ENVIRONMENTAL HEALTH SERIES
Air Pollution
METHODS OF MEASURING
AND MONITORING
ATMOSPHERIC SULFUR DIOXIDE
U.S. DEPARTMENT OF HEALTH, EDUCATION, AND WELFARE
Public Health Service
-------
METHODS OF MEASURING
AND MONITORING
ATMOSPHERIC SULFUR DIOXIDE
Seymour Hochheiser
U.S. DEPARTMENT OF HEALTH, EDUCATION, AND WELFARE
Public Health Service
National Center for Air Pollution Control
l
Cincinnati, Ohio 45237
August 1964
-------
The ENVIRONMENTAL HEALTH SERIES of reports was established
to report the results of scientific and engineering studies of man's en-
vironment: The community, whether urban, suburban, or rural, where he
lives, works, and plays; the air, water and earth he uses and reuses;
and the wastes he produces and must dispose of in a way that preserves
these natural resources. This SERIES of reports provides for professional
users a central source of information on the intramural research activities
of the Centers in the Bureau of Disease Prevention and Environmental
Control, and on their cooperative activities with State and local agencies,
research institutions, and industrial organizations. The general subject
area of each report is indicated by the letters that appear in the publica-
tion number; the indicators are
AP - Air Pollution
RH - Radiological Health
UIH - Urban and Industrial Health
Triplicate tear-out abstract cards are provided with reports in the
SERIES to facilitate information retrieval. Space is provided on the cards
for the user's accession number and additional key words.
Reports in the SERIES will be distributed to requesters, as supplies
permit. Requests should be directed to the Center identified on the title
page.
Public Health Service Publication No. 999-AP-6
-------
FOREWORD
Use of uniform analytical procedures for the determination of air pollutants
would eliminate many of the difficulties presently encountered in the evaluation
of data produced by the various analysts in the field of air pollution control.
It is very difficult to assess and compare the nature and amount of air pollu-
tants present in different environments when many and various methods are
employed, each subject to different interferences and to inherent and systematic
errors. So that the data reported may be more meaningfully utilized by people
engaged in the various aspects of air pollution, it is desirable to have uniform
operating procedures.
A literature review of methodology relating to the measurement of atmospheric
sulfur dioxide and a description of recommended methods are presented in this
report.
Selection of the methods described in detail herein for manual and automatic
sampling and analysis of atmospheric sulfur dioxide was based on informa-
tion currently available. It is recognized that further information on the meth-
odology is desk-able. It is also recognized that new, more nearly ideal methods
may be developed. Further research and investigation to those ends is to be
desired.
This publication of the Division of Air Pollution, Public Health Service, is in-
tended to serve as a resource document for those involved in measurement of
pollution and in research on new or improved methods, and for those who
seek to bring about widespread agreement in matters concerning measurement
of pollution.
Vernon G. MacKenzie
Chief, Division of Air Pollution
Bureau of State Services
Public Health Service
U. S. Department of Health,
Education, and Welfare
-------
ACKNOWLEDGEMENTS
Grateful appreciation is extended to those individuals who gave unselfishly
of their tune and effort in reviewing this report and in contributing many help-
ful suggestions. The author wishes to acknowledge particularly the assistance
and support provided by: Members of the Interbranch Chemical Advisory
Committee, Division of Air Pollution; by Dr. Aubrey P. Altshuller, Gilbert L.
Contner, Thomas R. Hauser, Charles L. Punte, Jr., Dr. Bernard E. Saltzman,
Stanley F. Sleva, Mario Storlazzi, Elbert C. Tabor; and by Austin N. Heller
and Jean J. Schueneman, Technical Assistance Branch, Division of Air Pollution;
and by John S. Nader, Laboratory of Engineering and Physical Sciences,
Division of Air Pollution.
IV
-------
CONTENTS
ABSTRACT vii
CRITERIA FOR SELECTION OF
RECOMMENDED METHODS 1
Manual Sampling and Analysis 1
Sensitivity 1
Specificity 1
Reproducibility 1
Stability of Reagents and Products 2
Collection Efficiency 2
Simplicity and Directness 2
Difficulty in Calibration 3
Automatic Instrumental Methods 3
Speed of Reaction 3
Temperature Coefficient 3
Instrument Drift 4
Flow or Pressure Regulation 4
Instrument Maintenance Requirement 4
RECOMMENDED METHODS 4
Recommended Methods for
Manual Sampling and Analysis 4
West and Gaeke Method 4
Introduction 4
Reagents 5
Apparatus 6
Analytical Procedure 6
Preparation of Calibration Curve 8
Discussion of Procedure 8
Hydrogen Peroxide Method 9
Introduction 9
Reagents 9
Apparatus 10
Analytical Procedure 10
Discussion of Procedure 11
Recommended Procedure and Specifications
for Automatic Monitoring Instrument 12
Electroconductivity Analyzer 12
Introduction 12
Reagents 12
Apparatus 13
Calibration 14
Procedure 15
Instrument Performance Specifications 16
Discussion 17
-------
COMPARISON OF METHODS CONSIDERED 18
Manual Methods 18
Instrumental Methods 23
MANUAL METHODS CONSIDERED IN
ADDITION TO RECOMMENDED METHODS 23
Colorimetric Methods 23
Fuchsin-Formaldehyde Method 23
Stratman Method 26
Barium Chloranilate Method 26
Zinc Nitroprusside Method 26
Astrazone Pink Method 27
Diazo Dye Method 27
Thorium Borate-Amaranth Dye Method 28
Indirect Ultraviolet Determination of SO 2 by Means of
Plumbous Ion 28
Ferrous-Phenanthroline Method 29
Polarographic Method 29
lodimetric Methods 30
Iodine Method 30
Direct Iodine Method 30
lodine-Thiosulfate Method 31
Turbidimetric Method 31
Cumulative Methods 32
Lead Peroxide Candle Method 32
Test Paper Method 34
Detector Tubes 35
INSTRUMENTAL METHODS CONSIDERED
IN ADDITION TO RECOMMENDED METHODS 35
Potentiometric Methods 35
Photometric Methods 37
Analyzer "A" 37
Analyzer "B" 38
Analyzer "C" 39
Analyzer "D" 40
Air lonization Method 41
REFERENCES 43
vi
-------
ABSTRACT
A literature review of methodology relating to the measurement of
atmospheric sulfur dioxide, a detailed description of recommended
methods, and criteria for selection of recommended methods are pre-
sented in this report. This publication is intended to serve as a resource
document for those involved in measurement of pollution and in research
on new or improved methods, and for those who seek to bring about
widespread agreement in matters concerning measurement of pollution.
-------
CRITERIA FOR SELECTION OF
RECOMMENDED METHODS
Manual Sampling and Analysis
Evaluation of methods for the measurement of air pollutants was based
upon an examination of systematic characteristics such as: Sensitivity, specificity,
reproducibility, stability of reagents and products, collection efficiency, sim-
plicity and directness, and difficulty of calibration.
SENSITIVITY
The quantity of air needed to be sampled to provide a detectable amount
of the air pollutant is a function of the sensitivity of the analytical technique.
To measure fluctuation in pollution amounts during short-term fumigations
it is necessary to use methods capable of detecting extremely small quantities
of the gaseous pollutant. When integrated values over a longer time interval
are desired a less sensitive method is applicable. Nevertheless, methods ca-
pable of sensing short-term fluctuations in gas concentrations produce more
meaningful data, and a highly sensitive method is, therefore, most often
desirable.
SPECIFICITY
The effect of interfering materials present in the concentration range anti-
cipated is an important consideration in the selection of suitable air-monitoring
methods. In many instances it is possible to eliminate interfering materials by
selective absorption or by other chemical reactions and thus avoid errors in
analysis. At times it may be desirable to monitor with a nonspecific method
that measures a group of compounds that have similar chemical properties
and produce the same physiological response. A knowledge of individual
components is, however, extremely valuable in interpretation of the data.
REPRODUCIBILITY
Reproducibility is a measure of the reliability and stability of the equip-
ment, reagents, and technique employed in the method. The accuracy of the
method can be no better than its reproducibility. Relatively good reproducibility
does not, however, necessarily indicate relatively high accuracy. Accuracy,
which is a measure of the deviation from a true value, is determined by
measuring known amounts of a material and observing the correspondence
between the measured and the true values. In air analysis wherein many
-------
METHODS OF MEASURING
methods rely upon the removal of the pollutant from the air mass, accuracy
is a function of collection or separation efficiency, flow metering, analytical
variation, and other sources of error.
STABILITY OF REAGENTS AND PRODUCTS
Reagents and absorbents that have comparatively long shelf lives are
advantageous. Their use eliminates the need for frequent reagent preparation
and calibration. Absorbents that are easily susceptible to changes in com-
position and reactivity owing to light, temperature, turbulence, or air oxidation
may produce serious errors in the analysis. If reaction products formed in air
sampling are unstable, the method becomes unsuitable for use when there is a
tune lag between sampling and analysis, and the method cannot, therefore,
be employed in certain automatic sampling equipment such as sequential
samplers, though it may be satisfactory for automatic, continuous analyzers.
COLLECTION EFFICIENCY
The collection efficiency of methods that require scrubbers for the collection
of a gaseous impurity depends upon the chemical nature of the absorbent and
the pollutant, the physical condition during gas-liquid contact, and the samp-
ling rate, which in effect controls the time of contact between the phases. The
latter condition is particularly applicable to the physical retention techniques
used in adsorption processes. Methods selected for aerosol collection depend
largely upon the size of the particulates in the air or gas stream. To avoid
using multiple separators in a sampling system, a collection efficiency of greater
than 95 percent is generally required of one collector. It would, however, be
permissible to use a sampling system having a relatively low collection effi-
ciency provided that the desired sensitivity, reproducibility, and accuracy are
obtainable. One method of determining collection efficiency is to connect similar
collectors in series downstream and measure the ratio of pollutant occurring
in the second and third units to that in the upstream unit. This is not an
absolute method of determining efficiency since there may be a systematic
threshold concentration below which no reaction occurs. Collection efficiency
is determined by introducing known amounts of gas or aerosol at different
dilutions. A method that has been previously calibrated and results in relatively
high collection efficiency may be used as a standard for comparison of another
method provided that the methods are both subject to the same interferences.
SIMPLICITY AND DIRECTNESS
When a simple technique gives the same degree of accuracy as a more
complicated and time-consuming method, one would obviously chbo.se; the
simpler technique, except, perhaps, when equipment costs render prohibitive
the use of the simpler method.
-------
AND MONITORING SO 2
Indirect methods are subject to greater inaccuracies owing to calibration
difficulties and should be avoided whenever possible. Factors that influence
simplicity are the number and difficulty of unit operations and the type of
laboratory and field sampling controls required.
DIFFICULTY IN CALIBRATION
An ideal method would be one in which a specific compound of known
composition is formed that is not subject to temperature, light sensitivity,
or other instabilities and that can be measured directly and without inter-
ference from other compounds. In this event the quantity of product formed
is directly related to the concentration of the reactants if a quantitative reaction
is assumed. To determine the efficiency of reaction, a known quantity of the
pollutant is introduced into the system at various concentrations corresponding
to those existing in ambient air, and the amount recovered is determined. The
techniques available for this type of standardization are dynamic dilution and
static dilution. The effect of interferences can also be determined by similar
techniques. After primary calibration of the system in the manner indicated
above, secondary calibration methods can be employed, that is, standard
solutions, conductivity cells of known resistance, and so forth.
Automatic Instrumental Methods
All the factors pertinent to the manual measurement of pollutants apply
to measurement in continuous, automatic recording instruments. Other im-
portant parameters are: Speed of reaction, temperature coefficient, instrument
drift, flow or pressure regulation, and instrument maintenance requirement.
SPEED OF REACTION
The time required to attain reaction equilibrium is an important considera-
tion in the choice of reagent to be used in continuous analyzers, particularly
in colorimetric recorders. Many colorimetric methods require a definite time
for complete reaction, and it may become necessary, therefore, to alter reagent
composition or calibrate at a definite time after sampling prior to complete
reaction, to attain conditions consistent with instrument response. Instrument
response, which is a function of flow rate and distance traveled by the air
sample and reagent prior to contact with the sensing mechanism, should
be minimal.
TEMPERATURE COEFFICIENT
The instrument should be calibrated at different temperatures corresponding
to actual operating conditions to determine the effect of temperature so that
proper corrections can be applied. Instrumental methods employing reagent and
-------
METHODS OF MEASURING
detection systems that vary as little as possible with temperature should be
selected. Many methods of analysis and systems of detection are sensitive
to temperature, and thermostatic regulation is required to minimize this source
of error.
INSTRUMENT DRIFT
Changes in zero and span or gain settings should be minimal to avoid
errors in the recordings and frequent calibration. Limitation of drift to a
maximum of 1 percent in a 24-hour period would be desirable. Excessive
electronic noise interferes with the interpretation of results. Unfortunately,
in many instruments, noise level varies directly with sensitivity so that noise
level will affect the maximum sensitivity attainable. Limitation of noise level
to a maximum of 1 percent of full scale would be desirable.
FLOW OR PRESSURE REGULATION
Nonuniform airflow or pressure, and reagent flow, where appropriate, may
produce serious errors in the analysis since the concentration recorded is a
function of the quantity of air sampled (except in closed-path instruments).
This is especially important when comparatively low flows are employed, since
changes in flow would result in a higher percentage of error. For this reason,
constant-flow devices should be employed, and flow rates should be automat-
ically recorded. Calibration curves of variation of concentration with flow
should be prepared, and the instrument should be operated under sampling
conditions least affected by flow variations.
INSTRUMENT MAINTENANCE REQUIREMENT
The instrument should be constructed in such a manner as to require the
minimum amount of attention and should be designed for simple maintenance.
Parts that require more frequent replacement or servicing should be easily
accessible. Complete instructions and diagrams for maintenance and trouble-
shooting should be provided. No more than 1 day per week of attended
operation would be desirable.
RECOMMENDED METHODS
Recommended Methods For
Manual Sampling and Analysis
WEST AND GAEKE METHOD l
Introduction
The West-Gaeke method is applicable to the determination of sulfur dioxide
(SO2) in outside ambient air in the concentration range from about 0.005 to
-------
AND MONITORING SO 2
5 ppm1 SO2 in the air sample is absorbed in 0.1 molar sodium tetrachloro-
mercurate. Nonvolatile dichlorosulfitomercurate ion is formed in this process.
Addition of acid-bleached pararosaniline and formaldehyde to the complex
ion produces red-purple pararosaniline methylsulfonic acid, which is deter-
mined spectrophotometrically.2 The system obeys Beer's law up to about
10 microliters of 862 per 10 milliliters of absorbing solution. This method
is more sensitive than the hydrogen peroxide method and is not subject to
interference from other acidic or basic gases or solids such as SO3, H2S04,
NHs, or CaO; the analysis should, however, be completed within 1 week
after sample collection, and the concentrations of ozone and NO2 should be
less than that of the SO2. 3'4'5
Reagents
All chemicals used must be A. C. S. analytical-reagent grade.
Absorbing Reagent, 0.1 Molar Sodium Tetrachloromercurate. Dissolve
27.2 grams (0.1 mole) mercuric chloride and 11.7 grams (0.2 mole) sodium
chloride in 1 liter of distilled water. (CAUTION: Highly poisonous; if spilled
on skin, flush off with water immediately.) This solution can be stored at room
temperature for several months.
Pararosaniline Hydrochloride (0.04%), Acid Bleached. Dissolve 0.20 gram
of pararosaniline hydrochloride in 100 milliliters of distilled water and filter
the solution after 48 hours. This solution is stable for at least 3 months if
stored in the dark and kept cool. The pararosaniline used should have an
assay of better than 95 percent and an absorbance maximum at 543 or
544 millimicrons. Pipette 20 milliliters of this into a 100-milliliter volumetric
flask. Add 6 milliliters of concentrated HC1. Allow to stand 5 minutes, then
dilute to mark with distilled water. This solution should be pale yellow with
a greenish tint. It can be stored at room temperature in an amber bottle for
a week or for about 2 weeks if refrigerated.
Formaldehyde, 0.2 Percent Dilute 5 milliliters of 40 percent formaldehyde
to 1,000 milliliters with distilled water. Prepare weekly.
Standard Sulfite Solution. Dissolve 640 milligrams sodium metabisulfite
(assay 65.5% as SO2) in 1.0 liter of water. This yields a solution of approxi-
mately 0.40 milligram per milliliter as SO2. The solution should be standard-
ized by titration with standard 0.01 normal I2 with starch as indicator, and
should be adjusted to 0.0123 normal. Then 1 milliliter = 150 microliters
SO2 (25° C, 760 millimeters Hg). Prepare and standardize freshly.
Starch Solution (Iodine Indicator), 0.25 Percent Make a thin paste of
1.25 grams of soluble starch in cold water and pour into 500 milliliters of
boiling water while stirring. Boil for a few minutes. Keep in glass, stoppered
bottle.
-------
METHODS OF MEASURING
Standard Iodine Solution, 0.01 Normal. Dissolve 12.69 grams of resublimed
iodine in 25 milliliters of a solution containing 15 grams of iodate-free KI;
dilute to the 1,000-milliliter mark in a volumetric flask. Rpette exactly 100
milliliters of this 0.1 normal solution and dilute to 1,000 milliliters in a vol-
umetric flask with 1.5 percent KI. This solution can be used as a primary
standard if the weighing is carefully done, or it can be checked against a
standard thiosulfate solution. This solution should be stored in an amber
bottle, refrigerated, and then standardized on the day of use.
Apparatus
Absorber. An all-glass midget impinger or other collection device capable
of removing SO2 from an air sample using 10 milliliters of absorbing reagent
should be used. (Among the suppliers of midget impingers are Corning Glass
Company and Gelman Instrument Company.)
Air Pump. The air pump should be capable of drawing 2.5 liters per minute
through the sampling assembly.
Air-Metering and Flow Control Devices. Metering and control devices should
be capable of controlling and measuring flows with an accuracy of ± 2
percent. The flow meter should be calibrated for variations in reading with
temperature and pressure of the airstream so that the appropriate corrections
can be applied.
Thermometer (or other Temperature-Measuring Device). The thermometer
should have an accuracy of :± 2°C.
Mercury Manometer (or other Vacuum-Measuring Device). The manometer
should have an accuracy of 0.2 inch Hg.
Spectrophotometer or Colorimeter. Color-measuring devices should be cap-
able of measuring color intensity at 560 millimicrons, in absorbance cells
1 centimeter or larger.
Analytical Procedure
Collection of Samples. Set up a sampling train consisting of, in order,
absorber, trap to protect flow device, flow control and metering devices, tem-
perature and vacuum gauge, and air pump. All probes and tubing upstream
from the bubbler should be pyrex glass, stainless steel, or teflon. Butt-to-
butt connections may be made with tygon tubing. The downstream flow meter-
ing device can be empirically corrected to atmospheric conditions/by conducting
a dummy run with an upstream flowmeter inline that is open to the atmosphere.
Pipette exactly 10 milliliters of absorbing reagent into the absorber. Aspirate
the air sample through the absorber at a rate of 0.2 to 2.5 liters per minute
-------
AND MONITORING SO 2
(depending upon the concentration of S02 in the atmosphere and the sampling
time desired). The sampling time may vary from a few minutes to 24 hours.
For 24-hour sampling the absorber selected should be capable of containing
20 milliliters or more of absorbing reagent. For best results, the sampling time
and rate should be chosen to provide a concentration of approximately 2 to 4
microliters of SO2 in 10 milliliters of the absorbing reagent. The dichloro-
sulfitomercurate formed may be stored for 3 days with only a slight decrease
in strength (about 1 percent per day). If samples are stored for longer periods,
a correctionfactor should be applied. Thesamplemay be stored in the collection
device or transferred to a stoppered glass or polyethylene container.
Analysis. If a mercury precipitate is present owing to the presence in the
air sample of inorganic sulfides, thiols, or thiosulfates, it may be removed by
filtration or centrifugation. To the clear sample, adjusted to 10 milliliters with
distilled water to compensate for evaporative losses, add 1.0 milliliter of
acid-bleached pararosaniline solution and 1.0 milliliter of the 0.2 percent
formaldehyde solution and mix.
Treat a 10-milliliter portion of unexposed sodium tetrachloromercurate
solution in the same manner for use as the blank. If the collecting reagent
remains exposed to the atmosphere during the interval between sampling and
analysis, the blank should be exposed in the same manner.
Allow 20 minutes for maximum color development and read the absorbance
at 560 millimicrons in a spectrophotometer with the blank as reference.
Calculations. Convert the volume of air sampled to the volume at standard
conditions of 25°C, 760 millimeters Hg:
(P- Pm) 298.2
Vs - V x 29 9? x (t + 273.2)
Vs - Volume of air in liters at standard conditions
V = Volume of air in liters as measured by the meter
P = Barometric pressure in inches of mercury
Pm - Suction at meter in inches of mercury
T = Temperature of sample air in degrees centigrade.
Ordinarily the correction for pressure is slight and may be neglected.
Compute the microliters of S02 in the sample by multiplying the absorbance
by the slope of the calibration plot. Then the concentration is:
0_ , , n 1 SO o
ppm SO ^ by volume = • • z
-------
METHODS OF MEASURING
Preparation of Calibration Curve
Pipette exactly 2 milliliters of standard sulfite solution into a 100-milliliter
volumetric flask and dilute to mark with absorbing reagent. This final solution
contains 3.0 microliters SC>2 per milliliter.
Add accurately 0.5-, 1.0-, 1.5-and 2.0-milliliter portions of the dilute standard
sulfite solution to a series of 10-milliliter volumetric flasks and dilute to the
marks with absorbing reagent. Continue with the analysis procedure given
above.
Plot the absorbance (optical density) as the ordinate against the microliters
of SO2 per 10 milliliters of absorbing solution on rectangular coordinate
paper. Compute slope of straight line best fitting the data.
Discussion of Procedure
The error for the combined sampling and analytical technique is ± 10
percent in the concentration range below 0.10 ppm with increasing accuracy
with concentration in the range 0.1 to 1 ppm.
The measurements should be reported to the nearest 0.005 ppm at con-
centrations below 0.15 ppm and to the nearest 0.01 ppm above 0.15 ppm.
03 and NO 2 interfere if present in the air sample at concentrations greater
than SO2.4 Interference of NO2 is eliminated by including 0.06 percent sulfamic
acid in the absorbing reagent.6 This may, however, result in a different cali-
bration curve of lower sensitivity and in greater losses of SO2 on storage of
the sample for more than 48 hours after sample collection. NO2 interference
may also be eliminated by adding o-toluidine subsequent to sample collection. 7
Heavy metals, especially iron salts, interfere by oxidizing dichlorosulfito-
mercurate during sample collection. This interference is eliminated by including
ethylenediaminetetracetic acid in the absorbing reagent.1 Sulfuric acid or
sulfate do not interfere. There is no experimental evidence to indicate that
SO 3 interferes; it probably hydrolizes preferentially in the absorbing reagent
to form H2S04 rather than combines with sodium tetrachloromercurate to
form the dichlorodisulfitomercurate complex ion. If the latter reaction should
prevail, 803 would interfere positively. If large amounts of solid material
are present, a filter may be used advantageously upstream; however, a loss
of SO2 may occur.8
The color produced is independent of temperature in the range 11 to 30°C
and is stable for 3 hours.
Much difficulty with the method has been caused by the use of impure
pararosaniline hydrochloride.9 A commercial brand is now available that is
-------
AND MONITORING SO 2
specially selected for this procedure (Fisher Scientific Company, catalog No.
P-389). The purity of the reagent may be estimated by comparing the slope
of the calibration plot with the value 0.15 absorbance unit per microliter
(obtained with 1-centimeter cells in a Gary spectrophotometer), which corres-
ponds to a molar absorptivity of 36,700.
HYDROGEN PEROXIDE METHOD
Introduction
This method is applicable to the determination of SO 2 in outside ambient
air in the concentration range from about 0.01 to 10.0 ppm. SO2 in the air
sample is absorbed in 0.03 normal hydrogen peroxide (H2O2) reagent (ad-
justed to about pH 5).10'11 The stable and nonvolatile sulfuric acid formed
in this process is titrated with standard alkali. The method requires only simple
equipment and can be performed by analysts having lesser skills; it is pre-
ferable to the West-Gaeke method if S02 is the principal acid or basic atmos-
pheric gaseous pollutant and if long storage of samples (greater than 1 week)
prior to analysis is required.
Reagents
All chemicals used must be A. C.S. analytical-reagent grade.
Absorbing Solution, Hydrogen Peroxide, 0.03 Normal, pH 5. Dilute 3.4
milliliters of 30 percent H2O2 solution to 2 liters with distilled water. Deter-
mine the alkalinity of the solution by taking a 75-milliliter portion, adding
3 drops of mixed indicator, and adding approximately 0.002 normal HC1
or HNO3 from a buret until the indicator turns pink (pH 5). Calculate the
amount of acid necessary to adjust the acidity of the bulk of the absorbent
and add the required amount. The zero blank, obtained by titrating
75 milliliters of the adjusted reagent with 0.002 normal NaOH to the indicator
equivalence point (green), should be not more, than 2 drops. The reagent
is stable at room temperature for at least 1 month.
Mixed Indicator, 0.1 Percent. Dissolve 0.06 gram bromocresol green and
0.04 gram methyl red in 100 milliliters of methanol. When stored in an
amber bottle at room temperature the reagent is stable for at least 6 months.
Standard Sulfuric Acid Solution, 0.002 Normal. Prepare this solution by
appropriate dilution of concentrated sulfuric acid. Standardize by the gravi-
metric barium sulfate method with a 200-milliliter portion or with a primary
standard such as Na2B407-10 H2O. This reagent maybe stored indefinitely
without change in strength.
-------
10 METHODS OF MEASURING
Standard Sodium Hydroxide Solution, 0.002 Normal. Prepare 2 liters of
this solution by dilution of 1 normal sodium hydroxide with freshly boiled
(CO2-free) distilled water. Standardize as follows: Pipette 25 milliliters of stand-
ard sulfuric acid solution into an Erlenmeyer flask, add 3 drops of mixed
indicator solution, and titrate with the sodium hydroxide reagent contained
in a buret to the green equivalence point. Store the reagent in a polyethylene
or other alkali-resistant bottle and restandardize bimonthly.
1 ml of 0.002 N NaOH = 64 /ig SO2 = 24.47 pi SO2 (25"C, 760 mm Hg).
Apparatus
Absorber. A standard all-glass impinger or fritted bubbler is acceptable
(capacity about 300 milliliters). (Among the suppliers are Corning Glass
Company and Fisher Scientific Company.)
Air Pump. The pump should be capable of drawing 1 cfm through the
sampling assembly.
Thermometer (or Other Temperature-Measuring Device). Thermometer should
be capable of controlling and measuring flows with an accuracy of ± 2
percent. The flow meter should be calibrated for variation in reading with
temperature and pressure so that the appropriate corrections can be applied.
Thermometer (orOtherTemperature-MeasuringDevice). Thermometer should
have an accuracy of ± 2°C.
Mercury Manometer (or Other Vacuum-Measuring Device). Manometer
should have an accuracy of 0.2 inch Hg.
BureL A buret of 25- or 50-milliliter capacity graduated in 0.1-milliliter
subdivisions, preferably with teflon plug, should be capable of measuring
volume with an accuracy of 0.05 miUiliter.
Analytical Procedure
Collection of Samples. Set up a sampling train consisting of, in order,
impinger, trap to protect flow device, flow control device, flow-metering device,
temperature and vacuum gauge, and air pump. Measure 75 milliliters of
absorbing reagent into the large impinger. Aspirate air through the bubbler
at a rate of 1 cfm for 30 minutes. Note the readings of the vacuum gauge
and thermometer. The downstream flow-metering device can be empirically
corrected to atmospheric conditions \ by conducting a dummy run with an
upstream flow meter inline that is open to the atmosphere. If an integrated
24-hour aur sample is desired the sampling rate may be reduced to 1 liter
per minute. For'SO 2 concentrations of 0.3 ppm and greater the strength of the
-------
AND MONITORING SO 2
11
standard alkali may be increased or the sampling time shortened. For con-
centrations greater than 0.8 ppm a second impinger should be connected in
series so that a recovery efficiency of 98 percent is maintained. All probes
and tubing upstream from the impinger should be pyrex glass, stainless steel,
or teflon. Butt-to-butt connections may be made with short lengths of tygon
tubing. The collected sample will not decompose on standing; consequently,
the solution may be titrated long after sample collection. The sample may be
stored in the impinger, which has been stoppered or transferred to a stoppered
glass or polyethylene container.
Titration. Add three drops of mixed indicator solution and titrate the solu-
tion with standard 0.002 normal sodium hydroxide until the color changes
from red to green. A reagent blank is titrated in the same manner, and this
result (which should be less than 0.1 milliliter) is subtracted from the sample
titer.
Calculations. Convert the volume of air sampled to the volume at standard
conditions of 25°C, 760 mm Hg:
V y_ (P-Pm) :: 298.2
vs ~ v x 29.97 (t + 273.2)
Vs - Volume of air in liters at standard conditions
V = Volume of air in liters as measured by the meter
P = Barometric pressure in inches of mercury
Pm =• Suction at meter in inches of mercury
T = Temperature of sample air in degrees centigrade.
Results are computed on the basis of the following reaction:
S02 + H202 o H2S04
Thus the net titer of 0.002 normal NaOH (in milliliters) multiplied by
24.47 gives the microliters of sulfur dioxide. Then the concentration is:
ppm SO2 by volume =~£
Vs
Discussion of Procedure
The error for the combined sampling and analytical technique is ± 10
percent in the concentration range below 0.1 ppm with increasing accuracy with
concentration in the range 0.1 to 1 ppm. The measurements should be reported
to the nearest 0.01 ppm.
The presence in the air sample of strong acidic gases other than SO 2 or
reactive acid solids such as HC1 and NaHSOs gives erroneously high results,
whereas the presence of alkaline gases or reactive basic solids such as NH3
and CaO gives erroneously low results. H2SO4 does not interfere since it is
not appreciably separated from the airstream owing to its small particle size
except, perhaps, when the relative humidity is greater than 85 percent, which
could result in particle sizes of greater than 1 micron. SOs gas, if present,
-------
12 METHODS OF MEASURING
would be a positive interference. Sulfates do not interfere. CO2 does not inter-
fere since it is not absorbed in the acid-absorbing reagent. If large amounts of
solid material are present, a filter may be employed advantageously upstream;
however, a loss of SOa may. occur. 8 The extent of this loss would depend
upon the composition of the particulate matter and the nature and retentive
capacity of the filter used. The acid base indicator is not included in the ab-
sorbing reagent because it tends to decompose during sampling, which leads
to unreproducible results.
Recommended Procedure and Specifications
for Automatic Monitoring Instrument
ELECTROCONDUCTIVITY ANALYZER*
Introduction
This method is applicable to the continuous, automatic sampling, analyzing,
and recording of SO2 concentrations in outside ambient air hi the concentration
range from about 0.01 to 2 ppm. The upper limit can range from 1 to 20
ppm, depending upon reagent flow and airflow rates and on electronic ampli-
fication. Air is continuously admitted to the absorber where the SO2 in the
airstream is removed by a continuously flowing liquid absorbent. The electrical
conductivity of the resulting solution is continuously measured and recorded,
and the readings obtained are proportional to the concentration of S02 in
the sampled air. The method is not specific for SOa since other soluble elec-
trolyte-forming gases and solids affect the results.
Reagents
All chemicals used must be A. C. S. analytical-reagent grade.
Distilled Water. For use as absorbing solution, distilled water should be
prepared by passing it through a cation-anion exchange resin.
Hydrogen Peroxide-Sulfuric Acid, Alternate Absorbent, 2 x 1C"3 molar
1 x 10'5 normal ^804. Add 2.3 mUliliters of 3 percent H2O2 solution
per liter of 1 x 10'5 normal
* It is not intended to imply that only analyzers using electroconductivity
detectors are suitable for automatic monitoring of SO2 in air. At the time
of this report, electroconductivity analyzers most nearly fulfilled the criteria
for evaluation and the recommended instrument performance specifications
set forth herein. Further research on and development of automatic air-
monitoring instruments incorporating other principles of detection such as
potentiometry, photometry, air ionization, thermal conductivity, and so forth
should be encouraged as well as electroconductivity.
-------
AND MONITORING SO 2 13
2, 4, 5 Trichlorophenate (Dowicide B) or Any Suitable Fungicide. Add 2
milligrams of absorbing reagent per liter.
Calibration Reagent A reagent should be prepared corresponding to 1
ppm SO2 under the conditions of air sample flow rates employed. Add the
calculated amount of 0.1 normal H2S(>4 solution to 1 liter of unexposed
absorbing reagent.
Apparatus
The apparatus should consist of a suitable assembly of sampling probe,
absorber, regulating and recording device for airflow, regulating and re-
cording device for liquid flow, ah- pump, liquid-metering pump or constant-
head device with capillary tube for dispensing the absorbent at a constant
rate, ther mo started cabinet, conductivity electrodes, and conductivity recorder.
Sampling Probe. The sampling probe should be made of pyrex glass,
316 stainless steel or teflon tube with intake end turned down, and a loose
glass wool filter to remove large particulate matter. The inline prefilter should
be mounted indoors, exterior to the instrument, or, if necessary, inside the
thermostatted cabinet, to prevent condensation of water vapor, which would
absorb SO2 and result in serious losses.
Absorber. A venturi scrubber or any reagent-air-contacting system capable
of a scrubbing efficiency of 98 percent or more is acceptable.
Measuring, Regulating, and Recording Device for Airflow. A flowmeter, needle
valve, or other flow-measuring, flow-regulating device capable of measuring
flows with an accuracy of ± 2 percent should be used. The airflow rate should
be monitored continuously by means of suitable mechanical and electronic
circuitry to cause characteristic markings to appear on the conductivity re-
corder chart.
Measuring, Regulating, and Recording Device for Liquid Flow. A metering
pump, flowmeter, and needle valve or device capable of measuring flows with
an accuracy of ± 2 percent is acceptable. The liquid flow rate should be
monitored continuously by means of suitable mechanical and electronic cir-
cuitry to cause characteristic markings to appear on the conductivity recorder
chart.
Air and Liquid Pumps. Any air pump and liquid pump or combination
air-liquid pump is acceptable that is capable of drawing air at a rate of 5
liters per minute and liquid at a rate up to 30 milliliters per minute through
the absorption-analyzing system under conditions of continuous operation.
To assure satisfactory performance, the pump should have a much greater
capacity, about 20-liters-per-mmute airflow under the conditions of operation.
-------
14 METHODS OF MEASURING
Thermostatted Cabinet. The reagent feed lines, absorption column, and
conductivity cells should be enclosed in an insulated compartment thermo-
statically maintained at a temperature a few degrees higher than the highest
ambient temperature expected. The temperature of the reagent in the con-
ductivity cells should be continuously monitored by means of suitable electronic
circuitry to cause characteristic markings to appear on the conductivity recorder
chart.
Conductivity Electrodes. Two sets of platinum dip electrodes of suitable
dimensions, one pair to measure the conductivity of the unreacted reagent and
the other that of the reacted reagent, should be provided.
Conductivity Recorder. A zero-to-10 millivolt, potentiometric, strip chart
recorder with 30-day chart and scale graduated from zero to 100 and chart
speed of 1 or 2 inches per hour, or any instrument capable of recording the
differential output of the conductivity cells corresponding to an SC>2 concen-
tration range of zero to 2 ppm with an accuracy of ± 1 percent of full scale
should be provided.
Reagent Reservoir and Delivery Bottle. Any bottle with sufficient volume
to contain a 1 week's supply of reagent can be used; the reagent should be
protected from air pollutants by the insertion of a soda-lime charcoal column
on the air inlet line.
Switch-controlled Electronic Check. A switch-controlled manual check con-
taining the proper resistance to cause a deflection of the recorder corresponding
to a concentration of 1 ppm SO2 should be used.
Switch-controlled Electronic Zero Check. A check should be used to simulate
a differential conductivity between the reference cell and a known resistance
equivalent to zero ppm SOa- This also serves as a check on the purity of
the unreacted reagent.
Calibration
Static Calibration, Standard Solutions. The instrument may be calibrated
by sulfuric acid solutions of known composition corresponding to definite
atmospheric SOz concentration hi the range zero to 2 ppm. Solutions cor-
responding to 0.5, 1.0, 1.5, and 2.0 ppm S02 are prepared by the addition
of calculated amounts of 0.1 normal H2 864 to the absorbing reagent. The
static methods described below may be used when the absorption efficiency
is known to be greater than 98 percent.
1. Establish instrument zero by introducing the unexposed absorbing
reagent in both the reference and sample conductivity cells; then the
-------
AND MONITORING SO 2
standard solutions are substituted for the absorbing reagent in the
sample conductivity cell only. The instrument reading is checked against
the standard reagent, and if necessary, the instrument is adjusted by
means of the span control to indicate the correct concentration.
2. The standard solution is substituted for the absorbing reagent and is
introduced directly into the system as in normal operation. SO2-free
air, obtained by passage of air through a drying tower containing
ascarite or soda lime, is admitted to the analyzer under the same con-
ditions as in actual sampling. The instrument reading is checked against
the standard reagent, and the necessary corrections are made. Absorbing
reagent is maintained in the reference cell.
Dynamic Calibration, Standard Gas Mixture. The dynamic calibration
methods described below should be employed to take into account the scrubbing
efficiency of the absorbing column under flow conditions. Standard air-SO2
mixtures may be prepared in a rigid test chamber, compressed gas cylinder,
or collapsible mylar or other inert plastic bag and then introduced into the
analyzer. The mixtures are prepared by diluting a measured quantity of pure
SO2 gas with a known volume of SO2-free air. A measured quantity of S02
gas may be introduced into the test chamber through a rubber diaphragm
by means of a hypodermic syringe and needle. In the compressed-gas-cylinder
technique, the dilution of SO2 is accomplished by introducing a measured
amount of SO2 by a hypodermic syringe into a partially evacuated, stainless
steel cylinder and compressing the mixture by addition of air contained in
a compressed-air cylinder at high pressure until the desired pressure is reached.
When a rigid test chamber is employed, corrections should be applied for
the diminution of gas concentration resulting from the dilution of the test gas
by influent air during sampling, or a flexible plastic bag can be put inside
the chamber to receive replacement air. The concentration of SO 2 in the gas
streams prepared by the above methods should be calibrated by the West-
Gaeke or hydrogen peroxide methods.
The instrument may also be calibrated against a manual method such
as the West-Gaeke or the hydrogen peroxide method. Atmospheric air or
synthetically produced air-S02 mixtures are introduced simultaneously into
the analyzer and the manual absorber. The instrument record is adjusted to
read S02 concentrations as determined by the manual method.
Fluctuations in air-h'quid flow rate and temperature may occur. Calibration
curves showing the effect of these variations on recorder reading should be
prepared so that appropriate correction factors can be applied.
Procedure
(a) When a fresh supply of absorbing solution is installed in the apparatus,
any air bubbles that may form in the reagent feed lines must be
removed.
-------
16 METHODS OF MEASURING
(b) Check the airflow and liquid flow rates and temperature of the reagent
in the conductivity cells and make necessary adjustments.
(c) Check the ink supply to the pens in the recorder. Check the recorder
battery and output range by suitable electronic test equipment.
(d) Lubricate the pump and motors as required.
(e) Calibrate the instrument once a week or more or less frequently as
required.
(f) Interpret the charts by using a calibration curve on a section of the
recorder paper, or integrate under the curve by using a planimeter.
Correct for deviations from the standard curve due to variations in
temperature and airflow or liquid flow. Hourly concentrations may be
used to calculate time-concentration values. Short-time peak concen-
trations are measured as required.
Instrument Performance Specifications*
(a) A reproducibility of ± 1 percent of full-scale deflection over a 24-
hour period,
(b) an instrument accuracy of ± 2 percent of full-scale deflection over
a 24-hour period,
(c) linear response in all ranges of concentration (90% response time
not to exceed 1 minute for the concentration range zero to 2 ppm),
(d) electronic drift not to exceed 1 percent of full scale in 24 hours,
(e) zero to 2 ppm full-scale range with a sensitivity of 0.01 ppm,
(f) sufficient control of temperature, reagent, and sample flows to assure
the accuracy of the calibration technique.
* Elecfroconductivity analyzers are available from the following manufac-
turers: Beckman Instrument Company, Davis Emergency EquipmentCompany,
Leeds & Northrup, Instrument Development Company, Mervyn-Cerl Ltd.,
and distributed by Gelman Instrument Company. Not all models supplied
by manufacturers meet these performance requirements. In the case of some
of the manufacturers none of the models available as of January 1964
meet specifications.
-------
AND MONITORING SO 2
Discussion
Electroconductivity, which is measured in terms of the resistance of the
solution between two electrodes immersed in it, is a property of all ionic solu-
tions and is not specific for any particular compound. It is dependent upon
the number and type of ions dissolved in solution. Soluble gases that yield
electrolytes in solution cause the greatest interference. All hydrogen halides
present would be measured. Except near special sources of contamination,
these gases are, however, seldom present in air in appreciable amounts in
comparison with S02. Weak acidic gases such as H2S cause practically no
interference because of their slight solubility and poor conductivity. If the
water is free of bases, the carbon dioxide content of air causes no interference.
Alkaline gases, such as ammonia, interfere by neutralizing the acid and yield
comparatively low results because the transport number of the hydrogen ion
is several tunes greater than that of other cations. Similarly, lime dusts or
other basic solids, if absorbed, would cause comparatively low results for
SO2. Neutral and acidic aerosols such as sodium chloride or sulfuric acid
would give comparatively high results depending on their solubility, ioniza-
tion, and the ability of the absorption system to remove them from the air-
stream, which, in this method, is very poor unless particle size is relatively
large. Since the particle size of sulfuric acid mist is small (less than 1 ^)
except perhaps when the relative humidity is greater than 85 percent, it is not
measured appreciably in this method. A special absorber and different operat-
ing parameters are required for effective collection of sulfuric acid mist. A
loose glass wool filter is used to minimize maintenance time requirements for
cleaning of absorber and conductivity cells, and reducing the interference of
particulate matter. SO3 gas, if present, would result in a positive interference.
Instrument response is a function of gas concentration, airflow and liquid flow
rate, and conductivity cell constants. Response time is a function of liquid
flow rate and the distance and volume between the absorber and the con-
ductivity cells.
A change in temperature of 2°F will alter the conductivity of a strong
electrolyte by approximately 2 percent; consequently, for accurate operation,
the absorption column and conductivity cells should be enclosed in an in-
sulated compartment thermostatically maintained at a temperature a few
degrees higher than the maximum ambient temperature expected.
12
Perley and Langsdorf reported that, at SO2 concentrations of 2 ppm,
amounts of CO2 as high as 2,000 ppm had no influence on the resultant
SO2 measured. H2S concentrations of 25 ppm result in a positive reading of
0.2 ppm. Since this concentration is rarely approached in atmospheric sampling
the interference of this weak acid is not appreciable.
Yocom et al.13 reported on the effect of hydrogen chloride on the operation
of an electroconductivity analyzer. Concentrations of hydrogen chloride in the
-------
METHODS OF MEASURING
range 1.2 to 11.4 ppm were collected at an efficiency of 75 percent. The
relationship between concentration of HC1 and its equivalent concentration
in ppm SO2 was 1:2.9. From these considerations it was shown that appreci-
able interference is caused by the presence of HC1, as would be the case with
all hydrogen halides and other strong electrolytes.
Jacobs, Braverman, and Hochheiser 14 adapted an electroconductivity
analyzer to the measurement of SO2 in outside air in the concentration range
zero to 0.95 ppm full scale. A dynamic calibration procedure is described
by the authors. Synthetic air-SO2 mixtures are introduced into the instrument
and into a bubbler containing the hydrogen peroxide reagent simultaneously.
In this manner the instrument is standardized in terms of the acidimetric
hydrogen peroxide method.
Reece, White, and Drinker 5 adapted the electroconductivity analyzer to
the analysis of CS 2 and H2S by attaching a combustion furnace and oxidizing
these gases to SO2. The apparatus was also used for the analysis of sulfur
compounds and chlorinated hydrocarbons by Thomas et al.16 Pyrolysis and
oxidation to HC1 and SOg were accomplished in a silica combustion tube by
using a platinum catalyst.
A detergent solution may be added to the absorbing reagent to help main-
tain uniformity of h'quid flow and prevent the accumulation of dust in the flow
system. 17
Commercially available electroconductivity analyzers designed for the mea-
surement of SO2 in air are described in a book about air-sampling
instruments, is
Recommended instruments and procedures for automatic monitoring of SO2
in air are contained in published methods manuals on atmospheric sampling
and analysis. 19,20
COMPARISON OF METHODS
CONSIDERED
Manual Methods
Hochheiser and co-workers 21 compared the hydrogen peroxide, iodine,
and fuchsin-formaldehyde methods for the determination of SO 2 in air by
taking atmospheric samples in New York City and found that all were equally
reliable (Table 1). In the peroxide method, the strongly acidic component
-------
AND MONITORING SO 2 19
in the air is determined rather than acidity attributable to SO2 alone, but
there is little difference in the results obtained by these methods, particularly
in the lesser concentrations, because the major acidic pollutant is SO 2 .
Table 1. COMPARISON AMONG METHODS OF DETERMINING SO2,ppm
Peroxide method Iodine method Fuchsin method
0.30
0.30
0.27
0.23
0.15
0.15
0.15
0.14
0.11
0.09
0.07
0.05
0.05
Average
0.15
0.26
0.23
0.22
0.22
0.14
0.12
0.11
0.11
0.09
0.09
0.07
0.06
0.05
0.13
0.25
0.22
0.22
0.22
0.12
0.10
0.09
0.09
0.07
0.06
0.06
0.05
0.04
0.12
In accordance with arecommendationoftheworkingparty to study methods
of measuring air pollution used by the organization for European Economic
Co-operation, the United Kingdom delegation made comparisons among four
recognized methods of estimating SO 2 on a simple routine basis. 22 The West-
Gaeke, peroxide, Stratman, and direct iodine methods were compared by
means of atmospheric samples obtained at a measuring site in a city center
during the winter period.
Table 2 shows that the hydrogen peroxide and the West-Gaeke methods gave
fair agreement when used in routine measurement. The Stratman method
yielded erratic results in that in some instances it gave good agreement with
the two methods mentioned above, but sometimes its results varied considerably
from those of the other two. Table 2 shows the tendency of the iodine method
to read consistently high. This tendency appears to be owing to carryover
of iodine from solution by the air stream.
-------
20 METHODS OF MEASURING
Table 2. COMPARISON AMONG METHODS OF DETERMING SO2,
3 (1 ppm-3,000/*g/m3)
West- Hydrogen Stratman Direct
Gaeke peroxide iodine
465 475 495 605
575 630 180 930
245 200 285 410
245 210 430 390
425 330 465 490
380 450 480
840 785 850 1,090
150 205 215 400
245 270 160 530
310 240 235 295
730 620 215 880
175 155 80 240
Average
390 368 298 550
Welch and Terry23 reported that values obtained by the H2O2 method
were approximately 30 percent lower than values found by the West-Gaeke
method for concentrations up to 0.65 ppm. The relationship between the
West-Gaeke and the H2O2 method was established by using synthetic mixtures
of SO 2 in air and a dynamic calibration procedure. It is believed that this
should be investigated further since the collection efficiency of the fl^Oz method
is generally considered to be greater than 90 percent. Relatively close correla-
tion between the electroconductivity analyzer and the West-Gaekeprocedure was
obtained on synthetic air-SO2 mixtures in the range of 1 to 3 ppm.
Paulus, Floyd, and Byers ^ compared the polarographic method of analysis
with the fuchsin-formaldehyde colorimetric method. The polarographic results
consistently averaged from 2 to 7 percent less than the colorimetric results,
depending upon the concentration range. A wider deviation was obtained in
the lower concentration range.
Terraglio and Manganelli4 studied the variability of the H2,O2, West-
Gaeke, and iodine methods. A comparative study over the concentration
range of 0.10 to 1.3 ppm SO 2 prepared synthetically in a test/chamber showed
that the average results obtained by the aeidimetric and colorimetric methods
were approximately the same but results of a single determination could vary
significantly.
-------
AND MONITORING SO2
21
Comparative recoveries by the iodimetric method were found to be lower
than the values obtained by the above-mentioned methods. The relative per-
centage recovery by the acidimetric method compared with the West-Gaeke
method was 106 percent at a concentration of 0.1 ppm and decreased to
97 percent at 1.3 ppm. For the iodimetric method, the relative percentage
recovery increased from 70 percent of the colorimetric value at 0.1 ppm to
84 percent at 1.3 ppm S02- The presence of ozone-oxides of nitrogen inter-
fered with all three methods as follows:
a. Colorimetric method resulted in reduced recovery owing to bleaching
of the sample by oxides of nitrogen and ozone.
b. Acidimetric method resulted in increased recovery owing to titration of
oxides of nitrogen dissolved in trapping solution.
c. Iodimetric method resulted in increased recovery owing to variation in
the method of oxidation of the sulfur dioxide (Table 3).
Table 3. INFLUENCE OF OZONE-NITROGEN OXIDES ON THE
DETERMINATION OF SO2
Ozone
added,
NO 2
added,
SO2
added,
ppm
0.01
0
0
0.01
0.08
0
0
0.07
0
0
0.07
0.25
0
0
0
0.47
0.46
0.47
0.46
0.47
0.46
S02
Colorimetric
0
0.47
0.46
0.44
0.38
0.47
0.46
recovered,
Iodimetric
0
0.25
0.38
0.20
0.36
0.26
ppm
Acidimetric
0.03
0.48
0.44
0.53
0.69
0.49
0.51
Selection of the manual methods currently used by the Division of Air
Pollution was based upon an evaluative review of available literature in-
formation reported herein and upon the experience of field investigators.
Reference to Table 4 and the material that follows it shows that of all
the manual methods reviewed the West-Gaeke and hydrogen peroxide methods
best approach the criteria for evaluation set forth herein. The West-Gaeke
method has a higher sensitivity, but its calibration and analytical procedure
are more difficult. The absorbents used and the products formed in the sep-
aration and concentration of the pollutant from the air mass are stable in
both methods; though the product formed in the hydrogen peroxide method
is more stable; either one may, therefore, be employed with automatic se-
-------
Table 4. COMPARISON AMONG MANUAL METHODS FOR THE ANALYSIS OF SO; IN AMBIENT AIR
Method
Color imetric
1. West-Gaeke
2. Fuchs in-
fo rmaldehyde
3. Stratman
4. Barium
chloranilate
Impregnated test papers
5. Zinc nitro-
prusside
6. Astrazone pink
Sulfate ion
7. Diazo dye
8. Thorium
borate-amaranth
9. Ultraviolet
determination
Acidimetric
10. Hydrogen
peroxide
1 1 . Polarographic
lodimetric
12. Iodine
1 3. Direct iodine
14. Iodine thiosulfate
15. Turbidimetric
Cumulative
16. Pb02 candle
Sensitivity,
ppm
0.005
0.01
0.01
0.05
0.5
0.05
0.20
0.5
0.03
0.01
0.02
0.01
0.10
0.40
0.40
0.02 mg SOj/100
. ir.2/rlay
Accuracy, + %,
combined error
sampling and
analytical
technique
10
10
--
—
20
—
10
20
—
10
10
10
15
10
10
15
Reproduci-
bility, +%
3
3
—
--
10
—
3
10
—
3
3
3
5
3
3
7
Inter- Sampling
ferences rate, Collection
(Known) liters/min. efficiency, %
NO,, O- 1 to 4 95 to 100
NO ,0 ,S, 1 to 4 90 to 100
SH.S2O3
H,0,H2S, 1 to 5 90 to 100
PO4, F, Cl 20 to 30 90tolOO
HC1,H2S, 0.30
CS2
only at
high cone
0.25
PO4,Cl,SQj 1 to 4 90 to 100
F, P04,HCO3,SO3 1 to 4 90 to 100
H2S, H,SO.
Acid and alkaline 3 to 28 90 to 100
gases
Oxidants 24 90 to 100
Oxidizing and re- 20 to 30 85 to 100
ducing materials
Oxidizing and re- 3 85 to 100
ducing materials
Oxidizing and re- 3 85 to 100
ducing materials
Fluorides 20 to 30 90 to 100
Particulates
.
--(2 weeks1 to 3
.nonths1 exposure)
Sta-
bility
of ab-
sorb-
ents,
days
90
90
90
30
90
14
90
90
30
90
90
1
1
90
90
Stability
of pro-
duct,
days
3
2
—
90
2 hours
Read
directly
90
90
90
2
2
Read
directly
1
90
90
to
to
2
W
H
ffi
o
O
cc
O
S,
w
cr>
S
Z
O
-------
AND MONITORING SO 2 23
quential sampling equipment. Comparable results are obtained by sampling
and analyzing known amounts of synthetic mixtures of SO 2 in air. Both
methods are nonspecific, though the West-Gaeke method is more selective,
and further studies concerning the relative effects of interfering materials are
necessary.
Instrumental Methods
The performance parameters of the automatic monitoring instruments are
summarized in Table 5. Of the analyzers currently commercially available,
some models of those employing the electroconductivity detection principle
best approach the criteria for evaluation. Any instrument employing other
methods of detection, such as potentiometry, photometry, and ionization,
that meet the performance criteria established for the electroconductivity ana-
lyzer would also be acceptable. The experience of field investigators has
demonstrated that the operating parameters specified for the automatic instru-
mental method selected for use that are detailed herein are necessary for
satisfactory operation of the equipment over a reasonable period of unat-
tended operation.
The method is nonspecific since any soluble, electrolyte-forming gases and
solids affect the result. It has, however, been • found that, except hi special
situations, the measurements recorded by these analyzers predominantly result
from atmospheric S02-
MANUAL METHODS CONSIDERED IN ADDITION
TO RECOMMENDED METHODS
Colorimetric Methods
FUCHSIN-FORMALDEHYDE METHOD25
SO 2 in the atmosphere is removed and concentrated by scrubbing through
a sampling solution of 0.1 normal sodium hydroxide and 5 percent glycerol.
The subsequent determination of SO2 is based on a color reaction first
developed by Steigmann.26 The chromogenic reagent consists of a mixture
of basic fuchsin, sulfuric acid, and formaldehyde, which develops a red-violet
color in the presence of sulfurous acid.
-------
to
Table 5. COMPARISON AMONG AUTOMATIC GAS ANALYZERS FOR THE CONTINUOUS RECORDING OF ATMOSPHERIC SO,
Principle of operation3 Absorbent
Electroconductivity
1. Analyzer "A" De-ionized water
2, Analyzer "B" Hydrogen peroxide-
sulfuric acid
3. Analyzer "C" 1 1
4. Analyzer "D"
Potentiometric Potassium bromide-
sulfuric acid
Photometric
1. Analyzer "A" Starch- iodide
2. Analyzer "B11 . j
3. Analyzer "C"
~*^ 4. Analyzer "D" Pararosaniline-
; ^ formaldehyde
- - «—
Air ionization Aerosol formation
a. Described in what follows.
Range Sensi-
covered, tivityf
ppm PPTi
0 to 2 0.01
0 to 5 0.05
0 to 2 0.01
0 to 1 0. 02
0 to 10 0.10
0 to 1 0.01
0 to 1 0. 01
0 to 0.5 0.01
0 to 5 0.02
0 to 10
Reprodu- Stability
cibility, Interference of reagent.
pprn days
Extremely stable; reagent
0.02 Soluble ionic materials (i.e., is circulated through
0.10 I' 30
0.02 u 30
0.05 11 30
0. 20 Oxidants and reductants Extremely stable; reagent
i.e., H_S, mercaptans, is circulated through
NO2l O^, etc., and purification system
olefins , phenols
Oxidants and reductants 7
i.e., H2S, NO2, etc. _
II
II 7
NO2 and O3 if present 14
in appreciable
quantities
..
M
H
a
o
a
o
1
a
2
3
o
-------
AND MONITORING SO2 25
The absorption maximum is at 570 millimicrons, and the color is
independent of temperature in the range 23 to26°C. Above this range the effect
of temperature on the absorbance of the sample gives increasingly erroneous
results. A 30-minute color development ther mo started at 25° C was suggested.
The extent of color stability for longer periods was not reported.
Inorganic sulfides, thiols, and thiosulfates interfere with the determination
and may be removed as a mercury precipitate by treatment with saturated
mercuric chloride prior to the addition of the colorimetric reagent.27
The method has a sensitivity of 0.01 ppm with a 40-liter air sample scrubbed
through 10.0 milliliters of absorbing solution. The minimum amount detectable
is 0.1 microgram per milliliter of solution. At a sampling rate of 20 liters per
hour with a midget fritted bubbler, the collection efficiency is close to 100 per-
cent. Stang et al., 28 using a Greenburg-Smith all-glass impinger containing
100 milliliters of 1 percent glycerol in 0.05 normal sodium hydroxide and
sampling at 1 cubic foot per minute, obtained results indicating from 93 to
99 percent efficiency based on the amount of 802 collected in two impingers
in series.
Qft
Moore, Cole, and Katz reported that NO 2, if present in the same con-
centration range as SO2, produces a negative interference owing to the
bleaching effect of NO2 on the fuchsin-formaldehyde sulfite color. Corrections
for the effect of NO2 on the colorimetric 862 values were obtained by con-
current determination of SOg by the conductimetric and colorimetric methods
and of NO2 by the Saltzman method.
Paulus, Floyd, and Byers 24 found it necessary to establish a new standard
curve with each new batch of colorimetric reagent They also reported on the
effect of aging, light, and agitation on the collected sample. Solution strength
was determined after the first, third, and sixth days of sample collection.
Average losses of 6 percent were found after 6 days. The samples that showed
losses after the first and third days were mostly in the lowest concentration
range. No losses due to sunlight or artificial light were found. The effect of
agitation is important when samples are shipped by mail. Losses due to
agitation are comparable to those due to the aging experiments and are,
therefore, not hastened by this condition.
The fuchsin-formaldehyde method for the determination of SO 2 in air is
recommended and reported in the ACGIH manual of recommended methods.30
An average deviation of 8 percent was found. According to ACGIH, when
concentrations of nitrogen oxides higher than those of SO2 are anticipated,
the polarographic method should be used.
-------
26 METHODS OF MEASURING
STRATMAN METHOD31
Atmospheric SO2 is initially absorbed on silica gel and then reduced with
hydrogen to H2S on a platinum contact catalyst at 700 to 900°C. The H2S
formed is passed into a bubbler containing 2 percent ammonium molybdate
in 0.4 normal sulfuric acid. The resulting blue-violet molybdenum complex
is then determined colorimetrically with the aid of a Ziess Opton S57 filter.
According to the amount of reagent used, 1 to 200 micrograms of SO2 can
be determined. Efficiency of removal of atmospheric SO2 is higher than 90
percent at a sampling rate up to 5 liters per minute. If the quantity of air
sampled contains more than 300 milligrams of water, it must be dried by
passage through a preliminary drying tower containing phosphorous pento-
xide. It was found that pressures up to 200 millimeters Hg and temperatures
between 18 and 40°C do not affect the collection efficiency. The method has
a sensitivity of 0.01 ppm with a 40-liter air sample when the H2S produced
in the catalytic desorption process is absorbed hi 10 milliliters of the ammon-
ium molybdate reagent. The interference of SO3 is eliminated by preceding the
silica gel absorption equipment by a bubbler containing phosphoric acid. The
Stratman method is particularly suitable for relatively short-term sampling
since moisture interferes when samples are obtained over a longer time.
oo oo
BARIUM CHLORANILATE METHOD0 '**
The method is based on the reaction of solid barium chloranilate with
sulfate ion at pH 4 in 50 percent ethyl alcohol to liberate highly colored
acid-chloranilate ion. The concentration is determined spectrophotometrically
with the absorption peak at 530 millimicrons. Atmospheric SO 2 is removed,
concentrated, and oxidized to sulfate by scrubbing through 0.5 percent aqueous
H2O2 solution. A buffer solution, 95 percent ethyl alcohol and 0.1 gram
barium chloranilate, is added and the mixture is shaken for 10 minutes. The
excess barium chloranilate and the precipitated barium sulfate are removed
by filtration. The method has a sensitivity of 0.05 ppm with a 1,000-liter air
sample scrubbed through 25 milliliters of absorbing solution. Kanno 32
reported a collection efficiency of close to 100 percent at a sampling rate of 5
liters per minute. The residual presence of H2O2 and CO2 did not interfere with
the colorimetric method. The accuracy and the effect of interfering materials
were not reported. Phosphates, fluorides, and chlorides are known to interfere
in the chloranilate procedure, and a preliminary separation would be required.
A method for the conversion of the gravimetric lead peroxide method to
colorimetric with the use of barium chloranilate is also described by the author.
i
ZINC NITROPRUSSIDE METHOD34
Air is aspirated at the rate of 300 milliliters per minute through a test
paper supported in a suitable holder. The determination of S02 is based on
-------
AND MONITORING SO 2 27
the formation of a brick-red stain, which is compared with a standard stain
chart or with a disc of standard tints. The test papers are prepared by soaking
strips of filter paper in ammoniacal zinc nitroprusside solution and drying
at a temperature not exceeding 40°C. The dried test papers are stored in a
stoppered container in the dark and are moistened by spraying with water
immediately before use. It was found that dry zinc nitroprusside papers are
stable indefinitely whereas papers prepared with glycerol decompose after
about 4 weeks of storage.
With a 360-milliliter air sample, the range of concentrations determinable
with an accuracy to within :±20percentis 1 to 20 ppm. Hands and Bartlett34
stated that concentrations outside these limits can be measured by increasing
or decreasing the size of the sample. The stains are stable for about 2 hours.
The effect of hydrogen chloride, hydrogen sulfide, and carbon disulfide on
color production was investigated. Hydrogen chloride produced a discoloration
while hydrogen sulfide produced a positive stain, although this effect was
observed only at relatively large concentrations. Carbon disulfide at con-
centrations up to 500 ppm yielded no stain.
In an investigation of the starch-potassium iodide-potassium iodate test
paper method35 the authors found that, unless strict control was exercised
during the preparation of the test paper, nonuniform papers resulted. This
made the calibration of the procedure difficult.
ASTRAZONE PINK METHOD36
Air is drawn through a wet test paper at a rate of 250 milliliters per minute
until bleaching of the impregnating reagent is effected, whereupon the volume
of air taken to produce this effect is noted. The test reagent consists of Astra-
zone Pink FG (Bayer), sodium bicarbonate, and glycerol and is stable in the
dark for 2 weeks. The dye solution alone is unchanged after several months.
One drop of this reagent is placed on the filter paper and allowed to spread
before sampling. The method has a sensitivity of 0.05 ppm SO2. No informa-
tion is reported regarding the specificity or accuracy of the method.
DIAZO DYE METHOD
Klein37 described a colorimetric method for the determination of sulfate
ion that may be applicable to air analysis. The sulfate in the sample is pre-
cipitated as benzidine sulfate with benzidine hydrochloride, purified, dissolved
in 0.2 normal HC1, diazotized, and coupled with N-1-naphthylethylenediaminedi-
hydrochloride after excess nitrous acid is destroyed. The resultant purple color
is read in a colorimeter with a green filter. The color produced is stable for
at least 12 hours. Phosphate and chloride interfere in the determination. Phos-
-------
28 METHODS OF MEASURING
phate should be completely removed. If the ratio of chloride to sulfate is
greater than 30, incomplete -precipitation of benzidine sulfate occurs. The
sensitivity of the method is 0.05 milligram in 15 milliliters of sample solution,
or to translate this to air sample concentration, the sensitivity would be 0.2
ppm for a 90-liter air sample collected in a midget impinger containing H2O2-
absorbing reagent. Comparison with the gravimetric sulfate method shows good
correlation of results. The analytical error is -± 2 percent.
THORIUM BORATE-AMARANTH DYE METHOD
A colorimetric procedure for determining sulfate ion, described by Lambert
et al. ,38 uses an insoluble thorium borate-amaranth dye reagent. This pro-
cedure may be applicable to air analysis. Sulfate releases dye molecules from
the solid reagent in direct proportion to the concentration of the dye measured
at 521 millimicrons. Interference by fluoride, phosphate, and bicarbonate is
eliminated through the use of lanthanum ion and a weak acid cation exchange
resin. It was found that the color produced was independent of the amount
of reagent added and a function only of the sulfate ion in solution. Never-
theless, it was found necessary to prepare a calibration curve for each new
batch of reagent. Reaction time is not a factor in the range 1 to 10 minutes.
With a 10-millimeter cell path, the sensitivity of the method is 0.2 milligram
sulfate contained in 10 milliliters of sample with a reproducibility of 10 per-
cent. For a 90-liter air sample obtained by collection and oxidation of SO^
to SO4 the sensitivity would be 1 ppm. No doubt the sensitivity of the method
could be increased by increasing the cell path.
INDIRECT ULTRAVIOLET DETERMINATION OF SO2 BY
MEANS OF PLUMBOUS ION 39
SO 2 is precipitated as lead sulfite with 1 milliliter of lead acetate solution
(100 fig/ml) and is determined indirectly as the plumbous ion remaining in
solution at 208 millimicrons. Plumbous ion absorbs at 208 millimicrons
(molar absorbance 8,210) and this permits a sensitivity of 13 micrograms
of plumbous ion per 0.1 absorbance unit (equivalent to 8 ^g SOz) to be
attained. The lead precipitate is sufficiently insoluble in water at room tempera-
ture for plumbous ion not to be spectrophotometrically determinable from its
saturated aqueous solution.40 This method would be sufficiently sensitive for
atmospheric analysis and may be applicable to air analysis by scrubbing an
air sample through a suitable absorbent and subsequently using the indirect,
ultraviolet, spectrophotometer method.
OQ
Blinn and Gunther found that under the conditions specified, two moles
of SO 2 are needed to precipitate 1 mole of plumbous ion. Acid gases such
as H2S and H2SO4, which precipitate plumbous ion from aqueous solutions,
and other materials, which absorb strongly in the 250^ to 190-millimicron
region, would interfere.
-------
AND MONITORING SO 2 29
FERROUS-PHENANTHROLINE METHOD
Stephan and Lindstrom 41 developed a direct color method for determining
SO 2 in the atmosphere. The air sample is passed through a wet scrubber con-
taining ferric-phenanthroline reagent. Sulfite ion oxidizes ferric ion to ferrous
and produces the colored ferrous-phenanthroline complex ion.
The color produced is stable for about 3 days. Nitrogen dioxide and ozone
do not interfere. The lower limit of detection is 0.05 ppm for a 100-liter air
sample collected for 50 minutes in 75 milliliters of absorbing reagent. A
serious disadvantage of the method is that the relatively high temperature
sensitivity during sampling necessitates the use of a constant water bath or
other regulatory device to control temperature to ± 0.5°C. Because of the
stability of the product formed, this method can, however, be adapted for
use with a sequential sampler or for collection of an integrated 24-hour sample
provided that the sampling unit is thermostatted.
94.
POLAROGRAPHIC METHOD
SO 2 in .air is removed and concentrated by scrubbing through an absorbing
solution, contained in a standard, all-glass, Greenburg-Smith impinger, con-
sisting of 2 percent glycerol in 0.05 normal sodium hydroxide at a rate of 24
liters per minute for 30 minutes. Subsequently an acetate buffer (pH 4) is
added, and the combined solution is deaerated by bubbling nitrogen through
the sample contained in an air electrolysis vessel. The flow of nitrogen is then
stopped, and a polarogram is made from -0.35 to -1.00 volt. A sensitivity
of 0.006 microampere per millimeter is used.
For a 30-minute air sample contained in 75 milliliters of absorbent, the
sensitivity is 0.02 ppm. Smaller concentrations could be determined, depending
largely on the ability to measure the inked lines on the polarogram. Air
samples can be determined with an accuracy of ± 10 percent and a repro-
ducibility of 3 percent.
42
According to Kolthoff and Miller only one of the two tautomers of
sulfurous acid is reducible at the dropping mercury electrode, and at pH 4,
there is less of the reducible than of the nonreducible tautomer present. The
fuchsin-formaldehyde colorimetric determination involves a similar situation
in that only one of the tautomers results in the formation of a red color. The
same conditions of collection efficiency and reagent stability also apply since
the scrubbing media used in the methods are identical.
Sulfur compounds generally do not interfere with the polarographic method.
Cystine does not interfere with the analysis but reacts with the SO2 while in
-------
30 METHODS OF MEASURING
the collecting medium. (The disulfide group is reduced to sulfhydryl by SO2.)
Nitrites are reduced at the dropping mercury electrode at a more negative
potential than SO 2 and presumably would interfere only if present in very
large concentration.
lodimetric Methods
IODINE METHOD 43
in the air is removed and concentrated by scrubbing through a 0.1
normal sodium hydroxide solution contained in a midget or standard impinger
at a sampling rate of 0. 1 and 1 cfm respectively. The solution is subsequently
acidified and titrated with a standard 0.001 normal iodine solution. This
method of sampling is not particularly suitable for field work unless a mobile
laboratory is available that permits titration of the sample within 24 hours
after collection. Negligible losses of SO2 strength result from the storage of
the sample solution in the concentration range reported for a 24-hour period. 21
For a 30-cubic-foot air sample collected at 1 cfm for 30 minutes, a sen-
sitivity of 0.01 ppm is obtained. Thereproducibilityfor the combined analytical
procedure is 10 percent in the range 0.10 to 1.3 ppm with the relative standard
deviation decreasing with increasing concentrations. 4 It was found that re-
coveries by the iodimetric method in the range of concentration of 0.10 to
1.3 ppm were lower than by West-Gaeke or hydrogen peroxide methods.
Oxidizing materials such as nitrogen dioxide and ozone would result in a de-
crease in recovery, or amount of SOg measured, and the presence of reducing
materials such as H2S in the airstream would result in a positive interference.
DIRECT IODINE METHOD
44
Pearce and Schrenk described a method in which SO 2 in air is ab-
sorbed and oxidized directly in a neutral starch-KI-iodine solution. Air is
sampled through a midget impinger at 0.1 cfm. The time required for dis-
appearance of the blue color is noted and the amount of SO 2 present is
calculated from the amount of iodine originally present. If sampling time is
too long, the results are relatively high owing to loss of iodine by aeration
of the absorbing solution. The loss is decreased with increasing amounts of
KI. A recovery of better than 83 percent is reported; and, is independent of
concentration. The method is nonspecific and is applicable hi the presence
of relatively small amounts of other oxidizing or reducing materials. A sensi-
tivity of 0.1 ppm is attained for a 0. 5-cubic-foot air sample collected for
5 minutes.
-------
AND MONITORING SO 2 31
Kitano 45 utilized a second bubbler downstream connected in series and
containing standard thiosulfate solution. Exposure to carbon dioxide and
oxygen did not affect the strength of the thiosulfate solution. In this manner
any iodine lost during aeration would be trapped by the second bubbler
and could therefore be accountable. The bubbler containing the iodine-ab-
sorbing reagent was maintained at 0°C. All-glass tubing was used to prevent
any losses of volatilized or entrained iodine solution. If a hand pump is
used, the method is useful for a rapid, direct approximation of SO 2 levels
in air since no accessory equipment is required in the analysis.
IODINE-THIOSULFATE METHOD 36'46-48
SO 2 hi air is collected by absorption in a fritted bubbler containing 50
milliliters of 0.005 normal iodine at 5 liters per minute for 10 to 30 minutes.
The amount of iodine consumed is proportional to the amount of SO 2 collected
and is determined by titration with 0.005 normal thiosulfate solution. It was
found that the error due to evaporation of liquid is relatively small as is
that due to loss of iodine if25percent KI is used. For a 30-minute air sample,
a sensitivity of 0.4 ppm is attained. This may be increased by reducing the
strength of the iodine and thiosulfate reagents. Griffin and Skinner 48 suggested
the use of an air blank obtained by simultaneously sampling through a bubbler
preceded by a soda-lime tower to remove all SO 2 at the source.
TURBIDIMETRIC METHOD 49>5°
Matty and Diehl 50 used a phototurbidimetric method developed by Corbett 49
for the measurement of SO2 in flue gas. SO2 is removed from the gas stream
by absorption in hydrogen peroxide reagent and is subsequently determined
as sulfate turbidimetrically in an acid isopropyl alcohol medium. The turbidity
is determined 5 minutes after the addition of 1 milliliter of 1 percent BaCl2
solution. Since optical density changes with time owing to the degree of com-
pleteness of precipitation and changes in crystal size, the time elapsed between
the addition of precipitant and photoelectric measurement is critical and should
be standardized. The range of concentration determinable is 0.1 to 0.4 milli-
gram of SO 4 per 25 milliliters when photometric measurement is made with
a light path of 25 millimeters. A very close correlation with the gravimetric
sulfate procedure was obtained over this range of concentration. A 30-cubio-
foot gas or air sample may be determined with a sensitivity of 0.4 ppm by
using these calibration data. The sensitivity of the method may be increased
by increasing the length of the cell path or by sampling for a longer period
of time to obtain a larger sample. The addition of barium chloride in solid
rather than in solution form was found to yield a more lasting suspension of
barium sulfate. 51 Volmer and Frohlich 52 stabilized the suspension with an
acid-glycerine-alchol-gum arable mixture at a pH of 3.2.
-------
32 METHODS OF MEASURING
Mathers 40 described a turbidimetric method in which SO 2 is collected in
a neutral 1 percent lead acetate solution. The amount of PbSOs formed is
determined turbidimetrically at a wavelength of 600 millimicrons. The sen-
sitivity of the method is 0.1 milligram as SO3. Materials other than SO2 that
form insoluble compounds with lead ion interfere in the determination although
PbSOa has a lower solubility product than do lead derivatives of other
commonly occurring gases and aerosols such as hydrogen sulfide and sulfuric
acid.
Cumulative Methods
LEAD PEROXIDE CANDLE METHOD
The lead peroxide (PbO2) method of measuring the extent of atmospheric
pollution by SO 2 was developed in England by the Department of Scientific
and Industrial Research (DSIR) in 1932 and has been used extensively there
ever since; it has been applied only recently in this country. 53>54
The object was to provide an index of the activity of SO 2 in the atmos-
phere as a measure of an aging effect on fabric and buildings and of its
effect on the corrosion of metals. The method is based on measuring the
sulfation caused by gaseous SO2 in ambient air by exposing PbO2 paste.
It is a cumulative method similar to the usual measurement of dust fall. The
candle used in England consists of a porcelain cylinder about 10 centimeters
in circumference. A 10 x 10-centimeter piece ofcotton gauze is wrapped around
the porcelain form as reinforcement and the active reagent is applied. The
active reagent is applied in the form of a paste consisting of 8 grams of
PbO2 in about 5 milliliters of a gum tragacanth solution prepared by dis-
solving the gum in ethyl alcohol and diluting with distilled water. The candle
is exposed in a shelter, which protects the reactive surface from rain, for a
period of 1 month; shorter or longer exposure periods may be used depending
upon the SO2 activity of the atmosphere. After exposure, the material is stripped
from the candle with sodium carbonate, and the amount of sulfate is deter-
mined by the standard gravimetric procedure. The results are reported as milli-
grams of SOa per 100 square centimeters of PbO2 per day.
Wilsdon and McConnell 55 indicated that the rate of sulfate formation is
proportional to SO2 concentrations in the atmosphere, at least up to 15 per-
cent conversion of the reactive material. From experiments in a wind tunnel,
the rate of reaction was found to vary inversely as the 4th root of the wind
velocity. An increase in temperature of 1°C increased the reaction rate about
-------
AND MONITORING SO 2 33
0.4 percent. The reaction rate also increased considerably when the surface
was wet. Conversion to PbSO4 was found to be a function of PbOz particle
size. The authors noted a change in reaction rate with different batches of
PbO2. In the work done in England, a large batch of PbO2 sufficient to last
for several years was obtained. The results obtained by this method corre-
lated very well with data secured by other methods (volumetric) when the results
were corrected for wind velocity and temperature. These experiments were
conducted at SO2 concentrations (30 to 300 ppm) much larger than those
in the atmosphere.
Parker and Richards estimated that errors of sampling and analysis
are about 10 percent. Eight PbC>2 cylinders were exposed simultaneously for
a period of 6 months during 1948-1949. The mean rate of sulfation was
2.6 milligrams of SOa per 100 square centimeters per day and the standard
deviation of one observation was 7 percent of the mean.
Thomas and Davidson 57 employed PbOg cylinders to obtain relative
sulfation values at selected sites in the vicinity of large, coal-burning steam
plants. No deterioration in the rate of reactivity of Pb02 with SO.2 was noted
in periods of exposure as long a-> 4 months. A relatively low degree of cor-
relation was obtained for sulfai'-m rates and SO2 dosage as measured by
the Thomas Autometer. This w .is attributed principally to the typically low
average SO 2 dosage at most sites near a single source. At the site of maximum
exposure, the average 862 concentration was 0.02 ppm. Rather than compare
field values with a sealed laboratory control, control cylinders were operated
at remote sites 60 to 70 miles distant from a sulfur dioxide source. Sulfation
rates varied from 0.02 to 0.04 milligram of SO 3 per 100 square centimeters
per day. This value of about 0.03 milligram of SOa per 100 square centi-
meters per day is considered to be a realistic value for clean air, which is
an order of magnitude greater than that established from sealed-source lab-
oratory controls. The basic cylinder employed at TVA consists of an ordinary
8-ounce, short-form glass jar. Eight grams of PbO2 paste are painted on a
100-square-centimeter band of cotton gauze stapled around the glass jar.
Freshly coated cylinders are dried overnight hi a'desiccator and screwed into
the smaller of two concentric, bradded and soldered-metal jar tops. This
assembly is screwed into a wide-mouthed, 32-ounce glass jar, which serves as
a convenient carrier for shipment and storage. At the site of the field station,
the inner small jar with PbOj coating is inverted and screwed into a jar
top permanently mounted on the base of a louvered shelter. The shelter is
mounted on a 4-foot post. Supports such as utility poles may be used.
Foran et al. 58 found that measurement of SO2 activity by means of
PbO2 candles was well suited to measuring relative concentrations of SO2 in
conjunction with metal corrosion studies. Severity of corrosion of zinc and stain-
-------
34 METHODS OF MEASURING
less steel panels closely correlated with SO2 dosage as measured by the Pb02
candle method. Wilkins 59 compared the relative values of S02 concentrations
determined by the H202 and the Pb02 candle methods. A close correlation of
sulfation rate with S02 concentration was obtained. The conversion factor
from ppm to sulfation rate was milligrams per 100 square centimeters per
day x 0.04 = -SO2, ppm. It was found that the factor by which the PbO2
reading must be multiplied to give the concentration of SO2 varied from month
to month at any given site and from site to site for any given month.8 Yearly
means for each of seven sites in and around London were obtained by divid-
ing the concentration of SO2 in micrograms per cubic meter by the rate of
sulfation of Pb02 in milligrams SO3 per 100 square centimeters per day by
the H2O2 method. The yearly means of the sites varied from 63 to 172 with
an average value of 112. These results show no simple connection between
concentration of SO2 and PbO2 readings. The author concluded that PbO2
readings at any one site should not be used to give an indication of change
in concentration from one month to another. Comparisons between one year
and another are also not very precise, though they may be useful in defining
areas of gross pollution. Nevertheless, if trends over a number of years are
considered, for example, by a comparison of one 5-year average with the
next or of 5-year running averages, the variation due to wind, weather, etc.,
tends to become small and the measurement of SO2 more precise; the 5-year
average has long been recommended by the DSIR for this purpose.
In any district of limited size, for example, the area surrounding a particular
source such as a power station, it is a reasonable assumption that the cylinders
would be exposed to the same weather conditions—wind, humidity, temperature—
so that the rate of sulfation should bear the same relation to concentrations
of SO 2 for each. The pattern of concentration so obtained should, therefore,
be valid, even though the absolute value for each month can be obtained only
by comparison with data obtained from other SO2apparatus.
TEST PAPER METHOD
Buck 60 described a cumulative method for the determination of SO2.
Filter paper is impregnated with a mixture of KHCOa - H2O- glycerine and
the treated paper is then exposed to ambient air for 100 hours. After exposure
the material is stripped from the filter paper, and the amount of sulfate is
determined by the standard gravimetric procedure. Further investigation is
necessary to determine sulfation values relative to SO2 concentration.
Pate et al .61 used membrane filters impregnated with potassium bicarbonate
to collect SOj in aspirated air samples. A 95 percent collection efficiency
was obtained in the concentration range 0.1 to 10 ppm.
Hugen 62 sampled SO2 in ah- using Whatman No. 1 filters impregnated
with potassium hydroxide and glycerol. A collection efficiency of greater than
95 percent was obtained at relative humidities of greater man 25 percent,
-------
AND MONITORING SO 2 35
but below 25 percent, collection efficiencies dropped sharply. S02 was sub-
sequently analyzed by washing the filter with water, neutralizing with HC1,
and applying the West-Gaeke colorimetric method. The stability of the po-
tassium sulfite formed on the filter was poor in the presence of water vapor,
but the filter could be stored for 2 weeks without appreciable oxidation if the
filters were stored in a dry atmosphere.
go
Detector Tubes
Gas detector tubes are generally nonspecific and not sufficiently sensitive
to measure concentrations of pollutants found in ambient air. The present
state of gas-detecting tubes is that they are semiquantitative devices useful
for preliminary survey and screening in industrial hygiene work. Test kits
are being used more and more in industrial hygiene investigations because
they are relatively inexpensive and capable of detecting hazardous concen-
trations immediately so that corrective measures can be taken. Detector tubes
are, however, subject to interferences, and the findings are frequently estimates
only, and abuse is, therefore, possible in the hands of untrained personnel.
Techniques for the calibration of gas-detecting tubes are described by
Kusnetz et al. M
INSTRUMENTAL METHODS CONSIDERED
IN ADDITION TO
RECOMMENDED METHODS
Potentiometric Method 20'65-66
The apparatus consists of a sampling probe, flow control device, absorption-
titration cell, current-generating electrodes, oxidation-reduction-sensing elec-
trode system, amplifier, milliampere recorder, and gas pump. Air is drawn
continuously through the titration cell at a fixed rate of approximately 1 liter
per minute. The zero level is automatically recorded periodically by passage
of the air sample through a charcoal-soda lime filter. SO 2 in the measured
air stream is absorbed in an acidified bromide solution contained in the
-------
36 METHODS OF MEASURING
titration cell. The instrument is initially adjusted to generate continuously a
comparatively low level of bromine in the acid-bromide reagent. A pair of
electrolyzing electrodes is used in which bromine is generated at one electrode
and hydrogen is evolved in the second electrode. Any compound in the air
stream that is oxidized by bromine will proportionately reduce the initially
selected bromine concentration. This reduction in bromine concentration changes
the oxidation reduction potential of the reagent, which is immediately sensed
by the appropriate sensor electrode system. This, in turn, electronically calls
for generation of sufficient additional bromine to maintain the original bromine
concentration. The electric current required to generate this additional bromine
is a measure of the reducing gas in the atmosphere.
Oxidizable sulfur compounds other than SO2 such as H2S, mercaptans,
organic sulfides, and disulfides are recorded by the analyzer. Some gases
such as olefins, diolefins, and phenolic compounds would be titrated to a
limited degree. The presence of these interferences would yield relatively high
results for SO2. Chlorine, bromine, chlorine dioxide, nitrogen dioxide, or
ozone would reduce the bromine demand and would be manifested by com-
paratively low results for S02- It would be possible to conduct a prior sep-
aration of an atmospheric mixture of sulfur-containing compounds before
passing the sample into the analyzer. 67 An automatic multiple-selector valve
operating on a timed sequence could pass the air sample sequentially through
various filters and scrubbers as follows:
1. Bismuth subcarbonate-H2SO4 solution to remove H2S;
2. potassium dichromate solution to remove H2S and 862;
3. alkaline CdSO4 solution to remove H2 S, SO2, and RSH;
4. activated charcoal-soda lime to remove all reactive constituents.
Pyrolysis offers the possibility of converting mixtures of sulfur gases to
either H2S or SO2. Field and Oldach 6S converted a mixture of sulfur com-
pounds to H2S by passing a sampled stream of air with hydrogen over alumina
at 900° C. Thomas et al. 69 oxidized H2S, mercaptans, and other sulfur
compounds to SO.2 by passing the sampled air through a silica combustion
tube containing an electrically heated, spiral, platinum wire.
The range of SO2 detectable by the instrument is 0.1 to 10 ppm with a
sensitivity of 0.1 ppni and a reproducibility of 0.2 ppm. A 90 percent response
to a change in concentration is effected in 30 seconds.
Nader and Dolphin 70 have developed a circuit modification that increases
instrument sensitivity by a factor of 10. This has, however, resulted in ex-
cessive background noise level and zero drift. Development of an accessory
circuit to arrange for automatic adjustment of the generating current is currently
in process and will permit the unattended, continuous monitoring of atmos-
pheric SO 2 at the increased sensitivity. McKee and Rollwitz '71 pursued a
similar modification and increased instrument sensitivity to one scale division
-------
AND MONITORING SO 2 37
per 0.01 ppm SO2; full-scale deflection corresponded to 0.75 ppm. A potentio-
metric recorder was substituted for the recording milliameter. The noise level
was reduced to a satisfactory level by an electronic filter. Zero drift was still
the greatest difficulty encountered, and occasionally no record was obtained
when the zero point drifted off the scale.
The apparatus is calibrated against standard mixtures of inert gas with
SO2. The calibrating mixtures may be prepared in a tank of known capacity
pressured to from 100 to 200 psig containing metered volumes of SO2 gas.20
S02 cylinders prepared in this manner will decrease in strength by 5 to 10
percent in 2 months and should therefore be used within this time. Standard
SO2 mixtures may also be prepared hi gas test chambers or in collapsible
mylar or teflon bags and then fed into the analyzer at the normal rate of
flow. Several calibration points within the full-scale reading of the instrument
should be obtained.
Carpenter and Sparkman 72 adapted the analyzer for field mobile sampl-
ing in a motor vehicle and in a helicopter. The analyzer has also been oper-
ated in a helicopter by Gartrell and Carpenter 73 in the study of dispersion
patterns by measuring SC>2 concentration in a plume.
Photometric Methods
ANALYZER"A"
Katz ' described a continuous, automatic analyzer for SO2 that employs
photoelectric cells and a recording potentiometer to indicate the increase in
light transmission of blue starch-iodine solutions after aspiration with con-
taminated air. The instrument has a range of 0.01 to 1 ppm or more, de-
pending upon the volume and concentration of solution in the absorbers and
the amount of air sample passed through the solution. This instrument is
not commercially available. The gas bubblers consist of pyrex salvarsan tubes
of 350-milliliter capacity. The instrument operates on a 2-minute time cycle,
the solution in one absorber being aspirated at a measured flow rate for
this interval after which the air is passed through the second absorber, by the
opening and closing of appropriate, cam-operated, poppet valves. The as-
piration is continued alternately for eight successive operations hi each absorber.
At the end of this period of about 32 minutes, the solution hi each absorber
is drained successively, and fresh solution is delivered from a stock bottle
of 20-liter capacity. The light transmission through the blue starch-iodine
-------
38 METHODS OF MEASURING
solution is recorded during the 2-minute quiescent period following each as-
piration cycle by means of a recording potentiometer with a scale range
of 0 to 50 millivolts. The change in concentration of the blue starch-iodine
solution is determined by using two photronic cells to measure the light trans-
mission through each absorber from a constant source supplied by a 20-
watt lamp. The light source is mounted midway between the absorbers, and
the photo cells are mounted behind each one. Output current from the cells
is passed through a standard resistance box, and the voltage drop across
the O-to-500-ohm terminals is measured by means of the recording potentiometer.
The recorder is calibrated by determining the potentiometer reading for
a series of starch-iodine solutions varying in normality from 8 x 10"^ to 2
x 10"5 during the operation of the analyzer with SO2 -free air. Reproducible
results are obtained on iodine solutions of equivalent normality. Experiments
indicate that the starch-iodine solution used remains stable after aspiration
with SO2 -free air at rates of about 10 liters per minute for periods of 30
minutes. Only a high grade of starch, which gives the characteristic blue color
with dilute iodine solution, is suitable for this work. Grades that yield a purple
tinge are unsatisfactory. The stock solutions are stable for 1 week when stored
in a dark, cool room provided the vessels are maintained in a clean, sterile
condition.
Sulfur trioxide, sulfuric acid mist or sulfate, and unsaturated hydrocarbons
do not interfere in this method. Oxidizing and reducing materials such as
H2 S and NOj do, however, interfere, and the method is not applicable in
atmospheres where these materials are likely to be present in appreciable
quantities.
ANALYZER"B"
Adams, Dana, and Koppe 76 developed a versatile, continuous, automatic,
photometric analyzer based on the dosimeter sampling principle. A small
volume of reagent sensitive to a particular ion or class of compounds is
circulated continuously in the air-reagent contactor system until a preselected
concentration of the pollutant under study is accumulated. When this has
been reached, the instrument automatically discharges the spent reagent and
injects a measured volume of fresh reagent. Sampling is then continued until
another cycle is completed by the accumulation of the standard quantity of
pollutant. The pollutant concentrations may be recorded as either a tape
record of the time necessary to accumulate each equivalent quantity of the
pollutant under study or as a continuously integrated recorder chart of the
rate of accumulation of the pollutant during each dosimeter period. This
instrument is suitable for the automatic recording of any pollutant for which
a suitable colorimetric reagent can be developed.
A starch-iodine reagent is used for the photometric determmaltion of atmos-
pheric SO2. It was found that the partially bleached blue color was more un-
stable under conditions of continuous aeration than was the unbleached blue
-------
AND MONITORING SO 2 39
color. A reasonably stable reagent that may be aerated at approximately
0.1 cfm for periods of more than 12 hours was produced by the addition of
small quantities of N-acetyl-p-aminophenol and mannitol. It was also found
that the blue color of the reagent was extremely sensitive to temperature.
Furthermore, the color was found to deteriorate rapidly when stored in the
dark at elevated temperatures of 120°F. To minimize these effects, the dosi-
meter cabinet was thermostatted at 75° F with a refrigerative compressor to
maintain this temperature. Light absorbance measurements are conducted with
a 50-watt projection lamp and an optical filter peaking at 575 millimicrons.
The prepared reagent is stored in a black reagent bottle in the automatic
analyzer. A drop of mercury is kept in the storage bottle to act as a fungicide.
The reagent has been found to remain stable up to 1 week when handled in
this manner. A soda lime-activated charcoal trap on the reagent bottle's air
inlet protects the reagent from possible contamination by air pollutants.
A countercurrent type of air-reagent-contacting system is employed in which
a small batch of reagent is continuously circulated through an optical flow
cell. Small evaporation losses are continuously sensed and dropwise additions
of distilled water are automatically made to maintain the original volume.
The sampled air is drawn through a rotameter, and the air volume is reg-
ulated by means of a built-in air bypass on the vacuum side of the air
pump. Air entering under suction lifts the reagent, which has fallen by gravity
through the optical path, up through the contacting column. The scrubbed
air is exhausted through a tube at the top of the system to the vacuum pump,
and the reagent is maintained within the optical cell by means of the capillary
tip at the lower end of the cell. The cycling of the reagent with the stream of
continuously sampled air continues until:
(1) A pre-selected concentration of the pollutant is accumulated, or
(2) a maximum time over which the reagent remains stable is exceeded, or
(3) the reset button is manually operated.
Further investigation should be conducted to establish the practicality of
the system under field conditions for the measurement of SO2. Instrument
performance data have been supplied by the manufacturer for the measure-
ment of atmospheric fluorides but no such specifications have been made
available as yet for SO2 analysis.
ANALYZER"C"
The Portable SO2 Meter was developed in the Central Electricity Research
Laboratory by Cummings and Redfearn. 77 862 in air reacts with a starch-
iodine reagent in a countercurrent absorption column. The amounts of light
absorbed by the unchanged and the partially decolorized reagent are com-
pared by photoelectric cells connected to a galvanometer. It was found that
the reagent as recommended by Katz 74 was stable for several days when
kept in stoppered polyethylene bottles. The light absorption of batches of
-------
40 METHODS OF MEASURING
reagent made over a period of 1 year varied by less than 1 percent. With
the countercurrent absorption column and the rates of air and reagent flow
used for this instrument, no iodine was volatilized from the solution by the
incoming air.
The starch-iodine reagent contained in a 1-liter aspirator bottle, which is
fitted with a constant-head device, flows by gravity through a control valve
into the bottom of a liquid cell. From here, the reagent passes through a
rotameter, which indicates the flow rate to the top of the absorption column
where it flows down the spiral path. To avoid corrosion by the starch-iodine
reagent, a korannite float and tantalum spring float stops are used.
Air is drawn by means of a DC blower into the bottom of the absorption
column where the SOg reacts with the reagent. If a check on the zero is re-
quired, air is first drawn through soda-lime by means of stopcocks. Air leaves
the top of the absorber via a rotameter and blower. The partially decolorized
reagent flows from the bottom of the absorber into the bottom of the glass
liquid cell and then runs to a waste bottle.
Comparison of the absorbance of the reacted to unreacted reagent is made
in a photocell compartment consisting of a single light source (6 volts, 0.2
amp), condensers to form parallel beams of light, red filters, and two photo-
cells mounted behind the glass cells. The unbalanced output of the photocells
is measured by a galvanometer. The meter is sensitive to 0.01 ppm in the
range 0 to 0.50 ppm. This range may be extended by reducing the flow of
air to the instrument; the sensitivity decreases in proportion to the increase in
range. The readings obtained must be regarded as 2-minute running averages
because there is a time lag of about 2 minutes before a steady reading is
obtained for a particular SO 2 concentration.
If the instrument is operated in a motor vehicle, power can be drawn from
the 12-volt battery. For use away from roads a 12-volt battery can be in-
corporated within the instrument case.
This instrument was designed for rapid measurement at various points in
a selected area; thus, there is always an operator with the instrument, and
a recorder is unnecessary. Since the total output of the photocell is about
0.25 millivolt, it would not be practical to feed the output to a normal recorder.
ANALYZER"D"
Helwig and Gordon 78 converted an automatic, conductimetric, SO 2 analyzer
to an automatic, continuous-recording, colorimetric analyzer by replacing the
conductivity cells with colorimeter cells having a 2-centimeter path and a mer-
cury lamp light source. A synchroverter was added and necessary circuit
changes were made to use a 0- to 10-millivolt recorder. This instrument is
not commercially available.
-------
AND MONITORING SO 2 41
The pararosaniline-formaldehyde reagent developed by West and Gaeke 1
was selected as the chromophoric reagent for use in the automatic recorder.
It was found that the rate and magnitude of the color response to SO2 were
greater in the absence of sodium tetrachloromercurate. Further increase in
sensitivity was obtained by decreasing the dye concentration to one half of
that suggested by the authors. The mixed reagent was stable for 2 weeks.
Collection efficiency of the countercurrent absorption column varied with
airflow rate. At an airflow rate of 0.25 liter per minute, the collection effic-
iency was found to be 95 to 97 percent. Efficiency dropped to 73 to 75 per-
cent at a flow rate of 1 liter per minute. A system was established to detect
SO 2 in the concentration range 0 to 5 ppm. By varying reagent and sample
flow, the scale can be reduced or expanded.
Static calibration of the instrument was conducted with standard solutions
of sodium meta bisulfite equivalent to 1 to 5 ppm of SO 2 on the basis of an
airflow of 0.25 liter per minute and a reagent flow of 3.3 milliliters per
minute.
An instrument sensitivity of 0.02 ppm and excellent reproducibility were
reported in the l-to-5-ppm range of concentration. NO2 and Oa in concen-
trations of less than 1 ppm produced negligible interference on SOz
measurement.
Air lonization Method
The aerosol ionization detector consists essentially of an ionization chamber
with a stainless steel cylinder serving as an outer electrode and a heavy wire
as the inner electrode. Fifty to 100 micrograms of radium is placed along the
surface of the cylinder to serve as an alpha source. The two electrodes are
under a voltage potential difference that causes a flow of ion current between
them. The presence of small concentrations of particulate matter causes a
marked drop in the flow of ion current.
A compensated detection system is used in the analyzer. The sample stream
is divided between two similar ion chambers. The difference in ion current
produced by the unreacted and reacted sample stream is detected by an elec-
trometer measuring circuit and is fed into a suitable recorder.
-------
42 METHODS OF MEASURING
To use this principle for the determination of gases and vapors, it is neces-
sary to form small particles from the material being analyzed. Three methods
may be used to accomplish this: (1) Reaction of the material with a reagent
to produce particulate matter, (2) pyrolysis or irradiation with ultraviolet
light, or (3) a combination of both.
The instrument is sensitized to SO 2 by pyrolysis in the presence of CuO.
A gas-solid reaction occurs and copper sulfate is formed.
An instrument range of 0 to lOppm SO2 is obtained with this sensitization
method. The method is nonspecific since any material capable of salt formation
with CuO at elevated temperatures will interfere. Additional investigation is
required concerning instrument performance under conditions of continuous
air monitoring, reproducibility, sensitivity, and the relative effects of interfering
materials.
-------
AND MONITORING SO 2 43
REFERENCES
1. West, P. W., Gaeke, G. C., Fixation of Sulfur Dioxide as Disulfitomer-
curate (II), Subsequent Colorimetric Estimation, Anal. Chem., 28:1916.
1956.
2. Nauman, R. V., West, F. W., Tron, F., Gaeke, G. C., Spectrophoto-
metric Study of the Schiff Reaction as Applied to the Quantitative
Determination of Sulfur Dioxide, Anal. Chem., 32:1307. 1960.
3. McCaldin, R. O., Hendrickson, E. R., Use of a Gas Chamber for Testing
Air Samplers, J. Am. Ind. Hyg. Assoc., 20:509. 1959.
4. Terraglio, F. P., Manganelli, R. M., Laboratory Evaluation of 862
Methods, Anal. Chem., 34:675. 1962.
5. Perry, W. H., Tabor, E. C., National Air Sampling Network Measure-
ment of SO 2 and N02, Arch. Environmental Health, 4:44. 1962.
6. West, P. W., Ordoveza, F., Elimination of Nitrogen Dioxide Interference
in the Determination of Sulfur Dioxide, Anal. Chem., 34:1324. 1962.
7. Zurlo, N., Griffini, A. M., Measurement of Sulfur Dioxide Content of
the Air in the Presence of Nitrogen and Heavy Metals, Med. d. Lavoro,
5:330. 1962.
8. Craxford, S. R., Slimming, D. W., Wilkins, E. T., The Measurement of
Atmospheric Pollution: The Accuracy of the Instruments and the Signifi-
cance of the Results, Proceeding of the Harrogate Conference. London,
England. 1960.
9. Pate, J. B., Lodge, J. P., Wartburg, A. F., Effect of Pararosaniline in
the Trace Determination of Sulfur Dioxide, Anal. Chem., 34:1660. 1962.
10. Jacobs, M. B., The Chemical Analysis of Air Pollutants, Interscience,
New York. 1960.
11. Jacobs, M. B., Greenburg, L., Sulphur Dioxide in New York City Atmos-
phere, Ind. Eng. Chem., 48:1517. 1956.
12. Perley, G. A., Langsdorf, B. T., Problems in the Recording of Sulfur
Dioxide in Polluted Atmospheres, Chap. 69, Proceedings of the United
States Technical Conference in Air Pollution, L. C. McCabe, ed., McGraw-
Hill, New York. 1952.
13. Yocum, J. E., Richardson, R. L., Saslaw, X. M., Chapman, S., The
Effect of Hydrogen Chloride on the Operation of the Thomas Autometer,
Proceedings 49th Annual Meeting of the Air Pollution Control Association,
Buffalo, N. Y. 1956.
-------
44 METHODS OF MEASURING
14. Jacobs, M. B., Braverman, M. M., Hochheiser, S., Ultrasensitive Con-
ductometric Measurement of Sulfur Dioxide in Air, Instrument Society
of America Meeting, Paper No. 56-32-3, Sept. 1956.
15. Reece, G. M., White, B., Drinker, P., Determination and Recording of
Carbon Disulfide and Hydrogen Sulfide in the Viscose-Rayon Industry,
J. Ind. Hyg. Tox., 22:416. 1940.
16. Thomas, M. D., Ivie, J. O., Abersold, N. N., Hendricks, R. H., Automatic
Apparatus for Determination of Small Concentrations of Sulphur Dioxide
in Air Application to Hydrogen Sulfide, Mercaptans, and Other Sulfur
and Chlorine Compounds, Ind. Eng. Chem. Anal. Ed., 15:287. 1943.
17. Cummings, W. G., Redfearn, M. W., Instruments for Measuring Small
Quantities of Sulfur Dioxide in the Atmosphere, J. Inst. of Fuel, 30:628.
1957.
18. Air Sampling Instruments, American Conference of Governmental In-
dustrial Hygienists, Cincinnati, Ohio. 1962.
19. Recommended Methods in Air Pollution Measurements, California State
Department of Public Health, Berkeley, Calif. 1960.
20. ASTM Standards on Methods of Atmospheric Sampling and Analysis,
American Society for Testing Materials, Philadelphia, Pa. 2nd Ed., 1962.
21. Hochheiser, S., Braverman, M. M., Jacobs, M. B., Comparison of
Methods for the Determination of Atmospheric Sulphur Dioxide, presented
at Metropolitan L. I. Subsection, New York Section, ACS Meeting in
miniature, Feb. 1955.
22. Paper EPA/AR/4283, Routine Methods for Estimating Sulphur Dioxide
in the Air, Organization for European Economic Cooperation, European
Productivity Agency, Paris, France. 1961.
23. Welch, A. F., Terry, J. P., Development in the Measurement of Atmos-
pheric Sulfur Dioxide, J. Am. Ind. Hyg. Assoc., 21:316. 1960.
24. Paulus, H. J., Floyd, E. P., Byers, P. H., Determination of Sulphur
Dioxide hi Atmospheric Samples. Comparison of a Colorimetric and a
Polarographic Method, J. Am. Ind. Hyg. Assoc. Quarterly, 15:4. 1954.
25. Urone, P. F., Boggs, W. E., Acid-Bleached Fuchsin in Determination of
Sulfur Dioxide in the Atmosphere, Anal. Chem., 23:1517. 1951.
26. Steigmann, A., A New Color Reaction for Sulfurous Acid, The Thiol
Group and Formaldehyde, Anal. Chem., 22:493. 1950.
27. Grant, W. N., Colorimetric Determination of Sulphur Dioxide, Ind. Eng.
Chem. Anal. Ed., 19:245. 1947.
-------
AND MONITORING SO 2 45
28. Stang, A. M., Zatek, J. E., Robson, C. D., A Colorimetric Method for
the Determination of Sulfur Dioxide in Air, Am. Ind. Hyg. Assoc.
Quarterly, 12:5. 1951.
29. Moore, G. E., Cole, A. F. W., Katz, M., The Concurrent Determination
of Sulphur Dioxide and Nitrogen Dioxide in the Atmosphere, J. Air
Poll. Control Assoc., 7:25. 1957.
30. Determination of Sulphur Dioxide in Air, Fuchsin-Formaldehyde Method,
American Conference of Governmental Industrial Hygienists, Methods
Manual, Cincinnati, Ohio. 1958.
31. Stratman, H., Micro analytical Methods for the Determination of Sulphur
Dioxide, Mikrochimica Acta, 6:668. 1954.
32. Kanno, S., The Colorimetric Determination of Sulfur Oxides in the Atmos-
phere, Intern. J. Air Poll., 1:231. 1959.
33. Bertolocini, R. J., Barney, J. E., Colorimetric Determination of Sulfate
Using Barium Chloranilate, Anal. Chem., 29:281. 1957.
34. Hands, G. C., Bartlett, A. F., A Field Method for the Determination of
Sulphur Dioxide in Air, Analyst, 85:147. 1960.
35. Dept. of Sci. and Ind. Research. DSIR Methods for the Detection of Toxic
Gases in Industry, Leaflet No. 3, H. M. Stationery Office, London,
England. 1939.
36. Liddell, E. F., A Reagent for Sulphur Dioxide, Analyst, 80:901. 1955.
37. Klein, B., Microdetermination of Sulfate. A Colorimetric Estimation of
the Benzidine Sulfate Precipitate, Ind. Eng. Chem. Anal. Ed., 16:536.
1944.
38. Lambert, J. L., Yaida, S. K., Grother, M. P., Colorimetric Determination
of Sulfate Ion, Anal. Chem., 27:800. 1955.
39. Blum, R. C., Gunther, F. H., Indirect Ultra-Violet Spectrophotometric
Determination of Sulphur Dioxide by Means of Plumbous Ion, Analyst,
86:675. 1961.
40. Mathers, A. P., Rapid Determination of Sulphur Dioxide in Wines, J.
Assoc. of Agric. Chem., 32:745. 1941.
41. Stephen, B. G., Lindstrom, F., Spectrophotometric Determination of Sulfur
Dioxide Suitable for Atmospheric Analysis, Anal. Chem., 36:1308. 1964.
42. Kolthoff, I. M., Miller, C. S., The Reduction of Sulfurous Acid at the
Dropping Mercury Electrode, J. Am. Chem. Soc., 63:2818. 1941.
43. Smith, R. B., Fries, B. S. T., Portable Motor-driven Impinger Unit for
Determination of Sulfur Dioxide, J. Ind. Hyg., 13:338. 1931.
-------
46 METHODS OF MEASURING
44. Pearce, S. J., Schrenk, H. H., Determination of Sulfur Dioxide in Air
by Means of the Midget Impinger, Bureau of Mines RI 4282. 1948.
45. Kitano, Y., Takakuwa, H., Determination of Hydrogen Sulfide and
Sulfur Dioxide in Air, Errors in lodometry, Japan Analyst, 3:7. 1954.
46. Jacobs, M. B., The Analytical Chemistry of Industrial Poisons, Hazards
and Solvents, Interscience, New York. 1959.
47. Elkins, H. B., The Chemistry of Industrial Toxicology, Wiley, New
York. 1959.
48. Griffin, S. W., Skinner, W. W., Small Amounts of Sulfur Dioxide in the
Atmosphere. Improved Methods for the Determination of Sulphur Dioxide
when Present in Low Concentrations in Air, Ind. Eng. Chem., 24:862.
1932.
49. Corbett, P. F., A Phototurbidimetric Method for the Estimation of Sulfur
Trioxide in the Presence of Sulfur Dioxide, J. Soc. of Chem. Ind., 67:227.
1948.
50. Matty, R. E., Diehl, E. K., Measuring Flue Gas Sulfur Dioxide and
Sulfur Trioxide, Power, 101:94. 1957.
51. Treon, J. F., Crutchfield, W. E., Rapid Turbidimetric Method for the
Determination of Sulfates, Ind. Eng. Chem. Anal. Ed., 14:119. 1942.
52. Volmer, W., Frohlich, F. Z., Turbidimetric Determination of Sulfate,
Anal. Chem., 126:414, 1944.
53. Department of Scientific and Industrial Research, The Investigation of
Atmospheric Pollution, 18th Report, 1931-1932, H. M. Stationery Office,
London, England.
54. Ibid, 20th Report, 1933-1934.
55. Wilsdon, B. H., McConnell, F. J., The Measurement of Atmospheric
Sulfur Pollution by Means of Lead Peroxide, J. Soc. Chem., Ind., 53:385.
1934.
56. Parker, A., Richards, S. H., Instruments Used for the Measurement of
Atmospheric Pollution in Great Britain, Chap. 67; Proceeding of the
United States Technical Conference hi Air Pollution, L. C. McCabe, ed.,
McGraw-Hill. 1952.
57. Thomas, F. W., Davidson, C. M., Monitoring Sulfur Dioxide with Lead
Peroxide Cylinders, J. Air Poll. Control Assoc., 11:24. 1961.
58. Foran, M. R., Gibbons, E. V., Wellington, J. R., The Measurement of
Atmospheric Sulfur Dioxide and Chlorides, Chemistry in Canada, 10:33.
1958.
-------
AND MONITORING SO 2 47
59. Wilkins, E. T., Air Pollution in London Smog, Mech. Eng., 76:426.
1954.
60. Buck, V. M., Methods for the Determination of Hydrogen Fluoride and
Sulfur Dioxide in the Atmosphere, Staub, 21:227. 1961.
61. Pate, J. B., et al., The Use of Impregnated Filters to Collect Traces of
Gases in the Atmosphere, Anal. Chem. Acta., 28:341. 1963.
62. Hugen, C., The Sampling of Sulfur Dioxide in Air with Impregnated
Filters, Anal. Chem. Acta, 28:349. 1963.
63. Gisclard, J., The Use of Detectors and Test Kits in Industrial Hygiene
investigations, AMA Arch. Ind. Health, 21:250. 1960.
64. Kusnetz, H. L., Saltzman, B. E., Lanier, M. E., CaEbration and Eval-
uation of Gas Detecting Tubes, Amer. Ind. Hyg. Assoc. J., 21:361. 1960.
65. Dickinson, J. E., The Operation and the Use of Titrilog and the Auto-
meter, Proceedings 49th Annual Meeting of Air Poll. Control Assoc. 1956.
66. Giever, P. M., Cook, W. A., Automatic Recording Instruments As Applied
to Air Analysis, Arch, of Ind. Health and Occ. Med., 21:233. 1960.
67. Washburn, H. W., Austin, R. R., The Continuous Measurement of Sulfur
Dioxide and Hydrogen Sulfide Concentrations by Automatic Titration,
Chap. 72, Proceedings of the United States Technical Conference in Air
Pollution, L. C. McCabe, ed., McGraw-Hill. 1952.
68. Field, E., Oldach, C. S., Conversion of Organic Sulfur to Hydrogen
Sulfide for Analysis, Ind. Eng. Chem. Anal. Ed., 18:668. 1946.
69. Thomas, M. D., Ivie, J. O., Abersold, N. N., Hendricks, R. H., Automatic
Apparatus for Determination of Small Concentrations of Sulfur Dioxide
in Air. Application to Hydrogen Sulfide, Mercaptans and Other Sulfur
and Chlorine Compounds, Ind. Eng. Chem. Anal. Ed., 15:287. 1943.
70. Nader, J. S., Dolphin, J. L., Improved Titrilog Sensitivity, J. Air Poll.
Control Assoc., 8:336. 1959.
71. McKee, H. C., Rollwitz, W. L., Improved Titrilog Sensitivity, Field
Performance and Evaluation, J. Air Poll. Control Assoc., 8:338. 1959.
72. Carpenter, S. B., Sparkman, R. E., Adaptation of Titrilog for Field
Mobile Sampling, J. Air Poll. Control Assoc., 5:195. 1956.
73. Gartrell, F. E., Carpenter, S. B., Aerial Sampling by Helicopter, A Method
for the Study of Diffusion Patterns, J. Amer. Meteorol. Soc., 12:215. 1955.
-------
48
74. Katz, M., The Photoelectric Determination of Atmospheric S.ulphur Dioxide
by Dilute Starch-Iodine Solutions, Chap. 71, Proceedings of the United
States Technical Conference in Air Pollution, L. C. McCabe, Ed., McGraw-
Hill, New York. 1952.
75. Katz, M., Photoelectric Determination of Atmospheric Sulfur Dioxide
Employing Dilute Starch-Iodine Solution, Anal. Chem., 22:1040. 1950.
76. Adams, D. F., Dana, H. J., Koppe, R.K., Universal Air Pollutant
Analyzer, USPHS Contract No. 66512. Robert A. Taft Sanitary Engineer-
ing Center, Cincinnati, Ohio. 1957.
77. Cummings, W. G., Redfearn, M. W., Instruments for Measuring Small
Quantities of Sulfur Dioxide in the Atmosphere, J. Inst. of Fuel, 30:628.
1957.
78. Helwig, H. H., Gordon, C. L., Colorimetric Methods for Continuous
Recording Analysis of Atmospheric Sulphur Dioxide, Anal. Chem., 34:1660.
1962.
79. Strange, J. P., Ball, K. E., Barnes, D. O., Continuous Parts Per Billion
Recorder for Air Contaminants, Proceedings of the Air Poll. Control
Assoc., Cincinnati, Ohio. May 22-26. 1960.
GPO 937-643
-------
BIBLIOGRAPHIC: Hochheiser, Seymour. Methods of mea-
suring and monitoring atmospheric sulfur dioxide. PHS
Publ. No. 999-AP-6. 1964.48pp.
ABSTRACT: A literature review of methodology relating to
the measurement of atmospheric sulfur dioxide, a detailed
description of recommended methods, and criteria for
selection of recommended methods are presented in this
report. This publication is intended to serve as a resource
document for those involved in measurement of pollution
and in research on new or improvedmethods, and for those
who seek to bring about widespread agreement in matters
concerning measurement of pollution.
BIBLIOGRAPHIC: Hochheiser, Seymour. Methods of mea-
suring and monitoring atmospheric sulfur dioxide. PHS
Publ. No. 999-AP-6. 1964. 48 pp.
ABSTRACT: A literature review of methodology relating to
the measurement of atmospheric sulfur dioxide, a detailed
description of recommended methods, and criteria for
selection of recommended methods are presented in this
report. This publication is intended to serve as a resource
document for those involved in measurement of pollution
and in research on new or improvedmethods, and for those
who seek to bring about widespread agreement in matters
concerning measurement of pollution.
BIBLIOGRAPHIC: Hochheiser, Seymour. Methods of mea-
suring and monitoring atmospheric sulfur dioxide. PHS
Publ. No. 999-AP-6. 1964.48pp.
ABSTRACT: A literature review of methodology relating to
the measurement of atmospheric sulfur dioxide, a detailed
description of recommended methods, and criteria for
selection of recommended methods are presented in this
report. This publication is intended to serve as a resource
document for those involved in measurement of pollution
and in research on new or improvedmethods, and for those
who seek to bring about widespread agreement in matters
concerning measurement of pollution.
ACCESSION NO.
KEY WORDS:
Sulfur Dioxide
Measurement and
Monitoring
Recommended Methods
Criteria for
Selection
Literature Review
Air Pollution
Manual and Automatic
Methods
ACCESSION NO.
KEY WORDS:
Sulfur Dioxide
Measurement and
Monitoring
Recommended Methods
Criteria for
Selection
Literature Review
Air Pollution
Manual and Automatic
Methods
ACCESSION NO.
KEY WORDS:
Sulfur Dioxide
Measurement and
Monitoring
Recommended Methods
Criteria for
Selection
Literature Review
Air Pollution
Manual and Automatic
Methods
------- |