Chromatography Flashcards
What is chromatography used for?
Separating a mixture of soluble substances and to identify them.
What are the two phases of chromatography?
The mobile phase and the stationary phase.
What is the mobile phase?
Where the molecules can move. This is always a liquid or a gas.
What is the stationary phase?
Where the molecules can’t move. This can be a solid or a really thick liquid.
When do the components in the mixture separate out?
As the mobile phase moves over the stationary phase - they all end up in different places in the stationary phase.
Why do the components end up in different places in the stationary phase?
Because each of the chemicals in a mixture will spend different amounts of time dissolved in the mobile phase and stuck to the stationary phase.
What determines the speed a chemical moves through the stationary phase?
How it ‘distributes’ itself between the two phases.
In paper chromatography, what is the stationary phase?
A piece of filter paper.
In paper chromatography, what is the mobile phase?
A solvent (e.g. water or ethanol).
What is the method for paper chromatography?
1) Draw a line near the bottom of the paper - this is the baseline. (Use a pencil to do this - pencil marks are insoluble and won’t move with the solvent as ink might.) Put a spot of the mixture to be separated on the line.
2) Put some of the solvent into a beaker. Dip the bottom of the paper (but not the spot) into the solvent.
3) Put a watch glass on the top of the beaker to stop any solvent from evaporating away.
4) The solvent will start to move up the paper. When the chemicals in the mixture dissolve in the solvent, they will move up the paper too.
5) You will see the different chemicals in the sample separate out, forming spots at different places on the paper. (If one of your components is insoluble in the mobile phase, it won’t move - it’ll stay as a spot on the baseline.)
6) Remove the paper from the beaker before the solvent reaches the top. Mark the distance the solvent has moved (the solvent front) in pencil.
The amount of time the molecules spend in each phase depends on what two things?
- How soluble they are in the solvent.
- How attracted they are to the stationary phase.
How does a higher solubility in the solvent affect the time spent in the mobile phase than stationary phase?
Molecules with a higher solubility in the solvent (and which are less attracted to the paper) will spend more time in the mobile phase than the stationary phase.
What will visually happen if molecules spend more time in the mobile phase than the stationary phase?
The molecules will be carried further up the paper.
