Rf Values in Chromatography Flashcards

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In chromatography, what does the term Rf stand for?

Rf stands for retention factor. It is a number that shows how far a spot has moved on the chromatogram.

What is the formula for calculating the Rf value of a spot in paper chromatography?

Rf = distance moved by the spot ÷ distance moved by the solvent front.

From which line on the paper do you measure distances when working out an Rf value?

You measure from the start line (the pencil baseline) up to the centre of the spot and up to the solvent front.

Why does an Rf value have no units and is always between 0 and 1?

It has no units because it is a ratio of two distances. It is between 0 and 1 because a spot cannot move further than the solvent front.

On a chromatogram a dye spot moves 3.0 cm and the solvent front moves 6.0 cm. What is the Rf value of the dye?

Rf = 3.0 ÷ 6.0 = 0.50.

How can you use Rf values to identify an unknown substance?

Measure its Rf value, then compare it with known Rf values for other substances in the same solvent. A matching Rf suggests the unknown is that substance.

How can Rf values show that a coloured ink is a mixture?

If the ink separates into several spots, you work out an Rf value for each spot. Different Rf values show there are different substances in the ink.

Why must you use the same solvent and temperature when comparing Rf values?

Rf values depend on the solvent and the temperature. If these change, the Rf values change and you cannot compare results fairly.

What does it mean if a substance has a high Rf value compared with another substance on the same chromatogram?

A higher Rf means the substance moved further with the solvent. This shows it is more soluble in the solvent or less attracted to the paper than the other substance.