf (spot distance ÷ solvent front)
- Not using centre of spot for measurement
- Giving units for Rf (Rf is unitless)
- Mixing up solubility order
High-Yield ATP Tips
- Always measure centre of spot
- Measure solvent front immediately
- Calculate Rf to 2 decimal places
- Compare with known standards for identification
- Describe trend: closer to solvent front → more soluble
- Use pencil baseline only
Core Scientific Principle
- Rf values quantify relative movement of substances on stationary phase
- Paper chromatography exploits:
- Solubility in solvent
- Polarity differences
- Correct Rf calculation and interpretation:
- Guarantees high ATP marks
- Allows identification of unknown compounds
Written and Compiled By Sir Hunain Zia, World Record Holder With 154 Total A Grades, 7 Distinctions and 11 World Records For Educate A Change O Level And IGCSE Chemistry Full Scale Course