Cambridge O Level Chemistry · Syllabus 5070 · Experimental Techniques and Chemical Analysis
Retention Factor
What is Retention Factor?
The retention factor, Rf, is the distance travelled by a substance divided by the distance travelled by the solvent, with both distances measured from the same baseline. Because it is a ratio of two lengths it has no unit, and for an ordinary chromatogram its value lies between 0 and 1. Rf depends on the solvent and conditions as well as on the substance, so values are only compared between chromatograms run under the same conditions.
This definition is part of the Experimental Techniques and Chemical Analysis chapter in Cambridge O Level Chemistry.
Retention Factor in context
Experimental techniques and chemical analysis cover how a chemist gathers and interprets evidence: choosing the right apparatus for a measurement, running a titration to find a reacting volume, using paper chromatography to separate and identify substances, separating and purifying mixtures by filtration, crystallisation or distillation, and running the qualitative analysis tests that identify anions, cations, gases and metal ions. A titration uses a volumetric pipette to measure a fixed volume and a burette to add a variable volume until an indicator shows the end-point, giving a titre as final reading minus initial reading. Chromatography separates soluble substances by how far they travel in a solvent, measured as the retention factor, Rf.