5.2 Paper Chromotography Flashcards
What is chromatography?
Chromatography is an analytical method used to separate the substances in a mixture. You can then use it to identify the substances.
What is the mobile phase in paper chromatography?
Where the particles of the phase can move. This is always a liquid or a gas.
What is the stationary phase in paper chromatography?
Where the particles of the phase can’t move. This can be a solid or a very thick liquid.
In paper chromatography, different chemicals will have different ____________ between the same phase - they’ll spend different amounts of time dissolved in the mobile phase or attracted to the stationary phase
distributions
What does the mobile phase do to the stationary phase?
The mobile phase moves through the stationary phase, and anything dissolved in the mobile phase moves with it.
The _____ time a chemical spends in the mobile phase, the further through the stationary phase it’ll move.
more
In paper chromatography, what is the stationary phase?
Filter paper
In paper chromatography, what is the mobile phase?
The solvent
In paper chromatography, what does the amount of time the molecules spend on each phase depend on?
How soluble they are in the solvent and how attracted they are to the ppaer
Describe how to do the paper chromatography practical (8)
1) Draw a line near the bottom of a sheet of filter paper. (Use a pencil to do this - pencil marks are insoluble and won’t dissolve in the solvent).
2) Add a spot of the test substance to the line and place the sheet in a beaker of solvent so that the solvent is just below, but not touching the spot.
3) The solvent used depends on what’s being tested. Some compounds dissolve in water, or in solutions where water is in the solvent (aqueous solutions), but sometimes other solvents (known as non-aqueous solutions) like ethanol, are needed.
4) Place a lid on top of the container to minimise the evaporation of solvent.
5) The solvent seeps up the paper, carrying the test substance with it.
6) When the solvent has nearly reached the top of the paper, take the paper out of the beaker, mark the point that the solvent reached with a pencil and leave the paper to dry.
7) The end result is a pattern of spots called a chromatogram.
8) If the chromatogram has more than one spot, the substance is impure. If it contains only one spot it may well be pure (but you’ll have to check that only one spot’s produced in other solvents too.)
What are the two different ways of using locating agents to make colourless chemicals visible, on the chromatogram?
- If the mixture contains amino acids, you can spray ninhydrin solution on the completed chromatogram, which should turn the spots purple.
- You could also dip the completed chromatogram into a jar containing a few iodine crystals. Iodine vapour sticks to the chemicals on the paper and they’ll show up as purple spots.
How do you find the Rf?
distance travelled by solute/distance travelled by solvent
How can the Rf value of a substance change?
It can change if the method used in the chromatography experiment changes, for example if a different solvent (or concentration of solvent), a different temperature or a different stationary base is used.
What could you change in your experiment if the spots on your chromatogram don’t separate properly?
The conditions: different solvent, concentration of solvent, different temperature, different stationary base
When might repeating a chromatography experiment with different conditions, be useful? (2)
- This can be useful if you get spots on a chromatogram that don’t separate properly - repeating the chromatography experiment with different conditions could cause them to separate.
- It’s also useful if two reference compounds have similar Rf values in a particular solvent. If you run the experiment again in conditions where the reference compounds have very different Rf values, you’ll be able to identify which one’s in the mixture
