Separation of Thorium from Lanthanum and Other Elements by Cation Exchange Chromatography at Elevated Temperatures F. W. E. Strelow and A. J. Gricius National Chemical Research Laboratory, Pretoria, Republic of South Africa CATION EXCHANGE EQUILIBRIUM constants and therefore also distribution coefficients are temperature dependent (1-4). While elevated temperatures have been used reasonably often to obtain improved separations through improved kinetics as in separations of the rare earth elements (5-3, only a few deliberate attempts seem to have been made to manipulate the temperature with the aim of obtaining larger separation factors. Powell and Burkholder (8) have shown that the separation factors for the Gd-Eu and Eu-Sm pair in cation exchange using EDTA solutions could be increased from 1.1 to 1.4 and from 1.4 to 1.8, respectively, by increasing the temperature from 25 to 92 " C ; and Koprda and Fojtic (9) have investigated the effect of temperature on the separation of Cr(II1) and Mn(I1) by anion exchange of the chloride complexes. In both cases, use is made of the effect of temperature on the complex equilibria involved. No deliberate attempt seems to have been made to exploit temperature effects in the exchange of simple hydrated cations with the aim of obtaining larger separation factors. The reason for this probably is the fact that the temperature effects observed in thermodynamic ion exchange studies have been relatively small (2-4,
16.1
ml eluate Figure 1. Elution curve La-Th with 4.OMHCl at 50 "C Thirty milliliters of AGSOW-XS, 200 to 400 mesh, resin (9.5 X 2.0 cm). Flow rate 3.0 f 0.3 mlimin. One millimole of each element (8.8
10-12).
Attempting to improve the kinetics of the separation between thorium and lanthanum by working at elevated temperatures, we were able to observe an unusually large temperature effect on the thorium peak indicating not only improved kinetics, but a substantial increase in the distribution coefficient. A study of the influence of temperature on the cation exchange distribution coefficients of thorium, lanthanum, and zirconium therefore was undertaken, and a method which could separate lanthanum from thorium more effectively on large and especially on small columns was developed.
ml eluate Figure 2. Elution curve La-Th with 4.OM HCl at 22 "C Thirty milliliters of AGSOW-XS, 200 to 400 mesh, resin (9.5 X 2.0 cm). mow rate 3.0 + 0.3 ml/min. One millimole of each element
EXPERIMENTAL Reagents and Apparatus. The resin used was the AGSOWX8 sulfonated polystyrene cation exchanger supplied by Bio-Rad laboratories of Richmond, Calif. Resin of 200to 400-mesh particle size was used for column and of 100to ZOO-mesh for batch experiments. Borosilicate glass tubes of 1.5-cm diameter and 30-cm length, or 2.0-cm diameter (1) G. Dickel and A . Meyer,Z. Elekrrochem., 57,901 (1953). (2) 0.D . Bonner and L. L. Smith,J. Phys. Chem., 61,1614 (1957). (3) 0.D. Bonner andR. R. Pruett, ihid.,63,1417 (1959). (4) K. A . Kraus and R . J. Raridon, ibid.,63, 1901 (1959). (5) B. H. Ketelle and G . E. Boyd, J . Amer. Chem. Soc., 69, 2800 (1947). (6) E. C. Freiling and L. R. Bunney, ibid.,76, 1021 (1954). (7) G. R. Choppin and R. J. Silva, J. Znorg. Nucl. Chem., 3, 153 (1956). (8) J. E. Powell and H. R. Burkholder, J. Chromntogr., 29, 210 (1967). (9) V. Koprda and M. Fojtic, Chem. Zcesri, 22,333 (1968). (10) 0. D. Bonner and R. R. Pruett, J. Phys. Chem., 63, 1420 (1959). (1 1) 0. D. Bonner and J. R. Overton, ibid.,65,1599 (1961). (12) 0. D. Bonner, G. Dickel, and H. Briimmer, 2. Phys. Chem. (Fraukfurtam Main), 25,81 (1960). 1898
and 35-cm length, fitted with a fused-in sinterplate of No. 2 porosity and a stopcock at the bottom and a B14 or B19 ground-glass joint, respectively, at the top were used as columns. The columns were surrounded by warm water jackets of about 3- and 4-cm outer diameter, respectively. A Philips sequential automatic 1220C X-ray spectrometer with a gold target X-ray tube and a Li F220 diffraction crystal was used for the determination of small amounts of lanthanum and thorium. The chemicals used were of A.R. quality. Distribution Coefficients. Two and a half grams of dry AG50W-X8 resin were equilibrated with 1.25 millimoles of thorium, or 1.67 millimoles of lanthanum, or 2.50 millimoles of zirconium in 250 ml of aqueous HC1 of the desired concentration by shaking for 24 hours in a G F L controlled temperature shaking bath at the desired temperature. The resin was separated by filtration without delay, and the amounts of the elements in both resin and solution were determined by suitable analytical methods. From the analytical results, equilibrium distribution coefficients
D =
amount of element in resin amount of element in solution
were calculated.
ANALYTICAL CHEMISTRY, VOL. 44, NO. 11, SEPTEMBER 1972
ml solution gram dry resin
Table I. Concn HCL, M
0.5 1 .o
2.0 3.0 4.0 5.0
20 "C 1460 230 49.7 21.3 12.3 9.5
Distribution Coefficients with AG50W-X8 Resin in HCI at Various Temperatures La Th Zr 40 "C 60 "C 20 "C 40 "C 60 "C 20 "C 40 "C > 1oj 36000 71000 > 105 1800 18400 1610 3850 10600 2590 3760 290 1760 252 295 509 282 409 61 196 54 83 150 45.9 104 26.3 79 23.5 18.1 10.8 92 68 14.9 51 13.3 7.5 4.7 53 73 10.9 42.6 10.0
60 "C
> 105 25200 833 133 30.9 11.9
Table 11. Separation Factors with AG50W-X8 Resin in HCI at 20" and at 60 "C Th 01 La
Concn HCI, M
20 "C 12.6 7.63 3.93 3.71 4.12 4.50
0.5 1 .o
2.0 3.0 4.0 5.0
ml eluate Figure 3. Elution and breakthrough curves for La-Th with 4.OMHCI at various temperatures Ten milliliters of AGSOW-X8,200 to 400 mesh, resin (5.4 X 1.5 cm). Flow rates from 1.2 to 2.0 i= 0.3 ml/min. One milligram of La and 15 mg of Th
The coefficients presented in Table I show an increase with temperature, which for thorium and zirconium is substantially larger than for lanthanum. The separation factor Th aLn which is shown in Table I1 therefore also shows an increase. Elution Curves. Figure 1 shows an elution curve for the La-Th pair (1 millimole of each) using a column of 30 ml (10 grams) of AG50W-X8 resin of 200- t o 400-mesh particle size and 4 M HC1 as eluting agent. The resin column was 9.5 cm in length and 2.0 cm in diameter and the flow rate was 0.3 ml per minute. Water of 50 "C maintained at 3.0 was passed through the jacket of the column. For comparison, Figure 2 shows a n elution curve for the same elements at room temperature (about 25 "C) using the same column, eluting agent, and flow rate. Figure 3 shows break-through curves for 15.0 mg of thorium when only 1.00 milligram of lanthanum was present. The column contained only 10 ml (3.3 gram) of AG50W-X8 resin of 200- to 400-mesh particle size and the resin column was 5.4 cm in length and 1.5 cm in diameter. Elution was carried out with 4 M HC1 using a flow rate of 1.2 i 0.3 ml per minute at 18 "C and 2.0 i 0.3 ml per minute at 40, 50, and 60 "C. Separation of Synthetic Mixtures. Large amounts of standard solutions of thorium and one other element in 0.1M HCl were measured out, mixed, and passed through a column containing 30 ml of AG50W-X8 resin as described under elution curves. The elements were washed onto the column with 0.1M HCl, and lanthanum and the other elements then were eluted with 350 ml of 4.OM HC1 at a flow rate of 3.0 + 0.5 ml per minute while water of 50 "C was passed through the jacket of the column. After excess of acid had been removed by evaporation, the amounts of the elements in the eluate were determined by suitable methods. The resin then was ashed and thorium determined complexometrically
*
60 "C 40.1 12.9 6.71 5.68
6.17 6.77
Table 111. Results of Quantitative Separationsa Taken Found Th, mg Other element, mg Th, mg Other element, mg 138.1 i 0 . 1 231.3 i 0 . 2 138.1 231.4 La 13.82 + 0.03 13.81 231.4 i 0.2 231.4 La 23.16 i 0.04 276.3 A 0.3 276.2 23.14 La 156.0 i 0 . 2 231.3 i. 0 . 3 156.1 231.4 Gd 172.4 A 0.2 231.3 i 0.2 172.4 231.4 Yb 88.53 i 0.11 231.4 Y 88.58 231.5 I!= 0.2 46.78 f 0.06 46.81 231.4 A 0 . 2 231.4 Ti(IV) 55.59 f 0.07 55.56 231.4 f 0.3 231.4 Fe(II1) 27.13 i 0.04 27.12 231.3 i 0 . 2 231.4 A1 40.30 f 0.05 40.28 231.3 i 0.2 231.4 Ca 24.52 i 0.04 24.53 231.5 =t0 . 2 231.4 Mg Th, mg La, pg Not determined 19.9 Not determined 20.3 15.0 Not determined 50.3 15.0 50.0 Not determined 50.9 15.0 50.0 a Results in the first part of the table are means of triplicate determinations. Th, mg 15.0
La,pg 20.0 20.0
as has been described before (13). The results are presented in the first part of Table I11 and the analytical methods are summed up in Table IV. Trace Amounts of Lanthanum. The columns contained 15 ml (5 grams) of resin and were 8.0 cm in length and 1.5 cm in diameter. Lanthanum was eluted with 300 ml of 4.OM HCl at 60 "C and a flow rate of 2.0 + 0.3 ml per minute. The amount of lanthanum in the eluate was coprecipitated with 250 kg of ferric iron and with the aid of a filter funnel filtered onto a filter paper disk of 3.0-cm diameter, forming a round and even precipitate spot of 1.7-cm diameter. After drying the paper, the amounts of lanthanum were determined by X-ray fluorescence using a Philips sequential automatic 1220 G X-ray spectrometer with a gold target X-ray tube, a Li F220 diffraction crystal, and the La-line. The sensitivity of this method was about 0.1 pg of lanthanum. The results are presented in the second part of Table 111. (13) F. W. E. Strelow, ANAL.CHEM., 33,1648 (1961).
ANALYTICAL CHEMISTRY, VOL. 44, NO. 11, SEPTEMBER 1972
0
1899
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 on 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. Gd, 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
