Chromatography Rf value: how to calculate it
An Rf value is the distance a substance moves up the paper divided by the distance the solvent moves. Rf = distance moved by substance ÷ distance moved by solvent. Measure both distances from the start line, called the origin, in the same unit. The Rf value has no unit, and it always lies between 0 and 1.
How paper chromatography separates a mixture
You put a small spot of the mixture on a start line near the bottom of the paper. A solvent then runs up through the paper and carries the mixture with it. The solvent is the mobile phase, because it moves. The paper holds the stationary phase, which stays put.
Each substance in the mixture spends part of its time dissolved in the solvent and part stuck to the paper. The balance between the two phases is different for each substance, so they travel at different speeds. A mixture spreads out into separate spots.
A pure substance gives a single spot, whatever solvent you use. The spots of a mixture may separate in one solvent and not in another. If the substances have no colour, a locating agent makes the spots show up.
Measuring the two distances
- Distance moved by the substance: from the origin to the centre of the spot.
- Distance moved by the solvent: from the origin to the solvent front, the furthest line the solvent reached.
Put both numbers into the calculator, choose mm or cm for each and it shows the working. If your two measurements are in different units, change one to match first. Because the spot never passes the solvent front, the answer can never be bigger than 1.
Using Rf values to identify a substance
A substance has its own Rf value in a given solvent. Run your unknown sample beside known reference substances on the same paper. A spot with the same Rf value as a reference, in the same solvent, could be that substance.
Compare values from the same solvent only. AQA points out that one compound gives different Rf values in different solvents. It also asks for answers to a sensible number of significant figures.
What each board asks
AQA prints the Rf formula in its specification for both GCSE Chemistry (8462) and Combined Science (8464). Its required practical uses paper chromatography to separate coloured substances and work out their Rf values. Edexcel asks you to identify substances by working out Rf values, in 1CH0 and 1SC0, and its core practical looks at inks.
OCR Gateway (J248) covers paper and thin layer chromatography, and names gas chromatography as another method. OCR Twenty First Century (J258) asks you to calculate Rf values and compare them with reference samples. Only AQA prints it as a formula. OCR Gateway expects you to recall it, and the ratio above works for every board.
At KS3, chromatography is one of the simple ways to separate a mixture. Rf values start at GCSE. If you teach this practical, our teachers page covers using the site in a lesson.
Worked examples
Rf value
- Rf = distance moved by substance ÷ distance moved by solvent
- Both distances are in cm, so no change is needed.
- Rf = 3.6 ÷ 8.0
- Rf = 0.45
0.45
Rf value
- Rf = distance moved by substance ÷ distance moved by solvent
- Rf = 42 ÷ 75
- Rf = 0.56, a number between 0 and 1 as expected
0.56
Rf value
- The units differ. Change the solvent distance to mm: 6.4 cm × 10 = 64 mm
- Rf = distance moved by substance ÷ distance moved by solvent
- Rf = 24 ÷ 64
- Rf = 0.375, which is 0.38 to 2 decimal places
0.375 (0.38 to 2 decimal places)
Common mistakes
- Turning the fraction upside down. Solvent distance on top gives an answer above 1, which cannot be right.
- Measuring to the top edge of the spot. Measure to its centre.
- Measuring from the bottom of the paper. Both distances start at the origin line.
- Dividing millimetres by centimetres. Match the units before you divide.
- Comparing Rf values from two different solvents.
Measure spots and work out Rf values in Rf Race. Other ways to identify a substance are on the flame test colours page, and the chemistry hub lists every page in this set.
Questions people ask
Can an Rf value be greater than 1?
No. A substance cannot travel further than the solvent carrying it, so the top number is always the smaller one.
What if a spot stays on the start line?
Its Rf value is 0, because it did not move. It may not dissolve in that solvent. OCR notes that some substances are insoluble in water, so chemists use other solvents.
Does an Rf value have a unit?
No. Both distances share a unit, so the units cancel when you divide.