that the behavior is less than ideal. At least part of the deviation from ideality is due to the inefficient column that was used, because of its short length, low capacit?:, and large-sized and nonuniform pa'cking. For some systems, nonideality may be due to limited solubility of the metal (In) or chemical interaction (Sn) in the mercury film. Hecause of its outstanding amenability to control by aelection of the elution potential., chromatography by electrodeposition is a promising method for analytical separations. However, until column preformance can be improved. and until the nonideal be-
havior of some metals can be circumvented, the full advantages of this new chromatographic method will not be realized. ACKNOWLEDGMENT
h stimulating discussion of the feasibility of the process with Professors L. B. Rogers and J. IT. Ross a t the Massachusetts Institute of Technology in .ipril 1959 is acknowledged. LITERATURE CITED
(1) Blaedel, W. J., Strohl, J. H., AKAL. CHEM.36, 445 (1964). ( 2 ) Ibid., p. 1245.
(3) Kemula, W., Kublik, Z., Galus, Z., A\7ature 184, 179.5 (1959). ( 4 ) Keulemans, A . I. lI., "Gas Chromat,ography," 2nd ed., Reinhold, Kew L., Brubaker, R . I,., Enke, C. G., AXAL.CHEM.35, 1088 (tO63j. ( 6 ) Sidgewick, X.I-,,',Chyniical E1ement)s and Their Compounds, Oxford I-niv. Press, S e w York, 1950.
RECEIVEDfor review hugu-it 5, 1964. Arcepted October 14, 1964. This work was support,ed by (:rant So. AT( 11-1 )10232,froni the I'. 8. Atomic Energy Cornmission. Taken in part from a thesis submitted by John H. Strohl in partial fulfillment of the requirements for the Ph.1). degree at the Universit,>- uf Wisconsin, January 1964.
Simultaneous Determination of Oxygen and Nitrogen in Refractory Metals by the Direct Current Ca r bo n-Arc, Gas Chro ma to g ra phic Te c hniq ue ROYCE K. WINGE and VELMER A. FASSEL lnstitute for Afomic Research and Department o f Chemistry, lowa State University, Ames, lowa
b The d.c. carbon-arc, gas chromatographic technique has been applied to the simultcrneous determination of the oxygen and nitrogen content of the refractory metals. With the aid of a platinum flux, the oxygen and nitrogen are liberated from the metal as carbon monoxide and molecular nitrogen into a static helium atmosphere. An aliquot of the resulting gas mixture i s then passed through a gas chromatograph and the respective peak heights are related to the oxygen and nitrogen contents of the metal. Analytical data olbtained from 1 2 different base metals suggest that single oxygen and nitrogen analytical curves can b e used for determining these impurities in most refractory metals and alloys.
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of oxygen and nitrogen commonly occur in the refractory metals ai; solid solutions in the interstices of the metal lattice or as inclusions of the oxides or nitrides of one or more of the m.etals present in the specimen. Molecular gases may be entrapped in voids in some metals, but this type of occurrence is not likely in the refractory metals (8). When oxygen and nitrogen are present as interstitial impurities, they usually exert a profound effect on the physical and mechanical properties of the host metal. ;\s a ronsequence, there is an el-er-increasing interest in analytical methods for the determination of these RACE IMPURITII:S
impurities in metals and alloys. Sitrogen is commonly determined by variants of the classical Kjeldahl procedure (15) with acceptable accuracy and precision. However, as more corrosion-resistant alloys are being developed. complete dissolution of the sample by the acid solvents is becoming increasingly difficult. Uncertainties also exist as to whether all of the combined nitrogen is quantitatively converted to ammonium salts for all of the alloy systems encountered. I n view of these considerations it is desirable to explore other techniques for performing the nitrogen determinations. Various modifications of the vacuumfusion technique (16, 17) have been applied successfully to the determination of the total combined oxygen content of many metals. I n principle, nitrogen can be determined simultaneously, but the analytical data so obtained have been repeatedly questioned (1, 13, 1 4 ) . The various factors which appear to contribute to the frequently observed low vacuumfusion nitrogen results have been discussed by several investigators (1, b j 9). I n 1957, Booth, Bryant, and Parker ( I ) concluded "that the vacuum-fusion technique, as presently operated, does not furnish very reliable nitrogen figures for those elements forming very stable nitrides." On the other hand, there have been scattered reports that vacuum or inert gas fusion nitrogen results on several refractory metals were in accord with
the Kjeldahl values. I n all instances, these determinations involved a platinum-bath environment a t teniperatures equal to or exceeding 1900" C. Thus, successful comparisons were found for Zircalloy 2 a t 1900" C. (8); for zirconium a t 1900" C. ( d ) ,and 1950" C. (10); and for niobium a t 1900°C. (11). One of the distinct advantages of the d.c. carbon-arc extraction technique is that the molten globule attains local temperatures in the 3000" C. range. This feature plus the precipitous temperature gradient in the molten globule appears to contribute to the rapid extraction of the oxygen and nitrogen content of many metals. The success achieved in simultaneously determining the oxygen and nitrogen content in low and high alloy steels by an arc extraction, gas chromatographic technique suggested its further application to the determination of these impurities in the refractory metals. EXPERIMENTAL
Apparatus and Procedure. The experimental facilities and their mode of operation have been described (3). The procedure for preparing the samples followed the practices outlined previously ('7). The degassing procedure used for this study was as follows. .liter loading the de3ired number of supporting electrodes into the chamber, they and the inner surfaces of the chamber are degassed by arcing each electrode for 30 seconds at 25 ampere:; in a helium supporting atmosphere at 250 torr. All of the electrodes are deVOL. 37, NO. l , JANUARY 1965
* 67
b6.4mm--(
L7.9mm -1 Figure 1 .
Electrode assembly
gassed without terminating the arc discharge as it readily jumps from one electrode to the next as the turntable is rotated. After all the electrodes have been arced in this manner, the chamber is evacuated while the inner surfaces are still hot. The chamber is then brought to atmospheric pressure with helium, the lid is opened, and the samples are loaded as rapidly as possible. Some readsorption of atmospheric oxygen and nitrogen occurs during sample loading, but these gases are effectively removed by conducting the arc discharge to an auxiliary electrode for two 2-minute periods. High blank conditions, such as atmospheric leakage into the chamber are readily detected before samples are arced by monitoring the chamber degassing with the gas chromatograph. Preliminary Experiments. Prior studies in our laboratories on the emission spectrometric determination of oxygen in metals by the d.c. carbon-arc technique have shown
Table 1. Experimental Parameters 0.30 f 0.03 grams Sample weight Platinum flux weight a ) hIo, S b , Tb, Th, V,Y, and 1.0 ==! 0.1 gram low alloy steel b) Cr, Hf, Ta, Ti, Zr, and high al1.5 f 0.1 gram loy steel Sample electrode United Carbon Co., Spectro-Tech grade, as shown in Figure 1 Supporting atmosphere Helium, 680 torr Arcing current 15 amperes Arcing interval 20 seconds Gas mixing period 60 seconds I'olume of gas aliquot transferred to chromatograph 5 cc.
68
ANALYTICAL CHEMISTRY
t h a t the simple expedient of arcing some metallic specimens in graphite electrodes does not lead to reproducible extraction of the oxygen and nitrogen contents ('7). This problem was solved by using an electrode assembly which provides a plat.inum bath after the arc is init.iated ( 7 ) . Platinum is commercially available with low oxygen and nitrogen content and possesses several unique properties which make it an ideal reaction medium for rapid and reproducible gas evolut,ion. I t readily forms alloys with ot,her met,als, so that even the most refract,ory specimens fuse rapidly. The solubility of carbon in the metal appears to be in t,he optimal range. Finally, its boiling point is high. I t is appropriate to recall that the carbon-arc discharge converts a portion of the extracted carbon monoxide and nitrogen into other chemical species (3). The technique of calibration should therefore meet' two conditions; namely, the fractional loss of the gases should be the same for various base metals, and the calibration scheme must correct for the losses incurred. Data presented by Fassel and Goetzinger ( 6 ) provide strong experimental evidence that quantitative oxygen extraction can be achieved with the platinum reaction medium and that losses of carbon monoside in the arc colunin are equivalent for most metals. Analogous to the observations already made for ironbase metals (S), these dat,a lead to the expectation that single, congruent analyt.ica1 curves can be established for refractory met,als as well. The previous spectroscopic studies (6, 7 ) delineated the experimental conditions under which maximal extraction of the oxygen content can be achieved with the platinum-flux electrode assembly. The geometry of the supporting electrode and receptacle used in the present study is shown in Figure 1. Factorial st,udies on the sample-to-platinum-flux-weight ratio required for maximal gas chromatographic peak heights for carbon monoxide and nitrogen showed t,hat the ratio differed for oxygen and nitrogen and varied from metal to metal. Maximal extraction behavior was obtained with the platinum flux weights listed in Table I. K h e n the loaded electrodes are arced a t 15 amperes in helium at 680 torr, about 3 to 5 seconds are required to melt the sample-platinum pellet. The metals alloy rapidly and evolut'ion of carbon monoxide and nitrogen occurs during the next 5 to 10 seconds. A typical gas evolution curve is shown in Figure 2. To obtain these data, arc extractions on individual samples were conducted for 10, 15, 20, 30, or 60 seconds. After allowing 60 seconds for homogeneous distribution throughout the arc-supporting atmosphere, the extracted gases were examined gas chromatographically. I t is apparent that even a t 10 seconds, nearly maximal amounts of carbon monoxide and nitrogen are evolved. In pract'ice, the arc extractions are continued for 20 seconds to assure complete extraction from
A NITROGEN 0
LJ
0
I
10
Figure 2.
I
20
I
OXYGEN (CARBON MONOXIDE)
I
30 40 ARC TIME ISEC.)
I
50
I
60
I
Gas evolution curve for thorium
fractious specimens and to assist in the uniform mixing (by thermal convection) of the ext'racted gases with the supporting atmosphere. Figure 2 also shows that continuat'ion of the arc discharge beyond 20 seconds leads to a significant loss of the extracted gases. This is in agreement with t'he findings of Evens and Fassel (3) who attributed the loss to gettering of carbon monoxide and nitrogen by metal vapor and also to the react'ion of nitrogen with carbon vapor to form cyanogen. ANALYTICAL CALIBRATIONS A N D RESULTS
The experimental parameters employed in the calibrations are summarized in Table I. Examples of typical chromatograms are shown in Figure 3. The first peak in each chromatogram results from a pressure disturbance caused by introduction of the sample. These recordings illustrate the use of an attenuator on the chromatograph for extending the useful range of the measurements. For the blank recordings, no attenuation was used. .It this sensitivity, 1-inm. peak height corresponded to approsimately 0.6 pg. (2 p.p.m. in a 0.3-pram sample) of oxygen or nitrogen. The oxygen blank ranges from 8 to 15 pg., approximately 60% of which originates in the platinum. The nitrogen blank ranges from 1 to 2 pg. .I platinum blank was run with each chaniberload of samples and all peak heights used in this study were blank corrected. Figure 3 also shows. that both the nitrogen and carbon monoxide peaks are recorded within 100 seconds after the sample is introduced into the chromatograph. The observed carbon monoxide peak heights for 12 different metals are correlated with vacuum-fusion oxygen data in Figure 4. The two curves are actually coincident but were separated to show the numerous data collected for the various metals. Figure 5 shows the correlation of the observed nitrogen peak heights with Kjeldahl data for eight different metals. Each point in these figures represents the average of two or more analyses. Steel was included in this study because of the
GO
I
TERBIUM 0.055 X N 0.313% 0
NlOB'lUM 0.078 X N
0.033960
BLANK / P l B$TH ELECTRODE)
A x 0 45
100
TIME (SEC.1
Figure 3.
Typical chromatograms
Attenuation factors, where used, a r e listed beside the peaks
availability of samples which have been analyzed by other methods. All peak heights for these figures were normalized to correspond to a 0.300gram sample weight. Before drawing conclusions from the data plotted in Figures 4 and 5, it is appropriate to note that uncertainties may exist in the vacuum-fusion values for oxygen and in the Kjeldahl values for nitrogen which were used to plot the points. Therefore, some of the scatter of points in these figures may be attributed to these errors. Figure 4 shows that all of the experimental points for oxygen cluster rather uniformly along a single congruent curve with no apparent systematic deviations for any of the metals. The concordance of these data indicates t h a t the small amount of carbon monoxide lost during the platinum-bath extraction is equivalent for a variety of metals, even though I
they differ markedly in volatility and reactivity toward carbon monoxide. A corollary of this conclusion is that a single analytical curve appears to be applicable to the determination of oxygen in most refractory metals or alloys. The conclusions stated above for oxygen appear to apply to the nitrogen data in Figure 5 with one exception; the points for titanium fall considerably below the curve. T h a t the observed low points reflect incomplete extraction of nitrogen during the initial 20-second arcing was confirmed by extraction of considerable quantities of nitrogen
during second and third re-arcings of the globule. Several factors may contribute to this anomalous behavior. The environmental conditions may not be optimal for the quantitative decomposition of Ti-X bonds (in solution or as the nitride) or for the quantitative transfer of the nitrogen from the melt to the gas phase. The equilibrium nitrogen content of the platinum-titanium globule may be greater and the rate at which this equilibrium is attained may be slower than for other base metalplatinum alloys. Further considerations of these factors must be deferred until more definitive data on the mechanism, kinetics, and thermodynamics of nitrogen evolution from platinum alloys are on hand. Precision study data are shown in Table 11. The relative standard deviation of the peak heights wa? calculated from ten or more individual determinations on as many different days. Sample homogeneity as well as instrumental variations contributes to the observed relative standard deviation of the results. The relative standard deviation values given in Table I1 favorably reflect the analytical adequacy of this technique.
Table II.
Precision Data
Relative Concentration, standard wt. yc deviation, 7, Oxy- Nitro- Oxy- Nitro Sample gen gen gen gen Thorium 0.110 0.016 7 . 0 5 . 5 Vanadium 0.036 0.298 7 . 8 7 . 9 Yttrium 0.266 0.034 6 . 4 7 . 2 Zirconium 0.118 , . . 3.1 . . .
I
I
I aa
QK)
0 CHROMIUM 4 NIOBIUM X STEEL I TERBIUM 0 THORIUM 0 TITANIUM 0 VANADIUM . PYTTRIUM
I
,
0 CHROMIUM 0 MOLYBDENUM x STEEL V TbNTALUM
0 TITANIUM
P YTTRIUM
L o /
10 ROCI
/ I aa
I
01 OXYGEN CONCENTRATION 111 X )
Figure 4.
1
n
D
NITROOEN CWCENTRATIOW
10
Oxygen analytical curve
I frt.16)
Figure 5.
Nitrogen analytical curve VOL. 37, NO. 1 , JANUARY 1965
69
DISCUSSION
The data plotted in Figures 4 and 5 adequately demonstrate the practical utility of this analytical technique. These results constitute the first successful simultaneous determination of oxygen and nitrogen in an extended list of refractory metals with time requirements considerably less than those of other approaches which have been applied. Loading of the chamber with 8 electrodes, degassing, and loading of the samples require about 20 minutes. The extraction and analysis require only an additional 3 minutes per sample. Thus, the average time required per sample is about 6 minutes. Single samples niay be added to the outgassed electrode and chamber system through the side port. I f the chamber is flushed simultaneously with helium, subsequent outgassing is not required. I n this way, the total time required for determining both oxygen and nitrogen on a single sample is only about 4 minutes. The potentialities of extending the sensitivity of this technique several
orders of magnitude have been discussed (3). L-nder the operating conditions described in this paper>the blank for oxygen imposes the primary limitation on sensitivity. rl considerable reduction in the oxygen blank can be effected by using “low oxygen platinum” now available from the Baker Platinum Division of Engelhard Industries (12).
LITERATURE CITED
(1) Booth, E., Bryant, F. Analyst 82, 50 (1957).
S.,Parker, .A,,
(2) Elwell, W. T., in “Determination of Gases in Aletals,” Iran and Steel Inst. Spec. Rept. 68, 19-42, 1960. (3) Evens, F. AI., Fassel, 1.. -4.,ASAL. CHEM.35, 1444 (1963). (4) Everett, hl. R., rlnalyst83, 321 (1958). ( 5 ) Fassel, V . -4.) Evens, F. >I., Hill, C. C., AKAL.CHEM.36, 2115 (1964). (6) Fassel, 1.. A , , Goetzinger, J. R., Spectrochim. i l c t a , in press. ( 7 ) Fassel, V. A., Gordon, W. A,, A K A L . CHEM.30, 179 (1958). (8) Goward, G . W.,Pratt and Whitney Aircraft, Sorth Haven, Conn., private communication, 1963.
( 9 ) Ihida, AI., Japan Analyst 8, 786 ~ x, m j - l_ !. _ .
(10) Holt, B. D., Goodspeed, H. T., ANAL.CHEM.35, 1510 (1963). (11) lIcKinley, T. D., E. I. I>u Pont de Semours and Co.. LVilminrton. 1)el.. private communication, IXXI (12) McKinley, T. I)., Englehard Ind. Tech. Rd1. 2, Y o . 4, 140 (1962). (13) llallett, 11. W., Talanta 9, 133 (1962). (14) lIallett, Ll. W., Griffith, C. B., .Am. Soc. dletals 46, 375 (1954). ( 1 5 ) Sational Academy of Sciences, ”Report of the Panel on Analytical Problems In Refractory Jletals,” Rept. MAB-154-M( l ) , \-olume 11, pp. 17-22, 1959. (16) Parker, A , in “Determination of Gases in Metals,” I ~ o n Steel Inst. (London) Spec. Rept.(‘68, 64-74, (1960). (17) Still, J. E., in Determination of Gases in Metals,” Iron Steel Inst. (London) Spec. Rept. 68, 43-63 (1960).
RECEIVEDfor review August 10, 1964. Accepted October 23, 1964. X o r k performed in the Ames Laborator) of the U. S. .Atomic Energy Commission.
Determination of Subtoxic Concentrations of Phosgene in Air by Electron Capture Gas Chromatography L. J. PRIESTLEY, Jr., F. E. CRITCHFIELD, N. H. KETCHAM, and J. D. CAVENDER Research and Development Department, Chemicals Division, Union Carbide Corp., South Charleston, W. Vu.
b A gas chromatographic method for measuring subtoxic concentrations of phosgene in air is described. The method utilizes an electron capture detector which is extremely sensitive to phosgene. The combination of a gas chromatographic column and the electron capture detector provides a high degree of specificity for phosgene. Hydrogen chloride concentrations as high as 1% in air do not interfere. This method could possibly b e ured as the basis for developing a continuous monitoring system for detecting and measuring phosgene in the atmosphere.
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use of phosgene as an industrial cheniical demands instrumentation continuously to monitor air for this contaminant. No completely satisfactory instrument has been reported to date. An analyzer utilizing a colorimetric reaction has been described ( 2 ) . This type of instrument is bulky, expensive, and coinplicated to maintain. This laboratory has developed a gas chromatographic method which can detect and measure phosgene in air in the range of’ 1 p.p.b. to 2 p.1i.m. and which the authors feel is a possible basis HE: INCREASING
70
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ANALYTICAL CHEMISTRY
for the development of instrumentation for monitoring phosgene in the atmosphere. The method utilizes the electron capture detector (3) which is sensitive to most halogenated compounds. Under the conditions used in the laboratory, a determination can be made every 6 minutes. The method will detect and measure concentrations of phosgene on the order of 1000 times less than those of physiological interest. The threshold limit value for daily 8-hour exposure to phosgene has been set at 1 p.1i.m. by volume in air (4).The odor of phosgene can be detected by some people at a level of 0.5 p.p.m. or less, but this ability varies widely. EXPERIMENTAL
Apparatus. An herograph Model A-350-I3 gas chromatograph, equipped with a n electron capture detector, was used for this T\ork. The recorder was a 13rown -0.05 to +1.05 mv. equipped with a Disc integrator. The column consisted of two meters of 4.T-mIn. 1.d. aluminum tubing packed with 307, by \\eight Flexol plasticizer 10-10 (didecyl phthalate, Cnion Carbide Corp.) coated on 100- to 120-mesh
GC-22 Super Support (Coast Engineering Laboratory). The column was operated isothermally at 50’ C. The flow rate of nitrogen carrier gas was 50 cc. per minute. The potential applied to the detector was 90 volts. Procedure. Samples of known concentrations of phosgene in air were prepared in a dynamic triple dilution system (Figure I), placed in a fume hood. This system consists of a series of three glass mixing chambers in which the phosgene is progressively diluted with air. Measured flows of phosgene and air are injected into the first stage of the system where they are mixed as they flon. through baffle plates. A measured flon of this mixture is then allowed to enter the second stage and the surplup material is vented. I n the second stage of the system, the mixture is further diluted by a measured flow of air. If a further dilution is desired, the same procedure is followed with the third stage. Samples can be taken at the venting points of all three stages. .ill flows are measured by Fischer & Porter Co. precision bore flowrator tubes;. For calibration purposes, the compressed air used in the dilutions should be as dry as possible, and hypodermic syringes used to inject samples into the
