37.1.1—And understand the terms

The stationary phase stays fixed (for example alumina on a solid support); the mobile phase is the solvent that moves through it. A sample travels from the baseline, while the solvent front marks the furthest solvent position.

A compound repeatedly partitions between phases according to polarity and intermolecular attraction. Its Rf is distance travelled by the spot divided by distance travelled by the solvent front.

If a spot moves 3.0 cm and the solvent front moves 6.0 cm, Rf = 0.50. Measure both distances from the baseline.

Rf has no units and should be between 0 and 1; do not measure from the edge of the paper or compare values from different solvent systems without care.