Chromatography separates mixtures based on how strongly each component is attracted to the stationary phase versus how readily it dissolves in the mobile phase (solvent). The more soluble a substance is in the solvent, the further it travels up the paper or plate.
Calculating the Rf Value
The Rf value (retention factor) is a fixed value for a substance under given conditions. It is calculated using:
Rf values are always between 0 and 1 (they have no units). An Rf of 1.0 would mean the substance moved with the solvent front; an Rf of 0 would mean it didn't move at all.
Pure vs Impure Substances
Chromatography reveals purity through the number of spots produced:
- One spot → the sample is likely a pure substance
- Multiple spots → the sample is a mixture (impure)
Identifying Unknown Substances
An unknown substance is identified by running it alongside known reference substances on the same plate in the same solvent. If the unknown produces a spot at the same Rf value (same distance) as a reference substance, they are likely to be the same compound.
This deep dive covers Chromatography and Rf Values within Instrumental Analysis for GCSE Chemistry. Revise Instrumental Analysis in Chemical Analysis for GCSE Chemistry with 6 exam-style questions and 6 flashcards. This topic appears regularly enough that it should still be part of a steady revision cycle. It is section 2 of 12 in this topic. Use this deep dive to connect the idea to the wider topic before moving on to questions and flashcards.
Practice questions for Instrumental Analysis
Which formula correctly calculates the Rf value in chromatography?
State two advantages of using instrumental methods of analysis compared with traditional chemical tests.