3.3.16 - chromatography
No description yet. This curated deck is ready for daily study.
Card preview (24)
thin layer chromatography (TLC) allows us too…
separate and identify unknown compounds
stationary phase of TLC and mobile phase
Stationary phase - doesn’t move - silica or alumina mounted on a glass/metal plate a pencil base line is drawn and drops of mixtures added Mobile phase - moves - solvent or eluent
what is done with stationary phase of TLC
placed in the solvent - the base line must be above the solvent level leave until solvent has moved up to near the top of the plate remove, mark the solvent front and allow to dry
how does TLC work
works by mixture spots dissolving in the solvent some chemicals in the mixture may not dissolve as much and stick to the stationary phase quickly what we are left with is a chromatogram
how can we identify chemicals
using their positions on the chromatogram
how do we see a colourless compound/ spot on a chromatogram
colourless compounds can be seen using iodine or fluorescent dues and uv light
how are fluroscent dyes and uv light used to see colourless compounds
adding a flurosecent dye to silica/alumina can be seen using a uv lamp the colourless spots on the chromatogram will block any glow from the fluorescent dye you can then draw round these spots to mark where they are
how is iodine used to see colourless compounds
place the chromatogram in a sealed jar with a few iodine crystals the iodine vapour sticks to the chemicals on the plate dying them purple
iodine vapour when using iodine to see colourless compounds is known as a…
locating agent
Size of sample in chromatography and what can be done after separation
Only small sample After seperation pure components can be further analysed
TLC adv
Simple and quick
TLC method
a) Wearing gloves, draw a pencil line 1 cm above the bottom of a TLC plate and mark spots for each sample, equally spaced along line. b) Use a capillary tube to add a tiny drop of each solution to a different spot and allow the plate to air dry. c) Add solvent to a chamber or large beaker with a lid so that is no more than 1cm in depth d) Place the TLC plate into the chamber, making sure that the level of the solvent is below the pencil line. Replace the lid to get a tight seal. e) When the level of the solvent reaches about 1 cm from the top of the plate, remove the plate and mark the solvent level with a pencil. Allow the plate to dry in the fume cupboard. f) Place the plate under a UV lamp in order to see the spots. Draw around them lightly in pencil. g) Calculate the Rf values of the observed spots.
Why use a pencil line
Won’t dissolve in solvent
Tiny drop used bc
Too big a drop will cause different spots to merge
Depth of the solvent why
If solvent to deep it will dissolve the sample spots from the plate
Why use a lid
Prevent evaporation of toxic solvent
Dry in fume cupboard bc
Solvent is toxic
Uv lamp used bc
the spots are colourless and not visible
Rf =
Distance traveled by spot ————————————— Distance travelled by solvent
TLC works dep on the
affinity of the component molecules for the mobile phase compared to the stationary phase.
The greater the solubility of the component molecules in the solvent…
the further the component molecules move up the TLC plate.
Solubility of the component molecules depends on the …
intermolecular forces between solvent and component molecules.
After you calc Rf value you …
Compare to a known reference value to identify substance Known Rf value must be for same solvent
2 dimensional chromatography
In order to separate a complex mixture that has components of different solubility in solvents, it may be necessary to do chromatography with two different solvents. A spot of the mixture on a TLC plate is first separated with one solvent. Then the TLC plate is rotated 90° and the plate is placed in a second solvent for a second separation to take place