V O L U M E 20, NO, 1 1 , N O V E M B E R 1 9 4 8 (8) Kitson, R. E., and Mellon, M ,G., IND.ENG.CHEM.,ANAL.ED., 16,379 (1944). (9) Koenig, R. A., and Johnson, C. R., Ibid., 14,155 (1942). (10) Mehlig, J. P., Ibid., 7, 27, 387 (1935); 9, 162 (1937). (11) Misson, G., Chem. Ztg., 32, 633 (1908). (12) Murray, W.M.,Jr., and Ashley, S. E. Q., IND.ENG.CHEM., ANAL.ED.,10,1 (1938). (13) Musakin, A. P., 2. anal. Chem., 105,351 (1936). (14) Schroder, R . , Stahl u. Eisen, 38, 316 (1918).
1073 (15) Snell, F. D., and Snell, C. T., “Colorimetric Methods of Analysis,” Vol. I , p. 485, New York, D. Van Nostrand Co., 1936. (16) Willard, H. H., and Center, E. J., IND.ENG.CHEM.,AXAL.ED., 13,816 (1941). (17) Yoe, J. H., and Hill, W.L., J . Am. Chem. Soc., 50,745 (1928). RECEIVEDMay 8, 1948. Presented before the Division of Analytical and Microchemistry at the 113th Meeting of the AXERICAKCHmrIcAL SOCIETY Chicago, Ill.
Determination of Rare Earth Elements Yttrium in Uranium Compounds E L G. SHORT
AND
W. L. DUTTON, Imperial Chemical Industries, Ltd., Widnes, England
A method has been developed for the determination of rare earth elements together with yttrium in uranium and its compounds. The greater portion of the uranium is separated from the rare earths by taking advantage of the solubility of uranium nitrate in ether. The rare earths are then precipitated as fluorides and subsequently purified as hydroxides. The final determination is carried out spectrographically. On 50 grams of sample, 0.2 part per lo6of gadolinium can be detected by this method and for some of the other rare earths the sensitivity is even higher. Gadolinium can be detected with greater sensitivity by increasing the quantity of sample used.
N
0 CHEIIIC.iL method of determining rare earth elements
and yttLium in uranium compounds which in any way approaches the required limit of sensitivity, has been published and it was decided that the spectrograph offered the only practicable solution to the problem. For the quantitative determination of minimum quantities of any element, an ideal spectrographic technique requires that the solution of the element to be examined be quite free from foreign elements, which affect the evcitation of the element under investigation in an unpredictable manner. These requirements immediately raiied a chemical problem of some difficulty-viz., the separation of microgram amounts of rare earths from 20 or more grams of uranium and their subsequent isolation as chlorides in a solution containing as few other elements as possible. The investigation of the chemical qeparation and of the spectrographic technique proceeded to a certain evtent simultaneously as described below. SPECTROGR4PHIC DETERMIN4TION OF R 4 R E EARTH ELEMENTS
The spectrographic work vias carried out with a Hilger Automatic Littrow Model spectrograph furnished with quartz and glass trains, The quqrtz train was used with the wave-length setting 6000 to 2975 A., in preference to the greater dispersion given by changing over to the glass train, a t the higher wave lengths, since the former enabled the characteristic lines of all the rare earths to be photographed with only one setting. Spectroscopically pure samples of all the rare earth oxides except thulium were obtained from Adam Hilger, Ltd., and standard solutions were prepared to contain 10 micrograms of element per ml. Suitable amounts of these solutions were evaporated off in depressions drilled in Hilger H.S. copper rods 7 mm. in diameter and their spectra photographed using a 5-ampere direct current arc, the arc gap being 4 mm. The arc was fed from a 220-volt line, sufficient resistance being introduced to limit the current to the above amount; the voltage drop across the arc was 45 t o 50 volts. The optical arrangements used are described in detail below. In general, the minimum amount detectable was found to be approsimately 20 micrograms for each element. By the use of copper electrodes 5 mm. in diameter and the same exposure, the sensitivity was increased to 5 micrograms, but further decrease in electrode size or increase in current strength gave no improvement on this figure, owing to melting of the electrodes and heavy increase in background. A standard plate was then prepared for
each rare earth element in steps of 10 up to 50 micrograms. Above this latter amount the intensity of the characteristic lines was too great for quantitative work. The amount of rare earth element in an actual test XTas determined by direct visual comparison of the intensities of the characteristic lines a i t h these standard plates. The lines used for the identification and estimation of each element are detailed in Table I, together with “coincident” lines which are limited to strong lines of other elements. Wave lengths are quoted from the N.I.T. tables ( 2 ) . The effect of foreign elements in the solution to be evaporated on the electrode had been noted in previous work. In general, foreign elements diminish considerably the intensity of the rare earth lines, but the quantitative effect varies from element t o element. Of cations likely to be present, 1 mg. of aluminum was found to affect the rare earth results very little, whereas a similar amount of thorium was much more objectionable. Of the acid radicals, fluoride and sulfate were particularly undesirable. The tests of Table I1 were made by addition of other elements to standard rare earth solution, followed by evaporation on the electrode and arcing, etc. The results shoa clearly the depressing effect of extraneous ions. Calcium had a depressing effect and was also objectionable, because two of its main lines almost coincided with the two gadolinium lines usually used for analysis. EXTRACTlON OF RARE EARTH COMPLEXES WITH ORGANIC SOLVENTS
The technique of solvent extraction of metallo-organic complexes has given such favorable results in many cases that this seemed the most desirable method of chemical separation, if a suitable reagent could be found. h large number of organic compounds were tried, but although in certain cases the rare earths were extracted from pure solutions, the addition of the reagents necessary to keep the large amount of uranium present in the aqueous layer prevented the extraction of the rare earths themselves. The following reagents were thoroughly investigated: cupferron a t various pH values, sodium diethyldithiocarbamate, 0-benzoin oxime, sebacic and other organic acids, 0-naphthol, and a-nitroso @-naphthol.
ANALYTICAL CHEMISTRY
10?4 Table I. Element Gadolinium Samarium Europium Praseodymium
Lines Used for Identification
Wave Lengths Used 3345.99 3350.10 3362.24 3568.26 3592.59 3634.27 4594.02 4627.12 4661.88 (3908.03 3908.43)
+
4100.75 4179.42 (4189.52 4191.62)
+
Xeodymium
3951.15 (4012.25 4012.70)
+
4- 4156.27)
(4156.08
4303.57
Holmium
3398.98 3453.13 3891.02
Erbium
3372.75 (3498.71 4- 3409.10) 3692.65
Terbium
3509.17 3561.74 (3702.85 3703.92) 3874.18 3131.26
+
Thulium
(3425.08 4- 3423.63) 3462.20 (3761.33 Dyeprosium
+ 3761.92)
3407, SO 3531.71 3645.42 4000.45 4211.72
Ytwium
3600.73 3710.29 3788.70 4374.94
Ytterbium Lutecium
(3289.37 f 3289.85) 3694.20 3987.99 3077.60 3397.07 3472.48 3560.80 3801.53 4012.39 (4133.8 4137.65)
Cerium
+
(4186.6 f 4187.32)
Lanthanum
3792.77 3949.11 4086.71
Coincident Lines Ca 3344.51 Ca 3350.21 Ca 3361.92 Lu 3567.84 Pd 3634.70
Mo 3635.14 Co 4594.63
Eu Cr La Yt Cr Gd Fe
So 3907.48 Co 3909.93 Nb 4100.92
3907.11 3908.76 4099.54 4102.38 4179.26 4190.15 4191.44
Eu 4011.68 Eu 4012.82 Co 4013.95 Mo 4155.58 Fe 4157.79
+
C u 4179.51 Gd 4190.79
Fe 3907.94
I n 4101.77
+ Gd
F e 3951.17
4191.08
Mo 4011.97
F e 4013.80
Ce 4012.39 Fe 4013.82
Zr 4156.24
F e 4156.80
Zr 4302.89 Pr (4303.14 f 4303.59) Fe 3399.34 Co 3453.51 La 3453.17 Zr 3890.32 Sm 3891.18 F e 3891.93 Tb 3372.72 Pd 3373.00 R h 3498.73 Ru 3498.94 S m 3692.22 V 3692.23 Ho 3692.65 T b 3692.95 Pr 3693.36 Ru 3509.72 Co 3509.84 Ce 3560.80 Co 3560.89 Co 3702.24 V 3703.58 Co 3874.00 Dy 3874.00 E u 3130.74 Nb 3130.79 Cd 3133.17 E u 3425.02 Dy 3425.06 Co 3461.18 Xi 3461.65 Co 3462.80 Sm 3760.05 Fe 3760.05 Fe 3763.79 Fe 3407.46 Gd 3407.60 Er 3531.71 h l n 3531.85 Cu 3645.23 So 3645.31 La 3645.41 P r 4000.19 Cr 4001.44 F e 4210.35 Ag 4210.94 -4g 4212.68 Pd 4212.95 Zr 3601.19 F e 3709.25 Sm 3710.87 Fe 3787.88 Dy 3788.45 F e 3790,lO Nb 3790.15 Gd 4373.84 Sc 4373.46 hln 4374.95 Sm 4374.98 R h 3289.14 Pt 3290.22 Sm 3694.00 Fe 3694.01 Gd 3987.22 Er 3987.95 Fe 3078.02 Eu 3396.68 Rh 3396.85 T m 3397.50 R h 3472.25 S i 3472.55 Co 3560.89 T b 3561.74 Sn 3801.00 Cr 4012.47 Nd 4012,25 Fe 4132.06 Li 4132.29 Fe 4134.68 Re 4136.45 Gd 4184.26 Lu 4184.25 Fe 4187.04 D g 4186.81 Fe 4187.80 .\lo 4188.32 Gd 4190.45 Fe 3795.00 Tm3795.77 F e 3948.78 P r 3949.44 F e 3949.96 Gd 4086.65 Co 4086.31 Co 4092.39 Gd 4090.42
.ittempts to use a similar technique in the extraction of uranium, leaving the rare earths in the aqueous layer, were also a failure, as reagents that removed the uranium also removed the rare earths.
T m 3453.66 Zr 3891.38
Co 3373.23 P r 3499.09 R h 3692.36 Co 3693.11 Ni 3510.34 Dy 3563.15 Co 3704.06
M o 3132.69
Ho 3425.35 R h 3462.04 Pr 3761,87
Pt 3408.13 T b 3645.45 Cr 4211.3.5 T b 3711.74 P r 3788.93 M n 3790.22 R h 4374.80 Fe 4375.93 F e 3290.99 F e 3695.05 La 3988.52 Bi
3397 21
Dy 3563.15
Fe Re Nb Fe Tm Pr
4013.82 4133.42 4137.10 4184.90 4187.62 4189.52
Gd 3796.39
200 micrograms, in the presence of large amounts of uranyl nitrate. From an analytical point of view the solubility of the rare earth oxalates is comparatively high and it is unlikely that this procedure can ever by developed to deal with amounts of rare earths of the order 10 to 20 micrograms. A l o m r limit of 100 to 200 micrograms would necessitate the use of 500-gram samples per test. This amount was thought to be a disadvantage and to be adopted only if absolutely unavoidable. Precipitation as Hydroxide with Ammonia. The insolubility of the rare earth hydroxides was found to be very marked, as 50-microgram amounti: could be quantitatively recovered from 400 ml. of aqueous solution. Uranium is also precipitated by ammonia, but in the presence of salicylic acid, a deep red colored soluble salicylo complex is produced when the solution is made alkaline with ammonia. The amount of salicylic acid required to prevent precipitation is high, 5 grams of uranium octoxide requiring 50 grams of acid. The presence of this large amount of ammonum salicylate increased the solubility of the rare earth hydroxides to such an extent that recovery of microgram amounts was no longer possible. The ammonia precipitation in the presence of salicylic acid, although useless for the primarv separation of rare earths, later proved to be of value for their purification from small amounts of iron, aluminum, calcium, and uranium. Precipitation as Fluoride. During the precipitation of microgram amounts of rare earth as fluoride, the same difficulty was encountered as in the ammonia precipitation-via., that amounts which could be quantitatively recovered from pure aqueous solutions were not recoverable in presence of 10 to 20 g r a m of uranium. The recovery was, however, better than in the case of ammonia -e.g., by direct precipitation with hydrofluoric acid from 150 ml. of solution containing 20 gram< of uranyl nitrate and 50 micrograms each of gadolinium, samarium, and europium, the following recoveries were obtained (micrograms) : gadolinium 20, samarium 20, europium 25. Some preliminary separation of the bulk of the uranium appeared to be necessary. This was accomplished by extraction of an ether solution of nitrates with small amounts of water, whereby the rare earths passed completely into the aqueous layer, togethar with a little uranium.
E u 3949.59
Twenty grams of pure UBOSwere converted into solution containing as little free acid as possible. Ten microgram; each of gadolinium, samarium, and europium were added, the solution was evaporated to dryness on a steam bath, and the solids were dissolved in 80 ml. of ether. This solution was twice shaken in a separating funnel with sufficient water to give an aqueous layer of about 1 ml. and the combined aqueous layers were run into the original beaker. This solution was precipitated with hydrofluoric acid as described below and the precipitate was examined for rare earths.
Pd 4087.34
nitrate . . .... ..
DIRECT PRECIPITATION OF RARE EARTHS
Attention was then turned to various reagents which would precipitate the rare earths directly from aqueous solution. Precipitation a s Oxalate. The classical method, precipitation as oxalate, proved inadequate for amounts of rare earths less than
Recovery was practically quantitative and similar results were obtained when the experiment was repeated on larger quantities of rare earths, showing the ether extraction procedure to be sound in principle.
1075
V O L U M E 2 0 , N O . 11, N O V E M B E R 1 9 4 8 Table 11.
Effect of Foreign.Elements Found ( + 1 Mg. Al)
Y
Y
Nil Trace
Gd 20 S m 20 E u 20
10 5-10 10 15 15 13-20 Found ( + 2 t o 3 1Ig. HF)
Found (2 to 3 X g . H S S O I ) Si1 Si1 Si1
Gd 50 Sm 50 Eu 50
Precipitation of Hare Earths with Ammonia and Salicylic Acid
Table 111.
Added
Found ( + 1 Mg. T h )
(Volume 10 ml.) Found, $0.1 Gram of Salicylic
Y
5 5
10-15
Added
7
Y
