Required practical 12 · Chemistry · Chemistry Paper 2, both tiers

Paper chromatography

Separate coloured substances using paper chromatography and use Rf values to distinguish pure from impure substances.

The exam trap. Measure both distances from the start line, and only compare Rf values in the same solvent.

Variables

Independent (you change): The ink or food colouring tested
Dependent (you measure): The distance moved by each spot and its Rf value
Control (you keep the same): The same solvent
The same paper
The same starting line and spot size
Solvent below the starting line

Draw a start line in pencil about 2 cm from the bottom of the paper.
Why Pencil does not dissolve in the solvent; ink would run.
Put small, concentrated spots of each sample on the line.
Why Small spots give clear, separate results.
Stand the paper in a beaker with solvent below the start line, and cover.
Why Spots below the solvent would wash into it; a lid stops the solvent evaporating.
Remove the paper before the solvent reaches the top and mark the solvent front.
Why You need the solvent distance to calculate Rf.
Let it dry and measure from the start line to the centre of each spot and to the solvent front.
Why Both distances must be measured from the same line.
Calculate Rf = distance moved by substance ÷ distance moved by solvent.
Why Rf identifies a substance under the same conditions.

Hazard	How to reduce the risk
Some solvents are flammable.	Keep away from flames and ventilate.

A pure substance gives one spot in every solvent; a mixture gives more than one.
Matching Rf values under the same conditions suggest the same substance: ink X contains dyes A and C.

Measuring distances from the bottom edge of the paper.
Instead Measure from the start line.
Drawing the start line in ink or putting it below the solvent.
Instead Use pencil and keep the line above the solvent.
Comparing Rf values from different solvents.
Instead Rf values only compare under the same conditions.