Chem 226 / Dr. Rusay
Gas Chromatography & Fractional Distillation
Toluene / Cyclohexane
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Experiment, Part 1: Gas Chromatography
1. Read Lab Guide: pp. 167-175
2. Complete the prelab form
3. Obtain an unknown. Analyze your individual unknown sample which is a mixture of
cyclohexane and toluene. Report the mass percent results for each.
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GC Analysis:
(a) Start Logger Pro Software by opening the GC icon on iMac desktop. Check Data
collection parameters to ensure a run time of 5 min. Screen should show a blank graph,
volts vs time.
(b) Zero the detector so that it reads between 0.003-0.006 using the zero control knob on the
GC. (The knob is at the bottom right.) Do not adjust any other settings!! Note: If the
instrument is set below 0.003, the controls will be off-scale and the data will not be
useable.
(c) Inject ~1 µL of standard into the GC through injector port for the carbowax GC column
and click on Logger Pro data collection.
(d) When you have finished the run, use Logger Pro to integrate the area under the peaks and
determine the retention time for each peak. Add an annotation box to provide the GC
parameters: column temperature, helium flow rate, column type.
(e) Print your data and be sure to put your name on the chromatograph, which can be
included in the annotation box. Do not save the file to the hard drive, but you may save it
in the Desktop folder named Temporary. Calculate the relative mass percentages for the
components.
[GC Thermal Conductivity Detector correction factor: cyclohexane: 1.11,
toluene: 1.05. To convert to relative mass multiply the peak area by the
respective correction factor.]
Experiment, Part 2: Fractional Distillation / Gas Chromatography
Read Lab Guide: pp. 136-138
Separation: Work in pairs. Place 0.200 mol of cyclohexane and 0.200 mol of toluene in a 100-mL
boiling flask. Do this as accurately as possible. NOTEBOOK: Records should be kept in both
partner’s notebooks; record data in a data table. Assemble an apparatus for fractional distillation,
using the vigreaux column and a small beaker as a receiver for the forerun. Clean, dry, weigh, and
number four fraction collectors with a capacity of 20 mL or more with tight-fitting caps. Have ready
a smaller container to collect the HETP sample.
Watch the vapors rise in the column as you heat the boiling flask. When they reach the still head,
reduce the heating rate enough to keep the ring of condensing vapors between the top of the column
packing and the sidearm for several minutes, so that the vapor composition can stabilize before any
distillate is collected. Distill the liquid slowly and discard the first few drops of distillate. Collect the
next 5 - 10 drops in the HETP vial and cap it tightly. Quickly replace the vial with the first fraction
collector, record the distillation temperature, and distill at a rate of not more than 20 drops per
minute. When the distillation temperature reaches 85oC, replace the first fraction collector with the
second one and cap the first one tightly. When the distillation temperature reaches 97oC, switch to the
third collector. When the temperature reaches 107oC, remove the heat source and let all the liquid that
remains in the column drain into the boiling flask. After it has cooled down, transfer the contents of
the boiling flask to the fourth collector. At this point, you should have fractions covering the
following boiling ranges:
1.) 81-85oC 2.) 85-97oC 3.) 97-107oC 4.) 107-111oC
Analysis:
Accurately weigh the four fractions. NOTEBOOK: record your data. Analyze each fraction
plus the HETP sample by gas chromatography as done previously.
Report:
Determine the corrected peak areas. NOTEBOOK: Show calculations: 1) the percentage (by
mass) of cyclohexane and toluene in each fraction, 2) the mass of cyclohexane and toluene in
each fraction, and 3) the respective percentage ( on a mole basis) of cyclohexane and toluene
in each fraction. Provide a data table for the calculated results. Plot the mole percent for each
fraction (on the x- axis) as a function of boiling temperature, using the midpoints of the
appropriate boiling ranges (on the y-axis). Draw a smooth, curve connecting the data points
for cyclohexane and another for toluene. Calculate the number of theoretical plates provided
by your fractional distillation apparatus; then calculate the HETP of your column. Provide 3
suggestions of improving the efficiency of your separation. Turn in your gas chromatograms
(or photocopies) with your report. Answer the post lab on-line questions.
Relative
Affinities
Stationary Phase
OV-1, SE-30, DC200
OV-17
Carbowax 20M
DEGS
OV-275
DC-710
Maximum T
o
C
Alkenes &
Aromatics
Alcohols, Phenols
& Carboxylic
Acids
Aldehydes,
Ketones, Ethers &
Esters
350
16
55
44
350
250
225
275
300
119
322
496
629
158
536
746
872
General Purpose
162
368
590
763