6.3.1 - Chromatography and Qualitative Analysis

Question 1

What is chromatography?

Answer 1

Analytical technique used to separate components of a mixture.

Question 2

What does chromatography use?

Answer 2

A mobile phase (moves in a definite direction) and stationary phase (fixed).

Question 3

In TLC what is the stationary phase?

Answer 3

A solid e.g. aluminium oxide coating a sheet of glass.

Question 4

In TLC what is the mobile phase?

Answer 4

A liquid e.g. a solvent which moves vertically up the plate.

Question 5

In gas chromatography what is the stationary phase?

Answer 5

Liquid or solid on an inert solid support, e.g a long chain alkane coating the inside of a coiled tube.

Question 6

In gas chromatography what is the mobile phase?

Answer 6

A carrier gas e.g. inert gas like nitrogen.

Question 7

How does separation occur?

Answer 7

The components of the mixture have different attractions for the solid and liquid phases. The greater the interaction, the more the components are slowed. As the components travel at different speeds they are separated.

Question 8

How does separation occur in TLC?

Answer 8

The stronger the absorption (attraction) to the stationary phase, the slower they are carried by the mobile phase.

Question 9

How does separation occur in GC?

Answer 9

The greater the solubility in the stationary phase, the slower they are carried by the mobile phase.

Question 10

What does the Rf value show?

Answer 10

The distance moved by the component compared to the solvent.

Question 11

Rf =

Answer 11

Distance moved by the component / distance moved by the solvent front.

Question 12

Why can you use the Rf values to identify the components of a mixture?

Answer 12

The Rf values for a particular substance are always the same if the conditions are the same.

Question 13

How else can you compare TLC?

Answer 13

A chromatogram of the mixture can be run alongside samples of known substances to allow easy identification by comparison.

Question 14

What is retention time?

Answer 14

The time taken for a component to pass from the column inlet to the detector.

Question 15

Why can retention times be used to compare using GC?

Answer 15

Compounds have different retention times. Moreover the area under the peak is proportional to the concentration of each component.

Question 16

How are calibration curves used?

Answer 16

• Standard solutions of known concentration are prepared.
• GC are obtained for each standard solution.
• The peak integration is found for each.
• A graph of peak area against concentration is plotted (calibration curve).
• Compare the peak area of the compound being investigated with the calibration curve under the same conditions to find the concentration.

Question 17

Test for alkenes?

Answer 17

Add bromine water - should decolourise (orange to colourless)

Question 18

Test for haloalkanes?

Answer 18

Add ethanol and then silver nitrate - chloroalkane = white precipitate, bromoalkane = cream precipitate, iodoalkane = pale yellow precipitate.

Question 19

Test for phenols?

Answer 19

Firstly add NaOH to identify whether it is acidic - should form a colourless solution. To identify the phenol add a solid carbonate such as sodium carbonate - carbonates only react with strong acids so nothing should happen.

Question 20

Test for carboxylic acids?

Answer 20

Add carbonate such as sodium carbonate - solution will fizz and carbon dioxide gas produced will turn limewater cloudy.

Question 21

Test for carbonyl group?

Answer 21

Add Brady’s reagent, 2,4-dinitrophenylhydrazine, shake and a bright orange precipitate should be produced.

Question 22

Test for aldehydes?

Answer 22

Tollen’s reagent - silver mirror produced.

Question 23

How is Tollen’s reagent made?

Answer 23

Add silver nitrate solution, then sodium hydroxide solution. A light brown precipitate should form. Add drops of dilute ammonia solution until the brown precipitate dissolves completely.

Question 24

Test for primary/secondary alcohols?

Answer 24

Add acidified potassium dichromate and warm the mixture in a hot bath. Orange to green. Remains orange for tertiary alcohols.

Question 25

How can the concentration of a component be determined in gas chromatography?

Answer 25

Measure peak area for different concentrations. Plot calibration curve of peak area against concentration. Read off peak area of unknown to find concentration.