Science Experiments

Chlorophyll Chromatography

Sun, 21st Mar 2010

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What you Need

Filter papers

Filter paper - white coffee filters work well, as does the paper used for posh tea bags.

Nail polish remover

Nail varnish remover which contains acetone - often the cheap brands do.

A jar

A jar

Leaves

Some leaves, ideally thin but still juicy ones, eg spinach or chrysanthemum leaves.

A penny

A small narrow smooth sided coin

What to do

Cut a piece of filter paper about 1cm wide and a bit longer than your jar is tall.

Place your leaf over the top of the strip and roll the coin across the strip about 2cm from the bottom.

Now add a little acetone (nail varnish remover) to the bottom of your jar, half a cm is plenty.

Suspend your strip of paper so the bottom end is sticking a few mm into the acetone. you can hold it up by folding the top end of the strip over forming a hook which will rest over the lip of the jar.

Wait a few minutes, with any luck you should see an interesting effect.

Try the experiment with a strongly coloured leaf like a purple one.

Hints:

- Chromatography is pretty  much a black art, and is very dependent on the exact type of paper you are using and the solvent you are using, so if it doesn't work try a different type of paper.

- To get more pigment into the paper and produce a better result you can add some leaf juice, let the paper dry out, and then add some more - repeat as many times as you have the patience for.

- Don't let the paper touch the side of the jar, or all the solvent will move up through the gap between the paper and the glass bypassing your sample.

- Doing the experiment inside a jar seems to be important - probably because it gets filled with acetone vapour and so reduces the evaporation rate of the acetone.

- The experiment only seems to work well on filter paper of one sort or another - probably because it is designed not to stick to things where as most other forms of paper are designed to stick to inks, or what you spilt on the table.

What may happen

You should find that the acetone slowly rises up the paper, and when it reaches the green mark it takes some of the colour with it, and seperating out the colours.

 

Doing chromatography on chlorophyl

You should find that may purple leaves still contain green pigments - otherwise they couldn't photosynthesize.

Why does it happen?

This is a technique called chromatography, it works because when you put the paper into the solvent, the solvent is drawn up into the paper by surface tension. This means that there is a flow of liquid over the paper. Some substances spend more time in the solvent and others spend more time on the paper, so they move at different speeds, and get separated out.

 

Chromatography Start

Moving solvent

Colours having moved

The particles of the dyes all start off in one place

When the solvent arrives, some of them spend more time in solution that others.

The ones that spend the largest proportion of the time in the solvent move the quickest.

This is a very powerful technique which is widely used by chemists to separate out mixtures, both analytically to find out what is in the mixture and to separate out different products of a reaction. The speed of each substance is very dependent on the solvent and the substrate (in this case filter paper) so it is a case of finding the right combination of the two to separate out the substances you want to separate.

Why was there more than one colour of green?

 

Chlorophyll absorption spectra

Showing the two different absorption spectra of Chlorophyll a and b  LewiƄki, Holak @ wikipedia

This particlular set  of substrate and solvent has been optimised to separate out different forms of chlorophyll - the pigment which gives plants their colour and is sued in photosynthesis (apparently the ideal combination is dry acetone moving over icing sugar, but dry acetone is hard to get hold of at home). There are in fact 4 types of chlorophyll each having a different absorption spectra. There are two main types found in plants, both which look green but one bluer and the other yellower, so you see two greens. This means that the plant can absorb a greater range of colours of light, and wastes less of the spectrum.

Dave Ansell

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