Chromatography…

I wrote a little about chromatography when I discussed simple separation techniques previously but I thought it might be interesting to look at it in more detail.

Paper chromatography uses a solvent to separate out the coloured components of a mixture. The separation occurs because of physical differences between the coloured molecules themselves. The solvent moves up the paper by capillary action as the solvent molecules are attracted to the cellulose molecules in the paper. The dissolved dyes also interact with the cellulose either causing them to travel quickly or slowly. Some dyes are more soluble than others and this affects the speed with which they travel up the paper. When I say speed, I mean relative to the solvent which obviously reaches the top of the paper first.

Different solutes have an R_ƒ value depending on how well they travel up the paper in a given solvent. The R_ƒ value is the ratio of the distance travelled by the dye to the distance travelled by the solvent. A value of 0 would indicate that the substance has not moved at all whereas a value of 1 would mean it had moved the same distance as the solvent in the same time.

In the picture above, the solvent has moved 5.0cm but the dye has only travelled 2.5cm so its R_ƒ value is 0.50 in that solvent. It is possible that different solutes may have the same R_ƒ value and then you either need to use a different solvent or a different form of chromatography.

Other methods of chromatography include thin layer, column, gas, liquid and ion exchange chromatography to name but few. In these methods it is not necessarily colours that are being separated. Individual compounds can be detected by their R_ƒ values. This is how illegal substances can be detected in an athlete’s urine sample. Chromatography is a key analytical technique used by chemists and other scientists to analyse mixtures of compounds.

Questions…

1. Ink is a mixture of a dye dissolved in a medium such as water. Is the dye the solute or solvent?
2. In paper chromatography, how does the solubility of a dye affect how far it travels?
3. The R_ƒ value of a substance is the ratio of what two distances?
4. Name a type of chromatography other than paper chromatography.
5. What is cellulose and what part does it play in your diet?
6. What sorts of illegal substances might an athlete take and why would they take them?