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7.2.1 Sample Preparation. After collection, if a precipitate is observed in the sample, remove it by centrifugation.

7.2.1.1 30-Minute and 1-Hour Samples. Transfer the sample quantitatively to a 25ml. volumetric flask; use about 5 ml. distilled water for rinsing. Delay analyses for 20 minutes to allow any ozone to decompose.

7.2.1.2 24-Hour Sample. Dilute the entire sample to 50 ml. with absorbing solution. Pipet 5 ml. of the sample into a 25-ml. volumetric flask for chemical analyses. Bring volume to 10 ml. with absorbing reagent. Delay analyses for 20 minutes to allow any ozone to decompose.

7.2.2 Determination. For each set of determinations prepare a reagent blank by adding 10 ml. unexposed TCM solution to a 25-ml. volumetric flask. Prepare a control solution by adding 2 ml. of working sulfiteTCM solution and 8 ml. TCM solution to a 25-ml. volumetric flask. To each flask containing either sample, control solution, or reagent blank, add 1 ml. 0.6 percent sulfamic acid and allow to react 10 minutes to destroy the nitrite from oxides of nitrogen. Accurately pipet in 2 ml. 0.2 percent formaldehyde solution, then 5 ml. pararosaniline solution. Start a laboratory timer that has been set for 30 minutes. Bring all flasks to volume with freshly boiled and cooled distilled water and mix thoroughly. After 30 minutes and before 60 minutes, determine the absorbances of the sample (denote as A), reagent blank (denote as A.) and the control solution at 548 nm. using 1-cm. optical path length cells. Use distilled water, not the reagent blank, as the reference. (NOTE! This is important because of the color sensitivity of the reagent blank to temperature changes which can be induced in the cell compartment of a spectrophotometer.) Do not allow the colored solution to stand in the absorbance cells, because a film of dye may be deposited. Clean cells with alcohol after use. If the temperature of the determinations does not differ by more than 2° C. from the calibration temperature (8.2), the reagent blank should be within 0.03 absorbance unit of the y-intercept of the calibration curve (8.2). If the reagent blank differs by more than 0.03 absorbance unit from that found in the calibration curve, prepare

a new curve.

7.2.3 Absorbance Range. If the absorbance of the sample solution ranges between 1.0 and 2.0, the sample can be diluted 1:1 with a portion of the reagent blank and read within a few minutes. Solutions with

higher absorbance can be diluted up to sixfold with the reagent blank in order to obtain onscale readings within 10 percent of the true absorbance value.

8. Calibration and Efficiencies.

8.1 Flowmeters and Hypodermic Needle. Calibrate flowmeters and hypodermic needle (8) against a calibrated wet test meter.

8.2 Calibration Curves.

8.2.1 Procedure with Sulfite Solution. Accurately pipet graduated amounts of the working sulfite-TCM solution (section 6.2.9) (such as 0, 0.5, 1, 2, 3, and 4 ml.) into a series of 25-ml. volumetric flasks. Add sufficient TCM solution to each flask to bring the volume to approximately 10 ml. Then add the remaining reagents as described in 7.2.2. For maximum precision use a constant-temperature bath. The temperature of calibration must be maintained within ±1° C. and in the range of 20° to 30° C. The temperature of calibration and the temperature of analysis must be within 2 degrees. Plot the absorbance against the total concentration in μg. SO, for the corresponding solution. The total ug. SO, in solution equals the concentration of the standard (Section 6.2.9) in μg. SO2/ml. times the ml. sulfite solution added (μg. SO2=μg./l. SO2×ml. added). A linear relationship should be obtained, and the y-intercept should be within 0.03 absorbance unit of the zero standard absorbance. For maximum precision determine the line of best fit using regression analysis by the method of least squares. Determine the slope of the line of best fit, calculate its reciprocal and denote as B. B2 is the calibration factor. (See Section 6.2.10.1 for specifications on the slope of the calibration curve). This calibration factor can be used for calculating results provided there are no radical changes in temperature or pH. At least one control sample containing a known concentration of SO, for each series of determinations, is recommended to insure the reliability of this factor.

8.2.2 Procedure with SO. Permeation Tubes.

8.2.2.1 General Considerations. Atmospheres containing accurately known amounts of sulfur dioxide at levels of interest can be prepared using permeation tubes. In the systems for generating these atmospheres, the permeation tube emits SO, gas at a known, low, constant rate, provided the temperature of the tube is held constant (±0.1° C.) and provided the tube has been accurately calibrated at the temperature of use. The SO, gas permeating from the tube is carried by a low flow of inert gas to a mixing chamber where it is accurately diluted with SO-free air to the level of interest and the sample taken. These systems are shown schematically in Figures A2 and A3 and have been described in detail by

O'Keeffe and Ortman (9), Scaringelli, Frey. and Saltzman (10), and Scaringelli, O'Keeffe, Rosenberg, and Bell (11).

8.2.2.2 Preparation of Standard Atmospheres. Permeation tubes may be prepared or purchased. Scaringelli, O'Keeffe, Rosenberg, and Bell (11) give detailed, explicit directions for permeation tube calibration. Tubes with a certified permeation rate are available from the National Bureau of Standards. Tube permeation rates from 0.2 to 0.4 μg./minute, inert gas flows of about 50 ml./minute, and dilution air flow rates from 1.1 to 15 liters/minute conveniently give standard atmospheres containing desired levels of SO, (25 to 390 μg./m. 3; 0.01 to 0.15 p.p.m. SO2). The concentration of SO2 in any standard atmosphere can be calculated as follows:

Where:

C=(PX103)/(Ra+R1)

C=Concentration of SO2, μg./m. at reference conditions.

P Tube permeation rate, μg./minute. R=Flow rate of dilution air, liter/minute at reference conditions.

=

R, Flow rate of inert gas, liter/minute at reference conditions.

8.2.2.3 Sampling and Preparation of Calibration Curve. Prepare a series (usually six) of standard atmospheres containing SO, levels from 25 to 390 μg. SO2/m.3. Sample each atmosphere using similar apparatus and taking exactly the same air volume as will be done in atmospheric sampling. Determine absorbances as directed in 7.2. Plot the concentration of SO, in μg./m.3 (x-axis) against A'A. values (y-axis), draw the straight line of best fit and determine the slope. Alternatively, regression analysis by the method of least squares may be used to calculate the slope. Calculate the reciprocal of the slope and denote as B.

8.3 Sampling Efficiency. Collection efficiency is above 98 percent; efficiency may fall off, however, at concentrations below 25 μg./m.3. (12, 13)

9. Calculations.

9.1 Conversion of Volume. Convert the volume of air sampled to the volume at reference conditions of 25° C. and 760 mm. Hg. (On 24-hour samples, this may not be possible.)

VR=VX(P/760)×(298/t+273)

VR Volume of air at 25° C. and 760 mm. Hg, liters.

V = Volume of air sampled, liters. P-Barometric pressure, mm. Hg. t=Temperature of air sample, °C.

9.2 Sulfur Dioxide Concentration. 9.2.1 When sulfite solutions are used to prepare calibration curves, compute the concentration of sulfur dioxide in the sample:

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p.p.m. SO2 =μg. SO2/m. 3×3.82× 10-4 10. References.

(1) West, P. W., and Gaeke, G. C., "Fixation of Sulfur Dioxide as Sulfitomercurate III and Subsequent Colorimetric Determination", Anal. Chem. 28, 1816 (1956). (2) Ephraims, F., "Inorganic Chemistry." p. 562, Edited by P.C.L. Thorne and E. R. Roberts, 5th Edition, Interscience. (1948).

(3) Lyles, G. R., Dowling, F. B., and Blanchard, V. J., "Quantitative Determination of Formaldehyde in Parts Per Hundred Million Concentration Level", J. Air Poll. Cont. Assoc. 15, 106 (1965). (4) Scaringelli, F. P., Saltzman, B. E., and Frey, S. A., "Spectrophotometric Determination of Atmospheric Sulfur Dioxide", Anal. Chem. 39, 1709 (1967). (5) Pate, J. B., Ammons, B. E., Swanson, G. A., Lodge, J. P., Jr., "Nitrite Interference in Spectrophotometric Determination of Atmospheric Sulfur Dioxide", Anal. Chem. 37, 942 (1965).

(6) Zurio, N. and Griffini, A. M., "Measurement of the SO, Content of Air in the Presence of Oxides of Nitrogen and Heavy Metals", Med. Lavoro, 53, 330 (1962).

(7) Scaringelli, F. P., Elfers, L., Norris, D., and Hochheiser, S., "Enhanced Stability of Sulfur Dioxide in Solution", Anal. Chem. 42, 1818 (1970).

(8) Lodge, J. P. Jr., Pate, J. B., Ammons, B. E. and Swanson, G. A., "Use of Hypodermic Needles as Critical Orifices in Air Sampling," J. Air Poll. Cont. Assoc. 16, 197 (1966).

(9) O'Keeffe, A. E., and Ortman, G. C., "Primary Standards for Trace Gas Analysis", Anal. Chem. 38, 760 (1966). (10) Scaringelli, F. P., Frey, S. A., and Saltzman, B. E., "Evaluation of Teflon Permeation Tubes for Use with Sulfur Dioxide", Amer. Ind. Hygiene Assoc. J. 28, 260 (1967). (11) Scaringelli, F. P., O'Keeffe, A. E., Rosenberg, E., and Bell, J. P., "Preparation of Known Concentrations of Gases and Vapors with Permeation Devices

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APPENDIX B-REFERENCE METHOD FOR THE DETERMINATION OF SUSPENDED PARTICULATES IN THE ATMOSPHERE (HIGH VOLUME METHOD)

1. Principle and Applicability.

1.1 Air is drawn into a covered housing and through a filter by means of a highflow-rate blower at a flow rate (1.13 to 1.70 m. 3/min.; 40 to 60 ft./min.) that allows suspended particles having diameters of less than 100 μm. (Stokes equivalent diameter) to pass to the filter surface. (1) Particles within the size range of 100 to 0.1μm. diameter are ordinarily collected on glass fiber filters. The mass concentration of suspended particulates in the ambient air (ug./m. 3) is computed by measuring the mass of collected particulates and the volume of air sampled.

1.2 This method is applicable to measurement of the mass concentration of suspended particulates in ambient air. The size of the sample collected is usually adequate for other analyses.

2. Range and Sensitivity.

2.1 When the sampler is operated at an average flow rate of 1.70 m. 3/min. (60 ft. / min.) for 24 hours, an adequate sample will be obtained even in an atmosphere having concentrations of suspended particulates as low as 1 μg./m. ". If particulate levels are un

usually high, a satisfactory sample may be obtained in 6 to 8 hours or less. For determination of average concentrations of suspended particulates in ambient air, a standard sampling period of 24 hours is recommended.

2.2 Weights are determined to the nearest milligram, airflow rates are determined to the nearest 0.03 m. /min. (1.0 ft. 3/min.). times are determined to the nearest 2 minutes, and mass concentrations are reported to the nearest microgram per cubic meter. 3. Interferences.

3.1 Particulate matter that is oily, such as photochemical smog or wood smoke, may block the filter and cause a rapid drop in airflow at a nonuniform rate. Dense fog or high humidity can cause the filter to become too wet and severely reduce the airflow through the filter.

3.2 Glass-fiber filters are comparatively insensitive to changes in relative humidity, but collected particulates can be hygroscopic. (2)

4. Precision, Accuracy, and Stability.

4.1 Based upon collaborative testing, the relative standard deviation (coefficient of variation) for single analyst variation (repeatability of the method) is 3.0 percent. The corresponding value for multilabora

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