Radiochemical Determination of Neodymium, Praseodymium, And Cerium in Fission Products HENRY G. PETROW Tracerlab, Inc., Boston TO, Mass.
A
S PART of this laboratory's research program, analytical
procedures applicable to the radiochemical determination of rare earths formed in the fission of natural uranium were investigated. This note is concerned with a procedure for the determination of praseodymium (Pr143), cerium (Ce144), and neodymium (Kd147). Cerium, which has a stable quadrivalent state, can be determined by application of the procedure of Boldridge and Hume (2). Neodymium and praseodymium, however, are more difficult to separate. They have been successfully separated by workers on the Manhattan Project ( 6 , 6 , 8 , 1 0 , 1 1 )and subsequent investigators (S, 4, 7 , Q), through the application of synthetic cation exchange resins. These procedures, however, are not easily adaptable to the routine determination of neodymium and praseodymium, and a simplification of the analytical scheme was felt to be desirable. It mas decided initially to limit the number of variables as much as possible. Colloidal Dowex 50, batch L2111-42, in the ammonium form, was used throughout. The column length and diameter were chosen as 35 em. and 11 mm., respectively. About 15 mg. each of neodymium and praseodymium were added as carriers. (The carrier solutions were standardized as the oxalate.) A flow rate of 0.5 to 0.8 ml. per minute was used in all cases. In order to study the behavior of promethium, tracer quantities of promethium-147 were added. To facilitate further the taking of data, tracer quantities of 11.0-day neodymium-147 and 13.6-day praseodymium-143 were added. Automatic plots of elution curves were taken with a liquid counter in conjunction with a pen and ink recorder. Several organic acids were tested as eluting agents, including citric, lactic, malic, and tartaric acids. The optimum separation for the fixed conditions earlier described was obtained with 4.25% lactic acid, p H 3.30 f 0.02. The elution pattern obtained is shown in Figure 1.
through the bed until 320 ml. in all have been collected. Examine each subsequent 10-ml. fraction for neodymium by heating the fraction to boiling, adding oxalic acid, and cooling the solution in an ice bath. As soon as neodymium breakthrough is detected, collect the next 80 ml. and set aside. This fraction contains about 80% of the neodymium. Discard the next 20 ml. of eluate and collect a 90-ml. praseodymium fraction. In order to recover the rare earths from the elutriant, heat the solution to boiling, add 10 ml. of saturated oxalic acid, and again bring the solution t o a boil. Chill the solution in an ice bath. Filter the precipitate onto a weighed filter disk and wash well with water, alcohol, and ether. Dry to constant weight by removing any remaining ether under vacuum. Mount the precipitate and count. To prepare the resin for subsequent analyses, pass 100 ml. of 5% citric acid, pH 7 , through the bed. Wash the resin well with water, and store for further use. DISCUSSION OF RESULTS
In Table I are listed yields and counting rates obtained from a triplicate analysis performed on a sample of uranium irradiated for 7 days in the Oak Ridge pile. Also listed are the half lives found for praseodymium-1.13 and neodymium-147. In both cases, samples with counting rates of about 10,000 counts per minute were decayed through 8 half lives. Because praseodymium-143 is a pure beta emitter whereas neodymium-147 is a beta and gamma emitter, the praseodymium samples were examined for gamma activity. A sample of praseodymium with a counting rate of 200,000 counts per minute counted 2 counts per minute through 400 mg. per sq. em. of aluminum absorber. -4sample of neodymium with a counting rate of 100,000 counts per minute counted 250 counts per minute through the same absorber. This would indicate a maximum
Table I.
PROCEDURE
Before performing the analysis, age the sample for 9 days to allow the 33-hour cerium-143 t o decay in to its daughter, the 13.6-day praseodymium-143. Dissolve the uranium in nitric acid, add about 10 mg. of neodymium carrier, and dilute the solution to an appropriate volume in a volumetric flask. To an aliquot of the solution, add 15 mg. each of neodymium, praseodymium, and cerium carrier as well as 30 mg. of zirconium. Separate out the pure rare group using the fluoride technique of Ballou (1). Dissolve the hydroxide precipitate obtained in 8 ml. of concentrated nitric acid and remove the cerium as ceric iodate according to the method of Boldridge and Hume (2). This same procedure can be further applied t o the separated cerium to give a pure product in excellent yield, Pour the supernatant from the ceric iodate precipitation into a centrifuge tube and cautiously neutralize the solution with concentrated ammonia t o precipitate the neodymium and praseodymium iodates. Centrifuge and discard the supernatant. Dissolve the precipitate in 2 to 3 ml. of concentrated nitric acid, 1 drop of concentrated hydrochloric acid, and 5,drops of 30% hydrogen peroxide. Boil the solution to volatilize any iodine formed and to destroy the excess peroxide. Transfer the solution to a small beaker and evaporate t o a few tenths of a milliliter. Dilute the solution to 5 ml. with water. Prepare a 35-em. column of colloidal Dowex 50, 11 mm. in diameter, using 4.25% lactic acid, pH 3.30, to slurry the resin. Carefully transfer the 5 ml. of rare earth solution to the resin column and adsorb a t a flow rate not exceeding 0.8 ml. per minute. When the liquid has drained, add 2 ml. of the lactate solution, being careful not to disturb the resin bed. When these 2 ml. have drained, fill the column with the lactate elutriant, attach a reservoir of the solution, and continue t o pass the elutriant
5
Irradiated Uranium Analyses Half Life, Days 11.1 11.0 11.0 13.6 13.6 13.7
Count Rate5 Yield, 0 '7 Nd-1 5,498 83.1 Nd-2 79.1 5,559 Nd-3 80.3 5,491 Pr-1 76.3 13,896 Pr-2 73.3 13,703 77.3 Pr-3 13,918 71.3 Ce-1 1,683 73.6 Ce-2 1.689 Ce-3 1,716 75.2 Corrected for yield and self-absorption.
5 0 0 0 7 -
4000'
P 3 30001 I I
3k ; ' z 2000-
I
1
I
1
I
1 -
i/,
I
PM
1
2
-
- -.N3
/ I pa
I
0
u
I
\I
\
1000 I
I MILLILITERS OF E L U T R I A N T PASSED
Figure 1.
Elution of Promethium, Neodymium, and Praseodymium from Dowex 50 With 4.25% lactic acid, pH 3.30
1514
1515
V O L U M E 26, NO. 9, S E P T E M B E R 1 9 5 4 neodymium impurity in the praseodymium of 0.4%. Since the very low gamma counting rate of the praseodymium sample could easily be bremsstrahlung rays produced by the interaction of beta particles and the aluminum absorber, there is reason to believe that the purity of the praseodymium fraction exceeds 99.6%. I n order to check the purity of the neodymium fraction, a sample of inactive neodymium and active praseodymium containing 100,000 counts per minute of activity were analyzed by the lactate technique. The final sample had a counting rate of 800 counts per minute and an 80% yield indicating about 1% praseodymium contamination. While this error does not exceed the error involved in counting, recycling would give neodymium 99.9% pure. ACKNOWLEDGMENT
The author wishes to express his thanks to Rose Tirrell, Walter Small, and John Goresh for their contributions. Special thanks are due to J.-4. Marinsky for his valuable advice and instruction.
LITERATURE CITED
(1) Ballou, N. E., Natl. Nuclear Energy Ser., Div. I V , Vol. 9, Paper 292 119.51). ~~. .-,. (2) Boldridge, W. F., and Hume, D. N., Ibid., Paper 294 (1961). (3) Fitch, F. T., and Russell, D. S., ANAL.CHEM.,23, 1469 (1951). (4) Fitch, F. T., and Russell, D. S., Can. J. Chem., 29, 363 (1951). (5) Harris, D. H., and Tompkins, E. R., J . Am. Chem. Soc., 69, 2792 (1947). (6) Ketelle, B. H., and Boyd, G. E., Ibid.,69, 2800 (1947). (7) Spedding, F. H., and Fulmer, E. I.. Ibid., 72, 2354 (1950). (8) Spedding, F. H., Fulmer, E. I., Butler, T. A., Gladrow, E. M., Gobush, M., Porter, P. E., Powell, J. E., and Wright, J. M., Ibid.,69, 2812 (1947). (9) Spedding, F. H., Fulmer, E. I., Butler, T. .4.,and Powell, J. E., Ibid.. 72. 2349 (1950). (10) Spedding, F. H., boigt, A. F., Gladrow, E. M.,Sleight, N. R., Powell, J. E., Wright, J. M., Butler, T. A., and Figard, P., Ibid.,69,2777 (1947). (I1) Ibid., P. RECEIVED for review August 6, 1953. Aocepted June 1, 1954.
Determination of Density of Small Fragments WILLIAM PRIMAK and PAUL DAY Chemistry Division, Argonne National Laboratory, Lemont,
T
HE extension of the flotation method of Hutchison and Johnston for determining crystal densities to the determination of the densities of small fragments in the fractional milligram weight range is shown to involve an entirely different set of limitations on precision from the original method. Control of temperature and freedom from convection and vibration no longer limit the precision, but rather the viscosity of the flotation liquid, the size of the particle, the length of observation of the crystal, and the heat diffusivity of the vessel and liquid. The adhesion of foreign matter becomes of greater importance while the presence of voids becomes a lesser problem. INTRODUCTION
The method of determining the density of a crystal by determining the temperature a t which it will neither float nor sink in a liquid whose density and temperature coefficient are determined was developed into a method of high precision by Hutchison and Johnston (4, 6 ) . Using crystal fragments 3 to 4 mm. on edge (3) and organic liquids with a temperature coefficient about 10-8 gram per cc. per degree, these authors determined the flotation temperature to 0.002' C. and found that they could compare densities to about 5 X 10-6. Thus their precision was limited by their temperature control. When the method is extended to smaller particles, other factors limit the precision. The flotation vessel can be decreased in diameter t o eliminate convection difficulties. The motion of the particle in the flotation chamber is observed with the aid of a telescope fitted with a cross hair, while the chamber is set in a thermostat. Khen the temperature of the thermostat is adjusted so that the density of the liquid is close to that of the particle, the hydrostatic force is small and the particle executes viscous motion according to a generalized Stoke's law applicable to its shape. I t attains a velocity, T',
V
=
kAA/p
where A is the difference in density between solid and liquid, A is a measure of the size of the particle with the dimensions of area, p is the viscosity, and k is a constant having a value near 70 when the other quantities are in c.g.s. units. [This equation for the steady-state motion follows by equating the viscous drag to the hydrostatic force (2). If A is taken to be the square of the
111. principal dimension, k must be adjusted by a shape factor. Alternatively, an A suitable to the size and shape of the particle may be chosen. The equation as written is probably good to within a factor of 2 without these refinements and is entirely adequate for this discussion of precision where orders of magnitude are involved.] In accordance with Johnston and Hutchison's procedure, the temperature of the bath is successively altered to reverse the motion and decrease the speed of the particle-Le., to reverEe the sign of A and decrease its magnitude. Assuming perfect instantaneous temperature control, the smallest difference in density which can eventually be detected, Am, is seen to be Am = pS/ktA where 6 is the smallest reliable displacement which one can detect and t is the effective time of observation. I n practice, in the absence of stirring, unless the diameter of the flotation chamber is small and its walls are thin, the bath temperature can be altered very much more quickly than the flotation liquid responds; hence the heat diffusivity of the flotation chamber becomes a factor, and its thermal relaxation time must be subtracted from the total time of observation to obtain t. [The effect of the thermal lag of the chamber probably explains the character of the flotation curve presented by Johnston and Hutchison ( S ) . ] In practice, the determinations become tedious when t exceeds 5 X lo2 seconds (although in some of the present work, times as long as 3 X lo3 seconds were employed). . Zreasonable value for 6 is 0.1 cm. For organic liquids 1.1 may be 10-2 poise; hence for Hutchison's fragments ( 3 )A,,, is a fraction of 10-6, smaller than can be conveniently obtained by regulating the bath temperature. On the other hand, for a fragment 0.04 cm. across corresponding to a temperature de(about 40-mesh) A, is termination of 0. l ' C., and temperature regulation more precise than to within a few hundredths degree is pointless! Thus, the who gave no low precision quoted by Lewis and McDonald (a), experimental details, probably does not represent a lack of care but is intrinsic in the the hydrostatic methods. In working with particles of density beyond the range of organic liquids it is necessary to use an aqueous solution like thallous formatemalonate (9). At the density of diamond the viscosity of the solution a t about 30" C. is about 0.06 poise and a t the density of corundum about 0.44 poise, and A,,, is correspondingly incremed.
