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).
The standard error estimate, S,, is ~k0.28ppm. The neutral lower than the infrared analyses if the curve is drawn through zero. The equation for zero intercept is:
KI analyses are calculated t o be 7%
Ozone (KI) ppm = 0.929 ozone OR) ppm
(2)
with a standard error, S,, of d=0.32 ppm. These data show that the reaction between ozone and iodide, commonly written as: 0 8
+ 2H+ + 21-
= 0 2
+ PI20 +
I2
(3)
is correct. This is in agreement with the recent findings of Hodgeson et al. (6). Since we did not use concentrations below 2 ppm of ozone, we cannot entirely rule out the possibility that the stoichiometry may be altered at lower concentrations. However, earlier work on gas phase reactions involving olefins and ozone has strongly suggested that the iodide-ozone reaction is stoichiometric. Wei and Cvetanovic (7) have observed 1:1 stoichiometry between olefins and ozone when the reactions were carried out in an inert nitrogen atmosphere. Although their ozone purity was measured iodometrically, the ozone was purified by repeated bulb to bulb distillation in uacuo. The amount allowed to react with the olefins was measured volumetrically. In an air atmosphere, more olefin than ozone is consumed and a mechanism has been written to explain this behavior (8). It is difficult to explain the 1:1 stoichiometry between olefins and ozone if the iodometric method for ozone is nonstoichiometric. There is also good agreement with the 1-olefin-ozone rate constants obtained by various groups employing different methods for measuring ozone (9, 10). (6) J. A. Hodgeson, R. E. Baumgardner, B. E. Martin, and K . E. Rehme, ANAL.CHEM., 43, 1123 (1971). (7) Y . K. Wei and R. J. Cvetanovic, Can. J . Chem., 41, 913 (1963). (8) J. J. Bufalini and A. P. Altshuller, ibid., 43, 2243 (1965). (9) A. P. Altshuller and J. J. Bufalini, Photochem. Photobiol., 4, 97 (1965). (10) P. A. Leighton, “Physical Chemistry: Vol. IX. Photochemistry of Air Pollution,” Academic Press, New York, N. Y . , 1961.
Figure 1. Ozone as measured by 1% neutral potassium iodide and by long path infrared
The NO2 equivalent method for ozone developed by Saltzman and Gilbert (11) shows that the iodine equivalent and nitrogen dioxide equivalent for ozone are in very good agreement. If the stoichiometry of iodide-ozone were 1.5:1, then the NO2 equivalent method should give lower values than the iodide. Analyses of the data given by S a l t m a n and Gilbert (12) show that at lower ozone concentrations, the nitrogen dioxide equivalent method gives slightly higher concentrations. As pointed out by the authors, the lower ozone concentrations given by the iodide method rnay arise from iodine losses.
RECEIVED for review January 18, 1971. Accepted March 18, 1971. (11) B. E. Saltzman and N. Gilbert, Amer. Ind. H y g . Ass. J., 20, 379 (1959). (12) B. E. Saltzman and N. Gilbert, ANAL.CHEM., 31, 1914 (1959).
Determination of Submicrogram Quantities of Mercury in Pulp and Paperboard by Flameless Atomic Absorption Spectrometry Douglas C. Lee and Clarence W. Laufmann Analytical and Test Department, Weyerhaeuser Company, Longuiew, Wash. 98632
SINCEAN ANALYTICAL method was needed for determining submicrogram quantities of mercury in pulp and paperboard, several methods were evaluated. One of the major problems encountered by two of the three methods commonly used for this analysis is digestion of a large amount of organic material without loss of mercury. Chemical oxidation of the sample using sulfuric and perchloric acid ( I ) or nitric and sulfuric acid (2) followed by dithizone extraction is susceptible to mercury loss through equipment absorption or through volatilization of mercury due to too rapid oxidation. The
length of time, complex digestion equipment, and the number of samples that can be analyzed concurrently are also drawbacks. A spectrophotometric method using a Parr oxygen bomb for digestion has been used (3), but suffers from the above-mentioned drawbacks as well as giving high and erratic blank values on the bomb and absorbing solution. Neutron activation analysis is extremely sensitive and reasonably accurate but requires a great deal of time and sophisticated equipment, In the present operational proceclure, the loss of mercury
(1) P. C. Bethge, Anal. Chim. Acta, 10, 317 (1954). (2) A . Johanssen, Su. Paperstidit., 53, 231 (1950).
(3) L. G. Bonchard!, and B. L. Browning, Toppi, 4 1 ( l l ) , 669 (1958). ANALYTLCAL CHEMISTRY, VOL. 43, NO. 8,JULY I971
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