Environ. Sci. Technol. 1986, 20,517-520
Table IV. Feasibility of Determination of Nitrite
samplea Yamato River Kizu River Uji River Katsura River Yodo River
IN,* x 105 M without with SAc SA difference 26.1 13.2 3.5 19.0 11.7
25.2 12.8 3.2 16.7 11.0
NO;, x lo6 M colorimetricd
0.3 2.3 0.7
Samples were taken on Nov 12, 1985. bIN inorganic nitrogen. SA: sulfanilic acid. dSulfamide/N-(1-naphthy1)ethylenediamine method.
were derived. The difference between the two results for each sample was in fairly good agreement with the value obtained by the standard colorimetric method for nitrite (3). This fact indicates that nitrite is stoichiometrically reduced to ammonia by titanium(III),and therefore, the present method can also be applied to the determination of nitrite. Registry No. NO3-, 14797-55-8;NH3, 7664-41-7; H 2 0 , 7732-
L i t e r a t u r e Cited (1) Stanley, W. D.; Hobbie, E. J. Limnol. Oceanogr. 1981,26, 30-42. (2) Knowles, R.; Lean, D. S.; Chan, Y. K. Limnol. Oceanogr. 1981,26,855-866. (3) “Standard Methods for the Examination of Water and Wastewater”, 16th ed.; American Public Health Association: Washington, DC, 1985; pp 391-406. (4) Connors, J. J.; Beland, J. J.-Am. Water Works Assoc. 1976, 68, 55-56. (5) Kamphake, J. L.; Hannah, A. S.; Cohen, M. J. Water Res. 1967,1, 205-216. (6) Anderson, L. Anal. Chim. Acta 1979, 110, 123-128. (7) Tanaka, K. Bunseki Kagaku 1982, 31, T107-Tl12. (8) Cresser, S. M. Analyst (London) 1977, 102, 99-103. (9) Aoki, T.;Uemura, S.; Munemori, M. Anal. Chem. 1983,55, 1620-1622. (10) Lindroth, P.; Mopper, K. Anal. Chem. 1979,51,1667-1674. (11) Small, H.; Miller, T. E. Anal. Chem. 1982, 54, 462-469. (12) “Standard Methods for the Examination of Water and Wastewater”, 16th ed.; American Public Health Association: Washington, DC, 1985; pp 379-382. Received for review J u n e 10, 1985. Accepted January 3,1986.
Measurement of Total Reduced Sulfur Compounds in Ambient Air Neil R. McQuaker,” Glenn E. Rajala, and Davld Pengllly Environmental Laboratory, Ministry of Environment, Vancouver, British Columbia V6S 2L2, Canada
Methods for the determination of total reduced sulfur (TRS) compounds in the ambient air based on coulometric detection (Philips Model PW 9700 analyzer) and thermal oxidation followed by detection using pulsed fluorescence (Teco Model 43 analyzer) have been evaluated. Analytical response factors, relative to HzS,were determined for both the individual TRS compounds and compounds such as terpenes and carbonyl sulfide that may be a potential source of interference. The results for COS and terpenes indicate that in a typical monitoring situation normally encountered concentrations of these compounds are not expected to cause significant measurement bias. The results for the individual TRS compounds indicate that while variations in TRS composition are not a factor in assessing measurement bias for the thermal oxidation/pulsed fluorescence method, they are a factor for the Philips coulometric method; i.e., increasing positive measurement bias may be introduced as the TRS composition shifts toward relatively less H2S. Philips-Teco comparison data collected at a single site in the vicinity of three operating kraft pulp mills are compatible with these expectations. Introduction
Total reduced sulfur or TRS may be defined as [TRS] + [CH3SH] + [(CH3),S] + B[(CH,),S,], and reference methods for its measurement in the ambient air have not been established by Federal Regulatory Agencies in either Canada or the United States. However, in many regional jurisdictions where there are kraft pulp mills there is a requirement, often compliance related, to measure ambient levels of TRS. Often, the method of choice has been coulometric detection using the Philips Model P W 9700 analyzer (I). In this method a scrubber is used to remove SO2 from the original sample; the sample then passes into a titration cell where the response to TRS is referenced to H,S. Details of the titration procedure ap= [HzS]
pear elsewhere (I),but the essential element involves titration of the sample with bromine which is electrolytically generated. An alternate method, based upon thermal oxidation of TRS to SO2, has recently become available. A scrubber is used to remove SO2 from the original sample; the sample then passes into a thermal oxidizer where TRS is oxidized to SOz and then detected by using pulsed fluorescence. One of the available instruments satisfying the analytical requirements of this method is Teco Model 43 (2). Current regulatory limits for TRS in the province of British Columbia, Canada, are specified as follows: existing mills, 20 ppb hourly average; new or upgraded mills, 5 ppb hourly average (3). The regulatory limits of 20 and 5 ppb compare with the detection limit of 2 ppb provided by each of the instrumental methods identified above. The Philips Model PW 9700 analyzer is no longer manufactured, and because of its unavailability increased monitoring activities will, more and more, have to depend upon the alternate thermal oxidation/pulsed fluorescence method. This immediately raises the question of compatability between the two methods and is related to the larger question of what measurement biases, if any, are inherent to each method. The answers to these questions have implications in terms of interfacing data provided by the two methods as well as ‘the suitability of the individual methods as possible reference procedures. Data that would assist in answering these questions are evidently not available, and in the present work we have attempted to remedy this by (i) collecting Teco-Philips comparison data, at a single site, in the vicinity of three adjacent kraft pulp mills and (ii) assessing known potentials for measurement bias by determining analytical response factors relative to H2S for both the individual TRS compounds and compounds that have a potential for interference. This latter group included (i) COS which under overload conditions
0 1986 American Chemical Society
Environ. Sci. Technol., Vol. 20, No. 5, 1986
Table I. Summary of Experimental Variables
permeation rate, ng/min max concn, ppb (Teco 43) max concn, ppb (Philips PW 9700)
342 N/A' N/A
1368 100 100
comoound (CH3)2S (CH3),Sp 81 40 100
91 30 80
248 100 100
125 30 75
338 80 100
"N/A, not applicable. Table 11. Summary of Analytical Response Factors analyzer
Philips PW9700° Barton 4OOb Teco 43 (1600 O F ) Teco 43 (1500 O F )
0.52 0.25 1.00
response factor (CH3)zSz cos 2.47 0.33 2.00 2.00
N/A 0.25 0.07
0.03 N/A 0
0.07 N/ A
"The observed values are the mean of at least six determinations (see text). Deviations from the mean values did not exceed f0.03 for the TRS compounds and f0.02 for the balance of the compounds. *These values were obtained elsewhere (6).
may be present in emissions from kraft mill recovery boiler stacks and (ii) terpenes which are present in the relief gases from the digestor and may be emitted if there is incomplete removal during the condensation cycle. The compounds chosen for an assessment of terpene interference were a-pinene and limonene. This is because they are representative of the terpenes appearing in the recovered turpentine; i.e., a-pinene is the predominant bicyclic terpene, and limonene is the predominant monocyclic terpene ( 4 ) .
Experimental Section Philips Model PW 9700-Teco Model 43 Comparison Study. For the Philips Model PW 9700-Teco Model 43 comparison study, the two analyzers were collocated on the rooftop of a building that was located within 10 km of three operating kraft pulp mills. The analyzers shared a common sample inlet which was also used as a common calibration point. The scrubbers used to remove SO2were those supplied by the instrument manufactures. Detailed specifications are proprietary in nature, but it is known that the active ingredient is a chemically treated molecular sieve. Scrubber efficiency was verified by using a concentration of 2000 ppb of SO2 which exceeds maximum ambient concentrations by at least a factor of 2. The Philips Model PW 9700 was operated at the prescribed sampling rate of 300 mL/min, and the Teco Model 43 was operated at a sampling rate of 800 mL/min. In the case of the Teco 43 the in-line STI thermal oxidizer was operated at a temperature of 1600 O F . Instrument full scale settings for both analyzers were adjusted to 200 ppb of H2S. The instruments were calibrated at 2-week intervals using standards provided by the dilution of H2S cylinder gas using zero air and mass flow control. These standards have an accuracy of at least *3%, and details of the calibration protocols providing this accuracy appear elsewhere (5). In addition, both analyzers were equipped with onboard permeation devices that provided calibration standards of approximately 140 ppb. These standards were used every 24 h, during the zero/span cycle, to monitor calibration drift. Data acquisition was achieved with a Soltec Model 1242 dual pen strip-chart recorded and was supplemented by a Sum-X Model SX-450 data logger which could be accessed from a centrally located microcomputer (DEC Model PRO 350) via modems and dial-up telephone lines. Data reduction procedures provided for correction for any calibration drift. Philips Model PW 9700-Teco Model 43 Response Factors. For the evaluation of the Philips Model PW 9700 518
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and Teco Model 43 analytical response factors, the operating parameters for the two analyzers were as described. Reference gases of known concentration were provided by using a permeation system; i.e., permeation devices, calibrated gravimetrically, were maintained at constant temperature under a flow of nitrogen; this flow was diluted by using zero air and mass flow control to provide reference gases of known concentration and an accuracy of at least f2%. Details of the permeation system appear elsewhere (5). The permeation devices themselves were obtained from Metronics Inc. and included gases with the calibrated permeation rates summarized in Table I. All permeation devices were maintained at 25 "C with the exception of the a-pinene and limonene permeation tubes which were maintained at 72 "C. The analyzers were calibrated with H2S calibration gases provided by the permeation system, and the analytical response of the balance of the compounds was then determined at selected concentrations of X , 2 / 3 X and l l 3 X . The maximum concentration, X , appears in Table I and was selected as either 100 ppb or the maximum concentration delivered by the permeation device, whichever was lower. Determinations at each concentration were carried out at least twice.
Results and Discussion Analytical Response Factors. Table I1 summarizes the results for analytical response factors provided by both the Teco and Philips analyzers as well as the Barton titrator. The Barton results were obtained elsewhere (6) and are included for the purposes of comparison (see below). In all cases, analyzer calibration is referenced to H2S,and the analytical response factor, for a particular compound, is the ratio of the observed ppb value and the calculated ppb value assuming 100% response, i.e. observed ppb response factor = calculated ppb Response factors for CH,SH, (CH3)#, and (CH3I2S2 which deviate from the reference values of 1, 1,and 2 may result in a measurement bias if these compounds are present at a significant concentration level relative to H2S. Nonzero response factors for compounds providing a direct interference (e.g., terpenes and COS) may result in a positive measurement bias if these compounds are present at significant concentration levels in the ambient air. Philips Coulometric Method. The Philips analyzer shows nonzero response factors for both a-pinene and
Table 111. Summary of Cell Conditions
analyzer Philips PW 9700 Barton 400
parameter sample flow rate through cell, cell cell acid mL/min temp: "C content
0.10 N (KBr)
0.36 N (H8Od 2.94 N (HBr)
factors. Note that the cell conditions cited in Table I11 may vary slightly from analyzer to analyzer. This variation may produce small deviations in the analytical response factors. However, for the Philips analyzer such deviations have not been found to exceed f 5 % Thermal Oxidation/Pulsed Fluorescence Method. Two sets of results for the Teco 43 appear in Table 11-one with the thermal oxidizer operated at the 1600 O F temperature, used in the comparison study, and one with the thermal oxidizer operated at 1500 OF. The thermal oxidizer itself is a 1.0 cm diameter quartz tube combustion chamber maintained at a preselected temperature, and when the Teco 43 is operated at a sampling rate of 800 mL/min, it has a sample retention time of 1.0 s. Note that for the two different operating temperatures specified it was determined that there was complete conversion of H2S to SOz by obtaining a response factor of unity for SOz. The Teco analyzer shows a nonzero response factor for COS (see Table 11). This is not surprising because we would expect some oxidation of COS to SOz. However, the reduction in the response factor from 0.25 to 0.07 as the temperature is lowered from 1600 to 1500 O F is dramatic. This shows the desirability of operating the thermal oxidizer at the lower temperature in those rare cases where COS may be a factor. Note that it was found that the temperature of the STI thermal oxidizer could be controlled to within only k25 O F , and for this reason it is necessary to operate the thermal oxidizer at a mean temperature at least 25 OF above the cutoff temperature necessary to achieve complete oxidation of the TRS compounds. If, however, the current supplied to the heating element were to be routed through a feedback circuit that was referenced to a thermocouple response, it should be possible to control the operating temperature to at least f 2 O F . This would allow operation of the thermal oxidizer at a mean temperature much nearer the cutoff temperature for complete oxidation of TRS compounds, and hopefully, there would be a concomitant reduction in the COS response factor that would yield a value below the 0.07 result obtained at 1500 O F . If a significant interference is defined as one which exceeds the detection limit of 2 ppb, then the observed response factors of 0.25 and 0.07, obtained when the thermal oxidizer is operated at 1600 and 1500 O F , show that COS would have to respectively be present at levels of 8 and 29 ppb to produce a significant measurement bias. The observed TRS response factors for CH3SH, (CH3),S, and (CH&S2 (see Table 11) agree with the reference values of 1, 1, and 2. These results confirm the complete oxidation of TRS to SOz and mean that, for the thermal oxidation/pulsed fluorescence method, variations in TRS composition are not a significant factor in assessing measurement bias. Philips Model PW 9700-Teco Model 43 Comparison Study. During the 2-month comparison study (Dec 1984-Jan 1985) data provided by 185-paired hourly averages on the interval 5-50 ppb were collected. These data, where required, have been corrected for calibration drift for each 24-h sampling sequence (see Experimental Section). Neither the zero nor span drifts were found to exceed 2 ppb. Checks on the scrubber efficiencies at the beginning and the conclusion of the experiment verified that SO2 was being quantitatively removed from the original sample. The data were analyzed by using a linear regression with the ( x , y) data points defined as (Philips, Teco); see Figure 1. The resulting curve had a slope of 0.855, an intercept of -0.14 ppb, and a correlation coefficient of 0.977. Note that the deviation of the correlation
2.94 N (HBr)
Cell temperature for Barton 400 assumes ambient conditions of 22 "C.
limonene (see Table 11). This is not surprising since these compounds have a double bond(s) across which bromine substitution may occur; i.e., a-pinene has a single double bond and limonene has two double bonds. The higher response factor for limonene, as compared to a-pinene (i.e., 0.07 vs. 0.03), may reflect the greater opportunity for bromine substitution associated with limonene. Note that limonene, C10H16,is monocyclic, and five other naturally occurring terpenes have the limonene skeleton are known to exist; a-pinene, C10H16,is bicyclic and one other naturally occurring terpene, @-pinene,having the same skeleton is known to exist. Given that the predominant terpene, in the vicinity of a kraft pulp mill, is expected to be apinene ( 4 ) , it is probably safe to assume that terpenes, when considered in combination, will have an effective response factor not exceeding 0.05. This means that if we define a significant interference as one which exceeds the detection limit of 2 ppb, then the combined concentration of terpenes necessary to produce a significant measurement bias is 40 ppb. The observed TRS response factors of 1.35, 0.52, and 2.47 for CH3SH,(CH3&3, and (CH3)zS2(see Table 11)show marked deviations from the reference values of 1, 1, and 2 and suggest the potential for significant measurement bias. However, when H2S is the predominant TRS compound present, this potential will be minimized. For example, if H2S emitted from the recovery boiler stack dominates total TRS emissions, an example ambient TRS composition for H2S/CH3SH/(CH3)2S/(CH3)zSz might be 12:3:1:1 (7), and this composition when applied to the observed TRS response factors yields a measurement bias of +5.8% for the Philips analyzer. If, however (due to, for example, installations for strong black liquor oxidation), H2S is no longer the predominant TRS compound present, measurement bias may indeed be significant. This is the case for the monitoring site chosen for the Philips-Teco comparison study of this work, and on the basis of a 90% reduction in recovery boiler H2S,an example ambient TRS composition at the monitoring site might be 1:3:1:1. This composition when applied to the Philips TRS response factors yields a measurement bias of +14.9%. It is instructive to compare the TRS response factors provided by the two coulometric methods-the Philips analyzer used for ambient monitoring and the Barton titrator used for source monitoring. In all cases, the response factors for the individual TRS compounds are remarkably different (see Table 11). This apparent anomaly is undoubtedly related to those parameters affecting reaction kinetics, i.e., sample residence time in the cell, cell temperature, and reagent concentration in the cell. For each parameter there are marked differences as shown in Table 111, and it is perhaps not surprising that the combined effect of the interaction of the parameters affecting reaction kinetics is to produce significantly different response
Envlron. Sci. Technol., Vol. 20, No. 5, 1986
40 N v)
P H I L I P S PW 9700 RESULTS.
p p b H2S
Figure 1. Philips PW 9700 results vs. Teco 43 results.
coefficient from unity is related to both analyzer precision and fluctuations in ambient TRS composition about a mean composition. The deviation from unity for the slope indicates that the Philips analyzer is biased high by +14.5% when compared to the Teco analyzer. The intercept of less than 1ppb is not considered significant in assessing bias, and in the absence of other interferences, the factors contributing to the observed bias will be limited to the following: (i) positive bias of Philips analyzer relative to Teco analyzer provided by deviation of Philips TRS response factors from the reference values; (ii) positive bias of Philips analyzer relative to Teco analyzer if ambient terpene concentration exceeds 40 ppb; (iii) negative bias of Philips analyzer relative to Teco analyzer if ambient COS concentration exceeds 8 ppb. Note that the 8 ppb value is raised to 29 ppb when the thermal oxidizer temperature is reduced from 1600 to 1500 OF. Provided neither factor ii nor iii is significant, it is probably safe to assume that the observed bias of +14.5% is largely due to factor i. The assumption that factor iii is not significant, in contributing to the observed bias, is probably a good one since the recovery boiler stacks in the vicinity of the monitoring site are not known to operate under overload conditions. Similarly, the assumption that factor ii is not significant is also probably a good one since even if incomplete removal of terpenes during the condensation cycle is allowed for, the ambient concentration of terpenes would seldom, if ever, be expected to exceed the 40 ppb value at which interference would become significant. Summary. Ideally, a reference method would have (i) response factors of zero for all potential interfering compounds and (ii) TRS response factors corresponding to the reference values of 1, 1, and 2 for CH,SH, (CH3)2S,and
Envlron. Sci. Technol., Vol. 20, No. 5, 1986
(CH3),S2. Criterion i is not satisfied by either the Philips coulometric method or the thermal oxidation/pulsed fluorescence method. However, in a typical monitoring situation normally encountered concentrations of COS and terpenes are not expected to cause significant measurement bias. Criterion ii is satisfied by the thermal oxidation/ pulsed fluorescence method but not the Philips coulometric method. This means that whereas variations in the TRS composition are not a factor in assessing measurement bias for the thermal oxidation/pulsed fluorescence method, they are a factor for the Philips coulometric method. For example, if the TRS composition for H2S/ CH3SH/(CH,),S/(CH,),S2 shifts from 12:3:1:1to 1:3:1:1, the measurement bias shifts from $5.8 to 14.9%. In summary, the results of this work suggest that a candidate reference method for ambient TRS would include the following elements; (i) removal of SO2 from the original sample, (ii) thermal oxidation of TRS to SOz, and (iii) detection of SO2using a suitable analytical technique. The analytical technique itself should provide a detection limit that is compatible with regulatory requirements. If the regulatory limit is 20 ppb, a detection limit of 2 ppb could be considered satisfactory. However, if the regulatory limit is 5 ppb, then the detection limit should be approximately 0.5 ppb. Recent refinements to the pulsed fluorescence techniques (Teco Model 43A) evidently show promise of achieving the 0.5 ppb detection limit (8).
Literature Cited (1) “Instruction Manual-Monitor PW9700”; N.V. Philips: Eindhoven, The Netherlands, 1974. (2) “Instruction Manual-Model 43 Pulsed Fluorescence Analyzer”; Thermo Electron Corp.: Hopkinton, MA. (3) “Pollution Control Objectives for the Forest Products Industry of British Columbia”. Ministry of Environment, Province of British Columbia, 1979. (4) Wend, H. F. J. “Kraft Pulping Theory and Practice”; Lockwood Publishing Co.: New York, NY, 1967. (5) McQuaker, N. R.; Pengilly, D.; Haboosheh, H. “Calibration and Operation of Continuous Ambient Air Monitors”. Ministry of Environment, Province of British Columbia, 1983. (6) “Installation and Operation Manual, Model 400 Titrator”; I T T Barton: Monterey Park, CA. (7) Environmental Protection Agency “Environmental Pollution Control, Pulp and Paper Industry, Part I, Air: EPA 625/7-76-007”. U.S. EPA: Cincinnati, OH, Oct 1976. (8) Fleming, B., C. D. Nova Ltd., private communication, 1985. Received for review February 11, 1985. Revised manuscript received July 25, 1985. Accepted December 23, 1985.