Anal. Chem. 1980, 52, 1565-1570
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Analysis of the Gaseous Components of Reactions by Fourier Transform Infrared Spectrometry David D. Saperstein Merck, Sharp and Dohme Research Laboratories, P.O. Box 2000, Rahway, New Jersey 07065
This paper describes a continuous monitor for the total gaseous effluent from a heterogeneously catalyzed chemical reaction. The technique is made possible by the speed, sensitivity, and resolution of a commercial, mid-infrared Fourier transform (FT) instrument. Full spectra are obtained and stored for later analysis. With this method, one can Identify unexpected products and reactions, obtain semiquantitative analyses, and observe moderately fast changes In reaction performance. Examples are given.
A number of useful infrared spectrometric techniques provide information about heterogeneous, gassolid, catalytic reactions. These include studies on the bulk catalyst (I-3), studies of adsorbed molecules on the catalyst (341, and, more recently, in.situ studies of the reaction, e.g., reference 7 . Infrared (IR) spectrometry studies on the bulk catalyst can be used to show chemical and structural composition ( I ) , concentration of surface and bulk hydroxyl groups (2) and changes in the catalyst composition, if any, during and after use ( 3 ) . Studies of adsorbed species either in vacuo (4-6)or in situ (7) can lead to identification of reaction intermediates and the nature of their binding to the catalyst. The Fourier transform infrared spectrometer (FT-IR) has facilitated the investigation of a number of catalysts and catalytic reactions (8)because it has greater innate sensitivity and resolution than dispersive instruments (8, 9). However, one of the features of FT-IR, and computerized dispersive instruments, that is not usually realized in catalytic studies is the ability to subtract two spectra and obtain a difference spectrum which shows only the dissimilar components. Failure of the subtract feature of FT-IR is due t o the myriad of solvent-solvent, solventsolute, and solutesolute interactions which cause pertubations of the isolated molecule spectrum (IO). In these cases, subtraction of two related spectra results in the formation of derivative-like features ( I I , I 2 ) , which tend to obliterate small, neighboring bands in the spectrum. This paper describes a technique, which is an extension of the methodology of several research groups (I3-15), that makes full use of computer subtraction for catalysis studies. With this technique, we monitor the total products and reactants using, without major modification, the gas cell-light pipe, commonly referred to as a light pipe (€9,designed for the GC-IR technique (16). Thus, the heated effluent from a tubular microreactor is passed continuously through the heated light pipe, and the FT-IR spectra are obtained. Because the reaction is outside of the IR path, we are not restricted to a certain catalyst size or composition or to a specific reactor design; there are no wavelength limitations due to catalyst absorption, and the recorded spectrum is, within good approximation, the superposition of the spectra of the individual molecular species. Thus, products and reactants present in only small quantities can be identified and monitored semiquantitatively. One of the important features of the technique is that no chemical separation of components is necessary prior t o analysis. This permits reaction infor0003-2700/80/0352-1565$01 .OO/O
mation to be recorded a t intervals of 1 s or less depending on the cycle time of the interferometer. In the past two years we have uncovered a number of unexpected reactions ( I 7) by this technique and have developed insights into the behavior of several catalysts of interest.
EXPERIMENTAL The apparatus is shown schematically in Figure 1. The heart of the instrument is an FT-IR (Nicolet 7199) spectrometer equipped with a light transmitting, gas flow tube (8, 16). This tube measures 2.5 mm i.d. X 42 cm, ca. 2 cm3, and is gold coated for IR transmission. A liquid nitrogen cooled HgCdTe detector (Infrared Associates) was used routinely for all measurements. The KBr windows are held in place with a high temperature, high vacuum sealant (Vacseal, Space Environment Laboratories), and the entire tube is enclosed in a heated block. The windows must be sealed to prevent gas leakage into the instrument and to accurately measure flows. The reactor bed and adjacent inlet lines are positioned in the oven of a Varian 3700 gas chromatograph, and thus can be heated to a uniform temperature. A thermocouple inside of the reactor monitors the bed temperature. The reactor can differ from the oven temperature significantly during rapid heating or cooling and during exothermic reactions. Because the reactor is inside the GC oven, the GC-IR capability of the instrument can be used without time-consuming changes to the apparatus. The gas lines connecting the oven and the light pipe and exiting the light pipe are heated to prevent condensation, plugging, and possibly unwanted reaction of the condensed products and reactants. Calibrated flow meters on all the gas inlet lines and the gas outlet line are essential for quantitative work. Prior to running an experiment, a test gas, e.g., helium, is metered into the system and the inlet and outlet flows are compared. In this way, leaks can be found and sealed, and weak plugs can be eliminated. Heavy plugs can be recognized by an unexpected increase in reactor pressure. Liquid samples can be metered into the injector block of the gas chromatograph and thereby vaporized. In a typical experiment (Figures 2-4), one observes IR bands for the products and unreacted starting material according to their partial pressures or relative flow rates in the light pipe. Quantitation of a component of the gas is achieved by relating the height of one or more of its bands, for a given resolution, to concentration through the use of external standards. The detailed procedure is described below for the reaction of a substance, which is a liquid at room temperature, with a catalyst at an elevated temperature. The liquid is pumped at a pre-set rate into the GC injector heated to the reactor or other temperature. Within the injector body, the liquid can be diluted with a gas, e.g., He, air, or N2,whose flow is metered. The gas serves three purposes: (i) it establishes control of the average reactant-catalyst contact time, (ii) if necessary, it provides a convenient way of adding one of the and (iii) it dilutes the IR absorbing species so reactants, e.g., 02, that the detector can accurately measure absorbances. After preheating, the gas flows through a tubular, no. 316 stainless steel reactor--'/, inch o.d., 3/16 inch i.d. This type of reactor was chosen for convenience. In a more complex experiment, additional substances, which might react with the unvaporized liquid, could be metered directly into the oven and mixed with the gas stream prior to or directly over the catalyst bed. Following contact with the bed, the total gas stream passes through heated '/,,-inch 0.d. tubes, ca. 1 to 2 cm3 in order to minimize dead volume, to the light pipe where its infrared spectrum is recorded. The absorbance of the effluent gas is measured quasi-continuously by the FT-IR: a 0.7-s measurement per 1-s interval for Q 1980 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 52,NO. 1 1 , SEPTEMBER 1980
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Flgure 1. Schematic of our FT-IR apparatus for methanol conversion
4000-650 cm-' at 4 cm-' resolution. Typically, 16 interferograms, taking 15 s, are co-added and stored in one disk file. By averaging 16 spectra, we increase the S / N without usually losing information about transient effects. During the progress of the experiment, we monitor flow rates, temperature, and the average IR absorbance measured in pre-selected wavelength regions (Chemigram, Nicolet Instrument Corp.). After completion of the experiment, the disk files containing the data are apodized, transformed, and phase corrected using software provided by Nicolet. These single beam spectra are ratioed to a spectrum of the light pipe either taken prior to the experiment or collected during the experiment when the flow of one or more reactants is shut off. The transmittance spectra are routinely converted to absorbance units. These absorbance spectra can then be examined one at a time or for steady-state measurements averaged to improve S / N before examining, subtracting, and plotting. During very long experiments, e.g., greater than 1 h, the computer disk is quickly filled with interferograms. Since storage is limited to approximately 390 spectra, the number is reduced by storing only spectra that show changes, via examination of the average IR absorbance in the Chemigram of the effluent. Sometimes changes in the Chemigram are obscured, and the data are saved during specific time periods, e.g., the first 5 min out of every 20, in a lengthy reaction. A t the conclusion of the experiment, the Nicolet software provides a mapping between file number and the times that they were recorded. No attempt was made to directly use the interferograms, during or after data collection, to choose the spectrum to be saved or examined. Use of the interferograms to aid in the processing of the data is under active (18, 19) investigation elsewhere and will probably improve the speed and reliability of the technique described here. Although we have found that obtaining good quantitative results, i.e., much less than a 5% error relative to the measured concentration of a gas, has often been difficult to achieve, we have always been able to observe reaction trends, and when all elements of the system are controlled, reproducible results are obtained. In some cases, band intensities do not increase linearly with increasing concentration. Factors contributing to the observed nonlinearity are band absorbances which are much greater than 0.5 and resolution effects ( 8 , Z O ) . In general quantitation might be improved by including an IR absorbing, nonreactive substance, perhaps CF4, in the carrier gas. This substance would serve as an internal standard eliminating vagaries of lebks, erratic flow, etc. In addition, high solute absorption can be avoided by increasing the carrier gas flow, or, if possible, decreasing the path length of the light pipe. Reliability can be improved by taking band contour data, rather than peak height data, from more than one band.
RESULTS AND DISCUSSION Our FT-IR technique can be used to identify and measure components of any gas phase source. However, because of the availability of certain unusual catalysts, we have con-
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1200
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Figure 2. Spectra showing the conversion of methanol to methane. (a)No catalyst, liquid flow: 15 ILlmin; (b) catalyst, 15 pL/min. Catalyst: reactant: 0.41 g/mL methanol in chromium cobalt molybdate (27); water; reactor bed: 420 "C; 1.5 cm X 0.48 cm i.d., 16-30 mesh particles, surface area ca. 15-20 m2/g; light pipe: 300 O C ; instrument resolution: 4 cm-', and He diluent gas ca. 80 cm3/min. Absorbance units derive from a computer ratio (see text)
centrated our efforts on catalytic experiments. The first set of Figures (2, a and b, and 3, a-c; catalytic conversion of methanol to methane) demonstrate the ability to identify products and obtain product selectivities, reaction activation energies, and material balances by altering the feed composition and temperature of the reaction. Figures 4, a-d (catalytic reaction of CO + HJ,show the ease of obtaining performance data even though the catalyst is poisoned rapidly. In the last part of this section, experiments are described (a) in which isobutylene was identified when it was formed in the unexpected reaction of acetone with CaO at 450 "C and (b) in which a quantitative accounting of the hydrolysis of 4cyanothiazole is obtained. T h e conversion of methanol to methane belongs in the general field of energy related chemistry (17,22). The general features of this reaction are shown in Figures 2a (reactants a t 300 "C) and 2b (products and unreacted starting material a t 300 "C). The spectrum shown in Figure 2a was taken without any catalyst and shows the superposition of a water spectrum, bands from 1200 to 2100 cm-' and 3200 to 4000 cm-', and a methanol spectrum, bands a t 1030, 1380, 1430, 2050, 2800 to 3000, and 3680 cm-'. Except for a very small amount of C 0 2 showing a t 2349 cm-', no other species are apparent. These COz bands typically arise (23) from small changes of the CO, concentration in the interferometer and optics during the course of the experiment. The intensities of the water and methanol bands are typical of a 1-1 mixture and can be
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Flgure 3. Quantitative conversion of methanol over chromium cobalt molybdate (27). Vapor = 2 . 1 mol YO methanol 4.7 mol YO water, and He is the balance. Flow = 38 cm3/min at 298 "C. Light pipe = 3 0 0 "C. Catalyst = 0.30 g, 16-30 mesh. Instrument resolution = 4 cm-'. (a) Absorbance spectra of effluent with reactor bed at 204 "C, 253 OC,and their computer difference. (b) Absorbance spectra of effluent with reactor at 302 "C ( 5 X expansion), 351 "C and 400 "C after methanol and water spectra have been subtracted. (c)Summary of methanol conversion CH,OH conversion, (X) CH,O appearance, (V)CH, appearance, (0)COP appearance, and (A)CO appearance data: (0)
used to estimate their relative concentrations after reaction, Figure 2b. The spectrum shown in Figure 2b was taken at 300 "C after the gas passed through a bed of chromium cobalt molybdate (21) held at 420 "C. A comparison of Figures 2a and b shows a number of features. It is apparent that a reaction has taken place: not only are the intensities of the bands for methanol and water lower in Figure 2b but new bands have been formed. A band appeared a t 3014 cm-' indicative of methane, and bands appeared a t 2349 and 667 cm-' indicative of COS. In addition, CO is observed, 2145 cm-', and another C,H-containing molecule is likely, possibly formaldehyde, with absorptions at ca. 2760, 2815, and 1770 cm-l. Possible equations describing the conversion of methanol to methane a t atmospheric pressure and elevated temperatures are shown below:
CH30H
-
CO CO
HZCO
+ 3H2
+ Hz
-
4
CO
+ 2H2
CH4 + H 2 0
+ HzO + C02 + H,; water shift 2co = c + coz C + H,O CO + H2
(1) (2)
(3) (4)
(5)
The first three equations can be added to yield Equation 6:
4CH30H
+
3CH4 + CO? + 2H20
(6)
which describes the conversion in the absence of side reactions such as the formation of formaldehyde and ethylene. Of particular significance in Figure 2b is the ratio of the amount of methane to all carbon-containing products, i.e., CHI, COz, CO and some H2C0. The higher this ratio, the better the catalyst selectivity, and, therefore, FT-IR experiments which measure the change in selectivity with flow, catalyst composition or temperature provide insights into the catalysis. For example, in a separate experiment when the volume ratio of methanol to water was changed from 0.5 to 0.2, we observed that the methane selectivity dropped 15%. This trend was borne out by independent gas analysis ( 1 7 ) . Another reaction feature shown in the spectrum in Figure 2b is the small number of products. Besides CH4, C02, CO and some evidence for HzCO, only starting materials are seen. Alkenes such as
C2H4,having a very strong band at 949 cm-' (IO),and alkanes such as CzHs, having a strong band at 821 cm-' (IO),were not present in measurable quantities. The only major product not observed was H2 which is IR inactive (IO). When flow rates and peak absorbances are controlled, some additional catalysis results can be obtained as shown in Figures 3a-c for the same reaction of methanol over chromium cobalt molybdate. In these experiments the bed temperature was changed step-wise from 240 to 400 "C. A visual comparison of the data in Figure 3a, 204 "C with that of 253 "C, shows almost no difference between the spectra. However, when the 204 "C spectrum is subtracted from the 253 "C spectrum, the spectrum of H2C0 stands out. Furthermore, as the temperature of the reactor is raised to 400 "C, the amount of formaldehyde reaches a maximum selectivity of approximately 93% and then falls to unobservable levels, Figure 3b. In contrast, the amount of CH4 formed is negligible at low temperatures and accounts for ca. 6 M 5 % of the carbon a t higher temperatures, 350 to 400 "C. Using external standards and relating peak heights to mol % (&lo% of the measured concentration) in the gas stream, we can make an Arrhenius plot of the disappearance (conversion) of reactants and the appearance of products, Figure 3c. In these experiments, the role of the water is unclear since no significant changes (less than 3% variation) in its band intensities were observed. The data shown in Figure 3c may be complicated at high methanol conversion by the increased possibility of sequential reactions. Nevertheless, from the general features of Figure 3c, it seems likely that the same reaction step is rate determining for the disappearance of methanol and for the formation of H2C0. This step has an overall activation energy of 10 kcal/mol assuming, for calculation ease, zero-order reaction kinetics (24). The equivalence of their rate-determing steps is evident by the similarity of the temperature dependence of the disappearance of methanol and the appearance of H&O. Using a similar argument, we conclude that a different reaction step is rate determining for the high temperature products, CHI and COz, which show an activation energy of ca. 30 kcal/mol assuming zero-order kinetics (24). While further work, such as concentration dependence, would be needed to substantiate these hypotheses, it is quite intriguing to derive this much in one experiment. In support of these hypotheses, we note that microanalysis of the used catalyst showed less than 0.2% of the total methanol carbon present on the catalyst after the
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Figure 4. Spectra showing Fischer-Tropsch synthesis of methane from CO and H,. (a) 16 spectra averaging 0 to 15 s after reaction onset, (b) 15 to 30 s, (c) 30 to 45 s, (d) 300 to 315 s. Catalyst: fresh chromium cobalt molybdate: reactants, CO: 17 cm3/min:H:, ca. 30 cm3/min: reactor bed: 400 OC, 10.5 cm X 0.48 cm i.d., 16-30 mesh particles; light pipe: 300 O C ; instrument resolution: 4 cm-', and catalyst pretreated with H, for 30 min at the reaction temperature, 400 O C experiment; i.e., Equations 4 and 5 do not favor carbon deposition. The direct manufacture of methane from CO and Hz, e.g., derived from coal and water, may be an important source of natural gas in the future. This well-known Fischer-Tropsch reaction (25, 26) was tested in our apparatus with a fresh sample of chromium cobalt molybdate (21). The experiment was run to demonstrate that Equation 2 was valid for this catalyst. The spectrum of the starting material is not included since only the P and R bands typical for CO (center a t 2145 cm-') are present. Figures 4a-d show that in addition to unreacted CO, the main components of the reaction are CH4 (e.g., bands a t 3014, 1350, and 1302 cm-'1 and COP (e.g., bands a t 2349, 670 to 750, and 3560 to 3760 cm-'). There is some absorption in the 280&3000 cm-' region not due to CH4. This is probably due to a very small amount of a number of C,Hcontaining compounds (27) because no other diagnostic bands are present. Unlike Figure 2b, there is no evidence for formaldehyde in Figures 4, a-d. One unexpected feature of the CO + H2 reaction can be seen by comparing the CHI to the total carbon products in the four spectra. Total carbon products in Figure 4a (+15 s) equal 60%; in Figure 4b (15-30 s) they equal 58%; in Figure 4c (30-45 s) they equal 54%; and in Figure 4d (5 min later) they equal 55%. The decrease in the CH4/(CH4 + COz) ratio indicates a loss of catalyst selectivity for CH4 during the first 30 s of reaction. It is quite probable that the CH4/(CH4+ COz) ratio was even higher at very short times, i.e., (f2 s. These observations
are very important because they show that the catalyst is being altered, e.g., coked, by the reaction on a time scale that might have been missed if only GC measurements were used. Although our measurements were not repeated on a fresh sample of the catalyst at 1-s time intervals, such measurements are quite feasible and can serve to provide a closer estimate of peak catalyst selectivity. Because many industrial Fischer-Tropsch reactions produce a myriad of alkane and alkene products (25, 26), we were intrigued by the lack of recognizable species, other than CHI and COz, in Figures 4, a-d. In fact, only by raising the temperature of the catalyst bed to 450 "C can another product be observed, namely, ethylene a t 949 cm-'. Even at that temperature, the amount of ethylene is small. Unfortunately, increasing the bed temperature has the deleterious effect on this catalyst of decreasing the methane selectivity. While we offer no explanation for the observed catalysis, it is noteworthy that we can facilely differentiate among catalysts by their product distribution in a given reaction. The effect of temperature on selectivity was further assessed for the same reaction using a related cobalt molybdate catalyst. With that catalyst, when the bed temperature was held a t 450 OC, the CH4/(CH4 + COz) ratio was 47% whereas when the bed temperature was reduced to 300 "C, the CH4/(CH4 + COZ) ratio increased to 53 %. These measurements were taken after the initial peak activity of the catalyst has passed and, hence, may be somewhat reduced from their maximum values. Nevertheless, they show that differences in catalyst perform-
ANALYTICAL CHEMISTRY, VOL. 52, NO. 11, SEPTEMBER 1980
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Table I. Hydrolysis of 4-Cyanothiazolea experiment no catalystb CoMoO,, 360 " C CoMoO,, 420 " C Cr2C02M0201,,C 360 " C Cr,Co,Mo,O,,, 420 "C
4-methylthiazole, 4-cyanothiazole, 804 crn-l, % 822 cm-', % 100 104 103 107 122
-
thiazole, 796 c m - ' , % 100 42 53 64 95
100 51 33 36 3
-
average 109 Equimolar solution of 4-cyanothiazole and 4-methylthiazole in water (0.10 M). min adiusted to 100%. Ref. 21. ance for this reaction can also be measured by our FT-IR technique. In another set of experiments, an unknown product was obtained from an unexpected, high temperature (450 "C) reaction of acetone, water, and calcium oxide. The reaction was unexpected because acetone and calcium oxide did not react in the absence of the water at the same temperature and pressure. The unknown product had a sharp Q branch at 888 f 1 cm-' and almost symmetrical P and R branches with no rotational fine structure at 4 cm-' resolution. Other spectral features included methane, which had the strongest product bands in the spectrum, C02, and unreacted acetone. Examination of the used CaO showed that most of the COPfrom the reaction was absorbed forming CaC03. Because acetone has some absorption in the 800-1000 cm-' region, this contribution to the spectrum was removed via spectral subtraction before the peak positions and band shapes of the unknown product were recorded. After chemical and spectral consideration of ketene, ethylene oxide, propylene oxide, ketene dimer, and butadiene, all of which have very strong band(s) in the 850-900 cm-l region, we showed that isobutylene was the progenitor of our 888 f 1 cm-' band. It was identified by matching the 800-950 cm-' region of an authentic sample (Matheson) with the same region in the unknown. In subsequent studies, we showed that the unobserved acetone dimerization products, diacetone alcohol and mesityl oxide, probably gave rise to the isobutylene rather then any intermediate involving ketene. This was deduced from the observation that mesityl oxide and water reacted with CaO to form 40 times more isobutylene than is formed by the equivalent reaction of acetone, water, and CaO. Formation of isobutylene from acetone and its dimerization products has been reported over strongly acidic catalysts (28,291 but has not been reported to our knowledge over strongly basic catalysts (30, 31). The ammoxidation of 4-methylthiazole to form 4-cyanothiazole, an important industrial process (II), is accompanied by the formation of water. Finding catalysts to increase the selectivity of the ammoxidation reaction is hampered by the possibility of side reactions. One such is the hydrolysis of the product to form thiazole, ammonia, and carbon dioxide. Table 1 compares two similar catalysts and shows that one is a lot more destructive to the desired product. It is noteworthy that these data were obtained from overlapping bands of the three thiazole compounds and yet clearly demonstrate the activation energy difference between the two catalysts. These analyses were facilitated by the wavelength and photometric accuracy of the FT-IR technique (8). For identification problems, we have found it necessary to record our own reference spectra, because the majority of infrared measurements have been made on condensed phases, e.g., Reference 32. These spectra can serve as a guide but are not suitable for precise identification because there is typically a 20 cm-' shift in band position from the liquid or solid to the gas phase, and there can be distinct changes in band shapes as well, e.g., the carbonyl of succinimide (32, 33). It should
-
4-cyanothiazole and thiazole, % -93 86 100 98 -
94 Peak height of standard, ca. 1 0 pm/
be noted that the finite resolution of an FT-IR instrument generates an uncertainty in band position (8). Such uncertainty has an effect on the identification of an unknown when its bandwidths (FWHM) are as small or smaller than the instrument resolution (16, 20). This problem can be minimized by working at increased resolution with a penalty of slower data acquisition and Fourier transform rates and with larger data storage requirements. Other distortions of the true spectrum can occur with FT-IR. For a more detailed explanation see Reference 20. In addition to the high temperature gas phase reactions that we have described already, many other catalytic reactions, e.g., total and selective oxidations, reduction of nitrogen oxides, would benefit by our FT-IR technique. It is easy to imagine that in some of these reactions, unexpected chemistry will be revealed. There is also a potential for noncatalytic applications as well, e.g., monitoring products and their rates of formation from pyrolytic reactions has been demonstrated (23,14), examining photochemical or radiochemical reactions, and even studying liquid or solid phase reactions by changes in the associated vapor are possible. We have used the technique to measure the rate and quantity of sorption and desorption of water and other gases from metal oxides. These studies are quite similar to work recently reported (14). A feature of importance to us was that the species of interest, e.g., water, can be measured without interference from or complication by other molecules desorbing or sorbing. We hope to extend our studies to liquid phase reactions. There, of course, solvent-solute, solute-solute interactions, and intense solvent absorptions interfere with the identification and quantitation of components. However, in some chemical reactions, e.g., solvents which are not very polar, the benefits of the FT-IR method-no separation, no sample handling, computer subtraction, etc.-will be a welcome tool for chemical analysis.
ACKNOWLEDGMENT I thank the many members of the Merck research staff who encouraged the development of this technique. Without their support and helpful criticisms, this work would never have been accomplished. Special thanks goes to Alan Rein, Julie Elward-Berry, Hugh Woodruff, Alan Douglas, Elizabeth Otterbein, Erwin Schoenewaldt, and Seemon Pines.
LITERATURE CITED (1) Lutz, H. D. Spectrochim. Acta 1968, 24 2107. (2) Ward, J. W. "Molecular Sieve Zeolltes", Adv. Chem. Ser. 1971, No 101, 380. (3) Kiselev, A. V.; Lygin, V. I. "Infrared Spectra of Surface Compounds", Keterpress: Jerusalem, 1975. (4) Kokes. R. J.; Dent, A. L. J . Am. Chern. SOC. 1970, 92. 6709. (5) Low, M. J. D.; Gocdsel, A. J.; Takezawa, N. Environ. Sci. Techno/.1871, 5, 1191. (6) Schoonheydt, R. A,; Lunsford, J. H. J . Catal. 1972, 26, 261. (7) Ekerdt, J. G.; Bell, A . T. J . Catal. 1979, 58, 170. (8) Griffiths, P. R. "Chemical Infrared Fourier Transform Spectroscopy", J. Wiley and Sons: New York, 1975; pp 15. 195, 202, 209, 269, and 333. (9) Green, D. W.; Reedy, G. T. "Fourier Transform Infrared Spectroscopy. Application to Chemical Systems", Ferraro, J. R., Basile, L. J., Eds.; Academic Press: New York, 1978; p 143. (1 0) Herzberg, G. "Molecular Spectra and Molecular Structure. 11. Infrared and Raman Spectra of Polyatomic Molecules"; Van Nostrand Reinhold
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Anal. Chem. 1980, 52, 1570-1574 Co.: New York, 1945; p 251, 295, 326, 344, and 534. Hirschfeid, T.; Kizer, K. Appl. Spectrosc. 1975, 29, 345, Lynch, P. F.; Brady. M. M. Anal. Chem. 1978, 50, 1518. Liebman, S.A.; Alhstrom. P. H.; Wffiths. P. R. Appl. Spectrosc. 1976, 30, 355. Fenner, R. A. Appl. Spectrosc. 1980, 3 4 , 174. Lephardt, J. 0.; Koga, Y.; Sugie, M.; Kondo, S.; Saeki, S. Bunseki Kagaku 1979, 28,298. Griffiths, P. R. "Fourier Transform Infrared Spectroscopy", Vol 1, Ferrero, J. R., Basile, L. J., Eds.; Academic Press: New York, 1978; p 143. Saperstein. D. D. U.S. Patent 4 182926, 1980. Hanna, Alan; Marshall, John C.; Isenhour, T. L. J. Chrornatogr. Sci. 1979, 77.434, and references therein. Delaney, M. F.; Uden, P. C. J. Chromatogr. Sci. 1979, 17, 428. Hirschfeld, T. "Fourier Transform Infrared Spectroscopy", Vol 2, Ferraro, J. R., Basile, L. J., Eds.: Academic Press: New York, 1979; p 193. Elion, G. R.; Klink, A. E. U.S. Patent 4055511, 1977. Nagy, J. B.; Gilsen, J. P.; Derouane, J. J. Mol. Catal. 1979, 5 , 393 and references therein. Ferraro, J. R.; Basile, L. J. "Fourier Transform Infrared Spectroscopy", Vol 1, Academic Press: New York, 1978; p 283, Figure 5.
(24) Carberry. J. J. "Chemical and Catalytic Reaction Engineering", McGraw-Hill: New York, 1976. (25) Mills, G. A.; Steffgen, F. W. Catal. Rev. 1973, 8 , 159. (26) Inui, T.; Funabiki, M.; Suehiro, M.; Sezune, T.; Iwana. T. Nippon Kagaku Kaishi 1978, 4 , 517. (27) Colthup, N. B.; Daly, L. H.; Wiberly, S. E. "Introduction to Infrared and Raman Spectroscopy"; Academic Press: New York, 1964; Chapter 5, p 364. (28) Kurganova, S.Y.; Rudenko, A. P.; Babndin, A. A. Zh.Org. Khirn. 1968, 2 , 804. (29) McAllister, S.H.; Bailey, W. A.; Bolton, C. M. J. Am. Chem. Soc. 1940, 62,3211. (30) Scheidt, F. M. J . Caral. 1964, 3 , 372. (31) Tada. A. Bull. Chem. Soc. Jpn. 1975, 48, 1391. (32) Pouchert, C. J., Ed., "The Aldrich Library of Infrared Spectra"; Aldrich Chemical Co: Milwaukee, Wis., 1970. (33) Vandef, A,; Bouche, R. Spectrosc. Lett. 1979, 72,371.
RECEIVED for review February 6,1980. Accepted June 2,1980.
Determination of Chromium Speciation in Natural Waters by Electrodeposition on Graphite Tubes for Electrothermal Atomization Graeme E. Batley" Analytical Chemistry Section, Australian Atomic Energy Commission, Lucas Heights, N.S. W., 2234 Australia
Jaroslav P. Matousek Department of Analytical Chemistry, University of New South Wales, P.O. Box 7, Kensington, N.S. W., 2033 Australia
An analytical technique has been developed for the study of chromium speciation In natural waters based on the atomlzation of electrodeposited species. Matrix Interferences can be overcome and preconcentratlon achieved by the electrodeposition of chromium with mercury, onto pyrolytic graphlte-coated tubular furnaces, using a flow-through assembly. At pH 4.7, using a deposttion potential of -1.8 V vs. SCE, both Cr(V1) and Cr(II1) are reduced and accumulated as metallic chromium. At the same pH, but at -0.3 V vs. SCE, only Cr(V1) is selectively reduced to Cr( 111) which accumulates by adsorptlon. Using the labile-bound discrimination of the electrodeposition technique combined with an ultraviolet Irradiation step, Cr(V1) was found to be dominant In the samples studied, wlth most Cr(V1) present as labile forms.
cation-exchangeable chromium with Cr(III), since Cr(OH)4-, predicted to be the dominant form of Cr(II1) in seawater (5, IO), will be retained in the anion-exchangeable fraction. The nonexchangeable fraction will consist of neutral polymeric species or chromium adsorbed or occluded in colloidal species and unable to penetrate the resin pore network. Inherent in all these procedures is the need to separate the metal species before analysis from the sodium chloride matrix, since it can interfere in any direct atomic absorption measurements. We have shown previously (11)that the separation and preconcentration of cobalt and nickel species from seawater can be readily achieved by their electrodeposition, in the presence of added mercuric ions, onto a graphite tube prior to the atomization of the deposited metals. This paper describes further studies of the electrodeposition-atomization technique and examines its potential for the analysis of chromium species in seawater.
Because of the difference in toxicity of hexavalent and trivalent chromium to aquatic biota, there have been many attempts in recent years to discriminate between species in these two oxidation states in natural waters ( I ) . Trivalent chromium is recognized as essential to mammals for the maintenance of an effective glucose, lipid, and protein metabolism. In the hexavalent state, however, chromium can diffuse, as Cr0:-, through cell membranes, when it can oxidize and bind to other important biological molecules with toxic results. Most of the early studies of chromium speciation (2-6) relied upon the ability of ferric hydroxide to selectively coprecipitate Cr(II1) ( 2 , 3 ) ,with Cr(V1) being determined after reduction, usually using SO2. Selective solvent extraction of Cr(V1) from seawater was reported by De Jong and Brinkman (7). Chromium(V1) was quantitatively extracted by Aliquat-336 from weakly acidic (pH 2) samples, while Cr(II1) was extracted after oxidation by ammonium persulfate. More recently, ion-exchange separations have been investigated (8) and these form the basis of a field technique proposed by Shuman and Dempsey (9). It is not possible, however, to equate anion-exchangeable chromium with Cr(V1) species or 0003-2700/80/0352-1570$0 1.OO/O
EXPERIMENTAL Apparatus. Electrodepositions were carried out using a Princeton Applied Research model 174 polarographic analyzer, and the cell assembly is shown in Figure 1. The cell, constructed
from Perspex, consisted of a threaded top section containing the three electrodes and gas inlet tubes and a detachable base section capable of holding 25 mL of solution. Pyrolytic graphite-coated tubes (9 mm x 3 mm i.d.) supplied by Ultra Carbon Corp., Bay City, Mich., were used as replaceable cathodes. For experiments on deposit distributions, tubes supplied by Varian Techtron were also used. A tube was held by a polythene screw in the recessed end of the Teflon tube. Pressure contact was made with the end of the graphite electrode, by a platinum wire inserted through a separate hole in the Teflon tube. The pyrolytic graphite coating was scraped from the upper end of the electrode to ensure good electrical contact. During electrolysis, the graphite cathode should be fully covered by solution. The anode consisted of a coil of platinum wire. A ceramic plug salt bridge was used to connect a Beckman No. 39178 fibre-junction, saturated calomel electrode (SCE). Nitrogen flow could be regulated either above or under the solution via Pyrex glass inlet tubes. Solution was circulated through the cathode and returned to the cell at 2.2 mL s-l using a Masterflex pump drive (model 7544-80) and head (model 7015-20). C
1980 American Chemical Society
