An Inexpensive High Pressure Optical Absorption Cell for IR-VIS-uv studies V. E. Rodgers' and C. A. Angell Purdue University, West Lafayette, IN 47907 The power of optical spectroscopy to provide structural insight into molecular systems at equilibrium, follow the process of equilibration following perturbations, and monitor changing concentrations of species in response to changing temperature or solvent composition is well recognized. ( I ) In order to promote changes in the chemical state of a system, the temperature variable is frequently employed in spectroscopic studies. The use of the independent variable, pressure, is usually considered the province of the specialists. This is unfortunate, because in many ways the effects of pressure on equilibrium states and rates of approach to equilibrium is as important as the effect of temperature, and the origin of the pressure effect is often more easily understood. Avoiding pressure is also unnecessary because it is possible, with very little in the way of financial resources or svecialized facilities, to study pressure effects up to 2200 Lars. This contribution describes an optical cell suitable for high pressure studies between at least -130°C and +150°C which can he assembled for about $50.00 and which is simple to use. Ferraro and Basile (2) give an excellent review of other instrumental techniques for obtaining optical spectra at high pressures. The Apparatus The body of the cell, Figure 1, is a stock item Aminco (American Instrument Company, Arlington Heights, IL 60005, Catalog No. 466D) bigh pressure 100,000 psi stainless steel "cross" ($15.00). T o increase the light flux, one bore is drilled from an original 1/18 in. to 1' 4 in. (or to 3/s in. with decreased safety factor) diameter. In our work this block bas been used for long periods at pressures up to 3 kbar (300 mPa, 2960 atm, 43,500 psi) with no sign of failure. However, for student use, 20,000 psi, the pressure easily reached with a crude hydraulic .pump, (3) should probably not be ex~eeded.~ The high pressure resistant optical windows for the cell are fabricated very conveniently by taking advantage of the form of the high pressure gland nuts supplied with the cross. These are made with a 318 in. internal space, intended for a threaded pressure line collar, into which 3/s in. clear sapphire parallelfaced cylinders (General Ruby and Sapphire Inc., 50 E. 42nd Street, New York, NY 10017) can be slipped. The sapphire windows rest on the 1mm wide load-hearing edge of the gland nut where a seal can he formed hy means of a 8/mo in. thick lead foil or 2/l,m in. thick copper foil gasket which sits between the sapphire window and the load-hearing edge of the gland nut. The saonhire window is retained in the gland nut by an ~
in a 6 w five minutes and IS uiallr in :ll,mt an houri. 'I'hc gland nut.; runtnining the zilpphirr a i d w a are t h m screwed krectly into the bod; of t h e d l (the bigh pressure cross) and a nut-to-cross high pressure seal is made by tightening the window assembly down onto a 318 in. i.d. neoprene O-ring, Figure 1.Two such windows are used for IR studies and a third window a t 90° to the incident beam may be used for Raman studies. The pathlength (maximum value = 3 cm) may he adjusted by adding sapphire or glass spacers to fill up the bore 602
Journal of Chemical Education
TO High Pressure Pump And Gauge
Sample Reservoir
TDermocouple
nigh Pressure Sfainlesr Steel Tubing High Reemre Stainless Steel Crorr
Connector N u t
\ L e a d Foil G
/
Epoxy Se< I
plug ( W h e n Not Using A r A Raman Cell)
Figure 1. Optical absorption cell. volume, thereby giving any desired optical path from 0.1 mm to 3 cm. This simple arrangement can withstand at least 3000 bar, though much lower limits are recommended for student work (see footnote 2). The sample can usually be added directly through the pressure input port. In our studies we included a length of larger bore input tubing, containing excess sample to provideL a reserve of sample to cover any losses incurred while start-up leaks are detected and eliminated, and to allow for sample compression. Pressure can he generated using a hand operated hydraulic pump (3) and is measured using any convenient bigh pressure gauge (we use a Heise Bourdon Gauge accurate to f2 bar). For temperature control, the cell may be wrapped with heating wire (nichrome or chromel-alumel thermocouple wire) and enclosed in an aluminum cooling block. The temperature may be measured using a thermocouple and 304 stainless steel sheathing (0.1 mm in wire diameter, 0.5 mm in sheath diameter, OMEGA Engineering, Inc., Stanford, CT 06907) and slipped through the plug in the gland nut directly into the sample as described by Angell and Williams, (4) or by embedding the thermocouple in the body of the pressure cell as shown in Figure 1(in which case the assumption is made that the temperature of the sample is the same as that of the stainless steel cell). Experimental Demonstration of Principles Absorption spectra have been routinely obtained for temperatures ranging from -130" to +lOO°C. An example is
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Present address: Department of Physical Sciences, Mississippi University for Women, Columbus, MS 39701 Provided no gases are introduced, pressures of this magnitude present only mild safety problems. Oil leaks themselves are not dangerous, but objects such as thermocouple sheaths which might be sealed into the high pressure system can become dangerous projectiles if the seal fails.
1.30
1.60
1.50
4 0
WAVELENGTH
1.70
/lm
Figure 2. Absorption spectra of the OH-overtone region of 25%
HOD in D20
soiution from 1.30 to 1.80 fim (For details of interpretation. see C. A. Angeil. in "Water: A ComprehensiveTreatise." F. Franks. (Edit04 Plenum Publishing GO.,VOI. 7, 1982, Ch. 1.) C. A. Angeil and V. E. Rodgers, J. Chem. Phys. (in press), 1983.
shown in Figure 2, where we display the results of a series of near infrared absorption spectra of the uncoupled OH-overtone vibration of HDO in a 25% HDO in D20 solution at different temperatures. These measurements were made at a steady pressure of 2150 bar. A case more suitable for student experiments is discussed below. There are many applications for high pressure techniques. The consequences of pressure change are more readily conceptualized by students than those of temperature change and it is, therefore, preferable, in principle, to illustrate thermodynamic responses, such as those described by LeChatelier's principle, using pressure-induced volume changes. Good reviews on the effects of pressure on chemical processes have been presented by LeNoble ( 5 )and Hagen. ( 6 ) An i m ~ o r t a nreason t whv the influence of temperature on. for instance, the position ofequilibrium in a solution chemical reaction is emphasized more than the influence of Dressure. i; rh.11 rht. ttluprrnturc d r p e n d t n . ~is nlurt. 1:n\ily ~ ~ I U U I I s t r a t d ill the l:rh
