CHROMATOGRAPHY

So, chromatography, the first thing you'd want to consider (which will apply throughout the rest of this topic) is the property of solubility.
The whole point and use of chromatography is to be able to separate out different chemicals that you'd find in a mixture of substances i.e. ink. If you can understand that, understanding the topic won't be much harder.

How it works - Paper Chromatography

The way chromatography works is that you separate the mixture using the different chemicals' levels of solubility (i.e. how well they can dissolve in a solvent).

In simple terms, in paper chromatography, as the solvent (something that dissolves another substance) seeps up the paper (the paper absorbs it), the solvent carries the chemicals with them. The diagram below will help:

 

 

From the diagram, 'a' is the distance that the solvent (usually water) has travelled (or has been absorbed) up the paper. 'b' represents the distance that one of the chemicals has dissolved up the paper with the solvent.
The base line is where all the chemicals are placed evenly to dissolve and be tested.

And all of this happens inside a large measuring cylinder or container.


What I mean by 'be tested' is that the diagram also shows a formula regarding 'Rf', this is used to indicate what type of chemical it is.
You calculate the Rf value by dividing the distance 'a' by the distance 'b' (on the image) and that value can tell you (from a database) what type of chemical that is.

And that's how you distinguish chemicals from a mixture/substance.

Step by Step Paper Chromatography;

1. Draw a line near the bottom of a sheet of filter paper, not regular paper - (this is the Base line)
2. Add spots of the different chemicals/dyes to the base line at equal intervals (as shown in the picture above.)
3. Now place the filter paper in a container of shallow solvent (i.e.water - depending on how well the chemical can dissolve) - Make sure that the solvent doesn't touch the chemical spots because, the chemicals.
4. Place a lid on the container to stop the solvent from evaporating.
5. The solvent will seep up the paper carrying the chemicals with it.
6. The chemicals will move at different rates depending on how soluble they are/how well they can dissolve.
7. The end result will show a pattern of spots called a Chromatogram.

How Chromatograms are used to identify Dyes.

A diagram to show chromatograms for all dyes.

Scenario: say you have an unknown brown dye and you're trying to find out what type of chemicals are present in the dye, you would use chromatography!

1. Firstly you make a chromatogram for the brown dye - (you would repeat the steps above for the brown dye)
2. Then you would make chromatograms for other dyes that you suspect would be in the brown dye. (say you chose purple, green and yellow.)
3. Finally you'd compare all the chromatograms and see which dyes are in the brown dye.

So from the diagram above we can conclude that the yellow and the purple dyes are both in the brown dye because on the chromatogram the purple and yellow chemicals line up level with all of the brown chemicals and they are of the same colour.

Thanks for reading! Hope this clarified any uncertainties in chromatography. :)

References:

Paddy Gannon - 2005CGP - Edexcel Chemistry - Pg 6 - Chromatography.