Tracer Techniques in Sulfur DioxideAir Pollution Studies Apparatus and Studies of Sulfur Dioxide Colorimetric and Conductometric Methods PAUL URONE, JAMES B. EVANS,’ and CLAUDIA M. NOYES Department o f Chemistry, University o f Colorudo, Boulder, Colo.
b Analytical tracer techniques for studying the role of sulfur dioxide in air pollution problems were developed. An apparatus was built which made it possible to measure directly microgram quantities of sulfur dioxide tagged with S35. The tagged sulfur dioxide was followed from its original pure state to its mixture at high dilution in purified air, to the sampling medium, and finally to the quantitation step, and was measured by proportional tube counting and scintillation solution counting techniques. Colorimetric and conductometric methods for determination of sulfur dioxide were evaluated by radiochemical tracer techniques. Colorimetric standards prepared from measured quantities of gaseous SO2 gave greater color intensities than standards prepared from equivalent amounts of standardized bisulfite solutions. Tracer techniques showed that conductometric methods became unreliable at concentrations below 1 p.p.m., even though purified diluting air was used, and sampling methods lost efficiency below 0.1 p.p.m. for both colorimetric and the conductometric methods. A second bubbler in series was not able to capture the S3502 lost by the first bubbler. This indicates that series bubblers cannot be used for evidence of absorbing solution efficiencies. Correlation of colorimetric and radiochemical analytical techniques can give useful supplementary information on the reactions of SO2 in controlled atmospheres.
M
chemical and physical methods of analysis have been proposed for studying the role of sulfur dioxide as one of the more common air pollutants (2, 4, 6, 8-10). I n general, quantitation of the actual pollutant has depended upon the comparison of the over-all chemical resultant of the air sample with the equivalent reaction of a carefully controlled laboratory Present address, National Standards Foundation, Inc., Boulder, Colo. 1104
ANY
ANALYTICAL CHEMISTRY
“standard.” I n question are the efficiencies of the sampling processes and the effects of concentrating the pollutant by many orders of magnitude into a liquid or solid medium in the presence of equally concentrated cocontaminants. Radiochemical techniques furnish a powerful tool for studying these effects, and sulfur dioxide readily lends itself to this type of analysis. Radioactive SS5O2is easily handled, has a convenient half life of 87.1 days, and is a weak beta emitter with an energy of 0.167 m.e.v. It is commercially available in high specific activity, ordinary glassware contains its radiation, and its air path is only a few inches. Using vacuum techniques, i t can be readily purified, measured, and transferred from one vessel to another. Because of the weakness of the 6-ray emitted, the quantitative measurement of P502requires special techniques. The techniques are similar to those used for the measurement of CI4, and in practice both internal proportional gas and liquid scintillation counting have been successfully used for S35 determinations (3, 6). EXPERIMENTAL
Apparatus. Figure 1 shows the apparatus used to purify, measure, and mix sulfur dioxide with a n y desired amounts of air in one, two, or three stages. T h e mixtures could be stored in a n intermediate or final stage, or could be sampled at will. The Microvol (7) permitted the direct measurement of microgram quantities of sulfur dioxide with a n accuracy of 1% or better. Essentially, it is a McLeod gauge containing a trap in which liquid nitrogen is placed. When the system is under vacuum, a condensable gas released into the system will collect on the trap. The mercury is allowed to rise to the neck of the collection chamber, and the liquid nitrogen is then removed by blowing warm air into the trap. After the condensable gas evaporates, the mercury is allowed to rise further, compressing the gas into the calibrated measuring capillaries. Mer-
cury levels for the pressure-volume measurement were determined with a telescope mounted on a cathetometer. -4 tube containing the radioactive sulfur dioxide (Nuclear-Chicago Corp., 30 mc. per mmole) was sealed into position (upper right, Figure 1). With the system closed but under vacuum of less than 1 micron, one or more aliquots were transferred to any one of several freeze-out points. The transfers were to the Microvol for volumetric measurement and then to the working flask, or to one of the bulbs for sealing off for scintillation counting, to the P20s tube for drying, or to the proportional tube for counting, etc. The transfers mere quantitative, and were complete in less than 10 minutes, providing that air leaks mere not present. The working flask (Figure 1) was used to dilute the highly active S3502 with untagged sulfur dioxide, so that larger quantities of sulfur dioxide could be used without involving excessive counting rates. The volume of the flask was calibrated and found to be 1082.1 i 0.1 ml. at 25’ C. The side arm for manometrically measuring the pressure in the flask had a volume of 0.347 ml. per cm. After evacuation, the working flask was charged with approximately 200 mm. of SOz. This was purified by a freezing and pumping technique at liquid nitrogen and dry ice temperatures. Enough aliquots of the S3602were then transferred to the flask to give adequate count rates for the planned experiments. Two 20-liter Mylar bags (G. T. Schjeldahl Co., Northfield, Minn.) were attached to the gas-air mixing portion of the apparatus with Teflon connections. SO2-air mixtures were made either from aliquots taken directly from the S3502 reserve or from the working flask. The sampling ports were used for filling additional sampling bags or containers. The S3jactivity of the bag mixtures was checked frequently by transferring an aliquot from the gas pipet to the evacuated proportional tube connected to the sampling port. Purified air was used to flush the lines leading to the proportional tube. A glass injection port sealed with a silicone rubber septum was built into the air stream for the injection of hydro-
Ca SO4
ilil
SAMPLING
EA1
Figure 1.
carbons or other compounds of interest by means of a microsyringe. For simplicity, the diluting air train is shown in an abbreviated schematic form. Pressurized air is passed through a large anhydrous calcium sulfate column, followed by passage through indicating silica gel, activated charcoal, and Molecular Sieve 5A. Two flowmeters were used to measure either inlet or outlet flow rates for the mixing and sampling processes. Each flowmeter had several replaceable capillary restrictions, each of which was calibrated with a wet-test meter, water, or mercury displacement from calibrated glassware as needed. All connections were fused glass or glass to glass. All stopcocks were greased with Apiezon lubricants. The gas pipet was calibrated, and its volume was 44.37 ml. at 25' C. With it a known volume of the bag mixtures could be repeatedly and reproducibly taken for consecutive sample studies. For preparing dynamic flow mixtures, an aliquot in the pipet was bled into an air stream at rates ranging from a few minutes to more than 4 hours by controlling the flow of mercury from the leveling bulb by means of the stopcock at the lower end of the pipet. The mercury head, or level, was manually adjusted to keep the same height. The long capillary neck prevented turbulent mixing of the air stream with the contents of the pipet. The sampling ports were equipped with standard tapered ball joints for midget bubblers, the proportional counting tube, or vacuum for flushing out the system. Procedures. COLORIMETRIC. Solutions were prepared according to t h e West-Gaeke and t h e Helwig-Gordon methods (2, 4 , 6, IO). For the WestGaeke method, the gaseous samples were collected in 15.0 ml. of the NazHgC14 (TCM) solution in a fritted-glass midget bubbler. Sampling rates ranged from 300 to 450 ml. per minute. T o 10.0 ml. of the absorbing solution, 1.0 ml. each of the pararosaniline and formaldehyde solutions were added.
ETER
Apparatus for making S3j02-airmixtures
The color was allowed to develop for 30 minutes (to its maximum color) in a water bath at 25.0" f 0.1' C. The absorbance was measured a t 560 mp with a Beckman Model DU spectrophotometer using 1-cm. cells. The remainder of the sampling solution was used for scintillation counting. For the Helwig-Gordon method, 15.0 ml. of the reagent solution were used for sampling. Color development was completed in a water bath a t 25.0' C. CONDUCTOMETRIC. Solutions were prepared according to Thomas (8). The sample was collected in 15.0 ml. of the Hz02-HzS04 absorbing solution in a midget bubbler as above. Ten milliliters of this solution were diluted to 25.0 ml. with more absorbing solution to give a sufficient volume to cover the electrodes. The remainder of the undiluted absorbing solution was used for scintillation counting. Conductance was measured at 25.0' C. with an Industrial Instruments Model RC-16B1 bridge, using a dip-type cell with 0.1 cell constant. PROPORTIONAL TUBE COUNTING. Samples were transferred to the evacuated proportional counting tube (Nancy Wood Counter Laboratory, Chicago) at the position shown in Figure 1 or at one of the sampling ports. Samples were taken either directly from the Microvol or from the bag mixtures by means of the gas pipet. The proportional tube was designed for counting in the air samples without use the S3602 of a counting gas. The tube was made of stainless steel, 16 inches long by 1inch 0.d. Its internal volume was approximately 200 ml. The potential on the tube was carefully controlled a t approximately 2300 volts to give a constant counting rate with a standardized reference counting tube. The counting efficiency increased linearly with the log of the applied potential. The potential selected was 50 to 100 volts below the continuous discharge limit and resulted in approximately S070 counting efficiency. The tube was evacuated and flushed with clean air several times before final evacuation and
filling with the sample. If background counts tended to build up, they were reduced with evacuation and flushing or with isotopic exchange using nonradioactive SO2. All samples were adjusted to barometric pressure (ca. 630 mm. at this altitude) with clean air if needed. SCINTILLATION COUNTING.The scintillation solution was made as follows: 75 grams of naphthalene, 7.0 grams of PPO (2,5-diphenyloxazole), 300 mg. of dimethyl POPOP [lI4-bis-(4-methyl5-phenyloxazolyl)-benzene], and 1000 ml. of 1,4-dioxane. The dioxane was purified by passing it through activated alumina, and the naphthalene was recrystallized. T o 18 ml. of scintillator solution in a counting vial 2 ml. of aqueous sample (conductometric solution, T C M , or other) were added. The samples were counted with a Packard Tri-Carb liquid scintillation spectrometer, Model 3003. The counting chamber was kept at 2' C., and counting periods were sufficient to give 10,000 counts or for a minimum of 10 minutes. Counting efficiencies were 72% for S35 in TCM solutions and 77% for S35in conductometric solutions. Foil-lined screw caps react with T C M solutions and cannot be used. White plastic snap caps and ordinary glass vials were found satisfactory. Difficulties due to freezing or phase separation of samples were not encountered. With T C M solutions, the range of miscibility with the scintillator solution was between 10 a n d 2274 by volume of TCM; with aqueous samples, 0 to 21% was miscible. STUDIES OF COLORIMETRIC METHODS ANALYSIS
OF
T o prepare for the radiochemical study, nonradiochemical studies were made of the pararosanilineformaldehyde-sulfur dioside color reaction in both the West-Gaeke and Helwig-Gordon solutions. Bisulfite solutions were prepared and standardized with iodine-thiosulfate in the usual manner. Colorimetric absorption VOL. 37, NO. 9, AUGUST 1965
1105
0 400
3
-
0300-
Y
2 0200e 4
0.100
0 :GASEOUS
-
SO2 STANDARDS
A :BISULFITE
0
0.2
04
SO2, pqlml. of
06
STANDARDS
08
10
COLORIMETRIC SOLUTION
Figure 2. Comparison of standard curves obtained from gaseous SO? samples and bisulfite standard solutions
curves were prepared by diluting the standardized bisulfite solutions with sodium tetrachloromercurato (TCM) solutions. I n addition a standard colorimetric curve was prepared from gaseous sulfur dioxide samples measured with the Microvol and transferred to tetrachloromercurato (TCM) absorbing solutions by means of the small gas transfer bulbs of the vacuum apparatus. Figure 2 compares the colorimetric absorption curves obtained by the two methods. The color reactions for the gaseous SO, samples have higher absorption values than those for the bisulfite solutions. Apparently, the lower color intensities are due to loss of sulfite activity in the dilution process despite the use of reagent chemicals and fresh double-distilled water. Bag Mixtures. Bag mixtures of sulfur dioxide and air were made with both nonradioactive and radioactive sulfur dioxide. T h e concentrations in the bags were followed for long periods of time using colorimetric, proportional tube, and scintillation counting techniques. Figure 3 shows the rate of loss of sulfur dioxide above radioactive decay in two Mylar bags as measured by the different techniques used. The rate of loss for each of the bags is relatively consistent and amounts to approximately 2.5% per day. Proportional tube techniques indicate greater SO2 losses than the colorimetric or scintillation techniques. It is not known whether this is real or due to some change in the proportional gas counting tube. When mixtures of air and sulfur dioxide were made in a new bag there was a n initial rapid loss of sulfur dioxide which settled down to the rates noted above. As a consequenpe, all new bags were treated with a n approximately 500-p.p.m. mixture for a few days before use. At this time it is not known what happens to the sulfur dioxide that is lost from the bags. No counts were obtained from air taken from a bag one month after its last use. When pieces of a used bag were soaked in T C M 1 106
0
ANALYTICAL CHEMiSTRY
\
BAG 1 : MIX No. 17
c TIME , DAYS Figure 3. Comparison of concentration changes of SOn-air mixtures in Mylar bags All A. B. C.
counts corrected for radioactive decay and all lines displaced for slope comparison Proportional tube countlng Colorimetric anolysis Scintillation counting
solution, no color and less than 0.1 c.p.m. per sq. cm. were obtained when the solution was analyzed. When pieces of the bag were suspended in scintillation solution and placed in the counting chamber, as many as 17 c.p.m. per sq. cm. were observed. There may be some diffusion through the bag walls, but the preceding facts plus the fact that new bags cause a more rapid loss of SOz,indicate that at least part of the sulfur reacts with the bag ( I ) . Sampling Efficiency. The sampling efficiency of the tetrachloromercurato sampling method for the colorimetric determination of sulfur dioxide (4, IO) was checked in the following manner: First 10 to 15 liters of air-S3602 mixtures were prepared by taking a calculated number of aliquots of the sulfur dioxide from the working flask and flushing them into the Mylar bag. The concentration of sulfur dioxide in the bag was adjusted so that one gas pipet aliquot (44.37 ml.) of the mixture contained enough sulfur dioxide to give an absorbance of approximately 0.200 when sampled and analyzed colorimetrically. This amounted to having approximately 75 to 100 p.p.m. of SOpin the bag. An aliquot from the bag was quantitatively transferred to the gas pipet. The upper stopcock was closed and the air lines were flushed with filtered air. A midget fritted-glass bubbler containing 15.0 ml. of the tetrachloromercurato absorbing solution was connected to one of the sampling port outlets. The air flow was adjusted between 300 and 450 ml. per minute and measured. The upper stopcock of the gas pipet was opened, and the mercury
from the leveling bulb was allowed to displace the SOz mixture in the air stream. The time for the transfer was measured with an electric timer. The volume of air passing through the bubbler from the beginning to the end of the displacement period was calculated, and the concentration of the SO2 in the sampled air was found by multiplying the bag concentration by the pipet-air stream volume ratio. All glass lines were carefully flushed until no radioactivity could be detected in test samples. Low concentration samples were run before higher concentration samples to prevent contamination from adsorption effects. Reproducible results were obtained a t all but the very low concentrations, where some loss was observed. For these low concentrations a second midget frittedglass bubbler connected in series with all-glass connections was used. Table I summarizes the findings from colorimetric and scintillation analyses of the samples prepared in the above manner. Judging from both the colorimetric absorbance values and the scintillation counts, the colorimetric sampling method becomes relatively inefficient in concentration ranges below 0.1 p.p.m. Oxidation, or other chemical interference, of the SO2 is ruled out because of the simultaneous drop in the color and the scintillation counts, The tracer technique definitely establishes a loss due to the sampling method rather than a chemical interference. Also indicated is the fact that once a concentration is too low for efficient sampling, a second bubbler in series is unable to capture the lost SO2effectively. For example, samples 187 and 188 (Table I) were exposed to the same
~
Table 1. 120
1
I
d
Sample Number 187 188 189 190 191 192 193 195 194 Table
II.
Colorimetric and Scintillation Analyses of Comparable Air Samples Taken at High and Low SOaConcentrations
Taken, pg. SO2 5 5 5.5 5.3 5.3 5.3 5.3 3.9 3.9 3.9
Calcd., p.p.m. 0.033 1.16 0.023 2.3 0.018 1.5 0.067 0.72 1.45
Absorbance
~
Scintillation, c.p.m.
1st
2nd
bubbler
bubbler
0.116 0.136 0.117 0.132 0.109 0.131 0.098 0.099 0.099
0,000 ... 0,000
...
0.000 ... ... , . .
...
2176 i 13 2533 f 14 2183 i 14 2443 f 15 2098 i 13 2440 i 18 1636 f 14 1802 i 14 1794 f 14
65 f 6 ... 93 f 7 , . .
2 i0.5
... , . .
... ...
Efficiency of Conductometric Method for Measurement of Atmospheric Sulfur Dioxide
(16.4-rg. SO2 samples a t increasing dilution) MICROGRAMS
Sample
Total air vol., ml.
p.p.m.
so*,
Specific conductance
Scintillation, c.p.m.
264 261 262 263
3,430 26,700 44,700 82,600
1.8 0.23 0.14 0.08
2 . 5 7 x 10-5 2.55 x 10-5 2.69 x 10-5 2.81 x
1203 1219 1207 1037
SO2
Figure 4. Standardization curves for typical batch of conductivity sampling solution
total amount of SO,(5.5pg.), but sample 187 which was diluted with air to 0.033 p.p.m. showed a drop of 15y0in absorbance and 14% in scintillation counts. A second bubbler in series with the first bubbler captured only 65 of the 357 c.p.m. lost by sample 187. Helwig and Gordon (2) also used the formaldehyde-pararosaniline reagent but omitted the sodium tetrachloromercurate from the sampling solution, since they found t h a t greater color response was obtained without it. They were adapting the colorimetric method to continuous recording analysis, which did not require prolonged stability of the dissolved sulfur dioxide. Their method in fact gave a greater color response, but the color was less stable and reproducible than that of the West method. CONDUCTOMETRIC METHODS
Sulfur dioxide may be determined by capturing it in an acidified hydrogen peroxide solution, which oxidizes it to sulfuric acid, and measuring the conductance of the resulting solution (8). This method was less satisfactory than the colorimetric methods. More sulfur dioxide was required per sample to give sufficient precision, and reproducibility was only fair. The method is, of course, not specific for sulfur dioxide, since any atmospheric component which dissolves in the absorbing solution to give ions will also increase conductivity and consequently be measured as sulfur dioxide. It is convenient for automatic instrumentation, however, and is a popular method of analysis. Analytical methods were developed for the determination of known quantities of sulfur dioxide in air using a conductometric method and a correlating scintillation counting method. Various known volumes of an analyzed
SOz-air mixture stored in one of the Mylar bags were passed directly and slowly through a fritted-glass midget bubbler containing conductometric absorbing solution. The conductance of each solution was measured and then the solution was analyzed by the developed scintillation method. Figure 4 shows the results obtained from a typical batch of conductometric absorbing solution and a high activity S3jOz-air mixture. A nearly linear relationship was obtained between the conductivity of a given solution and the sulfur dioxide concentration. The scintillation method gave a linear relationship and extrapolated to zero. Sampling Efficiency. T h e efficiency of t h e absorbing solution for conductometric analysis was comparable to t h a t of the tetrachloromercurato solution of the West method (IO). For samples to be analyzed by scintillation counting, the conductometric solution is to be preferred. A greater range of volumes of the solution can be used, and there is no interference from the tetrachloromercurato ion. Table I1 compares the data obtained from a series of samples wherein a constant amount (16.5 pg.) of sulfur dioxide was diluted with increasing volumes of air. The sampling method was that described above, except that a conductometric absorbing solution was used. The absorbing solution was the one used for Figure 4 but from a different batch. Since the amount of sulfur dioxide was the same in each of the samples (Table 11), the specific conductance should have remained the same. However, samples diluted with air to 0.14 and 0.08 p.p.m., respectively, showed increasing conductivity. The scintilla-
tion counts, on the other hand, showed little change in efficiency down to 0.14 p.p.m. and a definite loss of sulfur dioxide when the sample was diluted to 0.08 p.p.m. To reflect the loss of sulfur dioxide, the conductivity of the absorbing solution should have dropped. Instead it increased. This indicated that the conductometric method was subject to interference by substances other than sulfur dioxide, Consequently, under practical air pollution situations, the determination of sulfur dioxide by the conductometric method should be evaluated with caution because of the nonspecificity of the method as demonstrated in this case. LITERATURE CITED
(1) Conner, U '. D., Nader, J. S., Am. Ind. Hyg. Assoc. J . 25, 291 (1964). (2) Helwig, H. L., Gordon, C. L., ANAL. CHEM.30,1810 (1958). (3) Llerritt, W. F., Hawkings, R. C., Ibid., 32, 308 (1960). (4) Nauman, R. I-.,West, P. W., Tron, F., Gaeke, G. C., Jr., Ibzd., 32, 1307 (1960). ( 5 ) Pate, J. B., Lodge, J. P., Jr., Wartburg, A. F., Ibzd., 34, 1660 (1962). ( 6 ) Radin, N. S.. Fried, R., Ibid., 30, 1926 (1958). d e r s n n R. T.. ISD. ENG.CHEM., _ _76 , (1943).
, X D., .4bersold, J. N., 129). (.9,) Urone. P'.. Boars. W. E., ANAL. CHEM. 23, 15'17 ( i E i j . (10) West, P. W., Gaeke, G. C., I b i d . , 2 8 , 1816 (1956).
RECEIVED for review September 21, 1964. Accepted June 7, 1965. Presented in part before the Division of Water and Waste Chemistry, 149th Meeting, ACS, Detroit, Lfich,, April 1965. Study made possible by National Science Foundation Grant No. G15703 and USPHS Division of Air Pollution Grant N o . A P 00357-01. VOL. 37, NO. 9, AUGUST 1965
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