Paper Chromatography
Summary: Paper chromatography separates small amounts of coloured solutes (dyes, inks) based on their different solubilities in a solvent. Rf values identify substances. A pure substance gives a single spot. Tags: igcse chemistry chromatography practical separation Created: 2026-07-14 Last Updated: 2026-07-14
What Is Paper Chromatography?
A technique used to separate and identify small amounts of solutes dissolved in a solvent. The solutes move at different rates up the paper depending on their relative attraction to the stationary phase vs the mobile phase.
Key Terminology
| Term | Definition |
|---|---|
| Stationary phase | The chromatography paper (specifically, the water adsorbed onto the paper’s cellulose fibres) |
| Mobile phase | The solvent that moves up the paper (e.g., water, ethanol, propanone) |
| Solvent front | The furthest distance reached by the solvent |
| Chromatogram | The paper after the chromatographic separation is complete (shows spots/bands) |
| Rf value | Retention factor — a ratio comparing how far a substance travels relative to the solvent |
Method (Step by Step)
- Draw a pencil line about 2 cm from the bottom of the chromatography paper — this is the baseline/origin
- Use pencil, NOT pen — pencil graphite is insoluble and won’t dissolve in the solvent
- Apply small spots of each sample on the baseline using a capillary tube; label each spot in pencil
- Place the paper in a beaker/chromatography tank with a small amount of solvent — the solvent level must be below the baseline
- Cover the beaker (lid/watch glass) to prevent solvent evaporation
- Allow the solvent to rise up the paper by capillary action (this is development)
- When the solvent front is about 1-2 cm from the top, remove the paper
- Immediately mark the solvent front with a pencil line
- Allow the paper to dry
- Circle each spot in pencil and measure distances
Why Solvent Level Must Be Below the Baseline
If the solvent level touches or covers the spots, the samples will dissolve into the solvent reservoir instead of moving up the paper. The spots would be washed off, and no separation would occur.
Why Pencil, Not Pen?
Pen ink is a mixture of dyes that would also dissolve and separate — it would contaminate the chromatogram and give false spots. Pencil graphite is insoluble in all common chromatography solvents.
Rf Value (Retention Factor)
Formula
Rf = distance moved by substance / distance moved by solvent front
- Rf is a ratio — no units
- Always between 0 and 1 (substance can’t travel further than the solvent)
- Rf is characteristic for a given substance in a given solvent at a given temperature
Worked Example
A chromatogram shows:
- Solvent front: 12.0 cm from baseline
- Blue spot: 7.2 cm from baseline
- Red spot: 4.8 cm from baseline
Rf (blue) = 7.2 / 12.0 = 0.60 Rf (red) = 4.8 / 12.0 = 0.40
Factors Affecting Rf
| Factor | Effect on Rf |
|---|---|
| Solvent | Different solvents → different Rf for same substance |
| Temperature | Affects solubility → slight Rf variation |
| Paper type | Different porosity/structure |
| Concentration | Generally does NOT affect Rf (but very high concentration can cause tailing) |
Interpreting Chromatograms
Purity
- Single spot → substance is pure
- Multiple spots → substance is a mixture
- A pure substance has a sharp, well-defined melting/boiling point
Identification
- Compare the Rf of an unknown spot with the Rf of a known reference run on the same chromatogram
- If Rf values match, the substances are likely the same
- Run multiple known substances alongside the unknown for comparison
Example: Food Colouring Analysis
A brown food colouring is spotted alongside known reference dyes:
- Brown spot separates into yellow, red, and blue spots
- Yellow matches Rf of tartrazine
- Red matches Rf of carmoisine
- Blue matches Rf of brilliant blue
- Conclusion: Brown colouring is a mixture of three dyes
Why Do Substances Separate?
Two factors compete:
| Factor | Description | Effect |
|---|---|---|
| Solubility in mobile phase (solvent) | How well the substance dissolves in the moving solvent | Higher solubility → travels further (higher Rf) |
| Attraction to stationary phase (paper) | How strongly the substance adsorbs to the water/paper | Stronger attraction → travels less (lower Rf) |
A substance partitions itself between the two phases — it spends some time dissolved in the mobile phase (moving up) and some time adsorbed to the stationary phase (staying still). The ratio determines how far it travels.
Applications of Chromatography
| Application | Description |
|---|---|
| Food testing | Identify permitted/non-permitted food colourings |
| Forensic science | Analyse ink from documents, drugs testing |
| Pharmaceuticals | Check purity of drugs/medicines |
| Environmental monitoring | Detect pollutants in water/soil |
| Amino acid analysis | Separate amino acid mixtures |
| Photosynthesis research | Separate photosynthetic pigments (chlorophyll a, b, carotenoids) |
Locating Agents
Some substances (amino acids, sugars) are colourless after chromatography. A locating agent is used to make them visible:
- Ninhydrin spray — reacts with amino acids to give purple/brown spots
- UV light — some colourless substances fluoresce under UV
- Iodine vapour — absorbed by many organic compounds to give brown spots
Comparison with Other Separation Techniques
| Technique | Separates | Based On |
|---|---|---|
| Paper chromatography | Solutes in a solution (dyes, inks, amino acids) | Solubility in mobile phase vs adsorption to paper |
| Fractional distillation | Miscible liquids | Difference in boiling points |
| Crystallisation | Soluble solid from solution | Solubility change with temperature |
| Filtration | Insoluble solid from liquid | Particle size |
Common Question Types
Type 1: Calculate Rf (1-2 marks)
- Measure (or read given) distance of substance and solvent front
- Rf = d_substance / d_solvent
- Give to 2 decimal places, no units
Type 2: Purity Analysis (1-2 marks)
- “Is substance X pure? Explain.”
- Answer: No — it gave more than one spot / Yes — only one spot visible
- Alternatively: compare chromatogram spots of pure vs impure sample
Type 3: Method Description (3-4 marks)
- Frequency: ~30% of Papers 5/6
- Draw baseline in pencil → apply spots → solvent below baseline → develop → mark solvent front → dry → calculate Rf
Type 4: Explain Why… (1-2 marks)
- “Why is pencil used, not pen?”
- “Why must the solvent level be below the baseline?”
- “Why does substance A travel further than substance B?”
Common Mistakes
- Using pen to draw baseline — ink dissolves and runs
- Solvent covering the spots — samples wash off into solvent
- Spotting too much sample — spots smear/tail, making Rf measurement inaccurate
- Not marking solvent front immediately — solvent evaporates and you lose the reference distance
- Forgetting that Rf has no units — it’s a ratio
- Saying Rf > 1 is possible — substance cannot travel further than the solvent
- Not stating that Rf depends on solvent/temperature when comparing literature values
Key Facts to Memorize
- Baseline in PENCIL (NOT pen)
- Solvent level BELOW the baseline (spots must stay dry before development)
- Rf = d_substance / d_solvent (between 0 and 1, no units)
- Pure substance = single spot; mixture = multiple spots
- Identical Rf in the same solvent = same substance
- Stationary phase = paper (water on cellulose); mobile phase = solvent
- Higher solubility in mobile phase → travels further (higher Rf)
Related Notes
- Separation Techniques — Overview of all separation methods
- Experimental Techniques — All practical techniques
- Qualitative Analysis — Chemical tests for substances
- IGCSE-Chem-Index
Past Paper Sources
- 0620/62 May/June 2018 Q1: Chromatogram analysis, Rf calculation (4 marks)
- 0620/52 Oct/Nov 2019 Q2: Describe chromatography experiment for food colourings (5 marks)
- 0971/62 May/June 2022 Q1(b): Why pencil used for baseline? Solvent must be below spots? (3 marks)
- 0620/63 Oct/Nov 2021 Q3: Rf calculation and purity determination (3 marks)
IGCSE Chemistry (0620/0971) wiki. Common practical topic in Papers 5/6.