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and pressure is extremely difficult and is not ordinarily done. However, the accuracy of the measurement will be improved if meaningful corrections can be applied. If storage is necessary, refrigerate at 5° C. (see 4.2).

7.2 Analysis.

7.2.1 Sample Preparation. After collection, if a precipitate is observed in the sample, remove it by centrifugation. 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. 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. opti cal 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 Hypodermis 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 ug. SO, for the corresponding solution. The total ug. SO, in solution equals the concentration of the standard (Section 6.2.9) in ug. SO,/ml. times the ml. sulfite solution added (ug. SO,=kg./1. SO,x 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 Bz. B. is the calibration factor. (See Section 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. 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 dilut. ed 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. ug, SO2/m."=[((A-A.) (10) (B,))/ VR]XD and Saltzman (10), and Scaringelli, O'Keeffe, Rosenberg, and Bell (11).

A=Sample absorbance. Preparation of Standard Atmos- Ao=Reagent blank absorbance. pheres. Permeation tubes may be prepared 10'=Conversion of liters to cubic meters. or purchased. Scaringelli, O'Keeffe, Rosen- Vr=The sample corrected to 25° C. and 760 berg, and Bell (11) give detailed, explicit di mm. Hg, liters. rections for permeation tube calibration. B.=Calibration factor, wg./absorbance unit. Tubes with a certified permeation rate are D=Dilution factor. For 30-minute and 1available from the National Bureau of hour samples, D=1. For 24-hour samStandards. Tube permeation rates from 0.2 ples, D=10. to 0.4 ug./minute, inert gas flows of about 50 ml./minute, and dilution air flow rates

9.2.2 When SO, gas standard atmosfrom 1.1 to 15 liters/minute convenientlypheres are used to prepare calibration give standard atmospheres containing de- curves, compute the sulfur dioxide in the sired levels of SO, (25 to 390 ug./m. 3; 0.01 to sample by the following formula: 0.15 p.p.m. SO2). The concentration of SO,

SO2, ug./m."=(A-A,)x By
in any standard atmosphere can be calculat-
ed as follows:

A=Sample absorbance.

A.= Reagent blank absorbance.

C=Concentration of SO2, ug./m. ’ at refer-

9.2.3 Conversion of ug./m." to p.p.m. = If ence conditions.

desired, the concentration of sulfur dioxide P=Tube permeation rate, ug./minute. may be calculated as p.p.m. SO, at reference RorFlow rate of dilution air, liter/minute

conditions as follows: at reference conditions.

p.p.m. SO,=ug. SO2/m. 'x3.82x10-4 R;=Flow rate of inert gas, liter/minute at reference conditions.

10. References. Sampling and Preparation of (1) West, P. W., and Gaeke, G. C., "Fixation Calibration Curve. Prepare a series (usually of Sulfur Dioxide as Sulfitomercurate six) of standard atmospheres containing SO, III and Subsequent Colorimetric Deterlevels from 25 to 390 ug. SO2/m.". Sample mination", Anal. Chem. 28, 1816 (1956). each atmosphere using similar apparatus (2) Ephraims, F., “Inorganic Chemistry," p. and taking exactly the same air volume as 562, Edited by P.C.L. Thorne and E. R. will be done in atmospheric sampling. De Roberts, 5th Edition, Interscience. termine absorbances as directed in 7.2. Plot (1948). the concentration of SO, in ug./m. 3 (x-axis) (3) Lyles, G. R., Dowling, F. B., and Blanagainst AA, values (y-axis), draw the chard, V. J., "Quantitative Determinastraight line of best fit and determine the tion of Formaldehyde in Parts Per Hunslope. Alternatively, regression analysis by dred Million Concentration Level", J. the method of least squares may be used to Air Poll. Cont. Assoc. 15, 106 (1965). calculate the slope. Calculate the reciprocal

(4) Scaringelli, F. P., Saltzman, B. E., and of the slope and denote as Bx.

Frey, S. A., "Spectrophotometric Deter8.3 Sampling Efficiency. Collection effi

mination of Atmospheric Sulfur Dioxciency is above 98 percent; efficiency may

ide", Anal. Chem. 39, 1709 (1967). fall off, however, at concentrations below 25

(5) Pate, J. B., Ammons, B. E., Swanson, G. ug./m.". (12, 13)

A., Lodge, J. P., Jr., “Nitrite Interfer9. Calculations.

ence in Spectrophotometric Determina9.1 Conversion of Volume. Convert the

tion of Atmospheric Sulfur Dioxide", volume of air sampled to the volume at ref

Anal. Chem. 37, 942 (1965). erence conditions of 25° C. and 760 mm. Hg. (On 24-hour samples, this may not be possi

Zurio, N. and Griffini, A. M., "Measure

ment of the SO, Content of Air in the ble.)

Presence of Oxides of Nitrogen and VR=VX(P/760)x(298/t+273)

Heavy Metals", Med. Lavoro, 53, 330

(1962). VR=Volume of air at 25° C. and 760 mm. Hg,

Hp (7) Scaringelli, F. P., Elfers, L., Norris, D., liters. V=Volume of air sampled, liters.

and Hochheiser, S., "Enhanced Stability

of Sulfur Dioxide in Solution". Anal. P=Barometric pressure, mm. Hg.

Chem. 42, 1818 (1970). t=Temperature of air sample, °C.

(8) Lodge, J. P. Jr., Pate, J. B., Ammons, B. 9.2 Sulfur Dioride Concentration.

E. and Swanson, G. A., “Use of Hypoder9.2.1 When sulfite solutions are used to mic Needles as Critical Orifices in Air prepare calibration curves, compute the con Sampling," J. Air Poll. Cont. Assoc. 16, centration of sulfur dioxide in the sample:

197 (1966),

(9) O'Keeffe, A. E., and Ortman, G. C., "Pri.

mary Standards for Trace Gas Analy.

sis", Anal. Chem. 38, 760 (1966). (10) Scaringelli, F. P., Frey, S. A., and Saltz

man, 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

Calibrated Gravimetrically". Anel.

Chem. 42, 871 (1970). (12) Urone, P., Evans, J. B., and Noyes, C.

M., "Tracer Techniques in Sulfur Dioxide Colorimetric and Conductiometrie

Methods”, Anal Chem. 37, 1104 (1965). (13) Bostrom, C. E., “The Absorption of

Sulfur Dioxide at Low Concentrations (p.p.m.) Studied by an Isotopic Tracer Method", Intern. J. Air Water Poll. 9. 33 (1965).

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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.”/min.; 40 to 60 ft./min.) that allows suspended particles having diameters of less than 100 um. (Stokes equivalent diameter) to pass to the filter surface. (1) Particles within the size range of 100 to 0.1um. diameter are ordinarily collected on glass fiber filters. The mass concentration of suspended particulates in the ambient air (ug./m.") is computed by measuring the mass of collected particulates and the volume of air sam pled.

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./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 ug./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./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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