(577) Wetherill, G. W., J. Geophys. Res. 69,4403 (1964). (578) White, G., Scholes, P. H., Metallurgia 70, 197 (1964). (579) Wilson, A. AT., PLNAL. CHEM.38, 1784 (1966). (580) Wilson, A. M., AkFarland, 0. K. Ibid., 36,2488 (1964). (581) Wiltshire, N. D., Nancarrow, P. C., Ludvig, I., B.H.P. Tech. Bull. 8, 34 (1964). (582) Winefordner, J. D., T’eillon, C., ANAL.CHEM.36,943 ( 1964). (583) Wittick, J. J., liechnitz, G. A,, Zbid., 37,816 (1965). (584) Rohlmann, E., Geologie 13, 845 (1964). (585) Wood, D. F., 4dams, M. R., Anal. Chim. Acta 31, l!j3 (1964). (586) Wood. D. F.. Jonss. J. T., Analust 90; 125 (1965). ’ (587) Ibid., p. 638. (588) Wood, R., Richards, L. A., Ibid., p. 606. (589) Yablochkin, T‘. I)., Sudebno-Med. Ekspertiza, Min. Zdnwookhr SSSR 6 , 45 45 ilFlfi.?) (1963). (590) Yaguchi, H., Kajixara, T., Bunseki i! Kagaku 14,785 (1965)
(591) Yamada, Y., Radioisotopes (Tokyo) 13,32 (1964). (592) Yatrudakis, S. >I., Zhdanov, A. K., Uzbeksk. ,Khim.,Zh. 8,23 (1964). (593) Yatsimirskii, K. B., Morozova, R. P., Voronova, T. A , , Gershkovich, R. XI., Zh. Analit. Khim, 19,705 (1964). (594) Yatsimirskii, K. B., Zakharova, L. A., Neorgan. Khim. 8,96 (1963). (595) Yen, J., Yung, C., Liu, H., Hua Hsueh Hsueh Pao 30,562 (1964). (5961 Yoshida. RI..Kitamura., X.., Bunseki ‘ Kigaku 14,323 (1965). (597) Yoshimori, T., AIiwa, T., Takemura, T., Ito, N., Takeuchi, T., Ibid., 11,1243 (1962). (598) Yoshimura, C., Uno, S., Noguchi, H., Ibid., 12,42 (1963). (599) Yuasa, T., Ibid., 11,449 (1962). (600) Yule, H. P., Ax.4~.CHEM.37, 129 (1965). (601) Zabiyako, T’. I., Sharapova, G. N., Tr. Ural’sk. il’auchn.-Issled. Khim. Inst. 11,14 (1964). (602) Zakharov, 31. S., Stromberg, A. G., I z v . Tomsk. Polztekhn. Znst. 112, 143 (1963). (603) Zhelonkina, L., Zheenbaev, Zh., Karikh, F. G., Polovikov, A. I., Engel’sht, T’. S., Zzv. Akad. Nauk Kirg
SSR, Ser. Estestv. i Tekhn. Nauk 5, 99 (1963). (604) Zhigalkina, T. S., Lev, R. S., Cherkesov, A. I., Tr. Astrakhansk. Tekhn. Inst. Rubn. Prom. i Khoz. 8 . i23 (1962). (605) Zhivopistsev, V. P., Chelnokova, &/I. N., Uch. Z a p . Permsk. Gos. Univ. 111,162 (1964). (606) Zhuravlev,, G. I., Ryzhkova, L. I., Zh. Analit. Khzm. 18,930 (1963). (607) Zindel, E., Zeiher, R., Z. Anal. Chem. 195,27 (1963). (608) Zitnansky, B., Sebestian, I., Hutnicke Listy 18,274 (1963). (609) Zmijewska, W.,Kozminska, D., Chem. Anal. (Warsaw) 9,469 (1964). (610) Zolotukhin, V. K., Gavrilpuk, A. I., Galanets’, Z. G., Pasichnik, 0. AI., Visn. L’vavs’k. Derrh. Univ., Ser. Khim. 1963, p. 73. (611) Zopatti, L. P., Pollock, E. N., Anal. Chim. Acta 32, 178 (1965). 1612) Zotin. AI. A.. Uch. Zaw.. . Permsk. ‘ Gds. Univ: 25,53 (1963). (613) Ibid., 111,82 (1964). - I
WORKperformed in the Ames Laboratory of the U S . Atomic Energy Commission.
Simultaneous Determination of Oxygen and Nitrogen in Metals by Carrier-Gas Fusion IExt raction Wayne E. Dallmann arid Velmer A. Fassel, Institute for Atomic Research and Deparfment o f Chemisfry, Iowa Sfafe University, Ames, Iowa
A platinum-tin reaction medium has been applied successfully to the carrier-gas fusion determination of both the oxygen ancl nitrogen contents o f 19 different bcise-metals. The composition o f the reaction medium ranged from 80 (Pt):20 (Sn) a t extraction temperatures in the region 1725-1950°C, to 95 (Pt):5 (Sn) for extraction temperatures greater than 1050°C. Quantitativct accuracy was demonstrated b y concordance of carrier-gas fusion results with values obtained b y other analytical techniques such as Kjeldahl chemical, caustic fusion, isotope dilution, vacuum fusion, and hydrogen reduction; the theoretical oxygen contents of synthetic metal-oxygen standards; and values certified b y the National Bureau o f Standards or developed in cooperative ASTM analytical programs. A detailed description o f the carriergas fusion, gas chromatographic analytical facility employed in this study is given. In this apparatus, a pyrolytic boron nitride thimble i s employed to keep the operating blanks within tolerable limits.
0
NITROGEN impurities are commonly present in trace amounts in metals, occurring either as XYGEX ASD
solid solutions in the interstices of the metal lattice, as oxide and nitride inclusions, or in some cases, as trapped molecular gases. The presence of oxygen and nitrogen as interstitial inipurities normally results in profound effects on the physical, mechanical, and electrical properties of the host metal. For production control and metallurgical evaluation of such effects, accurate and precise analytical determinations on these impurity contents in metals and alloys are required. The nitrogen impurity content is normally determined by classical chemical dissolution procedures (24, 52), usually with acceptable accuracy and precision. I n some cases, notably silicon steels and many of the recently developed corrosion-resistant alloys, the chemical technique frequently fails to provide quantitative results on the nitrogen content. TThether the low nitrogen recoveries are due to incomplete sample dissolution or to nonquantitative conversion of the nitrogen to ammonium salts has not been clearly established. I n view of these uncertainties, it is appropriate to refine existing analytical techniques or develop new ones which may be generally applicable to the nitrogen determination. The possibility of utilizing high-tem-
perature fusion extraction techniques for determining the nitrogen content of metals along with the oxygen impurity has been previously considered by many investigators. The classical vacuum fusion (56) and the carrier-gas fusion (47, 49) techniques both involve high-temperature fusion of the metal specimen in a graphite crucible, so that carbon reduction and thermal decomposition reactions may occur as follows:
There is now general agreement that quantitative formation and extraction of carbon monoxide from most refractory metals can b e achieved if optimal environmental conditions are provided in the fusion crucible. Usually, molecular nitrogen is also evolved under the same experimental conditions. However, the validities of nitrogen results obtained in this way, especially for the refractory metals, have been clouded for many years by inadequate knowledge on the quantitative release and recovery of the molecular gas from the melt. VOL. 39,
NO. 5,
APRIL 1967
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Table 1.
Summury of Prior lnvestigutions on Fusion Extraction Determinution of Nitrogen Temperature (” C) 1800 1900 1900
hletal Ti Zircalloy Zr Zr, Steels, UN Steels Steels Nb Nb, Ta
1950
1850 1850 1900 2300
Steels
1950-2050 1900
Steels Steels
1900 1600
M O
Fusion conditions Iron bath, vacuum fusion Pt bath, Pt flux, vacuum fusion P t bath, 15-mg samples, vacuum fusion P t bath, carrier-gas fusion Pt bath, vacuum fusion P t bath, P t flux, vacuum fusion P t bath, P t flux, vacuum fusion Pt bath, carrier-gas fusion Pt bath, vacuum fusion No bath, single sample per crucible, carrier-gas fusion No bath, carrier-gas fusion No bath, single sample per crucible, vacuum fusion
SIMULTANEOUS DETERMINATION OF OXYGEN AND NITROGEN BY FUSION EXTRACTION
For the quantitative determination of both nitrogen and oxygen by fusion extraction techniques, it is essential that: 1. A reaction medium is employed
which accomplishes quantitative formation of carbon monoxide and quantitative dissociation of metalnitrogen bonds. 2. The terminal or equilibrium solubility of carbon monoxide and nitrogen in the reaction medium is a negligible fraction of the total. 3. Quantitative transfer of the evolved gases to the analytical system is effected without significant bulk or dispersal gettering losses. 4. Requirements 1, 2, and 3 are achieved under tolerable blank conditions. With reference to requirement 1, there is ample theoretical evidence in the literature (2, 20, 48) that even the most stable metal-nitrogen bonds should be decomposable at standard fusion extraction temperatures, especially those obtained under carrier-gas fusion conditions. There is also ample experimental evidence to support these conclusions. The summary in Table I shows some of the experimental conditions under which previous investigators have reported quantitative nitrogen recoveries. I n spite of these studies, the authors of recent critical reviews on this subject have written as follows:
“In general, vacuum fusion nitrogen values for metals other than steels amount to no more than numbers obtained incidental to oxygen determinations. . . No concentrated effort has been made to determine conditions required for quantitative recovery of nitrogen from refractory metals by vacuum fusion.” (40) 134 R
ANALYTICAL CHEMISTRY
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‘Wtrogen values obtained by vacuum fusion are still, however, regarded with suspicion. This is particularly true for the reactive metals (Ti, Zr, Hf), and to a lesser extent, the refractory metals (V, Xb, Ta, Cr, Mo, W).” (20) The views expressed by these authors accurately reflect the experiences of many analysts who have not found it possible to achieve quantitative nitrogen recoveries, even under the recommended environmental conditions. This suggests that one or more of the other requirements were not completely fulfilled. With reference to requirement 2 listed above, Goward’s (20) detailed theoretical treatment of vacuum fusion and extraction processes for the determination of nitrogen strongly suggests that the root of the problem is the difficulty of extracting nitrogen down to its equilibrium concentration in the melt. Goward’s conclusions are supported by Ihida’s observation (27) that 30 to 40 ppm of nitrogen remain unextracted from iron baths at 1850’ C. According to Ihida’s calculations, this residual nitrogen corresponds to an equilibrium nitrogen pressure of 7.6 mm a t 1600’ C, decreasing to 6.3 mm at 1900’ C. Kraus’ (32) theoretical deductions and experimentally determined mass transfer coefficients showed that the transport of dissolved gases from the interior of the melt to and through the melt-gas phase boundary indeed was the rate limiting step. Using a 20-minute extraction period as a criterion for a useful analytical method, Goward (20) also concluded that the transfer of nitrogen from the melt to the gas phase could be rate limiting. I n harmony with these conclusions was the observation of Somiya et nl. (50), that nitrogen recovery on steels was not quantitative unless the vacuum extraction time was extended considerably beyond that required for complete oxygen recovery. Because the thermodynamic environment (composition and temperature of fusion media), can influence the rate of
gas transfer, i t is not surprising that almost all reported instances of improved or quantitative recovery of nitrogen have involved trial and error modifications of bath compositions (8, 14, 15, 17, 31, 33, 48, 4 4 , and temperatures (14, 15, 19, 40, 43)’ until maximal extraction was achieved, Thus, Peterson and Beerntsen (44) found that nitrogen in lanthanum could be quantitatively extracted with a nickel bath at 1600’ C, while recovery with a n iron bath was low even at 1850’ C. Xuramatsu (42) found that recovery of nitrogen in molybdenum was complete when a platinum bath was used, but was incomplete when nickel, nickel-tin, and platinum-nickel baths were employed. Lemm (83) demonstrated that quantitative recovery of nitrogen in steel was possible when a new, small graphite crucible was used for each sample extraction under a carrier-gas atmosphere. Koch and Lemm ( S I ) later showed that nitrogen recovery progressively decreased when several steel samples were extracted in the same carrier-gas fusion crucible. However, recovery on a large number of alloyed and unalloyed specimens was quantitative when the crucible was changed after each extraction. Gerhardt, Kraus, and Frohberg (17) found that quantitative extraction of nitrogen from steels was very rapidly achieved when each sample was flused with a Ni-Ce-C alloy which contained approximately 1% cerium. According to Gerhardt et al., the cerium caused precipitation of spheroidal graphite, which affected the viscosity of the melt considerably less than the normally precipitated flake graphite. An interesting feature of the furnace employed by Gerhardt et al. (17) was the “spinning crucible.’’ After each sample extraction, the crucible was emptied by spinning it at a speed high enough to eject the melt by centrifugal force. The ejected melt \vas deposited on a graphite shield which surrounded the crucible. Fischer and Steinrucken (15) showed that nitrogen recovery on zirconium and titanium, and on their nitrides, was only 507, complete when a platinum reaction medium was used. They attributed the low results to a n insufficient supply of carbon in the melt. Recovery was increased substantially when they employed a nickel bath, but results were still low and tended to further decrease with each successive sample extraction. This effect was attributed to the dissolution of a n excessive amount of carbon in the nickel melt, which resulted in the rapid formation of graphite flakes a t the melt surface. Fischer and Steinrucken (15) found that a composite platinum-nickel reaction medium provided the highest nitrogen recovery, although quantitativeness still was not demonstrated, as
R KJELDAHLCHEMICAL VACUUM
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Figure 2. Carrier-gas fusion crucible assembly with the pyrolytic boron nitride thimble
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Figure 1. Studies on vacuum fusion reaction media for determination of nitrogen in lanthanide metals
recoveries amounted to 85-90%. Fassel and eo-workers ( 1 4 ) showed that quantitative determina,tions of nitrogen in steel were not provided by the vacuum fusion, iron reaction medium, but that quantitative recovery wa5 achieved by application of a platinum bath, platinum flux procedure. The experiences of many investigators in the past are reflected in the graphical recovery d a h shown in Figure 1 for the nitrogen contents in several lanthanide elements. It is seen that a platinum-tin bath a t B vacuum fusion extraction temperaturf of 1700' C provided essentially quani itative release of nitrogen, as evidenced by concordance of the results with Kjeldahl chemical data. I n contrast, a platinum bath a t 1900' C consistently yielded incomplete recovery. Thus, the presence of tin in the reacticn medium contributed to more effici:nt extraction of the nitrogen contents of these metals at a lower fusion temperature than was required for a simple platinum bath. This is not the first otiservation on the beneficial effects of tin in the reaction medium. Previous in ;estigators ( 1 , 3, 4,6, 8, 11, 12, 18, 19, 2'1, 36, 40, 50, 55) have shown that the presence of tin in various reaction media contributed to the more efficient and reproducible extraction of the oxygen content as well. The role played by ths tin is not clear although a number 0' desirable functions have been attributed to it (12). There is considerable evidence that it is difficult to extract nitrogen d o a n to its equilibrium level in many melts. Therefore, the increased fluidity of the platinum-tin melt (21, 51) and the sweeping (1) and stirring (21) action
provided by the tin as part of its evaporates appear to be important functions. A third requirement for quantitative analysis is that irreversible adsorption of the extracted gases through bulk (34) or dispersal gettering processes (11, 53, 55) be negligible. The rate of vaporization of certain metals and carbon from the fusion crucible may be the major contributing factor, and under vacuum fusion conditions appreciable amounts of metal and carbon are volatilized. Other things being equal, the amount of materials volatilized is directly related to the temperature of the crucible. The summary of data in Table I shows that extraction temperatures required for the vacuum fusion determination of nitrogen are in the 1600' C to 2000' C range. There is general agreement that gettering losses become serious beyond this temperature range. However, a t these temperatures, even with the use of the platinum-tin reaction medium, we were unable to achieve quantitative recovery of nitrogen from several refractory metals. If kinetic factors play an important role in the extraction process, then an increase in operating temperature should improve extraction. The simplest expedient for operating a t higher temperatures while keeping metal vaporization within tolerable limits is to increase the ambient pressure in the extracting system. This is precisely what is done in the carrier-gas fusion technique, which, as shown in Table I, has been successfully applied to several metal systems. The data to be presented later show that the combination of a platinum-tin reaction medium and extraction temperatures ranging from
1725" C to 2400' C under carrier-gas fusion conditions, has made it possible for us to determine simultaneously and quantitatively, the oxygen and nitrogen contents of 19 different metal systems. An additional requirement for the quantitative determination of oxygen and nitrogen a t the trace level is a low operating blank. High oxygen blanks are normally associated with the carriergas fusion technique when operating temperatures above approximately 2000" C are employed. After thorough degassing of the conventional crucible assembly, the principal source of this blank is the carbon monoxide formed by the high-temperature reaction of the graphite or carbon-black insulation with the quartz crucible-thimble, according to the reaction:
Si02
+ 2C
+ 2CO
+ Si
(2)
This problem was solved b y employing a pyrolytic boron nitride thimble in the manner previously described ( 7 ) . h detailed drawing of the cruciblethimble assembly is shown in Figure 2. Since the publication of our earlier note, we found that extended periods a t temperatures of 2200' C or above resulted in a gradual increase of the nitrogen blank when 15 to 25 grams of fusion melt were present in the crucible. EXPERIMENTAL
The experimental approach used in this study was based on the determination of the oxygen and nitrogen contents of a large variety of base metals under various carrier-gas fusion conditions, To assess the quantitative accuracy of the results obtained, the nitrogen and oxygen values were compared, respectively, to those obtained by the Kjeldahl chemical technique and by standard vacuum fusion procedures. The nitrogen contents of the same samples were also determined by vacuum fusion procedures in order to evaluate their applicability to this determination. During the course of these studies, a consistent pattern of results developed. This pattern is ilVOL. 39, NO. 5, APRIL 1967
135 R
lustrated in the bar graph recovery data shown in Figure 3. I t is seen that platinum-tin reaction media a t selected carrier-gas extraction temperatures consistently yielded maximal nitrogen recoveries which were in concordance with the Kjeldahl results, within experimental error. These intercomparative results are considered in more detail below.
Carrier-Gas and Vacuum Fusion Extraction Apparatus. A diagram of the carrier-gas extraction and analysis facility is shown in Figure 4. Pertinent experimental details and operating conditions are summarized in Table 11. The helium carrier-gas which flushes through the furnace area is purified by means of a coiled column of 13X Molecular Sieve maintained a t liquid nitrogen temperature. This purified helium enters the fusion-extraction zone by means of the gas manifold assembly connected to the bottom of the Vycor furnace jacket. Samples contained in the storage arms are introduced into the fusion crucible by means of a large ball valve. The impurity gases extracted from the sample are carried by the purified helium stream into the collection trap shown in Figure 5. At liquid nitrogen temperature, the 13X
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Figure 3. Studies on carrier-gas fusion reaction media for determination of nitrogen in metals Molecular Sieve contained in the trap effectively removes the impurity gases from the carrier-gas stream. After completing the sample extraction and gas
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collection, the collection trap is switched from the furnace carrier-gas stream into the chromatograph helium stream, and the liquid nitrogen is removed from the
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ANALYTICAL CHEMISTRY
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trap. The collection trap is then heated for 12-15 seconds by the circuitry d e tailed in Figure 4. Upon heating the trap, the collected impurities are immediately released and are flushed onto the chromatographic separation column. J u s t prior to elution 01' the nitrogen impurity from the separation column, the collection trap is switched back into the furnace carrier-gas stream. This action returns the recorder base line to zero from the deflection induced b y pressure disturbances during t i e trap switching and heating procedures. The impurity gases are separated and detected b y conventional gas chromatographic procedures. The areas under the recorded impurity peaks are automatically integrated and printec out. Integrator response to nitrogen and carbon monoxide was calibrated directly in terms of counts ,lg-I of impurity gas. The calibrations were achieved b y introducing known amounts of nitrogen or carbon monoxide into the carrier-gas stream ahead of the furnace assembly. The calibrzted volumes were measured in the gas manifold assembly. A typical calibratioii curve for the nitrogen impurity is shown in Figure 6. The calibration datz. summarized in Figure 6 were o b t a n e d at different crucible temperatures with the furnace assembly in various stmitesof cleanliness. The location of calibration points on the 45" curve demcnstrate that the various crucible temperatures and furnace conditions did not influence the
calibrations in any systematic manner; the collection of the nitrogen impurity in the trap from the helium carrier stream was quantitative; and the blank corrections employed were accurate.
Table II. Experimental Apparatus and Procedures for Simultaneous Determination of Oxygen and Nitrogen in Metals by the Carrier-Gas Fusion Technique
Heating facility
Crucible assembly Furnace assembly
Furnace carrier-gas Helium purification
Crucible degassing and bath addition
Bath conditioning and degassing
Sample preparation
4
Platinum flux blanks
Figure 4. Carrier-gays extraction and analysis experiment0 I facility A = Helium supplies; Bureau of Mines, G r a d e
B = C =
D = E =
F = G = H = I =
I = K =
I = M =
N = O = P = Q =
R =
s= T = U =
V = W X Y Z
= = = = AA =
A Helium purification t:olumn Hoke flow-control needle valve Mathesan shut-off valves Mathesan two-way pressure regulators Hake toggle valves Calibration dry-nitrogen supply; Linde Air Products Calibration carbon monoxide supply; Matheson, CP Welch mechanical vacuum pump Mercury manometel, Gas manifold Induction generator Work coil Vycar furnace jacket Crucible assembly 1-inch Jamesbury ball valve; NRC Equipment Corp. Sample storage arms Sighting prism and optical flat %-inch Jamesbury ball valve; NRC Equipment Corp. Glass-wool filter p h g Precision gos sam,ding valve-collection trap assembly Flow meter Gas chromatograph Collection trap heater unit Recorder Printing integrator Electrical circuit for collection trap heater
These calibrations are absolute and have a high degree of sensitivity under the experimental conditions used, amounting to approximately 85 counts pg-l of nitrogen. The carbon monox-
Furnace blanks Collection of evolved impurity gases
Gas chromatograph Separation column Chromatograph carrier gas Column and detector temperature Detector voltage Recorder Recorder sensitivity Recorder response Recorder chart speed Integrator Integrator calibration
Lepel High Frequency Laboratories, Inc., high frequency induction generator, Model T-5N-1, with nominal power output of 5 kw and frequency of 400 i 100 kc. Maximum power output coupling to work load achieved with maximum tank-coil setting and an %turn, 4-inch diameter work coil Ultra Carbon Corp., graphite crucible C-625 and graphite funnel F-703, or equivalent, cut down to dimensions specified in Figure 2. Design based on Guldner-Beach furnace ( 2 2 ) . Crucible assembly components floated on Ultra Carbon Corp., UCP-2, - 200-mesh graphite powder insulation, or equivalent, within specially fabricated "pyrolytic" boron nitride thimble ( 7 ) which is suspended by plat,inum wire hooks within air cooled Vycor jacket Purified helium, 275-300 cc min-' Furnace carrier-gas purified by passing it through a 12-foot coiled length of l/a-inch 0.d. copper tubing packed with approximately 50 grams of 20-50 mesh 13X Molecular Sieve maintained at liquid nitrogen temperature. Sieve originally activated by heating in vacuo at 180-190' C for 24 hours Crucible temperature raised to 2450-2500' C over 1-hour period and degassed for 45 minutes. Temperature reduced to 1750' C and plat.inum-tin bat'h materials added slowly and intermittently to crucible. Crucible and bath then gradually brought to the desired operat.ing temperature Conditioning metal added to bath when operating temperature is reached. When operating temperature exceeds 1900" C, conditioner is added at 1900" C, after which desired temperature is gradually attained. Degassing of entire composition a t operating temperature is then conducted for 30-46 minutes Careful filed and cleaned samples fluxed with 12-gauge platinum wire, platinum capsules formed from 1-mil foil, or both Platinum wire: < 2 ppm nitrogen; 5-8 ppm oxygen. Platinum foil:
