a41
Anal. Chem. 1906, 58. 841-843
by EGA-IR. Comparison of the monomer evolution profiles suggested that the zip length was substantially reduced, but only a small amount of thermally unstable end groups was formed. The remaining monomer was presumably either volatilized during irradiation or reacted to form nonvolatile products. The COz profiles were similar, though that from the irradiated material was broader. The methanol profiles suggested that one reaction which produced methanol between 300 and 350 "C did not occur in the irradiated material, though this analysis is approaching the S I N limit and needs to be viewed with caution. As well as the applications described here, we have used EGA-IR for characterization of vulcanized elastomers, epoxies, polyesters, and phenolics. The ability to analyze small samples, under conditions comparable with those used in thermal analysis work, has been very valuable. The technique is useful for the analysis of highly filled samples t h a t are difficult t o analyze by conventional transmission spectroscopy.
ACKNOWLEDGMENT We thank P. J. Burchill for samples of PMMA and S. D. Boyd for design and construction of the programmable temperature controller.
Registry No. PMMA (homopolymer),9011-14-7; PVC (homopolymer),9002-86-2; DER 322,25085-99-8; poly(tetramethy1ene oxide) (SRU), 25190-06-1;toluene-2,4-bis(N,N-dimethyl)urea, 17526-94-2. LITERATURE CITED (1) Liebman, S. A,; Ahlstrom, D. H.; Griffiths, P. R. Appl. Specfroc. 1978, 30, 355-357. (2) Lephardt, John 0.; Fenner, Robert A. Appl. Spectrosc. 1980, 3 4 , 174-185. (3) Lephardt, John 0.; Fenner, Robert, A. Appl. Spectrosc. 1981, 35, 95-101. (4) Fenner, Robert A.; Lephardt, John 0. J . Agric. Food Chem. 1981, 29, 846-849. (5) Lephardt, John 0. Appl. Spectrosc. Rev. 1982, 78,265-303. (6) Solomon, Peter, R.; Hamblen, David 0.; Carangelo, Robert M.; Markham, James R.; Di Tantaro, Marie 0. ACS Fuel Chem. Prepr. 1984, 2 9 , 83-88. (7) Roush, P. B.; Luce, J. M.; Totten, G. A. Am. Lab. (Fairfield, Conn.) 1983, 75, 90-93. (8) Davidson, Richard G.; Mathys, Gary I. J . Appl. Polym. Sci. 1983, 28, 1957-1968. (9) McNeill, Ian C. J . Polym. Scl. A7 1968, 4 , 2479-2485.
Received for review September 6,1985. Accepted November 22, 1985. This work was presented in part a t "POLYMER 85", an International Symposium on Characterization and Analysis of Polymers, Melbourne, Australia, Feb 11-14, 1985.
Fourier Transform Infrared Reflectance Spectrometry System for Studying Moderately Fast Reactions Melvin P. Miller,* Elwin C. Penski, and Raymond E. Miller Physical Organic Branch, Chemical Division, Research Directorate, US.Army Chemical Research and Development Center, Aberdeen Proving Gound, Maryland 21010-5423
Equlpment has been designed and fabricated to permit reaction rate data to be obtalned on exothermlc, varlabie temperature, solld/iiquid chemical reactlons that are 80 % complete In less than 10 s. A Fourier transform infrared (FTIR) spectrometer and attenuated total reflectance (Am)reactlon cell are used. Details are presented for the deslgn and construction of the temperature recordlng system for the quasiadiabatic reactor. This system allows klnetlc data on reactions to be obtalned based on the correlation of reaction time, temperature, and spectral data. Reaction temperatures may be measured with a precision of 0.03 O C . Eight spectra may be obtalned in 1 s. The advantages of this procedure for studying chemical reactions are discussed and demonstrated with a model solid-ilquid reaction: the spontaneous exothermic reaction of solid sulfur with trlisopropyl phosphite. A temperature profile and preliminary accounts of the overall reaction features are presented.
When it is necessary to spectroscopically study reactions that cannot be repeated many times and for which the critical steps occur in fractions of a second, an apparatus for highspeed infrared (IR) spectrometry t h a t can store dozens of spectra in seconds is required. This is possible only through modern computer data acquisition, experimental control, and data storage. In order t o study reactions in bulk, where the
sample is more than a millimeter thick, reflection techniques are required. The purpose of this paper is to provide a description of the apparatus and results of tests of a Fourier transform infrared (FTIR) reflectance system for studying moderately fast chemical reactions under quasi-adiabatic conditions. Classical chemical kinetics studies usually involve maintaining the reactants and products a t a constant temperature and measuring changes in concentrations as a function of time. Often, reactions are chosen to work well with some analytical technique for determining concentration. Conversely, when practical reactions are studied, the analytical technique must be chosen to fit the conditions of the reaction of interest. The simplifying use of a thermostat may no longer be appropriate. Following a single variable such as density, refractive index, pressure, or a spectroscopic peak intensity is only possible when the reaction is extremely well understood, which is often not the case in practical chemistry. Early spectroscopic studies of the kinetics of rapid reactions made use of ultraviolet and visible (UV-vis) spectrometry. More recent work has made use of IR spectrometry. Durana and Mantz (1) reviewed the IR literature several years ago. A few other reports on relevant technique and cell design have been published (2, 3). Since the reactions of interest to us imposed a large number of constraints, study of the reaction kinetics called for an innovative solution. These are rapid, highly exothermic reactions between solids and neat liquids. As is the case with
This article not subject to U S . Copyright. Published 1986 by the American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 58, NO. 4, APRIL 1986 AIR INLET
a
TEFLON DISK I N POSITION
EXTERNALLY THREADED INTERNAL THREADS
O-RING
I
I
COUPLE
PRISM MOUNT
b
r "GLISPEG SURCPCE5
Flgure 1. (a) Diagram of reaction cell with cut-away view showing Teflon disk prior to start of reactlon (not drawn to scale). (b) Perspective view of zinc selenide ATR prism (not drawn to scale).
most practical reactions, they are run under conditions where
the temperature varies. T h e UV-vis spectra provide insufficient information for kinetic studies, as is the case of most organic reactions. T h e mixing, thermal variations, and high absorptivity of t h e reactants pose numerous restrictions eliminating the use of IR transmission spectrometry. Relaxation or time-resolved methods are ruled out by the fact that the reactions are irreversible and are not easily repeated under the conditions of interest. For such reasons this report deals with a n IR spectrometer and temperature measuring system combined with a mixing cell of adequate dimensions fitted with an attenuated total reflectance (ATR) element (4) for studying nonisothermal, one-shot reactions in real time.
EXPERIMENTAL SECTION Reaction Cell. The reaction cell used is constructed entirely of Teflon and consists of two major parts designed to be threaded together, Figure la. The lower portion, designed to hold the liquid reagent, consists of a cylindrical chamber in which a thermocouple and the zinc selenide ATR prism are mounted. A diagram of the ATR prism is shown in Figure lb. Located on the sides of the lower portion of the cell are two ports that serve as an inlet for the liquid reagent and as a vent for pressure release. All tubing leading to the cell is made of Teflon and connected to the cell via Kel-F threaded or Luer-Lok adapters. A syringe connected to the inlet tubing is used to fill the cell with the liquid reagent. The upper portion of the cell, s h o w in Figure la, is cylindrical and designed to hold a solid reactant, held in place by means of a friction-tight Teflon disk. This disk, designed to be dislodged by a burst of air from a manually operated syringe connected to
the top side of the upper chamber via a Teflon tube, provides the means by which the reaction is initiated. The disk is suspended by a thread to prevent it from interfering with the mixing process occurring within the liquid reagent pool. Instrumentation. A Nicolet Instrument Corp. (NIC) Model 7199D Fourier transform infrared spectrometer was employed in these studies. Data acquisition and interferometer control were through a NIC Model 1180 data system supported by a Diablo Model 44 dual disk drive with a Zeta Model 160 plotter. The optical bench consisted of a Michelson interferometer with a KBr beam splitter and a mercury-cadmium telluride (MCT) detector, allowing operation in the 4000-600 cm-I region. Temperature Measuring System. When rapid temperature changes are recorded, two factors that can affect the accuracy of the temperature-time response curve are the time constant of the sensing device and the time constant of the recording device. Thermocouples with time constants of less than a tenth of a second are readily available commercially. Strip chart recorders with full-scale response times of 0.3-0.5 s are also common. Combinations of thermocouples and strip chart recorders are suitable for recording temperature changes occurring on a time scale of a few seconds. Such systems have been used routinely in this laboratory. In anticipation of the possibility of encountering reactions too rapid to be followed with a thermocouple/strip chart recorder system, we undertook the design of the system described in this paper. Our goal was a system whose response was limited by neither the sensor nor the recorder used. Preliminary chemical reaction studies in our laboratory indicated that significant temperature changes could be expected in times as short as 0.1 s. To sense such changes, we selected a fine gauge Chromel-Alumel thermocouple whose junction was bare but whose heads were insulated by alumina protection tubes. The thermocouple was sealed into a threaded stainless steel gland with epoxy cement. Epoxy cement was also applied to the points where the thermocouple wires emerged from the protection tubes. This assembly was installed in the reaction cell through a threaded port, Figure 1. The thermocouple was calibrated against a calibrated mercury-in-glass thermometer at the ice point, at ambient air temperature, and a t the boiling point of water, as well as a t intermediate points in a thermostated water bath. The calibration gave a very good linear fit. Microscopic examination of the bare thermocouple tip showed no signs of attack by the reagents used even after numerous reactions. We selected a two-channel digital storage oscilloscope to record the thermocouple signal. The Nicolet Instrument Corp. Model 206 is capable of storing up to 4096 data points, with a sampling time as short as 500 ne per data point. To achieve maximum usefulness from the temperature measurements, it is necessary to correlate the time scale of the temperature measurements with that of the spectral information. Precise correlation was achieved by using the output from a phototransistor to trigger the oscilloscope. (The device used was similar to FPT-100 and is available from Radio Shack as Catalog No. 276-130.) The transistor was activated by the sweep of light of the Nicolet 7199D spectrometer. The output was fed to one channel of the digital oscilloscope. This signal was used to trigger the other channel of the oscilloscope, which was receiving the output from the thermocouple. The trigger signal, as well as the signals from all successive spectrometer sweeps, were stored in the oscilloscope. Because the thermocouple output is below the level that can be used by the oscilloscope, the use of an amplifier becomes necessary. We selected an Omega Engineering, Inc., Model Omni-IIB multirange thermocouple amplifier for this purpose. This is a battery-powered instrument with a built-in reference junction. It was used on the lOOX range. To obtain permanent records of the oscilloscope traces, the stored data were output to a Hewlett-Packard Model 7015B X-Y recorder. The stored signals could be expanded in either or both directions prior to plotting, and the coordinates of significant features of the temperature-time curves could be read by positioning a cursor on the oscilloscope screen.
ANALYTICAL CHEMISTRY, VOL. 58, NO. 4, APRIL 1986
X-Y Recorder
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n
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(Thermocouple
Phototransistor
Figure 2. Schematic of temperature (channel a) and spectra time (channel b) measuring system. Figure 4. Temperature profile with positions of spectra a, b, and c shown in Figure 3. Caution: This reaction should be considered hazardous. It occurs with explosive speed producing heat and toxic products. As a minimum it should be run in a hood, behind a shield, with venting of the gases produced.
C)
b)
a) I
I
I
,
1050
920
790
660
Wavenumbers, cm-I Figure 3. Sample spectra a, b, and c obtained during the course of the reaction recorded at 3.90, 7.25, and 7.99 after start of data collection and at temperatures of 25.6, 175.9, and 196.3 OC, respectively. These spectra correspond to the points labeled in Figure 4.
A schematic drawing of the complete system is shown in Figure 2.
Reagents. The reagents used in this study were commercial sulfur, recrystallized from benzene, and triisopropyl phosphite (TIP) obtained from Aldrich Chemical Co., lot no. 062477. Procedure. A small stirring bar was placed in the Teflon reaction cell and the upper portion of the cell containing the solid was secured to the cell body. This assembly was secured atop the magnetic stirrer motor, attached by means of positioning pins to a beam condenser designed by Harrick Scientific Corp. and placed within the spectrometer sample chamber. Leads from the thermocouple, magnetic stirrer, and the Teflon tubes described above were connected to the cell. All wires and tubing left the spectrometer sample chamber via openings in the chamber cover. The sample chamber was then closed and allowed to purge for several minutes. A background spectrum of the empty cell was obtained, after which liquid reagent was injected into the cell. Typically in these experiments, about 0.48 g of sulfur was placed in the upper part of the cell, and 5 mL of neat TIP was put in the lower part of the cell. After the magnetic stirrer was turned on, the spectrometer was started in the rapid scan data collection mode, in turn triggering the data collection by the oscilloscope as described earlier. At the desired time, the disk was dislodged, introducing the sulfur to the reaction chamber.
RESULTS AND D I S C U S S I O N For the purpose of testing the apparatus we chose the reaction of TIP with solid sulfur, which yields O,O,O-triisopropyl phosphorothioate (TIPS) (5). The reaction is rapid and highly exothermic and produces spectral changes that are readily followed and characterized by using published band assignments (6-8). Reactions between mixtures of approximately equal moles of T I P and sulfur are about 80% complete in 10 s and produce temperatures in excess of 200 “C. Typical absorbance changes for these reactions are shown in Figure 3, while the corresponding temperature profile is shown in Figure 4. The vertical traces in Figure 4 resulted from the phototransistor output, and each corresponds to a spectral scan. The labeled points in Figure 4 correspond to the spectra in Figure 3. We are currently examining a kinetics model for this reaction that will permit us to interpret the data in a more detailed manner. At this point we have demonstrated the ability to follow the course of a rapid nonisothermal solid/ liquid reaction in real time and to precisely correlate spectral and thermal data with a simple optical interface. ACKNOWLEDGMENT The authors thank Robert J. Grula, John B. Samuel, and John J. Callahan for their advice. Registry No. TIP, 116-17-6; S, 7704-34-9. LITERATURE CITED (1) Durana, J. F.; Mantz. A. W. “Fourier Transform Infrared Spectroscopy”; Ferraro, J. R., Basile, L. J., Eds.; Academic Press: New York, 1979; Vol. 2, Chapter 1. (2) Crouch, S. R.; Holler, F. J.; Kotz, P. K.; Beckwith, P. M. Appl. Specfrosc.Rev. 1977, 13, 165-259. (3) Mantz, A. W. Appl Opt. 1978, 17, 1347-1351. (4) Harrlck, N. J. “Internal Reflection Spectroscopy”; Interscience: New York, 1967. (5) Chernlck, C. L.; Pedley, J. B.; Skinner, H. A. J. Chem. SOC. 1957, 1851-1854. (6) Thomas, L. C. “Interpretation of the Infrared Spectra of Organophosphorus Compounds”; Hyden: New York, 1974. (7) Thomas, L. C. “The Identification of Functional Groups in Organophosphorus Compounds”; Academic Press: New York, 1974. (8) Socrates, G. “Infrared Characteristic Group Frequencies”; Wiley: New York, 1980; Chapter 17.
RECEIVED for review April 11, 1985. Resubmitted November 7, 1985. Accepted November 7, 1985.
