Library / Foundations: Chemistry / FCH-12
Foundations: Chemistry
Rf value in chromatography
How far a substance moves relative to the solvent in chromatography.
- Ref
- FCH-12
- Rev
- v1
- Field
- Foundations: Chemistry
- Result
- Rf
Measure both distances from the same start line (the spot centre and the solvent front). Rf has no unit and is always between 0 and 1.
A substance that sticks to the paper moves only a short way; one that dissolves well travels nearly as far as the solvent.
Calculator
- Percentage of solvent distance40 %
- Distance behind the solvent front5.4 cm
What drives the result
Each variable is moved across its typical range while the others stay at the values above. The longest bar is the key driver.
- Rf values identify substances when compared against known standards run on the same plate and solvent.
- A different solvent changes the Rf, so only compare values from the same conditions.
Chart data as tables
Ranked variables
| Variable | Typical range | Lowest result | Highest result | Swing () | Local step | Change in result |
|---|---|---|---|---|---|---|
| s Distance moved by solvent | 1 to 30 cm | 0.12 | 3.6 | 3.48 | +10% | −0.0364 |
| d Distance moved by substance | 0.1 to 30 cm | 0.01111 | 3.333 | 3.322 | +10% | +0.04 |
Sweep of Distance moved by substance
| Distance moved by substance (cm) | Rf value |
|---|---|
| 0.1 | 0.01111 |
| 3.837 | 0.4264 |
| 7.575 | 0.8417 |
| 11.31 | 1.257 |
| 15.05 | 1.672 |
| 18.79 | 2.088 |
| 22.52 | 2.503 |
| 26.26 | 2.918 |
| 30 | 3.333 |
Sweep one variable
Variables and how each one contributes
Rf value
What the equation gives you. Everything else feeds it.
Distance moved by substance
Distance from the baseline to the centre of the spot. Directly proportional.
Distance moved by solvent
Distance from the baseline to the solvent front. Inversely proportional.
Worked example with the values above
- Substance distance: 3.6 cm; solvent distance: 9 cm.
- Divide: 3.6 / 9.
- Rf = 0.4.
Assumptions and limits
- Same solvent and paper or plate.
- Distances measured from the same baseline.
Common mistakes
- Measuring from the bottom of the paper instead of the baseline.
- Giving a value above 1.
Statistics and uncertainty
Reading the centre of a spot to ±1 mm on a 36 mm movement is about 3% on Rf.
Where it is used
- Forensic and food dye analysis.
- Checking product purity.
Maths you need
- Ratios: part over whole
- Measuring: distances in cm