Scientific Principles & Theory
Scientific Background: Chromatography is a separation technique used to distinguish the components of a mixture based on their relative solubility in a mobile phase (solvent) and their affinity to a stationary phase (paper).
As the solvent ascends the paper by capillary action, it carries the dyes with it. Dyes that are more soluble in the solvent travel faster and further up the paper, whereas dyes with a higher affinity for the paper bind more strongly and travel slower. The Rf value is a constant for a specific compound under defined conditions:
Experimental Variables
Independent Variable
The composition of the ink samples / food colourings.
Dependent Variable
The Rf values of the separated components.
Control Variables
Type of chromatography paper, solvent type (distilled water), starting line position.
⚠️ Lab Risk Assessment
| Hazard | Associated Risk | Control Measure |
|---|---|---|
| Solvent (if using ethanol/organic solvents) | Flammable and volatile | Keep away from Bunsen burners; conduct in a well-ventilated room or fume cupboard. |
| Dyes / food colourings | Staining of skin/clothing or mild irritant | Avoid direct skin contact; wash hands if contact occurs. |
Apparatus & Procedure
Required Apparatus
- Chromatography paper
- Beaker (250 cm³)
- Pencil and ruler
- Capillary tubes or toothpicks (to apply spots)
- Glass rod or wooden splint
- Paper clips
- Solvent (distilled water or ethanol)
- Ink samples or food colourings
Step-by-Step Procedure
- Draw a horizontal line in pencil 2 cm from the bottom of the chromatography paper. This is the baseline.
- Use a toothpick or capillary tube to place a small, concentrated spot of each ink sample on the pencil line. Label each spot in pencil below the line.
- Allow the spots to dry. Ensure the spots are small to prevent them from spreading or smudging horizontally.
- Pour the solvent into the beaker to a depth of about 1 cm. The solvent level must be below the baseline pencil line.
- Suspend the chromatography paper inside the beaker from a glass rod or splint using a paper clip. Ensure the paper does not touch the sides of the beaker.
- Cover the beaker with a lid or watch glass to prevent solvent evaporation and keep the air saturated with solvent vapour.
- Allow the solvent to rise up the paper until it is about 2 cm from the top.
- Remove the paper from the beaker and immediately draw a pencil line marking the furthest point the solvent reached. This is the solvent front.
- Allow the chromatogram 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.
Fig 1. Laboratory experimental setup for Core Practical 1.13.
Sample Data & Calculations
This representative dataset illustrates the values typically obtained when carrying out this experiment in the laboratory:
| Spot Component | Distance Travelled by Spot (cm) | Distance Travelled by Solvent (cm) | Rf Value |
|---|---|---|---|
| Solvent Front | - | 8.20 | 1.00 |
| Red Dye A | 3.45 | 8.20 | 0.42 |
| Blue Dye B | 6.90 | 8.20 | 0.84 |
| Yellow Dye C | 5.15 | 8.20 | 0.63 |
Data Processing & Analysis
- Formula: Rf = Distance moved by spot / Distance moved by solvent front
- Rf (Red Dye A) = 3.45 cm / 8.20 cm = 0.42 (no units)
- Rf (Blue Dye B) = 6.90 cm / 8.20 cm = 0.84 (no units)
- Rf (Yellow Dye C) = 5.15 cm / 8.20 cm = 0.63 (no units)
Conclusion & Evaluation
Chemical Explanation: Saturated solutions are heavily dependent on temperature. Heating shifts solubility limits, allowing more solute to form coordinate bonds or ion-dipole interactions with solvent molecules. When cooled, the reverse process happens and solute precipitates out.
Experimental Error Analysis
| Error Type & Source | Effect on Final Result | Mitigation Strategy |
|---|---|---|
| Systematic Error Baseline drawn in ink rather than pencil |
The ink from the baseline will dissolve in the solvent and run up the paper, contaminating the spots. | Always draw the baseline in pencil because graphite is insoluble in chromatography solvents. |
| Random Error Spots washing off into the solvent |
If the solvent level is higher than the baseline, the spots will dissolve in the beaker rather than ascending the paper. | Ensure the solvent depth is strictly below the level of the pencil baseline. |
Exam Practice
A student wants to investigate if a sample of food dye contains a banned coloring agent. Design an experiment using paper chromatography to compare the sample against three reference dyes, explaining how the results would show if the banned dye is present.
View Model Answer & Mark Scheme
Model Answer (6/6 Marks):
- Preparation: Draw a pencil line 2 cm from the bottom of the chromatography paper. Place 4 distinct marks along the line.
- Application: Spot the unknown food dye on the first mark, and the three reference dyes (including the banned dye) on the remaining marks. Keep spots small and concentrated.
- Development: Suspend the paper in a beaker with a solvent (e.g. water). The solvent level must be below the pencil line. Cover the beaker with a lid.
- Completion: Allow the solvent to run up near the top. Remove the paper and immediately mark the solvent front with a pencil. Let the paper dry.
- Analysis: Measure spot distances and the solvent front distance. Calculate the Rf values for all dyes.
- Conclusion: Compare the Rf values and vertical positions of the spots. If the unknown sample produces a spot with the exact same Rf value and color as the banned dye, the sample contains the banned substance.
Examiner Tip:
Always mention the use of pencil for the baseline and solvent front, and explain that the solvent level must be below the spots to prevent them from dissolving into the bulk solvent.