In the Laboratory edited by
Cost-Effective Teacher
Harold H. Harris University of Missouri—St. Louis St. Louis, MO 63121
A Passive Sampler for Determination of Nitrogen Dioxide in Ambient Air
Dan Xiao,* Lianzhi Lin, and Hongyan Yuan College of Chemical Engineering, Sichuan University, Chengdu 610065, China; *
[email protected] Martin M. F. Choi and Winghong Chan* ChemEdu Limited and Department of Chemistry, Hong Kong Baptist University, Kowloon Tong, Hong Kong SAR, China; *
[email protected] In most college chemistry laboratories, few gas-sampling tests are performed because the gas-sampling devices are complicated and costly. However, gas sampling is an important skill not only for students in chemistry and biology but also for students in toxicology and geology (1). It is essential for
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Figure 1. The schematic diagram of the passive sampler: (1) inlet for the absorbing solution, (2) outlet for the absorbing solution, (3) sampler body, (4) plastic cover, (5) PTFE membrane, and (6) absorbing solution reservoir.
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environmental pollution detection. Nitrogen dioxide, NO2, produced as a byproduct of high temperature flames in internal combustion engines (2) can form acid rain and photochemical smog that pollute the environment. The determination of NO2 concentration normally requires sampling devices that are expensive and inconvenient. In general, two major types of sampling devices are available. The first is an active sampler that needs suction pumps to draw fixed volumes of air through an absorbing solution (3, 4). The second is a passive sampler that requires a relatively long sampling time (5, 6). Sometimes NO2 can also be monitored in situ by expensive instruments (7). To circumvent these problems, a fast passive sampler with a large exposed surface area has been developed to determine NO2 in ambient air. This device is simple, lightweight, and inexpensive. It does not require an auxiliary sampling pump. The total sampling time for NO2 is short, less than fifteen minutes. Experimental Procedure
Design of the Passive Sampler A schematic diagram of the passive sampler is shown in Figures 1 and 2 and a photograph of the sampler is shown in Figure 3. The sampler consists of two pieces of PTFE microporous membranes,1 two plastic covers, and a plastic sampler body.2 The sampler covers (60-mm o.d., 50-mm i.d.)
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Figure 2. A three-dimensional illustration of the passive sampler: (1) sampler body, (2) PTFE membrane, (3) plastic cover, (4) stainless steel net, (5) solution inlet, and (6) solution outlet.
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Figure 3. A photograph of the passive sampler.
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In the Laboratory
fix the membranes on the sampler body. The sampler body is a cylindrical disc (60-mm diameter and 6-mm thick) with two holes drilled (2.5 mm) through each side. Two vertical, hollow channels are made from the top of the disc to join the two holes at the sides of the disc. Each side of the disc is then covered with a PTFE microporous membrane.1 One of the channels acts as the inlet for an absorbing reagent solu-
2NO2(g) ⴙ H2O(l)
Preparation of the Absorbing Reagent The absorbing reagent was prepared by dissolving 4.0 g sulfanilamide,3 10 g tartaric acid,4 0.1 g EDTA5 in 400 mL of warm deionized water. The solution was cooled, transferred to a 1.0-L volumetric flask containing 90 mg N-(1-naphthyl)ethylenediamine dihydrochloride6 and diluted to the mark. The solution was stable for several months, if it was kept in a brown bottle at 4 ⬚C (8).7
ⴙ HNO2(aq)
HNO3(aq)
ⴙ
NH2
N
N
HNO2(aq) ⴙ
SO3H
SO3H
sulphanilic acid
diazonium intermediate
NH2 ⴙ
N
NH2
N
HN
HN
ⴙ
N
SO3H
diazonium intermediate
N
azo dye
N-(1-naphthyl)ethylenediamine dihydrochloride
SO3H
Scheme I. Chemical reactions in the passive sampler.
1.2
Absorbance
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Figure 4. Absorbance of the azo dye that is produced from the reaction of known concentrations of NO2 with 3.8 mL of the absorbing reagent.
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Chemistry of the Absorption Reaction The chemistry of the absorption of NO2 by the absorbing reagent is shown in Scheme I (9). Nitrogen dioxide in air is collected by the passive sampler. When NO2 and the absorbent reagent are brought together, a pink colored solution results. The color is due to the formation of an azo dye complex. Since the complex is the only colored species in the system, the concentration of NO2 can be determined by spectrophotometry. The absorbance is directly proportional to the concentration of the colored constituent. Calibration of the Passive Sampler An aliquot, 3.8-mL, of the absorbing reagent was injected into the sampler and the sampler was then positioned in an enclosed environment of known NO2 concentration. After standing for 15 min, the absorbing reagent was transferred to a 1-cm glass cuvette and the absorbance of the solution at 540 nm was taken on an UV–vis spectrophotometer.8 The experiment was repeated by placing the sampler in other enclosed environments of known NO2 concentrations. The absorption intensity was plotted against the NO2 concentration (Figure 4). It was observed that the absorbance value was directly proportional to the NO2 concentration.9 The preparation of gaseous NO2 standards can be performed by many methods (10). In our work, NO2 was prepared by mixing known concentrations of sodium nitrite3 with concentrated sulfuric acid3 releasing gaseous NO2 to an enclosed environment. The NO2 level was then determined by a standard method (4). This standard method employed an active sampler10 containing an absorbing reagent to absorb the NO2. In brief, a stream of air was continuously passed through an absorbing solution at 400 mL兾min for 15 min. Afterwards, the absorbance value of the solution was calculated and determined from a calibration curve using known concentrations of sodium nitrite.3
0.6
NO2 Concentration / (mg/m3)
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tion and the other is the outlet. The two membranes are the crucial parts of the sampler as they allow air to diffuse through and react with the absorbing solution. The PTFE membrane is about 50-µm thick with submicrometer-sized pores distributed through it and the exposed surface area of each membrane is 19.6 cm2. An absorbing reagent reservoir is formed between the membranes and the sampler body, which can hold 3.8 mL of absorbing reagent. A stainless steel net is positioned adjacent to each membrane to support the membrane.
Hazards As the absorbing reagent is irritating and toxic, students must wear splash goggles and gloves to avoid contact with the absorbing solution and all other chemicals.
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In the Laboratory
Results and Discussion The concentration of NO2 in the sampler can be determined by the following equation C = (A − A0)BS D where C is the concentration of NO2 in the ambient air; A is the absorbance value of the absorbing solution; A0 is the absorbance value of the absorbing solution without exposing to NO2; BS is the calibration factor obtained from the calibration curve of the sampler, which is the reciprocal of the slope of the calibration curve; and D is the dilution factor of the absorbing solution. The dilution factor is needed when the absorbance value of the absorbing solution is large and does not fall within the linear calibration range for NO2; the absorbing solution has to be diluted to ensure that the absorbance value lies within the linear calibration range. In the passive sampler used here, A0 = 0.007 and BS = 0.58 mg兾m3. The concentrations of NO2 in various sampling sites were determined by the passive sampler. The results compared well with a standard method (4) and are displayed in Table 1. The error was less than 7%. The limit of detection was determined to be 7 ppb for 3 standard deviations at the blank (10). As shown in Figure 4, the measurement system has a good linearity within the NO2 concentration range of 26– 560 µg兾m3. Interference studies of the proposed method were done. It was found that ambient air pollutants such as sulfur dioxide, hydrogen sulfide, and hydrogen chloride did not produce any interference on the method. Conclusion Nitrogen oxides emissions from power stations, industries, and motor vehicles are the major sources of NO and NO2. Nitrogen dioxide is formed from oxidation of NO emitted from fossil fuel combustion. The concentrations of most air pollutants follow the diurnal pattern of traffic. For instance, higher levels of NO2 are usually observed in the early morning and the evening rush hours when there are more traffic and human activities. Likewise, the lowest concentrations often occur from midnight to dawn when the traffic is at its minimum (11). College students can perform the sampling and analysis of NO2 at roadside. Afterwards, they can compare and discuss the NO2 level under high and low traffic situations. The experiment provides hands-on experience for college students to gain understanding of air pollutants and environmental analysis. Our passive sampler is easy to fabricate and can be used repeatedly for at least several months. The proposed passive sampler provides a convenient, simple, and fast method for NO2 determination. Furthermore, this experiment can readily be modified for determinations of other air pollutants such as formaldehyde and sulfur dioxide for hands-on experience for students studying environmental pollution problems.
Table 1. Determination of NO2 Concentration in Various Sampling Sites Using the Proposed Passive Sampler and Standard Methods
1. PTFE membranes were purchased from Dongguan Plastic Company, Guangzhou, China. Alternative sources for PTFE membranes are Dupont, Wilmington, DE; Porex, Fairburn, GA; and NeoMecs, Eden Prairie, MN.
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Proposed Method
Standard Method
Error (%)
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46
4.3
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38
36
5.6
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93
87
6.9
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118
121
᎑2.5
2. Plastic covers and sampler body were fabricated in the workshop of Sichuan University, Chengdu, China. 3. Sulfanilamide, sodium nitrite, and concentrated sulfuric acid were obtained from Shanghai Chemical Reagent Company, Shanghai, China. 4. Tartaric acid was purchased from the First Factory of Shanghai Reagent, Shanghai, China. 5. EDTA was purchased from Chemical Reagent Factory of Chongqing, Chongqing, China. 6. N-(1-naphthyl)-ethylenediamine dihydrochloride was purchased from Aldrich, Milwaukee, WI. 7. The absorbing reagent has to be at room temperature before use. 8. Model 722 UV–vis spectrophotometer was obtained from Shanghai Instrument Factory, Shanghai, China. 9. Before each use, the sampler should be rinsed with distilled deionized water. The sampler has to be calibrated beforehand. After calibration, it can be used repeatedly for many times without re-calibration. 10. A SKC PCXR4 Universal Sampler was purchased from SKC, Eighty Four, PA.
Literature Cited 1. 2. 3. 4.
5. 6. 7. 8.
9.
Notes
NO2 Concentration/(µg/m3)
Sampling Sites
10. 11.
Weidenhamer, J. D. J. Chem. Educ. 1997, 74, 1437. Driscoll, J. A. J. Chem. Educ. 1997, 74, 1424. Nakano, N. Anal. Chim. Acta 1996, 321, 41. ASTM committee, Method no. D1607-91 http:// www.astm.org/cgi-bin/SoftCart.exe/DATABASE.CART/ REDLINE_PAGES/D1607.htm?E+mystore (accessed May 2005). Shooter, D. J. Chem. Educ. 1993, 70, A133. Gibson, L. T.; Cooksey, B. G.; Littlejohn, D.; Tennent, N. H. Anal. Chim. Acta 1997, 341, 1. Lal, S.; Patil, R. S. Environ. Monit. Assess. 2001, 68, 37. Cui, J; Wang, Q.; Wang, H. The Methods of Atmosphere Pollution Detection; Chemcial Engineering Press: Beijing, China, 1997; p 820. Harrison, R. M.; Perry, R. Handbook of Air Pollution Analysis; Chapman & Hall: London, 1986. Namiesnik, J. J. Chromatogr. 1984, 300, 79. Miller, J. N.; Miller, J. C. Statistics and Chemometrics for Analytical Chemistry; Pearson Education: Essex, United Kingdom, 2000; p 120.
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