EPA-650/4-75-022
April 1975
Environmental Monitoring Series
EVALUATION OF A CONTINUOUS
COLORIMETRIC METHOD
FOR MEASUREMENT
OF NITROGEN DIOXIDE
IN AMBIENT AIR
U.S. Environmental Protection Agency
Office of Research and Development
National Environmental Research Center
. Research Triangle Park, N.C. 27711
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EPA-650/4-75-022
EVALUATION OF A CONTINUOUS
COLORIMETRIC METHOD FOR MEASUREMENT
OF NITROGEN DIOXIDE
IN AMBIENT AIR
by
John H. Margeson and Robert G. Fuerst
Quality Assurance and Environmental Monitoring Laboratory
Program Element No. 1HA327
ROAP No. 26AAF
U.S . ENVIRONMENTAL PROTECTION AGENCY
Office of Research and Development
Research Triangle Park, North Carolina 27711
April 1975
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EPA REVIEW NOTICE
This report has been reviewed by the Office of Research and Development,
EPA, and approved for publication. Mention of trade names or commercial
products does not constitute endorsement or recommendation for use.
RESEARCH REPORTING SERIES
Research reports of the Office of Research and Development, U.S. Environ-
mental Protection Agency, have been grouped into series. These broad
categories were established to facilitate further development and applica-
tion of environmental technology. Elimination of traditional grouping was
consciously planned to foster technology transfer and maximum interface
in related fields. These series are:
1. ENVIRONMENTAL HEALTH EFFECTS RESEARCH
2. ENVIRONMENTAL PROTECTION TECHNOLOGY
3. ECOLOGICAL RESEARCH
4. ENVIRONMENTAL MONITORING
5. SOCIOECONOMIC ENVIRONMENTAL STUDIES
6. SCIENTIFIC AND TECHNICAL ASSESSMENT REPORTS
9. MISCELLANEOUS
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 quanti-
fication of environmental pollutants at the lowest conceivably significant
concentrations. It also includes studies to determine the ambient concen-
trations of pollutants in the environment and/or the variance of pollutants
as a (unction of time or meteorological factors.
Copies of this report are available free of charge to Federal employees,
current contractors and grantees, and nonprofit organizations - as
supplies permit - from the Air Pollution Technical Information Center^
Environmental Protection Agency, Research Triangle Park, North
Carolina 27711; or for a fee from the National Technical Information
Service, 5285 Port Royal, Springfield, Virginia 22161.
Publication No. EPA-650/4-75-022
11
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ACKNOWLEDGMENTS
The authors wish to thank Mr. Ray Ballard and the other
members of EPA's Human Studies Laboratory for modifying and providing
a Technicon IV Air Analyzer and for assisting in its initial
preparation.
The authors also wish to thank Mr. Larry Purdue of the Quality
Assurance and Environmental Monitoring Laboratory for providing two
Technicon IV Air Analyzers and Mr. Vinson Thompson also of QAEML for
assistance in rectifying mechanical problems with the instruments.
iii
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CONTENTS
Page
List of Figures • v
Abstract - vi
Conclusions vii
•X..
I Introduction 1
II Experimental 3
III Results and Discussion 9
IV Future Work 23
V References 24
APPENDIX Tentative Method for Determination of Nitrogen
Dioxide in Atmosphere (Continuous Colorimetric
Procedure) 27
iv
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LIST OF FIGURES
Figure Page
1 Nitrogen Dioxide Permeation Rate of NBS-EPA
Device No. 16-3 4
2 Comparison of Dynamic and Static Calibration Using
Lyshkow Absorbing Solution 12
3 Comparison of Dynamic and Static Calibration Using
Modified Saltzman Absorbing Solution 12
4 Typical Scan of Unreacted NEDA Solution that Gave
a Broad Absorption Peak at 320 nm 16
5 Typical Scan of Unreacted NEDA that Gave Minor
Absorption Peaks, as well as a Major Absorption
Peak at 320 nm 16
6 Wavelength of Maximum Absorbance for Lyshkow
Absorbing Solution 17
7 Wavelength of Maximum Absorbance for Modified
Saltzman Absorbing Solution 17
8 Stability of Net Absorbance Developed by Lyshkow
Absorbing Solution 20
9 Stability of Net"Absorbance Developed by
Modified Saltzman Absorbing Solution 20
10 Stability of Lyshkow Absorbing Solution 21
11 Stability of Modified Saltzman Absorbing
Solution 21
A-l Typical N02 Atmosphere Generation System 39
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ABSTRACT
A continuous colorimetric procedure for the measurement of
nitrogen dioxide in ambient air was evaluated. The evaluation included
laboratory experiments, using two different azo-dye-forming absorbing
solutions in a Technicon instrument, to test the reliability of
calibration techniques. Other procedures that are important in the
use of the method were evaluated, and a literature search was con-
ducted to identify possible interferents.
The results show that static calibration is unreliable; dynamic
calibration using a reliable NOg-generation system is required.
Ozone was found to be a significant negative interferent.
i
A detailed method write-up, based on dynamic calibration
specifications, was prepared to describe the use of the continuous
colorimetric procedure.
The results of a collaborative test of this method will be
the subject of a separate report. .
VI
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CONCLUSIONS
The Information developed as a result of this evaluation was
sufficient to allow preparation of the detailed method write-up
on the use of a continuous colorimetric method for N02 measurement
that appears in the Appendix. "The major conclusions of the'
evaluation are that dynamic calibration is required and that ozone is
a significant negative interferent. The procedures recommended by
the Technicon Corporation, manufacturer of the instrument used in
this test, do not meet these requirements, in that static calibration
is recommended.
vii
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I. INTRODUCTION
On July 14, 1972, the Administrator of the U.S. Environmental
Protection Agency withdrew the EPA-promulgated reference method
for measuring atmospheric concentrations of nitrogen dioxide (NO^)
because of demonstrated inadequacies in the procedure. Research
2 3
conducted both within and outside EPA revealed that the collection
efficiency of the promulgated method varied and that a positive
nitric oxide (NO) interference occurred. After the withdrawal,
EPA selected three tentative procedures to replace the original
method: a continuous colorimetric (Saltzman) procedure, a continuous
chemiluminescence procedure, and a manual arsenite procedure.
The report presents the results of an evaluation of the
continuous colorimetric procedure by the Methods Standardization
and Performance Evaluation Branch (MSPEB) of the Quality Assurance
and Environmental Monitoring Laboratory (QAEML). A detailed description
of the procedure is given in the Appendix.
Within EPA, MSPEB is responsible for standardizing the measure-
ment methods used in determining compliance with the national ambient
air quality standards for various pollutants. This standardization
process includes the development of a method description (write-up)
that is clearly written and technically accurate based on a combination
of literature review, review of existing write-ups, and laboratory
evaluation of the procedure.
If a method proves to be reliable after the above evaluation,
it can be subjected to a collaborative test designed to determine
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its precision (repeatability and reproducibility) and its accuracy
(bias). The collaborative test is the final phase of the standardi-
zation process and is a measure of the performance of the method in
actual use.
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II. EXPERIMENTAL
The specific reagents, the method of preparation of the
absorbing solutions, and the dynamic calibration procedures used
in the laboratory evaluation were the same as those described in the
method write-up (see Appendix). Other pertinent experimental details
not contained in the write-up are described, in the following paragraphs.
GENERATION OF DYNAMIC N02 ATMOSPHERES
Nitrogen dioxide atmospheres were generated using a permeation
device and known amounts of clean dilution jair. This procedure
"e A-
6, 7
(Figure A-l) has been described by O'Keeffe and Ortman and Scaringelli
et al.'
The FEP-Teflon-sleeve, glass-reservoir permeation device
(No. 16-3) developed by the National Bureau of Standards (NBS) and
Q
EPA was used as the test source of nitrogen dioxide.
g
This device was calibrated gravimetrically and had a permeation
rate of 1.233 ± 0.005 yg N02/min (95 percent C.I.) at 25.2 ± 0.1 °C.
The constancy of the permeation rate for this device was established
as shown by the data in Figure 1.
The temperature of the device was controlled by a water-jacketed
condenser, which was maintained at 25.2 ± u.l °C.by a constant-
temperature bath (Forma Temperature, Jr.). The NOg was flushed from
the condenser with extra-dry-grade nitrogen at approximately
50 cm /min after it had passed through a column of molecular sieve
(6-16 mesh, type 4-A) and indicating Driente.
The dilution air used in the test was compressed house air that
had been passed through an air filter (Wilkerson Corporation
3
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IU
u
C3
UJ
PERMEATION RATE = 1.233 ± 0.005 jug N02/min
5.36
5.35
3200
6400 9600
12800 16000
TIME, minutes
19200 22400 25600 28800 32000
Figure 1. Nitrogen dioxide - permeation rate of NBS-EPA Device No. 16-3.
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Model No. 1237-2F) to remove dirt particles and aerosols, though
indicating silica gel (6-16 mesh) for drying, through treatment
with ozone (O-j)to convert NO to N02, and finally through activated
charcoal (6-16 mesh) to remove N02 and hydrocarbons.
A blank, consisting of a 24-hour sampling of dilution air
plus flushing nitrogen with no permeation device in the system,
showed a N02 concentration of less that 2 yg N02/m . Thus, the
generation system was free from interferences.
The concentration of N02 generated was calculated as described
in the Appendix.
STATIC GENERATION OF N02
The static generation of N02 was accomplished by adding known
concentrations of NaNOo to the different absorbing reagents. The
NOo concentration was calculated as follows:
1. The N02 equivalence of the stock NaN02 solution was:
0.0812 g NaN09 1 yl NO,
£ x ^ = 40 ui N09/ml (1)
1000 ml 2.03 x 10"b g NaN02 *
2. The NaN02 to N02 conversion factor used in equation 1
was derived as follows:
By the ideal gas law, 1 mole of N02 would occupy a
volume of 24.47 £ at 25.0 °C and 760 mm Hg. Therefore,
10"6 moles N02 = 24.47 x 10"6 i N02.
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-6 - 12
10 D moles N02 0.72 mole N02
A
24.47 x 10'6 I NO- mole N0?
6
69.0 g NaN02 2.03 x 10'° g NaN02
X mole N02= ul N02
3. This procedure, which is the one recommended by Technicon,
assumes 100 percent collection efficiency for N02 and quantitative
conversion of N02 to azo dye. Side reactions resulting in
nitric oxide generation have been shown to occur in this reaction.
Calibration standards were prepared by adding different
volumes of working NaN02 solution to 100 ml of absorbing solution
to produce the desired levels of N02. The volume to be added was
calculated from the following relationship:
1.6 yl N02 Volume of working solution
iiil x 100 ml
Solution flow rate, ml/min N02 yl (3)
x Air flow rate, i/min = ~T~ = N02 ppm
REAGENTS
1. NaN02- ACS reagent grade NaN02 having an assay of
99.9 percent was dried for 1 hour at 105 °C before use.
2. Stock NaN02 solution (40 yl N02/ml). Dry NaN02 (0.081281 g)
was dissolved in distilled water and diluted to 1 l in a volumetric
flask. The concentration was adjusted for less than 100 percent
assay.
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3. Working NaN02 solution (1.6 pi N02/ml). Ten ml of
stock solution was diluted to 250 ml with distilled water.
CONTINUOUS COLORIMETRIC INSTRUMENTS
Two Technicon IV Air Monitors were used. The plumbing (air
and solution flow rates) of one was modified to allow use of the
Saltzman absorbing solution. The other instrument was not modified
and was used with the Lyshkow absorbing solution, for which the
Technicon IV was designed. The Saltzman absorbing solution contains
0.5 percent sulfanilic acid, 5.0 percent acetic acid, and
0.005 percent N-(l-Naphthyl)-ethylenediamine dihydrochloride (NEDA).
The Lyshkow absorbing solution contains 0.15 percent sulfahllamide,
1.5 percent tartaric acid, 0.005 percent NEDA, and 0.005 percent
2-naphthol-3,6-disulfonic acid disodium salt. The procedures used
to prepare these aqueous absorbing solutions are given in the
Appendix, sections 6.8.1 and 6.8.2.
The operating procedures recommended by the manufacturer were
followed in using the instruments. This included maintaining the
ratio of the air flow rate to the solution flow rate at 1050.
Typical examples of flow rates used with the Lyshkow absorbing
O 0
solution were 316 cnr/min (air flow rate) and 0.301 cnr/min
(solution flow rate); with the modified Saltzman absorbing solution,
typical values were 390 cm3/min (air flow rate) and 0.370 cm3/min
(solution flow rate).
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TEMPERATURE CYCLING OF ABSORBING SOLUTIONS
Temperature cycling was carried out by immersing the solutions
in a constant-temperature bath (Forma Temperature Jr.) and manually
adjusting the temperature.
WAVELENGTH OF MAXIMUM ABSORBANCE
The wavelength of maximum absorbance was determined manually
with a spectrophotometer (Beckman Model B).
8
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III. RESULTS AND DISCUSSION
Continuous colorimetric methods for measuring NO? are based on
absorption of N02 from the air into an acid solution containing a
diazonium salt precursor and a coupling agent. A dye is produced
and the color intensity, which is directly proportional to the NOo
concentration, is measured in a colorimeter. The resulting electri-
cal signal is then transmitted to a recorder where the concentration
is determined from the recorder chart and the calibration curve.
Absorbed N02 is first converted to nitrous acid (MONO). The
HOMO then reacts with e.g., sulfanilic acid to produce a diazonium
salt, which then couples with N-(l-Naphthyl)-ethylenediamine
dihydrochloride (NEDA) to produce a pink azo1 dye. Saltzman12
first recognized that this reaction could be applied to determine
N02 levels in the atmosphere. He developed a manual procedure
using an absorbing solution of sulfanilic acid, NEDA, and acetic acid.
13
Thomas ° developed an apparatus for continuous determination of N02
using the Saltzman reagent. Saltzman modified the original
absorbing solution, mainly by reducing the acetic acid concentration.
Lyshkow ' reduced the instrument response time to less than one
minute by adding a disodium sulfonate to a Saltzman-type absorbing
solution.
Users of continuous colorimetric methodology for NOo measure-
ments indicate that both the modified Saltzman and Lyshkow absorbing
solutions are being used. Accordingly, both were included in this
evaluation.
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Because it would be prohibitive from a time-cost standpoint
to evaluate all of the available continuous colorimetric instruments
with the two absorbing solutions, the MSPEB evaluation used one
rather popular instrument, the Technicon IV.
DYNAMIC VERSUS STATIC CALIBRATION
An accurate calibration of any method is, of course, extremely
important. Dynamic procedures are generally preferred over static
ones because the former simulates actual use conditions whereas the
latter does not. Both procedures are in use, however, with continu-
ous colorimetric methods for NOg. Because Technicon recommends
static calibration, it was decided to compare static calibration
with an accurate dynamic procedure as a reference.
A reliaDle NOg permeation device was used as the basis for
dynamic calibration, against which the results of the static
calibration were compared. The experiments involved calibrating
an instrument by both dynamic and static procedures as described
in the experimental section. Both modified Saltzman and Lyshkow
absorbing solutions were used. A minimum of five calibration
points were generated over the 0 to 0.4 ppm (modified Saltzman)
and 0 to 0.25 ppm (Lyshkow) range. (No significance should be
attached to the use of different ranges with the two absorbing
solutions; equal ranges could have been used.) The millivolt
response was plotted against N02 concentration for the dynamic and
10
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static procedures, and the data were fitted to a straight line by
the method of least squares. The results are shown in Figures 2 and 3
for the Lyshkow and the modified Saltzman absorbing solutions,
respectively; equations for the calibration curves are also included.
The results show that, with both absorbing solutions, the
slopes of the static calibration curves were significantly different
from those obtained by dynamic calibration. The dynamic slope in
Figure 2 is 15 percent less than the static slope. A repeat of this
experiment using a different Technicon IV instrument showed a
difference of twice the first values (-31 percent). To determine if
all of the N02 was being absorbed in the dynamic calibration, the
air vented from the concurrent solution-air glass contact coils
(absorbing system) was sampled using a manual N02 method that
involved placing a midget impinger containing absorbing solution in
line. The results showed that 0.08 ppm of the NOo from a 0.25 ppm
calibration atmosphere was in the vented air. Thus, the collection
efficiency of this second instrument was approximately 70 percent
(and presumably higher with the first instrument but still less
I O
than the assumed 100 percent). Pierce has shown that the design
dimensions of this type of absorption system have a critical effect
on collection efficiency.
In the experiment shown in Figure 3, where yet another
Technicon IV instrument was used, the dynamic slope is 17 percent
greater than the static slope. Thus, the above relationship of
the slopes has been reversed.
11
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65.0
52.0
39.0
26.0
13.0
0.0
i i i i r
DYNAMIC-STATIC
DYNAMIC x 100 = -15 percent
I I I I
I I I
0.0 0.025 0.050 0.075 0.100 0.125 0.150 0.175 0.200 0.225 0.250
N02 CONCENTRATION, ppm
Figure 2. Comparison of dynamic and static calibration using Lyshkow absorbing solution.
65.0
DYNAMIC-STATIC _„„ .,
DYNAMIC "100 =+17 percent
0.0
0.0 0.05 0.10 0.15 0.20 0.25 0.30 0.35 040 0.45 0.50
N02 CONCENTRATION, ppm
Figure 3. Comparison of dynamic and static calibration using modified Saltzman absorbing solution.
12
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All of these results suggest that differences in dynamic ana static
calibrations can depend on individual instrument design. Because
different makes of instruments would invariably possess a variety of
absorber designs as well as differing solution and air flow rates
(all of which would affect N02 collection to different degrees),
it would be virtually impossible to develop a universal correction
factor that would make static and dynamic calibrations agree, thus
1 Q
allowing use of static calibration in a standard method. Higuchi,
for example, performed dynamic and static calibrations on 12 different
analyzers and found that differences varied significantly from
19
instrument to instrument.
Dynamic calibration eliminates collection efficiency errors
and the use of stoichiometric factors (which might be incorrect)
because, whatever the errors are, they would be the same in sampling
and calibration and therefore cancel. Accordingly, dynamic calibration
using a reliable NC^ source was specified as the calibration procedure
in the method write-up. See section 8 of the Appendix for details.
RESPONSE TIME
In developing a continuous colorimetric method, rapid color
formation is important because the rate of color formation affects
the response time of the instrument. When analyzed manually, both
the modified Saltzman and the Lyshkow absorbing solutions reached
97 percent of full color development after 3 minutes. With the
Technicon IV, however, both absorbing solutions required 15 minutes
to reach the same level of color development.
13
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Thus, the rapid response time reported for the Lyshkow absorbing
solution, and the potentially faster response time of the modified
Saltzman solution, were not obtained with the continuous instrument
tested.
INTERFERENCES
No laboratory experiments were carried out to determine
chemical interferences in this method because considerable work
12
has already been done. Saltzman recognized ozone as a negative
20
interferent in the manual procedure. Baumgardner et al. and
12
Clark et al. quantitated this effect for the continuous procedure
and found the interference to depend on the 03/N02 ratio; ratios of 1:1,
2:1, and 3.5:1 produced interferences of 5, 19, and 38 percent,
13
respectively. Thomas showed that alkyl nitrites are positive
interferents. The concentration of this species in ambient air is
probably low enough for the interference to be minor, however.
A 30:1 ratio of SCL to 0., slowly bleaches the color of the dye in
12
the manual procedure, and this effect may be applicable to the
continuous procedure. In practice, the interference would probably
be very minor because of the high ratio required.
Ozone, therefore, can be a significant interferent in the
continuous colorimetric method.
PURITY OF NEDA
Midwest Research Institute, during its work under an EPA
contract to collaboratively test methods for measuring NOg in ambient
14
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air, investigated the effects of NEDA purity on the wavelength of
maximum absorbance (A max) of the dye developed in a diazotization-
22
coupling type of reaction. The contractor found that aqueous NEDA
with one major absorption peak at 320 nm, over the range 260-400 nm
(Figure 4), consistently gave a X max of 542 nm. The method specified
maximum absorbance at 540 nm. NEDA showing minor absorption peaks
in addition to the major peak at 320 nm (Figure 5) shifted the X max
to 530 nm. This shifting of the A max was undoubtedly caused by
impurities in the NEDA that absorb light between 260 and 320 nm.
Based on these findings, a specification for purity of NEDA
was developed (see Appendix, section 6.3).
WAVELENGTH OF MAXIMUM ABSORBANCE
The wavelength of maximum absorbance of the dye in a colorimetric
method should be the same as the wavelength utilized in calibration
and measurement. If these differ significantly, a loss in sensitivity
will result. The colorimeter in the Technicon IV instrument uses
a 560 nm optical filter.
The x max of the Lyshkow and modified Saltzman absorbing
solutions was determined manually for comparison with the 560 nm
value. This was done by adding a NaN02 solution to the two absorbing
solutions, prepared according to the specifications in the Appendix -
which include using pure NEDA, allowing 15 min for color development,
and determining x max as described in the experimental section. The
results in Figures 6 and 7 show a X max of 542 and 547 (540-553) nm
15
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0.1 —
200
250 300 350
WAVELENGTH, nm
1.0
— 0.9
0.8
0.7
0.6
0.5
— 0.4
0.3
0.1
250 300 350
WAVELENGTH, nm
400
Figure 4. Typical scan of unreacted NEDA
solutions that gave a broad absorption peak
at 320 nm.22
Figure 5. Typical scan of unreacted NEDA
solutions that gave minor absorption peaks,
as well as a major absorption peak at 320 nm.22
16
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0.220
III
450 465 480 495 510 525 540 555 570 585 600
WAVELENGTH, nm
Figure 6. Wavelength of maximum absorbance for Lyshkow absorbing solution.
0.220
0.176
0.132
u
<
oa
oc
0
0.088
0.044
1 I I I I
I I I I I
450 465 480 495 510 525 540 555 570 585 600
WAVELENGTH, nm
Figure 7. Wavelength of maximum absorbance for modified Saltzman absorbing solution.
17
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for the Lyshkow and the modified Saltzman absorbing solutions,
respectively. From these data, measurement at 560 nm amounts to
a loss in sensitivity of approximately 13 and 7 percent, respectively,
for the two solutions. (Use of impure NEDA caused a larger loss
in sensitivity.)
Thus, for optimum sensitivity, instruments using these solutions
should use a 540 nm filter in the colorimeter.
STABILITY OF ABSORBING SOLUTIONS
The use of a continuous colorimetric method often involves
exposure of the absorbing solution to changing environmental
conditions that could affect stability. Exposure can occur in
transporting solutions from the laboratory to a field monitoring
site and during operation of the instrument in the field. Accordingly,
the effect of subjecting the Lyshkow and the modified Saltzman
absorbing solutions to simulated ambient temperature conditions
was measured. The experiments involved placing volumetric flasks
of the solutions in a constant temperature bath, protected from
light to simulate use conditions, and varying the temperature
over a 24-hr period as follows: 4 hr at 27 °C (81 °F), 4 hr at 30 °C
(86 °F), and 16 hr at 22 °C (72 °F). This cycle was repeated
every 24 hrs. The effect on stability was determined by measuring
the net absorbance of the solutions (absorbance developed by adding
a NaNOg solution - blank absorbance) as a function of time. These
18
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same measurements were made on solutions exposed to two laboratory
conditions at 22 °C--storage on the laboratory bench top
exposed to light and storage in the dark.
The net absorbance data were plotted, on an expanded scale, as
a function of time for all three conditions and fitted to a straight
line by the method of least squares. The results are shown in Figures 8
and 9 for the Lyshkow and the modified Saltzman absorbing solutions,
respectively. The absorbance of the blanks as a function of time was
plotted separately (see Figures 10 and 11).
The net absorbance, which is what the instrument measures, shows
no significant change under any of the three conditions up to the
limits of the study (14 days for temperature stability and 27 days for
storage conditions). The largest change was .an increase of 0.004
absorbance units when the solutions were stored exposed to light.
This change was below the lower detectable limit of the instrumental
procedure, and, therefore, was not significant in terms of the method.
The absorbance of the blank for the modified Saltzman solution
showed no significant change under any of the exposure conditions
after 27 days. The Lyshkow solution showed an increase in absorbance
of 0.02 after the same period of exposure. Because the net absorbance
was not significantly increased, however, this color development
by the blank does not appear to represent a potential source of error
in the method.
19
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0.236
0.232 —
0228 —
e/j
00
18 21 24 27
0.224 —
Figure 8. Stability of net absorbance developed by Lyshkow absorbing solution.
0.232
0 3 6 9 12 15 18
TIME, days
Figure 9. Stability of net absorbance developed by modified Saltzman absorbing solution.
20
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0.030
0.024 —
0.018 —
u
Z
<
i
0.012 —
0.006 —
0.012
U
I
CA
CO
4
0.006 —
6
9
21
12 IS 18
TIME, days
Figure 10. Stability of Lyshkow absorbing solution.
24
27
6
9
12 15 18
TIME, days
Figure 11. Stability of modified Saltzman absorbing solution.
24
27
30
TEMPERATURE CYCLING (O)
30
21
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USE OF WETTING AGENT
Because Lyshkow recommended the use of a wetting agent In the
absorbing solution, a non-ionic wetting agent (0.1 percent Triton X-100)
was added to the solution. The addition of the wetting agent produced
an unstable recorder response (very high noise level) when this solution
was used to sample an N02 atmosphere. The unstable response was un-
doubtedly caused by air, in the foam produced by the addition of the
wetting agent, affecting the signal from the colorimeter. Accordingly,
it is recommended that wetting agents not be used in this method.
22
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IV. FUTURE WORK
In spite of the limitations of the procedure, it was decided to
complete the standardization process and subject the continuous
colorimetric method to a collaborative test. Because the method is
likely to continue in wide use for some time, it should be used to
its fullest capabilities; a collaborative test will help to define
these capabilities.
The collaborative test was carried out during the week of July 29,
22
1974, under EPA contract 68-02-1363. Analysis of the data and
reporting of the results are in progress.
23
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V. REFERENCES
1. Title 40-Protection of Environment. Federal Register. 36_:22396-
22397, November 25, 1971.
2. Mauser, T.R. and C.M. Shy. Environmental Science and Technology.
6.: 890-894, 1972.
3. Merryman, E.L. et al., Environmental Science and Technology,
7.: 1056-1059, 1973.
4. Title 40-Protection of Environment. Federal Register. 38(110):
15174, January 8, 1973.
5. O'Keeffe, A.E. and G.C. Ortman. Analytical Chemistry, 318:760,
1966. p. 760 (1966).
6. Scaringelli, F.P., S.A. Frey, and B.E. Saltzman, Journal
of the American Industrial Hygience Association. 28:260, 1967.
7. Scaringelli, F.P., A.E. O'Keeffe, E. Rosenberg, and J.P. Bell.
Analytical Chemistry. 42:871, 1970.
8. NBS Technical Note 585. National Bureau of Standards, Washington,
D.C. p. 26. Available from: Superintendent of Documents, Govern-
ment Printing Office, Washington, D.C. 20402.
9. Rook, H.L., R.G. Fuerst, and J.H. Margeson. Progress Report:
EPA-NBS Study to Determine the Feasibility of Using NOg Permeation
Devices as Standards, December 1972-January 1973. (Unpublished.)
10. Technicon Auto Analyzer Methodology Air Monitor IV, Industrial
Method #136-71AP/Preliminary Data Released Dec. 1972. (Unpublished.)
11. Huygen, C. Analytical Chemistry. 42_: 407-409, 1970.
12. Saltzman, B.E. Analytical Chemistry. 26:1949-1955, 1954.
13. Thomas, M.D., et al. Analytical Chemistry. 28:1810-1816, 1956.
14. Saltzman, B.E. Analytical Chemistry. 32:135-136, 1960.
15. Lyshkow, N.A. Journal of the Air Pollution Control Association.
15:481-484, 1965.
16. U.S. Patent 3, 375, 079.
17. Fuerst, R.G. and J.H. Margeson. An Evaluation of the TGS-ANSA
Method for Measurement of N02. Methods Standardization Branch,
Environmental Protection Agency, Research Triangle Park, North
Carolina 27711.
24
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18. Pierce, L., Y. Tokiwa, and K. Nishikawa. Journal of the Air
Pollution Control Association. 1_5_(5):204, May 1965.
19. Higuchi, J.E., et al. A Straightforward Dynamic Calibration
Procedure for Use with NOX Instruemnts. (Presented at the
APCA Convention. Denver. June 9-13, 1974. Preprint No. 74-13.)
20. Baumgardner, R.E., T.A. Clark, J.A.- Hodgeson, and R.K. Stevens.
Determination of an Ozone Interference in the Continuous Saltz-
man Nitrogen Dioxide Procedure. (Unpublished.)
21. Clark, T.A., et al. Instrumentation for the Measurement of
Nitrogen Dioxide. (Presented at ASTM-EPA Symposium on Instru-
mentation for Monitoring Air Quality. Boulder. September 1973.
Published in ASTM Special Technical Publication 555, June 1974.)
22. Collaborative Testing of Methods for Measurements of NOg in Ambient
Air. Vol. 1. Report of Testing. Midwest Research Inst., Kansas
City, Mo. Prepared for Environmental Protection Agency, Research
Triangle Park, N.C. under Contract No. 68-02-1363. Publication No.
EPA-650/4-74-019-a. June 1974.
25
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APPENDIX.
TENTATIVE METHOD FOR DETERMINATION
OF NITROGEN DIOXIDE IN ATMOSPHERE*
(CONTINOUS COLORIMETRIC PROCEDURE)
October 1974
*A tentative method is one which has been carefully drafted from
available experimental information, reviewed editorially within
the Methods Standardization and Performance Evaluation Branch
and has undergone extensive laboratory evaluation. The method
is still under investigation and therefore is subject to revision.
27
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1. Principle and Applicability
l!l The method is based on the reaction of N02 in acid media
to produce nitrous acid (HONO) with subsequent diazotization and
coupling. N02 in ambient air is continuously absorbed in a solution
of diazotizing-coupling reagents to form an azo-dye that absorbs
light, with a maximum absorbance at approximately 540 nm. The
transmittance, which is a function of the N02 concentration, is
measured continuously in a colorimeter, and the output is read on a
recorder or a digital voltmeter.
1.2 The method is applicable to the continuous determina-
tion of nitrogen dioxide in ambient air.
i
2. Range and Sensitivity
o
2.1 Typical ranges are 0 to 470 pg/m (0 to 0.25 ppm),
0 to 940 pg/m (0 to 0.50 ppm), and 0 to 1880 yg/m (0 to 1.0 ppm).
Beer's law is obeyed throughout this range.
2.2 For optimum sensitivity, the wavelength specification
of the filter in the colorimeter should correspond to the wave-
length of maximum absorbance of the dye. This may not be the
case in some instruments. Therefore, the dye should be scanned
and the wavelength of maximum absorbance determined. If the filter
is not within ± 10 nm of the wavelength maximum obtained by scanning
the dye, the filter should be replaced by one that meets this
specification.
28
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The stability of both solutions is unchanged after
temperature cycling to simulate ambient conditions (up to 30? C
for 4 hours per day for 7 days).
5. Apparatus
5.1 Continuous N02 analyzer. Sample air is drawn through
a gas/liquid contact column at an accurately determined flow rate
concurrent to a controlled flow of absorbing reagent. The sample
inlet line prior to the absorber column should be constructed of
either glass or Teflon. The absorber column must be carefully
i
designed and properly sized because N02 is somewhat difficult to
absorb. The colored solution is passed through a colorimeter
where the transmittance is measured continuously.
5.1.1 Probe. Glass or Teflon, with inverted poly-
propylene or glass funnel at the end.
5.1.2 Installation. Instruments should be installed on
location and demonstrated, preferably by the manufacturer, to
meet or exceed the specifications described in the addendum.
5.2 Calibration. The calibration apparatus and its use
is described in Section 8. Additional components follow:
5.2.1 Dilution air and flushing air (or N2). This can be
compressed (house) air or cylinder air. It should be purified by
passing through silica gel for drying, and through activated
charcoal (6-14 mesh) and molecular sieve (6-16 mesh, type 4A)
to remove any NC^ and hydrocarbons.
29
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5.2.1.1 Purity. Test the purity of the dilution and flushing
air by operating the instrument in the zero mode until a stable
baseline is obtained. Connect the dilution or flushing air tube to the
air intake of the gas/liquid contact column and operate the
instrument in the ambient mode. If the response changes by more
than twice the noise level, the air is impure. Correct before
proceeding.
o
5.2.2 Flow meters. One each with ranges of 0-100 cm /min ,
0 to 1 fc/min. and 0 to 20 fc/min is required.
5.2.2.1 Calibration. This can be accomplished with a bubble
flow meter or a wet test meter. With a stopwatch, determine the
o
rates of air flow (cm /minJ through the flow meter at a minimum
of four different ball positions. Plot ball positions versus
flow rates.
5.2.3 Thermometer. Graduated in 0.1° intervals over the
range 20 to 30° C.
6. Reagents
6.1 Sulfanilamide [4-(H2N)CgH4S02NH2]. Melting point
165-167°C.
6.2 Sulfanilic Acid Monohydrate, [4-CNH2)CgH4S03H-H20].
ACS reagent grade. Either the monohydrate or anhydrous form
can be used, provided the degree of hydration is known. If the
degree of hydration is not known, recrystallize from water and
dry over night at 120°C. 4 This will give the anhydrous
material.
30
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3. Interferences
3.1 Recent studies1 have shown that ozone can produce.
a negative interference, the magnitude of which depends on the 03
to N02 ratio. In the study cited, 03/N02 ratios of 1:1, 2:1 and
3.5:1 produced interferences of 5, 19, and 38 percent, respectively.
3.2 Alkyl nitrites are positive fnterferents. The
magnitude of the interference depends on the structure of the
alkyl nitrite.2
3.3 A 30: 1 ratio of S02 to NOp slowly bleaches the color
3
of the azo-dye in the manual procedure, and this effect may
be applicable to the continuous procedure.
4. Precision, Accuracy and Stability
4.1 No data are available on precision and accuracy.
4.2 Air bubbles can accumulate in the optical cell and
will cause an erratic response. This instability can be mini-
mized by increasing the air and solution flow rates. The ratio
of the air to solution flow rate should be maintained at the
value recommended by the manufacturer (see Section 7).
4.3 The modified Saltzman absorbing reagent (Section 6.8.1)
is stable for 1 month under laboratory conditions (22°C;
exposed to light). The Lyshkow solution (Section 6.8.2) develops
an absorbance of approximately 0.02 after 1 month under laboratory
conditions. The net absorbance (absorbance developed by adding a
NO solution - blank) is unchanged after 1 month.
31
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6.3 N-(l-Naphthyl)-ethylenediamine dihydrochloride (NEDA).
An aqueous solution should have one absorption at 320 nm, over t.hp
range 260-400 nm.
6.4 Tartaric Acid. ACS Reagent grade.
6.5 Glacial Acetic Acid. ACS Reagent grade.
6.6 2-Naphthol-3-6-disulfonic acid disodium salt.
[HOC-iQMSO-jNaK] Technical Grade. This compound is also known
by its trivial name, R-salt.
6.7 Nitrite-free distilled water. Mix the water with
absorbing solution. Absence of any visible pink coloration indicates
that the water is of acceptable quality. If the solution turns pink,
redistill the water in an all-glass srtfll after adding a crystal
of potassium permanganate and barium hydroxide.
6.8 Absorbing Solutions. Either the modified Saltzman
solution or the Lyshkow ' modification of the Saltzman solution
can be used.
6.8.1 Modified Saltzman absorbing solution. 0.5 percent sulfanilic
acid, 5.0 percent acetic acid, 0.005 percent NEDA. For 1 liter of solution
prepare as follows: dissolve 5.52 g of sulfanilic acid monohydrate
(or 5.00 g of the anhydrous material} in hot distilled water and
allow to cool to room temperature. Add 50 ml of glacial acetic
acid followed by 0.050 g of NEDA. Dilute to 1 liter with distilled
water.
6.8.2 Lyshkow solution. 0.15 percent Sulfanil amide, 1.5 percent
Tartaric acid, 0.005 percent NEDA and 0.005 percent 2-Naphthol-3,6-di-
sulfonic acid disodium salt. For 1 liter of solution, dissolve 15.Og of
32
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tartanc acid, 1.50 g sulfanil amide, 0.050 g of 2-naphthol-3,6-
disulfonic acid disodium salt, and 0.050 g NEDA in 500 ml of
distilled water. Dilute to 1 liter with distilled water.
7. Procedure
Allow the instrument to warm-up in accordance with
the manufacturer's instructions and until a stable baseline is
obtained. Turn pumps on and adjust the air and absorbing reagent
flow rates and their ratio to the recommended values. Verify the
air flow rate by measurements with the 1 A/min flow meter.
Calibrate the instrument as described in Section 8.
!
8. Calibration
8.1 General Description. A dynamic calibration is carried
out by generating synthetic atmospheres from tfie output of a reliable
l^-permeation device and determining the instrument response. Instru-
ment response is then plotted against M^ concentration to obtain
a calibration curve.
8.2 NOp-Permeation Device. Obtain or prepare a reliable
N02-permeation device with a permeation rate of approximately
1.0 vigN02/min. The following precautions must be observed in pre-
paring NOg-permeation devices:
o
1. The N02 used to fill the device must be dry.
2. The filling operation must be carried out in a
dry atmosphere to ensure that water is not introduced when the tube
is filled.
33
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3. The N02 should be pure, assay 99 percent or greater.
4. All seals in the device should be free of leaks.
5. The permeation rate should be checked gravi-
metrically as follows:
a. Allow the device to reach temperature equilibrium
in the NOg-atmosphere generation system (Section 8.3). This will
be attained over-night, in most cases.
b. Weigh the device periodically and record the time.
(Transport the device from the atmosphere generation system to
the balance area in a dessicator.)
c. All weighings should be carried out at the same
relative humidity - 10 percent. The time of exposure of the device to
the atmosphere during weighing should be constant (± 30 sec.)
from weighing to weighing. This technique cancels any weight gain
caused by moisture - NOg reactions at the effusing surface and gives
a reliable measure of the NOg-welght loss.
d. The time interval between weighings will depend
on balance sensitivity. With a sensitivity (standard deviation
at the mass being weighed) of 40 yg, weighing at 24-hour intervals
will produce reliable weight losses.
e. Plot device weight (in micrograms) on the y-axis
versus cumulative time (in minutes) on the x-axis. Obtain sufficient
data (at least five well-spaced points) to establish the slope
of the line, which is the permeation rate in gg/min. Determine the
slope algebraically or by regression analysis.
f. The permeation rate should be constant and in
reasonable agreement with the suppliers or other previous valve.
34
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8.3 NOg-Atmosphere-Generation System. This consists of
an NOp-permeation device contained in a water-jacketed condenser
which is connected to a constant-temperature bath. A homogenous
N02 in air atmosphere is produced by flushing the NOp, effusing
from the calibrated NCL-permeation device, into a mixing bulb where
it is further diluted with dilution air. Figure A-l shows a diagram
of this system with suggested specifications for the component
parts. The following key specifications must be met to ensure
the generation of reliable calibration atmospheres:
8.3.1 Temperature control must be maintained to within + 0.1° C.
of a fixed value.
8.3.2 Flushing and dilution air. These must be dry and
free of N02 (see section 5.2.1).
8.3.3 A Kjeldahl connecting bulb with a volume of at least
3
150 cm is required to obtain adequate mixing of N02 and dilution
air.
8.3.4 Connections that are in contact with N02 must be of glass
or Teflon. Rubber tubing may be used for flushing and dilution air
i
connections. Tygon tubing should not be used. Systems for preparation
of calibration atmospheres have been described in detail by O'Keeffe
9 10
and Ortman; Scaringelli, et al.; and Scaringelli, Rosenberg, and
4
Rehme. Commercial calibration systems using the permeation tube tech-
nique are now available.
35
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8.4 N0« Atmospheres. Allow the N02 atmosphere generation system
to equilibrate for at least one hour with flushing and dilution air flow-
ing. Generate a calibration gas equal to 80 + 5% of full scale by adjust-
ing the dilution flow rate. Calculate the exact concentration from the
following relationship: ,
P x 10J
C = (A-l)
D + F
where:
3
C = NOp concentration, yg/m
P = NOg permeation rate, yg/min.
F = Flushing air flow rate, ft/min.
D = Dilution air flow rate, £/min.
10 = Factor to convert liters to cubic meters.
Sample the atmosphere until a stable response is obtained and record
.the response. Generate four additional concentrations of approximately
10, 20, 40, and 60 percent of full scale and determine the response.
(The NOp permeation rate and highest workable dilution air flow rate may
necessitate a higher initial concentration.)
8.5 Other reliable dynamic procedures for generating NOp can be
used. For example, gas phase titration of excess NO with 03, and
12
analyzed cylinders of NO^ in N^ that are stable.
8.6 Calibration Curve. Plot the concentration of N02 in micrograms/
cubic meter (x - axis) against'instrument response (y - axis), and draw the
line of best fit. Some instruments are designed to give a linear and some
a non-linear response.
36
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8.7 Frequency of Calibration. The calibration should be
checked daily by spanning the calibration curve at 80 percent of full
scale. Spanning by generation of a dynamic standard is preferred.
However, if field use of the instrument makes this impractical,
a static-calibration check can be carried out by adding a solution
of nitrite ion, NOg (as NaNOpj, to the absorbing solution to
generate the dye. (Most instruments have a static calibration
mode through which solutions can be introduced.) CAUTION: Static
and dynamic calibrations may not agree; therefore, if static
spanning is to be used, a static reference point should be
established at the time of calibration.
9. Calculations
9.1 N02 Concentration. This is read directly from the
calibration curve. A 1-hour or longer average concentration is
reported. Electronic or electro-mechanical integration, equal
area averaging, a pi am'meter, paper weighing techniques, or the
average of a digital output can be used to obtain the average
concentration.
9.2 The NOp concentration can be converted to ppm as
fol1ows:
ppm N02 = ug N02/m3 X 5.32 X 10"4 (A-2)
37
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9.3 Air Volume. The volume of air sampled is not corrected to
standard temperature and pressure because of the uncertainty associated
with average temperature and pressure valves.
10. References
1. Clark, T.A., et al. Instrumentation for the Measurement of
Nitrogen Dioxide. (Presented at the ASTM-EPA Symposium on Instru-
mentation for Monitoring Air Quality. Boulder. September 1973.)
2. Thomas, M.D., et al. Automatic Apparatus for Determination of
Nitric Oxide and Nitrogen Dioxide in the Atmosphere, Anal. Chem.
28_: 1810-1816, 1956.
3. Saltzman, B.E. Colorimetric Microdetermination of Nitrogen Diox-
ide in the Atmosphere. Anal. Chem., 26_: 1949-1955, 1954.
4. Scaringelli, P.P., E. Rosenberg, and K.A. Rehme. Comparison of
Permeation Devices and Nitrite Ion as Standards for the Colori-
metric Determination of Nitrogen Dioxide. Environ. Sci. Tech.
4:924-929, 1970.
5. Saltzman, B.E. Modified Nitrogen Dioxide Reagent for Recording
Air Analyzers. Anal. Chem. 32_:135-136, 1960.
6. Lyshkow, N.A. A Rapid and Sensitive Colorimetric Reagent for
Nitrogen Dioxide in Air. J. Air Pol. Control Assoc. 15;481-484,
1965.
7. U.S. Patent 3, 375, 079.
8. National Bureau of Standards Technical Note No. 585. National
Bureau of Standards, Washington, D.C.
9: O'Keeffe, A.E., and 6. C. Ortman. Primary Standards for Trace Gas
Analysis. Anal. Chem. 38:760, 1966.
10. Scaringelli, P.P., E. O'Keeffe, E. Rosenberg, and J.P. Bell.
Vapors with Permeation Devices Calibrated Gravimetrically. Anal.
Chem. 42:871, 1970.
11. Title 40-Protection of Environment. Federal Register. 36:22392-
22396, November 25, 1971.
12. Higuchi, J.E. et al. A Straightforward Dynamic Calibration Pro-
cedure for Use with NOX Instruments. (Presented at the APCA Con-
vention. Denver. June 9-13, 1974. Preprint No. 74-13.)
38
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FLOWMETER, 0 TO 15 liters/min
RUBBER TUBING
11.0mml.D.
GLASS CONDENSER
3 ft. 1/4 in.
COPPER TUBING
DILUTION AIR
68cm
THERMOMETER
PERMEATION DEVICE
TYGON TUBING
KJELDAHL MIXING-^
BULB
WATER CIRCULATING PUMP
CONNECTOR (FOR SAMPLING TRAINK
VENT TO HOOD
TEFLON STOPCOCKS. 6mm
i
RUBBER TUBING
FLOWMETER,
0 TO 100 cm3/min
FLUSHING AIR
OR NITROGEN
CONSTANT-TEMP. BATH
±0.1°C
GLASS MANIFOLD'
Figure A-1. Typical IM02 atmosphere generation system.
39
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ADDENDUM
A. Performance Specifications for Continuous Color1metr1c
Atmospheric Analyzers.
Range Multiple
Noise 0.005 ppm
Lower Detectable Limtt 0.01 ppm
Zero Drift
12 Hour ± 0.02 ppm
24 Hour ± 0.02 ppm
Span Drift - 24 hour 0.02 ppm
Lag Time • 20 minutes
Rise Time, 95 percent 15 minutes
Fall Time, 95 percent 15 minutes
B. Definitions of Performance Specifications.
Range - Minimum and maximum concentrations which the system
shall be capable of measuring.
Noise - Spontaneous, short-duration deviations in the instru-
ment output about the mean output, which are not caused by input
concentration changes.
Lower Detectable Limit - The minimum pollutant concentration
«
which produces a signal of twice the noise level.
Zero Drift - The change in instrument output over a stated
time period of unadjusted continuous operation . when the input
concentration of pollutant fs zero.
40
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Span Drift - The change in instrument output over a stated
time period of unadjusted continuous operation when the input
pollutant concentration Is a stated upscale value.
Lag Time - The time interval between a step change in input
concentration at the instrument Inlet to the first observable
corresponding change equal to twice the noise in the instrument
output.
Rise Time - The time interval between initial response and
95 percent of final response after a step increase in input concentration.
Fall Time - The time interval between initial response and
95 percent of final response after a step decrease in input concentration.
41
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TECHNICAL REPORT DATA
{Please read Instructions on the reverse before completing)
I REPORT NO
_EPA-650/4-75-022
.1 HILL AND SUUIITLt
12.
Evaluation of Continuous Colon'metric Method for
Measurement of Nitrogen Dioxide in Ambient Air
3 RECIPIENT'S ACCESSICWNO.
5 REPORT DATE
ril 1975
6 PERFORMING ORGANIZATION CODE
7 AUTHOR(S)
8. PERFORMING ORGANIZATION REPORT NO
John H. Margeson and Robert G. Fuerst
9 PERFORMING ORGANIZATION NAME AND ADDRESS
U. S. Environmental Protection Agency
Quality Assurance and Environmental Monitoring
Laboratory, Research Triangle Park, North Carolina
27711
10. PROGRAM ELEMENT NO.
1HA327
11. CONTRACT/GRANT NO.
12. SPONSORING AGENCY NAME AND ADDRESS
U. S. Environmental Protection Agency
Office of Research and Development
Washington, D. C. 20460
13. TYPE OF REPORT AND PERIOD COVERED
Final
14. SPONSORING AGENCY CODE
15. SUPPLEMENTARY NOTES
16. ABSTRACT
A continuous colorimetric procedure for the measurement of nitrogen dioxide
in ambient air was evaluated. The evaluation included laboratory experiments, using
two different azo-dye-forming absorbing solutions in a Technicon instrument, to
test the reliability of calibration techniques. Other procedures that are important
in the use of the method were evaluated, and a literature search was conducted to
identify possible interferents.
The results show that static calibration is unreliable; dynamic calibration
using a reliable NOo-generation system is required. Ozone was found to be a signifi-
cant negative interrerent.
A detailed method write-up, based on dynamic calibration specifications, was
prepared to describe the use of the continuous colorimetric procedure.
The results of a collaborative test ot this method will be the subject of a
separate report.
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.lDENTIFIERS/OPEN ENDED TERMS C. COSATI Field/Group
Colorimetric procedure
Nitrogen dioxide
18. DISTRIBUTION STATEMENT
19. SECURITY CLASS (This Report)
Unclassified
21. NO. OF PAGES
48
Unlimited
20. SECURITY CLASSVTTib page)
Unclassified
22. PRICE
EPA Form 2220-1 (9-73)
42
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