Worked example 1
A spot travels 3.6 cm while the solvent front travels 8.0 cm. Calculate the retention factor.
Try it first: Confirm both distances are measured from the same pencil start line.
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Gases, Solutions & SpectroscopyMixturesContent level: Core 18 min
Separating Mixtures and Chromatography
What you'll be able to do: Choose a separation technique from the properties of a mixture and interpret a chromatogram using retention factors.
Introduction
Mixtures are separated by exploiting a difference in physical properties. Chromatography is the most versatile of these methods, and it rests on a single competition between two phases.
These are recommended, not required. You can start this lesson at any time.
Learning objectives
- Match a separation technique to the property difference it exploits
- Explain the roles of the stationary and mobile phases
- Calculate a retention factor from a chromatogram
- Relate intermolecular attraction to how far a component travels
Lesson
Choosing a technique
Filtration separates an insoluble solid from a liquid using a difference in particle size. Distillation separates miscible liquids using a difference in boiling point. Chromatography separates dissolved components using a difference in how strongly each is attracted to the stationary phase compared with the mobile phase. Identify the property that differs and the method follows.
How chromatography works
A stationary phase, such as the paper or a coated plate, stays fixed while a mobile phase, the solvent, moves through it. Every component is pulled two ways at once. A component that is held tightly by the stationary phase lags behind; one that prefers the solvent is carried along and ends up far up the plate.
Paper chromatography uses a polar stationary phase, so polar components hydrogen bond to it and travel only a short distance.
Retention factor
The retention factor, Rf, is the distance travelled by the component divided by the distance travelled by the solvent front, measured from the same starting line. It has no units and always lies between zero and one. Under identical conditions, Rf is characteristic of a substance and can be used to identify it.
Rf = distance moved by component / distance moved by solvent front
Reading a chromatogram
Count the spots to count the components: a pure substance gives one spot. Compare an unknown with a reference run on the same plate, because Rf depends on the solvent and the temperature. A spot that matches both the reference position and its colour is strong evidence of identity.
Use a second solvent to test co-migration
Two spots that travel together in one chromatographic solvent may be the same compound, but they may also be different compounds with coincidentally similar attraction to both phases. A second solvent system can change their relative migration. State the stationary and mobile phases when comparing retention factors; one Rf value alone does not prove identity or purity.
Matching Rf in one run is evidence, not a unique chemical fingerprint.
Key ideas
Definition
Stationary phase
The fixed material, often polar paper or silica, through which the solvent moves.
Definition
Mobile phase
The moving solvent that carries components up the plate.
Definition
Retention factor
A unitless ratio between zero and one that characterises how far a component travelled.
Rule
Attraction rule
Stronger attraction to the stationary phase means a shorter distance and a smaller Rf.
Equation
Retention factor
Rf = d_component / dsolvent
- d_component = distance from the start line to the spot centre
- dsolvent = distance from the start line to the solvent front
Worked examples
Worked example 2
Two dyes are run on polar paper. Dye A has Rf = 0.85 and dye B has Rf = 0.20. Which dye is more strongly attracted to the paper?
Try it first: Decide what a small Rf says about where the component spent its time.
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Worked example 3
Unknown dye X and standard dye S each give Rf = 0.42 on one paper chromatogram. Can you conclude they are identical?
Try it first: Ask whether two different dyes could co-migrate.
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Common mistakes
Reporting an Rf greater than one.
Why it's wrong: A component cannot outrun the solvent that carries it.
Check instead: Recheck which distance is on the bottom of the fraction.
Drawing the start line in ink.
Why it's wrong: The ink dissolves and runs, contaminating the chromatogram.
Check instead: Always mark the start line in pencil.
Starting the plate with the spot below the solvent level.
Why it's wrong: The sample dissolves straight into the reservoir instead of travelling.
Check instead: Keep the solvent level below the start line.
After the lesson · Question bank
Independent practice
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What you should now know
Filtration, distillation, and chromatography each exploit a different physical property: particle size, boiling point, and relative attraction to two phases. In chromatography, a component that interacts more strongly with the mobile phase travels further and has a larger retention factor, calculated as the distance moved by the component divided by the distance moved by the solvent front.
- Each separation technique exploits one differing physical property
- Chromatography balances attraction to the stationary and mobile phases
- Rf is the component distance divided by the solvent front distance
- Rf is always between zero and one
- Compare unknowns with references run on the same plate
Sources and further reading
This lesson is original Chem Help content. No external sources were adapted.