United States
Environmental Protection
Agency
Environmental Monitoring
Systems Laboratory
Research Triangle Park NC 27711
Research and Development
EPA-600/S4-84-025 May 1984
Project Summary
A New Audit Method for EPA
Reference Method 6
R.K.M. Jayanty
A simple, inexpensive, and accurate
method for evaluating and/or auditing
sampling and analytical phases of
EPA Source Reference Method 6 was
developed. The method uses a known
amount of a chemical compound in the
form of a tablet or pill (or placed in a
capsule) to generate sulfur dioxide
(SO2) quantitatively by reaction with an
acid. The reaction takes place in a
compact glass impinger system that
can be taken to the field. The SOa
generated in test runs was collected and
analyzed using the Method 6 procedure.
The SO2 generation was quantitative
and recoveries were found to be 94 ± 5
percent. The time to complete the
reaction was less than 15 min at a flow
rate of 1 L/min, but the recommended
sampling time was 45 min. The tablets
prepared gravimetrically were found to
be stable over a 6-month period. The
between-laboratory results obtained
showed close agreement with the
expected concentrations based on
calculations from the stoichiometric
reaction. The estimates of repeatability
(or within-laboratory precision) were
±5.0 mg 95 percent of the time for the
two concentration levels tested. The
reproducibility (or between-laboratory
precision estimate) was within ±5.9 mg
at the low range and ±23.8 mg at the
high range about 95 percent of the
time. The maximum bias observed for
the method was 0.5 percent.
This Project Summary was developed
by EPA's Environmental Monitoring
Systems Laboratory, Research Triangle
Park. NC, to announce key findings of
the research project that is fully docu-
mented in a separate report of the same
title (see Project Report ordering
information at back).
Introduction
The U.S. Environmental Protection
Agency (EPA) is interested in developing
audit materials for all EPA source
reference methods. These audit materials
are used to conduct performance audits
during stationary source measurements.
EPA is currently providing liquid sulfate
standards as audit materials for EPA
Source Reference Method 6, which is
used to determine SOz emissions from
stationary sources. However, these liquid
sulfate audit standards are useful only for
evaluating the analytical phase of Method
6. Currently, sampling phase procedures
are evaluated only by a systems audit; i.e.,
someone observing the sampling proce-
dures in the field. Therefore, a method or
device that can be used as an audit
material for the evaluation of both the
sampling and analytical phases of the
source reference methods is needed.
Research Triangle Institute (RTI) was
contracted by the Quality Assurance
Division of EPA's Environmental Moni-
toring Systems Laboratory, Research
Triangle Park, North Carolina, to develop
a simple method or device for evaluating
and/or auditing both the sampling and
analytical phases of various source
reference methods in the field. The main
objectives of this study were to develop a
device for auditing both the sampling and
analytical phases of EPA Method 6 and to
establish its accuracy, precision, and
stability. The method uses a known
amount of a chemical compound (sodium
bisulfite or sodium sulfite) mixed with
an inert binder (polyvinyl pyrollidine) and
formed into a tablet or pill to generate 862
quantitatively by reaction with sulfuric
acid. The SOz generated in test runs was
collected and analyzed using the Method
6 procedure as published in the Federal
-------
Register (Vol. 42, No. 160, August 1977).
The audit method developed was also
evaluated at two concentration levels by
six different laboratories to determine its
repeatability, reproducibility, and accuracy.
The various activities performed during
the study and the results obtained are
discussed in this report.
Experimental Procedures
The preliminary experiments to gener-
ate 862 from the chemical reaction of
sodium bisulfite and sulfuric acid were
conducted using the experimental system
shown in Figure 1. Initially, a known
amount of sodium bisulfite was placed
in a three-way, stoppered, round-bottomed
flask and dilute sulfuric acid (10 percent)
was added slowly from a burette. Dry
nitrogen or air was bubbled through the
solution at a flow rate of 1 L/min to
sweep the released SO2 into a glass
manifold. Glass wool was used to remove
the aerosol vapors. The generated SO2
was monitored continuously with a SO2
source analyzer (TECO Model 40 fluores-
cent analyzer) to determine the time of
completion of the reaction. Within 10
minutes the reaction was completed. In
later experiments, the SOz gas generated
from the chemical reaction was collected
into an evacuated Tedlar bag. After 30
minutes of gas flow into the bag, the
contents of the bag were further diluted
with zero air and analyzed for SOz using
the source analyzer. The analyzer was
calibrated with a cylinder gas of SOz
referenced to the National Bureau of
Standards Standard Reference Materials.
The experiments were repeated using
various amounts of sodium bisulfite and
different concentrations of sulfuric acid.
The sampling train component parts,
except for the SOz gas generator, were
the same as those in the EPA Method 6
train. The sampling probe was not
required and hence not included in the
train. A modified midget impinger (Ace
Glass, Inc., Model #7544-35) was used
as an SOz generator in place of a test-site
stack sample to generate a gas sample
(SOz) from the chemical reaction.
The sample collection procedure was
followed as described in EPA Method 6
except for the following modifications.
The gas sample was drawn through the
SOz generator for 45 minutes. The tablet
generally took 15 minutes to dissolve
completely; however, the sample collection
was continued for an additional 30
minutes. Since ambient air was used to
draw the sample from the generator, a
blank run (without a tablet) was performed
for 45 minutes and impinger solutions
were analyzed.
Connected to an
Evacuated Tedlar
Bag or SO2
Monitor Intake
3-Way Round-
Bottomed Flask
Figure 1. Preliminary SOi generation system.
The impinger solutions were analyzed
after each run for sulfate (converted form
of SOz collected) by ion chromatography
instead of barium perchlorate-thorin
colorimetric titration as described in EPA
Method 6. However, it was established
initially that both analytical procedures
gave identical results.
Because the amount of sodium bisulfite
or sodium sulfite required to generate
source concentrations of SOz (—400- to
1,000-ppm levels) is so. small, an inert
binder (polyvinyl pyrollidine, PVP) was
used for mixing. The mixture was made in
the form of a tablet or pill. The tablets or
pills were made using either a KBr die
(commonly used to make KBr pellets for
infrared spectrophotometric studies) or a
tablet press. The tablets or pills were
made by adding to the die a layer of
binder, a weighed amount of sodium
bisulfite, and another layer of binder.
Several hundred pounds of pressure was
applied using a hydraulic press to form
the tablet. The PVP binder was found to
be inert and water-soluble and, because
the chemical compound was contained
between the two layers of binder, the
tablets could be handled safely. The
binder also helped reduce the rate of
reaction of SOz generation. Tablets were
made with varying amounts of sodium
bisulfite to generate different concentra-
tions of SO2.
Collaborative Testing
Procedures
From those laboratories having contracts
with EPA, six were selected for the
collaborative study based on their past
experience in source testing. They were
Engineering-Science, Inc., Entropy Envi-
ronmentalists, Inc., PEDCo Environmen-
tal, Inc., Radian Corporation, TRC-Envi-
ronmental Consultants, Inc., and TRW-
Environmental Engineering Division.
Seven tablets representing two concen-
tration levels were shipped to each of the
six laboratories for analyses. Of the seven
tablets, three were low concentration (60
mg SO2) and four were high concentra-
tion (120 mg SO2). The true concentra-
tions of the tablets were based on
theoretical concentrations calculated
from gravimetric preparations and stoi-
chiometric chemical reaction. A SOz
generator (modified midget impinger),
general instructions to perform the
experiments, and a data sheet for
reporting the results were also sent to
each laboratory. Each laboratory was
provided with the true concentration for
one tablet and was asked to analyze the
true concentration tablet first; if the
measured value were within ±10 percent,
they were to analyze the remaining six
tablets. The trial run was thought
necessary because most laboratories had
not used this method before. The labora-
tories were informed that these samples
were not for an audit but were for an
evaluation of the developed method. The
types of compounds in the tablets were
not disclosed.
Results and Discussion
The measured SOz concentrations
from the preliminary Tedlar bag experi-
ments and the corresponding expected
concentrations from the stoichiometric
chemical reaction are shown in Table 1.
The results show that the percent
difference obtained ranged from 20 to 25
percent low between the concentration of
SOz measured and the expected SOz
-------
concentration from the stoichiometric
reaction for various amounts of sodium
bisulfite. The SOz generation was also
found to be independent of flow rate
(varied 0. 1 to 1 .4 L/min) and the amount
of sulfuric acid. The sulfuric acid must be
slightly in excess of the amount of
sulfuric acid. The sulfuric acid must be
slightly in excess of the amount actually
required for completion of the reaction.
The lower measured SO: concentrations
were probably a result of the combination
of loss of SOz in the bag plus loss of wet
S02 on the glass system, but the actual
reasons are not known nor were they
determined. However, the percent differ-
ence was constant within experimental
error for various amounts of sodium
bisulfite.
The results obtained for different
amounts of sodium bisulfite (ranging
from 10 to 800 mg) using the described
method are shown in Table 2. As the
results indicate, the percent difference
between the measured concentration
and the expected concentration ranged
from 2 to 7 percent; i.e., SO2 recoveries
obtained were between 93 and 98
percent. Lower SOz recoveries were
obtained for small amounts of sodium
bisulfite (10- to 40-mg range). Similar
results were obtained for sodium sulfite.
However, the S02 recoveries were found
to be poor for sodium metabisulfite. Since
ambient air was used to draw the sample
from the generator, a blank run (without a
tablet) was performed to test the ambient
air. Analysis results showed a negligible
amount of SOz. The SOz generated from
the material was found to be independent
of flow rate and total volume of sample
collection.
A study was also performed to determ ine
the storage stability of the prepared
tablets over a 6-month period at room
temperature. The prepared tablets were
analyzed periodically. The results of this
study are shown in Table 3. These results
demonstrate that the tablets are stable
within experimental error over the 6-
month period. The stability study is still in
progress.
The results of the analyses of the
samples by the different laboratories and
the expected concentrations are shown
in Table 4. The analysis results, in
general, showed close agreement between
the expected values. For example, results
of analyses of the low concentration
tablets ranged from -4 to +7 percent
different from the expected value. Like-
wise, the results of analyses of the high
concentration tablets ranged from -5 to
+8 percent different from the expected
Table 1. Bag Results
Amount of Amount of SOz Amount of SOz
rVaHSOs expected" measured
(mg) Ippm) (ppm)
102.6 650 507
199.9 804 646
2OO.5 863 66O
2OO.6 8O1 604
202.9 869 676
2O1.4 799 652
207.5 1O03 794
2O2.3 842 • 712
202.5 739 593
4O1.9 944 • 721
"Calculated from stoichiometric chemical reaction.
*Prm>nt rtiffevt-nff = lOO>r Measured SOz concentrate - Expected SOz concentrate
Expected SOz concentrate
Table 2. Results of SOz Generated from the Prepared Tablets* b
Weight of Amount of SOz ' Amount of SOz
NaHSOa expected measured
(mg) (mgf (mg)
12.9 7.9 . 5.2
4O.1 24.6 21.9
50.6 31.1 31.3
10O.4 61.8 57.6
101.6 62.5 61.5
102.9 63.3 59.6
202.9 124.9 120.3
401.9 247.3 235. 1
802.3 493.7 464.4
114.4" 38.5 23.9
204.3' 103.8 105. 1
'Flow rate 1 L/min for 45 minutes.
"3 to 5 mL of 10 percent HzSO* added.
^Calculated from stoichiometric chemical reaction.
"Percent riiff prince =inn * Amount of SOz measured - Amount of SOz expected
Amount of SOz expected
"Sodium metabisulfite was used.
'Sodium sulfite was used.
Table3. Stability Study Results
Amount of SOz Amount of SOi
No. of expected measured
days (mg) (mg)'
0 122.7 121.9
15 123. 1 125.5
29 123.0 124.9
48 123.2 126.8
68 121.7 124*
94 122. 1 126S
180 121.5 125.3
'Percent difference - 10O ..Amount ^oOzmeasured- Amount of SOzexpected
Percent
difference"
-21.9
-19.7
-23.5
-24.5
-22.2
-18.4
-20.8
-15.4
-19.8
-23.6
Percent
difference"
-34.2
-11.0
+0.6
-6.8
-2.0
-5.8
-3.7
-2.0
-5.9
-37.9
+1.3
Percent
difference"
-0.65
+1.95
+ 1.54
+2.90
+2.20
+3.55
+3.13
Amount of SOz expected
value. Two values reported from two Conclusions
laboratories were discarded during the
statistical analysis because the labora- A new audit method that is simple and
lories identified problems during the SOz inexpensive was developed for evaluating
generation. Blank values reported by the both the sampling and analytical phases
laboratories were also negligible. of EPA Method 6 in the field. The SOz
-------
generation was found to be quantitative
and the recoveries were found to be 94 ±
5 percent over the 40- to 800-mg range of
sodium bisulfite. The method was also
evaluated at two concentration levels
with six different laboratories to determine
its repeatability, reproducibility, and bias.
The interlaboratory results obtained
showed close agreement with the expected
concentrations based on calculation from
the stoichiometric reaction. The repeata-
bility within a laboratory was found to be
within ±5.0 mg 95 percent of the time for
the two concentration levels tested. The
reproducibility between laboratories was
within ±5.9 mg at the low range and
±23.8 mg at the high range about 95
percent of the time. The maximum bias
observed was 0.5 percent by comparison
with the expected concentrations. The
SOa generation from the material was
found to be independent of flow rate and
total volume, and the prepared tablets
were stable at room temperature over a
6-month period. It is recommended that a
performance audit using this new audit
method for EPA Method 6 be used during
future source SC>2 emissions analyses as
a routine quality assurance procedure.
Table 4. Interlaboratory Results (mg)
Lab 1
RV
EV
Lab 2
RV
EV
Lab 3
RV
EV
Lab 4
RV
EV
Lab 5
RV
EV
Lab 6
RV
EV
Known
119.7
121.8
126.5
122.2
114.2
122.2
123.2
122.7
116.1
121.8
116
122.4
1
59.9
60.3
64.5
60.3
a
-
63.0
60.5
60.1
59.9
58.3
60.9
Low
2
59.1
60.4
61.5
60.2
58.2
60.7
62.3
604
60.7
60.0
a
-
3
60.4
60.3
57.9
60.4
58.1
60.7
61.9
60.5
62.0
603
60.2
60.8
1
119.0
122.2
129.4
122.0
118.7
121.9
128.4
122.4
1167
122.1
117.0
122.4
High
2
119.9
122.3
128.0
121.8
115.8
122.1
124.9
122.6
119.6
122.1
118.0
122.3
3
122.6
122.1
131.7
121.9
115.6
122.0
1256
1225
120.5
122.2
118.0
122.3
RV - Report value.
EV = Expected value.
"Values discarded.
R. K. M. Jayanty is with Research Triangle Institute, Research Triangle Park, NC
27709.
Robert G. Fuerst is the EPA Task Manager (see below).
The complete report, entitled"A New Audit Method for EPA Reference Method 6,"
(Order No. PB84-172 097; Cost: $8.50, subject to change) will be available only
from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Task Manager can be contacted at:
Environmental Monitoring Systems Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, NC 27711
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