Alizarin Red S concn.,
x 105~ 0.41 0.24 5.8 1.03 0.60 4.5 4.12 2.38 3.6 8.24 5.95 3.2 20,60 11.90 2.9 0.3M HCI04,scan speed 0.1 volt rnin-l. M
a
Table 11. Precision Study Zirconium concn., Relative standard deviationb M X Free dye wave Complex wave 10.7 5.8 3.3 3.0 2.5
Seven determinations.
conium determination. It should be noted that the acid effect becomes more important at low concentrations of free dye. The precision of the method for various levels of zirconium and Alizarin Red S concentrations is shown in Table 11. The large relative standard deviation noted in the results at the lowest zirconium concentration is mainly due to the fact that, at the level of current sensitivity required, the complex wave begins to merge with the following water decomposition wave. This makes exact measurements difficult. Therefore, at very low zirconium concentrations it is advantageous to use derivative voltammetric techniques.
The following metals were tested at the 200-ppm level and found not to interfere in the determination of 5 ppm of zirconium : Ag(I), Al(III), Be(II), Bi(III), Ca(II), Cd(II), Co(II), Cu(II), Hg(II), Li(I), Mg(II), Mn(II), Mo(VI), Ni(II), Pb(II), Pr(III), Sb(V), Sn(IV), Th(IV), Tl(I), U(VI), W(VI), and Zn(I1). Ce(1V) and Cr(V1) interfere, but reduction with hydroxylamine perchlorate prior to addition of the Alizarin Red S reagent eliminates their interference. Iron (111) reduces the height of the free dye wave, but does not interfere with the Zr-ARS complex wave. Vanadium(V) at the 10-ppm level shows no interference, but at the 50-ppm level causes the zirconium values to be low by about 10%. Hafnium reacts in an identical manner to zirconium. At the 500-ppm level the following anions had no effect on the zirconium determination: C1-, CH3COO-, NO3-, and Sodp2. Both fluoride and phosphate cause serious negative errors, but since fluoride would normally be removed by the perchloric acid fuming step in the procedure, phosphate is the only serious interference. Oxygen does not affect the anodic electrode reactions, so deaeration is unnecessary. RECEIVED for review September 14, 1966. Accepted December 23, 1966. Research sponsored by the U. S. Atomic Energy Commission under contract with the Union Carbide Corp.
X-Ray Spectrographic Determination of Rare Earths in SiIica-AIumina CataIysts Irving C. Stone, Jr., and Kenneth A. Rayburn W. R . Grace and Co., Washington Research Center, Clarksuille, Md. 21029 THE DETERMINATION of rare earths by classical chemical analysis requires from six to 24 hours to complete. The most common method ( I ) involves an oxalate precipitation and calcination to the oxide to determine the total rare earth content. The accuracy of the method depends on the aging time of the oxalate precipitate and is sensitive to small pH changes. X-ray spectrometry has been used to determine yttrium in rare earth solutions (2), yttrium, thorium, and the rare earths in ore fractions (3),and trace rare earths in high purity rare earths (4). We have developed an x-ray spectrographic method which requires less than 2 hours for a single determination of the five most abundant rare earths present in silica-alumina catalysts. The data yield individual rare earth contents which may be summed for a total rare earth value.
(1) R. C. Vickery, "Analytical Chemistry of the Rare Earths," p. 60, Pergamon Press, New York, 1961. (2) R. H. Heidel and V. A. Fassel, ANAL.CHEM., 30, 176 (1958). ( 3 ) F. W. Lytle, J. I. Botsford, and H. Heller, Bur. Mines Report Incest., 5378 (1957). (4) F. W. Lytle and H. H. Heady, ANAL. CHEM., 31,809 (1959).
356
ANALYTICAL CHEMISTRY
EXPERIMENTAL
Standards. Rare earth stock solutions are prepared from the calcined oxides at concentrations of 0.03 gram/ml for CeOz, Laz03, and Nd203, and 0.01 gram/ml for PrGOliand Sm203. One hundred milliliters of 10% HCl are added to 24.00 grams of appropriate silica-alumina base material in a 400-ml beaker. Varying amounts of the stock solutions are then added followed by 100 ml of hot, saturated oxalic acid solution. The solution is evaporated, without boiling, to dryness. Constant stirring is necessary to obtain thorough mixing. The dry powder is then heated for 1 hr at 200" C, slowly raised to 1000" C, and held for 1 hr. Recovery should be 98 % or better. Samples. Samples are ground to pass 325 mesh and dried for 10 min at 200" C. Pellet Preparation. Pellets are prepared of standards and samples by mixing Elvanol (Du Pont) 71-17, a polyvinyl alcohol (PVA), at a samplelbinder ratio of 2/1. The PVA was obtained from E. I. Du Pont, Niagara Falls, N. Y . Samples are mixed in a Spex mixer mill before being pelletized at 40,000 psi in a 1'/,-inch diameter mold. About 0.5 gram of PVA is added to the bottom of the die before addition of the sample for increased pellet strength. Four 6-gram pellets of each standard and two pellets of each sample were prepared. Instrument Conditions. A General Electric XRD-6 vacuum spectrometer equipped with gas flow proportional counter, W-Cr dual target x-ray tube, LiF analyzing crystal, 0.030inch beam slit, and 0.005-inch receiving slit was used. The
pulse height selector a a s set to eliminate background noise only. Analytical lines chosen were the L-a lines for Ce and La, and the L-13 lines of Nd, Pr, and Sm. The W target was operated at 60 kv, 60 ma (full wave rectification) with a vacuum of less than 10-1 torr. Sufficient counts were accumulated in 100 sec under these conditions for statistical rare accuracy. One set of Four pellets of about 4 weight earth oxide (REO) is used to check the peak positions and counting rate variations at least once a day. Chemical Analysis. One gram of sample or standard is dissolved in HF and fumed with HC104. The sample is diluted with distilled water and excess saturated oxalic acid solution added. The pH is adjusted to 1.7 and aged overnight before filtering and washing. The rare earth oxalate precipitate is then dried and fired to the oxide at 1000" C before being weighed ( 1 ) .
Table I. Analysis of Standards, Wt. % Rare Earth Oxides Theoretical Chemical 1.12 1.93 3.08 3.93 4.03 5.19 6.72 7.03
z
Table 11. Analysis of Samples, Wt. Sample Chemical 55 IO 4B 9B 5D 7B 51
RESULTS
Standards. Table I compares the theoretical amount of total REO added and I he chemical analysis on a number of the standards. The deviations between the two values fall within the two sigma (2u) standard deviation of the chemical method (Table 111). The count rate data were used for a least squares analysis on an IBM 360/30 computer. Linear calibration curves were obtained of uncorrected counting rates cs. milligrams of rare earth oxide per gram 01' sample. Samples. Table I1 compares the chemical and x-ray methods for a number of samples. The x-ray results are obtained by summation of the individual REO'S. Table 111 compares the replicate analysis data of two different samples for precision and accuracy. Instrumental precision was evaluated from a set of four standard pellets counted about 100 times. A relative standard deviation of 0.7 % was obtained. Sample preparation precision was evaluated from data similar to Table I11 for a standard preparation, A relative sta idard deviation of 0.5 % was found. DISCUSSION
Standards. The chemical analysis of the standards indicates that the prepsration method is valid. The deviations between the two values fall within the 20 standard deviation of the chemical method. Interferences. Matri Y effects were investigated by preparing standards with the addition of about 3 % (with relation to the total rare earth cxide content) gadolinium and yttrium oxides. These had no effect on the x-ray determinations of the other rare earths. The chemical method, on the other hand, did not recover these two oxides along with the other rare earths. The same results were obtained when 3 and 20 (relative) erbium and ytterbium oxides, respectively, were added. Evidently the proper conditions are not present for the formation of these oxalates. Fe, Ni, and V were also tested for possible absorption effects. No interference was found for either method up to about 2% addition. The data used for the least squares analysis were also used for a regression analysis. The results showed that the data fit the calculated curves well.
1.11,l.lO 1.91, 1.97 3.01, 3.07 4.03, 4.02 4.09, 4.09 5.30, 5.31 6.74, 6.72 7.02, 6.99
Rare Earth Oxides X-Ray
0.24 1.60 2.68 2.98 2.99 3.62 5.65
0.20 1.51 2.66 3.09 3.09 3.62 5.44
Table 111. Precision and Accuracy Determination, Wt. Rare Earth Oxides Sample 9B Sample 5D Run No. Chemical X-Ray Chemical X-Ray I L
3 4
Average Std dev Re1 std dev, Mean error Re1 error,
2.96 2.98 2.95 3.02 2.98 0.04 1.4
...
3.09 3.05 3.05 3.15 3.09 0.04 1.3 1 0 .l i $3.7
2.96 3.04 3.01 3.00 2.99 0.06 2.0 ...
z
3.13 3.07 3.15 3.04 3.09 0.05 1.6 +O. 10 f3.3
Samples. The precision of the two methods indicates that the x-ray method offers some improvement. The oxalate precipitate, however, must be aged overnight to attain the precision noted in Table 111. Experiments conducted using shorter aging times resulted in larger standard deviations as the aging time was reduced. The x-ray method is considered superior to the chemical method in time required for analysis, precision obtained, and additional data obtained for relative concentrations of individual rare earths. ACKNOWLEDGMENT
We thank G. H. Germuth for the chemical analyses and G. R. Klinedinst and M. J. Randall for assistance in preparation of standards and data recording. E. M. Glocker and R. E. Raver assisted in the interpretation of the computer calculations. RECEIVEDSeptember 21, 1966. Accepted December 27, 1966. Division of Analytical Chemistry, 152nd National Meeting, ACS, New York, N. Y . ,September 1966.
VOL. 39, NO. 3, MARCH 1967
357
