Table IV. Analytical Methods Used Element
Method
La, Gd, Yb, Y Complexometric titration with EDTA at pH 5.5; xylenol orange indicator. Th Complexometric titration with EDTA at pH 2.5; xylenol orange indicator. AI Complexometrically with DCyTA, backtitration with ZnSOa at pH 5.5; xylenol orange indicator. Complexometric titration with EDTA in excess Ca ammonia; methylthymol blue indicator. Complexometrically with EDTA at pH 10; Mg calmagite indicator. Reduction with stannous tin and titration with Fe(II1) dichromate; barium diphenylamine indicator. Ti(IV) Differential spectrophotometry as hydrogen peroxide complex.
DISCUSSION
Cation exchange distribution coefficients of thorium and zirconium in hydrochloric acid show a temperature gradient which is considerably larger than those normally encountered in the exchange of hydrated cations, and which also is considerably larger than that of lanthanum, the most strongly adsorbed of the rare earth elements. As a result the separation between thorium and lanthanum, which a t 20 "C is not satisfactory in 4 M HC1 for 1-millimole amounts o n a 30-ml column of AG50W-X8 resin of 200- to 400-mesh particle size (Figure 2) is improved very substantially at 50 "C (Figure 1).
The separation factor for the Th-La pair increases from 4.12 at 20 "C t o 6.17 at 60 "C in 4 M H C l (Table 11). The peak for lanthanum is sharp and shows little tailing at 50 "C, while a t 20 "C using 3M HCl(I4), which also provides a satisfactory separation, much more tailing is encountered and larger elution volumes are required for quantitative recoveries. Recoveries of lanthanum and thorium from analysis of synthetic mixtures are excellent (Table 111). As little as 20 pg of lanthanum can be separated from 15 mg of thorium on a 15-ml resin column and determined by X-ray fluorescence. Less than 1 pg of thorium was found with the lanthanum fraction in three cases and 3 pg in one case. Since thorium interferes in the determination of some other rare earths by X-ray fluorescence, this is of importance. When a 10-ml resin column was used, between 10 and 30 pg of thorium were found with the lanthanum, the thorium apparently starting t o leak through a t a sub-ppm level. G d , Y ,Yt, Ti(IV), Fe(III), AI, Ca, and Mg are separated quantitatively together with lanthanum from thorium. Other elements were not investigated, but from known distribution coefficients at room temperature ( 1 9 , it seems reasonable to assume that all elements except hafnium and zirconium and those which do form insoluble precipitates in the eluting agent either as insoluble chlorides or by hydrolysis should be separated from thorium by the described procedure. RECEIVED for review December 28, 1971. Accepted May 12, 1972. (14) F. W. E. Strelow, ANAL,CHEM.,31, 1201 (1959). (15) Ibid., 32, 1185 (1960).
Determination of Lanthanum in Cobalt-Base Alloys by X-Ray Fluorescence Spectrometry F. J. Haftka' Union Carbide European Research Associates, Brussels, Belgium
COBALT-BASE ALLOYS containing about 20 % chromium, 20 nickel, and sometimes also about 10% tungsten as principal elements are well known as materials which are very corrosion resistant even a t high temperature. In order t o improve still further the properties of such alloys ( I ) by the addition of rare earths, a n analytical method had to be developed for the determination of lanthanum in concentrations ranging from 0.01 t o 0.2 %. Although lanthanum cannot be considered as a favorable element for X-ray fluorescence spectrometry under normal conditions, this latter seemed to be more attractive than wetchemistry methods with the well known problems in rareearth analysis. In the absence of reliable standards, the analytical problem has been attacked using several available samples which had been analyzed by wet-chemistry methods. Present address, Schweizerische Aluminum AG Forschungsinstitut, CH 8212, Neuhausen am Rheinfall/Switzerland. (1) C. D. Desforges, H. Hatwell. P. L. Moentack, W. De Sutter, N. Terao, unpublished data, 1972. 1900
The first measurements seemed to indicate that some of the chemical values could not be correct. In order to have an independent confirmation of this, the alloys had been tested by emission spectrometry with intyrupted arc excitation and the 2-m ARL-spectrograph (3.4 A/mm) using the 3337.49 lanthanum line. This method confirmed the first measurements. Later the reason for the discrepancy between the spectrochemical results and those obtained by wet-chemistry was shown to be due to the chemical interference of cerium present in some of the samples.
A
EXPERIMENTAL
Spectrometric Conditions. Preliminary tcsts showed that the solution method could not be considered because of the low concentration of lanthanum. This method would facilitate the calibration but would introduce matrix problems due to the difficult acid treatment of the very rcsistive alloys. The work has therefore been dirccted to cxcitation in the solid state, where only an exceptional small surface with a diameter of 15 mm was available for irradiation.
ANALYTICAL CHEMISTRY, VOL. 44, NO. 11, SEPTEMBER 1972
Tube
W 52/20 W 52/20 W 52/20 W 50120 W 50120 Cr 50/20 Cr 50120
w5om
w 50'20
Table I. Line Intensity, Background Intensity, Line to Background Ratio, and Background Gradient under Different Spectrometric Conditions BackBackground Line/ Collimator, Atmoground, gradient LaK,, background c/sec/grad Crystal Counter Volt. sphere cjsec cjsec wn 0.24 LI F: scintil. 1020 I20 850 160 air 240 air scintil. 15 270 0.45 850 160 120 topaz 40 1170 0.23 850 480 air 270 topaz sci nt il. LaL, Li F 1650 27 0.89 2 G FC 160 air 24 Li F L 1650 48 1.87 90 C FC 160 vac. 25 Li F air G FC 160 1650 0.68 17 1, 5 1.29 I.iF GFC 1650 51 ... 66 160 vac. 1 1650 15 5.6 Li F vac. + discr. G FC 160 84 3 LiF G FC I650 3.9 vac. discr. 335 480 85
+
Since there was a n irradiable sample area smaller than the irradiated :ire;i in the focal plane ( 2 ) , a diaphragm must be ~ i s c dso that the position of the sample can be reproduced. A very pure tungsten (or copper) sheet of 0.1 mm thickness (supplied by Metallwerke Plansee, Reutte, Austria) and with ii circular opening of 1 1 nim in diameter has been introduced into the sample holder (Philips PW 1527,'20) exactly in the place of the Mylar film. Tungsten has been chosen because 01' the low b:ickgrounil produccd by this heavy element. Since it is also used us anti-cathode material and is one of the constituents of the samples, there is no additional danger 01' line interference. Spectroscopically, lanthnnum can be determined either by its most intense K- or by its L-radiation. The corresponding excitation voltages are about 60 and 10 kV. Excitation tubes, crystals, counters, atmosphcres. collimators, and discriminator settings have to be chosen adequately or investigated evperimentally in order to get the best line-to-background ratio and :I sufficient counting rate. Table I shows the results for intensity, line-to-background ratio, and background gradient. The adjustment of the goniometer ~ ' a thc \ sariic Ibr :ill lincs. With K,-radiation, the tungsten tube gives better results than the chromium tube, with scintillation counter and normal air atmosphere in both cases. It follows also from Table I that the tungsten tube gives better results for the ewitation of the L,, line, so that the chromiuni tube need not be ionsidercd ftlrthcr. As regards the influence of the crystal. LiF, the most ~isedcrystal for normal work in X-ray fluorescence spectrometry, is suitable both for the K, and thc L, lines. Despite the higher intensity of the K line, there arc three points in favor of the L, line: better line-tobackground ratio. lower background gradient, and lower possibility of disturbance by other rare earths. In view of thc cstreme 20-positions of the K, and L, lines for LiF, other crystals have been tested. For K,-radiation, the topaz crystal gave half the intensity but double the lineto-background ratio and a smaller gradient. Because of its better resolving power. the topaz crystal is recommended if the K, line is to be measured in the presence of cerium. For L,-radiation the graphite crystal (3) is highly recommended under the condition that a good discriminator be available, which can eliminate the second ordcr WL,31line overlapping due to the insuflicient angular dispersion of the graphite crystal in this wavelength region. Considering a11 the results of Table I, it was decided to use the La L,,-line for normal analytical work and the La K, line for the special studies mentioned later. The spectro..
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.
~
.-
.
( 2 ) I-. J. f-l;ifrka. I'rocecding of Philips Colloquium on X-Ray Spcctroiiictr>..13russels. 1961. ( 3 ) F. J . Haftka. Proceedings of the XIVtli Colloquium Spectro-
scopium International. Heidelberg. 1971.
Table 11. Spectrometric Conditions for Lanthanum Analysis General: Philips X-ray fluorescence generator PW 1010 Philips X-ray fluorescence vacuum spectrometer PW 1540 Philips X-ray fluorescence W-tube 50 K V / 2 0 rnA PW 1559 Philips X-ray fluorescence Cr-tube 50 KV/20 rnA PW 2148101 Philips goniometer PW 1050 PW 1548 Philips gas density compensator Philips collimator changer 160 prn and 480 p m PW 1543 PW 1527120 Philips samplc holder (aluminum with tungsten diaphragm and cular opcning of I 1 min) S peci a I for La line for K, line crystal topaz Li F counter scintillation 850 V gas flow prop. counter 1650 V L1 RLE discriminator ... amplification 400 basic voltage 33 V window 20 V
metric conditions provided for the two cases are given in Table 11. Standard and Sample Preparation. Since spectrometric methods depend on reliable standards, which were not available in this case, it was decided to prepare these by the following way. Three samples showing nearly the same composition of the principal elements (Co 60z, Cr 2 0 z , Ni 2 0 x ) and some other elements in minor concentration (Fe, M n 0.1-1 %) have been chosen with diferent levels of concentration of lanthanum (roughly estimated) for transfer into the oxidized or salt state. This can be done either directly by a heat treatment (1000 "C) in oxygen or a n acid treatment followed by a heat treatment at lower temperature (400 "C) to a n anhydrous salt mixture. A sufficiently larger sample containing no La should be manipulated in the same way. This material serves as a blank and also as base material for the preparation of two La-doped samples. The three metallic samples will be calibrated with the aid of these latter samples via the oxidized material. It is essential for the complete oxidation of the sample to prepare very fine powder-like particles. This could be done using a special milling tool (Pferd-Ruggeberg Type X0610/4) on a cobalt-tungsten--carbide base with a cross-cut structure. The speed of the milling tool should be high ( e . g . , 25000 rpm). Every high-speed grinder motor (200-500 watt) is suitable for this. Care must bc taken to protect the fine powder (
