Sulfur trioxide permeation tube for calibration of sulfuric acid

from the liquid S03 permeation tube through a midget bubbler, the resulting sulfuric acid aerosol was efficiently collected by a quartz fiber filter a...
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Sulfur Trioxide Permeation Tube for Calibration of Sulfuric Acid Measurement Methods Russell N. Dietz" and Robert F. Wieser Department of Energy and Environment, Brookhaven National Laboratory, Upfon, New York 1 1973

Sulfur trioxide permeation tubes were prepared and calibrated for use in evaluating the effectiveness of flue gas sampling equipment designed for determination of sulfuric acid in fossil fueled combustion sources. By diluting and passing the vapors from the liquid SO3 permeation tube through a midget bubbler, the resulting sulfuric acid aerosol was eff lclently collected by a quartz fiber filter and titrated after washing. Over the 45 to 80 O C temperature range, the SO, permeation rate varied exponentially from about 2 to 35 kL/min with a repeatability of f8%. At a constant temperature, the repeatablllty was better than f3 %. The SO, permeation tube had a projected refrigerated storage lifetime of 30 years, an operating lifetime of 1 month at 65 O C or 1 year at 35 OC, and a permeation rate suttable for callbrating H2S04flue gas monitors. As a generator for simulating atmospheric H2S04aerosol concentrations, a permeation wafer device emitting SO3 at 0.1 pL/min was expected to have a 10-year operating lifetime at 65 O C .

The measurement of sulfuric acid in the atmosphere as well as in the emissions from stationary and automotive combustion sources has received renewed interest primarily because of potential adverse health effects from inhalation of the acid aerosol and its atmospheric reaction products. Each of the techniques for making those measurements a t some point requires validation by the use of a calibrated H2S04source. For calibrating combustion source monitoring equipment, the H2S04generator must be capable of providing sufficient mass to attain a concentration of from 1 to 200 mg/m3 in a total flow of 1 to 15 L/min, that is, from 1 pg/min to 3 mg/ min-typically, 0.05 to 0.5 mg/min. Lower source rates are needed for the calibration of equipment used to determine H2S04in the ambient air, typically 0.5 to 5 pg/min. In the other extreme, health effects studies conducted with animals in exposure chambers generally require high concentrations (10 to 200 mg/m3) at high flows (0.1 to 1 m3/min) or, typically, source strengths of 5 to 50 mg/min. A list of a number of techniques (1-12) that have been used to generate a wide range of sulfuric acid vapor and aerosol concentrations in air is presented in Table I. In flue gas studies (13-15) at Brookhaven, the calibration of sulfuric acid measurement equipment was first attempted by nebulizing and evaporating sulfuric acid solutions (method 6 in Table I), but adequate mass balances proved to be unattainable. Subsequently, evaporation of sulfuric acid solutions, slowly dripping down a heated glass tube (method 51, gave inconsistent mass rates and poor mass balances. This paper describes the successful fabrication and calibration of sulfur trioxide, SO3,permeation tubes (16)for calibrating (17) flue gas sulfuric acid monitoring equipment (cf. method 9 in Table I) and discusses the potential application of SO3 permeation devices as a standard reference material for generating H2S04concentrations a t combustion source and atmospheric levels. EXPERIMENTAL The permeation tubes, fabricated essentially by the procedure of O'Keeffe (18),used 0.25-in. (6.4-mm) outside diameter FEP (fluorinated ethylene propylene) copolymer tubing with a wall 0003-2700/79/0351-2388$01.00/0

thickness of 0.030 in. (0.8 mm). With one end of the tube plugged with a 0.5-in. (13-mm) long FEP rod, about 1 mL of stabilized liquid SO3 (Baker or Allied Chemical) was pipetted into the tube while all materials were contained within a glove bag (Instruments for Research and Industry, Model SS-1) purged with dry N2 at 1 L/min. The other end of the tube was then plugged and crimped, providing an effective inside length, that length exposed to the SO3liquid and vapor, of about 82 mm. Until ready for use, the SO3 permeation tubes were stored over silica gel in a refrigerated glass jar. To measure the rate of permeation of SO3vapors, the tube was placed in a small vessel purged with dry Nz (dried with molecular sieve 5A that had been activated at 400 "C for 16 h) as shown schematically in Figure 1. The oven, a concentric tube circulating air bath (19),was capable of operating from 30 to 80 "C, usually stable to within better than f0.1 "C. Dry N2 at a flow of about 60 mL/min, preheated by the 0.125-in. (3-mm) stainless steel tubing, diluted and transported the permeated SO3vapors through an electrically heated (100 "C) 0.125-in. (3-mm) FEP Teflon line into the calibration assembly (14) consisting of two midget bubblers separated by a fiter. The frst midget bubbler, each of which contained 15 mL of 80% isopropyl alcohol solution, converted the incoming SO3vapor to HzS04aerosol, a portion of which was retained in the bubbler and the balance collected by the quartz fiber filter (Pallflex QAO 2500, heat treated and phosphoric acid washed) assembly, which has been shown to be better than 98% effective ( I 7) for recovering HzS04aerosol. When not being used as a sulfuric acid source, the heated Teflon line terminating in the transfer fitting was connected to a midget impinger containing Drierite, in series with a bubbler containing sodium hydroxide solution. To start and stop a calibration run, the transfer fitting was simply moved from the top of the Drierite midget impinger to the top of the midget bubbler and then back again at the end of the appropriate time interval (usually 30 min). In that way, the Nz flow over the SO3 source was always at steady state. A typical sulfur trioxide permeation tube was contained in the glass and stainless steel vessel shown in Figure 2. At the conclusion of a calibration run, each of the midget bubblers and the filter assembly were separately washed with 5% isopropyl alcohol washing solution and titrated with 0.02 M NaOH from a 5-mL buret with 0.01-mL resolution to a pH end point of nominally 5.5 (Beckman Expandomatic, Model 76A).

RESULTS AND DISCUSSION A total of six SO3 permeation tubes were fabricated and four were tested. The first, the only one filled by vapor condensation rather than by pipetting liquid SO3, did not contain sufficient material for extensive measurements. The other five permeation tubes were filled in February 1978 by the procedure described and stored under refrigeration. The first of those five, permeation tube no. 2, was calibrated by the method described with the exception of the use of a switching valve in lieu of simply moving the heated Teflon line and transfer fitting. The valve (Whitey, SS-41XS2) cause the SO3emission results to have a repeatability of only f35% over the temperature range from 31 to 79 O C . That particular tube was placed in the permeation oven on February 21,1978, and removed on January 29, 1979. During that time, permeation rate measurements were performed infrequently because of other programmatic efforts. However, two interesting points could be made. On the second day in service (cf. Table 11),with the tube at the 72 "C temperature which minimized the effect of adsorption on the switching valve, the measured 'C 1979 American Chemical Society



Table I. Techniques for Generating Sulfuric Acid Vapor and Aerosol Air Standards

method 1

H, so, mass rate, mgimin

H,SO, source

bubbling through liquid SO,

dilution flow rate,



aerosol diameter, pm

concn., mgim'

potential application


0.2-0.4 exposure chambers 0 .O3-0.04 exposure chambers 1 0 0 - 3 0 0 0 0.2-2 exposure chambers



1 2 3

1.1- 1 . 3

2 3 4



catalytic oxidation of so; t o so, nebulizer (0.2 t o 1 8 M H 2 S 0 , ) fuming H,SO,

0.3-30 0.1-2


evaporate H,SO, solution




nebulize and evaporate H,SO, solution flame decomposition of H,SO, solution nebulizer ( 0 . 0 0 1 t o 0.1 M H.SO,I perrheacion of SO,: permeation tube






0.00 5-0.05

permeation wafer

exposure chambers 4 calibrate emissions 5 monitors 20-200 vapor calibrate emissions 6, 7 monitors 10-100 0.3-0.5 calibrate emissions 8, 9 monitors 0.2 1-3.5 calibrate ambient 1 0 , 11 air monitors 0.05-5 0.01-0.05 calibrate ambient 12 air monitors 1-100












calibrate emissions this work monitors calibrate ambient projected air monitors




Table 11. Long Term Performance of SO, Permeation Tube (SO; Permeation Tube N o . 2 ) SO, permeation rate ( P ) , no. of

av. temperature,


days in service



2123178 1/29/79

2 342

3 3

Determined from In P

= 29.451


71.9 56.0



1.3 0.1




101.9 i 8.0 28.7 r 0.4

96.4 = 8.2 29.0 t 2.5

85871T K , which was derived for SO, permeation tube no. 4. ~~

HEA-EC _t ---L N L I ~ E

= l L T E R PSSENS-"

Table 111. Stability of SO, Permeation Rate (SO, Tube No. 3 at 55.9 i 0 . 2 '(2, 30-Min Sampling) permeation hours H ,SO titrated, pequiv in rate, service bubbler 1 filter bubbler 2 total Mequiv/h ~

Figure 1. Schematic of calibration procedure

18 19 43 45 67 143 164 165 190 191

5.2 4.6 4.7 5.3 4.8 5.2 5.2 4.3 4.8 5.9

6.8 7.4 7.8 6.8 7.8 7.0 7.7 7.8 7.5 6.2

Averaged permeation rate

Figure 2. SO,

permeation tube in dilution apparatus

0.0 0.0 0.0 0.0 0.0 0.1 0.0 0.0 0.0 0.0 = 24.6 t

12.0 12.0 12.5 12.1 12.6 12.3 12.9 12.1 12.3 12.1

24.0 24.0 25.0 24.2 25.2 24.6 25.8 24.2 24.6 24.2

0.6 pequiv/h.

permeation rate, within experimental repeatability, agreed with the calculated rate, the latter of which was computed from the correlation of the data from permeation tube no. 4. Similarly, after 342 days in service, 3 days after the switching valve was removed and after the tube was more than 95% depleted of SO3liquid, the measured and calculated permeation rates were in excellent agreement. These results confirmed the long term usefulness and reliability of the SO3 permeation tube as a satisfactory standard reference material. Secondly, the operating temperature history, and therefore the emission rate history, was used to compute the accumulated mass emission from day 1 t o day 342 (the last day of use). The total emission was calculated to be 2.06 g, very close to the initial filled level of 2.1 f 0.1 g. Thus, the SO3 permeation tube had a reproducibly predictable performance as long as any liquid remained. Sulfur trioxide permeation tube no. 3 was calibrated by the described test procedure over a temperature range from 46 to 81 "C, resulting in permeation rates ranging from about




8C l








i 7--7;

4 \

1 i

The apparent activation energy of 17.1 kcal/mol was 36% greater than that for SOz of 12.6 kcal/mol(24),but subtracting the heats of vaporization of 9.7 and 5.4 kcal/mol, respectively, for SO3 and SO,, gave actual permeation activation energies that were nearly identical: 7.4 kcal/mol for SO3 and 7.2 kcal/mol for SOz. All SO3permeation rate measurements were taken after the tubes had been at temperature for a minimum of 16 h. It was found, however, that only 2 h after raising the oven temperature from 40 to 73 "C, the measured rate was less than 10% below its steady-state value, which was attained within 4 to 5 h from the original temperature adjustment. Details of the use of these SO3 permeation tubes for calibrating flue gas monitoring equipment (17)will be described elsewhere.


2 L









29 30 1000/1, I/'K



1 1 32


Flgure 3. Effect of temperature on SO3 permeation rate

10 to 180 pequivlh (2 to 36 FL/min). The reproducibility of the calibration technique and the stability of the SO3 permeation source were exemplified by the results shown in Table 111. Over a period of 8 days, the measured permeation rate had a relative standard deviation of less than 4 ~ 3 %for the 10 measurements. The resolution of each titration was f O . 1 pequiv. Since the bubbler and filter were separately determined, the overall resolution of each rate determination was f 0.4 pequivlh, a significant factor in the average value of 24.6 f 0.6 pequiv/h. The data also showed that, on the average, the midget bubbler retained only about 40% of the recovered acid. The absence of H2S04in the second bubbler confirmed the greater than 98% efficiency of the filter, which had previously been confirmed by the use of two filters in series ( I 7). As was done for permeation tube no. 2, the mass emission history was summed over the period of use of permeation tube no. 3 (from January 30, 1979, to March 1, 1979), giving an accumulated emission of 1.92 g of SO3 in good agreement with the filled amount (2.1 f 0.1 9). Unlike tube no. 3 which had only been calibrated at temperatures set in ascending order, SO3 permeation tube no. 4 was calibrated a t temperatures repeatedly varied from 45 to 74 "C after initially heating to a maximum temperature of 80 "C. The permeation rate measurements obtained were plotted in Figure 3, where the solid line represented the straight line least-mean-square fit with a coefficient of determination (rz)of 0.993 (cf. equation in Table 11). From the slope, an apparent activation energy of 17.1 kcal/mol was obtained with a 95% confidence interval of f0.7 kcal/mol. That was slightly less than the 18.4 4Z 0.4 kcal/mol obtained for permeation tube no. 3 (the dashed line in Figure 3). Note that the two filled points obtained for tube no. 4 a t 65 "C, the first data from that tube, coincided with the dashed line for tube no. 3. The observed temperature aging or conditioning effect was noted in earlier studies at Brookhaven for NOz permeation devices (19) and had been observed in work at the National Bureau of Standards (20-22). Once aged by temperature cycling, the repeatability of Brookhaven NO2 permeation devices (19,23) was generally better than f l % . The repeatability of SO3permeation tube no. 4 was only to within 4Z8% in the measurements presented in Figure 3 because of variability in the running speed of the oven fan.

The dynamic range of the SO3 permeation tubes fabricated for this study is given in Table I (method 9). At a temperature of 65 "C, the nominal permeation rate of 60 pequiv/h (48 pg of HzSO, per minute) diluted a t the rate of 1 L/min would provide a known sulfuric acid concentration of 48 mg/m3 or about 1 2 ppm by volume. Since the permeation tube contained at least 2.0 g, it would be able to maintain that source rate for a period of about 1 month. Tubes could be made conveniently up to about twice the length used in order to double the lifetime a t the same mass emission rate. Refrigerated (3 "C)storage of these permeation tubes in a capped glass jar containing Drierite for a period of more than 1 year had no demonstrated effect on their subsequent performance. At that temperature, the solid gamma form SO3with a melting point of 16.8 "C (25) would have a permeation rate of about 0.12 pglrnin corresponding to a storage lifetime of more than 30 years! As implied in Table I, a sulfur trioxide permeation wafer device, fabricated in the fashion of an acceptable NOz wafer device consisting of a 0.030-in. (0.8-mm) thick wafer with a diameter of about 0.25 in. (6.4 mm) and having an effective surface area of 0.15 cm2 (24,26),would have a permeation rate about of that of the SO3 permeation tube, which had an effective surface area of 14.3 cm2, Such devices have been fabricated a t Brookhaven as calibrated sources for H2S, SO2, NH3, NO,, and SF6 (19) and should be feasible for SO,. The advantage of the SOj permeation wafer device over the permeation tube would be the ability to more readily, and for a longer period of time, provide H,SO, concentrations in a range simulating that to be expected in the atmosphere. For example, a t a dilution flow rate of 10 L/min of humidified air and a temperature of 65 "C, the expected concentration would be about 12 ppb (48 pg/m3). Another order of magnitude reduction in concentration could be achieved by controlling the permeation wafer device temperature at about 35

"C. Calibration of a sulfur trioxide permeation wafer device would be performed by the approach described in this paper for SO3permeation tubes. The lower permeation rates would necessitate much longer calibration runs, that is, about 48 h fur the wafer device compared to 30 min for the permeation tubes. The special absolute pressure method (19,24) designed to calibrate low rate wafer devices in about 2 h would not be applicable to SO3because of its strong adsorptive and reactive nature. A permeation wafer device containing about 2 g of SO3 would be expected to have an operating lifetime of about 10 years at 65 "C.During the lifetime of either the permeation tube or wafer device, the rate of permeation is only dependent on the partial pressure of the SO3and the temperature of the device. As long as any liquid remains, the permeation tube has been shown to have a constant source rate a t constant


temperature. I t is expected that a permeation wafer device containing SO3would perform as well as one containing NOz (23),which was shown to behave according to a derived exponential temperature dependence which varied by less than 1% in 3'12 years. Compared with other methods of generating known sulfuric acid aerosol concentrations in air (1-12), the user of SO3 permeation devices, either high rate tubes or low rate wafer devices, would have numerous advantages, including (1) convenient 4 order-of-magnitude dynamic range of mass emissions (from about 0.02 pg/min to 0.2 mg/min); (2) subsequent savings in prepurified and filtered dilution air; (3) reproducibility of mass emission rate; (4) independent calibration of the mass emission rate; ( 5 ) extreme simplicity of use; (6) minimal hazards compared to working with pure SO3 or concentrated H2S04solutions; (7) long source lifetimes of from months to tens of years depending on mass emission rate; (8) rapid attainment of new steady state emission conditions (-2 h); (9) ease of independently setting H2S04concentration and relative humidity; and, perhaps most importantly, (10) some flexibility in selecting size distribution and aerosol concentration somewhat independent of each other. With regard to the last point, the manner in which the H2S04aerosol is generated can affect its ultimate equilibrated humidified size a t any given mass per unit volume concentration. At a generated H2S04concentration of 10 pg/m3 at about 50% relative humidity, the size of the HzS04aerosol derived from condensing H2S04vapor would be expected to be larger (3),perhaps 0.9 to 1.2 pm MMD (8),and more closely simulating actual atmospheric aerosol size distributions, than those derived from the nebulization and subsequent partial evaporation of very dilute acid solutions (method 8 in Table I), Le., about 0.15 pm from a 0.002 M H2S04solution ( 4 ) . Furthermore, the aerosol from condensed H2S04vapor a t 10 pg/m3 would be very much larger than the equilibrated H2S04 aerosol size of 0.0035 km t o be expected from the self-nucleation of SO3 vapors with water vapor (2,3). Sulfuric acid aerosol size in the later case appeared to be proportional to the square root of the SO3 concentration ( 2 , 3 ) whereas from the nebulization and subsequent complete (12)or partial ( 4 ) evaporation of dilute acid aerosol, the equilibrated humidified aerosol size was proportional to about the third (12)or fourth ( 4 ) root of the molarity of the generating solution. Only in the case of the self-nucleation of H2S04vapor was large sized aerosol formed ( 3 ) ,and in a size (- 1 pm) which did not vary with concentration over 2 orders of magnitude (8). A consequence of this dependence of aerosol size on the manner of preparation is that the SO3permeation device can be used to generate sub-tenth micrometer sized aerosol by condensation of SO3 vapor with water vapor or about unity micrometer sized aerosol by first converting the SO3 vapor to H2SO4 vzpor and then condensing.


ACKNOWLEDGMENT Appreciation is expressed t o Margaret W. Greene for preparing the permeation tubes and the dilution vessel, to Irvin A. Meyer for fabrication of the bubblers and filter assembly, and to Ted D'Ottavio and Roger Tanner for helpful discussions.

LITERATURE CITED Chang, Daniel P. Y.; Tarkington, Brian K. -1. Am. I d . wg. Assoc. 1977, 38, 493-7. Chang, Daniel P. Y.; Tarkington, Brian K.; Dwall, Timothy R. J . Air Pollut. Control Assoc. 1978, 28, 1137-9. Gillespie, G. R.; Johnstone, H. F. Chsm. Eng. Prog. 1955, 51, 74F-8OF. Cavender, F. L.; Williams, J. L.; Steinhagen, W. H.; Woods, D. J . Toxicol. Environ. Health 1977. 2, 1147-59. Cheney, J. L; Fortune, C. R . Anal. Lett. 1977, IO, 797-816. Lisle, E. S.;Sensenbaugh, J. D. Combustion 1985, 36, 12-16. Maddalone, Ray F.; Newton, Steve F . ; Rhudy, Richard G.; Statnick, Robert M. Presented at 70th Air Pollution Control Association Meeting, Toronto, Canada, June 20, 1977; No 77-49.2. Rapaport, E.; Weinstock, S.E. Experientia 1955, 1 1 , 363-4. Carabine, M. D.; Maddock, J. E. L. Atmos. Environ. 1978, 10, 735-42. Thomas, Ronnie L.; Dharmarajan, Venkatram; West, Philip W. Environ. Sci. Technol. 1974, 8, 930-35. Maddalone, Ray F.; Thomas, Ronnie L.; West, Philip W., Environ. Sci. Technol. 1976, IO, 162-8. Tanner, Roger L.; D'Ottavio, Theodore W. Unpublished work, Brookhaven National Laboratory. Dietz, Russell N.; Garber, Robert W. "Power Plant Flue Gas and Plume Sampling Studies"; Brookhaven National Laboratory, December 1978; BNL 25420. Dietz, Russell N.; Wieser, Robert F.; Newman, Leonard. In "Workshop Proceedings on Primary Sulfate Emissions from Combustion Sources", August 1978; EPA-600/9-78-020a, pp 3-25. Ret. 14, EPA-600/9-78-02Ob, pp 239-70. O'Keeffe, A. E.; Ortman, G. C. Anal. Chem. 1966, 38, 760-63. Dietz, Russell N. Presented at Engineering Foundation Conference on Stack Sampling and Stationary Source Emission Evaluation, April 1, 1979; unpublished work, Brookhaven National Laboratory, O'Keeffe, Andrew E. Anal. Chem. 1977, 49, 1278. Dietz, R. N.; Smith, J. D. In "Calibration in Air Monitoring", ASTM STP 598; American Society for Testing and Materials, 1976, pp 164-79. Hughes, Ernest E. ISA Reprint 74-704, Instrument Society of America, October 1974. Rook, Harry L.; Hughes, Ernie E.; Fuerst, Robert S.; Margeson, John H. Am. Chem. SOC. Div. Environ. Chem. Prepr. 1974, 14(1), 321-8. Hughes, E. E.; 1 mk, H. L.; Deardorff, E. R.; Margeson, J. H.; Fuerst, R. G. Anal. Chem. 1977, 49, 1823-9. Dietz, Russell N.; Smith, James D. Unpublished work, Brookhaven National Laboratory. Dietz, Russell N.; Cote, Edgar A,; Smith, James D. Anal. Chem. 1974, 46, 315-8. Moeller, Therald. I n "Inorganic Chemistry"; Wiley and Sons: New York, 1952: Chaoter 14. DO 528-30. Dietz; Russell N.; Smith, James D. Am. Chem. Soc:. Div. Environ. Chem. Prepr. 1975, 15(2), 40-43.

RECEIVED for review July 19, 1979. Accepted September 13, 1979. This work was performed under the auspices of the United States Department of Energy under Contract No. EY-76-C-02-0016. By acceptance of this article, the publisher and/or recipient acknowledges the U S . Government's right to retain a nonexclusive, royalty-free license in and to any copyright covering this paper.