toluene. The column efficiency in each case was calculated from the toluene elution curve by using Equation 3. The experimental conditions and results are given in Figures 10 and 11. These results show that for very small sample volumes the column efficiency increases as the percentage of liquid in the stationary phase is decreased. The reasons for this have been fully discussed by Cheshire and Scott (3). For relatively large samples higher column efficiencies may be obtained by increasing the amount of liquid in the stationary phase. This effect is largely due t o the fact that the value of the effective plate volume, v, increases as the amount of liquid in the stationary phase is increased. Although V, and VI decrease and increase, resrectively, in the same proportion as the amount of liquid in the stationary phase is increased, the effecVJK, tive plate volume, V = V, will increase, because K is usually much smaller than unity. At the same time, however, the resistance of the solute to mass transfer increases as the thicknrss of the liquid film is increased
+
(3) and this effect ultimately overrides the effect of the increase in the effective plate volume. Consequently, there is a limit to the amount of liquid which should be used for the effective separation of large samples. This effect is shown in Figure 10. The optimum amount of liquid used in the stationary phase will depend on the characteristics of the support (6). The effective plate volume may be taken as a measure of the relative amount of liquid in the stationary phase if K remains constant. Using Equations 12 and 13 the column efficiency has been p!otted against the sample volume for various values of u (calculated from the experimentally obtained limiting final retention volumes and no values). The results are shown in Figure 11. The experimental and theoretical curves depicted in Figure 11 show excellent agreement. ACKNOWLEDGMENT
One of the authors (V. P.) is indebted to African Explosives and Chemical Industries, Ltd., for the award of a research fellowship.
LITERATURE CITED
(1) Atkinson, E. P., Tuey, G. P., “Gas
Chromatography,” D. H. Desty, ed.,
p. 270, Butterworths Scientific Publica-
tions, London, 1958.
( 2 ) Carle, D. W.,John T., “Design and A plication of Preparative Scale Gas
CEromatography,” Beckman Instruments, Inc., Fullerton, Calif., 1958. (3) Cheshire, J. D., Scott, R. P. W., J . Inst. Petrol. 44,74-9 (1958). (4) Keulemans, A. I. M., “Gas Chromatography,” p. 100, Reinhold, Sew York, 1957. (5) Ibid.. D. 113. (6j Ibid.; b. 144. ( 7 ) Porter, P. E., Deal, C. H., Stross, F. H., J . Am. Chem. SOC.78, 2999 (1956). (8) Van de Craats, F., private communication, reported by Keulemans, A. I. M., “Gas Chromatography,” p. 65, Reinhold, Sem York, 1957. (9) Van Deemter, J. J., Zuiderweg, F. J., Klinkenberg, A., Chem. Eng. Scz. 5 , 271 (1956). (10) Whitham, B. T., “Vapour Phase Chromatography,” D. H. Desty, ed., D. 194. Butterworths Scientific Publicationt?, London, 1957. RECEIVED for reviex October 29, 1957. Accepted October 5, 1959.
Ultraviolet Determination of Nitrogen Dioxide as Nitrite Ion A. P. ALTSHULLER and A. F. WARTBURG Air Pollution Engineering Research, Roberf A. raft Sanitary Engineering Center, Public Health Service, U. S. Department o f Health, Education, and Welfare, Cincinnati 26, Ohio
b A method for determining nitrogen dioxide concentrations in the 0.01 weight % range and above uses ultraviolet absorption at 355 mp of the nitrite ion formed when nitrogen dioxide is absorbed in aqueous alkaline solutions of potassium or sodium hydroxide. The absorption maximum of nitrite ion a t 21 1 mp with an absorptivity of 5800 mole-‘ liter cm.-l offers possibilities for determining nitrogen dioxide concentrations in the parts per million range. A synthetic mixture of nitrogen dioxide in nitrogen, in a gas cylinder, was analyzed by both ultraviolet analysis and a gravimetric procedure involving absorption of nitrogen dioxide in aqueous potassium hydroxide solution in a series of bubblers. G r a b samples were absorbed in alkaline solution, and then analyzed by ultraviolet analysis. The results of the ultraviolet analyses by both methods of collection were in good agreement, giving a nitrogen dioxide concentraThe gravition of 3.4 weight metric procedure gave somewhat lower results.
yo.
174
ANALYTICAL CHEMISTRY
T
Griess-Ilosvay type coupling methods and the phenoldisulfonic acid method are used widely to determine concentrations ranging from less than 0.1 to above 1000 p.p.ni. of nitrogen dioside or nitric oxide plus nitrogen dioxide in air pollution and industrial hygiene work. The Griess-Ilosvay type procedures are especially useful at very low concentrations (5,9), but i t is not clear how the equivalence of nitrogen dioxide to nitrite varies over the total concentration range of interest (9,5, 9). The phenoldisulfonic acid method, as ordinarily used, is time-consuming, and its accuracy a t about 50 p.p.m. and below has been questioned (9). I n determining the amount of nitrogen dioxide in a cylinder gas mixture of nitrogen dioside in nitrogen, the direct conversion of nitrogen dioxide to nitrite ion and the measurement of the optical absorbance of the nitrite ion a t 355 mp are simple, rapid, and reproducible. No problem of the equivalence of nitrogen dioxide to nitrite exists for this procedure. The reaction for conversion of nitrogen dioxide gas to nitrite ion in HE
strongly alkaline solution has the following uncomplicated stoichiometry (4): 2x02 20H- + IiOtNOaH20
+
+
+
This method should be useful for concentrations of nitrogen dioxide from 0.01 to 10% and above. The authors have measured the molar absorptivities of nitrite ion in hydrochloric acid solutions, in distilled water at p H 5.6, in 0.01M sodium hydroxide solutions, and in 10% potassium hydroxide solutions between 200 and 400 mfi. The molar absorptivity at 355 mM was used to determine the concentration of nitrogen dioxide in nitrogen in a gas cylinder by absorption of the nitrogen dioxide in potassium hydroxide solutions contained in a series of three midget bubblers and in grab sampling flasks. The concentration of nitrogen dioxide also was measured by use of an absorption train which permitted determination of both the weight gain and the change in the ultraviolet absorption of the potassium hydroxide solutions. All these measurements confirm the utility and simplicity of this method at concentra-
tions of nitrogen dioxide above 100 p.p.m. EXPERIMENTAL RESULTS
The molar absorptivities of nitrite ion were determined on a series of five dilutions of 0.455 gram of nitrite in 100 ml. of stock solution of 10% aqueous potassium hydroxide. -4few measurements were made of the molar absorptivities of nitrate ion at p H 5.6 and 11.6 and of nitrite ion and nitrous acid a t p H 1.05,2, and 3.65. All these measurements as well as those on the nitrogen dioxide gas mixture were made between 2000 and 4000 A. using a Cary Model 11 snectroDhotometer with 1-cm. absorption ceiis. The first series of measurements on the concentration of the nitrogen dioxide in nitrogen from a commercially prepared cylinder gas mixture was made by passing the gas stream through a series of three bubblers, The first two bubblers each contained 20 ml. of 10% aqueous potassium hydroxide solution and the third bubbler contained 20 ml. of the Griess-Ilosvay type coupling reagent (1-3). A flow rate of 30 ml. per minute was used to try to ensure good collection efficiency. The pressure drop in this series of bubblers was determined to be equal t o 1.8 inches of mercury. If the flow rate is corrected for this pressure drop, the weight percentages of nitrogen dioxide are increased approximately 3%. The second series of measurements was made by passing the gas stream into a series of two midget bubblers, each containing 20 ml. of 10% aqueous potassium hydroxide solution and, finally, through a tube containing phosphorus pentoxide mixed with glass beads to remoye any water vapor carried over from the potassium hydroxide solutions. The bubblers were weighed before and after passage of the gas stream. The ultraviolet absorption a t 355 nip of the potassium hydroxide solutions also was measured. I n a third series of measurements grab sampling was done using several variations of procedures, which can be divided into three operations of the following order: Operation A. Evacuate flask. Operation B. Add 10 ml. of 2: % potassium hydroside to flask. Operation C. Add nitrogen dioxide to flask. Procedurc I I1 IIIa IIIb 5
Molar Absorptivity xitrite concn,, e, Mole-] Liter Cm.-' MolelLiter 15 min. 45 min. 23.0 22.8 0.0989 22.4 22.2 0.0808 23.0 23.0 0.0593 0.0396 23.4 23.5 0.0198 24.3 24.3 Average 23.2 233.2 Standard deviation 0.7 0 8 RESULTS AND DISCUSSION
The measurements of the molar absorptivity of the nitrite ion a t 355 * 1 mp in 10% potassium hydroxide solutions are shown in Table I. A small number of measurenients n ere also made of the optical absorbance of nitrite solutions between 200 and 400 mp a t p H 11.7, 5.6. 3.65, and 1.05. At p H 11.7 and 5.6, as in 10% potassium hydroxide solutions, the absorption is due to nitrite ion, which possesses a broad absorption maximum at 355 mp and a shoulder in the vicinity of 300 mp. I n 0.01M potassium hydroxide solutions (pH 11,7), and in distilled water (pH 5.6) the molar absorptivities were found to be 24 and 23 mole-' liter cm.-', respectively. Kortiini (6) obtained a molar absorptivity of 22 mole-' liter em.-] a t p H 11.7. Friedman's plot of molar absorptivity LIS. wave length (3) shows the value to be somewhat less than 25 mole-' liter cni.-' .-I molar absorptivity of 23.0 mole-' liter em.-' at 357 mp in 0 . l R sodium hydroxide was reported by Addison, Gamlen. and Thompson (1). I t appears from these data that the molar absorptivity for nitrite ion a t 355 mp is 23.0 i 1.0 mole-' liter cni.-' in the pH range above p H 5. The shoulder in the vicinity of 300 mp shows an inflection point just a t 300 mp Yith a molar absorptivity of 9.2 mole-' liter cm.-1 The nitrate ion has an absorption maximum at 302 mp with a molar absorptivity of 7.0 mole-] liter cm.-' The nitrate ion makes no appreciable contribution to the optical absorbance a t 355 mw.
Step 1
Step 2
Step :3
B. 107, KOH
A B. 1070 XOH C
c c
A A A A A
C B. 570 KOH C B. 10% KOH IV Ca B. 10% KOHb After addition, flask was flushed with nitrogen dioxide mixture. Displaced gas allowed t o escape. IIIC
b
Table I. Molar Absorptivity of Nitrite Ion at 355 Mp in 10% Potassium Hydroxide Solutions
glass stopcocks at'each end.
I n acid solutions somewhat below p H
5, the characteristic fine-structured bands of nitrous acid begin to appear. At p H 3.65 in hydrochloric acid solution, three maxima are observed a t 347, 357, and 368 mp with molar absorptivities of 24.5, 28, and 23.5 mole-' liter cm.-I, respectively. At p H 1.1 in hydrochloric acid solution, five absorption maxima occur a t 337, 346.5, 358, 370, and 384 mw with molar absorptivities of 17, 26.5, 37, 38, and 22 mole-' liter cm.-I, respectively. The intensity maxima increase in value soniem-hat as the pH decreases. Kortum (6) reported similar fine-structured bands a t p H 1.3, but his two most intense maxima had molar absorptivities of 54 mole-' liter cm.-' Longstaff and Singer ( 8 )reported similar banded structures in 4 X sulfuric acid, 4M perchloric acid, and 0.05iv nitric acid nith the two most intense maxima having molar absorptivities between 50 and 60 mole-' liter cm.-' It is felt that the lack of stability and reactivity of nitrous acid made sampling and analysis in alkaline solution more dmirable (4, 8 ) . The nitrite ion a t p H 7 also has an absorption maximum at 211 =t 1 mp with a molar absorptivity of 5800 i:400 mole-' liter cm.-l This value was found to be constant between 1 X and 1 x mole liter-1. Friedman (3) reported a maximum a t 211 mp and a molar absorptivity just under 6000 mole-] liter cm.-' He found the intensity of the maximum to be constant mole between 7 x 10-5 and 6 X per liter. At p H 2 the authors also observed an absorption maximum a t 211 mp, but its molar absorptivity was around 2000 mole-' liter cm.-l This band may be attributed to nitrous acid. This short wave length maximum in alkaline solution can be used to determine nitrogen dioxide down to a concentration of about 1 p.p.m. I n this spectral region the usual single quartz monochromator is just beginning to be subject to problems resulting from low available source energy and appreciable stray light effects, but with care this type of monochromator should be adequate for determinations employing the 21 1-mp absorption peak. Single-prism quartz monochromators have h e m modified to permit measurements belox 200 mp and double-prism and prismgrating instruments can be used around 210 mp without difficulty. Consequently, the absorption peak of 211 mp for the nitrite ion offers possibilities in terms of a simple. rapid, and direct method for determining part per million concentrations of nitrogen dioxide. The results obtained in the dynamic measurements of nitrogen dioxide concentration are given in Table 11. The gas volumes listed have been corrected for pressure drop in the system. The weight per cent of nitrogen dioxide was obtained from the relationship: VOL. 32, NO. 2, FEBRUARY 1960
175
Table II.
Run SO.
Dynamic Measurements of Nitrogen Dioxide Concentration in Gas Cylinder Optical Absorbance at 355 M p 2nd Bubbler. Gas YoL, XOZ) Litersa ccTotal Wt. 70 1st bubbler 2nd bubbler Total
1 2 3
0.02 0.06 0,015 0.00 0.01 0.00 0,005 0,005
0.465 0.465 0.48 0.465 0.50 1.025 1.02 1.01
4
5 6 7 8
0.485 0.525 0.495 0,465 0.51 1.025 1.025 1.015
3.28 3.56 3.35 3.15 3.45 3.44 3.44 0.5 2.05 3.39 .\\wage 3.38 Standard deviation 0,13 4
1.01
I t 14.7 p.8.i. and 70" F.
where A is the optical absorbance a t 355 mp, JI is the molecular weight of the nitrite ion, VI is the liquid volume of potassium hydroxide solution in liters, 2 is the conversion factor, accounting for the 2 molecules of nitrogen dioxide that are needed to produce 1 molecule of nitrite ion, t is the molar absorptivity of nitrite ion in mole-] liter em.-', VN, is the volume of nitrogen in liters passed through the solutions corrected to 70" F. and 14.7 p.s.i., and d h lis the corresponding density in grams per liter, Collection efficiency in the first bubbler mas good. I t can be assumed that essentially all the material which passed through the first bubbler was captured by the second bubbler containing the 10% potassium hydroxide solution or by the third bubbler containing the Griess-Ilosvay type reagent. Appreciably less than 1% of the total amount of nitrogen dioxide captured was detected in the third bubbler, which mas used in only a fen- runs. A collection efficiency of above 90% in the first
Table 111.
Grab Sampling Measurements of Nitrogen Dioxide Concentration in Gas Cylinder
Flask S o . 1
Piocedure"
I I T
I
I1 IIIa
IIIh IIIC 2
bubbler has been reported for nitrogen dioxide collect.ion in 0 . W sodium hydroxide solution a t a flow rate of 1.3 liters per minute using two bubblers in series (b'i. The concentrations involved were below 0.1 p.p.ni. of nitrogen dioside. -4 collection efficiency of 94 to 99% has been found for a Griess-Ilosvay t'ype collecting reagent in a midget bubbler using a flor rate of 0.4 liter per minute and 0.3 to 0.4 p.p.m. of nitrogen dioxide (9). The nitrogen dioxide concentration in this latter work was predet'eriniiied by use of a dilution system for prepamtion of known amounts of nitrogen dioxide. The results of these investigations on collection efficiencies were obtained a t higher flow rates but at' milch lower concentrations of nitrogen dioxide. However, they further support, the conclusion hhat the collection efficiencies ohtained in the present work are high. The weight percentages obtained for nit,rogen dioxide as listed in Table I1 were completely reproducible, giving an average d u e of 3.47, with a standard deviation of ~ 0 . 1 % . I n t'he esperiments listed as runs 6>7 ) and 5 both t'he ultraviolet aljsorbance and the n-eight
I I I IIIa
IIIb IIIC
IT
Absorbance a t 355 RIP 0.20 0,255 0.28 0,248 0 23 0.26 0.27 0.25 0.345 0.27 0.265 0.31 0 27 0.27
Corrected
Gas Volume,* Cc.
266 266 266 266 268 268 268 290 289 289 292 292 292 292 Average Standard deviation
S'ariations discur-sed in detail in experimental section.
* At 14.7 p.s.i. and 70" F. 176
ANALYTICAL CHEMISTRY
NO?, Wt. yo 3.75
3.33.6 3.15 2.95 3.3 3.45 2.95 4.1 3.2 3.1 3.65 3.15 3.15 3.35 0.3
change were determined. Agreement between the two types of measurements was only fair; the \\-eight change determinations gave nitrogen dioxide values between 2.7 and 2.9%. The weight change determinations were complicated by the necessity of making corrections for losses due to carry-orer of water vapor. For these corrections, the phosphorus pentoxide tube was inserted into the train, but the relatively large water vapor transfer compared to the small w i g h t gain due to absorption of nitrogen dioxide limited the accuracy of this met hod, Results of the grab sampling measurements are given in Table 111. The corrected volumes are those obtained after subtracting the volume of liquid solution and a small volume of residue air. The weight per cent of nitrogen dioxide was calculated from the formula given above. The over-all average value for the per cent nitrogen dioxide from the grab sampling measurements is in good agreement with the average value from the dynamic measurements using bubblers. If any real difference exists internally within the grab sampling measurements or in comparison of these measurements with those obtained in bubblers, it is that the use of 0.1X potassium hydroxide solution (which is 0.56 weight %) gives somewhat lower values for nitrogen dioxide than those obtained usiiig 5 or lOyopotassium hydroside solution as the collection medium. This method will not determine nitric oxide alone because the oxide is weakly absorbed in aqueous alkaline solution and will not be converted directly to nitrite ion. If sufficient time elapses between sampling and analysis while the nitric oside exists in the presence of air or oxygen in a sampling container, varying degrees of conversion of nitric oxide to nitrogen dioxide will occur through oxidation, depending on the time of contact and concentrations involved. I n nitric oxide-nitrogen dioxide mixtures, absorption and conversion to nitrous acid will be influenced by the fast reaction: S O 'g)
KO1 ( 9 ) t HzO ( g ) S HSOz (g) ( 4 , 7 , 10)
This reaction probably would affect all methods for the analysis of nitric oxidenitrogen dioxide mixtures to some extent, inasmuch as it influences the net stoichiometry. The possible implications of this reaction in the analysis of nitrogen oxide mixtures from combustion sources should be carefully examined, I n atmospheric analysis, the reaction of nitric oxide plus nitrogen dioxide with n-ater vapor is not significant because of the very low concentrations of the nitrogen oxides ( 7 ) . The interference of water vapor, car-
bon monoxide. and carbon dioxide with the nitrite ion peak at 355 mp is completely negligible. Similarly, chloride. bromide. hydroxide, nitrate, sulfate, and carbonate anions do not absorb above 300 mp. Water-soluble organic acids, esters, and saturated aldehydes and ketones nil1 not interfere with the nitrite ion absorption band, because they have weak absorption bands below 300 nip. Sulfur dioxide as sulfite ion has an absorption band Tvit1-i a maximum in a k a . line aqueous solution a t 225 mp, but the residue absorption at 355 mp is negligible. The hypochlorite ion, ‘210-. which is formed vhen chlorine dissolves in alkaline solution, interferes. This ion has an iiiteiise absorption maximum a t 290 nip and the residue absorptivity a t 355 m u is about half that of the nitrite ion ( 8 ) . Iodide ion could interfere if partial oxidation occurred to form
iodine, because the resulting triiodide ion absorbs a t 352 mp. If the f l o ~rate is increased to 1 to 2 liters per minute with high collection efficiency maintained ( 5 ) ,nitrogen dioxide nitrogen concentrations of a few hundred parts per million can be analyzed by using about 15- to 30-minute sampling periods. At concentrations much below this range the flow rates would have to be excessively high or the sampling period inconveniently long. Consequently, the present method is applicable principally to the analysis of (1) synthetic nitrogen dioxide mixtures, especially those with nitrogen dioxide concentrations in the 0.1 to 10% concentration range, and (2) high temperature combustion sources from which the nitrogen oxide concentrations are likely to be several hundred parts per million and above.
LITERATURE CITED
(1) Addison, C. C., Gamlen, G. X., Thompson, R., J . Chem. SOC.1953, 338. ( 2 ) Altshuller, A. P., Schwab, C. -I., , i S A L . CHEX. 31,314 (1959). (3) Friedman. H. L., J . Chem. Phys. 21, 320 (1953). (4) Gray, P., Yoffe, -1.D., Chert. Rezs. 55, 1069 (1955). (5) Jacobs, 31. B., Hochheiser, S., -%SAL. CHEX.30,426 (1958). (6) Kortum, G., 2. physii. Chem. B43, 418 (1939). ( 7 ) Leighton, P. .I.,Perkins, TI-. -1, “Photochemical Secondary Reactions in Urban Air,” Air Pollution Foundation Rept. 24 (August 1958). (8) Longstaff, J. V. L., Singer, I
