Paper & TLC

Question 1

What is chromatography use for?

Answer 1

used to seperate and analyse the substances present in a mixture

Question 2

What does the ability of the components to stick to the stationary phase in chromatography depend on?

Answer 2

will depend on the polarity of the stationary phase and the component molecules

Question 3

What does the rate of movement of each component in chromatography depend on?

Answer 3

it depends on
- how strongly the component absorbs onto the stationary phase
- how readily the component dissolves in the mobile phase

Question 4

Why do the components separate in chromatography?

Answer 4

they separate because the undergo absorption and desorption to different degrees

Question 5

What two phases do all methods of chromatography have?

Answer 5

all methods of chromatography have
- stationary phase
- mobile phase

Question 6

How does paper chromatography work?

Answer 6

• components are swept upwards over the stationary phase (the paper) by the solvent
• the components undergo a continual process of adsorption to the solid stationary phase, followed by desorption and dissolving into the mobile phase

Question 7

What is adsorption?

Answer 7

the gathering of molecules or substances to the surface of a solid or liquid

Question 8

What is desorption?

Answer 8

when the component dissolves into the liquid

Question 9

What is absoption?

Answer 9

particles and substances soak into a surface

Question 10

What is the stationary phase in thin layer chromatography?

Answer 10

• a fine powder such as alumina (aluminium oxide), spread on a glass or plastic plate
• Silica gel

Question 11

is thin layer and paper chromatography qualitative or quantitative?

Answer 11

• Both are QUALITATIVE analysis in which the chemicals present in the mixture are identified

Question 12

How do paper and thin-layer chromatography work:

Answer 12

1. a small spot of the solution of the sample to be analysed is placed on one end of the chromatography paper or plate (the position of this spot is called the origin)
2. the paper or plate is then placed in a container with solvent
3. the solvent rises up the paper or plate and the components of each sample separate depending on their attraction to the stationary phase and their solubility in the solvent

Question 13

Why must the origin be placed a little above the level of the solvent?

Answer 13

• so that the components can be transported up the paper or plate and not dissolve into the liquid in the container

Question 14

What are two ways to identify the components of a mixture?

Answer 14

1. include standards of known chemicals
2. calculate retardation factor of the sample and compare these with the Rf of known samples

Question 15

How to calculate retention factor:

Answer 15

distance the component travelled from the origin
DIVIDED BY
distance the solvent front travelled from the origin

Question 16

Factors that can change the Rf value of a chemical:

Answer 16

• changes in temperature
• the type of stationary phase
• the amount of solvent vapour around the paper or plate
• the type of solvent

Question 17

What are some pros of paper chromatography?

Answer 17

• cheap
• little preparation
• more efficient for polar and water-soluble compounds
• easy to handle and store

Question 18

What are some pros of thin-layer chromatography?

Answer 18

• detects smaller amounts
• better separation of less polar compounds
• corrosive materials can be used
• a wide range of stationary phases are available

Question 19

How do you make colourless compounds visible on a chromatogram?

Answer 19

• inorganic compounds fluoresce and become blue when viewed under ultraviolet light
• for other compounds the chromatogram can be sprayed with a chemical that reacts to form coloured or fluorescent compounds e.g. ninhydrin reacts with amino acids to give and blue and brown coloured compounds that are easily detected on the chromatogram

Question 20

How does column chromatography work?

Answer 20

• the solid stationary phase is packed into a glass column
• the sample mixture is applied carefully to the top of the packed solid, and a solvent, which acts as the mobile phase, is dripped slowly onto the column from a reservoir above
• a tap at the bottom of the column allows the solvent (eluent) to leave the column at the same rate as it enters at the other end

Question 21

What factors determine which method of chromatography to use?

Answer 21

• properties of the components being separated
• amount of sample available for analysis
• concentration of the component in the sample
• sensitivity of the technique
• time taken for analysis
• cost of equipment

Question 22

What are the typical substances used in paper and thin layer chromatography?

Answer 22

• polar, water-soluble substances

Question 23

What are the typical samples used in paper and thin-layer chromatography?

Answer 23

• drug detection
• dyes in food

Question 24

What are some advantages of paper and thin-layer chromatography?

Answer 24

• very cheap
• only basic laboratory equipment
• easy to perform

Question 25

What are some disadvantages of paper and thin-layer chromatography?

Answer 25

• poor precision and accuracy