chromatography and qualitative analysis

Question 1

what are the basic principles of all kinds of chromatography

Answer 1

• a family of separation techniques that depend on the principle that a mixture is separated
• if it is dissolved in a solvent and this mobile phase is passed over solid (the stationary phase)

Question 2

what is the mobile phase

Answer 2

carries the soluble components of the mixture

Question 3

what relationship between a sample and the mobile phase makes the sample move faster

Answer 3

more soluble components / components with more affinity to the solvent move faster

Question 4

what does the stationary phase do?

Answer 4

holds back components of the mixture that are attached to it

Question 5

what relationship between a sample and the stationary phase makes the sample move slower?
what kind of bonding does this often involve?

Answer 5

• more affinity for the stationary phase means that a component moves slower
• often attracted by hydrogen bonding

Question 6

how are substances separated by chromatography

Answer 6

• if suitable stationary/mobile phases are chosen
• the balance between affinity for the mobile phase and affinity for the stationary phase is different for each component of the mixture
• they move at different rates and are separated over time

Question 7

why will different substances show different Rf values

Answer 7

• they are bonded differently and have different polarities
• more polar bonds mean longer retention time or small Rf
• since hydrogen bonding/dipoles are attracted more strongly to the stationary phase

Question 8

what does TLC stand for?

Answer 8

thin layer chromatography

Question 9

what is the stationary phase in TLC

Answer 9

plastic/glass/metal sheet or plate coated in silica (SiO2) or alumina (Al2O3)

Question 10

what are the advantages of TLC over paper chromatography

Answer 10

• runs faster
• smaller amounts of a mixture can be separated
• TLC plates are more robust than paper

Question 11

how can you observe colourless spots

Answer 11

• shine UV light on them
• or spray with a developing agent
• developing agent for amino acids ninhydrin turns amino acid spots from colourless to purple so they can be seen

Question 12

how do you calculate the Rf value?

Answer 12

• measure the distance from the initial line (that the mixture was spotted onto) to the solvent front, and the distance from the initial line to the spot
• calculate Rf using Rf = distance moved by spot / distance moved by solvent front

Question 13

what does Rf value stand for

Answer 13

• retention factor: a measure of the rate of movement of a component through the chromatography apparatus
• a ratio between the rate of movement of the solvent and that component

Question 14

how could you confirm the identity of a substance from its Rf value

Answer 14

• compare your Rf value to accepted values Rf for that substance run in the same solvent and set-up
• if they match then identity is confirmed

Question 15

what is the stationary phase in gas-liquid chromatography

Answer 15

• powder, coated with oil
• packed into a long, thin, capillary tube
• coiled and place in an oven, the temperature of which can be varied

Question 16

what is the mobile phase in gas-liquid chromatography

Answer 16

• carrier gas
• inert N2 or He

Question 17

what do you measure in gas-liquid chromatography

Answer 17

• retention time
• different components of the mixture take different amounts of time to move through

Question 18

what are the advantages of GLC (gas-liquid chromatography)

Answer 18

• very sensitive
• GC can detect minute traces of substances in foodstuffs and link oil pollution on beaches to the specific tanker the oil came from

Question 19

what are GLC’s uses (gas-liquid chromatography)

Answer 19

• tests athletes and horses blood
• urine for drugs

Question 20

how can you use GC or GCMs to identify substances

Answer 20

• match the gas-chromatograph to that of a known substance under the same conditions
• retention time should exactly match
• substance’s identity can be confirmed by mass spectrometry, NMR or infrared spectroscopy

Question 21

how does GCMS work

Answer 21

• gas chromatography is run
• retention time is recorded
• then the mixture is run through a mass spectrometer
• fragmentation pattern/molecular ion peak confirms identity

Question 22

how do you test for alkenes? what is the result

Answer 22

• shake with bromine water
• the result is bromine water is decolourised (orange to colourless)

Question 23

how do you test for haloalkanes? what is the result?

Answer 23

• add NaOH and warm
• acidify with HNO3
• add AgNO3
• result: precipitate of AgX (Cl = white, Br = cream, I = yellow)

Question 24

how do you test for alcohol? what is the result?

Answer 24

• add acidified K2CR2O7 and reflux
• result: colour change from orange to green, no change for tertiary alcohols

Question 25

how do you test for aldehydes? what is the result

Answer 25

• warm with fehlings solution, result: red brick ppt forms from blue solution
• warm with tollens reagent, result: silver mirror (Ag ppt) forms

Question 26

how do you test for carboxylic acids? what are the results

Answer 26

• add Na2CO3
• result CO2 given off - effervescence

Question 27

how do you test for phenols?

Answer 27

• weak acidity so a neutralisation reaction
• reacted with NaOH but no reaction with CO3 2-

Question 28

how do you test for carbonyl compounds

Answer 28

react with 2,4 DNP and an orange precipitate should form