X-Ray Fluorescence Ion Exchange Method for Determination of Lanthanum, Cerium, and Praseodymium in Carbon Steels A. T.Kashuba and C. R. Hines Jones and Laughlin Steel Corp., Graham Research Laboratory, 900 Agnew Road, Pittsburgh, Pa. 15230 A method has been devised for simultaneously determining the concentrations of La, Ce, and Pr in carbon steels. The samples are dissolved and their solutions sent through small amounts of ion exchange resin. The dried resin is pelletized and the L, X-ray fluorescence intensities of the rare earths are obtained. The intensities are compared to a curve obtained from previously prepared elemental standards. Precision of the standard curves i s 8-14 rg, and detection limits are 0.002-0.0040/,, based on a 2-gram steel sample.
THEUSE of rare earths in steel has been primarily for inclusion shape control. The rare earth sulfides tend to form spherical or elliptical particles. They lessen the amount of manganese sulfides, which form long, brittle stringers in the rolling direction. This leads to directional variations in strength ( I ) . With the lower concentrations of sulfur found in modern steels, smaller rare earth additions are needed to achieve this control of sulfide inclusion shape. This, in turn, calls for methods for the determination of the rare earths with more precision and lower detection limits. In the past, most determinations of rare earths in steels have been limited t o Ce. Lev and Koutun (2) used a double oxalate precipitation followed by ignition to CeO?. This technique is limited by both the ccprecipitation of many of the minor constituents of steel and a high limit of detection. The incomplete precipitation of cerium has also been noted in several instances. For determinations of Ce a t the concentration range of interest, this leads to values of lower accuracy. Several methods involving the spectrophotometric determination of the Ce 8-hydroxyquinoline complex have also been attempted. The incomplete separation of Ce from Fe, Mn, and V species has caused interferences at the analytical wavelength. Westwood and Mayer (3) have used the precipitation of manganese ferrocyanide from alkaline media t o aid in this separation. Vanadium still interferes and cannot be removed in this manner. Goffart ( 4 ) has titrated Ce(II1) with permanqanate in neutral pyrophosphate and detected the appearance of excess M n O c reagent spectrophotometrically. The Mn(I1)-pyrophosphate complex interferes with the Mn04- absorption, hence absorbance L'S. titrant added must be plotted t o obtain the end point. The presence of any oxidizable material interferes. Working in the Ce concentration range of interest calls for the use of small volumes of dilute reagent, which unnecessarily degrades precision. Direct emission spectrographic and X-ray fluorescent techniques are limited by lack of suitable standards. Spectrochemical methods using the above mentioned separation schemes are limited by the difficulty of quantitatively recovering the fine grained cerium precipitates. The recent (1) L. Luyckx, J. R . Bell, A. McLean, ancl M. Korchynsky, Met. Tram., 1, 3341 (1970). (2) I. E. Lev and M. S. Koutun, Zuwdsk. Lub., 28, 273 (1962). (3) W. Westwood and A. Mayer, Auulyst, 73, 275 (1948). (4) G. Goffart. Anal. Chirn. Actu 2, 140 (1948).
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review of Eremin and Martyshova (5) supplies a much fuller list of recently developed methods for the determination of Ce. They cover analyses of a variety of substrates, including steels. Green (6) has successfully separated Ce from Mn, V, and Fe by using an ion exchange resin and a solution medium of methanol, acetic acid, water, and nitric acid. He eluted the Ce as the aqueous chloride and then proceeded t o the 8hydroxyquinoline method. In preparation for studying the method of Green, it was considered helpful to determine the position of the elements along the column before the elution step. Collin (7) has previously used ion exchange resin as a matrix for the determination of Sr impurities in Ca solutions. The method involves the use of a small volume column, and the recovery of the Sr impregnated resin after passing the Ca solution through it. The resin is washed with acetone and air dried. It is then mixed with boric acid and pressed into a pellet. The Sr X-ray fluorescent intensity of the pellet is taken, and related to the intensity of similarly prepared standards. For the study of the retention of the rare earths, it was decided to dispense with the washing and drying steps t o preserve the ordering of the rare earths on the column. Details of the experiment may be found below. EXPERIMENTAL
Reagents. The analytical grade reagents used were concentrated nitric acid, absolute methanol, glacial acetic acid, and a 1 :1 solution of Hz02. The anion exchange resin used was Bio-Rad 1-X10, 50-100 mesh, chloride form, and the matrix material was Whatman fibrous cellulose powder, CF 11. The wash solution was a solution of 1 :3 nitric acid. The conditioning solution consisted of 1 part wash solution, 5 parts glacial acetic acid, a n a 12 parts methanol. Apparatus. The X-ray fluorescence spectrometer used was of the type described in Table I. The column used was a 12-mm 0.d. borosilicate glass tube of sufficient length to accommodate the resin volume, fitted at the top with a large volume reservoir (250 ml), and fitted at the bottom with a 5-mm 0.d. delivery tube. Resin Preparation. Place a glass wool plug at the bottom of the column, add 1.5 grams of 1-X10 resin in a water slurry. Top with a glass wool plug. When the reservoir is drained of water, add 100 ml of wash solution. When this drains, add 100 ml of conditioning solution. Sample Preparation. The directions are based o n a 2-gram sample of steel. Adjust the proportions so that the final solution volume approximates 450 ml/gram of sample. To a 2-gram sample of millings, add 15 ml of water. Add 12.5 ml of concentrated nitric acid in small portions, waiting for subsidence of reaction between additions. After the last ( 5 ) Yu. G. Eremin and T. I. Martyshova, Zuoodsk. Lab., 36, 769 (1970). (6) H. Green, Metullurgia, 76, 223 (1967). (7) R. L, Collin. ANAL.CHEM., 33, 605 (1961).
ANALYTICAL CHEMISTRY, VOL. 43, NO. 13, NOVEMBER 1971
Table I. X-Ray Spectrographic Conditions Siemens Sequential X-Ray Spectrometer Instrument SRS-1
Target Voltage Current Crystal Detector Slit Analytical lines 28
Counting time
Cr 40 KV 40 mA Li F Flow proportional counter
0.4" La 82.85" 0 . 4 min
Ce La,,, 78.96"
Pr La,,,
15.42"
addition, bring the solution to a boil and allow complete reaction; then cool. If additions of rare earth solutions are desired, add now. Add 5 drops of 1 :1 HIOz to reduce any cerium present to Ce(III), and to aid in the solution of any residues. With water, bring the solution volume to 30 ml in a graduated cylinder, then pour into a 1-liter flask. Using 600 ml of methanol, wash the solution beaker and graduate and add the washings to the flask. Add 255 ml of glacial acetic acid. Mix and add to reservoir of column when the conditioning solution has drained from it. The flow rate should be no more than 15 ml/min. Flow rate variations and channeling of solution have no measurable effect on rare earth retention. Do not let the liquid level drop below the level of the resin bed at any time. X-Ray Fluorescence Measurements. When the steel solution has passed through the column, drain completely. Transfer the resin and plugs to a porcelain crucible and dry at 120 "C for 1-2 hours, or until the odor of acetic acid disappears. Remove the glass wool, taking care to leave all resin beads behind. Add 1.5 grams of cellulose powder, mix well and press at 60,000 psi into a 1-inch diameter pellet. For this work, the L,1,2 lines were used and their first order 2 0 angles using a LiF analyzing crystal are: La, 82.87'; Ce, 78.98"; Pr, 75.42". RESULTS AND DISCUSSION
For the initial study of the position of the individual elements along the column, a 25-ml (approximate wet volume) column was made by stacking 9 separate 1.5-gram charges of resin, each separated by a glass wool plug. A solution of 2 grams of rare earth-free steel (NBS 15 g) was doped with a volume of solution containing approximately 0.2 mg of the follcwing rare earths: La, Ce, Pr, Nd, Sm, Eu, Gd, Er, Yb, and Lu. The rare earth solution was prepared by dissolving the oxides in dilute boiling nitric acid. HzO, was added as an aid in dissolving CeOt.. The steel solution as passed through the column corresponded to a 0.1% total rare earth steel, with 0.01 % of each element. A separate pellet was made from each charge of resin, and each pellet was scanned for X-ray fluorescence intensity from 2 f3 = 46' to 2 0 = 84" at a rate of 2'/min. The slit was set at 0.4' for maximum sensitivity. Visual inspection of the scans showed evidence of La, Ce, and Pr being retained on the topmost column segment, while Sm and G d appeared in the three topmost segments. The upper sections showed no detectable Er, Yb, or Lu. N d and Eu appeared on the shoulders of the very intense Cr K, and Mn K,, respectively. No work could be done with them at this level. Yb appeared on the shoulder cf the less intense Ni K, line, but appeared in most of the later segments, as did Er and Lu. Therefore, it was decided to limit the determinations to La, Ce, and Pr. Standard curves were made for these three elements. Separate solutions of Ce(NH4)2(N03)6. 6 H 2 0 , La203, and
2
0.1 0.2 0.3 0 . 4 0.5 0.6 0.7
0
mg La
Figure 1. X-Ray fluorescent intensity of La us. mg of La added Pr203 were made with dilute nitric acid. Each contained 1.00 m g h l of rare earth ion. Columns were made with two separate 1.5-gram portions of resin, separated as above. To each 2-gram sample of rare earth-free steel, all three rare earth solutions were added to effect proper doping at the various levels tested. Figure 1 shows the result of integrating the La L,,,, lines (unresolved at the collimator setting used) for 0.4 minute. The upper curve is a plot of intensities observed (the average of at least two integrations) cs. milligrams of La added. The error bars for this and subsequent figures represent i l CJ. The curve is a least-squares linear fit of the average intensity observed for each addition. The lower area enclosed by dotted lines is the range (i1 u) of the average blank intensity, obtained from the lower 1.5-gram resin charge in each case. There is little evidence for La breakthrough from the upper 1.5-gram charge, since all blank intensities fall approximately within the = t l CJ limit. The average deviation cf the points from the curve is 8 pg, which is of the same order as the expected error of addition of La to the steel solution. Detection limit, as calculated from the average blank intensity value plus 3 c is 34 p g , or less than 0.002% (wt) of a 2-gram sample. Figure 2 is a similar presentation of the data for the case of /
10,
98INTENSITY, ARBITRARY UNITS
-
3
0
1
1
1
0.1 0.2 0.3
"
I
l
0.4 0 . 5 rng C e
'
l I 0.6 0.7 0.8 0.9
0
Figure 2. X-ray fluorescent intensity of Ce L'S. mg of Ce added
ANALYTICAL CHEMISTRY, VOL. 43, NO. 13, NOVEMBER 1971
1759
-*
-
I-
/’
[
7 1
INTENSITY’, ARBITRARY UNITS
Table 11. La and Ce Determinations of NBS 361 Sample weight, g Ce, % La, Z 2.50 0.0032 0.0012 6.00 0.002s 0.0008 Av 0.0028 0.0010 Std dev 0,0005 O.OOO5
T
-
6
I/
5O
0.1 0.2 0.3 0 . 4
0.5 0.6
rng Pr
Figure 3. X-Ray fluorescent intensity of Pr cs. mg Pr added Ce additions to rare earth-free steel. Again, the top curve is a least-squares fit of integrated intensity 6s. Ce addition. The bottom area represents the blank variation. While there may seem to be more evidence for Ce appearing in the second resin charge, most intensities appear in the It1 r range. All are well within 1 2 r , weakening any arguments against quantitative Ce retention. Calculations performed as above show the average deviation from the curve to be 14 pg and the detection limit to be 76 pg, or less than 0.004z of a 2-gram sample. Figure 3 presents the observations pertaining to Pr. In this region of the spectrum, background intensity is beginning t o rise because of the nearness of the intense Cr K, line. The variations in this background level begin to affect the precision of the measurement of the weak Pr La line. However, no indication of Pr breakthrough to the second resin charge is evident. Calculations similar t o the above show average deviation from the curve to be 14 pg, and the detection limit t o be 68 pg, or about 0.003 of a 2-gram sample. Testing the accuracy of the method is made difficult by the lack of steel standards whose rare earth concentrations are agreed upon. Recently, the National Bureau of Standards has issued a new series of cerium-containing steels. One of them, NBS 361, bears a note for informational purposes only that indicates a cerium concentration of 0.001 3-0.005 %. La and Ce were determined in this sample using two different sample weights, with the final solution volumes adjusted appropriately. The analysis results appear in Table 11. Table I11 contains the results of La and Ce determinations o n differing sample weights of a rare earth-containing steel. Total amount of steel dissolved in the solution remained constant, but the rare earth contents were varied by using different proportions of rare earth-bearing and rare earthfree steels. The standard deviations of the average La and Ce concentrations are similar, and the relative standard deviations (RSD’s) are identical, but do not ccmpare favorably with the average deviations obtained from the curves described above. Of the many explanations traditionally given for low analytical precision, the most common has been inhomogeneity of the trace element. I n the system under study, this explanation has merit. The rare earth metals are being added to a n oxygen-poor steel in order to preferentially form sulfide inclusions. Under these conditions, La and Ce should behave quite similarly in a chemical sense, and yet not be homogeneously distributed. Since both are determined
z
1760
e
Table 111. La and Ce Determinations in a Rare Earth-Bearing Steel RE-Tree Sample steel weight, addition, Ce, La/Ce grams grams La, Z 5 2.00 0.0 0.013 0.019 0.68 2.00 0.0 0.013 0.017 0.76 1.00 1 .00 0.018 0.026 0.69 0.50 1.50 0.012 0.018 0.67 AV 0.014 0.020 0.70 Std dev 0.003 0.004 0.04 Re1 std dev 20 20 7; 6% rare earth in sample, excluding effects of added rare earthfree steel additions.
z
Table IV. Duplicate Determinations of La and Ce in Rare Earth-Bearing Steels La,
Sample I1 14
17 19 20
26
a
Detn 2 0.003
0.001 0.001
0.001
0.017 0.020 0,025
0.039 0,023 33 0,034 34 0,005 I-gram sample. 20 30
z
Detn 1 0,004
0.005 0.014 0.016 0.026 0.042 0.020 0.034
