Stoichiometry of Ozone-Iodide Reaction: Significance of Iodate Formation Edward P. Parry’ and Don H. Hern North American Rockwell Science Center, Thousand Oaks, Calif. 91 360
The oxidation of iodide by ozone generally produces iodine, and it is this stoichiometry on which some ozone analytical methods are based. However, it is shown that the oxidation of iodide to iodate is thermodynamically feasible, and significant iodate is obtained a t higher ozone concentrations, particularly if the ozone is bubbled through a borosilicate glass frit into the iodide solution. The possible catalytic effect of the frit is considered and a simple technique to test for possible iodate formation is presented. The oxidation of iodide to iodine by ozone with subsequent titration of the iodine formed is the classical method for the determination of ozone (Byers and Saltzman, 1959; Hodgeson et al., 1971). Not only is this method the calibration method for most other techniques--e.g., chemiluminescence-it is also the reference method adopted by the Environmental Protection Agency (Air Quality Criteria 1970), and is the basis for several automated atmospheric monitors. However, there has been some recent controversy over the stoichiometry of the reaction between ozone and iodide. Boyd et al. (1970) have recently claimed an 1 2 / 0 3 ratio of 1.5 in the reaction
.
and have postulated a mechanism to explain their results. Ingols e t al. (1959) have published results which indicate that more iodine is liberated a t low p H values than a t higher values, and suggested that “oxygen or some intermediary substance might be responsible for the increasing iodine as the hydrogen ion concentration increases. ” In rebuttal to Boyd’s work, Hodgeson et al. (1971), and Kopczynski and Bufalini (1971) have more recently reaffirmed the stoichiometry for the reaction as 1:l. We present experimental evidence of possible reasons for some of this discord and suggest the sensitivity of this reaction to some extraneous influences. The oxidation of iodide to iodate is thermodynamically favorable and is shown to occur under some experimental conditions. Two procedures have been proposed for determination of ozone. In the alkaline procedure, preferred when the exposed reagent solution must be stored before titration, the stoichiometry is not 1:l and a correction factor of 1.54 must be used to obtain results comparable to the neutral titration. This is postulated as being caused by formation of hypoiodite, iodate, and/or possible iodite in basic solution (Byers and Saltzman, 1959). The most common procedure involves the collection of the ozone in a neutral ( p H 7 ) potassium iodide solution and subsequent titration a t p H 7 with thiosulfate. In our determination of relatively high concentrations of ozone (100-400 ppm), we bubbled the gas through a glass frit into a neutral iodide solution. Very little iodine was formed, but upon acidification, up to 10 times as much 1 T o whom
correspondence should be addressed.
iodine as found in the initial titration was liberated. I t was suspected that this was iodate, based on the work of Willard and Merritt (1942). This was confirmed by polarography where identical half-wave potentials were obtained for the sample solution and a similar solution in which iodate was added. The glass frit was then removed and the gas carrying ozone was allowed to bubble through an open glass tube into the neutral iodide collection solution. Two bubblers were used, but it was found a t the flow rates used, all the ozone reacted in the first one. Figure 1 shows the ratio of 12/(1031 2 ) as a function of concentration of ozone, with and without a frit. The results indicate a detinite catalytic effect by the frit for conversion to iodate. Ozone is a very strong oxidizing agent, and the standard free energy (AFo) of the reaction
+
3 0 3 + I-+3
0 2
+ 103-
(2)
is independent of p H and equal to -135 kcal. The standard free energy of the reaction
is -201 kcal, and is favored by increasing pH. Thus, for both reactions a very large driving force to form iodate is found; it is indeed rather surprising that the oxidation stops a t iodine. As can be seen from the data of Figure 1, at low concentrations of ozone, even in the presence of a glass frit, little oxidation to iodate is found. With higher concentrations of ozone, the catalytic effect of the frit becomes evident. Even a t 50 ppm ozone, u p to about 20% iodate can be formed, if a frit is used, and a t higher concentrations (>200 ppm) small amounts of iodate are formed even without a frit. Thus, the interference of 103- in the determination of atmospherically important concentrations of ozone (-0.5 ppm) by titration in neutral solution does not appear to be significant, but workers should be aware of the large driving force to form iodate and possible catalysis of the reaction. 121
I
I
I
I
I
I I
1 I
~
1
I 4
I 7
I I 100
1 200 03(pprnl
Figure 1. Ratio of iodine to oxidation with ozone
1
300 (IO3-+I21
I
400
I 500
total iodate plus iodine formed by
Volume 7 , Number 1, January 1973 65
The large catalytic effect of the frit toward oxidation to iodate is not understood a t this time, unless it provides a high surface area for a surface-catalyzed reaction. Both borosilicate glass and quartz frits were found to give essentially the same results. However, the fact that the oxidation of iodide to iodate by ozone can be easily catalyzed should suggest that caution be exercised by investigators, if possible catalytic materials are present in the sample. It is recommended that after titration of the neutral solution, the sample be deaerated, acidified, and then titrated again if iodine is formed. This is a simple means to check for possible iodate formation. Finally, although no catalytic materials appear to be present, a possible explanation for the higher amounts of iodine liberated by ozone a t lower p H values, as found by Ingols, may involve formation of iodate. Most investigators (Byers and Saltzman, 1959; Ingols et al., 1959) seem to consider the formation of iodate as important only in
alkaline solutions. With catalysts present and with moderate ozone concentrations, iodate is formed in significant amounts in neutral solution.
Literature Cited Air Quality Criteria for Photochemical Oxidations, USD HEW PHS, Nat. Air Pollut. Contr. Admin., Publ. No. AP-63, Washington, D.C., March 1970. Boyd, A. W., Willis, C., Cyr, R., Anal. Chem., 42,670 (1970). Byers, D. H., Saltzman, B. E., Aduan. Chem. Ser., 21,93 (1959). Hodgeson, J. A., Baumgardner, R. E . , Martin, B. E., Rechme, K. A,, Anal Chem., 43,1123 (1971). Ingols, R. S., Fetner, R. H., Eberhardt, W. H., Aduan Chem. Ser., 21, 102 (1959). Kopczynski, S . L., Bufalini, J. J., Anal. Chem., 43, 1126 (1971). Willard, H . H., Merritt, L. M., Jr., Ind. Eng. Chem. Anal. Ed., 14,489-90 (1942). Receiued for reuieu August 7, 1972. Accepted November 10, 1972.
Self-ventilated Chambers for ldentification of Oxidant Damage to Vegetation at Remote Sites Paul R. Miller’ and Ronald M. Yoshiyama Pacific Southwest Forest and Range Experiment Station, Forest Service, U.S. Department of Agriculture, Riverside, Calif. 92502
H Self-ventilated chambers have been developed as a means of identifying both acute and chronic oxidant air pollution damage to vegetation in remote locations. No electricity is required for operation. Two chambers are used-one with activated carbon filters to filter all incoming air and the other with no filters. Ventilation is provided by the natural convection of warmer air in the plant enclosure chamber a t the bottom upward through the cooler stack. The air is exhausted through a wind-driven, rotating ventilator which further enhances airflow.
Plant species sensitive to specific air pollutants are useful for detecting pollutants and evaluating their effects in the field (Weinstein and McCune, 1970). The sensitivity of this method can be improved by exposing indicator plants in force-ventilated field enclosures in which air filters exclude the suspected pollutant in one of two companion enclosures (Costonis and Sinclair, 1963; Dochinger et al., 1965; Miller et al., 1963). But forced ventilation depends on the availability of electrical power and limits the choice of sites where controlled plant exposures can be done. This note describes self-ventilating plant chambers, based, in part, on Businger’s (Van Wijk, 1966) concept, which do not require electricity and may be used to detect air pollution injury to vegetation in remote areas.
Experiment a 1 The self-ventilated plant chambers consist of three main sections (Figure 1): (1) a 3 X 3-ft square enclosure 1To whom correspondence should be addressed. 66
Environmental Science & Technology
with lyz-ft vertical panels and a pyramid-shaped roof with a 1-ftz opening for the metal stack; (2) a vertical stack of 16-gauge galvanized sheet metal, 1 X 1 X 8 ft tall with an internal damper; and (3) a rotating ventflator (Western Engineering and Manufacturing Co.). The wooden frame members, carbon filter enclosures, and metal stacks were painted white. The plastic film used to glaze the chamber was an 0.008-in. thick, transparent, pliable, vinyl plastic (Kreen, Union Carbide). The 18 X 18 X 1Ys-in. carbon filter panels each with 7 lb of granular activated charcoal were Model PAA absorbers (Barneby-Cheney). Each filtered air chamber had eight filter panels. Four were sealed with duct tape in each of the filter enclosures. Air temperatures were measured in August 1970 outside and inside each chamber and stack. Temperature was measured every 30 min. with iron-constantan thermocouples a t four locations: in the chamber, a t the top and bottom of the stack, and in the ambient air. The airflow rate in the stack was measured about 24 in. above its base with a thermo-anemometer (Anemotherm air meter). Total oxidant concentrations inside and outside of the chambers were measured with an ozone meter (Mast Model 724-2). The airflow characteristics of the chambers were first tested a t Riverside by comparing flow in black vs. white painted stacks. Stacks painted black were warmer and resulted in much lower airflow rates relative to white stacks. Only white stacks were employed in subsequent tests. The air-filtering ability of the chambers was tested by exposing oxidant-sensitive herbaceous plants during a period when ambient total oxidant concentration was high. The plant species selected were grown from seed in a greenhouse provided with carbon-filtered air. Pinto beans were transferred to the chambers 14 days after seeding,
