Evaluation of Arsenite-Modified Jacobs-Hochheiser Procedure Earl L. Merryman,' Chester W. Spicer, and Arthur Levy Battelle Columbus Laboratories, Columbus, Ohio 43201
w T h e Jacobs-Hochheiser ( J - H ) method is being used to determine integrated NO:! and N O levels (after oxidation of N O to NOz) in the 1-15 pphrn range in indoor and outdoor environments. Cnder controlled experimental conditions, the absorption of small quantities of NO2 in a series of bubblers containing 0.10 or 0.25N N a O H varied considerably resulting in poor reproducibility of d a t a . However, the addition of u p to 0.100/0 by weight of sodium arsenite to the absorbing solutions greatly improved the collection efficiency, the reproducibility, and the accuracy of the d a t a . N O interfered with t h e NO2 absorption process in, the presence of arsenite; a correction factor was determined which can be used when the N O concentration is known, A C O z effect on t h e p H of the absorbing solutions was observed and taken into account in determining NO2 levels. Water vapor or CH4 had little or no effect on t h e NO2 collection and analysis process. T h e J - H procedure, modified with sodium arsenite a n d corrected for S O . a p pears to yield accurate and reproducible integrated SO2 values. At present, t h e only practical method for obtaining 24-hr average NO:! measurements a t ambient levels is the Jacobs-Hochheiser (1958) method. T h e method has several desirable features: It is simple to use, the equipment is relatively inexpensive, and it appears to have the potential to accurately measure NO2 levels in the 1-15 pphm range required for atmospheric monitoring. This range is particularly desirable for correlation with t h e present n a tional air quality standard for NO:! of 5 p p h m . T h e main drawback to the Jacobs-Hochheiser ( J - H ) method, as presently used, is the variable collection efficiency encountered in the NO2 absorption process. Purdue e t al. (1972) a n d Rigdon and Crawford (1971) report a n overall collection efficiency for XO2 of 35% using fritted tube samplers. Also, the d a t a of Purdue et al. indicate no nitrate ion formation in t h e solution (i.e., a stoichiometry of one), while Rigdon and Crawford report that nitrates are formed in the basic solutions. Morgan e t al. (1967) report a n efficiency of about 50% for fritted tubes and a stoichiometry of 0.625 for the NO:! reaction in NaOH. T h e varying efficiencies and the conflicting d a t a on the stoichiometry of the reaction bring out the need for further study of the J - H method, especially since an alternative low-cost integrated analytical method is presently not in the offing. In line with t h e above studies, it appears t h a t t h e addition of small amounts' of any of the following substances to t h e N a O H solutions markedly improved t h e NO2 collection efficiency: XeOa, O s 0 4 (Rigdon and Crawford, 1971), a n organo salt with triethanol amine (Huygen and Steerman, 1971), sodium arsenite (Christie et al., 1970), and guaiacol (Nash, 1970). In general, the efficiency increased to well over 90%. Although the additives do improve the collection of NO2 a t specified levels, more detailed examinations are needed with respect to their effectiveness in the low NO2 concentration ranges and t o possible interferences with the NO:! collection and/or analysis procedure. In t h e pres-
ent paper, the J - H method is examined in the 4-15 pphm NO:! concentration range in t h e absence of additives, and then in the presence of sodium arsenite and various gaseous or volatile components likely to be encountered in the atmosphere-e.g.. NO, C O z , H20, and C H 4 . A negative interference from SO2 and methods for its elimination have been reported (Christie et al., 1970).
Experim en t n 1 NO2 S a m p l i n g Source. T h e nitrogen dioxide-air mixtures in this study were obtained from a low-flow permeation tube which provided NO2 levels of from 5-25 pphm in t h e sample gas stream. S:!.dried through Drierite and silica gel, was passed over the permeation tube suspended in a Pyrex container in a constant temperature bath; the entrained SO2 then entered a 200-liter Pyrex chamber (Figure l a ) . T h e 200-liter Pyrex chamber served as a constant s a m pling source of NO:! for the J - H units. Compressed breathing air was used to further dilute the NO2 in the chamber. T h e air contained approximately 320 ppm CO2 and no measurable N O or NOz. Total gas flow ( t h a t is, Nz. NOz, or air) into the chamber was 1.95 l./min. Carbon dioxide was removed from the compressed air when necessary by directing the diluting air stream through Ascarite prior to contact with the NOz-Nz mixture. An Atlas colorimetric instrument was used as a backup analyzer to monitor changes in NO2 and N O concentrations in the chamber. All N O concentrations were determined by first oxidizing the N O to NO2 with potassium dichromate a b sorbed on glass fiber filter paper and then using the usual methods for NO2 analysis (Saltzman method or absorption in basic solution). Jacobs-Hochheiser S a m p l i n g Units. A schematic. of the Jacobs-Hochheiser sampling unit used in these experiments is shown in Figure l b . Sulfuric acid was used to dry the gas stream before it entered the p u m p . Gas flow rates
200 111.1 mlxinp chamber
o
A p p o r o t u s l o r Generating Conrlonl NO,
1056
Environmental Science 8 Technology
J - H units
Source
Sample
Pres$urs Gape
A
Vent
y-1 H:: BSIlOW8
b NO,
To whom correspondence should be addressed
+ To
Pump
Absorplion Apporolus
Figure 1. NO2 generating
and sampling apparatus
and pressures were measured at the beginning and end of each run, and the average was used to calculate total gas volume. T h e sampling solution was 0.10Nor 0.25N sodium hyd.roxide, or simply distilled water. The arsenite modification of the Jacobs-Hochheiser procedure consisted of adding 0.033 to 0.10 wt ’70 of sodium arsenite to the sodium hydroxide solution prior to sampling. T h e colorimetric determination .of nitrite in the basic solution has been described by Jacobs and Hochheiser (1958). However. different lots of the coupling agent. N(1-naphthy1)ethylenediaminedihydrochloride, can give o.d. readings which differ by nearly 20%. It is therefore important that standard calibration curves be made up with the same chemical reagents as those used in developing the field samples. A very small quantity of hydrogen peroxide was added to the sample solutions prior to color development to oxidize any SO2 to sulfate and thus minimize SO2 effects.
Re.suIts a n d D k c u s s i o n The unmodified and modified version of the J - H method were evaluated under controlled laboratory conditions. The results of these experiments are presented and discussed in the sections below. Unmodified Jacobs-Hochheiser Method. In the unmodified J - H method. the collection efficiency, the effect of pH, and the effect of different type bubblers on NO2 analysis were examined. The usefulness and reliability of efficiency factors were also considered in the calculation of total XOZ values. NO2 Absorption in S o d i u m H?,dro.ride Solutioras. Table I presents data showing NO:! absorption at two different pH levels, 13.0 and 13.4. In these experiments. the sample gas stream contained 13.8 pphm NO2 and approximately 320 ppm of C O z . The d a t a in Table I show that the a b sorption process with either orifice tubes or frits is very often nonproducible in the more commonly used 0.10N NaOH solutions: the experimental X02 values ranged from 4.8-12.5 pphm for a sample gas stream containing 13.8 pphm 5 0 2 . Attempts to improve the absorption process by increasing the p H of the solutions to 13.4 (0.25N NaOH) resulted in no noticeable improvement in the accuracy of the data. In fact, the data at the higher p H appear to be less reliable than the d a t a a t p H 13.0. Reproducibility. however, isslightly better at the higher p H level. It is concluded from the variation in NO2 d a t a in Table I that the unmodified J - H procedure cannot be considered a quantitative method for NO2 analysis, even under idealized laboratory conditions. Efficient? C a / c u / a t i o n s from t h e J - I f Ilata. T h e efficiency factor of a single scrubber is defined as the amount of NO2 removed from the sample gas stream divided by the amount of NO2 originally in the gas stream (referred to in this work as the “actual efficiency”). A reproducible. or high efficiency factor is needed to calculate accurately the amount of NO2 originally in the sample gas stream. Two efficiency factors have been examined in this work. One is the actual efficiency, mentioned above. and the other the “apparent efficiency”; both are discussed below. T h e average actual efficiency of the NO2 absorption process was 13.67” for the orifice tubes and 29.0% for the fritted tubes (Table I ) . T h e higher efficiency observed with the fritted tubes no doubt reflects greater solution contact area of the gases when using these tubes. T h e large variations in the actual efficiency values observed in the above data certainly raise questions as to the validity of the d a t a using this method. Large errors no doubt would appear in much of the d a t a .
T h e apparent efficiency values recorded in this work were obtained from the equationA/T = ( A - R ) / A where
A = amount of NO2 collected in tube A (1st tube) B = amount of NO2 collected in tube B (2nd tube) T = total NO2 initially in t h e sample gas stream The above equation is derived from the assumption that the same fraction of NO2 is collected in each bubbler. This apparent efficiency factor, which is calculated from experimental d a t a only. is recorded as a percentage value in Table I. For the total amount of NO2 in the original gas stream. the equation is solved for T . These values are recorded m column 6 of the table. As seen in the table. the factor as used here does very little to improve the accuracy of the d a t a in the unmodified J - H method. As will be seen later. it is used with some success in the modified procedure. The J - H Method with Sodium Arsenite. Since the present J - H method appears to be unsatisfactory for obtaining accurate or reproducible NO2 data. the procedure was modified, after the work of Christie et al. (1970). to include sodium arsenite (SaAsOz) in the NaOH solutions. Experiments were carried out to evaluate the arsenite method a t low NO2 levels and to adapt it to field routine. In addition, the effects of NO, COz. CH4. and H 2 0 on SO2 analysis were examined. The effects of flow rates and pH on NO2 absorption were also explored. Removal of NO2 from a N02-Air Mixture. Mixtures of NOz-air were passed through N a O H solutions containing 0.033-0.10% NaAsO2. The normality of the sodium hydroxide solutions ranged from 0.00-0.25N. Data for 0.10N solutions, shown in Table 11, indicate a significant improvement in the quantitative determination of NO2 as compared to the results obtained in the absence of NaAsOz. T h e average pphm value from samplers containing 0.10N N a O H and 0.1(7c NaAsOz is 14.5 (column L5) and the maximum percent deviation from the average is 4.8%. The average experimental value of 14.5 pphm NO2 is also in good agreement with the gravimetrically determined NO2 permeation tube value of 13.8 pphm. Orifices and frits appear equally effective in collecting N 0 1 . A stoiTable I. NO2 Data from Jacobs-Hochheiser Method (13.8 pphm in sampling stream, approximately 24-hr sampling period) Pphm NO2 collected Efficiency. % PH,’ bubblera
13.010 13.0/0 13.010 13.O/F 13.O/F 13.O/F 13.010 13.010 13.010 13.O/F 13.O/F 13.O/F
A
B
1st tube
2nd tube
3.30 3.15 2.41 8.47 4.61 4.81 2.04 2.60 2.21 4.93 5.94 4.62
2.70 1.66 4.89 4.02 3.65 2.89 1.99 3.37 2.20 3.41 4.99 3.97
Pphm NO2
APparent* Actual‘
18.2 47.2 NAg
52.6 20.7 39.9 2.3 NA
0.4 30.8 15.9 14.2
17.2 16.4 12.6 44.2 24.0 25.1 10.6 13.5 11.5 25.7 31.0 24.1
Calcd totald
Exptl total‘
Fed. reg’
18.1 6.7
6.00 4.81 7.30 12.49 8.26 7.70 4.03 5.97 4.41 8.34 10.43 8.59
6.79 6.48 4.95 17.42 9.48 9.89 4.20 5.35 4.55 10.14 12.22 9.50
NA
16.1 22.2 12.0 87.4 NA
556.3 16.0 37.4 32.5
a 0 = orifice tubes F = fritted tubes Calculated from the relationship [ ( A - B ) / A ] X 100 (see text) CCalculatedby the Federal Register Total pphm = A / [ ( A - B) A ] = A2 ( A - B ) method for a single tube Total pphm NO2 from columns 2 and 3 The pphm NO2 calculated by the procedure recommended in the Federal Register These values are obtained from the present work by multiplying the NOz values in column 2 by 2 06-1 e 0 7 2 1 0 35 g Not applicable negative values
’
Volume 7, Number 11, November 1973
1057
Table I I . NO2 D a t a in Presence of N a A s 0 2 (13.8 pphm NO2 in sampling stream, -24-hr sampling period, pH = 13.0, 0.10% NaAs02, unless noted otherwise) Pphm NOz collected
Total pphm NOz'
Runo no.
A
E
Efi % b
X
1-0 2-0 3-0 4-F 5-F 6-F
12.11 12.85 12.69 11.94 13.72 12.13
1.99 1.40 1.27 1.97 0.57 2.46
83.6 89.1 90.0 83.5 95.9 79.7
14.5 14.4 14.1 14.3 14.3 15.2
14.10 14.25 13.96 13.91 14.29 14.59
7-Fd 8-Fp 9-0' 10-0g
9.01 7.54 8.85 7.92
1.96 4.14 3.33 3.55
78.2 45.2 62.3 55.2
11.5 16.7 14.2 14.3
10.97 11.68 12.18 11.47
Y
0 = orifice, F = fritted tubes. [ ( A - E ) : A ] X 100. Column 5 values = A z ; ( A - E ) , column 6 values = ( A E ) . 0.067% NaAsO? 0.033% NaAsO2. 0.067% NaAsO?. g 0.033% NaAs02
+
Table I l l . Flow Rate and p H Effects on NO2 Absorption (13.8 pphm NO2 in sampling stream, -24-hr sampling period, 0.10% NaAsOz) Pphm NO? collected
Flow
Total pphm N02('
pH/ bubbler"
rate, cc,'min
A
E
Eff %*
13.410 13.4/0 13.4/0 13.41F 13.4/F 13.4/F
56 242 465 61 159 496
8.19 11.68 10.62 14.30 11.72 10.31
0.08 0.40 1.10 3.45 0.33 1.69
99.0 96.6 89.7 75.9 97.2 83.6
8.3 12.1 11.8 18.8 12.1 12.3
8.27 12.08 11.72 17.75 12.05 12.00
1.19 1.73 4.71 2.77
1.07 2.17 1.79 2.59
10.3
11.5
0.0
0.0
62.0 6.6
7.6 42.0
2.26 3.90 6.59 5.36
7.010 7.0/0 7.O/F 7.0/F
X
Y
0 = orifice, F = fritted tubes. [ ( A - B ) / A ] X 100. values = A z / ( A - E ) . column 7values = ( A f 8). (I
chiometric factor of 0.72 was used to calculate NO2 values in all runs containing SaAsOz. Lowering the amount of h'aAsO2 in the N a O H solutions (Run Nos. 7-10) reduced t h e amount of NO2 collected but still resulted in acceptable NO2 values if the apparent efficiency factor in column 4 is used to calculate the total NOz. In this case. the apparent efficiency factor is useful. However. at efficiency values >8O'70. the improvement in NO2 values is marginal when compared to the sum of NO2 collected in Tubes A and R (column 6 ) . Of course. if only one bubbler is used to collect the NOz. a n actual efficiency factor must be determined and used with the modified .J-H method. T h e average actual efficiency for collecting NO2 in 0.101V N a O H with 0.10% Nails02 is 91.1'70, as determined from the d a t a in Table 11. Flow-Kate and p H Effects. T h e SO2 d a t a in Table 111, show that low flow rates give erratic results. T h e higher flow rates. above 159 cc/min give excellent reproducibility in t h e presence of 0 . 1 7 ~NaAsOz and 0.23N K a O H . T h e average value in column 6 at the higher flow rates is 12.1 pphm and maximum deviation from the average is 2.5%. The total pphm values in column 6 are, however, about 1870 below those obtained with 0.10R; N a O H solutions with 0.107~NaAsOz. T h e d a t a with NaAsOz indicate that 0.10N NaOH ( p H = 13.0) solutions are a t least as effective as 0 . 2 5 N NaOH ( p H = 13.4) solutions in collecting S O z . Under neutral conditions ( p H = 7.0). however, the d a t a were neither accurate nor reproducible. bringing out the importance of pH in removing NO2 from the gas streams. T h e p H of t h e 0.10,V N a O H solutions after 24 hr of sampling was 2 9 in all samples checked. Effect of N O on NO2 Anal,vsis. Data from previous investigations (Pigford and Sherwood. 1952: Bartok et al.. 1969) have shown that N O may actually interfere with a n accurate NO2 analysis. T h e overall effect is to produce
Table I V . Experimental D a t a , N O and N O 2 Average efficiency, %'
-
Orifice
Frit
~Run no.
NO, pphm
Apparent
Actual
NOz pphmb APparent
Actual
Orifice
Frit
87.2 99.0 80.3 95.4 85.6 110.1
13.9 15.2 15.5 16.0 15.7 19.1
16.0 16.1 16.4 19.4 17.2 24.0
5 6
0 7 7 14 14 29
82.4 84.3 66.4 78.5 81.8 71.6
80.9 93.8 72.7 88.4 78.7 96.3
77.3 87.5 69.4 69.8 81.8 65.0
86.8 94.5 103.5 111.9
9.1 9.8 11.2 10.9
9.0 10.7 12.1 13.4
107.4 94.2 87.4 104.0
4.5 4.1 4.1 4.8
5.2 4.5 4.8 6.8
I
I2 3
NO2 = 14.2 Pphm 1 2 3 4
4.3 pphrn NO, in air 9 =9.6 pphm NO, in air A =14.2 pphm NO, in air
0
Column 6
a
a
O
F
FRITS
O
NOr = 9.6 Pphm 7 8 9 10
0 4.8 14.4 19.2
91.7 88.2 79.3 85.4
86.5 89.7 92.7 96.9
92.9 85.1 82.0 80.3
NOr = 4.3 Pphm 11 12 13 14
0 2.2 4.3 11.5
88.8 92.8 81.2 73.4
94.2 77.2 76.3 81.5
89.1 90.8 79.1 65.8
a Average of three samples taken at each NO concentration.
values from efficiency factor ( A
1058
- B)/A.
Environmental Science & Technology
NOz
't '0
ORIFICES
4
8
12
16
20
24
NO, PPHM Figure 2. NO interference in presence of NO2
28
Table V. Effect of CO2 on NO2 Analysis Average efficiency, Yoa Pphm Sample no.
1 2 3 4
Orifice
NO2
NO
13.2 13.2 4.3 4.3
0 0 2.2 1.8
Average total NOp" pphm
Frit
Pprn C O z
Apparent
Actual
Apparent
Actual
Orifice
0 290 290 2000
97.6 92.4 89.3 39.3
94.7 83.0 77.2 32.6
98.8 95.5 90.8 43.6
97.1 88.3 94.2 62.2
12.8 11.9 4.0 3.7
Total NO>' collected. pphm
Frit
Orifice
13.0 12.2 4.5 6.1
Frit
12.8 11.8 4.0 2.2
a Values represent an average of three samples at each sampling point NO2 values obtained using efficiency factor i A - 8 )' A in Tubes A and 5 in analytical train
13.0 12.2 4.4 4.2
Sum of NO2 collected
Table V I . Effect of CH4 and Water Vapor on NO2 Analysis (NO2 = 11.5 pphm, NO = 12 pphm in gas stream unless noted otherwise; -24-hr sampling period) Average efficiency, O h n
corrected Pphm NOz for
__.-~___--
Sample 1C 2
3 4
Relative humidity
__
