Locating Agents & Rf Values
Section: 12. Experimental Techniques & Chemical Analysis | Syllabus: Cambridge IGCSE Chemistry (0620)
Locating Agents (Supplement)
Paper chromatography can also separate mixtures of soluble colourless substances, using a suitable solvent and a locating agent.
A locating agent is a chemical that reacts with an otherwise invisible (colourless), separated substance to produce a visible coloured spot, revealing its position on the chromatogram.
Without a locating agent, the separated spots of a colourless mixture cannot be seen at all. Knowledge of specific locating agents is not required - only that a locating agent is needed whenever the substances being separated are colourless.
Worked Example
- Question (based on a real exam question): which mixture would require a locating agent to be used in its paper chromatography - two colourless substances, two insoluble substances, two different-coloured substances with the same Rf value, or two substances that are the same colour?
- Answer: two colourless substances - their spots are invisible on the paper without a locating agent to react with them and produce visible colour.
The Rf Value (Supplement)
The Rf value (retention factor) is how far a substance has travelled up the paper, relative to how far the solvent has travelled.
Rf = 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.
- Answer: Rf = 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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