Rf value calculator for chromatography

A spot moves 3.6 cm and the solvent front moves 8.0 cm. $R_f = 3.6 \div 8.0 = 0.45$.

To find a distance: if a dye has $R_f = 0.6$ and the solvent moves 9 cm, the dye moves $0.6 \times 9 = 5.4$ cm.

In the same solvent, a substance always has the same Rf. Compare the Rf of an unknown spot with known substances run on the same paper: a match suggests they're the same substance. A pure substance gives a single spot in all solvents; a mixture separates into several spots.

The paper is the stationary phase and the solvent is the mobile phase. Each substance is shared between them: one that's more attracted to the solvent (more soluble) travels further and has a higher Rf, while one more attracted to the paper stays nearer the start line.

Draw the start line in pencil (ink would run), spot the samples on it, and stand the paper in a beaker with the solvent below the line so the samples don't dissolve into the solvent. Put a lid on to stop the solvent evaporating, and remove the paper before the solvent reaches the top, marking the front straight away.

• Measuring from the bottom of the paper instead of the start line.
• Measuring to the top of the spot instead of its centre.
• Getting an Rf above 1 (the distances are the wrong way round).

Practise with our Rf value questions.

• Measure in the same unit for both distances (mm or cm); the units cancel.
• Give Rf to 2 decimal places unless told otherwise.
• If two substances have the same Rf in one solvent, try another solvent to check whether they're really the same.
• Explain a result in terms of attraction: higher Rf, more soluble in the solvent.