Chemistry practical 12 - chromatography

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Why are plastic gloves worn when drawing the pencil line on the plate?
To prevent contamination of the plate with substances on the hands (e.g. amino acids)
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Why is a pencil used to draw on the chromatography plate instead of a pen?
The pencil line will not dissolve in the solvent.
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Why are only small drops of each solution used?
Because drops that are too big will cause different spots to merge.
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Why should the solvent be no more than 1cm deep in the beaker?
If it is too deep it will dissolve the sample spots from the plate.
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Why is a lid placed on the beaker?
To prevent evaporation of the toxic solvent - also creates a region saturated with solvent vapour so no solvent is wasted.
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Why are the results more accurate if the solvent is allowed to rise to near the top of the plate?
Gives maximum separation of the dye components so small differences in their relative solubilities are more apparent.
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Why should the TLC plate be dried in a fume cupboard?
Solvents are often toxic.
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Which 2 methods could be used to make colourless spots visible?
1) Shine a UV lamp and circle the spots in pencil 2) Use a developing agent e.g. ninhydrin that stains the sample components.
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What are the 4 main advantages of TLC over paper chromatography? *
1) Runs faster 2) Smaller amounts of mixtures can be separated 3) Spots spread out less 4) Plates are more robust than paper
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What is used as the stationary phase in column chromatography?
A powder e.g. silica or aluminium oxide.
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Where is the solvent added to the column?
At the top
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How are the components of the mixture separated in column chromatography?
Solvent (eluent) runs down column. Components of the mixture move at different rates and are collected separately in flasks at the bottom.
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What can be done to get a better separation?
Use more than one solvent. Or, in TLC, use two - way chromatography (plate turned 90 degrees and run again w/ different solvent).
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What is the main advantage of column chromatography?
Large amounts of mixture can be separated and collected (e.g. a mixture of amino acids).
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What is used as the stationary phase in gas chromatography?
A powder coated with oil, packed into or coated onto the inside of a long capillary tube.
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What is the capillary tube placed inside?
An oven with variable temperature.
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What is used as the mobile phase in gas chromatography?
An unreactive gas, e.g. nitrogen or helium.
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How are the components of the mixture separated in GC?
They have different retention times (most soluble components travel fastest through the capillary tube and leave first).
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What is the main advantage of GC?
Extremely sensitive.
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What can be used as the detector for a gas chromatography system?
A mass spectrometer.
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Other cards in this set

Card 2

Front

Why is a pencil used to draw on the chromatography plate instead of a pen?

Back

The pencil line will not dissolve in the solvent.

Card 3

Front

Why are only small drops of each solution used?

Back

Preview of the front of card 3

Card 4

Front

Why should the solvent be no more than 1cm deep in the beaker?

Back

Preview of the front of card 4

Card 5

Front

Why is a lid placed on the beaker?

Back

Preview of the front of card 5
View more cards

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