Table V. Removal of NOP with Aqueous Solutions of Organic Reagents

Reagent €1 ( NOz) Remarks

(1) pAminophenol 0.91 0.03 molll. (2) Neutral red 0.98 0.1 mol/l., NO was evolved (3) Xanthine 0.87 0.05 mol/l. + NaOH (0.05 mol/l.) (4) Hydroquinone 0.57 0.1 mol/l. (5) Pyrogallol 0.61 0.1 mol/l. (6) Methyl red 0.57 0.1 mol/l. (7) Crystal violet 0.81 0.01 mol/l., NO was evolved (8) Methyl orange 0.94 0.025 mol/l. (9) Pyoctanin blue 0.73 Saturated in water (10) Phenolphthalein 0.87 0.01 mol/l. + NaOH (0.06 mol/l.)

ergy of the reaction (8, 9). The efficiencies obtained a t the initial period of the removal are plotted in Figures 1 and 2 against the standard redox potentials of possible half reactions in which the reagents take part. The redox potentials (10) used in Figures 1 and 2 are listed together with their half reactions in Tables I11 and IV, respectively. The reagents which have lower standard redox potentials than -1.4 V are effective for NO removal, whereas the efficiency of the NO1 removal is increased as the redox potential of the reagent is increased.

Organic Reagents. Table V shows the results of NO2 re- moval with aqueous solutions of various organic reagents. The organic reagents are also favorable for NO2 removal. As Figure 3 shows, the relation similar to the plots in Figure 2 holds also for the organic redox system.

The standard redox potential of the reagents is therefore an important parameter for the prediction of the efficiency of NO, removal when either an organic or inorganic reagent is employed. The oxidation-reduction reaction (the electron transfer from or to nitrogen oxides) should then be involved in the removal as one of the rate-determining steps.

Literature Cited (1) Ganz, S. N., Lokshin, M. A,, Priklad. Khim., 30,1525 (1957). (2) Ganz, S. N . , J . Appl . Chem. U S S R , 29,1107 (1956).

1.0 I ~ ” “ ’ ” ’

0

I I

4

-1 0 1

S t a n d a r d Redox P o t e n t i a l ( v o l t )

Figure 3. Standard redox potentials of organic reagents vs. efficiencies of NO2 removal Numbers in figure: see Table V. Standard redox potentials: Clark ( 7 7). Those for methyl red, crystal violet, methyl orange. pyoctanin blue, and phenolphthalein not available. Concentration of reagent: 2, 4, 5, 0.1 mol/l.; 1, 0.03 mol/l.; 3, 0.05 mol/l.

Stain Method for Measurement of Drop Size

Lung Cheng Pittsburgh Mining and Safety Research Center, Bureau of Mines, 4800 Forbes Avenue, Pittsburgh, Pa. 15213

w A theoretical relation between water drop and stain di- ameters, D and S, presented elsewhere is in agreement with the empirical expression of D = a s b , where a and b are con- stants. The present theoretical value of a depends upon im- pact velocity and the natures of the liquid and target surface, and the value of b equals jiS. For large drops moving with ter- minal velocities, the value of a becomes insensitive to impact velocity, and the value of b becomes 2/3 . Values of D are pre- dicted to within f16% for a wide range of drop sizes and ve- locities and to within f 2 % for large drops moving a t their terminal velocities.

A popular method for measuring the size of liquid drops is the stain technique wherein the drop is allowed to impact onto a target surface and leave a permanent impression or stain.

(3) Ganz, S. N., Mamon, L. I., ibid., 30,391 (1957). (4) Wendel, M., Pigford, R. L., AIChE J., 4,249 (1958). ( 5 ) Ganz, S. N.,J . ADDL Chern. USSR. 26.927 (1953). (6) Kato, S., Chem. &g., Jpn . , 1086 (1973). ( 7 ) Lowell, P. S., Parsons, T. B., “A Theoretical Study of NO, Ab-

sorDtion Usine Aaueous Alkaline and Drv Solvents”. Technical Report, OAP eont‘ract No. EHSD 71-5, 1i71.

(8) Wells, P. R., Chem. Reo., 39, 171 (1962). (9) Leffler, J. E., Grunwald, E., “Rates and Equilibria of Organic

Reactions”, Wiley, New York, N.Y., 1963. (10) Latimer, W. M., “The Oxidation States of the Elements and

Their Potentials in Aqueous Solutions”, Prentice-Hall, New York, N.Y., 1952.

(11) Clark, W. M., “Oxidation Reduction Potentials of Organic Systems”, Williams & Wilkins, Baltimore, Md., 1960.

Received for review November 21,1975. Accepted August 31, 1976. Work supported in part by a grant from the National Research Zn- stitute for Pollution and Resources.

Stains have been produced, for example, on solid surfaces coated with a layer of lampblack (1,2), magnesium oxide ( 3 ) , or liquid-sensitive films ( 4 , 5 ) , on Ozalid paper ( 6 ) , and on filter papers (7-11). The drop and stain diameters, D and S, are reportedly related by the expression

D = asb (1)

where a and b are empirical constants. Numerical values of a and b appear to depend upon the specific experimental conditions (namely, the impact velocity of the drop and the nature of the target surface), Le., a ranges from 0.33 to 0.47 (6-8, IO), while b ranges from 0.67 to 0.93 (6-10). While b = 0.67 according to “simple” geometry (presumably, the simple geometry assumes the volume of the impacted drop is pro- portional to the square of the stain diameter) (7,8,10) and b = 1.0 for the impacting drop taking a form of a spherical cap (12,13), and a is predicted by the spherical model in terms of

192 Environmental Science & Technology

the contact angle between the liquid and the target surface and is not predicted by the “simple” model, a general theo- retical model for a and b formulated in terms of the impact velocity of the drop and the natures of the liquid and target surface does not appear to have been previously given. This paper presents such a relation in terms of a previously re- ported model (14) for liquid drops impacting a target sur- face.

Consider a spherical liquid drop impacting a smooth, inert solid surface. Upon impact the drop tends to flatten into a disc and undergo dampened oscillatory motion. If the stain di- ameter is equal to the maximum diameter of the flattened drop, the ratio of the stain-to-drop diameter is proportional to the Weber number raised to power (14), and thus it can be shown that

where u is the surface tension of the liquid against air, p w is the density of the liquid, V is the impact velocity, and C is the ratio of the experimental to the theoretical stain diameter and denotes departure from the theoretical model. [To follow the symbols of Equation 1 which are familiar to stain workers (6, 8-10), a change of symbols of D to S and d to D from Equation 6 of ref. 14 was made.] Figure 1 plots experimental values for water drops impacting onto a bituminous coal surface (14). The observed value of b was ‘$, and was independent of drop size and velocity over the noted range of conditions, thus agreeing with Equation 2. The observed value of a was 0.2-0.6 and was independent of drop size but dependent upon drop velocity. Values of a ranging from 0.2 to 0.6 as shown in Figure 1 were agreeable for impact velocities of 1000-1500 cm/s and less agreeable for velocities outside this range, e.g., 0.71 vs. 0.60, Le., C = 1.1-1.2 for the coal surface for an impact velocity of 250-450 cm/s and C = 1.2-1.5 for beeswax, cellulose acetate, and glass surfaces (14) for conditions for which high-speed photographs indicated that the impacting drop formed a reasonably flat disc. A smaller value of C was observed for a higher impact velocity, and photography indicated departure from a flat disc. Jarman (9) reported that b = 0.82-0.86 but did not report a value for a .

Most experimental studies of the stain method used drops falling at their terminal velocities. For a liquid drop larger than 1.5 mm in diameter, the terminal velocity (15) is

3.0 gD(p , - p a ) 112

V = [ Pa I where g is the gravitational acceleration and p a is the density of the air. In this case,

(3)

[Deformation of water drops falling a t terminal velocity was reported by, for example, Magono (16). We estimated the sphericity of the deformed drop to be >0.9 and a ratio of the maximum impact force to the air resistance to be 6 X lo?; the deformation of the incident drop thus has a negligible effect on the stain diameter.] The theoretical value of b now has a constant value of ?(j and is independent of drop size and ve- locity, agreeing with the results reported by Marshall et al. (7) and Fournier D’Albe and Hidayetulla ( 8 ) and approximately agreeing with the value of 0.74 reported by Engelmann (6) and 0.75 by Magarvey (10). The theoretical value of a is 0.43 for water if C = 1.0, agreeing with the reported values of 0.43 (6, 7 ) (the value of a = 1.0 reported in ref. 7 appears to be an error) and 0.47 ( B ) , but not with the reported value of 0.33 (10). (However, the data in ref. 10 seemingly can also be satisfac- torily matched by D = 0.43 S2l3.) These four references used absorbing paper as the target surface, and the close match

; I

a 5

0 4 0

3

-F 330 0 I 500 + 2000 * 2500

between theory and experimental data implies negligible in- teraction between the impacting drop and the absorbing target surface during the fi2 ms involved in the first spreading of the impacting drop (14).

The theoretical model giving Equations 2 and 3 assumes the liquid drop impacts an inert surface with sufficient velocity to spread into a flat disc, and such a model cannot explain b larger than ’Y5. I t is, however, likely (12, 23) that the value of b approaches 1.0 for a small, low-velocity drop as a limiting approximation.

Measurement of drop size is inherently important to such problems as scavenging of airborne particles by raindrops ( 6 ) , suppression of dust by water sprays (14), and elimination of drop carryover from cooling towers which may damage the environment ( 1 1 ) . While a calibration curve relating D and S must be experimentally determined if an appreciable in- teraction with the target surface is expected or if the drop size or velocity is outside the present range (Figure l), the present flat-disc model hopefully provides a clearer understanding of the stain technique and should be sufficiently precise for most purposes. Thus, even ignoring the effect of C, one obtains the drop size within f16% from Equation 2 for drop sizes from 0.2 to 1.44 mm and drop velocities from 100 to 2500 cm/s, and within f 2 % from Equation 3 for large drops (1.5 to -7 mm, the critical size for drop breakup, e.g., ref. 8 ) moving with terminal velocities. Somewhat surprisingly, the present model also appears to be useful for a filter-paper target surface.

Acknowledgment

suggestions during preparation of the text.

Literature Cited (1) Lee, D. W., “The Effect of Nozzle Design and Operating Condi-

tions on the Atomization and Distribution of Fuel Sprays”, NACA Rep. No. 425, 1932.

(2) Strazhevski, L., Tech. Phys. USSR, 4,978 (1937). (3) May, K. R., J . Sci. Instrum., 22, 128 (1950). (4) Liddell, H. F., Wootten, N. W., Q. J . R. Meteorol. Soc., 83, 263

(5) Farlow, N. H., French, F. A,, J . Colloid. Sei., 11, 177 (1967). (6) Engelmann, R. J., “The Hartford Raindrop Sampler and Selected

(7) Marshall, J. S., Langille, R. C., Palmer, M. Mck., J . Meteorol., 4,

( 8 ) Fournier D’Albe, E. M., Hidayetulla, M. S., Q. J . R. Meteorol. SOC.,

The author thanks Welby G. Courtney for his valuable

(1957).

Spectra”, AEC Res. and Dev. Rep. HW-73119, 1962.

186 (1947).

81,610 (1955).

Volume 11, Number 2, February 1977 193

(9) Jarman, R. T., ibid., 82,352 (1956). (10) Magarvey, R. H., J . Meteorol., 14,182 (1957). (11) Martin, A., Barber, F. R., Atmos. Enuiron., 8,325 (1974). (12) Ford, R. E., Furmidge, C. G. L., “Impact and Spreading of Spray

Drops on Foliar Surfaces”, Sci. Chem. Ind. Monograph 25, Symp. on Wetting, pp 417-432,1967.

(13) Bikerman, J. J., Ind. Eng. Chem., 13,443 (1941). (14) Cheng, L., Ind. Eng. Chem., Process Des. Deu., in press

(1977).

pp 2-6, Wiley, New York, N.Y., 1953. (15) Giffin, E., Muraszew, A., “The Atomization of Liquid Fuels”,

(16) Magono, C., J . Meteorol., 1 1 , ~ (1954).

Received for reuiew March 15, 1976. Accepted September 21, 1976.

PAN Measurement in Dry and Humid Atmospheres

William A. Lonneman U.S. Environmental Protection Agency, Environmental Sciences Research Laboratory, Research Triangle Park, N.C. 277

The effects of water on the gas chromatographic (GC) analysis of peroxyacetyl nitrate (PAN) are not observed in our laboratory studies. A series of PAN calibrations demonstrates the independent nature of water and PAN on our GC systems. On the basis of these results, the water anomaly observed by others is probably unique to their system.

A recent publication by Holdren and Rasmussen ( I ) dealt with the effect of moisture on peroxyacetyl nitrate (PAN) analyses. In brief, the authors reported on a series of experi- ments that were performed with a variety of GC-electron capture systems which demonstrated a reduced response for PAN when samples were a t relative humidities of 30% or lower. They observed as much as a tenfold decrease in PAN response when the relative humidity of the sample air was near 0%. The authors suggest that a possible cause for this water anomaly was a column sample interaction.

In the past 10 years, we have performed many GC-electron capture analyses for PAN and PAN-type compounds in both ambient atmospheres and smog chamber irradiation studies of hydrocarbon-nitrogen oxides mixtures. In those studies air samples of PAN were analyzed a t relative humidities ranging from 30 to 100% with no apparent water vapor effect upon the PAN response. The lack of such an effect, however, was more clearly demonstrated during instrument standardization where PAN standards in both humid and dry atmospheres were analyzed with no apparent effect on PAN response. Presented here are the results of three such calibrations.

Experimental and Discussion The GC system for PAN used in our smog chamber studies

was a 9 ft X in. o.d. glass column packed with 10% Carbowax 600 on Gas Chrom Z. This length of GC column was used to permit resolution of alkyl nitrate products. Retention times for PAN and water vapor with this system were 8.8 and 23 min, respectively. The GC system was operated a t 30-min intervals to analyze the chamber contents. A typical procedure for standardization was to replace one of the 30-min chamber samples with a series of PAN calibrations. These standard- izations were performed by the dynamic dilution of calibrated cylinders of PAN with dry nitrogen. An example of such a calibration is shown in Figure 1. In this case, a 10.2 ppm PAN standard was dynamically diluted by a factor of 170 with dry nitrogen to produce a PAN sample of 60 ppb which was in- jected onto the GC column. Dry nitrogen or air was the pre- ferred diluent since either eliminated the necessity of waiting for the resolution of a water peak. In this way, PAN standards could be repeated several times during this 30-min time pe- riod.

Generally, the first PAN standard sample was injected onto the GC immediately after the resolution of the PAN peak from

I / METHYL NITRATE

1

Figure 1. Dynamic calibration for PAN using 10.2 ppm standard cylinder of PAN in dry nitrogen and cylinder of prepurified grade nitrogen, Feb- ruary 2, 1971 Column used was 270 cm X 3.2 mm 0.d. glass column packed with 10% Car- bowax 600 on Gas Chrom 2

the previous chamber sample. This permitted a 14-min period for the elution of the first PAN standard peak before the elution of the water peak from the previous chamber sample. A second PAN standard sample was injected onto the GC column immediately after the elution of the first PAN stan- dard peak. A graphic illustration of these standards is shown in Figure 1. In effect, Figure 1 shows two identical 60 ppb PAN samples on the GC column, one with water a t approximately 50% relative humidity from the previous chamber sample with the possibility of a water-PAN interaction and one without water and no possibility of a interaction occurring.

If PAN and water interactions occurred in the GC system, one would expect to see a definite difference in response be- tween the two PAN standard samples; however, no difference was observed. Similar calibrations were performed many times during a two-year chamber program with PAN concentrations ranging from 25 to 300 ppb with no water anomalies ob- served.

In field study calibrations, the usual procedure for PAN standardization is the syringe injection of an aliquot from a concentrated PAN standard bag into a second bag containing a metered volume of dry air or nitrogen. The concentrated PAN bags are calibrated by IR absorptivities ( 2 ) . For example, in one specific case, a 10-cc aliquot of a 108 ppm PAN sample was injected into a Tedlar (PVF) bag containing 100 1. of dry nitrogen. Since Tedlar is permeable to water from the sur- rounding atmosphere, the water content inside the bag in- creased with storage time. Results of two such calibrations are given in Figures 2 and 3. Since the permeation rate of water through Tedlar is 2.8 g/(lOO in.*) (24 h) (mil) a t 39.5 “C (3) , it takes very little time for the relative humidity in the bag to

194 Environmental Science & Technology