Chromatography

Question 1

what is chromatography?

Answer 1

the separation of components in a mixture which can be achieved by exploiting the physicochemical properties of the molecules concerned

Question 2

How does chromatography separate molecules?

Answer 2

Based on their degree of interaction between a stationary phase and a mobile phase.

Question 3

What is thin layer chromatography?

Answer 3

Quickly separate and visualise components of a mixture with simple equipment.

Question 4

Explain thin layer chromatography.

Answer 4

• A TLC plate made from a thin strip of aluminium coated with a solid stationary phase such as silica.
• Mixtures of component to be analysed are dissolved and then applied near the bottom of the plate.
• A solvent (mobile phase), is drawn up the plate by capillary action carrying the components with it. Separation occurs according to their different physicochemical properties.
• Separated components are viewed under UV or with chemical stain.

Question 5

Explain what silica is used for in the TLC mechanism.

Answer 5

Silica is used as the stationary phase. The silanol groups make the stationary phase polar (hydrophilic).

Question 6

Explain the TLC mechanism.

Answer 6

• The mobile phase consists of non-polar and slightly polar solvents
• The polarity of the components is determines how far they travel on the TLC plate.
• A very polar compound interacts strongly with the stationary phase, but not very strongly with the mobile phase therefore doesn’t travel far on the plate.

Question 7

What is ninhydrin used to show and what colour does it develop?

Answer 7

Used: Amino acids
Pink - primary amine
Brown - secondary amine

Question 8

What are phosphomolybdic acid and potassium permanganate used to show and what colour do they develop?

Answer 8

These are oxidising agents.

Used: alcohols, amines, sulphides - organic material
Yellow/brown spots

Question 9

What is bromocresol green stain used for?

Answer 9

Used: Acidic compounds - carboxylic acids
Green

Question 10

What is lodine vapour used for and what colour does it develop?

Answer 10

Used: high affinity for unsaturated and aromatic compounds.
Brown spots

Question 11

What is 2,4-Dinitrophenylhydrazine used for and what colour does it develop?

Answer 11

Used: Aldehydes
Orange spots.