NOTES
Improved PhenoldisulfonicAcid Method for Determination of NO, from Stationary Sources Herman H. Martens,’ Louis A. Dee, and John T. Nakamura Air Force Rocket Propulsion Laboratory, Edwards AFB, Edwards, Calif. 93523
Fred C. Jaye Environmental Protection Agency, Research Triangle Park, N.C. 2771 1
The accuracy and precision of the Environmental Protection Agency-mandated NO, analysis method were adversely affected by several uncontrolled variables. These were identified as silicate turbidity, carbon dioxide evolution, and chloride interference. Data reported herein show that silicates apparently adsorb a portion of the nitrophenoldisulfonic acid complex, while carbon dioxide evolution is responsible for loss of nitric acid during the anhydrous nitration step. In addition, the formation and escape of nitrosyl chloride are the mechanism of chloride interference. Problems due to turbidity and carbon dioxide are eliminated, whereas nitrosyl chloride formation is minimized by prior precipitation of chloride as lead chlorofluoride. Acceptable accuracy and precision were obtained for the determination of nitrate ion even in the presence of chloride.
Use of the currently mandated method for sampling and analysis for NO, (NO + N02) from stationary sources (Method 7, Federal Register, 1971) has resulted in excessive analysis time and poor accuracy and precision. These difficulties were encountered, even though verified laboratory gas standards were used which contained only NO or NO2 in nitrogen or helium. Method 7 consists of two parts: a sampling method in which an evacuated 2-liter borosilicate glass vessel containing 25 ml of absorbing solution (0.03% H202 in 0.1 N H2S04) is rapidly filled with sample gas and then allowed to stand for 16 hr to provide complete conversion of NO, to nitrate ion. The second part is an analysis method wherein the absorbing solution is quantitatively transferred to a borosilicate glass vessel, made alkaline with NaOH, and evaporated to dryness. The dry residue is reacted with phenoldisulfonic acid (PDSA), diluted with water, neutralized with concentrated NHIOH, and finally, the resulting yellow nitrochromogen is measured spectrophotometrically. Margolis and Driscoll ( 1972) have thoroughly discussed the reaction mechanism and kinetics of the NO, reaction with the acidic peroxide solution. In addition, they have suggested a means of significantly decreasing this reaction time by addition of excess oxygen to the sample vessel. Coulehan and Lang (1971) significantly reduced both the reaction and the analysis times for measurement of NO, in diesel engine exhausts (500-1000 ppm NO,). The reaction time was reduced by decreasing the gas sample volume to only 250 ml and continuously mixing the sample and absorbing solution. They reduced the analysis time by elimination of the evaporation step and substitution of an aqueous PDSA nitration technique, resulting in a small To whom correspondence should be addressed. 1152
Environmental Science & Technology
loss of sensitivity. Since Berger et al. (1970) reported that NO, emissions from stationary sources range from 20-1500 ppm, application of this technique to low NO, emission sources (e.g., gas-fired boilers) may be difficult. Some stationary sources contain hydrogen chloride (Bartok et al., 1971), and the interference to PDSA analyses due to chloride ion is reported (APHA, 1971; Chamot et al., 1911). Method 7, however, makes no provisions for the elimination of such. This report describes several sources of error in the Method 7 analysis procedure and the resolution of same. Experimental
Apparatus. Calibrated borosilicate glass 2-liter roundbottom flasks were used for gas sampling. A Cary Model 14 recording spectrophotometer equipped with matched 1-cm cells was used to measure the absorbance of the nitro-PDSA chromogen a t 405 nm. Platinum crucibles were used for evaporation of the absorbing solution. Reagents. All reagents were ACS grade except where noted. The PDSA was prepared in accordance with either the APHA (APHA, 1971) or EPA (Federal Register, 1971) directions. The PbF2 (K and K Laboratories) contained a large quantity of nitrate ion, and was extracted repeatedly with water until the extract contained less than 1 ppm N o s - . The purified PbF2 was then dried a t 130°C and 20 mmHg. Gas Standards. Gas mixtures of NO or NO2 with nitrogen (99.99% N2 min) or helium (99.995% He min) were prepared by partial pressure measurement using a differential pressure gauge (Wallace and Tiernan, Model FA145). Mixtures containing 1% NO were verified by mass spectrometric analysis (Bell & Howell CEC 21-490). More dilute mixtures of the above were prepared by dynamic flow dilution of the 1% standards. A 256-ppm NO/He mixture was prepared by partial pressure measurement and verified by gas chromatography. All flow measurements were performed using a gas buret and electric stopclock. Since calculated and verified values agreed well (no difference at the 95% confidence level), a 211-ppm NOIN2 gas standard was also prepared by partial pressure measurement and used without verification (the appropriate PV product was used for the N2). Two analysis procedures resulted from the following investigations; the first based on the absence of chloride when the effluent from gas- and oil-fired combustion sources are sampled, and the second based on the presence of chloride when the effluent from coal-fired power plants or smelters (ferrous and nonferrous) are sampled. Common to both procedures is the use of 25 ml of a neutral 3% hydrogen peroxide (prepared fresh weekly by dilution of 90% HzO2) absorbing solution in the 2-liter sample flask.
Analysis Procedure No. 1 (Chloride Absent). A 5-ml aliquot of the 25-ml absorbing solution was made alkaline with a drop of 1N NaOH and evaporated to dryness in a 15-20 ml platinum crucible. The residue was reacted with 2 ml of the PDSA, allowed to stand 2-5 min to dissolve all solids, and was transferred to a 100-ml volumetric flask with three 10-ml washings of distilled water. The volumetric flask was swirled in an ice bath while 8 ml of the 28% NH3 (concd. NH40H) were added from a Mohr pipet to develop the yellow color. The solution was diluted to 100 ml with distilled water prior to analysis. Analysis Procedure No. 2 (Chloride Present). The absorbing solution in the flask was transferred to a 50-ml plastic bottle and was made alkaline with 5 drops of 1N NaOH. About 0.1 gram of lead dioxide (PbO2) was added to decompose the H202, and after 30 min 20 ml of this solution were transferred to a 25-ml volumetric flask and diluted to the mark. A 10-ml aliquot of this solution was transferred to a 12-ml centrifuge tube containing -0.1 gram of lead fluoride (PbF2), shaken, placed in an ice bath for 10 min, and centrifuged in a clinical centrifuge a t 2000-3000 rpm for 10 min. The tube was returhed to the ice bath, and a 5-ml aliquot of the supernatant was transferred to a Pt crucible containing 2 drops of 1N NaOH. This solution was evaporated to between 1 and 2 ml on a 100°C hot plate. Then 2 ml of PDSA reagent were added and gently mixed. The crucible was returned to the hot plate for a minimum of 10 min (Coulehan and Lang, 1971) to complete the nitration reaction. The contents of the crucible were transferred to a 100-ml volumetric flask and the neutralization with NH3 was the same as in procedure No. 1. Calibration Standards. A concentrated standard (1 ml = 1000 pg NO2) was prepared by dissolving exactly 2.1980 grams of dried K N 0 3 in distilled water and diluting the mixture to exactly 1 liter. A working standard of 100 pg NOz/ml was prepared by a ten-fold dilution of the above and both were stored in tightly capped plastic bottles. Calibration solutions (100, 200, 300, and 400 pg N02) were prepared by appropriate additions of working standard to either platinum crucibles or plastic bottles and all were treated exactly as the samples. In addition, 300 pg of C1were added to each calibration solution prior to adding the PbF2, when procedure No. 2 was used.
Results and Discussion A calibration curve was prepared as specified in Method 7 by addition of known quantities of 100 pg NO2 working standard solution to 25-ml portions of acidic hydrogen peroxide. Large variabilities in absorbance a t each concentration were noted. Observed turbidity and gas evolution were believed to be related to this unacceptable scatter. Turbidity. An example of the effects of turbidity on absorbance measurements is shown in Table I. The turbidity, when present, was removed by filtration prior to the absorbance measurement. The decrease in absorbance apTable I. Effect of Turbidity Filtration on 300 pG NO, Calibration Standards
a
Turbidity level=
Color developrnentb as p g NO?
Clear Slightly turbid Slightly turbid Very turbid Very turbid
300 275 258 120 106
Prior t o filtration.
b
After filtration.
peared to be directly proportional to the quantity of turbidity removed. I t was noted that the glass evaporating dishes became progressively more etched with use, indicating that silicates were the source. Apparently some of the chromogen, nitro-PDSA, was adsorbed on the silicate particles and was lost during filtration. Substitution of 15-20-ml platinum crucibles for the glass evaporation dishes ensured clear solutions henceforth. Gas Evolution. Acceptable precision for calibration curves was still not obtained, even though the turbidity problem had been eliminated (Figure 1). Gas evolution had been noted during the anhydrous nitration step, as mentioned earlier. From a comprehensive study of the sources of errors (Chamot et al., 1911), it was postulated that carbon dioxide may be added to the standards via the neutralization of the acidic peroxide with 1N NaOH. The hypothesis was tested by adding known amounts of 1M Na2C03 to nitrate standards (250 pg NO2) and omitting the acidic peroxide. Table I1 shows the effect of increasing amounts of the Na2C03, while 10 replicates, also containing 250 pg NO2 and no co32- or acid, yielded as absorbance of 0.37 ( s y = 0.005). The precision of this latter data is quite acceptable. Thus, Table I1 confirmed Chamot’s findings that C02 evolution is a source of error likely due to co-evaporization of anhydrous HN03. This problem is not encountered when using the PDSA method for the determination of nitrate in potable water (APHA, 1971), since large amounts of NaOH for neutralization of the sample are unnecessary prior to evaporation. The absorbing solution was consequently modified by increasing
T
0 7 r
‘c m $
04
T 0 3 t
O2
t
I
P YE NO2 m l
Figure 1. Variability of
standards
s = standard deviation n = number of replicates per concentration
Table II.
Effect of COzon2 5 0 p G Calibration Standards
1 N NaC03 added,
Absorbance
ml
0 0.1 0.2 0.4 0.8
Table 111.
Equiv.
NO1
255 ~. 222 222 218 204
0.39
~
0.34 0.34 0.33 0.31
Effect of Chloride on 250 WGNO?Standards
Chloride added, p g
0 250 0 300
NO: recovered, # g Replicates
Anhy. nitration
Aq. nitration
4
250 rt 6a 218 rt 20a
250 i ga 221 i 7a 250 zt 15 231 i 15
4 8 8
... ...
PbF2omitted.
Volume 7, Number 13, December 1973
1153
0 180
t
40
30 0
I
I
100
200 PPm
Figure 2. NO
I
I 300
500
400
NO
recovery vs. concentration
Sixteen-hourabsorption time with 3% neutral
TIME-HOURS
H202
the peroxide concentration to 3% and omitting the HzS04. Four sets of 5-ml aliquots, (using this modified absorbing solution) to which standards were added, yielded a regression equation of > = 0.15 x - 0.003, where ? = absorbance, x = 100-400 pg N02/100 ml, and s, = A0.0025 (analysis procedure No. 1). Chloride. The chloride interference to the PDSA method has been amply documented (Chamot et al., 1911; APHA, 1971). Its presence as hydrogen chloride in stack gases from coal-burning power plants is known (Bartok et al., 1971). Method 7, however, makes no provisions for minimizing or eliminating the possible interference which may be caused by chloride. To determine the mechanism of chloride interference, dry NaCl and NaN03 were placed in a small glass vial fitted with a side arm and closures. The vial was evacuated and 1 ml of PDSA was introduced through the side arm. The evolved gases were allowed to enter an evacuated 10-cm gas cell and the infrared spectrum of the evolved gas (5000-650 cm-l) showed strong absorption bands a t 1760 cm-1 (5.7 p ) and 1780 cm-l (5.6 1)corresponding to the published absorption bands for NOC1. In addition, the presence of NO2 was also detected, presumably owing to the presence of residual HzO and 0 2 which would decompose NOCl to yield HC1 and NOz. Procedure 2 was devised to minimize the NO3- loss due to chloride ion. It was impossible to eliminate the interference completely as can be seen from Table 111, where both procedures were compared in the absence and presence of chloride. Significant differences between the means of the two sets without PbFz are noted; however, no significant difference exists a t the 95% confidence level between the means of the replicates when PbFz was added. While chloride is not completely removed, it is fixed a t a constant low concentration regardless of the original level in the sample. The following equation illustrates the equilibrium conditions between PbClF and its ions: PbClF e Pb2+
+ C1- + F -
(1)
The PbFz contributes excess Pb2+ ions, thus driving the equilibrium to the left and the residual C1- concentration is about 10-6M. NO and NO2 Gas Sample Recovery. Seven replicates of the 256 ppm NO/He standard were sampled using 3% neutral peroxide in the 2-liter flasks. After 16 hr, the samples were analyzed using procedure No. 1 and an average of 236 ppm NO was recovered (s = A7 ppm). This represents a 92% recovery. In addition, four replicates of the 211-ppm NO/Nz source were analyzed by procedure No. 1, and the NO recovered averaged 205 ppm (s = *11 ppm). Recoveries of a series of concentrations ranging 1154
Environmental Science & Technology
Figure 3. NO and NO2 absorption vs. time 0 256 ppm NO/He source
Unverified NOP/NPsource Absorption into neutral 3% H 2 0 2
from 58-540 ppm NO (each point represents the average of four replicates) are shown in Figure 2. It is apparent that 16 hr is an insufficient reaction time if the NO concentration is below 220 ppm NO owing to the kinetic considerations (Margolis and Driscoll, 1972). The grab sample technique of Method 7 is therefore not recommended for sources containing less than 200 ppm NO. When only the reaction time prior to analysis was varied for NO and NOz (Figure 3), it is evident that the slow gas phase oxidation of NO to NO2 is the rate-controlling step for the conversion of NO to NOj- (Margolis and Driscoll, 1972). The NO2 to N03- conversion proceeds readily if sufficient peroxide is available. Maximum conversion to NO3- is obtained within 3 hr from the N02/N2 source. When 2400 ppm of HC1 gas were added to the 211-ppm NO/Nz source by dynamic flow dilution, six replicates 15 ppm NO analyzed by procedure No. 2 averaged 197 (average recovery of 93%). This level of recovery is comparable to that obtained in the absence of chloride. From the foregoing data procedure No. 2 can be recommended for all stack gas samples regardless of the chloride content. Procedure No. 1 can be used for gas samples in the absence of chloride, but the analysis time is somewhat longer because of the evaporation step. Evaporation to dryness is unnecessary here also, if the aqueous nitration technique is used for both the standards and samples. Literature Cited
*
American Public Health Association, “Staylard Methods for the Examination of Water and Wastewater, 13th ed., pp 234-7, Washington, D.C., 1971. Bartok, W., Crawford, A. R., Skopp, A., Chem. Eng. Progr., 67, 64 (1971). Berger, A. W., Brummer, K., Driscoll, J. N., Funkhouser, J., Margolis, G., Valentine, J., “Improved Chemical Methods for Sampling and Analysis of Gaseous Pollutants from the Combustion of Fossil Fuels,” Walden Research Corp., Environmental Protection Agency APTD 1291, February 1970, Research Triangle Park, N.C. Chamot. E . M.. Pratt. D. S.. Redfield. H. W.. J. Amer. Chem. Soc., 33,366 (1911). Coulehan, B. A , , Lanp, H. W.. Environ. Sei. Technol., 5 , 163 (1971). Federal Register, Method 7 , Vol. 36, pp 24891-3, December 23, 1971. Margolis, G., Driscoll, J. N., Enuiron. Sei. Technol., 6, 727 (1972).
Received for review March 12, 1973. Accpeted September 12, 1973. This work was partiall? funded the Environmental Protection Agene?, under an interagent?, agreement with the Air Force Rocket Propulsion Laboraton:
