Anal. Chem. 1981, 53, 2123-2127
It is instructive to consider the manner in which the use of a mercury(I1)--sulfite reagent (25-29)allows the order of addition tlo be reversed. In neutral solution the mercury(11)-sulfite complex appears to be more stable than the formaldehydebisulfite adduct, and the latter compound does not form. Upon acidification by the addition of the pararosaniline reagent, the mercury(I1)-sulfite complex decomposes to liberate sulfur dioxide. As noted, however, the acidity prevents the sulfur dioxide from reacting with the formaldehyde (31, 32). CONCLUSIONS The advantages of the optimized pararosaniline method described here are significant enough to recommend broadscale adoption of this method of determining formaldehyde in aqueous solution. Ease of use, greater sensitivity, and selectivity allow simpllified determination of formaldehyde in air from nonindustrial indoor environments. The excellent precision of the method is not a factor in making such determinations as the overall precision is defied by fluctuations in the formaldehyde levels and the sampling strategy. ACKNOWLEDGMENT The help of Nils Peterson and Linda Jencks is gratefully acknowledged.
LITERATURE CITED Andersen, I.; Lundgvist, (3. R.; Molhave, L. Atmos. Environ. 1975, 9 , 1121. Breysse, P. A. Environ. lfeakh Safew News 1977, 26, 1-13. Consumer Product Safety Commission, Technical Workshop on Formaldehyde, April 9-1 1, 1980, National Bureau of Standards, Gaithersburg, MD. Consumer Product Safety Commision Fed. Reglst. 1980, 45, 3403 1-34033. Garry, V. F. “Formaldehyde in the Home: Some Environmental Health Perspectlves”, M.S. Thesis, School of EnvironmentalHealth, Universlty of Mlnnesota, Minneapolis, MN, 1979. Bureau of Prevention, Wisconsin Dlvision of Health, Formaldehyde Case File Summary, Oct 23, 1978, Madison, WI. General Industry Safety and Health Standards; Table 21, p 506, U S . Department of Labor Occupational Safety and Health Administration OSHA 2206 (29 CFR 1910). Revised Jan 1976. Lin, C.; Anaclerio, R. N.; Anthon, D.W.; Fanning, L. 2.; Holloweii, C. D. ”Abstracts of Papers”, 178th National Meeting of the American Chem-
211!3
ical Society, Washington, DC, Sept 1979; American Chemical Society: Washington, DC, 1976; INOR 59. (9) American Public Health Association Intersociety Commlttee ”Methods of Air Sampllng and Analysis”, 2nd ed.; Katz, M., Ed.; American Public Health Association, Washington, DC, 1977; pp 300-307. (10) Natlonal Institute of Occupational Safety and Health “Manual of Aneiytlcal Methods”, 2nd ed.; 1977; Vol. 1, 125-1, 125-9. (11) West, P. W.; Sen, 8. 2. Z . Anal. Chem. 1956, 753, 177-183. (12) Altshuller, A. P.; Miller, D. L.; Sleva, S. F. Anal. Chem. 1961, 38, 821-625. (13) Cares, J. W. Am. Ind. Wg. Assoc. J . 1968, 28, 405-410. (14) Krug, E.; Hirt, W. Anal. Chem. 1977, 49, 1865-1867. (15) Schlff, H. Ann. Chem. Pharmacoi. 1866, 740, 92. (18) Stelgmann, A. J. Proc-Sac. Chem. Ind., Chem. Eng. Group 19412, 61, 18. (17) Grant, W. M. Anal. Chem. 1947, 19, 345. (18) Kozlyaeva, T. N. Zh. Anal. Khim. 1949, 4 , 75. (19) Atkin, S. Anal. Chem. 1950, 22, 947. (20) Urone, P. F.; Boggs, W. E. Anal. Chem. 1951, 23, 1517. (21) Paulus, H. J.; Floyd, E. P.; Byers, D. H. Am. Ind. W g . Assoc. 1954 75, 4. (22) Moore, 0. E.; Cole, A. F.; Katz, M. J. Air Pollut. Control Assoc. 1957, 7, 25. (23) West, P. W.; Gaeke, G. C. Anal. Chem. 1956, 28, 1816. (24) West, P. W. Atmos. Envlron. 1976, 10, 835. (25) Lyles, G. R.; Dowllng, F. B.; Bianchard, V. J. J. Alr Pollut. Control ASSOC.1965, 75, 106-108. (26) Lahmann, F.; Jander, F. Gesund.-Ind. 1968, 89, 18-21. (27) Yunghans, R. S.; Munroe, W. A. Automation in Analytical ChemlstSy, Technicon Symposia, 1965, New York, Mediad, 1966, pp 279-284. (28) Walker, J. K. “Formaldehyde”; R. E. Krleger Publishing Co: Huntington, NY, 1975; pp 486-487. (29) Hitchin, E. R.; Wllson, C. B. Bulhi. Sci. 1987, 2 , 59-82. (30) Schlesinger, G.; Miller, S. L. J. Am. Chem. Sac. 1973, 95, 3729-3734. (31) Skrabal, A.; Skrabal, R. Monatsch. Chem. 1936, 6S, 11-41. (32) Sorensen. P. E.: Andersen. V. S. Acta Chem. S a n d . 1970. 24. 1301-1306. (33) Vogh, J. W. Anal. Chem. 1971, 43, 1618-1624. (34) Nauman, R. V.; West, P. W.; Iron, F.; Gaeke, G. C. Anal. Chem. 1960, 32, 1307-1311. (35) Rumpf, P. Ann. Chim. (Parls) 1935, 3 , 327. (36) Cordes, E. H.; Jencks, W. P. J. Am. Chem. SOC. 1962, 841, 43 19-4328. (37) Dasgupta, P. K.; DeCesare, K.; Ullrey, J. C. Anal. Chem. 1980, 52?, 1912. .
I
RECEIVED for review March 20,1981. Accepted July 20,1981.. This work was supported by the Assistant Secretary for Conservation and Solar Energy, Office of Buildings and Community Systems, Buildings Division of the US. Department of Energy )under Contract No. W-7405-ENG-48,.
Laser Raman Spectrometry for the Determination of Crystalline Silica Polymorphs in Volcanic Ash Dennis R. Gage and Sherry 0. Farweli” Deparfment of Chemistty, University of Idaho, Moscow, Idaho 83843
Laser Raman spectrometry (LRS) was examined for its utility In the identification and quantlflcatlon of the three common crystalline silica polymorphs (Le., quartz, cristobailte, and trldymite) In Mt. St. Helens volcanic ash. Unilke the X-ray dlffractlon and infrared spectra, the Raman spectra of these three polymorphs contaln speclflc Identification bands which are completely resolved from one another and from other bands due to the ash matrix. For example, the malor Raman band for quartz is at 464 cm-I while the malor band for crlstobalite Rs at 416 cm-’. Thus, the Raman spectra provide a hlgh degree of speclation since the peaks are separated by 48 cm-‘ and the peak widths at half-height are -5 cm-I. The Raman methodology described In this paper exhiblted a detectability in the 0.5-1.0 wt % range for quartz and cristobalite in the untreated ash matrix. 0003-2700/81/0353-2123$01.25/0
The application of lasers as intense monochromatic sourceu in modern Raman spectrometers has been primarily responsible for the resurgence in Raman spectrometry since the early 1970s. Whereas, laser Raman spectrometry (LRS) has been subsequently employed as an important technique in t h e 0 retical studies of molecular structure, its reported analytical applications ( I ) have remained quite limited despite severall articles which have accentuated its quantitative potentialsi (2-4). Furthermore, the analytical literature contains even fewer reports concerning quantitative applications of LRS for solid samples. Notable exceptions are the LRS determinationsi of anatase in rutile TiOz pigments (5), sulfate impurities in NaN03 reagent samples (6),and styrene monomer in b u t a diene/styrene latexes (7). Our investigation into the analytical capabilities of LRS 0 1981 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 53, NO. 13, NOVEMBER 1981
was initiated by our search for an independent method for the qualitative and quantitative determination of crystalline silica in Mt. St. Helens volcanic ash (8). A review of the available LRS literature indicated that LRS spectra were probably capable of distinguishing the three common crystalline silica polymorphs, i.e., quartz, cristobalite, and tridymite (9-13). If so, then LRS might be a satisfactory alternative to the more standard infrared (IR) and X-ray diffraction (XRD) techniques which had provided somewhat ambiguous qualitative information in these particular ash analyses due to matrix interferences (1,14,151. In addition, the potential for acquiring quantitative data via LRS existed since the scattered Raman intensity is directly proportional to the concentration of the species which generates that specific Raman line (2). Therefore, this paper describes our experimental methods, experiences, and concomitant results for the application of laser Raman spectrometry to the determination of three crystalline silica polymorphs in solid samples which are difficult to analyze by other techniques because of their physical nature and/or matrix interferences.
EXPERIMENTAL SECTION Materials and Sample Preparation. The volcanic ash samples, in addition to the standard samples of quartz, cristobalite, and tridymite were obtained from the National Institute for Occupational Safety and Health (NIOSH) as part of their round-robin testing program. This round-robin program was organized because of the recent controversy concerning the crystalline silica content of Mt. St. Helens volcanic ash (14-16). These ash samples and crystalline silica standards had been cycloned by NIOSH to obtain the respirable fraction, i.e., those ash particles whose aerodynamic equivalent diameters were 110 pm (19). Four other quartz samples with particle diameter ranges of 1 5 pm, 510 pm, 115 pm, and 530 pm were obtained through the courtesy of Pennsylvania Glass Sand Corp. These latter four quartz samples were utilized in the investigation of particle size effects on the intensities of the analyte Raman lines. The KNOs used in the LRS pellets was AR grade from Baker Chemicals. Samples Were prepared for LRS analysis by pressing approximately 300 mg of sample into 13 mm diameter pellets via the application of 200 kg/cm2pressure in a conventional infrared pellet die (8). Spiked ash samples were mixed for 90 s in a Wig-L-Bug before being pressed into pellets. Instrumentation. The LRS instrument was a Spex system with a Spectra-PhysicsModel 164-03argon ion laser and a Spex No. 14018 double monochromator containing 1800 grooves/mm holographic gratings blazed at 500 nm. A Hamamatsu Model R-955 photomultiplier (PM) that was cooled to -30 "C was used for signal detection. The PM signal was processed by a Spex digital photometer operated in the photon-counting mode. All spectra were obtained by using the 514.5-nm line of the argon ion laser with the nonlasing plasma lines removed by the use of a narrow band-pass interference filter. Sample pellets were placed in the Spex No. 1445A solid sample spinner accessory which is equipped with a variable dc motor that allows spinning rates up to approximately 8ooo rpm. The spinner was positioned in the sample illuminator compartment of the spectrometer in a manner such that the pellet was oriented at a 60" angle to the incident laser beam. The Raman scatter was viewed at an angle of 90" to the laser beam and the sample pellet was spun at 1000 rpm in the laser beam path. Specific instrumental parameters such as laser power as measured at the sample, slit widths, scan rates, integration times, etc. will be presented with the accompanying data in the Results and Discussion section. RESULTS AND DISCUSSION Unlike the IR spectra of quartz and cristobalite which contain relatively broad absorption bands at almost identical wavenumbers (9-13), the corresponding LRS spectra show sharp, well-resolved peaks. The individual spectra recorded for these two polymorphs on our LRS system are reported in another recent article in this journal (8)and consequently are not repeated in this paper. As illustrated in the spectra of
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Figure 1. LRS spectrum of NIOSH tridymite standard: laser power, 180 mW at sample; time constant, 5 s; scan rate, 0.2 cm-'/s; slits, 5 cm-'; full scale, 10K counts; zero suppress, 9.4 X 100.
ref 8, these two LRS spectra contain three peaks in the region from 0 to 500 cm-'. For quantitative purposes, the most intense Raman peak in each spectrum was used, hence, the 464-cm-' peak of quartz and the 416-cm-l peak of cristobalite. Note that the peaks for these two Raman lines are separated by 48 cm-l which is more than sufficient to prevent quantitative problems due to band overlap. The spectrum obtained on our LRS system for the NIOSH tridymite standard is shown in Figure 1. Unlike the spectra for quartz and cristobalite, the tridymite spectrum in Figure 1contains a number of peaks in the low wavenumber region. Whereas the correlations between our experimental LRS spectra for quartz and cristobalite were identical with literature spectra (9-13), there was minimal correlation between the tridymite spectra. According to Sosman (In,tridymite is often a silica material of indefinite properties unless the particular sample's thermal history is known in considerable detail. This statement by Sosman is based on the fact that "tridymite" is usually a mixture of two independent phases-tridymite S and tridymite M. Tridymite M changes monotropically into tridymite S, which is the stable tridymite variety above 867 "C and below 1470 "C a t atmospheric pressure, but the M to S conversion rate is appreciable only above approximately 900 "C. Therefore, natural samples of tridymite are often mixtures of the two independent M and S tridymites and the M to S ratios will vary depending upon the sample's thermal history. Since tridymites M and S have distinct X-ray patterns and different crystalline structures, samples containing various ratios of tridymites M and S would be expected to show quite different Raman spectra. This lack of agreement in the various reported Raman spectra of tridymite was also noted by Etchepare et al. (13). These authors suggested that the different tridymite spectra were probably due to structural differences of the various low temperature forms of tridymite. Tridymite S can exist in six known phase modifications (S-I through S-VI) and tridymite M in three (M-I through M-111) (17). These various high-low types of polymorphism for tridymite add to the apparent confusion regarding tridymite in geological samples. The practical consequence of these dissimilar LRS tridymite spectra is the lack of a reliable analytical standard(s) which in turn means the accurate identification and quantification of tridymite in the ash samples were not possible by LRS. Therefore, the data in this paper only pertain to the determinations of quartz and cristobalite. Although there are four other known prin-
ANALYTICAL CHEMISTRY, VOL. 53, NO. 13, NOVEMBER 1981
cipal crystalline phases of silica besides quartz, cristobalite, tridymite S, and tridymite M, the phases of coesite, keatite, stishovite, and silica W are not common in geological samples (17, 18). In our initial work an attempt was made to use the standard LRS technique of containing the powder sample in a transparent glass capillary tube. However, quartz and cristobalite are relatively poor scatterers, and although the spectra of the concentrated standards could be obtained by this experimental procedure, no sharp Raman bands were observed from the ash samples. In fact, under conditions of high scale expansion, the 300-to 500-cm-' region of interest was obscured by a broad structureless band from the glass capillary tube itself. Furthermore, quantification would be difficult with this capillary tube method of sample illumination since the intensity of the signal would be a function of how tightly the powders were tamped into the capillary tube. Finally, the use of capillary sample tubes places a minimal amount of sample in the laser light path and such a small amount of sample illumination may not truely represent the bulk of the sample, particularly in the case of spiked samples. Pressing the powder samples into pellet form alleviated several of these foregoing problems. These pellets could be held in the LRS solid sample spinner and the face of the pellet exposed directly to the laser beam. This procedure eliminated the interference problems associated with a sample container such as the glass capillary tube. Because the pellets were formed under the same die ]pressure,the samples were packed much more uniformly than they could be via the capillary tube method. The sample spinner was positioned in the illuminator compartment of the Ramari spectrometer such that the laser beam was focused on a spot located about 5 mm off the axis of pellet rotation. The laser was focused to a spot approximately 2 mm in diameter; hence, the illuminated area was an annulus with a 5 mm radius and a width of 2 mm. This sample illumination area corresponds to approximately 50 mm2which is about 20 times the area illuminated in the initial capillary tube arrangement. Rotation of the sample at loo0 rpm served to average out local inhomogeneities that occurred in spiked samples and helped to minimize particle orientation effects (6). The volcanic ash samples were also varying shades of gray and, consequently,absorbed some of the 514.5-nm laser power. Rapid rotation of the sample pellet dissipated the heat created by this absorption and, therefore, higher laser powers could be used without degrading the samples. Several scanning electron microphotographs of sample pellets revealed no evidence of physical alteration of the pellet surfaces after extended exposure to the LRS laser beam. Pellets of the ash samples and of the quartz and cristobalite standards exhibited an initially high LRS background signal due to fluorescence. This fluorescence intensity decayed with time when the sample was continuously exposed to the laser beam, and the decay rate was more rapid at higher laser powers. Such fluorescence is fairly common and has been noted and explained by several authors (3,19-21). The high fluorescence background did not reappear when the laser was turned off and on provided the sample pellet remained in its original position in the spinner. In the case of concentrated standards, a period of 0.5-1 h in the laser beam was usually sufficient to reduce the background to a level where good spectra could be obtained. However, when the intensities of the signals of interest were small relative to the background, as was the case with the ash samples themselves, an exposure period of several hours was required before the background subsided to a level where spectra could be obtained. As previously mentioned, both quartz and cristobalite are relatively inefficient at Raman scattering. The relative
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Flgure 2. LRS spectrum of a pellet containing 10% KNO9 and 90% quartz: laser power, 150 niW at sample; time constant, 1 8; scan rate, 0.5 cm-'/s; slits, 5 cm-'; full scale, 50K counts; zero suppress, 29.1
x 100.
scattering efficiency for these two crystalline silica polymorphs is also demonstrated in Figure 2 where the standard pellet contained 90% quartz: and 10% KN03, yet the 464-cm-' quartz peak and the 1052-cm-l KN03 peak are approximately of equal intensity. Our previously reported spectrum (8) of a pellet containing 50% quartz and 50% cristobalite revealed that quartz was more efficient at Raman scattering than cristobalite by a factor of approximately 2. Due to a limited supply of the quartz and cristobalite standards, the dependence of the Raman peak height on concentration was demonstrated on our LRS system using KNOBas the analyte. A series of pellets ranging in concentration from 5% to 25% KN03 in a volcanic ash matrix were prepared. This series of KN03/ash pellets contained no internal standard for compensation of the fluctuations in laser power. Nevertheless, a plot of these uncompensated intensity data resulted in a linear relationship between analyte concentration and peak height. Whereas various effects of particle sizes on Raman scattering efficiencies have been reported (21,22), our investigations with pellets made from quartz of different particle sizes, Le., particle diameters of 1 5 pm, 110 pm, 115 pm, and 130 pm, revealed no significant dependence of the peak height with particle size. This independence of scattering intensity with particle size is probably accounted for by the fact that these samples did not consist exclusively of 5 pm, 10 pm, 15 pm, and 30 pm particles. Instead, these particle sizes of the quartz standards represent limiting diameters above which particles were rejected during sizing. Consequently, the bulk of the sample changes very little in going from a so-called 5-pm quartz sample to a 10-pm fraction. As usual, the actual ash samples were more difficult to analyze than the Standards. The LRS spectra of the NIOSH ash samples under identical conditions of scale expansion and integration times used for obtaining adequate spectra of the standards showed only a small amount of sample fluorescence and no discernible peaks. However, by using wider slits, zero suppression, scale expansion, extended integration, and slow scan rates, it was possible to increase the detectability and observe structure in the region from 350 to 550 cm-l. Figure 3 shows the LRS spectrum of a typical NIOSH volcanic ash sample obtained under these high sensitivity conditions. The small peak in Figure 3 at, 416 cm-' corresponds to the Raman peak of cristobalite, the larger peak at 464 cm-' corresponds to the Raman peak of quartz, and the peak at 512 cm-' is believed to be due to the ash matrix which is predominantly
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ANALYTICAL CHEMISTRY, VOL. 53, NO. 13, NOVEMBER 1981 416
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Figure 3. LRS spectrum of NIOSH volcanic ash sample. Laser power, 150 mW at sample; time constant, 20 s; scan rate, 0.1 cm-'/s; slits, 8 cm-'; full scale, 500 counts; zero suppress 11.3 X 100.
plagioclase feldspar (23). This 512-cm-l peak was present in all of the NIOSH ash samples; therefore, addition of KN03 as an internal standard was not necessary since this 512-cm-' peak could be used as the standard reference peak for the subsequent intensity ratios which are then independent of fluctuations in laser power. This particular analytical procedure corresponds to the direct comparison method since the reference peak is from another component in the sample (24). As stated earlier, a reliable tridymite standard was not available nor was there very good correspondence between other tridymite spectra reported in the literature (13). One common feature, however, of these various tridymite spectra is the presence of numerous relatively intense peaks in the low wavenumber region from 100 to 380 cm-l. Whereas the LRS determination of tridymite in the ash samples could not be specifically performed, none of the spectra for the ash samples were found to contain any peaks in this low wavenumber region other than those attributable to quartz and cristobalite. Recording LRS spectra under the experimental conditions of maximum sensitivity as discussed in the preceding paragraph is difficult and obviously requires an extremely stable Raman system. For example, our laser was originally cooled directly from the building water supply; however, changes in water pressure and flow rate caused by other water use in the building produced small fluctuations in the laser power which were greatly amplified under the high sensitivity condition. This source of noise was eliminated by installing a closed loop, pump-driven cooling system for the laser. Since random noise is proportional to the square root of the intensity of the signal at the detector, wider slits mean greater light throughput and, therefore, greater noise which in turn necessitates longer integration times. These conditions, unfortunately, produce a sloping base line in the low wavenumber region due to stray laser light. Longer integration, of course, requires slower scan rates and a good rule of thumb is that the resolution should be scanned in no less than three or four time constants. Therefore, the scan rate should be about 0.1 cm-l/s for a resolution of 8 cm-l and an integration time of 20 s. Under these conditions, the spectrum shown in Figure 3 was acquired
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Figure 4. LRS spectrum of the same volcanic ash sample as in Figure 3 but spiked with 10% cristobalite. Experimental conditions are the same as in Figure 3 except zero suppress, 10.8 X 100.
in approximately 35 min. Once again this emphasizes the need for an extremely stable LRS system. The limiting source of noise in our system was discovered to be variations in the spinning rate of the sample which caused small fluctuations in the stray light level. Thus, this particular LRS instrument was capable of producing very repeatable data; e.g., 50 scans on one of the NIOSH ash samples produced an intensity ratio, Iw/1512,based on peak height of 0.62 with a h0.04 standard deviation. In order to quantify the amounts of quartz and cristobalite in volcanic ash, we employed the method of standard addition. Figure 4 shows the LRS spectrum of a volcanic ash sample spiked with 10% cristobalite, and as illustrated, the peak at 416 cm-l is enhanced. Several similar LRS scans were obtained on each sample and the corresponding intensity ratios, 1464/1512 and 141a/1512, were calculated for both spiked and unspiked samples. The concentrations of quartz and cristobalite in the ash samples were then calculated taking into account the dilutions due to the addition of the spike. Quartz and cristobalite concentrations calculated relative to different spike concentrations of the same ash sample were found to differ by only a few tenths of a percent. The described LRS methodology had a detectability of 1wt % for cristobalite and 0.5 wt % for quartz in the volcanic ash samples. The total quartz and cristobalite content of the four NIOSH ash samples was found by the LRS analyses to range from 4% to 11%. These LRS data for quartz and cristobalite are in agreement with the overall results reported for other spectroscopic techniques in the recent NIOSH round-robin testing program (8,16). (The interested reader is referred to Table I in this issue's Analytical Approach article for more specific quantitative LRS results regarding the proportions of quartz and cristobalite in these ash samples.) Consequently, the LFL9 technique proved to be particularly valuable as an independent corroborative method in the determination of crystalIine silica in volcanic ash since no pretreatment of the samples was required (8). Hopefully, this application of Raman spectrometry to a particularly intriguing problem will cause analytical chemists to consider LRS as a valuable quantitative tool for future extension to other systems. ACKNOWLEDGMENT The contributions of the round-robin volcanic ash samples from D. D. Dollberg at the NIOSH laboratory in Cincinnati, OH, and the quartz samples from Pennsylvania Glass Sand
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Corp. are gratefully acknowledged. LITERATURE CITED Gardiner, D. J. Anal. Chem. 1980,52,96R. Irish, D. E.; Chen, H. Appl. Specfrosc. 1971,25, 1. Cody, C:. A.; Darlington, R. K. The Spex Speaker 1980,25, 1. Skoog, D. A,; West, D. M. “Principles of Instrumental Analysis”, 2nd 6d.;Saunder College: Philadelphia, PA, 1980;pp 270-272. Capwei!, R. J.; Spagnoio, F.; DeSesa, M. A. Appl. Spectrosc. 1972,
26, 537. Irish, D. E.; Riddell, J. D. Appl. Specfrosc. 1974,28,481. Wancheck, P. I.;Welfram, L. E. Appl. Spectrosc. 1976, 30, 542. Farwell, S.0.; Gage, D. F1. Anal. Chem. 1981,53,1529 A. Scott, J. F.; Porto, S. P. 8. Phys. Rev. 1967, 161,903. Bates. ,I. E.: Quist, A. S. J. Chem. Phvs. 1971,56, 1528. (llj Bates, J. B. J. Chem. Phys. 1872,S i , 4042. (12) Etchepare. J.; Merian, M.; Smetankine, L. J . Chem. Phys. 1974,60,
1873. (13) Etchepare, J.; Merian, M.; Kaplan, P. J. Chem. Phys. 1978, 68, 1531. (14) Anal. Chem. 1980,52, 1138 A. (15) Anal. Chem. 1980,52, 1272 A. (16) Dollberg, D. D.; Bolyard, M.; Sweet, D. V.; Carter, J. W.; Stettler, L. E.; Geracl, C. L. “Mount St. Helens Volcanic Ash: Crystalllne Sllica Analysis”; National Institute for Occupational Safety and Health: Cincinnati, OH, 1980.
(17) Sosman, R. E. “The Phases of Silica”; Rutgers University Press: New Brunswick, NJ, 1965. (18) Mason, B. ”Principles of Geochemistry”, 3rd ed.; Wiiey: New Yo&, 1966.
(19) &i;M.
J.; Hendra, F’. J.; Watsan, D.
frosc. 1971,25, 42:).
S.;Peacock, L. J. Appl. Spec-
(20) Cunningham, A O 7n K. M.; Coldberg, M. L.; Weiner. E. R. Anal. Chem. 1971, T I ,
,
1.
(21) Saije. E. J. Appl. Crystallogr. 1973, 6 , 422. (22) Chen. P. A.; Tlng, C. H. Phys. Lett. 1972,4 , 339. (23) Makovsky, L. E. Appl. Specfrosc. 1973,27, 43. (24) Cullity, B. D. “Elements of X-ray Diffraction”; Addison-Wesley: Reading, MA, 1967;pp 388-396.
RECEIVED for review May 26,1981. Accepted August 24,1981. Partial support for this work was provided by special funding from the Region X Office of the Environmental Protection Agency. We are grateful for the prompt actions on funding by Dick Thiel, Bill Schmidt, and Jon Schweiss of EPA Region X. Funds for the LRS instrumentation were provided by NSF Grant No. CHE-7727395.
Determination of Macrocyclic Compounds in Solution by Thermometric: Titration against Metal Cations; John D. Lamb,” James E. King, James J. Christensen, and Reed M. Izatt Departments of Chemistry and Chemical Engineering and the Thermochemical Institute, Brigham Young University, Provo, Utah 84602
The thermornetrlc Utration technlque was employed to determine concentratlons of macrocyclic ligands In solution by titration with catlon solutions. A series of aqueous 18-crown-6 solutlons (2 mL each) of concentratlons ranging from 0.1 M to 1 mM was titrated with standard Ba(CIO,), solution using an lsoperlbol calorlmeter equipped with a 4-mL reactlon vessel. The technique accurately determined the ligand concentratlon at or above 1 mM. Thermometrlc titratlons In methanol were also performed by using a 25-mL reactlon vessel to determlne several macrocycles using K+ and Ag+ as titrates. Llgands could be determlned In this manner If for the reactlon In the solvent used log Kexceeded 3.5 and AH was sufflclently different from 0. The technique was also used to determine the partition coefflcients of macrocycles between water and chloroform. The same thermometric titration method may be used to determlne metal Ion concentratlons If the macrocycle concentration Is known.
Macrocyclic compounds such as the crown ethers and cryptands have received considerable attention in recent years because of their ability to selectively bind cations and to solubilize cation salts in nonpolar media (1-5). These features of macrocycle chemistry have prompted their use in phase transfer catalysis, membrane transport of cations, solvent extraction of‘ cations, and other solution applications. In general, the compounds have been studied using the purest form available without attempts to standardize solutions for macrocycle concentration. One of the most promising methods for accurately determining solution concentrations of these compounds is the subject of this paper, namely, thermometric titration to an end point against a metal cation. This procedure provides a convenient means not only to establish solution concentrations of macrocyclic compounds when 0003-2700/81/0353-2127$01.25/0
studying their interactions with cations but also to determine partition coefficients of these compounds when used in phase transfer catalysis, solvent extraction, and membranes. A particular advantage of the method is that titrations can be carried out readily in imost solvents, the only requirements being that the reactants be soluble in the solvent and that heat effects due to solvent vaporization be limited. A useful corollary to the method in the determination of Rb+, Cs’, and other metal ions in solution by titration against standard macrocycle solutions. The thermometric titration method has been used to measure equilibrium constants and enthalpy changes for the reaction of a large number of macrocyclic ligands with various cations. The equilibrium constant values for these reactions are very dependent on the solvent in which they are measured. In general, as the donicity of the solvent decreases, the equilibrium constants increase (4). By use of the thermometric titration technique, if the equilibrium constant has a value of approximately 3.5 or higher, a reaction thermogram may be used to determine an end point from which the ligand concentration may be accurately determined if the cation concentration is known. In this paper, we shall illustrate this procedure and point out some of its potential applications.
EXPERIMENTAL SECTION Thermometric titrations were performed with a Tronac Model 450 isoperibol calorimeter equipped with a 4-mL Dewar-type reaction vessel and a 2.5-mL buret (Hamilton syringe type) driven by either a 4.0 or 1.0 rpm Hurst synchronous motor. Prior to the titration, a calibrated Hamilton gastight syringe was used to deliver 2.066 (h0.0013)mL of the crown ether solution into the reaction vessel. The calorimetric data (temperature in mV from a thermistor in a Wheatstone bridge) were recorded on a HewlettPackard Model 7100B strip chart recorder. All measurements were made in a room specially thermostated to 21.1 (-+0.5)O C . During the titration, titrant continued to be delivered beyond the end point so that approximately 0.5-0.8 mT., was delivered in total. 0 1981 American Chemical Society
