Fluorescent X-Ray Spectrographic Determination of Tantalum in Commercial Niobium Oxides WILLIAM J. CAMPBELL and HOWARD F. CARL Eastern Experiment Station, Bureau of Mines,
U. 5. Department of the Interior, College Park, Md.
Recent modifications in fluorescent x-ray spectrographic instrumentation permit the resolution of the tantalum La1 line from the usual interfering lines without a prohibitive loss of intensity. Tantalum Lal line intensities as high as 750 counts per second can be obtained from a sample of 5’j% tantalum oxide in niobium oxide by the use of a molybdenum target x-ray tube, a quartz analyzing crystal, and a 4-inch Soller slit collimator. The residual tantalum content of “high-purity” niobium oxide was determined by use of standards prepared from tantalum-free zirconium oxide. Hand-mixed samples, when properly prepared, are as suitable as chemically precipitated samples. The effects of possible impurities, such as stannic oxide, ferrous oxide, titanium dioxide, and sodium sulfate on the analysis were calculated from adsorption data and determined experimentally. Only titanium dioxide was found in commercial niobium oxides in amounts that would result in a measurable analytical error. Fluorescent x-ray spectrography is a rapid and reliable method of analysis. It has an accuracy within It5’j’o of the amount present in the usual concentration range of 0.5 to 10% tantalum oxide and a lower limit of detection of approximately 0 . 0 3 7 ~tantalum oxide, and requires an average time per analysis of about 10 minutes, after preparation of standards.
five evenly spaced thin plates was placed in the original collimator location between the sample and analyzer. A quartz analyzing crystal (2d = 3.636 A.) was used to provide sharp lines and to improve resolution. Quartz crystals have a relatively low reflectivity for higher order lines, and therefore the secondorder niobium K spectral intensity is decreased. The most satisfactory detector was the standard argon-filled Geiger tube, as it is very efficient for the tantalum La lines of longer wave length but relatively insensitive to the shorter niobium K lines.
Table I.
Reproducibility of -4nalysis of a Commercial Niobium Oxide TazOs, Wt. %
Detn. NO.
1 2 3 4 5 6 7 8 9
10 Av.
D
URING the past several years the x-ray laboratory of the Eastern Experiment Station, Bureau of Mines, has received numerous requests to determine the tantalum content of commercial grades of niobium oxides. A survey of the literature indicated a need for a suitable method of analyzing such samples. The analysis of niobium and tantalum ores has been performed successfully by fluorescent x-ray spectrographic methods (5, 7 ) . and recently both equipment and techniques have been considerably improved. Therefore, a rather intensive study of the effects of impurities and sample preparation on the x-ray spectrographic method was undertaken. INSTRUMENTATION
As pointed out in the authors’ original paper (6),the determination of low concentrations of tantalum oxide was restricted because of low intensities and insufficient resolution of interfering spectral lines, At that time the only practical solution to the interference problem was to excite the tantalum spectrum a t 18 kv., a value below the critical voltage for the niobium K spectrum ( 2 ) . Many restrictive features of the original Norelco 90’ spectrometer have been modified to provide resolution of closely adjacent spectral lines without appreciable loss of intensity ( 4 ) . The tungsten x-ray tube was replaced by one having a molybdenum anode, resulting in a more efficient excitation of tantalum, a reduction in background by a factor of 2 , and less interference from NbKa lines. The nickel tubing collimator between the x-ray tube and the analyzing crystal was replaced by a Soller slit collimator (4 inches long with 0.005-inch spacing) between the analyzer and the detector. An open-tube collimator vith
2.67 2.77 2.67 2.72 2.67 2.73 2.64 2.72 2.64 2.76 2.70
Deviation, Deviation -0.03 +0.07 -0.03 +0.02 -0.03 +0.03 -0.06 +0.02 -0.06 +0.06
0.04
% -1.1 +2.5
-1.1 +0.7 -1.1
$1.1
-2.2 $0.7 -2.2 +2.2
1.5
With these changes in instrumentation, intensities of TaLa, of 750 counts per second could be obtained from 5% tantalum oxide in niobium oxide. [Xormally loner intensities were used t o reduce counting losses arising from the nonlinear response of the Geiger tube a t high counting rates.] The degree of resolution obtained from a mixture of equal parts of niobium and tantalum oxides is shown in Figure 1. GENERAL AXALYTICAL THEORY AND PROCEDURE
In a two-component system, such as tantalum oxide in niobium oxide, the intensity of the tantalum Lal is a direct function of the tantalum concentration. At low concentrations of tantalum oxide (
