The spectral lines used ($), analytical range, and precision of the determinations are listed in Table 11. The reproducibility of the methods is expressed as the coefficient of variation, and is based on 30 determinations run individually over a several week period. Typical results are shown in Table 111. Some data are given on a doped standard sample as well as a comparison with chemical results obtained on some unknowns. The method as set up has been used mainly to analyze tungsten blue oxide. The effect on the accuracy of the analysis of converting tungsten metal powder to tungstic oxide a t 750’ C. was investigated. The resultant oxide had a different particle size and density than the standard samples. The spark pro-
cedure gave reproducible results which were not accurate, especially for the silicon and aluminum determinations in the upper part of the analytical ranges. This work showed the dependence of accurate answers on consistent physical and chemical properties of the samples for the spark analysis. The arc procedure, however, gave good results for the potassium determination. Samples which could be converted to tungstic oxide with no loss of potassium gave acceptable answers for this element. This procedure could be applied to the determinations of other trace elements such as calcium, magnesium, molybdenum, and sodium. However, the presence of these and other residual trace impurities is being determined by spectrographic quantitative methods in
other processing steps of the tungsten from the ore to the metal powder. LITERATURE CITED
(1) Gentry, C. H. R., hlitchell, G. P.,
hfetallurgia 46,47-51 (1952). (2) Harrison, G. R., “M.I.T. Wavelength Tables,” Wilev, New York. 1939. (3) Lounamaa, Nulo, Spectrbchim. Acta 4. 400-12 (19511.
(4)’Moore, C. E.,‘ “Ultraviolet hIultiplet Table,” Natl. Bur. Standards, Circ. 488, Section 2 (1952). (5) Nelson, R. C., M i n e s M a g . 47, 68-T3 (1957). (61 Phehne, J. M., Congr. groupe. auance. mbthod. anal. spectrog. prod. mCt. 7th Congr. 1947, 51-6, Paris. (7) Smithells, C. J., “Tungsten,” 3rd ed., Chapman & Hall, London, 1952.
RECEIVEDfor review May 22, 1958. Accepted October 30,1958.
Spectrographic Analysis of Tungsten Metal Powder RUDOLPH DYCK and THOMAS 1. VELEKER Chemical & Metallurgical Division, Sylvania Electric Products Inc., Towanda, Pa. )Aluminum, calcium, iron, magnesium, potassium, and silicon in tungsten metal powder are determined by a spectrographic method. Metal powder samples are buffered with a graphite mixture containing the internal standard and arced with high voltage excitation. An extremely refractory matrix is formed during arcing resulting in a marked depression of the tungsten spectrum and an enhancement of the impurity elements. Nickel and lithium are used as internal standards. The method is simple, precise, sensitive, and much more rapid than chemical analysis. It has been applied successfully to routine control of these elements in tungsten metal and covers the following ranges: calcium 0.000 1 to 0.005%, magnesium 0.0001 to 0.005%, silicon 0.0002 to 0.1 %, iron 0.0001 to 0.02%, aluminum 0.00005 to 0.015~0,and potassium 0.003 to
0.1%.
E
small quantities of impurities can significantly affect the physical characteristics of tungsten (6, 6). The properties of high purity tungsten can be modified to meet industrial needs by adding trace amounts of certain elements. Such additives, however, as well as the residual impurities, in the tungsten must be closely controlled; for this reason the tungsten process depends largely on sensitive reliable analytical techniques. XTREMELY
390
ANALYTICAL CHEMISTRY
The chemical methods developed for this purpose tend to be time-consuming, and their complexity often requires an experienced analyst for routine operation. The emission spectrograph is generally an ideal tool for such analyses. However, tungsten like other refractory metals is characterized by an extremely complex emission spectrum and it gives rise to an intense continuum. The resulting line interferences and background frequently preclude an accurate analysis. Lounamaa (S) and Gentry and Mitchell (8) have partially eliminated these difficulties by mixing tungstic oxide with high purity graphite before arcing. Essentially, they use a technique of differential volatilization in which the refractory tungsten carbide is formed in situ, and all impurities which exhibit higher volatilization rates volatilize out before an intense tungsten spectrum appears. These workers analyze tungstic oxide, which has the advantage of providing a matrix that is readily obtainable from most tungsten compounds. Several important advantages are gained from analyzing the metal itself, The tungsten spectrum can be suppressed much more effectively by arcing the metal in the presence of graphite. Tungstic oxide, although similarly affected, is volatile in relation to the metal; and in spite of dilution with graphite, will emit an intense tungsten spectrum during the initial
part of the burn. Gentry and Ifitchell (2) avoid this difficulty by a short preburn, but this results in an appreciable loss in sensitivity, even for elements of medium volatility. Also, the density difference between the metal and the oxide favors greater sensitivity in the metal, the dense metal offering a significant advantage. Finally, for metallic samples, this method avoids a conversion to the oxide. EQUIPMENT
Spectrograph. Bausch & Lomb Optical Co. large littrow with quartz optics and a spherical lens in front of the slit. Source. High voltage alternating current, 2200 volts, 2.5 to 15 amperes. Plates. Eastman Type 33 for, all elements except potassium, for which an Eastman type 1L was used. All plates developed in D-19 for 2l/2 minutes. Densitometer. YSL projection-type microphotometer. Electrodes. Three-fourths X ‘/4 inch cone-shaped high purity graphite electrodes with craters I/* inch deep and inch in diameter as sample-bearing electrodes, and 3/4 X inch coneshaped counterelectrodes. The electrodes were shaped to these specifications and purified after shaping by United Carbon Products Co., Inc. PROCEDURE
Sample Preparation. One and one quarter grams of tungsten metal powder and 0.20 gram of a graphitenickel mixture (15 parts of high purity
graphite t o 1 part of spectrographically pure nickel powder) are weighed and mixed thoroughly. (The Crescent Wig-L-Bug, Spex Industries, Inc., is ideal for this.) This mixture is used for the analysis of all the elements except potassium. One and one quarter grams of tungsten and 0.20 gram of a lithium sulfate-graphite mixture (equal parts of pure lithium sulfate powder and high purity graphite) are vveighed and mixed for the potassium analysis. The sample mixtures are well tamped into the electrode craters. A smooth cap flush with the top edge of the crater is desirable. Samples are run in triplicate, which necessitates the preparation of a set of three electrodes per sample for each spectral region.
Figure 1. curves
c
Working
r
Conditions for analysis are given in Table I.
Photometry. Line pairs (4) listed in Table I1 are read on the densitometer using an 8-micron slit width. Logarithmic intensity ratios are computed in the conventional manner using emulsion calibration curves obtained by an eight-step sector method. The analytical ranges for all the elements are given in Table 11.
I
I
1
I
PREPARATION OF STANDARDS
Because there were no tungsten standards in the desired ranges, a set of synthetic standards was prepared. A pure blank material was obtained by reduction of a high purity tungstic ovide to the metal powder. The resulting powder was washed with hydrofluoric and hydrochloric acids to remove 3,s many surface contaminants as possible, and finally washed with dpionized mater until the wash water was essentially neutral. After the mashed powder had been thoroughly dried and blended, impurities were added volumetrically. Iron, aluminum, magnesium, calcium, and potassium were added as chloride solutions. Silicon was added as the alkali silicate solution. The amounts of standard solution added to the metal were PO calculated that the powder was only partially moistened. Contact between doping solutions and the side of the vessel was scrupulously avoided. The dried standards were then passed through a hydrogen reduction furnace iinder the conditions employed routinely in the final reduction stage of tungsten, to approximate more closely the physical form of the impurities as it occurs in iinknown metal powder samples. There nas no evidence from analytical curve data that impurities were lost during the reduction step. The residual impurities in the blank material were estimated according to the procedure described by Duffendack and Wolfe (1). Table 111 gives the per cent of impurities in the blank metal
powder. The contribution of impurities in the electrode material and the buffer to the estimated residual was considered negligible, because none of the impurities as detectable when electrode blanks with buffer were arced under the conditions employed.
graphite mixtures photographed a t 30second intervals using high voltage alternating current excitation. The spectral intensity of the tungsten is very low initially, while the impurities exhibit peak intensities during this part of the burn. This is ideal, as interferences are minimized while impurity sensitivities are enhanced. For example, the enhancement of calcium in this procedure is so great that the range could be extended well below the limit given if a purer blank material were available.
RESULTS AND DISCUSSION
The behavior of the impurities under consideration is illustrated by moving plate studies (Figure 2 ) of tungsten-
Table 1.
Amperage Analytical gap, mm. Exposure, sec. Source to slit distance, em. Spectral region, A. Slit width, Rotating sector
1
45 62.5 2400-3200 20 None
Table 11.
AI
Ai Fe Fe Si Si Ca Mg K
A.
(4)
3092.713 2575.100 2483.027 2491.155 2506.899 2435.159 3933.666 2795.53 7664.907
Ca, hlg 5 1
60 62.5 2600-4000 20 ‘/4
open
K 5 1
60 62.5 3500-7800 25 Full open
Line Pairs and Analytical Ranges
Wave Length, Element
Conditions of Analysis
Fe, AI, Si 7
Internal Standard, -4. Ni 2805.083 Ni 2805.083 Ni 2805.083 Ni 2805.083 Ni2805.083 Ni 2805.083 Ni 3080.755 Ni 3080 755 Li 4971.990
Range, % 0.0001-0.0025
0.002 -0.01 0.0001-0.005 0.002 -0.02 0.0002-0.02 0.01 -0.1
0.0001-0.005 0.0001-0.005 0.003 -0.1 ~
~~~
VOL. 3 1 , N O . 3, MARCH 1959
391
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30
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c
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00 W
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80
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I00 0-30
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I 90-120
1 I 120-150 1%-100
EXPOSURE INTERVAL IN
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I 180-210 210-240
0-30
1
I
30-60 60-90
I
90-120 120-150 150-180 100-210 2 0 - 2 4 0
EXPOSUREINTERVAL IN SECONDS
SECONDS
Figure 2, Moving plate with high voltage alternating current excitation
Figure 3.
Moving plate with direct current excitation
The formation of tungsten carbide
(WC) probably accounts for this marked suppression of tungsten. X-ray diffraction patterns support this assumption. The residues after arcing were examined by Debye-Scherrer x-ray patterns and no evidence of metallic tungsten was found, whereas a very strong diffraction pattern attributable to CU-WC was identified. With direct current excitation, bhe suppression of tungsten is very much less. This is evident from a comparison of moving plate studies carried out with direct and alternating current sources (Figures 2 and 3). I n addition, the intensity peaks of the impurities occur much closer to the tungsten peak with direct current excitation which has an unfavorable effect on the line to background ratio. I n effect, high voltage alternating current provides significantly better sensitivities than direct current. This method could readily be modified to include other impurities. h n other internal standard could be substituted if nickel, for example, were to be determined. Gentry and Mitchell ( 2 ) used cobalt, which should give comparable results, although in this procedure several interferences, calcium 3933 and silicon 2506, made it undesirable as a n internal standard.
Table 111.
Residual Impurities
Residual Impurity Estimated, yo Ca 0.0001 0.0001 Mg Si 0.0002 Fe 0.00015 Al 0.0001 The working curves obtained from these synthetic standards were essentially linear (Figure 1). Except for the potassium determination, background corrections were not applied. Table IV. Comparison of Spectrographic and Chemical Analyses of Iron
Iron,% Chemical Spectrographic 0.0018 0.0015 0.0043 0.0049 0.0021 0.0021 0.0005 0.0005 0.0025 0.0030 0.00075 0.00090
Sample Number 1 2
3 4 5 6
392
ANALYTICAL CHEMISTRY
Precision and Accuracy Data
Element, % Ele- Added Found, menta residual av. Si 0.025 0.025 0 0018 0 0016 Fe 0 0052 0 0055 0 00047 0 00043 A1 0 0050 0 0050 0 00042 0 00036 CB 0.0017 0.0017 0 00030 0.00027 Mg 0 0017 0 0016 0 00030 0 00028 Kb 0 025 0 024 5 Sixteen analyses. * Ten analyses.
+
d
=
n
=
Coefficient of Variation, % 8.4 9 7 12 0 12 4 6 4 12 7 14.7 11.1
19 8 17 9 12 1
difference of determination from mean namber of determinations LITERATURE CITED
could be made only for iron (Table IV), because chemical analyses in these ranges were not available for the other elements. Data in Table V compare analytical values with theoretical values of synthetic standards. Coefficients of variation are also given, calculated according to the formula:
PRECISION AND ACCURACY
Normally, the accuracy of a procedure is evaluated by comparison with another method. Such a comparison, however,
Table V.
where C
=
average % concentration
(1) Duffendack, 0. S., Wolfe, R. A., I N D . ENG.CHEX., ANAL. ED. 10, 161 (1938). (2) Gentry, C. H. R., Mitchell, G. P., Metallurgia 46, 47-51 (1952). (3) Lounamaa, Kulo, Spectrochim. Acta 4,400-12 (1951). (4) “M.I.T. Wavelength Tables” (Harrison, G. R., ed.) Wiley, New York, 1939. (5) Nelson. R. d.,Mines Mao. 47. 68-73 ‘ j1957). ‘ (6) Smithells, C. J., “Tungsten,” 3rd ed., p. 131, Chapman & Hall, London, 1952. ”
RECEIVEDfor review May Accepted October 30, 1958.
I
22,
1958.
