chromatography explained

Did you know?

Chromatography gets its name ( Chroma="colour" in Greek) from the fact that mixtures which initially look like one uniform colour can be separated to produce many different colours. Not all chromatograms are coloured.

Colour separation

Chromatography is another important separation technique. Several different types of chromatography exist:

paper chromatography, thin layer chromatography, gas chromatography

All chromatography involves a mobile phase and a stationary phase. The mobile phase moves through the stationary phase. The solutes which are more soluble (in the mobile phase) will travel faster through it than other less soluble solutes. This fact is used in chromatography to separate and identify the separate components in a mixture.

Butterfly or chromatogram?

This chromatogram was made from five spots of ink, a filter paper and water.

1.11 Activity 1. Separating spinach

Students should:

1.11 understand how a chromatogram provides information about the composition of a mixture

1.12 understand how to use the calculation of Rf values to identify the components of a mixture

1.13 practical: investigate paper chromatography using inks/food colourings

Watch the video "separating spinach"and then the animation "identity check".

1.11 Activity 2. Running a chromatogram

Watch the video and work through the animation and then answer the questions below.

Why is the solvent depth kept low?

The solvent level is kept below the level of the pencil line so that the inks do not get directly dissolved out of the paper and into the solvent. If the level is below the pencil line the solvent will travel up the paper and dissolve the inks as it moves up through the paper.

Why is the line drawn on the chromatogram in pencil ...

The line is drawn in pencil because pencil does not dissolve in the solvent. Pencil "lead" is actually graphite ( a form of carbon) and is not very soluble. If pen was used to draw the line and crosses, the line would dissolve in the solvent and travel up the paper and would confuse the results obtained.

Why are Rf values useful?

The Rf value of a substance can provide a way of identifying a substance. The Rf value obtained for a particular (pure) substance is a characteristic of the substance. By comparing the Rf value of an unknown substance with those of known substances under the same conditions, the unknown can be itentified.

List the three coloured inks in order of solubility

The ink which is most soluble in the solvent is the one which travels the furthest up the paper ( ink C) . Thus the order of solubility of the three inks starting with the most soluble is C > A > B (least soluble).

In paper chromatography what is the stationary phase?

The paper is the stationary ( or static) phase in paper chromatography.

In paper chromatography what is the stationary phase?

The paper is the stationary ( or static) phase in paper chromatography.

1.11 Activity 2. Identity check

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Rf stands for retention factor. It gives a measure of how fast a given substance passes through the chromatogram.

The Rf value of a given substance can help to identify it.

Substances which dissolve better in the mobile phase will travel faster and have a higher retention factor.

A different method

This method uses sticks of chalk as the stationary phase and an alcohol/ water mixture as the mobile phase. As with all chromatography the different components are separated because certain components in the mixture are more soluble than others. Watch the video and answer the questions.

What does this procedure show about the composition of the various inks used?
Why would it not be easy to obtain accurate Rf values using a method like this?
What is the solvent ( mobile phase ) used in this method?