rp7 Use of chromatography to investigate the pigments isolated from leaves of different plants,

Describe how pigments from a leaf of a plant can be isolated with paper chromatography

1. Crush leaves with solvent to extract pigments

2. Draw a pencil line on filter / chromatography paper, 1 cm above bottom

• Use a capillary tube to spot a concentrated dot of pigment extract onto the line.

• Allow it to dry and reapply for stronger results.

4. Stand paper in boiling tube of (organic) solvent below point of origin

5. Add lid to reduce evaporation of the solvent. (solvent moves up, carrying dissolved pigments)

6. Remove before solvent reaches top and mark solvent front with pencil

Explain why the origin should be drawn in pencil rather than ink.

● Ink is soluble in solvent

● So ink would mix with pigments / line would move

Explain why the point of origin should be above the level of the solvent.

● Pigments are soluble in solvent

● So would run off paper / spots dissolve into solvent

Explain why a pigment may not move up the chromatography paper in one solvent.

May be soluble in one solvent but insoluble in another

Describe how pigments can be identified

● Rf value = distance moved by spot / distance moved by solvent front

● Compare Rf value to published value

Explain why the solvent front should be marked quickly once chromatography paper is removed.

Once solvent evaporates, solvent front not visible

Explain why the centre of each pigment spot should be measured.

Standardises readings as pigment is spread out

● So allows comparisons to be made

Explain why the obtained Rf values were similar, but not identical, to the published values.

● Different solvent / paper / running conditions may affect Rf value

Explain why Rf values are used and not the distances moved by pigment spots.

● Solvent / pigment moves different distances (further is more soluble)

● Rf value is constant for same pigment / can be compared

The pigments in leaves are different colours, suggest why this is an advantage to the plant.

 Absorb different/more wavelengths of light for photosynthesis

phases

• Stationary Phase: The chromatography paper or TLC plate.

• Mobile Phase: The solvent that carries the pigment mixture.

• Solvent front: where the solvent stops

Why stop it early?

If the solvent reaches the top, you cannot accurately mark the solvent front, making it impossible to calculate Rf values.

Why dry the chromatogram?

Ensures that measurements are accurate, and that pigment movement has fully stopped.
Wet pigment may continue to diffuse, blurring bands and reducing resolution.

Describe how pigments can be identified

● Rf value = distance moved by spot / distance moved by solvent front

● Compare Rf value to published value