B. A. Thrush University of Cambridge Cambridge, England
I
I
The Recombination of Oxygen .- Atoms in II Discharge Flow System
The chemical reactions of free atoms produced in electric discharges and the associated chemiluminescent reactions have been known for a t least fifty years. Early workers in this field include Strutt (Lord Rayleigh), Wood, and Harteck; but apart from studies of hydrogen atom recombination made in the 1930iO's, most of the investigations gave qualitative rather than quantitative information. In the last ten years there has been a strong renewal of interest in the field. This has been partly due to the development of new methods for measuring atom concentrations based on chemiluminescence and gas phase titration. The main reasons for the increased activity have beeii its importance in elucidating the chemistry of the upper atmosphere, reentry problems, new methods of chemical synthesis and the mechanism of combustion reactions. Most experiments on the reaction3 of free atoms generated in an electric discharge are conducted a t relatively low pressures (1-5 mm Hg) in a flow tube through which the discharge products are rapidly pumped. The experiment described in this communication measures the rate of oxygen atom recombination in the presence of molecular oxygen and of nitric oxide, and is a simplified version of the apparatus used by Kaufmanl to study these reactions. The reader is also referred to a comprehensive review article, also by Kaufrnaq2 on the use of flow systems to study oxygen atom reactions. This experiment is used in the practical physical chemistry course for final year undergraduates at Cambridge. It has proved remarkably trouble-free, and its popularity with the students stems from the use of cherniluminescence to follow oxygen atom recombination so that the student obtains a clearly visual picture of the extent of the reactions he is studying. Theory
At low oxygen atom concentrations, the reactions removing them are all first order with respect to oxygen atoms, i.e., O+O,+M=Os+M 0
0
+ 0, = 01f 0%
+ wall
=
'/POI
(1)
KAUFMAN, F.,Prog~.Reaction Kinetics, 1, 3 (1961).
I
=
I o [ O ][ N O ]
Nitric oxide produces a catalyzed removal of oxygen atoms: O+NO+M=NO?+M
(4)
O+N0,=NO+0*
(5)
but the amount of nitric oxide present is not appreciably reduced since k5 >> kn[M]. This means that for a given amount of NO added, the intensity of the airafterglow is proportional only to the oxygen atom concentration a t any point in the flow line. The intensity of this emission is measured with a photomultiplier cell which has the linearity and high sensitivity needed for a kinetic study. The presence of a little (about O.lyo) NO in the discharge products (which usually occurs without the addition of NO, due to a trace of nitrogen in most commercial oxygen) shows the oxygen atom decay due to reactions (1-3) without contributing significantly to it, and the oxygen atoms are observed to decay more rapidly at higher pressures as reaction (1) becomes more important. Addition of more nitric oxide enhances the air afterglow emission and increases the rate of oxygen atom decay so that the intensity of the air afterglow emission a t points a long way downstream will increase to a maximum and then decrease as more nitric oxide is added upstream. The kinetics will be derived for the apparatus illustrated in which known amounts of nitric oxide can be added to the system. Both the apparatus and the kinetic analysis can be simplified by the omission of the nitric oxide supply line and using the trace of nitric oxide formed in the discharge to follow the oxygen atom decay. For the determination of rate constants, it is simpler to work entirely in concentration units rather than partial pressures. If x is the distance down the flow tube corresponding to a reaction time t from the point corresponding to t = 0, then
(2)
(3)
The rate constants of these reactions can therefore be determined by measurement of relative oxygen atom concentrations. This is most conveniently done by adding a little nitric oxide to the discharge products, producing the yellow-green "air afterglow" emission from electronically excited NO2. The intensity (I) of
' KAUFMAN, F., PTOC. ROC Soe., A 247, 123 (19.58).
this emission is proportional to the product of the NO and 0 concentrations hut independent of the total
where u is linear flow velocity in cm/sec, A is cross sectional area of flow tube in cm2, ZF is total flow in moles/sec, and Zc is total concentration in moles/cc, and for each component (c./F.) = (ZclZF) a
CLYNE, M. A. A,, AND TARUSA, B. A,, Proe. Roy. Soe., A 269,
404 (1962).
Volume 41, Number 8, August 1964
/
429
The flow rates (F) are determined from flow meter readings, and since the bulk of the gas present is undissociated oxygen, it can be assumed that M in equations (1) and (2) is always O2 and that Zc and ZF are equal to the total oxygen flows. Since k2>> kl[M] and ka >> k4[M],the integrated rate equation for oxygen atom decay has the form In[010 - h[Ol
=
(2k1[011 [MI
+ 2kr[NOl [MI + kd t
substituting for t and [O] and putting [MI obtain
=
+
+
+
H + 0 2 + M =HOz+M O+HO~=OH+0? O+OH=H+O1
to be important in the products. With the apparatus described values of kl between lo1' and 2 X 10'4 cm6
Dicchorge Row system.
430
/
journal of Chemical Education
-
ks
[02], we
Thus, logarithmic plots of the decay of the air-afterglow down the flow tube give straight lines from which the quantity (2k1[02I2 2kr[NO][O1] ka] can be determined. If this quantity is plotted against [NO] a t constant total pressure a straight line of slope 2kr[02] and intercept (k1[O2I2 k3) is obtained. Good values of k4 are readily obtained, but measurements a t several pressures are needed to determine reliable values of kl and k3. I n the version of the experiment without additions of nitric oxide the term involving k4is neglected, the slope of the first order decay plots is proportional to (kl [O2]1 k,) and if this quantity is plotted against [02j2 a good straight line of slope kl and intercept kais obtained. The published values of k4 range from 4 X cm6 mole-'sec-' to about ten times this value. The higher values appear to be associated with the use of moist oxygen which yields enough hydrogen atoms for the catalytic recombination
+
mole-'sec-' are normally obtained, in good agreement with the most commonly quoted values. The published values of kh show a much smaller scatter, and this apparatus yields values close to the accepted rate constant of 2 X 10'6 cm6 mole-2sec-1. The value of ka depends on 7 the fraction of wall collisions leading to recombination. For a cylindrical flow tube of radius r, it can easily be shown that if 7 is small =
(rdl(2r)
where Cis the kinetic mean velocity. For clean Pyrex glass rinsed with dilute HF, y is usually about 10-6 giving k3 1 sec-'. Apparatus
The apparatus, which should be installed in a room with efficient blinds, is constructed from Pyrex glass. Oxygen atoms are generated in a U-shaped discharge tube made from a piece of glass tubing about 30 cm long and 2 cm diameter. The hollow aluminum electrodes are crimped to tungsten wires sealed through the glass. This discharge is operated by a 10-kv neon sign transformer fed from a Variac. This should be used a t the lowest setting which will maintain a stable discharge to avoid complications due to high oxygen atom concentrations. The tubes feeding oxygen to the discharge should each be about 50 cm long to prevent the discharge passing through them. The metal needle valves should be adequately grounded as the discharge may spread to them if it is operated a t too low a pressure. The discharge products flow through a light trap into the horizontal flow line which is mounted about 150 cm long and 2 to 2.5 cm id. A jet for the introduction of nitric oxide is sealed into the flow lime a t the upstream end, to ensure good mixing this jet should be carefully centered, about 1rnm diameter, and sharply rather than gently tapered. This jet and the associated apparatus for handling nitric oxide are omitted if the simplified form of the apparatus is constructed. At the downstream end of the flow line is a n oil manometer to measure the total pressure, which is typically between 1 and 4 rnm Hg. A bulb is fitted to one side of this manometer to eliminate errors caused by outgassing of the oil used. The flow of gas to the pumping trap and vacuum pump is controlled by a large stopcock, and a piece of silver foil can be conveniently inserted into its bore to remove the remaining oxygen atoms thereby preventing ozone formation in the trap. A single stage rotary pump with a rated capacity of 2-3 l/sec is suitable for this experiment. The oxygen and nitric oxide flows are controlled with fine needle valves; Edwards type LBlA have proved satisfactory. The flows are measured with capillary flow meters using silicone oil manometers. These flow
meters arc fittcd with shorting taps \\.hicll should be left open except when measurements are being taken, to minimize the danger of silicone oil being aecidrntally blown out of the manometers. For a uniform capillary tube of radius 1. aud leugth I , the Poisn~illeequation for the molar flow rate ( F ) is
To find F, it is therefore necessary to know the total pressure pl as well as the pressure drop Ap in the flow meter. I t is most convenient to take cylinder oxygen at slightly above atmosphere pressure, pass it through a packed trap cooled with solid C02 to remove excess water (to reduce H atom formation) and then iuto thc flow meter using an open ended mercury manometer to measure the pressure relative to that of the atmosphere. A typical oxygen flow would be about 100 pmole/sec and the flow meter can be calibrated against a commercial instrument or by using a capillary tube of known dimensions and calculating the calibration constant (I
