1424
Anal. Chem. 1980, 52. 1424-1426
CONCLUSIONS
for the loan of the lock-in amplifier used in this study.
T h e present double beam-in-time system is a viable configuration which yields good quality PAS spectra. A simple, yet workable cell design, coupled with a linear scanner allows the double beam-in-time mode to be utilized. The simplicity of design offers a substantial reduction in cost since only one microphone, one preamplifier, and one lock-in amplifier are used and construction is straightforward. The double beamin-time configuration permits the use of the ratio mode which provides compensation for variations in the source. Reference overcompensation can occur although this affects only the curvature of t h e base line and not the reproducibility or sensitivity. T h e system described is also easily converted to single beam and differential modes when required. Although the spectra observed are adequate, the acquisition of a more sensitive microphone is expected t o yield a substantial improvement in S/N.
LITERATURE CITED (1) Monroe, D. M.; Reichard, S. H. Anal. Chem. 1977, 4 9 , 119-131. (2) Adams, M. J.; King, A. A.; Kirkbright, G. F. Ana/ysf(London) 1978, 101, 73-85. (3) O'Haver, T. C. "Trace Analysis: Spectroscopic Methods for Elements", Winefordner, J. D., Ed.; W h y : New York, 1976; Chapter 2. (4) Jenkins, F. A.; Whie, H. E. "Fundamentals of Optics"; McGrawHill: New York, 1976; Chapter 21. (5) Rosencwaig, A.; Gersho, A. J. A@. Phys. 1978, 4 7 , 64-69. (6) Eaton, H. E.; Stuart, J. D. Anal. Chem. 1978, 50, 587-591. (7) O'Haver, T. C. J. Chem. Educ. 1972, 4 9 , A131-A221. (8) O'Haver, T. C.; Epstein, M. S.; Zander, A. T. Anal. Chem. 1977, 4 9 , 458-461. (9) Rosencwaig, A. Rev. Sci. Insfrum. 1977, 48, 1133-1137. (10) Blank, R. E.; Wakefield, T. W. Anal. Chem. 1979, 51, 50-54.
RECEIVED for review April 8, 1980. Accepted May 16, 1980. This work was presented in part at the Sixth Annual Meeting of the Federation of Analytical Chemistry and Spectroscopy Societies, Philadelphia, Pa., September 1979.
ACKNOWLEDGMENT The authors thank Princeton Applied Research Corporation
Determination of Nitrogen Oxides and Nitric Acid Vapor by Infrared Spectrometry Jan B. Lefers"' and Pieter J. van den.Berg Laboratory of Chemical Technology, Delft University of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands
of nitrogen oxides with water vapor, nitric acid vapor and/or nitrous acid vapor can be formed ( 8 , 9 ) ,especially at higher concentrations of nitrogen oxides. Very little information, however, can be found in the literature concerning the quantitative analysis of NO, NO2, Nz04,and nitric acid vapor in such gas mixtures. Such information may be of importance for the manufacture of nitric acid and for pollution control purposes. Using infrared absorption Fontanella (10) studied the determination of NO a t a wavenumber of 1915 cm-', of NOz a t 1606 cm-', and nitric acid vapor a t 1326 cm-' in the stratosphere using the sun as source. This method is not applicable a t higher concentrations of NOz because of the strong overlap of the nitric acid vapor and the NOz absorption band. In this work, a method has been developed for t h e determination of NO, NOz, NZO4, and nitric acid vapor in gas mixtures a t concentrations which occur in the manufacture of nitric acid.
A quantitative analytical method for the determination of NO, NO2, N204,and nitric acid vapor In gas mixtures has been developed by infrared spectrometry. The foilowing peaks are used 1908 cm-' (NO), 2908 cm-I (NO,), 2980 cm-' (NZO,), 3160 cm-I (N204),and 895 cm-' ("0,). At rather high concentrationsof HN03,the 2980 cm-I Nz04absorption band shows a small overlap with a weak nitric acid band. I n that case, the N,O4 concentration should be determined with the 3160 cm-' peak or calculated from the equilibrium constant at known NO2 concentrations and temperature. In gas mixtures Containing nitrogen oxides and water, nitrous acid vapor could be detected, especially at high NO/NO, ratios.
For pollution control purposes, much attention has been paid to the determination of nitrogen oxides and several analysis methods have been developed (1-5). The disadvantage of most methods is that they are not applicable in the higher concentration range which occurs in the manufacture of nitric acid. Infrared spectrometry, however, can also be used for the determination of nitrogen oxides a t higher concentrations. Infrared absorption coefficients of NOz and NO for pollution control have been measured as a function of the optical path length and the temperature (6). Guttman (7) investigated integrated absorption intensities of pure NOz a n d N204 a t temperatures of 50 "C u p to 100 "C and a t pressures up to 2 MPa. The results of Guttman indicate that Beer's law is valid. Often water vapor is also present in gas mixtures containing nitrogen oxides. Owing to the reaction
EXPERIMENTAL All spectral measurements were carried out on a Perkin-Elmer Model 117 infrared spectrophotometer. The infrared absorption gas cell was constructed of glass with an inner diameter of 3.5 cm and a path length of 10.0 cm. Silver chloride windows were cemented on the gas cell which was kept in all experiments at a constant temperature of 25.0 *C by a thermostat. A gas loading system was used t o evacuate the gas cell and t o charge and make up gas mixtures of known composition. Details of the procedure have been published elsewhere (20). The points on the calibration curve for nitric oxide were obtained by blending a known amount of nitric oxide (Matheson Gas Products, purity: 99.2%) with dry nitrogen till a cell pressure of 0.1067 MPa (= 80 cm Hg) was attained. The small amounts of nitrous oxide and nitrogen dioxide which are present in com-
Present address: KEMA, Utrechtseweg 310, Arnhem. The Netherlands. 0003-2700/80/0352-1424$01 .OO/O
e
1980 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 52, NO,.9, AUGUST 1980
1425
Table I. Calibration Curves for NO, NO,, N,O,, and Nitric Acid Vapor at 25.0 "C and 0.1067 MPa measured range, wavenumber, calibration curve MPa cm cm-I NO
A A A A A A
1908 1908 2908 2960 3120 895
NO NO1
NP, N1°4
HNO,
= 1.3918
J&T
+
0.005 - 0.12 0.005 - 0.015 0.005 - 0.075 0.005 - 0.035 0.005 - 0.035 0.001.- 0.007
- 0.065
= 7.0186 €'NO b = 8.3739 €'NO, b
0.00210 4- 0.00137 = 5.794 PN,o, b - 0.01245 = 3.902 PN,o, b - 0.00194 = 71.897 P"o, b - 0.0153
Table 11. Amounts of Nitrogen Oxides and Nitric Acid Vapor Found after Partial Oxidation of NO in the Presence of Water Vapor
no.
a
After Partial Oxidation PNO.,kPa PHNO,, kPa (895 c m - ' ) (1980 cm-')
P N , supplied, kPa
P N O , kPa (2904 cm-')
P N ~ O , kPa , (3160 cm-')
12.67 11.85 12.21 13.37 11.48 10.91 12.57 10.87 12.07
4.60
1.51 2.48 2.60 1.93 2.53 0.56 1.01 2.47 0.85
5.95 6.12 5.45 6.12 3.00 4.07 5.60 3.68
-
---a
5.27 0.771 0.817 3.80 0.423 6.27 6.40 0.147 6.64
0.091 0.091 ---a
0.111 ---a a
...
0.156
...a
PN2 O37
kPa
dev. in mass balance, %
0.131 0.024 0.026 0.107 0.013 0.097 0.135 0.004 0.126
5.4 -0.3 0.6 -0.3 2.3 -3.0 1.5 -0.2 1.7
Nitrous acid vapor found in the gas sample. 3500
Wavenumber ( c m - 1 ) 3000 2500
2000
1800
1600
Flgure 1. Infrared absorption spectrum of a gas mixture containing NO, NO2, N,O.,
mercial nitric oxide were small enough to be neglected. Gas mixtures of NO2 which is in equilibrium with N204,were prepared by oxidizing a known amount of NO in the gas cell with dry oxygen at a pressure of 0.1067 MPa. The oxidation of NO is complete within a few minutes and the partial pressures of NO2 and Nz04 were calculated by means of the equilibrium constant measured by Bodenstein and Boes ( 1 2 ) . Nitric acid vapors were prepared by bubbling dried nitrogen gas through concentrated nitric acid solutions (Merck analytical grade) which were kept at a constant temperature of 20 O C . To prevent condensation of the nitric acid vapor and the water vapor, the gas stream was then heated to 25.0 "C and continuously led through the gas cell. The presence of some water vapor did not influence the infrared absorption measurements. The nitric acid concentration in the nitrogen gas stream was changed by varying the concentration of the nitric acid solution. The concentration of nitric acid vapor in the gas stream was determined by adding a known amount of 0.1 N alkali to a gas sample and then analyzing on the nitrate and nitrite content with a colorimetric method. In this method, the nitrate is reduced to nitrite by a copper-cadmium reductor column. The nitrite ion then reacts with sulfanilamide under acidic conditions to form a diazo compound. This compound was coupled with N-1-naphthylethylenediaminedihydrochloride to form a reddish-purple azo dye (12). In the samples, the nitrite concentrations were always very small compared to the nitrate
1400
1200
1000 ,
800
600
I -
and nitric acid vapor
concentration (less than 0.1% of the nitrate content).
RESULTS AND DISCUSSION For each component (NO, NO2,N204,and nitric acid vapor) the absorbance ( A )was measured as a function of the optical path length ( P h )a t wavenumbers at which no absorbance of other components were found. The absorbance of each component was determined with the base-line method. T h e calibration curves were found by regression analysis and are presented in Table I. From this table, it can be concluded that for NO, Beer's law is valid only at low optical path lengths (€0.015 M P a cm). At higher optical path lengths, the absorbance falls off as the square root of the optical path length. Campani e t al. (6) found a much lower absorbance for NO at 1908 cm-' compared with the results presented here. No explanation could be given for this difference, but it may be caused by parameters such as slit width, scanning speed, and gain setting. From Table I, it can be further concluded that the calibration curves of NO2,Nz04,and nitric acid vapor agree with Beer's law. The calibration curve of the 2960 cm-' N204 peak does not go through the origin. This may be due to the small overlap of the NOz band and the Nz04band. A direct comparison of the results concering the absorbance of NO2 and N204at wavenumbers of, respectively, 2908 cm-' and 3160
1426
Anal. Chem. 1980, 52, 1426-1429
cm-' with Guttman's measurements (7) is difficult since his results were measured a t temperatures of 50 O C up to 100 "C. Neglecting the influence of the temperature on the absorbance a t 50 "C, our results agree well with those of Guttman (7). It should be noted that at high concentrations of nitric acid vapor (optical path length >0.003 MPaScm) in gas mixtures containing NOz and Nz04, a small overlap of the 2960 cm-I N z 0 4absorption band with a weak nitric acid band was observed. Therefore, the determination of Nz04 in the presence of high nitric acid vapor concentrations should be carried out with the 3160 cm-' N z 0 4absorption peak or calculated from the equilibrium constant a t known NOz concentrations. In order to study the accuracy of this method, gas mixtures containing NO, NOz, Nz04, and nitric acid vapor were analyzed. These gas mixtures were prepared by partial oxidation of a known amount NO with air in the presence of water vapor in the gas sample cell. After the partial oxidation the gas sample cell was pressurized with nitrogen to 0.1067 MPa. The following reactions and equilibria may occur in the gas sample cell: value of the equilibrium constant at 25.0 O C , MPa-'
+ 0, 2 N 0 , 2N0, 2N,O4_, 2 N 0 , + H,O 2HN0, + HO NO + NO, Z N,O,+ NO, + NO + H,O 2HN0, 2NO
ref.
-f
0.654
11,13
0.001 30
0.0517
18 13
0.140
14-17
The concentrations of NO, NOz, Nz04,and nitric acid vapor in the gas sample cell were determined with infrared absorption using the calibration curves. The small amounts of NZO3which are present in the gas mixtures were calculated using the equilibrium constant. A representative record of the infrared absorption spectrum of such a gas mixture is given in Figure 1.
Small amounts of water vapor did not produce serious optical interference. The initial amount of NO supplied before the reaction was compared to the amount of nitrogen oxides and nitric acid vapor after reaction had occurred in the gas sample cell and equilibrium had been attained. From Table 11, it can be concluded that the deviation in the mass balance is small. In the spectra an absorption band of nitrous acid vapor a t 850 cm-' (19) was sometimes found. The occurrence of the absorption band of nitrous acid can be explained by equilibrium ( 5 ) and its presence is a function of the ratio of the NO concentration and the NOz concentration in the gas sample cell. Nitric acid vapor was detected only in the gas sample a t rather low NO concentrations compared to the NOz concentrations.
LITERATURE CITED Saltzman, B. E.; Cuddeback, J. E. Anal. Chem., 1975, 4 7 , 1. Saltzman, B. E.; Burg, W. R . Anal. Chem. 1977, 49, 1. Allen, J. D.; Phil, M. J . Inst. Fuel 1973, 46, 123. Lievens, F. Rapp. Cent. Etude Energ. Nucl. B.L.G. 1973, 480. Forweg, W. V . D . I . ( V e r . Dfsch. Ing.) 1974, 2 4 , 247. Campani, P.; Fang, C. S . ; Prengle, H. W. Appl. Spectrosc. 1072, 26, 372. Gunman, A. J . Quant. Spectrose. Radiaf. Transfer 1961, 2 , 1. Enghnd, C.; Cwcoran, W. H. Ind. Eng. Chem., Fundam. 1974, 13,373. England, C.; Corcoran, W. H. Ind. Eng. Chem., Fundam. 1975, 14, 55. Fontanella. J. C. Off. Natl. D'EtUdes Recherches Adrosoafiales, 1074, Note technique no. 235. Bodenstein, M.; BOBS. F. Z Phys. Chem. 1922, 100, 68. Technicon AutoAnalyzer 11, Industrial method no. 230-72AITentative 1974. Hisatsune, I. C. J . Phys. Chem. 1061, 65,2249. Ashmore, P. G.; Tyler, B. J. J . Chem. SOC. 1961, 1017. Wayne, L. G.; Yost, D. M. J . Chem. Phys. 1951, 19,41. Karavaev, M. M.; Skortsov, G. A., Russ. J . Phys. Chem. 1962. 36,566. Waldorf, D. M.; Babb, A. L. J . Chem. Phys. 1963, 39,432. Nonhebel, G. "Gas Purification Processes for Air Pollution Control"; Newnes Butterworths: London, 1972; Chapter 5 . Jones, L. H., Badger, R. M.; Moore, G. M. J . Chem. Phys. 1051, 19. 1599. Lefers, J. B. Ph.D. Thesis, Delft University of Technology, Delft, The Netherlands, 1980.
RECEIVED for review July 27, 1979. Accepted May 5 , 1980.
Mixed Immobilized Enzyme-Porous Electrode Reactor W. J. Blaedel" and Joseph Wang Department of Chemistry, University of Wisconsin-Madison,
Madison, Wisconsin 53706
A flowthrough electrochemical cell has been constructed to evaluate an enzyme-porous electrode combination, composed of a Sepharose-bound enzyme which Is packed into a Reticulated Vltreous Carbon (RVC) disk. The effects of various experimental parameters upon the electrode response are descrlbed. Rapid response (15 s) Is obtained due to the lntlmate contact between the enzyme and the electrode surface. High sensitivity accrues to the large surface area, permlttlng substrate measurements to be made at the micromolar concentration level.
T h e application of immobilized enzyme electrodes in chemical analysis systems has greatly increased during the past decade. The enzyme electrodes have generally been 0003-2700/80/0352-1426$01 .OO/O
formed by holding a thin layer of the insoluble enzyme matrix (organic gel or polymer) over the electrode, or by trapping the enzyme in or on a semipermeable membrane that covers the electrode. The principles, applications, and recent developments of enzyme electrodes have been reviewed recently (1-3). Enzyme electrodes can be classified as amperometric or potentiometric. Although the amperometric electrode was the first enzyme device reported ( 4 ) ,its development has lagged that of the potentiometric electrode until recently ( 2 ) . The amperometric device, which usually operates a t a small ratio of electrode surface to solution volume, usually electrolyzes only a small fraction of the electroactive species involved in the biochemical reaction. The amperometric enzyme electrodes are usually limited by their long response time due to the thickness of the enzyme layer or the membrane through which species must diffuse. Further improvement of their 'C 1980 American Chemical Society
