Purity and chromatography

In chemistry a pure substance is a single element or compound, not mixed with anything else. Pure substances melt and boil at exact temperatures, so melting points can test purity. A formulation is a useful mixture made to a recipe, such as a paint or a medicine. Paper chromatography separates the substances in a mixture such as an ink, and the Rf value of each spot helps to identify it.

Key facts

  • A pure substance has a sharp melting point. Impurities lower the melting point and make it melt over a range of temperatures.
  • Formulations are mixtures with measured amounts of each component: fuels, cleaning agents, paints, medicines, alloys, fertilisers and foods.
  • Paper chromatography: a spot of the mixture goes on a pencil start line; the solvent (mobile phase) moves up the paper (stationary phase) and carries the substances different distances.
  • Rf = distance moved by the substance ÷ distance moved by the solvent. It is always less than 1.
  • A pure substance gives one spot in every solvent. Substances with the same Rf in the same solvent are probably the same.
  • Draw the start line in pencil (ink would dissolve) and keep the solvent below it.

Worked examples

Example 1

The solvent moved 8.0 cm and a spot of dye moved 3.2 cm. What is the Rf value of the dye?

  1. Measure both distances from the start line.
  2. Divide the distance moved by the spot by the distance moved by the solvent. Rf = 3.2 ÷ 8.0 = 0.40
  3. Rf has no unit, and it is always between 0 and 1.

Answer: 0.40

Example 2

A food colouring X is tested with four known dyes A, B, C and D. Which dyes are in X? Is X pure?

  1. X gives two spots, so it is a mixture: it is not pure.
  2. One spot is at the same height as A, and the other as C.
  3. So X contains dyes A and C, and not B or D.

Answer: A and C; X is not pure

Example 3

Pure aspirin melts at 136 °C. Sample P melts at 136 °C and sample Q melts from 128 °C to 133 °C. Which is pure?

  1. A pure substance melts at one exact temperature.
  2. P melts sharply at 136 °C, the melting point of pure aspirin: it is pure.
  3. Q melts lower and over a range of temperatures: it contains impurities.

Answer: P