Literature Cited Altshuller, A. P., Intern. J . Air Water Pollut. 10,713 (1966). Altshuller, A. P., Bufalini, J. J., Photochem. Photobiol. 4, 97 (1965). Altshuller, A. P., Cohen, I. R., Intern. J . Air Water Pollut. 7, 787 (1963). Altshuller, A. P., Klosterman, D. L., Leach, P. W., Hindawi, I. J., Sigsby, J. E., Jr., Intern. J. Air Water Pollut. 10, 81 (1966). Berliner, E., Berliner, F., J . Ainer. Chem. Soc. 72,222 (1950). Caplan, J. D., “Smog Chemistry Points the Way to Rational Vehicle Emission Control,” Society of Automotive Engineers International West Coast Meeting, Vancouver, B. C., Canada, Aug. 1965. Cox, J. D., Tetrahedron 19,1175 (1963). Cvetanovic, R. J., Can. J . Chem. 38, 1678 (1960). Dimitriades, B., Whisman, M. L., Division of Water, Air, and Waste Chemistry. 155th Meeting. ACS. San Francisco, Calif., April 1968.’ Egloff, G., “Physical Constants of Hydrocarbons,” Vol. 11, Rheinhold. New York. 1940. Glasson, W. A., Tuesday, C. S., J . Amer. Chem. Soc. 85,2901 (1963). Glasson, W. A., Tuesday, C. S., Division of Water, Air, and Waste Chemistry, 153rd Meeting, ACS, Miami Beach, Fla., April 1967. Glasson, W. A., Tuesday, C. S., Division of Water, Air, and Waste Chemistry, 149th Meeting, Detroit, Mich., April 1965. Haagen-Smit, A. J., Fox, M. M., Ind. Eng. Chem. 48, 1484 (1956). Heuss, J. M., Glasson, W. A,, ENVIRON. SCI.TECHNOL. 2,1109 (1968). -I
Hurn, R. W . , Dimitriades, B., Fleming, R. D., “Effect of Hydrocarbon Type on Reactivity of Exhaust Gases,” Society of Automotive Engineers, Chicago, Ill., May 1965. Kopczynski, S. L., Intern. J . Air Water Pollut. 8,107 (1964). Korth, M. W., Stahman, R. C., Rose, A. H., Jr., J. Air Pollut. Contr. Ass. 14, 168 (1964). Leighton, P. A., “Photochemistry of Air Pollution,” Academic Press, New York, 1961. McReynolds, L. A,, Alquist, H. E., Wimmer, D. B., “Hydrocarbon Emissions and Reactivity as Functions of Fuel and Engine Variables,” Society of Automotive Engineers, Chicago, Ill., May 1965. Prager, M. J., Stephens, E. R., Scott, W. E., Ind. Eng. Chem. 52, 521 (1960). Schuck, E. A., Doyle, G. J., Rept. No. 29, Air Pollution Foundation, San Marino, Calif., Oct. 1959. SteDhens. E. R.. Scott. W. E.. Proc. Amer. Petrol. Inst. 42111. 665 (1962). ’ Stevenson, H. J. R., Sanderson, D. E., Altshuller, A. P., Intern. J. Air Water Pollut. 9. 367 (1965). Trotman-Dickenson, A. F. Birchard, J. R., Steacie, E. W. R., J . Chem. Phys. 19,163 (1951). Tuesday, C. S., in “Chemical Reactions in the Lower and Upper Atmosphere,” R. D. Cadle, Ed., Interscience, New York, 1961, p. 15. Tuesday, C. S., Arch. Enciron. Health 7,72 (1963). Wei, Y. K . , Cvetanovic, R . J., Can. J . Chem. 41,913 (1963). Wright, F. J., 10th Symposium (International) on Combustion, Combustion Institute, Pittsburgh, Pa., 1965, p. 387. Receiced for reeiew December 16, 1968. Accepted June 3, 1970. Presented in part at the Dicision of Water, Air, and Waste Chemistry, 150th Meeting, ACS, Atlantic City, N.J., Sept. 1965.
Comparison of Permeation Devices and Nitrite Ion as Standards for the Colorimetric Determination of Nitrogen Dioxide Frank P. Scaringelli, Ethan Rosenberg, and Kenneth A. Rehme Public Health Service, Environmental Health Service, National Air Pollution Control Administration, Cincinnati, Ohio 45226
rn The most commonly used procedure for determination of the oxides of nitrogen in air is the colorimetric method of Saltzman. The ratio of the quantity of dye produced from nitrogen dioxide to that produced from nitrite ion has been the subject of considerable controversy. Permeation tubes were used to prepare standard atmospheres at realistic levels, 0.02 to 1.8 p.p.m. of NOn. Precautions are enumerated for the use of permeation devices in preparing standard concentrations of NO? in air. After hundreds of analyses we established that 1 mole of nitrogen dioxide is equivalent to 0.764 i 0.005 mole of nitrite ion with use of the unmodified Saltzman reagent. _ _ _ _ ~
~
T
he most widely used procedure for determining oxides of nitrogen in the atmosphere is the one described by Saltzman (1954). This method is one of the most sensitive colorimetric procedures for nitrogen dioxide. It has several other advantages: all of the reagents are incorporated into one solution, the solution is an efficient collection medium for gaseous nitrogen dioxide, and the color is partially developed during sampling. The latter is particularly advan924 Environmental Science & Technology
tageous when unknown concentrations of NOz in air must be measured, since the sampling time can be adjusted to provide sufficient color for an accurate photometric measurement. Since the method’s inception, however, considerable controversy has raged concerning the stoichiometric factor involved in the reaction. Saltzman standardized the colorimetric procedure with sodium nitrite and compared the quantity of color produced with nitrite with that produced with nitrogen dioxide. His analyses showed that 1 mole of nitrogen dioxide was equivalent to 0.72 mole of nitrite ion. Since that time, values of the stoichiometric factor ranging from 0.5 to 1.00 have been reported (Gill, 1960; Kooiker, Schuman, et al., 1963; Saltzman and Wartburg, 1965; Shaw, 1967; Stratmann and Buck, 1966). Buck and Stratmann (1967) maintain that the stoichiometric factor varies with concentration and approaches 1 at levels of NOncomparable to that found in the atmosphere. By definition, stoichiometry is the science of combining proportions; therefore, by this definition the stoichiometric factor, as reported by Stratmann and Buck (1966), is probably correct-Le., 1 mole of nitrogen dioxide is equivalent to one mole of nitrite, if an efficiency of collection and reaction of 100% is obtained. To avoid ambiguity, we will define an
efficiency factor, as reported herein, to be the efficiency of reaction or reaction yield under experimental conditions. The variation in stoichiometric factor-really, efficiency factor-reported by other investigators may be explainable by differences in reagents or experimental conditions. Compounding the problems of evaluating the stoichiometric factor are the difficulty of efficiently collecting the oxides of nitrogen and the lack of a procedure to produce primary gaseous standards of N02-a problem because of its high reactivity. For these reasons, color formed in the Saltzman procedure is usually calibrated against aqueous nitrite solutions. With the advent of permeation tubes (O'Keeffe and Ortman, 1966, 1969; Scaringelli, Frey, et a/., 1967) for thepreparation of known concentrations of gaseous pollutants in air, it seemed feasible to use this technique to determine the efficiency of the reaction involved in the Saltzman method. Heretofore, evaluations of the efficiency factor were done with static systems or at NOz concentrations higher than that in the atmosphere. Investigators who did determine the factor at low levels had to work with dilution systems, with chemically evaluated secondary standards, or both. Using permeation devices, which were calibrated gravimetrically, we were able to prepare concentrations of NO? in air down to the parts-per-billion range. After hundreds of analyses in which problems were resolved as they arose, we found that the efficiency factor remains constant at a value of 0.764 i 0.005. Experimental Apparatus. For preparation of permeation devices, detailed descriptions are given by O'Keeffe and Ortman (1966, 1967) and by Scaringelli, Rosenberg, et a/. (1970). For gravimetric calibration of permeation devices, see Scaringelli, Rosenberg, et a / . (1970). DETECTOR. Mast ozone meter, or Mast NO2 meter, electrometer, and potentiometric recorder, 1 mV. ABSORBERS. All-glass bubblers with frits of maximum pore size of 60 p (Ace Glass, catalog no. 7530-05, designation C , 18/7-29/42). FLOWMETERS. Capacities of flowmeters 0 to 1 liter and 0 to 10 liters, calibrated against a wet-test meter. A dry-test meter, 1 liter per revolution, was calibrated with a spirometer. REAGENTS.All reagents were prepared from analytical grade chemicals in nitrite-free water unless otherwise specified. The distilled water was redistilled from an all-glass apparatus containing crystals of potassium permanganate and barium hydroxide. Reagents were prepared as described by Saltzman (1965), with the following exceptions. COUPLING AGENT.N-( 1-Naphthyl)-ethylenediamine dihydrochloride (Matheson, Coleman and Bell); 1.00 g. was dissolved in 100 ml. of 1 acetic acid instead of water. This stabilized the coupling agent for periods greater than several months when kept in the dark. ABSORBING REAGENT. Sulfanilic acid was recrystallized from water, filtered, and dried overnight at 120" C. Sulfanilic acid can occur as the anhydrous, monohydrate, and dihydrate, depending upon the temperature of recrystallization. This chemical, obtained from various suppliers, indicated various degrees of hydration. The absorbing reagent was prepared by dissolving 5 g. of sulfanilic acid (5.52 g. of monohydrate) in about 800 ml. of hot doubly distilled water. After the solution cooled, 140 ml. of glacial acetic acid and 20 ml. of the solution of the coupling agent were added. The solution was brought to 1-liter volume with doubly distilled water. This procedure is more efficient than the one described by Saltzman
(1954), because sulfanilic acid dissolves with difficulty in acetic acid solution at room temperature. STOCKSOLUTION OF SODIUM NITRITE.0.2 g. per liter. This solution was standardized repeatedly for a month accordiiig t o Procedure 2 in Kolthoff and Sandell (1948). N o appreciable change in concentration was discernible. WORKING SOLUTIONS OF SODIUM NITRITE.Dilute solutions, 0.02 g. per liter, of nitrite for preparation of the static calibration were checked repeatedly with a colorimetric procedure. This procedure was a modification of the permanganate procedure of Kolthoff and Sandell (1948). The absorbance of a dilute acid permanganate solution was measured before and after the addition of the dilute nitrite solution. The decrease in absorbance of permanganate solution was proportional to the concentration of nitrite ion. Dilute solutions of nitrite remained stable for at least several months. Analyses. Several hundred samples were analyzed over a period of two years with several permeation devices. All absorbance readings were made after at least 0.5-hr. color development. Measurements were made against a distilled water blank with the spectrophotometer set at 550 nm. At the same time, measurements were taken of the unexposed absorbing reagent and of the aerated solution in the second bubbler. Permeation tubes of the single-walled variety containing liquefied NO, were prepared according to the procedure of O'Keeffe and Ortman (1966). When these tubes were not satisfactory, double-walled and triple-walled tubes and drilled rods of FEP Teflon were prepared. Subsequently, O'Keeffe and Ortman (1967) provided us with microbottles. These devices were all calibrated gravimetrically at 25" C. with the apparatus and procedure of Scaringelli, Rosenberg, et a/. (1970). When the permeation devices reached steady state, as indicated by the regression plot, one of the devices was selected and inserted into the condenser of the apparatus as described by Scaringelli, Rosenberg, et al. (1970). This apparatus consists of a constant-temperature bath capable of maintaining the temperature within *0.1" C. A water pump in the bath circulated water through the condenser and maintained the temperature at 25" C. + 0.1". A thermometer inside the condenser and upstream of the permeation device checked the temperature of the gas stream A stream of dry cylinder air, conditioned to the temperature of the bath, passed over the device, picked up the permeated NOz, and carried it to the dilution stream. The dilution stream of dried, purified air was regulated with a Conoflow and the rate of flow was measured with a dry-test meter that had been calibrated with a spirometer. Both streams were thoroughly mixed in a Kjeldahl trap and exhausted to a hood. Two glass tees downstream of the mixer served as sampling ports. One port was attached to the bubblers and the other to the inlet of the Mast instrument. The latter provided a continuous measurement of NO, in the air stream. With this apparatus, we prepared a wide range of concentrations from parts-permillion to parts-per-billion of NO, in air. T o provide a continuous measure of the concentration of NO, and to increase the sensitivity of the Mast, we amplified the current from the Mast with an electrometer and fed the signal to a 1-mV potentiometric recorder. The Mast instruments were calibrated by varying the flow of dilution air that mixed with a constant flow of cylinder air that passed over the permeation device. The current in microamperes was plotted against concentration in p.p.m. The permeation rate from the device was determined gravimetrically. COLLECTION SYSTEM. After the concentration of NO2 was constant, as indicated by the Mast instrument, the samples Volume 4, Number 11, November 1970 925
were absorbed into two all-glass bubblers in series. The frits of the bubblers were checked frequently for pore size by the procedure described by Saltzman (1969, particularly if low values of NO, were obtained. Those bubblers that had or developed pore sizes larger than 60 p were rejected. Pore sizes of these frits changed with age. Bubblers containing the absorbing reagent were weighed before and after collection of the sample to correct volume shrinkage caused by the volatility of the acetic acid and water. Flow through the bubblers was held constant at a rate of 315 ml./min. by a critical orifice (hypodermic needle, Lodge, Pate, et al., 1966). The needles were calibrated under simulated sampling conditions with a wet-test meter. At this flow rate, the collection efficiency was 100 %, as indicated by the absence of pink color in the second bubbler. The hypodermic needle was protected froin plugging by a trap containing a membrane filter. A flow meter was placed in line to ensure that the flow rate did not change. Sampling times were arranged so that the absorbances of the solutions were at least 0.1 for a 1-cm. optical cell. When sampling times were impractical with this cell because of low levels of NOs, we used a cell with a pathlength of 5 cm. Gill (1960) used Pb(NO& to prepare solutions of NOs in air. Stratmann, Buck, et a/. (1966) also used Pb(NO& to calibrate the Griess-Saltzman reagent. We decided to check this approach. The apparatus consisted of a cylinder of oxygen, a molecular sieve trap, a breech connector, a programmable furnace, and a quartz tube containing a platinum gauze. The bore of the quartz tube was large enough to accommodate a small platinum boat upstream of the gauze. A ball-joint was attached to the outlet side of the quartz tube with Teflon to accommodate either the fritted bubbler or the Mast instrument. We prepared a dilute solution of 0.4x Pb(NO&; transferred a small aliquot, 2 to 12 J., with a microsyringe to the platinum boat; and evaporated the water at 110 O C. With the oxygen flowing through the system a t a rate of 140 ml./min. we attached the Mast to the outlet of the system and obtained a baseline. We then opened the breech, inserted the boat, and manually programmed the temperature of the furnace. A thermocouple touching the outer wall of the quartz tube and connected to a potentiometric recorder, 50 mV, gave a continuous measurement of the temperature of the quartz tube with time. Simultaneously, the current output of the Mast instrument was recorded. A Gaussian peak appeared on the recorder attached to the Mast instrument. It indicated that the temperatures of initial and energetic decomposition (peak response) were 192" C. i 15" and 271" C. i 20", respectively. Therefore, the concentration of NOu in the oxygen stream was not constant, but varied from 0 to 1 p,p.m., depending upon the size of the sample. These experiments were repeated at various programming rates. After the best conditions were selected, we replaced the Mast with two fritted bubblers in series and repeated the experiments about 20 times.
Results and Discussion Permeation Devices. During the course of this study, approximately two years, over 40 permeation devices were Table I. Typical Results Obtained with Permeation Devices Permeation rate, Relative p g ./min. Device error, Double-walled 3 . 1 4 i 0.026 0.83 Triple-walled 0.258 f 0.005 1.94 Drilled rod 0.593 i 0.010 1.69 Microbottle 0.454 i 0.007 1.54 926 Environmental Science & Technology
calibrated by periodic weighings over several months. Gross weight of the devices was plotted against time before, during, and after their use in gaseous calibration of the GriessSaltzman reagent for NO*. These devices should be used during the period of steady state only. Some devices showed a slight curvature with time in the region of steady state. The reasons for the deviation from linearity cannot be thoroughly explained by existing data; it could be caused by memory effects or other physical phenomena peculiar to NO,. In any event, since the curvature is slight, these devices can be used if calibration is continued during the period of use. The slope of the line during steady-state conditions was evaluated statistically by linear regression with the leastsquares method. This treatment of the data gives the average slope of the line and the confidence limits to be placed on the value of the slope. Table I gives a few typical examples of the statistical data. In general, with a permeation rate of 0.1 to 4.0 pg./min., the relative deviation of the slope ranged from 0.5 to 3 z . The lower the output of the devices, the greater was the relative deviation. Single-walled tubes were not satisfactory for producing low levels of NOz with the described system. The permeation rate through the Teflon wall was too high, despite the low partial pressure of NO, (b.p. 21.15' C.) inside the tube. This indicates that NOz has greater solubility in FEP Teflon than d o other gaseous air pollutants. When flow rates of diluent air exceeded 15 liters/min., a pressure buildup occurred along the tubing in the system. Therefore, the dilution system could not be used unless larger-bore glass tubing was incorporated into the system. The use of larger tubing, however, would not be practical because large quantities of air would have to be purified. Reducing the permeation rate by operating at lower temperatures was unsatisfactory. Apparently, appreciable sorption occurs along the walls of the glass tubing. The permeation devices, therefore, were calibrated and standardized at 25 O C., close to room temperature. To reduce the permeation rate, we devised thicker-walled tubes, and O'Keeffe and Ortman supplied us with microbottles. Calibration of the Mast Instruments. Figure 1 shows the calibration of the Mast instruments. Initially, we used the Mast ozone meter. When the Mast NO, meter arrived, we calibrated it. Sensitivity for NO, was lower and standing current was higher with the NO, meter (line B) than with the Mast ozone meter (line A). No attempt was made to optimize the reagent or electrochemical conditions of the Mast instruments. These instruments were used according to the instructions of the manufacturer, except for the amplification of the current t o provide higher sensitivity. With the Mast attached t o one of the sampling ports, we calibrated the instrument by varying the flow rate of the diluent air. This flow rate was measured accurately with a dry-test meter. The flow rate of cylinder air was kept constant by means of a critical orifice operating a t critical pressure. Cylinder air was necessary because even small amounts of moisture reacted with NO2 on the surface of the Teflon, reducing the concentration of NO, in the gas stream. Once the NOn was in gas phase, no reaction with moisture was discernible even with a longer reaction time. A plot (not shown) of the current produced by the Mast 6s. the reciprocal of the air flow rate gives a straight line. Therefore, the permeation rate was constant and flow rate was measured accurately. Figure 1 also shows the results of simultaneous analyses, electrochemical (line A) and colorimetric (line C ) , of the gas
?.C-
I
1
COMPARATIVE ANALYSES &MAST (03) ..B
- -C MAST WOz) -0-
5 +-
1.6
-
1.4
-
1.2
-
COLORIMETRIC
4
&
1.0-
W
9
hc
0.8
-
0
0.2
0.4
0.6
0 8
1.0
1.2
1.4
1.6
ADDED N02, ppm
Figure 1. Simultaneous colorimetric and electrochemical analyses with permeation devices stream prepared with a double-walled permeation tube. With this system, we were able to produce concentrations of NO2 in air from 0.2 to 1.6 p.p.m. The amount of NOz in p.p.m. was computed from the permeation rate divided by the sum of the dilution flows of the air streams. The y-axis at the left indicates the amount of current produced by the particular concentration of NOz added. The sampling rate of the Mast was 140 ml./min. The y-axis at the right indicates the quantity of color formed per minute with the Griess-Saltzman reagent under exact conditions, but with a collection flow rate of 3 16 ml./min. Although two different chemical reactions are involved in the analyses, both yield linear responses. Hence, there is no change in the efficiency factor from 0.2 to 1.6 p.p.m. Stoichiometric Factor. I n determination of the stoichiometry of the Griess-Saltzman, the initial data, before corrections were applied, indicated a changing factor approaching 1, as stated by Stratmann, Buck, et al. (1966). Then we found that appreciable errors were resulting with long sampling time. A long sampling time was necessary to obtain sufficient color for accurate analyses at parts-per-billion levels of NOz. We decided to record the weight of the bubblers containing the reagent before and after collection of the sample. The average rate of evaporation, as determined from weight loss, was 7.6 mg./min. This average rate was computed from 70 analyses and was linear from 0 to 400 min. Therefore, for a sampling time of 2 hr. (120 min.), the effect would be to shrink the volume of the solution approximately 10%. The average concentration of acetic acid in the volatilized material was 7 Z v./v., as determined by collection in NaHCOa and back titration. The concentration of acetic acid in the reagent was 1 4 z , hence, the acidity of the absorbing solution was increasing. The net effect of volume shrinkage is to indicate a concentration of NO? that is high by at least 10% for a 2-hr. sample. Longer sampling times would yield a proportionally greater error. This difference does not account for the differences of approximately 25 % in the efficiency factor.
Another problem also tends to increase the value of the efficiency factor. In operation at a sampling rate of 315 ml./min. or below, with a fritted bubbler having a maximum pore size of 60 p, no pink color appears in the second bubbler. A yellow color does appear in the second bubbler after prolonged aeration or exposure to intense light. The quantity of yellow color produced does not correlate well with sampling time and appears to be caused by light rather than aeration. This color is probably a result of destruction or oxidation of the coupling agent. Similar exposure of the diazo dye, which was formed from the reagent and gaseous NOz produced only a slight fading of color (reduction in total absorbance of the solution). In the measurement of the absorbance of two bubblers in series, one would normally expect to add the absorbance of the second bubbler to that of the first, for this would indicate a low collection efficiency. If this yellow color is due to destruction of the reagent and if the same yellow color is produced in the first bubbler, the absorbance of the second bubbler should be subtracted from, not added to, that of the first bubbler. The obvious solution, if this is the case, is to shield the bubblers from light and to collect the sample at a flow rate of less than 400 ml./min. for periods of less than 1 hr. At these flow rates no pink color appears in the second bubbler. Under these conditions, therefore, an increase in absorbance of the second bubbler is not the result of the diazo reaction with nitrite ion. For more exact results, a second bubbler should be used at these flow rates and the absorbance of the yellow color at 550 nm. subtracted from the absorbance of the first bubbler. The magnitude of the error produced by the yellow color can vary from 0 to 40 %. Precise results are obtainable for the analyses of NOr under carefully controlled conditions. Under these conditions, the empirical value for the efficiency factor remains constant. The recommended parameters for improving the precision of the analyses for NO2 are tabulated in Table 11. No attempt was made in this study to optimize reagent parameters or to investigate side reactions. Several modifications of the Saltzman reagent have resulted from studies in which the objective was to increase the rate of color formation rather than reaction yield. In fact, Stratmann and Buck (1966) evaluated the stoichiometric factor with the modified Saltzman
Table 11. Recommended Parameters for the Analysis of Nitrogen Dioxide 1 hr. or less 400 ml.jmin. or less 15 ml.
Collection time Flow rate Volume of absorbing reagent Bubbler, fritted
60-p pore size. Use two bubblers in series or shield from light 0.01 to 0.3 p.p.m. Higher concentrations use shorter sampling times or larger volume of absorbing reagent 550 nm No pink color; subtract absorbance of yellow color at 550 nm. from the total absorbance of the first bubbler 5-cm. pathlength for absorbances below 0.1 absorbance units
Range
Wavelength, X Second bubbler
Optical cell ~
Volume 4, Number 11, November 1970 927
reagent. One would, therefore, hesitate to compare the results. There is, however, some experimental evidence which indicates n o nitrates are formed when low concentrations of NOn dissolve in alkaline solution. This would seem to substantiate a theoretical stoichiometry of one, but we would like t o see a more definitive study. Some nitric oxide evolves from the solution during the collection of NOn, but reported values are generally a few percent of the total NOs. Our treatment of the data was as follows: The parts-permillion of NO2found as nitrite were plotted against the partsper-million of NOL added, computed from the gravimetric data of the permeation tube. (Parts-per-million values are directly proportional t o the number of moles.) The only assumption is that qualitatively the dye produced from nitrite is identical to the dye produced by NOs. From this plot, the slope of the line is equal to the stoichiometric factor for the overall reaction. If the slope is 1, then 1 mole of NOs is equivalent t o 1 mole of nitrite ion. In Figure 2, we show this type of plot in the range of 0.02 to 0.3 p.p.m. from the data obtained with drilled rods as a typical example. Over 30 colorimetric analyses were included in this experiment. The volume of absorbing reagent was increased t o 15 ml. to minimize the shrinkage correction, to increase contact time, and to provide sufficient quantity for colorimetric analyses in a 5-cm. cell. Corrections for the destruction of the reagents were made in the manner previously indicated. The time of sampling varied from 20 to 60 min. For NOL concentrations less than 0.08 p.p.m., a 5-cm. optical cell was used to measure the absorbance, since the absorbances of these solutions were usually less than 0.1. The air that came in
o,2,11-,~,---.T--T-c.20
t
~-
D R I L L E D ROD NUMBER 1 CO L OR1hl E T R 1 C A N A L Y SES 0 .SINGLE POINTS * . M U L T I P L E POINTS
@ . l a b
1 Figure 2. Colorimetric analyses with drilled rod no. 1
"T
1
Figure 3. Summary of stoichiometric factor with various permeation devices 928 Environmental Science & Technology
-1
'
STRATMANN
I
I
I
METHOD
'
I
I
/.-- --1 /
A D D E D N 3 2 [ A S P b l N O 3 1 2 ] , p9
Figure 4. Colorimetric analyses with lead nitrate
contact with the drilled rod or microbottle was dried cylinder air. The slope of the regression equation was 0.746 f 0.005 for the drilled rod. Other permeation devices gave similar results. Finally, we plotted the data from over 30 analyses made with various permeation devices selected a t random. The complete stoichiometric factor for each determination was plotted against the respective concentration range. All analyses were performed under carefully controlled conditions. The results are summarized in Figure 3. The dotted line represents the statistical mean and the solid lines indicate the 95 confidence interval. The value of the efficiency factor was constant, except in the very low parts-per-million range around 0.02 p.p.m. These latter values were low, probably because of reduced collection efficiency of the reagent or bubbler. However, no pink color was found in the second bubbler. Figure 4 is a plot of pg. NOs found as nitrite ion cs. the pg. of NO2 added as Pb(NO& These data were the results of our attempt to reproduce the experiments of Stratmann, Buck, et al. (1966). The major difference in the experimental conditions was that the bubbler used by Stratmann, Buck, et al., was not available to us. We did, however, thoroughly evaluate the system by programming the heat applied to the furnace at 23" C. per min., and by continuously monitoring the output of NOLwith the Mast as modified. The response of the Mast indicated a Gaussian-type curve with increasing temperature. Therefore, the concentration of NO, in the oxygen was not constant but varied from 0 to 1 p.p.m. The colorimetric analyses, in effect, gave the sum of the NOn in these concentration ranges. Samples were collected in 15 ml. of reagent and the absorbance was corrected for the oxygenated reagent blank. The result of linear regression of the data again indicated a slope or efficiency factor of 0.75 i 0.037. Calibration with lead nitrate may result in serious problems. Elevated temperatures downstream of the platinum catalyst decompose the NOz to NO. The rate of reoxidization of the nitric oxide with air at low concentrations is extremely slow. Hence, losses will result since the detectors or reagent d o not respond to nitric oxide. The efficiency factor determined with over 60 samples under the most carefully controlled conditions is 0.764 + 0.005. This determination was done for the first time at known partsper-billion concentrations of NOn. We were unable to reproduce the results of Stratmann, Buck, et al. (1966) with Pb(NO&. With our approximate version of their experimental system, our results for the efficiency factor were 0.75.
Acknowledgment The authors thank John P. Bell for calculations of standard deviation by computer. Literature Cited Buck, M., Stratmann, H., Stauh 27, 11-15 (1967). Gill, W. E., Amer. Ind. Hyg. Ass. J . 21, 87-96 (1960). Kolthoff, I. M., Sandell, E. B., “Textbook of Quantitative Inorganic Analysis,” rev. ed., Macmillan, New York, 1948, p. 603. Kooiker, R. H., Schuman, L. M., Chan, Y. K., Arch. Environ. Health7,13-32, (1963). Lodge, J. P., Pate, J. B., Ammons, B. E., Swanson, G. A,, J . Air Poll. Control Ass. 16, 197-200 (1966). O’Keeffe, A. E., Ortman, G. C., Anal. Chem. 38,760-63 (1966). O’Keeffe, A. E., Ortman, G . C., ibid., 39, 1047 (1967). O’Keeffe, A. E., Ortman, G . C., ibid., 41, 1598 (1969). Saltzman, B. E., Anal. Chem. 26, 1949-55 (1954). Saltzman, B. E., ‘Selected Methods for the Measurement of
Air Pollutants,” Public Health Service Publication No. 999AP-11, pages C-4 and C-5, (1965). Saltzman, B. E., Wartburg, A. F., Anal. Chem. 37, 1961-64
(1969. ~S c a r i n g h , F. P., Frey, S. A., Saltzman, B. E., Amer. Znd. Hyg. ASS.J . 28,260-266 (1967). Scaringelli, F. P., Rosenberg, E., O’Keeffe, A. E., Bell, J. P., Anal. Chem. 42 (87). 1970. Shaw. J. T., Atmis. Environ. 1, 81-85 (1967). Stratmann, H., Buck, M., Air, Water Pollut. 10, 313 (1966). Thomas, M. D., MacLeod, J. A., Robbins, R . C., Goettelman, R. C., Eldridge, R . W., Rogers, L. H., Anal. Chem. 28, 1810-16 (1956). \-
Received for review February 24,1970. Accepted June 15, 1970. Presented in part at the Division of Water, Air, and Waste Chemirtry, 157th Meeting, ACS, Minneapolis, Minn., April 1969. Also, presented in part at the 11th Conferencen j Methods in Air Pollution and Industrial Hygiene Studies, Uniwr rity of California, Berkeley, Calif., March 30, 1970. Mention of any conimerrial product in this paper does not constitufe endorsement by the National Air Pollution Control Administration.
Nitrogen Isotope Fractionation in Soils and Microbial Reactions C. C. Delwiche and Pieter L. Steyn University of California, Davis, Calif. 95616
The isotopic composition of soil nitrogen in profile was examined and the extent of nitrogen isotope discrimination in various microbial reactions of the nitrogen cycle determined. The abundance of nitrogen-15 at various depths in profile differed among the soils examined. These differences appeared to be a function of soil texture or texture-dependent factors, and the possibility of isotope fractionation by chromatographic processes is likely. Because of the many factors contributing to variations in isotopic composition of soil nitrogen, the extent of enrichment with nitrogen-15 cannot be considered a direct measure of the degree to which nitrogen cycling has taken place in a given soil. In the fixation of nitrogen, the oxidation of ammonium ion to nitrite by Nitrosomonas and the assimilation of ammonium ion by the several species examined all showed some discrimination in favor of the lighter (nitrogen-14) isotope.
T
he rare stable isotope of nitrogen, N16, constitutes approximately 0.366 atom of atmospheric nitrogen (Nier, 1950). This value is relatively constant in the atmosphere (Dole, Lane, et al., 1954), but the isotope distribution can differ significantly in other nitrogenous forms, particularly in biological systems. Work in these laboratories and elsewhere (Cheng, Bremner, et al., 1964) indicated not only a variation in the abundance of the rare stable isotope of nitrogen from one soil to another, but also differences in distribution in profile. These minor variations in abundance become of concern because of their bearing on isotopic tracer studies of nitrogen fixation, and the potential errors they might introduce into any determination of fixation rates. The purpose of the present study was to determine in greater detail the nature of these variations in
isotopic composition, to examine some of the factors contributing to their existence, to evaluate the significance of such variations as they affect observations on nitrogen cycling, and to examine their potential contribution to our understanding of the biogeochemistry of nitrogen. Materials and Methods Nitrogen was determined by a modification of microKje1dah1 methods essentially as described by Bremner (1965) with use of a boric acid receiver with no internal indicator and titrating to the end point with a p H meter. The distillate was then evaporated to near dryness and converted to nitrogen gas by oxidation with alkaline hypobromite. Two successive liquid nitrogen traps were used to remove possible volatile contaminants, and the isotope ratio was determined on a modified Consolidated-Nier Model 21-201 mass spectrometer with some alterations in circuitry and in mechanical features, including the substitution of an ion-getter pump for the diffusion pump with which the instrument was originally equipped. All samples were referred to a standard of ammonium chloride which in turn had been calibrated against atmospheric nitrogen. Since the purpose of the present study was to make comparative observations on isotope distribution, no effort was made to determine the absolute abundance of N15 in the atmosphere. The value of 0.366 atom “5, determined by Nier (1950) and established to be essentially constant by the work of Junk and Svec (1958), was used. Results Isotope Distribution in Profile. Studies were made of the isotopic composition of nitrogen in profile by using several soils of northern California which were part of a concomitant study of the rate of nonsymbiotic nitrogen fixation under field Volume 4, Number 11, November 1970 929
