A conservation scientist wants to identify and separate the pigments present in a sample of ancient ink recovered from a historical manuscript. The scientist decides to use paper chromatography to analyse the sample. Explain how paper chromatography works and how it can be used to identify the pigments in the ink sample.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
The ancient ink sample is dissolved in a suitable solvent. A spot of the ink solution is placed at the bottom of a sheet of chromatography paper. A reference mixture containing three known pigments is also spotted onto the same paper. The paper is then placed in a beaker containing a small volume of solvent.
Model answer (5 marks)
Paper chromatography separates components by capillary action: the solvent rises up the paper.
The ink solution is applied as a spot at the bottom of the paper. As the solvent climbs, each pigment moves a different distance because of its different solubility in the solvent and its different affinity for the paper.
Each pigment therefore gives a distinct spot at a characteristic position. The Rf value of a pigment is calculated as
Rf = (distance travelled by the spot) ÷ (distance travelled by the solvent front).
The Rf values of the spots from the ink are compared with the Rf values of the reference pigments spotted on the same sheet. Matching Rf values identify the pigments present in the ink.
The ink solution is applied as a spot at the bottom of the paper. As the solvent climbs, each pigment moves a different distance because of its different solubility in the solvent and its different affinity for the paper.
Each pigment therefore gives a distinct spot at a characteristic position. The Rf value of a pigment is calculated as
Rf = (distance travelled by the spot) ÷ (distance travelled by the solvent front).
The Rf values of the spots from the ink are compared with the Rf values of the reference pigments spotted on the same sheet. Matching Rf values identify the pigments present in the ink.
Examiner tips
- Show the step‑by‑step process: solvent rise, differential movement, spot formation, Rf calculation, comparison with reference.
- Use the exact terms: capillary action, solubility, affinity, Rf value, solvent front.
- Mention that the reference spots are on the same paper to ensure identical conditions.
Common mistakes
- Failing to explain why pigments move different distances (solubility/affinity).
- Calculating Rf incorrectly or omitting the solvent‑front distance.
- Not stating that the reference pigments are spotted on the same paper for comparison.
Mark scheme (5 marks)
- The solvent travels up the chromatography paper by capillary action
- Different pigments have different solubilities in the solvent / different attractions to the paper, so they travel different distances up the paper
- Each pigment produces a spot at a characteristic position / the Rf value for each pigment is constant under the same conditions
- The Rf value is calculated as the distance travelled by the spot divided by the distance travelled by the solvent front
- The Rf values of the spots from the ink sample are compared to the Rf values of the known reference pigments to identify which pigments are present in the ink
Key terms in this question
Related
- All Edexcel A-Level Chemistry (9CH0) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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