A Simple Infrared Cell for Gases Gary A. Mabbott University of St. Thomas, 2115 Summit Avenue, St. Paul, MN 55105
Recently, Amey (1)described an imaginative experiment i n this Journal i n which introductory chemistry students record infrared spectra of gases that they collected in toy balloons. We have tried this experiment in the lab part of our environmental chemistry course. The students enjoyed t h e experience and commented t h a t they especially liked having the opportunity to learn about and use the FTIR spectrometer. There is one aspect of the exercise that we changed. Students collected a mixture of nitrogen oxides that are generated from a reaction of nitric acid with metallic copper. As the original article points out, these gases attack rubber, a n d within five t o 10 minutes t h e balloon breaks. Although the exposure that a n individual student receives when a balloon breaks in h i s h e r hand (away from the hood) may not he serious, this practice seemed to contradict the precautions that we had been stressing about safety. We also worried about the cumulative effects of these oxidizers on our instrument. We still collected the nitrogen oxides in a balloon in order to demonstrate (in the hood) the impressive effect these gases have on rubber, but we used a simple homemade gas cell for recording the IR spectrum. The cell can be made quickly with a pair of standard IR salt plates and a few inexpensive materials. Figure 1 shows the construction of the cell. The chamber consists of a 2-in. piece of standard PVC water pipe ( 314-in. stock). The salt plates are held in place by rubber septa (also called rubber sleeve stoppers or serum bottle stoppers in some catalogs). The larger, #9 septum (made for use with 24/40 standard taper ioints: Aldrich catalog # 212.439-7) fits tightly into the pipe. A #8 cork bore makes a 1
