Unstable Products from Gas-Solid Reactions in Potassium Bromide Pellets Continuous Infrared Monitoring R. A. Friedel, J. A. Queiser, and E. C. Moroni Pittsburgh Energy Research Center, Bureau of Mines, U S . Department of the Interior, Pittsburgh, Pa.
IN THE STUDY of unstable reaction products, it is essential that the method of investigation and the preparation of the sample must not result in the destruction of these unstable products. The purpose of this paper is to describe a spectral method of investigating unstable reaction products continuously without danger of decomposing them. For the investigation of organic derivatives of metal carbonyls (e.g., the Vaska compound, IrCl(CO)[P(C~H,)31z(I) (1)) for their possible utilization as sorbents for SOz in plant stack gases, it has been advantageous to study the reactions involved by means of infrared spectrometry ( 2 ) . The infrared sampling method usually preferred for relatively unstable substances has been mineral oil mulls, the preparation of which does not involve the introduction of considerable energy into the system. Thus, the possibility of destruction of unstable species is lessened. This technique is necessarily a batch method, for samples must be extracted from the reaction system at intervals and prepared for investigation by infrared. The method does not permit one to follow the course of a reaction continuously; also, quantitative results are difficult to achieve. Moreover, solid organometallic complexes react with SO2 to produce more than one bonded adduct (2), of which the more weakly bonded are destroyed even by the mineral oil mull preparation technique. Two other infrared spectral sampling methods also were considered for the present work: Spectral examination of a thin layer of powdered sample (3), and preparation by evacuation and pressing of a mixture of KBr and the sample into an optically clear pellet. The latter technique is the most popular method at present. The thin layer technique suffers from a considerable amount of scattered energy and the production of low quality spectra. The preparation of halide pellets has the difficulty that structures that are not stable may suffer some loss during the evacuation and grinding procedures for preparation of the pellets. Because of advantages in quantitativeness the halide method was applied to the investigation of the reaction of SO2 with various solid sorbents, one of which was the Vaska compound, IrC1(CO)[P(CsHJ312 (I). During the course of the reaction of SO2 and the sorbent I, samples were withdrawn at intervals for investigation by infrared. Because of difficulties involved in the grinding and evacuation operation it became apparent that variable losses of SOz were producing considerable scatter of the data. It was obvious that an in situ method was desirable for obtaining accurate and reproducible data, and also to afford continuous analysis of the reacting system. In spectral studies of coal in halide pellets, it was known that water could be eliminated from (1) L. Vaska and S. S. Bath, J . Amer. Chem. SOC.,88, 1333 (1966). (2) E. C. Moroni, R. A. Friedel, and Irving Wender, J. Organometal. Chem.,21,23 (1970). (3) L. H. Little, “Infrared Spectra of Adsorbed Species,” Academic
Press, New York, N. Y., 1966.
Figure 1. Infrared gas cell Cell spacer is a l/le-inchO-ring pierced with inlet and outlet hypodermic needles for gas flow. Small crescent beside O-ring centers halide
pellet in the optical beam
the coal by simple heating of the pellet at 105 O C . The quantitative changes in the coal sample contained in the pellet were easily observable by infrared spectrometry (4). The decrease in the hydroxyl absorption band with heating could be followed in situ (5). Conversely, when heat was removed and the pellet was exposed to air, uptake of water by the dried coal in the pellet occurred, and could also be followed in situ spectroscopically. Thus it was demonstrated that HzO molecules could be emitted through, as well as reenter through, interstices of the particles of powder that make up the compressed halide pellet. It was decided to determine whether or not a similar gas-solid reaction could be carried out with SO2 and an appropriate solid sorbent imbedded in a KBr pellet. EXPERIMENTAL
A simple, thin gas cell was constructed from two KBr windows and a l/le-inch thick O-ring as the cell spacer. It was desirable to make the cell space as short as possible, to accommodate the KBr pellet with little or no dead space; this was necessary in order to minimize the interference of the spectrum of SOngas (Figure 1). A plain KBr pellet was put in the sample of the gas cell, centered in the source beam, and two hypodermic needles were inserted through two holes drilled in the O-ring to provide inlet and outlet for SOz or other gases. SO2 was allowed to flow slowly through the system for a considerable length of time; with a plain KBr pellet in the cell no absorption due to SO2 was observable. With this result as a blank, a pellet of the iridium carbonyl derivative I was placed in the sample space and SO2 again was allowed to flow through the gas cell. Spectral changes ap(4) H. H. Tschamler, in “Chemistry of Coal Utilization,” H. H. Lowry, Ed., John Wiley & Sons, New York, N. Y., 1963, p 72. (5) R. A. Friedel, in “Applied Infrared Spectroscopy,”D. N. Kendall, Ed., Reinhold, New York, N. Y., 1966, p 316. ANALYTICAL CHEMISTRY, VOL. 43, NO. 6, MAY 1971
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Figure 2. Partial infrared spectra of halide pellet of: A . IrCI(CO)[P(CBH6)3J2;B. A after reaction with SO2 in situ; C. B after further reactions with SO2 (pellet
removed from cell). Arrows indicate bands produced by SOs reaction. * indicates a highly unstable complex-SO2 band which exists only in an SO2 atmosphere
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SOz, hours
Figure 3. Infrared absorption us. time of exposure to SOz for the spectral bands involved in the iridium complex-S02 reaction. Two absorption bands decrease with exposure to SOz
peared rapidly due to the reaction of SOZwith the sorbent, but complete reaction requires many hours. Absorbance values were measured from the infrared charts. The reproducibility of absorbance values for a given sorbent sample in a KBr pellet is =t4z. For different KBr pellets of the sorbent the reproducibility of absorbance values is
*7z. RESULTS AND DISCUSSION
With this in situ method results were observable, quantitatively, throughout the course of the gas-solid reaction. Infrared absorption bands appear for strongly bound SO2 in the sample as well as loosely bound SOz. Some of the spectral bands were new bands; with the batch process of preparing KBr pellets, SO2 had become lost from the complex and some of the associated absorption bands never appeared. 762
ANALYTICAL CHEMISTRY, VOL. 43, NO. 6, MAY 1971
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Only with the in situ technique were the spectral absorption bands produced by weakly bound SO2 observable and accurately determinable. Even with the mild preparation of mineral oil mulls, some of the absorption bands due to weakly bound SO2 did not appear. Sample absorption bands of the complex and of the SOz products are shown in Figure 2. Arrows indicate the bands produced by the reaction of SO2. The two absorption bands at high frequencies, 1288 and 1152 cm-' are asymmetric stretching vibrations of SO2 molecules, coordinated or associated with the iridium complex. All the other 6 bands, indicated by arrows, are the result of structures produced by SO2 molecules weakly bound to the iridium complex. The weakly bound (associated) SO2can be removed from the complexin the pellet by simply flushing it out of the complex with inert gas; the bands of SO2 coordinated strongly with the complex are not removed by the flushing process. Figure 3 presents curves of absorbance data collected for various absorption bands of the sorbent-SO2 complex, using the in situ method. Very smooth curves were obtained in all cases; these would not have been possible with the batch process of pellet preparation. Two of the absorbance curves in Figure 3, for 838 and 442 cm-1, indicate decreasing absorbance values; these bands are assignable to the sorbent. Reaction with SO2 apparently produces changes in the structure of the sorbent which removes, or shifts, the 838 and 442 cm-' bands. An additional indication of the reactions that are occurring are found in the color changes which are known to occur in the reaction of SO2with the iridium complex. The same color changes were observed for the in situ reaction in the KBr pellet. The initial color of the unreacted complex I is yellow; after reaction with SOz the color changes to green (2). Additional information on the reaction is available; a check on the total amount of SOz taken up can be made by measuring the weight of the pellet before and after reaction has occurred. Another application of this method is demonstrated (Figure 4), namely, the uptake of H 2 0 vapor through the interstices of KBr pellets of coal (Figure 4). The absorbance of a fresh pellet of coal was measured at 3300 crn-'. Then the pellet was heated at 150 O C in uucuo and the absorbance at zero time was measured. From this point the pellet of
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dried coal was exposed to room air; the uptake of atmospheric water by the coal in the pellet begins immediately. The data in Figure 4 were obtained by measuring absorbances from successive spectral scans. The reaction could also be followed by setting the spectrometer to follow the intensity changes of the 3300 cm-1 absorption band as a function of time. In conclusion, this simple method is considered very
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useful for observing gas-solid reactions at moderate temperatures, particularly when the product involved is not of great stability, as for example in the reactions of organometallic complexes with Nz, Hz, and 0 2 , and other common gases.
RECEIVEDfor review July 24, 1970. Accepted February 3,1971.
Determination of Refractive Indices of Highly Alkaline Solutions Using Holographic Interferometry V. S. Srinivasan Chemistry and Chemical Engineering Laboratory, TR W Systems Group, Redor do Beach, Calif. 90278
IN THE COURSE of our work with the diffusion of ions by holographic interferometry, a need arose to determine the refractive indices of highly alkaline potassium hydroxide solutions. We were reluctant to expose prisms of a precision refractometer to the highly corrosive solutions. We conceived the idea of extending the holographic interferometric technique for the determination of refractive indices of corrosive solutions using disposable liquid prisms. Though there are existing interferometric methods for the determination of refractive indices of solutions, the advantages of the holographic approach lie in the fact that the random phase variations attendant upon poor quality prisms are automatically compensated, while more elaborate procedures are required to eliminate spurious fringes in a conventional interferometric approach. In the stored-beam interferometric technique (1-3) the hologram of the object (scene) is made in the usual way. After photographic processing, the singly exposed hologram ( 1 ) R. E. Brooks, L. 0. Heflinger, and R. F. Wuerker, Appl. Phys. Lett., 7 , 248 (1965). ( 2 ) L. 0 . Heflinger, R. F. Wuerker, and R. E. Brooks, J. Appl. Phys., 37, 642 (1966). (3) A. N. Zaidel, G. V. Ostrovskaya, and Y. I. Ostrovsky, Sou. Phys.-Tech. Phys., 113, 1153 (1969).
wavelength) in the original is replaced precisely (within holographic system and reilluminated. What is seen through the illuminated hologram is the superposition of the waves from the actual object and the synthetically generated waves from the recording. If the hologram is placed exactly in the original position and no other changes have occurred to the object since the recording, the reconstructed and the direct waves will be identical (except for a 180’ phase shift) and no interference fringes are seen. However, if the hologram or one of the optical elements is moved so as to change the angle between the regenerated and actual beams, finite fringes result. The angle between the beams can also be changed by varying the refractive index of the medium through which the object beam is passing. Our “refractometer” is based on the use of this phenomenon. The object is a liquid prism containing initially a reference solution. After replacing the exposed hologram precisely in the holographic system, no fringes are visible. However, if the reference solution is replaced with a liquid of different refractive index, fringes appear. The spatial frequency of the fringes can be quantitatively related to the relative change in the refractive index (4). (4) R. J. Collier, E. T. Doherty, and K. S . Pennington, Appl. Phys. Lett., 7 , 223 (1965). ANALYTICAL CHEMISTRY, VOL. 43, NO. 6, MAY 1971
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