Measurement of Sulfuric Acid Aerosol, Sulfur Trioxide, and the Total Sulfate Content of the Ambient Air Ronnie L. Thomas, V. Dharmarajan, G. L. Lundquist, and Philip W. West* Chemistry Department, Coates Chemical Laboratory, Baton Rouge, La. 70803
Sulfuric acid, sulfur trioxide, and soluble sulfate salts can be determined after collection on perimidylammonium bromide impregnated filters. The droplets of acid react topochemically with the solid reagent to form the perimidylammonium sulfate which can be pyrolyzed to produce stoichiometric amounts of sulfur dioxide. The SOz is readily determined as a measure of the acid aerosol. The critical feature is the stabilization of the sulfuric acid at the moment of collection whereby its reaction with coexisting pollutants such as Fez03, AIZO3, and PbO is inhibited. This reaction with co-pollutants during sampling has been an inherent source of error in all previous methods for determining ambient acid aerosol concentration. Total sulfate levels can be determined by a relatively easy modification of the procedure. The detection limit for sulfuric acid or its soluble salts is 0.3 pg as Sod2- and the working range is 1 to 50 pg.
A novel method has been developed for the collection, stabilization, and determination of sulfuric acid aerosol. Previous methods have suffered from errors inherent in the sampling step whereby coexisting pollutants such as FesOa, A1203, and PbO react with the acid, thus neutralizing a t least part of it to form salts. In the ambient atmosphere, antagonists can coexist because of the great dilution of the system. Sampling and concentrating the system have been requisites in all sulfuric acid aerosol studies to date and the resultant intimate mixing of concentrated pollutants has led to sample distortion. T h e new method utilizes the topochemical reaction of the acid droplets with filters coated with perimidylammonium bromide. T h e reaction to form the stable perimidylammonium sulfate is essentially instantaneous and thus inhibits any reaction between the acid aerosol and collected co-pollutants. Salts of sulfuric acid, except for ammonium sulfate, do not interfere because, as solids, they do not undergo topochemical reactions. However, salts can be included by a simple modification in the procedure to provide values for total sulfate as well as for acid sulfate. Sulfuric acid aerosol pollution dates to man's first use of sulfur-bearing fuels. Although sulfur dioxide is the predominate oxidation product of sulfur, it is readily converted to sulfur trioxide in the presence of oxygen, moisture, and sunlight ( 1 ) . Sulfur trioxide combines with moisture to produce sulfuric acid aerosol which is much more toxic than sulfur didxide (2). In addition to its effect on health, sulfuric acid is highly corrosive and the resultant damage to materials costs millions of dollars each year. Unlike sulfur dioxide which is readily dispersed in air because of its mobility as a gas, sulfuric acid aerosol can linger and cause localized stagnation with obvious damage to animal and plant life and to metal and stone structures. T h e increased use of high content sulfur-bearing fuels and the advent of the catalytic converter, which converts sulfur dioxide to trioxide, must cause an increase in sulfuric acid pollution. T h e health and economic impact of such pollution requires that methods for the determination of sulfuric acid aerosol and its salts be developed.
A review of methods for sampling, separating, and determining of sulfuric acid aerosol has recently been reported by Dharmarajan e t al. ( 3 ) .T h e determination of total sulfate can be done by several different methods (4-7). Maddalone et al. have proposed a sulfate method based upon the thermal reduction of perimidylammonium sulfate (8).This method offers good sensitivity and reliability. When combined with a collection procedure which would stabilize the sulfuric acid aerosol, the thermal reduction method would be superior to any other method reported to date. Methods for the collection, stabilization, and determination of sulfuric acid aerosol and for the collection and determination of total sulfate are now proposed. T h e methods are based upon the reaction of sulfate with perimidto yield perimidylylammonium bromide, (PDA-Br) (8), ammonium sulfate, (PDA)2S04, which undergoes thermal reduction a t 400 OC, to yield sulfur dioxide which is easily determined by using the West-Gaeke procedure (9). No separation of sulfuric acid aerosol from other sulfate species is necessary for a sulfuric acid aerosol determination unless large quantities of ("&SO4 are present. T h e critical feature of the process is the stabilization of sulfuric acid a t the moment of sample collection, whereby its reaction with coexisting pollutants is inhibited. An air sample is collected on two PDA-Br impregnated glass fiber filters using open face filter holders placed side by side. One filter is directly pyrolyzed (8) and the other is treated with PDA-Br in methanol followed by pyrolysis. T h e sulfur dioxide evolved during the pyrolysis of the first filter is due only to the presence of sulfuric acid aerosol. Upon addition of reagent solution to the second filter, sulfate salts dissolve, ionize, and precipitate as (PDA)2S04 which is indistinguishable from the (PDA)2S04 formed by the topochemical reaction of H2S04 with PDA-Br during collection. Thus, the sulfate regardless of origin, reacts with reagent to provide a measure of total sulfate.
EXPERIMENTAL Reagents and Materials. A partial list of reagents and materials is given in Ref. 8 and 10. Gelman Spectro Grade Type A glass fiber filters were used for impregnation. Apparatus. A partial apparatus list is given in Ref. 8 and 10. The filter holders had open faces of 13-mm diameter. A Model 125 VF ultrasonic generator was used to generate ammonium sulfate. Procedure. Gelman Spectro Grade Type A glass fiber filters were impreganted by immersion in 10 ml of 1.8%PDA-Br in methanol. The filter was cut so that it just f i t inside a 100-mm Petri dish and, after 5 min, the dish and filter were placed in an oven at 80 "C to evaporate the methanol. Once the solvent had evaporated, the filter was cut into an appropriate size for sampling. A 16-mm diameter cork borer was used for cutting out filters for our work. The filter discs were placed in two open face filter holders and samples were collected. The same volume of air was pulled through each filter holder by maintaining the same rotameter reading following each filter. After the samples had been collected, one of the filters was removed, pyrolyzed, and the resultant SO2 trapped in 10 ml of 0.1 M sodium tetrachloromercurate(I1). The second filter was removed and placed in a 5 mm X 20 mm i.d. Petri dish. Two hundred ~1 of 0.5% PDA-Br solution were added to the center of the filter. The sample was allowed to stand for 5 min and then placed in the oven ANALYTICAL CHEMISTRY, VOL. 48, NO. 4 , APRIL 1976
639
Table I. Filter Efficiency of Impregnated Glass Fiber Filters Compared to Fluoropore
Table 11. Relative Standard Deviation of Direct Pyrolysis Analysis of Generated Aerosol Samples pg H,SO,
pg H,SO, collected
Group
1 2
3 4
Fluoro pore
Impregnated glass fiber filters
3.0 4.4 4.4 3.6
6.3 7.6 7.9 6.9
0.42 2.9 7.2 16.4 30.1 78.4
Re1 increased efficiency, %
110 73 80 91
Re1 s t d dev, %
9.5 6.9 9.0 3.0 3.7 2.7
-___
at 80 O C until all the methanol had evaporated. The dish and filter were then removed, pyrolyzed for 3 min, and the resultant SO2 was determined by using the West-Gaeke procedure (9).From the SO2 evolved from the respective filters, sulfuric acid and the total amount of sulfate was calculated. The SO2 obtained from direct pyrolysis is due solely to sulfur trioxide and/or sulfuric acid aerosol and possible contributions from (NH4)2S04. The second sample gives total sulfate since the added PDA-Br in solution ensures complete precipitation of the salts which were not included in the topochemical reaction of the droplets of the sulfuric acid with PDA-Br.
R E S U L T S A N D DISCUSSION T o test the feasibility of using PDA-Br impregnated glass fiber filters, it was first necessary t o determine t h e percentage recovery of sulfuric acid and other sulfates from t h e filters. This was accomplished by spiking impregnated filters with 10-pg amounts of sulfate as H2S04 and NazS04, respectively. Additional PDA-Br was added to each sample followed by pyrolysis. Results show that 101% of t h e H2SO4 was recovered while 97% of the Na2S04 was recovered. Thus, none of the sulfate and/or SO2 produced during thermal decomposition was sequestered within t h e filter. T h e optimization of PDA-Br impregnation was accomplished by adding various amounts of t h e reagent to filters followed by collection of a standard amount of generated sulfuric acid aerosol. T h e collected H2SO4 was then determined to establish t h e level of impregnation required for maximum filter efficiency without introducing a subsequent large amount of organic debris t h a t would be produced during the final pyrolysis of the filters. A comparison of the efficiency of impregnated and nonimpregnated glass fiber filters was made. T h e impregnated filters were approximately 25% more efficient than t h e non-impregnated. This is probably due mainly to the stabilization of sulfuric acid aerosol by PDA-Br although reduction in pore size and t h e coating of the alkaline sites of the glass fiber filters may also contribute significantly. Table I clearly shows t h a t the impregnated glass fiber filters are much more efficient than Fluoropore filters which previously have been shown to be t h e most efficient for sampling the acid aerosol ( 1 1 ) . I n each group, the same amount of air was pulled through both filters. Thus, if t h e filter efficiencies were identical, both filters should contain the same volume of generated acid. T h e increased efficiency of impregnated glass fiber filters will aid the chemist in obtaining ambient concentrations that approach true values. A study of the recovery of sulfuric acid aerosol in direct pyrolysis was performed. Ten generated sulfuric acid aerosol samples were collected. Five were directly pyrolyzed and the other five were treated with solutions of additional reagent and used for total sulfate determinations. By comparing the total sulfate content (H2S04 content) and t h e amount of H2SO4 calculated from direct pyrolysis, the per640
ANALYTICAL CHEMISTRY, VOL. 48, NO. 4, APRIL 1976
Table 111. Relative Standard Deviation of Total Sulfate Analysis of Generated Sulfuric Acid Aerosol Samples pg H W ,
Re1 std dev, %
0.45 3.3 7.6 15.9 28.7 75.4
15.5 9.4 11.0 7.0 5.9 7.9
centage of the sulfuric acid aerosol t h a t reacted topochemically to form (PDA)2S04could be calculated. Tables I1 and I11 give t h e results of this study. It is very important t o note t h e excellent recovery of sulfuric acid aerosol below 5 pg. I n microdiffusion, recovery is very poor at this level ( 1 2 ) . An average of 96 f 5% recovery was obtained for sulfuric acid aerosol over the 0.5 t o 78 pg range. T o check on possible interferences due to sulfate salts, sodium sulfate was selected for study as a typical example. Upon direct pyrolysis of impregnated filters to which more than 1000 pg of powdered Na2S04 had been added, none indicated any interference. Therefore, sulfuric acid aerosol values can be calculated from t h e amount of SO2 evolved by direct pyrolysis of impregnated filters:
H2S04 wg H2S04 = pg SO2 evolved X -
so2
Total sulfate is due to sulfate from HzS04 plus sulfate derived from its salts, e.g., NazS04, ZnS04, (NH4)2S04, etc. It should be noted t h a t (NH4)2S04, even as dry particulate, generated ultrasonically, interferes with the method, probably by decomposing to form SO3 which reacts with PDABr during the pyrolysis step to form transitory (PDA)2S04. Attempts to remove (NHJ2S04 by a low temperature reduction at 300 to 350 "C failed to alleviate the problem. However, because (NHJ2S04 is readily hydrolyzed to H2SO4 in moist air or the lung, it may be realistic to include it with SO3 and H2S04 as a n environmental hazard. Sulfur trioxide is effectively stabilized with PDA-Br impregnated filters. Passage of SO3 through such filters and subsequent pyrolysis proved that (PDA)2S04 is formed. T o test the efficiency of t h e filters toward SO3, 80 pg of SO3 was passed through the filters followed by an aqueous PDA-Br trap. T h e resulting filter efficiency was found to be 100 5%. Samples on impregnated filters are treated (moistened) with additional PDA-Br solution to yield sulfate ions from all sulfate-containing species. Thus, the ( P D A h S 0 4 formed represents total sulfate salts. Pyrolysis therefore yields SO2 as a measure of total sulfates.
+
Calculation of sulfate salts can be made by expressing the results of both H2S04 and the total sulfate in terms of
-
Table V. Loss of H, SO, Aerosol during Microdiffusion
Table IV. Detection Limitsa
Pg H,SO,
Blank absorbance values Direct pyrolysis m e t h o d
Total sulfate m e t h o d C
0.065 0.075 0.074 0.063 0.063 0.060 0.063 0.061 0.068 0.062 0.070 Av 0.066
0.017 0.085 0.086 0.082 0.077 0.087 0.087 0.079 0.093 0.079 0.082 0.087 Av 0.084 aThe detection limit is defined to be the pg of SO, corresponding to two standard deviations of the blank. b o = 0.005. Relative standard deviation = 7.6%. C U = 0.005. Relative standard deviation = 6.0%. Detection limits = 0.3 Pg
so,-.
(or any other logical common form) and calculating the salt values from the difference, e.g.,
H,SO, collected alone
H,SO, collected followed by metal oxide collection (approximately 5 pg each Pb, Al, Fe)
3.0 4.4 3.6 4.4
nil nil nil nil __~--____--___
Table VI. Stabilization of H, SO, Aerosol by PDA-Br
H, SO, collection, pig
H,SO, collection followed by metal oxide collection (10-30 pg each of Pb, Al. Fe) p g
13.5 13.3 15.9 17.0 16.2 16.8 15.8 15.6 Av 15.5 * 1 . 4
14.0 16.8 14.5 15.3 Av 15.2 I1 . 2
pg salt sulfate, as S042- = pg total S042- - pg Sod2- from H2S04 Table IV shows the detection limits for the direct pyrolysis (HzS04) and total sulfate methods. A detection limit of 0.3 pg sulfate is obtained for both methods. Although the detection limits are low enough that small air samples can be collected, topochemical reactions between H2SOl and other species are still possible. T h e reaction of sulfuric acid aerosol with other basic species is inevitable unless the acid can be stabilized immediately upon collection. To show that topochemical reactions have presented problems in the past in the determination of HzS04 aerosol, the following experiment was conducted. Eight samples were generated and collected on Fluoropore filters. Four contained only H2S04 and four contained H2S04 plus approximately 5 pg each of Pb, Al, and F e as their oxides collected on top of the aerosol ( 1 3 ) .All samples were microdiffused and pyrolyzed in the standard way outlined by Maddalone et al. (12). Table V shows the results of analyses. No microdiffusion was observed in samples containing metal particulates. Undoubtedly, all of the sulfuric acid reacted with metal salts before microdiffusion occurred. Therefore, although microdiffusion is selective for HzS04, quantitative destruction of acid can occur because of the presence of reactive co-pollutants and thus invalidate the results. Table VI shows the results of analyses of generated samples containing the same amount of acid collected of impregnated filters. As indicated in Table VI, part of the samples had Al, Pb. and Fe as their oxides in the 10-30 pg range collected following H2S04 aerosol collection. Data indicate that all of the H2S04 aerosol was stabilized ,by the topochemical reaction with PDA-Br and no reaction with co-pollutants occurred. T h e precision of the method is less than normal. T h e decrease in precision was probably due t o a variation in aspiration rate of the feed solution used during generation. I t should be noted that 200 pg of sulfur dioxide gave a positive interference of 0.5% in terms of sulfate. This can normally be ignored in a sulfuric acid and total sulfate determination.
CONCLUSION T h e stabilization of sulfuric acid aerosol by PDA-Br which results in the elimination of topochemical reactions between sulfuric acid aerosol and other basic species on the filter is definitely the salient feature of our method. No tedious and time-consuming separation step for sulfuric acid is necessary. Samples are simply collected on PDA-Br impregnated filters placed in two open face filter holders. T h e filters are then pyrolyzed after addition of reagent solution to one of the filters. Samples are pyrolyzed for 3 min and one sample can be run every 4 min. T h e impregnated glass fiber filters are approximately 90% more efficient for H2S04 aerosol than Fluoropore filters and are much easier to work with. Although sulfuric acid and sulfates, especially in complex mixtures, are difficult to measure a t any time, their sampling prior to measurement is an even greater challenge. I n gaseous atmospheres, the coexistence of antagonistic pollutants has defied all previous attempts to isolate (sample) sulfuric acid without its undergoing change due to neutralizing reactions. Consideration has been given to in situ measurements but, to date, we have been unable to see any practical approach that would avoid sample collection and concentration; consequently this is the first approach toward a reliable method for the determination of sulfuric acid aerosol and sulfur trioxide in the ambient atmosphere.
LITERATURE CITED (1) P. A. Leighton, "Photochemistry of Air Pollution", Pergamon Press, London, 1957. (2) T. R. Lewis, M. 0. Amdur, M. 0. Fritzhand. and K. I. Campbell, "Toxicology of Atmospheric Sulfur Dioxide Decay Products", Environ Prof. Agen. ( U . S . )Pub/. AP-11 (1972). (3) V. Dharmarajan, R. L. Thomas, R . F. Maddalone, and P. W. West, Sci. Total Environ., 4, 279 (1975) ANALYTICAL CHEMISTRY, VOL. 48, NO. 4, APRIL 1976
641
s.
(4) H. N. Schaefer, Anal. Chem., 39, 1719 (1967). (5) L. Dubois, C. J. Baker, T. Teichman, A. Zdrojewski, and J. L. Monkman, Mikrochim Acta ( Wen), 269 (1969). (6) F. L. Ludwig and E. J. Robinson, Coli. Sci., 20, 571 (1965). (7) J. B. Davis and G. Lindstrom, Anal. Chem., 44, 525 (1972). (8) R. F. Maddalone, G. L. McClure, and P. W. West, Anal. Chem., 47, 316 (1975). (9) P. w. West and G. C.Gaeke, Anal. Chem., 28, 1816 (1956). (10) R. L. Thomas, V. Dharmarajan, and P. W. West, Environ. Sci. Techno/.. 8, 930 (1974). (1 1) R. F. Maddalone, Ph.D. Dissertation, Louisiana State University, Baton Rouge, La.. December 1974.
(12) R . F. Maddaione, R . L. Thomas and P. W. West, submitted to Environ. Sci. Techno/. (13) V. Dharmarajan and P. W. West, Anal. Chim. Acta, 69, 43 (1974).
RECEIVEDfor review September 15, 1975. Accepted December 29, 1975. This research was supported by the National fkience Foundation Grant No. GP-18081 and RANN (NSF) Grant No. GI-35114x1.
Determination of Suspended and Dissolved Uranium in Water R. L. Fleischer"' General Electric Research and Development Center, Schenectady, N. Y. 1230 1, and National Center for Atmospheric Research, Boulder, Colo. 80303
A. C . Delany National Center for Atmospheric Research, Boulder, Colo. 80303
Individual droplets of water can be analyzed for uranium down to less than 0.01 pg/l. using readily available neutron doses. By separately counting randomly arrayed and clustered tracks, the dissolved uranium can be separated from that which is suspended in particulate matter.
Measurements of uranium in environmental surface water can be complicated by the presence of a variable mass of suspended uranium-bearing particles. T h e abundance of such particles is in turn affected by many factors, including flow velocities, degree of turbulence, and the local geology and vegetation. In this note, we describe a technique for analytically separating the uranium that is located in suspended, particulate matter from t h a t which is present in solution. T o do so, we utilize the nuclear track technique that has been outlined for sensitively measuring the uranium content of water droplets (1) and supply necessary experimental detail.
EXPERIMENTAL PRINCIPLES Water droplets are allowed to evaporate on a substrate that is a nuclear track detector ( 2 ) such as Lexan or Ansco polycarbonate. As sketched in Figure 1, this leaves a thin residue of nonvolatile constituents, including uranium. Subsequent exposure to 4 thermal neutrons per cm2 in a nuclear reactor induces fission of a fraction u 4 of the uranicm2) is the cross section um atoms, where u ( = 4.2 X for fission. Appropriate etching of the detector after irradiation dissolves material along the damage tracks and thus allows them to be visualized, as is illustrated in Figures 2 and 3. Ordinary water droplets normally evaporate from the surface of Lexan polycarbonate in the manner shown in Figure 1. T h e diameter remains constant as the droplet becomes thinner. T h e resultant character of the distribution of the deposited residue is displayed in the track array in Figure 2, a rim of thicker deposition a t the edge of the droplet site and a center region of essentially uniform del Present address, General Electric Research and Development Center, Schenectady, N.Y. 12301.
642
*
ANALYTICAL CHEMISTRY, YOL. 48, NO. 4, APRIL 1976
position. Particulate concentrations of uranium are indicated by clusters of tracks, such as the single cluster near the center of Figure 2 or the many clusters in Figure 3, which shows the track pattern in the center of a water drop residue t h a t contained suspended solids. When using a detector with an etching efficiency of one, the weight concentration of uranium c is given by
c = Nm/VN,u$
(1)
where N is the total number of tracks in the pattern from a drop of volume V . N o = Avogadro's number, 6.02 X 1023 and m = atomic weight of uranium, 238.03. For a convenient neutron dose of 1016/cm2and a 52-pl volume, such as we have used here, the concentration can be written
c (pg/l.) = 1.81 X
N (tracks)
(2)
T h e conclusion from Equation 2 is that a droplet that yielded 100 tracks would have contained 18 X g U/g water, a value that is known with f10% accuracy (at one standard deviation). T h e accuracy is limited primarily by two quantities in Equation 1, the neutron dose 4, which we measure by counting fission tracks induced in a precalibrated glass dosimeter of known uranium content (2, 3 ) , and by the number of tracks N , which can be increased, if necessary, either by evaporating multiple or larger drops of water or by increasing the neutron fluence. Although Equation 1 is best used by counting the entire N tracks associated with a drop, often high track abundances make it cumbersome to do so. In most cases, the drop pattern is circular and can be well described in terms of a constant number of tracks per unit length ( N L )along the rim of the distribution and a constant number of tracks per unit area ( N A )in the interior. If the radius of the circular track distribution from the droplet is R , and the radial nonuniformity near the rim has a width 6 t h a t is much less than R ,
N = ~ T R N+L T ( R - ~ ) ' N A
(3)
Where the droplet is not accurately circular, as in the slight ellipticity and flattening of the track pattern in Figure 2, straightforward variations on Equation 3 can be used. Ex-
