Distillation and Gas Chromatogaphy
Distillation and Gas Chromatogaphy
Air condenser
Micro distillation head
Conical reaction vial
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Separation and Purification Methods
for Organic Chemistry
1. Recrystallization
2. Extraction
Air condenser
3. Distillation
4. Chromatography
Micro distillation head
Conical reaction vial
Advantages of Distillation
Simple equipment if boiling point
difference is large
Separate liquids with small boiling point
difference (~ 1o C) with complex
equipment.
Adaptable to large scale separations.
Example – petroleum refining.
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Limitations of Distillation
Volatile compounds only. Many organic
compounds have quite high boiling points.
(Applicability increased by using vacuum
distillation at low pressure.)
Compounds must be heat stable.
High losses with small amounts except
under special circumstances.
Some mixtures form non-separable
azeotropes.
Distillation
Distillation is the process of heating a
mixture to the point that it boils – that is, the
vapor pressure of the liquid equals the
atmospheric pressure, and vapor (bubbles)
will start forming below the surface of the
liquid. If the resulting vapor is condensed
(by cooling), the condensed liquid will often
contain more of one component of the
mixture than others. This allows purification
by distillation.
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Figure 1. Vapor-Liquid Composition for an Ideal Mixture of Two Liquids
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_
_
_
Vapor
_
_
100
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_
_
_
Temperature
OC
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_
_
90
_ d
_
_
b
_
_
_
_
_
_
a
Liquid
c
x
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80
_
I
I
100% A
0% B
I
I
I
I
I
I
I
_
_
_
0% A
100% B
40% A
60% B
Fractional Distillation Apparatus
thermometer or
thermocouple
fractionating
column
Distillation flask
with heating mantle
collection
flask
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Theoretical Plates Needed
b.p. Difference, oC
35
30
20
10
7
4
2
# Plates Required
5
8
13
22
35
65
100
Simple Micro-scale Distillation
In today’s lab we are going to use a micro-scale
distillation apparatus that has a very short
column. This apparatus has only 1-2
theoretical plates. It can effectively separate
only liquids with widely varying boiling points.
However, this apparatus allows you to
experience partial separation by distillation.
We will monitor the extent of separation by
analyzing the several fractions (portions) of the
distilled material.
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thermocouple
screw cap
to adjust height
of thermocouple
hole
open to the
atmosphere
Air
Condenser
D istillation
H ead
Side Arm
with cork
C
insulated
Conicle
Reaction
Vial
Distillation Procedures
In a distillation, we need to collect the condensing
liquid in portions , called fractions. In an ideal
separation of two compounds, we would need to
collect only three fractions – one for the lower
boiling component; one for the higher boiling
component; and one
‘middle’ fraction as
the last of compound
A distills and the
first of compound B
distills.
fraction 3
b.p.
fraction 2
fraction 1
amount distilled
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Distillation Fractions
In today’s distillation, you will collect three
fractions of about 0.8 mL each. You may
need to empty material from the collection
well more than one time to obtain this
amount of material for a fraction.
Record the temperature range of the
thermocouple during the collection of each
of the three fractions.
Your fourth fraction is the material still
remaining in the distillation vial.
Collection of Distillate
When the collection well at the bottom of
the bulb of the distillation head is full,
remove the cork from the sidearm and
quickly transfer the liquid with a Pasteur
pipet into a clean, dry (no liquid present)
sample vial. Replace the cork and continue
collection of distillate.
Note: It will take a while for the
thermocouple temperature to correctly read
the distillation temperature.
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Compounds to be Distilled
Hexane -
Heptane Hexane -
Octane -
b.p. 69 oC
b.p. 98 Oc
b.p. 69 oC
b.p. 126 oC
Obtaining Mixture
To obtain your mixture for distillation, you
need to take a conical vial to Room 2203,
where a student worker will dispense an
appropriate amount into your vial.
YOU MUST WEAR GOGGLES TO
OBTAIN YOUR SAMPLE!
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Gas Chromatography
micro syringe
detector
injection port
Microsyringes
Gas chromatographs analyze quite small
samples. In order to get small samples
injected into the injection port, you will use
a microsyringe. This microsyringe measures
one to ten microliters. You will inject 2.0
microliters of sample today.
Read page 51 in the lab manual on Sample
Preparation and Introduction before doing
any GC analyses.
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Procedures for GC Analysis
As soon as you have collected your first
distillation fraction, one student should take
it to the instrument room and run the first
chromatogram. You may analyze ONLY
one fraction at a time at the GC. When you
have finished one analysis, go back to the
lab and send your partner to take the next
fraction to the end of the line to obtain a GC
of that fraction!
Gas Chromatograms
The output from a gas chromatography
analysis is referred to as a chromatogram.
It is a plot of detector signal value versus
time.
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Gas Chromatograms
The x axis is units of time since time of
injection. The y-axis is signal intensity.
The distillates today will have different
retention times related to their boiling
points.
signal
intensity
time
Gas Chromatograms
It is the area under each peak that is related
to the amount of that material that passed
through the column. Today’s GC
instruments will automatically calculate
peak areas for each signal.
signal
intensity
time
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Objective
Once you have obtained GC data for your
four samples (3 distillate fractions and 1 pot
residue), you will need to obtain data from a
pair of students that distilled the other
mixture.
Record their observed retention times and
relative peak areas for their four GC
samples.
Objectives
Make sure you discuss in your laboratory
reports the differences observed in the level
of separation with each of the mixtures.
Which mixture gave better separation?
Why?
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Procedures and Equipment Setup
Start your distillation BEFORE you
take melting points for your
extraction products. Melting points
can be taken while students are
waiting in line for last GC fractions.
Be sure to set up your distillation
apparatus in front of your mini-hood
intake. Don’t pollute the lab rooms
with alkanes!
Clean-up
Make sure glassware, particularly the
distillation vial, is cool before placing it in
contact with water or acetone. Heat stress
cracks this vial very easily.
Your first rinse of your distillation head and
conical vial should be collected in a beaker
or flask and disposed of in the container
labeled for this purpose in the hood.
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Clean-up – Extraction products
IMPORTANT!!!!!!!! Do NOT
dispose of ANY chemicals or filter papers
that have been in contact with chemicals
into the trash bins!!!! Any TA observing
this should deduct points for a safety
violation.
You endanger the health of custodial
workers when you do this.
No Chemicals into trash!!!
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