Experiment 6
Simple and Fractional Distillation

250
250
200
200
250
200
150
150
150
100
100
100
Vapor Pressure vs Temperature of Water
Vapor Pressure vs Temperature of Water
Vapor Pressure vs Temperature of Water
50
0
0
0
0 20 40 60 80 100 120 140
20
20
40 60 80
80
100 120 140
140

What is and what do the following measure?
vapor pressure
The vapor pressure of a pure substance is the pressure exerted by the substance against the external pressure which is usually atmospheric pressure. Vapor pressure is a measure of the tendency of a condensed substance to escape the condensed phase. boiling point:
When the vapor pressure of a liquid substance reaches the external pressure, the substance is observed to boil. normal boiling point:
When the external pressure is atmospheric pressure, the temperature at which a pure substance boils is called the normal boiling point .

D
H vap is the energy necessary to vaporize a mole of a pure substance
B vap
A vap
D
H vap
A
D
H vap
B
A liq
B liq

I f mixing liq A and liq B and mixing gas A and gas B results in no heat of mixing in both the liquid and gas phase, the system is considered to be ideal Then:
P
A obs
=

A
P o
A
; P
B obs
=

B
P o
B where P
A obs
; P
B obs mixture; is the observed vapor pressure in the

A;

B is the mole fraction of A, B, ...

A
= n
A
/ (n
A
+n
B
)

B
= n
B
/ (n
A
+n
B
) and P o
A
; P o
B are the vapor pressures of pure A and B

A diagram of a simple distillation apparatus liq vapor liq
This process is referred to as one theoretical plate

At what temperature will a 1:1 molar mixture of cyclohexanemethylcyclohexane boil?
P
T
= P
A obs
+ P
B obs
P
T obs
=

A
P o
A
+

B
P o
B

A
=

B
= 0.5
1 Atm = 101 kPa

Vapor Pressures of Pure Cyclohexane (C); Methylcyclohexane (M)
T /K C/ kPa M/ kPa
300 14.1
305 17.6
310 21.7
315 26.5
320 32.2
325 38.8
330 46.5
335 55.3
340 65.4
345 77
350 90
6.7
8.5
10.6
13.2
16.2
19.8
24
28.9
34.6
41.2
48.7
T /K C/ kPa M /kPa
354 101.3
55.4
360 121.3
66.9
362 128.5
71.1
365 139.9
77.9
370 160.5
90.2
373 174 101.3
380 208.8
119.3
385 236.7
136.4
390 267.3
155.3
395 300.9
176.2
400 337.5
199.1

C = Cyclohexane; M = Methylcyclohexane
T/K C/ kPa M/ kPa
300 14.1
6.7
305 17.6
310 21.7
8.5
10.6
315 26.5
320 32.2
325 38.8
330 46.5
13.2
16.2
19.8
24
335 55.3
340 65.4
345 77
350 90
28.9
34.6
41.2
48.7
T/K C/ kPa M /kPa
354 101.3
55.4
360 121.3
66.9
362 128.5
71.1
365 139.9
77.9
370 160.5
90.2
373 174 101.3
380 208.8
119.3
385 236.7
136.4
390 267.3
155.3
395 300.9
176.2
400 337.5
199.1

At what temperature will a 1:1 molar mixture of cyclohexanemethylcyclohexane boil?
P
T
= P
A obs
+ P
B obs
P
T obs
=

A
P o
A
+

B
P o
B

A
=

B
= 0.5
1 Atm = 101 kPa
T/K C /kPa M /kPa
362 128.5
71.1
P
T obs
= 0.5*128.5 +0.5*71.1 = 64.3 + 35.6 = 99.9 kPa ≈ 1 atm

What is the composition of the vapor at this temperature?
T/K C /kPa M /kPa
362 128.5
71.1
P
C
V
C
= n
C
RT ; P
M
V
M
= n
M
RT n
C
/n
M
= P
C
/P
M
=
128.5/71.1 = 1.8/1

An apparatus for fractional distillation
5 theoretical plates can be identified

A look at a schematic diagram of a gas chromatograph:

Instrumental components
Carrier gas
The carrier gas must be chemically inert. Commonly used gases include nitrogen, helium, argon, and carbon dioxide. The choice of carrier gas is often dependant upon the type of detector which is used. The carrier gas system also contains a molecular sieve to remove water and other impurities.
Sample injection port
For optimum column efficiency, sample sizes should be small and should be introduced onto the column as a "plug" of vapor. The most common injection method is where a microsyringe is used to inject sample through a rubber septum into a heated port at the head of the column. SPME is an alternative method for introducing your sample. For packed columns, sample size ranges from tenths of a microliter up to 20 microliters. In preparative GC, sample sizes as much as a mL can be used in certain cercumstances.
Capillary columns, on the other hand, need much less sample, typically less than 10 -3 mL.
For capillary GC, split/splitless injection is used.

Columns
There are two general types of column, packed and capillary . Packed columns contain a finely divided, inert, solid support material (commonly diatomaceous earth ) coated with liquid stationary phase. Most packed columns are 1.5 - 10m in length and have an internal diameter of 2 - 6mm. Liquid coatings vary depending on polarity. Most are non-volatile waxes or organic liquids.
Capillary column
Fused silica open tubular columns have much thinner walls than the glass capillary columns, and are given strength by the polyimide coating.
These columns are flexible and can be wound into coils. They have the advantages of physical strength, flexibility and low reactivity.
Temperature range of use varies from room temperature to about 300 °
C, depending on the coating. Cyclodextrin is bonded in chiral columns.

It is not unusual for a capillary column to have more an efficiency characterized by more than 100,000 theoretical plates.

Detectors
There are many detectors which can be used in gas chromatography. Different detectors will give different types of selectivity. A non-selective detector responds to all compounds except the carrier gas, a selective detector responds to a range of compounds with a common physical or chemical property and a specific detector responds to a single chemical compound. The FID is mostly commonly used.
Flame ionization (FID)
Mass flow
Hydrogen and air Most organic cpds.
10
0 pg
7
1
0
Thermal conductivity
(TCD)
Concen tration
Reference Universal 1 ng
7
1
0
Electron capture (ECD)
Concen tration
Make-up Halides, nitrates, nitriles, peroxides, anhydrides, organometallics 50 fg
5
1
0
Nitrogenphosphorus
Mass flow
Hydrogen and air Nitrogen, phosphorus 10 pg
6
1
0
Flame photometric
(FPD)
Mass flow
Hydrogen and air possibly oxygen
Sulphur, phosphorus, tin, boron, arsenic, germanium, selenium, chromium
10
0 pg
3
1
0
Photo-ionization
(PID)
Concen tration
Make-up Aliphatics, aromatics, ketones, esters, aldehydes, amines, heterocyclics, organosulphurs, some organometallics
2 pg 7
1
0
Hall electrolytic conductivity
Mass flow
Hydrogen, oxygen
Halide, nitrogen, nitrosamine, sulphur