Chromatography Identifies by Rf Value, Not Colour

What students typically write

“The unknown dye is Sunset Yellow because it produced a yellow spot on the chromatogram.” Or: “We identified the substance by its colour on the paper.”

Students focus on what they can see (colour) and ignore the quantitative measurement (Rf value) that actually provides the identification.

Why it is wrong

Colour is not a unique identifier. Many different chemical substances produce spots of the same colour. Two yellow food dyes might look identical to the eye but travel different distances on the chromatography paper because they have different molecular structures and different affinities for the paper and the solvent.

The Rf value provides the identification:

Rf = distance moved by substance / distance moved by solvent front

Each substance has a characteristic Rf value for a given solvent and paper combination. If an unknown spot has the same Rf value as a known reference substance run on the same chromatogram under the same conditions, they are the same substance.

Additionally, many substances are colourless. Amino acids, sugars, and other important compounds produce no visible spots. These require a locating agent (such as ninhydrin spray for amino acids) to reveal their positions. Without Rf values, there would be no way to identify them at all.

What the mark scheme actually wants

A typical Paper 6 question: “Use the chromatogram to identify which dye is present in the food sample. Show your working.” [2]

Marking points:

1. Rf value calculated correctly for the unknown spot (1 mark)
2. Identification by matching the Rf value to a reference dye (1 mark)

Simply naming a colour without calculating Rf scores 0. The examiner wants to see the measurement and the comparison.

Worked example: wrong vs right

Question: A chromatogram shows an unknown spot that has travelled 3.6 cm. The solvent front has travelled 6.0 cm. Reference dyes have the following Rf values: Red 2 = 0.45, Yellow 5 = 0.60, Blue 1 = 0.72. Identify the unknown.

Wrong answer: “The unknown is Yellow 5 because the spot looks yellow.” This scores 0. No calculation, identification based on colour.

Right answer: “Rf = 3.6 / 6.0 = 0.60 (1). This matches Yellow 5, so the unknown is Yellow 5 (1).” This scores 2/2. The Rf value is calculated and matched.

How to avoid this mistake

Always calculate the Rf value. On Paper 6, measure distances with a ruler from the pencil baseline to the centre of the spot and to the solvent front. Divide. Compare with reference values. That is the method. Colour may help you check your answer but never replaces the calculation.

For the full chromatography method including paper and thin-layer techniques, see chromatography and Rf. The identifying unknowns page covers other analytical methods alongside chromatography.