Paper ChromatographyOxford AQA IGCSE Chemistry: Revision notes
Section 1
What is paper chromatography used for?
Paper chromatography is a technique used to analyse and separate the substances present in a mixture, particularly ones dissolved in a solution. It is commonly used to identify the different dyes in food colourings, or the different components in inks.
Section 2
What are the stationary and mobile phases?
Chromatography always involves two phases:
- The stationary phase does not move - in paper chromatography, this is the chromatography paper itself.
- The mobile phase moves through the stationary phase, carrying the components of the mixture with it - this is the solvent (often water, but other solvents can also be used).
Separation happens because different components in the mixture have different solubilities in the solvent: substances that are more soluble travel further up the paper, while less soluble substances stay closer to the baseline.
Section 3
How is a paper chromatography separation carried out?
- Draw a pencil baseline near the bottom of a sheet of chromatography paper (pencil is used because it will not dissolve in the solvent and interfere with results).
- Place small spots of the sample(s) on the baseline.
- Stand the paper in a shallow layer of solvent, making sure the baseline itself stays above the solvent level.
- Allow the solvent to travel up the paper, carrying the dissolved substances with it at different rates.
- Remove the paper once the solvent has nearly reached the top, and mark the solvent front before it evaporates.
If the baseline is drawn below the solvent level, or in ink rather than pencil, the sample spots (or the baseline mark) will simply dissolve straight into the solvent and ruin the separation.
Section 4
How are components identified using Rf values?
Each substance travels a set distance relative to the solvent, which can be expressed as an Rf value:
Rf = distance travelled by substance / distance travelled by solvent
An Rf value is always between 0 and 1. Because a given substance has a consistent Rf value in a particular solvent, comparing the Rf value of an unknown spot with reference values (or running known substances alongside the unknown on the same paper) allows components to be identified.
If a dye travels 6 cm while the solvent front travels 10 cm, its Rf value is 6 divided by 10 = 0.6.
Section 5
What does the number of spots tell us?
A pure substance produces only one spot on a chromatogram, whereas a mixture produces multiple spots, each corresponding to one component. This makes chromatography useful for checking whether a substance is pure, as well as identifying what a mixture contains.
Must Know
- Paper chromatography separates the substances in a mixture based on differing solubility.
- The stationary phase is the paper; the mobile phase is the solvent (not always water).
- The baseline must be drawn in pencil and kept above the level of the solvent.
- Rf = distance travelled by substance / distance travelled by solvent, always between 0 and 1.
- A pure substance gives one spot; a mixture gives multiple spots.
- Comparing Rf values (or running known substances alongside) identifies unknown components.
That's the notes covered.
Carry on to the next subtopic.