Chromatography required practical

Aim: To use paper chromatography to separate the coloured substances in inks or food colourings and calculate their R_f values to help identify them.

Variables

• Dependent: Distance moved by each spot (used to calculate its R_f value)
• Control: Type of chromatography paper; Solvent used; Temperature; Height of the start line above the solvent

Equipment

• Chromatography paper
• Beaker (250 cm³) with a lid or watch glass
• Pencil and ruler
• Capillary tubes or fine glass rods
• Samples of inks or food colourings
• Solvent (water)
• Glass rod or split bung to hang the paper

Method

1. Draw a straight line in pencil about 2 cm from the bottom of the chromatography paper.
2. Using a capillary tube, put a small spot of each sample on the pencil line, well spaced apart; let each spot dry and add more to make it concentrated.
3. Pour water into the beaker to a depth of about 1 cm, so the level is below the pencil line.
4. Hang the paper in the beaker so the bottom dips into the water but the spots stay above it, and cover the beaker with a lid.
5. Leave until the solvent has moved about three-quarters of the way up the paper.
6. Take the paper out and immediately mark the solvent front with a pencil line.
7. Allow the paper to dry.
8. Measure the distance from the start line to the solvent front and from the start line to the centre of each spot.

Safety

• Wear eye protection when handling solvents.
• If a flammable solvent such as ethanol is used instead of water, keep it away from naked flames and use a ventilated room.
• Take care with glass capillary tubes, which can break easily.

Results and calculations

Record the distance moved by the solvent front and by each spot in a table, then calculate R_f = distance moved by substance ÷ distance moved by solvent (always a value less than 1, with no units). Compare the pattern of spots and the R_f values with those of known reference substances run in the same solvent to identify the unknowns.

Common mistakes and improvements

• Drawing the start line in ink, which runs up the paper and mixes with the samples; always use pencil because graphite is insoluble.
• Putting the start line below the solvent level, so the samples dissolve into the solvent in the beaker; keep the line above the solvent.
• Forgetting to mark the solvent front before it evaporates; mark it as soon as the paper is removed.
• Spots too large or too close together so they spread into each other; use small, concentrated spots with space between them.

Exam tips

• Calculate R_f values accurately and give them to 2 significant figures or 2 decimal places as asked; measure to the centre of each spot.
• Explain that a pure substance gives a single spot in all solvents, while a mixture separates into two or more spots.
• Explain separation: substances move different distances because they have different solubility in the solvent (mobile phase) and different attraction to the paper (stationary phase).
• Explain why the lid is used (to stop the solvent evaporating) and why a spot may stay on the start line (it is insoluble in that solvent).
• Identify an unknown by matching its spots and R_f values to reference substances.