Determination of Oxygen and Nitrogen by Inert Gas Fusion with Impulse Heating C. G.
Goldbeck, S . P. Turel, and
C. J. Rodden
U.S.Atomic Energy Commission, New Brunswick Laboratory, New Brunswick, N . J.
AN “IMPULSE HEATER,” described by Vasserman and Turovtseva ( 1 ) for the rapid determination of oxygen in certain refractory metals, carbides, and nitrides, is a water-cooled chamber containing a graphite crucible through which a high current is passed for short durations of time. A sample in the crucible is heated quickly during a firing to a high temperature. This heater appeared to offer several advantages over the commonly used induction heater when applied to inert-gas fusion analysis: more rapid outgassing; no need for insulation surrounding the crucible; higher reaction temperatures. Furthermore, the sample remains completely within the capped crucible with only the gaseous products escaping. Because of these desirable features, this laboratory investigated the applicability of this heating unit used with a gas chromatographic measuring system to the determination of both oxygen and nitrogen in certain materials not tested by Vasserman and Turovtseva. Some of these materials such as beryllium and thorium present problems in conventional inert-gas fusion analysis. The gas chromatography technique allowed the simultaneous determination of both oxygen and nitrogen in contrast to the oxygen determination alone by a carbon monoxide infrared absorption procedure used by the Russian workers. In addition, a modification of the shape of the crucible and electrodes contributed significantly to the firing efficiency of the heater unit. EXPERIMENTAL
Apparatus. The impulse heater is essentially identical to that developed by Vasserman and Turovtseva. It consists of a water-jacketed stainless steel cylindrical chamber about 4 inches tall and 3 inches 0.d. with two cylindrical side arms 1 inch 0.d. and 1 l/2 inches long. One side arm, functioning as a viewing port, has a glass window sealed in on the end; the other fitted with a threaded cap, protrudes into a small glove box filled with helium during operation. The crucible is loaded into the chamber through the port in the glove box, and is handled for insertion and positioning between the electrodes by means of a spring-loaded clamp mounted on a rod. The crucible is held between two water-jacketed copper electrodes faced with molybdenum plugs. The upper electrode is fixed to the chamber lid, the lower is attached to the bottom of the chamber by an expandable spring-loaded bellows mounted externally below the chamber to permit the insertion of the crucible and to hold it snugly against the upper electrode. O-rings are used between the chamber body, the bolted-down lid, and the threaded cap of the loading port. Insulated inserts of laminated Bakelite are used in the bolt holes of the lid. The crucible size was increased over that originally described so that larger samples could be accommodated. They are made from spectrographic-grade graphite rods and are 10 mm in diameter and 18 mm long. The sample cavity measures 5 mm in diameter and 9 mm deep. Caps are made
(1) A. M. Vasserman and Z. M. Turovtseva, Zh. Analit. Kim., 20, 1359 (1965).
5 mm deep with a 2-mm projection into the sample cavity to give a secure fit. The molybdenum faces are convexshaped with a 1/2-inch radius, and the crucible ends are hollowed out to the same radius so that the crucible can be positioned easily. Power is furnished through a 20 kVA variable-step transformer adjusted to give 10 volts on the secondary, with a variable choke on the primary to limit the current. The choke consists of 80 turns of covered solid-core No. 8 wire around an 8-inch high, 41/2o.d., 1/2-inchthick ceramic tube filled with iron rods. The induction should be varied to meet the requirements of the particular system. Electrical connections are made with No. 1 welding cable to the upper electrode pole on the chamber lid and the bottom electrode pole under the bellows. Contact is made through a relay timer which can provide firing times up to 30 seconds. The Plexiglas glove box, roughly 15 X 14 X 20 inches, contains a supply of crucibles, samples, and a Roller-Smith balance. The measuring system is a gas chromatograph (F and M Model 700) with a thermal conductivity detector, a 24-inch molecular sieve 5A column and a Perkin-Elmer peak area printing integrator. The carrier gas is helium, dried by a silica gel column. The helium reaction gas is purified by a zirconium sponge column at 700 *C followed by a trap of 8 turns of a 2-inch coil containing molecular sieve 5A immersed in liquid nitrogen. When the carbon monoxide is measured directly (along with the nitrogen), a freeze-out loop consisting of a 20-inch U-shaped column of molecular sieve 5A in liquid nitrogen is placed before the chromatograph. If it is desired to measure the oxygen alone as carbon dioxide, the carbon monoxide is oxidized in the usual way with an 18-inch column of copper oxide and the cold trap is omitted. Gas lines of 1/4-inchcopper tubing leading to and from the heating chamber are insulated electrically from the chamber by means of 4-inch lengths of polypropylene tubing placed next to the heating chamber. The plastic segment leading from the chamber contains glass wool as a graphite-dust filter. Procedure. OUTGASSING.Start the reaction gas flowing through the impulse heater at 600 to 700 ml/minute bypassing the freeze-out loop. Maintain gas pressure at 30 lbs/inch2. Place a crucible in position and fire three 12-second bursts, allowing the crucible to cool for a few seconds between bursts. Sweep the heater chamber for 10 minutes. BLANK. Remove the crucible, open it momentarily in the glove box, and replace in the heater. Sweep out through the bypass for 5 minutes, then switch the reaction gas to flow for 1 minute through the loop immersed in hot water. Immerse the loop in liquid nitrogen and after 1 minute, fire the crucible for 12 seconds. Sweep the heater chamber for 8 minutes. Switch the reaction gas to flow through the chromatographic column. Replace the liquid nitrogen on the loop with hot water and record the carbon monoxide and nitrogen peak areas. ANALYSIS.Load the crucible with a weighed sample, fire, sweep, and record the peak areas as in the blank determination. With beryllium and beryllium oxide samples, add about 100 mg of tin powder to the crucible before outgassing. CALIBRATION.Record the peak areas of tank carbon monoxide and nitrogen added through a sample loop. To check the entire apparatus, fire weighed samples of U308 to obtain the carbon monoxide peaks. VOL. 40, NO. 8, JULY 1968
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Table I. Inert Gas Fusion Analyses by Impulse Heater-Gas Chromatograph Sample size, mg Oxygen found, % Nitrogen found, 40-50 3.12, 3.16
