6.5.5: Chromatography Flashcards
Chromatography
use
To separate components of a mixture.
Mobile phase
Where the molecules can move.
Always a liquid or gas
Stationary phase
Where the molecules can’t move.
Solid or a liquid on a solid support
TLC
Thin layer chromatography
Thin layer chromatography
Mobile ands stationary phases
Mobile phase, a liquid solvent eg:ethanol
Stationary phase, a thin layer of silica of alumina fixed to a glass of metal plate
Alumina and Silica
Aluminium oxide
Silicon dioxide
The baseline
A pencil line at the bottom of the TLC plate where small drops of each mixture are put.
Why should the level of the solvent be below the baseline?
So the sample doesn’t dissolve away
Why is the top of the beaker in TLC covered with a watch glass?
To stop the solvent evaporating
Where should the TLC plate be left to dry and why?
A fume cupboard
So toxic fumes are prevented from escaping
Methods to reveal colourless chemicals such as amino acids
2
Use of fluorescent dye
Use of iodine vapour
How does use of fluorescent dye reveal colourless chemicals?
A fluorescent dye is added to the silica/alumina layer.
The plate is put under a UV lamp and draw around the dark patches in pencil = where the chemicals are.
How does use of iodine vapour reveal colourless chemicals?
Expose the chromatography to iodine vapour (a locating agent) which will stick to the chemicals and show as purple spots.
Rf value equation
Dt = distance travelled
Rf value = dt. by spot/ dt by solvent
How is the distance travelled by the spot measured?
Measure from the baseline to the verticle centre of the spot
What can cause different Rf values
3
Different:
Temperature
The composition of the TLC plate
Different solvent
Look up Rf values…
In a table of standard Rf values
Column chromatography use
Mostly used to purify an organic product.
Station phase in column chromatography
A glass column packed with a solid, absorbent material such as aluminium oxide coated with water- called a slurry.
Mobile phase in column chromatography
A solvent is run slowly and continually through the column.
Rt
Retention time
Why do components have different retention times
Depends on how soluble they are in the mobile phase and how strongly they are absorbed in the stationary phase.
