Infrared Spectroscopy A Serviceable KBr Press Using Disposable "Blotting Paper" Pellet Holders Designed for Student Use Henry E. Gerry Montana State University, Bozeman, MT 59717
Rhoda E. R. Craig1 Kalamazoo College, Kalamazoo, MI 49007 For the identification via infrared spectroscopy of unknown solid compounds at the microscale level the K B r window technique is superior because less sample is used in t h e analysis. The optimum condition for this technique is t h e use of 2 m g of sample a n d 100m g of dried, spectral-guide KBr. I n order t o use the K B r window technique in the student laboratories we have designed a heavy-duty K B r press t h a t will allow undereraduate students to prepare K B r windows rapidly a n d efficiently even in conditionswhere the prevailine room temuerature is 75 "C a n d the relative humidity is u p t o 80%. T h e essential components for the K B r window preparation are: 1. KBr press consisting of a modified 20-ton (205) Roper Whitney Ball Bearing Puneh with standard base (model #110030200); ratchet attachment (model #138031190) and pipe handle (model #138032026). Figure 1shows the side view of the press. The central shaft has been machined evenly to allow for the placement of the KBr pellet dies. 2. Dies (Fig. 2) constructed from two tool steel plates, each 2 X 1 x 11%in. The bevelled corners are formed by measuring % in. from the corner on each side and shearing off the resulting piece of metal shaped in an isosceles triangle. Two holes each of '/&-in.diameter to receive fitted pegs are drilled along the 2-in. centerline, 'Is to 318 in. from each end of the die. The top die has nontapered %in. holes
Flgure 2. KBr dies, peg (to scale),and "bloning paper" holder.
while the bottom die has %-in. holes that taper 1mm in diameter for the last in. of the hole. The pegs (Fig. 2), which are 1 in. in length, have a convex end at the top and a concave end at the bottom. The peg is tapered to correspond to the bottom die. The concave surface facilitates removal of the pegs manually from the die with a nail punch should they become jammed. The inner die surfaces have been highly polished and may he maintained in this condition by the use of "cn~cuacloth", ultrafinc wet or dry sanding paper, or poliuhing grit compound. To ohtnin clear, godrlunlity KHr windows, the die surfaces must retain their hrehh - ..~oliahedrond~tion.When dies are stored for an extended period of time, the polished surfaces should he protected with a heavy grease coating to prevent pitting or rust formation. 3. "Blotting Paper" KBr Window Holders. A heavyweight, hard-surface sheet of white blotting paper should he cut into 1-in. squares using a paper cutter. A hole for the KBr window is then cleanly punched using a Whitney-Jensen XX Bench Punch with mounting base (model # 135010001) and a 1' 4- X 71,&. type-M die. 4. Magnetic KBr Pellet Holder or V-Mount Pellet Holder to receive the KBr blotting paper window in the infrared spectrophotometer. T h e procedure for t h e K B r window preparation is as follows: 1. Weigh 2 mg of the solid sample into a dispwahle, polystyrene vial; add a clear plexiglas hall-pestle and 100mg (estimate) of dried, speetral-grade KBr. Thoroughly grindlmix the sample with the KBr in a WIG-L-BUG. 2. Center the punched blotting paper on the bottom die; add and smooth the KBr/sample in the punched area using a spatula. Place the top die on the sample with the correct peg alignment. 3. Place the dies and sample in the KBr press. 4. Apply firm pressure to the handle of the KBr press for 1-2 min, then release, remove and separate the dies carefully, transfer the "hlotting paper" holder immediately to the infrared holder, and
Figure 1. Side view of KBr press.
616
Journal of Chemical Education
Presented before the Microscale Organic Laboratow Session at the 9th Bienntal Conference on Chemical Eoucation, ,uly 29, 1986, Bozeman. MT. Presentation numoer MC-03. Author to whom inquiries should be addressed.
record thespectrum. We have found that the blottingpaper wrndow srts in the V-mount holder sntisfortorily. nispost of the blotting paper sample.
If the intensity of your infrared spectrum is unsatisfactory, the dilution factor may be altered by adopting the following procedure: If your sample is too concentrated,carefully break a portion of the central section of the KBr window out, combine it with more KBr, and repeat steps 2-4. If your sample is too dilute, use your KBr window plus additional sample and repeat steps 2-4. Care must be exercised in removing any or all of the KBr window from the blotting paper holder in order to avoid contamination of the new KBr pellet with blotting paper fragments. Blotting paper has an infrared spectrum. However, in making approximately 1000 KBr pellets aver the past three years we have not experienced any blotting paper contamination in our infrared spectra. We have found that the disposable nature of the supplies
prevents contamination of samples and students obtain excellent results in a neat area. The rugged construction and quick, easy use of the press Drovides results com~arableto considerablv more ex~ensive hydraulic presses for pressing, pelletizing, and cornparting samples for infrared analysis. The ovcrall cost of the 20-ton hall hearing punch, the Cl'hitneydensen bench punch, and two sets of custom-designed dies is approximately $500. Acknowledgment RERC would like to express appreciation to T h e Society for Analytical Chemists of Pittsburgh (SACP) and The Spectroscopy Society of Pittsburgh (SSP) for the award of a 1986 Pittsburgh Conference Memorial National College Grant.. which funded all nurchases associated with the fore. going project, and to the Joyce Foundation for a grant providing financial support allowing faculty release time.
Volume 66
Number 7
July 1989
617
