Purity and Chromatography » Chromatography

Separating a mixture with chromatography

Have you ever seen a black felt-tip pen leak onto wet paper? The black ink spreads out and splits into different colours. That is chromatography in action.

Chromatography is a way to separate mixtures. It can also give information that helps us identify substances. It works because different substances in a mixture are carried along at different speeds.

Key terms:

• Chromatography: a method used to separate the substances in a mixture.
• Stationary phase: the phase that does not move. In paper chromatography this is the paper.
• Mobile phase: the phase that moves. In paper chromatography this is the solvent, which soaks up the paper.
• Chromatogram: the paper with the separated spots on it, after the experiment.
• Rf value: a number that compares how far a substance moved with how far the solvent moved.

How paper chromatography works

A small spot of the mixture is placed on a line near the bottom of a piece of chromatography paper. This starting line is called the origin. The bottom edge of the paper is then placed in a solvent. The solvent soaks up the paper and carries the substances in the mixture with it.

Separation depends on how the substances are distributed between the two phases. A substance that spends more time in the mobile phase moves further up the paper. A substance that spends more time stuck to the stationary phase stays closer to the origin. Because each substance has its own balance between the two phases, the mixture splits into separate spots.

Pure or impure?

A chromatogram can show whether a substance is pure.

• A pure compound produces a single spot.
• A mixture may separate into two or more spots.

One solvent is not always enough to be sure. The compounds in a mixture may separate into different spots in one solvent, but stay together in another. So a result with one solvent could be misleading. A pure compound, however, will produce a single spot in all solvents. That is why chemists often test with more than one solvent.

Calculating Rf values

The distance a spot travels depends on the substance and on the solvent. To compare results fairly, we calculate the Rf value.

Rf = distance moved by substance ÷ distance moved by solvent

• Measure the distance moved by the substance from the origin line to the centre of the spot.
• Measure the distance moved by the solvent from the origin line to the solvent front, the highest point the solvent reached.
• Use the same units for both distances, so the units cancel. An Rf value has no units.

An Rf value is a ratio, so it is always between 0 and 1. A spot that stays on the origin has an Rf of 0. A spot that moves with the solvent has an Rf of 1.

Using Rf values to identify substances

Different compounds have different Rf values in different solvents. So an Rf value can help to identify a compound. If you know the Rf values of some substances in a particular solvent, you can compare them with your unknown.

Example: a table says compound X has an Rf of 0.45 in a certain solvent. Your unknown spot has an Rf of 0.45 in that solvent. It could be compound X. An Rf of 0.80 would rule it out.

Two different compounds can sometimes have the same Rf value in one solvent. This is why a match is used as evidence to help identify a substance, not as absolute proof.

Exam-style question

A student runs a chromatogram of a green ink. It forms two spots. Spot A moves 2.4 cm and spot B moves 5.6 cm. The solvent front moves 8.0 cm. (a) Is the ink pure? (b) Calculate the Rf value of spot B. (c) Spot A has an Rf of 0.30 in this solvent. A table says a dye called Y has an Rf of 0.30 in this solvent. What can the student say?