Paper Chromotography Flashcards
what is chromatography?
-an analytical method used to separate the substances in a mixture
-you can then use it to identify the substances
what are the 2 phases in chromatography?
1) a mobile phase
2) a stationary phase
what is a mobile phase?
where the molecules can move- this is always a liquid or a gas
what is a stationary phase?
where the molecules cant move. This can be a solid or really thick liquid
what happens during chromatography?
1)During a chromatography experiment the substances in the sample constantly move between the mobile and stationary phases s an equilibrium is formed between both
2)the mobile phase moves through the stationary phase and anything dissolved in the mobile phase moves with it. How quickly a chemical moves depends o how it is distributed between the 2 phases (whether it spends more time in the mobile phase or the stationary phase)
3)The chemicals that spend more time in the mobile phase than the stationary phase will move further
How many spots will a pure substance form during chromatography?
1 as there is only one substance in the sample
What is the result of chromatography analysis called?
a chromatagram
How do yo carry out paper chromatography?
1) draw a line near the bottom of a sheet of filter paper (use a pencil to do tis as pencils are insoluble and won’t dissolve in the solvent)
2)add spot of ink to the line and place the sheet in a beaker of solvent e.g. water (however the solvent depends on what is being tested some other compounds dissolve in solvents like ethanol). Make sure the ink is not touching the solvent as you do not want it to dissolve into it
3)place a lid on top of the container to stop the solvent evaporating
4)the solvent will seep up the paper carrying ink with it. Each different dye in the ink will move up the paper at a different rate so the dyes will separate out and each will form a spot in a different place
5)when the solvent has nearly reached the top of the paper take the paper out of the beaker and leave it to dry leaving a chromatogram
what does the amount of time the molecules spend in each phase in paper chromatography depend on?
1)how soluble the are in the solvent
2)how attracted they are to the paper
*molecules with a higher solubility in the solvent are less attracted to the paper and will spend more time in the mobile phase and will therefore be carried further up the paper
what is an Rf value?
-the ratio between the distance travelled by the dissolved substance (the solute) and the distance traveled by the solvent
-the further through the stationary phase a substance moves the larger the Rf value
How do you calculate the Rf value?
Rf= distance travelled by substance (this is the distance from the baseline to the centre of the spot)/distance travelled by solvent
Why is chromatography usually carried out?
-it is often carried out to see if a certain substance is present in a mixture
-to do this you run a pure sample of that substance (a reference)alongside the unknown mixture and if the Rf values of the reference and one of the spots in the mixture match then the substance may be present)
what does the Rf value depend on?
-the Rf value is dependent on the solvent- if you change the solvent the Rf vaue for a substance will change
-you can test both the mixture and the reference in a number of different solvents. if the Rf value of the reference compound matches the Rf vaue to one of the spots in the mixture in all the solvents it is likely the reference compound is present but if the value is only the same in some solvents the reference compound is not present
