Separation of Alcohols
PA State Standards:
1.2.11.A
Read and understand the central content of informational texts and documents in all
academic areas.
 Use teacher and student established criteria for making decisions and drawing
conclusions.
2.4.11.E
Demonstrate mathematical solutions to problems.
3.4.10.A
Explain concepts about the structure and properties of matter.
 Apply knowledge of mixtures to appropriate separation techniques.
3.7.10.A
Identify and safely use a variety of tools, basic machines, materials and techniques to
solve problems and answer questions.
3.7.12.A
Apply advanced tools, materials and techniques to answer complex questions.
Introduction:
Chromatography is a separation technique utilizing a stationary phase and a mobile phase.
In gas chromatography, the stationary phase is a solid material packed in a coiled column,
and the mobile phase is air.
The gas chromatograph is an instrument that allows a chemist to separate a mixture of
gases or volatile liquids. Separation is based on the size, structure, polarity of the
molecules.
Retention time is used to identify the components, and the area under the curve is used to
determine the relative quantities of the components. In this experiment, a HAYSEP-D
column is used to separate 2-propanol, ethanol, and methanol.
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2-propanol
Ethanol
Methanol
Boiling Point 78 C
Boiling Point 82C
Boiling Point 65C
Vocabulary:
Area under the curve – used to determine the relative amount of the components
areaofcomponentpeak
%component 
 100
totalarea
Retention Time – the amount of time necessary for a component to move through the
column
Guiding Questions:
Please answer the following questions before beginning the lab.
1. Which of the three alcohols will have the shortest retention time?
2. Which of the three alcohols will have the longest retention time?
Equipment/Materials:
air gas chromatograph
computer with Peak Simple software
printer
all power cords and cables
10 microliter syringe
vial of methanol
vial of ethanol
vial of 2-propanol
unknown samples of alcohol
Safety:
- Always wear safety glasses in the lab.
- Handle the syringes carefully. The syringes are very dangerous, especially when
filled with hazardous chemicals. They also break easily. Do not allow the
syringe to roll off of the lab bench. When injecting the sample, push the plunger
straight so that it does not bend.
- The gas chromatograph is like an oven, so the port may be hot. Use caution.
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Procedure:
1. Turn on the GC and start the Peak Sample software. A data acquisition system
popup should appear with phrases like initializing, waking up, signing on, setting
modes, and calibrating. If you get a popup saying can’t wake – check power and
cable, try one of the following options.
a. Check to make sure that the cable between the GC and computer is
properly attached and that the GC is on.
b. Close the program. Unplug the USB cord from the computer, then plug it
in again. Restart the program. If it still won’t wake, try the next option.
c. Click on the Edit Menu, then select Overall. In the COM Port/USB device
number box, try a number from 1 – 5. Keep trying until the data
acquisition system begins to function.
2. Click on edit menu, then select channels. Under Channel 1, pick Temperature.
Click on the numbers to highlight them, then click Change. Change the numbers
as follows:
Start 150C
Hold 10 minutes
Ramp 0 degree / min
Until the temperature is 150C
3. Be sure the time scale starts at zero and ends at 10 min. Use the arrows to adjust.
4. Clean the syringe by rinsing it 10 -- 15 times with one of the standards. With the
plunger fully depressed, place the needle into the sample. Slowly draw up the
plunger to obtain a sample in the syringe. Remove the syringe from the sample.
Discharge this sample into the sink or onto a Kimwipe. Depress the plunger, and
put the syringe needle back into the sample. Draw up a second sample and
discharge it. Repeat. Do not push on the plunger when the needle is inside a
sample.
5. Place the syringe needle back into the sample and withdraw over 1 microliter.
Remove the needle from the sample bottle, then depress the plunger to the 1
microliter line. Wipe the needle with a Kimwipe.
6. Click on the Z or  button on the run screen to zero the current.
7. Insert the needle carefully into the injection port of the GC until the entire needle
is inside the instrument.
8. Inject the sample and start the data collection simultaneously by depressing the
syringe plunger and pressing the space bar on the computer at the same time.
9. When the sample peak of a standard returns to baseline, the run may be stopped by
pressing the end button on the computer. After pressing end, print the graph. Be
sure to label the graph.
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10. Repeat steps 4-9 for the other 2 known alcohols and the unknown alcohol.
When running an unknown sample, be certain to let the data collect long
enough to get all possible peaks.
Data:
Standards
Retention time
2-propanol
Ethanol
Methanol
Unknown 1
Retention Time (min)
Identity of Peak
Retention Time (min)
Identity of Peak
Retention Time (min)
Identity of Peak
Peak 1
Peak 2
Peak 3
Unknown 2
Peak 1
Peak 2
Peak 3
Unknown 3
Peak 1
Peak 2
Peak 3
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Calculations:
1. Calculate the area of each peak in the unknown. Treat each peak as a triangle to find
its area. Find the total area by summing the areas of the individual peaks. (Note: if
the computer is set to determine the area, that value from the print-out may be used.)
2. Use the areas if the peaks to determine the percent composition of the mixture.
3. If given the correct answers, calculate the percent error for you calculations.
Unknown 1
Component
Area
Percent Composition
Percent Error
Area
Percent Composition
Percent Error
Area
Percent Composition
Percent Error
2-propanol
Ethanol
Methanol
Unknown 2
Component
2-propanol
Ethanol
Methanol
Unknown 3
Component
2-propanol
Ethanol
Methanol
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Questions:
1. Why was it important to run known samples of possible components in the
unknown mixture?
2. If a supposedly pure sample were properly injected into the gas chromatograph
and several peaks were observed, what can be concluded about the sample?
3. Why is it important to clean the syringe between samples?
4. Why is it important to wipe the needle before injecting the sample into the
instrument?
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Separation of Alcohols
Teacher Notes
Time for Completion:
The lab can be completed in a single period, ideally 50 – 60 minutes. More time is
necessary for calculations.
Target Grade Level:
This lab is primarily for Chem I or Chem II, but it may be adapted for applied or practical
chemistry by omitting some calculations.
Objective:
The objective is to determine the percent composition of an unknown mixture of alcohols
using gas chromatography.
Major Concepts:
This lab can be taught during units on mixtures, organic chemistry, polarity, and
separation techniques.
Preparations:
Unknown mixtures can be made by mixing two or three of the standards in any ratio.
Sample Data:
Standards
Retention time (min)
2-propanol
2.7
Ethanol
1.8
Methanol
1.1
Unknown 2
Retention Time
Identity of Peak
Peak 1
0.85
Methanol
Peak 2
1.8
Ethanol
Peak 3
3.4
2-propanol
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Sample Calculations:
1. Calculate the area of each peak in the unknown. Treat each peak as a triangle to find
its area. Find the total area by summing the areas of the individual peaks. (Note: if
the computer is set to determine the area, that value from the print-out may be used.)
Since the syringe has only 2 significant digits, only 2 are reported here.
2. Calculate the percentage of each component in the unknown.
areaofcomponentpeak
 100
totalarea
1900
%ethanol 
 100
5600
%ethanol = 32%
%component 
3. If given the correct answers, calculate the percent error for you calculations.
The correct answer is that all alcohols are in equal proportions, or ~33% each.
actual  exp erimental
percenterror 
*100
actual
1
percenterror  *100
33
percent error = 3.0%
Unknown 2
Component
Area
Percent Composition
Percent Error
Methanol
1800
32%
1%
Ethanol
1900
34%
1%
2-propanol
1900
34%
1%
Answers to Questions:
1. Why was it important to run known samples of possible components in the
unknown mixture?
This was necessary to determine the retention times in order to identify the
components in the unknown mixture.
2. If a supposedly pure sample was properly injected into the gas chromatograph and
several peaks were observed, what can be concluded about the sample?
It contained some volatile impurities.
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3. Why is it important to clean the syringe between samples?
Any trace of the previous substance will be detected by the gas
chromatograph and produce an additional peak.
4. Why is it important to wipe the needle before injecting the sample into the
instrument?
If any sample is on the outside of the needle it will be vaporized and travel
through the column before the sample is injected, thus producing double
peaks.
Extensions:
This lab serves to familiarize the students with the GC before they move onto more
complex labs with the instrument. Other alcohols can be used for the lab, but the lab
should be tested before the students try it, as some alcohols are harder to separate using
the HAYSEP-D column. Column temperatures may need to be adjusted for better
separation (lower temperature), or for faster retention times (higher temperature).
Temperature should not be changed during the lab as it will affect the retention times
which are used for identification of the components in the unknown.
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Pre-Test for Separation of Alcohols Using a Gas Chromatograph (Key)
1. What aspect of the peak gives the relative amount of a component?
A.
B.
C.
D.
Width.
Area.
Height.
Retention time.
2. What aspect of the peak gives the relative identity of a component?
A.
B.
C.
D.
Area.
Retention time.
Height.
Width.
3. Sometimes the peaks overlap. What action may solve this problem?
A.
B.
C.
D.
4.
Changing the nature of the column packing.
Lowering the temperature of the column.
Increasing the length of the column.
All of the above.
Why is it important to wipe the needle before making an injection?
A.
B.
C.
D.
A wet needle may clog the injection port.
A wet needle may slip out of the injection port.
A wet needle may cause a distorted peak or contamination peak.
The instrument may not register a peak.
5. What does the carrier gas do?
A.
B.
C.
D.
Pack the column.
Heat the column.
Move the sample through the column.
Saturate the column with liquid before the experiment begins.
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