Chem 226 / Dr. Rusay
Gas Chromatography & Fractional Distillation
Toluene / Cyclohexane
An earlier laboratory exercise, Toxicity, Health & Safety, required you to use an MSDS, which
provided information for toluene. On your report form using an MSDS for cyclohexane answer the
following questions as part of the pre lab.
1. Is flammability a concern? If so, how should a mishap be handled?
2. Is inhalation a cause for concern? What precautions should you take handling
cyclohexane? Briefly explain.
3. Can prolonged exposure cause toxic effects? If there were a risk of chronic exposure
what measures could be taken to deal with the risk?
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Experiment, Part 1: Gas Chromatography, (#s 1 and 2 are to be completed and reported on the prelab form before obtaining your unknown in #3.)
1. Read Lehman: Exp. 6 pp. 54-59; OP-32.
2. Calculate retention times (Rf values) and relative area percents for the components in
chromatogram unknown provided to you. Identify each of the components in the
unknown chromatogram by their retention times. Refer to:
http://chemconnections.llnl.gov//organic/Chem226/Labs/GC/GC-table.htm
3. Obtain an unknown. Analyze your individual unknown sample which is a mixture of
cyclohexane and toluene. Report the mass percent results for each on the pre-lab form.
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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 the potential reads between 0.003-0.006(V) 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 (1-1.5 µL depending on the GC used.) of standard into the GC through
injector port for the DC-710 GC column (on the right side) 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. For an example refer to:
http://chemconnections.llnl.gov/Organic/Chem226/Labs/GC/GC-logger.htm
(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
temporarily save it on the Desktop. Calculate the relative mass percentages for the
components.
[GC Thermal Conductivity Detector correction factor: cyclohexane: 1.11,
toluene: 1.05, (benzene 1.00). To convert to relative mass multiply the peak
area by the respective correction factor.]
Experiment, Part 2: Fractional Distillation / Gas Chromatography
Read Lehman: Exp. 6 pp. 54-59; OP-27.
Answer questions assigned on the Web assignment page in the final report.
Separation: (Variation of Lehman Procedure; you may work in pairs on this part.) 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. Pack a distilling column with the column
packing provided. Measure the height of the packing to the nearest millimeter. Assemble an apparatus for fractional
distillation, using 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 and the remaining weight in the distillation pot.
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 and complete the pre-lab form with your results.
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