Literature Cited (1) Schuldiner,J. A., Anal. Chem., 23 (ll),1676-80 (1951). (2) Matthews, P. J., J . Appl. Chem., 20,87-92 (1970). (3) Wasik, S.P., J . Chromatogr. Sci., 12,845-48 (1974). (4) Frankenfeld, J. W., Schulz, W., “Identification of Weathered Oil Films Found in the Marine Environment”, DOT Contract No. CG-23,035-A,Exxon Government Research Lab, Linden, N.J., 1974 (availableto the public through the National Technical Information Service, Springfield, Va. 22151, Accession No. ADA-015883). (5) Zeeuw, R. A., Anal. Chem., 40,915-18 (1968). (6) Zeeuw, R. A., ibid., pp 2134-38.
(7) Dept. of Transportation, US.Coast Guard, Office of Research and Development, Washington, D.C., “Oil Spill Identification System”, Report No. CG-D-41-75(available to the public through the National Technical Information Service,Springfield,Va. 22151, Accession No. ADA-003803).
Received for review M a y 9, 1975. Accepted March 22, 1976. T h e mention of brand names i n the preceding report does not constitute endorsement by the U S . Government. T h e opinions or assertions contained herein are t h e private ones of the writers and are not to be construed QS official or reflecting the views of the Commandant or the Coast Guard at large.
Comparison of Ozone Determinations by Ultraviolet Photometry and Gas-Phase Titration W. B. DeMore” and M. Patapoff Jet Propulsion Laboratory, Pasadena, Calif. 91 103
A comparison of ozone determinations based on ultraviolet absorption photometry and gas-phase titration (GPT) shows good agreement between the two methods. Together with other results, these findings indicate that three candidate reference methods for ozone, UV photometry, IR photometry, and G P T are in substantial agreement. However, the G P T method is not recommended for routine use by air pollution agencies for calibration of ozone monitors because of susceptibility to experimental error. The absolute accuracy of ozone calibration methods has been of considerable interest to environmental chemists in recent years. Much of this activity stems from a report by Boyd and coworkers ( 1 )which questioned the accuracy of the neutral-buffered iodometric method of ozone analysis. Since the latter method forms the basis of the Federal Register method ( 2 )for calibration of ozone monitoring instruments, a number of groups have attempted to evaluate the accuracy of iodometric methods with reference to independent standards (3, 4 ) . Somewhat different versions of the iodometric method previously used in California have recently been examined by the Ad Hoc Oxidant Calibration Committee ( 5 )of the California Air Resources Board (CARB) and also by the Statewide Air Pollution Research Center (6), University of California, Riverside, Calif. (SAPRC). These studies and others (7,8) show that the neutral-buffered KI method reads high by 10-30%, depending on details of the procedure. One problem encountered in connection with the foregoing evaluations was the fact that relatively few methods for absolute ozone measurement were available, and the reliability and intercomparability of the various methods had not been well established. The principal candidate methods are ultraviolet photometry, infrared photometry, and gas-phase titration (GPT). The UV and IR methods have recently been compared in joint tests conducted by the CARB Oxidant Calibration Committee and the SAPRC group (5, 6). The results showed excellent agreement (within approximately 2%) between the two methods, which are completely independent. The situation was less clear, however, with respect to the G P T method. For example, the G P T results disagreed by 9% with the UV standard in the CARB Oxidant Committee tests ( 5 ) .Also, the GPT method was previously reported to be in good agreement with the neutral-buffered KI method ( 3 ) ,and the latter method is now known to be incorrect.
In the present work we have attempted to resolve this discrepancy by conducting further comparisons of the UV and G P T analyses. The results show good agreement between the two methods and provide further substantiation of the intrinsic accuracy of all three reference methods.
Experimental According to the UV method, the O3 concentration is given (in ppm) by lo6 T I, Odppm) = -log 273 Pkl It where:
T = temperature, K
P
= total pressure, atm
k = extinction coefficient of ozone, base 10, units cm-l atm-’ (STPI 1 = path length, cm I , = intensity with carrier gas only I t = intensity with O3 present The extinction coefficient for O3 a t 254 nm has been measured by several groups (9-14) using a variety of physical methods to measure the 0 3 concentration. Table I lists the results of these studies and the methods used. The manometric method is based on pressure measurements of gaseous 0 3 , requiring in a t least one case ( 9 )a substantial and somewhat uncertain correction for decomposition. The method of decomposition stoichiometry depends on the pressure change which accompanies the decomposition:
2 O3
30
2
Clyne and Coxon (13)determined O3 concentrations in a flow tube by means of titration with NO, so that their method was essentially equivalent to the GPT. The value used in the present work was 135 cm-l atm-l, which we consider to be the preferred value on the grounds that the lower values may have been the result of O3 decomposition. Photometric Apparatus and Methods. There are three components to the photometric apparatus, the light source, the absorption cell, and the light detector (Figure 1).A PenRay low-pressure mercury lamp (Ultraviolet Products) was used as a source of the 254-nm light. A Y4-m Jarrel-Ash monochromator was used to spectrally isolate the 254-nm line. (This component is not strictly necessary and can be replaced Volume 10, Number 9, September 1976
897
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Table 1. Ozone Extinction Coefficient Measurements OZONE METER, DASIB I M 3 - A H
k,
i r-
HEC 1200
cm-' atm-1 STP,
Authors
Ref
Inn and Tanaka (1953) Hearn (1961)
9 10
133 134
DeMore and Raper
11
135
12 13 14
132 136 (250 nm) GPT
(1964) Griggs (1968)
Clyne and Coxon (1968) Becker et al. (1974)
base 10
135
/-
Method
Manometric Decomposition stoichiometry Decomposition stoichiometry Manometric
hi
Manometric
N O , N2
t
-& I METER
03, AIR
MTER
HASIINGS O-Yrm Y S r n
HASTING8
0-503 %CEm
~ ~ o - i m r ~ m
Figure 2.
Figure 1. Schematic diagram of
by an interference filter to isolate the 254-nm line.) Collimation of the light beam was aided by use of a 25-cm focal length quartz lens. Stability of the light source was enhanced by operation from a constant voltage transformer and by enclosing the lamp in an aluminum block for increased thermal inertia. The absorption cell is constructed of 2-in. 0.d. Pyrex tubing, and has a path length of 5.00 m. Quartz windows are attached a t each end by means of epoxy cement, with care being taken to minimize internal exposure of cement. The 254-nm light emerging from the cell is monitored with a Hammamatsu-type R166 solar-blind, side-on photomultiplier tube. The photomultiplier is operated a t 340 V. The photomultiplier output voltage is measured across a 10-MR resistor and is read with a four-digit voltmeter. A 0.4-pfd condenser is placed in parallel with the resistor to obtain a suitable time constant. Ozone was generated in a stream of cylinder air by means of a photochemical generator of conventional design, employing a Pen-Ray low-pressure mercury lamp with sliding shield to vary the O3 concentration. The gas flow through the cell, usually 5-7 l./min, enters near one end of the cell and exits a t the opposite end. I t has been established experimentally that there is no appreciable gradient of ozone concentration in the cell. Teflon tubing of lh-in. 0.d. was used for all connections. To facilitate rapid data acquisition during the comparisons with the G P T method, a Dasibi Model 1003-AH was calibrated against the UV photometer, thus serving as a secondary or "transfer standard". The following procedure was used for calibration of the Dasibi meter. A "span factor" of 56.6 was placed in the instrument, because this value gives approximate agreement with the absolute photometer under the temperature and pressure conditions of our laboratory (T= 23 f 2 "C, P = 0.96 atm) (730 torr). The Dasibi instrument was connected to the photometer a t midlength of the absorption cell. With carrier gas flowing, photomultiplier output voltages were read periodically during the calibration procedure and were plotted as a function of time. A t intervals, I t , readings were taken at ozone concentrations in the range 0.05-0.85 ppm of 03,and the time of the readings was recorded. Simultaneously, readings were taken with the Dasibi meter. For calculation of the ratio Zo/Zt, the appropriate values of I , were taken from the graph of I , vs. time. This procedure accounted for the drift in I,, which was small (about 0.5% per hour) but 898
Schematic diagram of gas-phase titration apparatus
ultraviolet ozone photometer
Environmental Science & Technology
not negligible. Dark current readings were measured but were negligible with the solar-blind photomultiplier tube and light intensity used in this work. During the calibration, the atmospheric pressure and room temperature were recorded. The relationship between the photometer readings and the Dasibi readings was then expressed as a linear least-squares equation: Os(phot) = (0.999 f 0.003)03(Dasibi) - (0.007 f 0.002) where the uncertainties refer to 95% confidence limits. These small error limits are indicative of the high precision of the readings taken from both instruments. T o use the above equation a t a different ambient pressure, the span factor would be changed in inverse proportion to the pressure relative to the calibration pressure. In practice, such corrections were unnecessary because pressure fluctuations were less than 1%.Temperature corrections to the span factor were also not required since temperature changes in the laboratory were less than f 2 "C. GPT Apparatus and Methods. A schematic diagram of the G P T apparatus is shown in Figure 2. The NO/N2 mixture was obtained from a Union Carbide cylinder having a nominal concentration of 52.3 ppm NO. Calibration of this mixture relative to a standard NBS cylinder (45.9 ppm, fl.l%estimated upper limit of error, 95% confidence limit) indicated a true concentration of 54.3 ppm, and the latter figure was used in the calculations. The O3/air flow was obtained from the source described above. Both flows were measured with mass flow meters (Hastings-Teledyne),which were calibrated for N2 and air immediately prior to each daily use by means of bubble meters. The mixing and reaction vessels consisted of two 1-1. bulbs placed in series. The effluent gas was sampled for 0 3 with the calibrated Dasibi instrument, and the NO was monitored with an MEC Model 1200 chemiluminescent NO meter. The response of the NO detector was slightly nonlinear in the range of interest, 0.5-2.5 ppm NO. Therefore, the response was calibrated each day by preparation of known NO concentrations based on flow measurements. The quantities measured in the GPT experiments were the changes in 0 3 and NO concentrations, A 0 3 and ANO. Based on unit stoichiometry for the reaction 0 3
the relations are
+ NO
+
NO2 + 0
2
A 0 3 = AN0
(03i - 03f)= (NOi - NO/) where 03iand O3f are the initial and final 0 3 concentrations, and similar definitions hold for NO. In practice, excess NO was used, so that the residual O3 concentrations were small, but not always negligible. The presence of excess NO also precludes possible error due to secondary reaction of O3 with NOg, because the rate constant for the 0 3 NO reaction is about 500 times greater than that of the O3 NO2 reaction a t room temperature (14). The quantity 03iwas obtained from the following expression:
+ +
Table II. Summary of Results NO. DDm Initial
Final
lnlilal
Final
ANO, ppm
A031 ppm
a a a
2.025 2.010 2.015 2.240 2.365 2.080 1.115 1.130 1.130 1.130 1.130 2.270 2.270 2.270
1.505 1.400 1.245 1.625 1.755 1.330 0.815 0.575 0.425 0.440 0.750 1.545 1.325 1.900
0.519 0.599 0.762 0.602 0.610 0.767 0.311 0.600 0.781 0.738 0.394 0.738 0.950 0.374
0.005 0.005 0.009 0.001 0.005 0.009 0.009 0.032 0.057 0.050 0.0‘10 0.005 0.005 0.003
0.520 0.610 0.770 0.615 0.610 0.750 0.300 0.555 0.705 0.690 0.380 0.725 0.945 0.370
0.5 14 0.594 0.753 0.601 0.605 0.758 0.302 0.568 0.724 0.688 0.384 0.733 0.945 0.371
b b
b C C
C C C
The quantities F a i r and F N O are the respective flow rates of the O3/air and NO/N2 mixtures, and ( 0 3 i ) m e a s is the O3 concentration as measured with the NODI2 flow turned off. Since O3f was measured with both flows on,
Both NO‘ and NOf were obtained from the calibration data of detector output vs. NO concentration. For purposes of calibration, the NO concentrations were obtained from the equation
NO =
NO Fair
x
NOcylind
+ FNO
where NOcy*lnd is the concentration of NO in the standard cylinder. The O3/air flow rates were in the range 4-5 l./min, and the NO/N2 flow rates were in the range 50-250 cc/min, thus giving NO concentrations of about 0.5-2.5 ppm. The O3 concentrations were in the range 0.3-0.95 ppm. Results and Discussion
The data, shown in Table 11, show good agreement with the predicted relation, AN0 = Ao3. A linear least-squares treatment gives A 0 3 = (1.00 f 0.05)ANO
- (0.00 f 0.01)
This result shows that 0 3 determination by UV photometry is in good agreement with results obtained by the G P T method, which is traceable to the standard NO/N2 mixture prepared by the Bureau of Standards. Similar conclusions were recently reported by Hodgeson ( 7 ) and by Paur (8). It may therefore be concluded that the three candidate methods for 0 3 measurement, UV photometry, IR photometry, and GPT, are in substantial agreement, since previous comparisons (5,6)have demonstrated equivalence of the UV and IR methods. All three methods are in poor agreement with the neutral-buffered KI procedure (5,6),indicating that the latter method is not suitable for calibration of ozone monitors. The question arises as to which methods may be recommended as replacements for the iodometric method. The simplest and most accurate method is to use a Dasibi instrument which has been calibrated against an ultraviolet photometer by a procedure such as that described in the present work. This method was recommended by the CARB Oxidant Calibration Committee and has been in use in California since June 1,1975. The IR photometric method requires relatively complex and expensive apparatus and therefore will probably not find extensive use as a laboratory standard for ozone measurement.
03, ppm
Seriesa
d d d
Each series represents data taken during one day.
Our experience indicates that the GPT method is accurate in principle, but that in practice, excessive care must be exercised to obtain accurate results. The susceptibility of this method to error is shown by the fact that it has in the past been reported (3)to agree well with the neutral-buffered KI procedure, which is now known to be incorrect. Further, the G P T method showed an error of 9% in tests conducted by the CARB Oxidant Calibration Committee ( 5 ) .The reasons for these previous erroneous results are not known but may be associated with difficulties in the measurement of flow rates. In any case, the method is not sufficiently reliable for routine use as a calibration standard. L i t e r a t u r e Cited (1) Boyd, A. W., Willis, C., Cyr, R., Anal. Chem., 42,670 (1970). (2) Federal Register, 36 (228), 22392 (November 25,1971). (3) Hodgeson, 3.A,, Baumgardner, R. E., Martin, B. E., Rehme, K. E., Anal. Chern., 43,1123 (1971). (4) Kopcynski, S. L., Bufalini, J. J., ibid., p 1126. (5) DeMore, W. B., Romanovsky, J. C., Feldstein, M., Hamming, W. J., Mueller, P. K., “Comparison of Oxidant Calibration Procedures’’, final report of the Ad Hoc Oxidant Measurement Committee of the California Air Resources Board, February 3,1975; also presented a t the ASTM Symposium on Calibration Problems and Techniques, University of Colorado, Boulder, Colo., Aug. 5-7, 1975. (6) Pitts, J. N., McAfee, J. M., Long, W. D., Winer, A. M., Enuiron. Sci. Technol., submitted for publication. (7) Hodgeson, J. A,, ASTM Symposium on Calibration Problems and Techniques, University of Colorado, Boulder, Colo., Aug. 5-7,1975. (8) Paur, R. J., ibid. (9) Inn, E.C.Y., Tanaka, Y., J. Opt. SOC.Am., 43,820 (1953). (10) Hearn, A. G., Proc. Phys. Soc., 78,932 (1961). (11) DeMore, W. B., Raper, O., J . Phys. Chem., 68,412 (1964). (12) Griggs, M., J . Chem. Phys., 49,857 (1968). (13) Clyne, M.A.A., Coxon, J. A., Proc. Roy. Soc., A303, 207 (1968). (14) Becker, K. H., Schurath, U., Seitz, H., Int. J . Chem. Kinet., VI, 725 (1974).
Receiued for review October 6,1975. Accepted March 22,1976. T h i s paper presents the results of one phase of research carried out at the Jet Propulsion Laboratory, California Institute of Technology, under Contract No. NAS7-100, sponsored by the National Aeronautics and Space Administration.
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