Required Practical 12: Chromatography

Question 1

Chromatography

Answer 1

Analytical technique by which a chemical mixture is separated into it components

Question 2

Mobile Phase

Answer 2

Moves and carries soluble components of mixture, i.e. solvent in TLC or eluent in column chromatography

Question 3

Stationary Phase

Answer 3

Attracts components of mixture, i.e. filter paper or silica plate in TLC

Question 4

Thin Layer Chromatography

Answer 4

A silica or aluminium oxide plate is coated with solid and solvent moves up plate

Question 5

Explain how components of a mixture are separated by TLC

Answer 5

• mixture is adsorbed onto plate which is polar
• plate placed in solvent which moves up plate due to capillary action
• balance between affinity of mixture to mobile and stationary phase in order to provide great enough separation

Question 6

Rf

Answer 6

Retardation Factors = distance moved by spot/distance moved by solvent

Question 7

Compare paper and thin layer chromatography

Answer 7

• both have liquid mobile phase
• both separate mixture in liquid state
• TLC is less time consuming
• separation is more sharp for TLC
• stationary phase is more robust for TLC (silica plate vs paper)

Question 8

Suggest applications of TLC

Answer 8

• test purity of a substance
• check composition of a mixture
• determine end point of a reaction

Question 9

Column Chromatography

Answer 9

A column is packed with powdered silica and eluent moves down column

Question 10

Suggest why column in column chromatography has a mineral wool plug

Answer 10

Allows passage of solvent but not powder

Question 11

Suggest how a mixture is separated by column chromatography

Answer 11

Components of mixture run down column at different rates so have different retention times based on their relative affinity for the mobile and stationary phase

Question 12

Gas Chromatography(-Mass Spectrometer)

Answer 12

A column is packed with solid or solid coated by liquid and a gas is passed through column under pressure at high temperature

Question 13

Suggest how components of a mixture are identified by gas chromatography

Answer 13

Components have different retention times so leave column at different times to be detected and identified by mass spectrometer

Question 14

Suggest applications of gas chromatography-mass spectrometer / high pressure liquid chromatography

Answer 14

• environmental monitoring of pollution
• blood/urine drug tests for athletes
• food/beverage safety and quality analysis
• forensics

Question 15

Suggest how components with similar Rf values can be accurately identified using TLC

Answer 15

• run chromatography a second time using a different solvent
• rotate plate 90 degrees
• check if any component is masked by another

Question 16

Suggest ways organic molecules which are colourless can be identified on a chromatogram

Answer 16

• ninhydrin spray for amino acids

- ultraviolet light for other organic molecules

Question 17

Describe a method for thin layer chromatography

Answer 17

• Hold TLC plate with plastic gloves
• Draw a thin 1.5cm pencil line from the bottom of the plate
• Use a capillary tube to apply a small drop of solution onto mid point of pencil line
• Allow spot to dry completely
• Place TLC plate in tank with line above solvent level and cover lid
• When solvent nears top remove plate from solvent
• Mark solvent line immediately (volatile solvent)
• Allow to dry in fume cupboard (toxic solvent)
• Locate spots using UV light or developer spray
• Calculate Rf values

Question 18

Suggest why it is essential to wear gloves when handling TLC plate

Answer 18

Avoid contamination of plate from amino acids transferred from fingerprints

Question 19

Suggest why it is not essential for solvent front to reach near the top of plate but recommended

Answer 19

• Rf value is ratio of lengths

- the longer the distance measured the smaller the percentage uncertainty

Question 20

Suggest why different amino acids have different Rf values

Answer 20

• different R groups

- different solubility/ affinity for solvent

Question 21

State what determines the distance travelled by a spot in TLC

Answer 21

Relative affinity to stationary AND mobile phase (not solubility in solvent alone)

Question 22

Explain why component has a higher Rf value with a solvent mixture of hexane and ethyl ethanoate compared to hexane alone

Answer 22

• solvent is more polar
• component has higher affinity for mobile phase
• travels further along TLC plate

Question 23

Explain why component has a different retention time to another in column chromatography

Answer 23

• polarity of each component is different based on the type of bonding present
• more polar components have a greater affinity for the stationary phase so higher retention time