This restricts the sample size, resulting in a lowered effective sensitivity. An alternate technique could be the use of glacial HOAc as the extraction solvent. If the material were dissolved directly into the acid, the reagent concentration could be adjusted accordingly, and considerable sensitivity could be gained. Because the Br determination can be accomplished in the presence of large excesses of C1, this method, coupled with a C1 analysis technique, can be used for measuring Cl/Br ratios. Acknowledgment
The authors thank John B. Pate for providing the highvolume air filter samples. Literature Cited
Cosgrove, J. F., Bastian, R. P., Morrison, G . H., Anal. Chem. 30,1872 (1958).
Duce, R. A,, Wasson, J. T., Winchester, J. W., Burns, F. J., J. Geophys. Res. 68, 3943 (1963). Duce, R. A,, Winchester, J. W., Radiochim. Acta 4, 100 (1965). Duce, R. A., Winchester, J. W., VanNahl, T. W., J. Geophys. Res. 70. 1775 (1965). Duce, R. A , , Woodcock, A. H., Moyers, J. L., Tellus 19, 369 (1967). Feigl, F., “Spot Tests in Inorganic Analysis,” 5th ed., Transl. R. E. Oespar, Elsevier, New York, 1958, p. 262. Hunter, G., Goldspink, A. A,, Analyst 79,467 (1954). Lininger, R. L., Duce, R. A., Winchester, J. W., Matson, W. R., J . Geophys. Res. 71, 2457 (1966). Lodge, J. P., Jr., Pate, J. B., Science 153,408 (1966). Saltzman, B. E., Anal. Chem. 33, 1100 (1961). Winchester, J. W., Duce, R. A., Naturwissenschaften 54, 110 (1967). Received for review June 23, 1970. Accepted September 25, 1970. The National Center for Atmospheric Research is sponsored by the National Science Foundation.
Nitric Acid and the Nitrogen Balance of Irradiated Hydrocarbons in the Presence of Oxides of Nitrogen Bruce W. Gay, Jr. and Joseph J. Bufalini Division of Chemistry and Physics, National Air Pollution Control Administration, Environmental Health Service, US. Public Health Service, US. Department of Health, Education, and Welfare, Cincinnati, Ohio 45226
In the photooxidation of systems containing hydrocarbons and nitrogen oxides in air, nitrogen balances have been poorLe., the amount of nitrogen consumed cannot be accounted for as products. Often, however, only gas-phase organic nitrogen products are considered; when surface-adsorbed products are analyzed, the nitrogen balances are greatly improved. Nitric acid is the principal surface product. It is believed to be formed primarily by hydrolysis of a nitrogen pentoxide intermediate on wall surfaces.
L
aboratory studies of the photooxidation of hydrocarbons in the presence of nitrogen oxides have satisfactorily explained many characteristics of photochemical smog. Many investigators agree concerning hydrocarbon reactivities, types of carbon-containing products, and amounts of oxidant formed in irradiations of particular hydrocarbon systems. Good carbon balances have been reported for irradiated systems in which the hydrocarbons were ethylene (Altshuller and Cohen, 1964), propylene (Altshuller et al., 1967), and 1-butene and trans-2-butene (Schuck and Doyle, 1959; Tuesday, 1961). In the past few years, greater interest has focused on nitrogen balance. Most determinations have not accounted for all of the nitrogen. In the photooxidation of propylene, Altshuller et al. (1967) reported that only 35 to 70% of the nitrogen consumed could be accounted for as products. With ethylene, only 13% of the nitrogen could be accounted for as methyl nitrate and nitrogen dioxide (Altshuller and Cohen, 1964). Similar results were observed with 422 Environmental Science & Technology
aromatics. With toluene, Altshuller et al. (1970) could explain only 10 to 20% of the nitrogen consumed as peroxyacetyl nitrate (PAN). With m-xylene, 10 to 75 % of the nitrogen originally present could be accounted for as PAN. In the work reported here, this lack of nitrogen balance has been resolved for various hydrocarbon-nitrogen dioxide systems. One earlier attempt to explain the nitrogen balance was in terms of a mechanism for the formation of molecular nitrogen. The data reported indicated that irradiation of nitrogen dioxide in a system with ethylene resulted in the formation of molecular nitrogen (Bufalini and Purcell, 1965). We performed studies to confirm these earlier findings and to extend the analyses to other hydrocarbon systems. The experimental results do not confirm the earlier findings concerning molecular nitrogen, but they do account for most of the nitrogen in the systems. Experimental
For the initial molecular nitrogen experiments, a 22-liter borosilicate flask, having a silvered outer surface and a double-walled borosilicate well, was used as a static reactor. A high-intensity Hanovia no. 679A36 mercury lamp fitted into the well and was cooled by flowing water between the double walls of the well. Molecular nitrogen and N20 were analyzed by vapor-phase chromatography with a helium photoionization detector. Other irradiations were made in the chamber described earlier (Altshuller and Cohen, 1964). This chamber is fitt$d with GE-F42-T6 black lamps with energy maximum at 3660 A. Temperature was maintained at 25 “C f 2 O during the irradiation. A plastic bag of 100 liters, fabricated from Teflon FEP
film, or a 72-liter borosilicate flask was used as the reactor and placed inside the irradiation chamber. The light intensity of the chamber was measured in terms of the rate of NO2 photolysis in nitrogen (Tuesday, 1961). The first-order dissociation constant for NOz in the 72-liter flask was 0.4 min-'. There was no way of conditioning the bag reactor other than purging with air. The 72-liter flask was washed before each irradiation with aqueous cleaning solution, 300 ml of acetone, and several times with large amounts of distilled water. The flask was allowed to dry in the open atmosphere. The flask, after being installed in the irradiation chamber, was equipped with a large magnetic Teflon-finned stirrer and a head with several glass and Teflon lines which served as inlets and exhausts, some connected to gas chromatographs. After the head was affixed and lines closed, the flask was evacuated with a mechanical pump and refilled with tank air. Hydrocarbons were determined by gas-liquid chromatography with use of a boiling point column, alkyl nitrate, and peroxyacetyl nitrate separated on a 10% Carbowax 600 column. All were detected with a flame-ionization detector. Carbon dioxide and carbon monoxide, separated on a Porapak Q column, were converted to methane by reducing with hydrogen over heated nickel. The resulting methane was detected with a flame-ionization detector. Nitrogen dioxide was measured by the method of Saltzman (1954). Formaldehyde in the gas phase was determined with chromotropic acid (Altshuller et al., 1961). Research-grade gases, hydrocarbons, and chemicals were used without further purification. The relative humidity of the tank air used was 5 %. Nitrate and nitrite in solution were determined by the method of Mullin and Riley (1955). The concentrations of formaldehyde formed in the hydrocarbon photooxidations did not appear to interfere with the reduction of nitrate by this method. The organic and inorganic nitrate and nitrite were collected in a 0.03N NaOH solution, which was used to scrub the gas phase and wash the reactor surface. A gaseous sample was withdrawn periodically during the irradiation for chromatographic analysis. Nitrogen dioxide in the gas phase was determined at the onset and termination of the irradiation. Two hundred milliliters of alkaline solution, injected into the flask at the termination of the irradiation, washed nitrate and nitrite from the surface and any nitrogen dioxide, alkyl nitrate, and PAN from the gas phase. The alkaline solution remained in the stirred flask for 12 to 16 hr before being withdrawn for nitrate and nitrite analyses. At the end of some irradiations, the gaseous products were withdrawn and scrubbed through alkaline solution. Nitrate and nitrite analyses of this resulting solution gave the gasphase nitrogen constituents. The flask was evacuated and refilled with tank air. Alkaline solution was injected into the flask to wash the surface. The total nitrate and nitrite of the system is the sum of that in the gas phase and that adsorbed on the surface.
Results and Discussion Molecular Nitrogen. Initially we attempted to detect molecular nitrogen as one of the products of the reaction. In these experiments helium and oxygen were the diluent gases. Initial concentrations of ethylene of 60 ppm and NO, of 15 ppm were used. It became obvious after several experiments that molecular nitrogen was not a product. Although as little as a 0.05 ppm change in nitrogen concentration could have been detected, no change in the background level of -3 pprn was
observed. N o N 2 0 was detected in the analysis of the gas phase, When the flask was evacuated and washed with Saltzman solution, less than 2 % of the nitrogen in the system could be accounted for. Inorganic Nitrate and Nitrite. Since no molecular nitrogen or NzO could be found as end-products, we turned our attention to the possibility of inorganic nitrate and nitrite formation, Ethylene was used since carbon balances have been good in irradiation with oxides of nitrogen. Data from the photolyses of ethylene-NOz systems are shown in Table I. When 21 ppm of ethylene and 3.75 ppm of NOn in tank air were irradiated for 5 hr, 25.6 ppm of carbon or 12.8 ppm of ethylene reacted. The products observed were 10.24 ppm of CO, 3.9 ppm of C0,,10.7 ppm of formaldehyde, and 0.3 ppm of methyl nitrate. No detectable peroxyacyl nitrates were observed. The total observed concentration of carbon as products was 25.14 ppm. This value represents a good carbon balance and indicates that no significant amount of nitrogen is present in carbon-containing compounds. Nitrate analysis of an alkaline solution used to scrub a withdrawn gas sample indicated a concentration of 0.3 ppm, which arises from the methyl nitrate previously measured chromatographically. Nitrite analysis indicated a concentration of 0.1 ppm resulting from the remaining NO, in the gas phase. Analysis of the alkaline solution used to wash the reactor surface accounted for 3.36 ppm of nitrate and 0.25 ppm of nitrite. When the flask was evacuated following the irradiation of trans-2-butene (Table II), the high percentage of nitrite observed in the alkaline wash resulted from NOz and PAN-type compounds adsorbed on the reactor surface. In experiments where the flask was not evacuated, the nitrite observed was the combined total resulting from nitrogen dioxide and PANtype compounds both in the gas phase and adsorbed on the reactor surface. The basic hydrolysis of PAN gave nitrite on a mole-per-mole basis (Nicksic et al., 1967). The nitrite and nitrate alkaline hydrolysis products of the nitrogen-containing constituents, in the gas phase and on the reactor surface, are listed in Table 111. Alkaline hydrolysis of 6 ppm of NO2 in a 72-liter flask produced nitrite as the product. When 200 ppm of NO, was hydrolyzed in the same volume of alkaline solution in a 250-ml flask, equal molar amounts of nitrate and nitrite were observed. All the experiments involving hydrocarbon photooxidation were conducted in the low-ppm range of NOZ. The alkaline hydrolysis of NO, resulted only in nitrite formation. When 6 ppm of NO, was irradiated in air for 4 hr in the 72-liter flask, 5.4 ppm of NO2 remained in the gas phase. After evacuation and washing of the surface with alkaline solution, 0.3 ppm of nitrate and 0.3 ppm of nitrite were detected. Although this aspect was not investigated in detail, replicate runs yielded the same results. These findings suggest that the small amounts of nitrogen dioxide adsorbed on the reactor surface are hydrolyzed by adsorbed water to nitrate and nitrite prior to washing of the wall with alkaline solution. Data from the photolyses of 1,3-butadiene, propylene, 1-butene, and trans-2-butene are listed in Table 11. The result of the photolysis of trans-2-butene in the Teflon FEP bag is also shown. In this system 3.13 ppm of PAN and 1.88 ppm of methyl nitrate were detected. The nitrogen balance for 1,3-butadiene accounted for 92 to 100% of the nitrogen. For both propylene and 1-butene, essentially all of the nitrogen in the system was accounted for. When only gas-phase, nitrogen-containing compounds were Volume 5. Number 5, May 1971 423
Irradiation time, hr 5 5 5 5 6
Hydrocarbon 1,3-Butadiene
Propylene
1-Butene trans-2-Butene
a
Initial HC
Table I. Photooxidation of Ethylene with Nitrogen Dioxide Initial Reacted Remaining Total nitrate Nos HC NOz from wash
Total nitrite from wash
Alkyl nitrate
0.25 0.30 0.18 ... 0.29
0.3 0.4 0.3 0.2
PPm
21 .o. 19.6 13.7 11.8 16.9
3.75 4.78 4.57 5.60 4.02
12.8 13.6 12.8 7.27 8.63
0.1 0.1 0.1 0.12
3.36 4.28 4.17 5.23 3.67
...
...
Table 11. Photooxidation of Olefins with Nitrogen Dioxide Initial Initial Reacted Remaining Nitrate Nitrite Nitrate in Nitrite in Irradiation HC NOz HC NO2 from wash from wash gas phase gas phase time, min PPm 0.50. ... ... 18.2 3.95 13.5 0.65 2.805 60 0.30 2.45 12.2 ... ... ... 13.0 3.12 75 14.3 0.80 ... ... 0.36 3.93 16.0 5.16 75 16.2 5.40 14.6 0.87 ... ... 0.36 4.10 75 0.70 ... ... 3.75 22.1 4.90 18.3 ... 120 4.47 15.4 ... 0.90 ... ... 16.9 5.50 120 1.20 ... ... 16.3 6.40 15.3 ... 5.00 120 1.3 ... 19.1 5.85 14.2 0.72 ... 4.4 70 0.57 1.5 ... ... 5.5 7.10 14.7 90 14.6 1.1 ... ... 3.1 13.0 4.10 13.0 ... 120 3.8 1.2 ... ... 150 22.0 5.24 17.0 0.40 3.35 1.2a 1.22 1.45 21 .o 7.25 17.2 0.56 90 20.0 0.97 0.76. 1 . 4 4 ~ 2.90 1.20 22.6 6.29 30 0.75 18.1 5.48 18.4 0.430 1 . 4 0 ~ 2.54 1.07 30 0.990 3.10 1.10 0.63 0.56a 16.1 5.75 16.3 30 ... ... 3.13c-1.88d 6.20* 14.2b 0.75b 16.1b ..* 30b
The flask was evacuated before washing with sodium hydroxide solution. Irradiated in Teflon bag. PAN in Teflon bag. Methyl nitrate in Teflon bag.
Table 111. Alkaline Hydrolysis of Products Reactant Product Nitrogen dioxide in gas phase nitrite Alkyl nitrate in gas phase nitrate PAN in gas phase nitrite Nitric acid on surface nitrate nitrite Nitrous acid on surface Nitrogen dioxide adsorbed nitric and nitrous on surface acid
considered in the photolyses of trans-2-butene carried out in the flask, 76 to 82% of the nitrogen could be accounted for. Tuesday (1961) investigated the photooxidation of truns-2butene and accounted for all of the nitrogen as PAN, methyl nitrate, and nitrogen dioxide. However, the adsorptivity of PAN was arbitrarily adjusted to 6.38 X ppm-l m-1 to yield a proper carbon and nitrogen balance. If the more recent absorptivity value of 10.3 X ppm-l m-l (Stephens, 1964) is applied, the nitrogen balance for Tuesday’s studies becomes approximately 78% at the end of the irradiation. This value is well within our observed range for gaseous nitrogen-containing products using the glass reactor. In the photolysis of trans-2-butene in a Teflon FEP reactor, 93 of the nitrogen-containing compounds was found in the gas phase. This larger percentage may be due to the lower surface adsorption of oxides of nitrogen on Teflon FEP compared to glass. 424 Environmental Science & Technology
Data for the photooxidation of various hydrocarbons with nitrogen dioxide are listed in Table IV. The nitrogen balance for 2-methyl-2-butene was relatively good with 97% of the nitrogen accountable. Poorer nitrogen balances were observed with m-xylene (-90 %) and isopropylbenzene (72 to 80%). Since only one run was made with m-xylene and two with isopropylbenzene, the experimental error could not be determined. It is possible that the unaccountable nitrogen is present in a carbon-containing polymer on the surface of the reactor. Kopczynski (1964) found that only 50 % of the carbon could be explained in terms of observed products in the photooxidation of mesitylene with oxides of nitrogen. Upon cleaning the reaction vessel, Kopczynski evaporated the cleansing solution and observed a yellow tacky deposit. Under infrared scrutiny, this deposit showed nitrate, acyl, and nitro bands. Since we did not test for nitro-organics, it is entirely possible that the nitrogen may be found in this form. Mechanism for Nitric Acid Formation. Photooxidation of hydrocarbons in the presence of nitric oxide has shown (Altshuller and Cohen, 1964; Altshuller et al., 1967; Schuck and Doyle, 1959) that a good nitrogen balance can be maintained until the nitric oxide is completely oxidized to nitrogen dioxide. After this point, the nitrogen balance becomes increasingly poor with increasing time of irradiation. At the NOz maximum, ozone formation becomes important. At this state in the reaction nitric acid can also be produced by the following mechanism:
0 s
+ NOz
-+
NO3 f 02
(1)
Table IV. Photooxidation of Various Hydrocarbons with Nitrogen Dioxide Initial HC
Initial
Reacted
19.5
6.1
11 .o
22
21.7
5.7
23
19.8
1
19.2
Irradiation time, hr 15.5
Hydrocarbon m-Xylene Isopropylbenzene Isopropylbenzene 2-Methyl-2butene
NO3
+ NO2
+ NzOs
Nitrate from wash
Nitrite from wash
0.6
4.3
1.2
18
0.1
4.17
0.4
6.2
19
...
4.3
0.2
6.0
17
0.8
2.5
3.3
NOz
+ HzO
+
2”03
(2)
(3)
When 6.0 ppm of NOn and 1.6 ppm of O 3were reacted in the 72-liter reactor in the dark for 4 to 6 hr, gas-phase analysis did not indicate formation of nitrate. Evacuation of the reactor, refilling with tank air, and washing with alkaline solution yielded 3.1 ppm of nitrate and 0.1 ppm of nitrite. The equilibrium concentrations of NpOrand H N 0 3 formed in the photooxidation of ethylene can be estimated by using the equations given by Leighton (1961). The NpOsconcentration is in the pphm range, and the nitric acid concentration is about lo5 pphm. This estimated equilibrium concentration of nitric acid is three orders of magnitude higher than the original concentration of NO2. Leighton made this same observation--i.e., the equilibrium concentration of nitric acid is much too large. Occurrence of a concentration this large would suggest that nitric acid is formed in the gas phase and that all of the nitrogen oxide should form nitric acid. Since nitric acid is not detected in the gas phase, it seems reasonable to assume that N z 0 5is adsorbed and hydrolyzed on the reactor surface to nitric acid. If nitric acid is formed by Reaction 3 in the gas phase, it must then be rapidly adsorbed on the reactor surface. To test whether adsorption occurs, nitric acid was vaporized in an airstream into the reaction vessel. After 3 hr, nearly all of the nitric acid was still in the gas phase. This does not preclude the possibility that nitric acid, once formed, is immediately adsorbed on the surface, since when vaporized with the airstream aerosols of nitric acid could be produced. The experiment strongly suggests that nitric acid can exist for some time in the gas phase, with the further implication that Reaction 3 is too slow to occur in the gas phase. The following reaction has also been considered as a possible source of nitric acid: OH
+ NO2
M +“ 0 3
Remaining NOz PPm
The nitrogen pentoxide can be adsorbed on the reactor surface and hydrolyzed by water also adsorbed on the surface. NzOs
HC
(4)
Nicolet (1965) postulated this reaction as being of importance in the upper atmosphere. We, however, excluded Reaction 4 on the basis of experimental results. Since hydroxyl radicals apparently are responsible for a large fraction of the hydrocarbon consumption (Bufalini et al., 1971), Reaction 4 should occur very early in the reaction scheme. In our experiments, the nitrogen balance is very good up to the time of the nitrogen dioxide maximum. Reaction 4, therefore, cannot be important. This is compatible with observed rate constants for hydroxyl
radicals. The rate constant for Reaction 4 is 2 X cme/ mole2 sec (Niki, 1970); the rate constant for O H C2H4 reaction is 10-11 cc/mole sec (Greiner, 1969). At atmospheric pressure, Reaction 4 is three orders of magnitude slower than the OH C2H4 reaction. For higher olefins, the rate constant is even larger (Greiner, 1969) and the difference even greater. Reaction 4 can occur at the latter stages of reaction when the hydrocarbon is nearly consumed, but the concentration of OH radicals will also be very low at this time. It is possible that NzOs formed in the gas phase is adsorbed on the surface of the flask and is not hydrolyzed before the alkaline wash solution is introduced. There is, however, enough water adsorbed on the surface from the tank air (5 relative humidity) to fully hydrolyze all adsorbed NpOa. Therefore, the reaction of nitrogen pentoxide with water (Reaction 3) on the surface of the reaction vessel appears to be the most plausible.
+
+
Literature Cited Altshuller, A. P., Cohen, I. R., Intern. J. Air Water Poll. 8,611 (1964). Altshuller, A. P., Kopczynski, S. L., Lonneman, W. A., 1, 899 Becker, T. L., Slater, R., ENVIRON.SCI. TECHNOL. (1967). Altshuller, A. P., Kopczynski, S. L., Lonneman, W. A., SCI. TECHNOL. Sutterfield, F. D., Wilson, D. L., ENVIRON. 4,44 (1970). Altshuller, A. P., Miller, D. L., Sleva, S. F., Anal. Chem. 33, 621 (1961). Bufalini, J. J., Gay, B. W., Jr., Kopczynski, S. L., ENVIRON. SCI. TECHNOL. (1971), in press. Bufalini, J. J., Purcell, T. C., Science 150, 1161 (1965). Greiner, N. R., University of California, Los Alamos Scientific Laboratory, Los Alamos, N. M. Private communication to H. Niki, Scientific Research Staff, Ford Motor Co., 1969. Kopczynski, S. L., Intern. J. Air Water Poll. 8, 107 (1964). Leighton, P. A,, in “Photochemistry of Air Pollution,” Academic Press, New York, 1961, pp 189, 192. Mullin, J. B., Riley, J. R., Anal. Chim. Acta 12, 464 (1955). Nicksic, S. W., Harkins, J., Mueller, P. K., Atmos. Environ. 1, 11 (1967). Nicolet, M., J. Geophys. Res. 70, 679 (1965). Niki, H., Scientific Research Staff, Ford Motor Co., Dearborn, Mich. 48121, private communication, 1970. Saltzman, B. E., Anal. Chem. 26, 1949 (1954). Schuck, E. A,, Doyle, G. J., Air Pollution Foundation Report no. 29, San Marino, Calif., October 1959. Stephens, E. R., Anal. Chem. 36, 928 (1964). Tuesday, C. S., in “Chemical Reactions in the Lower and Upper Atmosphere,” Cadle, R. D., Ed., Interscience, New York, 1961, pp 1 4 9 . Received for reciew April 20, 1970. Accepted October 28, 1970. Paper was presented in part at the Division of Water, Air, and Waste Chemistry, 158th Meeting, ACS, New York, N . Y., September 1969. Volume 5, Number 5, May 1971 425
