Required practical

Chromatography

Aims

To investigate how paper chromatography can be used to separate and tell the difference between coloured substances.

Method

  1. Draw a pencil line across the chromatography paper, 1 to 2 cm from the bottom.
  2. Use a pipette or capillary tube to add small spots of each ink to the line on the paper.
  3. Place the paper into a container with a suitable solvent in the bottom.
  4. Allow the solvent to move through the paper.
  5. Before the solvent reaches the top of the paper, mark the height reached by the solvent with a pencil. It's best to do this on both the left hand side, and right hand side, of the paper.
  6. Remove the chromatogram immediately.
  7. Allow the chromatogram to dry, then measure the distance travelled by each spot and by the solvent.
Core practical of dipping paper with in marks on a pencil line into a solvent.

Results

Record your results in a suitable table. For example:

InkSpot colourDistance travelled by spot (mm)

Analysis

1. Calculate the Rf value of each spot:

Rf = \frac{\textup{distance~travelled~by~substance}}{\textup{distance~travelled~by~solvent}}

2. Compare the Rf values and colours of each spot in the inks. Describe their similarities and differences.

Evaluation

Question

Explain why the distances travelled by each spot were measured in mm, rather than in cm.

Measurements in millimetres are more precise than those in centimetres. They will give more precise Rf values.

Hazards, risks and precautions

It is important in this practical activity to use appropriate apparatus and methods. This includes the safe use and careful handling of substances. Eye protection must be worn.

Evaluate the hazards and the precautions needed to reduce the risk of harm. For example:

HazardPossible harmPossible precaution
Harmful solventSkin irritationAvoid skin contact, eg wear gloves
Harmful solventBreathing difficultiesEnsure adequate ventilation or carry out the chromatography in a fume cupboard

Separation of photosynthetic pigments by chromatography

Chloroplasts usually contain two types of chlorophyll. They also contain several other types of pigment that capture light falling on leaves.

Mixtures of these pigments from chloroplasts can be separated using paper chromatography.

The pigments can be extracted from leaves by grinding the leaves in organic solvents with a pinch of sand to break up the plant cells.

The pigments can be separated using a mixture of organic solvents.

A chromatogram showing the photosynthetic pigments of a grass
A chromatogram showing the photosynthetic pigments of a grass