the resins, the lignite resin was the only one that yielded phenols in signscant amounts. This result is in line with the observation that lignite tar has a much higher proportioyi of Dhenols than the other tars (2). The general structures of’the resins are in line indicated by these with the conclusions drawn from ring analysis, including ring arrangement,
infrared spectra, and ultraviolet spectra of the lignite, subbituminous, and bituminous resins (3). LITERATURE CITED
(1) Davis, J. D., in “Chemistry of Coal Utilization,” H. H. Lowry, ed., Vol. 1, p. 834-47, Wiley, New York, 1945. arr, Clarence, Jr., in “Chemistry of Coal Utilization,” H. H. Lowry, ed.,
(~YK
SupplementaW v0l.j PP. 539-79, W k , , 3 ~ ~ $ & ~ Jr., ; ~ ~ ~ ~ ,J. R,, Estep, P. A,, Fuel 41, 167 (1962). (4) Ibid.. 42.211 (1963). (5j Ode,‘W.‘H., Selvig; W. A., U. S. Bur. Mines, Rcpt. Invest. 3748 (1944).
RECEIVED for review April 12, 1963. Accepted May 29, 1963. Division of Petroleum Chemistry, 144th Meeting, ACS, Loa Angeles, Calif., March 1963.
Simultaneous Determination of Oxygen and Nitrogen in Steels by a D.C. Carbon-Arc, Gas Chromatog ra p hic Tec hnique F. MONTE EVENS’ and VELMER A. FASSEL Institute for Atomic Research and Department of Chemistry, Iowa State University, Ames, Iowa
b A d.c. carbon-arc discharge in a static helium atmosphere rapidly extracts the oxygen and nitrogen content of the metal sample as carbon monoxide and molecular nitrogen. An aliquot of the resulting gas mixture is transferred to a commercial gas chromatograph, where a Molecular Sieve column separates the individual components. A sensitive thermal conductivity cell detects the carbon monoxide and nitrogen in the effluent stream. For the operating conditions described, the concentration range of 0.003 to 0.1 weight % oxygen and nitrogen can be measured with a relative standard deviation ranging from 12.5 to lo%, depending on the concentration. For production control operations, a single sample analysis can be concluded in less than 5 minutes.
D
the past decade increasing demands have been placed on the analytical chemist to provide rapid, sensitive, and accurate procedures for determining the residual oxygen and nitrogen content of metals. The advent of high-punty-oxygen steel-refining operations has increased the need to know quickly, so that corrective measures can be applied during the production operations (1, 3, 10). Recent modifkations of the vacuum fusion (2, 9) and inert gas fusion (17) techniques have made it possible to reduce to 10 minutes the time required for oxygen and nitrogen determinations in B sample. However, the validity of the nitrogen results obtained with these techniques has been repeatedly questioned (la, 19,16; 19, page 355). 1
URINQ
Present address, Continental Oil Co.,
Ponca City, Okla.
1444
ANALYTICAL CHEMISTRY
It has been demonstrated that a d.c. carbon-arc discharge in a pure, rare gas atmosphere can effect the quantitative liberation of the oxygen and nitrogen content of metals a t a faster rate than has been achieved in a furnace fusion (4, 6-8). Aside from the advantage of exceedingly rapid liberation of the oxygen and nitrogen contents, the d.c. carbon-arc extraction process possesses three additional distinctive features:
cedure based on the combination of conventional gas chromatographic measurements with d.c. carbon-arc extraction of oxygen and nitrogen. With this procedure it is possible to determine simultaneously the oxygen and nitrogen of low and high alloy steels with an elapsed time requirement of less than 5 minutes.
Electrode configurations can be employed which assure that the arc discharge rests directly on the molten sample. Under these conditions, the temperature of the anode spot is equal to the boiling point of the melt, which has been estimated to exceed 3000” C. Thus, carbon reduction or nitride decomposition reactions, which may not occur a t 2000° C., may proceed with vigor a t 3000” C. (19). The precipitous temperature gradient of approximately 1500” C. over a linear distance of only a few millimeters causes vigorous convective stirring of the melt, facilitating the rapid evolution of gases formed in the globule. This behavior is in sharp contrast to the languid reaction medium characteristic of furnace fusions. The arc is a more thermally isolated high-temperature source than a furnace. As a consequence, it is easier and faster to degas the experimental facilities to tolerable blank levels.
Apparatus. The vacuum chamber, illustrated in Figure 1, was originally constructed by the National Research Corp., Cambridge, Mass., b u t was extensively modified for our purposes. The top assembly is hinged on the back side t o provide access t o the inside of the chamber. The insulated electrical terminal (Carborundum Co., Model 95.0056) is silver-soldered into the vertical port. The vacuum seal between the top and bottom of the chamber is achieved by a conventional O-ring seal. Air tightness is assured by applying pressure a t four equally spaced points along the circumference of the chamber top with toggle-type clamps (not shown in the figure) attached to the base of the chamber. The horizontal ports were sealed with glass windows compressed against neoprene O-rings by threaded brass sleeves. A high vacuum coupling (Central Scientific Co., Model 94235-3), inserted into and sealed to one of the horizontal p0rt.s by a silver-solder connection, provided a gas sampling port. I n addition to water-cooling cavities in the top of the chamber, copper coils of appropriate diameter are soldered to the external chamber surface to provide adequate cooling of the base. A rotatable platform accommodating eight electrodes is attached to the bottom plate with a pin. A graphite washer separates this platform from the floor of the chamber. Magnetic inserts in the platform permit its rotation with a magnet held outside
Our continuing studies on the d.c. carbon-arc extraction technique have shown that, under optimal environmental conditions, oxygen and nitrogen are quantitatively extracted from the melt. Thus, to complete the analysis, it is only necessary to determine the cmbon monoxide and molecular nitrogen content of a suitable aliquot of the atmosphere in which the extraction is performed. This paper describes a pro-
EXPERIMENTAL
-
QZlSTAINLESS STEEL nBOROSILICATEG L A S S I GRARIITE Ed TANTALUM ITL?iaBOiLER PLATE BReSS OR PLATED B R C X BZZZ WATER COOLING KOVAR ALLOY e NEOPRENE o RINGS
I
PURIFIED HELIUM
D-C
VAdiUM++A
CARBON
ARC
I
Figure 2. Schematic diagram of arc extraction chamber and gas chromatograph assembly
-
--__-__2
3 SCDLt I "
Figure 1.
6
4
bit
GPS S A M R I N G VALVE AVD GAS CHROWAlOGRAFli
il
Schematic diagram of arc extraction chamber
the chamber. A single counter electrode serves for many extractions. The high-vacuum system employed to evacuate the chamber is similar to those previously described (4). The procedures for outgaising of the electrodes and chamber, loading of the electrodes, and samp e arcing are the same as described previously (4), except that electrode loading is done through the side port in this chamber. The extraction chamber is attached to the gas chromatograph (Beckman Instruments, Inc., Model GC-2a) in the manner shown schematically in Figure 2. The connection is made through a n appropria1,e combination of a high-vacuum coupling (Central Scientific Co., Model 94235-3), Swagelok tube fitting (Crawford Fitting Co., Models 400-6-3-516 and 400-7-2-316), toggle valve (Hoke, Inc.), and '/l-inch 0.d. annealed copper tubing. The 3/16inch stainless steel inlet tube extending from the rear of the chromatograph to the gas-sampling valve is standard equipment for the instrument and is indicated by the shaded tubing shown in Figure 2. A single helium gas cylinder is used to supply both the chromatograph and the arc extraction chamber. The chromatograph is stabilized with helium flowing through valve 1 into the internal components and exhausted to the atmosphere a t the thermal conductivity cell exhaust ports. Valve 2 is closed during this oFeration. During the stabilization period, the desired number of sample elxtrodes are positioned on the rotar,y electrode platform and outgassed (4). Valves 3 and 4 are opened to permit evacuation of the chamber and one loop of the gassampling valve. When the internal pressure of the chamber and sampling loop is reduced to less than 10-4 Torr, valves 3 and 4 are closed and valve 2 is opened to admit a measured quantiby of helium into the chamber. A variable constriction, placed adjacent t o valve 2 and on the helium supply side, controls the rate of gas flow into the evacuated
chamber. This arrangement prevents chromatogram base line shift, which would result from an instantaneous decrease in helium forepressure a t the front of the capillary orifices. After the arc extraction operation, valve 4 is opened to expand a 5-ml. aliquot of the chamber gas into the evacuated sampling loop. A rapid 90" rotation of the sampling valve handle makes it possible for the carrier gas stream to sweep the sample onto the column. This rotation of the gas-sampling valve simultaneously places the alternate sample loop in position for a repetition of the evacuation sequence described above. Within experimental error, the two sampling loops are identical in volume, thus allowing equivalent quantities of gas to be transferred from the extraction chamber. Thus, the extraction cycle of the next sample can be conducted while the chromatographic analysis of the preceding sample is completed. A typical analysis is completed by observing recorder deflections (E. H. Sargent Co., potentiometric recorder, Model SR) as the individual components pass through the thermal conductivity cell. Sample Reparation. Solid samples were cut into cubes of appropriate dimension and abraded with a fine steel file to remove surface contamination. Samples in the form of chips were pressed into l/r-inchdiameter cylinders by x briquetting press (Applied Research Laboratory, Model No. 52) operated at 8000 p.s.i. Preliminary Experiments. A d.c. arc initiated between the graphite cathode and a steel sample supported by a graphite electrode of suitable geometry serves to melt the sample, extract the oxygen and nitrogen impurities as carbon monoxide and molecular nitrogen, and equilibrate the liberated gases with the surrounding inert gas atmosphere by convective stirring (4, 6-8). The spectroscopic studies in the authors' laboratories have repeatedly
0.6,
Figure 3.
Electrode geometry
Dimensions in mm.
shown that the rate a t which the carbon monoxide and molecular nitrogen are extracted and the completeness of the extraction are critically dependent on the environmental conditions in the electrode receptacle ('7). It is desirable to achieve the following beneficial conditions: rapid carbon dissolution from the receptacle walls, so that the carbon reduction reaction proceeds a t a maximal rate; dissolution of an optimal amount of carbon, since an excess tends to form a viscous globule which may trap the gases formed; complete dissolution of the receptacle walls so that the arc rests on the globule; and minimal sample volatilization in order to reduce gettering possibilities. The electrode geometry shown in Figure 3 satisfies these four conditions to a high degree when 1-gram samples are arced in a helium atmosphere a t a pressure of 680 Torr and a t an arcing current of 15 amperes. A factorial study on the rates of evolution of the carbon monoxide and nitrogen and the time required for uniform mixing of the extracted gases in the chamber revealed that a steadystate condition is achieved for most alloys during a 60-second extraction cycle. A few high alloy steel samples required 90-second extraction periods to produce maximal gas chromatographic peak heights for nitrogen, indicating VOL. 35, NO. 10, SEPTEMBER 1963
e
1445
that the extraction of this impurity proceeds at a slower rate for some steel compositions. These observations confirm the spectroscopic results (6). To make the analytical calibrations independent of the steel composition, a standard 90-second extraction cycle is employed. Since continuation of the arc discharge beyond the time required to achieve a steady-state condition leads to significant loss (see below) of some of the extracted gases, precise timing of the arcing cycle is essential. A 60-second gas cooling period after the extraction leads to a substantial improvement in precision. K h e n synthet,icgas mixtures are arced in the chamber and transferred t,o the gas chromatograph immediately after termination of the arc discharge, the peak heights show a relative standard deviation of =+=-tj%. This variation ca.n be reduced to less than +2% by alloLi-ing the gases to cool for 60 seconds prior to admitting them into the gas chromatograph. ANALYTICAL CALIBRATIONS A N D DETERMINATIONS
The pertinent experimental conditions employed in this study are summarized in Table I. Typical chromatogram? obtained under the experimental conditions outlined in Table I are illustrated in Figure 4. The deflection shown at zero time is produced by the interruption of the carrier gas flow rate when the gas-sampling valve is rotated. This deflection serves as a convenient time reference point for the chromatographic recording. Since reproducible and symmetrical elution bands are obtained, the simple expedient of measuring the peak heights is used. For quantitative calibrations it may
Table 1.
seem sufficient, a t first glance, to introduce into the chamber measured volumes of helium gas containing accurately determined nitrogen and carbon monoxide impurities This simple approach neglects the fact that the carbon arc discharge converts a portion of the extracted carbon monoxide and nitrogen into other chemical species. There is, first of all, the well known reaction betmen carbon and nitrogen to form cyanogen, as evidenced by the emission of cyanogen band spectra in the arc discharge (6). Atomic lines of oxygen and nitrogen are also emitted in the arc column ( 6 ); conaequently some of the evolved molecule^ are dissociated into the constituent xtum. Since a small amount of metal \ q ) u r is also present in the arc column during the evolution of carbon monoxide and nitrogen, the reaction of these gases with metal atoms is another likely process. Thus, some loss of the evolved gases ir expected. The degree of loss can be approximately ascertained by introducing helium containing carbon monoxide and nitrogen impurities into the extraction chamber and noting the peak heights obtained before and after conducting an arc discharge in the mixture. A typical set of data is shown in Table 11. .ilthough it i\ possible, in principle, t o determine empirical corrections for these losses, a. more definitive experimental approach is to observe peak heights for a set of steel samples of widely ranging composition. If the oxygen and nitrogen content of thebe samples is accurately known, an unequivocal evaluation can then be made on the reproducibility of loss during the extraction and on the potential scope of application of this technique to steel
8. Gas Extraction 1.0 i 0.1 (measured concentrations normalized to 1.0-gram basis by multiplication xith appropriate conversion factor) He, a t pressure of 680 Torr, reproduced to =kl% Cnited Carbon Co., Spectro-Tech grade Undercut graphite electrode as shown in Figure 3 8-mm. diameter pointed graphite rod, 22 mm. long
2. Supporting atmosphere 3. Type of electrodes Anode Cathode 15 4. Arcing current, amperes 90 5 . Srcing interval, seconds 60 6 . Gas cooling period, seconds 7 . Volume of gas aliquot transferred 5 , reproduced to i=170 to chromatograph, cc. B. Gas Chromatograph Determination 1. Helium forepressure, p.s.i. 25 2. Carrier gas flow rate at column 7 2 exit, cc./minute \folec.ulur Sieve 5.1 (Tindt. Co.) 3. Column packing :I -4. Column lensth, feat 7u 5 Column temperature, C. 5 ( ' 1 5 of total output) 6. Detector output attenuation 7 . Recorder chart speed, /z inchlminute 1 (full scale) 8. Recorder span, mv. 1 (full scale) 9. Recorder response, second
ANALYTICAL CHEMISTRY
I
156 pprn N 380 ppm 0
co
71 pprn N 2 8 3 ppm 0
3 2 0 ppm N 47 ppm 0
BLANK
~
0 09 2 2 TIME (MIN)
Figure 4. Typical gas chromatograms
Table II. Loss of Carbon Monoxide and Nitrogen during Arcing Process
Gas chromatographic peak heights, mm. After arciyg ~Steel glqbule Befpre Graphite in arcing anode anode
Combined Arc Extraction and Gas Chromatographic Operating Parameters
1. Sample size, gram
1446
CO
CO
NZ
114 329
106 264
92 309
I301
3t
7 %
i
:L--%$iiP%e*+j OXYGEN CONCENTRATION ( W T Yo)
Figure 5. Analytical curve for determination of oxygen in steels
Table 111.
Sample 1. NBS8h 2. NBS 152 3. XBS l l l b 4. KBS 122c 5 . NBS lOld
6. NBS 129a 7. NBS 461 8. NBS 462 9. KBS 463
10. Type 321
11. Type347 12. T"ype430 13. NBS 1040
14. KBS 1042 15. NBS 1043 16. XBS 1045 17. SBS 1047 18. E 19. Type302 20. Sickel steel
Steel Samples of Known Oxygen and Nitrogen Content
Composition Bessemer stee .(chips)," 0.5% hln Basic open hearth (chips)," 0.8% Mn, 0.2% Si Ni-110 steel (SAE 4620), (chips)," 1.87, Ni, 0.3% Mo 0.370 Si, 0.77, Mn Cast iron (chips)," 0.5% Mn, 0.6% Si 18% Cr, 9% Ni (chips),a 0.77, Mn, 0.5% Si Bessemer (SAE X1112) (chips)," 0.?7, Mn, 0.3% S Low alloy, 0.4% hln, 0 . 3 7 , Mo, l.i% Xi Low alloy, 0.9% N n , 0 37, Si, 0.77, Ni, 0.77, Cr Low alloy, 1.15% &In, 0 4% Si, 0.4% Ni, 0,1570 Ta 0.4% Cu, 0 . 2 % Zr, 0.1% M o 18% Cr, 9% Ni, 27, &In, 1.07, Si, 0.3% Ti 187, Cr, 7Y0 Xi, 2% Mn, 1% Nb 16% Cr, 1.07, Mn, 1.0% Si Low carbon, 0 . 3% Mn Bessemer, 0 . 770 Rln Low carbon, 0.6% Mn Medium carbon, 0.541, Mn Low carbon, 0.45%M n Low alloy 18% Cr,'9% Xi, 0 . 7 % Mn 2 . 8 % Ki, 0.7CjC Aln, 0.3% Si, 0.9% Cr, 0.2yo V, 0 . 2 %
Concentration, Xitrogen 0.017 0.0038 0.0050 0.0040
0.024 0.014 0.0071 0.0092 0.0072 0,010 0.084
0.032 0.0037 0.016 0.0050 0.0050 0.0040 0.0043 0.064 0.0072
weight 7, Peak heights, mm. Oxygen Nitrogen Oxygen 98.5 54.0 0.038 76.0 9.5 0.031 55.0 13.5 0.028 0.056 0.027 0.069 0.028 0,0082 0.0095
