References
for Ambient Air SamDline and Analvsis. No. 3: Health Lab. Sei., 7, 267-9 (1970). SteDhens, E. R., Price, M. A., J. Colloid Interface Sei., 39, 272-86 .
Archer. M. D.. Clark. S. D.. Thillv. J. E.. Tannebaum. S. R.. Science 174,1341-3 (1971). Begeman, C. R., Colucci, J. M., “Analysis of Carcinogenic Air Polh a n t s , ” National Cancer Institute Monograph -No. 9, U.S. Department of Health, Education and Welfare, pp 17-55 (19621. Buell, B. E., Anal. Chem., 39,756-61 (1967a). Buell, B. E., ibid., 762-4 (1967b). Cundiff, R. H.. Markunas. P. C., ibid., 28,792-7 (1956). Fritz, J. S., ibid., 25,407-11 (1953). Jacobs, M. B., “The Chemical Analysis of Air Pollutants,” Interscience, NewYork, N.Y., p 413 (1960). Lundgren, D. A., J. Air Pollut. Contr. Ass., 20,603-8 (1970). Saltzman, B. E., Chairman Intersociety Committee on Methods
I
(igw. Streuli, C. A., Anal. Chem., 30,997-1000 (1958). Treadwell, F. P., Hall, W. T., “Analytical Chemistry,” 9th ed., vol. 2, p 494, Wiley, New York, N.Y., 1942. U.S. Environmental Protection Agency, “Air Quality Criteria for Hydrocarbons,”No. AP-64,pp 3-13 (1970). Wimer, D. C., Anal. Chem., 30,7740 (1958). Received for reuieu: November 30, 1972. Accepted March 27, 1973. This study uus conducted under contract PH 43-NCI-68-1030 within the Special Virus Cancer Program of the National Cancer Institute, National Institutes of Health, Public Health Service.
CORRESPONDENCE
Stoichiometry of Ozone-Iodide Reaction: Significance of Iodate Formation SIR: Recently Parry and Hern [Enuiron. Sci. Technol., 7, 65 (1973)l found that iodate is formed in the reaction of ozone and iodide. This is considered important because the standard method for ozone determination is based on the liberation of iodine from a neutral potassium iodide solution according to Equation 1:
+
+
+
+
0, 212H’ @ I, H20 0, (1) The 1 : l ratio of ozone to iodine given in Equation 1 has been questioned by Boyd et al. [Anal. C h e m . , 42, 670 (1970)], who found a ratio of 1.5:l. However, the 1:l ratio was confirmed by two groups: Kopczynski and Bafalini [Anal. Chem., 43, 1126 (1971)] and Hodgeson et al. [Anal. Chem., 43, 1123 (1971)]. In their paper Parry and Hern incorrectly state this reaction as: 20, 21- =+ 30, I, (2)
+
+
When using either borosilicate glass or quartz frits to introduce the ozone into the’iodide solution, Parry and Hern found that not only was iodine formed but also iodate. Two possible pathways for iodate formation were given: 30, I30, IO3(3) and 50, + I? + H,O + 50, + 210~- + 2 ~ (+4 )
+
+
In acidic solution, iodate will oxidize iodide to iodine:
10,- f 516H+ 312 + 3H20 (5) Consideration of the stoichiometry of Equations 1, 3, 4, and 5 shows that if the iodide solution is acidified just before the determination of iodine, no stoichiometric error will be involved. The iodine formed via Equation 5 will be
equal to that which would have been formed according to Equation 1 from the ozone used in Equation 3 or 4. Therefore, if the determination of iodine is carried out under the proper conditions, no error will result from the formation of iodate. This knowledge will be most useful to people working with ozone concentrations higher than those found in polluted atmospheres (-0.5 ppm). Parry and Hern found that a t very low ozone concentrations iodate formation does not appear to be significant. At “higher” ozone concentrations, the iodine formed is usually titrated with sodium thiosulfate. Acidification just before titration is a simple matter and will ensure the decomposition of iodates. Lawrence R . Schmitz Department of Chemistry University of Montana M,ssou,a,M o n t 5980, SIR: The gist of Mr. Schmitz’s comments makes a point which, though not specifically stated in the paper, is evident from the data. The total amount of iodine formed after acidification of the collection solution was taken in our paper as a measure of the ozone, as a little study of Figure 1 would indicate. [See abscissa of Figure 1 where the O3 concentration was measured by the total (103- + LJ.1 We, therefore, agree with Schmitz’s comments but feeithe point he raises was implicit in the original paper. Edward P. Parry North American Rockwell Science Center Thousand Oaks, Calif. 91 360
Volume 7 , Number 7, July 1973
647
