Paper chromotography Flashcards
What is chromatography + used for
An analytical method used to separate the substances in a mixture = identify substance
The different phases of chromatography
Mobile phase = where the molecules can move, this is always a liquid or a gas
Stationary phase = where the molecules can’t move - this can be a solid or a really thick liquid
details on the phases 1-2
1) During a chromatography experiment, the substances in the sample move between mobile and stationary phase - equilibrium is formed between the 2 phases
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 on how it’s distributed between the two phases- whether it spends more time in MP / SP
details on the phases 3-4
3) The chemicals that spend more time in mobile phase than stationary phase will move further through the stationary phase.
4) The components in a mixture will normally separate through the stationary phase, so long as all the components spend different amounts of time in the mobile phase.
The number of spots may change in different solvents as the distribution of the chemical will change depending on the solvent. A pure substance will only ever form one spot in any solvent as there is only one substance in the sample
During chromatography, What is the stationary and mobile phases.
Stationary phase = chromatography paper
mobile phases = solvent (water, ethanol )
The amount of time the molecules spend in each phases depend on:
2 things
- how soluble they are in the solvent
- how attracted they are to the paper
Molecules with higher solubility in the solvent = less attracted to paper = spend more time in mobile phase = carried further up the paper
RF value practical steps 1-2
1) The results of chromatography analysis is called chromatogram
2) An Rf value is the ratio distance travelled by the dissolved substance and the distance travelled by the solvent. The further through stationary phase a substance moves = Larger Rf value
Rf = distance travelled by the substance (B) / the distance travelled by the solvent (A)
RF value practical steps 3-4
3) chromatography is often carried out to see if a certain substance is present in the mixture .To do this you run a pure sample of that substance along the unknown mixture.
If Rf value of reference and one of the spots in mixture match = the substance may be present
4) The Rf value is dependent on the solvent- If you change the solvent the Rf value of the substance will change .You can test both the mixture and the reference in a number of different solvent
If the Rf value of the reference compound matches the Rf value of one of the spots in the mixture in all the solvents, then its likely the reference compound is present in the mixture
If the spots in the mixture and the spot in the reference only have the same Rf value in some of the solvents, then the reference compound isn’t present in the mixture.
