6.3.1: 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 a 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 attracted to it.

Question 5

What relationship between a sample and the stationary phase that make 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. Thus, 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 smaller Rf value, 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 that paper

Question 11

How can you observe colourless spots?

Answer 11

Shine UV light on them.
Or spray with a developing agent (e.g. ninhydrin turns amino acid spots from colourless to purple, so they can be seen) (heating needed with ninhydrin)

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 (100m long, 0.5mm diameter).
Coiled and placed 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 e.g. 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?

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?

Answer 19

Test athletes’ and horses’ blood and urine for drugs

Question 20

How can you use GC or GCMS to identify substances?

Answer 20

Match 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 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, 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 (aq) and warm, acidify with HNO3, add AgNO3 (aq)
Result: precipitate of AgX (for Cl=white, for Br=cream, for I=yellow)

Question 24

How do you test for alcohols? What is the result?

Answer 24

Add acidified K2Cr2O7 (potassium dichromate(VI)) and heat
Result: colour change from orange to green for 1 (to the power of 0) and 2 (to the power of 0) alcohols (note: no change for 3 (to the power of 0) alcohols)

Question 25

How do you test for aldehydes? What is the result? (2 ways)

Answer 25

1. Warm with Fehling’s solution, result: brick red ppt forms (from blue solution)
2. Warm with Tollens’ reagent, result: “silver mirror” (Ag(s)ppt) forms

Question 26

How do you test for carboxylic acids? What is the

result?

Answer 26

Add Na2CO3 (aq), result: CO2 (g) given off effervescence

Question 27

How do you test for phenols?

Answer 27

By weak acidity - there is a neutralisation reaction reacted with NaOH but no reaction with CO32-

Question 28

How do you test for carbonyl compounds?

Answer 28

React with 2,4- DNP and an orange precipitate should form