Required Practical 6: Paper Chromatography

Scientific Principles & Theory

Scientific Background: Chromatography is an analytical method used to separate the substances in a mixture based on their distribution between a mobile phase (solvent) and a stationary phase (chromatography paper).

Substances that are more soluble in the mobile phase will travel further and faster up the stationary phase. The Rf (retention factor) value is a ratio that is characteristic for a specific compound in a given solvent system, allowing identification:

\(R_f = \frac{\text{Distance travelled by substance}}{\text{Distance travelled by solvent front}}\)

Experimental Variables

Independent Variable

The food colouring dye samples applied to the baseline.

Dependent Variable

The Rf values of the separated dye spots.

Control Variables

The type of chromatography paper, the solvent used (distilled water), and the starting baseline position.

Lab Risk Assessment

Hazard	Associated Risk	Control Measure
Organic solvents (if used instead of water)	Flammable and vapours can cause dizziness	Keep solvent containers capped; avoid naked flames; perform in a well-ventilated area.
Dye solutions	Staining skin and clothing	Avoid direct skin contact; wash hands thoroughly after use.

Apparatus & Procedure

Required Apparatus

Chromatography paper (strip or sheet)
Beaker (250 cm³) or chromatography tank
Glass rod or wooden splint
Paper clips
Pencil and ruler
Capillary tubes
Samples of known food colourings (A, B, C, D) and an unknown mixture (U)
Solvent (distilled water)

Step-by-Step Procedure

Draw a straight horizontal line in pencil exactly 1.0 cm from the bottom of the chromatography paper. This is the baseline.
Use a capillary tube to place a small, concentrated spot of each known dye and the unknown mixture on the baseline. Label them in pencil below the line. Let them dry.
Pour the solvent (water) into the beaker to a depth of about 0.5 cm. The liquid level must be below the pencil baseline.
Suspend the paper in the beaker using a glass rod and paper clip. Ensure the paper does not touch the sides of the beaker.
Cover the beaker with a watch glass or lid to prevent evaporation.
Allow the solvent to rise up the paper until it is about 1 cm from the top. Do not move the beaker during this run.
Remove the paper and immediately draw a pencil line indicating the furthest level reached by the solvent (the solvent front).
Allow the paper to dry completely.
Measure the distance from the baseline to the solvent front, and the distance from the baseline to the centre of each separated spot.
Calculate the Rf value for each dye spot.

Fig 1. Laboratory experimental setup for Required Practical 6.

Sample Data & Calculations

This representative dataset illustrates the values typically obtained when carrying out this experiment in the laboratory:

Dye sample	Distance from baseline to spot (cm)	Distance from baseline to solvent front (cm)	Rf value
Solvent Front	-	8.50	1.00
Blue Dye A	5.10	8.50	0.60
Red Dye B	3.40	8.50	0.40
Yellow Dye C	6.80	8.50	0.80

Rf value formula: Rf = Distance travelled by substance / Distance travelled by solvent front.
Rf of Blue Dye A = 5.10 cm / 8.50 cm = 0.60 (no units).
Rf of Red Dye B = 3.40 cm / 8.50 cm = 0.40 (no units).
Rf of Yellow Dye C = 6.80 cm / 8.50 cm = 0.80 (no units).
Compare these Rf values with the spots produced by the unknown sample U. If sample U produces spots with Rf values 0.40 and 0.60, it contains red dye B and blue dye A.

Conclusion & Evaluation

Chemical Explanation: Evaluating the experimental outcomes against known values ensures validity. Understanding the source of systematic and random deviations allows for better experimental designs in future trials.

Experimental Error Analysis

Error Type & Source	Effect on Final Result	Mitigation Strategy
Systematic Error Baseline drawn with an ink pen instead of pencil	The ink from the pen will dissolve in the solvent and run up the paper, creating foreign dye spots and invalidating the chromatogram.	Always draw the baseline in pencil. Graphite is insoluble in chromatography solvents and will not run.
Random Error Spots applied too large or too concentrated	Dye spots spread out horizontally and bleed into each other, preventing clear separation.	Use a fine capillary tube or toothpick to apply tiny, highly concentrated spots. Allow them to dry completely.

Exam Practice

Practise this practical

Write your answer and the AI examiner checks it against the mark scheme, showing exactly which marks you earned and what to add.

[6 marks]

Describe how paper chromatography can be used to identify if an unknown food colouring contains a banned yellow dye. Detail the steps you would take and how you would analyze your results.

View Model Answer & Mark Scheme

Marking guidance: award 1 mark for each correct, clearly explained step in the model answer below, up to 6 marks.

Model Answer (6/6 Marks):

Baseline preparation: Draw a straight horizontal line in pencil 1.0 cm from the bottom of the chromatography paper. Pencil is insoluble, so it will not run with the solvent.
Spot application: Place a tiny, concentrated spot of the unknown colouring and a spot of the banned yellow dye side-by-side on the baseline. Label them in pencil. Let them dry.
Solvent setup: Place the paper in a beaker containing a small depth (0.5 cm) of solvent (water). Ensure the solvent level is strictly below the baseline to prevent the spots from dissolving into the beaker.
Development: Cover the beaker with a lid to keep the air saturated. Allow the solvent to ascend near the top. Remove the paper and immediately mark the solvent front in pencil.
Measurements: Once dry, measure the distance from the baseline to the solvent front and the distance to the centre of the separated spots.
Data processing: Calculate the Rf value for both the banned dye and the spots from the unknown. If the unknown sample produces a spot with the exact same Rf value and colour as the banned yellow dye, it contains the banned substance.

Examiner Tip:

Make sure to explicitly state that the solvent level must be below the baseline, that the baseline is drawn in pencil, and how Rf values are calculated and compared.

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