Reinvestigation of the Jacobs-Hochheiser Procedure for Determining Nitrogen Dioxide in Ambient Air Larry J. Purdue, Jared E. Dudley,' John B. Clements,2 and Richard J . Thompson Division of Air Quality and Emission Data, National Air Pollution Control Administration, Environmental Health Service, U.S. Public Health Service, U S . Department of Health, Education and Welfare, Cincinnati, Ohio 45226
Problems with the determination of ambient air concentrations of NOn by absorption in NaOH solution (JacobsHochheiser procedure) lead t o a reinvestigation of the method. Permeation tubes were used t o generate atmospheres containing known amounts of NOz a t ambient air levels which were sampled by the Jacobs-Hochheiser procedure. It is shown that nitrate ions are not generated on sampling these atmospheres and no account need be made for their presence in using the Jacobs-Hochheiser procedure. The NanCO3 generated on sampling must be carefully considered in a n analytical procedure involving a hydrazine reduction t o determine nitrate ions. Overall analytical efficiency for the Jacobs-Hochheiser procedure, as carried out in this investigation, is 35 %. Surface active agents cannot be used, because of excessive foaming. It is shown that fritted glass bubblers give higher precision than glass-restricted orifices.
N
itrogen dioxide, a widespread air pollutant of major importance, can be determined in ambient air using dilute sodium hydroxide as a collection medium. The procedure, commonly referred t o as the Jacobs-Hochheiser procedure, has been automated (Jacobs and Hochheiser, 1958) but has found its widest application as an integrated sampling method for use in a network operation, such as the National Air Surveillance Network, where relatively long sampling periods (up t o 24 hr) and return of the samples t o a central laboratory are required. These requirements prohibit the use of the more common procedure (Saltzman, 1954) in which a fairly unstable azo dye is formed directly with nitrogen dioxide during sampling. Poor collection efficiency, variable stoichiometry (Morgan et al., 1967), and poor correlation with the continuous monitoring method based on the Saltzman procedure, strongly suggested that the integrated JacobsHochheiser method for ambient air nitrogen dioxide determination deserved reinvestigation. Integrated Jacobs-Hocliheiser Method
I n the integrated Jacobs-Hochheiser method, air containing nitrogen dioxide is drawn through a dilute (approximately 0.1N) sodium hydroxide solution for a sampling period of up t o 24 hr. This operation supposedly produces a dilute solution of sodium nitrite and sodium nitrate in the absorbing solution. The ambient air concentration of nitrogen dioxide is determined by analyzing for nitrite ion using the classical diazotization and coupling procedure t o form a deeply colored azo dye. The transmittance of the resulting azo dye solution is Present address: University of Idaho. Moscow, Idaho 83833. Present addre5s: Environmental Protection Agency, Research Triangle Park, N.C. 2771 1. To whom correspondence should be addressed. 152 Environmental Science & Technologg
measured and compared t o the transmittance of solutions of properly developed sodium nitrite standards made up in the same concentration of sodium hydroxide. The degree of conversion of nitrogen dioxide gas t o nitrite ion must be known in order t o calculate the concentration of nitrogen dioxide in the air samples. Morgan et al. (1967) determined the degree of conversion by sampling ambient air a t 13 urban sites, analyzing for sodium nitrite and sodium nitrite plus sodium nitrate, then calculating the ratio of the nitrite alone to the total nitrite plus nitrate. They found the ratio t o be 0.63. To account for unabsorbed nitrogen dioxide, Morgan et al. (1967) independently determined the collection efficiency by passing ambient air through a series of five absorbing solutions and comparing the nitrite plus nitrate in the first absorber with the total nitrite plus nitrate in the other four. They found the collection eficiency t o be 50%. The National Air Surveillance Network used the results of the Morgan et al. (1967) study t o develop a n analytical method for determining nitrogen dioxide concentrations in ambient air. The data previously collected by the National Air Surveillance Network showed that the degree of conversion is subject to wide variation and can, therefore, be a significant source of error. If, however, the collected nitrate were reduced t o nitrite before analysis so that nitrite and nitrate would both be determined, this source of variation should be significantly reduced and a decrease in experimental error should result. The work reported here described experiments designed t o obtain an improved nitrogen dioxide determination using a nitrate reduction procedure. Procedure Standard Nitrogen Dioxide Atmospheres. Permeation tubes (O'Keeffe and Ortman, 1968; Scaringelli et al., 1970) were used to generate sub-ppm atmospheres of nitrogen dioxide. These levels of nitrogen dioxide were prepared by passing a slow stream (ca. 200 ml/min) of dry, NOn-free air over a calibrated permeation tube held a t an appropriate temperature and diluting the resulting airstream with dry, N02-free air t o the four concentrations shown in Table I. In contrast t o the
Table I. Conditions for Generating Known Atmospheres of Nitrogen Dioxide Nitrogen dioxide Nitrogen dioxide Air dilution concentration, permeatioii Permeation tube temp, "C rate, pglmin rate, I./min pg/m3 (ppm)" 5.45 0.726 8.854 82 (0.044) 5.45 0.726 4.774 152 (0.081) 15.95 1.797 8.854 203 (0.108) 15.95 1.797 4,714 376 (0.200) pg/m3 = (pg'rniti);(l.!min) 10-4 zit 25°C a n d 760 mtn Hg.
X 103; pprn
= (pgIm3)
X 5.32 X
experience of other workers (Scaringelli et al., 1970), no difficulty was encountered in using nitrogen dioxide permeation tubes a t temperatures down t o 5.45OC. Air Sampling. The National Air Surveillance Network's 24-hr integrated gas sampler (Morgan et al., 1967) was used t o collect samples from the standard atmospheres. Upturned, 70- t o 100-p sintered glass frits and glass-restricted orifices (0.35 i 0.05 m m i.d.) were each evaluated as dispersers for the airstream entering the collection reagent (0.25N NaOH). A surfactant dispersing agent (Carbowax 6000) was also evaluated as a n aid to increase nitrogen dioxide absorption but it gave inconsistent results a t the levels of Carbowax that could be used without causing excessive foaming. Ten samples, each collected over a 24-hr period a t a sampling rate of 200 ml/min, were taken for each run. Analysis. All analyses were done by a n automated procedure. To determine nitrite plus nitrate, a copper-catalyzed, basic hydrazine reduction procedure based o n the method of Kamphake et al. (1967) was developed to give equal responses for equimolar amounts of both nitrite and nitrate. The same sodium hydroxide solution used to absorb the nitrogen dioxide was used as the source of base for the hydrazine reduction. The flow patterns and reagent concentrations are shown in Figure 1. T o determine nitrite alone, the analysis was carried out without nitrate reduction by substituting distilled water for hydrazine. Sodium nitrite and sodium nitrate standards were made up to contain 0.25, 0.50, 1.0, 1.5, and 2.0 pg NOz-/ml.
Results and Discussion Reevaluation of the Integrated Jacobs-Hochheiser Method. Initial work in the reevaluation was directed to reducing the nitrate to nitrite before analysis, thus eliminating variability in the ratio of these two ions as a source of error. Four atmospheres containing sub-ppm levels of nitrogen dioxide (Table I) were :ampled, and the amount of nitrite and nitrite plus nitrate present in each collected sample was determined. Initially, our results agreed with Morgan et al. (1967), but then we realized an important error had been committed in both procedures because the effect of carbon dioxide in air had been ignored. Independent investigation showed that in a basic hydrazine reduction procedure, the analytical response for either nitrite or nitrate is higher in sodium carbonate than in sodium hydroxide, apparently due t o differences in the rates of destruction of the nitrite by hydrazine. Since the carbon dioxide of air leads t o the formation of sodium carbonate in the absorbing solutions, it is obvious that careful consideration must be given t o the composition of the absorbing solution after sampling in order t o obtain an accurate analysis. Previous procedure always had prepared nitrite standards in sodium hydroxide, but t h e above findings show that it is important that the standards be prepared in mixtures of sodium hydroxide and sodium carbonate which duplicate the composition of the collected sample. Sodium bicarbonate was shown t o be absent and did not have t o be considered. Analyses were rerun with and without hydrazine, using properly prepared standards, and nitrite and nitrite plus
39
/
"*
SAMPLE' AIR
PEROXIDE (0.025%)
2'00
Cum4 5 4 0 (18.5 mJlittr) 3
an
REAGENT
TIME DELAY
1.60
r
FLOW CELL
'CONTAINS BASE NECESSARY FOR REDUCTION
COLORIMETER 15 mm VI
RECORDER
Figure 1. Automated analysis flow pattern for nitrogen dioxide Sample time, 1 min Washout time, 2 min Diazotizing-coupling reagent: 20 g / l . sulfanilamide and 2.0 g/l. N-(l-napth) 1)ethglene diamine dihydrochloride in 10% ( V / V ) phosphoric acid in distilled water Volume 6, Number 2, February 1972 153
Table 11. Stoichiometric Factor as a Function of Nitrogen Dioxide Concentration. Nitrogen dioxide concentration, 4 m 3 (wm) Stoichiometric factorh 82 (0 044) 0.98 0 97 152 (0 081) 203 (0.108) 1.03 376 (0 200) 1.02 a Standards prepared in solutions containing 9.5 g,l. sodium carbonate and 2.5 811. sodium hydroxide. pig NOZ-lml pg NOz-/ml f p g NOs-iml
Table 111. Recovery of Nitrogen Dioxide from Standard Atmospheres NO2levels, wg/m3 (ppm)a ~~
82 (0.044)
152 (0.081)
316 203 (0.108) (0.200)
~~~~~
~~~~~
Orifice Frits Orifice Frits Orifice Frits Frits Average, (X)O 2 2 . 3 29.7 26.0 53.8 2 0 . 4 78.0 1 1 . 5 Re1 std devia1 5 . 4 1 9 . 9 25.2 1 0 . 8 2 5 . 4 1 1 . 9 1 5 . 6 tion, (SJ Recovery,(R)d 27.2 3 6 . 2 1 7 . 1 35.4 1 0 . 1 3 8 . 5 30.6 a Calculated from: pg N0z’m3 = (C X A),’V, where C = concentration of nitrite ion, pglrnl; A = volume of collection reagent, ml; and V = volume of air sampled, m3. b Replicate of IO. c Srcl= ( S j X ) x 100, S = std dev. d R = (X/NOzlevel) X 100.
+
nitrate redetermined. The ratio N02-/(N02NOr-), which is defined as the stoichiometric factor, was recalculated for each sample (Table 11). Since there was no difference in response between analysis for nitrite and for nitrite plus nitrate, a stoichiometric factor of one was obtained. This leads t o the conclusions that no nitrate is formed during sample collection and that overall variation can be accounted for by collection efficiency and analytical variation. Crecelius and Forwerg (1970) reported similar results in which they find that NO2 in the sub-ppm range does not generate nitrate ion. A nitrate reduction procedure is, therefore, not only unnecessary but also impractical for use in network sampling because the amounts of sodium hydroxide, sodium bicarbonate, and sodium carbonate would also have t o be determined for each sample in order t o make accurate evaluations. Additional Procedural Evaluations. In addition t o eliminating the error due to variability in the stoichiometric factor, two other factors were investigated with hopes of improving the analysis. The first of these, the incorporation of Carbowax 6000, in the collecting solution t o aid nitrogen dioxide absorption, gave inconsistent results at the levels of Carbowax that could be used without excessive foaming. The second factor investigated was the precision and efficiency of the device for dispersing the airstream in the collecting reagent. Glassrestricted orifices and sintered glass frits were compared.
154 Environmental Science & Technology
In spite of quality control problems, difficulty in network use, and expense, past practice in air sampling for nitrogen dioxide indicates that fritted-glass devices are preferred for dispersing the airstream in the collecting reagents (Morgan et al., 1967; Saltzman, 1954). Meadows and Stalker (1966) reported that glass-restricted orifices provide higher analytical precision, albeit lower collection efficiency, than do fritted-tip devices. It was reasoned that some collection efficiency could be sacrificed for a gain in precision and that restricted-orifice dispersers deserved further investigation. The experimental results, shown in Table 111, clearly show that frits are more efficient for nitrogen dioxide collection than are glass-restricted orifices. In addition, precision as shown by relative standard deviation, is also better with frits than with orifices, a finding contradictory t o the one reported by Meadows and Stalker. This difference in precision may be a consequence of the use of different types of fritted-tip bubblers. Meadows and Stalker used 30- to 50-p polyethylene-fritted bubbler, which gave poor precision (32.7 re1 std dev). The 70- to 100-p glassfritted bubblers used for this study gave relatively good precision (14.5 re1 std dev). From the data of Table 111, an overall analytical efficiency of 3 5 z for the integrated Jacobs-Hochheiser method, as used in the National Air Surveillance Network, was calculated by averaging the four recovery values determined when frits were used t o disperse the airstream. The report of a 15% conversion of nitrogen dioxide to nitric oxide on absorption into sodium hydroxide contributes an additional reason for the low efficiency of measured nitrogen dioxide absorption (Buck and Stratmann, 1967). Low efficiency remains the chief weakness of the integrated Jacobs-Hochheiser method for ambient air sampling and analysis of nitrogen dioxide, but, a t present, there is no other practical method available for measurement of this pollutant in a network operation where analysis must be delayed.
z
z
Acknowledgnien t We express our sincere appreciation t o Richard E. Enrione for helpful advice on this problem and to Philip Greenland and Barbara Bonfield for valuable technical assistance. Literature Cited Buck, M., Stratmann, H., Staub., 27,ll-15 (1967). Crecelius, H . J., Forwerg, W., ihid., 30,23-5 (1970). Jacobs, M. B., Hochheiser, S., Anal. Clzem., 30, 426-8 (1958). Kamnhake, L. J., Hannah, S. A., Cohen, J. M., Water Res., 1,205-16(1967). Meadows. F. L.. Stalker. W. W.. Amer. Ind. H y g . Ass. J., 27, 556-9 (1966). Morgan, G . B., Golden, C., Tabor, E. C., J . Air Pollut. Contr. Ass., 17, 300-4 (1967). O’Keeffe, A. E., Ortman, G. C., Anal. Chem., 38,760-3 (1968). Saltzman. B. E., Anal. Clzem., 26,1949-55 (1954). Scaringelli, F. P.. O’Keeffe, A. E., Rosenberg, E., Bell, J. B., Anal. Chem., 42, 871-6 (1970). Scaringelli, F. P., Rosenberg, E., Rehme, K. A , , E N ~ I R O N . Scr. TECHNOL., 4,924-9 (1970). Receiced for reciew October 27, 1970. Accepted July 16, 1971.
