COM MUN ICATlON
Further Observations on the Ferrous Ammonium Thiocyanate Reagent for Ozone I. R. Cohen and J. J. Bufalini Chemical and Physical R/D Program, National Center for Air Pollution Control, Public Health Service, U. S. Department of Health, Education, and Welfare, Cincinnati, Ohio 45226
Results and Discussion Ferrous ammonium thiocyanate reagent was used for analysis of small amounts of ozone. The molar absorptivity was approximately constant at ozone levels above 2 p.p.m. (v./v.), but increased very rapidly at levels below 0.5 p.p.m. The maximum absorptivity is approximately 30,000 at ozone concentrations below 0.1 p.p.m. Bubbler frit size and collection rate were also important.
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he authors have been concerned with the nature of oxidants produced photochemically in model systems. Recent publications from this laboratory have shown that with the judicious choice of methods, ozone, peracids, alkyl hydroperoxides, hydrogen peroxide, and PAN-type compounds can be determined quantitatively (Cohen, Purcell, et a/., 1967; Purcell and Cohen, 1967; Altshuller, Cohen, et a/., 1966). During some recent work on the photooxidation of formaldehyde-NO, mixtures, the authors have been unable to make meaningful quantitative measurements of the amount of oxidant produced. This observation has led to a reinvestigation of the ferrous ammonium thiocyanate colorimetric method for ozone. Experimental The reagents and chemicals, as well as the procedure employed in this work, were described in a previous paper (Cohen, Purcell, et al., 1967). The data shown in Figure 1 were obtained by bubbling dilute ozone concentrations through 10 ml. of ferrous ammonium sulfate solution (0.1 gram of ferrous ammonium sulfate and 1.0 ml. of 6 N sulfuric acid diluted to 200 ml. of water). The color development step was effected with a 2-ml. aliquot of a 5 0 z (weight by volume) aqueous solution of ammonium thiocyanate. The absorbance was read at 480 mM.
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Further work with the method has shown that molar absorptivity, em, is a function of ozone concentration. Figure 1 shows the molar absorptivity over a range of ozone concentrations as measured by the ferrous ammonium thiocyanate method. The molar absorptivity is approximately constant at ozone levels above 2 p.p.m. (v,/v,), but it increases very rapidly at levels below 0.5 p.p.m. The maximum absorptivity is approximately 30,000. Why the method does not follow Beer's law with an oxidant such as ozone whereas it does so with n-butyl hydroperoxide, tert-butyl hydroperoxide, and hydrogen peroxide is unknown at present. The method exhibits characteristics unlike those observed with the neutral KI method with which low ozone concentrations give low absorptivities (Saltzman and Gilbert, 1959). The effect of bubbler frit size was also investigated. At an ozone concentration of 0.1 p.p.m., the molar absorptivities were 8000 and 24,000 at maximum pore diameters of 290 and 50 microns, respectively. [Pore diameter (microns) was measured by use of the equation I-( = 30 sip, where s is the surface tension of water (dynes per centimeter) and p is the pressure in millimeters of Hg. This equation is derivable by equating the forces responsible for the internal pressure with those of the surface tension of water operating on a hemisphere.] These observations are again contrary to those associated with the neutral KI method, which gives better results with a large pore diameter. Although the molar absorptivity increases with decreasing ozone concentration in the region usually encountered in polluted air, the ferrous ammonium thiocyanate method probably can be useful. For example, the method can be employed in the field and the color developed many days after collection of a sample. One laboratory has developed the color after 14 days (Clements and Morgan, 1967) and obtained reasonable values for the oxidant concentrations. N o other known colorimetric method is as sensitive and still as stable. Therefore, if highly accurate ozone measurements are desired, a calibration curve should be made for use of the method at the lower levels of ozone. Frit size and collection rate should also be considered with such a calibration.
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