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 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. 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 can you observe
colourless spots?

Answer 12

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 13

How do you calculate the Rf
value?

Answer 13

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 14

What does Rf
value
stand for?

Answer 14

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 15

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

Answer 15

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 16

What is the stationary phase
in gas-liquid
chromatography?

Answer 16

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 17

What is the mobile phase in
gas-liquid chromatography?

Answer 17

Carrier gas, inert e.g. N2
or He

Question 18

What do you measure in
gas-liquid chromatography?

Answer 18

Retention time; different components of the
mixture take different amounts of time to move
through

Question 19

What are the advantages of
GLC?

Answer 19

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 20

What are GLC’s uses?

Answer 20

Test athletes’ and horses’ blood and urine for
drugs

Question 21

What are GLC’s uses?

Answer 21

Test athletes’ and horses’ blood and urine for
drugs

Question 22

How can you use GC or
GCMS to identify
substances?

Answer 22

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 23

How does GCMS work?

Answer 23

Gas Chromatography is run, retention time is
recorded, then mixture is run through a Mass
Spectrometer. Fragmentation pattern/molecular
ion peak confirms identity.

Question 24

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

Answer 24

Shake with bromine water, result is bromine
water is decolourised (orange to colourless)

Question 25

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

Answer 25

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 26

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

Answer 26

Add acidified K2Cr2O7
(potassium
dichromate(VI)) and heat
Result: colour change from orange to green for
1
0
and 20
alcohols (note: no change for 30
alcohols)

Question 27

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

Answer 27

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 28

How do you test for
carboxylic acids? What is
the result?

Answer 28

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

Question 29

How do you test for phenols?

Answer 29

By weak acidity - there is a neutralisation
reaction reacted with NaOH but no
reaction with CO3
2-

Question 30

How do you test for
carbonyl compounds?

Answer 30

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