On the Thorium Content of Primary Standard Grade Ammonium Hexanitratocerate: An Optical Emission Spectrographic Study Velmer A. Fassel, Raymond J. Jasinski, Edward L. DeKalb, and William V. Lucas Institute far Atomic Research and Department of Chemistry, Iowa State Unioersity, Ames, Iowa
The results of a study on the emission spectrometric determination of thorium in cerium compounds are presented, and a procedure for the quantitative determination of thorium is described. With this procedure, the Thol content of ammonium hexanitratocerate can be determined quantitatively down to 30 ppm and detected down to 16 ppm. The failure of prior investigators to detect the presence of appreciable amounts of thorium impurity by emission spectrometric methods in primary standard or reference grade cerium compounds is attributed to line misidentifications in the published wavelength tables of cerium. The thorium contents of a series of typical “Reagent,” “Primary Standard,” and “Standard of Reference” samples of ammonium hexanitratocerate are tabulated and the significance of these results in terms of the use of this compound as a primary standard is discussed.
BECAUSETHE SALTS OF QUADRIVALENT CERIUM are Strong, versatile oxidizing agents, they have found extensive use as reagents for preparing standard solutions in oxidimetry (1-5). Ammonium hexanitratocerate has, in fact, been recommended as a primary standard reagent (4). One of the important requirements of a primary standard is that it must be available in pure form, or in a state of known purity. G. F. Smith and associates (3-5) have established the conditions for the preparation of a chemically pure form of ammonium hexanitratocerate from a low grade mixture of ceric and cerous oxides and mixed oxides of lanthanum, praseodymium, and neodymium. It has been stated ( 4 ) that repeated emission spectrometric examination of the purified product failed to detect the presence of thorium. However, Salutsky, Kirby, and Quill (@, Kirby (7), and Smith and Mongan (8) have demonstrated the presence of thorium in commercial analytical reagent grade cerium salts, “Specpure” compounds, and reagent grade ammonium hexanitratocerate. These investigators employed three independent methods for detecting the presence of thorium-namely, separation and identification of the radium224 daughter product, x-ray fluorescence spectrometry, and neutron activation analysis. In this paper the results of a study on the emission spectrometric determination of thorium in cerium compounds are presented, and procedures for the quantitative determination of thorium in a cerium matrix are described. It is also postu(1) J. Martin,J. Am. Chem. SOC.,49, 2133 (1927). (2) H. H. Willard and P. Young, Ibid.,50, 1322 (1928). (3) G. F. Smith, V. R. Sullivan, and G. Frank, IND. ENG.CHEM., ANAL.ED., 8, 449 (1936). (4) G. F. Smith and W. H. Fly, ANAL.CHEM., 21, 1233 (1949). ( 5 ) G. F. Smith, “Cerate Oxidimetry,” G. Frederick Smith Chemical Co., Columbus, Ohio, 1942. (6) M. L. Salutsky, H. W. Kirby, and L. L. Quill, ANAL.CHEM., 27, 1960 (1955). (7) H. W. Kirby, Ibid.,29, 1599 (1957). (8) G. W. Smith and D. M. Mongan, Infern. J. Appl. Radiatioir and Isotopes, 16, 81 (1965).
lated that line misidentifications in the published wavelength tables of cerium was the contributing cause for failure of prior investigators to detect the presence of significant thorium impurity in “Reagent,” “Primary Standard,’’ and “Standard of Reference” grade cerium compounds by observation of their optical emission spectra. EXPERIMENTAL
Line Misidentification in Cerium Wavelength Tables. The complex emission spectrum of cerium coupled with the difficulty of preparing cerium free of the other rare earths and thorium can readily lead to errors in identifying the thousands of lines in the spectrum. If cerium matrices free of the suspected impurity are not available, it is difficult t o establish whether the presence of weak spectral lines at wavelengths characteristic of persistent lines of the impurity are really caused by the impurity, by the matrix cerium, or possibly both. A critical examination of the optical emission spectra of samples collected from ion exchange column fractionations of highly purified rare earths has provided definitive information for making these differentiations (9). A comparison of the spectra of highly purified cerium matrices obtained from ion exchange column separations with the spectra obtained from “Primary” and “Standard of Reference” grade ammonium hexanitratocerate revealed the presence of a considerable number of atomic emission lines in the cerate salts which were not detected in the ion exchange fractions. The wavelengths of most of these lines were coincident with listings of weak cerium lines in the MIT Wavelength Tables (IO). However, these wavelengths were also found to be coincident with the more persistent lines of thorium. The absence of these lines in highly purified cerium therefore established the presence of significant amounts of thorium in the cerate salts. Moreover, these observations suggest that thorium was not suspected as an impurity in the cerium matrices used in preparing the spectra on which the wavelength measurements reported in the MIT Wavelength Tables (IO)were made. Excitation Conditions and Sample Form. The optical emission spectrum of thorium is even more complex than that of cerium, and may be characterized as having thousands of lines of comparatively uniform intensity. To attain even moderate detection limits for thorium, recourse must be taken to the most sensitive excitation technique, oiz., dc arc excitation. Because these excitation conditions are most readily applicable to samples in the form of oxides, this sample form was employed in all of our studies. Selection of Analytical Line Pairs. The persistent lines of thorium given in the NBS Tables of Spectral Line Intensities (9) V. A. Fassel, H. D. Cook, L. C. Krotz, and P. W. Kehres, Spectrochim. Acta, 5, 201 (1952). (10) G. R. Harrison, “MIT Wavelength Tables,” Wiley, New York, 1939 and 1956. VOL. 40, NO. 2, FEBRUARY 1968
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Table I. Strong Thorium Emission Lines All thorium wavelengths and intensities taken from reference (11) Waveiength, Intensity Remarks A 4019.13 300 CN and Ce interferences 110 Very weak Ce interference on high 2837.30 wavelength $de Ce 2837.289 A is a misidentification 3469.92 3392.03 3741.19 4381.86 4391.11
95 90 90 90
3180.20 41 16.71
75 75
2832.31
70
3351.23
70
3402.70 3434.00 3609.44 3256.28 3262.67
70 70 70 65 65
3291.74 4069.20 3325.12 3839.74 4108.42 3188.23 3435.98 3721.82 3675.57 4085.04 4086.52 4094.75 4282.04
65 65 60
80
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60
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Weak CN interference, Ce interference on low wavelength side Ce interference Ce interference CN interference Ce interference Weak Ce interference on high wavelength side Weak Ce interference CN interference Ce interference CN interference CN interference Ce interference CN interference CN interference Ce interference CN interference CN interference CN interference Ce interference to low wavelength side
.Ox
(11) W. F. Meggers, C. H. Corliss, and B. F. Scribner, “Tables of Spectral-Line Intensities-Arranged by Elements,” National Bureau Standards, Monograph 32, Part I, U. S. Government Printing Office, Washington, D. C., 1961. (12) V. A. Fassel and E. L. DeKalb, “The Metal Thorium,” H. A. Wilhelm, Ed., American Society for Metals, Cleveland, 1958, Chap. 22. 0
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(10)
Ce and CN interference Ce interference CN interference Ce interference CN interference, Ce interference on both sides Weak CN interference CN interference, Ce 4116.713 A is a misidentification ( I O ) Most sensitive, interference free line. Ce 2832.307 A is a misidentification
(11) are listed in Table I, with our observations concerning actual interferences and misidentifications in the MIT Wavelength Tables. The strongest line which we believe t o be the best compromise between maximal detection sensitivity and minimal interference is at 2832.31 A. Because excitation potential data for weak cerium lines are not available, the cerium internal standard lines at 2832.568 and 2834.744 A were selected on the basis of wavelength proximity and intensity. Synthetic Calibration Standards. A series of synthetic standards were prepared by combining nitric acid solutions of thorium and cerium in the proper proportions, evaporating t o dryness, and igniting t o the oxide at 900” C . These standards ranged in thoria content from the residual present in the in thoria. The thorium solution was cerium base up t o 1 prepared from Ames Laboratory thorium metal. This material was analyzed spectrographically (12) and found to be free of significant impurity contamination. The cerium stock
322
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ANALYTICAL CHEMISTRY
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Table 11. Operating Conditions for the Determination of Thoria in Ceria Electrode assembly Upper electrode: Cathode, 3-mm diameter, high purity graphite rod (National Carbon L3803, Ultra Carbon U40) Lower electrode: Anode, ASTM shape S-4 (15), (National Carbon L-4012 AGKSP, Ultra Carbon 1988) Sample charge: 16 mg of 1 : 1 blend of CeO, and powdered graphite Analytical gap : 4 mm Excitation conditions Current: 18 A None Pre-arc : Sample arced to complete consumption Exposure time: plus 10 seconds, totaling approximately 75 seconds Jarrell - Ash 3.4-meter Wadsworth Spectrograph mounting grating spectrograph W/mm Rulings: Secopd Order used: 2.5 A/mm Reciprocal linear dispersion: 20 microns Slit width: Filter: None Photographic photometry Emulsion: Eastman Spectrum Analysis # 1 , developed in D-19 for 4 minutes at 20“ c Emulsion calibration: Two step sector, preliminary curve method (15) Analytical lines Internal ConcenImpurity standqrd tration Concentration line, A line, A index, % range, % 0.025a 0.01-0. l a Th 2832.31 Ce 2832,568 0.3 0.05-1 . O Th 2832.31 Ce 2834.744 a Background correction necessary. Background reading is taken between the two lines. See Figure 2.
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INH4)2 Ce(N03)g PREPARED FROM THORIUM-FREE BASTNAESITE BASE MATERIAL FOR SYNTHETIC CALIBRATION STANDARDS
0.01% T h o 2 IN C e 0 2
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d
Figure 2. Microphotometer tracings of the thoriumcerium analytical line pair
solution was prepared from ceric oxide which had been ,purified by ion exchange separation (13, 14). Spark source mass spectrometric analysis of this oxide indicated that the thorium content was less than 2 ppm by weight. This material was also analyzed spectrographically and found to be free of significant contamination by common impurities. The ceric oxide was dissolved in a mixture of nitric acid and hydrogen peroxide. Excitation Conditions. The pertinent data on the various excitation parameters are summarized in Table 11. A moving plate study of the degree of selective volatilization is shown in Figure 1. Although some improvement may result from a 10-second pre-arc period, a satisfactory coefficient of variation has been obtained by integrating the intensities over the entire arcing cycle. A coordinate or graphical zero-intercept (16,17) plot of the calibration data gave a residual thorium content of approximately 50 ppm, which was not in agreement with the “less than 2 ppm” value found by mass spectrometry. An examination of microphotometer tracings (Figure 2) of a set of spectra obtained from highly purified ceric oxide matrices revealed two factors which may account for this discrepancy. There is a very weak unidentified interfering line located on the high wavelength side of the thorium analysis line. Because this line is consistently evident at the same intensity level in all of the spectra of the highly purified preparations, its origin is (13) J. E. Powell and F. H. Spedding, Chem. Eng. Prog., Symp. Ser., 55, 101 (1959). (14)F. H. Spedding and J. E. Powell, in “Ion Exchange Technology,” F. C. Nachod and J. Schubert, Eds., Academic Press, New York, 1956,Chap. 15. (15)“Methods for Emission Spectrochemical Analysis, 4th Edition,” American Society for Testing and Materials, Philadelphia, 1964. (16)0.S. Duffendack and R. A. Wolfe, IND. ENG.CHEM., ANAL. ED., 10, 161 (1938). (17)W. C.Pierce and N. H.Nachtrieb, Ibid.,13,774(1941).
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Figure 3. Analytical curves for the determination of thorium in cerium probably from the cerium matrix or a component of the CN band system. The recordings in Figure 2 also show that the background readings, which are most conveniently made between the analytical line pair, may not completely correct for the actual background contributions under the thorium analysis line. Whether these two contributions actually account for the observed apparent residual cannot be concluded from these data. Because only 2 ppm of thorium residual was found by mass spectrometric measurements and the error in this estimation was not greater than a factor of two, we have not applied a residual correction to the analytical calibrations, The analytical curves are shown in Figure 3. Precision and Accuracy. The precision of analysis was determined from 10 exposures, on individual plates, of two samples containing 0.03% and 0.30% thoria. Relative standard deviations of 4.2% and 3.3 %, respectively, were measured. Table I11 shows a comparison of analytical results obtained by three different laboratories on a series of samples provided by G . F. Smith. The spectrographic commercial consulting laboratory and the radiochemist who performed the activation analyses were commissioned by G . F. Smith to provide the referee analytical data. Although there is some disagreement between the optical emission and activation data, the excellent agreement between the independent spectrographic results provides strong experimental verification on the quantitative accuracy of these techniques. Thorium Content of “Primary Standard” and “Standard of Reference” Grade Ammonium Hexanitratocerate. Table IV summarizes the analytical data observed on commercial
Table 111. Comparison of Analytical Results All values are reported in per cent thorium in ammonium
Sample 1 Sample 2 Sample 3 Sample 4
hexanitratocerate Spectrographic analysis Commercial Iowa State Consulting University Laboratory 0.124 0.125 0.041 0,046 0.011 0.014 0.004
