How does paper chromatography work

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Related Articles. By Rachel M Johnson. By Danida. By Robert Sacchi. By Kelley Marks. By Doug West. By Linda Crampton. By Colin Quartermain. By Howard Allen. You can therefore think of paper as being cellulose fibres with a very thin layer of water molecules bound to the surface.

It is the interaction with this water which is the most important effect during paper chromatography. Non-polar molecules in the mixture that you are trying to separate will have little attraction for the water molecules attached to the cellulose, and so will spend most of their time dissolved in the moving solvent. Molecules like this will therefore travel a long way up the paper carried by the solvent.

They will have relatively high R f values. On the other hand, polar molecules will have a high attraction for the water molecules and much less for the non-polar solvent. They will therefore tend to dissolve in the thin layer of water around the cellulose fibres much more than in the moving solvent.

Because they spend more time dissolved in the stationary phase and less time in the mobile phase, they aren't going to travel very fast up the paper. The tendency for a compound to divide its time between two immiscible solvents solvents such as hexane and water which won't mix is known as partition.

Paper chromatography using a non-polar solvent is therefore a type of partition chromatography. A moment's thought will tell you that partition can't be the explanation if you are using water as the solvent for your mixture.

If you have water as the mobile phase and the water bound on to the cellulose as the stationary phase, there can't be any meaningful difference between the amount of time a substance spends in solution in either of them. All substances should be equally soluble or equally insoluble in both. If water works as the mobile phase as well being the stationary phase, there has to be some quite different mechanism at work - and that must be equally true for other polar solvents like the alcohols, for example.

Partition only happens between solvents which don't mix with each other. Polar solvents like the small alcohols do mix with water. In researching this topic, I haven't found any easy explanation for what happens in these cases. Most sources ignore the problem altogether and just quote the partition explanation without making any allowance for the type of solvent you are using. Other sources quote mechanisms which have so many strands to them that they are far too complicated for this introductory level.

I'm therefore not taking this any further - you shouldn't need to worry about this at UK A level, or its various equivalents. Producing a paper chromatogram You probably used paper chromatography as one of the first things you ever did in chemistry to separate out mixtures of colored dyes - for example, the dyes which make up a particular ink.

The diagram shows what the plate might look like after the solvent has moved almost to the top. R f values Some compounds in a mixture travel almost as far as the solvent does; some stay much closer to the base line. For each compound it can be worked out using the formula: For example, if one component of a mixture travelled 9. What if the substances you are interested in are colorless? Two way paper chromatography Two way paper chromatography gets around the problem of separating out substances which have very similar R f values.

The final chromatogram would look like this: Two way chromatography has completely separated out the mixture into four distinct spots. How does paper chromatography work? Simple chromatography is carried out on paper. A spot of the mixture is placed near the bottom of a piece of chromatography paper. The paper is then placed upright in a suitable solvent , such as water. As the solvent soaks up the paper, it carries the mixtures with it. She has written for a variety of online destinations, including Peternity.

She holds a Bachelor of Arts in communication from the University of Rochester. What Is Contained in a Permanent Marker? Simple Chromatography Experiments. How to Separate the Components of Ink. Food Coloring Experiments. How Does Ink Diffuse in Water? How to Separate Ink From Water.

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