Paper Chromatography
Section: 12. Experimental Techniques & Chemical Analysis | Syllabus: Cambridge AS Level Physics 9702
What is Paper Chromatography? Chromatography A technique used to separate and identify the components in a mixture based on their different rates of movement through a medium. Paper Chromatography A type of chromatography that uses paper as the stationary phase and a solvent as the mobile phase to separate dissolved substances.
Key Principle: Different substances move at different speeds through the paper because they have different levels of attraction to the paper and the solvent. More soluble substances travel further up the paper.
Key Terms in Chromatography Term Definition Example in Paper Chromatography Stationary Phase The phase that does not move Chromatography paper (cellulose) Mobile Phase The phase that moves through the stationary phase Solvent (e.g., water, ethanol) Solvent Front The furthest point reached by the solvent The top edge of the wet area on the paper Baseline The starting line where samples are placed Pencil line drawn near the bottom of the paper Spot A small sample of the substance being tested Ink or food coloring dot on the baseline Apparatus Required Equipment Purpose Chromatography paper Acts as the stationary phase Beaker or boiling tube Container for the solvent Solvent (e.g., water, ethanol) Acts as the mobile phase to carry substances up the paper Pencil To draw the baseline (pen ink would dissolve and affect results) Ruler To measure distances for Rf value calculations Capillary tube or pipette To apply small, concentrated spots of the sample Watch glass or lid To cover the beaker and prevent solvent evaporation Interactive Diagram: Labeled diagram showing chromatography apparatus setup including beaker with solvent, chromatography paper suspended above solvent level, baseline with spots, and watch glass cover.
Should show how paper is positioned so baseline is above solvent level. Method for Paper Chromatography Step-by-Step Procedure: Draw the baseline: Use a pencil to draw a horizontal line about 1-2 cm from the bottom of the chromatography paper.
Never use pen as the ink will dissolve in the solvent. Apply the samples: Use a capillary tube to place a small spot of each substance on the baseline. Keep spots small and concentrated. Label the spots: Use pencil to mark which spot is which (lightly, on the side or top of the paper).
Prepare the solvent: Pour a small amount of solvent (about 0.5-1 cm deep) into a beaker. The solvent level must be below the baseline. Place the paper: Carefully lower the paper into the beaker so that the bottom edge is in the solvent but the baseline and spots are above the solvent level.
Cover the beaker: Place a watch glass or lid on top to prevent evaporation of the solvent and create a saturated atmosphere. Allow separation: Leave the apparatus undisturbed. The solvent will rise up the paper by capillary action, carrying the dissolved substances with it.
Remove the paper: When the solvent front is about 1 cm from the top of the paper, remove the paper from the beaker. Mark the solvent front: Immediately use a pencil to mark the position of the solvent front before it evaporates.
Dry the chromatogram: Allow the paper to dry completely in air or use a hairdryer on a cool setting. Analyze the results: Observe the separated components and calculate Rf values if required. Important Safety and Technique Points What to Do Why It's Important Use pencil for baseline and labels Pencil won't dissolve in the solvent (unlike pen ink which would contaminate results) Keep spots small and concentrated Small spots give sharper, clearer separation; large spots spread and overlap Ensure baseline is above solvent level If spots touch the solvent, they will dissolve directly into it rather than separating Cover the beaker with a lid Prevents solvent evaporation and creates a saturated atmosphere for even solvent movement Mark solvent front immediately The solvent front may evaporate and become invisible after the paper is removed Never touch the paper with fingers Oils and substances from skin can contaminate the chromatogram Don't move the beaker during the run Movement can disturb the solvent flow and cause uneven separation Use appropriate solvent in well-ventilated area Some solvents (like ethanol) are flammable and produce fumes Interpreting Chromatograms What the Results Tell You: Number of spots: Each spot represents a different component in the mixture.
A single spot indicates a pure substance; multiple spots indicate a mixture. Position of spots: Different substances travel different distances up the paper. The distance traveled depends on the substance's solubility in the solvent and attraction to the paper.
Color of spots: The color helps identify what the substance might be (e.g., in food coloring or ink analysis). Comparing unknowns: If an unknown spot travels the same distance as a known substance (and has the same color), they are likely the same substance.
Interactive Diagram: Example chromatogram showing separation of a mixture (e.g., blac…
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