Rf Value
Summary: The Rf value (retention factor) is the ratio of the distance travelled by a substance to the distance travelled by the solvent front in paper chromatography. It is used to identify unknown substances. Rf = distance travelled by substance ÷ distance travelled by solvent. Tags: igcse chemistry definition practical-skills chromatography Created: 2026-07-14 Last Updated: 2026-07-23
The Rf value (retention factor or retardation factor) is a numerical value used in paper chromatography to identify and compare substances. It represents how far a substance travels relative to the solvent front.
The Rf Equation
Rf = distance travelled by substance ÷ distance travelled by solvent
Both distances are measured from the origin line (pencil line where the sample was spotted).
How to Calculate Rf
Solvent front: ↑
| • | distance travelled by solvent = 10 cm
| • | (measured from origin to solvent front)
| • A | distance travelled by spot A = 6 cm
| • | Rf(A) = 6 ÷ 10 = 0.6
| • B | distance travelled by spot B = 3 cm
|• | Rf(B) = 3 ÷ 10 = 0.3
—————— origin line
Properties of Rf Values
| Property | Explanation |
|---|---|
| Always < 1 | The substance cannot travel further than the solvent front |
| Dimensionless | Has no units (cm ÷ cm = ratio) |
| Characteristic | Each substance has a specific Rf for a given solvent and temperature |
| Independent of distance | Same Rf regardless of how long the chromatogram runs |
Using Rf Values to Identify Substances
- Run the chromatogram
- Measure and calculate Rf for each spot
- Compare with Rf values of known substances run under the same conditions (same solvent, same temperature)
- If Rf matches → substance is likely the same
Important: Rf values depend on:
- The solvent used
- The temperature
- The type of paper
Rf values can only be compared if conditions are identical.
Pure vs Impure Substances
- A pure substance produces one spot on the chromatogram → one Rf value
- An impure substance (mixture) produces multiple spots → multiple Rf values
Key Facts
- Rf = distance travelled by substance ÷ distance travelled by solvent
- Always a value between 0 and 1
- Has no units (it’s a ratio)
- Each substance has a characteristic Rf for a given solvent and temperature
- Compare Rf to known values to identify unknown substances
- A pure substance gives one spot → one Rf value
- Measurements taken from the origin line (pencil line)
See Also
This is a key term definition page — part of the IGCSE Chemistry (0620/0971) keyword glossary.