Paper Chromatography
Section: Chemistry | Syllabus: Cambridge IGCSE Combined Science (0653)
What is Paper Chromatography?
Paper chromatography is used to separate a mixture of soluble, coloured substances (such as the dyes in ink) using a suitable solvent.
- The stationary phase is the chromatography paper - it does not move
- The mobile phase is the solvent - it moves up through the paper by capillary action
- Different substances travel different distances up the paper, because they have different solubilities in the solvent and different attractions to the paper
Method
- Draw a pencil baseline near the bottom of the paper - never use pen, as the ink would dissolve and contaminate the results
- Apply small, concentrated spots of each sample onto the baseline
- Stand the paper in a shallow layer of solvent, with the solvent level kept below the baseline - if the spots themselves touch the solvent, they simply dissolve straight into it instead of separating
- Cover the container to keep the atmosphere saturated with solvent vapour
- Remove the paper once the solvent has risen most of the way up, and immediately mark the position of the solvent front in pencil before it evaporates
Interpreting a Chromatogram
- A pure substance produces a single spot; an impure substance (a mixture) produces two or more spots
- An unknown substance can be identified by running it alongside known substances on the same paper: if a spot from the unknown lines up at exactly the same height and colour as a spot from a known substance, the unknown is likely to contain that substance
Worked Example
- Question (based on a real exam question): a student runs an ink sample on the same chromatogram as four known dyes, 1-4. Dyes 2 and 4 each produce spots that line up exactly with spots produced by the ink; dyes 1 and 3 do not fully match. Which dyes could be present in the ink?
- Answer: dyes 2 and 4 - a dye can only be a component of the ink if every one of its spots lines up with a spot the ink itself produces.
Common Mistakes
- ❌ Using a pen to draw the baseline or label spots
- ✓ pen ink dissolves in the solvent and contaminates the chromatogram - always use pencil
- ❌ Letting the solvent level start above the baseline
- ✓ the sample spots would dissolve directly into the solvent rather than separating as the solvent rises past them
The Rf Value
The Rf value (retention factor) is how far a substance has travelled up the paper, relative to how far the solvent has travelled.
R_f = (distance travelled by substance/distance travelled by solvent)
- Both distances are measured from the baseline: to the centre of the spot, and to the solvent front
- Rf values are always between 0 and 1, and have no units
- Under the same conditions (same solvent, paper and temperature), a given substance always has the same Rf value - so Rf values can be used to identify an unknown substance by comparing it with known values
Worked Example
- Question: on a chromatogram, a spot has travelled 4.5 cm from the baseline, and the solvent front has travelled 7.5 cm from the baseline. Calculate the Rf value. [2]
- Answer: R_f = (4.5/7.5) = 0.60
Common Mistakes
- ❌ Measuring the distance travelled by the substance from the solvent front instead of from the baseline
- ✓ both distances in the Rf formula must be measured from the same starting point - the baseline
- ❌ Giving an Rf value greater than 1, or including units
- ✓ a substance can never travel further than the solvent itself, so Rf is always between 0 and 1; it is a ratio of two distances, so the units cancel and there are none in the answer
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