Paul A. Faeth' and A. F. Clifford Purdue University Lafayette, lndiono
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Praseodymium Oxide-Oxygen Generator
During the course of an investigation of the phase diagram of the Pr-0 s y ~ t e m , ~the - ~ relative ease with which oxygen could be removed from and combined with the oxides of praseodymium became apparent. The possibility of using this system as a source of research quantities of oxygen naturally occurred. I n addition, the possibility of the regeneration of the oxide reactant in situ was appealing. The simple apparatus shown in the figure was constructed to test the idea.
Trap A in the figure contained glass helixes and was used to prevent water vapor from entering the system. Trap B was constructed from fused silica (Vycor may be used) and contained approximately 20 grams of PrsOll. Trap C contained activated charcoal and was used to adsorb the generated oxygen. Trap D was a small Pyrex bubbler filled with a few ml of dibutyl phthalate; the bubbler was used to monitor the flow of air through trap B during the regeneration process. All stopcocks were of a high vacuum type. The oxide was conditioned initially by removing all impurity gases which were sorbed chemically or physically.5 A mercury diffusion pump provided the necessary vacuum, and vertical tube furnaces were used as the heating elements. Trap C was heated a t 200°C for several hours to aid in the removal of sorbed eases. --
This research was supported by the United States Air Force through the Air Force Office of Scientific Research of tho Air Force Research and Development Command, under contract no. AF 18(603)-45. Reproduct,ion in whole or in part, is permittcd for any purpose of the United States government. Present address: Lewis Research Center, 21000 Brookpark R o d . Cleveland 36. Ohio. * FERGUSON, R. E., GUTH,E. D., AND &RING,L., J. Am. Chem. Sac., 76, 3890 (1954). a HONIG, J. M., CLIFFORD, A. F., AND FAETR, P. A., J. InOTg. Chem., in press. FLETH, P. .4., PhD thesis, Purdue University, 1961. VAETH,P. A,, "Adsorption snd Vacuum Technique," University of Michigan Press, Ann Arbor, Mich., in press.
150 / Journal of Chemical Educofion
Trap B containing the oxide in vaeuo was heated to 900°C and the PraO,, was regenerated as mentioned below. At the beginning of the generation all stopcocks were open except no. 1, and the system was evacuated. To generate oxygen, stopcock no. 4 was opened to the previously evacuated storage flask; stopcock no. 3 mas closed and the temperature of the tube furnace around trap B was rais~dto between 700-900°C. Oxygen evolved slowly under these conditions and reached an equilibrium pressure of about 1.50-160 mm. after a few minutes. The composition at this point is -PrOl.n. To reduce the oxide further to a composition nearer PrOl.s, a Toepler pump may be conveniently used to pump the oxygen from the geuerator into the storage vessel. More simply trap C may be cooled with liquid air to cause the oxygen to adsorb on the charcoal, after which the oxygen can be distilled into the storage flask at a pressure greater than 150 mm. I n this case stopcocks nos. 3 and 5 arc closed and no. 4 is open to the storage vessel. To regenerate the oxide (PrsOn), the temperature (900°C) was maintained and stopcocks nos. 4 and 5 were closed; air was carefully introduced through no. 1 (no. 2 open) until atmospheric pressure was attained inside the system. Stopcocks nos. 2 and 3 were then closed and no. 5 was opened to the atmosphere through 0'. Compressed air from a service line was blown slowly through the system via the path 0-A-1-B-5.0'. The flow of air was monitored with bubbler D. The temperature of the furnace around trap B was then reduced to 450°C a t the rat,e of about 5'C/min. At 450°C the system mas allowed to equilibrate for 15 minutes after which time the temperature was reduced to 2OOoCand the system subsequentlyevacuated. The generator was then ready to produce more pure oxygen and the cycle was repeated. The purity of the generated oxygen was checked by analyzing samples of the gas with a Bendix Time-ofFlight mass spectrometer. Mass numbers through 50 were scanned, and there was uo indication that any other gaseous components were present except oxygen. The generation of oxygen using praseodymium oxide has several advantages: (1) The starting material may be regenerated in situ by simple and economical means. (2) The initial investment of Pr60,, is never lost as it is in the case of KMnOI, for example. (3) The actual time spent by the operator to perform the generation is small; the generation needs only a minimum of supervision since the evolution of gas is not necessarily rapid and is easily controlled. (4) The generation is relatively safe although caution should be exercised. The gas pressure never exceeds 150-160 mm. a t temperatures below 900°C. When using the liquid
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nitrogen trap method, should the liquid nitrogen become depleted by a n oversight, the oxygen adsorbed on the charcoal would desorb and recombine with the oxide and not build up an excessive pressure. To guard against a power failure (which would lower the oxide temperature rapidly) and subsequent liquid nitrogen depletion (which would raise the gas pressure), a safety device such as a mercury bubbler could be installed in the system. (5) The equipment needed for the generation is conveniently found in most laboratories with the possible exceptions of the silica tube and the Toepler pump.
The possibility of using the reduced oxide (PrsOa) as a "getter" for oxygen has not been extensively studied in this laboratory but this application seems promising. A sample of the reduced oxide, maintained a t 400450°C in the above system, removed completely a sample of oxygen which had been generated a t higher temperatures. The oxide might conveniently be used to remove oxygen from gas samples during certain gas analyses. A suggested additional use of the oxide is as a source of oxygen a t relatively constant pressure to be used in conjunction with the constant oxygen pressure studies of other systems.
Volume 40, Number 3, March 1963
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