CHEM431 Analytical Chemistry II
Ultraviolet/Visible Absorption Spectroscopy
Part B: Effect of Slit Width on Resolution and Comparison of Vapor Phase and Solution Phase
Spectra
In this section of the experiment you will observe the effect of slit width on the resolution of a
UV/visible absorption spectrophotometer. You will also compare the solution phase to the vapor phase spectra of benzene and observe the effect of substitution upon the 260 nm absorption band of the benzene molecule.
Procedure:
1. Carefully place a drop of benzene in the bottom of a dry quartz cuvette and cap it (be sure not to get benzene on the windows of the cell). Handle the benzene cautiously, within the hood only!
2. Place the cell into the sample beam of the Perkin-Elmer Lamdba-950 double beam spectrophotometer. Make sure the instrument is set to operate in auto-gain double beam mode.
With the slit width set at 0.5 nm, scan the spectrum between 230-290 nm (as the peaks are narrow, use a slow scan speed). If necessary, zero the instrument at 290nm before starting your scan.
3. Obtain at least 4 additional scans of the benzene vapor with the slit width set to values between
0.1-5.0 nm. Note that you should re-zero the instrument (at 290nm) each time you change the slit width.
4. Remove most of the benzene from the cell (dispose of the benzene waste in the appropriate waste container in the hood) and fill the cell with cyclohexane. Using a slit width of 0.5 nm, scan the spectrum of the solution between 230-290 nm.
5. Repeat steps 1 and 2 using toluene or ethylbenzene (or another substituted benzene) as the sample molecule.
Data Analysis (Effect of Slit Width):
Prepare a plot of peak width (or peak height ) versus slit width of the instrument for one or more peak(s) in the absorption spectrum of the benzene vapor.
Questions:
1. Why does the spectrum of the benzene vapor change with instrument slit width? What are the trade-offs when the slit width is varied?
2. How does the spectrum of benzene differ in comparing the solution and vapor phase spectra of the molecule? To what do you attribute this difference?
3. The molar absorptivity of benzene at the most intense band near 260 nm is approximately 200
L/mole-cm. Using this value, calculate the concentration of your mixture of benzene in cyclohexane.
4. What type of spectroscopic structure (information) are you observing in the ultraviolet absorption spectrum of the benzene molecule?
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CHEM431 Analytical Chemistry II
5. Comparing the spectra you obtained for benzene, toluene, or ethylbenzene, explain the observed wavelength shift in the absorption spectrum of the benzene derivatives.
6. Do you need to use pure cyclohexane in the reference cell when collecting these spectra? Why or why not?
7. Do you need to use quartz cuvettes for this experiment? Why or why not? rev. 1/17/06 kgo
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