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ANALYTICAL CHEMISTRY, VOL. 51, NO. 9, AUGUST 1979
less stability in the base line due to humidity changes might become a problem in long-term continuous measurement. Life Time. T h e Carbowax 1000 coating showed a reasonably long life time. T h e coated crystal gave almost the same sensitivity for the M N T vapor even after a month of use. It is concluded that this piezoelectric quartz crystal coated with Carbowax 1000 is useful for the detection of M N T with good selectivity and fast response, which are required for a T N T monitor, for example, in airports.
(11) (12) (13) (14) (15) (16) (17) (18) (19) (20) (21) (22) (23) (24) (25) (26)
LITERATURE CITED R. W. Dalton, J. A. Kohlbeck, and W. T. Bolleter, J . Chromatogr., 50, 219 (1970). D. G. Gehring and J. E. Shirk, Anal. Chem., 39, 1315 (1967). J. Yinon. H. G. Boettger, and W. T. Weber, Anal. Chem., 44, 2235 (1972). D. G. Gehring, Anal. Chem., 42, 898 (1970). G. E. Spangler and P. A. Lawless, Anal. Chem., 50, 884 (1978). F. W. Karasek, Anal. Chem., 46, 710A (1974). C. D. Chandler, J. A. Kohlbeck, and W. T. Boileter, J . Chromatogr., 64, 123 (1972). J. C. Hoffsommer and D. J. Glover, J . Chromatogr., 62, 417 (1971). J. Hlavay and G. G. Guilbault. Anal. Chem., 49, 1890 (1977). E. P. Scheide and R. B. J. Warnar, Am. Ind. Hyg. Assoc. J . , 39, 745 (1978).
(27) (28) (29)
W. H. King, Jr., Environ. Sci. Technol., 4, 1136 (1970). M. Janghorbani and H. Freund, Anal. Chem., 45, 325 (1973). M. W. Frechette and J. L. Fasching. Environ. Sci. Technol.,7, 1135 (1973). J. L. Cheney and J. B. Homoiya, Anal. Left., 8, 175 (1975). K. H. Karmarkar and G. G. Guilbault, Anal. Chim. Acta, 71, 419 (1974). E. P. Scheide and G. G. Guilbault, Anal. Chem., 44, 1764 (1972). K . H. Karmarkar, L. M. Webber, and G. G. Guilbault, Environ. Lett., 8, 345 (1975). K. H. Karmarkar and G. G. Guilbault, Anal. Chlm. Acta, 75, 111 (1975). J. Hlavay and G. G. Guilbault, Anal. Chem., 50, 1044 (1978). J. Hlavay and G. G. Guilbault, Anal. Chem., 50, 965 (1978). L. M. Webber, K. H. Karmarkar, and G. G. Guilbault, Anal. Chlm. Acta, 97, 29 (1978). G. Z. Sauerbrey, Z.Phys., 178, 457 (1964). W. H. King, Jr., Anal. Chem., 36, 1735 (1964). L. M. Webber and G. G. Guilbault, Anal. Chem., 48, 2244 (1976). C. P. Conduit, J . Chem. SOC.,3273 (1959). N. B. Jurinski, G. E. Podolak, and T. L. Hess, Am. Ind. Hyg. Assoc. J., 36,497 (1975). K. H. Karmarkar and G. G. Guilbault, Environ. Left., 10, 237 (1975). F. W. Karasek and J. M. Tiernay, J . Chromatogr.. 89, 31 (1974). J. Cheney. T. Norwood, and J. Homolya, Anal. Lett., 9, 361 (1976).
RECEIVED for review February 12,1979. Accepted May 7,1979. The authors gratefully acknowledge the financial support of the Army Research Office, in the form of Grant No. DAAG-77-G-0266, in carrying out this research project.
Assay for Arsenic Trioxide in Air Carroll A. Snyder* and Daniel A. k o l a New York University Medical Center, Department of Environmental Medicine, A . J. Lanza Laboratories, Long Meadow Road, Tuxedo, New York 10987
A quick, safe, and inexpensive method for the assay of arsenic trioxlde In air has been developed. The procedure utilizes an ultraviolet absorbing moiety that develops when As,O, is dissolved in alkaline solutions. Arsenic trioxide dust is removed from air by filtration and dissolved in 1 N sodium hydroxide solution. The resultant solution is read in an ultraviolet spectrophotometer at 222 nm against a sodium hydroxide blank. The calibration curve gives a linear response from 11 pg to 44 pg As,O, per mL NaOH. Absorptivity data and some concepts on the nature of the bonding responsible for the observed spectral data are also presented.
arsenic and oxygen atoms. Alkaline solutions were used to accelerate the dissolution of As203 and to minimize interference from As" compounds which will not absorb a t the chosen wavelengths in basic solutions above certain strengths. Comparisons of the absorptivities of As"' and AsVcompounds in solutions of varying base strengths were used to determine the optimal base concentration for the procedure and to shed some light on the nature of the ultraviolet absorbing moiety. For our purposes, a sample for analysis could be collected in 3 min. Sample preparation required only a n additional 2 min and, therefore, airborne As203levels could be determined in 5 m i n intervals.
EXPERIMENTAL T h e most ubiquitous of the arsenic compounds found in industrial environments seems to be arsenic trioxide and there are firm data linking exposure to this compound with excessive lung and skin cancer ( I ) . In spite of the evidence for arsenic-induced cancer in humans, attempts to reproduce cancer in animals by arsenic exposure have been largely unsuccessful ( I ) . In this laboratory efforts have been focused on the experimental induction of lung cancer by treatment of animals with controlled atmospheres of As203dust. These exposures require frequent monitorings of the test atmospheres in order to make real-time corrections in the generating systems so that constant exposure levels can be maintained. The current procedures for the assay of arsenic in air, while accurate and precise (2-5), are too time consuming and, therefore, make frequent, routine monitoring difficult. As part of the study investigating the effects of exposure to As203dust, a new procedure for determining concentration of airborne As203was developed. The procedure utilizes the ultraviolet absorbance of the da-pa bonding between adjacent 0003-2700/79/0351-1478$01 .OO/O
Instrumentation and Equipment. Ultraviolet spectra scans and absorptivities were determined on a UV-visible double-beam scanning spectrophotometer equipped with a strip chart recorder (Perkin-Elmer Model 525, Oak Brook, Ill. 60521). Calibration curves and routine atmosphere monitorings were performed on a UV-visible, single-beam spectrophotometer (Hitachi Model 100-40, Tokyo, Japan). Air samples were taken with a sample train consisting of: (1) a modified end-of-line Swinney filter holder containing a 0.5-inch (1.27 cm) Whatman 41 filter paper, (2) a standard diaphragm pump with a 10 L/min capacity, and (3) a wet test meter (GCA/Precision Scientific, Chicago, Ill.) to determine air sample volumes. Arsenic trioxide dust atmospheres were generated in a 128-L stainless steel exposure chamber (6) using a Wright dust feed (L. Adams, Ltd., London) with an in-line cyclone to remove large particles, thereby maintaining a flow of respirable-sized particles (mass median diameter 2.83.2 pm). The atmosphere generating system and exposure chamber were completely enclosed in a glove box hood. Procedures. The optimal absorptivities of the arsenic compounds As203,?;aAs02, and Na2HAs0,.iH20 were determined as a function of base concentration by adding appropriate amounts c 1979 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 51, NO. 9, AUGUST 1979
10
1
Table I. hmax of Na,HAs04~7H,0,NaAsO,, and As,O, Solutions as a Function of Base Strength
t 70 1
h,,,
8-
X 6i
2t
c
0'
lo-)
1479
~
Na,HAsO, 7H,O IO-'
io-'
lo+
io-'
,
ioa
IO'
NazHAs0,.7Hz0
Na,HAs04
2.00 1.00 2.50 X 10.' 6.00 x 1.60 x l o - ' 4.00 x L O O x 10-3 5.00 x io-,
0 0 0 0
0.00
Normality of NoOH
Figure 1. Absorptivities of Asz03, NaAsO,, and a function of base strength
normality of NaOH
as
of each compound into eight sodium hydroxide solutions of the following normalities: 2.00, 1.00, 2.50 X lo-', 6.00 X lo-', 1.60 X 4.00 X 1.00 X 5.00 X In this way the ratio of hydroxide ion concentration to arsenic concentration ranged from 1.33 X IO4to 2.98. All solutions were stirred for 72 h at room temperature in order to complete dissolution. The calibration curve of various concentrations of As203in 1 N NaOH was prepared from two stock solutions containing 0.1217 and 0.1135 g, respectively, of As203 per 100 mL of NaOH. Appropriate dilutions of both stock solutions were made so that final As203concentrations ranged from 55.0 pg/mL to 4.8 fig/mL. Because some of the atmospheres also contained 10 ppm sulfur dioxide, amounts of Na2S03were added t o each point in the calibration curve which reflected the SO2 concentration in the atmosphere. Sampling, Air samples were collected at 3 L/min for 3 min. The filter paper was then placed into 15 mL 1N NaOH in a closed vessel and sonicated for 2 min using a standard 80-kHz ultrasonic cleaner. Aliquots of the resultant solutions were then read on the single-beam spectrophotometer against appropriate blanks at 222 nm.
RESULTS The maximum absorbance wavelengths for Asz03 in NaOH were found to decrease with decreasing base strength. Absorptivities of all solutions were essentially the same except for depressions observed a t 2.50 X lo-' N, 6.00 x lo-' N, and 1.60 X lo-' N NaOH (Figure 1). Analogous results were obtained when solutions of NaAsOz in NaOH were scanned (Figure 1). The absorptivities of the NaAsO' solutions were about half the values obtained from the As203solutions a t a given base strength. Scans of ten replicate samples of each compound in 1 N NaOH gave identical spectral curves with absorptivities averaging 7.26 f a standard deviation (u) of 0.30 for As203and 3.63 f u = 0.17 for NaAsO'. This ratio is exactly 2.00/1.00 and reflects the ratio of As02- furnished in basic solution by each molecule. The absorptivities of Na2HAsO4.7H20as a function of base strength are presented in Figure 1. There were no maxima between 185 nm and 240 nm until base strength declined to 1.60 X lo-* N (Table I). As with the As"' solutions, the wavelength of maximum absorbance decreased with decreasing base strength. With these results, the 1 N NaOH solution was chosen for the analytical technique. This base strength is sufficient for quick dissolution of As203 a t room temperature, gives a high absorptivity, apparently does not generate an interfering absorbance from As", and is relatively safe for quick handling. The calibration curve of As203in 1N NaOH was linear over the range 44 fig/mL to 11 Fg/mL (12 points) with a linear correlation coefficient equal to 0.9995. The equation for the regression line was found to be Absorbance = 26.7 (fig As203/mL 1 N NaOH) - 0.4. At the sampling rate chosen (3 L/min), the collection efficiency of the 0.5-inch (1.27 cm)
208 203 196 191 189
nm As203 9 NaAsO , 223 222 218 214 211 207 203 202
-
Table 11. Comparison of U V Spectrophotometric Method and KMnO, Titrimetric Procedurea sample 1
2 3 4 5
Concentration, mg/m3 uv KMnO, 29.5 30.5 40.5 33.6 42.0 39.7
6
7 8 9 10 11
12 13 14 15 16 mean stand. dev.
34.5 33.2 34.0 31.5 38.9 33.6 28.7 34.3 26.4 27.4 34.3 5.8
33.0 3.5
a Samples of test atmosphere taken alternately for each mocedure.
Whatman filter was found to be essentially 100%. T h e accuracy of the spectrophotometric method was determined from a test aerosol generated at a nominal level of 30 mg/m3 and combined with 10 ppm sulfur dioxide. Air samples were taken over a 2-h period and alternately analyzed by the spectrophotometric technique and by a K M n 0 4 titration procedure (7). Eight air samples were assayed by each procedure. The titration procedure required 20 L of air to be filtered as opposed to 9 L for the spectrophotometric method. The results of these determinations are given in Table 11.
DISCUSS I ON The ultraviolet absorbance of Ai-0 compounds results from the partial d s - p s backbonding between adjacent arsenic and oxygen atoms. Although the literature is lacking in information concerning the ultraviolet absorbing characteristics of As-0 compounds, many of our findings were essentially predicted in a paper by Van Der Veken et al. (8)on the basis of calculated bond orders. The authors stated that increased formal negative charge on the As-0 moiety would render backbonding more difficult because of a concentration of negative charge on the periphery of the moiety. Thus rbond character would increase in the series: AsOd3-< HOAS03'< (HO),As021- < (HO),AsO. Our data show t h a t the absorptivity of Na2HAs04-7H20increases with decreasing base strength. Decreasing base strength results in a decline in the net charge on the arsenate ion and leads to greater d r - p r overlap.
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ANALYTICAL CHEMISTRY, VOL. 51, NO. 9, AUGUST 1979
The, , A of As203and NaAsOz solutions is a linear function of the log of the base concentration. The equation for the regression line is: , ,A
= 5.92 log [ N a O H ]
+ 222.52
with a linear correlation coefficient of 0.9993. Whether this relationship is based on pH or ion effects or both is not known. One possible explanation might be that dlr-pa overlap becomes more localized in solutions of weak base strength and therefore requires more energetic radiation to effect electronic transitions. In strongly basic solutions the predominate moiety may be: - - --
- ---
0-As-0
whereas in weakly basic solutions, the predominate moiety might be: HO-As=O It should be possible to modify this procedure to determine As203levels in some industrial atmospheres. The most recent OSHA recommended standard for As203is 10 pg/m3 (measured as As) as determined by one sample in an 8-h period (9). If ambient As203 concentration is close to 5 pg/m3 As, only 2 m3 of air would be required to collect sufficient sample to be on the linear section of the calibration curve. In addition, because only an inexpensive single-beam instrument is required, monitorings could be performed quickly and cheaply a t a number of different sampling stations by using several instruments. Since our test atmospheres contained no interfering impurities, a reading a t a single wavelength was sufficient. In industrial atmospheres, however, there may be light absorbing moieties which would interfere with this procedure. In this case, spectral scans could be taken. Because As203has little or no absorbance above 240 nm, the amount of interference from the contaminating compounds could be corrected for, provided the interfering compounds had at least one additional absorbance peak above 240 nm ( 1 1 ) . Finally, it has been pointed out by Lao et al. (IO) that attempts to filter As203from ambient levels near 0.6 pg/m3
would be fruitless because the vapor pressure of As203a t 25 "C is approximately this value. The method described herein was originally designed for synthetic atmospheres with concentrations many times greater than 0.6 pg/m3. In addition, the current exposure standard is more than 10-fold greater than the vapor pressure; therefore, a filtration procedure would seem to be an adequate method of sampling.
ACKNOWLEDGMENT The authors acknowledge the following persons who contributed to the work described herein: Edward Palmes for critical review, Reade A. Moulton and Kenneth Magar for assistance in chamber technology, G. Roger Sparling for graphic arts, and Dorothy J. Natalizio for manuscript preparation.
LITERATURE CITED "Medical and Biologic Effects of Environmental Pollutants. Arsenic". National Academy of Sciences, Washington, D.C., 1977. "Arsenic in Urine and Air". NIOSH Manual of Analytical Methods. U.S. Dept. HEW, NIOSH Publication No. 75-121, D Criteria Development, 1974, pp 139 (1-8). Tahm, Y.; Feldman. C. "The Determination of Traces of Arsenic: A Review", in "Arsenical Pesticides", Woolson, E. A,. Ed.; American Chemical Society: Washington, D.C., 1975. Fiorlno, J. A.; Jones J. W.; Capar, S. G. Anal. Chern. 1976, 4 8 , 120. Davis, P. H.; Dultide, G. R.; Griffin, R. M.; Matson W. R.; Zink. E. W. Anal. Chern., 1978,50, 137-43. Drew, R. T.; Laskin, S."Environmental Inhabtion Chambers", in "Methods of Animal Experimentation", Vol. IV; Academic Press: New York, 1973. "Determination of arsenic by titrimetric methods", in "Standard Methods of Chemical Analysis", 6th ed.; Furman, N. H., Ed.; D. Van Nostrand Company: Princeton, N.J., 1962: Vol. 1, pp 114-118. Van Der Veken, 8. L.; Vansant. F. K.; Herman, M. A. J . Mol. Struct. 1977,36. 225-32. Volume 1: "General Industry Standards and Interpretations" Change 4: June 22, 1978. Occupational Safety and Health Subscription Service. Lao, R. C.; Thomas, R. S . ; Teichman, T.; DuBois, L. Sci. TotalEnviron. 1974, 2,373-9. Snyder, C. A., "Analytical Techniques", p 12, in "Benzene Toxicity, A Critical Evaluabn," Laskin, Sidney: Goldstein, Bernard, Ed.; J. Tox. Envion. Health, Suppl. 2 , 1977.
RECEIVED for review February 12,1979. Accepted May 7,1979. This research is supported by Contract number NO1 CP 33260 from the National Cancer Institute and is part of a Center program supported by Grant number E S 00260 from the National Institutes of Environmental Health Sciences and Grant number CA 13343 from the National Cancer Institute.
Coulometric Determination of Aromatic Nitro Compounds with EIect rogenerat ed Chromium(I I) I. M. AI-Daher and B. G. Kratochvil" Department of Chemistry, University of Alberta, Edmonton, Alberta, Canada T6G 2G2
The scope of coulometrically generated chromium( 11) as a reagent for the determination of aromatic nitro compounds was investigated. Samples of the order the external 0.2 to 0.5 pnol could be determined with about 1% precision and accuracy for a number of substances; others reacted too slowly to permit direct determination. Reduction was in most instances to the corresponding hydroxylamine or amine. Several water-insoluble nitro compounds were determined after dissolution in acetonitrile or in mixtures of water and acetonitrile or ethanol.
T h e usefulness of aromatic nitro compounds in a variety of commercial products and as intermediates in chemical 0003-2700/79/0351-1480$01 .OO/O
synthesis makes the determination of this functional group of broad interest (I, 2). Analytical methods for the nitro group have centered on reduction to the corresponding amine or to one of several intermediate nitrogen oxidation states, generally either polarographically (3)or by addition of excess standard reductant, usually Ti3+or Cr2+,and back titration of the excess with a standard oxidant ( 4 , 5 ) . Some direct titrations have been investigated in a limited way (6). Polarographic methods tend to lack precision and require careful calibration curves, while indirect titrimetric methods necessitate storage and handling of oxygen-sensitive solutions. Electrochemical generation of strong analytical reductants by constant-current coulometry eliminates calibration curves, C 1979 American Chemical Society
