A New Method of Generation of Gases at Parts per Million Levels for Preparation of Standard Gases Yoshikazu Hashimoto and Shigeru Tanaka" Department of Applied Chemistry, Faculty of Engineering, Keio University, Hiyoshi, Yokohama 223, Japan
A new method of generation of gases a t parts per million (ppm) levels for gas standards was developed. This method enables generation of many sorts of gases including Son, NO, (NO and N&), H2S, HCN, and NH3 by a simple apparatus and simple operations. The rate of gas generation depends on pH, Concentration, and temperature of t h e solution for gas generation, which contains a specific solute and buffer agents. This method possesses precision and-accuracy equal to those of the permeation tube, and the variation of the concentration of generated gas was less than 3%. Gases generated by this method can be utilized for calibration of the measuring instruments, biological and medical experiments, and studies on chemical reactions in ambient air. T h e preparation of gases of known concentrations, especially a t parts per million (pprn) and sub-ppm levels, is a knotty problem, although it is a prime necessity in research of air pollution measurements and air chemistry. A classical and regular method of preparation of these gases is a dilution method ( I ) . In this method, the gases of interest are diluted with air or t h e other base gases at a definite ratio. This procedure appears to be easy and simple, but some difficulties are known to analytical chemists in avoiding losses of gases by absorption t o t h e walls of vessels and tubes. The other disadvantages are the expensiveness of apparatus and the possibility of the chemical change of gases while handling. A more convenient method, in which gases diffused from t h e polyfluorinated resin (Teflon, FEP) tubes are diluted with air of a known flow rate, was investigated by O'Keeffe et al. ( 2 , 3 ) , and this made the preparation of gases at ppm or sub-ppm levels simpler than the technique used heretofore. This permeation tube method is simple in both apparatus and operations, and, moreover, the apparatus is less expensive than the former. The most prominent advantage of this method is that it produces standard gases a t a n absolute basis with gravimetric measurement of the amount of gases from t h e permeation tube ( 4 ) . However, this permeation tube method requires t h e skill t o fabricate the tube if one wanted to prepare it by himself, and a si rict temperature control of the tube itself ( 4 ) .I n addition to these disadvantages, it takes, for some gases, a long time until the concentrations of t h e generated gas become constant in this (permeation tube) method ( 5 ) . T o improve some difficulties associated with the existing methods, we have developed a new technique, which is simple, less expensive, and easy t o operate. T h e apparatus of this technique consists of the usual laboratory wares, and the main part of t h e gas generator is an impinger modified a little bit for this purpose. A solution specific to the gas to be generated is placed in the gas generator, and the p H of t h e solution is made constant with buffer agents. Passing t h e air over the 0013-936X/80/0914-0413$01 .OO/O
surface of the solution a t a constant rate of flow, the gas, whose concentration is constant, is generated a t the outlet of the gas generator. Concentrations of the gas generated are controlled by changing the experimental parameters: t h e pH, the concentration and the temperature of solution, and the flow rate of the dilution gas or air. By controlling these parameters, the gas of a known concentration can be generated a t a wide range of concentrations (0.01 to 10 ppm). Experimental
Apparatus and Procedure. A schematic diagram of an apparatus for this method of gas generation is shown in Figure 1. T h e gas generator, the main part of this apparatus, is a Smith-Greenburg type impinger whose impaction nozzle is cut off short so as to make it keep a definite distance from the surface of the solution placed in the impinger. Figure 2 shows a detailed structure of the gas generator. T h e 60-mL solution in the impinger contains a specific solute to generate a gas, for instance, potassium cyanide for hydrogen cyanide, and the p H of the solution is kept constant by adding buffer agents. The other examples of the solute added were sodium hydrogen sulfite for sulfur dioxide, sodium sulfide for hydrogen sulfide, sodium nitrite for nitrogen oxides, and ammonium chloride for ammonia. T h e air is dried by passing through a desiccant (A), and traces of contaminants such as sulfur dioxide and nitrogen oxides are eliminated a t the purification bed (B) loaded with t h e peroxides of alkaline earths and are led to a preheater bottle (D) through a flow meter (C). T h e preheater and the gas generator are dipped in a water bath (.J) to keep them a t a definite temperature. After the temperature is adjusted, the air is led to t h e gas generator, where it takes u p a vapor mixture of water and gas generated from the solution specific to the gas of interest, and the air containing the vapor then leaves t h e generator bottle as a dilute gas mixture of interest. T h e concentration of gas thus prepared becomes constant in a short time, usually in 0.5 h, when a steady state of vaporization and elimination of the gas is reached over the surface of t h e solution in the gas generator. Calibration. Concentrations of the output gas from t h e generator were calibrated by analysis of t h e solution into which the gas from the outlet of the generator was absorbed. T h e trapping efficiencies of t h e solution for each gas at their concentration levels were confirmed to be a t least 98% using additional bubblers in a series after the first absorption bubbler. Sulfur dioxide, nitrogen oxides (nitrogen monoxide was converted to nitrogen dioxide in advance), and hydrogen sulfide were analyzed by absorption spectrophotometry (6-8). Hydrogen cyanide and ammonia were analyzed by ion selective electrode techniques (9, 1 0 ) . For continuous measurements of sulfur dioxide and nitrogen oxides, an electrolytic
@ 1980 American Chemical Society
Volume 14, Number 4, April 1980
413
1.5 B 0 =
- 1.0 E
c
z z
0
J Figure 1. Schematic diagram of the gas generator: A , desiccant; B, purification bed; C, flow meter; D, preheater; E, gas generator; F, trap; G, H, thermometer; I, gas generating solution; J, water bath
c 4 e z c
e
0.5
z 0 0 x
B Lc c
3
4,O
E.O 8.0 IO P H OF S?LUTI3N
12
Figure 3. Variation of gas concentration in air flow with pH of solution. ( 0 )Sop: NaHS03solution 500 pg/mL; temperature 25 O C ; air flow rate 300 mL/min. (a)NO,: NaN02 solution 500 pg/mL; temperature 25 O C ; air flow rate 900 mL/min. ( A ) H2S: Na2S solution 120 wg/mL; temperature 25 O C ; air flow rate 500 mL/min
Figure 2. Gas generator
conductivity analyzer (Tokyo Kogyo, Model 70-5) and a chemical luminescence analyzer (Horiba, Nitrogen Oxides Analyzer) were also used, respectively. The primary calibration of prepared gases by this method was obtained by measuring solute losses after generation of gases over a day. T h e solute losses were determined by the difference of the amount of solute in the solution during gas generation. The solute loss was converted to the gas generation rate, expressed as pg/(cm2.min).
PH OF NaNO, SOLUTION
Figure 4. NO, (NO and N02) concentration in air flow with pH variation of NaN02 solution: ( A ) NO,; (0)NO2; (a)NO; NaN02 solution 500 pg/mL; temperature 25 O C ; air flow rate 900 mL/min
Results pH of the Solution for Gas Generation. The relationships between the concentrations of generated gases and the pHs of solutions are shown in Figure 3. Characteristic variations of gas concentrations with p H are different for each gas-solution system. Sulfur dioxide begins to generate at p H -7, and the amount of generation increases with decreasing p H of the solution of NaHS03, and the amount of generation increases rapidly a t about p H 5 . The nitrogen oxides generate a t about p H 9 and lower from the NaN02 solution, and the amount increases rapidly a t about p H 6. With hydrogen sulfide, generation from the Na2S solution is at a higher p H , about 12.5 and lower, and it is facilitated below p H 10. Therefore, the generating solutions have to be kept a t a proper p H specific to the concentration required of each gas. For generation of gases in the range of 0.1-1 ppm, proper p H values of each solution are 5 to 6 for sulfur dioxide, 6 to 7 for nitrogen oxides, and 11 to 12 for hydrogen sulfide. In Figure 4, concentrations of nitrogen oxides (NO and NO2) are shown with the p H variation of the NaN02 solution. I t is interesting that the ratios of NO to NO2 of the generated NO, changed with the p H of the NaNOz solution. As is seen in Figure 4, overwhelming amounts of nitrogen oxides are occupied by NO2 a t p H 5, while the amount of each of NO arid NO2 becomes equal at p H 6, becoming double that of NO over NO2 a t p H 7 . Thus, the ratio of concentrations of NO to NO2 is possible by adjusting the p H of the solution. Concentration Relations between Gas and Solution. 414
Environmental Science & Technology
j
10'
loi
l
Z
5
10'
C O N C E N T U T I O N OF THE SOLUTi3N
I
-4/"1
Figure 5. Relation between concentration of the solution and concentration of generated gas. ( A ) HCN: pH 11.7; temperature 40 O C ; air flow rate 300 mL/min. (a)NO,: pH 5.0; temperature 25 O C ; air flow rate 900 mL/rnin. (0)SO2: pH 5.0; temperature 25 O C ; air flow rate 300 mL/ min
In Figure 5, the relation between concentrations of the generated gas and solution is shown in logarithmic scales. Their relation is also expressed in Equation 1. This equation implies that gas generation from the solution complies with Fick's law of diffusion. Under a given p H value of the solution, Equation 1is valid within a wide range of solution concentration, 10-103 pg/mL. Therefore, gases of a desired concentration, Cgas,can be prepared by simply varying the concentration of the solution, Cliq, in Equation 1 under a definite pH, temperature, and air flow rate.
Table 1. Experimental Parameters of Equation 1 in Concentration Ranges of the Solution from 10 to lo3 WmL k
n
system
NaHS03-S02 NaNO2-NOXa Na2S-H2S
1.24 0.73 0.63 0.90 0.64
KCN-HCN NH4CI-NH3
0.00025
0.015 0.007 0.009
0.021
pH
temp, O C
5.0 5.0 10.2 11.7 7.3
25.0 25.0 25.0
air flow, mLlmin
300 900 500 300 1000
40.0 25.0
NO, means a mixture of NO and NO2 whose concentration ratio (NO/N02) is 0.1. a
0
0
20
60
40
TIME
,
50
120
100
mln
Figure 7. Variation of SO2 concentration in air flow with time: (0) NaHS03; ( 0 )Na2S03;solution pH 5.0; 250 pg/mL; temperature 25 OC; air flow rate 300 mL/min
o-o-o-o-
E Q
,*-.-.-0-0-0-
-A-A-A-A-A-Az i L
z
v
::
?, '
01
3
I
I
I
I
I
30
60
90
120
150
TIME ,
I
180
TI:
Figure 6. Variation of gas concentration in air flow with time. (0)SO2: NaHS03 solution pH 5.0; 500 pg/mL; temperature 25 O C ; air flow rate 300 mL/min. ( 0 )NO,: NaN02 solution pH 6.0; 500 pg/mL;temperature 25 O C ; air flow rate 900 mL/min. ( A ) H1S: Na2S solution pH 10.2; 360 pg/mL; temperature 25 O C ; air flow rate 500 mL/min C,,dppm) = kCiiqn(pg/mL)
;
0 10
I
l
l
20
30
l
, 50
40
TEMPERATURE OF h3HS03 S O L U - I 3 N , i c
Figure 8. Variation of SO2 concentration in air flow with temperature of NaHS03 solution: NaHS03 solution pH 5.0; 500 pg/mL; air flow rate 300 mL/min
(1)
The parameters n and h in Equation 1are the slope of the line expressing relations of concentrations in the gas-solution system and the concentration of generated gas a t Cli, of 1 pg/mL, respectively. Some examples of the parameters n and h in Equation 1 are shown for various gases in Table I. As the parameter n increases so does the variation of Cgaswith Cliq. The parameter h also indicates the minimum gas concentration that we can achieve in this system. Stability of Generated Gases. In the first 30 min, the concentration of generated gas is not stable, but it becomes constant about 30 min after the start of gas generation (Figure 6). Variations of the concentrations of generated gas were less than 3% in the experirnental duration of 3 h: 2 , 3 , and 2% for 0.56 ppm of SO*, 0.36 ppm of NO, (NO and NO*), and 0.28 ppm of HzS, respectively. Thus, the preparation of standard gases, whose concentrations are known and very stable, can be easily attained by using the simple apparatus mentioned above, if the pH, concentration, and temperature of solution and the rate of air flow were kept constant. The stability of generated gas is dependent on the sort of solute used. With sulfur dioxide, the concentration of generated gas from the Na2S03 solution decreases gradually with time, although that from the NaHS03 solution was stable for several hours (Figure 7). This difference in stabilities of concentration must be caused by the difference in the rates of oxidation between SO32- and HS03-; SO3*- oxidized faster than HS03-. Therefore, it is important for the stable generation of the gas to select a proper solute for each gas of interest. Effect of Temperature. The temperature dependence of SO2 generation by the present technique is shown in Figure 8. Temperature influences the concentration of the generated gas, but the degree is much less than t h a t in the permeation tube technique. For instance, the temperature dependence of 0.56 ppm of SO2 is 0.02 ppm/OC between 20 and 30 "C. This
0 0
1000
500 A I R FLOW RATE ,
ml/nii
Figure 9. SO2 concentrations at various rates of air flow: NaHS03 solution pH 5.0; 500 pg/mL; temperature 25 OC means that the temperature of the gas-generating solution should be controlled to an accuracy of f0.3 "C, if the variation of gas concentration must be kept less than 1%. The major cause of the concentration change with temperature is the p H effect on an ionic equilibrium in the gasgenerating solution, in which the amount of molecules to be evaporated as the gas molecules is changed in an equilibrium with the ions of the same kind of molecules. The gas prepared contains the water vapor due to the vaporization of water from the solution; the relative humidity of the generated gas was about 20% a t the air flow rate, 300 mL/min, and the temperature of the solution was 25 "C. A lower humidity of the gas is obtained by decreasing the temperature of the solution. Air Flow Rate. The relation between the gas concentration and the air flow rate is shown in Figure 9. I t is natural to expect t h a t a higher flow rate of air results in a lower concentration of gas. However, as is seen from Figure 9, the decrease of gas concentration is smaller than is expected; the concentration ratios of SO2 decrease only 0.05 ppm, from 0.58 ppm at the air flow rate of 500 mL/min to 0.53 ppm at that of 1000 mL/min, Volume 14, Number 4, April 1980
415
leaving the other conditions for gas generation unchanged. It seems that a higher air flow rate in this range facilitates the generation of gas by removing the vapor from the surface of solution. Reaction Mechanisms of SO2 Generation. Reaction mechanisms of SO2 generation are shown in Equations 2,3, and 4: NaHS03 HS03H2S03
-
Na+
+ H+ +
+ HSO3-
(2)
erator is excellent and the generation sustains for several hours a t least. The concentration varies to a smaller extent with changes in temperature and air flow rate than with the permeation tube technique. The prepared gas contains some water vapor which is preferred for experiments meant to simulate the real atmosphere. In this paper, SOz, NO, NOz, HCN, HzS, and NH3 gas generation was described, but other gases such as hydrogen fluoride and carbon dioxide can also be prepared by the same technique.
H2S03
(3)
Literature Cited
(4)
Saltzman, B. E., Anal. Chern., 33,1100 (1961). (2) O'Keeffe, A. E., Ortman, G. C., Anal. Chern., 38, 760 (1966). 13) O'Keeffe. A. E.. Ortman. G. C.. Anal. Chem.. 39.1047 (1967) (4) Scaringelli,F. P., O'Keefe, A. E., Bosenberg, E., Bell, J. P., Anal. Chem.. 43,871 (1970). (5) Lucero, D. P., Anal. Chern., 43,1744 (1971). (6) West, P. W., Gaeke, G. C., Anal. Chem., 28,1816 (1956). (7) Saltzman, B. E., Anal. Chem., 26,1949 (1954). (8) Jacobs, M. B., Braverman, M. M., Hochheiser, S., Anal. Chern., 29,1349 (1957). (9) Frant, M. S., Ross, J. W., Riseman, J. H., Anal. Chern., 44,2227 (1972). (10) Thomas, R. F., Booth, R. L., Enuiron. Sci. Technol., 7, 523 (1973).
+
H20 t SO2
NaHS03 dissociates into Na+ and HS03- in a solution (Equation 2). If the p H of the solution is lowered by the addition of H+, the reaction tends to move to the right sides in Equations 3 and 4. I n other words, the enhanced production of SO2 due to the increased H+ can be explained on the basis of Equations 3 and 4. Conclusions
The new technique for preparation of gases of a known concentration presented here makes it possible to prepare gases for environmental measurements and studies. The apparatus and operations of this technique are simple. The stability of the concentration of gas generated from the gen-
(1)
Received for review November 17,1978. Accepted October 22,1979. The authors are grateful that this research was supported in part by the Takeda Foundation.
Correlation between the Concentrations of Polynuclear Aromatic Hydrocarbons and Those of Particulates in an Urban Atmosphere Takashi Handa", Yoshihiro Kato, Takaki Yamamura, and Tadahiro lshii Department of Chemistry, Faculty of Science, Science University of Tokyo, 1-3, Kagurazaka, Shinjuku-ku, Tokyo 162, Japan
Kyo Suda Hitachi Ltd. Central Research Laboratory, 1-280, Higashi-Koigakubo, Kokubunji, Tokyo 185, Japan
T h e atmospheric levels of polynuclear aromatic hydrocarbons (PAHs) and the size distrihution and number concentration of particles were measured a t several sites in the Tokyo metropolitan area. The concentrations of benzo[ghi]perylene ( B [ g h i ] P ) , benzo[a]pyrene ( B a P ) , perylene, chrysene, benz[a]anthracene (Bail), and pyrene were determined with a high-volume air sampler, and the number concentrations of particles ranging in size from 0.1 to 0.2 pm were determined by a n optical counter. For capturing the missing fractions of four-membered PAHs (pyrene, BaA, etc.), an improved collection system with traps cooled by liquid nitrogen was developed. Linear relationships between the contents of B[ghi]P, BaP, and perylene and particles were established. It was found t h a t the ratio of the atmospheric PAH level to the B[ghi]P level was in fairly good agreement with that based on the average PAH levels in automotive exhausts from 26 Japanese cars. I t is well known that polynuclear aromatic hydrocarbons (PAHs) exist in the atmospheric environment. PAH compounds are emitted from many sources including internal combustion engines. Experiments with animals ( 1 , 2 ) indicate that benzo[a]pyrene (BaP) and related compounds are carcinogenic. In addition, it was established t h a t automotive exhaust gases contain considerable amounts of very fine particles. There is no doubt that the automobile is the single most important source of atmospheric PAHs in big cities. 416
Environmental Science & Technology
Several investigators have shown that PAH content depends on the size of suspended particulate matter, especially when the size is below -3 pm (3,4 ) . This finding was further supported by the analysis of soils ( 5 ) .The relationship of traffic density to the atmospheric PAH concentration was also measured a t four Los Angeles sites (6). An improved optical counter with He-Ne laser as a light source (7, 8 ) , capable of detecting particles with diameters of 0.06-10.0 pm, was used to characterize the size distribution of t h e solids from different combustion sources (9, I O ) . This paper is primarily concerned with the correlation between the number concentration of suspended particles and PAH levels in an urban atmosphere as related to traffic. In addition, a n improved collection method for four-membered PAHs in the atmosphere is described. Experimental
Measurement of Particle Size Distribution. An improved optical counter was employed for the measurement of the size distribution and number concentration of particles suspended in air. The minimum detectable size of 0.06 p m in diameter was attained by minimizing the signal-to-noiseratio (S/N) for the scattered intensity of a single particle (8).T h e counter detects particle sizes in three ranges (0.06-0.1,O.l-1.0, and 1.0-10.0 p m in diameter) by varying the diameter of the incident beam in three steps (25 pm, 100 pm, and 1mm). Since the adjustment of the equipment for precision measurements is rather time consuming, the method was used only in a se0013-936X/80/0914-0416$01.00/0
@ 1980 American Chemical Society
