Greek Letters 7 = dimensionless diffusion time defined by Equation 5 p* = viscosity of a solution or suspension, g/cm-s psolvent = viscosity of a solvent, g/cm-s
Conclusions
Zinc acetate was the most efficient absorbent tested. At 2 1 “C, a 0.091-M solution of zinc acetate ensures more than 90% absorption of H2S in 15-min samples a t sampling rates of up to 10 l./min in the concentration range 100-800 ppm. The theory of Calvert and Workman ( 2 , 3 )does not predict the experimental observations. The neglect of a distribution in bubble sizes or the presence of precipitate at the surface of bubbles may be responsible for a discrepancy between the calculations and experimental results.
Literature Cited
Nomenclature a = bubble radius, cm CB = bulk concentration of chemical absorbent, mol/cm3
C,, = liquid interfacial concentration of HzS, mol/cm3
Dg = diffusivity of H2S in air, cmz/s DL = diffusivity of H2S in absorbent, cm2/s h~ = depth of aerated absorbent in an impinger, cm H = Henry’s law constant for H2S in aqueous solution, atm-cm3/mol k~ = liquid phase mass transfer coefficient with chemical reaction cm/s L = dimensionless mass transfer parameter defined by Equation 6 po = partial pressure of H2S in the gas, atm R = gas constant, 82.06 atm-cm3/mol-K T = temperature, K tg = residence time of bubble in absorbent, s u g = upward velocity of a bubble, cm/s
(1) Hauser, T. R., Shy, C. M., Environ. Sci. Technol., 6, 890 (1972). ( 2 ) Calvert, S., Workman, W., Talanta, 4,89 (1960). (3) Calvert, S., Workman, W., J . Am. Ind. Hyg. Assoc., 22, 318 (1961). (4) Sands, A. E., Grafius, M. A., Wainwright, H. W., Wilson, M. W., “The Determination of Low Concentrations of Hydrogen Sulfide in Gas by the Methylene Blue Method”, U S . Dept. Interior, Bureau of Mines Rep., R.I. 4547, September 1949. (5) Jacobs, M. D., Braverman, M. M., Hochheiser, S., Anal. Chem., 29,1349 (1957). (6) Buck, M., Stratmann, H., Staub, 24,241 (1964). (7) Mason, D. M., Hydrocarbon Process., 43,145 (1964). (8) Marbach, E. P., Doty, D. M., J . Agri.Food Chem., 4, 881 (1956). (9) Gioia, F., Astarita, G., Ind. Eng. Chern. Fundam., 6, 370 (1967). (10) Tavares da Silva, A., Danckwerts, P. V., Inst. Chem. Eng. Symp. Ser., 28, p 48, Inst. Chem. Eng., London, 1968. (11) Calderbank, P. H., The Chemical Engineer, CE 209, October 1967. (12) Perry, J. H., Ed., “Chemical Engineering Handbook”, pp 14-20, McGraw-Hill, 1963. (13) Einstein, A., “Investigations on the Theory of the Brownian Movement”, pp 49-54, R. Furth, Ed., Dover Pub., 1956. Received for review JQnUQry13, 1975. Accepted October 14, 1975.
Measurement of Sulfuric Acid Aerosol and Total Sulfate Content of Ambient Air Ray. F. Maddalone, Ronnie L. Thomas, and Philip W. West* Environmental Sciences Institute, Chemistry Department, Louisiana State University, Baton Rouge, La. 70803 W Sulfuric acid aerosol and total sulfates can be deter-
mined by formation of permidylammonium sulfate and subsequent thermal decomposition to form quantitatively sulfur dioxide. The SO2 may be determined by the WestGaeke procedure or by means of a flame photometric sulfur monitor. Airborne particulate samples are analyzed by isolating the H2S04 by microdiffusion, and total sulfate is determined by aqueous extraction of the particulate samples. A working range of 1-50 pg of sulfate can be handled conveniently. The determination (excluding microdiffusion) requires only a few minutes.
The case against sulfuric acid aerosol has been building over the years through evidence collectkd during air pollution incidents like those in the Meuse Valley, Donora, Pa., and London ( I ) . Although SO:! levels were high during these episodes, they did not approach levels suspected as toxic or for that matter, fatal. Because of that fact, reasons were sought to explain the deaths that occurred during the air pollution episodes. Fricket (2) was the first to point out a correlation between high particulate loading and high mortality rates. Pursuing this evidence, experts sought the agent or agents in the particulate fraction of the fogs that could account for the mortality rates. 162
Environmental Science & Technology
The oxidation of SO2 to H2S04 has been well documented ( 3 ) ,and the presence of H2S04 has been identified in the atmosphere. The toxicity of sulfuric acid in gross quantities is well known, but until recently the effect of sulfuric acid aerosol has not been investigated. Amdur (4, 5 ) found that animals exposed to various amounts of H2S04 aerosol had narrowed air passages caused by bronchosterosis, mucosal swelling, or increased secretion. As a result, people who already have difficulty breathing (the old, asthmatic, or heart patients) can be killed not by the direct corrosive qualities of sulfuric acid, but by the respiratory strain that HzSO4 places on an already stressed person. With the advent of the energy crisis, a compromise has been reached to allow the burning of fuels with higher sulfur contents. This measure of necessity is sure to increase the sulfur dioxide levels, and consequently, the sulfuric acid levels in the atmosphere. Because of the mobility of SO2 the formation of H2SO4 can occur downwind of SO2 sources. Recent studies have shown an increase in the acidity of rain water in areas remote from SO2 sources (6). The reason for this is the slow oxidation of SO2 emitted from high stacks as the gas is dispersed in the atmosphere. The most dangerous problem lies with direct sources of H2SO4. Because sulfuric acid is not as mobile as S02, localized concentrations of sulfic acid can build up. A potential
source of sulfuric acid is the catalytic converter. It has been demonstrated that the sulfur in gasoline will be oxidized in the catalytic converter to sulfuric acid, and yet, as Russell Train, the head of the EPA, admitted, "There is no method available for the measurement of HzSO4 aerosol." The methods that are used for the measurement of sulfuric acid are based on the measurement of protons (7-14) or the sulfate anion (15-19). Several attempts have been made to rationalize acidity measurements as HzSO4 measurement, but with the complexity of most environmental samples, the answers obtained can only be taken as a measure of the total acidity of the sample. Those who have sought to measure HzSO4 as sulfate have employed a separation step and any number of standard sulfate methods. Thus, the entire method is only as good as the separation step. Barton and McAdie's (19) isopropanol extraction for HPS04has been shown to extract NaHS04 and NH4HS04 (20). Leahy et al. (20) used benzaldehyde to extract H2SO4 but the procedure was tested using samples containing gross amounts of HzSO4 well above normal environmental samples. Dubois et al. (21) tried to separate sulfuric acid aerosol from the gross filter sample by diffusion from glass fiber filters. By coating the inside of the covering petri dish with NaOH, the HzSO4 was trapped and later determined as sulfate. Maddalone et al. ( 2 2 ) found that volatilizing HzSO4 from the gross filter sample was possible, but the use of high-volume sampling on glass fiber filters was unacceptable. Of the filters tested only Mitex (Teflon), Fluoropore (Teflon), or Poco graphite filters were acceptable. Scaringelli and Rehme (23) have proposed a unified approach to sulfuric acid aerosol determinations. This method is based on the vaporization of hot H2SO4 (as SOs) from glass fiber filters and its subsequent reduction over hot copper a t 500 "C. The apparatus used required preconditioning with sulfuric acid, had less than 100% conversion, and had a positive interference from ammonium sulfate. The successful development of a method for sulfuric acid aerosol determination depends on the proper combination of collection, separation, and determination of the acid from a gross air sample. Maddalone found that sulfuric acid could be separated from the Teflon filters by diffusion to a suitable trapping medium. During the process of heating the filter, the problem of topochemical reactions occurring on the surface of the filter between the collected particulate matter and sulfuric acid was noted. The probability of topochemical reactions occurring increases as the amount of sample collected increases, so a decision was made to collect a small air sample. By collecting small samples, the possibility of interfering reactions was reduced, but a sensitivity problem was created. Most methods bel come highly unreliable at the low concentrations of sulfuric acid expected when small (1m3 or less) air samples are collected. Maddalone et al. (24) developed a new method for sulfate determinations based on the pyrolysis of perimidylammonium sulfate [ (PDA)2S04] having a sensitivity limit of 0.1 pg. In this article, the combination of diffusion to PDA-Br-coated dishes, and subsequent pyrolysis of the (PDA)zS04 formed, will be proposed as a specific sulfuric acid aerosol method. Experimental
Reagents. Perimidylammonium bromide, 0.5%. Fifty milligrams of PDA-Br synthesized according to McClure (25) and recrystallized twice from methanol was added to 10 ml of distilled water or methanol. This solution was made fresh daily. Potassium sulfate stock solution. Freshly dried reagent
O-RING SEAL
QUARTZ
QUARTZ
ROD
G A s I N L E T/O UT LE T
Figure 1. Combustion tube and injector
grade potassium sulfate (1.81 g) was added to 1 1. of distilled water to give a solution 1 pg/pl as sulfate. Solutions containing 0.1 pg/pl of sulfate were made up by dilution of the stock solution. Sodium tetrachloromercurate (II), 0.1 M. Reagent grade mercury(I1) chloride (27.2 g) and reagent grade sodium chloride (11.2 g) were dissolved and diluted with distilled water (26). Formaldehyde solution, 0.2%. One-half milliliter of 40% formaldehyde solution was added to 100 ml of distilled water. This solution was made fresh daily. Pararosaniline hydrochloride. Reagent grade pararosaniline hydrochloride (0.64 g) was added to 240 ml of concentrated HC1, and diluted to 1 1. with distilled water. Twentyfive milliliters of this solution were diluted to 100 ml with distilled water to provide the working solution. Isopropanol. Bromophenol blue solution. Bromophenol blue, 0.50%; 0.05 g, was added to 100 ml volumetric and diluted to 100 ml with distilled water. Nitrogen-prepurified. Oxygen-prepurified, Apparatus. Spectrophotometer-Beckman DB. Pyrolysis oven. A Sargent microcombustion furnace (S21580) was modified to accept a 25-33 mm 0.d. quartz tube. The original heating coils were retained, but a No. 50122Type 1708-KSP Lindburg cylindrical heating block was added to the top hinge of the oven. The Lindburg was controlled by a separate Variac. Combustion train. At one end of a 33-mm 0.d. by 65-mm quartz tube a 33-mm 0.d. 34/45 female ground glass joint was attached. The other end of this 33-mm 0.d. quartz tube was fitted with an 11-mm 0.d. by 60-mm 18/9 Vycor male ball joint. A bubbler was connected to the quartz tube via 6-mm i.d. Teflon tubing, glass tubing, and a Pyrex 18/9 female ball joint (Figures 1 and 2). Injector system. A 34/45 Pyrex male ground glass joint was fitted on its center line with a 12-mm 0.d. by 30-mm Pyrex tube. This tube was blown with an inside depression halfway down its length enabling it to accept a Viton high temperature O-ring (11 mm 0.d.). A 9.5-mm 0.d. Teflon sleeve with a 6.5-mm center tap was inserted into the 12-mm 0.d. tube. A 6-mm 0.d. by 80-mm quartz rod with a fork designed to hold a 15-mm 0.d. dish at one end was inserted through the Teflon sleeve. By means of another Viton O-ring (6 mm i.d.) and Teflon washer, the system was made airtight. Finally, at 90" to the center line of the 34/45 Volume 10, Number 2, February 1976
163
male joint, a 10-mm piece of 7-mm 0.d. Pyrex tubing was added to the side of the joint to act as an inlet/outlet port (Figure 1). Pipets. Solutions were pipetted with a 10 or 50 ~1 Eppendorf pipet. Flowmeters. Lab-crest rotameters of appropriate ranges were used. These were accurate to f 2 % of full scale. Tape sampler. Gelman Instrument Co., Model 23000-1 tape sample was modified by the addition of a 17-mm diameter stainless steel mesh, a 12-mm i.d. (16-mm 0.d.) 0ring, and 8-mm i.d. (11-mm 0.d.) O-ring, and an 11-mm diameter stainless steel mesh. These parts were layered in the sampling port in the order listed to provide support for the Fluoropore filters. Without this buildup support, the filters would collapse under the pressures produced by the sampling rate employed. With the sample port modified in this manner, samples were collected over an effective diameter of approximately 12 mm. Glass dishes. Soft glass vials, 15 mm i.d. by 120 mm, were cut 3 mm from the bottom to provide microdiffusion dishes. Vacuum desiccator. Corning No. 3118 or equivalent. Filters. Fluoropore Teflon filters (47 mm) of 0.5-w pore size were used to collect the sulfuric acid aerosol. Teflon block. A 14 X 7 X 1 cm Teflon block was used to support the filters during microdiffusion. Ultrasonic cleaner. Fisher ultrasonic generator, Model CT, and cleaner, Model SS-0, were used to sonicate the air samples. Procedure The sulfuric acid generator of Thomas et al. (27) was used to produce a standard aerosol to test the recovery of the diffusion-pyrolysis method a t different levels of sulfuric acid. The resultant aerosol was sampled using a slightly modified Gelman tape sampler to produce samples containing from 2-40 gg of sulfuric acid on the sample spot. The Gelman tape sampler is normally expected to operate with a 2.5-cm paper tape as the sampling medium. A 47-mm Fluoropore filter cut in half was slid into the collection zone, and the sulfuric acid was collected on a 12-mm spot. A flow rate of approximately 4.0 l./min was maintained throughout the sampling period. Normally, 10 samples were collected containing like amounts of aerosol, so that five samples could be analyzed by the extraction procedure (Total Sulfate Method). Extraction Procedure (Total Sulfate Method). After the sulfuric acid samples were collected on the Fluoropore filter, a 16-mm diameter cork borer was used to cut away the excess filter surrounding the 12-mm sample spot. This filter disk was placed in a 10-ml beaker and approximately 0.5-1.0 ml of the extraction solvent was added. These beakers, along with a beaker containing a blank sample of Fluoropore, were placed in a Fisher ultrasonic cleaner and sonicated for 3 min. After sonification, the extract and a 0.5-ml rinse were transferred to a 5-ml volumetric flask. The sonification was repeated a total of three times for each sample. Methanol will extract most hydrogen sulfate salts and sulfuric acid (20). Since the laboratory samples contained only sulfuric acid, methanol was used as the extraction solvent. Besides the hydrogen sulfate salts, an actual air sample may contain ammonium sulfate, sodium sulfate, and possibly small amounts of calcium sulfate. The total sulfate can be extracted with a 50% methanol-water (v/v) solution. This mixed solvent is easier to evaporate than water alone, and will extract all the soluble sulfates present in the air sample. 164
Environmental Science & Technology
1
t
4:
m
a
0 VI
m
..WO.~
..
( P D A ) , SO, CALIBRATION
4: I-
z
. 2 00 .b
.r
CURVE
J
/
1
* 20
IO
30
F g so;
Figure 3. Pyrolysis calibration curve
After the samples have been extracted, 2 ml of the 5-ml extract were put into a 15 X 15 mm i.d. dish containing 250 ~1 of a 0.5% methanol solution of PDA-Br. The samples were allowed to stand for 10 min before they were placed in a vacuum desiccator maintained a t 80 "C. A water aspirator was used to produce a vacuum and remove the solvent. The dry samples were then pyrolyzed. With the pyrolysis oven set at 500 "C, the nitrogen flow rate at 500 ml/min, and 10 ml of 0.1-M tetrachloromercurate(I1) in the bubbler, the samples were ready to be pyrolyzed. By proper manipulation of the four-way valves and a 3-way stopcock, the nitrogen flow was fed through the injector, past the hot zone, and into the West-Gaeke trap (pyrolysis mode), or the flow was reversed so the nitrogen would pass through the three-way stopcock, back through the oven, and out the injector male joint (backflush mode). This flow system allowed the oven to operate continually while a nitrogen blanket was maintained in the quartz combustion tube. A typical pyrolysis cycle would be: 1. Pull the injector rod out of the hot zone. 2. Remove the tetrachloromercurate(I1) solution from the bubbler. 3. Reverse the nitrogen flow to the backflush mode. 4. Remove the entire injector system at the 34/45 joint. 5. Place a fresh sample on the injector fork. 6. Replace the injector in the 34/45 joint. 7 . Place the nitrogen flow in the pyrolysis mode. 8. Replace the tetrachloromercurate(I1) solution in the bubbler. 9. Slide the injector rod into the center of the oven. 10. Heat the sample for 2.5 min. This entire cycle, including replacing the West-Gaeke trap, would take approximately 3.5 min. After several pyrolysis runs, the quartz tube immediately outside of the oven on the bubbler side of the oven collected a small amount of organic debris. The Teflon and glass tubing leading to the bubbler were also coated with this material. The sulfur dioxide evolved during the pyrolysis was not absorbed on this material, but as a precaution the quartz tube was periodically cleaned. The quartz tube was disconnected from the system at the ball joint, and retracted into the oven. At the same time oxygen was fed into the quartz tube, and the temperature was raised to 700 "C. While the quartz tube was being heated, the rest of the combustion train was rinsed with acetone and allowed to air dry. Within 10 min, the combustion train was ready for use. Diffusion-Pyrolysis Method (Sulfuric Acid Aerosol Method). A 15-mm i.d. by 3-mm dish was filled with 250 p1
of a 0.5% methanol solution of PDA-Br. The solvent was evaporated in an oven at 80 "C, leaving a thin crystalline coating on the inside of the dish. A small amount of PDABr will crystallize on the rim of the dish, and should be removed with a cotton ,swab wetted with methanol. The crystalline coating was extremely cohesive and resistant to mechanical shock up to its melting point (265 "C). The filter containing the sample was placed on a Teflon block and the sample spot was covered with one of the inverted PDA-Br coated dishes. The Teflon block, filter, and dish were placed in a nitrogen purged oven at 125 "C for 2 h, after which the dishes were removed and directly pyrolyzed. By our knowing the amount of sulfate present (calibration curve), the diffusion efficiency at that level, and the sampling rate, the concentration of sulfuric acid aerosol in the ambient air could be calculated.
Results and Discussion Maddalone's method ( 2 4 ) for sulfate determination is based on the pyrolysis of perimidylammonium sulfate [(PDA)2SO,]. The sulfate is precipitated by addition of PDA-Br and the dried (PDA)zSO*heated to 400 "C where sulfur'dioxide is quantitatively released. The sulfur dioxide is trapped in 0.1-M tetrachloromercurate(I1) solution and analyzed by the West-Gaeke method (26). A calibration curve is made by plotting absorbance vs. known amounts of sulfate. Figure 3 is a typical calibration curve for (PDA)2S04 pyrolysis. Total Sulfate Method. Though the Total Sulfate Method is quite simple, there are several points to be considered while performing the procedure. It is important to ensure that the filter disk is placed sample side down in good contact with the extracting solution. If methanol is used (to remove only sulfuric acid and the common hydrogen sulfates), no special precautions are necessary, since it will wet the Fluoropore. Another significant step is the evaporation of the solvent once the sulfate has been precipitated. Although 2 ml of solvent can be removed easily in the heated. vacuum desiccator, smaller aliquots are desirable. A water solution of (PDA)2S04 will decompose slightly upon extended heating above 80 "C in air. The results obtained by pyrolyzing these samples will be lower than expected. Thus, if any discoloration of the precipitate occurs during the solvent removal, the samples should be discarded. Since there is no lack of sensitivity with the pyrolysis method, the size of the aliquot for most environmental samples can be adjusted downward. A working range of 1-50 pg sulfate in the aliquot is recommended. To test both the accuracy and precision of the extraction procedure, 10 pl of a 1 pg/l pl (as sulfate) sodium sulfate solution were added to one quarter of a 47-mm Fluoropore filter. There was some difficulty removing the last drops of the sulfate solution, since the Teflon surface is hydrophobic, and the pipet tips are designed to be emptied partially by capillary action. This problem was overcome by placing approximately 50 pl of deionized water on the filter, before adding the sulfate solution. Approximately 10 p1 of the water-sulfate solution were pulled into the pipet tip and flushed out. Once the sulfate solution was on the filter, the water was evaporated in an oven a t 80 "C, and the sulfate then extracted using 50% methanol solution. The results of the extraction were quite good, as 105% of the sulfate was recovered with a coefficient of variance of f3.6%. The recovery was greater than loo%, since the sample pipet tips were rinsed while the standard pipet tips did not appear to require any additional rinsing. The results
Table I . Coefficient of Variance for Extraction Procedure Analysis of Generated Aerosol Samples at Several Concentrations
8
Sulfuric acid extracted, ,ug
Coefficient of variance, O/O
5 9 28
i5.4
20
~8.2 56.8
