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_Chlorine Sampling Techniques _
By: John Hardin
The OSHA sampling and analytical method for chlorine is discussed in reference
9.1. The principle of sampling is described in reference 9.2. The analytical
procedure is based on an iodometric technique which uses a residual chlorine
electrode (RCE) for detection (9.3.). The validation of the chlorine method
consisted of the following experimental studies: Analysis of a total of
eighteen samples (six samples at each of the three test levels) which were
prepared by adding known amounts of standardized chlorine solution to 0.1%
sulfamic acid collecting solution. Analysis of a set of eighteen samples (six
samples at each of the three test levels) collected from dynamically generated
test atmospheres at approximately 0.5, 1, and 2 times the OSHA Permissible
Exposure Limit (PEL) of 1 ppm. Determination of the collection efficiency of
the 0.1% sulfamic acid sampling solution. Determination of potential
breakthrough when sampling. Determination of the storage stability over 30
days for collected samples. Comparison with an independent method. Assessment
of the method. The details with respect to each of these items are discussed
in the following sections. All experiments were based on the ability of the
method to collect and analyze a 15-L air sample for each concentration tested.
The sample preparation and analytical technique used during the method
evaluation follow that described in reference 9.4. A revised method (9.1.) is
also available.
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- Note: The revised version of the method (9.1.) differs in one major point
from reference 9.4.: Preparation of samples and standards for analysis. It is
now recommended to prepare samples and standards in 0.1% sulfamic acid
solutions rather than the deionized water previously used (9.5.).
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- 1. Generation A dynamic standard generator (Model 350, Analytical Instrument
Development Inc., Avondale, PA) was used for generating test atmospheres of
chlorine. The system is further described in reference 9.6. Two chlorine
permeation vials were used as the chlorine source. The permeation vials were
maintained at a constant temperature of 30 ± 0.1 °C. The chlorine
concentration was determined from the weight loss of the permeation vial over
a measured time period and was calculated according to the equation listed in
Appendix A. After the chlorine was produced it was then diluted with tempered
air so that a controlled concentration at 80% RH and 25 °C was achieved. Flow
rates for the generation system were measured with a soap bubble flow meter to
accurately determine dilution and the final concentration of the gas. Samples
were taken from a glass sampling manifold which was attached to the generation
system. The majority of samples were collected in sets of six at one of three
concentrations, about 0.5, 1, or 2 ppm chlorine. 2. Analysis Procedure:
Samples containing 0.1% sulfamic acid were spiked with standardized chlorine
solutions. This test determined the precision and accuracy of the analysis
portion of the method. Results: The results are shown in Table 1. Average
analytical recovery was 96.7% and the coefficient of variation (CV1) was 0.03
for the three sets of spiked samples. 3. Sampling and Analysis Procedure:
Samples were collected in 0.1% sulfamic acid solutions at three different
chlorine concentrations using the generation system described in Section 1.
Midget fritted glass bubblers (MFGB) were used to disperse the air samples in
the sulfamic acid solutions. This test determined the precision and accuracy
of the sampling and analysis portion of the method. Results: Sampling and
analysis data are presented in Table 2. The NIOSH Statistical Protocol (9.7.)
developed for evaluation of methods under the Standards Completion Program
(SCP) was used as a guide for the determination of precision and accuracy from
this data. The precision for this method using these statistical procedures
is: CV1 = 0.03 CV2 = 0.03 CVT = 0.03 The average recovery for sampling and
analysis over all levels was 99.4%. Any variation from 100% recovery was
probably related to difficulties in generating the atmosphere containing the
analyte at a given concentration rather than a true bias in the method. The
Bartlett's test for homogeneity of variances and an outlier test (9.7.) were
applied to this data. The sampling and analysis data passed the Bartlett's
test, indicating the CVs could be pooled for all sets of generated samples.
One data point in the third set (2 × PEL, n=5) was rejected as an outlier. 4.
Collection Efficiency Procedure: The collection efficiency (CE) of MFGBs
containing 0.1% sulfamic acid was assessed. A chlorine concentration of
approximately 2 ppm was generated and samples were collected in series using
the MFGBs. A flow rate of 1 L/min and a 15-min sampling period were used. The
amount of chlorine collected in each of the two bubblers connected in series
was measured. Results: The results are shown in Table 3. The CE of the single
bubbler was determined to be 1.00; therefore, a recovery correction or
sampling train is not necessary. 5. Breakthrough Procedure: Breakthrough is
defined as the time the effluent concentration in a second bubbler (containing
0.1% sulfamic acid) connected in series reaches 5% of the concentration of the
test gas mixture. A test for breakthrough was conducted at about 2 ppm and at
a 1 L/min sample collection flow rate. A sample set consisting of two bubblers
in series was used to collect samples for each time interval listed: 15, 30,
60, 120, 180, and 240 min. Results: No breakthrough occurred at this flow
rate, during these sampling times, or at the concentration tested. The results
are given in Table 4. 6. Storage Stability Procedure: A study was conducted to
assess whether chlorine samples collected at the 1 ppm PEL could be
successfully stored in the 0.1% sulfamic acid collection solution. After
sample collection using MFGBs, samples were transferred into 20-mL vials,
capped with Teflon-lined caps, and stored at ambient laboratory temperatures
in a dark environment for 1, 5, 15, or 30 days before analysis. Results:
Samples analyzed after 30 days were found to be within 3% of those analyzed
immediately. Thus, storage does not represent a source of bias that would need
to be corrected in the method. Storage stability data are presented in Table
5. 7. Independent Method (Analytical) An independent volumetric method (9.8.)
involving a thiosulfate titration was used to determine the concentration of
chlorine used for spiking. Chlorine gas was generated at a theoretical
concentration which would give a chlorine concentration of 21.70 µg/mL in a
specified volume of sulfamic acid. This value was calculated using the
equation given in Appendix A. The gas was collected in 0.1% sulfamic acid
solution which was then used as a chlorine stock solution for spiked samples
(Section 2.). All other samples were taken using the generation system.
Analyses of the stock solution were performed using the RCE and the
thiosulfate titration methods. The following results were obtained: Method
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- Chlorine (µg/mL)
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- Titration 20.45 RCE 20.92 The average value of 20.69 µg/ml from both methods
was used as the stock solution concentration for the spiked samples mentioned
in Section 2. 8. Conclusions This sampling and analytical method has been
shown to be precise and accurate at exposures near the OSHA PEL of 1 ppm when
using 15-L air volumes. Breakthrough or storage stability do not pose
significant problems under the conditions tested. The development and
evaluation of this method took place when the OSHA PEL for chlorine was a
Ceiling value (15-min samples were taken). Although long-term samples were
only taken during breakthrough studies, the data appears to indicate the
method is capable of taking samples for chlorine TWA assessments. Two 4-h
samples are recommended for TWA determinations of chlorine. When considering
the amount of chlorine collected (as mass), any samples taken near the TWA PEL
of 0.5 ppm will have slightly larger mass concentrations of chlorine than
those tested for this evaluation. In addition, during the evaluation no
breakthrough was noted after 240 minutes of sampling and collection efficiency
was excellent after 15 minutes of sampling. Both of these experiments were
conducted at approximately 4 times the TWA PEL of 0.5 ppm. 9. References 9.1.
Occupational Safety and Health Administration Technical Center: Chlorine in
Workplace Atmospheres by S. Edwards and J. Ku (USDOL/OSHA-SLTC Method No.
ID-101). Salt Lake City, UT. Revised 1991. 9.2. Takeuari, J. et al.:
Measurement of Chlorine in the Atmosphere. Osen Kenkyu (Air Pollution
Research) 9(2): 210 (1974). 9.3. Orion Research, Inc.: Residual Chlorine
Electrode, Model 9970 Instruction Manual. Cambridge, MA: Orion Research, Inc.,
1976. 9.4. Occupational Safety and Health Administration Analytical
Laboratory: OSHA Analytical Methods Manual (USDOL/OSHA-SLCAL Method No.
ID-101). Cincinnati, OH: American Conference of Governmental Industrial
Hygienists (Pub. No. ISBN: 0-936712-66-X), 1985. 9.5. Rando, R.J., and Y.Y.
Hammad: A Diffusive Sampler for Gaseous Chlorine Utilizing an Aqueous Sulfamic
Acid Collection Medium and Specific Ion Electrode Analysis. Appl. Occup.
Environ. Hyg. 5(10): 700-706 (1990). 9.6. Analytical Instrument Development,
Inc.: Model 350 Standards Generator Operation and Service Manual. Avondale,
PA: Analytical Instrument Development, Inc., 1977. 9.7. National Institute for
Occupational Safety and Health: Documentation of the NIOSH Validation Tests by
D. Taylor, R. Kupel and J. Bryant (DHEW/NIOSH Pub. No. 77-185). Cincinnati,
OH: National institute for Occupational Safety and health, 1977. pp. 7-11.
9.8. American Society for Testing and Materials: Standard Recommended
Practices for Apparatus, Reagents, and Safety Precautions for Chemical
Analysis of Metals (Annual Book of ASTM Standards, Part 12, E-50).
Philadelphia, PA: American Society for Testing and Materials, 1978.
_Bibliography _
9.8. American Society for Testing and Materials: Standard Recommended
Practices for Apparatus, Reagents, and Safety Precautions for Chemical
Analysis of Metals (Annual Book of ASTM Standards, Part 12, E-50).
Philadelphia, PA: American Society for Testing and Materials, 1978.
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