Chromatography

 

Separating dissolved solids using chromatography

Chromatography is a method of separating mixtures of substances that can dissolve in water.

This is especially easy when the dissolved substances have different colours. An ink or food colouring is usually made from more than one colour. Chromatography can be used to find out the colours in ink or food colouring.

 

Chromatography works because some of the colours dissolve more easily in water than others. Colours that dissolve more easily travel further up the paper.

There are two ways to do chromatography:

 

• standard chromatography
• radial chromatography

Standard chromatography

The photo shows an example of a paper with different colours from standard chromatography.

Four samples of ink were used. Looking at the photo from left to right, the first sample on the left is a mixture of colours (yellow and orange). The other three samples are single colours (blue, pink and yellow).

 

 

A chromatogram with different samples of ink.

 

Radial chromatography

Another way to separate colours in an ink is radial chromatography.

 

This is done by putting water on a spot of ink in the middle of a piece of filter paper. The colours spread out as they move away from the middle, rather than moving up as in standard chromatography.

 

 

Comparing chromatograms

Chromatography is often used to see if an unknown mixture has the same colours as pure pigments. This helps scientists to identify the colours in the mixture. Pure pigments that are used for this purpose are called standard reference pigments.

The piece of paper that is left after a chromatography experiment is called a chromatogram. A chromatogram shows the colours that were in the mixture.

 

🧪 Knowledge Check: Chromatography

Test your understanding of how chromatography works and how to interpret the results.

1. In paper chromatography, what is the 'mobile phase'?

• The chromatography paper
• The solvent that moves through the paper
• The pencil line drawn at the bottom

2. Why should the start line be drawn in pencil rather than ink?

• Pencil lead is more soluble in water.
• Pencil lead is insoluble and will not move or interfere with the results.
• It makes the Rf value easier to calculate.

3. If a substance produces three distinct spots on a chromatogram, what does this tell you?

• The substance is a pure compound.
• The substance is a mixture of at least three components.
• The experiment has failed.

4. How is the Rf value of a substance calculated?

• Distance moved by solvent ÷ Distance moved by substance
• Distance moved by substance ÷ Distance moved by solvent
• Distance from start line to the top of the paper

5. A substance that is highly soluble in the mobile phase will:

• Stay on the start line.
• Move a short distance up the paper.
• Move a long distance up the paper.

Click to Reveal Answers

1. The solvent that moves through the paper (The mobile phase carries the substances).
2. Pencil lead is insoluble (This ensures the start line stays fixed).
3. A mixture of at least three components (Each spot represents a different substance).
4. Distance moved by substance ÷ Distance moved by solvent (This ratio is always between 0 and 1).
5. Move a long distance up the paper (Greater solubility results in a higher Rf value).

 

 

Summary:

 

• Chromatography is a method of separating mixtures of substances that can dissolve in water.
• These are often substances that have different colours, such as food colourings, inks, dyes or plant pigments.
• Chromatograms can be used to compare known pigments with those in a mixture.
• On a chromatogram, one spot means that the substance is pure. An impure mixture produces two or more spots.