Analytical Applications of X-Ray Excited Optical Fluorescence Spectra Direct Determination of Fractional Parts per Million Amounts of Rare Earths in Thorium T. R. Saranathan,’ V. A. Fassel, and E. L. DeKalb Institute f o r Atomic Research and Department of Chemistry, Iowa State University, Ames, Iowa 50010
An X-ray excited optical fluorescence method for the direct quantitative determination of fractional part per million amounts of Pr, Sm, Eu, Gd, and Dy in nuclear grade thorium is described. Prior chemical separation of the rare earths is not required. The thorium oxide host is prepared by evaporating thorium nitrate solutions to dryness and igniting the residue at 1000 O C for three hours. Selected erbium lines are employed as internal standards to correct for the depressing effect caused by other impurities present in the sample. The concentration range for which analytical curves have been established are: 0.08-4 ppm for Pr, 0.12-6 ppm for Sm, 0.04-2 ppm for Eu, 0.1-1 ppm for 6 d , and 0.2-10 ppm for Dy. THORIUM HOLDS promise as a fertile nuclear material, especially in breeder reactor concepts ( I , 2). Jn view of the high capture cross-section possessed by several rare earths for slow neutrons [46000, 5600, 4300, and 950 barns for Gd, Sm, Eu, and Dy, respectively (3)],trace quantities of these elements in the thorium matrix can markedly reduce the neutron economy in such reactors. Thus it is essential that analytical methods capable of determining fractional part per million amounts of these rare earths be made available. I n the past most of the procedures which have been proposed were based on preconcentrating the rare earths by appropriate chemical separation techniques. The several techniques which have been used t o effect these separations and the optical emission spectrographic techniques for the analysis of rare earth concentrates have been reviewed (4-11). Recently Nelms and Vogel(12) showed that it was possible to 1
Present address, B.A.R.C., Trombay, Bombay-74, India.
(1) S. Glasstone, “Source Book on Atomic Energy,” D. Van Nostrand Company, Princeton, N. J., 1958, p 451. (2) J. P. Howe in “The Metal Thorium,” H. A. Wilhelm, Ed., American Society of Metals, Cleveland, Ohio, 1958, Chapter 1. (3) P. F. Nichols and E. M. Woodriff in “Nuclear Graphite,”
R. E. Nightingale, Ed., Academic Press, New York, N. Y., 1962, p 79. (4) C. J. Rodden in “Peaceful Uses of Atomic Energy,” Vol. 8, United Nations Publications, New York, N. Y.,1958, pp 197-205. ( 5 ) V. A. Fassel and E. L. DeKalb in “The Metal Thorium,” H. A. Wilhelm, Ed., American Society of Metals, Cleveland, Ohio, 1958,Chapter 22. (6) J. E. Powell in “Progress in the Science and Technology of the Rare Earths,” Vol. I, L. Eyring, Ed., Pergamon Press, Inc., Oxford, England, 1964. (7) M. W. Lerner and C. J. Petretic, ANAL.CHEM., 28,227 (1956). (8) C. G. Goldbeck in “Analysis of Essential Nuclear Reactor Materials,” C. J. Rodden, Ed., U. S. Government Printing Office, Washington, D. C., 1964, pp 959-86. (9) A. N. Zaidel, N. I. Kaliteevskii, A. N. Razumovskii, and P. P. Yakimova in “Rare Earth Elements (Extraction, Analysis, Applications),” D. I. Ryabchikov, Ed., Office of Technical Services, U. S. Dept. of Commerce, Washington, D. C., 1960, pp 272-79. (10) J. Loriers in “Progress in the Science and Technology of the Rare Earths,” Vol. I, L. Eyring, Ed., Pergamon Press, Inc., Oxford, England, 1964, pp 351-98. (11) H. J. Hettel and V. A. Fassel, ANAL.CHEM., 27, 1311 (1955). (12) J. R. Nelms and R. S. Vogel, Appl. Spectrosc., 27, 242 (1967).
determine these rare earths spectrographically, without prior chemical concentration, down t o 0.5 t o 1 ppm. Two superimposed exposures were employed using a direct current arc in a n atmosphere of 80% argon-20% oxygen. Silver chloride was used t o effect a carrier distillation from the anode cup. For many purposes this procedure should be of adequate sensitivity, but to satisfy more stringent requirements, a t least a n order of magnitude improvement in detectability is essential. As early as 1935, Wick and Throop (13) observed fluorescence lines of Pr, Sm, and T b impurities in T h o l under X-ray, ultraviolet light, and hydrogen flame excitation. Subsequently Tomaschek (14, 15) employed ultraviolet excitation and found intense fluorescence lines of Sm in various host crystals, including Tho,. Trofimov (16) employed these spectia as an analytical tool and found T h o 2 t o be a n excellent host for rare earth activators. As a n extension of this study, the same author (17) reported detection limits for seven rare earths in Tho, under ultraviolet excitation. Considerable interest has been created by recent reports of Low et al. (18, 19) and also Derr and Gallagher (20) on the use of X-rays for exciting optical fluorescence. The distinct advantages of X-ray over ultraviolet excitation are the possibility of observing fluorescence spectra from even higher energy levels than those excited by ultraviolet radiation; primary and secondary filters are not required; and ultraviolet absorption bands are no longer a prerequisite for the production of intense optical fluorescence. The application of X-ray excited optical fluorescence spectra of the rare earths to the detection of fractional ppm amounts of most of the rare earths in Y 2 0 1and in G d 2 0 3 has been described by Linares et a / . (21), Cosgrove et al. (22,23), and Burke and Wood (24).
(13) F. G. Wick and C. G. Throop, J. Opt. SOC.Amer., 25, 57 (1935). (14) R. Tomaschek, “Leuchten und Struktur Fester Stoffe,” R. Oldenburg, Munich, 1943. (15) R. Tomaschek, Ergeb Exakt. Naturw., 20, 268 (1942). (16) A. K. Trofimov, Bull. Acad. Sci. USSR, Phys. Ser., 21, 754 (1957). (17) Zbid., 25,453 (1961). (18) J. Makovsky, W. Low,and S. Yatsiv, Phys. Lett., 2,186 (1962). (19) W. Low, J. Makovsky, and S. Yatsiv in “Quantum Electronics-Paris 1963 Conference,” Vol. I, Columbia University Press, New York, N. Y., 1964, p 655. (20) V. E. Derr and J. J. Gallagner, ibid., p 817. (21) R. C. Linares, J. B. Schroeder, and L. A. Hurlburt, Spectrochim. Acta, 21, 1915 (1965). (22) J. F. Cosgrove, D. W. Oblas, R. M. Walters, and D. J. Bracco, Electrochem. Technol., 6 , 137 (1968). (23) R. J. Jaworowski, J. F. Cosgrove, D. J. Bracco, and R. M. Walters, Spectrockim. Acta, 23B, 751 (1968). (24) W. E. Burke and D. L. Wood in “Advances in X-Ray Analysis,” Vol. 11, J. B. Newkirk, G. R. Mallet, and H. G. Pfeiffer, Eds., Plenum Press, New York, N. Y., 1968, pp 204-13. ANALYTICAL CHEMISTRY, VOL. 42, NO. 3, M A R C H 1970
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Table I. Reproducibility with Internal Standardization Relative intensity or intensity ratio, 0.24 ppm Eu and 1.2 ppm Dy in Tho2 Line or Preparation line pair, A 1 2 3 4 Eu 5904 1.20 1.10 1.oo 0.63 Dy 5766 0.30 0.27 0.24 0.17 Er 5592 2.75 2.65 2.10 1.45 Eu 5904 ~0.44 0.41 0.48 0.43 Er 5592
'Or
Figure 2. X-Ray excited optical fluorescence spectrum of 12 ppm Sm in Thoz
1 I
T
a L.. c
I
Ill
I
Figure 1. X-Ray excited optical fluorescence spectrum of 7 ppm P r in Thos
Figure 3. X-Ray excited optical fluorescence spectrum of 2 ppm Eu in T h o z
The paper presented here deals with the quantitative determination of fractional ppm concentrations of Pr, Sm, Eu, Gd, and Dy in thorium and its compounds using the X-ray excited optical fluorescence technique. Erbium is used as the internal standard. The effect of other impurities on the determination of rare earths is reported in detail.
of 25-mm diameter and 2-mm depth. The powder was spread uniformly in the cup, compressed at a pressure of 8000
EXPERIMENTAL
The experimental equipment has been described in a previous paper (25). The thorium metal used was prepared and purified by the Metallurgy Division of the Ames Laboratory. A spark source mass spectrographic analysis for rare earth impurities, with detection limits in the range of 0.1 to 0.3 ppm, revealed only a fractional ppm amount of gadolinium. Rare earth solutions were prepared from high purity oxides using distilled water and reagent grade nitric acid. Preparation of Calibrating Standards and Samples. Thorium metal or oxide samples were dissolved in nitric acid containing a trace of fluorosilicic acid. The calibrating standard solutions were prepared by adding measured amounts of nitric acid solutions of the appropriate rare earth elements to aliquots of the master solution of pure thorium. The erbium internal standard (10 ppm by weight) was also added prior to conversion of the solutions to the powdered oxide. The calibrating standard solutions were evaporated to dryness, the oxides of nitrogen were driven off at about 300 "C,and the sample was placed into a preheated muffle furnace. The optimum time and temperature of ignition were found to be 3 hours at 1000 'C. The introduction of the sample into a preheated furnace provided greater spectral intensities. Samples to be analyzed were prepared in the same manner. For spectral observation, the resulting oxides were ground to a fine powder and transferred into an aluminum planchet (25) E. L. DeKalb, V. A. Fassel, T. Taniguchi, and T. R. Saranathan, ANAL.CHEM., 40,2082 (1968).
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ANALYTICAL CHEMISTRY, VOL. 42, NO. 3, MARCH 1970
pounds for one minute, and then loaded into the X-ray irradiation chamber. Internal Standard. The intensities of X-ray excited, rare earth optical fluorescence may be enhanced or suppressed by other impurities present in the host matrix (25). A technique for internally compensating for these effects and for other experimental variables has been described (25). This technique is based on relating impurity concentration to the intensity ratio of the fluorescence emitted by the analysis element and by another rare earth element added in constant amount to the host matrix. In the present procedure, erbium was selected as the internal standard or reference element. Ten ppm of erbium provided seven spectral lines of appropriate intensity. This amount exceeds by several orders of magnitude the Er content expected in typical purified thorium samples. Thus variations in the residual Er present in the thorium can be considered to exert a negligible effect on the total Er content of the prepared sample. DISCUSSION
Table I illustrates the effectiveness of erbium as an internal standard in compensating for day-to-day intensity variations. All of the samples in the table contained the same amount of rare earth impurities, but were prepared on different days. It is seen that the low spectral intensities obtained from sample No. 4 were effectively compensated when intensity ratios were measured. Fluorescence Spectra. In Tho2, the rare earths Pr, Sm, Eu, Tb, Dy, Ho, Er, and Tm fluorescence in the visible and G d in the ultraviolet region of the optical spectrum. The individual spectra of Pr, Sm, Eu, Gd, Dy, and Er in Tho2 are presented in Figures 1-6, while in Figure 7 the spectra of a mixture of the six elements in T h o z are shown. In these figures the photocurrent scale refers to the lower spectrum, which was recorded at a factor of ten less sensitivity thtn the upper spectrum. The wavelengths are accurate to 1 A.
*
10-
e-
a
t
6-
I.-
I
Figure 7. X-Ray excited optical fluorescence spectrum of five impurity rare earths and the Er internal standard in Tho2
WAVELENGTH
Figure 4. X-Ray excited optical fluorescence spectrum of 3 ppm Gd in Tho2
" 0 -
Figure 5. X-Ray excited optical fluorescence spectrum of u) ppm Dy in Thoz concentration of add8d impurity i n Tho2 (PPm)
Figure 8. Effect of impurities on fluorescent intensities and on intensity ratios
Figure 6. X-Ray excited optical fluorescence spectrum of 5 ppm Erin Tho2 It is indeed fortunate that the rare earths which are detected with the greatest sensitivity are precisely the ones which possess the highest neutron absorption cross-sections. The Pr determination is included to provide a n indication of the presence of the lighter rare earths which may be expected to occur in thorium in greater abundance than several of the heavier rare earths. In addition to the intense rare earth lines, all of the spectra show a broad band emission with a peak at about 4050 A. This peak is intense for pure thoria, but decreases in intensity if the host material was not prepared under optimal conditions, or if the total impurity content exceeds a few ppm. This band may be the same as has been observed after irradiation of T h o 2 with nuclear particles (26). Effect of Impurities. The seven spectral lines of erbium are not equally effective in compensating for the suppressive (26) C. E. Mandeville and H. 0. Albrecht, Phys. Rev. Ser. 2, 90, 992 (1953).
0 Ca
0 Ce
0Fe
AU
effect of additional impurity elements so that a careful selection of optimum line pairs was necessary. The effects of four impurities-Fe, Ca, Ce, and U-were studied in detail. Fe and Ca are representative of most of the common impurities that can be expected in the samples, while Ce and U are elements of the lanthanide and actinide groups which are most likely to be found in thorium. Four sets of T h o z standards were prepared containing known amounts of rare earths (0.96 ppm of Pr, 1.44 pprn of Sm, 0.48 ppm of Eu, 0.34 ppm of G d , and 2.4 ppm of Dy and with 10 ppm of Er as the internal standard) and differing amounts of the four impurities (Fe, Ca, Ce, U) in the range of 50 to 250 ppm. All four of the impurities suppressed rare earth fluorescent line intensities, as shown in the graphs in the top portion of Figure 8. The seven Er lines were not equally sensitive to these impurity suppression effects. The analytical line pairs selected on the basis of their effectiveness in compensating for impurity suppression effects and the concentration range found applicable for these line pairs ale listed in Table 11. The curves in the bottom part of Figure 8 show the response of these line pairs to changing impurity concentrations. Although every effort was made to select the best possible internal standard line, the observed intensity ratios were in several instances sensitive to impurities at concentrations of only 100 ppm, as shown in Figure 8. Since a precision of 1 1 5 % was considered adequate, the tolerance limits at that level of confidence are indicated in Table I11 for the four impurities studied. However, none of ANALYTICAL CHEMISTRY, VOL. 42, NO. 3, MARCH 1970
327
Element
Detection limit in ThOz, ppm
Pr
0.08
Pr
0.04
Sm
0.05
Eu
0.02
Gd
0.04
DY
0.10
DY
0.1
Analyticai line pair, A Pr 4597 Er 5195 Pr 5028 Er 5195 Sm 5707 Er 5465 Eu 5904 Er 5195 Gd 3132 Er 5592 Dy 5766 Er 5406 Dy 5838 Er 5592
Concentration of Gd or Sin in ThOp lpprn)
0.1
Table II. Analytical Data
1.0
05
50
I00
,
J00
Concentration range in Thoz, ppm 0.08-4.0 0.08-4.0 0.12-6.0 0.04-2.0 0.1-1 .o 0.2-10
Figure 9. Analytical curves
0.2-10
Table 111. Tolerance Limits for Impurities Rare earth determined
Fe
Maximum concentration of impurity tolerable," ppm Ca Ce
Pr >250 >250 Sm >250 100 Eu >250 100 Gd >250 100 >250 DY >250 a For an accuracy of f15 %.
>250 250 >250 100 >250
Table V. Accuracy of Analytical Results U
>250 250 250 100 >250
Table IV. R-values at Various Impurity Levels for Analytical Line Pairs"
Impurity Fe
ca Ce
Impurity concentration in Tho2, ppm 250 100 250 100 250
U
aR
=
R-values for =k 15% Accuracy Line Pairs Pr/Er Sm/Er Eu/Er Gd/Er Dy/Er 1.9
1.9 2.7
1.9 2.7
1.9 2.7
3.7
1.9 3.7
1.6 2.9
2.9
2.9
100 250 1.9 1.9 1.9 Intensity of Er 5465 in pure ThOz Intensity of Er 5465 in impure ThOz'
2.9 1.3 1.9
the impurities introduced intensity ratio variations greater than 30% at impurity concentrations up to 250 ppm. The G d determination was found to be the least precise because the analysis line is situated on the edge of the thorium band fluorescence, which renders the net intensity measurement less accurate. For this reason the effect of added impurities on the Gd-Er line pair may appear to be greater than for the other rare earth elements. When actual samples are analyzed, the analyst is concerned with the cumulative effect of all the impurities acting together. The possibility of there being 100 ppm or more of total impurities in highly purified thorium matrices is remote, especially if fractional ppm amounts of rare earth elements are sought. Fortunately, the presence of appreciable concentrations of any combination of impurities can be ascertained from the reduction in the intensity of either the T h o z band 328
ANALYTICAL CHEMISTRY, VOL. 42, NO. 3, MARCH 1970
Element Pr Sm Eu Gd DY a N.D.
=
Concentration in original sample, ppm
Added rare earth, Total concentration, ppm PPm Calcd Found
0.15 N.D."
0.96
N.D.
0.48 0.24 2.4
0.18 N.D. not detected.
1.44
1.11 1.44 0.48
0.42 2.4
0.90
1.35 0.41 0.45 2.4
or any one of the seven Er lines. The strongest Er line at
5465 A has been used for this purpose because its intensity is more reproducible than the T h o z band emission, and because a measurable intensity remains at a total impurity concentration of several thousand ppm. The ratio of the intensity of the Er 5465 line in a matrix of highly purified T h o z to the intensity of the same line in typical samples thus provides an indication of the overall purity of the sample. This ratio, or R-value, has been computed for the standard samples listed in Table 111. The R-values at which a rare earth determination accuracy of f15 % can be assured are listed in Table IV for the four impurity elements studied. Assuming that the four impurity elements are representative of all impurities, determinations of Pr, Sm, Eu, Gd, and Dy may be subject to an experimental error greater than f l 5 % if the R-values are greater than 1.3. Reactor-grade thorium materials would not be expected to contain impurity concentrations which would significantly affect the intensity ratios of analytical line pairs, and an impure sample could be diluted with pure Th to reduce the effect of impurities and still maintain useful sensitivity. Without internal standardization, it would be impossible to provide quantitative estimates of rare earth concentrations in the presence of other impurities, since it is clear in Figure 8 that the net intensities of the rare earth fluorescence lines are sensitive to impurity effects. The analytical curves obtained under the experimental conditions described in this paper are shown in Figure 9. The
and ranged from 10 to 15%, with the G d determination the least precise. The accuracy of the method could only be assessed by the method of known additions, the data for which are listed in Table V.
G d analytical curve has been corrected for a residual G d content of 0.1 ppm, as determined by the graphical extrapolation method (27). Precision and Accuracy. The coefficient of variation for the line pairs listed in Table I1 was calculated for the repetitive preparation of a standard sample on five different days,
RECEIVED for review September 22, 1969. Accepted December 11, 1969. Work performed in the Ames Laboratory of the u. s. Atomic Energy Commission, Iowa State University, Ames, Iowa.
(27) T. E. Beukelman and S. S. Lord, Jr., Appl. Spectres., 14, 12 (1960).
Bipolar Pulse Technique for Fast Conductance Measurements D. E. Johnson and C . G . Enke Chemistry Department, Michigan State University, East Lansing, Mich. 48823
The applications and limitations of several ac bridge techniques and models are analyzed, especially with respect to polarization, the parallel (C,) and series (c,) cell capacitances, and the frequency required. I n the case of using a phase-angle voltmeter as a null detector, the ideal frequency is shown to be proporA new technique, which is fast tional to (cpcs)-1'2. (40 psec), accurate (O.Ol%), wide ranged (100 Q-IMQ), and independent of C, and C,, has been developed to overcome some of the limitations of ac methods. This technique consists of applying consecutive constant voltage pulses of equal magnitude but opposite polarity to a standard conductance cell. The current/voltage ratio is measured at the end of the second pulse. Theoretical and actual errors are discussed. The rate of ethanolysis of acetyl chloride is studied and compared to results by others. An EDTA titration of Zn2+ in a highly buffered medium is followed conductometrically and an end-point change of approximately 40 pmhos out of a total conductance of 40,000 Nmhos is recorded.
Figure 1. AC conductance bridge Cell consists of C,,R,, and C,
RECENTLY, there has been wide interest in systems where conventional conductance techniques cannot be used, or can be used only with great difficulty. These systems include those with very high resistance (such as much non-aqueous work), with very low resistance (such as work in molten salts), or where platinized electrodes cannot be used (if surface adsorption is a problem). Furthermore, traditional bridge techniques are not readily applicable to systems which require continuous or instantaneous conductance measurements (such as in following reactions, titrations, flow systems, or ion exchange). The conductance techniques which are used today are quite limited in applications since each was developed for a particular model of the conductance cell. The conductance measurement is valid only as long as the model truly represents the cell. The most appropriate model, in turn, depends on the experimental conditions used, especially the cell design, resistance range, and frequency. The purpose of this paper is to show the limits of applicability of some of the models and methods which are commonly used for the measurement of conductance, and then to demonstrate a completely new technique which is not only less restricted in application than the traditional methods, but is also extremely fast (40 psec/ measurement) and accurate (0.01 %).
was not considered, and C, was used only to obtain a null (not as a correction factor). This method is still the method most commonly used, as it works quite well on aqueous solutions whose resistance is between 200 $2 and 10 kn when platinized electrodes are used. Jones and Josephs (2) extended this technique by using a modified Wagner ground to make meaningful measurements up to 60 k$2. They demonstrated that for bridge balance, the reactance in any arm must be balanced in another arm, and that resistive balance occurs only if the phase angle between the current and voltage is the same in two adjacent pairs of arms of the bridge. They concluded that the Kohlrausch method was the best approximation to resistive balance. They discounted the method of Taylor and Acree (3) who used a n inductor in series with the cell to compensate for C,. Adjusting C, and R, in Figure 1 until a null is obtained ideally causes an error of (RzCzw)-2,where w is the angular frequency ( 2 ~ f ) . The equations for balance also require:
or where
AC MODELS AND METHODS
In 1893, Kohlrausch ( I ) developed the ac bridge technique shown in Figure 1. In his model, Cp (parallel cell capacitance)
(1) F. Kohlrausch, Wied. Ann., 69,249 (1893).
CsRs/Rz"Cp Kz
=
+ Cz/Kz2
(2)
RzCfl.
Thus, knowing C, and w , it is possible to calculate the error term and compensate for it to obtain more accurate results.
(2) G . Jones and R. Josephs, J . Amer. Chem. SOC.,50,1049 (1928)~ (3) W. Taylor and S. Acree, ibid., 38,2403 (1916). ANALYTICAL CHEMISTRY, VOL. 42, NO. 3, MARCH 1970
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