Jay E. Taylor Kent State University Kent. Ohio
A Constant Pressure Gas Buret For determination of gas absorption rates
A
constant pressure gas buret and gas absorption reactor have been developed for the purpose of determining the rate of uptake of gases. With this apparatus a constant pressure of gas over the reacting solution, and correspondingly a constant concentration of gas in solution, is maintained at all times by a simple hydrostatic control. Although the buret so far has been used exclusively with oxygen reactions, it could well be applied to any slightly soluble gas.
Figure 1.
end has been fitted into Kel-F bearings. These bearings are conveniently made by indenting the glass to hold the Kel-F bearing and by heating the glass shaft to form a bearing in the Kel-F. The side tube E provides a means of introducing the liquid reactants. The threeway Y stopcock F permits filling of both the buret and reactor with oxygen from G. Tube H is connected to the constant pressure gas buret which serves as a source of metered oxygen during the course of the reaction. The essential parts of the constant pressure gas buret are seen at I-L in Figure 2. The other features provide only a means of extending the accuracy of pressure control, particularly to correct for changes in atmospheric pressure which would otherwise be the sole reference pressure. To fill the buret, an oxygen source is connected to stopcock G in Figure 1. Stopcock I is opened, and K is reversed from the position shown. Upon opening G to the buret, the liquid in the reservoir tube M is forced through the side tube and into the buret I-J. The side tube N also fills up to the same level as the buret. Stopcocks F and I are closed as soon as the
Reactor with rnagneticolly powered aerotion stirrer.
The reactor (Fig. 1) contains a magnetically powered aeration stirrer.' The magnet a t A is attached to the shaft of a low-powered 1500 rpm electric motor. The Teflon-covered bar magnet at B is held inside a short glass tube of appropriate diameter chosen to give a close press fit after sintering of the ends of the glass. The aeration stirrer, C-D, provides a continuous satur* tionIof oxygen in the solution by circulation of gaseous oxygen from the atmosphere over the solution into C and through D by centrifugal action. The '/v-in. Kel-F plate above C has a short bearing insert of the same material. This bearing is adjusted to the diameter of the glass tube and provides sufficient Aability so that the stirrer operates smoothly a t speeds up to about 1500 rpm. Actually, several modifications of the stirrer design have been used. The other designs provide bearings in the top and bottom of the reactor (above C and below D), and eliminate the bearing plate a t C. I n one case, a Kel-F stirring shaft tapered at both ends has been fitted into a tapered glass bearing made by dimpling the hot glass with a tapered steel rod in the appropriate locations. Alternatively, a glass shaft tapered on each 'TAYWR, 3. E., m n F E L ~ T. S , J., J. Am. Chem. Soc., 74,1331 (1952).
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Journal of Chernicol Education
Figure 2. Constant pressure go. buret with auxiliary reference bulb and pressure odiurtment rnechonirm.
buret is filled. Stopcock K is turned to the position shown, and F is opened to the atmosphere so that the liquid goes out of N and one or two bubbles of air enter the buret. The buret may be of any capacity, 10 ml or larger, with a minimum bore of I/, in. The reaction is begun by first flushing the air out of the reactor (Fig. 1). The liquid reactants, previously stored under nitrogen, are introduced through E. Stopcock F with the connection to the oxygen source removed is opened momentarily to connect the reactor to the atmosphere. Stopcock F is then turned to connect the reactor with the buret, and the rate of gas uptake is followed by observing the rate of fall of liquid level in buret. I-J. As the gas is absorbed the hydrostatic pressure between J and L forces the liquid downward into V. This liquid is replaced by air which bubbles in a t N. The pressure of gas in M and in the huret is always equal to atmospheric pressure plus that due to the head of liquid of height measured from the air inlet tip a t J to the liquid outlet tip at L. The pressure is maintained as long as the liquid level in the huret is above J . For runs of long duration, where there may be appreciable variation in the atmospheric pressure, 0T may be used to insure constancy of pressure. During the process of flushing the reactor and filling the buret, stopcocks P and R should be opened to permit a slow flow of oxygen through S so that the air is displaced and does not diffuse upward to mix with the oxygen from the buret. Stopcock Q should be similarly flushed. Stopcock R is then closed and Q opened allowing the pressures to equilibrate between bulb T and the buret. Stopcock Q is closed, establishing the reference pressure for the course of the experiment. The leveling bulbs at 0 are needed if the atmospheric pressure changes, as indicated by the liquid level in S. The relative levels of the two bulbs can be changed to offset the increase or decrease in pressure. Obviously t,he capacities of these bulbs should be large with re-
spect to the volume of the buret. However, the inside bulb is made flat on top to minimize the volume of gas so that it is less subject to atmospheric temperature deviations. It is extremely important that the reactor and buret assembly be immersed up to stopcock K in a good constant temperature bath. If the room temperature is nearly the same as the bath temperature, the buret from I-K need not be immersed. With a large temperature differential, a water jacket about the buret may be needed. The reason for this is particularly evident if the bath is much warmer than room temperature. The cooler liquid in the buret tends to d i s place the warmer liquid in the tip L. As oxygen is used the cooler liquid runs into M effecting a cooling of the gas in M. The result is an immediate contraction of the volume of gas in M and an apparent absorption of gas. Then, as the gas warms to the bath temperature, an expansion occurs as evidenced by a backing up of the liquid from the buret into tube N. This disrupts the hydrostatic balance and changes the pressure in both the buret and the reactor. If the bulbs and pressure reference O-T are used, the pressure may be checked at any time for disruptive influences such as failure of the constant temperature bath, etc. If O-T are not used, a backflow of buret liquid into tube N is always indicative of a disruptive effect. Any liquid may be used in the buret. Preferably, it should be the same liquid as that in the reactor. For example, the buret has been used successfully with both benzene and water. This buret has been used in three separate oxidation studies; these include the reactions of oxygen with cyclohexene in benzene solution, with cysteine in aqueous solution, and with 2-phenylbutane in aqueous suspension. Acknowkdgmnt. The apparatus was partially developed on a project sponsored by the National Institute of Heakh; this support is gratefully acknowledged.
Volume 42, Number 1 1 , November 1965
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