Anal. Chem. 1905, 57,737-740 (1 1) Sontheimer, H. "European Experience with Problems of Drinking
Water Quality. Safe Drinking Water: Current and Future Water Problems"; Russell, C. S., Ed.; Resources for the Future: Washington,
DC, 1978. (12) "Drinking Water and Health"; National Research Council, Natlonal Academy Press: Washington, DC, (1980 and 1982; Vol. 3 and 4). (13) Syrnons, J. M. Second Conference on Water "Chlorination: Environmental Impact and Health Effects", Gatlinburg, TN, 1977. (14) Bender, 0.F. Environmental Monitoring Series, EPA-60014-78-019, 1978. (15) Ruzicka, J.; Hansen, E. "Flow Injection Analysis"; Wiley: New York, 1981. (16) Gordon, G. Proceediqs of the American Water Works Assoclatlon, The 1982 Water Quallty Technology Conference 1982, 7 , 175-189. (17) Emmenegger,,F.; Gordon, G. Inorg. Chem. 1967, 6 , 633. (18) Kieffer, R. G.; Gordon, G. Inorg. Chem. 1968, 7, 235. (19) Tang, T.-F.: Gordon, G. Anal. Chem. 1080, 52, 1430.
737
(20) Ikeda, Y.; Tang, T. F.; Gordon, G. Anal. Chem. 1984, 56, 71. (21) Tang, Tsung-fei; Gordon, Gilbert Environ. Sci. Techno/. 1984, 18, 212. (22) Emerich, Dwight E. Ph.D. Thesis, Miami University, 1981. (23) Bray, W. C. J. Phys. Chem. 1903, 7, 92. (24) Nikolelis, D. P.; Karayannis, M. I.; Hadjiioannou, T. P. Anal. Chlm. Acta 1977, 9 4 , 415. (25) Ditz, H. Chem-Ztg. 1001, 2 5 , 727. (26) Rupp, E. Z . Anal. Chem. 1917, 56, 580. (27) Aeita, E. M.; Roberts, P. V.: Hernadez, M. J.-Am. Water Works Assoc. 1984, 76, 64-70. (28) Tang, T. F. M.S. Thesis, Miami University Oxford, OH, 1960. (29) Long, G. L.; Wlnefordner, J. D. Anal. Chem. 1983, 55, 712A-724A.
RECEIVED for review October 1, 1984. Accepted December 3, 1984.
Determination of Nitrate by a Flow System with a Chemiluminescent NO, Analyzer Kunio Yoshizumi* and Kazuyuki Aoki Tokyo Metropolitan Research Institute for Environmental Protection, 2- 7-1 Yuraku-cho, Chiyoda-ku, Tokyo 100, Japan Toshikazu Matsuoka Department of Applied Science, Tokyo Denki University, 2-2 Kandanishiki-cho, Chiyoda-ku, Tokyo 101, Japan Shukuji Asakura Department of Safety Engineering, Yokohama National University, 156 Tokiwadai, Hodogaya-ku, Yokohama 240, Japan
A high sensitivlty anaiytlcal method to determine nitrate in a solutlon by use of a NO, chemiluminescent analyzer was developed through a modlficationof Cox's original batch reactor methods. A flow sample Injection system was used and the acldlc reducing solution in the reactlon vessel was heated at 100 OC. The detection ilmit was improved to be 1 ng of NO,-/mL for a 5-mL solution sample. Nitrlc oxide recovery In an acidic solvent was observed to be 100% over the ranges of 55-68 wt % of sulfuric acid and 40-95 wt % of phosphoric acid solutions. The reactive species in the concentrated acidic solution was estimated to be HNO, and not NO,- or NO,'. Atmospherlc partlculate nltrate and gaseous nltrlc acid were measured at 1-h intervals with Teflon and polyamlde filters uslng the analytical method studied. I t was shown that the apparent total nltrate formatlon rate was 4.5%/h In a downtown neighborhood in Tokyo in summer.
Particulate nitrate and gaseous nitric acid in ambient air have been of recent concern as atmospheric photochemical reaction products. Moreover, the behavior of these species in the atmosphere is important to understand the fate of NO, emitted from combustion processes such as internal combustion engines, boilers, incinerators, and so on (1-7). Nitrate collected on a filter has been mostly sensitively analyzed by ion chromatography (8,9)and the Cd-NEDA (Cd reduction/N-( 1-naphthy1)ethylenediamine)method (10 , 1I ) . Thermal decomposition of nitrate to nitric oxide was also studied to determine nitrate by Moskowitz (12) and Spicer et al. (13). Recently, Cox developed highly sensitive analytical methods for nitrate and nitrite in a solution in which they were
reduced to nitric oxide and measured by a NO, chemiluminescent analyzer (14). The preparation of a sample is much simplier in his methods compared to other methods (8-1 1) because suspended material and color do not interfere. In this study, we modified Cox's methods (14)to eliminate the influence of impurity in reagents on the detection and to analyze rapidly a large number of samples such as atmospheric nitrates collected on filters. The optimum operating conditions were also investigated. Further, the analytical method was applied to the measurements of atmospheric particulate nitrate and gaseous nitric acid in a short time interval in downtown Tokyo.
EXPERIMENTAL SECTION Apparatus. A schematic diagram of the analytical system is shown in Figure 1 which is different from Cox's original batch reactor without external heating (14). Teflon tubing was used for all gas lines with glassware. High-purity nitrogen gas was used as a carrier at a flow rate of 200 mL/min. The reaction vessel was electrically heated to keep the reducing solution at appropriate temperature. Usually, a 0.5-mL sample was supplied to the reaction vessel by an injector. Nitric oxide produced in the reduction reaction in the vessel was measured with a Beckman Model 952 NO, chemiluminescentanalyzer. The nitric oxide mode of the analyzer was used. Acidic mist was removed by a cold trap consisting of an impinger in an ice water bath. The signal from the NO, analyzer was processed with a strip chart recorder and an electronic integrator. Reagents. Quantitative evaluation of samples was obtained by comparison with standards on the basis of peak area. Anhydrous potassium nitrate and sodium nitrite (Wako Pure Chemical) were used to prepare standard solutions of the respective anions. Known concentration of nitric oxide gas (Takachiho Chemical) was also used to calibrate the analytical system. Solutions of reagent grade 4 % ferrous ammonium sulfate and 2%
0003-2700/85/0357-0737$0 1.50/0 0 1985 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 57, NO. 3, MARCH 1985 ~
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Sulfuric acid, w t %
J
80
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Reaction vessel temperature
b
r
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Flgure 2. Effect of reaction vessel temperature on response of the analytical system: 0 , integrator response: 0, half width of response peak on a recorder.
ammonium molybdate in water were prepared for nitrate determination following Cox's original methods (14). High-purity water obtained by passing distilled water through an ion exchange column was used. Sulfuric acid or phosphorus acid (Wako Pure Chemical) was added to an equal volume mixture of these solutions to provide the proper reducing conditions as mentioned later whereas 57.6 wt % of sulfuric acid was used as a solvent in Cox's original method. Air Sampling. The collection of nitrate and nitric acid in the atmosphere was done by pumping ambient air at 20 L/min through a Teflon filter (SumitomoFP080) and a polyamide filter (Sartorius SM119) with 47 mm diameter in series. The Teflon filter collected particulate nitrate and the polyamide filter collected gaseous nitric acid. Sampling was made in I-h intervals on the roof of the Tokyo Metropolitan Research Institute for Environmental Protection in the summer of 1982. Extraction of nitrate ion from a filter was done by shaking it with 10 mL of high-purity water for 1 h. As for the Teflon filter, 0.5 mL of ethanol was impregnated before the addition of water. Extracted sample solutions were analyzed under the optimum conditions mentioned later.
RESULTS AND DISCUSSION Modification of Cox's Original Methods. The effect of the reaction vessel temperature on the analytical response is shown in Figure 2. The reducing solution was electrically heated to accelerate the reduction reaction of nitrate to nitric oxide. In this case, sulfuric acid in the reducing solution was set to be 60 wt %. The nitric oxide recovery was found to be 100% over all the temperature range. With the increase of the temperature, the half width of the response peak which closely related to the reduction rate in the vessel was observed to be smaller, which means that more rapid response was obtained. However, the evaporation of the solution also became too rapid. Therefore, the optimum vessel temperature
Figure 3. Effect of sulfuric acid concentration in reducing solution on analytical response: 0 , conversion efficiency of supplied nitrate to nitric oxide; 0, half wldth of nitrate response peak; A,conversion efficiency of supplied nitrite to nitric oxide: A, half width of nitrite response peak.
was concluded to be 100 O C in this experiment. Further, various amounts of nitrate solution, nitrite solution, and niric oxide gas were injected into the reaction vessel to calibrate the analytical system. The consistency among their responses was observed as a good linear calibration curve between 1 and 1000 ng of N03-/mL through an origin with a high correlation coefficient, 0.9997. In this study, after the base line response caused by impurity in the reducing solution was well minimized, a solution sample was injected into the flow reaction vessel shown in Figure 1. Therefore, the influence of the impurity on the response could be eliminated completely. The detection limit for nitrate was 1 ng of NOy/mL for a 5-mL solution sample which is more sensitive by a factor of about 5 and 100 when compared with Cox's original batch basis methods (14) and ion chromatography by Mulik et al. (9), respectively. Moreover, a sample could be supplied by an injector to our flow system without changing reducing solution on every sample injection. It resulted in more rapid operation in the analysis. The measurement time per sample was reduced to about 3 min. I t is an additional advantage for aerosol analysis that this method is free from the interference of ethanol in the concentration of 5%. This feature is useful to extract aerosol components on a nonhydrophilic Teflon filter into a water solution. Effect of Acid Concentration on Nitrate Conversion to Nitric Oxide. Figure 3 shows the effect of sulfuric acid concentration in the reducing solution on the conversion efficiencies of nitrate and nitrite to nitric oxide. It was found that supplied nitrate was converted to nitric oxide by 100% over the range of 55-68 wt % of sulfuric acid concentration. The reduction rates shown by the half width of the response peak were retarded in the range of sulfuric acid concentration less than 55 wt 70. High and low sulfuric acid concentrations seemed to disturb the reduction of nitrate to nitric oxide. Bayliss et al. (15) studied the distribution of nitrate species which consisted of nitrate ion (NO,?), nitric acid (HNO,), and nitronium ion (NO,+) in concentrated sulfuric acid solutions by spectrophotometry. Their results suggest that a reactive species in the reduction is nitric acid (HNOJ which was observed to be a dominant species in the corresponding sulfuric acid concentration range (15) although nitronium ion is a reactive species on aromatics nitration (16). On the other hand, the reduction efficiency of nitrite was observed to be 100% except in the range of more than 68 wt 7'0 of sulfuric
ANALYTICAL CHEMISTRY, VOL. 57, NO. 3, MARCH 1985
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400
i
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Phosphoric acid. w t %
Figure 4. Effect of phosphoric acid concentration in reducing solution on analytical response: 0 , conversion efficiency of supplied nitrate to nitric oxide; 0, half width of nitrate response peak; A,conversion efficiency of supplied nitrite to nitric oxide; A, half width of nitrite response peak.
acid concentration. The reduction rate is rapid enough over almost all the concentration range with a slightly retarded trend for concentrations of more than 68 wt %. Figure 4 shows the reduction efficiencies of nitrate and nitrite to nitric oxide by using phosphoric acid as a solvent. A comparison of Figure 3 and Figure 4 indicates that the reactivity characteristics in the sulfuric acid and phosphoric acid systems are qualitatively similar. One hundred percent of nitric oxide recovery in phosphoric acid solution was observed over the wider range of 40-95 wt %. The reduction rate became slower in the ranges of less than 63 wt % and more than 90 wt % of phosphoric acid concentration. These results could be explained by the fact that nitronium ion exists over a more narrow range in phosphoric acid due to lower hydrogen ion activity in phosphoric acid than in sulfuric acid (17). Therefore, nitric acid which is considered to be reactive in the reduction as mentioned before exists over a wider range in the phosphoric acid concentration. In summary, we obtained a wider reactive range on the reduction of nitrate by using phosphoric acid as a solvent in comparison with sulfuric acid. As for a routine analysis, phosphoric acid solvent can be concluded to be more useful because a greater number of sample injections could be done with 100% of nitric oxide recovery. Especially, 90 wt % of phosphoric acid is recommended as a solvent for the start condition of a routine analysis. Ambient Nitrate Measurements. The results of the application of the analytical method to the measurements of particulate nitrate and gaseous nitric acid in ambient air are shown in Figure 5 on the short time average basis with other related gaseous and meterological data which were obtained through a continuous air quality monitoring system by the Tokyo Metropolitan Government. The separation of nitrate and nitrite in the particulate was not made because the nitrite amount could be considered negligibly small as shown by Spicer (1). On the first day, August 24, 1982, about 3 m/s of south wind (typical seasonal wind in the neighborhood of Tokyo in summer) blew continuously through all the daytime. The values of pollutants as a whole represented low pollution levels. However, on the second day, August 25,1982, about 1m/s of weak north wind blow until about 11:OO JST and then shifted to 2-3 m/s of south wind. The particulate nitrate and gaseous nitric acid formation seemed to be based on photo-
Aug.24
Aug.25
Jpn Std Time
Figure 5. Diurnal pattern of nitrogenous compounds in ambient air in downtown Tokyo In summer: a, particulate nitrate (hatched) and gaseous nitric acid (nonhatched); b, PAN by gas chromatography (courtesy of Masataka Sofuku of the Tokyo Metropolitan Research Institute for Environmental Protection); c, wlnd speed and direction (The ordinate shows the magnitude of a wind speed. The direction of a arrow shows the direction of wind).; d, nitrogen dioxide (solid line) and nitric oxide (dashed line) by Sattzman method; e, oxidant by K I method (c-e, courtesy of Air Quality Monitoring Section of the Tokyo Metropolitan Government).
chemical reactions because PAN (peroxyacetylnitrate) and oxidant concentrations behaved similarily with them. Moreover, NO2values were observed to be inversely correlated with NO values. This is considered to be due to the consumption of NO by O3to form NOz according to NO + O3= NO2 02. Thus, the measured oxidant concentration became low because O3was simultaneously consumed. Nitrogenous species balance in the field, which have not been completely evaluated so far because of lack of measurement of particulate nitrate (18)or gaseous nitric acid (9), could be examined for the results given in Figure 5 by consideration of the following nitrogenous gas-nitrate distribution factor as a modification of the gas-particle distribution factor by Grosjean and Friedlander (20):
+
Fn =
+ particulate NO3- + gaseous NO3NO + NO2 + P A N + particulate NO3- + gaseous NO3PAN
In this case, terms of PAN and gaseous NO3- (which refers to gaseous nitric acid) are added because they are also nitrogenous species converted from NO, in the atmosphere. The distribution factors on our ambient measurements were found to be 1.71%, which is similar to an average value of 1.34% through the first day, during 90C-lO:OO JST and 15.1% during 1200-13:O JST on the second day. It is considered that under these meterological conditions, air pollutants were blown to Tokyo Bay until about 11:OO JST and then carried back to our sampling site which is located nearby without the significant addition of pollutants because sources in the bay were negligible. Thus, the apparent total nitrate formation rate is evaluated to be 4.5 % / h as an upper limit from the gradient
740
Anal. Chem. 1985, 57,740-743
of F n between 9:OO-1O:OO and 12:OO-13:OO ignoring NO, removal rate in the neighborhood of downtown Tokyo in summer. This is considered to be comparable to nitrate formation rates evaluated in the plume chemistry by Meagher et al. (41, Richars et al. (5),and Forrest et al. (6). In this case, the volatility of "*NO3 (21-23) and the reaction of sulfuric acid with nitrate (24),which have been discussed as artifact nitrate terms, might result in the decrease of particulate nitrate measurement on a Teflon filter and the increase of nitric acid measurement on a polyamide filter. However, shorter time interval sampling would suppress the artifact effects to the greater extent because it would decrease the contact time of sampled nitrate with ambient sulfuric acid and increase the possibility of keeping the equilibrium with respect to ",NO3 through the sampling time. The 1-h interval sampling in this study is thought to help avoid artifact production although 2 h to 1 days inverval samplings have been made so far in the low volume filer sampling (25-28). Registry No. NO3-, 14797-55-8.
LITERATURE CITED (1) Spicer, C. W. Atmos. Environ. 1977, 7 7 , 1089-1097. (2) Orel, A. E.; Seinfeld, J. H. Environ. Sci. Techno/. 1977, 7 7 , 1000- 1007. (3) Appel, B. E.; Kothny, E. L.: Hoffer, E. M.; Hidy, G. M.; Wesolowski, J. J. Envlron. Sci. Techno/. 1978, 72,418-425. (4) Meagher, J. F.; Stockburger, L., 111; Bonanno, R. J.; Bailey, E. M.; Luria, M. Afmos. Envlron. 1981, 75,749-762. (5) Richards, L. W.; Andersen, J. A,; Blumenthal, D. L.; Brandt, A. A. Atmos. Environ. 1981, 75,2111-2134. (6) Forrest, J.; Garber, R.; Newman, L. Atmos. Environ. 1981, 75, 2273-2282. (7) Spicer, C. W.; Howes, J. E., Jr.; Bishop, T. A.; Arnold, L. H.; Stevens, R. K. Atmos. Environ. 1982, 16, 1487-1500.
(8) Small, H.; Stevens, T. S.:Bauman, W. C. Anal. Chem. 1975, 4 7 , 1801-1809. (9) Mulik, J.; Puckett, D.; Williams, D.; Sawickl, W. E. Anal. Lett. 1978, 9 , 653-663. (10) Morris, A. W.; Riiry, J. P. Anal. Chim. Acta 1963, 29, 272-283. (11) Sandberg, J. S.;Levaggi, D. A.; Demandel, R. E.; Slu, W. J . Air folluf. Control Assoc. 1976, 26,559-564. (12) Moskowitz, A. H. "Particle Size Distribution of Nitrate Aerosols in the Los Angeles Air Basin"; EPA/600/3-77/053, 1977. (13) Spicer, C. W.; Schumaker, P. M.; Kouyoumijian, J. A.; Joseph, D. W. "Sampling Methodology for Atmospheric Particulate Nitrates"; EPA/ 60012-781367, 1978. (14) Cox, R. D. Anal. Chem. 1980, 52,332-335. (15) Bayliss, N. S.;Watts, D. W. Aust. J . Chem. 1963, 76,943-946. (16) Ingold, C. K.; Millen, D. J.; Poole, H. G. J . Chem. SOC. 1959, 2576-2587. (17) Bayliss, N. S.;Watts, D. W. Ausf. J . Chem. 1958, 9 , 319-329. (18) Spicer, C. W. Reaction of NOx in Smog Chambers and Urban Atmospheres, Formation and Fate of Atmospheric Nitrates, Workshop Proceedings; Barnes, H. M., Ed.; EPA-600/9-81-025, 1981. (19) Spicer, C. W.; Joseph, D. W.; Ward, G. F. "Studies of NOx reactions and O3 Transport in Southern California-Fall, 1976"; PB 83-200 360, 1983. (20) Grosjean, D.; Friedlander, S. K. J . Air folluf. Control Assoc. 1975, 25, 1038-1044. (21) Kadowaki, S.Atmos. Environ. 1977, 77, 671-675. (22) Stelson. A. W.; Friedlander, S. K.; Seinfeld, J. H. Atmos. Envlron. 1879, 73,369-371. (23) Yoshizuml, K.; Okita, T. J . Air follut. Control Assoc. 1983, 33, 224-226. (24) Harker, A. 6.; Richards, L. W.; Clark, W. E. Atmos. Environ. 1977, 77, 87-91. (25) Okita, T.; Morimoto, S.: Izawa, M. Atmos. Environ. 1976, 10, 1085-1089. (26) Forrest, J.; Tanner, R. L.; Spandau, D.; D'Ottavio, T.; Newman, L. Atmos. Environ. 1980, 74, 137-144. (27) Grennfelt, P. Afmos. Environ. 1980, 1 4 , 311-316. (28) Cadle, S.H.; Countess, R. J . ; Kelly, N. A. Atmos. Environ. 1982, 76, 2501-2506.
RECEIVED for review September 26,1984. Accepted November 19, 1984.
Estimation of Acidity in Rainfall by Electrical Conductivity Robert A. Stairs* and Jaleh Semmler' Department of Chemistry, Trent Uniuersity, Peterborough, Ontario, Canada K9J 7B8
Measurement of electrical conductivity of a sample of rain, or other natural freshwater, and of the same sample after passage through a bed of ion exchange resin in H+ form, allows the calculation of acidity (or alkalinity) and salinity. Results for 64 samples of rain, compared wlth those based on titration, are as follows: for acidity C(K)= -0.0070 -k 1.0809C(t), f0.0135, from -0.05 (alkaline) to 0.160 mequiv L-'; for salinity, C(K)= 0.0017 4- 0.9996C(t), f0.0191, from zero to 0.330 mequlv L-'. The method is suitable for automation.
The method described here was conceived in the context of an acid precipitation study. It is, however, adaptable with caution to the proximate analysis of freshwaters generally. The principle is simple. All ions contribute to the conductivity of a solution in direct proportion to their concentrations in equivalents per unit volume and their equivalent conductances. A solution containing a single ionic solute, such as hydrochloric acid, requires only a single conductance measurement for analysis. Two solutes, for instance HCl plus Present address: D e p a r t m e n t of Chemistry, U n i v e r s i t y of Waterloo, Waterloo, Ontario, Canada N2L 3G1.
NaC1, require two measurements. The second measurement is performed after the solution has been allowed to pass through a column of a strongly acidic, cation exchange resin, which will replace the sodium ions with hydrogen ions. The two conductances should conform to eq la,b where K~ K~ = hac, h,C, (la)
+
K~
=
hac,4- hac,
Ob)
and x2 are the measured conductances, A, and A, the equivalent conductances of HC1 and NaCl, and C, and C, the corresponding concentrations. The values of A, and A, should be appropriate to the conditions (temperature and concentration). If the solutions are sufficiently dilute, as rainwater may be expected to be, the limiting values Ao may be used without serious error. They are obtained from the single-ion values ioi, representative examples of which are listed in Table I ( I ) . If correction is necessary, a brief iteration may be used, with A a suitable function of the ionic strength. Rainwater usually contains appreciable amounts of an assortment of ions: Ca2+,Mg2+, K+, Na+, NO3-, C1-, and H+ or HC03- (not both), as well as COz. The value of Xo for H+ is uniquely large. Those for the other cations cluster around 0.062 (&O.Oll) S cmm2mequiv-l, and the anions (except OH- and HC03-) are similarly clustered, around 0.076 (f0.005). This leads to the expectation that treatment of
0003-2700/85/0357-0740$01.50/0 0 1985 American Chemical Society
",+,
