An Economical and Efficient Photochemical Box B. E. Arney, Jr., M. C. Banta, and Patrick Prouty Sam Houston State University, Huntsville, TX 77341 Photochemical experiments such as the photoreduction of benzophenone in 2-propanol demonstrate the interaction of light and matter in a manner that leads to a chemical change. This particular reaction is probably the most commonly used photochemical reaction because of its advantages: inexpensive reagents, virtually quantitative conversion to benzpinaeol, ability to use sunlight, and a facile workup.
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However, our point of concern is the practice of sending the test tube of reaction solution home with the student to react with sunlight. Though this provides the student a n opportunity to observe the progress of a reaction over a period of time, i t goes against the general wisdom of not permitting reagents to leave the lab. Trying to run the high number of samples through a Rayonnet Photochemical reactor or finding adequate, sunny window space that will be undisturbed for a week or more were not real options for us to preclude sending the samples home. The obvious solution for o w dilemma was to setup a UVlight source in a cabinet large enough to hold 4 M 0 samples in test tubes. Constructing the cabinet was a simple matter, but obtaining a UV-light source with sufficient output to provide reasonable reaction times (a week or less) was not cheap from the scientific supply companies. However, mercury vapor lamps (10&175 W) with requisite power supplies are available relatively inexpensively (under $50) a t many department stores under the guise of security lights. Figure 1is a drawing of the light box. I t was constructed using 314-in. plywood. The box has only two sides, a top, and a back. This was done so that the box could be moved easily for storage when it was not in use. The inner walls were covered with aluminum foil. Two cooling fans, taken from discarded electronic instruments, were mounted on opposite insides of the box a s shown. The one on the lower right front side forced air into the box, and the fan located near the upper back corner forced air out of the box. The 175-W mercury vapor lamp, socket, ballast, and capacitor were taken from a security light. The wiring diagram for the light and fans is shown in Figure 2. The capacitor and ballast were mounted on the back of the box. A protective cover, made from the case of a discarded instrument, was
placed over the electronics to reduce the risk of electric shock because dangerous voltages are present. The socket for the lamp was inserted into a 2.25-in. hole drilled through the back so that the wiring was accessible from the back. The experiment used to test the apparatus was the photolysis of benzophenone. Two-and-one-half grams of benzophenone were dissolved in about 15 mL of warm isopropyl alcohol, followed by the addition of one drop of glacial acetic acid. The solution was then diluted to fill a 150 x 20 mm test tube and stoppered with cork. Ten such test tubes were placed a t various locations in the box. At about 2-h intervals, the test tubes were removed from the box and the precipitate was collected by filtration, dried, and weighed. The test tubes were then placed back into the box for further photolysis. Figure 3 is a graph of the average percent yield plotted versus time in minutes.
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Figure 2. Wiring diagram for the mercury vapor lamp and cooling fans.
Photolysis o f Benzophenone 80
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Figure 1. Diagram of light box.
Figure 3. Average percent yield versus time for the photoreduction of benzophenone.
Volume 71
Number 9 September 1994
797
