Determination of Nitrogen, Oxygen, and Hydrogen in Metals by Inert Gas Fusion A Manometric Method BEN D. HOLT and HARVEY T. GOODSPEED Argonne National laboratory, Argonne, 111.
b Capillary manometers are used for the simultaneous determination of microgram quantities of nitrogen, oxygen, and hydrogen in metals b y the inert gas fusion technique. The gases evolved from fusion of the sample are carried b y helium, first through hot copper oxide for oxidation of H2 to H20 and CO to CO1; then through capillary-manometer cold traps for the isolation and subsequent measurement of HzO (at 110" C.) and COz; and finally through activated charcoal at -196" C. for the removal of N2. The NB i s transferred b y a Toepler pump to a capillary manometer for measurement. The time required for a complete analysis i s about 25 minutes; less, if all three elements are not required. Sensitivity and precision are about the same as for the vacuum fusion method.
D
u s r r few years, the inert gas fusion method has been applied by workers in several laboratories t o the determination of microgram quantities of osygen in metals (3, 4,8 9, 1 1 ) . Generous bibliographies of literature references relative to the merits of inert gas fuqinn and vacuum fusion have been given by these and other authors. Carrier ga. techniquehave been used in the dpttmiination of hydrogen in steel by liot extraction (Z), and, more recently, inert gas fusion has been extended to the determination of nitrogen (or nitrogen and oxygen) in steels ( 7 ) . The apparatus used for measuring the gases of interest in these techniques have included open-well capillary manometers. gas chromatographic units, and electronically controlled, constant-temperature conductometric equipment. Because of the simplicity of construction and operation, the economy in initial cost and maintenance, and the adequate precision afforded in the microgram range of impurities, the authors have continued to use capillary manometers during these years of growing popularity of carrier gas techniques. Several manometers of the general type used by Smiley (IO), with the needle URING THE
1 5 10
ANALYTICAL CHEMISTRY
valves replaced by grooved stopcocks ( 5 ) ,have been used in oxygen analyses. h capillary, constant-volume manometer, designed for the measurement of water vapor pressure a t 110" C., has been in continuous use in a combustionmanometric method for the determination of hydrogen (6). A third type of capillary manometer has been applied more recently in the collection and measurement of noncondensable gases. I n this paper a description is given of how these three types of manometers have been employed in a single analytical train for the determination of the gases, €I2, CO, and Sz,evolved from the fusion furnace in the inert gas method. Purified helium sweeps the gaseous products from a graphite crucible, in which the metal sample is fused in a platinum bath a t 1950" C., through hot copper oxide to convert the Ha to H20 and the CO to COz, through the respective cold traps of the hydrogen and oxygen manometers for removal and measurement of H20 and COz, and finally through activated charcoal a t -196" C. where the Nz is retained for subwqiicnl transfer by Toepler pump t o the nitrogen manometer. EXPERIMENTAL
Apparatus and Reagents. A diapram of the ann11 tical train used in this work i:, slioivn in Figure 1. The iliain funrtions of the component parts were purification of the carrier gas, fusion of the sample, oxidation of the CO and Hz, and isolation and measurement of the carried gases. TRo IVelch Duoseal pumps, Model 1405-6, provided vacuum for t n o lines, one, 57,)for the exhaust of the analytical line, and thc other, V,, for the operation of mercury lifts. liquid nitrogen cold trap was proridpd on the V1 line to prevent hack-diffusion of vapors from thc meclianicd punip. The quality of the varuuni pro\ ided b) these puinps (about 10-3 nim. of Hg) was adequate without the aid of diffusion pumps. Tank helium !\as purified by passing it through a fused silica tube (not shonn), 1 inch in diameter X 12 inches long. filled n-ith uranium chips at 650" C' for tlie i e i i i o ~ a of l OA,H,O, CO, and CO,, and then through a U-tub(., CIA containing activated charcoal a t -196
C. for the removal of Sz.The U-t'ube, attached to the 4 w a y stopcock, SI, by
T
14
.
joints, was 12 inches long, and made from 8-mm. fused quartz tubing. Activated charcoal (8- to 14mesh, grade GG) supplied by Carbide and Carbon Chemical, filled the U-tube to a height of 5 inches. This reagent was preconditioned for use by evacuating and heating with a Fisher burner to a temperature sufficient to make it glow to medium redness. The line pressure of the helium stream was maintained slightly above I atmosphere by a manostat (not shown) consisting of two 4-liter bottles containing 4 liters of silicone oil. The graphite crucible, fused silica reaction tube, and borosilicate glass sample dumper, D, were all essentially the same as described by Smiley (IO), with some improvements in design. 4 , stopcock, Sz,was used to bypass the analytical train during operations of changing crucibles or of degassing the charcoal trap, C1. h drying tube, R, charged with Drierite and magnesium perchlorate, followed the fusion unit t'n remove moisture inadvert'ently introduced int,o the line. A stopcock, Sa,was used to switch n slow stream of oxygen through the copper oxide tube for overnight rejuvenation, leaving a slow stream of helium to flow out of the bubbler, B1, and osygen out of BS. The copper oxide (wire form) was heated to 420" C. in a fused quartz tube, 1 inch in dianiter X 8 inches long, by a set of replacement heating element's, manufactured by Hevi-Duty Equipment Co., hlilwaukee, Kis. The glass lines from stopcock S'S to the copper oxide tube and from the copper oxide tube to stopcock S d were maintained a t about 110" C. hy electrical heating tapes, H , to prexwlt. the ret'ention of moiiture produced I J ~ tlie oxidation of hydrogen in the aainldc. The H20 manometer and the COZ manometer', for measurement of hydrogen and osygen. reqpwtivel~-,have been described previouily ( 6 ) . 'I'he H2O nianonieter was calibrated in two ranges to measure hydrogen from 0.0 to 6.0 pg., and from 0 to 30 pg. On the more sensitive scale, 1 pg. of hydrogen was equivalent to 34 mm. of Hg. The CO, manometer measured oxygen in thr~ range of 0 t,o 500 pg. oil tlie niore >ensitive scale, with 10 pg. of osygen p ~ o diicing a manometer reading of 14 mni.
Table I.
UN
I+
Reaction
sample wt., mg.
"I
1.844 1.861 1.709 1.840 1.724 1,845
PI co2
I CuO F u r n a c e
fi ' i"
N2
M a nometet
v2
H 2 0 Manometer Figure 1.
Reaction furnace and analytical train
of Hg. The range oi the leas sensitive scale extended to 3000 pg. of oxygen. The charcoal trap, C?, used for collecting the Kzevolved from the sample was identical to C1. 1 three-my stopcock, &, followed ihe charcoal trap either to direct the helium stream out to the exhaust vacuum line during sample fusion; or, after fusion, to connect the charcoal bed containing the adsorbed nitrogen to the Toepler pump, PI, for transfer of the Kzto the Kzmanometer. This open-well, 1-mm capillary manometer illustrated in Figure 2, had two ranges provided by calibration marks above and below a smdl expansion bulb. Using the upper mark, the range was 0 to 650 pg. of nitrogen, and the lower inark, 0 to 3000 pg. The Toepler pump \vas operated autornatically by an tlectrically controlled solenoid valve. -1stopcock, SI6,was used for manual control when it was desired to confine the gas to the Nz inanometer for a measurement. This stopcock was grooved so that tfle movement of inercury up to the marks could be 1 ~reciselycontrolled. By manipulation of stopcock, S14,soml: of the collected SZcould be transferred to a sample bulb iur mass spectrometi.ic analysis as a t o t of its purity. Procedure. Evacuate the nitrogen manometer and the Toepler pump through stopcocks Llll, S12, and Sls. Close stopcock SI5. Evacuate charcoal trap CZ and close Slo. With stopcock Sd closed, evacuate the HzO manometer and the COZoxygen nmnometer. Close SS and slowly open 8 4 to establish manostatic pressure c'n the line up to stopcock 8 s . Open the side polt to the sample dumper D and, with excess helium (hupplied by the manostat) floa ing out of the port, introduce a cleaned and
neighed sample of metal. Close the port loosely, permitting helium to flush out beneath the cover, and after about 30 seconds tighten securely. Dump the sample into the crucible in which 8 grams of platinum has been melted and used in preceding procedure blank runs or analyses. Place a dry ice-acetone bath on the U-tube of the HzO manometer and liquid nitrogen baths on both the U-tube of the COz manometer and the charcoal trap, Cz. Open Cz t o the line a t stopcock Slo. Slowly open the grooved stopcock, Sa,until the helium flow (about 150 cc. per minute) produces a depression of the inercury column in the COz manometer of 5 cm. Heat the crucible a t about 1950" C. for 10 minutes. L4110wthe gas flow to continue for 2 additional minutes. I m m . PRESSURE C A P I L L A R Y STOPCOCK
Theoretical 103 103 96 102 96
Meas-
ured
103
102 103 96 103
98 101
Difference -1 0 +1 +1
rg
Close stopcock S 4 and completely open Sa. When the COz manometer shows 0.0 mm. of Hg. pressure, close SS and Sg; then turn SI1and SlZt o connect the charcoal trap, C2, to the I oepler pump. Start operation of thc automatic controls of t,lie Toepler pump. Heat the ch:rrcoal trap, Cr, n.itli a Fisher burner for 1 minute, uniformly raising the temperature of the charcoal, so that a t the end of the heating period the coals gloi$- with a dull red color (about 590" C.) in subdued light. Manipulate stopcocks Sl, Sg, and Ss to isolate the water condensed in the U-tube of the HzO manometer as previously described (6). Heat the U-tube, swiveled in the upward position, t o 110" C. using a heat gun, and read the pressure of the entrapped steam. Khile the LT-tubeof the HzO manometer is reaching thermal equilibrium in the above step, warm the U-tube of the COa manometer to room temperature and read the pressure of the entrapped 7
Manometer
Analysis of Uranium Mononitride Sitrogen, ______ pg.
,
coz.
These two measurements may be made in about 6 minutes, during which time the Nz may be quantitatively transferred from the charcoal bed to the Nzmanometer by concurrent pumping. By manual operation of the Toepler pump, raise the mercury level in the Sa manometer to the calibration mark below the expansion bulb. Take thc pressure reading if sufficient gas is present to produce a t least two significant figures. If not, raise the mercury level to the calibration mark above the expansion bulb for a reading of high sensitivity. From the weight of the sample and the data taken from the three manometers calibrated in micrograms of hydrogen, oxygen, and nitrogen, respectively, calculate the concentration3 of thp three irnpuritir5. RESULTS
CALIBRATION MARKS ON S T R A I G H T UNOISTORTED SECTIONS
76cm
Figure 2. Capillary manometer for noncondensable gases
The method was tested by aiialyniiig materials containing known quantitica of one or more of the three gases. Uranium mononitride samples, added to the crucible in platinum capsules, were analyzed for nitrogen. Table I shows a comparison of measured nitrogen with theoretical data based on previous analyses of this matwial for uranium and for nitrogen by gravimetric and by Kjeldahl methods, respectively. Table 11 sliows nitrogen and oxygen values obtained on Lome standard steel VOL. 35, NO, 10, SEPTEMBER 1963
151 1
samples supplied by the National Bureau of Standards, Washington, D.C. Although the NBS certified values, as supplied, contained only 3 decimal places, 4 decimal places are quoted in the experimental data, so that the precision of the results might not be obscured. I n Table 111 results obtained for all three gaseous impurities in zirconium metal are compared with values obtained by the methods indicated. I n this case, both sets of oxygen values were obtained by the inert gas fusion method, but in different apparatus with minor differences in analytical procedure. As a supplement to the hydrogen results shown in Table 111, 3 other metals were analyzed for hydrogen, the content of which had been determined by other methods. -4comparison of these data is shown in Table IV. Duplicate nitrogen determinations were made on 5 uranium ingots both by
Table 11.
the inert gas fusion method and by the Kjeldahl method. Comparing the results in the same sample order, the range of nitrogen content was
