Monday, 8 September 2014

Chromatography and why change is bad.

I remember with fondness doing paper chromatography experiments at school using Smarties. The food colour on the outside of the Smarties, especially the brown ones, would separate into different colours proving that - erm …

… well … you know …

That life was more complicated than it would appear on the surface? 

Felt pen chromatography in water
Reasons aside, I was recently trying to recreate this wonder of kitchen chemistry for a science fair event. I discovered three things. First, my memory of this may be clouded by time. Second that there is more to paper chromatography than I recalled. And lastly, that Smarties are not what they used to be. It would appear that the colours on the outside of Smarties no longer have the neon zing they used to. Instead, we now have hippy natural colours that are rubbish for chromatography. The dyes are so water soluble, they all run at the water front. Further more, felt-tip pens are now made with super-washable inks and you have the same problem. The different dyes just don't want to separate.

Before I go further, a digression to explain the science. In paper chromatography you take a strip of filter paper (or blotting paper or even clean, white newspaper) and make a spot of colour near one end. You then dunk the end in water which obligingly soaks up the paper and smears the colours out as it does. The key to where each colour ends up on the paper is determined by how much the dye molecules like to stick to the paper and how happy they are in a water solution. Water is a highly charged liquid, full of positive and negative charges and if the dye molecule is also charged, it will be happy in solution and will thus move further up the paper. Conversely, if the dye is uncharged, it will tend to stick to the paper more and not move. Because the world is messy, most dye molecules have some charged bits and some uncharged sections. The distance the dye molecule moves depends on how much of it is charged and how charged the liquid is that soaks up the paper.


Same colours with salt water
Fortunately, it is possible to recapture the bygone, halcyon-glory of childhood chromatography with the addition of a teaspoon of salt. If you add a bit of salt (1 to 10% by weight) to the water it becomes more charged with sodium and chloride ions. Now any dye molecule that has uncharged regions will tend to stay stuck to the paper. While modern dyes are all charged to an extent, adding the salt allows you to separate out the dyes based on more subtle variations.

In the example on the left, the red and yellow colour must be less charged than the blue, since they don't move much in salt water. The purpley mauve colour is somewhere in between.

So, not content with my success, I decided to take the whole set of 20 colour felt tips and examine them chromatographically. It was the obvious thing to do surely? Below is the glorious, technicolour result. Most significantly it proved to me that you could make every colour in the whole pack of felt tips using just four colours, as seen in the the chromatography strips labelled red, yellow, pink and light blue. I should point out that I didn't come up with the names for the colours. For that I deferred to my partner in this particular crime. I don't know what colour "mushroom" is, but I do now know what dyes it contains.

More felt pen chromatography with salt water

So, it turns out it is possible to recapture the chromatographic experiences of your youth, despite the terrible changes wrought on the dyes we use by parents and their desire for washable pens. You just need salt.

1 comment:

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Science TV Presenter, live show performer, writer, strange prop builder and all round Science Bloke. All opinions expressed are mine alone. Not the BBC's, just mine.