M. B. Kennedy and J. R. Lacher
University of Colorado Boulder, 80302
A Constant Pressure Gas Absorption Reactor
The gas absorption reactor described here can be used to study the rate of uptake of gases for gas-liquid reactions occurring at constant pressure in the temperature range 20-30°C. This reactor could also be used for gas solubility studies by determining the volume of gas absorbed per unit volume of liquid a t atmospheric pressure. The apparatus includes the following features: simple techniques for charging liquid sample holder and introducing liquid to reaction bulb; a modification which permits air-sensitive liquids to be used; and a sampling device which can be used to withdraw liquid or gas samples from the reaction flask. The general arrangement of the constant pressure gas buret and gas absorption reactor is shown in Figure 1. A detailed diagram of the gas absorption reactor is given in Figure 2. The gas buret apparatus consists of a water-jacketed U-shaped tube connected at the bottom to a mercury reservoir. The right-hand volume-measuring tube is a Fisher Precision Model gas buret with a capacity of 100 ml graduated in 0.1-ml divisions, and connected by a straight-bore 3-way stopcock at the top to t,he rest of the apparatus. The left-hand tube is an ungraduated replica of the buret, and sealed to a 2-way stopcock at the top. A thermostatic water bath is used to maintain the temperature of the water in jackets F and G constant with maximum deviations of *0.l0C. Two Teflon coat,edspin bars, 27 and 12 mm in length, are used to stir the liquid in t,he bath (G) and r e a h o n bulb, respectively. A suitable magnetic stirrer which is quite compact and gives off no heat is one manufactured by Tri-R Instruments. Also, in the case where the reaction bulb is suspended directly in the water bath, this stirrer can be immersed in the water and mounted beneath the flask. The Pyrex reaction bulb (- 300-ml capacity) is 15
cm long and 55 mm in diameter fitted with a glass spike approximately 2 cm from the bottom of the bulb. This bulb is, in turn, sealed to a 18-cm length of Tmbore heavy duty bearing (Ace Glass Co.), 19.05 mm i.d. A vertical side arm fitted with a small rubber serum stopple can be constructed for the purpose of withdrawing liquid or gas samples using a hypodermic syringe. Another possibility is to study the rate of uptake for a certain period of time using one liquid, and then to inject a small amount of a second liquid through the side arm with the use of a microliter syringe fitted with a long needle. In this way any alteration in the rate of reaction by another substance such as a catalyst could be studied. The central rod consists of a 43-cm length of heavy duty precision stirring rod (Ace Glass) designed to fit the Trubore bearing. The rod terminated at its lower end with a liquid sample holder 18 mm 0.d. and 46 mm long, giving a volume of 9.2 ml. Two lengths of 2-mm i.d. glass tubing passed through the hollow shaft of the rod terminating in two separate side openings. One tube, through which the gas enters, is sealed to a 2-way stopcock at the top of the rod, and the other tube is sealed to a side arm exit on the rod. This side arm consists of a 4-cm length of a capillary ball joint, 2 mm i.d. A flexible 50-cm length of Teflon tubing, 1.73 mm i.d. and 2.03 mm o.d., is inserted into a 4-cm length of a 2-mm i.d. capillary ball joint at one end and into a similar length of a 2-mm i d . capillary socket joint at the other end. This close-fitting tubing is sealed to the
GAS INLET
t
EXIT TRAPL
TRUBORE
@ ~.~ .W~ E
.~~ .
HOLDER GLASS
SPIKE
TEFLON COATED
SPIN 81\R
Figure 1 .
The condant presure gas buret and gar absorption reactor.
Figure 2.
Detoilr of the gas absorption reoctor.
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glass with Epoxy cement to ensure a gas-tight seal. With this arrangement the tubing could be clamped at one end to the side arm of the central rod and a t the other end to the gas buret. The experimental procedure is to fill the sample holder almost to the brim with the liquid, and then to apply a thin layer of grease (Apiezon L) around the rim. An inexpensive circular cover glass (Corning Glass Co.) 18 mm in diameter and 0.15 mm thick is then sealed over part of the rim, and after adding the last few drops of liquid to the brim with a syringe, slide the cover glass over the entire rim to give a liquid-tight seal. With practice the holder can be charged within a few minutes and in such a way that no air bubbles remain. If the liquid is air-sensitive it can be added to a standard 7-ml round-bottom polypropylene centrifuge tube in a dry box. The tube's dimensions are such (6 cm long, 14 mm id.) that it can be inserted into the rod sample holder to give a tight fit,and because its flanged lip has t,he same diameter as the cover glass (18 mm) a vaportight seal is easily achieved. After applying a light coating of Dow Corning high vacuum grease to the surface of the rod and bearing, the central rod is assemhled in the Pyrex reaction bulb and rotated several times to give a vapor-tight seal along the entire bearing surface. After the apparatus is assembled
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A is opened, and the gas is then allowed to flow through B, C, and exit at D. After flushing the system with the gas, the buret is connected to the system and filled with the gas by lowering E. A is closed, and the apparatus is then allowed to reach temperature equilibrium. When thc temperatures indicat,ed by TI and T2 are constant and nearly ident,ical, a first reading of the gas volume and atmospheric pressure is madc. The ampoule is then broken by pushing the cent.ral rod downward against the glass spike. Care is exercised to sec that the rod is returned to its initial position. As the gas is consumed in the reaction a constant pressure of gas over the reacting liquid is maintained by keeping the mercury levels equal in the buret and reference tube. On completing the experiment any toxic reactant gases can be flushed into the exit trap by streaming NI through t,he system. If the liquid has an appreciable vapor-pressure it may be necessary to perform blank experiments using the particular liquid and a relatively inert and insoluble gas such as N?. The number of moles of gas expelled into the system from evaporation of the liquid can be determined, and the appropriate correction thereby applied. This research has been supported by a National Science Foundation Grant.
