EPA-600/4-76-015
March 1976
Environmental Monitoring Series
MEASUREMENT OF ATMOSPHERIC SULFATES:
EVALUATION OF THE
METHYLTHYMOL BLUE METHOD
Environmental Monitoring and Support Laboratory
Office of Research and Development
U.S. Environmental Protection Agency
Research Triangle Park, North Carolina 27711
-------
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.
-------
EMSL/RTP No. 7
March 1976
MEASUREMENT OF ATMOSPHERIC SULFATES: EVALUATION OF THE
METHYLTHYMOL BLUE METHOD
by
Fred J. Bergman and Michael C. Sharp
Midwest Research Institute
425 Volker Boulevard
Kansas City, Missouri 64110
Contract No. 68-02-1728
Michael E. Beard
Quality Assurance Branch
Environmental Monitoring and Support Laboratory
U.S. Environmental Protection Agency
Research Triangle Park, North Carolina 27711
Prepared For
U.S. ENVIRONMENTAL PROTECTION AGENCY
OFFICE OF RESEARCH AND DEVELOPMENT
ENVIRONMENTAL MONITORING AND SUPPORT LABORATORY
RESEARCH TRIANGLE PARK, NORTH CAROLINA 27711
-------
DISCLAIMER
This report has been reviewed by the Environmental
Research Center, Research Triangle Park, Office of Re-
search and Development, U.S. Environmental Protection
Agency, and approved for publication. Approval does not
signify that the contents necessarily reflect the views
and policies of the U.S. Environmental Protection Agency,
nor does mention of trade names or commercial products
constitute endorsement or recommendation for use.
-------
FOREWORD
This program, "Measurement of Atmospheric Sulfates: Evaluation
of the Methylthymol Blue Method," is being conducted under the Environmental
Protection Agency (EPA) Contract No. 68-02-1728, which is Midwest Research
Institute (MRI) Project No. 3948-C. The program is concerned with methods
for analyzing the water soluble sulfates in ambient air samples. The ob-
jectives of the program were (a) to conduct a thorough literature survey,
(b) to select the two most promising methods, and (c) to subject the two
methods to a ruggedness test.
This is the final report of Phase III covering the ruggedness
test of the methylthymol blue procedure. Included as an appendix is a new
write-up of the method for sulfate analysis.
This program has as a,requirement the submission of two final re-
ports for Phase III; one for each of two methods evaluated during the pro-
gram. The final report covering the evaluation of the second method is be-
ing submitted separately.
This program is being conducted under the management of Mr. Paul
C. Constant, Jr., Head, Environmental Measurements Section of MRl's Physical
Sciences Division. The principal investigator is Mr. Fred J. Bergman. He
was assisted in the experimental investigation by Mr. William H. Maxwell.
The statistical analysis was performed by Mr. Michael C. Sharp.
Approved for:
MIDWEST RESEARCH INSTITUTE
L. J.(Shannon, Assistant Director
Physical Sciences Division
April 2, 1976
iii
-------
TABLE OF CONTENTS
Summary 1
I. Introduction 3
II. Research Program 3
A. Calibration Curve 5
B. Analytical Wavelength 9
C. Method Variables ^ 9
D. Development Time 12
E. Reagent Stability 12
F. Interferences 12
G. Air in the Ion Exchange Column 12
H. Pulse Suppressors 13
I. Tubing Materials 13
J. Ruggedness Tests 13
III. Method Strengthening 20
References 22
Appendix - Tentative Method for the Determination of Sulfates in the
Atmosphere (Automated Methylthymol Blue Procedure)
List of Tables
Table Title Page
1 Variation in Reagent Operating Parameters 4
2 Physical Operating Parameters 5
3 Index of Determination (r^) 6
4 Least Squares Curve Fit 7
5 Calculated Values and Percent Difference Using Constants
Obtained From a Least Squares Curve Fit 8
IV
-------
List of Tables' (Concluded)
Table Title
6 Variables in MTB Method 11
7 Ruggedness Test for Methylthymol Blue Method 14
8 Ruggedness Test of Dimension 20 15
9 Analysis of Variance of Ruggedness Test 17
10 Variable Effects in Sulfate Analysis 19
List of Figures
Figure Title Page
1 MTB System Absorbance 10
2 Schematic of Ruggedness Test 16
-------
SUMMARY
The methylthymol blue (MTB) procedure used by EPA for sulfate
analysis has been subjected to a ruggedness test. The method write-up has
been modified to strengthen the procedure where required. A new procedure
developed by EPA for extracting sulfates from the filter samples has been
incorporated in the write-up. The absorbance for the MTB procedure has
been established as hyperbolic. The method, as now presented, appears to
be reasonably rugged with the exception of phosphate interference, which
remains a problem. A potential approach for eliminating the phosphate in-
terference is presented.
-------
I. INTRODUCTION
Recent health effects studies have shown that the concentration
of sulfate in ambient air presents a significant hazard to human health.
The use of catalytic control devices in automobiles will possibly increase
the-level of atmospheric sulfate.
This has led to an increasing emphasis on the measurement of sul-
fate concentrations in ambient air. Since the data obtained may subsequently
be used to set an ambient air standard for sulfates and may influence auto-
motive standards for other emissions, it is important that the analytical
methods used for sulfate analysis be reliable.
Establishing the reliability of analytical methods is the respon-
sibility.of the Environmental Monitoring and Support Laboratory of the En-
vironmental Protection Agency.- One method of accomplishing this is to sub-
ject candidate methods to a ruggedness test as described by W. J. Youden and
Stow and Mayer.* The results of a ruggedness test then permits strengthen-
ing the methods and enables EPA to recommend the most reliable of the methods
evaluated.
The program for accomplishing the above goals was divided into
three phases. Phase I included a thorough literature search and an evalua-
tion of the various methods with a recommendation of the two most promising
methods. Phase II.covers a familiarization period and a detailed method
write-up for the two methods. Phase II includes the ruggedness test design
and investigation and a qualitative identification of the significant and
nonsignificant effects. This report presents a complete description of the
familiarization phase and ruggedness test of the methylthymol blue (MTB)
method of sulfate analysis. An MTB method write-up which incorporates the
results of the ruggedness test and editorial comments by EPA is shown in
the Appendix.
II. RESEARCH PROGRAM
During Phase I of this program, it became apparent that it would
be desirable to select the MTB method as one of the two methods for evalua-
tion. This selection was based on the following considerations: (a) the
Statistical Techniques for Collaborative Tests, by W. J. Youden; AOAC
Publication, Box 540, Benjamin Franklin Station, Washington, D.C. 20044,
and "Efficient Screening of Process Variables," by R. A. Stbwe and R. P.
Mayer, Ind. Eng. Chem.. 58:36-40 (1966).
-------
method Is well developed; (b) the National Air Surveillance Network (NASN)
is presently using the method; (c) there are a number of organizations fa-
miliar with the procedure; (d) the methpd is automated, which permits a
large number of samples to be run; (e) the method appears to produce accept-
able results; (f) the results of the familiarization and ruggedness test can
be applied to the method to strengthen it; and (g) most of the current sul-
fate investigators are employing the MTB method, making a comparison with a
possible alternate highly desirable.
The literature search shows 13 references associated with the MTB
method. Lingemen!/ presents a general description of automated analytical
procedures from a clinical standpoint but does not describe a sulfate method.
Kbrbl and PribilslZ/ report the development and investigation of methylthy-
mol blue as a metallochromic indicator. The remaining references°,"13/
-------
The physical operating parameters are shown in Table 2.
TABLE 2
PHYSICAL OPERATING PARAMETERS
Sample rate
Concentration range ug/ml
Precipitation delay coil
Length
Sample/wash
Color development coil
Length
Wavelength
Lazrus
30/hr
0.5-50
120 in.i/
240 in.i/
Not specified
8 in.
480 nm8-/
465 nmi/
Technicon
30/hr
0-300
43 in.
6:1
40 in.
460 run
NASN
30/hr
5-60
27 in.
1:2
27 in.
460 run
It was desirable to investiage all of the significant variations
in the procedure to determine their effects.
A. Calibration Curve
The least defined part of the MTB method appeared to be the cali-
bration procedure. It is generally agreed that a calibration plot does not
follow the Beer-Lambert law. Lazrus1 observation was that the plot was
slightly concave. Other investigators have reported the curve to be either
a power function, or in some cases, linear. This disagreement was considered
to be one of the more important aspects to be investigated in evaluating the
method.
A Technicon I Autoanalyzer module was assembled following the spe-
cifications given in the NASN procedure. Standards were prepared following
the same procedure and then they were analyzed. Various concentrations were
run in ascending, descending, and random orders. The peak height appeared
to be representative of the sample analyzed and not influenced by large shifts
in concentration either before or after a specific peak. The results of this
analysis produced over 1,000 calibration points. Random data was subject to
a least squares curve fit of six curves.
-------
Of the six curves, the best fit was obtained with a linear func-
tion, Y = A + (B-X); a power function Y *= A-X^; and a'hyperbolic function
Y = X/(A + BX). The index of determination (square of the correlation co-
efficient or r2) was always greater than 0.9 for these three curves and less
than 0.9 for the other three. The index of determination of seven typical
curves fits is shown in Table 3.
TABLE 3 .
INDEX OF DETERMINATION (r2)
Linear Function Power Function Hyperbolic Function
0.991039 0.997017 0.99838
0.976019 0.990774 0.99662
0.970671 0.992314 0.998299
0.955881 0.988047 0.99.8266
0.957649 0.989792 0.999408
0.97526 0.993453 0.999411
0.978717 0.996125 0.999777
Although r2 varies from run to run, the best fit is always ob-
tained with the hyperbolic function Y = X/(A + BX) followed by the power
function and then the linear function.
The values obtained from a least squares curve fit using typical
data is shown in Table 4 which includes calculated values for.constants A
and B.
The values of A and B in Table 4 were then used to calculate peak
heights and the percent error for each point using the three best curves.
The results of this operation are shown in Table 5.
• Table 5 presents a more complete view of the results. The best
overall fit is obtained with the hyperbolic function Y = X/(A + BX). The
power function fits slightly better at low values (peak height < 20) but
does not equal the hyperbolic at midrange arid high range. The linear func-
tion is inferior to both the hyperbolic and power function.
This- evaluation confirms that the calibration curve is nonlinear
and indicates that a hyperbolic function Y = X/(A + BX) is optimum for the
calculations.
-------
TABLE 4
Curve Type
LEAST SQUARES CURVE FIT
Index of
Determination
Linear
Exponential
Power
Hyperbolic
Hyperbolic
Hyperbolic
Y » A + (B-X)
Y = A-Exp(B-X)
Y « A-XB
Y = A + (B/X)
Y « I/ (A +BX)
Y «= X/(A +BX)
0.991039
0.890888
0.997017
0.755255
0.707708
0.99838
5.67479
17.8936
3.43963
113.005
5.13178E-2
0.317611
B
2.58614
4.50122E-2
0.942017
-567.619
-1.06356E-3
1.16851E-3
Y
X
Peak height
S07 Concentration in micrograms per milliliter
Values used were:
Y = 129.9, X = 50
Y = 110.2, X = 40
Y = 89.2, X = 30
Y = 61.1, X = 20
Y = 28.9, X = 10
Y = 15.6, X = 5
-------
TABLE 5
CALCULATED VALUES AND PERCENT DIFFERENCE USING CONSTANTS
X-Actual
50
40
30
20
10
5
X-Actual
50
40
30
20
10
5
X-Actual
50
40
30
20
10
5
OBTAINED
Y-Actual
129.9
110.2
89.2
61.1
28.9
15.6
Y-Actual
129.9
110.2
89.2
61.1
28.9
15.6
Y-Actual
129.9
110.2
89.2
61.1
28.9
15.6
FROM A LEAST SQUARES
Hyperbolic Function
Y-Calculated
132.966
109.784
85.0661
58.6542
30.3678
15.4581
Linear Function
Y-Calculated
134.982
109.12
83.2589
57.3975
31.5362
18.6055
Power Function
Y-Calculated
137.079
111.091
84.7198
57.8234
30.0974
15.6658
CURVE FIT
Percent Difference
-2.3
0.3
4.8
4.1
-4.8
0.9
Percent Difference
-3.7
0.9
7.1
6.4
-8.3
-16.1
Percent Difference
-5.2
-0.8
5.2
5.6
-3.9
-0.4
-------
B. Analytical Wavelength
Interference filters are normally available at 10-nm increments.
To determine if the optimum wavelength filter had been selected, a scan was
made of the systems absorbance. The results are shown in Figure 1. Curve
A is of the barium chelate of MTB at a pH of 12.4. Curve B consists of the
reagents at a pH of 2.8 prior to chelation. The wavelength of the spectro-
photometer was checked prior to the analysis using a holmium glass reference
standard.
It appears from an examination of the curve that the conversion
to the chelate by pH adjustment is essentially complete. Less than complete
conversion to the chelate would have been indicated by an absorption peak
occurring at a. 435 nm on the A curve. This was not observed.
Spectrophotometric methods are normally performed using the wave-
length where the absorption is at a maximum to obtain the greatest sensitivity.
The MTB procedure produces a broad absorption curve. The exact analytically
employed wavelength is therefore not critical. However, the analytical wave-
length should be selected so that measurements are made near the maxima and
within that portion of the curve where the slope is essentially zero.
If absorbance were to be measured where the slope of the curve is
larger, a small change in the wavelength would produce a significant change
in the absorbance. In the MTB procedure a change of 10 nm near the maxima
(for example from 455 to 465) will produce a change of only 1% in the absorb-
ance. A change of 10 nm where the slope is greater (for example from 500 to
510 nm) will produce a 3% change in the absorbance.
The absorbance curve indicates that any wavelength between 440 and
470 nm would meet the above criteria and that the normally employed 460-nm
wavelength should be satisfactory.
C. Method Variables
In preparation for the ruggedness tests, a list of variables and
test levels for the MTB method was compiled. This list is shown in Table 6.
There are actually 27 functions listed in the table under 19 general classi-
fications. The design of a ruggedness test for 27 functions was considered
undesirable. Each function was therefore examined for its suitability for
inclusion in the ruggedness test. If suitable, it was labeled yes under the
heading Valid for Test. Those labeled no were reserved for individual evalu-
ation and are discussed below.
-------
100
0)
o
c
o
_Q
_Q
<
Ba+2(MTB)pH 12.4
X Analysis
Ba+2(MTB)pH 2.8
0 I I I I I I I I I I I
390 400
425
450 460 475 500
Wavelength, nm
550
600
Figure 1 - MTB System Absorbance
10
-------
TABLE 6
VARIABLES IN MTB METHOD
1.
2.
3.
4.
5.
6.
7.
8.'
9.
10.
11.
12.
13.
14.
15.
16.
17.
Function
Ba/MTB ratio
Sample time
Wash time
MTB mixing time (coil)
NaOH color development time (coil)
Reagent age (stability) MTB
Sample age
Standard age (stability) (cold and dark)
Wash solution
Interferences
Ba
P04
Turbidity plus color
S=
S03=
Concentration S04=(ug/ml)
Reaction temperature, MTB
pH, MTB solution
NaOH Reaction
Air in ion exchange column
Sample/water dilution
Pulse suppressor,
MTB
NaOH
Air bubble rate, sample
MTB
Tubing materials
Ethanol concentration
Cations, 10 ug Fe + 10 Al + Ca
Valid
for
Test
Yes
Yes
Yes
No
No
Yes
No
No
No
Yes
Yes
Yes
No
No
Yes
Yes
Yes
Yes
No
Yes
No
No
Yes
Yes
No
Yes
Yes
Levels
+
1.00
40
80
29 turn
29 turn
1 day
1 day
1 day
EDTA
0
0
0
0
0
30
20°C
3.5
13.5
Yes
Yes
Yes
Yes
0.23
0.32
Si
400 ml
0
-
0.90
72
108
14 turn
14 turn
7 days
14 days
14 days
H20
30 ug
30 mg
Filter ex-
tract
30 ug
30 ug
120
30° C
2.0
12.0
No
No
No
No
0.32
1.00
Solv.
471 ml
30 Total
11
-------
D. Development Time
The analyzer was set up with a 29 turn coil inserted after MTB
addition in place of the 3-in. (14 turn) coil specified in the method write-
up. This allowed approximately double the normal time for barium sulfate
precipitation. Three series of sulfate standards were then analyzed and the
peak heights obtained were the same as those obtained with the 3-in. coil.
The same procedure was followed in evaluating the color development time,
again no change in the peak height was observed. It was therefore concluded
that the coils specified in the method are satisfactory.
E. Reagent Stability
The stability of the MTB solution was included in the ruggedness
test. The stability of the sample was given only a simple check. An ambient-
air sample was extracted according to the method write-up. A portion of the
extract was analyzed immediately and the remaining solution was stored in a
volumetric flask in the laboratory (=. 220C~) for 14 days. The sulfate concen-
tration remained unchanged.
A 30-ug/ml standard sample was stored in a walk-in cooler (5°C in
the dark) for 14 days. Analysis using a freshly prepared standard for cali-
bration gave a value of 31 ug/ml for the stored standard. The 30-ug/ml stan-
dards stored cold and in the dark appear to be stable for at least 14 days.
Other levels of sulfate standards were not checked.
F. Interferences
Interferences of major concern, based on earlier investigations,
were evaluated. These consisted of phosphates, turbidity and color, and the
cations Fe, Al, and Ca. The evaluation was conducted by including the inter-
ferences in the ruggedness test.
G. Air in the Ion Exchange Column
The inclusion of air bubbles in the ion exchange column appears
to produce erratic results when the bubble is moving. Smaller bubbles (di-
ameter less than that of the tubing that remains stationary) appears to have
no effect. Since bubbles are removed from stream prior to entry in column,
this is not normally a problem unless bubbles are accidentally admitted.
12
-------
H. Pulse Suppressors
We could observe no effect on the precision or accuracy of the
method related to the pulse suppressors in our system. Installing pulse
suppressors made a slight improvement in the base line (decreased noise)
but the precision was not significantly improved.
I. Tubing Materials
The analytical system was operated for extended periods of time
with both Solvaflex and silicon pump tubing. No effect was observed other
than the fact that the silicon pump tubing has a significantly longer life
time. At present, silicon tubing appears to be good for at least 6 weeks
of continuous daily operation.
J. Ruggedness Tests
The remaining functions listed in Table 6, which were not in-
vestigated individually, were evaluated using a ruggedness test. The test
format is shown in Table 7. Most of the functions investigated are self-
explanatory. The phosphate ion was investigated separately because of its
known potential to cause an interference. Theoretically, the phosphate in-
terference should be minimized since the precipitation is carried out at a
pH of 2.8. The cations Fe, Al, and Ca were combined into a single function
to simplify the ruggedness test. Color and turbidity were also combined
for the same reason.
The format of Table 8 is technically a ruggedness test of dimen-
sion 20. The levels of the 19 variables are listed in Table 8.
The experimental design was the fractional framework given by
Plackett and Burman (see Figure 2). However, the design did differ from
an ordinary ruggedness test in that 12 replicate analyses per run were per-
formed. In the normal ruggedness test (no replicates), the dummy "effects"
are used as residual variance; this is quite conservative since the degrees
of freedom are very small and the dummy effects contain interaction effects
of the real variables (if any). The residual could also be estimated from
the (pooled) replicate variance, although this is likely too liberal since
the replicates are not experiment replicates but only analytical replicates.
The analysis of variance table is shown in Table 9. The response
in this analysis is the difference between the actual sample value and the
calculated value. In other words, the method bias is always used as the re-
sponse (regardless of the level involved).
13
-------
No.
1
2
3
4
5
6
7
8
9
10 '
11
12
13
14
15
16
17
18
19
20
(g/500 ml)
A
+ 4.1
- 4.0
+ 4.1
+ 4.1
- 4.0
- 4.0
- 4.0
- 4.0
+ 4.1
- 4.0
+ 4.1
- 4.0
+ 4.1
+ 4.1
+ 4.1
+ 4.1
- 4.0
- 4.0
+ 4.1
- 4.0
(ml/100 ml)
B
+ 9.0
+ 9.0
- 8.3
+ 9.0
+ 9.0
- 8.3
- 8.3
- 8.3
- 8.3
+ 9.0
• 8.3
+ 9.0
- 8.3
+ 9.0
• + 9.0
+ 9.0
+ 9.0
- 8.3
- 8.3
- 8.3
("/looo"2?)
c
• 1.4659
+ 1.5000
+ 1.5000
- 1.4659
+ 1.5000
+ 1.5000
- 1.4659
- 1.4659
- 1.4659
- 1.4659
+ 1.5000
- 1.4659
+ 1.5000
- 1.4659
+ 1.5000
+ 1.5000
+ 1.5000
+ 1.5000
- 1.4659
- 1.4659
Mru
D
- 0.
- 0.
+ 0.
+ 0.
- 0.
+ 0.
+ 0.
- '0.
- 0.
- 0.
- 0.
+ 0.
.1301
1301
.1400
.1400
.1301
.1400
.1400
1301
1301
1301
.1301
.1400
- 0.1301
+ 0.
- 0.
+ 0.
+ 0.
+ 0.
+ 0.
- 0.
.1400
.1301
.1400
.1400
.1400
.1400
.1301
' °"°
E
+ 500
- 471
- 471
+ 500
+ 500
- 971
+ 500
+ 500
- 471
- 471
- 471
- 471
+ 500
- 471
+ 500
- 471
+ 500
+ 500
+ 500
- 471
Sample
Rate
(cc/min)
F
+ 1.00
+ 1.00
- 0.80
- 0.80
+ 1.00
+ 1.00
- 0.80
+ 1.00
+ 1.00
- 0.80
- 0.80
- 0.80
- 0.80
+ 1.00
- 0.80
+ 1.00
- 0.80
+ 1.00
+ 1.00
- 0.80
Water
Sample
+
+
+
-
-
+
+
-
+
+
-
-
-
-
+
-
+
-
+
-
• 1
G
1.60
1.60
1.60
1.40
1.40
1.60
1.60
1.40
1.60
1.60
1.40
1.40
1.40
1.40
1.60
1.40
1.60
1.40
1.60
1.40
H
+ 0.32
+ 0.32
+ 0.32
+ 0.32
- 0.23
- 0.23
+ 0.32
+ 0.32
- 0.23
+ 0.32
+ 0.32
- 0.23
- 0.23
- 0.23
- 0.23
+ 0.32
- 0.23
+ 0.32
- 0.23
- 0.23
MTB
/ 1
I
- 1.20
+ 1.40
+ 1.40
+ 1.40
+ 1.40
- 1.20
- 1.20
+ 1.40
+ 1.40
- 1.20
+ 1.40
+ 1.40
- 1.20
- 1.20
- 1.20
- 1.20
+ 1.40
- 1.20
+ 1.40
- 1.20
MTB
J
+ 1.20
- 1.00
+ 1.20
+ 1.20
+ 1.20
+ 1.20
- 1.00
- 1.00
+ 1.20
+ 1.20
+ 1.00
+ 1.20
+ 1.20
- 1.00
- 1.00
- 1.00
- 1. 00
+ 1.20
- 1.00
- 1.00
NaOH
(CC/min)
_
+
+
+
+
+
-
+
+
+
+
-
-
-
-
+
-
K
1.00
1.20
1.00
1.20
1.20
1.20
1.20
1.00
1.00
1.20
1.20
1.00
1.20
1.20
1.00
1.00
1.00
1.00
1.20
1.00
Analysis
L
+ 35
- 25
+ 35
- 25
+ 35
+ 35
+ 35
+ 35
- 25
- 25
+ 35
+ 35
- 25
+ 35
+ 35
- 25
- 25
- 25
- 25
- 25
MTB
Age
(days)
M
- 0
+ 1
- 0
+ 1
- 0
+ 1
+ 1
+ 1
+ 1
- 0
- 0
+ 1
+ 1
- 0
+ 1
+ 1
- 0
- 0
- 0
- 0
Sample
Cations
Time (UR/ml) ftio/mM
N 0
- 40-80
- 40-80
+ 72-108
-.40-80
+ 72-108
- 40^80
+ 72-108
+ 72-108
+ 72-108
+ 72-108
- 40-80
- 40-80
+ 72-108
+ 72-108 •
- 40-80
+ 72-108
+ 72-108
- 40-80
- 40-80
- 40-80
- 0
- 0
- 0
+ 30
- 0
+ 30
- 0
+ 30
+ 30
+ 30
+ 30
- 0
- 0
+ 30
+ 30
- 0
+ 30
+ 30
- 0
- 0
P
- 0
- 0
- 0
- 0
+ 30
- 0
+ 30
- 0
+ 30
+ 30
+ 30
+ 30
- 0
- 0
+ 30
+ 30
- 0
+ 30
+ 30
- 0
Color
and
Turbidity
Q
+ Yes
- No
- No
- No
- Ho
+ Yes
- No
+ Yes
- No
+ Yes
+ Yes
+ Yes
+ Yes
- No
- No
+ Yes
+ Yes
- No
+ Yes
- No
Sample SO^
(UK/ml) Dummy
R S
+ 40
+ 40 +
- 15 +
- 15
- 15
- 15
+ 40
- 15 +
+ 40
- 15 +
+ 40
+ 40 +
+ 40 +
+ 40 +
- 15 +
- 15
+ 40
+ 40 +
- 15 +
- 15
-------
TABLE 8
RUGGEDNESS TEST OF DIMENSION 20
Variable , Low Level (-) High Level (+)
A NaOH (g/500 ml) 4.0 4.1
B HC1 (ml/100 ml) 8.3 9.0
C BaCl2-2H20 (g/100 ml) 1.4659 1.5000
D MTB (g) 0.1301 0.1400
E Ethanol (ml) 471 500
F Sample rate (cc/min) 0.80 1.00
G Water rate (cc/min) 1.40 1.60
H Sample air rate (cc/min) 0.23 0.32
I MTB rate (cc/min) 1.20 1.60
J MTB air rate (cc/min) 1.00 1.20
K NaOH rate (cc/min) 1.00 1.20
L Analysis temperature (°C) 25 35
M MTB age (days) 0 1
N Wash/sample time 40/80 72/108
0 P04 (ug/ml) 0 30
P Cations (Fe, Al, Ca) (jig/ml) .0 30 of each
Q Color and turbidity No Yes
R Sample SOT concentrations (ug/ml) 15 40
S Dummy
15
-------
ABCDEFGHIJKLMNOPQRS
1 + + + + + + + + + + 1
2+ + + + +'+ + + + + 2
3 + + + + + + + + + + 3
4 + + + + + + + + ++ 4
6 + + + + + + + +++6
Qj_ i i _|_J_ _1.4_ » i i 0
9 + + ++ ++ -fc + + + 9
10+ + ++ ++ + + + + 10
11 + ++ ++ + + + ++11
12 + + + + + + + + + + 12
13 ++ ++ + + + + + +13
14 ++ ++ + + + + + + 14
15 ++ ++ + + + + + + 15
16 ++ ++ + + + + + + 16
17 + + +++ + + + + + 17
18 + + + + + + + + + +18
19 ++ + + + + + + + + 19
20 20
+ = high level
Blank = low level
Figure 2 - Schematic of Ruggedness Test
16
-------
-TABLE 9
ANALYSIS OF VARIANCE OF RUGGEDNESS TEST
Variable
A
0
F
J
H
E
C
G
B .
K
D
I
Q
L
P
M
N
S
(dummy)
R .
e*'
d.f.
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
320
M.S.
539.64
475.79
384.71
236.89
210.45
192.32
159.35
136.93
130.30
107.47
92.80
66.72
34.96
14.39
9.24
6.44
5.88
4.17
4.09
1.56
F-Testil/
345.02
304.20
245.97
151.46
134.55
122 .96
101.88
87.54
83.31
68.71
59.33
42 .66
22.35
9.20
5.90
4.12
3.76
2.67
2.61
F-Test-/
129.41
114.10
92.26
56.81
50.47
46 . 12 .
38.21
32.84
31.25
25.77
22.25
16.00
8.38
3.45
2.22
1.54
1.41
-
0.98
—
d/
Contribution— to
Total Variance (%)
19.2
16.9
13.7
8.4
7.5
6.8
5.7
4.9
4.6
3.8
3.3
2.4
1.2
0.5
0.3
0.2
0.2
0.1
0.1
a/ Based on the replicate analyses.
b_/ Using the replicate residual.
c_/ Using the dummy residual.
d/ Ignoring the e term, which is of trivial size, anyway.
Cumulative
Variance (%)
19.2
36.1
49.8
58.2
65.7
72.5
78.2
83.1
87.7
91.5
94.8
97.2
98.4
98.9
99.2
99.4
99.6
99.7
99.8
-------
The "replicate residual" and the "dummy residual" are both used for com-
pleteness. Fortunately, the two residuals are not very different in size.
Therefore, it appears plausible to clain that interactions among the real
variables are not present.to any large extent. .
For convenience, in Table 9 the variables are listed in decreas-
ing order of importance. Regardless of which column of F-values is "best,"
the contribution to the total variance is the statistical criterion for
evaluating the significance of the variables.
Table 9 shows that the three variables A, 0, and F account for
half of all the variability in the MTB method. The first seven variables
account for almost four-fifths of the variability in the data, and every
variable below accounts for less than 5%. Probably the first 7 or 10 var-
iables on the list are the only ones with a practical significance as far
as the MTB method is concerned. Unfortunately, this is still a rather long
list.
Although the primary objective of the ruggedness test is to de-
cide which variables are important, the actual magnitude of the effects is
also important. The ruggedness test analysis directly examines the differ-
ence in bias between high and low sulfate levels, i.e., labels a variable
significant if and only if the variation in bias is larger. It is also de-
sirable to examine the magnitude of the bias (without reference to changes
in bias between levels). Table 10 illustrates both quantities. For example,
variable A is significant in the statistical analysis because 2.20 - (-0.80)
= 3.00 is a large change in bias. However, the average (positive) bias is
also worth noting, i.e., the average calculated value is significantly less
than the average sample value. The ruggedness test shows that all of the
variables examined will produce analytical results that are high (positive
bias). No single variable produced a large effect. However, the combined
effect of a change in the flow rates (i.e., the quantity of each reagent
delivered) could be substantial. This change would most likely be observed
as a change in sensitivity. Since standards are analyzed before, during,
and after each analysis, a change in sensitivity should have a minimal af-
fect on the accuracy of the determination. One of the inherent advantages
of an automated analysis is the capacity to run numerous standards during
the analysis so that the response of the system is well defined for a given
period.
On the other hand, a substantial loss in sensitivity will have a
more serious affect on the accuracy of the method. Therefore, it is impor-
tant that near nominal flow rates be maintained.
18
-------
TABLE 10
VARIABLE EFFECTS IN SULFATE ANALYSIS
Variable
A
0
F
J
H
E
C
- G
B
K
D
I
Q
L
P
M
N
S
R
Average
a/
Response^/
(Ug/ml)
2.20
2.11
1.97
1.70
-0.23
-0.19
-0.11
-0.05
1.44
0.04
0.08
1.23
0.32
0.95
0.51
0.54
0.55
0.57
0.57
Average
a/
Response—
(ug/ml)
-0.80
-0.70
-0.56
-0.29
1.64
1.60
1.52
1.46
-0.03
1.37
1.33
0.18
1.09
0.46
0.90
0.87
0.86
0.84
0.84
A
(tag/ml)
' 3.00
2.81
2.53
1.99
1.87
1.79
1.63
1.51
1.47
1.33
1.25
1.05
0.77
0.49
0.39
0.33
0.31
0.27
0.27
at Response in sample value/calculated value.
b_/ Calculated from (A-100/2) .L average sulfulate level.
Average Bias of
Variable
5.45
5.11
4.60
3.62
3.40
3.25
2.96
2.75
2.67
2.42
2.27
1.91
1.40
0.89
0.71
0.60
0.56
0.49
0.49
19
-------
Although the basic flow rates are controlled by the automated
equipment and are beyond the control of the analyst, some components of the
system can affect the overall performance. Checks should therefore be made
of these components. These components include the pump tubing which should
be checked when installed and frequently thereafter to establish that the
proper delivery rates are being maintained. Pump tubing should be replaced
at periodic intervals and whenever the delivery rate changes. The pump
should be well maintained, the system should be frequently checked for re-
striction in the flow lines and the ion exchange column, and proper func-
tioning of the pulse suppressors. The.reagents should also be carefully
prepared since concentration changes will affect the quantity of reagent
delivered to the system, a change which is essentially the same as a change
in flow rates.
Other than the above considerations, the only other significant
variable was the interference produced by the phosphate ion. Currently this
is related to the composition of the sample and is beyond the control of the
analyst.
III. METHOD STRENGTHENING
The method write-up originally submitted has been .rewritten to
incorporate editorial and technical comments. The results of the rugged-
ness test were then used to strengthen the method weakness when possible.
This tentative final method write-up of the methylthymol blue procedure is
attached as an appendix.
The largest contributing actor was the NaOH concentration (final
pH). The specification on the NaOH concentration was not changed but the
flow rate of all reagent lines was modified to increase the delivery pre-
cision. A frequent check on the flow rates was also included. This general
method strengthening should correct variables A, F, J, H, E, C, G, B, K, D,
and I or the 11 most significant functions with the exception of the phos-
phate interference. The improvement obtained in the method by these modifi-
cations has not been evaluated.
It may be possible to remove the phosphate interferences using
lanthanum salts as recommended by Palaty,i2' calcium or zinc salts as rec-
ommended by Belcher!^/ and Jones. —' The applicability of these procedures
to the MTB method was not evaluated during this program. The remaining six
functions produce a variance of 2.6%. There is no solution apparent to fur-
ther correct function Q (color and turbidity) and P (cations). The function
L (analysis temperature) has been controlled by a change in the method write-
up.
20
-------
The function M (MTB age) is resolved in the method by the instruc-
tion "make fresh daily."
We have not corrected the write-up for N (wash/sample time) or R
(sample concentration). The effect of both of these functions are small—
0.2 and 0.1% variance. The decision not to strengthen the method for these
two functions was made on the basis of optimization. The effect of sample
concentration (0.1%) could be eliminated by limiting the method to a range
of 5 to 30 ug/ml. It is believed that the minor improvement obtainable does
not justify such a severe range limitation.
The sampled wash ratio was investigated further after the rugged-
ness test. An improvement can be obtained by going to a 20 sample/hr cam
with a 72-sec sample time and a 108-sec wash time. This timing arrangement
appears to be near the optimum for the system. However, a decrease in the
analysis rate of 10 samples/hr is probably not justified for a gain of only
0.2% variance.
21
-------
REFERENCES,
1. Lingemen, R., Standard Methods of Chemical Analysis, 6th ed., Vol. 3,
Part B, F. W. Welcher, Editor, D. Van Nostrand Company, Inc.,
Princeton, New Jersey, p. 975 (1966).
2. Korbl, J., "New Metallochromic Indicator of Complexon Type," Chem. Ind.,
p. 233 (1957).
3. Korbl, J., "Methylthymol Blue, a New Metallochromic Indicator of the
Complexon Type," Chem. Listy., 51, 302 (1957).
4. Korbl, J., "Methylthymol Blue, a New Metallochromic Indicator of the
Complexon Type," Chem. Listy.. 51. 1061 (1957).
5. Korbl, J., "Methylthymol Blue, a New Metallochromic Indicator of the
Complexon Type," Chem. Listy.. 51, 1304 (1957).
6. Korbl, J., "Methylthymol Blue, a New Metallochromic Indicator of the
Complexon Type," Chem. Listy.. 51, 1680 (1957).
7. Pribil, R., "Contributions to the Basic Problems of Complexometry - I,
The Blocking of Indicators and Its Elimination," Talanta, 3, 91-94
(1959). ~~
8. Lazrus, A. L., "A New Colorimetric Microdetermination of Sulfate Ion,"
Automation in Analytical Chemistry, Technicon Symposia, 1965,
Mediad, Inc., pp. 291-293 (1966).
9. Lazrus, A. L., "New Automated Microanalysis for Total Inorganic Fixed
Nitrogen and for Sulfate Ion in Water," Advan. Chem. Ser., 73, 164-
171 (1968). ~~
10. Morgan, G. B., "Automated Laboratory Procedures for the Analysis of
Air Pollutants," Anal. Instr.. 4, 101-112 (1966).
11. Bostrom, E. E., "Improvement in Automatic Colorimetric Determination
of Low Concentrations of Sulfate," Atmos. Envirn., 1(5):599 (1967).
12. Technicon, "Sulfate in Water and Wastewater," Technicon Industrial
Method No. 118-7IW/Tentative, Tarrytown, New York (1972).
22.
-------
13. "Determination of SuIfates—Automated Methylthymol Blue Method,"
National Air Surveillance Network Method, U.S. Environmental Pro-
tection Agency.
14. Palaty, V., "Determination of Sulfate," Chem. and Ind.. p. 176 (1960).
15. Belcher, R., "Substituted Benzidines and Related Compounds as Reagents
in Analytical Chemistry. Part XII. Reagents for the Precipitation
of Sulfate," J. Chem. Soc., pp. 1334-1337 (1953).
16. Belcher, R., "The Titrimetric Determination of Sulphate with 4-Amino-
4'-Chlorodiphenyl Hydrochloride as Reagent," Analyst, 81, 4-8 (1956),
17. Jones, A. S.., "A Submicro Method for the Estimation of Sulfur," Chem.
and Ind., 662(23):662-663 (1954).
23
-------
APPENDIX
TENTATIVE METHOD FOR THE DETERMINATION OF
SULFATES IN THE ATMOSPHERE (AUTOMATED
METHYLTHYMOL BLUE PROCEDURE)
-------
ENVIRONMENTAL PROTECTION AGENCY
QUALITY ASSURANCE BRANCH
ENVIRONMENTAL MONITORING AND SUPPORT LABORATORY
NATIONAL ENVIRONMENTAL RESEARCH CENTER
RESEARCH TRIANGLE PARK, NORTH CAROLINA 27711
Tentative Method for the Determination of
Sulfates in the Atmosphere (Automated
Methylthymol Blue Procedure)^-'
A tentative method is one which has been carefully drafted from
available experimental information, reviewed editorially within the
Methods Standardization Branch and has undergone extensive labora-
tory evaluation. The method is still under investigation and there-
fore, is subject to revision.
-------
1. Principle and Applicability
1.1 Ambient sulfates are collected by drawing air through a glass fiber
filter with a high-volume pump. The filters are extracted with water and
the extract is treated with barium chloride and methylthymol blue at a pH
of 2.8. After the barium sulfate precipitates, the pH is increased to
12.4 and the unreacted barium forms a chelate with the MTB. The uncomplexed
MTB remaining is then measured colorimetrically at 460 nm.
1.2 The method is applicable to the collection of 24-hr samples in the
field and subsequent analysis in the laboratory.
2. Range and Sensitivity
2.1 The range of the analysis is 5 to 60 ug SO^/ml. With a 50-ml
extract from 1/12 of the high volume filter collected at a sampling rate
of 1.7 M-Vminute (60 cfm) for 24 hr, the range of the method is 1 to 12 ug/M^.
The lower range may be extended up to 12-fold by increasing the portion of
the filter extracted. The upper limit may be increased by diluting the
sample with distilled water.
2.2 Using the procedure outlined, a concentration of 5 ug S07/ml
will produce a scale deflection of 1.6% of full scale (signal/noise ratio
of 2).
3. Interferences
3.1 Sulfides, sulfites, and phosphates produce a positive interference
which is dependent on the concentration of sulfate and the interferring ion.
3.2 The interferences from cations are eliminated by passing the sample
through an ion-exchange column.
-------
4. Precision and Accuracy
4.1 The relative standard deviations for sampling SOT concentrations
of 30 and 60 ug/M3 are 2 and 1%, respectively.
4.2 The accuracy of the method in the range of 0 to 10 ug/M3 is
-5.8%; 10 to 20 ug/M3 is -5.7%; 20 to 40 ug/M3 is -2.5%, and over 40 ug/M3
is -8.7%.
5. Apparatus
5.1 Sampling: Apparatus as specified in "Appendix B - Reference
Method for the Determination of Suspended Particulates in the Atmosphere
(High Volume Method)," Federal Register. 36_ (84): 8191-8194, April 30, 1971,
shall be used.
5.2 Analysis
5.2.1 Automatic Analyzer: An automated analytical system must
be used for the determination of water soluble sulfate by this method.
Alkaline solutions of methylthymol blue decompose on exposure to air.
The method therefore cannot be adapted to a manual procedure. The automated
system consists of the following components:
5.2.1.1 Sample Turntable: With a 30 sample/hr cam having a
1:2 sample to wash time ratio.
5.2.1.2 Proportioning Pump: Capable of supplying independently
variable flow rates in eight supply lines.
5.2.1.3 Mixing Coils: Three small 14-loop (7.5 cm long)
mixing coils.
5.2.1.4 Pulse Supressors: Two pulse supressors are required,
one on the methylthymol blue line and one on the sodium hydroxide line as
shown in Figure 1. 3
-------
omaii o Mixing v_on
Pulse Supressor
Ion -Exchange
•i
—i
V|
e_
Debubbler
»
• j
-^ Waste
Debubbler
1
-
-
W W
R R
Y Blue
O W
Y Y
Grey Grey
Grey Grey
Purple Black
Proportioning
Pump
-
-
1 '
0.60
0.80 c ,
^ ]-40 Wnt°r
_ 0.23 r , A.
1 .20 Methylrhymol
^ Blue
1 -00
_ 1 .00 Sodium
^ Hydroxide
— 2.90
Flow Rates,
ml/min
Colorimeter
15mm Tubular Flow Cell
460 nm
Figure 1 - Analytical Flow Diagram
-------
5.2.1.5 Ion-Exchange Column: Interferrlng heavy metals are
removed by the use of an ion-exchange resin. Use Dowex 50W-X8, sodium form,
300-850 urn (20-50 mesh). The resin should be stirred into distilled water
and the fines discarded before they can settle. The resin should be soaked
before use, at least overnight, and may be stored under distilled water
until used. To pack a column, a small piece of glass wool is inserted
in one end of a piece of plastic tubing 10 cm (4 in) long and 2.3 mm (0.09 in)
I.D. A rubber pipet bulb is attached to the end of the tubing containing ;
the glass wool plug. The other end of the tubing is placed in the
soaked resin container and the rubber bulb operated until the tubing is
filled with resin. The column must be free of trapped air after filling
with resin. The resin column should be replaced after a full day's use.
5.2.1.6 15 mm Flow Cell Colorimeter: A stable colorimeter
suitable for use at 460 run with a band width of no greater than 18 nm at
half height. Wavelength accuracy should be checked prior to use and quarterly
thereafter.
5.2.1.7 Recorder: Strip chart recorder or digital printer
matched to the colorimeter output.
5.2.1.8 Pump Tubing: Flow rated tubing of the capacities
shown in Figure 1.
5.2.2 Sonic Cleaner: Of suitable size to process the required
number of samples and at least 7.6 cm ( 3 in) deep.
5.2.3 Volumetric Flasks: 100, 200, 500, 1,000 ml capacity.
5.2.4 Pipets: 4, 5, 7, 10, 20, 25, 50-ml volumetric; 10 ml graduated
in 1/10 ml intervals.
-------
5.2.5 Pyrex Glass Wool
5.2.6 Plastic Tubing: 10 cm (3.94 in) and 2.3 mm (0.09 in)'I.D.
5.2.7 Rubber Pipet Bulb
5.2.8 Buchner Funnels: Buchner style 150 ml capacity with fine-
pore fritted glass filter. '
5.2.9 Vacuum Filtering Apparatus: Device which permits vacuum
filtering directly into the receiver. This consists of a bell jar with a
top opening, a side tabulation and a bottom plate. The Buchner funnel passes
through the top opening and is sealed to the bell jar with a stopper. The
bell jar should be tall enough to contain the polyethylene bottles used for
storing the samples. The vacuum connection is made using the side tubulation.
The filtering apparatus is shown in Figure 2.
5.2.10 Vacuum Pump: -Any device which can maintain a vacuum of
at least 64 cm of Hg. Mechanical pumps or water aspriators may be used.
5.2.11 Polyethylene Bottles: Bottles with a capacity of 60 ml
(2 oz) fitted with polyseal caps.
5.2.12 Glass Bottles (clear): 60 ml (2 fl oz) glass bottles with
polyseal caps.
5.2.13 Glass Bottles (brown); 500 ml glass bottles with polyseal
caps.
5.2.14 Graduated Cylinder; 10-ml capacity.
6. Reagents
6.1 Sampling
-------
Buchner Funnel with
Fritted Disc
To Vacuum
Pump
Polyethylene
Bottle
Bell Jar
Baseplate
Figure 2 - Vacuum Filtering Apparatus
-------
6.1.1 Filter Media; Filter media as specified in "Appendix B -
Reference Method for the Determination of Suspended Particulates in the
Atmosphere (High Volume Method)," Federal Register. 36. (84): 8191-8184,
April 30, 1971, shall be used. Each lot of filters should be analyzed
for sulfate and those producing a blank value > 1 ug/ml should be rejected.
6.2 Analysis
6.2.1 Sodium Hydroxide: ACS Reagent Grade.
6.2.2 Barium Chloride: ACS Reagent Grade.
6.2.3 Methylthymol Blue (MTB): 3', 3" -Bis[N,N-Bis (Carboxymethyl)
Amino] Methyl Thymolsulfone-phthalein Pentasodium salt. 96% minimum by
spectro analysis.
6.2.4 Ammonium Chloride: ACS Reagent Grade.
6.2.5 Concentrated Ammonium Hydroxide: ACS Reagent Grade, 28-
30% NH3.
6.2.6 EDTA Tetra Sodium Salt: Tetra sodium ethylenediamine
Tetraacetate, Technical Grade.
6.2.7 Sodium Sulfate: ACS Reagent Grade, anhydrous.
6.2.8 Concentrated Hydrochloric Acid; ACS Reagent Grade, 36.5-
38% HC1.
6.2.9 Ethanol: USP, 95%.
6.2.10 Sodium Hydroxide Solution (0.15 N): Dissolve 4.0 g of
sodium hydroxide in distilled water and make to 500 ml in a volumetric
flask.
6.2.11 Hydrochloric Acid Solution (1.0 N); Add 8.3 ml of concen-
trated hydrochloric acid to water in a volumetric flask and make to 100 ml.
-------
6.2.12 Barium Chloride Solution (0.006 M): Dissolve 1.466 g
of barium chloride dihydrate (BaC^'Zt^O) in distilled water and make to
1,000 ml in a volumetric flask.
6.2.13 Methylthymol Blue Solution: To 0.1301 g of MTB in a
500-ml volumetric flask add successively 25 ml of barium chloride solution,
4 ml of 1.0 N hydrochloric acid, and make to 500 ml with 95% ethanol.
Prepare fresh daily, and store in a brown glass bottle.
6.2.14 Buffer pH 10.1: Dissolve 6.75 g of ammonium chloride
(NH^Cl) in 500 ml of distilled water. Add 57 ml of concentrated ammonium
hydroxide (NH^OH) and dilute to 1,000 ml with distilled water. Adjust the
pH to 10.1 with additional NH4OH.
6.2.15 Buffered EDTA (wash solution); Dissolve 40 g of tetrasodium
EDTA in pH 10.1 buffer solution and make to 1,000 ml with additional
buffer solution.
6.2.16 Stock Sulfate Solution (1,000 ug S04/ml): Dissolve
1.4789 g of sodium sulfate (^SO^), which has been heated at 105°C for
4 hr and cooled in a dessicator over anhydrous magnesium perchlorate, and
dilute to 1,000 ml with distilled water. Store under refrigeration.
6.2.17 Blank Reagent Solution: In a 500 ml volumetric flask add
successively 25 ml of barium chloride solution, 4 ml of 1.0 N hydrochloric acid,
and make to 500 ml with 95% ethanol.
7. Procedure
7.1 Sampling: Sampling procedures as specified an "Appendix B -
Reference Method for the Determination of Suspended Particulate in the
Atmosphare (High Volume Method)," Federal Register, 3£ (84): 8191-8194
(30 April 1971), shall be used.
-------
7.2 Analysis
7.2.1 Sample Extraction: The filters are removed from the folder,
opened flat, and cut into 1.9 by 20.3 cm (3/4 x 8 in) strips using a paper
cutter. The filter should be cut with the particulates face up. One or more
filter strips are placed in a 60 ml (2 oz) glass bottle. Fifty milliliters
of distilled water is pipetted into each bottle which is then closed with a
polyseal cap. The samples are placed in the sonic bath, which should be
refilled with fresh cold tap water to the level of the liquid in the
bottles for each set of extractions. The sonic bath is operated for 30 minutes.
The extracts are immediately vacuum filtered using the Buchner funnels and
the vacuum filtering apparatus. The samples are filtered directly into
polyethylene bottles. The filters should not be washed or squeezed, and
the filtrates are not diluted. After filtering is complete, the polyethylene
bottles are capped with polyseal caps and stored upright until analyzed.
7.2.2 Sample Analysis: An automatic analyzer is employed for
analysis. A flow diagram and reagent flow rates are shown in Figure 1.
The absorbance is measured at 460 nm and a flow cell with a path length of
15 mm is employed. The sample turntable rate is 30 samples per hour with
a 1:2 sample to wash time ratio. The elapsed time between sample pickup
and the corresponding peak is approximately 6 minutes. The instrument should
be zeroed and spanned following the manufacturers directions.
The automatic analyzer is operated at the beginning of each day,
after a fresh ion-exchange column is installed and prior to the first sample
analysis, until a drift free baseline is obtained. This normally requires a
10
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minimum of 2 hr. This is a convenient period in which to run several series
of standards (see 8.2). When a stable baseline is obtained, a final series
of standards should be run. A blank filter strip sample should be inserted
with this set of standards. This will establish the blank absorbance data
required to calculate the final values. Analysis should be conducted in
a laboratory with reasonable temperature control since the method is moder-
ately temperature sensitive.
Approximately 10 ml of each sample to be analyzed should be trans-
ferred to a sample cup and placed in the turntable. Every tenth sample cup
should contain a 30 ug S07/ml standard. A second set of standards should
be run half way through the analysis. The baseline will remain noisy through-
out the day with some peak tailing. Peak height should be measured perpen-
dicular to the baseline to the highest part of the peak. Samples which
exceed the absorbance of the highest standard of the calibration curve
(60 ug S04/ml) are diluted until the concentration falls within the calibra-
tion range. When operating the automatic analyzer, air bubbles should not be
allowed to enter the ion-exchange column. If air bubbles become trapped,
the ion-exchange column should be replaced with a new column. A broadening
of the colorimeter output with a corresponding loss in peak height usually
indicates a performance decay in the pump tubing. At the first indication
of peak broadening, the pump tubing should be replaced. The use of silicon
rubber tubing in place of the standard pump tubing is highly recommended.
Standard pump tubing should be replaced every day if used. Other available
tubing has correspondingly longer life (=- 3 weeks) with silicon rubber tubing
11
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having performed satisfactorily for as long as 5 weeks. If a plasticized
tubing is used, it should be washed with acetone followed by distilled water
prior to its use.
If the extracts are highly colored or contain suspended particulate,
a blank must be run on the samples. This may be accomplished by replacing
the MTB solution (6.2.12) with blank reagent solution (6.2.16)
and performing the analysis for a second time. The absorbance values from
the sample blanks should then be used in calculating the final sulfate
concentration.
At the end of each day of analysis, a third series of standards
should be run, including a filter blank. A random 5-107» of the samples
should be rerun throughout the day to maintain internal quality assurance.
After completing the final analysis, the system should be cleaned
with the EDTA solution. With the analyzer operating, place the MTB line
and the NaOH line in distilled water for 2-3 minutes. Then transfer the
MTB and NaOH lines to the EDTA solution container for 10 minutes. All
liquid lines are then finally washed with distilled water for 15 minutes
before shutting down the analyzer. The sample line may be conveniently
washed during this operation by shutting off the turntable when the sample
probe is in the wash position. All liquid lines should be left filled with
water after the system has been washed. A coating will slowly develop on
the internal parts of the flow system. When the coating becomes noticible,
the mixing coils should be cleaned by pumping IN ammonium hydroxide through
the system. The rate at which the coating develops is variable depending on
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the nature of the samples being analyzed. A coating will slowly build up
on the flow cell windows which is not removed by the NH^OH wash. This build
up is indicated by a loss in colorimeter sensitivity and may be corrected
by washing the cell with IN HC1 followed by an acetone and then a water wash.
8. Calibration
8.1 High Volume Sampler: The high volume air sampler shall be calibrated
as specified in "Appendix B - Reference Method for the Determination of
Suspended Particulates in the Atmosphere (High Volume Method)," Federal
Register. 36_ (84): 8191-8194, April 30, 1971.
8.2 Automatic Analyzer
8.2.1 Flow Rates: The flow rates in the automatic analyzer
system should be checked when the system is originally set up and once a
week thereafter. It should also be checked when any systems substitutions
are made. Disconnect the specific line, as it leaves the pump, and insert
the line in a 10 ml graduated cylinder. Operate the pump for 2 minutes.
If the flow rate is in error by more than 5%, change the pump tubing and
recheck the flow.
The flow rates indicated in Figure 1 will produce the recommended
reagent ratios. Minor variation in flow can be tolerated as long as they
are constant since they will be corrected for in the calibration procedure.
8.2.2 Colorimeter Wavelength; The uncomplexed MTB at a pH of 12.4
has a maximum absorbance at 460 nm. If the colorimeter is of the interference
filter type, the wavelength accuracy should be checked prior to use and
quarterly thereafter. Maximum transmission of the filter should occur at
460 + 15 nm. During the accuracy check, the filter must be mounted perpendi-
cular to the light beam. Nonperpendicular mounting will produce an apparent
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shift in the wavelength. If a colorimeter (spectrophotometer) with a prism
or grating is used, the accuracy of the wavelength scale should be checked
with a reference standard or by determining the indicated wavelength of
minimum absorbance in the 460 nm area. This should be done under static
conditions (pump shut off) with the cell containing a MTB solution (6.2.13)
at a pH of 12.4.
8.2.3 Concentration Standards; Dilute 50.0 ml of stock sulfate
solution containing 1,000 pig SO^/ml to 500 ml with distilled water. This
intermediate sulfate solution contains 100.0 ug SO^/ml. Ptpet 5, 10, 10,
15, 20, 50, and 60 ml of the 100 jig S0^=/ml solution into 100, 100, 50, 50,
100, and 100 ml volumetric flasks and dilute to the mark with distilled
water. The solutions contain 5, 10, 20, 30, 40, 50, and 60 ug S0^~/ml,
respectively.
8.2.4 Calibration Curve: The absorbance of the reaction products
does not conform to Beer's Law. The plot of peak height versus concentration
is hyperbolic rather than linear - a hyperbolic curve is expressed by Y =
X/(A + BX). If Y equals peak height in millimeters and X equals the sulfate
concentration in micrograms per milliliters; typical values will be for A:
0.3 to 0.4, and for B: 5 x 10"4 to 1.5 x 10"3.
Three series of standards are run each day: one at the beginning
of analysis (after system is stable), one at the midpoint of analysis, and
one after the final analysis. Each of the three standard series are sub-
jected to a least squares hyperbolic curve fit. The average values for A
and B from the first and second standard series are used to calculate con-
centrations from unknown peak heights for analysis performed during this
14
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period. The peak heights for che 30 ug SoT/ml samples analyzed with the
unknowns in which the 30 ug standard fails to agree should be rerun. The
same procedure should be applied to the second half of the analysis using
the second and third series of standards.
8.2.5 Frequency of Calibration; The calibration should be per-
formed daily following the procedure outlined in 8.2.4.
9. Calculations .
9.1 High Volume Sampler; Calculations relating to the high volume
sampler shall be done in accordance with "Appendix B - Reference Method
for the Determination of Suspended Particulates in the Atmosphere (High
Volume Method)," Federal Register. 36(84):8191-8194, April 30, 1971.
9.2 Calculate Constants A and B
Y = X/(A + BX)
where A and B = Constants
Y = Peak height of standard, mm
X = Cone, of standard, S04/ml
9.3 Sulfate Concentration; Calculate concentration of SO* ug/ml
using values for constants A and B obtained in 9.2.
X- —
X 1-BY.
where A and B = Constants
Y = Peak height of unknown, mm
X = Cone. SC Ug/ml
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9.4 Calculate Total ]ig SO^ Collected
X V-[Wf
T = ~^T
where T = Total ug 864 collected, ug
X = Concentration of sample from 9.4, ug/ml
V;i = Volume of extraction liquid, ml (normally
50 ml)
Wf = Width of exposed filter, cm (normally 20.3 cm)
Ws = Width of Sample analyzed, cm (normally 1.9 cm)
9.5 Calculate Concentration of Ambient Sulfates
C = ^
V
where C = Concentration of ambient sulfates, ug/m^
T = Total ug SC£, ug from 9.5
V = Total volume sampled, nr from 9.2.2 of "Appendix B •
Reference Method for the Determination of Sus-
pended Particulates in the Atmosphere (High
Volume Method),"Federal Register, 36(84): 8191-
8194, April 30, 1971.
10. References
10.1 "Appendix B - Reference Method for the Determination of Suspended
Particulates in the Atmosphere (High Volume Method)," Federal
Register, 36(84):8191-8194, April 30, 1971.
10.2 Lazrus, A. L., K. C. Hill, and J. P. Lodge, "A New Colorimetric
Microdetermination of Sulfate Ion," Presented at the Technicon
Symposium, "Automation in Analytical Chemistry," New York,
New York, September 8, 1965.
10.3 Prince, J. F., "A Comparison Study of Glass Fiber Filter Extrac-
tion Technique," unpublished EPA report, February 1975.
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EMSL/RTP No. 7
TECHNICAL REPORT DATA
(Please read Instructions on the reverse before completing)
1. REPORT NO.
2.
3. RECIPIENT'S ACCESSION-NO.
4. TITLE AND SUBTITLE
MEASUREMENT OF ATMOSPHERIC SULFATES:
METHYLTHYMOL BLUE METHOD
EVALUATION OF THE
5. REPORT DATE
April 2, 1976
6. PERFORMING ORGANIZATION CODE
7. AUTHOR(S)
Fred J. Bergman and Michael C. Sharp
8. PERFORMING ORGANIZATION REPORT NO,
9. PERFORMING ORGANIZATION NAME AND ADDRESS
Midwest Research Institute
425 Volker Boulevard
Kansas City, Missouri 64110
10. PROGRAM ELEMENT NO.
1HD621
11. CONTRACT/GRANT NO.
68-02-1728
12. SPONSORING AGENCY NAME AND ADDRESS
Environmental Monitoring and Support Laboratory
Office of Research and Development
U.S. Environmental Protection Agency
Research Triangle Park. N.r. 77711
13. TYPE OF REPORT AND PERIOD COVERED
Final
14. SPONSORING AGENCY CODE
EPA-ORD
15. SUPPLEMENTARY NOTES
16. ABSTRACT
The methylthymol blue (MTB) procedure used by EPA for sulfate analysis has
been subjected to a ruggedness test. The method write-up has been modified to
strengthen the procedure where required. A new procedure developed by EPA for
extracting sulfates from the filter samples has been incorporated in the write-up.
The absorbance for the MTB procedure has been established as hyperbolic. The
method, as now presented, appears to be reasonably rugged with the exception of
phosphate interference, which remains a problem. A potential approach for eliminat-
ing the phosphate interference is presented.
7.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.lDENTIFIERS/OPEN ENDED TERMS
c. COS AT I Field/Group
Air pollution
Sulfate analysis
Methyl thymol blue
Ruggedness test
13B
18. DISTRIBUTION STATEMENT
RELEASE TO PUBLIC
19. SECURITY CLASS (ThisReport)
UNCLASSIFIED
21. NO. OF PAGES
46
20. SECURITY CLASS (Thispage)
UNCLASSIFIED
22. PRICE
EPA Form 2220-1 (9-73)
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