sieved it still tended t o plug up the column. The reactive selection was changed every 10-12 samples. Samples of 1 p1 were injected. Chemicals. The following were used : Span-60, sorbitan monostearate; Tween-60, sorbitan monostearate polyoxyalkylene ; Tween-80, sorhitan monoleate polyoxyalkylene: soda-lime beads, 30 mesh : Carbowax 20 M ; Chrotnosorb-T; and Itstant Whip. Procedure. A can of whipped cream was placed in a n acetone-dry ice bath. The g2s in the can was released through the nozzle after 30 minutes. The top of the can was then removed and 25 grams. of sample were placed in a 500-ml separatory funnel. Twenty-five milliliters of H20 and 50 ml of absolute alcohol were added and mixed thoroughly. One hundred twenty-five milliliters of ether were added with shaking for one minute. One hundred twenty-five milliliters of petroleum ether were added and shaking repeated for one minute. The aqueous alcohol layer was discarded. The ether layer (containing polysorbates) was washed with 25-ml portions of 1 :1 water-alcohol. ‘ f i e washings were discarded. ’The ether phase was transferred to a 400-ml beaker, evaporated to 10--15 ml, nnd made t o volume in a 25-ml volumetric flask. RESULTS AND DISCUSSION
Extracted polysorbates were saponified in the normal maniier and the saponification mixture was injected into the
chromatograph with a nonreactive column. In all cases, three peaks were obtained, which were the same as those obtained by hjecting tlie polysorbate directly onto the reactive column. A real sample, whipped crcam, was treated by the official method and then .injected into the chromatograph. The same peaks were presun!. but had retention times about 10 seconds longer because of the additicnal time in going through the reactiw c.olamn. The first peak to emerge was tile solvent---ether. The second peak was glyierol, whish is fornied when the glycerides are saponifid. The third peak was the polyol. Polysorbates are mixtures of the polyols of sorbitol and it was expected that these would also be separated. This was not the case. This has the advantage qf making the determination easier to quantitate bst t.he disdvantage oE not being able to identify each compma?t. It is possible that the polyols are further converted to sorbitol since the retention time of sorbitol is the same as the polyol peak: our attempts to prove this hRve thus far failed. It was found that the soda-lime beads had to be replaced after about 10 injections.
RECEIVEDfor review April 9, 1970. Resubmitted and accepted April 2, 1971. This work was supported by a grant from the National Science Foundation.
Stoichiometry in the Neutral lodometric Procedure fer Ozone by Gas-Phase Titration with Nitric Oxide J. A. Hodgeson, R. E. Baumgardner, B. E. Martin, and K. A. Rehme Environmental Protection Agency, Air Pollution Control Ofice, Divisiort of Chemisf r y iind Physics, P.O. Box 12055, Research Triangle Park, N . C., 27709 A RECENT ARTICLE by Boyd et al. ( I ) questioned the stoichiometry in the one per cent neutral buffered potassium iodide procedure for the determination of ozone. By making simultaneous ultraviolet absorption measurements of absolute ozone, Boyd found that the molar ratio between iodine released and ozone absorbed was 1.5. The stoichiometry that has been used for several years in air pollution measurements is 1 : l . This ratio is apparently based on the earlier report by Byers and Saltzman ( 2 ) of a laboratory study of the neutral K I procedure. The neutral buffered potassium iodide procedure with the 1 :1 stoichiometry has recently been adopted by the Environmental Protection Agency’s Air Pollution Control Office (APCO) as the reference technique for oxidant measurernents in the Federal criteria document on photochemical oxidants (3). If the factor of 1.5 is correct, then past ambient oxidant measurements have been high by 33 per cent. Because of present confusion (1) A. W. Boyd, C. Willis, and R . Cyr, ANAL. CHEM.,42, 670 (1970).
(2) D. H. Ryers and B. E. Saltzman, Aduan. Chem.Ser.,21,93--101 (1959). (3) “Air Quality Criteria for Photochemical Oxidants,” USDPIEW-
PHS, National Air Pollution Control Administration, Publication No. AP-63, Washington, D. C., March 1970.
over the issue, a n independent confirmation of the stoichiometry was urgently needed. We have utilized the fast reaction between nitric oxide and ozone in a dynamic gas-phase titration apparatus to provide an independent measure of absolute ozone concentration, along with simultaneous neutral K I measurements. A gasphase chemiluminescent-ozone detector first described by Nederbragt ( 4 , s ) provided a rapid determination of the titration curve and the end point. Comparison of absolute ozone determined in this manner with neutral K I measurements indicates thst the stoichiometry is 1.O, within experimental error. The details of our experiments are given below. EXPERIhIENTAL
A flow schematic of the titration apparatus is shown in Figure 1. The reactor wm a flow tube, 100 cm long by 4 cm
i.d., containing four sampling ports for sampling at different residence times. T’ne (>,zone source consisted of a n 8-in. Pen-Ray lamp (Ultraviolet Products, In:.), which irradiated purified air flowing through a qu:.rtz tube, 30 cm long hy (4) G. W. Nederbragt. A. Van Der Horst, and J. Van Duijn, Nufur?, 206, 87 (1965). ( 9 G. J. Werren and G . Babcock, Rei.. Sei. Zristrurn., 41, 280 (1970). A N A L Y T I X L CHEMISTRY, VOL. 43, NO. 8, JULY 1971
1123
HfiH VOLTACE
’
r
L
S16MAl
EM1 62565 PHOTOMULTIPLIER
WISDOW OR i n r m ALUMI LIMED
3/16 in. 1.0. TUBlM6 1/16 in. 1.0. TUBIMG IPYREX OR TEFLOMJ
OLOM
Figure 1. Flow diagram for gas-phase titration apparatus 25 mm i.d. At fixed air flow rate, the output of the source could be varied from zero to maximum with an adjustable aluminum sleeve that moved in and out over the lamp envelope. Flow rates through the source were 3 to 9 liters/ min, and the ozone concentration range was 0 to 20 parts per million (ppm) by volume. The air flow was measured before and after each titration with a 500-cc volumetric soapbubble meter or a calibrated wet test meter. The flow meter shown in the diagram was used to monitor ozone flow during a given titration experiment. Various samples containing dilute nitric oxide (104 ppm) in nitrogen from a pressurized cylinder were mixed with the ozone stream at the first inlet. The nitric oxide flow was metered through a flow meter (0 to 500 cc/min) that we calibrated with a soap-bubble meter. The nitric oxide was Matheson research grade and was used without further purification. The relative ozone concentration during the titration, from zero to excess nitric oxide flow, was followed by means of a Nederbragt-type ozone detector. This is a relatively unfamiliar apparatus, the response characteristics of which we recently reported (6). A schematic of the detector cell is shown in Figure 2. Ozonized air (1 liter/min) and ethylene (30 cc/min) are mixed as shown in a cell maintained at slightly less than atmospheric pressure with a small air pump. As chemiluminescence occurs, its intensity is monitored with a photomultiplier closely coupled to the mixing zone. The response is linear with ozone concentration, and is stable over prolonged periods; the sensitivity is less than 0.005 ppm and the response time is of the order of 1 second. The ozone concentration before and after each titration was determined by the one per cent neutral buffered potassium iodide procedure (7). The spectrophotometer used for absorbance measurements at 352 nm was calibrated with iodine solutions standardized against a primary arsenious oxide solution. Equivalent NOs formed during some of the titration experiments was determined by the Saltzman procedure (8). A conversion factor of 0.72 for NO2 to Nos- was used. (6) J. A. Hodgeson, B. E. Martin, and R. E. Baumgardner, 160th National Meeting, ACS, Chicago, Ill., (Sept. 1970), No. Watr-11. (7) B. E. Saltzrnan, “Selected Methods for the Measurement of Air
Pollutants,” Public Health Service Publication No. 999-AP-11, p D-1 (1965). (8) B. E. Saltzman, ibid., p C-4. 1124
ANALYTICAL CHEMISTRY, VOL. 43, NO. 8, JULY 1971
t
AIR 1W
Figure 2. Nederbragt chemiluminescent ozone detector
RESULTS AND DISCUSSION Standardization of Nitric Oxide Source. A cylinder containing nitric oxide at an approximately known concentration was prepared as follows. A 44.0-liter stainless steel cylinder was evacuated to 100 Torr over 48 hours with repeated nitrogen flushings. A measured volume of nitric oxide was introduced into the cylinder, which was then pressurized with Matheson prepurified nitrogen to a total pressure equivalent to 100 ppm nitric oxide by volume. The estimated uncertainty in the resulting nitric oxide concentration was d=lOX. This uncertainty was due almost entirely to the measured value of total cylinder pressure. A value of 110 ppm actually represented an upper limit inasmuch as prepurified nitrogen contains typically 5 ppm oxygen, according to Matheson specifications. At the cylinder pressure of 60 atmospheres, this concentration of oxygen would convert 10 ppm nitric oxide to nitrogen dioxide within a few days. Analysis of the cylinder contents, however, with a calibrated Mast NO2 meter and an electrochemical sensor sensitive to both NO and NO2 (9) indicated less than 1 per cent conversion of nitric oxide 2 weeks after preparation. No further decrease in nitric oxide concentration was observed during the period of experimentation. The cylinder described above was used in all subsequent titrations. As a further check on nitric oxide concentration, this cylinder was compared with two cylinders, purchased from Scott Laboratories, that contained calibrated nitric oxide in nitrogen. These cylinders were calibrated at Scott by infrared absorption analysis and were labeled 109 ppm and 1170 ppm. To compare the relative concentration of these cylinders and the long-term stability of the nitric oxide concentration, we used a chemiluminescence analyzer constructed by Ford Research Corporation and donated to APCO for evaluation. This analyzer is identical in principle to that recently described by Fontijn (10). The chemilum(9) R. Chand and R. V. Marcote, Dynasciences Corp., Chatsworth, Calif., Environmental Protection Agency, APCO Contract NO. CPA-22-69-118 (1970). (10) A. Fontijn, A. I. Sabadell, and R. J. Ronco, ANAL.CHEM.,42, 575 (1970).
inescent response of this instrument is linear with nitric oxide concentrations from 0.01 to 1000 ppm and is quite stable for prolonged periods. By comparing the ratio of the chemiluminescent response of our cylinder with the responses of both Scott cylinders, we calculated the content of our cylinder based on the labeled Scott concentrations. Both comparisons gave 104 ppm as the nitric oxide concentration of our cylinder and this was the number used in subsequent calculations. One final check made on the reliability of our nitric oxide concentration was a series of colorimetric Saltzman analyses of nitrogen dioxide produced during one of the titration experiments in which the initial ozone concentration was 1.6 ppm. The initial nitric oxide concentration was calculated from the plot of the concentration of nitrogen dioxide produced us. flow of nitric oxide added. The value obtained for our cylinder was 101 f 5 ppm. Titration Curves. In a given titration experiment, ozone at constant flow rate and concentration flowed into the reactor. Dilute nitric oxide from the standard cylinder was introduced at various flow rates and the decrease in ozone concentration was followed continuously at the end of the reactor by means of the Nederbragt detector. The following symbols are used to describe the titration curve.
I Co
Nederbragt response, nanoamperes (nA) = O3concn entering reactor, ppm Co' = O3concn leaving reactor, ppm Fo = O3flow, liters/min Cn = NO concn in cylinder, pprn Fn = NO flow into reactor, liters/min F f = Total flow in reactor = Fo 4-Fn, liters/min =
> FnCn
=
(2)
(FJF0)I
kCo - (kCn/Fo)Fn = Io - (kCn/Fo)Fn
=
F,, ce/min
Figure 3. 08-NO titration curve. Nederbragt response us. NO flow = 1.60 ppm,
C,
= 104 ppm
C,(KI), ppm 1.60 11 .o 11.4 18.2
F,,liters/min Fni,litersimin 4.41 2.94 3.04 3.00
0.0725 0.320 0.334 0.535
CdKUICo (NO equiv)
0.936 0.972 1.00 0.981
equivalent ozone concentration of 1.7 pprn. Data obtained from four titration curves at different ozone mass flow rates are given in Table I . All the data points from the linear portions of the above curves were fit by regression analysis to a single plot obtained from Equation 3 by the following change of variables:
I" = I'/Co(KI) = FJ/FoCo(KI) (3)
A plot of I' us. F, is linear over the region of excess ozone, with y-intercept = kCo and slope = -kCn/Fo. Thus the equivalent ozone concentration may be determined from the ratio of the intercept to the slope. Alternatively, the ozone concentration may be calculated by equating the two terms, Co X Fo = Cn X Fn'
To = 4.41 Ilters/min ColKll = 1.60 ppm C, = 104 pprn
(1 1
Substituting for Co' in Equation 1 and multiplying by the ratio, Ff/Fo,the response equation may be written as:
I'
I
I
I
Table I. Titration Data a t Different Ozone Mass Flow Rates
At excess ozone mass flow rate, Co'can be written as a function of initial ozone concentration, corrected for dilution, minus the equivalent concentration of nitric oxide added. Co' = (Fo/Ft)Co - (Fn/FJCn, for FoCo
I
I
F, = 4.41 liten/mln, Co(KI)
The response of the Nederbragt detector is directly proportional to relative ozone concentration under prescribed conditions of constant detector pressure and flow rates. I = k X Cot,k = constant (nA/ppm)
4
(4)
where F,' is the extrapolated x-intercept of the linear portion of Equation 3 and represents the flow of nitric oxide equivalent to the total flow of ozone. Figure 3 shows a typical titration curve in which 1.6 ppm ozone, measured by neutral K I , at 4.4 liters/min was titrated by 104 ppm nitric oxide. The deviation from linearity above 60 cc/min nitric oxide represents incomplete titration when the product of concentrations is small. The equivalent nitric oxide flow in this example was 72.5 cc/min, which yielded an
Fn' = Fn/FoCo(KI)
s
= CO(KI)/CO
I"
=
k/S
(4)
- kCnFn'
The factor, S, is determined by the ratio of the slope, -kC,, to the intercept, k / S . This factor is the ratio of ozone determined by neutral K I to absolute ozone concentration determined by the nitric oxide equivalence method. This ratio is also equal to the moles of iodine produced per mole of ozone absorbed in the chemical analysis. A least-squares analysis involving 24 data points yielded a value for S of 0.975 f 0.067. The uncertainty given is the sum of the standard deviation of the slope and of the intercept and an estimated uncertainty of f5 ppm in nitric oxide concentration. CONCLUSIONS
Our results confirm, within experimental error, a stoichiometry of 1.0 in the neutral buffered KI analysis for ozone ANALYTICAL CHEMISTRY, VOL. 43, NO. 8, JULY 1971
1125
concentrations greater than 1.6 ppm. The responses of the Nederbragt detector is a linear function of ozone concentration, as determined by neutral KI, from 0.05 to at least 30 ppm. This observation implies that the same stoichiometry exists for concentrations less than 1.6 ppm. If the stoichiometry were different at the lower concentrations, then the response characteristics of the detector would have to change in the sub-ppm range in a manner which would preserve the observed linear relationship. Our data are in agreement with recent work of Kopczynski and Bufalini ( I I ) , who related ozone measured by neutral K I to long-path infrared absorption analysis of ozone. The approximate data given by Byers and Saltzman ( 2 ) on the gas-phase titlation, O3 NOz, also indicated a stoichiometry of 1.0. An interesting consequence of this work has been our use
-+
-~
.~.
..
.-
.-.
(11) S.L. Kopczynski and J. J. Bufalini, ibid., 43, 1126(1971).
of a highly stable o.zone source and the Nederbrzgt.ozone detector to measure dilute concentrations of nitric oxide in air by gas-phase titration. Figure 3 indicates the wide linear range available by measuring the decrease in photomultiplier current, AI. Further studies are under way to investigate the utility of this approach. ACKNOWLEDGMENT The authors are grateful to mernbers of the Technical Staff, Research and Engineering Center, Ford Motor Company, for the chemiluminescent-nitric oxide monitor used in these experiments. RECEIVED for review January 18, 1971. Accepted April 1, 1971. Mention of company names or soinniercial products does not constitute endorsement by the Environmental Protection Agency.
Some Observations on Stoichiometry of Iodometric Analyses of Ozone at pH 7.0 Stanley L. Kopczynski and Joseph J. Bufalini Encironmentul Protection Agency, Dirision of Chemistry and Physics, Air Polluiion Control Ofice, 4676 Columbia Parkway, Cincinnati, Ohio 45226 A RECENT PUBLICATION by Boyd and coworkers ( I ) has shown that the stoichiometry of the ozone-iodide reaction is not 1 :l. Their work shows that ozone produced by the irradiation of gaseous oxygen with an electron accelerator releases approximately 1.5 molecules of iodine. This observation is indeed surprising since we in APCO have, for some time, been assuming 1 :1 stoichiometry for the 1 neutral buffered KI method for ozone as given by Byers and Saltzman ( 2 ) . Many atmospheric data have been obtained by the application of neutral KI as a reference method. Obviously, if the stoichiometry is incorrect, then the atmospheric data for oxidants are too high by a factor of 1.5. This paper is concerned with the stoichiometry of the neutral K I reaction with ozone. Our results d o not agree with those of Boyd and coworkers (I) and show that 1 :1 stoichiometry is valid within experirnental error.
z
EXPERIRIENTAL
Prepurified air was metered at a flow rate of 14 l./miii through an ozone generator containing five small mercury vapor 4W (GE OZ4Sll) lamps. The ballasts for the lamps were connected to a variable voltage transformer and then to a constant voltage transformer to ensure steady line voltage. The ozone concentration could be varied by either changing the voltage on the variable transformer or by turning off some of the lamps. The ozone once generated passed through a Teflon (Du Pont)-lined Perkin-Elmer gas cell having an oplical path of (1) A. W. Boyd, C. Willis. and R. Cyr, ANAL. CHEM., 42, 670 (1970). (2) D. H. Byers and B. E. Saltzman, A d m i . Chem. Ser., 21, 93 (1959).
1126 * ANALYTICAL. CHEMISTRY, VOL.. 43, NO. 8, JULY 1971
ten meters. After constant absorbance was obtained at 1054 cm-I, the ozone gas stream was split, part vented to the atmosphere (after the ozone was destroyed), while the other was collected in two impinger type bubblers containing a 1 neutral buffered K I solution. The flow rate through the K I solution was 433 cc/min. The absorbance of the liberated iodine was measured at 352 mp on a Beckman DU. The molar absorbancy employed was 24,200 l./mole cm (3). A Perkin-Elmer Model 621 spectrophotometer was employed for the infrared absorbancy measurements. A mechanical slit width of 750 p was used in most of the measurements. Reducing the slit to 500 p showed no difference in absorbance. A further reduction to 400 p showed that the absorbance is reduced by 3z. The spectral slit width used in this study is comparable t o that used by Hanst and coworkers ( 4 ) . The absorptivity of ozone at 1054 cm-1 employed in this study is 3.80 X ppm-l m-l (4). Since this absorptivity was obtained by pressure and volume changes (3, it is independent of the iodometric method.
z
RESULTS AND DISCUSSION The results of the optical and the iodide methods are shown in Figure 1. The regression equation when the intercept is not restricted is: Ozone ppm (KI)
=
0.947 ozone ppm (IR) - 0.218
(1)
(3) R. M. Hendricks and L. B. Larsen, Anier. brd. H y g . Ass. J . , 27, 80 (1966). (4) P. L. Hanst. E. R. Stephens, W. E. Scott, and R . C. Doerr, ANAL. CHEM., 33, 1113 (1961). ( 5 ) C. M. Birdsall, 4. C . Jenkins, and E. Spadinger, ibid., 24, 662 ( 1952).
