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(b) 260 micrograms per cubic meter (0.1. p.p.m.)—maximum 24-hour concentration not to be exceeded more than once per year, as a guide to be used in assessing implementation plans to achieve the annual standard.

(c) 1,300 micrograms per cubic meter (0.5 p.p.m.)-maximum 3-hour concentration not to be exceeded more than once per year. $ 50.6 National primary ambient air

quality standards for particulate

matter. The national primary ambient air quality standards for particulate matter, measured by the reference method described in Appendix B to this part, or by an equivalent method, are:

(a) 75 micrograms per cubic meterannual geometric mean.

(b) 260 micrograms per cubic metermaximum 24-hour concentration not to be exceeded more than once per year. $ 50.7 National secondary ambient air

quality standards for particulate matter. The national secondary ambient air quality standards for particulate matter, measured by the reference method described in Appendix B to this part, or by an equivalent method, are:

(a) 60 micrograms per cubic meterannual geometric mean, as a guide to be used in assessing implementation plans to achieve the 24-hour standard.

(b) 150 micrograms per cubic metermaximum 24-hour concentration not to be exceeded more than once per year. $ 50.8 National primary and secondary

ambient air quality standards for car

bon monoxide. The national primary and secondary ambient air quality standards for carbon monoxide, measured by the reference method described in Appendix C to this part, or by an equivalent method, are:

(a) 10 milligrams per cubic meter (9 p.p.m.)-maximum 8-hour concentration not to be exceeded more than once per year.

(b) 40 milligrams per cubic meter (35 p.p.m.)-maximum 1-hour concentration not to be exceeded more than once per year. $ 50.9 National primary and secondary

ambient air quality standards for

photochemical oxidants. The national primary and secondary ambient air quality standard for photo

chemical oxidants, measured and corrected for interferences due to nitrogen oxides and sulfur dioxide by the reference method described in Appendix D to this part, or by an equivalent method, is: 160 micrograms per cubic meter (0.08 p.p.m.)-maximum 1-hour concentration not to be exceeded more than once per year. $ 50.10 National primary and second

ary ambient air quality standard for

hydrocarbons. The hydrocarbons standard is for use as a guide in devising implementation plans to achieve oxidant standards.

The national primary and secondary ambient air quality standard for hydrocarbons, measured and corrected for methane by the reference method described in Appendix E to this part, or by an equivalent method, is: 160 micrograms per cubic meter (0.24 p.p.m.)-maximum 3-hour concentration (6 to 9 a.m.) not to be exceeded more than once per year. $ 50.11 National primary and second

ary ambient air quality standard for

nitrogen dioxide. The national primary and secondary ambient air quality standard for nitrogen dioxide, measured by the reference method described in Appendix F to this part, or by an equivalent method, is: 100 micrograms per cubic meter (0.05 p.p.m.)—annual arithmetic mean. APPENDIX A REFERENCE METHOD FOR THE Law is followed through the working range from 0.03 to 1.0 absorbance units (0.8 to 27 ug. of sulfite ion in 25 ml. final solution computed as SO2).


1. Principle and Applicability. 1.1 Sulfur dioxide is absorbed from air in a solution of potassium tetrachloromercurate (TCM). A dichlorosulfitomercurate complex, which resists oxidation by the oxygen in the air, is formed (1, 2). Once formed, this complex is stable to strong oxidants (e.g., ozone, oxides of nitrogen). The complex is reacted with pararosaniline and formaldehyde to form intensely colored pararosaniline methyl sulfonic acid (3). The absorbance of the solution is measured spectrophotometrically.

1.2 The method is applicable to the measurement of sulfur dioxide in ambient air using sampling periods up to 24 hours.

2. Range and Sensitivity. 2.1 Concentrations of sulfur dioxide in the range of 25 to 1,050 ug/m.3 (0.01 to 0.40 p.p.m.) can be measured under the conditions given. One can measure concentrations below 25 ug./m.3 by sampling larger volumes of air, but only if the absorption efficiency of the particular system is first determined. Higher concentrations can be analyzed by using smaller gas samples, a larger collection volume, or a suitable aliquot of the collected sample. Beer's

2.2 The lower limit of detection of sulfur dioxide in 10 ml. TCM is 0.75 mg. (based on twice the standard deviation) representing a concentration of 25 kg./m 80, (0.01 p.p.m.) in an air sample of 30 liters.

3. Interferences. 3.1 The effects of the principal known interferences have been minimized or eliminated. Interferences by oxides of nitrogen are eliminated by sulfamic acid (4, 5), ozone by time-delay (6), and heavy metals by EDTA (ethylenediaminetetraacetic acid, disodium salt) and phosphoric acid (4, 6,). At least 60 ug. Fe (III), 10 ug. Mn(II), and 10 ug. Cr(III) in 10 ml. absorbing reagent can be tolerated in the procedure. No significant interference was found with 10 ug. Cu (II) and 22 mg. V(V).

4. Precision, Accuracy, and Stability. 4.1 Relative standard deviation at the 95 percent confidence level is 4.6 percent for the analytical procedure using standard samples. (5)

4.2 After sample collection the solutions are relatively stable. At 22° C. losses of sulfur dioxide occur at the rate of 1 percent per day. When samples are stored at 5° C. for 30 days, no detectable losses of sulfur dioxide occur. The presence of EDTA enhances the stability of So, in solution, and the rate of decay is independent of the concentration of SO2. (7)

5. Apparatus.
5.1 Sampling.

5.1.1 Absorber. Absorbers normally used in air pollution sampling are acceptable for concentrations above 25 mg./m.3 (0.01 p.p.m.). An all-glass midget impinger, as shown in Figure A1, is recommended for 30-minute and 1-hour samples.

For 24-hour sampling, assemble an absorber from the following parts:

Polypropylene 2-port tube closures, special manufacture (available from Bel-Art Products, Pequannock, N.J.).

Glass impingers, 6 mm. tubing, 6 inches long, one end drawn to small diameter such that No. 79 jewelers drill will pass through, but No. 78 jewelers drill will not. (Other end fire polished.)

Polypropylene tubes, 164 by 32 mm. (Nalgene or equal).

5.1.2 Pump. Capable of maintaining an air pressure differential greater than 0.7 atmosphere at the desired flow rate.

5.1.3 Air Flowmeter or Criticai Orifice. A calibrated rotameter or critical orifice capable of measuring air flow within +2 per cent. For 30-minute sampling, a 22-gauge hypodermic needle 1 inch long may be used as a critical orifice to give a flow of about 1 liter/minute. For 1-hour sampling, a 23gauge hypodermic needle five-eighths of an inch long may be used as a critical orifice to give a flow of about 0.5 liter/minute. For 24 hour sampling, a 27-gauge hypodermic needle three-eighths of an inch long may be

used to give a flow of about 0.2 liter/minute. Use a membrane filter to protect the needle (Figure Ala).

5.2 Analysis.

5.2.1 Spectrophotometer. Suitable for measurement of absorbance at 548 nm. with an effective spectral band width of less than 15 nm. Reagent blank problems may occur with spectrophotometers having greater spectral band width. The wavelength calibration of the instrument should be verified. If transmittance is measured, this can be converted to absorbance:

6. Reagents.
6.1 Sampling

6.1.1 Distilled water. Must be free from oxidants.

6.1.2 Absorbing Reagent (0.04 M Potas. sium Tetrachloromercurate (TCM)). Dissolve 10.86 g. mercuric chloride, 0.066 g. EDTA (ethylenediaminetetraacetic acid, disodium salt), and 6.0 g. potassium chloride in water and bring to mark in a 1,000-ml. volumetric flask. (Caution: highly poisonous. If spilled on skin, flush off with water immediately). The pH of this reagent should be approximately 4.0, but it has been shown that there is no appreciable difference in collection eficiency over the range of pH 5 to pH 3.(7) The absorbing reagent is normally stable for 6 months. If a precipitate forms, discard the reagent.

6.2 An

6.2.1 Sulfamic Acid (0.6 percent). Dissolve 0.6 g. sulfamic acid in 100 ml. distilled water. Prepare fresh daily.

6.2.2 Formaldehyde (0.2 percent). Dilute 5 ml, formaldehyde solution (36–38 percent) to 1,000 ml. with distilled water. Prepare daily.

6.2.3 Stock Iodine Solution (0.1 N). Place 12.7 g. iodine in a 250-ml. beaker; add 40 g. potassium iodide and 25 ml. water. Stir until all is dissolved, then dilute to 1,000 ml. with distilled water.

6.2.4 Iodine Solution (0.01 N). Prepare approximately 0.01 N iodine solution by diluting 50 ml. of stock solution to 500 ml. with distilled water.

6.2.5 Starch Indicator Solution. Triturate 0.4 g. soluble starch and 0.002 g. mercuric iodide (preservative) with a little water, and add the paste slowly to 200 ml. boiling water. Continue boiling until the solution is clear; cool, and transfer to a glass-stoppered bottle.

6.2.6 Stock Sodium Thiosulfate Solution (0.1 N). Prepare a stock solution by dissolving 25 g. sodium thiosulfate (Na2S2O3.5H20) in 1,000 rnl. freshly boiled, cooled, distilled water and add 0.1 g. sodium carbonate to the solution. Allow the solution to stand 1 day before standardizing. To standardize, accurately weigh, to the nearest 0.1 mg., 1.5 g. primary standard potassium iodate dried at 180° C. and dilute to volume in a 500-ml. volumetric flask. To a 500-ml. iodine flask, pipet 50 ml. of iodate solution. Add 2 g. potassium iodide

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6.2.7 Sodium Thiosulfate Titrant (0.01 N). Dilute 100 ml. of the stock thiosulfate solu. tion to 1,000 ml. with freshly boiled distilled water. Normality=Normality of stock solution

x 0.100. 6.2.8 Standardized Sulfite Solution for Preparation of Working sulfite-TCM Solution. Dissolve 0.3 g. sodium metabisulfite (Na,S,03) or 0.40 g. sodium sulfite (Na Soz) in 500 ml. of recently boiled, cooled, distilled water. (Sulfite solution is unstable; it is therefore important to use water of the highest purity to minimize this instability.) This solution contains the equivalent of 320 to 400 ug./ml. of SO,. The actual concentration of the solution is determined by adding excess iodine and back-titrating with standard sodium thiosulfate solution. To back-titrate, pipet 50 ml, of the 0.01 N iodine into each of two 500-ml. iodine flasks (A and B). To flask A (blank) add 25 ml. distilled water, and to flask B (sample) pipet 25 ml. sulfite solution. Stopper the flasks and allow to react for 5 minutes. Prepare the working sulfite-TCM Solution (6.2.9) at the same time iodine solution is added to the flasks. By means of a buret containing standardized 0.01 N thiosulfate, titrate each flask in turn to a pale yellow. Then add 5 ml, starch solution and continue the titration until the blue color disappears.

6.2.9 Working Sulfite-TCM Solution. Pipet accurately 2 ml. of the standard solution into 2 100 ml volumetric flask and bring to mark with 0.04 M TCM. Calculate the concentration of sulfur dioxide in the working solution: ug SO,/mi- (A - B) (N) (32,000)

25 A=Volume thiosulfate for blank, ml. B=Volume thiosulfate for sample, ml.

N=Normality of thiosulfate titrant. 32,000=Milliequivalent wt. of SO2, Mg. 25=Volume standard sulfite solution,

ml. 0.02=Dilution factor. This solution is stable for 30 days if kept at 5° C. (refrigerator). If not kept at 5° C., prepare daily.

6.2.10 Purified Pararosaniline Stock Solution (0.2 percent nominal). Dye Specifications. The pararosaniline dye must meet the following performance specifications: (1) the dye must

have a wavelength of maximum absorbance at 540 nm, when assayed in a buffered solution of 0.1 M sodium acetate-acetic acid; (2) the absorbance of the reagent blank, which is temperature-sensitive (0.015 absorbance unit/°C), should not exceed 0.170 absorbance unit at 22° C. with a 1-cm. optical path length, when the blank is prepared according to the prescribed analytical procedure and to the specified concentration of the dye; (3) the calibration curve (Section 8.2.1) should have a slope of 0.030+0.002 absorbance units/ug. SO2 at this path length when the dye is pure and the sulfite solution is properly standardized. Preparation of Stock Solution. A specially purified (99–100 percent pure) solution of pararosaniline, which meets the above specifications, is commercially available in the required 0.20 percent concentration (Harleco*). Alternatively, the dye may be purified, a stock solution prepared and then assayed according to the procedure of Scaringelli, et al. (4)

6.2.11 Pararosaniline Reagent. To a 250ml. volumetric flask, add 20 ml. stock pararosaniline solution. Add an additional 0.2 ml. stock solution for each percent the stock assays below 100 percent. Then add 25 ml. 3 M phosphoric acid and dilute to volume with distilled water. This reagent is stable for at least 9 months.

7. Procedure.

7.1 Sampling. Procedures are described for short-term (30 minutes and 1 hour) and for long-term (24 hours) sampling. One can select different combinations of sampling rate and time to meet special needs. Sample volumes should be adjusted, so that linearity is maintained between absorbance and concentration over the dynamic range.

7.1.1 30-Minute and 1-Hour Samplings. Insert a midget impinger into the sampling system, Figure A1. Add 10 ml. TCM solution to the impinger. Collect sample at 1 liter/ minute for 30 minutes, or at 0.5 liter/minute for 1 hour, using either a rotameter, as shown in Figure Al, or a critical orifice, as shown in Figure Ala, to control flow. Shield the absorbing reagent from direct sunlight during and after sampling by covering the impinger with aluminum foil, to prevent deterioration. Determine the volume of air

B) (N) (32,000) x 0.02

*Hartmen-Leddon, 60th and Woodland Avenue, Philadelphia, PA 19143.

sampled by multiplying the flow rate by the time in minutes and record the atmospheric pressure and temperature. Remove and stopper the impinger. If the sample must be stored for more than a day before analysis, keep it at 5° C. in a refrigerator (see 4.2).

7.1.2 24-Hour Sampling. Place 50 ml. TCM solution in a large absorber and collect the sample at 0.2 liter/minute for 24 hours from midnight to midnight. Make sure no entrainment of solution results with the impinger. During collection and storage protect from direct sunlight. Determine the total air volume by multiplying the air flow rate by the time in minutes. The correction of 24-hour measurements for temperature 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 25ml. volumetric flask. Prepare a control solution by adding 2 ml. of working sulfite-TCM 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 to) 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 (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. SO2 in solution equals the concentration of the standard (Section 6.2.9) in ug. SO2/ml. times the ml. sulfite solution added (ug. SO2= ug./ml. SO2 X ml. added). A linear relationship should be obtained, and the y-inter 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 Bs. By 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 SO2 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

erence conditions of 25° C. and 760 mm. Hg. (On 24-hour samples, this may not be possible.)

P 298

760 t+273 Ve=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 con. centration of suifur dioxide in the sample:

(A-Ao) (103) (Bs) ug. SO2/m.3=



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 & 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). 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 ug./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 ug./m.3; 0.01 to 0.15 p.p.m. SO,). The concentration of So, in any standard atmosphere can be calculated as follows:

PX 108
C =

C = Concentration of SO2, ug./m.: at ref-

erence conditions. P =Tube permeation rate, ug./minute. Ra=Flow rate of dilution air, liter/minute

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

reference conditions. Sampling and Preparation of Cali. bration Curve. Prepare a series (usually six) of standard atmospheres containing SO, levels from 25 to 390 ig. So,/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 SO2 in ug./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 Bg.

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

9. Calculations.

9.1 Conversion of Volume. Convert the volume of air sampled to the volume at ref

A =Sample absorbance.
A.=Reagent blank absorbance.
103=Conversion of liters to cubic meters.
VR =The sample corrected to 25° C. and

760 mm. Hg, liters.
B. =Calibration factor, Mg./absorbance

D =Dilution factor.

For 30-minute and 1-hour samples,

D =1.

For 24-hour samples, D=10. 9.2.2 When SO2 gas standard atmospheres are used to prepare calibration curves, compute the sulfur dioxide in the sample by the following formula:

SO2, Mg./m.3=(A-Ao) XBg
A =Sample absorbance.
Ao=Reagent blank absorbance.
Bg= (See

9.2.3 Conversion of ug./m.s to p.p.m.=II desired, the concentration of sulfur dioxide may be calculated as p.p.m. SO, at reference conditions as follows:

p.p.m. so,=ug. So,/m.8 X 3.82 X 10+ 10. References. (1) West, P. W., and Gaeke, G. C., "Fixa

tion of Sulfur Dioxide as Sulfitomercurate III and Subsequent Colori. metric 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, Inter

science. (1948). (3) Lyles, G. R., Dowling, F. B., and Blanch

ard, 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).

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