KINETICSOF NITROUS OXIDEDECOMPOSITION BY MASS SPECTROMETRY
ethanol or from acetic acid gave purified material melting at 248-249' (lit.l0m.p. 249"). Thianthrene &dioxide was obtained from the reaction of thianthrene 5,lO-trioxide with zinc and acetic acid by the method of Gilman and Swayampati.ll Two recrystallizations from glacial acetic acid gave thianthrene 5-dioxide melting at 165.5-166' (fit.11 m.p. 168-169'). Thianthrene 5,l O-trioxide was prepared by the oxidation of thiant,hrene with chlorine in glacial acetic Recrystallization from 90% acetic acid re sulted in trioxide melting at 221-222' (lit." m.p. 221.5-222.5').
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Thianthrene 5,1O-i!etroxide was obtained by the oxidation of thianthrene with excess hydrogen peroxide in glacial acetic acid. The tetroxide was recrystallized from glacial acetic acid, m.p. 323-324' (lit.lob m.p. 321). (IO) (a) H. Baw, G. M. Bennett, and P. Dearnes, J. Chem. SOC.,680 (1934); (b) see F. Krdt and R. E. Lyons, Ber., 29, 435 (1896),for an earlier reference t o this method; (c) in both of these papers incorrect stereochemical assignments were made, and these were later corrected by the work of T. W. J. Taylor [J. C h . Soc., 625 (1935)1 and of Hosoya and Wood." (11) H.Gilman and D. R. Swayampati, J. Am. Chem. SOC.,77,5946 (1955). (12) K, Fries and W. Vogt, Ann., 381, 312 (1911).
Kinetics of the Nitrous Oxide Decomposition by Mass Spectrometry. A Study to Evaluate Gas-Sampling Methods behind Reflected Shock Waves
by Anthony P. Modica Avco Corporatwn, WiZminQton, Massachusetts
(Received February
4 , 1966)
The thermal decomposition of NzO in the temperature range 2400-3300'K. has been investigated in a mass spectrometer-shock tube apparatus to evaluate two sampling techniques, one involving a simple pinhole and the other a slender Pyrex nozzle protruding into the reflected region. It has been shown that below 3000OK. both methods yield nearly identical experimental results, comparing favorably with those of other investigators. Above this temperature, the effects of the cold end wall on the hot, shocked gas introduce a systematic error in the measured rate constant. The error is slightly less for the nozzle.
Introduction The shock tube has become a popular laboratory tool to prepare a gas rapidly and homogeneously for high temperature kinetic studies of fast chemical reactions. Until recently, exploitation of the shock tube has been restricted to choice of analytical techniques mainly involving optical spectroscopy, and even then it has been necessary to study simple systems with species of high extinction coefficients.l-a The advent of the time-of-fight mass spectrometer,
with its ability to follow the concentration of several species simultaneously in short resolution times (10 to 50 psec.), has now made it possible to study a greater variety of reactions behind reflected shock waves.416 ~~
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(1) H. B. Palmer and D. F. Hornig, J. C h . Phys., 26, 98 (1957). (2) M.Camac and A. Vaughan, ibid., 34, 460 (1961). (3) A. P.Modica and D. F. Hornig, "Kinetics of the Thermal Dissociation of N2F4 in Shock Waves," Report No. 357-275,Princeton University, Oct. 1963.
Volume 69,Number 6 June 1966
ANTHONY P. MODICA
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A technique, similar to that reported by Bradley and Kistiakowsky,‘ coupling a Bendix time-of-flight mass spectrometer to a shock tube, has been developed here to study at high temperatures relatively complex reactions which may be of interest to propellant combustion and re-entrv ablation. In order to “test” the apparatus, a hown kinetic system, the thermal decomposition of nitrous oxide, has been investigated. The objective of this study has been to evaluate experimentally in terms of end-wall effects ( i e . , the effects of the cold end wall on the temperature of the sampled gas) two gas-sampling techniques, one involving the’ usual pinhole and the other a slender conical nozzle protruding into the reflected region. Apparatus The operating principles of the Bendix time-of-flight mass spectrometer have been described elsewhere’ and may be summarized as follows. Ions are formed in a bunch by a pulsed electron beam, and the ions are then accelerated by a series of grids into a field-free drift tube. Because the ion velocities depend on the charge to m&ss ratios, the bunch separates into groups of differentm/e while moving down the tube. The time between the accelerating pulse and the arrival of each of the mass-resolved groups at the ion-multiplier detector is proportional to MI’* for ions of the same charge. The detector output is displayed on an oscilloscope which is triggered by the accelerating pulses. In the present research, the mass spectrometer is a Bendix Model 14-206 time-of-flight iastrunient. Timeintensity profiles of two individual peaks are selected and displayed on an oscilloscope (Tektronix 581) by using the electronic gates of the mass spectrometer to modulate the peak intensities of interest. Permsnent records are made with Polaroid fdm. The downstream end of the shock tube is used to prepare the high temperature reaction mixture, and sampling is performed by allowing the test gas to pass through the end wall directly into the mass spectrometer. The shock tube and end plate are secured to the ion source through the “fast reaction chamber” available on the commercial instrument. In a number of experiments, the end wall is a 0.0076 cm. thick brass foil havingeithera0.0051-or0.0076-cm. diameter pinhole in ita center. Also a small Pyrex nozzle similarly located and protruding into the reflected region has been used. The nozzle is fabricated from Pyrex capillary tubing which has been heated, drawn, and cut to desired specifications. Eastman cement (Type 910) is used to bond the nozzle to the end plate. A typical nozzle (Figure 1) has an entrance and exit diameter of 0.0076 and 0.0254 cm. and a
Figure 1. End plate with Pyrex sampling nozzle.
length of 0.475 cm. The nozzle characteristics are such that for an entrance to exit area ratio of 11.1, the exit to entrance temperature and pressure ratios are 8.3 and 200, respectively.’ Accordingly, the temperature and pressure of the shocked gas entering the nozzle was changed, for example, from 3000 to 360°K. and 0.24 to 0.0012 atm. (The background pressure in the mass spectrometer before shock arrival varied between 3 X lod and 9 X 10“ torr.) At the nozzle inlet the gas flow is sonic and becomes supersonic upon expansion. For a flow velocity of 1.02 mm./psec. (speed of sound in a 5: 100 N&Ar mixture at 3000”K.), the residence time of the reaction mixture in the nozzle was less than 5 psec. With the particle velocity taken as 1 mm./psec. for an average nozzle temperature of 168OoK.,the number of collisions with the nozzle wall was about 10” as compared to 1OIo (4) J. N. Bradley and G. B. Kistiskows!sy, 3. Chem. Phys.. 35, 256 (1961).
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J. Felmloe.
