EPA-600/4-76-024
May 1976
Environmental Monitoring Series
EFFECT OF TEMPERATURE ON STABILITY OF
SULFUR DIOXIDE SAMPLES COLLECTED BY THE
FEDERAL REFERENCE METHOD
Environmental Monitoring and Support Laboratory
Office of Research and Development
U.S. Environmental Protection Agency
Research Triangle Park, North Carolina 27711
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RESEARCH REPORTING SERIES
Research reports of the Office of Research and Development, U.S. Environmental
Protection Agency, have been grouped into five series. These five broad
categories were established to facilitate further development and application of
environmental technology. Elimination of traditional grouping was consciously
planned to foster technology transfer and a maximum interface in related fields.
The five series are:
1. Environmental Health Effects Research
2. Environmental Protection Technology
3. Ecological Research
4. Environmental Monitoring
5. Socioeconomic Environmental Studies
This report has been assigned to the ENVIRONMENTAL MONITORING series.
This series describes research conducted to develop new or improved methods
and instrumentation for the identification and quantification of environmental
pollutants at the lowest conceivably significant concentrations. It also includes
studies to determine the ambient concentrations of pollutants in the environment
and/or the variance of pollutants as a function of time or meteorological factors.
This document is available to the public through the National Technical Informa-
tion Service. Springfield, Virginia 22161.
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EPA-600/4-76-024
May 1976
EFFECT OF TEMPERATURE
ON
STABILITY OF SULFUR DIOXIDE SAMPLES COLLECTED
BY THE FEDERAL REFERENCE METHOD
by
Robert G. Fuerst, Frank P. Scaringelli, and John H. Margeson
Quality Assurance Branch
Environmental Monitoring and Support Laboratory
Research Triangle Park, North Carolina 27711
U.S. ENVIRONMENTAL PROTECTION AGENCY
OFFICE OF RESEARCH AND DEVELOPMENT
ENVIRONMENTAL MONITORING AND SUPPORT LABORATORY
RESEARCH TRIANGLE PARK, NORTH CAROLINA 27711
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DISCLAIMER
This report has been reviewed by the Environmental Monitoring
and Support Laboratory, U.S. Environmental Protection Agency, and
approved for publication. Mention of trade names or commercial
products does not constitute endorsement or recommendation for use.
11
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ABSTRACT
This report describes an evaluation of the effect of temperature on
the stability of collected samples according to the Environmental Protection
Agency (EPA) reference procedure for measurement of ambient sulfur dioxide.
3
This evaluation was carried out over the range of 35 to 278 yg SOp/m .
Collected samples decay at a critical temperature-dependent rate. The
rate of decay increases five-fold for every 10°C increase in temperature
over the range 20 to 40°C. The rate of decay is independent of concen-
tration over the range studied, and the decay reaction follows first-order
kinetics.
At 20, 30, 40 and 50°C (68, 86, 104 and 122°F) S02 is lost at the rate
of 0.9, 5.0, 25, and 74 percent per day, respectively.
A mathematical model was developed that allows sample decay to be
calculated if the temperature history of the sample is known.
Temperature specifications, and changes in the procedure necessary to
eliminate the decay problem are proposed.
in
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CONTENTS
Page
Abstract iii
List of Figures v
List of Tables vi
Acknowledgments vii
I Introduction 1
II Experimental 3
III Results and Discussion 5
IV Conclusions 19
V Recommendations 20
VI References 22
Technical Report Data Sheet 23
IV
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LIST OF FIGURES
Number Page
1. Effect of Temperature on Stability of Dichlorosulfitomercurate
Complex, 50°C 6
2. Effect of Temperature on Stability of Dichlorosulfitomercurate
Complex, 40°C 7
3. Effect of Temperature on Stability of Dichlorosulfitomercurate
Complex, 30°C 8
4. Effect of Temperature on Stability of Dichlorosulfitomercurate
Complex, 20°C 9
5. Determination of Order and Rate of Reaction 13
6. Effect of Temperature on Decay Rate Constant of Dichlorosulfito-
mercurate Complex 15
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LIST OF TABLES
Number Page
1. Effect of Temperature on Percent Decay Per Day . . 12
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ACKNOWLEDGMENTS
The authors wish to thank Dr. Joseph E. Knoll of the Quality Assurance
Branch, Environmental Monitoring and Support Laboratory, U.S. Environmental
Protection Agency, Research Triangle Park, North Carolina for helpful
discussions during this project.
Vll
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SECTION I
INTRODUCTION
On April 30, 1971, the Environmental Protection Agency (EPA) promulgated
in the Federal Register the national primary and secondary air quality
standards for sulfur dioxide (S02). At the same time, the reference method
for the determination of ambient concentrations of S02 was also described.
2
Previous research on the S02 reference method indicated that a stable
complex was formed when SOp in the ambient air was collected in a solution of
potassium tetrachloromercurate and subsequently analyzed for S02 concentration
with formaldehyde and pararosaniline. The complex formed, dichlorosulfito-
o
mercurate (S02-TCM), had suspected thermal instability (1 percent per day at
22°C), but sample refrigeration at 5°C resulted in no decay for up to 30 days.
4 5
With the completion of two collaborative tests ' of the SOp reference
method, no indication of any major effect of thermal instability was found.
This was due mostly to the experimental design of the tests, which did not
address itself to elevated temperatures of sampling, shipment, and storage
of SOp-TCM samples prior to analysis.
At present, it is a common practice to sample for ambient concentrations
of S02 in a thermostated sampling box (35°C) and then to transport the collected
24-hour S02 samples back to the laboratory for analysis in a non-temperature
controlled container. This could involve either direct transportation of
samples to the laboratory by staff personnel or transportation by the U.S. Postal
Service. Upon receipt at the laboratory, the samples are stored either in the
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dark at room temperature, or in a refrigerator. Thus, prior to analysis,
the collected samples are exposed to a variety of temperature conditions
for various lengths of time. Therefore, it became necessary to define the
effect of temperature and time on the stability of SCL-TCM samples, since
it had never before been investigated at elevated temperatures.
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SECTION II
EXPERIMENTAL
The specific reagents and procedures used for sample preparation and
analysis are those specified in the Federal Register for the determination
of S02 in the atmosphere (Pararosaniline Method). Other pertinent experi-
mental details not contained in that procedure are described in the following
sections:
SAMPLE PREPARATION
Sufficient quantities of five concentrations of S02 in TCM (0, 0.2, 0.4,
0.8, and 1.6 yg SO^/ml) were prepared by dilution of an SO^-TCM stock standard
prepared from sodium metabisulfite. Four sets of the five different concen-
trations were prepared by filling (~ 80 ml) each 100-ml polypropylene centrifuge
tube with a specific concentration and capping it with a polypropylene tube
closure. This is the same tube and closure recommended for use when sampling
for S02 in the ambient air for 24 hours.
TEMPERATURE CONTROL
Each set of tubes was placed in a constant temperature bath. Each bath
was thermostated at either 20, 30, 40 or 50°C. The specific temperature was
maintained within + 0.1°C. Each tube was submerged below the solution volume
line.
SAMPLE ANALYSIS .
After selected periods of time, portions of the thermostated samples were
taken, and rapidly cooled to 20°C in a constant temperature bath to prevent
further decay before analysis. Aliquots of these portions were analyzed
immediately for S02 concentration by the S02 reference method. This temperature
(20°C) was also the temperature used for dye formation in the analysis part of
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the procedure.
DATA ANALYSIS
Best fit regression analyses and equations were derived using a
programmable calculator-plotter.
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SECTION III
RESULTS AND DISCUSSION
In sampling and analysis of ambient concentrations of S0~ by the
Federal Register method, there were two areas where temperature and length
of exposure to various temperatures needed investigation. These areas were:
(1) the effect of temperature on the ability to collect a valid sample and
(2) the effect of temperature on the stability of the collected sample. We
chose the second area for our investigations, assuming that if the decay is
substantial at higher temperatures, we would need to define it statically
before any investigation of the dynamic collection system could be attempted.
This report then discusses only area two.
To define the effect of temperature on the determination of ambient
concentrations of S0£ collected in 0.04 M TCM, we needed to investigate four
areas: (1) the reaction rate at which the S02-TCM complex is decaying with
time at a specific ambient temperature, (2) the reaction order indicating
how the reaction rate varies with SOp concentration, or with another species
present, (3) a general outcome (decay) when any concentration of 50^ in TCM
is exposed to elevated ambient temperatures, and (4) a method to estimate the
effect of sample exposure to varying temperature.
RATE OF REACTION
. The rate of reaction (decay) of the SO^-TCM complex was determined by
analyzing aliquots of the thermostated samples for S02 concentration by
the SOo reference method, at various periods of time, and then plotting the
data to derive a best fit regression equation (Figures 1 to 4). Using this
technique the slope of the curve described the rate of decay. The general
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0.3
0.6 o.g
TIME, days
1.2
1.5
Figure 1. Effect of temperature on stability of dichlorosulfitomercurate
complex, 50°C.
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Figure 2. Effect of temperature on stability of the dichlorosulfitomercu-
rate complex, 40°C.
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1.6
oo
TIME, days
Figure 3. Effect of temperature on stability of dichlorosulfitomercurate
complex, 30°C.
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2.0
CM
o
C/l
2
<
cc
o
o
1.2
k = 0.009
14
21
TIME, days
28
35
Figure 4. Effect of temperature on stability of dichlorosulfitomercurate
complex, 20°C.
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equation best fitting these data is described by an exponential curve
of the formula
C = Ae'kt (1)
where
C = concentration measured, yg S02/ml
A = a constant, yg SCL/ml
k = rate of decay, day"
t = time, day
If we assume no decay has taken place at t = o(k = o), then C = A, which
defines A as the initial concentration, C , and the equation
C = CQe"kt (2)
will describe the data.
Comparing the rates of decay at each temperature (20, 30, 40 and 50°C),
we found the rate of decay at a given temperature to be independent of concen-
tration in the range of 0.2 to 1.6 yg S02/ml. This represents a 24-hour S02
sample concentration of 35 to 278 yg S02/m collected at a sample flow rate of
0.2 1/min.
The rate of decay of the 0.2 yg S02/ml sample at 50°C differs slightly
from the other rates of decay at that temperature. This effect is also
evident at the other temperatures, and is probably due to poor precision in
the measurements at this low concentration. This is evident when one considers
that the minimum sensitivity is approximately 0.14 yg S02/ml and that small net
losses (< 0.14 yg/ml) are being measured at this concentration.
10
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By determining an average rate of decay (k) at each temperature, we can
describe decay per day of a collected SCL-TCM sample at a specific temperature
(Table 1) with the equation
P = 100 (l-e"kt) . (3)
where
P = percent decay/day
100 = conversion factor
k = average rate of decay, day"
t = time of exposure to a specific temperature, day
The data in Table 1 indicate that, with this reaction, the average rate
of decay (k) increases about 5-fold for each 10°C rise in temperature.
ORDER OF REACTION
Although our data indicated exponential decay and therefore, a first order
reaction, we applied the Van't Hoff equation to the data as an independent
means of determining the order of the decay reaction. Van't Hoff stated that
the true order of reaction can be determined using a log-log plot of the
velocity of the reaction against different initial sample concentrations at a
specific temperature, e.g.
V = kcn (4)
In V = In k + n In c (5)
where:
In = logarithm, base e
V = velocity of reaction, yg S0?/ml-day
k = rate of reaction, day"
n = order of reaction (slope of line)
c = concentration of sample, yg S09/ml
11
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Table 1. EFFECT OF TEMPERATURE ON PERCENT DECAY PER DAY
Average rate constant Percent loss
•Temperature, °C of decay (k) per day
20 0.009 day"1 0.9
30 0.051 day"1 5.0
40 0.287 day"1 25.0
50 1.33 day"1 73.6
12
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1.000
-7.000
•2.000
-1.500
-1.000
-0.500
LOGARITHM CONCENTRATION. «| S02/ml
Figure 5. Determination of order and rate of reaction.
0.500
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If this plot gives a straight line, then the slope of the line is an
estimate of the order of the reaction with respect to initial sample concentration
and the y-intercept is an estimate of the logarithm of the rate of decay.
Our data indicate that the average order of reaction is 0.97 (Figure 5),
thus confirming that the decay reaction is first order.
GENERAL OUTCOME (DECAY)
Arrhenius showed that for certain reactions there is a linear
relationship between the logarithm of the reaction rate (decay in this case)
and the inverse of the absolute temperature at which the rate was measured.
Therefore, by determining rates of decay at a few specific temperatures, we
can derive an equation that will describe the rate of decay at any temperature
within the range measured.
After determining the average rates of decay at 20, 30, 40 and 50°C, we used
the Arrhenius equation (given below) to calculate an expression that would
describe the rate of decay at any temperature within this range.
k = Ae"E/RT (6)
where
k = rate of reaction (decay), day"
A = frequency factor, day"
E = energy of activation, calories/mole
R = gas constant, 1.987 calories/degree-mole
T = absolute temperature of reaction, °K
Taking logarithms, we have the expression
In k = In A-(E/R)(1/T) (7)
which has the form Y = b + mx
A plot of this equation, using the above decay data, is shown in Figure 6.
14
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c_n
0.5000
-0.6000
-1.7000
-2.8000
-3.9000
5.0000
0.00300 0.00310
Ink = 48.735-15661 (=f)
0.00320 0.00330
TEMPERATURE, "K"1
0.00340 0.00350
Figure 6. Effect of temperature on decay rate constant of dichlorosulfito-
mercurate complex.
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An Arrhenius equation will hold only if the activation energy remains
independent of temperature. This is the case as indicated by the linear
relationship shown in Figure 6 where
In k = 48.735-15661 (|) (8)
This equation can now be used to calculate the rate of decay at any tempera-
ture within the range 20 to 50°C and to estimate the rate of decay outside
this range.
This equation also indicates that the energy of activation of the decay
reaction is 31.2 kcal/mole.
Scaringelli3 found that at 22°C, the SC^-TCM complex decayed at the rate
of 1.0 percent per day and could be stored at 5°C for up to 30 days without any
noticeable decay. Using equation (8) we find that at 22°C the rate of decay is
1.3 percent per day and at the end of 30 days at 5°C a sample will have decayed
a total of only 1.5 percent. Thus, the current study confirms the values
previously reported.
EFFECT OF VARYING TEMPERATURE
Because field samples are exposed to different temperatures for various
periods of time, a mathematical model was developed to estimate total decay
that would be experienced under varying but known temperature conditions.
Using equation (2) and breaking the time of exposure (t) into parts of
a day, we have
C -Ce
Sn ~ V
16
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taking logarithms, we have
in Cm = In CQ -[(kltl)T + (k^) + (k + . . . (k^.)] (10)
In Cm = In CQ - Cz(k1t1)]T< (11)
where
Cm = concentration measured, yg SO^/ml
C = initial concentration, yg S02/ml
k. = rate of decay at a specific temperature, day~
t. = time of exposure to a specific temperature, day
T.J = a specific temperature, °K
Thus, by using the Arrhenius equation, we can determine the decay rate
(k.) for any temperature (T- ). By knowing the time of exposure (t.) to a
particular temperature, we can determine the overall effect of varying tempera-
ture on sample concentration. As practical examples of the use of this
equation, we have estimated the decay of collected SOp-TCM samples on
exposure to the temperature conditions of a typical spring and summer day at
the Research Triangle Park. If the initial concentration C is set equal to
1.0 yg S02/ml (174 yg SO^/m ), C will also represent the fraction of S02
remaining after decay for any concentration of S02 exposed to these conditions.
The calculations are:
Temperature Conditions, Spring Day
16 hours at 22°C (72°F)
4 hours at 25°C (77°F)
4 hours at 30°C (86°F)
*i
0.013
0.023
0.054
[z(k.t.)]j = 0.022
11 i
0.009
0.004
0.009
17
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In Cm = 0-0.022 = -0.022
Cm = 0.978 yg S02/ml
or a sample decay of 2.2 percent per day
Temperature Conditions, Summer Day " k^ k^t^
16 hours at 22°C (72°F) 0.013 0.009
4 hours at 30°C (86°F) 0.054 0.009
4 hours at 35°C (95°F) 0.124 0.021
[s(k.t.)]Ti = 0.039
In Cm = 0-0.039 = -0.039
Cm = 0.962 yg S02/ml
or a sample decay of 3.8 percent per day.
These examples are intended to show the weighted effect, of even short-
time exposure to temperature of 35°C or above on the overall decay rate
experienced by SOp-TCM samples.
18
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SECTION IV
CONCLUSIONS
The data generated in this study indicated that collected SOp-TCM samples
decay at a critical temperature-dependent rate. The rate of decay is much
greater than had been suspected and follows first order reaction kinetics.
If the temperature history of the collected sample is known, the decay can be
determined and corrected for by use of a mathematical model.
Since the SOp-TCM complex is formed during sampling, one would expect
that elevated temperatures would also cause decay during sampling. This
has recently been confirmed in our laboratory.
19
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SECTION V
RECOMMENDATIONS
The EPA Reference Method for measurement of S02 has been shown to be
a reliable procedure when used under more restrictive temperature conditions
4 5
than those in common field practice today. ' Therefore, to maintain the
integrity of field samples, we recommend that the solution temperature be
kept at 25°C or less during sampling and at 5°C throughout shipment and
storage prior to analysis. To meet these more restrictive temperature specifi-
cations, the following techniques are suggested as possible solutions.
REDESIGN OF GAS SAMPLER AND SHIPPING CONTAINER
This technique will in fact solve the decay problem altogether, if the
more restrictive temperature specifications mentioned above are met. Physically
separating the pump from the sampler, maintaining the temperature inside the
sampler below 25°C year-round and shipping the samples in a refrigerated con-
tainer will help meet these specifications.
RELOCATION OF SAMPLING SITE
If the sampling site could be relocated into an air conditioned room
( 22°C) and the sample maintained at room temperature, no significant decay
would take place during sampling. After sampling, the samples must still be
placed in a 5°C chamber during shipment and storage prior to analysis. This
solution would presumably require relocating a large number of sampling sites.
CONTINUOUS MONITORS
The use of continuous monitors would provide no possibility for sample decay
and also would give short-term pollutant profiles, but the initial cost per site
would be high. The instrument would also require temperatures of approximately
22°C year-round for proper operation.
20
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SAMPLE OF KNOWN CONCENTRATION
Instead of minimizing decay by temperature control, the decay can be
measured and corrected by exposure of a sample of known concentration to
the same environmental conditions as the field sample. This technique could
be used to determine sample decay after collection, but would not provide an
accurate measure of decay during sampling, because the concentration of the
field sample is unknown and changing during sampling.
21
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SECTION VI
REFERENCES
1. Federal Register 36:8186-8187. April 30, 1971.
2. Scaringelli, F. P., B. E. Saltzman, and S. A. Frey. Anal. Chem. 39:1709-1719,
December 1967.
3. Scaringelli, F. P., L. Elfers, D. Norris, and S. Hochheiser. Anal. Chem. 42;'
(14):1818-1820, December 1970.
4. McKee, H. C., R. E. Childers, and 0. Saenz, Jr. Collaborative Study of
Reference Method for Determination of Sulfur Dioxide in the Atmosphere
(Pararosaniline Method), Prepared by Southwest Research Institute, Houston,
Texas for the U.S. Environmental Protection Agency, Research Triangle Park,
North Carolina, No. PB-205-891, September 1971.
5. McCoy, R. A., D. E. Camann, and H. C. McKee. Collaborative Study of
Reference Method for Determination of Sulfur Dioxide in the Atmosphere
(Pararosaniline Method)(24-Hour Sampling). Prepared by Southwest Research
Institute, Houston, Texas, for U.S. Environmental Protection Agency, Research
Triangle Park, North Carolina, No. EPA-650/4-74-027, December 1973.
6. Laidler, K. 0. Chemical Kinetics, 2nd Ed., New York, McGraw-Hill Company,
1965. p. 15-16.
7. Arrhenius, S. Z., Physik Chem. 4:226, 1889.
22
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TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing]
1. REPORT NO.
EPA-600/4-76-024
4. TITLE AND SUBTITLE
EFFECT OF TEMPERATURE ON STABILITY OF SULFUR DIOXIDE
SAMPLES COLLECTED BY THE FEDERAL REFERENCE METHOD
6. PERFORMING ORGANIZATION CODE
3. RECIPIENT'S ACCESSION-NO.
5. REPORT DATE
Mav 1976
7. AUTHOR(S)
8. PERFORMING ORGANIZATION REPORT NO.
Robert G. Fuerst, Frank P. Scaringelli, and John H.
Margeson
9. PERFORMING ORGANIZATION NAME AND ADDRESS
U.S. Environmental Protection Agency
Office of Research and Development
Environmental Monitoring and Support Laboratory
Research Triangle Park, North Carolina 27711
10. PROGRAM ELEMENT NO.
1HD621
11. CONTRACT/GRANT NO.
12. SPONSORING AGENCY NAME AND ADDRESS
Environmental Monitoring and Support Laboratory
Office of Research and Development
U.S. Environmental Protection Agency
Research Triangle Park, North Carolina 27711
13. TYPE OF REPORT AND PERIOD COVERED
Final
14. SPONSORING AGENCY CODE
EPA-ORD
15. SUPPLEMENTARY NOTES
16. ABSTRACT
This report describes an evaluation of the effect of temperature on the stability of
samples collected according to the Environmental Protection Agency procedure for
measurement of ambient sulfur dioxide. This evaluation was carried out over the
range 35 to 278 micrograms per cubic meter of air sampled. Collected samples were
found to decay at a critical temperature-dependent rate. The rate of decay increases
five-fold for every 10 degree centigrade increase in temperature over the range 20 to
40 degrees. The rate of decay is independent of concentration over the range studied,
and the decay reaction follows first-order kinetics. A mathematical model was
developed that allows sample decay to be calculated if the temperature history of the
sample is known. Temperature specifications and changes in the procedures necessary
to eliminate the decay problem are proposed.
7.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.IDENTIFIERS/OPEN ENDED TERMS
Air pollution
Sampling
Mathematical model
Temperature
Sulfur dioxide
c. COSATl Field/Group
13B
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