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Store the sample at 5° +5° C until it is analyzed.

10.0 Analytical Calibration.

10.1 Spectrophotometer Cell Matching. If unmatched spectrophotometer cells are used, an absorbance correction factor must be determined as follows:

1. Fill all cells with distilled water and designate the one that has the lowest absorbance at 548 nm as the reference. (This ref. erence cell should be marked as such and continually used for this purpose throughout all future analyses.)

2. Zero the spectrophotometer with the reference cell.

3. Determine the absorbance of the remaining cells (Ac) in relation to the reference cell and record these values for future use. Mark all cells in a manner that adequately identifies the correction.

The corrected absorbance during future analyses using each cell is determining as follows: A=Abo-Ac (10) where: A=corrected absorbance, Abs=uncorrected absorbance, and Ac=cell correction.

10.2 Static Calibration Procedure (Option 1). Prepare a dilute working sulfiteTCM solution by diluting 10 mL of the working sulfite-TCM solution (Section 8.2.11) to 100 mL with TCM absorbing reagent. Following the table below, accurately pipet the indicated volumes of the sulfiteTCM solutions into a series of 25-mL volumetric flasks. Add TCM absorbing reagent as indicated to bring the volume in each flask to 10 mL.

(during the 30-minute period) in a temperature environment in the range of 20° to 30° C, which is controlled to +1° C. For increased precision, a constant temperature bath is recommended during the color development step. After 30 minutes, determine the corrected absorbance of each standard at 548 nm against a distilled water reference (Section 10.1). Denote this absorbance as (A). Distilled water is used in the reference cell rather than the reagant blank because of the temperature sensitivity of the reagent blank. Calculate the total micrograms SO, in each solution: mg SO,=V TCM/502 X CTCM/S02XD (11) where: VTCM/so3=volume of sulfite-TCM solution

used, mL; CTCM/502=concentration of sulfur dioxide in

the working sulfite-TCM, ug SO,/mL

(from equation 4); and D=dilution factor (D=1 for the working

sulfite-TCM solution; D=0.1 for the diluted working sulfite-TCM solution). A calibration equation is determined using the method of linear least squares (Section 12.1). The total micrograms SO, contained in each solution is the x variable, and the corrected absorbance (eq. 10) associated with each solution is the y variable. For the calibration to be valid, the slope must be in the range of 0.030 +0.002 absorbance unit/ ug SO2, the intercept as determined by the least squares method must be equal to or less than 0.170 absorbance unit when the color is developed at 22° C (add 0.015 to this 0.170 specification for each °C above 22° C) and the correlation coefficient must be greater than 0.998. If these criteria are not met, it may be the result of an impure dye and/or an improperly standardized sulfiteTCM solution. A calibration factor (B) is determined by calculating the reciprocal of the slope and is subsequently used for calculating the sample concentration (Section 12.3).

10.3 Dynamic Calibration Procedures (Option 2). Atmospheres containing accurately known concentrations of sulfur dioxide are prepared using permeation devices. In the systems for generating these atmospheres, the permeation device emits gaseous SO, at a known, low, constant rate, provided the temperature of the device is held constant ( +0.1° C) and the device has been accurately calibrated at the temperature of use. The SO, permeating from the device is carried by a low flow of dry carrier gas to a mixing chamber where it is diluted with SO2-free air to the desired concentration and supplied to a vented manifold. A typical system is shown schematically in Figure 4 and this system and other similar systems have been described in detail by O'Keeffe

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and Ortman; (19) Scaringelli, Frey, and Saltzman, (20) and Scaringelli, O'Keeffe, Rosenberg, and Bell. (21) Permeation de vices may be prepared or purchased and in both cases must be traceable either to a National Bureau of Standards (NBS) Standard Reference Material (SRM 1625, SRM 1626, SRM 1627) or to an NBS/EPA-approved commercially available Certified Reference Material (CRM). CRM's are described in Reference 22, and a list of CRM sources is available from the address shown for Reference 22. A recommended protocol for certifying a permeation device to an NBS SRM or CRM is given in Section 2.0.7 of Reference 2. Device permeation rates of 0.2 to 0.4 ug/min, inert gas flows of about 50 mL/min, and dilution air flow rates from 1.1 to 15 L/ min conveniently yield standard atmospheres in the range of 25 to 600 mg SO2/m' (0.010 to 0.230 ppm).

10.3.1 Calibration Option 2A (30-minute and 1-hour samples); Generate a series of

(12)

where: Co=concentration of SO, at standard condi

tions, ug/m; P,=permeation rate, ug/min; Qa=flow rate of dilution air, std L/min; and Qo=flow rate of carrier gas across perme

ation device, std L/min.

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VoxC Cox Q. x 10**

(14)

Be sure that the total flow rate of the standard exceeds the flow demand of the sample train, with the excess flow vented at atmospheric pressure. Sample each atmosphere using similar apparatus as shown in Figure 1 and under the same conditions as field sampling (i.e., use same absorbing reagent volume and sample same volume of air at an equivalent flow rate). Due to the length of the sampling periods required, this method is not recommended for 24hour sampling. At the completion of sampling, quantitatively transfer the contents of each impinger to one of a series of 25-mL volumetric flasks (if 10 mL of absorbing solution was used) using small amounts of distilled water for rinse (<5mL). If > 10 mL of absorbing solution was used, bring the absorber solution in each impinger to orginal volume with distilled H,O and pipet 10-mL portions from each impinger into a series of 25-mL volumetric flasks. If the color development steps are not to be started within 12 hours of sampling, store the solutions at 5° + 5° C. Calculate the total micrograms SO2 in each solution as follows:

where: t=sampling time, min; Vo=volume of absorbing solution used for

sampling (50 mL); Cr=desired concentration of SO, in the ab

sorbing solution, ug/mL; Ca=concentration of the standard atmos

phere calculated according to equation

12, ug/m3; and Q, =sampling flow rate, std L/min.

At the completion of sampling, bring the absorber solutions to original volume with distilled water. Pipet a 10-ml portion from each absorber into one of a series of 25-mL volumetric flasks. If the color development steps are not to be started within 12 hours of sampling, store the solutions at 5° +5° C. Add the remaining reagents for color development in the same manner as in Section 10.2 for static solutions. Calculate the total ug SO2 in each standard as follows:

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(13)

(15)

where: C,=concentration of SO, in the standard at

mosphere, ug/m3; 0,=sampling flow rate, std L/min; t=sampling time, min; Vn=volume of absorbing solution used for

color development (10 mL); and Vo=volume of absorbing solution used for

sampling, mL. Add the remaining reagents for color development in the same manner as in Section 10.2 for static solutions. Calculate a calibration equation and a calibration factor (BR) according to Section 10.2, adhering to all the specified criteria.

10.3.2 Calibration Option 2B (24-hour samples): Generate a standard atmosphere containing approximately 1,050 ug SO2) mo and calculate the exact concentration according to equation 12. Set up a series of six absorbers according to Figure 2 and connect to a common manifold for sampling the standard atmosphere. Be sure that the total flow rate of the standard exceeds the flow demand at the sample manifold, with the excess flow vented at atmospheric pressure. The absorbers are then allowed to sample the atmosphere for varying time periods to yield solutions containing 0, 0.2, 0.6, 1.0, 1.4, 1.8, and 2.2 ug SO2/mL solution. The sampling times required to attain these solution concentrations are calculated as follows:

where: V,=volume of absorbing solution used for

color development (10 mL). All other parameters are defined in equa

tion 14. Calculate a calibration equation and a calibration factor (B) according to Section 10.2 adhering to all the specified criteria. 11.0 Sample Preparation and Analysis.

11.1 Sample Preparation. Remove the samples from the shipping container. If the shipment period exceeded 12 hours from the completion of sampling, verify that the temperature is below 10° C. Also, compare the solution level to the temporary level mark on the absorber. If either the temperature is above 10° C or there was significant loss (more than 10 mL) of the sample during shipping, make an appropriate notation in the record and invalidate the sample. Prepare the samples for analysis as follows:

1. For 30-minute or 1-hour samples: Quantitatively transfer the entire 10 mL amount of absorbing solution to a 25-mL volumetric flask and rinse with a small amount (<5 mL) of distilled water.

2. For 24-hour samples: If the volume of the sample is less than the original 50-ml volume (permanent mark on the absorber), adjust the volume back to the original and disposed of using one of the procedures contained in Section 13. Until disposal, the discarded solutions can be stored in closed glass containers and should be left in a fume hood.

12.0 Calculations.

12.1 Calibration Slope, Intercept, and Correlation coefficient. The method of least squares is used to calculate a calibration equation in the form of: y=mx+b

(16) where: y=corrected absorbance, m=slope, absorbance unit/ug SO2. X=micrograms of SO2. b=y intercept (absorbance units).

The slope (m), intercept (b), and correlation coefficient (r) are calculated as follows:

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volume with distilled water to compensate for water lost to evaporation during sampling. If the final volume is greater than the original volume, the volume must be measured using a graduated cylinder. To analyze, pipet 10 mL of the solution into a 25-ML volumetric flask.

11.2 Sample Analysis. For each set of determinations, prepare a reagent blank by adding 10 mL TCM absorbing solution to a 25-mL volumetric flask, and two control standards containing approximately 5 and 15 mg SO2, respectively. The control standards are prepared according to Section 10.2 or 10.3. The analysis is carried out as follows:

1. Allow the sample to stand 20 minutes after the completion of sampling to allow any ozone to decompose (if applicable).

2. To each 25-mL volumetric flask contain: ing reagent blank, sample, or control standard, add 1 mL of 0.6% sulfamic acid (Section 8.2.1) and allow to react for 10 min.

3. Accurately pipet 2 mL of 0.2% formaldehyde solution (Section 8.2.2) and then 5 mL of pararosaniline solution (Section 8.2.13) into each flask. Start a laboratory timer set at 30 minutes.

4. Bring each flask to volume with recently boiled and cooled distilled water and mix thoroughly.

5. During the 30 minutes, the solutions must be in a temperature controlled envi. ronment in the range of 20° to 30° C maintained to † 1° C. This temperature must also be within 1° C of that used during calibration.

6. After 30 minutes and before 60 minutes, determine the corrected absorbances (equation 10) of each solution at 548 nm using 1. cm optical path length cells against a distilled water reference (Section 10.1). (Distilled water is used as a reference instead of the reagent blank because of the sensitivity of the reagent blank to temperature.)

7. Do not allow the colored solution to stand in the cells because a film may be deposited. Clean the cells with isopropyl alcohol after use.

8. The reagent blank must be within 0.03 absorbance units of the intercept of the cali. bration equation determined in Section 10.

11.3 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 the absorbance redetermined within 5 minutes. Solutions with higher absorbances can be diluted up to sixfold with the reagent blank in order to obtain scale readings of less than 1.0 absorbance unit. However, it is recommended that a smaller portion (<10 mL) of the original sample be reanalyzed (if possi. ble) if the sample requires a dilution greater than 1:1.

11.4 Reagent disposal. All reagents containing mercury compounds must be stored

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