sertion into the counter. In our experiments no interferences were observed from activate11 impurities, all the materials examined being relatively pure. I n an unknown matrix, however, activation of impurities could give rise to serious error, especially if positronemitting species were produced. In such cases decay-curve measurements or chemical separations would be necessary. While most organic materials are relatively free from impurities (iron, silver, bromine, copper, titanium, and scandium) likely to cause such interference, this consideration does limit the applicability of methods utilizing annihilation radiation. Anders ( I ) has described a method for the determination of iluorine based on produced the short half-life isotope W e J by the reaction F'9(~ia)N'~.The advantage of utilizing this reaction is due to the high energy (e.6 m.e.v.) of the emitted y-radiation which enables most interferences to be eliminated. Inter-
ference due to the reaction 0'6(~zp)N'~ was not observed by Anders since irradiation was carried out with neutrons produced by the bombardment of a beryllium target with 2-m.e.v. deuterons, the maximum energy of the resulting neutrons being less than the threshold energy necessary for the reaction. The accuracy attained by this technique was comparable to that of the present work. While being more specific than methods based upon the counting of annihilation radiation, procedures based on the F19(na)S16reaction suffer from the disadvantage that interference due to oxygen precludes the use of 14.0-m.e.v. neutrons which can be produced by relatively inexpensive equipment. Some of the results obtained in the present work are shown in Table 11. Since the accuracy attainable is highly dependent on the counting rate, best results are obtained with large samples, although with a detector crystal of the size used, it was convenient to limit the
sample volume to less than 2 ml. The higher neutron fluxes (wlOgn/sq.cm./sec.,) currently available should enable the accuracy of this method of fluorine estimation to be significantly increased. LITERATURE CITED
(1) Anders, 0. U., ANAL.CHEM.32, 1368 (1960). \ - - - - ,
(2) Atchison, G. J., Beamer, W. H., Ibid., 28,237 (1956). (3) Leveque, P., Proc. Int. Conf. Peaceful Uses At. Energy, (U. N.) New York, Vol. XV, paper 342 (1956). (4)Leveque, P., Goenvac, H., Bull. SOC. Chim.France, 1955, 1213. (. 5 .) Steele. E. L., Meinke, W. W., ANAL. CHEM.34, 185'(1962). ' (6) Sweetser, P. B., Zbid., 28, 1766 (1956). (7) Veal, D. J., Cook, C. F., Ibid., 34, 179 (1962).
ROBERT BLACKBURN Royal Military College of Science Shrivenham, Swindon Wiltshire, England RECEIVEDfor review July 26, 1963. Accepted October 27, 1963.
Specific Spectrophotometric Determination of Ozone in the Atmosphere SIR: Specific measurement of ozone in the oxidizing mixture of pollutants characteristic of Los Aiigeles, and, to an increasing extent, of other metropolitan areas, has engaged the attention of a number of investigators in the past. Early investigations were made using three techniques. Tqe oxidation of potassium iodide to free iodine has been the basis of a number of techniques, both manual (6, 17) and instrumental (4, 6, 10, 13). The oxidation of the leuco forms of several dyes, principally phenophthalin, has likewise been used both manually (7) and instrumentally (1). Both of these mffer from considerable lack of specificity, and in neither is the stoichiometry completely clear. The cracking of stressed rubber has been claimed to be specific for ozone (S),but some results seem to shed doubt on this fact, and in any case, the calibration of the method is eqtirely arbitrary. There is evidence that several previously unrecognized parameters would require very close control if accurate analyses are to be performed (8). More recently, several properties of ozone have been utilized in attempts to devise analytical systzms specific for that substance. Reiizetti and coworkers utilized a long-path ultravioletfilter photometer which compared the
absorption of the air in the path a t a wavelength in the Huggins bands of ozone with a wavelength lying outside these bands (14). McCully et al. (11) determined ozone thermometrically by its heat of decomposition on a suitable catalyst. Regener (12) developed an instrument based on the chemiluminescence of certain dyes in the presence of ozone. An instrument was developed based on the liberation of radioactive krypton by the oxidation of a hydroquinone clathrate (9). Saltzman and Gilbert (15 ) determined ozone by adding gaseous nitric oxide to the sampled air stream and measuring the resulting nitrogen dioxide. None of the above can be said to be completely satisfactory for ozone measurements in polluted atmospheres, either because of lack of specificity, slowness of response, instability of calibration, or uncertain stoichiometry. A chemical method that can be carried out with relatively simple laboratory equipment is badly needed for studies of ozone levels in the presence of other oxidants and reductants. One reaction which is apparently specific for ozone is ozonolysis. In this reaction, an ethylenic double bond is cleaved, generally yielding to oxygenated fragments. Under appropriate
conditions, one of these fragments is aldehydic and can presumably be determined by a suitable colorimetric reagent. The olefin selected for study was 4,4'-dimethoxystilbene. Ozonolysis of this compound under appropriate conditions should yield 1 mole of anisaldehyde, for which Sawicki (16) has published an extremely sensitive analytical method. Preliminary calculations showed that, if the initial ozonolysis step proceeded with good yield, the technique should be sufficiently sensitive to determine atmospheric levels of ozone. EXPERIMENTAL
Reagents and Apparatus. All the reagents were the purest available. All were used as received, except the tetrachloroethane which was purified by passage through a short column of sodium bicarbonate. The sampling reagent was a solution of 5 mg. of 4,4'-dimethoxystilbene in 100 ml. of sym-tetrachloroethane. Thp coupling reagent was a 5yGsolution of fluoranthene in chloroform. Trifluoroacetic acid and trifluoracetic anhydride were used as received. The mixed reagents were stable for about 1 week. The sampling was done in a modification of the "inverted bubbler" described VOL. 36, NO. 3, MARCH 1964
671
-
VACUUM
c
SAMPLE
COLLECTING. SOLUTION
Figure 1,
Special inverted bubbler
by Ehmert (6). This is shown in Figure 1 (Landay Scientific Glass, Boulder, Colo.). Sampling and Analysis. Three milliliters of the sampling solution were pipetted into the lower cups of two of the special bubblers. The two bubblers were connected in series, and air was drawn through them a t a rate between 0.10 and 0.15 liter per minute for a period of from 15 minutes t o 1 hour, depending upon the expected ozone concentration. At the end of the sampling period, 1 ml. of the reagent from each flask was transferred to a 10-ml. volumetric flask. To each were added 1 ml. of the chloroform solution of fluoranthene and 0.8 ml. of trifluoroacetic anhydride. This mixture was agitated strongly and then allowed to stand for 5 minutes. It was then diluted to 10 ml. with trifluoroacetic acid. The resulting solution from the first bubbler was blue in color. The absorption was determined a t a wavelength of 610 mp against a reagent blank. If any visible color appeared in the sample from the second bubbler, it was also measured and the total ozone determined. However, the collection efficiency of the first bubbler was generally near 98%, so the second solution could be discarded. The ozone concentration was then computed from a curve constructed by carrying known solutions of anisaldehyde through the analytical procedure. DISCUSSION
The method was calibrated by using Mylar bags of dilute ozone which were measured against neutral potassium iodide ( 2 ) . Comparison of this calibration with one obtained using a standard solution of anisaldehyde showed that the yield of the latter in the ozonolysis step was 98%. Thus, it seems likely that this method can be considered to
672
be absolute. While the stoichiometry of the potassium iodide-ozone reaction is uncertain a t low concentrations, it is the only presently accepted standard. It appears likely that, a t pH 7.0, and with sufficient precautions, this reaction is sufficiently stoichiometric to serve as a standard, despite an apparently contrary finding by Saltzman and Gilbert (15). The molar absorptivity of the colored product is 35,000 at the wavelength of maximum absorption (Figure 2). In 8 liters of air, therefore 1 part per hundred million of ozone can reasonably be detected. The vapors of trifluoracetic acid and trifluoracetic anhydride are highly corrosive to metals. Extreme care must be employed in filling spectrophotometer cells and in closing them tightly to prevent rapid corrosion of the cell compartment of the spectrophotometer.
ANALYTICAL CHEMISTRY
5000
Figure 2.
ANISALDEHYDE
r.\
'
1
,200
sew
6ooo
6xK) WAVELENGTH ( A I
6400
Figure 3. Comparison of ozone test (simulated using anisaldehyde) with reaction produced by photolysis products of azomethane
densation with fluoranthene, gives a sensitive and selective test for ozone in the presence of all known interfering substances and a t levels expected in the atmosphere. The use of trifluoroacetic acid and trifluoroacetic anhydride makes the test somewhat unpleasant and slightly hazardous for spectrophotometric equipment. However, the sensitivity, specificity, and the apparently precise stoichiometry of the ozonolysis step makes the technique extremely attractive for research on polluted atmospheres and on mixed photochemical systems. The absence of any fully proven referee method makes proof of the precise stoichiometry of this reaction difficult.
(A1
ACKNOWLEDGMENT
Absorption curve for test
,Reagent blank xxxxx and ooooo Ozone reaction ,
1-
7000
6000 WAVELENGTH
280
, ,,
Interferences. XOdetectable color was developed on sampling concentrations of the order of 1 p.p.m. of nitrogen dioxide, peroxyacetic acid, peroxyacetyl nitrate, or methyl hydroperoxide. Furthermore, known concentrations of ozone were recovered quantitatively in mixtures containing roughly equal amounts of ozone and sulfur dioxide, ozone and peroxyacetyl nitrate, and ozone plus nitrogen dioxide, all a t about 0.5 to 1.0 p.p.m. Preliminary findings suggest, however, that hydrogen sulfide may interfere with color development. The technique was used to attempt to determine ozone in the photolysis products of a mixture of azomethane and oxygen. No ozone was detected. However, after 10 hours of sampling, a broad absorption developed in the general region of the ozone maximum (Figure 3). This is probably caused by the products of indiscriminate free radical attack on the reagent molecule. CONCLUSIONS
The ozonolysis of 4,4'-dimethoxystilbene, with subsequent determination of the resulting anisaldehyde by con-
The authors acknowledge the assistance of Ellis Darley and the personnel of the University of California, Riverside, in performing several of the interference tests. The study of interference by methyl hydroperoxide and the use of the reagent in the study of the azomethane-oxygen system were done with the cooperation of Jack C. Calvert and Garnet R. ?vlch.lillan of the Department of Chemistry, Ohio State University. Eugene Sawicki and Arthur F. Wartburg, Robert A. Taft Sanitary Engineering Center, Cincinnati, Ohio, and Robert Cvetanovic of the Xational Research Council, Ottawa, Canada, contributed useful discussions. LITERATURE CITED
Pollution Foundation. Third Technical Progress Reporf, Rep& No. 17, p. 58, A.P.F., Los Angeles (1957). (2) Altshuller, A. P., Wartburg, A. F., Intern. J . Air Water Pollution 4 , 70 (1961). (3) Bradley, C. E., Haagen-Smit, A. J., Rubber Chem. Technol. 24, 750 (1951). (4) Brewer, A. W., Milford, J. R., Proc. Roy. SOC.London Ser. A 256, 470
(1 I Air \-,
~
i 1R6Ol. ( 5 ) Byers, D. H., Saltzman, B. E., Bm. Ind. Hz/g. Assoc. J . 19, 251, (1958). \ - - - - / -
(6) Ehmert, A., Meteorol. Rundschau 4,65, (1954). ( 7 ) Haagen-Smit, A. J., Brunelle, Margaret F., Intern. J. Air Water Pollution 1 , 51 (1958).
(8) Haagen-Smit,
A. J., Brunelle, Margaret F., Haagen-Smit, J. W.,
Rubber Chem. Technol. 32, 1134 (1959). (9) Hommel, C. O., Chleck, D., Brousaides, F. J., Nucleorics 19, 94, (1961). (10) Littman, F. E., Benoliel, R. W., ANAL.CHEM.25, 1480 (1953). ( 1 1 ) McCully, C. R., Roesler, J. F., Gordon, E. S.; Van Scoyoc, J. N., Carrigan, R. A., IRE Trans. Znstr. 1-10, 89 (1961).
(17) Smith, R. G., Diamond, P., Am. Znd. Hyg. Assoc. J . 13, 235 (1952).
(12) Regener, V. H., J. Geophys. Res. 65, 3975 (1960). (13) Regener, V. H. in Ozone Chemistry and Technology, Am. Chem. SOC.,Adv. Chem. Series 21. 124 (1959). (14) Renzetti, N. 'A., Romanovsky, J. C., J. Air Pollution Control Assoc. 6 , 154 (1956). (15) Saltzman, B. E., Gilbert, N., Am. Ind. H y g . Assoc. J. 20,379 (1959). (16) Sawicki, E., Stanley, T., Hauser, T., Chemist-Analyst 47,31 (1958).
HUMBERTO A. BRAVO JAMES P. LODGE, JR. National Center for Atmospheric Research Boulder, Colo. RECEIVED for review October 14, 1963. Accepted December 20, 1963. Division of Water and Waste Chemistry, 145th Meet.ing,ACS, New York, N. Y., September 1963.
Spectrophotometric Determination of Traces of Acetic Acid in Acetic Anhydride SIR: Some dye reagents have lately been developed in this laboratory for the detection of minute traces of acids, bases and salts to 10-6N) in benzene (2). The present paper reports the application of such a technique to the development of EL simple method for determining acetic acid content of acetic anhydride. The method in essence consists of dissolving acetic anhydride in dry benzene and measuring its acid content spectrophotometrically after reacting with a suitable dye reagent in benzene. EXPERIMENTAL
Reagents. A. Stock solution. One-tenth gram of the acetic anhydride sample (97% B.D.H. Laboratory Reagent) t o be analyzed is dissolved in exactly 10 ml. of dry benzene t o make ZL nearlv 0.1M solution. B. Acetic acid solution. Pure glacial acetic acid (not less than 99.5yo B.D.H. Laboratory Reagent) (0.06 gram) is taken in exactly 10 ml. of dry benzene t o make a 0.1N solution. C. Acetic anhydrjde solution of known acetic acid content (for checking the result). A known amount of water-Le., 0.5%, 1% 275, 3%, 4%is added to definite quantities of acetic anhydride (same sample as for A) in separate volumetric flatjks and is left for 7 days t o let the reaction go to completion. Rhodamine Reagenl . (Calcozine) Rhodamine 6Gx conclmtration (Color Index 45160) 3 to 4 ing., is dissolved in 2.5 ml. of a buffer (sodium hydroxidesodium phosphate) of pH 10 to 12 and is extracted immediately with 100 ml. of benzene. The reagent is preserved over solid caustic soda in the dark ( 2 , 3). For convenience in comparison, the absorbance of the rhodamine reagent is always adjusted t o 0.40 =k 0.005 a t 515 mp. The reagent does not de-
electric spectrophotometer contain ing a similar cell filled with benzene as reference. The absorbance is read at 515 mp. The measurement should be done quickly since the coefficient of expansion of benzene is very high. The dilution and mixing operation (with the dye reagent) should be done in a closed box saturated with benzene vapor. This is done a t a number of dilutions of the anhydride solution, A, t o have a linear curve of absorbance against concentration (Figure l a ) . Similar measurements are made on solution B a t various dilutions to obtain a standard curve for acetic acid (Figure lb). Calculation. From the slopes of the curves for A (anhydride) and B (acetic acid) (Figure l a and b) the
teriorate over months. This reagent can easily detect acetic acid as low a concentration as 10-5 molar in benzene. Procedure. ESTIMATION OF ACETIC ACID CONTENT. The stock solution A is first diluted with dry benzene to a concentration (about lO+M acetic anhydride) such that the pink color developed with the dye reagent is not too intense. Two milliliters of this diluted anhydride solution is mixed with 2 ml. of the dye reagent in a clean test tube and a pink color is developed immediately. For control, 2 ml. of benzene is mixed with 2 ml. of the dye reagent. The colored solution and the blank are then transferred in two separate 1-cm. cells to the cell compartments of a Hilger Uvispek photo-
Figure 1. Absorbance a t 515 rnp vs. concentration of ( a ) Macetic anhydride, (b) 1 O-4M acetic acid
t
01
I
I
I
I
I
I'0 1'5 2 '0 C O N C E N T R A T I O N , < ~ > ~ I O ' ~ / I (b) x i 0 4 m/l.
0'5
VOL. 36, NO. 3, MARCH 1964
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