1123
Anal. Chem. 1980, 5 2 , 1123-1125
possible. We thus use simultaneous notes described by Equation 1 rather than introduce harmonics. Finally, tapping our ability to recognize speech as a series of sounds and music as a series of notes, one can use sequences of sounds to increase the dimensionality of the representation. For example, two consecutive sequences can already handle 36 dimensions. A Synthesizer. T h e use of a computer to generate sounds in this work is simply to allow data manipulation and changing of the representation for testing. The computer is really not necessary, and its elimination may even be advantageous. A faster D / A converter with more bits will improve the resolution achieved here, but demands a corresponding increase in computation for the cosine function in Equation 1. This is particularly serious when 6 loudspeakers and 9 pitches are used. The alternative is to make use of discrete components. Each pitch can simply be derived from a voltage-to-frequency converter with appropriate output smoothing to convert to a cosine wave. A summing amplifier can then perform the function of Equation 1. The exponential forms in the Master Scheme section can be represented by anti-log amplifiers. Division of the amplitude into each of the six loudspeakers can be accomplished with standard analog division networks. Scaling of the d a t a vectors (most likely directly as input voltages from the instruments) requires simple op-amp circuitry, once the standard deviation or precision has been determined. Damping can be performed by a voltage-controlled amplifier tied to an anti-log circuit, which in turn monitors the discharge of a capacitor by a constant current specified by X,. Analog timing circuits can suffice in determining durations. T h e entire package probably will be significantly more convenient and less expensive than the corresponding computer-controlled version. C o m p a r i s o n s . We feel t h a t audio representation of multivariate analytical data, as shown here, is superior to the known visual methods. While audio methods do not require
a computer, visual graphics always do. T h e simplicity, dynamic range, orthogonality of the parameters, and presence of good standards are real pluses for the present scheme. Detailed statistical computation will always have its place in pattern recognition studies, but the present scheme is a viable alternative. ACKNOWLEDGMENT The author thanks William G. Tong, J. C. Kuo, and Steven D. Woodruff for participating in the evaluation process, Larry Steenhoek for discussions concerning voltage-to-frequency converters, and W.G.T. for helping in parts of the computer programming. LITERATURE CITED (1) Harper, A. M.; Duewer, D. L.; Kowalski, B. R.; Fashing, J. L. "Chemometrics: Theory and Application", Kowalski, B. R., Ed.; American Chemical Society: Washington D.C.. 1977; p 14. (2) Nie, N. H.; Bent, D. H.; Hull, C. H. "Statistical Package for the Social Sciences", 2nd 4.;McGraw-Hill: New York, 1975. (3) Dixon, W. J., Ed. "BMDP, Biomedical Computer Programs"; University of California Press: Berkeley, Calif., 1971. (4) Anderson, E. Technometrics 1960, 2, 387. (5) Pickett, R.; White, B. W. "Constructing Data Pictures"; Proceedings of the VI1 National Symposium of the Society for Information Display, 1966; p 75. (6) Siegal, J. H.; Goldwyn, R. M.; Friedman, H. P. Surgery 1971, 70, 232. (7) Daetz, D. "A Graphical Technique to Assist in Sensitivity Analysis"; unpublished report, 1972. (8) Chernoff, H. J. Am. Stat. Assoc. 1973, 68, 361. (9) McGill, J. R.;Kowalski, B. R. Anal. Chem. 1977, 4 9 , 596. (10) Speeth, S. D. J. AcousticalSoc. Am. 1961, 33, 909. (11) Kowalski, B. R.; Bender, C. F. J. Am. Chem. SOC.1972, 94,5632. (12) Eargle, J. "Sound Recording"; Van Nostrand: New York. 1976; p 34. (13) Kowalski, 8. R.; Schatzki, T. F.; Stross, F. H. Anal. Chem. 1972, 4 4 . 2176. (14) Stevenson, D. F. Archaeornetry 1971, 13, 17
RECEIVED for review November 1, 1979. Accept.ed March 24, 1980. This work was supported by the U S . Department of Energy, Contract No. W-7405-Eng-82, Office of Basic Energy Sciences, Division of Chemical Sciences (AK-01-03-02-3).
Gas Phase Ozone Evaluation by Thermal Decomposition Technique Vincenzo Caprio" and Pier Giorgio Lignola Istituto di Chirnica Industriale e Impianti Chimici, Universitg, P. le V. Tecchio, 80 125 Napoli, Italy
Amedeo Insola Laboratorio di Ricerche sulla Combustione, C.N.R., P. le V. Tecchio, 80 125 Napoli, Italy
An absolute method for gas phase ozone evaluatlon Is described. The method which is based on the measurement of pressure rises due to the ozone decomposition is sensitive to ozone contents of 0.02% by volume. Its reliability is verified by comparison with batch iodometric and UV analyses. Results also show the limits of the usually adopted iodometric analyses performed by bubbling the ozone stream through the K I solution.
During recent years there has been a marked increase of interest toward ozone with respect to its capability of acting 0003-2700/80/0352-1123$01 .OO/O
as a strong oxidant for the chemical treatment of wastes. Whereas some applications of ozone such as air deodorization and water disinfection are long since known and largely employed, only more recently has the attention of researchers and technicians been focused on chemical applications of ozone for pollution abatement. Its high reactivity as oxidant makes ozone capable of promptly destroying any oxidizable material even a t very low concentrations a t which the use of less expensive oxidants is affected by serious limitations. Thus more stringent limits of cleanliness such as those imposed from new standards can be achieved. I n this connection, one of the first and most serious problems encountered from investigators has been the evaluation 1980 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 52, NO. 7, JUNE 1980
liquid is realized by means of a capillary tube (volume: 0.15 mL) in order to keep its volume negligible in comparison with the volume of the decomposition chamber. T o evaluate the ozone content of the gaseous stream, this is admitted into the decomposition chamber and allowed to flow for such a time to eliminate the dilution effect of the preexisting gas. After the insulation of the reactor, the platinum wire is electrically excited up to its incandescence, from 15 to 20 s, which is sufficient to ensure the complete ozone decomposition according to the equation: 203
+
302
The duration of 20 s of heating time was chosen to prevent excessive pressure rises in the experimental device. After decomposition, the gas is allowed to cool at ambient temperature to eliminate any pressure disturbance due to heating. The use of a water bath, as a cooling medium, permits thermal stabilization to be achieved within 2 min. The pressure rise (AI’) due to the ozone decomposition is then measured by connecting the reaction chamber with the manometric system. This allows the ozone concentration t o be evaluated on a volume or molar percentage scale as: 222 co, = 100 P A
I I
Figure 1. Ozone analyzer
of ozone contents in the gaseous stream entering and leaving t h e pollutant solution under study. T h e question is not completely solved as shown from the controversy still existing in t h e literature a b o u t the iodometric methods of ozone analysis. This is the most adopted method and it i5 affected b y some uncertainties concerning its stoichiometry (1-3). Alternative methods, such as those based on t h e fast reaction of nitric oxide with ozone ( 4 ) and UV absorption photometry (5),although more reliable, are usually disregarded in comparison with t h e simpler iodometric methods. T h e nitric oxide method has t h e disadvantages of complexity and tediousness whereas t h e UV method requires a preliminary calibration normally performed by means of iodometric analysis. T h e object of this paper is to attract t h e attention of ozone concerned people t o t h e existence of a n absolute method of analysis for gaseous ozone which has t h e advantages of reliability, simplicity, and quickness. This method, firstly proposed by Jahn (6), is still ignored by t h e majority of people as shown by the lack of any reference in the more recent ozone literature. Therefore it appears interesting t o repropose this method by supporting it with t h e experience gained by an extended adoption of this technique (7-9).
EXPERIMENTAL The analytical apparatus used for gas-phase ozone evaluation is shown in Figure 1. It consists of a U-shaped Pyrex decomposition chamber with a volume of 100 mL. A platinum wire (id. = 0.1 mm, length = 30 cm) is located along the axis of the tube with its ends sealed into the glass walls and emerging into small dimples filled with mercury to ensure an electrical contact a t 15
v.
Two stopcocks permit the following operations: the connection of the decomposition chamber with the gas to be measured, its insulation after sampling, and its connection with a manometer operating a t constant volume. The junction between the decomposition chamber and the reference line of the manometric
in which PA represents the ambient pressure. From the above equation, it appears that the sensitivity of the method depends on the sensitivity of the manometric system used for pressure measurement. By assuming 1 mm as the sensitivity limit for pressure rise detection, an ozone content of approximately 0.02% by volume can be estimated if a manometric liquid such as dibutyl phthalate ( d = 1.05 g/mL) is used. This liquid also presents an adequate viscosity for a fast reading of the pressure excess. Ozone analyses are performed on ozonized streams of oxygen produced by an 0 2 0 B Elbe ozonator capable of being operated a t different voltages. The ozone percentage can be varied up to 5% by volume. Ozone contents less than 1% are obtained by diluting the ozonized stream with pure oxygen. The same ozone samples are also analyzed by means of the usual iodometric method by dispersing the gaseous streams into a 6% neutral buffered K I water solution and then titrating the I2 formed. A more rigorous iodometric method is also employed by which a previously evacuated and thermostated 0.513-L vessel is filled up to ambient pressure by ozone samples. Then the vessel is allowed to cool, thus reducing the inside gas pressure in order to introduce 16 mL of the 6% neutral buffered KI solution. This operation determines the appearance of an abundant mist, which is slowly absorbed even by submitting the sample to a vigorous shaking. After the time for mist absorption, the oxidized KI solution is quantitatively removed from the vessel and I2 titrated after the addition of 6 N H2S04down to pH 2. This batch procedure avoids the loss of any material due to the gas streaming through the iodide solution. Absorbancies of ozonized streams are also measured in a 1-cm optical path cell at 254 nm by means of a Perkin Elmer 402-UV spectrophotometer.
RESULTS AND DISCUSSION I t is well known that ozone decomposition t o oxygen is thermodynamically favored. T h e complete conversion of ozone can only be hindered by reaction kinetics which make the spontaneous decomposition at the ambient temperature too slow t o go t o completion in a short time (IO). However, this reaction could be opportunely accelerated by means of thermal and/or catalytic effects (11) t o ensure a satisfactorily rapid ozone decomposition. Therefore a n analytical method for ozone evaluation based on its decomposition can be theoretically foreseen. Measurements plotted in Figure 2 show t h a t t h e above requirements are actually fulfilled from t h e previously described apparatus. Results obtained by submitting each sample of ozonized oxygen at different durations of thermal decomposition (TD) show that actually decomposition is en-
ANALYTICAL CHEMISTRY, VOL. 52, NO. 7, JUNE 1980
'"
value 133 & 1 atm-' cm-' (12), reported in the literature. Iodometric analyses performed by bubbling the ozonized streams into the KI solution are compared with those obtained by the thermal decomposition method. T h e concentration values derived from the iodide procedure appear t o be systematically in defect in comparison with those obtained from pressure rise measurements, as shown by the least squares equation:
/ I IUCl
Figure 2. Pressure rises by ozone thermal decomposition vs. heating time
Table I . Ozone Molar Percentage thermal decomposition 4.26 3.79 3.46 3.06 3.05 2.44 2.35 2.11
2.10 1.85 1.66 1.51 1.40 1.07 1.04 1.04
1125
batch iodometric method 4.27 3.77 3.44 3.04 2.97 2.37 2.31 2.08 2.08
absolute difference
relative error, %
+0.01 -0.02
+0.2 -0.5 -0.6 -0.6
-0.02
1.59 1.48
-0.02 -0.08 -0.07 -0.04 -0.03 -0.02 -0.04 ~0.03 -0.03
1.38
-0.02
1.02
-0.05
1.01
-0.03
-1.4 -4.7 -2.9
1.03
4.01
-0.Y
1.81
-2.6
-2.8 -1.7
-1.4 -0.9 -2.1 + 1.9 -1.9
tirely achieved in a few seconds. Moreover, no ozone signal by decomposed gases is detected by means of UV monitoring. Experiments performed by introducing additional platinum wires in the reaction chamber do not show any appreciable reduction of the total decomposition time. Therefore any catalytic effect, even by platinum, should be irrelevant in comparison with the thermal effects. Ozone analyses performed by means of the above procedure are compared in Table I with those obtained by the batch iodometric method. There is a satisfactory agreement between the two series of measurements as indicated by the regression equation obtained by combining data from the two methods: [ 0 3 ] K I = 1 . 0 0 1 [ 0 3 ] T D - 0.03 (2) with a correlation coefficient R = 0.999, Results by UV measurements at 254 nm show a linear correlation with no appreciable scattering between absorbancies and concentration values. The least squares equation with zero intercept allows t h e ozone absorptivity to be calculated as 133.1 atm-l cm-' which is in agreement with the
On the basis of results obtained from the batch iodometric analyses, it can be inferred that these discrepancies are mainly due to the loss of oxidized products caused by mist leaving the KI solution. This is also confirmed by the increase of deviations a t increasing ozone concentration which also determines the production of larger amounts of mist. Attempts made for recovering the mist have not been successful. Even the use of two consecutive KI absorbers did not eliminate this complication. A strong limitation, therefore, arises in the use of iodometric analysis for ozone streams, especially for high ozone contents of the gaseous stream.
CONCLUSIONS T h e reaction of thermal ozone decomposition gives the opportunity of devising a simple and reliable method for gaseous ozone analysis. This method is unaffected by uncertainties of stoichiometry as is the case of the widely adopted iodometric analysis. The thermal ozone decomposition method exhibits greater precision than iodometric measurements as substantiated by UV measurements which me recognized for their unquestionable reliability. T h e autocalibration properties of the proposed method make it also suitable as reference for independent calibration of other ozone analyzers including those with higher ozone sensitivity. This is possible by properly diluting the tested ozonized streams. T h e method itself, although of not extraordinarily high sensitivity, is more t h a n satisfactory for analytical requirements by the many applications for which ozone contents higher than 1% are usually adopted. LITERATURE CITED Kopcznski, Stanley L.; Bufaiini, Joseph. Anal. Chem. 1971, 43, 1126-7. Boyd, Alan W.; Willis, C.; Cyr, R . Anal. Chem. 1970, 4 2 , 670-72. Pitts. J. N., Jr.; McAfee, J. M.; Long, W. D.; Winer, A. M. Environ. Sci. Technol. 1976, 10, 787-93. Hodgeson, J. A.: Baurngardner, R. E.; Martin, 6.E.; Rechen, K. A. Anal. Chem. 1971, 43, 1123-6. De More, W. B.;Patapoff, M. Environ. Sci. Technol. 1976, 10, 897-99. Jahn, S. Chem. Ber. 1910, 4 3 , 2319-21. Caprio, V.; Di Lorenzo, A.; Insoia, A. Chim. Ind. (Milan) 1969, 5 1 , 983-5. Caprio, V.; Insoia, A , ; Lignola, P. G.; Barbeila, R. Chim. Ind. (Milan) 1975, 5 7 , 311-15. Caprio, V.; Insoia, A.; Lignola, P. G. Combust. Sci. Techno/. 1979, 2 0 , 19-24. Axworthy, A. E., Jr.; Benson, S. W. Adv. Chem. Ser. 1959, 21, 388-97. Johnston, H. S. "Gas Phase Reaction Kinetics of Neutral Oxygen Species", 1968, 20, NSRDS-NBS. Rehrne, K. A. "Ozone: Analytical Aspects and Odor Control", Rice, R. G., Browning, M. E., Eds.; The International Ozone Institute: Syracuse, N.Y.. 1976; p 17-41.
RECEIVED for review 0ct.ober 22, 1979. Accepted February 1, 1980.
