Determination of nitrogen dioxide content of the atmosphere - Lab Report Example

ASTM D1607 - 91(2005) Standard Test Method for Nitrogen Dioxide Content of the Atmosphere (Griess-Saltzman Reaction) Introduction Griess-Saltzman reaction is used for the determination of nitrogen dioxide content of the atmosphere. An experiment has been conducted to determine nitrogen dioxide content in the atmosphere by using the ASTM D 1607 method. The objective of the experiment was to set up a reliable method for the sampling and analysis of nitrogen dioxide in ambient air. Principle An azo dye forming reagent is used to absorb nitrogen dioxide. Within 15 minutes, a stable red violet color is produced; which could be read visually or in a suitable instrument at 550 nm. Nitrogen dioxide reacts with sulfanilic acid and NEDA producing a colored azo dye (see figure 1), and the intensity of the reactant solution is measured spectrophotometrically (ASTM International 1). Figure 1. Greiss Reagent System (Promega 2) Method The method is used for manual determination of nitrogen dioxide in the atmosphere present in the range of 0.005-5 parts per million by volume or 0.01-10 µg/L. Sampling was conducted by the use of fritted bubblers (Lodge 389-394). Figure 2. Fritted Bubbler (ASTM International 2) A fritted glass bubbler has been illustrated in figure 2. The sample is absorbed in a 60 µm pore diameter fritted bubbler tube. Absorption efficiency is affected by porosity of the fritted bubbler and sampling flow rate. With a flow rate of 0.4 L/min or less and a maximum pore diameter of 60 µm, an efficiency of 95 percent could be obtained (Lodge 389-394). Apparatus include sampling probe, absorber, gas drying tube, air-metering device, thermometer, manometer, air pump, spectrophotometer, and stopwatch. Reagent grade chemicals have been used. Water free from nitrite and deionised according to specification D 1193 for type I or II reagent water has been used. Anhydrous sulfanilic acid has been used as the absorbing reagent, N-(1-Naphthyl)-Ethylenediamine Dihydrochloride stock solution (0.1 percent), Sodium Nitrite standard solution (0.0246 g/L) and NO2 permeation device were reagents and materials that were used in the experiment. 5.0012 g of anhydrous sulfanilic acid was dissolved in 1 L of water containing 140 mL glacial acetic acid. The process was gently heated to speed up the process. 20 mL of the of N-(1-naphthyl)-ethylenediamine dihydrochloride 0.1 % stock solution and 10 mL acetone were added, and diluted to 1 L. 0.1 g of the reagent was dissolved in 100 mL water (ASTM International 2). Calibration and Standardization The flowmeter was calibrated using practice D 3195. The gas meter was calibrated using test method D 1071. Standardization was based on observation. 0.82 mol of NaNO2 produced the same color as 1 mol NO2. 1 mL working standard solution contains 24.6 µg NaNO2. The amount of NO2 in given by (24.6/69.1)x(46.0/0.82), which is 20 µg NO2. Standard conditions of 101 kPa and 25C were taken, and the molar gas volume was 24.47L (ASTM International 3). Calibration Graph Graduated amounts of NaNO2 solution were added to a series of 25mL volumetric flasks upto 1 mL. The solutions were diluted to the marks with absorbing reagent, and mixed. Allowing 15 minutes for color development, the absorbance for each standard was read. The absorbances were plotted against micrograms of NO2/mL of absorbing reagent. The plot followed Beer’s law. The standardization factor K was determined (ASTM International 3). Table 1. Absorbances of Standards Sl No Volume (mL) NaNO2 (µg) NO2 (µg) NO2 (µg/1 mL) NO2 (µg/5 mL) NO2 (µg/25mL) Spectrophotometer Reading Absorbance 1 0 0 0 0 0 0 0.09 0 2 0.2 4.92 3.99 3.990 0.798 0.160 0.22 0.13 3 0.4 9.8 7.988 7.988 1.998 0.320 0.369 0.279 4 0.6 14.76 11.98 11.98 2.396 0.479 0.479 0.389 5 0.8 19.69 15.976 15.976 3.195 0.639 0.61 0.52 6 1.0 24.6 20 20 4 0.800 0.81 0.72 Figure 3. Absorbances of Standards vs NO2 (µg) A calibration curve has been illustrated in figure 4. Y axis represents absorbance and X axis represents NO2 in µg/ 1 mL. Figure 4. Absorbances of Standards vs NO2 (µg/1mL) A calibration curve has been illustrated in figure 5. Y axis represents absorbance and X axis represents NO2 in µg/ 5 mL. Figure 5. Absorbances of Standards vs NO2 (µg/5mL) A calibration curve has been illustrated in figure 6. Y axis represents absorbance and X axis represents NO2 in µg/ 25 mL. Figure 6. Absorbances of Standards vs NO2 (µg/25mL) Procedure A sampling train was assembled (see figure 7) using a fritted-tip absorber, mist eliminator, flow meter and pump. Connections were made with vinyl tubing. The flowmeter was kept free from spray or dust. Temperature and pressure difference were measured from the atmosphere. Using a pipette, 10 mL of absorbing reagent was entered into a dry fritted bubbler. Air sample was drawn through it at a rate of 0.4 L/min. The sample was drawn long enough to develop color. The temperature and air pressure were measured. After use, the bubbler was rinsed with water and dried. Discolored fritted tip was rinsed with water and dried. After sampling, the sample was transferred to a stoppered curvette and read in a spectrophotometer at 550 nm. Distilled water was used as reference. The absorbance of the reagent blank has been deducted from that of the sample (ASTM International 3-4). Figure 7. Sampling Train (ASTM International 2) Calculation Air volume sampled was converted to volume at standard conditions of 25C and 101.3 kPa using the expression given by (ASTM International 4): Vr =[VxP/101.3]x[298.15/T] Where: Vr= air volume sampled at standard conditions in L; V = air volume sampled at ambient conditions in L; P = ambient atmospheric pressure in kPa; T = ambient atmospheric temperature in K; 101.3 = standard atmospheric pressure in kPa; 290.15 = standard atmospheric pressure in K; Concentrations of NO2 in the sample were calculated using the expression given by: NO2 µg/m3 = (absorbance x K x 103 x v)/Vr Where: K = standardization factor; Vr = volume of air sample in L; 103 = L/m3; and v = volume of absorbing reagent in mL. Results The value of K was determined to be 13.704. Trial 1 comprised of two 69 minutes samples of air exposure in the hallway outside the laboratory. Trial 2 comprised of two 62 minutes samples in the laboratory. Trial 3 comprised of two 37 minute samples in the laboratory with explosive dimitrotoluene. Mass of dimitrotoluene used was 0.0113 g. Samples 1 and 2 were at room temperature and sample 3 was at 40C. The final trial was with 0.01 g of dimitrotoluene, and had four samples at 25C, 40C, 60C and 80C respectively. The results have been included in table 2. Table 2. Trial Results Trial Sample Absorbance Start Flow Rate (L/min) Stop Flow Rate (L/min) Average Flow Rate (L/min) Duration (min) Volume (L) 1 Blank 1 1 0.040 0.3980 0.3891 0.39355 69 27.15 1 2 0.049 0.3980 0.3891 0.39355 69 27.15 2 Blank 2 1 0.035 0.3871 0.3880 0.3876 62 24.03 2 2 0.033 0.3871 0.3880 0.3876 62 24.03 3 Blank 3 1 0.045 0.3985 0.3764 0.3875 37 14.34 3 2 0.065 0.3985 0.3764 0.3875 37 14.34 3 Blank 3 3 0.122 0.3805 0.3756 0.3781 37 13.99 4 Blank 4 1 0.045 0.4021 90 36.19 4 2 0.037 0.4041 90 36.37 4 3 0.044 0.4052 90 36.47 4 4 0.050 0.3720 90 33.48 The calculations have been illustrated in table 3. Table 3. Calculations Trial Sample Absorbance Volume (L) Vr =[VxP/101.3] x[298.15/T] NO2 µg/m3 = (absorbance x K x 103 x v)/Vr Sample NO2 (µg/m3) NO2 (µg/m3) 1 Blank 1 1 0.040 27.15 27.15x101.3/101.3x298.15/298.15 = 27.15 (0.040x7.3x103 x10)/27.15 107.55 (107.55+131.75)/2=119.65 1 2 0.049 27.15 27.15x101.3/101.3x298.15/298.15 = 27.15 (0.049x7.3x103 x10)/ 27.15 131.75 2 Blank 2 1 0.035 24.03 24.03x101.3/101.3x298.15/298.15 =24.03 (0.035x7.3x103x10)/24.03 106.32 (106.32+100.25)/2=103.28 2 2 0.033 24.03 24.03x101.3/101.3x298.15/298.15 =24.03 (0.033x7.3x103 x10)/ 24.03 100.25 3 Blank 3 1 0.045 14.34 14.34x101.3/101.3x298.15/298.15 =14.34 (0.045x7.3x103 x10)/ 14.34 229.09 (229.09+330.89)/2=279.99 3 2 0.065 14.34 14.34x101.3/101.3x298.15/298.15 =14.34 (0.065x7.3x103 x10)/ 14.34 330.89 3 Blank 3 3 0.122 13.99 13.99x101.3/101.3x298.15/313.15 =13.31 (0.122x7.3x103 x10)/ 13.31 669.12 669.12 4 Blank 4 1 0.045 36.19 36.19x101.3/101.3x298.15/298.15=36.19 (0.040x7.3x103 x10)/ 36.19 90.77 90.77 4 2 0.037 36.37 36.37x101.3/101.3x298.15/313.15=34.63 (0.037x7.3x103 x10)/ 34.63 77.99 77.99 4 3 0.044 36.47 36.47x101.3/101.3x298.15/333.15=32.64 (0.044x7.3x103 x10)/ 32.64 98.41 98.41 4 4 0.050 33.48 33.48x101.3/101.3x298.15/353.15=28.27 (0.050x7.3x103 x10)/ 28.27 129.11 129.11 The average NO2 for trial 1 and trial 2 were 119.65 µg/m3 and 103.28 µg/m3 respectively. In trial 3, average NO2 were 279.99 µg/m3 and 669.12 µg/m3 respectively. In the final trial, average NO2 were 90.77 µg/m3, 77.99 µg/m3, 98.41 µg/m3, and 129.11 µg/m3 respectively. Discussion Levels of NO2 measured ranged from 77.99 µg/m3 to 669.12 µg/m3. At ambient conditions NO2 levels were between 90.77 µg/m3 and 279.99 µg/m3. Increase in NO2 measured by the use of explosive in trial 3 was expected. During combustion processes, significant amount of nitric oxide may be produced by the combination of nitrogen from the atmosphere and oxygen. At ambient temperature levels NO could be converted to NO2 by oxygen and other atmospheric oxidants. Nitrogen dioxide could also be generated from processes involving nitric acid, nitrates, use of explosives, and welding. Nitrogen dioxide is significant in photochemical reactions such as smog-forming reactions, and could be deleterious to health, materials, visibility and agriculture (ASTM International 1). Interferences could be caused by thirty fold SO2 and bleaching the color, which could be overcome by the addition of acetone. Interferences could also be caused by ozone and peroxyacetyl nitrate, and other nitrogen oxides and gases present in polluted air. However, normal levels of ambient air are not a cause of concern (ASTM International 2). The method is highly sensitive. Careful work can result in a precision of 1 percent of the mean. Measurements of the volume of the air sample and absorbance of the color are limiting factors. The method is reliable for short sampling periods, such as a few hours, followed by immediate subsequent analysis. The method is highly sensitive to detect low nitrogen dioxide concentrations. It is not suitable for longer sampling periods, such as 24 hours (Lodge 389-394). References ASTM International. "Standard Test Method for Nitrogen Dioxide Content of the Atmosphere (Griess- Saltzman Reaction)." ASTM International D 1607 – 91(2000): 1-5. Print. Gold, A. “Stoichiometry of Nitrogen Dioxide Determination in Triethanolamine Trapping Solution.” Anal. Chem. 49 (1977):1448-50. Lodge, James. Methods of Air Sampling and Analysis. 3. USA: CRC Press, 1989. Print. Promega, "Griess Reagent System." Technical Bulletin TB229(2005): 1-8. Print. Read More