Interpreting Chromatograms and Rf Values

This lesson builds on paper chromatography by focusing on the detailed interpretation of chromatograms and the calculation and use of Rf values, as required by the Edexcel GCSE Chemistry specification (1CH0, Topic 1). You need to be confident in calculating Rf values, comparing them with reference data, determining purity, and understanding the factors that affect Rf values.

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Recap: The Rf Value Formula

The Rf value (retention factor) is calculated using:

Rf = distance moved by substance \u00f7 distance moved by solvent front

Both distances are always measured from the baseline (the pencil line at the bottom).

Key points:

• Rf values are always between 0 and 1.
• Rf values have no units (they are a ratio of two lengths).
• Each substance has a characteristic Rf value in a given solvent under given conditions.

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Calculating Rf Values: Step-by-Step

Step 1: Identify the Baseline

The baseline is the pencil line drawn at the bottom of the chromatography paper where the spots were originally placed.

Step 2: Identify the Solvent Front

The solvent front is the pencil line drawn to mark how far the solvent travelled up the paper.