4.6d thin layer chromatography

Question 1

What is chromatography?

Answer 1

• the technique used to separate the components present within a mixture
• it separates substances by making use of differences in their polarity or molecular size

Question 2

What does thin layer chromatography use?

Answer 2

a fine film of silica or aluminium oxide spread over glass, aluminium foil or plastic

Question 3

What is the process of this layer chromatography?

Answer 3

• a small sample of the mixture being tested is spotted onto the base (pencil) line off the chromatogram
• a solvent dissolves the compounds in the spot and carries the compounds up the chromatogram

Question 4

What does how far the commands are carried depend on?

Answer 4

how soluble the compounds are in the chosen solvent and how well they adhere to the plate

Question 5

What is normally required to visualise the spots on the chromatogram?

Answer 5

a developing agent or ultraviolet light

Question 6

What equation is used for Rf values to be calculated?

Answer 6

Rf = distance travelled by the sample/distance traveled by the solvent

Question 7

Explain Rf values and conditions

Answer 7

Under the same conditions (temperature, solvent and saturation levels) aa compound always has the same Rf value (with experimental error)

Question 8

How can the identity of a compound be confirmed?

Answer 8

• comparing the experimentally determined Rf values with a literature or known value determined under the same conditions
• making a direct comparison on a TLC plate between the compound being tested and the pure substance - a co-spot could be used

Question 9

What is TLC used for?

Answer 9

to assess the purity of substances

Question 10

Explain pure substances in TLC

Answer 10

• when spotted and developed on a TLC plate, a pure substance should appear as a single spot (some impurities may not be visible by TLC analysis)
• the presence of more than one spot shows that impurities are present